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Synthesis and Applications of Metal Oxide Nanowires

Permanent Link: http://ufdc.ufl.edu/UFE0021953/00001

Material Information

Title: Synthesis and Applications of Metal Oxide Nanowires
Physical Description: 1 online resource (189 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: films, mbe, nanostructures, oxides, pld, sensors, sno2, zno
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The one-dimensional nanostructured materials have attracted much attention because of their superior properties from the deducing size in the nanometer range. Among them, metal oxide materials provide a wide diversity and functionality in both theoretical study and applications. This work focused on the synthesis of metal oxide nanowires, and further investigated possible applications of nanostructured metal oxide materials. High quality ZnO nanowires have been synthesized by catalyst-assisted molecular beam epitaxy. The control of initial Au or Ag film thickness and subsequent annealing conditions is shown to provide an effective method for controlling the size and density of nucleation sites for catalyst-driven growth of ZnO nanorwires. For gas sensing applications, it is found that the sensitivity for detecting hydrogen is greatly enhanced by sputter-depositing metal catalysts (Pt and Pt) on surface. The sensors are shown to detect ppm hydrogen at room temperature using < 0.4 mW of power when using multiple nanowires. A comparison study of the hydrogen-sensing characteristics of ZnO thin films with different thickness and ZnO nanowires was studied. The Pt-coated single nanowires show a current response by approximately a factor of 3 larger at room temperature. Both types of sensors are shown to be capable of the detection of ppm hydrogen at room temperature with nW power levels, but the nanowires show different recovery characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen. The feasibility of a number of metal oxide nanowires has been synthesized by a high-pressure assisted pulsed laser deposition. The high density well-aligned metal oxide nanowires can be directly grown on substrate without metal catalysts. The results suggest the possibility of growing complex metal oxide nanostructures, including tailored heterostructures and aligned heretojunction arrays with PLD technique. The growth of epitaxial SnO2 on c-sapphire using pulsed laser deposition is examined. X-ray diffraction analysis shows that the films are highly a-axis oriented SnO2 with the rutile structure. The effects of Ga doping on SnO2 films were studied. The Hall data showed p-type behavior occurs only at specific growth condition, but converted back to n-type and degraded as time proceeds.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Norton, David P.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021953:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021953/00001

Material Information

Title: Synthesis and Applications of Metal Oxide Nanowires
Physical Description: 1 online resource (189 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: films, mbe, nanostructures, oxides, pld, sensors, sno2, zno
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The one-dimensional nanostructured materials have attracted much attention because of their superior properties from the deducing size in the nanometer range. Among them, metal oxide materials provide a wide diversity and functionality in both theoretical study and applications. This work focused on the synthesis of metal oxide nanowires, and further investigated possible applications of nanostructured metal oxide materials. High quality ZnO nanowires have been synthesized by catalyst-assisted molecular beam epitaxy. The control of initial Au or Ag film thickness and subsequent annealing conditions is shown to provide an effective method for controlling the size and density of nucleation sites for catalyst-driven growth of ZnO nanorwires. For gas sensing applications, it is found that the sensitivity for detecting hydrogen is greatly enhanced by sputter-depositing metal catalysts (Pt and Pt) on surface. The sensors are shown to detect ppm hydrogen at room temperature using < 0.4 mW of power when using multiple nanowires. A comparison study of the hydrogen-sensing characteristics of ZnO thin films with different thickness and ZnO nanowires was studied. The Pt-coated single nanowires show a current response by approximately a factor of 3 larger at room temperature. Both types of sensors are shown to be capable of the detection of ppm hydrogen at room temperature with nW power levels, but the nanowires show different recovery characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen. The feasibility of a number of metal oxide nanowires has been synthesized by a high-pressure assisted pulsed laser deposition. The high density well-aligned metal oxide nanowires can be directly grown on substrate without metal catalysts. The results suggest the possibility of growing complex metal oxide nanostructures, including tailored heterostructures and aligned heretojunction arrays with PLD technique. The growth of epitaxial SnO2 on c-sapphire using pulsed laser deposition is examined. X-ray diffraction analysis shows that the films are highly a-axis oriented SnO2 with the rutile structure. The effects of Ga doping on SnO2 films were studied. The Hall data showed p-type behavior occurs only at specific growth condition, but converted back to n-type and degraded as time proceeds.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Norton, David P.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021953:00001


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209c475fa25ea6b62b525a4c250956bd371b6449







SYNTHESIS AND APPLICATIONS OF METAL OXIDE NANOWIRES


By

LI-CHIA TIEN
















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2008


































2008 Li-Chia Tien

































To my family









ACKNOWLEDGMENTS

Many people have inspired, guided, helped, and laughed with during the 5 years I spent at

theUniversity of Florida, and I would like to thank them all for a great graduate school

experience. First, I would like to thank my advisor Dr. David Norton for his guidance, both

personally and professionally. His positive attitude, enthusiasm and patience inspired me during

different research projects. Throughout my doctoral work he encouraged me to develop

independent thinking and research skills. He continually stimulated my analytical thinking and

greatly assisted me with scientific writing. It has been a great pleasure to work with him. Also I

thank Dr. Steve Pearton and Dr. Fan Ren's guidance and support during these years. I would also

like to thank the members of my committee, Dr. Cammy Abernathy and Dr. Simon Phillpot for

their valuable advice.

Special thanks go to Young-Woo for teaching me and fitting me into the lab when I first

joined the group. He is always nice, patient and a pleasure to work with. Thank to Hung-Ta and

Sam for performing measurements and help in hydrogen sensor project. Also thank Hyun-Sik's

help in PL measurements. Mat's dedication on PPMS is greatly appreciated. I thank Kerry

Siebein from MAIC for her generous help in HR-TEM. I also thank Dr. John Budai for

performing HR-XRD on tin oxide thin film samples.

Finally, I thank all the members in Dr. Norton's research group, especially those who

helped and taught me when I was here in University of Florida. I really enjoy having small talk

with Patrick, it always enlighten me on American culture. I'll remember the great time sharing

laser, chambers, targets, experience and jokes. Finally, I would like to thank my parents and my

brother for their understanding, unconditional support and dedication throughout the years. A

special congratulation goes to Tze-Ning, my nephew born just before my final defense. I also

want thank my best friend Shih-Ying's support these years.









TABLE OF CONTENTS


page

A C K N O W L E D G M E N T S ..............................................................................................................4

L IST O F T A B L E S ...................... ............... ......................................................... . 8

LIST OF FIGURES ................................. .. ..... ..... ................. .9

A B S T R A C T ............ ................... ............................................................ 14

CHAPTER

1 INTRODUCTION ............... .......................................................... 16

2 L IT E R A TU R E R E V IE W ......................................................................... ........................ 19

2 .1 Intro du action ............................................................................... 19
2 .2 M material P rop erties.............................................................................. .....................2 0
2.2.1 Properties of ZnO ...........................................................................20
2.2.2 Properties of ZnM gO ......................................................... ............... 22
2 .2 .3 P properties of Sn0 2............................................................................. ... ........22
2.2.4 Properties of V O 2 ..............................................................................23
2.3 Synthesis of One-Dimensional Nanostructures........................ ......... ..........24
2.3.1 V apor-liquid-solid m echanism ...................................... ........................ 24
2.3.2 V apor-solid m echanism ............................................................................. 26
2.3.3 Laser ablation ................................. ........................... ...........26
2 .3 .4 T herm al ev aporation .............................................................. .....................26
2.3.5 Solution-based chem istry ........................................................ ............... 27
2.4 A applications of N anow ires .................................................. .............................. 28
2.4.1 Electrical applications ......................................................... .............. 28
2.4.2 O optical applications................................................. ... .... 29
2.4.3 Chemical and biochemical sensing ..........................................................30

3 EXPERIMENTAL DETAILS AND CHARACTERIZATION...........................................37

3.1 M materials Synthesis T techniques ......................................................... .....................37
3.1.1 M olecular beam epitaxy ........................................................................ .. .... 37
3.1.2 Pulsed laser deposition............. ............................ 38
3.2 C haracterization T techniques .............................................................. .....................39
3.2.1 Scanning electron m icroscope ........................................ ....... ............... 40
3.2.2 A tom ic force m icroscopy ...................................................................... .. .... 40
3.2 .3 X -ray diffraction ........... ........................................................ ...... .... ... ..4 1
3.2.4 Transm mission electron m icroscope................................................................ 42
3.2.5 Photolum inescence .............................................. ....... ........................ 43
3.2.6 Energy-Dispersive X-ray spectroscopy .................................... ............... 44









3.2 .7 H all m easurem ent ........................... .................... ... .. ...... .. ................44
3.3 Processing and D evice Fabrication .................................................... .. .................45
3.3.1 Electron beam lithography ........................................ .......................... 45
3.3.2 Sputter deposition ............... ........................................... ... .. ... 46
3.3.3 Fabrication of multiple nanowire devices............... ......... ..................47
3.3.4 Fabrication of single nanowire devices................................. ............... 47
3.3.5 Gas sensing measurement .................... ......... ....... ........... 47

4 NUCLEATION CONTROL AND SELECTIVE GROWTH OF ZNO NANOWIRES........54

4 .1 Intro du action ................................54............................
4.2 E xperim mental M ethods........................................... .. ........................... ............... 55
4.3 R results and D discussion ............... ......... ...... ...... ............ ...... .. ...... ... 56
4.3.1 Structural and optical properties of ZnO nanorods grown on Si ............ ......56
4.3.2 Growth mechanism ......................... ............................... 57
4.3.3 Nucleation control and site-selective growth........ ....... ................ 58
4.3.4 Structural and optical properties of ZnO nanowires grown on sapphire ..........59
4.4 Sum m ary and C conclusions .................................................. .............................. 61

5 ZNO NANOWIRES FOR HYDROGEN SENSING APPLICATIONS...............................74

5 .1 Introdu action ...................................... .................................................. 74
5.2 Experim mental M methods .............................................................. ................... 76
5.2.1 Synthesis and fabrication of ZnO nanowires sensors .................. .............76
5.2.2 Synthesis and fabrication of ZnO thin films sensors ................ ................77
5.2.3 Synthesis and fabrication of single ZnO nanowire sensors ...........................77
5.2.4 Synthesis and fabrication of Sn02-ZnO nanowire sensors ............................78
5.3 Results and Discussion .............................................. .. ............ ........ 78
5.3.1 Catalyst functionalized ZnO nanowires........................................................ 78
5.3.2 Room temperature hydrogen selective sensing with ZnO nanowires ...............80
5.3.3 Single ZnO nanow ire sensors....................................... ......................... 82
5.3.4 A comparison of ZnO thin film and nanowire sensors ...............................85
5.3.5 Surface functionalized Sn02-ZnO nanowire sensors......................................87
5.4 Sum m ary and Conclusions .................................................. .............................. 89

6 CATALYST-FREE GROWTH OF METAL OXIDE NANOWIRES .............................109

6 .1 Introdu action ............. ..... .... ............ ......................................................... 109
6.2 Experim mental M ethods.................................................................... ............... 112
6.2.1 ZnO nanowires growth .............. ................. ............... 112
6.2.2 ZnMgO nanowires growth .. .. ........................ .....................113
6.2.3 SnO2 nanorods grow th ... ... ............................................... .. .............. .. 114
6.2.4 VO2 nanowires growth.... ... .............................................. .. ............... 115
6.3 R results and D discussion ........................................ ................... ........... ............... 115
6.3.1 Synthesis and characterization of vertical-aligned ZnO nanowires..............15
6.3.2 Synthesis and characterization of ZnMgO nanowires ..................................119
6.3.3 Synthesis and characterization of SnO2 nanorods..............................121









6.3.4 Synthesis and characterization of VO2 nanowires .........................................123
6.4 Sum m ary and Conclusions ................................................ .............................. 125

7 EPITAXIAL GROWTH OF TRANSPARENT TIN OXIDE THIN FILMS .......................151

7 .1 In tro d u ctio n ........................................................................................................... 1 5 1
7.2 Experim mental M methods ............................................................... ............ 153
7 .3 R esu lts an d D iscu ssion .............................................................................. .................. 154
7.3.1 Properties ofundoped Sn02 thin film s ..........................................................154
7.3.1 Properties of gallium-doped SnO2 thin films...............................157
7.4 Sum m ary and C onclusions................................................. ............................. 160

8 CONCLUSION................ ..... .. .......... ........... ............... .. 174

L IST O F R E FE R E N C E S ......... .. ............ ........................................................................176

B IO G R A PH IC A L SK E T C H ........................................... ......... ................... .......................... 189









LIST OF TABLES


Table page

2-1 Properties of ZnO............ ................................ ......... 32

5-1 Relative resistance response of metal-coated multiple nanowires...............................91

5-2 Relative resistance response of Pd and Pt coated multiple nanowires..............................91

7-1 C candidate dopant atom s for Sn0 2....................................................................... ...... 161

7-2 Hall data of Sn02 thin films grown at different temperature .....................................161

7-3 Hall data of Ga-doped SnO2 films grown at different temperature .............................161

7-4 Hall data of Ga-doped SnO2 films grown at different oxygen pressure .......................161









LIST OF FIGURES


Figure page

2-1 Electronic, chemical and optical processes occurring on metal oxides that can benefit
from reduction in size to the nanometer range........................................ ............... 33

2-2 Crystal structure of wurtzite ZnO.. ......................... ................ ......................... 34

2-3 U nit cell of rutile SnO 2. ........................................................................... ............ ..... .... 34

2-4 V apor-Solid-Liquid (VL S) process......................................................... ............... 35

2-5 V apor-Solid (V S) process. ........................................................................ ....................35

2-6 The energy band diagram of oxide thin film materials................................................36

2-7 Gas sensing mechanism of ZnO nanowire................................. ...............36

3-1 M olecule beam epitaxy cham ber. ........................................................... .....................49

3-2 Pulsed laser deposition cham ber ............................................... ............................. 50

3-3 Typical Hall measurement setup and Van der Paul sample geometry.............................51

3-4 Photograph of gas sensor device...................................... .. .................................. 52

3-5 G as sensing m easurem ent system .............................................. ............................ 52

3-6 Process sketch for the fabrication of single ZnO nanowire devices. ................................53

4-1 Scanning electron microscope images of ZnO nanorods on a Ag coated silicon grew
at 4 0 0 C ........................................................................................... 6 2

4-2 X-ray diffraction pattern of ZnO nanorods grown on a 20A Ag coated Si02/Si
sub state at 4000C ....................................................... ................. 63

4-3 High resolution TEM image of a single ZnO nanorod and room temperature PL
spectra ......................................................... ................................... 64

4-4 25A Ag on SiO2/Si with different annealing temperature and time ................................65

4-5 25A Ag on Si3N4/Si with different annealing temperature and time...............................66

4-6 Density and average size of the resulting Ag clusters on SiO2......................................67

4-7 Density and average size of the resulting Ag clusters on Si3N4 .....................................68









4-8 Scanning electron microcope images of selectively grown ZnO nanorods on 25 A
A g/SiO 2. ........ ........ .................... ..................................................... 69

4-9 Scanning electron microscope images of 200-650 nm of Au clusters on sapphire...........70

4-10 Scanning electron microscope images of 50-150 nm of Au clusters on sapphire.............71

4-11 Scanning electron microscope images of ZnO nanowires on an Au coated c-sapphire. ...72

4-12 X-ray diffraction pattern of ZnO nanowires. ........ .............................................73

4-13 Room temperature photoluminescence spectra of ZnO nanowires .............. ...............73

5-1 M etal catalysts decorated ZnO nanowires....................................................................... 92

5-2 Time dependence of relative resistance response of metal-coated multiple nanowires ....93

5-3 Time dependence of resistance change of Pt-coated multiple ZnO nanowires ...............93

5-4 Time dependence of resistance change of Pd-coated multiple ZnO nanowies..................94

5-5 Time dependence of resistance of either Pd-coated or uncoated multiple ZnO
nanow ires ............... ..... ..... ............. ..........................................94

5-6 Relative response of Pd-coated nanowires as a function of H2 concentration in N2. ........95

5-7 Time dependence of resistance change of Pd-coated multiple ZnO nanowires ................95

5-8 Rate of resistance change after exposure to 500 ppm H2 in N2 wasmeasured at
different temperatures. ....................................... .... .. ..... .............. .. 96

5-9 Arrhenius plot of rate of resistance change after exposure to 500 ppm H2 in N2 .............96

5-10 Current-voltage (I-V) plot of uncoated or Pt-coated single ZnO nanowires measured
at room tem perature in pure N 2....................................................................... 97

5-11 Current-voltage (I-V) characteristics of Pt-coated ZnO single nanowires measured in
vacuum, air, N2 or 500ppm H2 in N2 ambients. ...................................... ...............97

5-12 Current versus time plot for single ZnO nanowires either with or without Pt coatings
and corresponding |ARI/R(%)-time plots.......................... ......... ..................98

5-13 Room temperature I-V characteristics from ZnO thin films of thickness 20 or 350 nm
measured in air before and after coating with Pt. ................................... ............... 99

5-14 Current as a function of time for Pt-coated ZnO thin films of different thickness..........100

5-15 Time dependence of current from Pt-coated ZnO nanowires and thin films................... 101









5-16 Change in current at fixed bias (0.5V) when switching to the H2-containing ambient
of either Pt-coated ZnO nanowires or thin films. ................................. .....................101

5-17 Scanning electron microscopy micrographs of SnO2-coated ZnO nanowires and EDX
sp e ctru m ....................... ......1.............. ........................................ 1 0 2

5-18 X-ray diffraction pattern from SnO2-coated ZnO nanowires. .......................................103

5-19 High resolution transmission electron microscope images of SnO2/ZnO nanowires..... 104

5-20 High resolution transmission electron microscope image of SnO2-coated ZnO
nanow ire ................................................................. ................ ..... 104

5-21 Energy-Dispersive X-ray Spectroscopy analysis of SnO2/ZnO nanowires...................105

5-22 Current-voltage (I-V) characteristics from SnO2-coated ZnO nanowires for two
different deposition tim es. ..................................................................... ....................106

5-23 Current-voltage (I-V) characteristics from SnO2-coated ZnO nanowires at room
tem perature or 400 C .......................... ...... ..................... ............ .........107

5-24 Current at fixed bias of-0.5 V and temperature of 400C as a function of time.............108

6-1 Scanning electron microscope images of well-aligned ZnO nanowires grown on a
Z nO thin fi lm tem plate..................... ..................................... .............. ........... 127

6-2 X-ray diffraction 0-20 scan of ZnO nanowires grown at 800C in 500 mTorr Ar. .........128

6-3 High resolution transmission electron microscope images of ZnO nanowires grown
on a ZnO thin film.................. ........................ ......... 129

6-4 Scanning electron microscope images of the ZnO nanowires grown at 800C...............130

6-5 Scanning electron microscope images of the ZnO nanords.......................... ..........131

6-6 Room temperature PL spectra of ZnO nanowires and near-band-edge-emission of
ZnO thin film and ZnO nanowires grown under different background ambient ...........132

6-7 Scanning electron microscope images of the ZnMgO nanowires grown at 800C in
500 m T orr A r. .............................................................................133

6-8 Energy-dispersive spectroscopy spectra for ZnMgO nanowires grown on sapphire at
800C in 500 m Torr A r. ...................... ........ ................ ... .....................134

6-9 X-ray diffraction 0-20 scan of ZnMgO nanowires grown on sapphire at 800C in 500
m T o rr A r. ........ ........ ......................................................................13 5

6-10 High resolution transmission electron microscope image of single ZnMgO nanowire.
.............................................................................................................. . 1 3 6









6-11 Scanning electron microscope images of the cored ZnMgO nanowires grown on
sapphire at 8000C in 500 m Torr Ar ....................................................... ............... 137

6-12 X-ray diffraction 0-20 scan of cored ZnMgO nanowires grown on sapphire on
sapphire at 8000C in 500 m Torr A r ....................................................... ............... 138

6-13 Transmission electron microscope image of single cored ZnO/ZnMgO nanowire .........139

6-14 X-ray diffraction patterns of SnO2 nanorods grown at 8000C in 500 mTorr oxygen..... 140

6-15 Energy-dispersive spectroscopy spectra of SnO2 nanorods grown on sapphire at
8 0 0 C ....................................................... ..................................... 14 1

6-16 Scanning electron microscope morphologies of SnO2 nanorods grewn on silicon .........142

6-17 Scanning electron microscope images showing the surface morphology and cross-
section of SnO2 nanorods deposited by pulsed laser deposition............... .......... 143

6-18 High resolution transmission electron microscope image of SnO2 nanorods................44

6-19 X-ray diffraction patterns of VO2 nanowires grown at 6000C in 500 mTorr oxygen......145

6-20 Energy-dispersive spectroscopy spectra for VO2 nanowires grown on silicon at
600C ............. .... ..................... .................................. ......... ...... 146

6-21 Scanning electron microscope images of VO2 nanowires on silicon grew at 600C.......147

6-22 High resolution transmission electron microscope image of VO2 nanowires ...............148

6-23 The photoluminescence spectra of V02 thin film and nanowires grew at different
oxygen pressure. ........................................................................ 149

6-24 Scanning electron microscope images of fabricated single nanowire device and I-V
characteristics of the indivudial V O2 nanow ire .......................................... .................150

7-1 X-ray diffraction patterns of SnO2 films deposited on (0001) A1203 at different
tem perature. .............................................................................162

7-2 a-axis constant as a function of growth temperature. ..................................................... 162

7-3 Rocking curve of (200) reflection of SnO2 films grown at 700C ................................163

7-5 The SnO2 crystal structure projection on the (100) plane, and in-plane epitaxial
grow th orientations. .......................... ...... ....................... .... .. ..... ........165

7-6 Growth rate of SnO2 films on (0001) A1203 as a function of temperature......................165

7-7 Atomic force microscope images of SnO2 thin film grown at 700C.................................166









7-8 Resistivity and carrier density of SnO2 films grown at different temperatures .............166

7-9 Transm mission spectra of the SnO2 film ................................................. .....................167

7-10 Energy-Dispersive X-ray Spectroscopy analysis of Ga-doped SnO2 thin film .............168

7-11 Comparison of a-axis constant as a function of growth temperature. ...........................168

7-12 X-ray diffraction patterns of Ga-doped SnO2 films deposited on (0001) A1203 at
different oxygen pressure .............................................................................. ......... 169

7-13 a-aixs constant of Ga-doped SnO2 films as a function of growth pressure. ....................169

7-14 Transmission spectra of the Ga-doped SnO2 film ................................. ............... 170

7-15 Hall plot (B-RH-B) of Ga-doped SnO2 film grew at 4000C in 50 mTorr oxygen............ 171

7-16 Hall plot (B-RH-B) of Ga-doped SnO2 film grew at 4000C in 50 mTorr oxygen............72

7-17 Hall plot (B-RH-B) of Ga-doped SnO2 film grew at 4000C in 20 mTorr oxygen and
annealed at 800C in oxygen for lh.......................................... ........................... 173









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

SYNTHESIS AND APPLICATIONS OF METAL OXIDE NANOWIRES

By

Li-Chia Tien

May 2008

Chair: David P. Norton
Major: Materials Science and Engineering

The one-dimensional nanostructured materials have attracted much attention because of

their superior properties from the deducing size in the nanometer range. Among them, metal

oxide materials provide a wide diversity and functionality in both theoretical study and

applications. This work focused on the synthesis of metal oxide nanowires, and further

investigated possible applications of nanostructured metal oxide materials.

High quality ZnO nanowires have been synthesized by catalyst-assisted molecular beam

epitaxy. The control of initial Au or Ag film thickness and subsequent annealing conditions is

shown to provide an effective method for controlling the size and density of nucleation sites for

catalyst-driven growth of ZnO nanorwires. For gas sensing applications, it is found that the

sensitivity for detecting hydrogen is greatly enhanced by sputter-depositing metal catalysts (Pt

and Pt) on surface. The sensors are shown to detect ppm hydrogen at room temperature using

<0.4 mW of power when using multiple nanowires. A comparison study of the hydrogen-sensing

characteristics of ZnO thin films with different thickness and ZnO nanowires was studied. The

Pt-coated single nanowires show a current response by approximately a factor of 3 larger at room

temperature. Both types of sensors are shown to be capable of the detection of ppm hydrogen at









room temperature with nW power levels, but the nanowires show different recovery

characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen.

The feasibility of a number of metal oxide nanowires has been synthesized by a high-

pressure assisted pulsed laser deposition. The high density well-aligned metal oxide nanowires

can be directly grown on substrate without metal catalysts. The results suggest the possibility of

growing complex metal oxide nanostructures, including tailored heterostructures and aligned

heretojunction arrays with PLD technique.

The growth of epitaxial SnO2 on c-sapphire using pulsed laser deposition is examined. X-

ray diffraction analysis shows that the films are highly a-axis oriented SnO2 with the rutile

structure. The effects of Ga doping on SnO2 films were studied. The Hall data showedp-type

behavior occurs only at specific growth condition, but converted back to n-type and degraded as

time proceeds.









CHAPTER 1
INTRODUCTION

"There's plenty of room at the bottom", a famous quote from Richard P. Feynman in 1959,

addressed the great interest and significance of nanotechnology. Nanotechnology is the ability to

manipulate individual atoms and molecules to produce nanostructured materials that have

applications in real world. Nanotechnology involves the production, design, and application of

physical, chemical and biological systems at scales ranging from individual atoms or molecules

to about 100 nanometers. It also involves the integration of the resulting nanostructures into

larger systems. There are basically two main approaches to create very small structures or

devices. The first approach is known as the "bottom up". The atomic or molecular building

blocks are put together to create bigger objects. With this approach, individual atoms or

molecules can be precisely placed by scanning probe microscopy or self-assembling. Materials

and devices are built from molecular components which assemble themselves chemically by

principles of molecular recognition or other techniques. The second approach is known as "top

down" approach. The macro-scale systems are converted into nano-scale ones by a series of

sequential reduction operations. The smallest features that can be created by the "top down"

approach depend on the tools used and the system operator's experience and skills. Therefore,

there are limitations in creating features smaller than 100 nanometers. In the "top-down"

approach, materials are constructed from larger entities without atomic-level control. Obviously,

the "top down" approach requires more work and produce considerable waste. In contrast, the

"bottom up" approach is time and waste efficient. However, precision and scalability are still big

challenges for the "bottom up" approach.

Nanostructures of metals, oxides and semiconductors have been studied intensely in the

last several years by different chemical and physical methods. 1-7 Interests in one-dimensional









nanotubes and nanowires had drawn much interest with the discovery of carbon nanotubes in

1991.8-11 Nanowires are one-dimensional, anisotropic structures, small in diameter, and large in

surface-to-volume ratio. Unlike other low-dimensional systems, nanowires have two quantum-

confined directions but one unconfined direction available for electrical conduction. This allows

nanowires to be used in applications where electrical conduction, rather than tunneling transport,

is required.12-14 The unusual electronic, optical, magnetic and chemical properties of nanowires

depend on their size has motivated intense research in this area. The great interest have resulted

in better understanding of the phenomena of quantum confinement,15'16 logical synthetic schemes

and fabrication of novel nano-electronic devices.2'12'13

Among all materials, oxide materials appear to show the most diverse range of

functionality. Metal oxides play a very important role in many research areas such as: chemistry,

physics, and materials science.17 The metal elements can form a large family of oxide

compounds. These elements can adopt many structure geometries with an electronic structure

that can exhibit metallic, semiconductor or insulator character. In technological applications,

oxides are used in the fabrication of microelectronic circuits, sensors, fuel cells, piezo-electric

devices, coatings for the passivation of the surfaces and as catalysts."

In this dissertation, we concentrate on the synthesis, characterization and applications of

one-dimensional metal oxide materials. The applications will be focused on their gas sensing

properties. Chapter 2 presents background and a brief literature review of current research on

metal oxide nanowires. In Chapter 3, experimental details such as synthesis, characterization and

processing techniques will be explained. The nucleation control and selective growth of ZnO

nanowires will be addressed in Chapter 4. Chapter 5 examines the applications of ZnO

nanowires in gas sensing. A comparison of thin films and nanowires based hydrogen sensor will









also be described. A catalyst-free growth method to prepare varieties of metal oxide nanowires

will be described in Chapter 6. Growth and characterization of undoped and Ga-doped SnO2 thin

films will be evaluated in Chapter 7 followed by conclusions in Chapter 8.









CHAPTER 2
LITERATURE REVIEW

2.1 Introduction

This Chapter provides an overview of general properties and recent research on metal

oxide nanowires. As a group of functional materials, metal oxides has a wide range of

applications, including transparent electronics,18'19 chemical sensors,20-22 piezo-

electronics,7'19'23'24 light-emitting diodes,25'26 etc. A basic understanding of the fundamental

properties of the metal oxide system is necessary for research and development towards practical

applications. Nanowires are one-dimensional, anisotropic structures, small in diameter and large

in surface-to-volume ratio. The physical properties are totally different than bulk, because of

their unique density of electronic states. The small diameters of nanowires are expected to

exhibit significantly different electrical, optical and magnetic properties. The electron-hole

interaction will have orders of magnitude enhancement in a nanostructure, due to the

dramatically increased electronic density of states. Figure 2-1 shows a few of the electronic,

chemical and optical processes occurring on metal oxides that can benefit from reduction in size

to the nanometer range. Efforts have been made on both developing synthetic methodologies for

the fabrication of nanowires, and devices based on their superior properties. The one-

dimensional oxide nanostructures are expected to possess novel characteristics and promising

applications for the following reasons:27

* A large surface-to-volume ratio means that a significant fraction of the atoms (or
molecules) can participate in surface reactions. The surface depletion region can be
changed dramatically by surface adsorbates. This is particular useful to gas sensing
applications.

* The Debye length (XD) for most semiconducting oxide nanowires is comparable to their
radius over a wide temperature and doping range. This causes their electronic properties to
be strongly influenced by processes at their surface. The nanowire's conductivity could
vary from a fully nonconductive state to a highly conductive state based on the surface. By
well controlled dimension, this could result in better sensitivity and selectivity.









* One-dimensional nanostructure oxides are usually stoichiometrically better defined and
have a greater level of crystallinity (usually single crystal) than the thin-film oxides.
Normally they are either with very small amount of defects or defect free structure.

* As the diameter of the nanowires is reduced to certain value, we can expect to see the
quantum effects.

* Low cost and low power consumption. One can expect the low power consumption
devices based on one-dimensional materials.

The Chapter is divided into three main sections. After a brief introduction to the materials

properties in the first section, the second section explores one-dimensional synthesis methods

and growth mechanism. The applications of one-dimensional nanowires in chemical sensing will

be addressed in the last section.

2.2 Material Properties

As a group of functional materials, metal oxides has a wide range of applications,

including transparent electronics, chemical sensors, piezo-electronics, light-emitting diodes, etc.

A basic understanding of the fundamental properties of the metal oxide system is necessary for

research and development towards practical applications. The general properties of metal oxide

materials studied in the dissertation including zinc oxide, zinc magnesium oxide, tin oxide and

vanadium oxide will be reviewed in this section. Their bulk properties provide a basic

understanding of materials and its possible applications.

2.2.1 Properties of ZnO

ZnO is a key technology material with numerous applications ranging from optoelectronics

to chemical sensors because of unique optical, electronic, and chemical properties. Table 2-1

shows a summary of basic physical parameters of ZnO.28 The lack of a center of symmetry in

wurtzite results in consequently piezoelectric and pyroelectric properties. The lattice parameters

of ZnO are a=0.32495 nm and c=0.52069 nm at 300K, with a c/a ratio of 1.602, which is close to

the 1.633 ratio of an ideal hexagonal close-packed structure as shown in Figure 2-2. The Zn









atoms are tetrahedrally coordinated to four O atoms, where the Zn d electrons hybridize with the

Op electrons. The oppositely charged ions produce positively charger (0001)-Zn and negatively

charged (0001)-O polar surfaces, resulting in a normal dipole moment and spontaneous

polarization along the c-axis.1 In addition, ZnO is a wide band-gap (3.37 eV) II-VI compound

semiconductor that is suitable for short wavelength optoelectronic applications. The high

excition binding energy (60 meV) in ZnO crystal can ensure efficient excitonic emission at room

temperature and room temperature ultraviolet (UV) luminescence.19 Moreover, ZnO is

transparent to visible light and can be made highly conductive by doping. Electron doping in

nominally undoped ZnO has been attributed to Zn interstitials, oxygen vacancies, or hydrogen.

ZnO is intrinsically an n-type semiconductor, owing primarily to the presence of oxygen

vacancies and/or zinc interstitials. The intrinsic defect levels that lead to n-type doping lay

approximately 0.01-0.05 eV below the conduction band.29 On the other hand, considerable effort

has been investigated to achieve p-type ZnO by incorporating group V elements.30-33 The reliable

and reproduciblep-type conductivity has not yet been achieved due to many issues. The

compensation of dopants by energetically favorable native defects such as zinc interstitials or

oxygen vacancies is one of obstacles.19 The low dopant solubility is another issue.29

Optical properties of ZnO have been extensively studied because of their promising

applications in optoelectronics.29 ZnO has an effective electron mass of -0.24 me, and a large

exciton binding energy of 60 meV. Furthermore, the lasing conditions can be further improved

with low-dimensional ZnO structures, which enhance the excition oscillator strength and

quantum efficiency.4 Therefore bulk ZnO has a small exciton Bohr radius (-2.34 nm). The

quantum confinement effect in ZnO nanowires could be observable at the scale of an exciton









Bohr radius. It has been reported by Gu et al. that the excition binding energy is significantly

enhanced due to size confinement in ZnO nanorods with diameter of-2 nm.16

2.2.2 Properties of ZnMgO

The realization of band-gap engineering to create barrier layer and quantum wells in the

device heterostructures is very important in optoelectronic applications. ZnMgO alloy is an

important material to construct the heterostructure or superlattice to obtain high performance

laser diode (LD) and light emitting diode (LED) devices.19'34 The ionic radius of Mg2+ (0.57 A)

and Zn2 (0.60 A) are comparable,35 alloying the ZnO phase with MgO has been investigated for

increasing the band gap ZnO. Theoretically, the band gap of ZnO (Eg = 3.4 eV) can be

modulated from 3.4 to 4.0 eV by doping with different amount of MgO (Eg = 7.8 eV). The

energy gap Eg(x) of the ternary semiconductor Zn,. MNu O is determined by the following

equation:29

Eg(x) = (l-x) Eg(ZnO)+ x Eg(MgO) bx(1-x)

where b is the bowing parameter and Eg(ZnO) and Eg(MgO) are the band-gap of ZnO and MgO,

respectively. The bowing parameter b depends on the difference in electronegativities of the ZnO

and MgO. In addition, MgO has a cubic structure (a=4.216 A) and it has been reported that MgO

segregates in the wurtzite ZnMgO lattice above 33% of Mg, limiting the maximum band-gap to

3.9 eV.29

2.2.3 Properties of SnO2

Tin oxide (SnO2) is a wide band gap (Eg=3.6 eV at 300 K) semiconductor material suitable

for multiple applications that include gas sensors, transparent conducting electrodes, and solar

cells.21'36'37 The thermodynamically stable crystal structure of SnO2 is rutile (tetragonal) with

lattice parameters a=4.737 A and c=3.186 A as shown in Figure 2-3. The crystal structure of

SnO2 belongs to the point group symmetry 4/mmm and space group P42/mmm with tin and oxygen









atoms in 2a and 4f positions. With a unit cell consisting of two tin atoms and four oxygen atoms,

with metal and oxygen atoms having an octahedral coordination.

SnO2 is suitable for multiple applications that include gas sensors, transparent conducting

electrodes, and solar cells. In sensor applications, SnO2 has been reported to display high gas

sensitivity and selectivity.38'39 The reduced size of nanostructured SnO2 provides a material with

a large surface-to-volume ratio. Gas sensors based on one-dimensional nanostructured SnO2 have

been reported to exhibit good selectivity, low detection limits, and short response and recovery

time.40-42 Several methods have been employed to prepare SnO2 nanorods including thermal

evaporation, thermal decomposition, solution-phase growth, and hydrothermal methods.4349

2.2.4 Properties of VO2

Among the metal oxides, vanadium oxides with various phases are of great interest and

have been extensively investigated for their distinctive properties.5055 Vanadium oxide (VO2)

has attracted great attention because of the metal to insulator transitions and reversible dramatic

changes in electrical and optical properties accompanied by a structural phase transition. V02

can exhibit a sharp (by a factor of 104-105) and fast (sub-picosecond) metal-insulator transition

close to room temperature (340 K).55 The metal-insulator transition is due to a small structural

distortion of the lattice from a low-temperature monoclinic (Ml, semiconducting phase) to a

high-temperature tetragonal rutile (R, metallic phase) structure, accompanied by large changes in

conductivity and optical properties from infrared (IR) transmission to reflecting.54 The structure

of the low-temperature monoclinic phase has the following unit cell dimensions: a=5.75 A,

b=4.53 A, c=5.38 A and P=122.6. For the high-temperature, rutile structure the lattice constants

are a=4.55 A and c=2.86 A, The monoclinic structure with the presence of V-V pairs along it's a

axis, where amonoclinic = 2rutile. During the transformation, the regular V-V separation of 2.86 A in

the tetragonal rutile structure transforms to an alternative V-V separation of 2.65 and 3.12 A









leading to a doubling up of the unit cell.56 It also makes it a promising material for the use in

device applications to achieve reliable electrical and optical switching operations. Moreover, B

phase VO2 was found to have good electrochemical performance, especially for use as an

electrode material for lithium batteries.57'5 It exhibits a maximum reversible capacity of about

320 mA h g-1 in the range 4 to 1 V in lithium cells.59'60 It has been reported that the operating

properties of batteries depend not only on the structure but also on the morphology of the

electrode components.61 Therefore, the great surface area of nanowire materials may play an

important role for electrochemical applications.

2.3 Synthesis of One-Dimensional Nanostructures

In this section we discuss the most common synthetic approaches that have successfully

developed obtaining high quality nanowires of a large variety of materials. In order to control the

diameter, aspect ratio and crystallinity, diverse techniques have been applied including laser

ablation, thermal evaporation, and solution-based growth, etc.62 According to the synthesis

environment, they can be divided into two categories: vapor phase growth and liquid phase

growth. A brief discussion of one-dimensional growth mechanism followed by various synthetic

approaches will be covered in this section.

Understanding the growth mechanism of one-dimensional growth is critical in controlling

nanostructures. Mechanisms for one-dimensional growth include Vapor-Liquid-Solid (VLS),

screw dislocation growth, catalyst-free self-nucleation growth, and vapor-solid (VS)

mechanisms.63 The VLS and VS mechanism are mostly accepted to interpret the growth process;

however, the detailed mechanisms are not yet understood in detail in many cases.

2.3.1 Vapor-liquid-solid mechanism

Among the various mechanisms, the VLS mechanism is well established and most widely

accepted. Control over the nanowire diameter and its morphology has been demonstrated by









VLS growth.64-69 The VLS mechanism was proposed by Wagner and Ellis in 1964 to explain the

growth of Si whiskers using Au as metal catalysts.70 As illustrated in Figure 2-4, the VLS

process consist basically 3 steps: (1) Formation of the liquid alloy droplet by heating up, (2)

crystal nucleation upon gas adsorption and supersaturation, and (3) axial growth from the

crystalline seeds to form nanowires. According to the VLS mechanism, a liquid phase is formed

initially, due to formation of a eutectic phase or the presence of a low-melting-point phase in an

alloy system. The liquid phase adsorbs the reactant gaseous species more efficiently than the

solid surface. On supersaturation of the liquid alloy, a nucleation center forms, and serves as a

preferred site for axial growth of a nanowire. The adsorbed gas reactants are then diffused

through the liquid phase to the solid/liquid interface, and the growth of the solid phase proceeds.

Because of the larger sticking coefficient of the reactants in the liquid, growth is much faster at

the solid/liquid interface compared to the solid/vapor interface.

The diameter of a nanowire via VLS growth is primarily determined by the liquid alloy

droplet, and the thermodynamic-limited minimum radius is given by:71

Rmin = 2LvVL/RT In s

Where CLV is the liquid/vapor surface free energy, VL is the molar volume of liquid, and s

is the vapor phase supersaturation.

Selection of metal catalyst species depends on the formation of a eutectic phase at the

temperature according to the phase diagram, as well as vapor/liquid/solid interfacial energies and

chemical stability in the reaction products. A wide variety of elemental semiconductor (Si and

Ge), binary compound semiconductor (GaN, GaAs, GaP, InAs) and oxide (ZnO, MgO, SnO2,

CdO, TiO2, In203 and Ga203) nanowires has been synthesized via the VLS method.62 Relatively

good control over the nanowire diameter and distribution has been achieved and reported.









2.3.2 Vapor-solid mechanism

There have been numerous reports on one-dimensional nanostructure growth from vapor

phase without metal catalyst, the growth mechanism usually refer to vapor-solid mechanism.72

Under high temperature condition, source materials are vaporized and then directly condensed on

the substrate placed in the low temperature region. Once the condensation process happens, the

initially condensed molecules form seed crystals serving as the nucleation sites. As a result, they

assist directional growth to minimize the surface energy. The process is illustrated in Figure 2-5.

With thermodynamic and kinetic considerations, the formation of nanowires could be

possibly through different models, including an anisotropic growth, Frank's screw dislocation

model and defect-induced model. In an anisotropic growth model, one-dimensional growth can

be also explained by the preferential reactivity and binding of gas phase reactants along specific

crystal facets, and also the desire for a system to minimize surface energies. In the dislocation

and defect-induced growth models, specific defects are known to have larger sticking

coefficients for gas phase species, thus resulting anisotropic growth at those sites. Although the

exact mechanisms for vapor-solid growth are not well understood, a variety of nanostructures

have been synthesized via this approach.66'73

2.3.3 Laser ablation

Laser-assisted catalytic VLS growth is a method used to generate nanowires under non-

equilibrium conditions.5'74'75 A target containing the catalyst and the source materials, plasma

plume containing both catalyst and source material is generated during the ablation. The

nucleation occurs on the substrate where nanowires grow.

2.3.4 Thermal evaporation

In thermal evaporation, a vapor reacts on the substrate to produce the desired product. In

the case of nanowires, the vapor-liquid-solid (VLS) method described previously usually applies,









where the substrate usually deposit with metal catalysts serve as seed layer. The seed layer reacts

with the source vapor material until it is saturated and the desired material starts to solidify and

grow outward from the catalyst. The growth parameters such as: partial oxygen pressure,

chamber pressure, substrate and growth temperature are crucial to the resulting structure of

product. By control the oxygen pressure and growth temperature, different nanostructures such

as nanorods, nanowires, nanorings, nanobelts and comb-like structures can be synthesized.76

The solid-vapor process is a relative simple setup used to synthesis a variety of metal

oxide nanostructures. The process are carried out in a horizontal tube furnace, which is

composed of a horizontal tube furnace, an alumina tube, a rotary pump system and a gas supply

and control system. The source materials (normally powders) are loaded on an alumina boat and

positioned at the center of the alumina tube, where the temperature is the highest. Substrates

were placed downstream for collecting growth products. A variety of metal oxide nanowires

(ZnO, SnO2, In203, CdO) with different nanostructures were successfully synthesized by thermal

evaporation process.43'71'77-80

2.3.5 Solution-based chemistry

One of the major disadvantages of high temperature approaches to one-dimensional

nanowire synthesis including the high cost of fabrication, high processing temperature and the

inability to produce metallic nanowires. As a result, a solution-based synthesis of nanowires with

controllable diameters without the use of templates, catalysts or surfactants has been

demonstrated.81-87 The relative low cost setup to synthesize one-dimensional nanostructure can

be realized via selective capping mechanism at low temperature. The kinetic control of the nano-

crystal growth by preferentially adsorbing molecular capping agents to specific crystal faces,

thus inhibiting growth of that surface, results in anisotropic growth. Because of the low









temperature process, the nanowires can be grown on inexpensive substrates, such as glass and

plastic substrate.88 It provides an important feature for device applications.

2.4 Applications of Nanowires

2.4.1 Electrical applications

The semiconductor industry continues to face technological (especially in lithography)

challenges as the device feature size is decreased, especially below 100 nanometers. The self-

assembly of nanowires might open a way to construct devices that do not rely on improvements

in photolithography. Devices made from nanowires have several advantages over those made by

photolithography. For example, a variety of approaches have been devised to organize nanowires

via bottom up approaches, thus the expensive lithography techniques are not required. In

addition, unlike traditional silicon processing, different semiconductors for example, metal

oxides, can be used simultaneously in nanowire devices to produce diverse functionalities.

Transistors made from nanowires could also hold advantages due to their unique

morphology. For example, in bulk field effect transistors (FETs), the depletion layer formed

below the source and drain region results in a source-drain capacitance which limits the

operation speed.89 However, in nanowires, the conductor is surrounded by an oxide and thus the

depletion layer cannot be formed. Thus, depending on the device design, the source-drain

capacitance in nanowires could be greatly minimized and possibly eliminated.90

Nanowires have also been proposed for applications associated with electron field

emission, such as flat panel displays, because of their small diameter and large curvature at the

nanowire tip, which may reduce the threshold voltage for electron emission.91-95 In this

application, the demonstration of very high field emission currents from the sharp tip of

nanowires has stimulated interest in this potential area of application for nanowires.









2.4.2 Optical applications

Nanowires also show promise for optical applications. One-dimensional systems exhibit

novel properties in their joint density of states, allowing quantum effects in nanowires to be

optically observable, sometimes even at room temperature. Since the density of states of a

nanowire in the quantum limit (small wire diameter) is highly localized in energy, the available

states quickly fill up with electrons as the intensity of the incident light is increased.34'68'96'97 This

filling up of the sub-bands, as well as other effects that is unique to low-dimensional materials,

lead to strong optical nonlinearities in quantum wires. Quantum wires may thus yield optical

switches with a lower switching energy and increased switching speed compared to currently

available optical switches.97

Light emission from nanowires can be achieved by photoluminescence (PL) or

electroluminescence (EL), distinguished by whether the electronic excitation is achieved by

optical illumination or by electrical stimulation across a p-n junction, respectively. PL is often

used for optical property characterization, but from an applications point of view, EL is a more

convenient excitation method. Light-emitting diodes (LEDs) have been achieved injunctions

between ap-type and an n-type nanowire and in superlattice nanowires withp-type and n-type

segments.312 The light emission was localized to the junction area, and was polarized in the

superlattice nanowire.

Nanowire photodetectors were also interesting applications. ZnO nanowires were found to

display a strong photocurrent response to UV light irradiation. The conductivity of the nanowire

increased by several orders of magnitude compared to the dark state. The response of the

nanowire was reversible, and selective to photon energies above the band-gap, suggesting that

ZnO nanowires could be a good candidate for optoelectronic switches.28'98'99









2.4.3 Chemical and biochemical sensing

Chemical and biological sensors have a great influence in the areas of personal safety,

public security, semiconductor processing, and the automotive and aerospace industries. Thin

film based semiconducting metal oxides as chemical sensing materials have been extensively

studied for a long time due to their advantageous feature, such as good sensitivity to the ambient

conditions and ease in fabrication.21'22'37'100-103 However, there are some critical limitations

difficult to overcome for thin-film-based sensing devices. The thin-film based devices have a

limited maximum sensitivity due to the limited surface-to-volume ratios. In the case of

polycrystalline thin film devices, only a small fraction of the species adsorbed near the grain

boundaries is active in modifying the electrical transport properties (Figure 2-6). Furthermore,

most thin-film devices are operated at high temperatures (200-600C) in order to achieve

enhanced chemical reactivity between sensor materials and surrounding gases. These drawbacks

bring inconvenience for practical applications. In contrast to thin-film-based devices, devices

based on one-dimensional nanostructures have great potential to overcome these fundamental

limitations.

Sensors for chemical and biochemical substances with nanowires as the sensing probe are

a very attractive application area. Nanowire sensors will potentially be smaller, more sensitive,

demand less power, and react faster than their macroscopic counterparts. For ZnO nanowires, the

most widely accepted model is based on the modulation of the depletion layer within ZnO due to

the chemisorption as illustrated schematically in Figure 2-7. When nanowires are exposure in

oxidizing gas environment, oxygen adsorbs on the exposed surface of ZnO, extracting an

electron from the conduction band, ionizes to O or 02 The O is believed to be dominant

among all:

02 (g) + 2 e- -- 20-(ads)









Consequently, depletion layers are formed in the surface as well as in the grain-boundary

regions of ZnO, causing the carrier concentration and electron mobility to decrease. These

chemisorbed 02- deplete the surface electron states and consequently reduce the channel

conductivity. The resistivity increases as well.

When exposed to reducing gases such as ethanol, the ethanol will react with the adsorbed

0:

CH3CH20H(ads) + 6 0 (ads) 2 CO2 + 3 H20 + 6 e

By releasing the trapped electron back to the conduction band and, then, both the carrier

concentration and the carrier mobility of ZnO increase. From the above-mentioned surface

reactions, one can expect that the factors, which can affect the interaction process between the

surface-reactive chemical species adsorbedd 0 ) and the target-gas molecules (ethanol for

example), would be of importance for the gas-sensing performances of ZnO devices.

Moreover, the more the effective surface area, the more the oxygen-adsorption quantities

and the higher the sensitivity of metal oxide sensors. Since the ZnO nanowires have a larger

surface area exposed to the air and a consequent higher quantity of the surface states related to

oxygen vacancies than that of the ZnO thin films, a higher gas sensitivity and a faster response

time of the ZnO nanowire sensors than those of the ZnO thin film devices are reasonable. This

eventually increased the conductivity of the ZnO nanowires. Because of the small diameter of

nanowires, it's expected that the bulk electronic transport properties will change. Based on these

properties, ZnO nanowires could be used as chemical and biochemical sensing materials.












Table 2-1. Properties of ZnO
Property

Lattice parameters at 300 K

ao

Co

ao co



Density

Stable phase

Melting point

Thermal conductivity

Linear expansion coefficient


Value


0.32495 nm

0.52069 nm

1.602

0.345

5.606 g cm3

Wurtzite

1975 C

0.6

ao: 6.5 x 10-6

co: 3.0 x 10-6

8.656

2.008

3.4 eV, direct

<106 cm-3

60 meV

0.24

200 cm2V-S-1


Static dielectric constant

Reflective index

Energy gap

Intrnsic carrier concentration

Exciton binding energy

Electron effective mass

Electron Hall mobility











local depletion
region J



depleted
region hv


Sc electro-negative
c;, iadsorbate


Figure 2-1. Electronic, chemical and optical processes occurring on metal oxides that can benefit
from reduction in size to the nanometer range.




























Figure 2-2. Crystal structure of wurtzite ZnO. The Zn atoms (orange) are tetrahedrally
coordinated to four O atoms (blue).


Figure 2-3. Unit cell of rutile SnO2,consisting of two tin atoms (Red) and four oxygen atoms
(blue), with metal and oxygen atoms having an octahedral coordination.


















Metal Catalyst S :Ni ration
Nucleation
E utectiI Forrration


Figure 2-4. Vapor-Solid-Liquid (VLS) process.


V i r




Nucleation


Growth


Supersal ration


Figure 2-5. Vapor-Solid (VS) process.


Nanowire


Nanorod


/










Grain Boundaries


\ A N7E
E,


Figure 2-6. The energy band diagram of oxide thin film materials. Showing the Schottky barrier
height at the grain boundary either with or without a chemically reducing ambient.


(a)


O, ambient

o-


(b)
CHOH ambient
CO2

C02 A


Depleted region


Depleted region


Figure 2-7. Gas sensing mechanism of ZnO nanowire. (a) The large surface depletion region
caused by oxidizing ambient (b) the small surface depletion region caused by
reducing ambient.


CO2


CO2









CHAPTER 3
EXPERIMENTAL DETAILS AND CHARACTERIZATION

3.1 Materials Synthesis Techniques

Any performance or property of a material mainly depends on the efficiency and precise

nature of the synthesis and the fabrication methods. The desirable properties of materials can be

realized by the control of dimension, size, morphology, microstructure and chemical composition

of materials. The ability to control these properties strongly depends on different synthesis

methods. In this dissertation, various metal oxide nanowires and thin films were synthesized via

two different approaches. Molecular beam epitaxy was used to synthesize ZnO nanowires by a

catalyst-assisted approach, while pulsed laser deposition was employed to grow epitaxial oxide

thin films and metal oxide nanowires without catalyst. The detail of materials synthesis

techniques are described in following sections.

3.1.1 Molecular beam epitaxy

Molecular beam epitaxy (MBE) is a material deposition technique capable of predictably

and reproducibly yielding material with very low impurity levels and precise thickness control.

Normally MBE requires ultra high vacuum and proceed at low growth temperature to prevent

possible contamination. MBE involves the generation of fluxes of constituent matrix and doping

species, the reaction take places at the heated substrate and form an ordered overlayer. Elemental

or compound constituents are heated or introduced into the chamber to generate mass transfer to

the substrate via the vapor phase. To maintain the high purity of flux, the ultra high vacuum is

needed. The fluxes of elements can be temporally modulated either by altering the evaporation

conditions or physically interrupting the beam using mechanical shutters.

The precision with composition and doping of a structure can be tailored by MBE. To

achieve this level of control, deposition rates are normally around less than one monolayer per









second. The low growth rates and growth temperatures have made MBE a superb technique for

growing complex hetero-epitaxial structures on atomic scale. The high vacuum provides a clean

environment and allows high quality films to be grown. The growth rate is dependent on the flux

of the cells and the growth temperature. Lower growth temperature and higher fluxes result in

amorphous of polycrystalline films, while higher growth temperature and lower fluxes can be

used to synthesize single crystal films.

The ZnO nanowires were synthesized using the catalyst-driven molecular beam epitaxy

method. The growth was carried out in a conventional MBE system (Figure 3-1). The base

pressure was pumped down to approximately 5x 10-8 mbar using a cryopump. An ozone/oxygen

mixture generated by ozone generator was used as the oxidizing source. The nitrogen-free

plasma discharge ozone generator yielded an 03/02 ratio on the order of 1-3%. The cation flux

was provided by Kundsen effusion cells using high purity Zn metal (99.9999%) as the source

materials. The Zn and 03/02 pressures were determined by a nude ionization gauge that was

placed at the substrate position prior to growth. The 03/02 pressure was fixed at 5x 10-6 mbar by

a leaking valve during growth. The Zn pressure was varied between 5x 10-7 and 4x 10-6 mbar

controlled by temperature.

3.1.2 Pulsed laser deposition

The technique of pulsed laser deposition (PLD) has been used to deposit high quality

films of materials for more than a decade.104 The technique uses high power laser pulses

(typically ~108 Wcm-2) to melt, evaporate and ionize material from the surface of a target. This

process produces a transient, highly luminous plasma plume that expands rapidly away from the

target surface. The ablated material is collected on a heated substrate on which it condenses and

the thin film grows.









It was found to have significant benefits over other film deposition methods. The

capability for stoichiometric transfer of material from target to substrate can be reproduced in the

deposited film. The high deposition rates can be achieved at moderate laser fluences. Since a

laser is used as an external energy source, the deposition can occur in both inert and reactive

background gases. However, the plasma plume created during ablation process is highly forward

directed, therefore the thickness of resulting film is highly non-uniform and the composition can

vary across the film. The deposition area is also relatively small.

The oxide nanowires and thin films were synthesized using the pulsed laser deposition

method. The growth was carried out in a commercial PLD system build by Neocera (Figure 3-2).

The ablation target was fabricated using high purity oxide powders. The target was pressed and

sintered at high temperature. A Lambda-Physik Compex 205 KrF excimer laser was used as the

ablation source. The laser produces a coherent beam with a 248 nm wavelength. A desired

repetition rate can be used to achieve different deposition rates, with target to substrate distance

of 2.5 cm and a laser pulse energy density of 1-3 J/cm2. The growth chamber exhibits a base

pressure of 10-6 Torr. Prior to growth, the target was cleaned in situ by pre-ablating with

approximately 2000 shots. The growth experiments were performed over a temperature range of

400-800C in a desired oxygen pressure.

3.2 Characterization Techniques

Different techniques were used to analyze the materials, including surface morphology,

structural, optical, chemical and electrical analysis. The surface analysis is preformed to know

the morphology and roughness of the surface. Structural analysis provides the crystal structure,

phases and defects information of materials. It also provides information about alignments

between materials and substrates. Optical analysis is useful to understand the optical properties

in transparent materials. Chemical analysis is useful in providing both qualitative and









quantitative information, which are very valuable to know doping amounts and contaminations.

Electrical analysis is important because the electrical properties directly determine the

performance of materials in device applications. With all these characterization techniques not

only provide a better understanding of materials but also are helpful to optimize the growth

conditions, producing better materials and devices.

3.2.1 Scanning electron microscope

The scanning electron microscope (SEM) is often the first analytical instrument used to

quickly look at a material. The SEM is used to observe surface and cross-section images of

nanowires samples. The topographical information such as diameters, length and growth

direction of nanowires can be collected quickly. The electron beam is focused by condense lens

into fine probe and scanned over a small area of the sample. The interaction between beam and

sample surface causes elastic and inelastic interactions resulting in emission of secondary

electrons. Due to their low energy, these electrons originate within a few nanometers from the

surface. By collecting secondary electrons, surface topography can be observed. The

measurements were performed in a JEOL 6335F system that uses a cold cathode field emission

electron source. The FE-SEM was operated at 15 keV in high magnifications. In addition, the

samples were not coated to prevent contamination and artificial features

3.2.2 Atomic force microscopy

Atomic force microscopy (AFM) provides information on the topography and

morphology of the surface. Inter-atomic forces between the atoms on the surface and those on

the tip cause the deflection of a micro-fabricated cantilever. Because the magnitude of the

cantilever deflection depends strongly upon the separation between the surface and tip atoms,

they can be used to map out surface topography with atomic resolution in three dimensions.









AFM Dimension 3100 (Digital Instrument, Inc.) was performed in tapping mode to obtain

surface topographic and roughness of metal oxide thin films.

3.2.3 X-ray diffraction

X-ray diffraction (XRD) is used to obtain structural information and determine the phases

of the sample. X-ray diffraction is commonly used to identify unknown substances, by

comparing diffraction data against a database maintained by the International Centre for

Diffraction Data. It may also be used to characterize heterogeneous solid mixtures to determine

relative abundance of crystalline compounds and, when coupled with lattice refinement

techniques, can provide structural information on unknown materials. Powder diffraction is also

a common method for determining strains in crystalline materials. The XRD measurements were

performed in a Philips APD 3720 system that uses a copper x-ray source. The source emits Cu

Kai X-rays with a 1.54056 A for diffraction. The radiation impinged on the sample and

undergoes constructive or destructive interference after reflecting from the sample. Constructive

interference will cause a peak at a particular 20 angle according to Bragg's law:

2dsinO =n

where d is the spacing between planes, A is the wavelength of the incident x-ray, n is the number

of whole wavelengths. For a single crystal, there are only specific orientations that satisfy the

Bragg's law. On the other hand, for a polycrystalline film with differently oriented grains,

diffraction peaks appear when those grains meet the diffraction conditions. Normally the full

width at half maximum (FWHM) of a diffraction peak can be used to determine the crystal

quality of the materials. For nanowires, the peak positions in the X-ray diffraction pattern can be

used to determine the chemical composition and the crystal phase structure of the nanowires.









High resolution XRD was performed on epitaxial thin film samples via a Philips X'pert

High Resolution X-ray Diffraction system in order to determine the thickness, distribution of

crystalline orientations, grain size and mosaic spread in crystalline materials.

3.2.4 Transmission electron microscope

Transmission electron microscope and high-resolution transmission electron microscopy

are powerful imaging tools for studying nanowires at the atomic scale. They usually provide

more detailed structural information than are seen in the SEM images. Transmission electron

microscope (TEM) is preformed to obtain the detail structural information. TEM is an imaging

technique whereby a beam of electrons is transmitted through a specimen, then an image is

formed, magnified and directed to appear either on a fluorescent screen to be detected by a CCD

camera. Because the wavelength of high-energy electrons is a fraction of a nanometer, and the

spacings between atoms in a solid is only slightly larger, the atoms act as a diffraction grating to

the electrons, which are diffracted. Therefore, some fraction of electrons will be scattered to

particular angles, determined by the crystal structure of the sample, while others continue to pass

through the sample without deflection. In the diffraction mode, the selected area diffraction

(SAD) is useful to identify crystal structures and examine crystal defects. It is similar to x-ray

diffraction, but unique in areas as small as several hundred nanometers in size can be examined,

whereas x-ray diffraction typically samples areas several centimeters in size. This is a very

useful technique to determine the crystal structure of nanowire materials. An analytical TEM is

one equipped with detectors that can determine the elemental composition of the specimen by

analyzing its X-ray spectrum or the energy-loss spectrum of the transmitted electrons.

High resolution transmission electron microscopy (HR-TEM) is an imaging mode of the

TEM that allows the imaging of the crystallographic structure of a sample at an atomic scale.

Because of its high resolution, it is an invaluable tool to study nano-scale properties of crystalline









material such as semiconductors and metals. At these small scales, individual atoms and

crystalline defects can be imaged. The high resolution also permits the surface structures of the

nanowires to be studied. It also serves as a powerful tool to observe core-sheath nanowire

structures by mass contrast imaging.

There are a number of drawbacks to the TEM technique. Many materials require

extensive sample preparation to produce a sample thin enough to be electron transparent, which

makes TEM analysis a relatively time consuming process with a low throughput of samples. The

structure of the sample may also be changed during the preparation process. Also the field of

view is relatively small, raising the possibility that the region analyzed may not be characteristic

of the whole sample. There is potential that the sample may be damaged by the electron beam,

particularly in the case of biological or radiation sensitive materials.

By coupling the powerful imaging capabilities of TEM with energy dispersive X-ray

spectrometer within the microscope, additional properties of the nanowires can be probed with

high spatial resolution. The elemental composition within the probed area can be determined.

This technique is particularly useful for the compositional characterization of superlattice

nanowires and core-shell nanowires. In this dissertation, HR-TEM images and selected area

diffraction (SAD) patterns of metal oxide nanowires were acquired by a JEOL 2100F

transmission electron microscope at 200 keV.

3.2.5 Photoluminescence

Optical methods provide an easy and sensitive tool for measuring the electronic structures

of a material, since optical measurements require minimal sample preparation.

Photoluminescence (PL), a powerful and nondestructive analysis technology, can reveal the band

structure and the carrier transport behaviors in materials. Photoluminescence refers to emission

of light resulting from optical stimulation. When an electron increases energy by absorbing light,









there is a transition from the ground state to an excited state. This excited state is not stable and

has to return to the ground state. In luminescence materials the released energy is in the form of

light, which is called as radiative transition. This emitted light is detected as photoluminescence.

PL is typically used to determine band gap of a semiconductor and identify impurities. A He-Cd

(325 nm) laser was used as the excitation source and photoluminescence was detected by a GaAs

PMT detector. All the measurements were performed in a wavelength of 340-800 nm at room

temperature.

3.2.6 Energy-Dispersive X-ray spectroscopy

Energy-dispersive X-ray spectroscopy (EDX) is an analytical tool predominantly used for

chemical characterization. EDX measures the emitted x-ray spectrum when sample is bombarded

by a high-energy radiation. The emitted spectrum consists of a series of peaks representative of

the type and relative amount of each element in the sample. The relative amount of each element

can be calculated by comparing the peak heights with standards and applying ZAF corrections,

where Z is the atomic number, A is absorption and F is the x-ray fluorescence. The EDX analysis

was performed inside JEOL 6335F fitted with liquid nitrogen cooled EDX detector.

3.2.7 Hall measurement

The Hall measurement is used to determine the electrical transport properties of a

material. The carrier type, mobility, density and electrical resistivity can be measured from it.

The Hall Effect provides a relatively simple, fast and low cost method to extract these electrical

properties.105

The Lorenz force is defined as a force exerted on a charged particle in an electromagnetic

field. For example, an n-type, bar shaped semiconductor shown in Figure 3-3. It is assumed that

a constant current I flows along the x-axis from left to right in the presence of a z-directed

magnetic field. Under Lorenz force electrons drift toward the negative y-axis and accumulate on









the side of the sample to produce an electrical surface charge. A potential drop cross the sample

called Hall voltage is formed. The induced electric field increases until it counteracts to the

opposite Lorenz force:


E = vx B=Bj
nq

where eE, is the induced electric field force, qvB is the Lorenz force, j, = nqv, is the total


current density. The Hall coefficient RH is defined as:


RH -
nq

For electrons the charge is q = -e, the hall coefficient is negative for n-type semiconductor while

positive indicates p-type semiconductor.

The mobility is defined as the coefficient of proportionality between velocity and electric

field:

v ]
v = RHo
E, nqEx

where cr is conductivity, which is proportional to mobility.

The resistivity, carrier type, and carrier concentration of samples were determined at

room temperature using a Lakeshore 7507 system with a 10 Tesla magnet. The samples were cut

into a 10 mm2 pieces. And indium dots are soldered onto corners to perform Van der Pauw

measurement.

3.3 Processing and Device Fabrication

3.3.1 Electron beam lithography

The electron beam lithography was used for single nanowire device fabrication. Instead

of photolithography, e-beam lithography provides accessibility to exposure desired patterns in









small area without a mask. The electron gun generates a beam of electrons with a suitable current

density. A single-crystal lanthanum hexaboride (LaB6) is used to for the electron gun. Condenser

lenses are used to focus the electron beam to a spot size 10-25 nm in diameter. Beam blanking

plates that turn the electron beams on and off, and beam deflection coils are computer controlled

and operated at MHz or higher rates to direct the focused electron beam to any location in the

scan field on the substrate. Since the scan field is much smaller than the substrate diameter, a

precision mechanical stage is used to position the substrate to be patterned.

The advantages of electron-beam lithography include the generation of sub-micrometer

resist geometries, highly automated and precisely controlled operation, depth of focus greater

than that available from optical lithography, and direct patterning on a semiconductor wafer

without using a mask. The disadvantage is that electron-beam lithographic machines have low

throughput. A RAITH 150 operated at 10 keV has been used to generate patterns for single

nanowire device fabrication; the details of device fabrication will be described later.

3.3.2 Sputter deposition

Sputtering is basically a simple PVD process to deposit thin films. The atoms are

removed from the surface of a solid by high-energy ion impacts. The plasma consisting of argon

ions and electrons is ignited between the source and substrate. The target material is placed on

the electrode with the voltage set to maximize the flux at the target. As the positively charged

argon ions bombard the target surface, the target material is removed from the surface. The

atoms impinge on the substrate and form a thin film. Sputter deposition has multiple advantages

over other techniques, including low cost, large area deposition, high throughput and good

uniformity. Sputtering is used extensively in the semiconductor industry to deposit thin films of

various materials in integrated circuit processing. A Kurt Lesker CMS-18 multi target sputter









deposition system was used to deposit metal contacts and metal catalysts on nanowires in this

dissertation.

3.3.3 Fabrication of multiple nanowire devices

By the nature of nanowires is highly cross-linked on dielectric substrates, the sensing

properties can simply measure the transport by put down electrodes on the substrate. Multiple

nanowire devices were fabricated by sputtered Al/Ti/Au electrodes on as-grown samples by a

shadow mask. The separation of electrodes was approximately 400 am. The samples were placed

on a holder and Au wires were bounded to the contact pads for current-voltage (I-V)

measurements. Figure 3-4 shows a typical gas sensor device mounted on the holder.

3.3.4 Fabrication of single nanowire devices

The fabrication process of single ZnO nanowire devices is illustrated in Figure 3-6. The

nanowires were released from the as-grown substrate by sonication in ethanol and then

transferred to Si substrates with 100 nm thermal oxide. The silicon substrates were deposited

with gold markers before nanowire dipersion. With the sputtered markers on the substrate, the

location of dispersed single nanowires can be addressed. The E-beam lithography followed by

metallization was used to pattern electrodes contacting both ends of a single nanowire. The

separation of electrodes was approximately 3[ m. Au wires were bounded to the contact pads for

current-voltage (I-V) measurements.

3.3.5 Gas sensing measurement

Transport DC-IV measurements were used to examine gas sensing properties on thin films,

single and multiple nanowires devices. The measurement has been performed in a tube furnace

system (Figure 3-5) connected with Hewlett Packard Model 4156 semiconductor analyzer. The

system equipped with a variety of different gas (02, 03, N2 and H2) and gas flow is controlled by

a mass flow controller. Before measurements, the chamber was pumping down to 10-5 Torr by a









turbo molecular pump. Current-voltage (I-V) measurement was used to examine the gas sensing

properties and a fix bias (0.5 V) was applied during the measurements. The furnace can be

heated up to 4000C to perform gas sensing measurements at high temperatures. The response (s)

was defined as the ratio of the electrical resistance in air (Ra) to that in a sample gas (Rg):

R
S=
Rg

The sensitivity (0) was defined as the change in resistivity (Rg- Ra) divided by resistance in dry

air (Rg):

Rg -R
R











OZONE INLET


ION GAUGE


e-GUN


MBE Chamber


Figure 3-1. Molecule beam epitaxy chamber.








Laser Beam


Target


Heatable Sample Stage


Oxygen Inlet


Substrate


Figure 3-2. Pulsed laser deposition chamber.


Qua


Laser Plume











V=O


V=VH


S


Figure 3-3. Typical Hall measurement setup and Van der Paul sample geometry.


v4-\























Figure 3-4. Photograph of gas sensor device








H-, inlet





TMP

Figure 3-5. Gas sensing measurement system.














Deposit thermal oxide on silicon


Deposit alignment marks


I na





I T-








[ _1 1i ] F I_


Disperse nanowires with solvent




Find and record coordinates of nanowires


Spin coat photoresist


Exposure patterns and develop


Deposit ohmic contacts


Lift Off


Figure 3-6. Process sketch for the fabrication of single ZnO nanowire devices.









CHAPTER 4
NUCLEATION CONTROL AND SELECTIVE GROWTH OF ZNO NANOWIRES

4.1 Introduction

One-dimensional nanostructures are potential candidates for a range of device applications

in nanoelectronics, optoelectronics and biosensors.3'4'7'12'14'106-111 Due to its wide band gap and

high excition binding energy (60 meV), ZnO has potential for a wide range of applications. It

also can form a wide range of nanostructures, including nanowires, nanorods, nanobelts,

nanocombs and nanorings.19 There is a strong interest in the development of ZnO nanowire

device structures for potential applications in high density arrays of low power field-effect

transistors (FETs), gas sensors, solar cells and UV detectors.2 28'112 It is clear that nanowires and

nanotubes are excellent candidates for this type of sensing, given their large surface-to-volume

ratios and low weight.27'40 The ability to detect hydrogen selectively at room temperature is

important because it avoids the use of on-chip heaters that add to the power consumption and

weight. The ability to control the nucleation site density for catalyst-driven growth of nanorods

makes them candidates for micro-lasers or memory arrays. A key requirement for exploiting the

potential of ZnO nanowires in these types of device applications is the control of both the density

and size of the metal catalyst dots used for nucleation of the ZnO growth and demonstration of

selective area growth of the nanowires.

The ZnO nanorods were synthesized by a catalyst-driven molecular beam epitaxy method.

The nanorods were found only nucleate on catalyst surface followed by the VLS mechanism. In

this Chapter, the density control of single crystal ZnO nanorods by varying both the annealing

time and temperature is studied. It is straightforward to control the Ag cluster size in the range of

8-65 nm diameter and the cluster density from 30 to 2500 mm2 using an initial Ag film









thicknesses of 25 A and either SiO2 or SiNx dielectrics on the initial Si wafer, followed by

annealing at 600-8000C for 1-30 min.

In an effort to grow ZnO nanowires with longer lengths for device applications, c-sapphire

substrates coated with gold were used. Similar procedures were preformed to produce Au cluster

size in the range of 50-650 nm in diameter. High density cross-linked ZnO nanowires were

synthesized when Au catalysts were in the range of 50-150 nm in diameter. By selecting proper

metal catalyst size and lattice matched substrate, high density ZnO nanowires were synthesized

by catalyst-driven molecular beam epitaxy. A morphology evolution from nanorods to nanowires

is observed when using Au on c-sapphire substrate. It provides alternative synthesis approach to

fabricate high aspect-ratio ZnO nanowires for device applications.

4.2 Experimental Methods

Two different metal catalysts (silver and gold) and substrates (silicon and sapphire) were

used in the study. The p-Si(100) substrates were coated with 100 nm of either SiO2 or SiNx

deposited by plasma-enhanced chemical vapor deposition and then coated with Ag films of 25 A

thickness using RF sputtering The c-plane sapphire substrates were coated with Au films of 30

A by RF sputtering. Annealing of the samples was carried out in the range of 600-800C for 1-

30 min under flowing N2 gas. The annealing process causes clusters to form small islands, whose

size and density were measured using scanning electron microscopy (SEM). These clusters act as

nucleation sites for growth of the ZnO nanorods. The site-selective growth of ZnO nanorods was

achieved by nucleating them on a patterned substrate coated with Ag. For nominal Ag

thicknesses of 25 A, discontinuous Ag clusters are realized after annealing. ZnO nanorods were

synthesized by molecular beam epitaxy (MBE) with a base pressure of 5 x 108 mbar. An

ozone/oxygen mixture generated by ozone generator was used as the oxidizing source. The

cation flux was provided by Kundsen effusion cells using high purity Zn metal (Alfa Aesar,









99.9999%) as the source materials. The Zn pressure was varied between 4x 10-6 and 2x 107 mbar,

while the pressure of the 03/02 mixture was 5 x 10-4 mbar. The growth temperature was varies

from 400 to 6000C and typical growth time was 2 h. After growth, the samples were

characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission

electron microscopy (TEM) and photoluminescence (PL).

4.3 Results and Discussion

4.3.1 Structural and optical properties of ZnO nanorods grown on Si

Figure 4-1 shows a typical top-view (a)-(c) and side-view (d) FE-SEM image of the as-

grown nanostructures on silver coated silicon at 400C. Different shapes of nanostructures

including whisker, needles and rods were observed in high density. The diameter and length of

the ZnO nanorods are approximately 20-60 nm and 1 m. At higher temperatures (500C),

nanorods are observed slightly longer in length (2 [tm) with lower density. Few nanorods were

observed at growth temperature over 600C. The growth temperature is found to have significant

effect on the dimension and density of resulting nanostructures.

The XRD pattern of the ZnO nanorods grown at 4000C is shown in Figure 4-2. The peaks

are indexed according to the hexagonal wurtzite structure of ZnO with lattice constants of

a=3.249 A and c=5.2038 A. No impurity or secondary phase peaks were observed. Although the

diffraction patterns show a randomly oriented polycrystalline material, a preferred (002)

orientation is observed. A further structural characterization is performed by TEM. The low

magnification TEM image of single ZnO nanorod is shown in Figure 4-3(a), which shows the

diameter is approximately 20 nm. Metal tip at the top of nanorod confirms the VLS growth

mechanism. Local energy-dispersive X-ray spectroscopy measurements indicate that the

terminating particle is Ag. This is similar to what is observed for other nanorod synthesis that is

driven by a catalytic reaction, where catalyst particles become suspended on the nanorod tip.









Evidence for termination of the ZnO nanorods tips with catalyst particles is also observed in

field-emission SEM images. The high resolution TEM image of a single ZnO nanorod shows

lattice fringes with [001] growth direction. The selected area electron diffraction (SAD) patterns

taken from a single ZnO nanorod indicates that the as-grown nanorods are single crystal with a

wurtzite structure.

The optical properties of the nanorods were examined using photoluminescence. A He-Cd

(325 nm) laser was used as the excitation source. At 300 K there was strong near-band-edge

emission at 375 nm as shown in Figure 4-3(d). This is consistent with luminescence reported for

near-band-edge emission in epitaxial films and larger diameter ZnO nanorods.113'114 A broad

weak, green emission at -520 nm was also present. This is typically associated with trap-state

emission attributed to singly ionized oxygen vacancies in ZnO.115

4.3.2 Growth mechanism

By the evidence of metal segregation at tips, the growth of nanostructures can be explained

by the vapor-liquid-solid (VLS) model as illustrated previously in Figure 2-4. The vapor-

liquid-solid (VLS) model, as an effective route to synthesis semiconductor nanowires was

proposed by Wagner and Ellis in 1964, has been widely used for the one-dimensional

nanostructure synthesis.70 The main features of VLS growth are that semiconductor nanowires

have metal or alloy droplets on their tips and these droplets define diameters and direct the

growth direction. The VLS growth can be divided into 4 steps: (1) mass transport of nanowire

growth species in vapor phase, (2) chemical reaction on the liquid catalyst droplet surface, (3)

diffusion in the liquid phase, and (4) crystallization at the liquid-solid interface. In this case,

nanorod only nucleates on metal catalyst and site-selective growth can be realized. The size of

metal catalyst determines the dimension of resulting structures.









ZnO nanoparticle formation via the internal oxidation of Zn in Ag/Zn alloys has previously

been reported.116 In these studies, oxygen is diffused into an Ag/Zn alloy, with nanoscale ZnO

precipitates forming in the bulk of the sample. For the present case of nanorod formation, the

reaction between ozone/oxygen flux and the Ag islands appears to result in surface and

subsurface oxygen diffusion in the metal island, perhaps involving the intermediate formation of

Ag20. Zn atoms impinging on the Ag island surface then diffuse either on the surface or in the

bulk of the island, where they react with the Ag20 to form ZnO. The solid solubility of Zn in Ag

is on the order of 25 wt.% for the temperatures considered in these experiments. Zn addition to

Ag significantly suppresses the melting point to 7100C at 25 wt.% Zn. The melting point of Zn is

420C. Based on these arguments, one might anticipate rather high diffusion rates for Zn in Ag

for the temperatures considered. It should also be noted that the addition of Ag during the growth

of complex oxide thin film has been reported to be effective in enhancing the oxidation process

for various oxide thin-film compounds.117

4.3.3 Nucleation control and site-selective growth

Figure 4-4 shows SEM micrographs of 25 A Ag films on SiO2/Si after different annealing

temperature and time: (a) 700C for 5 min; (b) 700C for 30 min; (c) 600C for 5 min; and (d)

800C for 5 min. The effect of the annealing is the creation of Ag clusters. The cluster size

increases with anneal temperature from 8 nm at 6000C to 30 nm at 8000C while the cluster

density decreases rapidly (by about a factor of 25) in this range. Similar data is shown in Figure

4-5 for the Ag films on SiNx/Si substrates. The same general trends are seen as with the SiO2/Si.

The variation of both cluster diameter and density are shown in Figure 4-6 and 4-7 for the

SiO2/Si and SiNx/Si templates, respectively. Note that we can control the cluster density in the

range of 100-2500 mm2 for SiO2/Si and 30-1900 mm2 for SiNx/Si using an initial Ag film

thicknesses of 25 A and annealing temperatures of 600-800C. The corresponding range of









cluster diameters is 8-30 nm on SiO2 and 10-65 nm on SiNx. At 700C, the annealing time has a

much stronger effect on cluster density than size in both cases, while the growth of the clusters

and associated decrease in density is much more significant at the higher temperatures, as

expected. Preliminary atomic force microscopy results show that the height of the clusters also

increases as the diameter increases.

To examine the selective area growth of ZnO nanorods on regions, we patterned parallel

lines by lift-off of e-beam deposited 25 A thick films with width 20 mm and separation of 450

mm were patterned. The samples were annealed at 800C for 1 min to create the Ag clusters and

then ZnO nanorods were grown, as described previously. FE-SEM micrographs taken at different

magnifications of the selectively grown wires are shown in Figure 4-8. The ability to control

both the wire density and location is useful in applications where, for example, the wires need to

be grown on an electrode for sensing or UV photodetection.

4.3.4 Structural and optical properties of ZnO nanowires grown on sapphire

From the previous experimental results, it's clear that nanowires grown on silicon substrate

tend to be short and tapered. Generally, 1-D nanomaterials with high aspect ratios are highly

desired for device applications. The low aspect ratio structures limited their applications in

device application. Therefore, efforts have been made on synthesis longer and relatively

untapered nanowires. Long nanowires provide higher surface-to-volume ratios and are beneficial

to device applications.

The choice of the substrate material is another concern in synthesis of 1-D materials.

Silicon has a lot of advantages such as low cost and easy process integration; however the large

lattice mismatch makes it difficult to control the growth. Epitaxial ZnO films have been realized

on various substrate orientations of sapphire (A1203) substrates, the small lattice mismatch makes

them suitable substrates for II-VI semiconductor growth.29 For a wide range of growth conditions,









c-axis oriented epitaxial ZnO films have been realized on c-plane sapphire. In the concern of

lattice mismatch, the c-sapphire substrates coated with gold were used for nanowire growth

substrates. Similar procedures were preformed to produce Au cluster size in the range of 50-650

nm in diameter. As shown in Figure 4-9, no 1-D growth observed when the Au clusters are larger

than 200 nm. The size of gold clusters slightly increased and surface became rough after growth,

which suggests some surface reaction occurred. When the sizes of Au catalysts are smaller than

150 nm, high density ZnO nanowires were synthesized as shown in Figure 4-10. Therefore, the

dimensions of Au catalysts have significant effect on nanowire growth. By selecting proper

metal catalyst size and lattice matched substrates, high density ZnO nanowires were synthesized

by catalyst-driven molecular beam epitaxy. A morphology evolution from nanorods to nanowires

is observed when using Au catalyst on c-sapphire substrate. It provides alternative synthesis

approach to produce high aspect-ratio ZnO nanowires for device applications.

Figure 4-11 shows top-view and side-view FE-SEM image of the ZnO nanowires grew on

c-plane sapphire at 500 and 600C. In contrast of pervious case (silicon), the nanowires grown on

sapphire have higher aspect ratios and uniform diameters. The nanowires nucleate on Au

particles on the surface and highly cross-linked together with c-axis orientation. The diameters of

the ZnO nanowire are approximately 20-60 nm and length up to 8[tm. At higher temperatures

(600C), nanorods are observed with slightly longer length (10[tm) with smaller diameters. As a

result, high growth temperature provides sufficient activation energy of crystallization results in

longer nanowires growth.

The XRD pattern of the nanowires grown on sapphire at 5000C is shown in Figure 4-12.

Only a strong (002) and weak (101) peaks are observed, which suggest a more uniform growth

direction and high density of nanowires. Figure 4-13 shows the room temperature









photoluminescence spectra of ZnO nanowires grown at 5000C on sapphire. Compared with

nanorods grown on Ag/Si, a much stronger near band edge emission at -380 nm with relative

small deep level emission, which suggest they are highly crystalline with relative low defects.

ZnO nanowires grew on Au/sapphire appears to have better quality and optical property than

those on Ag/Si. These high quality ZnO nanowires appear to be candidates for nano-electronics,

nano-sensor applications.

4.4 Summary and Conclusions

In summary, the synthesis and nucleation control of ZnO nanowires via VLS growth

mechanism is studied. The control of initial Ag film thickness and subsequent annealing

conditions is shown to provide an effective method for controlling the size and density of

nucleation sites for catalyst-driven growth of ZnO nanorods. By using Ag film thickness of 25 A

on SiO2 or SiNx layers on Si substrates, we have shown that annealing between 600 and 800C

creates Ag cluster size in the range of 8-30 nm diameter for Si02 and 10-65 nm for SiNx with a

cluster density from 100 to 2500 mm2 for Si02 and 30 to 1900 mm2 for SiNx. Conventional

optical lithography to create parallel Ag stripes shows that completely selective growth is

possible on either dielectric. High density cross-linked ZnO nanowires were synthesized when

Au catalysts were in the range of 50-150 nm in diameter. By selecting proper metal catalyst size

and lattice matched substrate, high density ZnO nanowires were synthesized by catalyst-driven

molecular beam epitaxy. A morphology evolution from nanorods to nanowires is observed when

using Au on c-sapphire substrate. It provides alternative synthesis approach to produce high

aspect-ratio ZnO nanowires for nano-device applications. Those nanowires will be applied for

gas sensing applications and described in the next Chapter.




































Figure 4-1. (a)-(b) Top view (d) side view FE-SEM images of ZnO nanorods on a Ag coated
silicon grew at 4000C.


MAII', sl 1 r1kV Xp(lo ''I WI)












MAIC A I V'OkV X0001) lpi.. INI j 1 A


SH 150kV M0000














(002)





>1
"t: (101)
C/-
C:

C (103)
100) (102)
(110)


30 40 50 60 70 80

29 (deg)

Figure 4-2. X-ray diffraction pattern of ZnO nanorods grown on a 20A Ag coated SiO2/Si
substrate at 4000C.


I I



























(d)


360 380 400 420 440
Wavelength (nm)


Figure 4-3. (a) Low magnification TEM image of single ZnO nanorod. (b) High resolution TEM
image of a single ZnO nanorod with lattice fringes. (c) Select area diffraction patterns
taken from a single ZnO nanorod showing the single crystal wurtzite structure. (d)
Room temperature PL spectra of as-grown ZnO nanorods.





































Figure 4-4. 25A Ag on SiO2/Si with different annealing temperature and time: (a) 700C for 5
minutes (b) 700C for 30 minutes (c) 600C for 5 minutes and (d) 800C for 5
minutes.


7000C for 5 min 100 nm












6000C for 5 min.


7000C for 30 min 100 nm

















"-' 0 foIr 5 min 0i








"lW" "1 r 5 min -- 0 i


Figure 4-5. 25A Ag on Si3N4/Si with different annealing temperature and time: (a) 700C for 5
minutes (b) 700C for 30 minutes (c) 600C for 5 minutes and (d) 800C for 5
minutes.


7000C for 30 min 100 nin



(d)







8000C for 5 min 100 nin

















40


20


60


40


20


600
600


S-*-Diameter
.-- Density
25A Ag on SiO2
7000C annealing



----------- M
-U



I =I I
S10 20 3(
Time (min)


700
Temperature (OC)


-


-


800


Figure 4-6. The top plot shows the density and average size of the resulting Ag clusters on SiO2
as a function of anneal time at 700C. The bottom plot shows density and average size
of the resulting Ag clusters on SiO2 as a function of anneal temperature for 5 min
anneals.


-- Diameter
--Density
25A on SiO2
5 min annealing


2000O


1500o


1000=


500

0








2500,

2000

1500

1000G

500

0












-*-Diameter -2000
60 -.-Density
25A Ag on SiNx
\- 1500
7000C annealing 1
40-
.-\ 1000

S20- Al OWS I
-. 500 b

0 0
ol--------------J

0 10 20 30
Time (min)




S -- 2000
60 = =-~* .

40 -
-1500

S-*-Diameter 1000 *
----Density
25A Ag on SiN
20- x 500 O
S 5 min annealing

U 0 I
00 U
600 700 800
Temperature (oC)

Figure 4-7. The top plot shows the density and average size of the resulting Ag clusters on Si3N4
as a function of anneal time at 700C. The bottom plot shows density and average size
of the resulting Ag clusters on Si3N4 as a function of anneal temperature for 5 min
anneals.



































Figure 4-8. Scanning electron microcope images of selectively grown ZnO nanorods on 25 A
Ag/Si02.




















































Figure 4-9. (a), (b) Scanning electron microscope images of 200-650 nm of Au clusters on
sapphire. (c), (d) resulting ZnO nanowires.


MAIL SF I V,:kV X000 :,.F" WO 1".;3


MAW:











































Figure 4-10. (a), (b) Scanning electron microscope images of 50-150 nm of Au clusters on
sapphire. (c), (d) resulting ZnO nanowires.


MAIL SF I V, kV X 000 wl) 1,.j


MAIL SF I !:,:kV X;`,(RIU 1010-11 WO ll,.;3



(d)











KA A 1 ". )kV















































Figure 4-11. (a), (b) Top view and side view scanning electron microscope images of ZnO
nanowires on an Au coated c-sapphire grew at 500C (c), (d) SEM images of ZnO
nanowires on an Au coated c-sapphire grew at at 6000C.


MAI" SF I i.,,!,V x WO 11 11


MAI" SF I W O 11 11









| Sapphire


2e (deg)

Figure 4-12. X-ray diffraction pattern of ZnO nanowires grown on a sapphire substrate at 6000C.


350


400 450 500 550


600


Wavelength (nm)


Figure 4-13. Room temperature photoluminescence spectra of ZnO nanowires grown on sapphire
at 6000C.


(002)







(101)









CHAPTER 5
ZNO NANOWIRES FOR HYDROGEN SENSING APPLICATIONS

5.1 Introduction

Solid-state gas sensors play an important role in environmental monitoring, chemical

process controlling and personal safety. Semiconductor metal oxide sensors have been widely

used due to their low cost and high compatibility with microelectronic processing.21'100 In the

case of the polycrystalline thin film devices, only a small fraction of the species adsorbed near

grain boundaries is active in modifying transport properties.103'118 The low surface-to-volume

ratios also result the limitations in their applications, some drawbacks such as low sensitivity and

slow response.

Recently, there is strong interest in developing hydrogen sensors for use with proton-

exchange membrane and solid oxide fuel cells for space craft and other long-term applications. A

key requirement for these sensors is the ability to selectively detect hydrogen at room

temperature with minimal power use and weight. It is clear that nanowires and nanotubes are

excellent candidates for this type of sensing, given their large surface-to-volume ratios and low

weight. The ability to detect hydrogen selectively at room temperature is important because it

avoids the use of on-chip heaters that add to the power consumption and weight. In the case of

hydrogen sensing with carbon nanotubes (CNTs), several groups have reported that use of Pd

doping or films or loading with Pd nanoparticles can functionalize the surface of nanotubes for

catalytic dissociation of H2 to atomic hydrogen.119-125 Of course, thin-film sensors of Si, GaAs,

InP, SiC, and GaN with Pd contacts have been used previously as hydrogen sensors.124

ZnO nanowires are attractive for a wide variety of sensing applications because of the ease

of synthesis, ability to readily transfer them to cheap substrates and their bio-safe characteristics.

ZnO nanowires and nanowires have shown potential for use in gas, humidity, and chemical









sensing. The ability to make arrays of nanowires with large total surface area has been

demonstrated with a number of different growth methods and a large variety of ZnO one-

dimensional structures has been demonstrated. To date, most of the work on ZnO nanostructures

has focused on the synthesis methods. The large surface area of the nanowires and biosafe

characteristics of ZnO makes them attractive for both chemical sensing and biomedical

applications. There are still many aspects of this approach that require work, including

quantifying the sensitivity, detection limits at room temperature, power consumption of the

sensors, and time response upon switching away from the H2-containing ambient.

Tin oxide (SnO2) n-type semiconductor sensors are widely used for detection of reducing

gases and carbon monoxide.37'39'126 In general, these sensors have suffered from relatively low

selectivity for different gases and long-term instabilities in their response. New directions

towards solving these problems include use of nanocrystalline thin films or use of catalysts to

increase dissociation of gases at lower temperatures. The sensitivity of SnO2 sensors can be

enhanced by reducing the nanocrystallite size below 10 nm.127 SnO2 operates at lower

temperatures (300C) and is sensitive to a wider range of gases in comparison with many other

thin film sensor candidates.128'129 This makes it possible to change its sensitivity to a specific

compound or group of compounds by the addition of the appropriate substances. For example,

previous work has shown that the sensitivity and selectivity of SnO2 to ethanol can be improved

by adding La203, Y, Pd or Pt128-130 and to CO can be improved by adding MoO3.131 SnO2 or

multilayers of SnO2, ZnO, TiO2 and WO3 with the addition of Pd coatings have been reported for

the detection of the mixtures of methanol and acetone.132-136 There has also been recent interest

in the use of ZnO nanowires for sensing. ZnO has been effectively used as a gas sensor material









based on the near-surface modification of charge distribution with certain surface-absorbed

species.137

In this Chapter, applications of ZnO nanowires as material for hydrogen sensors will be

addressed. The addition of sputter-deposited metal clusters to the surface of ZnO nanowires

produces a significant increase in detection sensitivity for hydrogen at room temperature. The

sensors are shown to detect ppm hydrogen at room temperature using <0.4 mW of power when

using multiple nanowires. When using a single ZnO nanowire sensor, the power consumption

can further pushing down to [tW range. Furthermore, a comparison study of the hydrogen-

sensing characteristics of ZnO thin films with different thickness and ZnO nanowires will be

described. Both types of sensors are shown to be capable of the detection of ppm hydrogen at

room temperature with nW power levels, but the nanowires show different recovery

characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen. The

use of single-crystal ZnO nanowires provide a convenient template for coating with SnO2 and the

resulting structure can be used to detect hydrogen at 4000C.

5.2 Experimental Methods

5.2.1 Synthesis and fabrication of ZnO nanowires sensors

The site-selective growth of ZnO nanowires was achieved by nucleating the nanowires on

a substrate coated with Au islands as has been described in Chapter 3. In general, ZnO nanowires

were synthesized by molecular beam epitaxy with a base pressure of 5 x 108 mbar using high

purity Zn metal (Alfa Aesar, 99.9999%) and an 03/02 plasma discharge as the source chemicals.

The Zn pressure was varied between 4x 10-6 and 2x 10-7 mbar, while the beam pressure of the

03/02 mixture was varied between 5x 10-6 and 5x 104 mbar. The growth time was -2 h at 5000C.

The typical length of the as-grown nanowire was 2-10 [am, with typical diameters in the range of

30-150 nm.









A shadow mask was used to pattern sputtered Al/Ti/Au electrodes on the ZnO

nanowire/A1203 substrates. The separation of the electrodes was 30 am. In some cases, the

nanowires were coated with metal catalyst thin films (-100 A thick) deposited by sputtering.

This forms clusters of metal with -70% coverage of the nanowire surface and rms roughness of

-80 A. Figure 5-1 illustrates the metal catalysts decorated ZnO nanowire. Au wires were bonded

to the contact pad for current-voltage (I-V) measurements performed at 250C in a range of

different ambients (N2, 02 or 10-500 ppm H2 in N2). Note that no currents were measured

through the discontinuous Au islands and no thin film of ZnO was observed with the growth

condition for the nanowires. The I-V characteristics from the multiple nanowires were linear

with typical currents of 0.8 mA at an applied bias of 0.5 V.

5.2.2 Synthesis and fabrication of ZnO thin films sensors

The ZnO thin films were grown by Pulsed Laser Deposition on c-plane sapphire substrates

at 4500C, as described previously. The thickness of thin film was varied from 20-350 nm. A 3%

03/02 mixture was used as background gas resulting lower carrier density (-1017 cm-3). The

films were nominally undoped with low n-type 1017 cm-3 carrier concentration. The Ohmic

contacts of sputtered Al/Ti/Au were patterned by a shadow mask. In some cases, the sensors

were coated with Pt thin films 10 A thick deposited by sputtering. Au wires were bonded to the

contact pad for current-voltage I-V measurements performed at 250C in air, N2 or 500 ppm H2 in

N2. No currents were measured through the discontinuous Au islands.

5.2.3 Synthesis and fabrication of single ZnO nanowire sensors

The nanowires were removed from the original substrate by sonication and transferred to

a Si substrate. The e-beam lithography was used to pattern sputtered Al/Ti/Au electrodes

contacting both ends of single nanowires on the Si substrates. The separation of the electrodes

was 10 am. In some cases, the nanowires were coated with discontinuous Pt cluster (10-20 A









thick) deposited by sputtering as shown in Figure 5-1. Au wires were bonded to the contact pad

for current-voltage (I-V) measurements performed at 250C in a range of different ambient

(vacuum, N2, 02 or 100-500 ppm H2 in N2).The I-V characteristics from the uncoated single

nanowires were linear with typical currents in the nA at an applied bias of 0.5 V.

5.2.4 Synthesis and fabrication of SnO2-ZnO nanowire sensors

The SnO2 layers were deposited on as-grown ZnO nanowires by PLD at 600C, a partial

pressure of 02 of 50 mTorr and 130 mJ of laser power with 1 Hz repetition rate, with two

different deposition times of 5 or 10 min. The pattern sputtered Al/Ti/Au electrodes contacting

both ends of multiple nanowires on the A1203 substrates using a shadow mask. The separation of

the electrodes was 3 am. Au wires were bonded to the contact pad for current-voltage (I-V)

measurements performed over the range 25-400C in a range of different ambients (N2, vacuum

or 500 ppm H2 in N2).

5.3 Results and Discussion

5.3.1 Catalyst functionalized ZnO nanowires

Previous results show that ZnO nanowires are not very sensitive to hydrogen at room

temperature.138 In order to enhance sensitivity at room temperature and realize gas sensing

applications, efforts have been working on surface fictionalizations. One method for increasing

hydrogen detection sensitivity is to use a catalytic metal coating or to actually dope the sensor

material with the transition metal. This leads to catalytic dissociation of H2 to atomic hydrogen,

which produces a sensor response through binding to surface atoms and altering the surface

potential.

A comparison study of different metal coating layers on multiple ZnO nanowires for

enhancing the sensitivity to detection of hydrogen at room temperature has been done. Figure 5-2

shows the time dependence of relative resistance change of either metal-coated or uncoated









multiple ZnO nanowires as the gas ambient is switched from N2 to 500 ppm of H2 in air and then

back to N2 as time proceeds. These were measured at a bias voltage of 0.5V. The first point of

note is that there is a strong increase (by approximately a factor of five) in the response of the Pt-

coated nanowires to hydrogen relative to the uncoated devices. The maximum response was

approximately 8%. There is also a strong enhancement in response with Pd coatings, but the

other metals produce little or no change. This is consistent with the known catalytic properties of

these metals for hydrogen dissociation. Pd has a higher permeability than Pt but the solubility of

H2 is larger in the former.139,140 Moreover, studies of the bonding of H to Ni, Pd and Pt surfaces

have shown that the adsorption energy is lowest on Pt.141 There was no response of either type of

nanowire to the presence of 02 in the ambient at room temperature.

Once the hydrogen is removed from the ambient, the recovery of the initial resistance is

rapid (< 20 s). By sharp contrast, upon introduction of the hydrogen, the effective nanowire

resistance continues to change for periods of >15 min. This suggests that the kinetics of the

chemisorption of molecular hydrogen onto the metal and its dissociation to atomic hydrogen are

the rate-limiting steps in the resulting change in conductance of ZnO.142 An activation energy of

12 kJ/mole was calculated from a plot of the rate of change of nanowire resistance. This energy

is somewhat larger than that of a typical diffusion process and suggests that the rate-limiting-step

mechanism for this sensing process is more likely to be the chemisorption of hydrogen on the Pd

surface. This reversible change in conductance of metal oxides upon chemisorption of reactive

gases has been discussed previously.142 The gas-sensing mechanisms suggested in the past

include the desorption of adsorbed surface hydrogen and grain boundaries in poly-ZnO143

exchange of charges between adsorbed gas species and the ZnO surface leading to changes in

depletion depth144 and changes in surface or grain-boundary conduction by gas









adsorption/desorption.145 We should also point out that the I-V characteristics were the same

when measured in vacuum as in air, indicating that the sensors are not sensitive to humidity.

The power requirements for the sensors were very low. The I-V characteristics measured at

25 C in both a pure N2 ambient and after 15 min in a 500 ppm H2 in N2 ambient. Under these

conditions, the resistance response is 8% and is achieved for a power requirement of only 0.4

mW. This compares well with competing nanotechnologies for hydrogen detection such as Pd-

loaded carbon nanotubes. Moreover, the 8% response compares very well to the existing SiC-

based sensors, which operate at temperatures > 100 C through an on-chip heater in order to

enhance the hydrogen dissociation efficiency.124 Figure 5-3 and 5-4 shows the response of Pt and

Pd coated nanowire sensor to 10-500 ppm H2 in N2. The Pt-coated sensors can detect 100 ppm

H2 while Pd-coated sensors can detect down to 10 ppm H2.

In conclusion, Pt is found to be the most effective catalyst, followed by Pd. The resulting

sensors are shown to be capable of detecting hydrogen in the range of ppm at room temperature

using very small current and voltage requirements and recover quickly after the source of

hydrogen is removed.

5.3.2 Room temperature hydrogen selective sensing with ZnO nanowires

A more detail study of hydrogen sensing with catalyst functionalized ZnO nanowires will

be discussed in this section. Figure 5-5 shows the time dependence of resistance of either Pd-

coated or uncoated multiple ZnO nanowires as the gas ambient is switched from N2 to various

concentrations of H2 in air (10-500 ppm) as time proceeds. There are several aspects of the data.

First, there is a strong increase (approximately a factor of 5) in the response of the Pd-coated

nanowires to hydrogen relative to the uncoated devices. The addition of the Pd appears to be

effective in catalytic dissociation of the H2 to atomic hydrogen. Second, there was no response of

either type of nanowire to the presence of 02 in the ambient at room temperature. Third, the









effective conductivity of the Pd-coated nanowires is higher due to the presence of the metal.

Fourth, the recovery of the initial resistance is rapid (20 s) upon removal of the hydrogen from

the ambient, while the nanowire resistance is still changing at least 15 min after the introduction

of the hydrogen. The reversible chemisorption of reactive gases at the surface of metal oxides

such as ZnO can produce a large and reversible variation in the conductance of the material.143

Fifth, the relative response of Pt-coated nanowires is a function of H2 concentration in N2.

The Pd-coated nanowires detected hydrogen down to 10 ppm with relative responses of

2.6% at 10 ppm and 4.2% at 500 ppm H2 in N2 after a 10 min exposure, as shown in Figure 5-6.

By comparison, the uncoated devices showed relative resistance changes of 0.25% for 500 ppm

H2 in N2 after a 10 min exposure, and the results were not consistent for lower concentrations.

The gas-sensing mechanisms suggested in the past include the desorption of adsorbed surface

hydrogen and grain boundaries in poly-ZnO144, exchange of charges between adsorbed gas

species and the ZnO surface leading to changes in depletion depth146 and changes in surface or

grain boundary conduction by gas adsorption/desorption.145 The detection mechanism is still not

firmly established in these devices and needs further study. It should be remembered that

hydrogen introduces a shallow donor state in ZnO and this change in near-surface conductivity

may also play a role.

Figure 5-7 shows the time dependence of resistance change of Pt-coated multiple ZnO

nanowires as the gas ambient is switched from vacuum to N2, oxygen or various concentrations

of H2 in air (10-500 ppm) and then back to air. These data confirm the absence of sensitivity to

02. The resistance change during the exposure to hydrogen was slower in the beginning and the

rate resistance change reached maximum at 1.5 min of the exposure time. This could be due to

some of the Pd becoming covered with native oxide, which is removed by exposure to hydrogen.









Since the available surface Pd for catalytic chemical absorption of hydrogen increased after the

removal of oxide, the rate of resistance change increased. However, the Pd surface gradually

saturated with the hydrogen and the rate of resistance change decreased. When the gas ambient

switched from hydrogen to air, the oxygen reacted with hydrogen right away, with the resistance

of the nanowires changed back to the original value instantly. Moreover, the data were recorded

at a power level of 0.4 mW which is low even in comparison with CNTs.123,125 This is attractive

for long-term hydrogen sensing applications.

The rate of resistance change for the nanowires exposed to the 500 ppm H2 in N2 was

measured at different temperatures as shown in Figure 5-8. Figure 5-9 shows the Arrhenius plot

of nanowire resistance change rate. An activation energy of 12 kJ/mole was calculated from the

slope of the Arrhenius plot. This value is larger than that of a typical diffusion process. Therefore,

the dominant mechanism for this sensing process is more likely to be the chemisorption of

hydrogen on the Pd surface.

In conclusion, Pd-coated ZnO nanowires appear well suited to detection of ppm

concentrations of hydrogen at room temperature. The recovery characteristics are fast upon

removal of hydrogen from the ambient. The ZnO nanowires can be placed on cheap transparent

substrates such as glass, making them attractive for low-cost sensing applications and operate at

very low power conditions.

5.3.3 Single ZnO nanowire sensors

In this section, we describe how the addition of sputter-deposited Pt clusters to the

surface of single ZnO nanowires produces a significant increase in detection sensitivity for

hydrogen at room temperature. The sensors are shown to detect ppm hydrogen at room

temperature using [tW of power. Figure 5-10 shows that the addition of the Pt-coatings increased

the effective conductivity of the nanowires by over an order of magnitude. Because the Pt films









are discontinuous as evidenced by both field-emission scanning electron microscopy and atomic

force microscopy, this suggests that the sputtering process itself changes the resistance of the

nanowires, most likely through the introduction of oxygen vacancies which are donor states in

ZnO.147,148 There was a strong increase approximately a factor of 5 in the response of the Pt-

coated nanowires to hydrogen relative to the uncoated devices. Figure 5-11 shows the I-V

characteristics of Pt-coated nanowires as a function of both the measurement ambient and the

time after exposure to 500 ppm H2 in N2. There are several aspects of the data. There was no

response of either coated or uncoated nanowires to the presence of 02 in the ambient at room

temperature and indeed the I-V characteristics were independent of the measurement ambient for

vacuum, air, or pure N2. By sharp contrast, the nanowires were sensitive to the presence of H2 in

the ambient, with the response being time-dependent. The nanowire resistance was still changing

at least 15 min after the introduction of the hydrogen. An Arrhenius plot of the rate of resistance

change for the nanowires exposed to the 500 ppm H2 in N2 for 10 min produced an activation

energy of 15 kJ/mol. This is larger than that expected for typical diffusion processes and suggests

that the rate-limiting step may be chemisorption of hydrogen on the Pt surface. The reversible

chemisorption of reactive gases at the surface of ZnO can produce a large reversible variation in

the resistance.142 In addition, atomic hydrogen introduces a shallow donor state into ZnO and this

may play a role in the increased conductance of the nanowires.147,149 The diffusion coefficient of

the hydrogen is also much faster in ZnO than in any other wide band-gap semiconductor. Note

the very low power consumption of the nanowire sensors, which is in the range 15-30 aW. This

is approximately a factor of 25 lower than multiple ZnO nanowires operated under the same

conditions and more than a factor of 50 lower than carbon nanotubes doped with Pd that were









used for hydrogen detection.123,125 The low power consumption is clearly of advantage in many

types of remote sensing or long-term sensing applications.

Figure 5-12 shows the time dependence of current (top) or relative resistance change

(bottom) in both the uncoated and Pt-coated nanowires exposed to 500 ppm H2 in N2. The

relative resistance responses were 20 and 50%, respectively, after 10 or 20 min exposure. By

comparison, the uncoated devices showed relative resistance changes of 2 and 3%, respectively,

after 10 or 20 min exposure. The resistance change during the exposure to hydrogen was slower

in the first few minutes, as is clear in Figure 5-13. This may be due to removal by the atomic

hydrogen of native oxide on the Pt. As the effective surface area of the Pt would increase as the

oxide was removed, the rate of change of resistance due to hydrogen adsorption should also

increase. At fixed voltage, the relative resistance change was linear as a function of hydrogen

content in the measurement ambient up to a few percent and then increased more slowly at

higher concentrations. This may indicate a saturation of bonding sites for hydrogen at high

concentrations. We have not yet investigated the long-term reliability and reproducibility of the

nanowire sensors, but this aspect will be a key for practical applications. We have measured the

time recovery characteristics of the single nanowires when hydrogen is removed from the

ambient and find the recovery is limited by the time needed to flush the hydrogen out of the test

fixture a few seconds and not by the nanowire response.

In summary, Pt-coated ZnO single nanowires are shown to selectively detect hydrogen at

room temperature with very low power consumption. The disadvantage of this approach relative

to using a network of multiple nanowires is the additional processing that is needed to contact a

single nanowire, but the power consumption is significantly (a factor of -25) lower.









5.3.4 A comparison of ZnO thin film and nanowire sensors

Previous work, ZnO nanowires have been demonstrated to have good sensitivity as sensing

material due to their larger surface area and high aspect ratio. However, to this point, there has

been no clear demonstration of improved detection sensitivity with nanowires compared to thin

films. In this section, we report on a comparison of the hydrogen-sensing characteristics of ZnO

thin films of different thicknesses and ZnO nanowires, both with Pt coatings. Both types of

sensors are shown to be capable of the detection of ppm hydrogen at room temperature with nW

power levels, but the nanowires show different recovery characteristics, consistent with the

expected higher surface coverage of adsorbed hydrogen.

Two types of ZnO were employed in these experiments: nanowires and thin films. The thin

films were grown by Pulsed Laser Deposition on sapphire substrates at 450 C, as described in

detail previously. The ZnO thickness was varied from 20-350 nm. Figure 5-13 shows the I-V

characteristics measured between the Ohmic contacts on the thin film ZnO samples of either 20

or 350 nm thickness, both before and after the Pt deposition on the surface. The current increase

as a result of the Pt deposition is approximately a factor of 2 for the thinnest sample and remains

in the nA range at 0.5 V bias, i.e., the power consumption is 4 nW at this operating voltage. The

effective conductivity of the Pt-coated films is higher due to the presence of the metal. At longer

Pt sputtering times, we would typically see a transition to much higher currents, as the Pt film

became continuous and the conductivity of the structure was no longer determined by the ZnO

layer itself.

Figure 5-14 (top) shows the time dependence of current change at 0.5 V bias on the Pt-

coated ZnO films of different thickness as the gas ambient is switched from N2 to 500 ppm H2 in

N2 and back to air as time proceeds. This data shows that the sensors are insensitive to N2 and

that there is a strong ZnO thickness dependence to the response to hydrogen. The bottom of









Figure 5-14 shows the change in current at 0.5 V bias when switching from N2 to the hydrogen-

containing ambient for the ZnO films of different thickness. At small thicknesses, the current

change is small, which is probably related to poorer crystal quality and also at large film

thickness where the bulk conductivity dominates the total resistance.

Figure 5-15 shows the time dependence of current in both the Pt-coated multiple ZnO

nanowires and the thin films, as the gas ambient is switched from N2 to 500 ppm H2 in N2 and

then back to air as time proceeds. It is clear that the nanowires have a much larger response

roughly a factor of 3, even for the optimal response for the thin films to the introduction of

hydrogen into the ambient compared to their thin film counterparts. This is consistent with the

expectation of a higher relative response based on their larger surface-to-volume ratio. Although

not shown here, there was no response of either type of sensor to the presence of 02 in the

ambient at room temperature. The recovery of the initial resistance is rapid 90%, 20 s upon

removal of the hydrogen from the ambient by either 02 or air, while the nanowire resistance is

still changing at least 15 min after the introduction of the hydrogen. The response is faster at

higher temperatures.

The nanowires show a slower recovery than the thin films, most likely due to the relatively

higher degree of hydrogen adsorption. The expected sensing mechanism suggested previously is

that reversible chemisorption of the hydrogen on the ZnO produces a reversible variation in the

conductance, with the exchange of charges between the hydrogen and the ZnO surface leading to

changes in the depletion depth.142,143,146 The conductivity of both the ZnO thin film and

nanowires did change when the ambient switched from N2 to air. Figure 5-16 shows the

maximum current change at 0.5V bias for exposure of the nanowires and thin films to the

500ppm H2 in N2. As discussed earlier, a key requirement in long-term hydrogen-sensing









applications is the sensor power consumption. Both the thin film and multiple nanowire sensors

can operate at 0.5 V bias and powers 4 nW. We have also demonstrated hydrogen sensing with

single ZnO nanowires at power levels approximately an order of magnitude lower than this, but

the devices show poorer long-term current stability than multiple nanowire sensors.

In conclusion, Pt-coated ZnO thin films and multiple nanowires both are capable of the

detection of ppm concentrations of hydrogen at room temperature. The thin films show optimum

responses to the presence of hydrogen at moderate thicknesses. The nanowires show larger

responses to hydrogen than the thin films, consistent with their large surface-to-volume ratios

and have the advantage in terms of flexibility of the choice of substrate.

5.3.5 Surface functionalized SnO2-ZnO nanowire sensors

In this section we show the use of single-crystal ZnO nanowires provide a convenient

template for coating with SnO2 and the resulting structure can be used to detect hydrogen at

400C. Since the metal oxides exhibit different sensing characteristics toward chemical or gas

species. Therefore, the rationale for this work is that it is straightforward to grow ZnO nanowires

and the method provides an approach for integrating a range of oxides with high surface-to-

volume ratio.

Figure 5-17 (top and center) shows scanning electron microscopy (SEM) micrographs of

the Sn02/ZnO structures while the bottom of the figure shows an energy dispersive X-ray (EDX)

spectrum. The latter shows characteristic Sn X-rays and confirms the presence of the Sn02 layer.

Figure 5-18 shows X-ray diffraction (XRD) spectra from the SnO2-coated ZnO nanowires. The

(200) peak from the SnO2 is readily detected, while the (301) peak is barely visible in the

samples deposited for 10 min. When take N in consideration with the electron microscopy

images discussed below, the data is consistent with the Sn02 being polycrystalline.









Figure 5-19 shows some high-resolution transmission electron microscopy (HR-TEM)

images from the hybrid structure. It is clear that the SnO2 is deposited on only one side of the

ZnO nanowires, the thickness of the SnO2 is around 10 nm and that these layers are

polycrystalline. A lattice image of the SnO2/ZnO structures is shown in Figure 5-20,

emphasizing the ZnO is single-crystal while the SnO2 is polycrystalline. EDX line scans shown

in figure 5-21 confirmed that the SnO2 was present on only one side of the ZnO, as expected

from the line-sight geometry in the PLD chamber.

To perform the gas detection measurements, the multiple nanowires were contacted and

mounted on a standard header. Figure 5-22 shows the current-voltage (I-V) characteristics from

SnO2-coated ZnO nanowires for two different deposition times (top) and time dependence of

current at fixed bias of 0.5 V as a function of measurement ambient (bottom). The devices with

the thinner SnO2 has a higher current, suggesting there is less surface depletion in the ZnO

nanowire under these conditions. It is not clear why the addition of another 5-10 nm of SnO2

should increase the effective resistance of the underlying ZnO nanowire. There was no response

at room temperature to the introduction of 500 ppm hydrogen into the measurement ambient and

the structure shows more drift in current in air relative to pure N2.

Figure 5-23 shows I-V characteristics from SnO2-coated ZnO nanowires at room

temperature or 400C (top) and current at fixed bias of 0.5 V as the wires are heated as a

function of time (bottom). The conductivity of the ZnO is clearly sensitive to temperature, with

the current being thermally activated with activation energy 0.42 + 0.11 eV. This probably

represents the ionization energy of the dominant interfacial state between the ZnO and SnO2

since pure ZnO nanowires without any coating exhibit lower activation energy for their

conductivity (0.11 eV).









Figure 5-24 shows the current at fixed bias of 0.5 V and temperature of 400C in the

SnO2/ZnO structures as a function of time as the ambient is switched from N2 to 500 ppm H2 in

N2 or vacuum. Upon exposing the structures to 500 ppm H2, the conductivity increases, but

neither the introduction of pure N2 or vacuum helps for obtaining 100% recovery of the initial

current prior to the introduction of the trace amounts of hydrogen. The sensor continues to show

drift in the current at fixed voltage, as generally reported for SnO2. For the initial detection of the

hydrogen, the sensitivity was high 70%. However, the effective sensitivity for subsequent

detection events is lowered due to the current drift. There are multiple possible detection

mechanisms for hydrogen in this hybrid structure, including the doping of ZnO by hydrogen

donors, desorption of adsorbed surface oxygen and grain boundaries in poly-ZnO143, exchange of

charges between adsorbed gas species and the ZnO surface leading to changes in depletion

depth144 and changes in surface or grain boundary conduction by gas adsorption/desorption.145

In conclusion, single-crystal ZnO nanowires were coated with SnO2 using pulsed laser

deposition and characterized with SEM, TEM, XRD and EDX. The SnO2 was polycrystalline,

with typical thickness of order 10 nm. The hybrid structure shows a strong sensitivity to 500 ppm

H2 in N2 at 400C. This approach provides a relatively straightforward method to integrate

different oxides on templates with large surface-to-volume ratio.

5.4 Summary and Conclusions

The applications of ZnO nanowires as material for hydrogen sensors were addressed. A

variety of different metal catalysts (Pt, Pd, Au, Ag, Ti and Ni) sputter-deposited on multiple ZnO

nanowires have been compared for their enhancement for detecting hydrogen at room

temperature. It is found that the sensitivity for detecting hydrogen is greatly enhanced by sputter-

depositing metal catalysts (Pt and Pt) on surface. Pt-coated ZnO nanowires can detect hydrogen

down to 100 ppm with relative response of 4%. Pd-coated ZnO nanowires can detect hydrogen









down to 10 ppm with a relative smaller response than Pt-coated devices. Approximately 95% of

the initial conductance after exposure to hydrogen was recovered within 20 s by exposing the

device to air. The sensors are shown to detect ppm hydrogen at room temperature using <0.4

mW of power when using multiple nanowires. When using a single ZnO nanowire coated with Pt

as sensing material, the power consumption can further pushing down to [tW range. These

sensors are not sensitive to oxygen, nitrogen, humidity and air at room temperature, suggests

high selectivity for hydrogen sensing applications. Furthermore, a comparison study of the

hydrogen-sensing characteristics of ZnO thin films with different thickness and ZnO nanowires

was described. The Pt-coated single nanowires show a current response of approximately a factor

of 3 larger at room temperature upon exposure to 500 ppm of hydrogen. Both types of sensors

are shown to be capable of the detection of ppm hydrogen at room temperature with nW power

levels, but the nanowires show different recovery characteristics, consistent with the expected

higher surface coverage of adsorbed hydrogen. Finally, SnO2 coated ZnO nanowires were used

as materials for hydrogen sensors. There was no response to 500 ppm hydrogen at room

temperature but showed a 70% response at 400C. The use of single-crystal ZnO nanowires

provide a convenient template for coating with SnO2 and the resulting structure can be used to

detect hydrogen at 4000C.









Table 5-1. Relative resistance response of metal-coated multiple nanowires as the gas ambient is
switched from nitrogen to 500 ppm of hydrogen in air.
Metal Catalyst |ARI/R (%)
Platinum (Pt) 8.49
Palladium (Pd) 4.26
Gold (Au) 0.66
Titanium (Ti) 0.39
Nickel (Ni) 0.28
Silver (Ag) 0.16

Table 5-2. Relative resistance response of Pd and Pt coated multiple nanowires as the gas
ambient is switched from nitrogen to different concentration of hydrogen in air.
Concentration (ppm) Pd-ZnO |ARI/R (%) Pt-ZnO |AR|/R (%)
10 2.41 0.25
100 3.14 4.67
250 3.79 6.44
500 4.28 8.50










Electrode Electrode





Metal catalyst

Electrode Electrode







Figure 5-1. Metal catalysts decorated ZnO nanowires.















6 Pd
Au
4 -Ag
-0- Ti
Ni
S2-


0 ....................................

0 5 10 15 20 25 30
Time (min)

Figure 5-2. Time dependence of relative resistance response of metal-coated multiple nanowires
as the gas ambient is switched from N2 to 500 ppm of H2 in air as time proceeds.



10 .-. I.
.10-500 ppm H2 Air
8 -
Pt-ZnO nanowires
S l 500 ppm
Sv- 250 ppm
100 ppm
4 -- 10 ppm

2-

0 NOO 3

0 5 10 15 20 25 30

Time (min)

Figure 5-3. Time dependence of resistance change of Pt-coated multiple ZnO nanowires as the
gas ambient is switched from N2 to various concentrations of H2 in air (10-500 ppm)
and then back to N2.



























0 5 10 15 20
Time (min)


25 30


Figure 5-4. Time dependence of resistance change of Pd-coated multiple ZnO nanowies as the
gas ambient is switched from N2 to various concentrations of H2 in air (10-500 ppm)
and then back to N2.


950 -


670

660

650

640


0 30 60 90 120 150


Time (min)


Figure 5-5. Time dependence of resistance of either Pd-coated or uncoated multiple ZnO
nanowires as the gas ambient is switched from N2 to various concentrations of H2 in
air (10-500 ppm) as time proceeds. There was no response to 02.


ZnO nanorod without Pd


2 N l o ppm Air looppm ir 250ppm Air pAi
H2 H H2 H

-





-*- ZnO nanorod with Pd
I I I I '













0.00


-0.01


-0.02
<1
-0.03 O1ppm

100ppm
-0.04 H 250ppm 500p
H H
I I IH, I H,
0 30 60 90 120 150

Time (min)



Figure 5-6. Relative response of Pd-coated nanowires as a function of H2 concentration in N2.


680

675

670

665

660

655

650

645


0 5 10 15 20 25 30

Time(min)


Figure 5-7. Time dependence of resistance change of Pd-coated multiple ZnO nanowires as the
gas ambient is switched from N2 to oxygen or various concentrations of H2 in air(10-
500 ppm) and then back to N2.












10
500 ppm H2 Air
8 Pd-ZnO nanorods
200C
S6- 1500C
6 10000C
6-
100C
500C
<_ 4 roomT

2 -

0

0 5 10 15 20 25 30
Time (min)

Figure 5-8. Rate of resistance change after exposure to 500 ppm H2 in N2 wasmeasured at
different temperatures.


0.0020 0.0025 0.0030 0.0035

1/T (1/K)

Figure 5-9. Arrhenius plot of rate of resistance change after exposure to 500 ppm H2 in N2.


-*- adsorption curve
- Arrhenius fitting

slope= -1420.00457
activation energy (E)= 11.805 kJ/mol


I I I I I I I


1 room T












3.0x10-5

2.0x10-5

1.0x10-5

0.0

-1.0x10-5


-2.0x105- jt

-3.0x105 '
-0.4 -0.2 0.0 0.2 0.4

Voltage (V)

Figure 5-10. Current-voltage (I-V) plot of uncoated or Pt-coated single ZnO nanowires measured
at room temperature in pure N2.


6.0x10-'

4.0x10-'

2.0x10-'

0.0

-2.0x10-5

-4.0x10-5

-6.0x10-5


-0.4 -0.2 0.0 0.2 0.4


Voltage (V)

Figure 5-11. Current-voltage (I-V) characteristics of Pt-coated ZnO single nanowires measured
in vacuum, air, N2 or 500ppm H2 in N2 ambients. The latter responses were time-
dependent.













SN2 500 ppm H2


2 2


single Pt-ZnO nanorod
single ZnO nanorod



S 5 10 15
Time (min)


5 10 15 20
Time (min)


Figure 5-12. Current versus time plot for single ZnO nanowires either with or without Pt coatings
(top) and corresponding |ARI/R(%)-time plots (bottom).


50


20













1.0x10-6


5.0x10-7

0.0


-5.0x10.7


-1.0x10-6


-0.4 -0.2 0.0 0.2 0.4


Voltage (V)


8.0x1 09
6.0x1 09
4.0x1 09
2.0x1 09
0.0
-2.0x10-9
-4.0x10-9
-6.0x10-9
-8.0x10-9


-0.4 -0.2 0.0 0.2 0.4


Voltage (V)



Figure 5-13. Room temperature I-V characteristics from ZnO thin films of thickness 20 or 350
nm measured in air before and after coating with Pt.













2.5x10.5


2.0x10 -


S1.5x105-


1 .0x10o-
S Pt-ZnO thin films
--- 350nm
-6 170nm
5.0x10 --- 40nm
-20nm

0.0
0 300 600 900 1200 1500 1800

Time (sec)


C

I


2.5x1 0-5


2.0x1 05


1.5x105


1.0x105


5.0x1 0-6


200


thickness (nm)


Figure 5-14. Current as a function of time for Pt-coated ZnO thin films of different thickness
cycled from N2 to 500 ppm H2 in N2 to air ambient (top) and change in current at
fixed bias (0.5V) when switching to the H2-containing ambient (bottom).


(Al) vs. film thickness































0 300 600 900 1200 1500 1800

Time (sec)


Figure 5-15. Time dependence of current from Pt-coated ZnO nanowires and thin films as the
gas ambient is switched from N2 to 500 ppm H2 in N2, then to air for recovery.


C

I


7.0x1 05

6.0x1 05

5.0x1 05

4.0x1 05

3.0x1 05

2.0x1 05

1.0x10 5

0.0


thickness (nm)


Figure 5-16. Change in current at fixed bias (0.5V) when switching to the H2-containing ambient
of either Pt-coated ZnO nanowires or thin films as the gas ambient is switched from
N2 to 500 ppm H2 in N2, then to air for recovery.


Snanowires (Al) vs. film thickness



-S
S

-S

*









































4x104


3x104


2x104


1x104


0


Al
Sn Zn
Azn


Energy (keV)


Figure 5-17. Scanning electron microscopy micrographs (top and center) of SnO2-coated ZnO
nanowires and EDX spectrum (bottom).


200 nm

























20 30 40 50 60 70

26 (deg)


30 33 35 38 40 43


2 6 (deg)


Figure 5-18. X-ray diffraction pattern from SnO2-coated ZnO
with the SnO2 being polycrystalline.


nanowires. The data is consistent




























Figure 5-19. High resolution transmission electron microscope images of SnO2/ZnO nanowires
showing deposition of SnO2 on one side of the nanowires.


Figure 5-20. High resolution transmission electron microscope image of SnO2-coated ZnO
nanowire.





































100nm Electron Image 1
















0 2 4 6 8 10 12 14 16 Sn2/ O 18 nan
IIl Scale 99 cts Cursor: -0.127 keV (0 cts) keV


Figure 5-21.Energy-Dispersive X-ray Spectroscopy analysis of SnO2/ZnO nanowires.












5x1 0-7
4x1 0-7
3x1 0-7
2x1 0-7
1x10~7
0
-1x10-7
-2x1 0-7
-3x1 0-7
-4x1 0-7
-5x1 0-7


SnO2 coated ZnO nanowires
- 10min deposition /
- -5min deposition



-0






-0.4 -0.2 0.0 0.2 0.4


Voltage (V)


0 N2 500ppm H2 Air
4x 107

< 3x10-7


- 2x10-7 -- -5min deposition
O 10min deposition

1x10-7


0 500 1000 1500 2000


Time (s)

Figure 5-22.Current-voltage (I-V) characteristics from SnO2-coated ZnO nanowires for two
different deposition times (top) and time dependence of current at fixed bias of -0.5V
as a function of measurement ambient (bottom).












2.0x10.7

1.5x10.7

1.0x10.7

5.0x10.8
0.0

-5.0x1 0-8

-1.0x10-7

-1.5x10.7

-2.0x10-7


-0.4 -0.2 0.0 0.2 0.4


Voltage (V)


2.Ox 104


1.5x104


1.OxlW-


5.0x10-5


5min deposition 4000C
SnO2 coated ZnO nanowires
760 torr, N2 ambient


3500C


2500C 3000C
600C 100 C 1500C
2000C
) 200 400 600 800 1000


Time (s)


Figure 5-23.Current-voltage (I-V) characteristics from SnO2-coated ZnO nanowires at room
temperature or 400oC (top) and current at fixed bias of-0.4 V as the nanowires are
heated as a function of time (bottom).












2.5x10 --4


2.0x104-


1.5x104


1.0x104-


5.0x105 -


0.0 I
0 300


600


900 1200 1500 1800


Time (s)


Figure 5-24.Current at fixed bias of -0.5 V and temperature of 400C as a function of time as the
ambient is switched from N2 to 500ppm H2 in N2 or vacuum.









CHAPTER 6
CATALYST-FREE GROWTH OF METAL OXIDE NANOWIRES

6.1 Introduction

The synthesis of one-dimensional (1-D) semiconductor nanostructures has attracted great

interest due the unique physical and chemical properties of these materials. There is significant

interest in one-dimensional semiconducting nanostructures due to their unique optical, electronic

and chemical properties. Electronic nanomaterials are being pursued as possible building blocks

in fabricating nanoscale electronics, optoelectronic, magnetic storage devices, and chemical

sensors.

Zinc oxide (ZnO) is a wide-band-gap n-type semiconductor with direct band gap of 3.37

eV that has been extensively studied due to its applicability in transparent electronics,33,150,151

chemical and gas sensors, 10'119'138,152-155 spin functional devices28'66'156159, Schottky diodes,160'161

nanoelectronics,112 and blue light-emitting diodes.25'26'162'163 The synthesis of ZnO nanowires has

been reported using a variety of methods, including thermal evaporation", molecular beam

epitaxy,164 solution-phase growth165 and hydrothermal methods.166 Vertically aligned ZnO

nanowires are potentially useful for vertical device fabrication, with proposed device

implementations that include light-emitting-diodes,163'167 dye-sensitized solar cells165'168 and

nanopiezoelectrics.7 Considerable effort has been made to fabricate aligned ZnO nanowires on

various substrates using either physical vapor deposition (PVD)169, chemical vapor deposition

(CVD)170'171 or metal-organic chemical vapor deposition (MOCVD).98'172 It remains challenging

to controllably grow well-aligned ZnO nanowires. In many cases, the growth of semiconductor

nanowires proceeds via a vapor-liquid-solid (VLS) growth mechanism that requires a metals

catalyst.70 However, metal catalysts can also serve as impurities in the nanowires, thus limiting

material properties.









Recent progress in semiconductor nanowire heterostructure synthesis provides possibilities

in developing high-performance electronic, optoelectric, and sensing devices.15,173-181 The

composition modulated nanowire heterostructures have great potential as building blocks for the

fabrication of high performance optoelectronics.15,182-184 Composition modulation in the radial

direction can efficiently confine both the carriers and emitted photons. For example, if a shell

layer in a coaxial nanowiere heterostructure has wider band gap energy and a lower reflective

index than a core layer, confinement of both carrier and photons in the core nanowire can be

significantly enhanced.15'176 Alloying the ZnO phase with MgO has been investigated for

increasing the bad gap of ZnO-based nanowires.29 Theoretically, the band gap of ZnO can be

modulated from 3.3 to 4.0 eV by doping with different amount of MgO.19,185 ZnMgO alloy is an

important material to construct the heterostructure or superlattice to obtain high performance

laser diode (LD) and light emitting diode (LED) devices.15'186 Previously, we have observed the

formation of various core/shell nanowires when adding Mg to the Zn and O flux during growth

of Au-catalyst nucleated ZnO nanowires by molecular beam epitaxy.183,187 However, a clean and

abrupt interface has not been produced because of spontaneous phase separation inducing self-

ordered formation of coaxial heterostructures.

Tin oxide (SnO2) is a wide band gap (Eg=3.6 eV at 300K) semiconductor material suitable

for multiple applications that include gas sensors38'188, transparent conducting electrodes189, and

solar cells.190'191 In sensor applications, SnO2 has been reported to display high gas sensitivity

and selectivity.38'127 The reduced size of nanostructured SnO2 provides a material with a large

surface-to-volume ratio.155 Gas sensors based on one-dimensional nanostructured SnO2 have

been reported to exhibit good selectivity, low detection limits, and short response and recovery

time.37'42'192-194 Several methods have been employed to prepare SnO2 nanorods including









thermal evaporation,6'195 thermal decomposition,45'196 solution-phase growth,47'197 and

hydrothermal methods.198 Among these, the thermal evaporation approach has been used to

synthesize a wide variety of one-dimensional materials.5 This often has involved the use of a

catalyst in which nanowire growth proceeds by a vapor-liquid-solid (VLS) mechanism.70

However, metal catalysts can serve as impurities in the nanowires, possibly forming defect states

that limit their application in devices.

Vanadium oxide (V02) nanowires have attracted great attention because of their metal to

insulator transitions and reversible dramatic changes in electrical and optical properties

accompanied by a structural phase transition.50'54'58'59 It also makes it a promising material for the

use in device applications to achieve reliable electrical and optical switching operations. VO2 can

exhibit a sharp (by a factor of 104-105) and fast (sub-picosecond) metal-insulator transition close

to room temperature (340 K).55 The metal-insulator transition is due to a small structural

distortion of the lattice from a low-temperature monoclinic (semiconducting phase) to a high-

temperature tetragonal rutile (metallic phase) structure, accompanied by large changes in

conductivity and optical properties from infrared (IR) transmission to reflecting.54 Moreover, B

phase VO2 was found to have good electrochemical performance, especially for use as an

electrode material for lithium batteries.57'5 It exhibits a maximum reversible capacity of about

320 mA h g-1 in the range 4 to 1 V in lithium cells.59'60 It has been reported that the operating

properties of batteries depend not only on the structure but also on the morphology of the

electrode components.61 Therefore, the great surface area of nanowire materials may play an

important role for electrochemical applications.155

In this chapter, we report the synthesis of metal oxide nanowires by high-pressure assisted

pulsed laser deposition. A variety of metal oxide nanowires (ZnO, SnO2, and VO2) can be









synthesized without metal catalyst. The doping effects of Mg in ZnO nanowires were also

examined. In the first section, the growth of well-aligned ZnO nanowires by high-pressure

assisted pulsed laser deposition (PLD) is reported. The nanowire growth requires a ZnO template

for nucleation, but proceeds without the use of any metal catalyst. The structure and properties of

the nanowires are characterized, revealing high quality single crystal ZnO nanowires. The effects

of growth temperature and background pressure on nanowire growth and properties are discussed.

The addition of Mg into ZnO has been examined as well. The resulting structures show the

segregation of Mg because of big lattice mismatch and limited solubility. By switching MgO and

ZnO targets during growth, a core-sheath structure is observed. The synthesis of aligned SnO2

nanorods is described in the second section. The nanorod morphology is observed for PLD

growth conditions that include a relatively high background pressure, high substrate temperature,

and a non-epitaxial relationship between the SnO2 and substrate. SnO2 nanorod growth is

achieved without the use of any metal catalyst. In the last section, high aspect ratio monoclinic

VO2 nanowires grew laterally on the silicon and c-sapphire substrates at 6000C in 500 mTorr

argon. The nanowires randomly nucleate on the surface with the diameter of 90-200 nm, length

up to 50 am. Since pulsed laser deposition is a convenient means for achieving stoichiometric

transfer in growing multi-element materials199, these results suggest the possibility of growing

oxide nanowires with complex crystal structures and/or multi-cation stoichiometry.

6.2 Experimental Methods

6.2.1 ZnO nanowires growth

Pulsed laser deposition was used for the ZnO nanowire growth. The ablation target was

fabricated using high purity ZnO (Alfa Aesar, 99.9995%). The target was pressed and sintered at

1000C for 12 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of

5 Hz was used, with target to substrate distance of 2.5 cm and a laser pulse energy density of 1-3









J/cm2. The growth chamber exhibits a base pressure of 10-6 Torr. In order to achieve well-

ordered ZnO nanowires, a thin (75-200 nm) ZnO template layer was grown on the c-plane

sapphire substrate prior to nanowire nucleation. The ZnO template film was c-axis oriented.

Prior to deposition, the substrates were ultrasonically cleaned with trichloroethylene, acetone and

methanol, followed by compressed N2 drying. The substrates were attached to the heater using

Ag paint. Prior to growth, the target was cleaned in situ by pre-ablating with approximately 2000

shots. The growth experiments were performed over a temperature range of 500-800C in a

background pressure of 150-500 mTorr. Three different gas ambient (02, Ar and 02/Ar mixture)

were used to investigate the effects of oxidation and gas-phase collisions in the formation of

nanowires. The typical growth time was 2 h. After growth, the samples were cooled under the

same gas ambient as was used during growth. The as-grown samples were characterized using

X-ray diffraction (XRD) (Philips 3720, Cu-Ka), field emission scanning electron microscopy

(FE-SEM) (JEOL 6335F) and high resolution transmission electron microscopy (HR-TEM)

(JEOL 2010F). The optical properties of the nanowires were examined using photoluminescence

at room temperature. A He-Cd (325 nm) laser was used as the excitation source.

6.2.2 ZnMgO nanowires growth

The synthesis of ZnMgO nanowires was carried out by pulsed laser deposition. The

ablation target was fabricated using powder mixture of high purity ZnO (Alfa Aesar, 99.9995%)

and MgO (Alfa Aesar, 99.95%) with Mg:Zn atomic ratios of 1:4. The target was pressed and

sintered at 1000C for 12 h in air. A KrF excimer laser was used as the ablation source. A

repetition rate of 5 Hz was used, with target to substrate distance of 2.5 cm and a laser pulse

energy density of 1-3 J/cm2. The growth chamber exhibits a base pressure of 10-6 Torr. The c-

plane sapphire was used as the substrate materials in this study. Prior to deposition, the substrates

were ultrasonically cleaned with trichloroethylene, acetone and methanol, followed by









compressed N2 drying. The substrates were attached to the heater using Ag paint. Prior to growth,

the target was cleaned in situ by pre-ablating with approximately 2000 shots. The growth

temperature was 800C in a background pressure of 500 mTorr of oxygen. The typical growth

time was 2 h. After growth, the samples were cooled under the same gas ambient as was used

during growth. The as-grown samples were characterized using X-ray diffraction (XRD) (Philips

3720, Cu-Ka), field emission scanning electron microscopy (FE-SEM) (JEOL 6335F) and high

resolution transmission electron microscopy (HR-TEM) (JEOL 2010F). The optical properties of

the nanowires were examined using photoluminescence at room temperature. A He-Cd (325 nm)

laser was used as the excitation source.

6.2.3 SnO2 nanorods growth

Pulsed laser deposition (PLD) was used for the SnO2 nanorod growth. The ablation target

was fabricated using high purity SnO2 (Alfa Aesar, 99.996%). The target was pressed and

sintered at 1300C for 16 h in air. A KrF excimer laser was used as the ablation source. A

repetition rate of 5 Hz was used, with a target to substrate distance of 2.5 cm and a laser pulse

energy density of 1-3 J/cm2. The growth chamber exhibits a base pressure of 10-6 Torr. Single

crystal p-silicon (100) and c-plane sapphire were used as the substrate materials in this study.

Previous work has shown that SnO2 can grow epitaxially on sapphire. Prior to deposition, the

substrates were ultrasonically cleaned with trichloroethylene, acetone and methanol, followed by

compressed N2 drying. The substrates were attached to the heater using Ag paint. Prior to growth,

the target was cleaned in situ by pre-ablating with approximately 2000 shots. The growth

experiments were performed over a temperature range of 700-800C in a background pressure of

500 mTorr. The typical growth time was 30 minutes. The as-grown samples were characterized

using X-ray diffraction (XRD) using a Philips 3720, field emission scanning electron microscopy









(FE-SEM) using a JEOL 6335F and high resolution transmission electron microscopy (HR-TEM)

using a JEOL 2010F.

6.2.4 V02 nanowires growth

Pulsed laser deposition (PLD) was used for the VO2 nanowire growth. The ablation target

was fabricated using high purity V205 (Alfa Aesar, 99.99%). The target was pressed and sintered

at 700C for 12 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of

5 Hz was used, with a target to substrate distance of 2.5 cm and a laser pulse energy density of

1-3 J/cm2. The growth chamber exhibits a base pressure of 10-6 Torr. Single crystal p-silicon

(100) and c-plane sapphire were used as the substrate materials in this study. Previous work has

shown that VO2 can grow epitaxially on sapphire. Prior to deposition, the substrates were

ultrasonically cleaned with trichloroethylene, acetone and methanol, followed by compressed N2

drying. The substrates were attached to the heater using Ag paint. Prior to growth, the target was

cleaned in situ by pre-ablating with approximately 2000 shots. The growth experiments were

performed over a temperature range of 500-700C in a background pressure of 500 mTorr. The

typical growth time was 2 h. The as-grown samples were characterized using X-ray diffraction

(XRD) using a Philips 3720, field emission scanning electron microscopy (FE-SEM) using a

JEOL 6335F and high resolution transmission electron microscopy (HR-TEM) using a JEOL

2010F. The optical properties of the nanowires were examined using photoluminescence at room

temperature. A He-Cd (325 nm) laser was used as the excitation source.

6.3 Results and Discussion

6.3.1 Synthesis and characterization of vertical-aligned ZnO nanowires

In general, nanowire growth was achieved via PLD growth at relatively high temperature

and high background pressure. Figure 6-1 shows cross-section and plan view FE-SEM images of

ZnO nanowire arrays grown at 8000C in 500 mTorr oxygen, 500 mTorr argon, or a 325 mTorr Ar









/ 175 mTorr 02 mixture. In the case of pure argon, the only oxygen supplied during growth was

from the ZnO in the ablation plume. Vertically well-aligned nanowires were observed by cross-

sectional FE-SEM, showing that the growth is highly c-axis oriented along the normal direction

of the substrate. At low magnification, a relatively uniform distribution of diameter is observed

for the nanowires. The diameters are around 50-90 nm. Moreover, the nanowires grow as a high

density array and are uniformly distributed over the entire substrate. For the samples in Figure 6-

1, the length of the nanowires was approximately 6 tm. At high magnification, nanowires with

smooth hexagonal facets can be observed. As expected, no catalyst particles are observed on the

tips of the nanowires, which indicates that the nanowire growth does not proceed by a vapor-

liquid-solid mechanism. The composition of the nanowires was investigated by Energy-

dispersive X-ray (EDX) analysis. The results indicate that the nanowires are composed of zinc

and oxygen with no significant impurities found in the EDX data.

The orientation and crystalline properties of the ZnO nanowires was characterized with

XRD and transmission electron microscopy (TEM). Figure 6-2 shows the x-ray diffraction

patterns of ZnO nanowires grown in pure argon. Two sharp ZnO (002) and (004) peaks with

high intensity dominate the diffraction patterns, consistent with ZnO nanowires that are highly

oriented along the c-axis. A relatively weak ZnO (110) peak is also observed. The patterns can

be indexed to the ZnO hexagonal wurtzite structure with lattice constants of a=0.325 nm and

c=0.512 nm. Additional structure characterization was carried out using HR-TEM. Figure 6-3(a)

shows low magnification images for parallel nanowires that were mechanically removed from

the substrate. The nanowires have a relatively uniform diameter (50-90 nm) and are a few

micrometers in length. Note that no metal particles are observed at the top or bottom of the

nanowires. These results are consistent with the FE-SEM observations as shown in Figure 6-1.









The image in Figure 6-3(b) shows that the nanowires are tapered and faceted at the ends. In order

to further investigate the structure, selected-area electron diffraction was performed on a single

nanowire. The pattern is consistent with a single crystal wurtzite structure. Figure 6-3(d) shows

the high-resolution transmission electron microscopy images, showing that the nanowire is

structurally uniform and contains few defects. In the high resolution images, lattice fringes show

lattice spacing of 0.26 nm, which corresponds to 1/2 the c-axis lattice constant, confirming that

the ZnO nanowires are oriented in the c-axis direction. Since the (001) planes of ZnO are the

closest packed plane, stacking along the c-axis is the most energetically favorable. This growth

direction is commonly observed in ZnO nanowires.71

The morphology and microstructure of the nanowires were examined as a function of

growth conditions, in particular total background pressure, oxygen pressure, and substrate

temperature. Figure 6-4 shows the FE-SEM images of samples deposited in pure oxygen at 02

pressures ranging from 150 to 500 mTorr. The growth temperature was 800C. All samples were

grown on a thin template layer of ZnO on the sapphire substrate. At a growth pressure of 150

mTorr 02, the deposited ZnO consists of a continuous, smooth thin film as seen in Figure 6-4(a)

and (b). Increasing the oxygen pressure to 300 mTorr resulted in the nucleation and growth of

oriented microcrystals with hexagonal facets as seen in Figure 6-4(c). The size of microcrystals

varies from 1 to 5 [tm, growing normal to the substrate. When the pressure was further increased

to 500 mTorr, the growth mode undergoes a transition from continuous thin film to highly

aligned nanowire growth. A highly dense array of nanowires with hexagonal facets is observed.

The nanowires are oriented with their c-axis perpendicular to the surface with relatively uniform

diameter and density. Note that very few nanowires were obtained when the pressure was further

increased to 1 Torr.









In addition to pressure, the effect of substrate temperature in the formation of nanowires

was also examined. Figure 6-5 shows the cross-sectional and plan-view FE-SEM images of ZnO

nanorods grown at an oxygen pressure of 500 mTorr at temperatures ranging from 550-800C.

At 550C (Figure 6-5(a) and (b)), the diameters of the nanowires are on the order of 500 nm. At

750C, the diameter is reduced to approximately 150 nm. At 800C, the diameter of the

nanowires is less than 100 nm. One explanation for this temperature dependency of nanowire

diamaeter relates to surface diffusion. A higher surface mobility is realized for higher growth

temperatures. High temperatures provide sufficient energy for deposited species from the

ablation target to migrate to low energy sites for growth. If substrate temperature is low (<650C),

surface species will remain at higher energy sites, thus yielding large diameter nanowires or

simply rough, granular films. In order to achieve one dimensional growth, it is important to

provide sufficient surface mobility for species to reach low energy nucleation sites.

The variation in optical properties for the ZnO nanowires grown in different background

ambients was investigated using room temperature photolumienscence measurements. Figure 6-

6(a) shows the typical room temperature PL spectra of the ZnO nanowires grown at 800C and

500 mTorr oxygen. A weak near-band-edge-emission in the UV region at 380 nm and a strong

broad band deep-level-emission at 520 nm is observed. The green band around 520 nm is

commonly attributed to deep-level or trap-state emission due to vacancies and/or interstitials of

zinc and oxygen in the crystal.115 In order to further investigate the origin of deep-level

emissions, PL measurements were carried out for nanowires grown using the three different

ambients (500 mTorr 02, 500 mTorr Ar and 175 mTorr O2/ 325 mTorr Ar mixture) at 8000C.

The near-band-edge emission was higher for nanowires grown using pure argon. However, the

deep-level emission was enhanced as well, suggesting a high density of defects due to the









oxygen deficient ambient. In order to investigate this further, nanowire grown using the

argon/oxygen mixture were examined. The near-band-edge emission intensity increased 20 times

relative to nanowires grown using pure oxygen. However, the broad green emission remained

unchanged in the spectra for all cases. A plot of PL spectra for nanowires grown in the different

background gases is shown in Figure 6-6(b). The results show that the deep-level emission

persists in all ambients considered. The large deep-level emission was also observed on high

temperature grown nanowires. Liu et al. attributed modification in ZnO nanowire PL properties

to size effects and oxygen stochiometry.200 Future work will examine the transport properties of

individual PLD-grown nanowires and compare their properties to those grown by other

techniques.201

6.3.2 Synthesis and characterization of ZnMgO nanowires

Figure 6-7 depicts the FE-SEM images of the formation of high density well-aligned

ZnMgO nanowires. The diameters of the nanowires are on the order of 90-120 nm. A cross-

sectional FE-SEM image of the ZnMgO nanowire on the sapphire substrate as shown in Figure

6-7(d) demonstrates that most of the nanowires were grown perpendicularly to the sapphire

substrate. The chemical composition of the nanowires was determined using energy dispersive

spectroscopy (EDX). Figure 6-8 shows a typical EDX spectra for pulsed laser deposited ZnMgO.

The elements detected are zinc, magnesium and oxygen. Structural characterization was

performed by XRD. Figure 6-9 shows the diffraction pattern of ZnMgO nanowires. In addition to

ZnO diffraction peaks, MgO (200) and (220) peaks were also detected. Additional peaks from

ZnO were also detected. The c-axis lattice constants calculated from the (002) peaks for the

ZnMgO nanowires (5.1656A) is slightly smaller than ZnO nanowires (5.199A), which may

result from the stress derived from secondary phase and MgO segregation.









Further structural characterizations of the ZnMgO nanowires were performed using the

HR-TEM analysis. Figure 6-10(a) illustrates TEM image of single ZnMgO nanowire. The lattice

fringes images indicate the segregation of secondary phase in ZnO matrix, resulting in a

polycrystalline structure with multiple growth orientations as illustrates in figure 6-10(b) and (c).

Furthermore, stacking faults were also observed in the lattice images in figure 6-10(d),

suggesting the inhomogeneous growth. Generally speaking, the solubility of Mg in ZnO depends

on the growth methods as well as conditions. In this case, the solubility limit of Mg in ZnO has

been exceeding, resulting defects and secondary phase growth.

In the efforts to increase the solubility of Mg and decrease defects of the nanowires, a

different growth recipe has been used. In this approach, a 30 minutes growth period of ZnO was

initially performed to provide seeding sites on the substrate, followed by switching between ZnO

and Zno.sMgo.20 target over during the growth (10 minutes each). Finally end up with a 30

minute growth period of ZnO. Figure 6.1 l(a)-(c) show the FE-SEM images of as-grown samples.

Note that small circles with diameter approximately 20-90 nm were found on the facets of

naowires. The ZnMgO nanowires have uniform diameter and well-aligned along c-axis as shown

in Figure 6-11(d). In contrast with previous case, XRD shows only c-oriented ZnO peaks in

Figure 6-12. No MgO or secondary phase peaks were found suggests a higher solubility and

incorporation of Mg into Zn sites. Furthermore, the c-axis lattice constants calculated from the

(002) peaks for the cored ZnMgO nanowires (5.1932 A) is very close to ZnO nanowires (5.199

A), which suggests a smaller stress inside the sample.

In order to further investigate the structural properties, HR-TEM was used to characterize

the sample. The HR-TEM images in Figure 6-13 clearly reveal the core-sheath structure of the

nanowires. In the Figure, the nanowire displays a difference in brightness intensity between the









core and sheath regions. The contrast across the diameter of the nanowire is predominantly mass

contrast, reflecting a difference in average atomic number (Z) of the core and sheath region. The

darker core region contains more Zn, while the lighter sheath region more Mg. The core and

sheath is approximately 80 nm and 40 nm in diameter respectively. The selected-area diffraction

pattern shows that the nanowire is with single crystal wurtzite structure, which is consistent with

XRD results. The HR-TEM lattice fringes image in Figure 6-13(d) confirmed the single crystal

structure. No segregated cluster of impurity phase appears via HR-TEM observation. The

absence of the diffraction peaks of MgO in the XRD and SAD patterns suggest that the Mg

incorporated within the ZnO nanowires by means of substituting Zn. There has been no

investigation of the transport properties of the heterostructured nanowires. They represent

opportunities to examine transport in electron-confining structures due to the larger band-gap

ZnMgO sheath.

6.3.3 Synthesis and characterization of SnO2 nanorods

The initial characterization focused on the phase formation and microstructure of SnO2

materials deposited at high temperatures and pressures. The XRD pattern shown in Figure 6-14(a)

is for SnO2 deposited on silicon at a temperature of 800C and oxygen pressure of 500 mTorr.

All of the peaks could be indexed to the tetragonal rutile structure of SnO2 with lattice constants

of a=4.3738 A and c=3.188 k from JCPDS file (41-1445). No impurity or secondary phase

peaks were observed. The nanorods show no preferred crystalline texture when deposited on

silicon. Figure 6-14(b) shows the XRD results for SnO2 deposited on sapphire using similar

deposition conditions. In this case, the deposited material is strongly a-axis textured. This is

consistent with an epitaxial relationship between SnO2 and c-plane sapphire. The chemical

composition of the deposited material was determined using energy dispersive spectroscopy









(EDX). Figure 6-15 shows a typical EDX spectra for pulsed laser deposited SnO2. The only

elements detected are tin and oxygen.

The morphology and microstructure of the deposited SnO2 were characterized by FE-SEM

and HR-TEM. Figure 6-16(a) shows a typical top-view FE-SEM image of the as-grown film

deposited at high temperature and background pressure on a silicon substrate. Nanorod growth is

observed. The materials grow by a columnar growth mode that is maintained for the duration of

the deposition. As shown in Figures 6-16(b)-(d), an aligned SnO2 nanorod array is evident in

cross-sectional FE-SEM. The diameters of the SnO2 nanorods are on the order of 50-90 nm. The

nanowire length is 1.5 [tm for the sample considered. Figure 6-17 shows FE-SEM micrographs

for SnO2 films deposited on c-plane sapphire. For materials nucleated directed on the sapphire at

high pressure (- 500 mTorr), a SnO2 nanorod microstructure was observed as seen in Figure 6-

17(a) and (b). Interestingly, if a thin nucleation layer is initially deposited at low pressure (50

mTorr) with high pressure deposition following, the microstructure does not yield distinct

nanorods. Instead, a dense, small grain polycrystalline film is observed as seen in Figure 6-17(c)

and (d). The lower pressure results in a higher density of densely spaced nucleation sites.

The HR-TEM image of laterally aggregated SnO2 nanorods grown on silicon is shown in

Figure 6-18(a). The selected area electron diffraction (SAD) pattern from an individual nanorod

with a [102 ] zone axis is shown in Figure 6-18(b). The SAD data indicates that the individual

as-grown SnO2 nanorods are single crystals with the rutile structure. The HR-TEM image in

Figure 6-18(c) shows lattice fringes near the edge of the nanorods indicating an interplanar

spacing is 2.3 A, corresponding to the (100) plane of a rutile SnO2 lattice. The lattice fringes in

Figure 6-18(d) show the two distinct spacing of 3.4 k and 3.2 k corresponding to (110) and (001)

planes respectively.









Since no catalyst was used in the synthesis, the growth of SnO2 nanorods cannot be

explained by a VLS mechanism. Instead, the nanorods grow via a simple columnar growth

mechanism. Note that the high background pressure (500 mTorr) was necessary in order to

achieve small grain columnar growth that persists over the duration of the deposition. In pulsed

laser deposition at such high pressures, the interaction between plume and gas molecules reduces

the plume-induced kinetic energy of the ablated species to negligible values. While the initial

nucleation of SnO2 on the silicon substrate follows a three-dimensional island growth mechanism,

subsequent growth proceeds by adhesion to existing sites, yielding one-dimensional vapor-solid

growth of SnO2 nanorods. The formation of distinct nanorods was also favored for deposition on

silicon where epitaxy does not occur.

6.3.4 Synthesis and characterization of V02 nanowires

The XRD pattern shown in Figure 6-19(a) is for V02 nanowires deposited on silicon at a

temperature of 600C and argon pressure of 500 mTorr. All of the peaks could be indexed to the

monoclinic B phase of V02 with lattice constants of a=12.03 k, b=3.693 k, c=6.42 k and

P=106.6 from JCPDS file (31-1438). No impurity or secondary phase peaks were observed. The

nanowires show no preferred crystalline texture when deposited on silicon. The chemical

composition of the deposited material was determined using energy dispersive spectroscopy

(EDX). Figure 6-20 shows a typical EDX spectra for pulsed laser deposited V02. The only

elements detected are vanadium and oxygen, which confirmed the catalyst-free growth.

The morphology and microstructure of the deposited V02 were characterized by FE-SEM

and HR-TEM. Figure 6-21 show typical top-view FE-SEM images of the as-grown nanowires

deposited at 6000C and relative high background pressure (500 mTorr argon) on a silicon

substrate. As shown in figure 6-21(a), a typical low-magnification FE-SEM image indicates that

the as-synthesized products consists a large quantity of nanowires with uniform diameter and









very high aspect ratio. The materials randomly nucleate on the surface and maintained for the

duration of the deposition. As shown in Figures 6-21(b)-(d), high density VO2 nanowires are

evident in top view FE-SEM. The diameters of the VO2 nanowires are on the order of 90-200

nm. The nanowire length is up to 50 [tm for the sample considered. Figure 6-21(f) shows side-

view FE-SEM micrographs for VO2 nanowires grew on silicon, which confirmed the randomly

nucleation growth. For materials nucleated directed on the sapphire at same condition, similar

microstructure was observed but with slightly larger diameters. This suggests the growth may not

be related to lattice mismatch between substrates. Although the exact growth mechanism is still

unclear, we suggest the growth follows a diffusion-based vapor-solid mechanism. Interestingly,

no nanostructures were observed when using a lower background pressure. The high background

pressure may still play an important role in the formation of VO2 nanowires in this case. Further

investigations are required to clarify the interplay between the surface energetic and surface

effects.

The HR-TEM image of as-grown VO2 nanowires grown on silicon is shown in Figure 6-22.

The selected area electron diffraction (SAD) pattern from an individual nanowire is shown in

Figure 6-22(b). The SAD data indicates that the individual as-grown VO2 nanowires are single

crystals with the monoclinic structure. The HR-TEM image in Figure 6-22(c) shows lattice

fringes near the edge of the nanowires indicating an interplanar spacing is 3.05 A, corresponding

to the (002) plane of a monoclinic V02 lattice. The lattice fringes in Figure 6-22(d) show the two

distinct spacing of 3.62 A and 3.05 A corresponding to (110) and (002) planes respectively.

The optical properties of as-grown VO2 nanowires were investigated by room temperature

photoluminescence. Figure 6-23 shows that the VO2 nanowires, in comparison with V02 thin

films grew at low oxygen background pressure (30 mTorr) and same temperature (600C),









exhibit a board PL peak centered at 2.44 eV. The board PL peak might be related to the

crystalline defects induced during the growth. The defects might be caused by the high

background pressure, resulting oxygen vancancies. On the other hand, no emission was observed

from V02 thin film sample grew at low oxygen pressure. This suggests the emission is related to

growth pressure and dimension of V02 material. Low temperature PL is required to investigate

the origin of the peak.

To further investigate the transport properties of VO2 nanowires, a single nanowire device

has been fabricated on a 1000A thick thermally grown SiO2/Si substrate. The fabrication process

is similar as described in Chapter 5. The electrodes (Ti/Al/Pt/Au 200/800/400/800 A) were

deposited by e-beam evaporation at room temperature without further annealing. Figure 6-24(a)

shows the FE-SEM image of single nanowire device. The distance between the two electrodes

was 13 tm. The measurement was performed in air ambient. At room temperature, the device

exhibited linear, symmetric current-voltage (I-V) characteristics as shown in figure 6-24(c). The

linear dependence can be observed in relatively wide current range from approximately 2.25 to -

2.25 [A, indicating good Ohmic contacts between the nanowire and electrodes. The diameter

size (160 nm) and the length (13 am) of the VO2 nanowire can be measured through FE-SEM

image as shown in figure 6-24(a) and (b), and the resistivity of the VO2 nanowire is calculated to

be 1.471x 103 Q m at room temperature.

6.4 Summary and Conclusions

A high-pressure assisted pulsed laser deposition has been applied to fabricate a variety of

metal oxide nanowires (ZnO, ZnMgO, SnO2 and VO2) without catalysts. Vertically well-aligned

ZnO and Zn M\,. O arrays were grown on c-sapphire substrates at 600-8000C. The nanowires

growth proceeds without employing catalysts for nucleation, although an epitaxial ZnO thin film

template is necessary in order to achieve uniform alignment. The ZnO nanowire diameters are as









small as 50-90 nm, with diameters largely determined by growth pressure and temperature.The

SnO2 nanowires with single crystal rutile structure were grown on silicon substrates at 7000C.

The growth of SnO2 nanorods on silicon begins as small grain columnar with a subsequent

vapor-solid growth mechanism at high pressure yielding one-dimensional SnO2 nanorods. The

diameter and length of the SnO2 nanorods are approximately 50-90 nm and 1.5 am, respectively.

High aspect ratio monoclinic VO2 nanowires grew laterally on the silicon and c-sapphire

substrates at 600C. The nanowires randomly nucleate on the surface with the diameter of 90-

200 nm, length up to 50[tm. A board emission peak was observed at 510 nm by room

temperature photoluminescence measurement. A single nanowire device has been fabricated to

measure transport properties of single VO2 nanowire. The device exhibits linear, symmetric I-V

characteristics and the resistivity of the nanowire is approximately 1.471 x 103 Q m. Further

investigations such as temperature dependant transport, optoelectronic response and gas sensing

measurements are needed for further device applications.

The metal oxide nanowires are attractive for numerous applications. This study provides a

relative convenient approach to synthesize a wide range of metal oxide nanowires. By choosing

proper growth parameters such as substrate materials, growth temperature and gas pressure, it

has been demonstrated that the present approach can be extended to obtain a large family of

semiconducting metal oxide nanowires. The results also suggest the possibility of growing

complex metal oxide nanostructures, including tailored heterostructures, with PLD.













































Figure 6-1. Scanning electron microscope images of well-aligned ZnO nanowires grown on a
ZnO thin film template in (a), (b) 500 mTorr pure oxygen, (c), (d) 500 mTorr pure
argon, and (e), (f) a 325 mTorr / 175 mTorr argon/oxygen mixture.


0.5 pm











0.5 pm










0.5 pm
























30 40 50 60 70


29 (deg)


Figure 6-2. X-ray diffraction 0-20 scan of ZnO nanowires grown at 800C in 500 mTorr Ar.










(a)












0.2 r


Figure 6-3. Low magnification (a), (b) TEM images ofZnO nanowires grown on a ZnO thin film
at 800C in 500 mTorr Ar. Also shown is (c) a selected area electron diffraction
pattern taken from a single ZnO nanowire, showing the single crystal wurtzite
structure, as well as (d) an HR-TEM image of a single ZnO nanowire showing lattice
fringes.


0002



10TO

















































Figure 6-4. Cross-sectional and top view scanning electron microscope images of the ZnO
nanowires grown at 800C in pure oxygen with oxygen background pressures of (a),
(b) 150 mTorr, (c), (d) 300 mTorr, and (e) (f) 500 mTorr.









130









































Figure 6-5. Cross-sectional and top view scanning electron microscope images of the ZnO
nanords grown under 500 mTorr of oxygen at different temperatures. (a), (b) 550C,
(c), (d) 7500C, (e), (f) 800C, respectively


(c)







500 nm


(e)







500 nm

























350 400 450 500 550 600
Wavelength (nm)


350 360 370 380 390
Wavelength


400
(nm)


650 700


410 420


Figure 6-6. Room temperature PL spectra of ZnO nanowires and near-band-edge-emission of
ZnO thin film and ZnO nanowires grown under different background ambient (a)
Room temperature PL spectra of ZnO nanowires grown at 800C in 500 mTorr
oxygen (b) near-band-edge-emission of ZnO thin film and ZnO nanowires grown
under different background ambient at 8000C.
































Figure 6-7. (a), (b) Top and (c), (d) cross-sectional view scanning electron microscope images of
the ZnMgO nanowires grown at 8000C in 500 mTorr Ar.


MAIC I 1 11, r4V X'!,90 2 ....... .......


(C)








MAIC S11 VIJkV Xfl000 1


MAIC S1 I MOW X-15.000














14000-
Zn

12000

10000

8000
C
0 6000-

4000-
Mg
2000 Zn
Zn
0-
0 5 10

Energy (keV)


Figure 6-8. Energy-dispersive spectroscopy spectra for ZnMgO nanowires grown on sapphire at
800C in 500 mTorr Ar.


















-Q
U ZnO(101


C






20 30 40


50 60 70 80


26 (deg)

Figure 6-9. X-ray diffraction 0-20 scan ofZnMgO nanowires grown on sapphire at 800C in 500
mTorr Ar.































.'~ ~~~ ~~ ,'. r"...
7 :

low.


L-P
.. ~,
.: v

:~ '1*;:'

~ ~
:mi~jni:
i:* ..


Figure 6-10. (a) High resolution transmission electron microscope image of single ZnMgO
nanowire (b)-(d) HR-TEM lattice fringes images indicate the segregation of
secondary phase in ZnO matrix. Defects such as stacking faults were also observed in
the lattice images as well.
















136








































Figure 6-11. (a)-(c) Top and (d) cross-sectional view scanning electron microscope images of
the cored ZnMgO nanowires grown on sapphire at 8000C in 500 mTorr Ar.


(a)











MAIC SH t!, GkV X', Ono WI) 1.1 Dn 1j I


(C)










MAIC S I I,0kV X8,000 100rr,


0





MAIR' SH tDkV X2000

(d)











MAIK' S 1 1, GkV X25 COO WI) 1 .1 Dmr, 1,mi

















suDsiraie

CU

.- ZnO(100)


nZnO(200)
C-e
ZnO(110)



20 30 40 50 60 70 80

2e (deg)
Figure 6-12. X-ray diffraction 0-20 scan of cored ZnMgO nanowires grown on sapphire on
sapphire at 800C in 500 mTorr Ar.










(a (b)











C, ,'


















Figure 6-13. (a), (b) Low magnification TEM image of single cored ZnO/ZnMgO nanowire (c),
(d) HR-TEM lattice fringes images indicate the mass contrast of core-sheath
structure.























30 40 50 60 70 80 90
2 8 (deg)


30 40 50 60 70 80 90
2 8 (deg)


(C) JCPDS 41-1445
80

60-

c 40-

20-

0-
30 40 50 60 70 80 90
2 8 (deg)

Figure 6-14. X-ray diffraction patterns of SnO2 nanorods grown at 800C in 500 mTorr oxygen
on (a) silicon and (b) sapphire. (c) diffraction intensities of rutile SnO2 from JCPDS
file.


140













6000
Sn

5000


4000
cl)
C
c 3000- Sn
0

2000

0 Sn
1000
Sn )sn
Sn
0_-
I I
0 5 10 15

Energy (keV)



Figure 6-15. Energy-dispersive spectroscopy spectra of SnO2 nanorods grown on sapphire at
8000C.
































v -- -













Figure 6-16. Scanning electron microscope morphologies of SnO2 nanorods grew on silicon. (a)
Top-view of as-grown nanorods. (b) (d) Cross-sectional views of as-grown SnO2
nanorods.




































Figure 6-17. Scanning electron microscope images showing (a) the surface morphology and (b)
cross-section of SnO2 nanorods deposited by pulsed laser deposition at 800C directly
on sapphire. Also shown are (c) the surface morphology and (d) cross-section of SnO2
deposited on sapphire under similar conditions but with a thin epitaxial SnO2
nucleation layer first grown at 50 mTorr.




















1'



20 'U


Figure 6-18. (a) High resolution transmission electron microscope image of SnO2 nanorods (b)
SAD patterns recorded from an individual nanorod with an electron beam along the
[102 ] direction. (c), (d) HR-TEM lattice fringes images indicate their single crystal
rutile structure.


(211















S(401)
d
-Q



C
>1 (312)
(022)

( (202) (112)
C
(402)
(110)



20 30 40 50 60
28(deg)





100 (b)

80

-0
S60-

40
C
(U
c 20

0-

20 25 30 35 40 45 50 55 60

2 e (deg)

Figure 6-19. X-ray diffraction patterns of VO2 nanowires grown at 600C in 500 mTorr oxygen
on (a) silicon. (b) Diffraction intensities of monoclinic VO2 (B) from JCPDS file.














20000 Si



15000


I.
C 10000
0

5000


0 V


0 5 10

Energy (keV)


Figure 6-20. Energy-dispersive spectroscopy spectra for VO2 nanowires grown on silicon at
6000C.























































Figure 6-21. (a)-(d) Top view scanning electron microscope images of VO2 nanowires on silicon
grew at 6000C (e) individual VO2 nanowire dispersed on the silicon substrate (f) side
view scanning electron microscope images of VO2 nanowires on silicon grew at
6000C.


MAIC SH IDkv X80,000 WI)Vio.- 100-



































'Zt





e'


Figure 6-22. (a) High resolution transmission electron microscope image of VO2 nanowires (b)
SAD patterns recorded from an individual nanowire (c), (d) HR-TEM lattice fringes
images indicate their single crystal monoclinic structure.














148











0.05


0.04


3 0.03

L-
0.02

C1)
C 0.01


0.00

400 500 600 700 800 900

Wavelength (nm)



Figure 6-23. The photoluminescence spectra of V02 thin film and nanowires grew at different
oxygen pressure.

































a>
U,
0


2.5x1 06

2.0x1 0-6

1.5x10-6

1.0x10-6

5.0x10-7
0.0

-5.0x10-7

-1.0x10-6

-1.5x10-6

-2.0x10-6

-2.5x10-6


-0.8 -0.4 0.0 0.4 0.8


Voltage (V)


Figure 6-24. (a), (b) Scanning electron microscope images of fabricated single nanowire device.
(c) I-V characteristics of the indivudial V02 nanowire measured in air ambient at
room temperature.


MAIC SF I VOkV X11,000 WI) 13 /,.m 100.m,









CHAPTER 7
EPITAXIAL GROWTH OF TRANSPARENT TIN OXIDE THIN FILMS

7.1 Introduction

In recent years, there has emerged significant interest in the epitaxial growth and properties

of functional oxide thin films.17'202 Functional oxide of interest include superconductors,203-205

ferroelectrics,206 dielectrics,207 phosphors,208'209 and semiconductors.210,211 The latter class of

oxides, namely semiconductors, has emerged as particularly interesting for sensors, thin-film

electronics, and photonics. Tin oxide (Sn02) is a wide band gap (3.6 eV) metal oxide

semiconductor with excellent optical transparency in the visible range.212 It possesses the rutile

(tetragonal) crystal structure with a=4.738 A and c=3.188 A. With a relatively high conductivity,

visible wavelength transparency, chemical stability and thermal stability in oxidizing

environments,213 tin oxide films are being explored for a number of applications. As a wide

band-gap semiconductor, SnO2 is attractive for use in photonic applications, such as solar cells,

where transparent electrodes are required.18'214 Sn02 thin films are used for gas sensor devices

based on changes in conductivity when exposed to selected chemical species.21,37,188 In addition,

there is also interest in the possibility of inducing ferromagnetism in Sn02 through transition

metal doping,215,216 an approach that is also being pursued for other wide band-gap

semiconductors.217'218

Sn02 thin films have been fabricated by a variety of techniques including sol-gel

method,219 electron beam evaporation,220 reactive sputtering,221 chemical vapor deposition222'223

and sputtering.36'224 One of the major challenges in synthesizing Sn02 thin films is the control

over oxygen stoichiometry. When deposition is carried out in vacuum conditions at high

temperatures, SnO2 films tend to be nonstoichiometric, frequently including metastable phases









such as SnO and Sn304.225 The existence of these metastable phases and relaxed crystal defects

will strongly affect the properties of the films.

While previous applications of SnO2 for sensors or transparent conductors have primarily

relied on polycrystalline material, many of the emerging applications for functional wide band-

gap semiconductors require highly crystalline epitaxial films. As such, understanding the effects

of growth parameters and substrate selection on the epitaxial growth of SnO2 is important.

Epitaxial growth kinetics can yield specific defect structures that significantly affect the oxide

thin film properties.226'227 This becomes increasingly important as targeted thin film structures

involve heterointerfaces, multilayers, or superlattices. 226-229

Similar to most metal oxide materials, undoped SnO2 is an n-type semiconductor because

of intrinsic defects (oxygen deficient or metal excess). The fabrication of high quality p-type

transparent conducting oxides (TCOs) is one the major challenges in the fabrication of p-n

junction based devices.230 As is well known that doping in semiconductor with selective

elements offers an effective approach to adjust the electrical, optical, and magnetic properties,

which is crucial for practical applications. A perusal of the periodic table suggests that possible

acceptor candidates for SnO2 include Group III elements such as B, Al, Ga, and In substituting

for Sn. Theoretically if effective substitution of Sn with group III elements, the p-type SnO2 can

be realized. However, only a few groups reported thep-type conductivity in SnO2 thin films up

to date.230-234 The difficulties can come from a variety of causes. Difficulty of obtaining p-type

SnO2 may due to its low dopant solubility and self-compensating process on doping.

Furthermore, dopants may be compensated by low-energy native defects, such as Sni and Vo or

background impurities. Low solubility of the dopant in the host material is also another issue.









Deep impurity level can also be a source of doping problem causing significant resistance to the

formation of shallow acceptor level.

Bagheri-Mohagheghi et al. reported the p-type conductivity on Li-doped Sn02 thin film by

spray pyrolysis.231 The heavily Li-doped Sn02 films (-2 wt.%; 37 at.%) show a carrier

conversion from electrons to holes. Because of similar ionic radius of Sn4+ (0.71 A) and Li+ (0.68

A), three holes are produced when the substation occurs. Ji et al. reported p-type In-doped Sn02

with In/Sn ratio of 0.2 by spray pyrolysis.232'234 Similar result has also been reported by Huang et

al. with Ga-doped SnO2 films (0.2%) by DC magnetron sputtering.233 However, the electric

properties of these films were not good because of the limitations of sol-gel method, such as poor

crystalline quality and poor process control.

Pulsed laser deposition (PLD) has been widely used in synthesis complex oxide thin films,

such as high To superconductors and perovskite oxides. PLD has the advantage of operating in a

reactive atmosphere over a wide range of oxygen pressure.104 In this Chapter, we study the

epitaxial growth of Sn02 thin films on (0001) sapphire by PLD, including the specific crystalline

orientation of this rutile structure on a hexagonal template. The effects of growth parameters on

the electrical transport property and surface morphology will be discussed as well. The effects of

gallium doping will be examined in the second section.

7.2 Experimental Methods

Pulsed laser deposition was used for thin film growth. The ablation target was fabricated

using high purity Sn02 (Alfa Aesar, 99.996%). For gallium doped thin films, the ablation target

was fabricated using powder mixture of high purity ZnO (Alfa Aesar, 99.9995%) and Ga203

(Alfa Aesar, 99.999%) with Ga:Zn atomic ratios of 1:99. The targets were pressed and sintered at

1300C for 16 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of

1 Hz was used, with target to substrate distance of 4 cm and a laser pulse energy density of 1-3









J/cm2. The growth chamber exhibits a base pressure of 10-6 Torr. Single crystal (0001) A1203

(sapphire) was used as the substrate material in this study. Prior to deposition, the A1203

substrates were ultrasonically cleaned with trichloroethylene, acetone and methanol, followed by

compressed N2 drying. The substrates were attached to the heater using Ag paint. Prior to growth,

the target was cleaned in situ by pre-ablating with approximately 2000 shots. Film growth

experiments were performed over a temperature range of 300-800C in an oxygen pressure

range of 20-150 mTorr. Film thickness ranged from 200-300 nm and typical growth time was 2

h. The deposited films were characterized using X-ray diffraction, atomic force microscopy and

field emission microscopy. Four-point van der Pauw Hall measurements were performed to

determine transport properties. A Perkin-Elmer Lambda 800 UV/Vis double-beam spectrometer

was used for optical absorption measurements.

7.3 Results and Discussion

7.3.1 Properties of undoped SnO2 thin films

Epitaxial SnO2 films on (0001) A1203 were realized for deposition at temperature as low as

400 C in an oxygen pressure of 50 mTorr. Figure 7-1 shows the X-ray diffraction (XRD) 0-20

patterns for SnO2 thin film prepared by PLD at different temperatures. The (200) and (400) SnO2

are the dominant peaks in all scans, which indicates that the films are highly a-axis oriented; i.e.

the principal out-of plane orientation is SnO2 (100) // A1203 (0001). The weak peaks for SnO2

(101) grains are also observed at higher temperatures. The results clearly show that epitaxial

growth of a-axis oriented SnO2 grains on the (0001) A1203 substrate is favored, placing the film

c-axis in the plane of the surface. As seen in Figure 7-2, the a-axis lattice parameter shows an

increase with increasing growth temperature. However, the a-axis spacing is consistently less

than that seen in bulk SnO2.









The epitaxial crystallinity of the films was confirmed by looking at both out-of-plane

rocking curves and in-plane p-scans. The rocking curve through the (200) plane for SnO2 film

grown at 7000C is shown in Figure 7-3, yields a full width half maximum (FWHM) of 0.0156,

which confirms that the film is highly oriented with the a-axis perpendicular to the surface. With

increasing growth temperature, the crystallinity significantly improved as reflected in

corresponding smaller FWHM values. Overall, the films exhibit good out-of-plane alignment of

the (100) planes.

The in-plane alignment of the SnO2 film is seen from the yp-scan in Figure 7-4. The film in-

plane mosaic (AO-10) is much larger than the out-of-plane mosaic. The in-plane alignment can

be described as SnO2 [010] // A203 <1120 > or equivalently SnO2 [001] // A203 <0110> and 600

rotations. The SnO2 films grow epitaxially on the (0001) A1203 substrate with three orientations

rotated 600 in plane with respect to each other due to the six-fold symmetry of the c-plane

surface of A1203. This epitaxial structure is consistent with the matching of the oxygen

octahedral arrangements existing on the SnO2 (100) surface and on the A1203 (0001) surface as

illustrated in Figure 7-5, resulting in the epitaxial growth of (100) oriented SnO2. This in-plane

variant structure with three different symmetry-equivalent orientations has also been observed on

films synthesized by metal-organic chemical vapor deposition (MOCVD)223 and sputtering

approaches.235 A likely explanation as to why the in-plane mosaic is much larger than the out-of-

plane is that the rotational mosaic is due to lattice matching between the film (tetragonal

symmetry) and the substrate rhombohedrall symmetry). On average the film (010) and (001)

planes are aligned with low-index sapphire directions. But then other low-index film planes are

not aligned. For example, the film (011) planes are 56.30 from (010) planes and 33.7 from (001)

planes. Thus, if these planes tend to align with low-index A1203 planes, they would be frustrated









by -40 on average. All crystal directions can not be aligned when a tetragonal film grows on a

rhombohedral (pseudo-hexagonal) c-axis substrate. Figure 7-6 shows the growth rate of SnO2

thin films at different temperatures. The growth rate is measured to be approximately 0.3-0.4 A/s.

For all growth temperatures, the growth rate shows a weakly linear relation to growth

temperature, suggesting that an increase in growth temperature enhances the SnO2 phase

formation.

The surface morphology of the SnO2 film was measured using atomic force microscopy

(AFM) measurements. AFM measurements were performed in air using a Veeco Nanoscope III.

All samples were scanned over a 5 ammx5 tm area. In Figure 7-7, the AFM images for SnO2

grown on sapphire at 7000C are shown. The surfaces showed a dense columnar structure with an

rms roughness of 16.76 A. With increasing temperature, the surface roughness increases,

reflected in the formation of large columns.

The resistivity and carrier concentration of the films were determined at room

temperature using Hall measurements (Lakeshore 7507). The Hall data is shown in Table 7-2.

Hall measurements showed that the epitaxial SnO2 films were n-type semiconductors with carrier

concentration varying from 4x 1017 cm-3 to 4.2x 1019 cm-3. Figure 7-8 shows the resistivity and

carrier concentration of SnO2 grown on sapphire as a function of deposition temperature. It has

been postulated that the conductivity is related to the existence of shallow donor levels near the

conduction band, formed by a large concentration of oxygen vacancies. The electrical conduction

in undoped SnO2 is associated with nonstoichiometry and with oxygen-related intrinsic

defects.189 The resistivity increase with growth temperature suggests that fewer oxygen vacancies

formed during high temperature deposition, resulting in lower carrier concentrations as well.









Optical absorption measurement was used to determine the band-gap of films. Figure 7-9(a)

shows transmission data for SnO2 film grew at 4000C in 50 mTorr of oxygen, showing a

maximum transmission of 70%. The band-gap of the film was calculated by (ahv)2-hv plot in

Figure 7-9(b). The band-gap was approximately 3.89 eV, which is in the range of the SnO2 thin

film reported else where.

7.3.1 Properties of gallium-doped SnO2 thin films

In order to optimize the growth conditions and examine the effects of Ga doping on the

films, different growth temperature and oxygen pressure were used. The EDX (Figure 7-10) was

used to examine the Ga dopant in the film, the detected elements are Al, Sn, O and Ga. No other

impurity or contamination was detected. The X-ray diffraction was used to examine the

crystallinity of the films grew at temperature from 400 to 7000C in 50 mTorr of oxygen. All the

SnO2 films showed (200) and (400) SnO2 peaks, which indicates that the films are highly a-axis

oriented, same as described in undoped SnO2 films previously. The results show that dopants do

not effect the epitaxial growth of SnO2 on the (0001) A1203 substrate. A comparison of a-axis

lattice parameter between undoped and Ga-doped SnO2 films shows an increase with increasing

growth temperature in both cases. However, the a-axis spacing is consistently less than that seen

in bulk SnO2. Note that the Ga-doped SnO2 films have slightly smaller a-axis constant in all

growth temperature. This might indicate that the Ga3+ does not substitute the Sn4+ sites, which

leads to the decrease of the lattice constants due to the larger size of Ga3+ (0.76A) compared to

Sn4+ (0.71 A). The effects of oxygen pressure on the films were further examined. Figure 7-12

shows the X-ray diffraction patterns of SnO2 films deposited at different oxygen pressure at

400C. The results show that the a-axis parameter (Figure 7-13) decreased with increasing

oxygen pressure, while the crystallinity increased with oxygen pressure. The a-axis parameter









was close to bulk at 10 mTorr of oxygen, suggests a loose structure which is consistent with a

larger full width half maximum (FWHM) value of (200) peak.

Optical absorption measurement was used to determine the band-gap of films. Figure 7-

14(a) shows transmission data for Ga-doped SnO2 film grew at 4000C in 50 mTorr of oxygen,

showing a maximum transmission of 80%. The band-gap of the film was calculated by (ahv)2-hv

plot in Figure 7-14(b). The band-gap was approximately 3.94 eV, which is close to value of

undoped SnO2 reported previously. The results show that no remarkable changes were found for

the band-gap of Ga-doped films.

The resistivity and carrier concentration of the Ga-doped films were determined at room

temperature using Hall measurements (Lakeshore 7507). The Hall data is shown in Table 7-3

and 7-4. Hall measurements showed that the Ga-doped SnO2 films in most case were n-type

semiconductors with carrier concentration varying from 4.8x 1015 cm3 to 4.2x 1019 cm3. In

general, the Hall data can be explained by considering the donor (intrinsic defects) and acceptors

(substation of Sn by Ga) in the films. At low temperature, the gallium atoms were not activated

as acceptors and the films were n-type because of large numbers of intrinsic defects. At high

temperature, the number of intrinsic defects decreased and results in high resistivity and low

carrier concentration. However, the gallium atoms may compensate with donor due to high

temperature and do not behave as acceptors. Interestingly, thep-type SnO2 film can be realized

at specific growth parameter (400C and 50 mTorr 02) and was reproduced again in same

condition. The optimum temperature and oxygen pressure was realized and Ga atoms were

activated and the film showedp-type characteristic with carrier concentration approximately 1019

c -3
cm









In order to further confirm the p-type behavior, a B-RH-B plot was used to determine the

Hall coefficient. In Hall measurement, the measured Hall voltage (Vmeasured) is given by:

Vmeasured = VH + Voffset + Vnoise

For Hall system, the recorded Hall voltage (Vmeasured) that calculates Hall coefficient (RH) for

single magnetic field polarity, includes noise (Vnoise) and offset voltages (Vnoise). However, the

noise and offset voltages are both magnetic field independent. If we plot the Hall coefficient

times magnetic field (B-RH) versus magnetic field (B), we can extract the actual Hall coefficient

(VH) by the slope of the linear fit line. The carrier concentration is given by the equation:

RH -
nq

Figure 7-15(a) shows the B-RH -B plot for Ga-doped SnO2 film (312 nm) grew at 4000C in 50

mTorr of oxygen, clearly showing a positive slope with Hall coefficient of 0.2291 cm3C-1. The

carrier concentration of film is 2.7x 1019 cm3, which is close to the results given by Hall system.

A similar approach has been used on another sample (184 nm) to confirm p-type characteristics

as shown in Figure 7-16(a). Although the data points were more scattered in this case, the slope

was also positive with carrier concentration of 3 x 1018 cm-3. The results show that the Ga-doped

SnO2 films were p-type at specific growth condition. However, the p-type behavior was not

stable and degraded as time proceeds. The Hall measurements were preformed again on the same

samples after one month. The carrier type was found convert from p-type to n-type after one

month (Figure 7-15(b) and 7-16(b)). Similar behavior was also observed on thermal annealed

Ga-doped SnO2 film shown in Figure 7-17(a), showing p-type with carrier concentration of

7x 1017 cm-3 after annealing at 8000C in oxygen for 1 h. The carrier type converted back to n-

type after 2 weeks (Figure 7-17(b)). The results suggest the instability of Ga dopants in the SnO2

films, similar results were found in other oxide materials.









7.4 Summary and Conclusions

In conclusion, epitaxial SnO2 thin films were realized on (0001) A1203 substrates using

pulsed laser deposition. X-ray diffraction shows that the films have a phase-pure rutile structure

and grow along the (100) plane. The epitaxial relationship can be described as SnO2 [010] //

A1203 <1120 > or equivalently SnO2 [001] // A1203 <0110> and 600 rotations. The undoped

SnO2 films were n-type semiconductor with carrier concentration varied from 4x 1017 cm-3 to

4.2x1019 cm3. The electrical transport properties are strongly dependent on the growth

temperature. The effects of Ga doping on SnO2 films were studied. The Ga-doped SnO2 films

were epitaxially grown on (0001) A1203 substrate with slightly smaller a-axis parameters. No

remarkable changes were found for the band-gap of Ga-doped films. The Hall data showed p-

type behavior occurs only at specific growth condition, but converted back to n-type and

degraded as time proceeds. More work is needed to study thep-type instability on Ga-doped

SnO2 films.












Table 7-1. Candidate dopant atoms for SnO2


Atom Valence Radius (A)
Sn +4 0.71
O -2 1.38
Li +1 0.68
In +3 0.94
Ga +3 0.76

Table 7-2. Hall data of SnO2 thin films grown at different temperature.


Resistivity
(ohm cm)
0.047
0.296
0.244
15.193
28.534


Hall Coefficient Carrier Density
(cm3/C) (1/cm3)
-0.149 4.2x1019
-0.728 8.6x1018
-0.703 8.9x1018
-10.3 6.1x1017
-16.417 4.0x1017


Hall Mobility
(cm2/VS)
3.1
2.4
2.9
0.67
0.58


Table 7-3. Hall data of Ga-doped SnO2 films grown at different temperature.


Carrier Density
(1/cm3)
6.9x1018
2.9x1019
7.1x1018
7.5x 1018
4.8x1015


Hall Mobility
(cm2/VS)
1.19
6.75
2.8
1.92
0.265


Table 7-4. Hall data of Ga-doped SnO2 films grown at different oxygen pressure.


Carrier Density
(1/cm3)
4.2x1019
2.7x1018
8.9x1018
6.1x1017


Hall Mobility
(cm2/VS)
6.09
8.36
1.04
2.32


Temp
(C)
300
400
500
600
700


Thickness
(nm)
205
239
223
237
279


Temp
(C)
350
400
500
600
750


Type


Thickness
(nm)
400
312
328
227
806


Resistivity
(ohm cm)
0.762
0.039
0.316
0.437
5130


Pressure
(C)
20
50
100
150


Type


Thickness
(nm)
260
184
163
113


Resistivity
(ohm cm)
0.011
0.337
6.175
36.568















r SnO

(101)
0700C


S5000C
S1 LJ V4000.C
300C

30 40 50 60 70 80
20 (degrees)

Figure 7-1. X-ray diffraction patterns of SnO2 films deposited on (0001) Al203 at different
temperature.


400 500 600
Growth Temperature (oC)

Figure 7-2. a-axis constant as a function of growth temperature.


700


Bulk











4.0x106


S3.0x106

0 6
2.Ox1O IFWIHM 0.0156'





0.0
*^ I
-0.2 -0.1 0.0 0.1 0.2
Omega (deg)

Figure 7-3. Rocking curve of (200) reflection of SnO2 films grown at 7000C.









4-scan through SnO2 (110)


0 L-
-180


-120 -60 0 60 120
6 (deg)


4-scan through SnO2 (101)


0 w-
-180


-120 -60 0 60 120
6 (deg)


Figure 7-4. p-scans (a) through the SnO2(110) and (b) through the SnO2(101) reflections.


5000

4000

3000

2000

1000


180


3000


2000


1000


180














(a)
[001]


[010] I

0 0


O




0 _


[0110]


Oxygen 0 Tin


[1120]


Figure 7-5. The (a) SnO2 crystal structure projection on the (100) plane, and (b) in-plane
epitaxial growth orientations by SnO2 (100) on sapphire (0001) plane.


0.40


0.36 k


0.32 I


0.28


300 400 500 600 700
Growth Temperature (oC)


Figure 7-6. Growth rate of SnO2 films on (0001) A1203 as a function of temperature.














20 pm

10 pm

0 pm


0 2.5 5.0
Scan Length (pm)


Figure 7-7. Atomic force microscope images of SnO2 thin film grown at 7000C.


102



} io-
1
10

10


1 1
rt in


300 400 500 600
Growth Temperature (oC)


700


1020



10191

10,

1o18!
10S
u~


Figure 7-8. Resistivity and carrier density of SnO2 films grown at different temperatures.


I I I I


I I110n17




















60


40-


i 20


0

2 3 4 5 6

Energy (eV)















Eg =3.89 eV

0 I
3.0 3.5 4.0 4.5 5.0

Energy (eV)


Figure 7-9. (a) Transmission spectra of the SnO2 film grew at 4000C in 50 mTorr oxygen. (b)
(ahv)2-hv plot shows the band-gap was approximately 3.89 eV.




















rt
Sn
E 3k
0 o
S2k
Sn
1k Ga Sn Sn
'a Sn
0

0 3 5 8 10

Kinetic Energy (keV)


Figure 7-10. Energy-Dispersive X-ray Spectroscopy analysis of Ga-doped SnO2 thin film grew at
400C in 50 mTorr 02


4.04 '- '- '- '- '- '- '
400 500 600 700

Growth temperature (oC)


Figure 7-11. Comparison of a-axis constant as a function of growth temperature.


Bulk




undoped

















SnO2
(101)
M ^-V\-40 mTorr

Si 30 mTorr

20 \ mTorr

10 mTorr


30 40 50 60 70 80 90

20 (degree)


Figure 7-12. X-ray diffraction patterns of Ga-doped SnO2 films deposited on (0001) A1203 at
different oxygen pressure.


4.72


4.70


4.68


4.66


10 20 30 40 50

Oxygen pressure (mTorr)


Figure 7-13. a-aixs constant of Ga-doped SnO2 films as a function of growth pressure.


Bulk




















0

.0


0
2 3 4 5
Energy (eV)


3.5 4.0 4.5 5.0


Energy (eV)


Figure 7-14. (a) Transmission
oxygen. (b) (ahv)2


spectra of the Ga-doped SnO2 film grew at 4000C in 50 mTorr
-hv plot shows the band-gap was approximately 3.94 eV.













-10.Ok (a) IuL oIedsU II UO- Io
*

-12.0k


O -14.0k
E
-16.0k

-r RH = 0.2291
m -18.0k H
n= 2.7x101 cm
R = 0.039 0 cm
-20.0k p = 6.75 (cm2/Vs)
I I I I I
-10000 -5000 0 5000 10000

B(G)



3.5k
2nd measurement on 09-18-06
3.0k
.(b)
S 2.5k

E 2.0k
CD
T 1.5k
m RH =-0.1258
1.0k n = 4.97x1019 cm-3
R = 0.043 0 cm
500.0 p- = 2.49 (cm2/Vs)

-10000 -5000 0 5000 10000
B(G)


Figure 7-15. Hall plot (B-RH-B) of Ga-doped SnO2 film grew at 400C in 50 mTorr oxygen, (a)
measured immediately after grown, (b) measured after 1 month.


*,I ,,, .. ^^ ,,,. .^^^ ^^ ^-* no ic n












-10k


-20k

-30k

b -40k

S-50k
(_
-i -60k
S* R =2.065
S-70k n =3x1018cm
R = 0.34 Q cm
-80k p = 8.36 (cm2/Vs)

-90k
-10000 -5000 0 5000 10000
B(G)



-20.0k
2nd measurement on 09-18-06
-25.0k
(b) *
-30.0k

-35.0k

E -40.0k

-45.0k RH = -0.9955
m -3
n = -6.28x1018 cm
-50.0k
R = 0.192 Q cm

-55.0k p = 3.39 (cm2/Vs) S

-10000 -5000 0 5000 10000
B(G)


Figure 7-16. Hall plot (B-RH-B) of Ga-doped SnO2 film grew at 400C in 50 mTorr oxygen, (a)
measured immediately after grown, (b) measured after 1 month.














-350.0k


--400.0k
CO
E
Y -450.0k
(D

-500.0k


-550.0k


12.0k


10.0k


8.0k


6.0k


4.0k


2.0k


0.0


1st measurement on 09-26-06


-10000 -5000 0 5000 10000

B(G)




S6 2nd measurement on 10-11-06


-10000 -5000


0 5000 10000

B(G)


Figure 7-17. Hall plot (B-RH-B) of Ga-doped SnO2 film grew at 400C in 20 mTorr oxygen and
annealed at 800C in oxygen for lh, (a) measured immediately after annealing, (b)
measured after 0.5 month.


-(a)


-S

RH = 8.7591
n =7.14x1017 cm-3
R = 8.84 Q cm
p = 1.13 (cm2/Vs)


(b)<
-0


s

RH =-0.42
19-3
n = -1.489x1019 cm3
-R = 8.84 0 cm
p = 0.0717 (cm2/Vs)









CHAPTER 8
CONCLUSION

This work focused on the synthesis of one-dimensional metal oxide nanowires and

hydrogen sensing applications. In the synthesis part, the control of initial Ag film thickness and

subsequent annealing conditions is shown to provide an effective method for controlling the size

and density of nucleation sites for catalyst-driven growth of ZnO nanorods. The completely

selective growth is possible on dielectric and silicon substrates. High density cross-linked ZnO

nanowires can be synthesized by selecting proper metal catalyst size and lattice matched

substrate. A high-pressure assisted pulsed laser deposition has been applied to fabricate a variety

of metal oxide nanowires (ZnO, ZnMgO, SnO2 and V02) without catalysts. Vertically well-

aligned ZnO and Zn Mu,. O arrays were grown on c-sapphire substrates at 600-8000C. The

nanowires growth proceeds without employing catalysts for nucleation, although an epitaxial

ZnO thin film template is necessary in order to achieve uniform alignment. This study provides a

relative convenient approach to synthesize a wide range of metal oxide nanowires.

For hydrogen sensing applications, it is found that the sensitivity for detecting hydrogen is

greatly enhanced by sputter-depositing metal catalysts (Pt and Pt) on ZnO nanowires surface. Pt-

coated ZnO nanowires can detect hydrogen down to 100 ppm with relative response of 4%. Pd-

coated ZnO nanowires can detect hydrogen down to 10 ppm with a relative smaller response

than Pt-coated devices. Approximately 95% of the initial conductance after exposure to

hydrogen was recovered within 20 s by exposing the device to air. The sensors are shown to

detect ppm hydrogen at room temperature using <0.4 mW of power when using multiple

nanowires. When using a single ZnO nanowire coated with Pt as sensing material, the power

consumption can further pushing down to [LW range. The sensors are not sensitive to oxygen,

nitrogen, humidity and air at room temperature, suggests high selectivity for hydrogen sensing









applications. A comparison study of the hydrogen-sensing characteristics of ZnO thin films with

different thickness and ZnO nanowires was described. The Pt-coated single nanowires show a

current response of approximately a factor of 3 larger at room temperature upon exposure to 500

ppm of hydrogen. Both types of sensors are shown to be capable of the detection of ppm

hydrogen at room temperature with nW power levels, but the nanowires show different recovery

characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen.

Finally, SnO2 coated ZnO nanowires were used as materials for hydrogen sensors. There was no

response to 500 ppm hydrogen at room temperature but showed a 70% response at 400C. The

use of single-crystal ZnO nanowires provide a convenient template for coating with SnO2 and the

resulting structure can be used to detect hydrogen at 400C. The results show that ZnO

nanowires have superior properties in gas sensing applications.

The epitaxial SnO2 thin films were realized on c-sapphire substrates using pulsed laser

deposition. X-ray diffraction shows that the films have a phase-pure rutile structure and grow

along the (100) plane. The epitaxial relationship can be described as SnO2 [010] // A1203

<1120 > or equivalently SnO2 [001] // A203 <01 10> and 600 rotations. The undoped SnO2 films

were n-type semiconductor with carrier concentration varied from 4x1017 cm3 to 4.2x 1019 cm3

The electrical transport properties are strongly dependent on the growth temperature. The effects

of Ga doping on SnO2 films were studied. The Ga-doped SnO2 films were epitaxially grown on

(0001) A1203 substrate with slightly smaller a-axis parameters. No remarkable changes were

found for the band-gap of Ga-doped films. The Hall data showedp-type behavior occurs only at

specific growth condition, but converted back to n-type and degraded as time proceeds.









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BIOGRAPHICAL SKETCH

Li-Chia Tien was born in Taipei, Taiwan, in 1976. He grew up in Taipei city until he

finished his high school education. With enthusiasm and great interest in Chemistry, he enrolled

at the Department of Chemistry at National Tsing Hua University (NTHU), Hsinchu, Taiwan in

1995. He received the B.S in chemistry in 1999, and continued graduate study in Department of

Materials Science and Engineering at the same institution. During the master program, he spent

two years working on surface science with Dr. Jenn-Chang Hwang and Dr. Tun-Wen Pi in

National Synchrotron Radiation Center (NSRRC) where he learned synchrotron photoemission

techniques and concepts of scientific research. During his MS, he published 4 journal articles in

the photoemission study of silicon surface. He received the M.S. degree in materials science and

engineering from National Tsing Hua University in 2001. After 20 weeks of military training,

mental and physical, he was commissioned second lieutenant in the army. From 2001 to 2003, he

served as a platoon leader in the military police corp. During this period, he learned discipline,

management and leadership.

From 2003, he began pursuing a doctoral degree in the Department of Materials Science

and Engineering at University of Florida. He was fortunate to join Dr. Norton's group in 2004

and begin research on semiconductor oxide materials. During his PhD, he was able to learn

different techniques including synthesis, processing and characterization of semiconductor

materials. He was involved in different projects such as, synthesis/characterization of metal

oxide nanowires, developing ZnO nanowires devices and synthesis/characterization epitaxial

oxide thin films by plused laser deposition (PLD) and molecular beam epitaxy (MBE). He also

developed a new catalyst-free method to synthesize metal oxide nanowires by plused laser

deposition. The results were published in approximately 24 journal articles and were presented in

7 international conferences. His future plan is to continue research in academia.





PAGE 1

1 SYNTHESIS AND APPLICATIONS OF METAL OXIDE NANOWIRES By LI-CHIA TIEN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2008

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2 2008 Li-Chia Tien

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3 To my family

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4 ACKNOWLEDGMENTS Many people have inspired, guided, helped, and laughed with during the 5 years I spent at theUniversity of Florida, and I would like to thank them a ll for a great graduate school experience. First, I would like to thank my advisor Dr. David Norton for his guidance, both personally and professionally. His positive attitude, enthusiasm and patience inspired me during different research projects. Throughout my doctoral work he encouraged me to develop independent thinking and research skills. He con tinually stimulated my analytical thinking and greatly assisted me with scientif ic writing. It has been a great pleas ure to work with him. Also I thank Dr. Steve Pearton and Dr. Fan Rens guidan ce and support during these years. I would also like to thank the members of my committee, Dr Cammy Abernathy and Dr. Simon Phillpot for their valuable advice. Special thanks go to Young-Woo for teaching me and fitting me into the lab when I first joined the group. He is always nice, patient and a pleasure to work with. Thank to Hung-Ta and Sam for performing measurements and help in hydrogen sensor project. Also thank Hyun-Siks help in PL measurements. Mats dedication on PPMS is greatly appreciated. I thank Kerry Siebein from MAIC for her generous help in HR-TEM. I also thank Dr. John Budai for performing HR-XRD on tin oxide thin film samples. Finally, I thank all the members in Dr. No rtons research group, especially those who helped and taught me when I was here in Univers ity of Florida. I really enjoy having small talk with Patrick, it always enlighten me on American culture. Ill remember the great time sharing laser, chambers, targets, experience and jokes. Finally, I would like to th ank my parents and my brother for their understanding, unconditional s upport and dedication throughout the years. A special congratulation goes to Tze-Ning, my nephew born just befo re my final defense. I also want thank my best friend Sh ih-Yings support these years.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........8 LIST OF FIGURES.........................................................................................................................9 ABSTRACT...................................................................................................................................14 CHAP TER 1 INTRODUCTION..................................................................................................................16 2 LITERATURE REVIEW.......................................................................................................19 2.1 Introduction............................................................................................................ ........19 2.2 Material Properties.........................................................................................................20 2.2.1 Properties of ZnO..............................................................................................20 2.2.2 Properties of ZnMgO........................................................................................22 2.2.3 Properties of SnO2.............................................................................................22 2.2.4 Properties of VO2..............................................................................................23 2.3 Synthesis of One-Dimensional Nanostructures............................................................. 24 2.3.1 Vapor-liquid-solid m echanism.......................................................................... 24 2.3.2 Vapor-solid m echanism.................................................................................... 26 2.3.3 Laser ablation ....................................................................................................26 2.3.4 Therm al evaporation......................................................................................... 26 2.3.5 Solution-based chem istry.................................................................................. 27 2.4 Applications of Nanowires............................................................................................28 2.4.1 Electrical applications....................................................................................... 28 2.4.2 Optical applications...........................................................................................29 2.4.3 Chemical and biochemical sensing................................................................... 30 3 EXPERIMENTAL DETAILS AND CHARACTERIZATION............................................. 37 3.1 Materials Synthesis Techniques....................................................................................37 3.1.1 Molecular beam epitaxy.................................................................................... 37 3.1.2 Pulsed laser deposition...................................................................................... 38 3.2 Characterization Techniques.........................................................................................39 3.2.1 Scanning electron microscope.......................................................................... 40 3.2.2 Atomic force microscopy.................................................................................. 40 3.2.3 X-ray diffraction................................................................................................41 3.2.4 Transmission electron microscope.................................................................... 42 3.2.5 Photoluminescence............................................................................................ 43 3.2.6 Energy-Dispersive X-ray spectroscopy............................................................ 44

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6 3.2.7 Hall measurement............................................................................................. 44 3.3 Processing and Device Fabrication ............................................................................... 45 3.3.1 Electron beam lithography................................................................................ 45 3.3.2 Sputter deposition.............................................................................................46 3.3.3 Fabrication of multiple nanowire devices......................................................... 47 3.3.4 Fabrication of single nanowire devices............................................................. 47 3.3.5 Gas sensing measurement.................................................................................47 4 NUCLEATION CONTROL AND SELECTIVE GROWTH OF ZN O NANOWIRES ........ 54 4.1 Introduction............................................................................................................ ........54 4.2 Experimental Methods...................................................................................................55 4.3 Results and Discussion..................................................................................................56 4.3.1 Structural and optical properties of ZnO nanorods grown on Si ......................56 4.3.2 Growth mechanism........................................................................................... 57 4.3.3 Nucleation control and site-selective growth.................................................... 58 4.3.4 Structural and optical propert ies of ZnO nanowires grown on sapphire .......... 59 4.4 Summary and Conclusions............................................................................................ 61 5 ZNO NANOWIRES FOR HYDROGE N SENSING APPLICATIONS ................................ 74 5.1 Introduction............................................................................................................ ........74 5.2 Experimental Methods...................................................................................................76 5.2.1 Synthesis and fabrication of ZnO nanowires sensors ....................................... 76 5.2.2 Synthesis and fabrication of ZnO thin film s sensors........................................ 77 5.2.3 Synthesis and fabrication of single ZnO nanowire sensors .............................. 77 5.2.4 Synthesis and fabrication of SnO2-ZnO nanowire sensors............................... 78 5.3 Results and Discussion..................................................................................................78 5.3.1 Catalyst functionalized ZnO nanowires............................................................78 5.3.2 Room temperature hydrogen selective sensing with ZnO nanowires ............... 80 5.3.3 Single ZnO nanowire sensors............................................................................82 5.3.4 A comparison of ZnO thin film and nanowire sensors..................................... 85 5.3.5 Surface functionalized SnO2-ZnO nanowire sensors........................................87 5.4 Summary and Conclusions............................................................................................ 89 6 CATALYST-FREE GROWTH OF METAL OXIDE NANOW IRES................................ 109 6.1 Introduction............................................................................................................ ......109 6.2 Experimental Methods.................................................................................................112 6.2.1 ZnO nanowires growth.................................................................................... 112 6.2.2 ZnMgO nanowires growth..............................................................................113 6.2.3 SnO2 nanorods growth....................................................................................114 6.2.4 VO2 nanowires growth....................................................................................115 6.3 Results and Discussion................................................................................................115 6.3.1 Synthesis and characterizatio n of vertical-aligned ZnO nanowires ................115 6.3.2 Synthesis and characte rization of ZnMgO nanowires .................................... 119 6.3.3 Synthesis and ch aracterization of SnO2 nanorods........................................... 121

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7 6.3.4 Synthesis and characterization of VO2 nanowires.......................................... 123 6.4 Summary and Conclusions.......................................................................................... 125 7 EPITAXIAL GROWTH OF TRANSPARENT TIN OXIDE THIN FILMS....................... 151 7.1 Introduction ................................................................................................................. 151 7.2 Experim ental Methods................................................................................................ 153 7.3 Results and Discussion ................................................................................................154 7.3.1 Properties of undoped SnO2 thin films...........................................................154 7.3.1 Properties o f gallium-doped SnO2 thin films.................................................. 157 7.4 Summ ary and Conclusions..........................................................................................160 8 CONCLUSION..................................................................................................................... 174 LIST OF REFERENCES.............................................................................................................176 BIOGRAPHICAL SKETCH.......................................................................................................189

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8 LIST OF TABLES Table page 2-1 Properties of ZnO..............................................................................................................32 5-1 Relative resistance response of metal-coated multiple nanowires.................................... 91 5-2 Relative resistance response of Pd and Pt coated multiple nanowires.............................. 91 7-1 Candidate dopant atoms for SnO2...................................................................................161 7-2 Hall data of SnO2 thin films grown at different temperature.......................................... 161 7-3 Hall data of Ga-doped SnO2 films grown at different temperature................................ 161 7-4 Hall data of Ga-doped SnO2 films grown at different oxygen pressure......................... 161

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9 LIST OF FIGURES Figure page 2-1 Electronic, chemical and optical processes occurring on m etal oxides that can benefit from reduction in size to the nanometer range................................................................... 33 2-2 Crystal structure of wurtzite ZnO...................................................................................... 34 2-3 Unit cell of rutile SnO2......................................................................................................34 2-4 Vapor-Solid-Liquid (VLS) process.................................................................................... 35 2-5 Vapor-Solid (VS) process..................................................................................................35 2-6 The energy band diagram of oxide thin film materials...................................................... 36 2-7 Gas sensing mechan is m of ZnO nanowire......................................................................... 36 3-1 Molecule beam epitaxy chamber....................................................................................... 49 3-2 Pulsed laser deposition chamber........................................................................................ 50 3-3 Typical Hall measurement setup a nd Van der Paul sam ple geometry............................... 51 3-4 Photograph of gas sensor device........................................................................................ 52 3-5 Gas sensing m easure ment system...................................................................................... 52 3-6 Process sketch for the fabrica tion of single ZnO nanowire devices. ................................. 53 4-1 Scanning electron microscope images of ZnO na norods on a Ag coated silicon grew at 400oC..............................................................................................................................62 4-2 X-ray diffraction pattern of ZnO nanorods grown on a 20 Ag coated SiO2/Si substrate at 400oC..............................................................................................................63 4-3 High resolution TEM image of a sing le ZnO nanorod and room tem perature PL spectra................................................................................................................................64 4-4 25 Ag on SiO2/Si with different annealing temperature and time.................................. 65 4-5 25 Ag on Si3N4/Si with different annea ling temperature and time................................. 66 4-6 Density and average size of the resulting Ag clusters on SiO2..........................................67 4-7 Density and average size of the resulting Ag clusters on Si3N4........................................68

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10 4-8 Scanning electron microcope images of selectively grown ZnO nanorods on 25 Ag/SiO2..............................................................................................................................69 4-9 Scanning electron microscope images of 200 nm of Au clusters on sapphire........... 70 4-10 Scanning electron microscope images of 50 nm of Au clusters on sapphire............. 71 4-11 Scanning electron microscope images of ZnO nanowires on an Au coated c -sapphire. ... 72 4-12 X-ray diffraction pattern of ZnO nanowires...................................................................... 73 4-13 Room temperature photoluminescence spectra of ZnO nanowires................................... 73 5-1 Metal catalysts decorated ZnO nanowires.........................................................................92 5-2 Time dependence of relative resistance respons e of metal-coated multiple nanowires.... 93 5-3 Time dependence of resistence chan ge of Pt-coated m ultiple ZnO nanowires................. 93 5-4 Time dependence of resistence chan ge of Pd-coated m ultiple ZnO nanowies.................. 94 5-5 Time dependence of resistance of either Pd-coated or uncoated multiple ZnO nanowires ...........................................................................................................................94 5-6 Relative response of Pd-coate d nanowires as a function of H2 concentration in N2.........95 5-7 Time dependence of resistance chan ge of Pd-coated m ultiple ZnO nanowires................95 5-8 Rate of resistance change after exposure to 500 ppm H2 in N2 wasmeasured at different temperatures........................................................................................................96 5-9 Arrhenius plot of rate of resist ance change after exposure to 500 ppm H2 in N2..............96 5-10 Current-voltage (I-V) plot of uncoated or Pt-coated single ZnO nanowires m easured at room temperature in pure N2..........................................................................................97 5-11 Current-voltage (I-V) char acteristics of Pt-coated ZnO single nanowires m easured in vacuum, air, N2 or 500ppm H2 in N2 ambients.................................................................. 97 5-12 Current versus time plot for single ZnO na nowires either with or without Pt coatings and corresponding | R|/R(%)-tim e plots........................................................................... 98 5-13 Room temperature I-V characteristics fr om ZnO thin films of thickness 20 or 350 nm measured in air before and after coating with Pt............................................................... 99 5-14 Current as a function of time for Pt-coa ted ZnO thin film s of different thickness.......... 100 5-15 Time dependence of current from Pt-coated ZnO nanowires and thin films................... 101

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11 5-16 Change in current at fixed bi as (0.5V) when switching to the H2-containing ambient of either Pt-coated ZnO nanowires or thin films............................................................. 101 5-17 Scanning electron microscopy micrographs of SnO2-coated ZnO nanowires and EDX spectrum....................................................................................................................... ....102 5-18 X-ray diffraction pattern from SnO2-coated ZnO nanowires..........................................103 5-19 High resolution transmission el ectro n microscope images of SnO2/ZnO nanowires......104 5-20 High resolution transmission el ectro n microscope image of SnO2-coated ZnO nanowire...........................................................................................................................104 5-21 Energy-Dispersive X-ray Spectroscopy analysis of SnO2/ZnO nanowires.....................105 5-22 Current-voltage (I-V) ch aracteristics from SnO2-coated ZnO nanowires for two different deposition times................................................................................................ 106 5-23 Current-voltage (I-V) ch aracteristics from SnO2-coated ZnO nanowires at room temperature or 400 C.......................................................................................................107 5-24 Current at fixed bias of -0.5 V and temperature of 400 C as a function of tim e............. 108 6-1 Scanning electron microscope images of well-aligned ZnO nanowires grown on a ZnO thin film tem plate.....................................................................................................127 6-2 X-ray diffraction -2 scan of ZnO nanowires grow n at 800C in 500 mTorr Ar. ......... 128 6-3 High resolution transmission electro n microscope imag es of ZnO nanowires grown on a ZnO thin film............................................................................................................129 6-4 Scanning electron microscope images of the ZnO nanowires grown at 800oC...............130 6-5 Scanning electron microscope images of the ZnO nanords............................................. 131 6-6 Room temperature PL spectra of Zn O nanowires and near-band-edge-em ission of ZnO thin film and ZnO nanowires grow n under different background ambient............. 132 6-7 Scanning electron microscope images of the ZnMgO nanowires grown at 800C in 500 m Torr Ar................................................................................................................... 133 6-8 Energy-dispersive spec troscopy spectra for ZnMgO nanowires grown on sapphire at 800C in 500 m Torr Ar.................................................................................................... 134 6-9 X-ray diffraction -2 scan of ZnMgO nanowires gr own on sapphire at 800C in 500 mTorr Ar. .........................................................................................................................135 6-10 High resolution transmission electron m icroscope image of single ZnMgO nanowire. ..........................................................................................................................................136

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12 6-11 Scanning electron microscope images of the cored ZnMgO nanowires grown on sapphire at 800C in 500 mTorr Ar. ................................................................................. 137 6-12 X-ray diffraction -2 scan of cored ZnMgO nanowires grown on sapphire on sapphire at 800C in 500 mTorr Ar. ................................................................................. 138 6-13 Transmission electron microscope im age of single cored ZnO/ZnMgO nanowire ......... 139 6-14 X-ray diffraction patterns of SnO2 nanorods grown at 800C in 500 mTorr oxygen...... 140 6-15 Energy-dispersive sp ectroscopy spectra of SnO2 nanorods grown on sapphire at 800C................................................................................................................................141 6-16 Scanning electron micros cope m orphologies of SnO2 nanorods grewn on silicon......... 142 6-17 Scanning electron microscope images showing the surface morphology and crosssection of SnO2 nanorods deposited by pul sed laser deposition...................................... 143 6-18 High resolution transmission el ectro n microscope image of SnO2 nanorods.................. 144 6-19 X-ray diffraction patterns of VO2 nanowires grown at 600C in 500 mTorr oxygen...... 145 6-20 Energy-dispersive sp ectroscopy spectra for VO2 nanowires grown on silicon at 600C................................................................................................................................146 6-21 Scanning electron microscope images of VO2 nanowires on silicon grew at 600oC.......147 6-22 High resolution transmission electro n microscope image of VO2 nanowires................. 148 6-23 The photoluminescence spectra of VO2 thin film and nanowires grew at different oxygen pressure...............................................................................................................149 6-24 Scanning electron microscope images of fabricated single nanowire device and I-V characteristics of the indivudial VO2 nanowire...............................................................150 7-1 X-ray diffraction patterns of SnO2 films deposited on (0001) Al2O3 at different temperature.................................................................................................................... ..162 7-2 a-axis constant as a function of growth tem perature....................................................... 162 7-3 Rocking curve of (200) reflection of SnO2 films grown at 700oC................................... 163 7-5 The SnO2 crystal structure projection on the ( 100) plane, and in-plane epitaxial growth orientations..........................................................................................................165 7-6 Growth rate of SnO2 films on (0001) Al2O3 as a function of temperature...................... 165 7-7 Atomic force microscope images of SnO2 thin film grown at 700oC.............................. 166

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13 7-8 Resistivity and carrier density of SnO2 films grown at different temperatures............... 166 7-9 Transmission spectra of the SnO2 film............................................................................ 167 7-10 Energy-Dispersive X-ray Spect roscopy analysis of Ga-doped SnO2 thin film............... 168 7-11 Comparison of a-axis constant as a functi on of growth tem perature.............................. 168 7-12 X-ray diffraction patterns of Ga-doped SnO2 films deposited on (0001) Al2O3 at different oxygen pressure.................................................................................................169 7-13 a-aixs constant of Ga-doped SnO2 films as a function of growth pressure..................... 169 7-14 Transmission spectra of the Ga-doped SnO2 film............................................................ 170 7-15 Hall plot (BRHB) of Ga-doped SnO2 film grew at 400oC in 50 mTorr oxygen............171 7-16 Hall plot (BRHB) of Ga-doped SnO2 film grew at 400oC in 50 mTorr oxygen............172 7-17 Hall plot (BRHB) of Ga-doped SnO2 film grew at 400oC in 20 mTorr oxygen and annealed at 800oC in oxygen for 1h.................................................................................173

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14 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SYNTHESIS AND APPLICATIONS OF METAL OXIDE NANOWIRES By Li-Chia Tien May 2008 Chair: David P. Norton Major: Materials Science and Engineering The one-dimensional nanostructured materials ha ve attracted much a ttention because of their superior properties from the deducing size in the nanometer range. Among them, metal oxide materials provide a wide diversity and functionality in both theoretical study and applications. This work focused on the synt hesis of metal oxide nanowires, and further investigated possible app lications of nanostructure d metal oxide materials. High quality ZnO nanowires have been synthe sized by catalyst-assisted molecular beam epitaxy. The control of initial Au or Ag film thickness and subsequent annealing conditions is shown to provide an effective method for contro lling the size and density of nucleation sites for catalyst-driven growth of ZnO nanorwires. For gas sensing applications, it is found that the sensitivity for detecting hydrogen is greatly enhanced by sputter-d epositing metal catalysts (Pt and Pt) on surface. The sensors are shown to de tect ppm hydrogen at room temperature using <0.4 mW of power when using multiple nanowir es. A comparison study of the hydrogen-sensing characteristics of ZnO thin films with differe nt thickness and ZnO nanowires was studied. The Pt-coated single nanowires show a current response by approximately a factor of 3 larger at room temperature. Both types of sens ors are shown to be capable of the detection of ppm hydrogen at

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15 room temperature with nW power levels, but the nanowires show different recovery characteristics, consistent with the expect ed higher surface coverage of adsorbed hydrogen. The feasibility of a number of metal oxid e nanowires has been synthesized by a highpressure assisted pulsed laser deposition. The hi gh density well-aligned metal oxide nanowires can be directly grown on substrate without metal catalysts. The results suggest the possibility of growing complex metal oxide nanos tructures, including tailored heterostructures and aligned heretojunction arrays with PLD technique. The growth of epitaxial SnO2 on csapphire using pulsed laser deposition is examined. Xray diffraction analysis show s that the films are highly a-axis oriented SnO2 with the rutile structure. The effects of Ga doping on SnO2 films were studied. The Hall data showed p-type behavior occurs only at specific growth condition, but converted back to n-type and degraded as time proceeds.

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16 CHAPTER 1 INTRODUCTION "Theres plenty of room at the bottom", a famous quote from Richard P. Feynman in 1959, addressed the great interest and significance of nanotechnology. Nanotechnology is the ability to manipulate individual atoms and molecules to produce nanostructured materials that have applications in real world. Nanotechnology involves the producti on, design, and application of physical, chemical and biological systems at scal es ranging from individual atoms or molecules to about 100 nanometers. It also involves the integration of the resulting nanostructures into larger systems. There are basically two main a pproaches to create very small structures or devices. The first approach is known as the "bottom up". The atomic or molecular building blocks are put together to create bigger obj ects. With this approach, individual atoms or molecules can be precisely placed by scanni ng probe microscopy or self-assembling. Materials and devices are built from molecular component s which assemble themselves chemically by principles of molecular recogni tion or other techniques. The s econd approach is known as "top down" approach. The macro-scale systems are c onverted into nano-scale ones by a series of sequential reduction operations. The smallest features that can be created by the "top down" approach depend on the tools used and the system operator's experience and skills. Therefore, there are limitations in creati ng features smaller than 100 nanometers. In the "top-down" approach, materials are constructed from larger en tities without atomic-lev el control. Obviously, the "top down" approach requires more work and produce considerable wast e. In contrast, the "bottom up" approach is time and waste efficient. However, precision and scalability are still big challenges for the "bottom up" approach. Nanostructures of metals, oxides and semiconduc tors have been studied intensely in the last several years by different chemical and physical methods.1-7 Interests in one-dimensional

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17 nanotubes and nanowires had drawn much interest with the discovery of carbon nanotubes in 1991.8-11 Nanowires are one-dimensional, anisotropic st ructures, small in diameter, and large in surface-to-volume ratio. Unlike other low-dimensional systems, nanowires have two quantumconfined directions but one unconfined direction available for electrical conduction. This allows nanowires to be used in applica tions where electrical conduction, ra ther than tunneling transport, is required.12-14 The unusual electronic, optical, magnetic and chemical properties of nanowires depend on their size has motivated intense research in this area. The great interest have resulted in better understanding of the phenomena of quantum confinement,15,16 logical synthetic schemes and fabrication of novel nano-electronic devices.2,12,13 Among all materials, oxide materials appear to show the most diverse range of functionality. Metal oxides play a very important ro le in many research areas such as: chemistry, physics, and materials science.17 The metal elements can form a large family of oxide compounds. These elements can adopt many structur e geometries with an electronic structure that can exhibit metallic, semiconductor or insu lator character. In tec hnological applications, oxides are used in the fabricati on of microelectronic circuits, se nsors, fuel cells, piezo-electric devices, coatings for the passivation of the surfaces and as catalysts.17 In this dissertation, we concentrate on the s ynthesis, characterizati on and applications of one-dimensional metal oxide materials. The app lications will be focused on their gas sensing properties. Chapter 2 presents background and a br ief literature review of current research on metal oxide nanowires. In Chapter 3, experimental details such as synthesis, characterization and processing techniques will be explained. The nucleation control and selective growth of ZnO nanowires will be addressed in Chapter 4. Chapter 5 examines the applications of ZnO nanowires in gas sensing. A comparison of thin films and nanowires based hydrogen sensor will

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18 also be described. A catalyst-free growth method to prepare vari eties of metal oxide nanowires will be described in Chapter 6. Growth a nd characterization of undoped and Ga-doped SnO2 thin films will be evaluated in Chapter 7 followed by conclusions in Chapter 8.

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19 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction This Chapter provides an overview of gene ral properties and recen t research on metal oxide nanowires. As a group of functional materials, metal oxides has a wide range of applications, including transparent electronics,18,19 chemical sensors,20-22 piezoelectronics,7,19,23,24 light-emitting diodes,25,26 etc. A basic understan ding of the fundamental properties of the metal oxide system is necessary for research and development towards practical applications. Nanowires are one-dimensional, anisot ropic structures, small in diameter and large in surface-to-volume ratio. The physical properties are totally different than bulk, because of their unique density of electronic states. The sm all diameters of nanowires are expected to exhibit significantly different electrical, optical a nd magnetic properties. The electron-hole interaction will have orders of magnitude enhancement in a nanostructure, due to the dramatically increased electronic density of st ates. Figure 2-1 shows a few of the electronic, chemical and optical processes occurring on metal oxides that can benefit from reduction in size to the nanometer range. Efforts have been ma de on both developing synthetic methodologies for the fabrication of nanowires, and devices ba sed on their superior properties. The onedimensional oxide nanostructures are expected to possess novel characteristics and promising applications for the following reasons:27 A large surface-to-volume ratio means that a significant fraction of the atoms (or molecules) can participate in surface reac tions. The surface depletion region can be changed dramatically by surface adsorbates. Th is is particular useful to gas sensing applications. The Debye length (D) for most semiconducting oxide nano wires is comparable to their radius over a wide temperatur e and doping range. This causes their electronic properties to be strongly influenced by processes at th eir surface. The nanowires conductivity could vary from a fully nonconductive state to a high ly conductive state based on the surface. By well controlled dimension, this could result in better sensitivity and selectivity.

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20 One-dimensional nanostructure oxides are usua lly stoichiometrically better defined and have a greater level of crystallinity (usually single crystal) than the thin-film oxides. Normally they are either with very small amount of defects or defect free structure. As the diameter of the nanowires is reduced to certain value, we can expect to see the quantum effects. Low cost and low power consumption. One can expect the low power consumption devices based on one-dimensional materials. The Chapter is divided into three main sections. After a brief introduction to the materials properties in the first section, the second secti on explores one-dimensi onal synthesis methods and growth mechanism. The applications of one-d imensional nanowires in chemical sensing will be addressed in the last section. 2.2 Material Properties As a group of functional m aterials, metal oxides has a wide range of applications, including transparent electronics, chemical sensors, piezo-electronics, light-emitting diodes, etc. A basic understanding of the fundamental propertie s of the metal oxide system is necessary for research and development towards practical appl ications. The general pr operties of metal oxide materials studied in the dissertation including zinc oxide, zinc magnesium oxide, tin oxide and vanadium oxide will be reviewed in this section. Their bulk properties provide a basic understanding of materials a nd its possible applications. 2.2.1 Properties of ZnO ZnO is a key technology m aterial with numerous applications ranging from optoelectronics to chemical sensors because of unique optical, electronic, and chemical properties. Table 2-1 shows a summary of basic physical parameters of ZnO.28 The lack of a center of symmetry in wurtzite results in consequently piezoelectric and pyroelectric pr operties. The lattice parameters of ZnO are a=0.32495 nm and c =0.52069 nm at 300K, with a c/a ratio of 1.602, which is close to the 1.633 ratio of an ideal hexa gonal close-packed structure as shown in Figure 2-2. The Zn

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21 atoms are tetrahedrally coordinated to four O atoms, where the Zn d electrons hybridize with the O p electrons. The oppositely charged ions produce positively charger (0001)-Zn and negatively charged (000 1)-O polar surfaces, resulting in a norm al dipole moment and spontaneous polarization along the c -axis.1 In addition, ZnO is a wide band-gap (3.37 eV) II-VI compound semiconductor that is suitable for short wa velength optoelectronic applications. The high excition binding energy (60 meV) in ZnO crystal can ensure effici ent excitonic emission at room temperature and room temperature ultraviolet (UV) luminescence.19 Moreover, ZnO is transparent to visible light and can be made highly conductive by doping. Electron doping in nominally undoped ZnO has been attributed to Zn interstitials, oxygen vacancies, or hydrogen. ZnO is intrinsically an n -type semiconductor, owing primarily to the presence of oxygen vacancies and/or zinc interstitials. The intrinsic defect levels that lead to n -type doping lay approximately 0.01.05 eV below the conduction band.29 On the other hand, considerable effort has been investigated to achieve p -type ZnO by incorporating group V elements.30-33 The reliable and reproducible p -type conductivity has not yet been achieved due to many issues. The compensation of dopants by energetically favorable native defects such as zinc interstitials or oxygen vacancies is one of obstacles.19 The low dopant solubility is another issue.29 Optical properties of ZnO have been exte nsively studied becaus e of their promising applications in optoelectronics.29 ZnO has an effective el ectron mass of ~0.24 me, and a large exciton binding energy of 60 meV. Furthermore, the lasing conditions can be further improved with low-dimensional ZnO structures, which enhance the excition os cillator strength and quantum efficiency.4 Therefore bulk ZnO has a small ex citon Bohr radius (~2.34 nm). The quantum confinement effect in ZnO nanowires c ould be observable at th e scale of an exciton

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22 Bohr radius. It has been reported by Gu et al. that the excition binding energy is significantly enhanced due to size confinement in Zn O nanorods with diameter of ~2 nm.16 2.2.2 Properties of ZnMgO The realization of band-gap engineering to cr eate barrier layer and quantum wells in the device heterostructures is very im portant in optoelectronic ap plications. ZnMgO alloy is an important material to construct the heterostru cture or superlattice to obtain high performance laser diode (LD) and light emitting diode (LED) devices.19,34 The ionic radius of Mg2+ (0.57 ) and Zn2+ (0.60 ) are comparable,35 alloying the ZnO phase with MgO has been investigated for increasing the band gap ZnO. Theo retically, the band gap of ZnO (Eg = 3.4 eV) can be modulated from 3.4 to 4.0 eV by doping with different amount of MgO (Eg = 7.8 eV). The energy gap Eg( x ) of the ternary semiconductor Zn1-xMgxO is determined by the following equation:29 Eg( x ) = (1-x ) Eg(ZnO)+ x Eg(MgO) bx (1x ) where b is the bowing parameter and Eg(ZnO) and Eg(MgO) are the band-gap of ZnO and MgO, respectively. The bowing parameter b depends on the difference in electronegativities of the ZnO and MgO. In addition, MgO has a cubic structure ( a=4.216 ) and it has been reported that MgO segregates in the wurtzite ZnMgO lattice above 33% of Mg, limiting the maximum band-gap to 3.9 eV.29 2.2.3 Properties of SnO2 Tin oxide (SnO2) is a wide band gap (Eg=3.6 eV at 300 K) semiconductor material suitable for multiple applications that include gas sens ors, transparent conducting electrodes, and solar cells.21,36,37 The thermodynamically stable crystal structure of SnO2 is rutile (tetragonal) with lattice parameters a=4.737 and c =3.186 as shown in Figure 2-3. The crystal structure of SnO2 belongs to the point group symmetry 4/mmm and space group P42/mmm with tin and oxygen

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23 atoms in 2a and 4f positions. With a unit cell co nsisting of two tin atoms and four oxygen atoms, with metal and oxygen atoms having an octahedral coordination. SnO2 is suitable for multiple applications that include gas sensors, transparent conducting electrodes, and solar cells. In sensor applications, SnO2 has been reported to display high gas sensitivity and selectivity.38,39 The reduced size of nanostructured SnO2 provides a material with a large surface-to-volume ra tio. Gas sensors based on one-d imensional nanostructured SnO2 have been reported to exhibit good selectivity, low de tection limits, and short response and recovery time.40-42 Several methods have been employed to prepare SnO2 nanorods including thermal evaporation, thermal decomposition, solutio n-phase growth, and hydrothermal methods.43-49 2.2.4 Properties of VO2 Among the metal oxides, vanadium oxides with various phases are of great interest and have been extensively investigated for their distinctive properties.50-55 Vanadium oxide (VO2) has attracted great attention because of the metal to insulator tran sitions and reversible dramatic changes in electrical and optical properties accompanied by a structural phase transition. VO2 can exhibit a sharp (by a factor of 1045) and fast (sub-picosecond) metal-insulator transition close to room temperature (340 K).55 The metal-insulator transition is due to a small structural distortion of the lattice from a low-temperat ure monoclinic (M1, semiconducting phase) to a high-temperature tetragonal rutile (R, metallic ph ase) structure, accompanied by large changes in conductivity and optical prope rties from infrared (IR) tr ansmission to reflecting.54 The structure of the low-temperature monoclinic phas e has the following unit cell dimensions: a=5.75 b=4.53 c =5.38 and =122.6o. For the high-temperature, rutile structure the lattice constants are a=4.55 and c =2.86 The monoclinic stru cture with the presence of V-V pairs along its a axis, where amonoclinic = 2 crutile. During the transformation, the re gular V-V separation of 2.86 in the tetragonal rutile st ructure transforms to an alternat ive V-V separation of 2.65 and 3.12

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24 leading to a doubling up of the unit cell.56 It also makes it a promising material for the use in device applications to achieve reliable electrical and optical switching operations. Moreover, B phase VO2 was found to have good electrochemical performance, especially for use as an electrode material for lithium batteries.57,58 It exhibits a maximum reve rsible capacity of about 320 mA h g-1 in the range 4 to 1 V in lithium cells.59,60 It has been reported that the operating properties of batteries depend not only on the structure but also on the morphology of the electrode components.61 Therefore, the great surface area of nanowire materials may play an important role for electrochemical applications. 2.3 Synthesis of One-Dimensional Nanostructures In this section we discuss the m ost common synthetic approaches that have successfully developed obtaining high quality nanowires of a larg e variety of materials. In order to control the diameter, aspect ratio and crys tallinity, diverse tec hniques have been applied including laser ablation, thermal evaporation, a nd solution-based growth, etc.62 According to the synthesis environment, they can be divided into two cat egories: vapor phase growth and liquid phase growth. A brief discussion of one-dimensional gr owth mechanism followed by various synthetic approaches will be covered in this section. Understanding the growth mechan ism of one-dimensional growth is critical in controlling nanostructures. Mechanisms for one-dimensiona l growth include Vapor-Liquid-Solid (VLS), screw dislocation growth, catal yst-free self-nucleation growth, and vapor-solid (VS) mechanisms.63 The VLS and VS mechanism are mostly acce pted to interpret the growth process; however, the detailed mechanisms are not yet understood in detail in many cases. 2.3.1 Vapor-liquid-solid mechanism Am ong the various mechanisms, the VLS mechanism is well established and most widely accepted. Control over the nanowire diameter and its morphology has been demonstrated by

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25 VLS growth.64-69 The VLS mechanism was proposed by Wa gner and Ellis in 1964 to explain the growth of Si whiskers us ing Au as metal catalysts.70 As illustrated in Figure 2-4, the VLS process consist basically 3 steps: (1) Formati on of the liquid alloy droplet by heating up, (2) crystal nucleation upon gas adsorp tion and supersaturation, and (3) axial growth from the crystalline seeds to form nanowires. According to the VLS mechanism, a liquid phase is formed initially, due to formation of a eutectic phase or the presence of a low-melting-point phase in an alloy system. The liquid phase adsorbs the reactan t gaseous species more efficiently than the solid surface. On supersaturation of the liquid a lloy, a nucleation center forms, and serves as a preferred site for axial growth of a nanowire. The adsorbed gas reactants are then diffused through the liquid phase to the solid/liquid interfac e, and the growth of the solid phase proceeds. Because of the larger sticking coefficient of the r eactants in the liquid, growth is much faster at the solid/liquid interface compared to the solid/vapor interface. The diameter of a nanowire via VLS growth is primarily determined by the liquid alloy droplet, and the thermodynamic-limit ed minimum radius is given by:71 Rmin = 2 LVVL/RT ln s Where LV is the liquid/vapor surface free energy, VL is the molar volume of liquid, and s is the vapor phase supersaturation. Selection of metal catalyst species depends on the formation of a eutectic phase at the temperature according to the phase diagram, as we ll as vapor/liquid/solid interfacial energies and chemical stability in the reac tion products. A wide variety of elemental semiconductor (Si and Ge), binary compound semiconductor (GaN, GaAs, GaP, InAs) and oxide (ZnO, MgO, SnO2, CdO, TiO2, In2O3 and Ga2O3) nanowires has been synthesized via the VLS method.62 Relatively good control over the nanowire diameter and di stribution has been achieved and reported.

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26 2.3.2 Vapor-solid mechanism There have been num erous reports on one-dim ensional nanostructure growth from vapor phase without metal catalyst, the growth mech anism usually refer to vapor-solid mechanism.72 Under high temperature condition, source materials are vaporized and then directly condensed on the substrate placed in the low temperature region. Once the condensation process happens, the initially condensed molecules form seed crystals serving as the nucleation sites. As a result, they assist directional growth to minimize the surface energy. The process is illustrated in Figure 2-5. With thermodynamic and kinetic consideratio ns, the formation of nanowires could be possibly through different models, including an an isotropic growth, Fra nks screw dislocation model and defect-induced model. In an anisotropic growth model, one-dimensional growth can be also explained by the preferential reactivity and binding of gas phase reactants along specific crystal facets, and also the desire for a system to minimize surface energies. In the dislocation and defect-induced growth models, specific de fects are known to have larger sticking coefficients for gas phase species, thus resulting anisotropic growth at those sites. Although the exact mechanisms for vapor-solid growth are not well understood, a variety of nanostructures have been synthesized via this approach.66,73 2.3.3 Laser ablation Laser-assisted catalytic VLS growth is a m ethod used to generate nanowires under nonequilibrium conditions.5,74,75 A target containing the catalyst a nd the source materials, plasma plume containing both catalyst and source mate rial is generated during the ablation. The nucleation occurs on the substr ate where nanowires grow. 2.3.4 Thermal evaporation In therm al evaporation, a vapor reacts on the substrate to produce the desired product. In the case of nanowires, the vaporliquid-solid (VLS) method described previously usually applies,

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27 where the substrate usually deposit with metal cataly sts serve as seed layer. The seed layer reacts with the source vapor material until it is saturated and the desired material starts to solidify and grow outward from the catalyst. The growth parameters such as: partial oxygen pressure, chamber pressure, substrate and growth temperat ure are crucial to the resulting structure of product. By control the oxygen pressure and grow th temperature, different nanostructures such as nanorods, nanowires, nanorings nanobelts and comb-like stru ctures can be synthesized.76 The solid-vapor process is a relative simple setup used to synthesis a variety of metal oxide nanostructures. The process are carried out in a horizontal tube furnace, which is composed of a horizontal tube furnace, an alum ina tube, a rotary pump system and a gas supply and control system. The source materials (norma lly powders) are loaded on an alumina boat and positioned at the center of the alumina tube, wh ere the temperature is the highest. Substrates were placed downstream for co llecting growth products. A vari ety of metal oxide nanowires (ZnO, SnO2, In2O3, CdO) with different nanostructures we re successfully synthesized by thermal evaporation process.43,71,77-80 2.3.5 Solution-based chemistry One of the m ajor disadvantages of high te mperature approaches to one-dimensional nanowire synthesis including the high cost of fabrication, high processing temperature and the inability to produce metallic nanowir es. As a result, a solution-based synthesis of nanowires with controllable diameters without the use of templates, catalys ts or surfactants has been demonstrated.81-87 The relative low cost setup to synt hesize one-dimensional nanostructure can be realized via selective capping mechanism at lo w temperature. The kinetic control of the nanocrystal growth by preferentially adsorbing mol ecular capping agents to specific crystal faces, thus inhibiting growth of that surface, results in anisotropic grow th. Because of the low

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28 temperature process, the nanowires can be grow n on inexpansive substrates, such as glass and plastic substrate.88 It provides an important f eature for device applications. 2.4 Applications of Nanowires 2.4.1 Electrical applications The sem iconductor industry continues to f ace technological (especially in lithography) challenges as the device feature size is decrea sed, especially below 100 nanometers. The selfassembly of nanowires might open a way to cons truct devices that do no t rely on improvements in photolithography. Devices made from nanowires have several advantages over those made by photolithography. For example, a variety of approach es have been devised to organize nanowires via bottom up approaches, thus the expensiv e lithography techniques ar e not required. In addition, unlike traditional silicon processing, di fferent semiconductors for example, metal oxides, can be used simultaneously in nanowir e devices to produce di verse functionalities. Transistors made from nanowires could al so hold advantages due to their unique morphology. For example, in bulk field effect tr ansistors (FETs), the depletion layer formed below the source and drain region results in a sourcedrain capacitance which limits the operation speed.89 However, in nanowires, the conductor is surrounded by an oxide and thus the depletion layer cannot be formed. Thus, de pending on the device design, the sourcedrain capacitance in nanowires could be great ly minimized and possibly eliminated.90 Nanowires have also been proposed for applications associated with electron field emission, such as flat panel displays, because of their small diameter and large curvature at the nanowire tip, which may reduce the thre shold voltage for electron emission.91-95 In this application, the demonstration of very high fi eld emission currents from the sharp tip of nanowires has stimulated interest in this potential area of application for nanowires.

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29 2.4.2 Optical applications Nanowires also show prom ise for optical ap plications. One-dimensional systems exhibit novel properties in their joint de nsity of states, allowing quantum effects in nanowires to be optically observable, sometimes even at room te mperature. Since the density of states of a nanowire in the quantum limit (small wire diamete r) is highly localized in energy, the available states quickly fill up with electrons as the in tensity of the incident light is increased.34,68,96,97 This filling up of the sub-bands, as well as other effects that is unique to low-dimensional materials, lead to strong optical nonlinea rities in quantum wires. Quantu m wires may thus yield optical switches with a lower switching energy and incr eased switching speed compared to currently available optical switches.97 Light emission from nanowires can be achieved by photoluminescence (PL) or electroluminescence (EL), distinguished by whet her the electronic excitation is achieved by optical illumination or by electrical stimulati on across a p-n junction, respectively. PL is often used for optical property charac terization, but from an applications point of view, EL is a more convenient excitation method. Light-emitting diodes (LEDs) have been achieved in junctions between a p-type and an n-type nanowire and in s uperlattice nanowires with p-type and n-type segments.3,12 The light emission was localized to the junction area, and was polarized in the superlattice nanowire. Nanowire photodetectors were also interesting applications. ZnO nanowires were found to display a strong photocurrent res ponse to UV light irradiation. The conductivity of the nanowire increased by several orders of magnitude compared to the dark state. The response of the nanowire was reversible, and sele ctive to photon energies above the band-gap, suggesting that ZnO nanowires could be a good candi date for optoelectronic switches.28,98,99

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30 2.4.3 Chemical and biochemical sensing Che mical and biological sensors have a great influence in th e areas of personal safety, public security, semiconductor processing, and the automotive and aerospace industries. Thin film based semiconducting metal oxides as chemi cal sensing materials have been extensively studied for a long time due to their advantageous feature, such as good sensitivity to the ambient conditions and ease in fabrication.21,22,37,100-103 However, there are some critical limitations difficult to overcome for thin-film-based sensi ng devices. The thin-film based devices have a limited maximum sensitivity due to the limited surface-to-volume ratios. In the case of polycrystalline thin film devices, only a small fraction of the species adsorbed near the grain boundaries is active in modifying the electrical transport properti es (Figure 2-6). Furthermore, most thin-film devices are operate d at high temperatures (200oC) in order to achieve enhanced chemical reactivity between sensor materials and surrounding gases. These drawbacks bring inconvenience for practical applications. In contrast to th in-film-based devices, devices based on one-dimensional nanostructures have gr eat potential to overc ome these fundamental limitations. Sensors for chemical and biochemical substanc es with nanowires as the sensing probe are a very attractive application area. Nanowire sensor s will potentially be smaller, more sensitive, demand less power, and react faster than their ma croscopic counterparts. For ZnO nanowires, the most widely accepted model is based on the modulat ion of the depletion layer within ZnO due to the chemisorption as illustrated schematically in Figure 2-7. When nanowires are exposure in oxidizing gas environment, oxygen adsorbs on the exposed surface of ZnO, extracting an electron from the conduc tion band, ionizes to O or O2 The O is believed to be dominant among all: O2 (g) + 2 e2O(ads)

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31 Consequently, depletion layers are formed in the surface as well as in the grain-boundary regions of ZnO, causing the carrier concentration and electron mobility to decrease. These chemisorbed O2 deplete the surface electron states and consequently reduce the channel conductivity. The resistivity increases as well. When exposed to reducing gases such as ethano l, the ethanol will react with the adsorbed O: CH3CH2OH(ads) + 6 O (ads) 2 CO2 + 3 H2O + 6 e By releasing the trapped electron back to the conduction band and, then, both the carrier concentration and the carrier mobility of Zn O increase. From the above-mentioned surface reactions, one can expect that th e factors, which can affect the interaction process between the surface-reactive chemical species (adsorbed O) and the target-gas mo lecules (ethanol for example), would be of importance for the gas-sensing performances of ZnO devices. Moreover, the more the effective surface ar ea, the more the oxygen-adsorption quantities and the higher the sensitivity of metal oxide se nsors. Since the ZnO na nowires have a larger surface area exposed to the air and a consequent higher quantity of the surface states related to oxygen vacancies than that of the ZnO thin films, a higher gas sensitivity and a faster response time of the ZnO nanowire sensors than those of th e ZnO thin film devices are reasonable. This eventually increased the conductivity of the ZnO nanowires. Because of th e small diameter of nanowires, its expected that th e bulk electronic transport properti es will change. Based on these properties, ZnO nanowires could be used as chemical and biochemical sensing materials.

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32 Table 2-1. Properties of ZnO Property Value Lattice parameters at 300 K a0 0.32495 nm c0 0.52069 nm a0 / c0 1.602 u 0.345 Density 5.606 g cm-3 Stable phase Wurtzite Melting point 1975 oC Thermal conductivity 0.6 Linear expansion coefficient a0 : 6.5 10-6 c0 : 3.0 10-6 Static dielectric constant 8.656 Reflective index 2.008 Energy gap 3.4 eV, direct Intrnsic carrier concentration <106 cm-3 Exciton binding energy 60 meV Electron effective mass 0.24 Electron Hall mobility 200 cm2V-1S-1

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33 Figure 2-1. Electronic, chemical and optical processes occurring on metal oxides that can benefit from reduction in size to the nanometer range.

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34 Figure 2-2. Crystal structure of wurtzite Zn O. The Zn atoms (orange) are tetrahedrally coordinated to four O atoms (blue). Figure 2-3. Unit cell of rutile SnO2,consisting of two tin atoms (Red) and four oxygen atoms (blue), with metal and oxygen atoms having an octahedral coordination.

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35 Figure 2-4. Vapor-Solid-L iquid (VLS) process. Figure 2-5. Vapor-Solid (VS) process.

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36 Figure 2-6. The energy band diagram of oxide thin film materials. Showing the Schottky barrier height at the grain boundary either with or without a chemically reducing ambient. Figure 2-7. Gas sensing mechanism of ZnO nano wire. (a) The large su rface depletion region caused by oxidizing ambient (b) the sm all surface depletion region caused by reducing ambient.

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37 CHAPTER 3 EXPERIMENTAL DETAILS AND CHARACTERIZATION 3.1 Materials Synthesis Techniques Any perform ance or property of a material mainly depends on the efficiency and precise nature of the synthesis and the fabrication met hods. The desirable propertie s of materials can be realized by the control of dimension, size, mo rphology, microstructure and chemical composition of materials. The ability to control these pr operties strongly depends on different synthesis methods. In this dissertation, various metal oxide nanowires and thin films were synthesized via two different approaches. Molecu lar beam epitaxy was used to synthesize ZnO nanowires by a catalyst-assisted approach, while pulsed laser deposition was employed to grow epitaxial oxide thin films and metal oxide nanowires without catalyst. The detail of materials synthesis techniques are described in following sections. 3.1.1 Molecular beam epitaxy Molecu lar beam epitaxy (MBE) is a material deposition technique cap able of predictably and reproducibly yielding material with very low impurity le vels and precise thickness control. Normally MBE requires ultra high vacuum and pr oceed at low growth temperature to prevent possible contamination. MBE involves the generation of fluxes of constituent matrix and doping species, the reaction take places at the heated substrate and form an ordered overlayer. Elemental or compound constituents are heat ed or introduced into the chambe r to generate mass transfer to the substrate via the vapor phase. To maintain the high purity of flux, th e ultra high vacuum is needed. The fluxes of elements can be temporally modulated either by altering the evaporation conditions or physically interrupting th e beam using mechanical shutters. The precision with composition and doping of a structure can be tailored by MBE. To achieve this level of control, deposition rates are normally ar ound less than one monolayer per

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38 second. The low growth rates and growth temperat ures have made MBE a superb technique for growing complex hetero-epitaxial structures on atomic scale. The high vacuum provides a clean environment and allows high quality films to be grown. The growth rate is dependent on the flux of the cells and the growth temperature. Lowe r growth temperature and higher fluxes result in amorphous of polycrystalline films, while highe r growth temperature and lower fluxes can be used to synthesize single crystal films. The ZnO nanowires were synthesized using the catalyst-driven molecular beam epitaxy method. The growth was carried out in a conve ntional MBE system (Figure 3-1). The base pressure was pumped down to approximately 5-8 mbar using a cryopump. An ozone/oxygen mixture generated by ozone generator was used as the oxidizing sour ce. The nitrogen-free plasma discharge ozone generator yielded an O3/O2 ratio on the order of 1%. The cation flux was provided by Kundsen effusion cells using high purity Zn metal ( 99.9999%) as the source materials. The Zn and O3/O2 pressures were determined by a nude ionization gauge that was placed at the substrate position prior to growth. The O3/O2 pressure was fixed at 5-6 mbar by a leaking valve during growth. The Zn pressure was varied between 50-7 and 4-6 mbar controlled by temperature. 3.1.2 Pulsed laser deposition The technique of pulsed laser deposition (P LD) has been used to deposit high quality films of materials for more than a decade.104 The technique uses hi gh power laser pulses (typically ~108 Wcm-2) to melt, evaporate and ionize material from the surface of a target. This process produces a transient, highly luminous pl asma plume that expands rapidly away from the target surface. The ablated mate rial is collected on a heated substrate on which it condenses and the thin film grows.

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39 It was found to have significant benef its over other film deposition methods. The capability for stoichiometric transfer of material fr om target to substrate can be reproduced in the deposited film. The high deposition rates can be achieved at moderate laser fluences. Since a laser is used as an external energy source, th e deposition can occur in bo th inert and reactive background gases. However, the plasma plume creat ed during ablation process is highly forward directed, therefore the thickness of resulting film is highly non-uniform and the composition can vary across the film. The deposition area is also relatively small. The oxide nanowires and thin films were synthesized using the pulsed laser deposition method. The growth was carried out in a commerci al PLD system build by Neocera (Figure 3-2). The ablation target was fabricated using high purity oxide powders. The target was pressed and sintered at high temperature. A Lambda-Physik Compex 205 KrF excimer laser was used as the ablation source. The laser produces a coherent beam with a 248 nm wavelength. A desired repetition rate can be used to achieve different de position rates, with targ et to substrate distance of 2.5 cm and a laser pulse energy density of 1 J/cm2. The growth chamber exhibits a base pressure of 10-6 Torr. Prior to growth, the target wa s cleaned in situ by pre-ablating with approximately 2000 shots. The growth experiment s were performed over a temperature range of 400oC in a desired oxygen pressure. 3.2 Characterization Techniques Different techniques were used to analyze the m aterials, including surface morphology, structural, optical, chemical and electrical analysis. The surface analysis is preformed to know the morphology and roughness of the surface. Structur al analysis provides the crystal structure, phases and defects information of materials. It also provides information about alignments between materials and substrates. Optical analysis is useful to understand the optical properties in transparent materials. Chemical analysis is useful in providi ng both qualitative and

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40 quantitative information, which are very valu able to know doping amounts and contaminations. Electrical analysis is important because the el ectrical properties directly determine the performance of materials in devi ce applications. With all thes e characterization techniques not only provide a better understandi ng of materials but also are helpful to optimize the growth conditions, producing better materials and devices. 3.2.1 Scanning electron microscope The scanning electron microscope (SEM) is of ten the first analytical instrument used to quickly look at a material. The SEM is used to observe surface and cross-section images of nanowires samples. The topographical informati on such as diameters, length and growth direction of nanowires can be co llected quickly. The electron beam is focused by condense lens into fine probe and scanned over a small area of the sample. The interaction between beam and sample surface causes elastic and inelastic inte ractions resulting in emission of secondary electrons. Due to their low ener gy, these electrons orig inate within a few nanometers from the surface. By collecting secondary electrons, surface topography can be observed. The measurements were performed in a JEOL 6335F sy stem that uses a cold cathode field emission electron source. The FE-SEM was operated at 15 keV in high magnifications. In addition, the samples were not coated to prevent contamination and artificial features 3.2.2 Atomic force microscopy Atom ic force microscopy (AFM) provides information on the topography and morphology of the surface. Inter-atomic forces between the atoms on the surface and those on the tip cause the deflection of a micro-fabricat ed cantilever. Because the magnitude of the cantilever deflection depends st rongly upon the separation between the surface and tip atoms, they can be used to map out surface topography with atomic resolution in three dimensions.

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41 AFM Dimension 3100 (Digital Instrument, Inc.) was performed in tapping mode to obtain surface topographic and roughness of metal oxide thin films. 3.2.3 X-ray diffraction X-ray diffraction (XRD) is used to obtain st ructural information and determine the phases of the sample. X-ray diffraction is commonl y used to identify unknown substances, by comparing diffraction data against a database maintained by the International Centre for Diffraction Data. It may also be used to charact erize heterogeneous solid mixtures to determine relative abundance of crystal line compounds and, when coupled with lattice refinement techniques, can provide structur al information on unknown materials. Powder diffraction is also a common method for determining st rains in crystalline materials. The XRD measurements were performed in a Philips APD 3720 system that uses a copper x-ray source. The source emits Cu K 1 X-rays with a 1.54056 for diffraction. The radiation impinged on the sample and undergoes constructive or destruct ive interference after reflecti ng from the sample. Constructive interference will cause a peak at a particular 2 angle according to Braggs law: 2dsin =n where d is the spacing between planes, is the wavelength of the incident x-ray, n is the number of whole wavelengths. For a single crystal, there are only specific orientat ions that satisfy the Braggs law. On the other hand, for a polycrysta lline film with differently oriented grains, diffraction peaks appear when t hose grains meet the diffraction conditions. Normally the full width at half maximum (FWHM) of a diffraction peak can be us ed to determine the crystal quality of the materials. For nanowires, the peak positions in the X-ray diffraction pattern can be used to determine the chemical composition and the crystal phase structure of the nanowires.

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42 High resolution XRD was performed on epitaxial thin film samples via a Philips Xpert High Resolution X-ray Diffraction system in order to determine the thickness, distribution of crystalline orientations, grain size and mosaic spread in crystalline materials. 3.2.4 Transmission electron microscope Transmission electron microscope and high-resolution transmission electron microscopy are powerful imaging tools for studying nanowires at the atomic scale. They usually provide more detailed structural information than ar e seen in the SEM images. Transmission electron microscope (TEM) is preformed to obtain the detail structural information. TEM is an imaging technique whereby a beam of electrons is tr ansmitted through a specimen, then an image is formed, magnified and directed to appear either on a fluorescent screen to be detected by a CCD camera. Because the wavelength of high-energy el ectrons is a fraction of a nanometer, and the spacings between atoms in a solid is only slightly larger, the atom s act as a diffraction grating to the electrons, which are diffracted. Therefore, so me fraction of electrons will be scattered to particular angles, determined by th e crystal structure of the sample while others continue to pass through the sample without defl ection. In the diffraction mode the selected area diffraction (SAD) is useful to identify crys tal structures and examine crystal defects. It is similar to x-ray diffraction, but unique in areas as small as several hundred na nometers in size can be examined, whereas x-ray diffraction typically samples areas several centimeters in size. This is a very useful technique to determine the crystal structur e of nanowire materials. An analytical TEM is one equipped with detectors that can determin e the elemental composition of the specimen by analyzing its X-ray spectrum or the energyloss spectrum of the transmitted electrons. High resolution transmission el ectron microscopy (HR-TEM) is an imaging mode of the TEM that allows the imaging of the crystallographi c structure of a sample at an atomic scale. Because of its high resolution, it is an invaluable tool to study nano-scale properties of crystalline

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43 material such as semiconductors and metals. At these small scales, individual atoms and crystalline defects can be imaged. The high resolu tion also permits the surface structures of the nanowires to be studied. It also serves as a powerful tool to observe core-sheath nanowire structures by mass contrast imaging. There are a number of draw backs to the TEM technique. Many materials require extensive sample preparation to produce a sample thin enough to be electron transparent, which makes TEM analysis a relatively time consuming process with a low throughput of samples. The structure of the sample may also be changed dur ing the preparation proce ss. Also the field of view is relatively small, raising the possibility th at the region analyzed may not be characteristic of the whole sample. There is potential that the sample may be damaged by the electron beam, particularly in the case of biologica l or radiation sensitive materials. By coupling the powerful imaging capabilities of TEM with energy dispersive X-ray spectrometer within the microscope, additional properties of the nanowires can be probed with high spatial resolution. The elemental compositi on within the probed area can be determined. This technique is particularly useful for th e compositional characterization of superlattice nanowires and core-shell nanowires. In this dissertation, HR-TEM images and selected area diffraction (SAD) patterns of metal oxide nanowires were acqui red by a JEOL 2100F transmission electron mi croscope at 200 keV. 3.2.5 Photoluminescence Optical methods provide an easy and sensitive tool for measuring the electronic structures of a material, sin ce optical measurements requir e minimal sample preparation. Photoluminescence (PL), a powerful and nondestruct ive analysis technology, can reveal the band structure and the carrier transpor t behaviors in materials. Photol uminescence refers to emission of light resulting from optical stimulation. When an electron increases energy by absorbing light,

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44 there is a transition from the ground state to an exc ited state. This excited state is not stable and has to return to the ground state. In luminescence materials the released energy is in the form of light, which is called as radiative transition. This emitted light is detected as photoluminescence. PL is typically used to determine band gap of a semiconductor and identify impurities. A He-Cd (325 nm) laser was used as the excitation sour ce and photoluminescence wa s detected by a GaAs PMT detector. All the measurements were pe rformed in a wavelength of 340 nm at room temperature. 3.2.6 Energy-Dispersive X-ray spectroscopy Energy-dispersive X-ray spectroscopy (EDX) is an analytical tool pr edominantly used for chemical characterization. EDX measures the emitt ed x-ray spectrum when sample is bombarded by a high-energy radiation. The emitted spectrum consists of a series of peaks representative of the type and relative amount of each element in the sample. The relative amount of each element can be calculated by comparing the peak height s with standards and applying ZAF corrections, where Z is the atomic number, A is absorption a nd F is the x-ray fluorescence. The EDX analysis was performed inside JEOL 6335F fitted wi th liquid nitrogen cooled EDX detector. 3.2.7 Hall measurement The Hall m easurement is used to determin e the electrical transport properties of a material. The carrier type, mobility, density and el ectrical resistivity can be measured from it. The Hall Effect provides a relativ ely simple, fast and low cost me thod to extract these electrical properties.105 The Lorenz force is defined as a force exerte d on a charged particle in an electromagnetic field. For example, an n-type, bar shaped semiconductor shown in Figure 3-3. It is assumed that a constant current I flows along the x-axis from left to ri ght in the presence of a z-directed magnetic field. Under Lorenz force electrons drif t toward the negative y-axis and accumulate on

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45 the side of the sample to produce an electrical surface charge. A potential drop cross the sample called Hall voltage is formed. The induced elec tric field increases unt il it counteracts to the opposite Lorenz force: nq jB BvExz zxy where yeE is the induced electric field force, Bqvx is the Lorenz force, x xnqvj is the total current density. The Hall coefficient HR is defined as: nq RH1 For electrons the charge iseq the hall coefficient is negative for n-type semiconductor while positive indicates p-type semiconductor. The mobility is defined as the coefficient of proportionality between velocity and electric field: H x x x xR nqE j E v where is conductivity, which is proportional to mobility. The resistivity, carrier type, and carrier concentration of samples were determined at room temperature using a Lakeshore 7507 system with a 10 Tesla magnet. The samples were cut into a 10 mm2 pieces. And indium dots are soldered onto corners to perform Van der Pauw measurement. 3.3 Processing and Device Fabrication 3.3.1 Electron beam lithography The electron beam lithography was used for single nanowire device fabrication. Instead of photolithography, e-beam lithography provides accessi bility to exposure desired patterns in

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46 small area without a mask. The electron gun generates a beam of electrons with a suitable current density. A single-crystal lanthanum hexaboride (LaB6) is used to for the electron gun. Condenser lenses are used to focus the electron beam to a spot size 10 nm in diameter. Beam blanking plates that turn the electron beams on and off, and beam deflection coils are computer controlled and operated at MHz or higher rate s to direct the focused electr on beam to any location in the scan field on the substrate. Since the scan fiel d is much smaller than the substrate diameter, a precision mechanical stage is used to position the substrate to be patterned. The advantages of electron-beam lithography include the generation of sub-micrometer resist geometries, highly automated and precisely controlled op eration, depth of focus greater than that available from optical lithography, and direct patterning on a semiconductor wafer without using a mask. The disadvantage is that electron-beam lithographic machines have low throughput. A RAITH 150 operated at 10 keV has been used to generate patterns for single nanowire device fabrication; the details of device fabrication will be described later. 3.3.2 Sputter deposition Sputtering is basically a sim ple PVD pro cess to deposit thin films. The atoms are removed from the surface of a solid by high-ener gy ion impacts. The plasma consisting of argon ions and electrons is ignited betw een the source and substrate. Th e target material is placed on the electrode with the voltage se t to maximize the flux at the ta rget. As the positively charged argon ions bombard the target surface, the target material is removed from the surface. The atoms impinge on the substrate and form a thin film. Sputter deposition has multiple advantages over other techniques, incl uding low cost, large area deposition, high throughput and good uniformity. Sputtering is used extensively in th e semiconductor industry to deposit thin films of various materials in integrated circuit processi ng. A Kurt Lesker CMS-18 multi target sputter

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47 deposition system was used to deposit metal cont acts and metal catalysts on nanowires in this dissertation. 3.3.3 Fabrication of multiple nanowire devices By the nature of nanowires is highly cro ss-linked on dielectric substrates, the sensing properties c an simply measure the transport by put down electrodes on the substrate. Multiple nanowire devices were fabricated by sputtere d Al/Ti/Au electrodes on as-grown samples by a shadow mask. The separation of electrodes was approximately 400 m. The samples were placed on a holder and Au wires were bounded to th e contact pads for current-voltage (I-V) measurements. Figure 3-4 shows a typical gas sensor device mounted on the holder. 3.3.4 Fabrication of single nanowire devices The fabrication process of si ngle ZnO nanowire devices is il lustrated in F igure 3-6. The nanowires were released from the as-grown substrate by sonication in ethanol and then transferred to Si substrates with 100 nm thermal oxide. The s ilicon substrates were deposited with gold markers before nanowire dipersion. With the sputtered markers on the substrate, the location of dispersed single nanowires can be addressed. The E-beam lithography followed by metallization was used to pattern electrodes cont acting both ends of a single nanowire. The separation of electrodes was approximately 3 m. Au wires were bounded to the contact pads for current-voltage (I-V) measurements. 3.3.5 Gas sensing measurement Transport D C-IV measurements were used to examine gas sensing prope rties on thin films, single and multiple nanowires devices. The meas urement has been performed in a tube furnace system (Figure 3-5) connected with Hewlett Packard Model 4156 semiconductor analyzer. The system equipped with a va riety of different gas (O2, O3, N2 and H2) and gas flow is controlled by a mass flow controller. Before measurements, the chamber was pumping down to 10-5 Torr by a

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48 turbo molecular pump. Current-voltage (I-V) meas urement was used to examine the gas sensing properties and a fix bias (0.5 V) was applied during the measurements. The furnace can be heated up to 400oC to perform gas sensing measurements at high temperatures. The response (s) was defined as the ratio of the electrical resistance in air (Ra) to that in a sample gas (Rg): g aR R s The sensitivity ( ) was defined as the change in resistivity (RgRa) divided by resistance in dry air (Rg): a agR RR

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49 Figure 3-1. Molecule beam epitaxy chamber.

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50 Figure 3-2. Pulsed laser deposition chamber.

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51 Figure 3-3. Typical Hall measurement setup and Van der Paul sample geometry. V=VHV=0 B B v F I 1 2 3 4

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52 Figure 3-4. Photograph of gas sensor device Figure 3-5. Gas sensing measurement system.

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53 Figure 3-6. Process sketch for the fabric ation of single ZnO nanowire devices.

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54 CHAPTER 4 NUCLEATION CONTROL AND SELECTIVE GROWTH OF ZN O NANOWIRES 4.1 Introduction One-dim ensional nanostructures are potential candidates for a ra nge of device applications in nanoelectronics, optoel ectronics and biosensors.3,4,7,12,14,106-111 Due to its wide band gap and high excition binding energy (60 meV), ZnO has poten tial for a wide range of applications. It also can form a wide range of nanostructu res, including nanowires, nanorods, nanobelts, nanocombs and nanorings.19 There is a strong interest in the development of ZnO nanowire device structures for potential applications in hi gh density arrays of low power field-effect transistors (FETs), gas sensors, solar cells and UV detectors.2,28,112 It is clear that nanowires and nanotubes are excellent candidates for this type of sensing, give n their large su rface-to-volume ratios and low weight.27,40 The ability to detect hydrogen selectively at room temperature is important because it avoids the use of on-chip heaters that add to th e power consumption and weight. The ability to control th e nucleation site density for cata lyst-driven growth of nanorods makes them candidates for micro-lasers or memo ry arrays. A key requirement for exploiting the potential of ZnO nanowires in thes e types of device appli cations is the control of both the density and size of the metal catalyst dots used for nucle ation of the ZnO growth and demonstration of selective area growth of the nanowires. The ZnO nanorods were synthesized by a catal yst-driven molecula r beam epitaxy method. The nanorods were found only nucleate on catalyst surface followed by the VLS mechanism. In this Chapter, the density cont rol of single crystal ZnO nanorods by varying both the annealing time and temperature is studied. It is straightforward to control the Ag cluster size in the range of 8 nm diameter and the cluster density from 30 to 2500 mm2 using an initial Ag film

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55 thicknesses of 25 and either SiO2 or SiNX dielectrics on the initia l Si wafer, followed by annealing at 600oC for 1 min. In an effort to grow ZnO nanowires w ith longer lengths for device applications, c-sapphire substrates coated with gold were used. Similar procedures were preformed to produce Au cluster size in the range of 50 nm in diameter. Hi gh density cross-linked ZnO nanowires were synthesized when Au catalysts were in the ra nge of 50 nm in diamet er. By selecting proper metal catalyst size and lattice matched substrat e, high density ZnO nanowires were synthesized by catalyst-driven molecular beam epitaxy. A morphology evolution from nanorods to nanowires is observed when using Au on c-sapphire substrate. It provides alternative synthesis approach to fabricate high aspect-ratio ZnO na nowires for device applications. 4.2 Experimental Methods Two different m etal catalysts (silver and gold) and substrat es (silicon and sapphire) were used in the study. The p-Si(100) substrates were coated with 100 nm of either SiO2 or SiNX deposited by plasma-enhanced chemical vapor de position and then coated with Ag films of 25 thickness using RF sputtering The c-plane sapphire substrates were coated with Au films of 30 by RF sputtering. Annealing of the samp les was carried out in the range of 600oC for 1 30 min under flowing N2 gas. The annealing process causes cl usters to form small islands, whose size and density were measured using scanning el ectron microscopy (SEM). These clusters act as nucleation sites for growth of the ZnO nanorods. The site-selective growth of ZnO nanorods was achieved by nucleating them on a patterned s ubstrate coated with Ag. For nominal Ag thicknesses of 25 discontinuous Ag clusters are realized afte r annealing. ZnO nanorods were synthesized by molecular beam epitaxy (M BE) with a base pressure of 5-8 mbar. An ozone/oxygen mixture generated by ozone generator was used as the oxidizing source. The cation flux was provided by Kundsen effusion cells using high purity Zn metal (Alfa Aesar,

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56 99.9999%) as the source materials. The Zn pressure was varied between 4-6 and 2-7 mbar, while the pressure of the O3/O2 mixture was 5-4 mbar. The growth temperature was varies from 400 to 600oC and typical growth time was 2 h. After growth, the samples were characterized by X-ray diffraction (XRD), sca nning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL). 4.3 Results and Discussion 4.3.1 Structural and optical propert ies of Z nO nanorods grown on Si Figure 4-1 shows a typical top-view (a)(c) and side-view (d) FE-S EM image of the asgrown nanostructures on silv er coated silicon at 400oC. Different shapes of nanostructures including whisker, needles and rods were obser ved in high density. The diameter and length of the ZnO nanorods are approximately 20 nm and 1 m. At higher temperatures (500oC), nanorods are observed slightly longer in length (2 m) with lower density. Few nanorods were observed at growth temperature over 600oC. The growth temperature is found to have significant effect on the dimension and density of resulting nanostructures. The XRD pattern of the ZnO nanorods grown at 400oC is shown in Figure 4-2. The peaks are indexed according to the hexagonal wurtzite structure of ZnO with lattice constants of a=3.249 and c=5.2038 No impurity or second ary phase peaks were observed. Although the diffraction patterns show a randomly oriented polycrystalline materi al, a preferred (002) orientation is observed. A furt her structural characterizati on is performed by TEM. The low magnification TEM image of single ZnO nanorod is shown in Figure 4-3(a), which shows the diameter is approximately 20 nm. Metal tip at the top of nanorod confirms the VLS growth mechanism. Local energy-dispersive X-ray sp ectroscopy measurements indicate that the terminating particle is Ag. This is similar to what is observed for other nanorod synthesis that is driven by a catalytic reaction, where catalyst particles become suspended on the nanorod tip.

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57 Evidence for termination of the ZnO nanorods tips with catalyst particles is also observed in field-emission SEM images. The high resolu tion TEM image of a single ZnO nanorod shows lattice fringes with [001] growth direction. The selected ar ea electron diffraction (SAD) patterns taken from a single ZnO nanorod indicates that the as-grown nanor ods are single crystal with a wurtzite structure. The optical properties of the nanorods were examined using photoluminescence. A HeCd (325 nm) laser was used as the excitation sour ce. At 300 K there was strong near-band-edge emission at 375 nm as shown in Fi gure 4-3(d). This is consistent with luminescence reported for near-band-edge emission in epitaxial f ilms and larger diameter ZnO nanorods.113,114 A broad weak, green emission at ~520 nm wa s also present. This is typically associated with trap-state emission attributed to singly ionized oxygen vacancies in ZnO.115 4.3.2 Growth mechanism By the evidence of m etal segregation at tips, the growth of nanostructures can be explained by the vaporliquidsolid (VLS) model as illustrated previously in Figure 2-4. The vapor liquidsolid (VLS) model, as an effective route to synthe sis semiconductor nanowires was proposed by Wagner and Ellis in 1964, has b een widely used for the one-dimensional nanostructure synthesis.70 The main features of VLS growth are that semiconductor nanowires have metal or alloy droplets on their tips and these droplets define diameters and direct the growth direction. The VLS growth can be divide d into 4 steps: (1) mass transport of nanowire growth species in vapor phase, (2) chemical r eaction on the liquid catalyst droplet surface, (3) diffusion in the liquid phase, and (4) crystallizat ion at the liquid-solid interface. In this case, nanorod only nucleates on metal cata lyst and site-selective growth can be realized. The size of metal catalyst determines the dimension of resulting structures.

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58 ZnO nanoparticle formation via the internal oxida tion of Zn in Ag/Zn alloys has previously been reported.116 In these studies, oxygen is diffused into an Ag/Zn alloy, with nanoscale ZnO precipitates forming in the bulk of the sample. For the present case of nanorod formation, the reaction between ozone/oxygen flux and the Ag islands appears to result in surface and subsurface oxygen diffusion in the metal island, perh aps involving the intermediate formation of Ag2O. Zn atoms impinging on the Ag island surface th en diffuse either on the surface or in the bulk of the island, where they react with the Ag2O to form ZnO. The solid solubility of Zn in Ag is on the order of 25 wt.% for the temperatures c onsidered in these experiments. Zn addition to Ag significantly suppresses the melting point to 710oC at 25 wt.% Zn. The melting point of Zn is 420oC. Based on these arguments, one might anticipate rather high diffusion rates for Zn in Ag for the temperatures considered. It should also be noted that the addition of Ag during the growth of complex oxide thin film has been reported to be effective in enhancing the oxidation process for various oxide thin-film compounds.117 4.3.3 Nucleation control and site-selective growth Figure 4-4 shows SEM m icrographs of 25 Ag films on SiO2/Si after different annealing temperature and time: (a) 700oC for 5 min; (b) 700oC for 30 min; (c) 600oC for 5 min; and (d) 800oC for 5 min. The effect of the annealing is the creation of Ag clusters. The cluster size increases with anneal temperature from 8 nm at 600oC to 30 nm at 800oC while the cluster density decreases rapidly (by about a factor of 25) in this range. Similar data is shown in Figure 4-5 for the Ag films on SiNX/Si substrates. The same general tr ends are seen as with the SiO2/Si. The variation of both cluster diameter and de nsity are shown in Figure 4-6 and 4-7 for the SiO2/Si and SiNX/Si templates, respectively. Note that we can control the cluster density in the range of 100 mm2 for SiO2/Si and 30 mm2 for SiNX/Si using an initial Ag film thicknesses of 25 and ann ealing temperatures of 600oC. The corresponding range of

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59 cluster diameters is 8 nm on SiO2 and 10 nm on SiNX. At 700oC, the annealing time has a much stronger effect on cluster density than size in both cases, wh ile the growth of the clusters and associated decrease in density is much more significant at the higher temperatures, as expected. Preliminary atomic force microscopy result s show that the height of the clusters also increases as the diameter increases. To examine the selective area growth of Zn O nanorods on regions, we patterned parallel lines by lift-off of e-beam de posited 25 thick films with wi dth 20 mm and separation of 450 mm were patterned. The samples were annealed at 800oC for 1 min to create the Ag clusters and then ZnO nanorods were grown, as described prev iously. FE-SEM microgra phs taken at different magnifications of the selectively grown wires ar e shown in Figure 4-8. The ability to control both the wire density and location is useful in applications where, for example, the wires need to be grown on an electrode fo r sensing or UV photodetection. 4.3.4 Structural and optical propert ies of Z nO nanowires grown on sapphire From the previous experimental results, its cl ear that nanowires grown on silicon substrate tend to be short and tapered. Ge nerally, 1-D nanomaterials with high aspect ratios are highly desired for device applications. The low aspect ratio structures limited their applications in device application. Therefore, efforts have been made on synthesi s longer and relatively untapered nanowires. Long nanowires provide higher surface-to-volume ratios and are beneficial to device applications. The choice of the substrate material is anot her concern in synthesi s of 1-D materials. Silicon has a lot of advantages such as low cost and easy process integrations; however the large lattice mismatch makes it difficult to control the growth. Epitaxial ZnO films have been realized on various substrate orientations of sapphire (Al2O3) substrates, the small lattice mismatch makes them suitable substrates for II-VI semiconductor growth.29 For a wide range of growth conditions,

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60 c-axis oriented epitaxial ZnO films have been realized on c-plane sapphire. In the concern of lattice mismatch, the c-sapphire substrates coated with gold were used for nanowire growth substrates. Similar procedures were preformed to produce Au cluster size in the range of 50 nm in diameter. As shown in Figure 4-9, no 1-D gr owth observed when the Au clusters are larger than 200 nm. The size of gold clusters slightly increased and surface became rough after growth, which suggests some surface reactio n occurred. When the sizes of Au catalysts are smaller than 150 nm, high density ZnO nanowires were synthesi zed as shown in Figure 4-10. Therefore, the dimensions of Au catalysts have significant effect on nanowire growth. By selecting proper metal catalyst size and lattice matched substrat es, high density ZnO nanowires were synthesized by catalyst-driven molecular beam epitaxy. A morphology evolution from nanorods to nanowires is observed when using Au catalyst on c-sapphire substrate. It pr ovides alternative synthesis approach to produce high aspect-ratio ZnO nanowires for device applications. Figure 4-11 shows top-view and side-view FE -SEM image of the ZnO nanowires grew on c-plane sapphire at 500 and 600oC. In contrast of pervious case (silicon), the nanowires grown on sapphire have higher aspect ratios and uniform diameters. The nanowires nucleate on Au particles on the surf ace and highly cross-linked together with c-axis orientation. The diameters of the ZnO nanowire are approximate ly 20 nm and length up to 8 m. At higher temperatures (600oC), nanorods are observed with slightly longer length (10 m) with smaller diameters. As a result, high growth temperature pr ovides sufficient activation energy of crystallization results in longer nanowires growth. The XRD pattern of the nanowires grown on sapphire at 500oC is shown in Figure 4-12. Only a strong (002) and weak (101) peaks are observed, which suggest a more uniform growth direction and high density of nanowires. Figure 4-13 shows the room temperature

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61 photoluminescence spectra of ZnO nanowires grown at 500oC on sapphire. Compared with nanorods grown on Ag/Si, a much stronger near band edge emission at ~380 nm with relative small deep level emission, which suggest they ar e highly crystalline with relative low defects. ZnO nanowires grew on Au/sapphire appears to ha ve better quality and optical property than those on Ag/Si. These high quality ZnO nanowires appear to be candidates for nano-electronics, nano-sensor applications. 4.4 Summary and Conclusions In summ ary, the synthesis and nucleation control of ZnO nanowires via VLS growth mechanism is studied. The control of initial Ag film thickness and subsequent annealing conditions is shown to provide an effective method for controlling the size and density of nucleation sites for catalyst-driven growth of ZnO nanorods. By using Ag film thickness of 25 on SiO2 or SiNX layers on Si substrates, we have shown that annealing between 600 and 800oC creates Ag cluster size in the ra nge of 8 nm diameter for SiO2 and 10 nm for SiNX with a cluster density from 100 to 2500 mm2 for SiO2 and 30 to 1900 mm2 for SiNX. Conventional optical lithography to create parallel Ag stripe s shows that completely selective growth is possible on either dielectric. High density cross-linked ZnO nanowires were synthesized when Au catalysts were in the range of 50 nm in diameter. By selecting proper metal catalyst size and lattice matched substrate, high density ZnO nanowires were synthesized by catalyst-driven molecular beam epitaxy. A morphology evolution from nanorods to nanowires is observed when using Au on c-sapphire substrate. It pr ovides alternative synthesi s approach to produce high aspect-ratio ZnO nanowires for nano-device appli cations. Those nanowires will be applied for gas sensing applications and de scribed in the next Chapter.

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62 Figure 4-1. (a)(b) Top view (d) side view FE-SEM images of ZnO nanorods on a Ag coated silicon grew at 400oC.

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63 304050607080 Intensity (a.u.)2 (deg)(100) (002) Si (101) (102) (110) (103) Figure 4-2. X-ray diffraction pa ttern of ZnO nanorods grown on a 20 Ag coated SiO2/Si substrate at 400oC.

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64 360380400420440 Wavelength (nm) PL Intensity (a.u.) Figure 4-3. (a) Low magnification TEM image of single ZnO nanorod. (b) High resolution TEM image of a single ZnO nanorod with lattice fri nges. (c) Select area diffraction patterns taken from a single ZnO nanorod showing the single crystal wurtzite structure. (d) Room temperature PL spectra of as-grown ZnO nanorods. (d)

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65 Figure 4-4. 25 Ag on SiO2/Si with different annealing temperature and time: (a) 700oC for 5 minutes (b) 700oC for 30 minutes (c) 600oC for 5 minutes and (d) 800oC for 5 minutes.

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66 Figure 4-5. 25 Ag on Si3N4/Si with different annealing temperature and time: (a) 700oC for 5 minutes (b) 700oC for 30 minutes (c) 600oC for 5 minutes and (d) 800oC for 5 minutes.

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67 0102030 0 20 40 60 Diameter Density 25 Ag on SiO2700 o C annealing Time (min)Cluster Daimeter (nm)0 500 1000 1500 2000 Cluster Density (#/mm2) 600 700 800 0 20 40 60 Diameter Density 25 on SiO25 min annealing Temperature (oC)Cluster Diameter (nm)0 500 1000 1500 2000 2500 Cluster Density (#/mm2) Figure 4-6. The top plot shows th e density and average size of th e resulting Ag clusters on SiO2 as a function of anneal time at 700 C. The bottom plot shows density and average size of the resulting Ag clusters on SiO2 as a function of anneal temperature for 5 min anneals.

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68 0102030 0 20 40 60 Diameter Density 25 Ag on SiNx700oC annealing Time (min)Cluster Diameter (nm)0 500 1000 1500 2000 Cluster Density (#/mm2) 600 700 800 0 20 40 60 Diameter Density 25 Ag on SiNx5 min annealingTemperature (oC)Cluster Diameter (nm)0 500 1000 1500 2000 Cluster Density (#/mm2) Figure 4-7. The top plot shows th e density and average size of th e resulting Ag clusters on Si3N4 as a function of anneal time at 700 C. The bottom plot shows density and average size of the resulting Ag clusters on Si3N4 as a function of anneal temperature for 5 min anneals.

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69 Figure 4-8. Scanning electron microcope images of selectively grown ZnO nanorods on 25 Ag/SiO2.

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70 Figure 4-9. (a), (b) Scanning electron micros cope images of 200 nm of Au clusters on sapphire. (c), (d) resu lting ZnO nanowires.

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71 Figure 4-10. (a), (b) Scanning electron microscope images of 50 nm of Au clusters on sapphire. (c), (d) resu lting ZnO nanowires.

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72 Figure 4-11. (a), (b) Top view and side view scanning electron microscope images of ZnO nanowires on an Au coated c-sapphire grew at 500oC (c), (d) SEM images of ZnO nanowires on an Au coated c-sapphire grew at at 600oC.

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73 3040506070 Intensity (a.u.)2 (deg)(002) (101) Sapphire Figure 4-12. X-ray diffraction pa ttern of ZnO nanowires grown on a sapphire substrate at 600oC. 350400450500550600 Intensity (a.u.)Wavelength (nm)3.31 eV Figure 4-13. Room temperature photoluminescence spectra of ZnO nanowires grown on sapphire at 600oC.

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74 CHAPTER 5 ZNO NANOWIRES FOR HYDROGEN SENSING APPLICATIONS 5.1 Introduction Solid-state gas sensors play an important ro le in environmental monitoring, chemical process controlling and personal safety. Semic onductor metal oxide sensors have been widely used due to their low cost and high comp atibility with microelectronic processing.21,100 In the case of the polycrystalline thin film devices, only a small fraction of the species adsorbed near grain boundaries is active in m odifying transport properties.103,118 The low surface-to-volume ratios also result the limitations in their applica tions, some drawbacks such as low sensitivity and slow response. Recently, there is strong interest in deve loping hydrogen sensors for use with protonexchange membrane and solid oxide fuel cells fo r space craft and other long-term applications. A key requirement for these sensors is the abil ity to selectively detect hydrogen at room temperature with minimal power use and weight. It is clear that nanowires and nanotubes are excellent candidates for this type of sensing, given their large surface-to-volume ratios and low weight. The ability to detect hydrogen selectivel y at room temperature is important because it avoids the use of on-chip heaters that add to the power consumption and weight. In the case of hydrogen sensing with carbon nanotubes (CNTs), se veral groups have reported that use of Pd doping or films or loading with Pd nanoparticle s can functionalize the surface of nanotubes for catalytic dissociation of H2 to atomic hydrogen.119-125 Of course, thin-film sensors of Si, GaAs, InP, SiC, and GaN with Pd contacts have been used previously as hydrogen sensors.124 ZnO nanowires are attractive for a wide variety of sensing applications because of the ease of synthesis, ability to readily transfer them to cheap substrates and their bio-safe characteristics. ZnO nanowires and nanowires have shown potenti al for use in gas, humidity, and chemical

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75 sensing. The ability to make arrays of nanow ires with large total surface area has been demonstrated with a number of different growth methods and a larg e variety of ZnO onedimensional structures has been demonstrated. To date, most of the work on ZnO nanostructures has focused on the synthesis methods. The larg e surface area of the na nowires and biosafe characteristics of ZnO makes them attractiv e for both chemical sensing and biomedical applications. There are still many aspects of this approach that require work, including quantifying the sensitivity, detection limits at room temperature, power consumption of the sensors, and time response upon switching away from the H2-containing ambient. Tin oxide (SnO2) n-type semiconductor sensors are widely used for detection of reducing gases and carbon monoxide.37,39,126 In general, these sensors have suffered from relatively low selectivity for different gases and long-term in stabilities in their re sponse. New directions towards solving these problems include use of nanoc rystalline thin films or use of catalysts to increase dissociation of gases at lower temperatures. The sensitivity of SnO2 sensors can be enhanced by reducing the nanocrystallite size below 10 nm.127 SnO2 operates at lower temperatures (300oC) and is sensitive to a wider range of gases in comparison with many other thin film sensor candidates.128,129 This makes it possible to change its sensitivity to a specific compound or group of compounds by the addition of the appropriate substances. For example, previous work has shown that the sensitivity and selectivity of SnO2 to ethanol can be improved by adding La2O3, Y, Pd or Pt128-130 and to CO can be improved by adding MoO3.131 SnO2 or multilayers of SnO2, ZnO, TiO2 and WO3 with the addition of Pd coatings have been reported for the detection of the mixtures of methanol and acetone.132-136 There has also been recent interest in the use of ZnO nanowires for sensing. ZnO has b een effectively used as a gas sensor material

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76 based on the near-surface modification of char ge distribution with cer tain surface-absorbed species.137 In this Chapter, applications of ZnO nanowires as material for hydrogen sensors will be addressed. The addition of sputter-deposited meta l clusters to the surface of ZnO nanowires produces a significant increase in detection sensiti vity for hydrogen at room temperature. The sensors are shown to detect ppm hydrogen at ro om temperature using <0.4 mW of power when using multiple nanowires. When using a single ZnO nanowire sensor, the power consumption can further pushing down to W range. Furthermore, a comparison study of the hydrogensensing characteristics of ZnO thin films with different thickness and ZnO nanowires will be described. Both types of sensors are shown to be capable of the detection of ppm hydrogen at room temperature with nW power levels, but the nanowires show different recovery characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen. The use of single-crystal ZnO nanowires provide a convenient template for coating with SnO2 and the resulting structure can be used to detect hydrogen at 400oC. 5.2 Experimental Methods 5.2.1 Synthesis and fabrication of ZnO nanowires sensors The site-selective growth of ZnO nanowire s was achieved by nucleating the nanowires on a substrate coated with Au islands as has been described in Chapter 3. In general, ZnO nanowires were synthesized by molecular beam ep itaxy with a base pressure of 5-8 mbar using high purity Zn metal (Alfa Ae sar, 99.9999%) and an O3/O2 plasma discharge as the source chemicals. The Zn pressure was varied between 4-6 and 2-7 mbar, while the beam pressure of the O3/O2 mixture was varied between 5-6 and 5-4 mbar. The growth time was ~2 h at 500C. The typical length of the as-grown nanowire was 2 m, with typical diameters in the range of 30 nm.

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77 A shadow mask was used to pattern sputtered Al/Ti/Au electrodes on the ZnO nanowire/Al2O3 substrates. The separati on of the electrodes was 30 m. In some cases, the nanowires were coated with metal catalyst thin films (~100 thick) deposited by sputtering. This forms clusters of metal with ~70% covera ge of the nanowire surface and rms roughness of ~80 Figure 5-1 illustrates the metal catalysts decorated ZnO nanowire. Au wires were bonded to the contact pad for current-voltage (I-V) m easurements performed at 25C in a range of different ambients (N2, O2 or 10 ppm H2 in N2). Note that no currents were measured through the discontinuous Au isla nds and no thin film of ZnO was observed with the growth condition for the nanowires. The I-V characteris tics from the multiple nanowires were linear with typical currents of 0.8 mA at an applied bias of 0.5 V. 5.2.2 Synthesis and fabricat ion of ZnO thin films sensors The ZnO thin films were grow n by Pulsed Laser Deposition on c-plane sapphire substrates at 450C, as described previously. The thicknes s of thin film was varied from 20 nm. A 3% O3/O2 mixture was used as background gas resulting lower ca rrier density (~1017 cm-3). The films were nominally undoped with low n-type 1017 cm-3 carrier concentration. The Ohmic contacts of sputtered Al/Ti/Au were patterned by a shadow ma sk. In some cases, the sensors were coated with Pt thin film s 10 thick deposited by sputteri ng. Au wires were bonded to the contact pad for current-volta ge I-V measurements performed at 25C in air, N2 or 500 ppm H2 in N2. No currents were measured thro ugh the discontinuous Au islands. 5.2.3 Synthesis and fabrication of single ZnO nanowire sensors The nanowires were removed from the origin al substrate by sonicati on and transferred to a Si substrate. The e-beam lithography was used to pattern sputtered Al/Ti/Au electrodes contacting both ends of single nanowires on the Si substrates. The separation of the electrodes was 10 m. In some cases, the nanowires were co ated with discontinuous Pt cluster (10

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78 thick) deposited by sputtering as shown in Figure 5-1. Au wires we re bonded to the contact pad for current-voltage (I-V) measur ements performed at 25C in a range of different ambient (vacuum, N2, O2 or 100 ppm H2 in N2).The I-V characteristics from the uncoated single nanowires were linear with typical currents in the nA at an applied bias of 0.5 V. 5.2.4 Synthesis and fabrication of SnO2-ZnO nanowire sensors The SnO2 layers were deposited on as-grown ZnO nanowires by PLD at 600oC, a partial pressure of O2 of 50 mTorr and 130 mJ of laser power with 1 Hz repetition rate, with two different deposition times of 5 or 10 min. The pa ttern sputtered Al/Ti/A u electrodes contacting both ends of multiple nanowires on the Al2O3 substrates using a shadow mask. The separation of the electrodes was 3 m. Au wires were bonded to the c ontact pad for currentvoltage (I-V) measurements performed over the range 25oC in a range of different ambients (N2, vacuum or 500 ppm H2 in N2). 5.3 Results and Discussion 5.3.1 Catalyst functionalized ZnO nanowires Previous results show that ZnO nanowires ar e not very sensitive to hydrogen at room temperature.138 In order to enhance sensitivity at ro om temperature and realize gas sensing applications, efforts have been working on su rface fictionalizations. One method for increasing hydrogen detection sensitivity is to use a catalytic metal coating or to actually dope the sensor material with the transition metal. This leads to catalytic dissociation of H2 to atomic hydrogen, which produces a sensor response through binding to surface atoms and altering the surface potential. A comparison study of different metal coa ting layers on multiple ZnO nanowires for enhancing the sensitivity to dete ction of hydrogen at room temperature has been done. Figure 5-2 shows the time dependence of rela tive resistance change of either metal-coated or uncoated

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79 multiple ZnO nanowires as the gas ambient is switched from N2 to 500 ppm of H2 in air and then back to N2 as time proceeds. These were measured at a bias voltage of 0.5V. The first point of note is that there is a strong incr ease (by approximately a factor of five) in the response of the Ptcoated nanowires to hydrogen relative to th e uncoated devices. The maximum response was approximately 8%. There is also a strong enhanc ement in response with Pd coatings, but the other metals produce little or no change. This is consistent with the known catalytic properties of these metals for hydrogen dissociation. Pd has a highe r permeability than Pt but the solubility of H2 is larger in the former.139,140 Moreover, studies of the bonding of H to Ni, Pd and Pt surfaces have shown that the adsorpti on energy is lowest on Pt.141 There was no response of either type of nanowire to the presence of O2 in the ambient at room temperature. Once the hydrogen is removed from the ambient, the recovery of the initial resistance is rapid (< 20 s). By sharp contrast, upon intr oduction of the hydrogen, the effective nanowire resistance continues to change for periods of >15 min. This suggests th at the kinetics of the chemisorption of molecular hydrogen onto the me tal and its dissociation to atomic hydrogen are the rate-limiting steps in the resulting change in conductance of ZnO.142 An activation energy of 12 kJ/mole was calculated from a plot of the rate of change of nanowire resistance. This energy is somewhat larger than that of a typical diffusi on process and suggests that the rate-limiting-step mechanism for this sensing process is more likel y to be the chemisorption of hydrogen on the Pd surface. This reversible change in conductance of metal oxides upon chemisorption of reactive gases has been discussed previously.142 The gas-sensing mechanisms suggested in the past include the desorption of adsorbed surf ace hydrogen and grain boundaries in poly-ZnO143, exchange of charges between adsorbed gas spec ies and the ZnO surface leading to changes in depletion depth144 and changes in surface or grain-boundary conduction by gas

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80 adsorption/desorption.145 We should also point out that th e I-V characteristics were the same when measured in vacuum as in air, indicati ng that the sensors are not sensitive to humidity. The power requirements for the sensors were very low. The I-V characteristics measured at 25 oC in both a pure N2 ambient and after 15 min in a 500 ppm H2 in N2 ambient. Under these conditions, the resistance response is 8% and is achieved for a power requirement of only 0.4 mW. This compares well with competing nano technologies for hydrogen detection such as Pdloaded carbon nanotubes. Moreover, the 8% response compares very well to the existing SiCbased sensors, which operate at temperatures > 100 oC through an on-chip heater in order to enhance the hydrogen dissociation efficiency.124 Figure 5-3 and 5-4 shows the response of Pt and Pd coated nanowire sensor to 10 ppm H2 in N2. The Pt-coated sensors can detect 100 ppm H2 while Pd-coated sensors can detect down to 10 ppm H2. In conclusion, Pt is found to be the most effective catalyst, followed by Pd. The resulting sensors are shown to be capable of detecting hyd rogen in the range of ppm at room temperature using very small current and vo ltage requirements and recover quickly after the source of hydrogen is removed. 5.3.2 Room temperature hydrogen selective sensing with ZnO nanowires A more detail study of hydrogen sensing with catalyst functionalized ZnO nanowires will be discussed in this section. Fi gure 5-5 shows the time dependence of resistance of either Pdcoated or uncoated multiple ZnO nanowires as the gas ambient is switched from N2 to various concentrations of H2 in air (10 ppm) as time proceeds. There are several aspects of the data. First, there is a strong increase (approximately a factor of 5) in the response of the Pd-coated nanowires to hydrogen relative to the uncoated devices. The additi on of the Pd appears to be effective in catalytic dissociation of the H2 to atomic hydrogen. Second, there was no response of either type of nanowire to the presence of O2 in the ambient at room temperature. Third, the

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81 effective conductivity of the Pd-coated nanowires is higher due to the presence of the metal. Fourth, the recovery of the ini tial resistance is rapid (20 s) upon removal of the hydrogen from the ambient, while the nanowire resistance is s till changing at least 15 min after the introduction of the hydrogen. The reversible chemisorption of reactive gases at the surface of metal oxides such as ZnO can produce a large and reversible variation in the conducta nce of the material.143 Fifth, the relative response of Pt-coa ted nanowires is a function of H2 concentration in N2. The Pd-coated nanowires detected hydrogen do wn to 10 ppm with relative responses of 2.6% at 10 ppm and 4.2% at 500 ppm H2 in N2 after a 10 min exposure, as shown in Figure 5-6. By comparison, the uncoated devices showed re lative resistance change s of 0.25% for 500 ppm H2 in N2 after a 10 min exposure, and the results were not consiste nt for lower concentrations. The gas-sensing mechanisms suggested in the pa st include the desorption of adsorbed surface hydrogen and grain boundaries in poly-ZnO144, exchange of charges between adsorbed gas species and the ZnO surface leading to changes in depletion depth146 and changes in surface or grain boundary conduction by gas adsorption/desorption.145 The detection mechanism is still not firmly established in these de vices and needs further study. It should be remembered that hydrogen introduces a shallow donor state in ZnO a nd this change in near-surface conductivity may also play a role. Figure 5-7 shows the time dependence of re sistance change of Pt-coated multiple ZnO nanowires as the gas ambient is switched from vacuum to N2, oxygen or various concentrations of H2 in air (10 ppm) and then back to air. Th ese data confirm the absence of sensitivity to O2. The resistance change during the exposure to hydrogen was slow er in the beginning and the rate resistance change reached maximum at 1.5 min of the exposure time. This could be due to some of the Pd becoming covered with native oxide, which is removed by exposure to hydrogen.

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82 Since the available surface Pd for catalytic chem ical absorption of hydrogen increased after the removal of oxide, the rate of resistance change increased. However, the Pd surface gradually saturated with the hydrogen and the rate of resistance change decreased. When the gas ambient switched from hydrogen to air, the oxygen reacted with hydrogen right awa y, with the resistance of the nanowires changed back to the original va lue instantly. Moreover, the data were recorded at a power level of 0.4 mW which is low even in comparison with CNTs.123,125 This is attractive for long-term hydrogen sensing applications. The rate of resistance change for the nanowires exposed to the 500 ppm H2 in N2 was measured at different temperatures as shown in Figure 5-8. Figur e 5-9 shows the Arrhenius plot of nanowire resistance change rate. An activatio n energy of 12 kJ/mole was calculated from the slope of the Arrhenius plot. This value is larger th an that of a typical diffu sion process. Therefore, the dominant mechanism for this sensing process is more likely to be the chemisorption of hydrogen on the Pd surface. In conclusion, Pd-coated ZnO nanowires ap pear well suited to detection of ppm concentrations of hydrogen at room temperatur e. The recovery charac teristics are fast upon removal of hydrogen from the ambient. The ZnO nanowires can be placed on cheap transparent substrates such as glass, making them attractive for low-cost sensing appl ications and operate at very low power conditions. 5.3.3 Single ZnO nanowire sensors In this section, we describe how the addition of sputter-de posited Pt clusters to the surface of single ZnO nanowires produces a signif icant increase in detection sensitivity for hydrogen at room temperature. The sensors ar e shown to detect ppm hydrogen at room temperature using W of power. Figure 5-10 shows that the addition of the Pt-coatings increased the effective conductivity of the nanowires by over an order of magnitude. Because the Pt films

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83 are discontinuous as evidenced by both field-em ission scanning electron microscopy and atomic force microscopy, this suggests that the sputtering process itself changes the resistance of the nanowires, most likely through the introduction of oxygen vacancies which are donor states in ZnO.147,148 There was a strong increase approximately a factor of 5 in the response of the Ptcoated nanowires to hydrogen relative to the uncoated devices. Figure 5-11 shows the I-V characteristics of Pt-coated nanowires as a f unction of both the measurement ambient and the time after exposure to 500 ppm H2 in N2. There are several aspects of the data. There was no response of either coated or uncoa ted nanowires to the presence of O2 in the ambient at room temperature and indeed the I-V characteristics we re independent of the measurement ambient for vacuum, air, or pure N2. By sharp contrast, the nanowires we re sensitive to the presence of H2 in the ambient, with the response being time-depende nt. The nanowire resistance was still changing at least 15 min after the introducti on of the hydrogen. An Arrhenius plot of the rate of resistance change for the nanowires exposed to the 500 ppm H2 in N2 for 10 min produced an activation energy of 15 kJ/mol. This is larger than that expected for typical diffusion processes and suggests that the rate-limiting step may be chemisorpti on of hydrogen on the Pt surface. The reversible chemisorption of reactive gases at the surface of ZnO can produce a large re versible variation in the resistance.142 In addition, atomic hydrogen introduces a shallow donor state into ZnO and this may play a role in the increased conductance of the nanowires.147,149 The diffusion coefficient of the hydrogen is also much faster in ZnO than in any other wide band-gap semiconductor. Note the very low power consumption of the nanow ire sensors, which is in the range 15-30 W. This is approximately a factor of 25 lower than multiple ZnO nanowires operated under the same conditions and more than a factor of 50 lower than carbon nanotubes doped with Pd that were

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84 used for hydrogen detection.123,125 The low power consumption is clearly of advantage in many types of remote sensing or l ong-term sensing applications. Figure 5-12 shows the time dependence of curre nt (top) or relative resistance change (bottom) in both the uncoated and Pt-c oated nanowires exposed to 500 ppm H2 in N2. The relative resistance responses were 20 and 50%, re spectively, after 10 or 20 min exposure. By comparison, the uncoated devices showed relative resi stance changes of 2 and 3%, respectively, after 10 or 20 min exposure. The resistance ch ange during the exposure to hydrogen was slower in the first few minutes, as is clear in Figure 5-13. This may be due to removal by the atomic hydrogen of native oxide on the Pt. As the effectiv e surface area of the Pt would increase as the oxide was removed, the rate of change of resistance due to hydrogen ad sorption should also increase. At fixed voltage, the relative resistan ce change was linear as a function of hydrogen content in the measurement ambient up to a few percent and then increased more slowly at higher concentrations. This may indicate a saturation of bonding site s for hydrogen at high concentrations. We have not yet investigated th e long-term reliability a nd reproducibility of the nanowire sensors, but this aspect will be a key for practical app lications. We have measured the time recovery characteristics of the single na nowires when hydrogen is removed from the ambient and find the recovery is limited by the time needed to flush the hydrogen out of the test fixture a few seconds and not by the nanowire response. In summary, Pt-coated ZnO single nanowires are shown to selectiv ely detect hydrogen at room temperature with very low power consumption. The disadvantage of this approach relative to using a network of multiple nanowires is the ad ditional processing that is needed to contact a single nanowire, but the power consumption is significantly (a factor of ~25) lower.

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85 5.3.4 A comparison of ZnO thin film and nanowire sensors Previous work, ZnO nanowires have been demo nstrated to have good sensitivity as sensing material due to their larger surface area and high aspect ratio. However, to this point, there has been no clear demonstration of improved detectio n sensitivity with nanowires compared to thin films. In this section, we report on a comparis on of the hydrogen-sensing characteristics of ZnO thin films of different thicknesses and ZnO nanow ires, both with Pt coatings. Both types of sensors are shown to be capable of the detecti on of ppm hydrogen at room temperature with nW power levels, but the nanowires show different re covery characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen. Two types of ZnO were employed in these experi ments: nanowires and thin films. The thin films were grown by Pulsed Laser Deposition on sa pphire substrates at 4 50 C, as described in detail previously. The ZnO thickness was va ried from 20 nm. Figure 5-13 shows the I-V characteristics measured between the Ohmic cont acts on the thin film ZnO samples of either 20 or 350 nm thickness, both before and after the Pt deposition on the surfa ce. The current increase as a result of the Pt deposition is approximately a factor of 2 for the thinnest sample and remains in the nA range at 0.5 V bias, i.e., the power co nsumption is 4 nW at this operating voltage. The effective conductivity of the Pt-coa ted films is higher due to the pr esence of the metal. At longer Pt sputtering times, we would typically see a tran sition to much higher curr ents, as the Pt film became continuous and the conductivity of the st ructure was no longer determined by the ZnO layer itself. Figure 5-14 (top) shows the time dependence of current change at 0.5 V bias on the Ptcoated ZnO films of different thickness as the gas ambient is switched from N2 to 500 ppm H2 in N2 and back to air as time proceeds. This data shows that the sensors are insensitive to N2 and that there is a strong ZnO thickness dependence to the response to hydrogen. The bottom of

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86 Figure 5-14 shows the change in curren t at 0.5 V bias when switching from N2 to the hydrogencontaining ambient for the ZnO films of different thickness. At small thicknesses, the current change is small, which is probably related to poorer crystal quality a nd also at large film thickness where the bulk conductivity dominates the to tal resistance. Figure 5-15 shows the time dependence of cu rrent in both the Pt-coated multiple ZnO nanowires and the thin films, as the gas ambient is switched from N2 to 500 ppm H2 in N2 and then back to air as time proceed s. It is clear that the nanowir es have a much larger response roughly a factor of 3, even for the optimal respon se for the thin films to the introduction of hydrogen into the ambient compared to their thin f ilm counterparts. This is consistent with the expectation of a higher relative response base d on their larger surf ace-to-volume ratio. Although not shown here, there was no response of eith er type of sensor to the presence of O2 in the ambient at room temperature. The recovery of the initial resistance is rapid 90%, 20 s upon removal of the hydrogen from the ambient by either O2 or air, while the nanowire resistance is still changing at least 15 min after the introdu ction of the hydrogen. The response is faster at higher temperatures. The nanowires show a slower rec overy than the thin films, most likely due to the relatively higher degree of hydrogen adsorption. The expected sensing mechanism suggested previously is that reversible chemisorption of the hydrogen on the ZnO produces a reversible variation in the conductance, with the exchange of charges betw een the hydrogen and the ZnO surface leading to changes in the depletion depth.142,143,146 The conductivity of both the ZnO thin film and nanowires did change when the ambient switched from N2 to air. Figure 5-16 shows the maximum current change at 0.5V bias for expos ure of the nanowires and thin films to the 500ppm H2 in N2. As discussed earlier, a key requ irement in long-term hydrogen-sensing

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87 applications is the sensor power consumption. Bo th the thin film and multiple nanowire sensors can operate at 0.5 V bias and pow ers 4 nW. We have also dem onstrated hydrogen sensing with single ZnO nanowires at power levels approximately an order of magnitude lower than this, but the devices show poorer long-term current stability than multiple nanowire sensors. In conclusion, Pt-coated ZnO thin films and multiple nanowires both are capable of the detection of ppm concentrations of hydrogen at room temperature. The thin films show optimum responses to the presence of hydrogen at modera te thicknesses. The nanowires show larger responses to hydrogen than the thin films, consistent with their large surface-to-volume ratios and have the advantage in terms of flexibility of the choice of substrate. 5.3.5 Surface functionalized SnO2-ZnO nanowire sensors In this section we show the use of sing le-crystal ZnO nanowires provide a convenient template for coating with SnO2 and the resulting structure can be used to detect hydrogen at 400oC. Since the metal oxides exhib it different sensing characterist ics toward chemical or gas species. Therefore, the rationale for this work is that it is straightforward to grow ZnO nanowires and the method provides an approach for integr ating a range of oxides with high surface-tovolume ratio. Figure 5-17 (top and center) shows scanning electron microscopy (SEM) micrographs of the SnO2/ZnO structures while the bottom of the fi gure shows an energy dispersive X-ray (EDX) spectrum. The latter shows characteristic Sn X-rays and confirms the presence of the SnO2 layer. Figure 5-18 shows X-ray diffracti on (XRD) spectra from the SnO2-coated ZnO nanowires. The (200) peak from the SnO2 is readily detected, while the (301 ) peak is barely visible in the samples deposited for 10 min. When take N in consideration with the electron microscopy images discussed below, the data is consistent with the SnO2 being polycrystalline.

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88 Figure 5-19 shows some high -resolution transmission electron microscopy (HR-TEM) images from the hybrid structure. It is clear that the SnO2 is deposited on only one side of the ZnO nanowires, the thickness of the SnO2 is around 10 nm and that these layers are polycrystalline. A lattice image of the SnO2/ZnO structures is shown in Figure 5-20, emphasizing the ZnO is single-crystal while the SnO2 is polycrystalline. EDX line scans shown in figure 5-21 confirmed that the SnO2 was present on only one side of the ZnO, as expected from the line-sight geometry in the PLD chamber. To perform the gas detection measurements, the multiple nanowires were contacted and mounted on a standard header. Figure 5-22 shows the currentvoltage (I-V) characteristics from SnO2-coated ZnO nanowires for two different de position times (top) and time dependence of current at fixed bias of 0.5 V as a function of measurement ambient (bottom). The devices with the thinner SnO2 has a higher current, suggesting ther e is less surface depletion in the ZnO nanowire under these conditions. It is not clear why the addi tion of another 5 nm of SnO2 should increase the effective re sistance of the underlying ZnO nanowire. There was no response at room temperature to the introduction of 500 ppm hydrogen into the measurement ambient and the structure shows more drift in current in air relative to pure N2. Figure 5-23 shows I-V characteristics from SnO2-coated ZnO nanowires at room temperature or 400oC (top) and current at fixed bias of 0.5 V as the wires are heated as a function of time (bottom). The condu ctivity of the ZnO is clearly se nsitive to temperature, with the current being thermally activ ated with activation energy 0.42 0.11 eV. This probably represents the ionization ener gy of the dominant interfacial state between the ZnO and SnO2 since pure ZnO nanowires without any coati ng exhibit lower activa tion energy for their conductivity (0.11 eV).

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89 Figure 5-24 shows the current at fixed bias of 0.5 V and temperature of 400oC in the SnO2/ZnO structures as a function of tim e as the ambient is switched from N2 to 500 ppm H2 in N2 or vacuum. Upon exposing the structures to 500 ppm H2, the conductivity increases, but neither the introduction of pure N2 or vacuum helps for obtaining 100% recovery of the initial current prior to the introduction of the trace amounts of hydrogen. The sensor continues to show drift in the current at fixed volta ge, as generally reported for SnO2. For the initial detection of the hydrogen, the sensitivity was high 70%. However, the effective sensitiv ity for subsequent detection events is lowered due to the current drift. There are multiple possible detection mechanisms for hydrogen in this hybrid stru cture, including the doping of ZnO by hydrogen donors, desorption of adsorbed surface oxygen and grain boundaries in poly-ZnO143, exchange of charges between adsorbed gas species and the ZnO surface leading to changes in depletion depth144 and changes in surface or grain boundary conduction by gas adsorption/desorption.145 In conclusion, single-crystal ZnO nanowires were coated with SnO2 using pulsed laser deposition and characterized with SEM, TEM, XRD and EDX. The SnO2 was polycrystalline, with typical thickness of order 10 nm. The hybrid structure shows a strong sensitivity to 500 ppm H2 in N2 at 400oC. This approach provides a relative ly straightforward method to integrate different oxides on templates with large surface-to-volume ratio. 5.4 Summary and Conclusions The applications of ZnO nanowires as mate rial for hydrogen sensor s were addressed. A variety of different metal catalysts (Pt, Pd, Au, Ag, Ti and Ni) sputter-deposited on multiple ZnO nanowires have been compared for their en hancement for detecting hydrogen at room temperature. It is found that the sensitivity for detecting hydrogen is grea tly enhanced by sputterdepositing metal catalysts (Pt and Pt) on surfa ce. Pt-coated ZnO nanowires can detect hydrogen down to 100 ppm with relative response of 4% Pd-coated ZnO nanowires can detect hydrogen

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90 down to 10 ppm with a relative smaller response than Pt-coated devices. Approximately 95% of the initial conductance after exposure to hydrogen was recovered within 20 s by exposing the device to air. The sensors are shown to dete ct ppm hydrogen at room temperature using <0.4 mW of power when using multiple nanowires. When using a single ZnO nanowire coated with Pt as sensing material, the power cons umption can further pushing down to W range. These sensors are not sensitive to oxygen, nitrogen, humidity and air at room temperature, suggests high selectivity for hydrogen se nsing applications. Furthermore, a comparison study of the hydrogen-sensing characteristics of ZnO thin films with different thickness and ZnO nanowires was described. The Pt-coated single nanowires show a current res ponse of approximately a factor of 3 larger at room temperature upon exposure to 500 ppm of hydrogen. Both types of sensors are shown to be capable of th e detection of ppm hydrogen at r oom temperature with nW power levels, but the nanowires show different recovery characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen. Finally, SnO2 coated ZnO nanowires were used as materials for hydrogen sensors. There was no response to 500 pp m hydrogen at room temperature but showed a 70% response at 400oC. The use of single-crystal ZnO nanowires provide a convenient templa te for coating with SnO2 and the resulting structure can be used to detect hydrogen at 400oC.

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91 Table 5-1. Relative resistance res ponse of metal-coated multiple nanowires as the gas ambient is switched from nitrogen to 500 ppm of hydrogen in air. Metal Catal y st | R | /R ( % ) Platinum ( Pt ) 8.49 Palladium (Pd) 4.26 Gold (Au) 0.66 Titanium (Ti) 0.39 Nickel (Ni) 0.28 Silver (Ag) 0.16 Table 5-2. Relative resistance response of Pd and Pt coated multiple nanowires as the gas ambient is switched from nitrogen to diffe rent concentration of hydrogen in air. Concentration (pp m ) P d -ZnO | R | /R ( % ) Pt-ZnO | R | /R ( % ) 10 2.41 0.25 100 3.14 4.67 250 3.79 6.44 500 4.28 8.50

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92 Figure 5-1. Metal catalysts decorated ZnO nanowires.

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93 051015202530 0 2 4 6 8 500ppm H2Air Time (min)| R|/R (%) Pt Pd Au Ag Ti Ni Figure 5-2. Time dependence of relative resistan ce response of metal-coated multiple nanowires as the gas ambient is switched from N2 to 500 ppm of H2 in air as time proceeds. 051015202530 0 2 4 6 8 10 Air 10~500 ppm H2Pt-ZnO nanowires 500 ppm 250 ppm 100 ppm 10 ppmTime (min)| R|/R (%) Figure 5-3. Time dependence of resistence change of Pt-coated multiple ZnO nanowires as the gas ambient is switched from N2 to various concentrations of H2 in air (10 ppm) and then back to N2.

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94 051015202530 0 1 2 3 4 5 Air 10 ~ 500 ppm H2| R|/R (%) Time (min)Pd-ZnO nanowires 500 ppm 250 ppm 100 ppm 10 ppm Figure 5-4. Time dependence of resistence change of Pd-coated multiple ZnO nanowies as the gas ambient is switched from N2 to various concentrations of H2 in air (10 ppm) and then back to N2. 0306090120150 640 650 660 670 950 960 Air Air Air Air500ppm H2250ppm H2100ppm H210ppm H2O2N2 ZnO nanorod with Pd ZnO nanorod without Pd Resistance(ohm)Time (min) Figure 5-5. Time dependence of resistance of either Pd-coated or uncoated multiple ZnO nanowires as the gas ambient is switched from N2 to various concentrations of H2 in air (10 ppm) as time proceeds. There was no response to O2.

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95 0306090120150 -0.04 -0.03 -0.02 -0.01 0.00 ZnO nanorod with PdAir Air Air Air O2500ppm H2250ppm H2100ppm H210ppm H2N2 R/R Time (min) Figure 5-6. Relative response of Pd-coa ted nanowires as a function of H2 concentration in N2. 051015202530 645 650 655 660 665 670 675 680 Resistance(ohm)Time(min) O2 N2 10ppm H2 100ppm H2 250ppm H2 500ppm H2 Figure 5-7. Time dependence of resistance change of Pd-coated multiple ZnO nanowires as the gas ambient is switched from N2 to oxygen or various concentrations of H2 in air(10 500 ppm) and then back to N2.

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96 051015202530 0 2 4 6 8 10 Air 500 ppm H2| R|/R (%)Pd-ZnO nanorods 200oC 150oC 100oC 50oC room T Time (min) Figure 5-8. Rate of resistance ch ange after exposure to 500 ppm H2 in N2 wasmeasured at different temperatures. 0.00200.00250.00300.0035 1 10 100 adsorption curve Arrhenius fitting slope= -1420.00457 activation energy (E)= 11.805 kJ/molroom T 50oC 100oC 150oC 200oC 1/T (1/oK) |dR|/dt Figure 5-9. Arrhenius plot of rate of resi stance change after exposure to 500 ppm H2 in N2.

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97 -0.4-0.20.00.20.4 -3.0x10-5-2.0x10-5-1.0x10-50.0 1.0x10-52.0x10-53.0x10-5 Single ZnO nanowire without Pt clusters with Pt clusters Current (A)Voltage (V) Figure 5-10. Current-voltage (I-V) plot of uncoa ted or Pt-coated single ZnO nanowires measured at room temperature in pure N2. -0.4-0.20.00.20.4 -6.0x10-5-4.0x10-5-2.0x10-50.0 2.0x10-54.0x10-56.0x10-5 Current (A)Voltage (V) air 1 min H2 vac 3 min H2 N2 5 min H2 10 min H2 15 min H2 Figure 5-11. Current-voltage (I-V) characteristics of Pt-coated ZnO single nanowires measured in vacuum, air, N2 or 500ppm H2 in N2 ambients. The latter responses were timedependent.

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98 05101520 0 10 20 30 40 50 60 500 ppm H2N2 Time (min)Current ( A) single Pt-ZnO nanorod single ZnO nanorod 05101520 0 10 20 30 40 50 single Pt-ZnO nanorod single ZnO nanorod500 ppm H2N2 | R|/R(%)Time (min) Figure 5-12. Current versus time plot for single ZnO nanowires either with or without Pt coatings (top) and corresponding | R|/R(%)-time plots (bottom).

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99 -0.4-0.20.00.20.4 -1.0x10-6-5.0x10-70.0 5.0x10-71.0x10-6 350nm ZnO film pre-coating post-coating Current (A)Voltage (V) -0.4-0.20.00.20.4 -8.0x10-9-6.0x10-9-4.0x10-9-2.0x10-90.0 2.0x10-94.0x10-96.0x10-98.0x10-9 Current (A)Voltage (V)20nm ZnO film pre-coating post-coating Figure 5-13. Room temperature I-V characteristic s from ZnO thin films of thickness 20 or 350 nm measured in air before and after coating with Pt.

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100 0300600900120015001800 0.0 5.0x10-61.0x10-51.5x10-52.0x10-52.5x10-5 Time (sec)Air500ppm H2N2 Current (A) Pt-ZnO thin films 350nm 170nm 40nm 20nm 0100200300400 0.0 5.0x10-61.0x10-51.5x10-52.0x10-52.5x10-5 thickness (nm) IH2I0 (A)( I) vs. film thickness Figure 5-14. Current as a function of time for Pt -coated ZnO thin films of different thickness cycled from N2 to 500 ppm H2 in N2 to air ambient (top) and change in current at fixed bias (0.5V) when switching to the H2-containing ambient (bottom).

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101 0300600900120015001800 0.00000 0.00001 0.00002 0.00073 0.00074 0.00075 0.00076 0.00077 0.00078 0.00079 0.00080 0.00081 Air500ppm H2N2 Current (A)Time (sec)Pt-coated ZnO nanowires 350 nm film 170 nm film 40 nm film 20 nm film Figure 5-15. Time dependence of current from Pt-coated ZnO nanowires and thin films as the gas ambient is switched from N2 to 500 ppm H2 in N2, then to air for recovery. 0100200300 0.0 1.0x10-52.0x10-53.0x10-54.0x10-55.0x10-56.0x10-57.0x10-5 nanowires IH2I0 (A)thickness (nm)( I) vs. film thickness Figure 5-16. Change in curre nt at fixed bias (0.5V) when switching to the H2-containing ambient of either Pt-coated ZnO nanowires or thin films as the gas ambient is switched from N2 to 500 ppm H2 in N2, then to air for recovery.

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102 051 01 5 0 1x1042x1043x1044x104 Sn Al Zn Zn Zn O CountsEnergy (keV) Figure 5-17. Scanning electron microsc opy micrographs (top and center) of SnO2-coated ZnO nanowires and EDX spectrum (bottom). 1 m 200 nm

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103 203040506070 SnO2(301) ZnO (110) ZnO (110)SnO2(200) SnO2(200)ZnO (101) ZnO (101) ZnO (100) ZnO (100)5 mins Intensity (arb.)2 (deg)10 mins 303335384043 SnO2(200) SnO2(200)ZnO (101) ZnO (101) ZnO (100) ZnO (100)5 mins Intensity (arb.)2 (deg)10 mins Figure 5-18. X-ray diffrac tion pattern from SnO2-coated ZnO nanowires. The data is consistent with the SnO2 being polycrystalline.

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104 Figure 5-19. High resolution transmission electron microscope images of SnO2/ZnO nanowires showing deposition of SnO2 on one side of the nanowires. Figure 5-20. High resolution transmissi on electron microscope image of SnO2-coated ZnO nanowire. Single crystalline Polycrystalline SnO2 layers

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105 Figure 5-21.Energy-Dispersive X-ra y Spectroscopy analysis of SnO2/ZnO nanowires.

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106 -0.4-0.20.00.20.4 -5x10-7-4x10-7-3x10-7-2x10-7-1x10-70 1x10-72x10-73x10-74x10-75x10-7 Current (A)Voltage (V) SnO2 coated ZnO nanowires 10min deposition 5min deposition 0500100015002000 0 1x10-72x10-73x10-74x10-75x10-7 Air 500ppm H2N2 Current (A)Time (s) 5min deposition 10min deposition Figure 5-22.Current-voltage (I-V ) characteristics from SnO2-coated ZnO nanowires for two different deposition times (top) and time depende nce of current at fixed bias of -0.5V as a function of measurement ambient (bottom).

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107 -0.4-0.20.00.20.4 -2.0x10-7-1.5x10-7-1.0x10-7-5.0x10-80.0 5.0x10-81.0x10-71.5x10-72.0x10-7 room T 400C Current (A)Voltage (V) 02004006008001000 0.0 5.0x10-51.0x10-41.5x10-42.0x10-4 400oC 350oC 300oC 250oC 200oC 150oC 100oC 60oC Current (A)5min deposition SnO2 coated ZnO nanowires 760 torr, N2 ambient Time (s) Figure 5-23.Current-voltage (I-V ) characteristics from SnO2-coated ZnO nanowires at room temperature or 400 C (top) and current at fixed bias of -0.4 V as the nanowires are heated as a function of time (bottom).

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108 0300600900120015001800 0.0 5.0x10-51.0x10-41.5x10-42.0x10-42.5x10-4 500ppm H2500ppm H2 Vac. N2N2Current (A) Time (s) Figure 5-24.Current at fixed bias of -0.5 V and temperature of 400 C as a function of time as the ambient is switched from N2 to 500ppm H2 in N2 or vacuum.

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109 CHAPTER 6 CATALYST-FREE GROWTH OF METAL OXIDE NANOW IRES 6.1 Introduction The synthesis of one-dimensional (1-D) semico nductor nanostructures has attracted great interest due the unique physical and chemical properties of these materials. There is significant interest in one-dimensional semiconducting nanost ructures due to their unique optical, electronic and chemical properties. Electronic nanomaterials are being pursued as possible building blocks in fabricating nanoscale electronics, optoelectronic, magnetic storage devices, and chemical sensors. Zinc oxide (ZnO) is a wide-band-gap n-type semiconductor with direct band gap of 3.37 eV that has been extensively studied due to its applicability in transparent electronics,33,150,151 chemical and gas sensors,108,119,138,152-155 spin functional devices28,66,156-159, Schottky diodes,160,161 nanoelectronics,112 and blue light-emitting diodes.25,26,162,163 The synthesis of ZnO nanowires has been reported using a variety of me thods, including thermal evaporation71, molecular beam epitaxy,164 solution-phase growth165 and hydrothermal methods.166 Vertically aligned ZnO nanowires are potentially useful for vertical device fabrica tion, with proposed device implementations that include light-emitting-diodes,163,167 dye-sensitized solar cells165,168 and nanopiezoelectrics.7 Considerable effort has been made to fabricate aligned ZnO nanowires on various substrates using either physical vapor deposition (PVD)169, chemical vapor deposition (CVD)170,171 or metal-organic chemi cal vapor depos ition (MOCVD).98,172 It remains challenging to controllably grow well-ali gned ZnO nanowires. In many cases, the growth of semiconductor nanowires proceeds via a vapor-liquid-solid (VLS ) growth mechanism that requires a metals catalyst.70 However, metal catalysts can also serve as impurities in the nanowires, thus limiting material properties.

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110 Recent progress in semiconductor nanowire hete rostructure synthesis provides possibilities in developing high-performance electronic, optoelectric, and sensing devices.15,173-181 The composition modulated nanowire hete rostructures have great potenti al as building blocks for the fabrication of high perf ormance optoelectronics.15,182-184 Composition modulation in the radial direction can efficiently confine both the carri ers and emitted photons. For example, if a shell layer in a coaxial nanowiere heterostructure has wider band gap energy and a lower reflective index than a core layer, confinement of both car rier and photons in th e core nanowire can be significantly enhanced.15,176 Alloying the ZnO phase with Mg O has been investigated for increasing the bad gap of ZnO-based nanowires.29 Theoretically, the band gap of ZnO can be modulated from 3.3 to 4.0 eV by dopi ng with different amount of MgO.19,185 ZnMgO alloy is an important material to construct the heterostru cture or superlattice to obtain high performance laser diode (LD) and light emitting diode (LED) devices.15,186 Previously, we have observed the formation of various core/shell nanowires when adding Mg to the Zn and O flux during growth of Au-catalyst nucleated ZnO nanowires by molecular beam epitaxy.183,187 However, a clean and abrupt interface has not been pr oduced because of spontaneous phase separation inducing selfordered formation of coaxial heterostructures. Tin oxide (SnO2) is a wide band gap (Eg=3.6 eV at 300K) semiconductor material suitable for multiple applications that include gas sensors38,188, transparent conducting electrodes189, and solar cells.190,191 In sensor applications, SnO2 has been reported to display high gas sensitivity and selectivity.38,127 The reduced size of nanostructured SnO2 provides a material with a large surface-to-volume ratio.155 Gas sensors based on one-dim ensional nanostructured SnO2 have been reported to exhibit good selectivity, low de tection limits, and short response and recovery time.37,42,192-194 Several methods have been employed to prepare SnO2 nanorods including

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111 thermal evaporation,6,195 thermal decomposition,45,196 solution-phase growth,47,197 and hydrothermal methods.198 Among these, the thermal evaporation approach has been used to synthesize a wide variety of one-dimensional materials.5 This often has involved the use of a catalyst in which nanowire growth proceed s by a vapor-liquid-solid (VLS) mechanism.70 However, metal catalysts can serve as impurities in the nanowires, possibly forming defect states that limit their application in devices. Vanadium oxide (VO2) nanowires have attracted great at tention because of their metal to insulator transitions and reversible dramatic changes in electrical and optical properties accompanied by a structural phase transition.50,54,58,59 It also makes it a promising material for the use in device applications to achieve reliable electrical and optical switching operations. VO2 can exhibit a sharp (by a factor of 1045) and fast (sub-picosecond) me tal-insulator transition close to room temperature (340 K).55 The metal-insulator transition is due to a small structural distortion of the lattice from a low-temperat ure monoclinic (semiconducting phase) to a hightemperature tetragonal rutile (metallic phase) structure, accompanied by large changes in conductivity and optical prope rties from infrared (IR) tr ansmission to reflecting.54 Moreover, B phase VO2 was found to have good electrochemical performance, especially for use as an electrode material for lithium batteries.57,58 It exhibits a maximum reve rsible capacity of about 320 mA h g-1 in the range 4 to 1 V in lithium cells.59,60 It has been reported that the operating properties of batteries depend not only on the structure but also on the morphology of the electrode components.61 Therefore, the great surface area of nanowire materials may play an important role for electrochemical applications.155 In this chapter, we report the synthesis of metal oxide nanowires by high-pressure assisted pulsed laser deposition. A variety of metal oxide nanowires (ZnO, SnO2, and VO2) can be

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112 synthesized without metal catalyst. The doping effects of Mg in ZnO nanowires were also examined. In the first secti on, the growth of well-aligned ZnO nanowires by high-pressure assisted pulsed laser deposition (PLD) is report ed. The nanowire growth requires a ZnO template for nucleation, but proceeds without the use of an y metal catalyst. The stru cture and properties of the nanowires are characterized, revealing high quality single cr ystal ZnO nanowires. The effects of growth temperature and background pressure on nanowire growth and properties are discussed. The addition of Mg into ZnO has been examined as well. The resulting structures show the segregation of Mg because of big lattice mismat ch and limited solubility. By switching MgO and ZnO targets during growth, a core -sheath structure is observed. The synthesis of aligned SnO2 nanorods is described in the second secti on. The nanorod morphology is observed for PLD growth conditions that include a relatively high background pressure, high substrate temperature, and a non-epitaxial relationship between the SnO2 and substrate. SnO2 nanorod growth is achieved without the use of any me tal catalyst. In the last sect ion, high aspect ratio monoclinic VO2 nanowires grew laterally on the silicon and c-sapphire substrates at 600oC in 500 mTorr argon. The nanowires randomly nucleate on the su rface with the diameter of 90 nm, length up to 50 m. Since pulsed laser depos ition is a convenient means for achieving stoichiometric transfer in growing multi-element materials199, these results suggest th e possibility of growing oxide nanowires with complex crystal structures and/or multi-cation stoichiometry. 6.2 Experimental Methods 6.2.1 ZnO nanowires growth Pulsed laser deposition was used for the Zn O nanowire growth. The ablation target was fabricated using high purity ZnO (Alfa Aesar, 99.9995%). The target was pressed and sintered at 1000oC for 12 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of 5 Hz was used, with target to substrate distance of 2.5 cm and a laser pulse energy density of 1

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113 J/cm2. The growth chamber exhib its a base pressure of 10-6 Torr. In order to achieve wellordered ZnO nanowires, a th in (75 nm) ZnO template layer was grown on the c-plane sapphire substrate prior to nanowire nuc leation. The ZnO template film was c-axis oriented. Prior to deposition, the substrates were ultrasonically cl eaned with trichloroe thylene, acetone and methanol, followed by compressed N2 drying. The substrates were attached to the heater using Ag paint. Prior to growth, the target was cleaned in situ by pre-ablating with approximately 2000 shots. The growth experiments were pe rformed over a temperature range of 500oC in a background pressure of 150 mTorr. Three different gas ambient (O2, Ar and O2/Ar mixture) were used to investigate the effects of oxidation and gas-phase collisions in the formation of nanowires. The typical growth time was 2 h. After growth, the samples were cooled under the same gas ambient as was used during growth. The as-grown samples were characterized using X-ray diffraction (XRD) (Philips 3720, Cu-K), field emission scanning electron microscopy (FE-SEM) (JEOL 6335F) and high resolution transmission electron microscopy (HR-TEM) (JEOL 2010F). The optical properties of the na nowires were examined using photoluminescence at room temperature. A He-Cd (325 nm) la ser was used as the excitation source. 6.2.2 ZnMgO nanowires growth The synthesis of ZnMgO nanowires was carried out by pulsed laser deposition. The ablation target was fabricated using powder mixture of high purity ZnO (Alfa Aesar, 99.9995%) and MgO (Alfa Aesar, 99.95%) with Mg:Zn atomic ratios of 1:4. The target was pressed and sintered at 1000oC for 12 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of 5 Hz was used, with target to substrate distance of 2.5 cm and a laser pulse energy density of 1 J/cm2. The growth chamber exhib its a base pressure of 10-6 Torr. The cplane sapphire was used as the substrate materials in this study. Prior to deposition, the substrates were ultrasonically cleaned with trichlor oethylene, acetone and methanol, followed by

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114 compressed N2 drying. The substrates were attached to th e heater using Ag paint. Prior to growth, the target was cleaned in situ by pre-ablating with approximately 2000 shots. The growth temperature was 800oC in a background pressure of 500 mTorr of oxygen. The typical growth time was 2 h. After growth, the samples were co oled under the same gas ambient as was used during growth. The as-grown samples were characterized using X-ray diffraction (XRD) (Philips 3720, Cu-K), field emission scanning electron mi croscopy (FE-SEM) (JEOL 6335F) and high resolution transmission electron microscopy (HRTEM) (JEOL 2010F). The optical properties of the nanowires were examined using photoluminescence at room temperature. A He-Cd (325 nm) laser was used as the excitation source. 6.2.3 SnO2 nanorods growth Pulsed laser deposition (PLD) was used for the SnO2 nanorod growth. The ablation target was fabricated using high purity SnO2 (Alfa Aesar, 99.996%). The target was pressed and sintered at 1300oC for 16 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of 5 Hz was used, with a target to substrate distance of 2.5 cm and a laser pulse energy density of 1 J/cm2. The growth chamber exhib its a base pressure of 10-6 Torr. Single crystal p-silicon (100) and c-plane sapphire were used as the substrate materials in this study. Previous work has shown that SnO2 can grow epitaxially on sapphi re. Prior to deposition, the substrates were ultrasonically cleaned with tr ichloroethylene, acetone and methanol, followed by compressed N2 drying. The substrates were attached to th e heater using Ag paint. Prior to growth, the target was cleaned in situ by pre-ablating with approximately 2000 shots. The growth experiments were performed over a temperature range of 700oC in a background pressure of 500 mTorr. The typical growth time was 30 minutes The as-grown samples were characterized using X-ray diffraction (XRD) using a Philips 37 20, field emission scanning electron microscopy

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115 (FE-SEM) using a JEOL 6335F and high resolu tion transmission electr on microscopy (HR-TEM) using a JEOL 2010F. 6.2.4 VO2 nanowires growth Pulsed laser deposition (PLD) was used for the VO2 nanowire growth. The ablation target was fabricated using high purity V2O5 (Alfa Aesar, 99.99%). The target was pressed and sintered at 700oC for 12 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of 5 Hz was used, with a target to substrate distance of 2.5 cm and a laser pulse energy density of 1 J/cm2. The growth chamber exhibits a base pressure of 10-6 Torr. Single crystal p-silicon (100) and c-plane sapphire were used as the substrate materials in this study. Previous work has shown that VO2 can grow epitaxially on sapphire. Prio r to deposition, the substrates were ultrasonically cleaned with trichloroethylene, acetone and methanol, followed by compressed N2 drying. The substrates were attached to the heater using Ag paint. Prior to growth, the target was cleaned in situ by pre-ablating with approximately 2000 shots. The growth experiments were performed over a temperature range of 500oC in a background pressure of 500 mTorr. The typical growth time was 2 h. The as-grown samples were characterized using X-ray diffraction (XRD) using a Philips 3720, field emission scanning electron microscopy (FE-SEM) using a JEOL 6335F and high resolution transmission electron microscopy (HR-TEM) using a JEOL 2010F. The optical properties of the nanowires we re examined using photoluminescence at room temperature. A He-Cd (325 nm) laser was used as the excitation source. 6.3 Results and Discussion 6.3.1 Synthesis and characterization of vertical-alig ned Z nO nanowires In general, nanowire growth was achieved vi a PLD growth at relatively high temperature and high background pressure. Figure 6-1 shows cross-section and pl an view FE-SEM images of ZnO nanowire arrays grown at 800C in 500 mTorr oxygen, 500 mTorr argon, or a 325 mTorr Ar

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116 / 175 mTorr O2 mixture. In the case of pure argon, the only oxygen supplied during growth was from the ZnO in the ablation plume. Vertically well-aligned nanowires were observed by crosssectional FE-SEM, showing that the growth is highly c-axis oriented along the normal direction of the substrate. At low magnification, a relativel y uniform distribution of diameter is observed for the nanowires. The diameters are around 50 nm. Moreover, the nanowires grow as a high density array and are uniformly di stributed over the entire substrate. For the samples in Figure 61, the length of the nanow ires was approximately 6 m. At high magnification, nanowires with smooth hexagonal facets can be observed. As expect ed, no catalyst particles are observed on the tips of the nanowires, which indicates that th e nanowire growth does not proceed by a vaporliquid-solid mechanism. The composition of the nanowires was investigated by Energydispersive X-ray (EDX) analysis. The results indicate that the na nowires are composed of zinc and oxygen with no significant impu rities found in the EDX data. The orientation and crystalline properties of the ZnO nanowires was characterized with XRD and transmission electron microscopy (TEM ). Figure 6-2 shows the x-ray diffraction patterns of ZnO nanowires grown in pure argon. Two sharp ZnO (002) and (004) peaks with high intensity dominate the diffraction patterns, consistent with ZnO nanowires that are highly oriented along the c-axis. A relatively weak ZnO (110) peak is also observed. The patterns can be indexed to the ZnO hexagonal wurtzite structure with lattice constants of a=0.325 nm and c=0.512 nm. Additional structure ch aracterization was carried out using HR-TEM. Figure 6-3(a) shows low magnification images for parallel nano wires that were mechanically removed from the substrate. The nanowires have a relative ly uniform diameter (50 nm) and are a few micrometers in length. Note that no metal part icles are observed at th e top or bottom of the nanowires. These results are cons istent with the FE-SEM observa tions as shown in Figure 6-1.

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117 The image in Figure 6-3(b) shows that the nanowires are tapered and faceted at the ends. In order to further investigate the structure, selected-a rea electron diffraction wa s performed on a single nanowire. The pattern is consistent with a single crystal wurtzite structure. Figure 6-3(d) shows the high-resolution transmission electron micros copy images, showing that the nanowire is structurally uniform and contains few defects. In the high resolu tion images, lattice fringes show lattice spacing of 0.26 nm, wh ich corresponds to 1/2 the c-axis lattice constant, confirming that the ZnO nanowires are oriented in the c-axis direction. Since the ( 001) planes of ZnO are the closest packed plane, stacking along the c-axis is the most energetical ly favorable. This growth direction is commonly obs erved in ZnO nanowires.71 The morphology and microstructure of the na nowires were examined as a function of growth conditions, in particul ar total background pressure, ox ygen pressure, and substrate temperature. Figure 6-4 shows the FE-SEM imag es of samples deposited in pure oxygen at O2 pressures ranging from 150 to 500 mTorr. The growth temperature was 800oC. All samples were grown on a thin template layer of ZnO on the sap phire substrate. At a growth pressure of 150 mTorr O2, the deposited ZnO consists of a continuous, sm ooth thin film as seen in Figure 6-4(a) and (b). Increasing the oxygen pressure to 300 mT orr resulted in the nuc leation and growth of oriented microcrystals with hexagonal facets as s een in Figure 6-4(c). The size of microcrystals varies from 1 to 5 m, growing normal to the substrate. When the pressure was further increased to 500 mTorr, the growth mode undergoes a tran sition from continuous thin film to highly aligned nanowire growth. A highly dense array of nanowires with hexagonal facets is observed. The nanowires are oriented with their c-axis perpendicular to the su rface with relatively uniform diameter and density. Note that very few nanowires were obtained when the pressure was further increased to 1 Torr.

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118 In addition to pressure, the effect of substr ate temperature in the formation of nanowires was also examined. Figure 6-5 shows the crosssectional and plan-view FE-SEM images of ZnO nanorods grown at an oxygen pressure of 500 mTorr at temperatures ranging from 550C. At 550C (Figure 6-5(a) a nd (b)), the diameters of the nanowir es are on the order of 500 nm. At 750C, the diameter is reduced to approximate ly 150 nm. At 800C, the diameter of the nanowires is less than 100 nm. One explanation for this temperature dependency of nanowire diamaeter relates to surface diffusion. A higher su rface mobility is realized for higher growth temperatures. High temperatures provide sufficient energy for deposited species from the ablation target to migrate to low energy sites for growth. If substrate temperature is low (<650oC), surface species will remain at higher energy sites, thus yielding large diameter nanowires or simply rough, granular films. In order to achie ve one dimensional growth, it is important to provide sufficient surface mobility for species to reach low energy nucleation sites. The variation in optical pr operties for the ZnO nanowires grown in different background ambients was investigated using room temp erature photolumienscence measurements. Figure 66(a) shows the typical room temperature PL spectra of the ZnO na nowires grown at 800oC and 500 mTorr oxygen. A weak near-band-edge-emissi on in the UV region at 380 nm and a strong broad band deep-level-emission at 520 nm is observed. The green band around 520 nm is commonly attributed to deep-level or trap-state emission due to vacancies and/or interstitials of zinc and oxygen in the crystal.115 In order to further investig ate the origin of deep-level emissions, PL measurements were carried out for nanowires grown using the three different ambients (500 mTorr O2, 500 mTorr Ar and 175 mTorr O2 / 325 mTorr Ar mixture) at 800oC. The near-band-edge emission was higher for na nowires grown using pure argon. However, the deep-level emission was enhanced as well, s uggesting a high density of defects due to the

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119 oxygen deficient ambient. In order to investig ate this further, nanowire grown using the argon/oxygen mixture were examined. The near-ba nd-edge emission intensity increased 20 times relative to nanowires grown using pure oxygen. Ho wever, the broad green emission remained unchanged in the spectra for all cases. A plot of PL spectra for nanowires grown in the different background gases is shown in Fi gure 6-6(b). The results show that the deep-level emission persists in all ambients considered. The larg e deep-level emission was also observed on high temperature grown nanowires. Liu et al. attribut ed modification in ZnO nanowire PL properties to size effects and oxygen stochiometry.200 Future work will examine the transport properties of individual PLD-grown nanowires and compare their properties to those grown by other techniques.201 6.3.2 Synthesis and characterization of ZnMgO nanowires Figure 6-7 depicts the FE-SEM images of the formation of high density well-aligned ZnMgO nanowires. The diameters of the nanow ires are on the order of 90 nm. A crosssectional FE-SEM image of the ZnMgO nanowire on the sapphire substrate as shown in Figure 6-7(d) demonstrates that most of the nanowires were grown pe rpendicularly to the sapphire substrate. The chemical composition of the nanow ires was determined using energy dispersive spectroscopy (EDX). Figure 6-8 shows a typical EDX spectra for pulsed laser deposited ZnMgO. The elements detected are zinc, magnesium and oxygen. Structural characterization was performed by XRD. Figure 6-9 shows the diffrac tion pattern of ZnMgO na nowires. In addition to ZnO diffraction peaks, MgO (200) and (220) peaks were also detected. Additional peaks from ZnO were also detected. The c-axis lattice constants calculated from the (002) peaks for the ZnMgO nanowires (5.1656) is slightly smalle r than ZnO nanowires (5.199), which may result from the stress derived from secondary phase and MgO segregation.

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120 Further structural characteri zations of the ZnMgO nanowires were performed using the HR-TEM analysis. Figure 6-10(a) illustrates TEM image of single ZnMgO nanowire. The lattice fringes images indicate the segr egation of secondary phase in ZnO matrix, resulting in a polycrystalline structure with multiple growth orie ntations as illustrates in figure 6-10(b) and (c). Furthermore, stacking faults were also obser ved in the lattice images in figure 6-10(d), suggesting the inhomogeneous growth. Generally speaking, the solubility of Mg in ZnO depends on the growth methods as well as conditions. In this case, the solubility li mit of Mg in ZnO has been exceeding, resulting defects and secondary phase growth. In the efforts to increase the solubility of Mg and decrease defects of the nanowires, a different growth recipe has been used. In this approach, a 30 minutes gr owth period of ZnO was initially performed to provide seeding sites on the substrate, followed by switching between ZnO and Zn0.8Mg0.2O target over during the growth (10 mi nutes each). Finally end up with a 30 minute growth period of ZnO. Figure 6.11(a)-(c) show the FE-SEM images of as-grown samples. Note that small circles with diameter appr oximately 20 nm were found on the facets of naowires. The ZnMgO nanowires have uni form diameter and well-aligned along c-axis as shown in Figure 6-11(d). In contrast w ith previous case, XRD shows only c-oriented ZnO peaks in Figure 6-12. No MgO or secondary phase peak s were found suggests a higher solubility and incorporation of Mg into Zn sites. Furthermore, the c-axis lattice constants calculated from the (002) peaks for the cored ZnMgO nanowires (5.1932 ) is very close to ZnO nanowires (5.199 ), which suggests a smaller stress inside the sample. In order to further investigat e the structural properties, HR -TEM was used to characterize the sample. The HR-TEM images in Figure 6-13 cl early reveal the core-s heath structure of the nanowires. In the Figure, the nanowire displays a difference in brightness intensity between the

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121 core and sheath regions. The cont rast across the diameter of th e nanowire is predominantly mass contrast, reflecting a difference in average atomic number (Z) of the core and sheath region. The darker core region contains more Zn, while th e lighter sheath region more Mg. The core and sheath is approximately 80 nm and 40 nm in diam eter respectively. The selected-area diffraction pattern shows that the nanowire is with single crystal wurtzite structure, which is consistent with XRD results. The HR-TEM lattice fringes image in Figure 6-13(d) confirmed the single crystal structure. No segregated cluster of impurity phase appears via HR-TEM observation. The absence of the diffraction peaks of MgO in th e XRD and SAD patterns suggest that the Mg incorporated within the ZnO nanowires by means of substituting Zn. There has been no investigation of the transport properties of the heterostructured nanowires. They represent opportunities to examine transport in electron-confining structures due to the larger band-gap ZnMgO sheath. 6.3.3 Synthesis and characterization of SnO2 nanorods The initial characterization focused on the pha se formation and microstructure of SnO2 materials deposited at high temperatures and pre ssures. The XRD pattern shown in Figure 6-14(a) is for SnO2 deposited on silicon at a temperature of 800C and oxygen pressure of 500 mTorr. All of the peaks could be indexed to the tetragonal rutile structure of SnO2 with lattice constants of a=4.3738 and c=3.188 from JCPDS file (41-1445). No impurity or secondary phase peaks were observed. The nanorods show no pref erred crystalline texture when deposited on silicon. Figure 6-14(b) shows the XRD results for SnO2 deposited on sapphire using similar deposition conditions. In this case, the deposited material is strongly a-axis textured. This is consistent with an epitaxi al relationship between SnO2 and c-plane sapphire. The chemical composition of the deposited material was determined using energy dispersive spectroscopy

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122 (EDX). Figure 6-15 shows a typical EDX spectra for pulsed laser deposited SnO2. The only elements detected are tin and oxygen. The morphology and microstructure of the deposited SnO2 were characterized by FE-SEM and HR-TEM. Figure 6-16(a) shows a typical top-view FE-SEM image of the as-grown film deposited at high temperature a nd background pressure on a silicon substrate. Nanorod growth is observed. The materials grow by a columnar growth mode that is maintained for the duration of the deposition. As shown in Figures 6-16(b)(d), an aligned SnO2 nanorod array is evident in cross-sectional FE-SEM. The diameters of the SnO2 nanorods are on the order of 50 nm. The nanowire length is 1.5 m for the sample considered. Figur e 6-17 shows FE-SEM micrographs for SnO2 films deposited on c-plane sapphire. For materials nucle ated directed on the sapphire at high pressure (~ 500 mTorr), a SnO2 nanorod microstructure was obs erved as seen in Figure 617(a) and (b). Interestin gly, if a thin nucleation layer is in itially deposited at low pressure (50 mTorr) with high pressure deposition following, th e microstructure does not yield distinct nanorods. Instead, a dense, small grain polycrystalline film is observed as seen in Figure 6-17(c) and (d). The lower pressure results in a highe r density of densely sp aced nucleation sites. The HR-TEM image of la terally aggregated SnO2 nanorods grown on silicon is shown in Figure 6-18(a). The selected area electron diffrac tion (SAD) pattern from an individual nanorod with a [10 2] zone axis is shown in Fi gure 6-18(b). The SAD data in dicates that the individual as-grown SnO2 nanorods are single crysta ls with the rutile structur e. The HR-TEM image in Figure 6-18(c) shows lattice fri nges near the edge of the nanorods indicating an interplanar spacing is 2.3 corresponding to the (100) plane of a rutile SnO2 lattice. The lattice fringes in Figure 6-18(d) show the tw o distinct spacing of 3.4 and 3.2 corresponding to (110) and (001) planes respectively.

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123 Since no catalyst was used in th e synthesis, the growth of SnO2 nanorods cannot be explained by a VLS mechanism. Instead, the na norods grow via a simple columnar growth mechanism. Note that the high background pres sure (500 mTorr) was necessary in order to achieve small grain columnar grow th that persists over the duration of the deposition. In pulsed laser deposition at such high pressures, the inte raction between plume an d gas molecules reduces the plume-induced kinetic energy of the ablated sp ecies to negligible values. While the initial nucleation of SnO2 on the silicon substrate follows a three-dimensional island growth mechanism, subsequent growth proceeds by adhesion to exis ting sites, yielding one-d imensional vapor-solid growth of SnO2 nanorods. The formation of distinct na norods was also favored for deposition on silicon where epita xy does not occur. 6.3.4 Synthesis and characterization of VO2 nanowires The XRD pattern shown in Figure 6-19(a) is for VO2 nanowires deposited on silicon at a temperature of 600C and argon pres sure of 500 mTorr. All of the peaks could be indexed to the monoclinic B phase of VO2 with lattice constants of a=12.03 b=3.693 c=6.42 and =106.6o from JCPDS file (31-1438). No impurity or secondary phase peaks were observed. The nanowires show no preferred crystalline texture when deposit ed on silicon. The chemical composition of the deposited material was determined using energy dispersive spectroscopy (EDX). Figure 6-20 shows a typical EDX spectra for pulsed laser deposited VO2. The only elements detected are vanadium and oxygen, which confirmed the catalyst-free growth. The morphology and microstr ucture of the deposited VO2 were characterized by FE-SEM and HR-TEM. Figure 6-21 show typical top-view FE-SEM images of the as-grown nanowires deposited at 600oC and relative high background pressure (500 mTorr argon) on a silicon substrate. As shown in figure 6-21(a), a typical low-magnification FE-SEM image indicates that the as-synthesized products consists a large quantity of nanowires with uniform diameter and

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124 very high aspect ratio. The materials randomly nucleate on the surface and maintained for the duration of the deposition. As shown in Figures 6-21(b)-(d), high density VO2 nanowires are evident in top view FE-SEM. The diameters of the VO2 nanowires are on the order of 90 nm. The nanowire length is up to 50 m for the sample considered. Figure 6-21(f) shows sideview FE-SEM micrographs for VO2 nanowires grew on silicon, which confirmed the randomly nucleation growth. For materials nucleated dire cted on the sapphire at same condition, similar microstructure was observed but with slightly larger diameters. This suggests the growth may not be related to lattice mismatch between substrates. Although the ex act growth mech anism is still unclear, we suggest the growth follows a diffusi on-based vapor-solid mechanism. Interestingly, no nanostructures were observed when using a lower background pressure. The high background pressure may still play an important role in the formation of VO2 nanowires in this case. Further investigations are required to clarify the interplay between th e surface energetics and surface effects. The HR-TEM image of as-grown VO2 nanowires grown on silicon is shown in Figure 6-22. The selected area electron diffraction (SAD) patte rn from an individual nanowire is shown in Figure 6-22(b). The SAD data indicate s that the individual as-grown VO2 nanowires are single crystals with the monoclinic structure. The HR-TEM image in Figure 6-22(c) shows lattice fringes near the edge of the nanowires indicating an interplanar spacing is 3.05 corresponding to the (002) plane of a monoclinic VO2 lattice. The lattice fringes in Figure 6-22(d) show the two distinct spacing of 3.62 and 3.05 corresponding to (110) and (002) planes respectively. The optical properties of as-grown VO2 nanowires were investigated by room temperature photoluminescence. Figure 6-23 shows that the VO2 nanowires, in comparison with VO2 thin films grew at low oxygen background pressu re (30 mTorr) and same temperature (600oC),

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125 exhibit a board PL peak centere d at 2.44 eV. The board PL p eak might be related to the crystalline defects induced dur ing the growth. The defects might be caused by the high background pressure, resulting oxygen vancancies On the other hand, no emission was observed from VO2 thin film sample grew at low oxygen pressu re. This suggests the emission is related to growth pressure and dimension of VO2 material. Low temperature PL is required to investigate the origin of the peak. To further investigate the transport properties of VO2 nanowires, a single nanowire device has been fabricated on a 1000 thick thermally grown SiO2/Si substrate. The fabrication process is similar as described in Chapter 5. Th e electrodes (Ti/Al/Pt/Au 200/800/400/800 ) were deposited by e-beam evaporation at room temp erature without further annealing. Figure 6-24(a) shows the FE-SEM image of single nanowire de vice. The distance between the two electrodes was 13 m. The measurement was performed in air am bient. At room temp erature, the device exhibited linear, symmetric current-voltage (I-V) characteristics as shown in figure 6-24(c). The linear dependence can be observed in relatively wi de current range from approximately 2.25 to 2.25 A, indicating good Ohmic contacts between th e nanowire and electrodes. The diameter size (160 nm) and the length (13 m) of the VO2 nanowire can be measured through FE-SEM image as shown in figure 6-24(a) and (b), and the resistivity of the VO2 nanowire is calculated to be 1.4713 m at room temperature. 6.4 Summary and Conclusions A high-pressure assisted pulsed laser deposition has been applie d to fabricate a variety of metal oxide nanowires (ZnO, ZnMgO, SnO2 and VO2) without catalysts. Vertically well-aligned ZnO and ZnxMg1-xO arrays were grown on c-sapphire substrates at 600oC. The nanowires growth proceeds without employing catalysts for nucleation, although an epitaxial ZnO thin film template is necessary in order to achieve unifo rm alignment. The ZnO nanowire diameters are as

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126 small as 50 nm, with diameters largely determ ined by growth pressure and temperature.The SnO2 nanowires with single crystal rutile struct ure were grown on silicon substrates at 700oC. The growth of SnO2 nanorods on silicon begins as small grain columnar with a subsequent vapor-solid growth mechanism at high pressure yielding one-dimensional SnO2 nanorods. The diameter and length of the SnO2 nanorods are approximately 50 nm and 1.5 m, respectively. High aspect ratio monoclinic VO2 nanowires grew laterally on the silicon and c-sapphire substrates at 600oC. The nanowires randomly nucleate on the surface with the diameter of 90 200 nm, length up to 50 m. A board emission peak was observed at 510 nm by room temperature photoluminescence measurement. A singl e nanowire device has been fabricated to measure transport properties of single VO2 nanowire. The device exhibits linear, symmetric I-V characteristics and the resistivity of the nanowire is approximately 1.4713 m. Further investigations such as temperature dependant tr ansport, optoelectronic response and gas sensing measurements are needed for further device applications. The metal oxide nanowires are attractive for numerous applications. This study provides a relative convenient approach to synthesize a wi de range of metal oxide nanowires. By choosing proper growth parameters such as substrate mate rials, growth temperature and gas pressure, it has been demonstrated that the present approach can be extended to obtain a large family of semiconducting metal oxide nanowires. The result s also suggest the possibility of growing complex metal oxide nanostructures, includ ing tailored heterostructures, with PLD.

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127 Figure 6-1. Scanning electron microscope images of well-aligned ZnO nanowires grown on a ZnO thin film template in (a), (b) 50 0 mTorr pure oxygen, (c), (d) 500 mTorr pure argon, and (e), (f) a 325 mTorr / 175 mTorr argon/oxygen mixture. ( a ) ( c ) ( b ) ( d ) ( e ) 1 m 1 m 1 m 0.5 m ( f ) 0.5 m 0.5 m

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128 20304050607080 (110) (004) Intensity (a.u.)2 (deg)(002) Sapphire K Figure 6-2. X-ray diffraction -2 scan of ZnO nanowires grow n at 800C in 500 mTorr Ar.

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129 Figure 6-3. Low magnification (a), (b) TEM images of ZnO nanowires grown on a ZnO thin film at 800oC in 500 mTorr Ar. Also shown is (c ) a selected area electron diffraction pattern taken from a single ZnO nanowire, showing the single crystal wurtzite structure, as well as (d) an HR-TEM imag e of a single ZnO nanowire showing lattice fringes.

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130 Figure 6-4. Cross-sectional and top view sca nning electron microscope images of the ZnO nanowires grown at 800oC in pure oxygen with oxygen background pressures of (a), (b) 150 mTorr, (c), (d) 300 mTorr, and (e) (f) 500 mTorr. 1 m ( a ) 5 m 5 m 5 m 5 m ( b ) ( c ) ( d ) ( e ) ( f ) 1 m

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131 Figure 6-5. Cross-sectional and top view sca nning electron microscope images of the ZnO nanords grown under 500 mTorr of oxygen at different temperatures. (a), (b) 550oC, (c), (d) 750oC, (e), (f) 800oC, respectively ( c ) ( d ) ( a ) ( b ) 1 m ( e ) 1 m 1 m 500 nm 2 m ( f ) 500 nm

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132 350400450500550600650700 Intensity (a.u.)Wavelength (nm) (a) 350360370380390400410420 Ar Ar/O2 O2 Thin film Intensity (a.u.)Wavelength (nm) (b) Figure 6-6. Room temperature PL spectra of ZnO na nowires and near-band-edge-emission of ZnO thin film and ZnO nanowires grow n under different background ambient (a) Room temperature PL spectra of ZnO nanowires grown at 800C in 500 mTorr oxygen (b) near-band-edge-emi ssion of ZnO thin film and ZnO nanowires grown under different background ambient at 800oC.

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133 Figure 6-7. (a), (b) Top and (c), (d) cross-sectional view scanning electron microscope images of the ZnMgO nanowires grown at 800C in 500 mTorr Ar.

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134 051 00 2000 4000 6000 8000 10000 12000 14000 CountsEnergy (keV)O Zn Zn Zn Mg Figure 6-8. Energy-dispersive sp ectroscopy spectra for ZnMgO na nowires grown on sapphire at 800C in 500 mTorr Ar.

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135 20304050607080 ZnO(110) ZnO(004)ZnO(101)ZnO(002) substrate Intensity (arb.)2 (deg)ZnO(102) MgO(220) MgO(200) Figure 6-9. X-ray diffraction -2 scan of ZnMgO nanowires gr own on sapphire at 800C in 500 mTorr Ar.

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136 Figure 6-10. (a) High resoluti on transmission electron micros cope image of single ZnMgO nanowire (b)-(d) HR-TEM lattice fringes images indicate the segregation of secondary phase in ZnO matrix. Defects such as stacking faults we re also observed in the lattice images as well.

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137 Figure 6-11. (a)(c) Top and (d) cross-sectional view scanning electron microscope images of the cored ZnMgO nanowires grown on sapphire at 800C in 500 mTorr Ar.

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138 20304050607080 ZnO(100) ZnO(200) ZnO(004) Intensity (arb.)2 (deg)ZnO(002) substrate ZnO(110) Figure 6-12. X-ray diffraction -2 scan of cored ZnMgO nanowires grown on sapphire on sapphire at 800C in 500 mTorr Ar.

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139 Figure 6-13. (a), (b) Low magnification TEM im age of single cored ZnO/ZnMgO nanowire (c), (d) HR-TEM lattice fringes images indi cate the mass contrast of core-sheath structure.

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140 30405060708090 (312) (202) (101) (200) (400) (222) (321) (301) (112) (310) (002) (220) (111) (211) Intensity (a.u.)2 (deg)Si Si(a)30405060708090 (202) (301) (310) (211) (400) (101) (200) Intensity (a.u.)2 (deg)(b) 30405060708090 0 20 40 60 80 100 Intensity2 (deg)JCPDS 41-1445(c) Figure 6-14. X-ray diffraction patterns of SnO2 nanorods grown at 800C in 500 mTorr oxygen on (a) silicon and (b) sapphire. (c) di ffraction intensities of rutile SnO2 from JCPDS file.

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141 0 5 10 15 0 1000 2000 3000 4000 5000 6000 Sn Sn Sn Sn SnCountsEnergy (keV)O Sn Figure 6-15. Energy-dispersive spectroscopy spectra of SnO2 nanorods grown on sapphire at 800C.

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142 Figure 6-16. Scanning electron mi croscope morphologies of SnO2 nanorods grew on silicon. (a) Top-view of as-grown nanorods. (b) (d) Cross-sectional views of as-grown SnO2 nanorods.

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143 Figure 6-17. Scanning electron microscope imag es showing (a) the su rface morphology and (b) cross-section of SnO2 nanorods deposited by pulsed la ser deposition at 800C directly on sapphire. Also shown are (c) the surface morphology and (d) cross-section of SnO2 deposited on sapphire under similar cond itions but with a thin epitaxial SnO2 nucleation layer first grown at 50 mTorr.

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144 Figure 6-18. (a) High resolution transmission electron microscope image of SnO2 nanorods (b) SAD patterns recorded from an individual nanorod with an electron beam along the [10 2] direction. (c), (d) HR-TEM lattice fri nges images indicate their single crystal rutile structure.

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145 2030405060 Intensity (arb.)2 (deg)(110) (002) (3 10) (2 02) (401) (4 02) (112) (312) (022)(a) 202530354045505560 0 20 40 60 80 100 Intensity (arb.)2 (deg)(b) Figure 6-19. X-ray diffr action patterns of VO2 nanowires grown at 600C in 500 mTorr oxygen on (a) silicon. (b) Diffraction in tensities of monoclinic VO2 (B) from JCPDS file.

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146 051 00 5000 10000 15000 20000 CountsEnergy (keV)V V O Si Figure 6-20. Energy-dispersive spectroscopy spectra for VO2 nanowires grown on silicon at 600C.

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147 Figure 6-21. (a)(d) Top view scanni ng electron microscope images of VO2 nanowires on silicon grew at 600oC (e) individual VO2 nanowire dispersed on the silicon substrate (f) side view scanning electron microscope images of VO2 nanowires on silicon grew at 600oC. ( b ) ( a ) ( d ) ( c ) ( f ) ( e )

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148 (a) (a) Figure 6-22. (a) High resolution transmission electron microscope image of VO2 nanowires (b) SAD patterns recorded from an individual nanowire (c), (d) HR-TEM lattice fringes images indicate their single crystal monoclinic structure. (110) 3.62 (002) 3.05 (d) (110) 3.62 (002) 3.05 (110) 3.62 (002) 3.05 (d)

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149 400500600700800900 0.00 0.01 0.02 0.03 0.04 0.05 Nanowires Thin film Intensity (arb. unit)Wavelength (nm)2.44 eV Figure 6-23. The photoluminescence spectra of VO2 thin film and nanowires grew at different oxygen pressure.

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150 -0.8-0.40.00.40.8 -2.5x10-6-2.0x10-6-1.5x10-6-1.0x10-6-5.0x10-70.0 5.0x10-71.0x10-61.5x10-62.0x10-62.5x10-6 Current (A)Voltage (V) Figure 6-24. (a), (b) Scanning el ectron microscope images of fa bricated single nanowire device. (c) I-V characteristics of the indivudial VO2 nanowire measured in air ambient at room temperature.

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151 CHAPTER 7 EPITAXIAL GROWTH OF TRANSPARENT TIN OXIDE THIN FILMS 7.1 Introduction In recent years, there has emerged significant in terest in the epitaxial growth and properties of functional oxide thin films.17,202 Functional oxide of intere st include superconductors,203-205 ferroelectrics,206 dielectrics,207 phosphors,208,209 and semiconductors.210,211 The latter class of oxides, namely semiconductors, has emerged as pa rticularly interesting for sensors, thin-film electronics, and photonics. Tin oxide (SnO2) is a wide band gap (3.6 eV) metal oxide semiconductor with excellent optical transparency in the visible range.212 It possesses the rutile (tetragonal) crystal structure with a=4.738 and c=3.188 With a relatively high conductivity, visible wavelength transparenc y, chemical stability and thermal stability in oxidizing environments,213 tin oxide films are being explored for a number of applications. As a wide band-gap semiconductor, SnO2 is attractive for use in photonic a pplications, such as solar cells, where transparent elec trodes are required.18,214 SnO2 thin films are used for gas sensor devices based on changes in conduc tivity when exposed to selected chemical species.21,37,188 In addition, there is also interest in the possibi lity of inducing ferromagnetism in SnO2 through transition metal doping,215,216 an approach that is also bein g pursued for other wide band-gap semiconductors.217,218 SnO2 thin films have been fabricated by a variety of technique s including sol-gel method,219 electron beam evaporation,220 reactive sputtering,221 chemical vapor deposition222,223 and sputtering.36,224 One of the major challenges in synthesizing SnO2 thin films is the control over oxygen stoichiometry. When deposition is ca rried out in vacuum conditions at high temperatures, SnO2 films tend to be nonstoichiometric, fr equently including metastable phases

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152 such as SnO and Sn3O4.225 The existence of these metastable phases and relaxed crystal defects will strongly affect the pr operties of the films. While previous applications of SnO2 for sensors or transparen t conductors have primarily relied on polycrystalline material, many of the emerging applications for functional wide bandgap semiconductors require highly crystalline epitaxial films. As such, understanding the effects of growth parameters and substrate se lection on the epitax ial growth of SnO2 is important. Epitaxial growth kinetics can yield specific defect structures that signifi cantly affect the oxide thin film properties.226,227 This becomes increasingly important as targeted thin film structures involve heterointerfaces, mu ltilayers, or superlattices. 226-229 Similar to most metal oxide materials, undoped SnO2 is an n-type semiconductor because of intrinsic defects (oxygen deficient or metal excess). Th e fabrication of high quality p-type transparent conducting oxides (TCO s) is one the major challenge s in the fabrication of p-n junction based devices.230 As is well known that doping in semiconductor with selective elements offers an effective approach to adjust the electrical, optical, and magnetic properties, which is crucial for practical a pplications. A perusal of the peri odic table suggests that possible acceptor candidates for SnO2 include Group III elements such as B, Al, Ga, and In substituting for Sn. Theoretically if effective substitution of Sn with group III elements, the p-type SnO2 can be realized. However, only a few groups reported the p-type conductivity in SnO2 thin films up to date.230-234 The difficulties can come from a vari ety of causes. Difficulty of obtaining p-type SnO2 may due to its low dopant solubility and self-compensating process on doping. Furthermore, dopants may be compensated by low-energy native defects, such as Sni and VO or background impurities. Low solubility of the dopant in the host material is also another issue.

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153 Deep impurity level can also be a source of doping problem causing significant resistance to the formation of shallow acceptor level. Bagheri-Mohagheghi et al. reported the p-type conductivity on Li-doped SnO2 thin film by spray pyrolysis.231 The heavily Li-doped SnO2 films (~2 wt.%; 37 at.%) show a carrier conversion from electrons to holes. Because of similar ionic radius of Sn4+ (0.71 ) and Li+ (0.68 ), three holes are produced wh en the substation occurs. Ji et al. reported p-type In-doped SnO2 with In/Sn ratio of 0.2 by spray pyrolysis.232,234 Similar result has also been reported by Huang et al. with Ga-doped SnO2 films (0.2%) by DC magnetron sputtering.233 However, the electric properties of these films were not good because of the limitations of sol-gel method, such as poor crystalline quality and poor process control. Pulsed laser deposition (PLD) has been widely used in synthesis complex oxide thin films, such as high Tc superconductors and perovskite oxides. PLD has the advantage of operating in a reactive atmosphere over a wide range of oxygen pressure.104 In this Chapter, we study the epitaxial growth of SnO2 thin films on (0001) sapphire by PLD, including the spec ific crystalline orientation of this rutile structure on a hexagonal template. The e ffects of growth parameters on the electrical transport property and surface morphology will be disc ussed as well. The effects of gallium doping will be examined in the second section. 7.2 Experimental Methods Pulsed laser deposition was used for thin film growth. The ablation target was fabricated using high purity SnO2 (Alfa Aesar, 99.996%). For gallium doped thin films, the ablation target was fabricated using powder mixture of high purity ZnO (Alfa Aesar, 99.9995%) and Ga2O3 (Alfa Aesar, 99.999%) with Ga:Zn atomic ratios of 1:99. The targ ets were pressed and sintered at 1300oC for 16 h in air. A KrF excimer laser was used as the ablation source. A repetition rate of 1 Hz was used, with target to substrate distance of 4 cm and a laser pulse energy density of 1

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154 J/cm2. The growth chamber exhib its a base pressure of 10-6 Torr. Single crystal (0001) Al2O3 (sapphire) was used as the substrate materi al in this study. Prior to deposition, the Al2O3 substrates were ultrasonically cleaned with tric hloroethylene, acetone and methanol, followed by compressed N2 drying. The substrates were attached to th e heater using Ag paint. Prior to growth, the target was cleaned in situ by pre-ablati ng with approximately 2000 shots. Film growth experiments were performed over a temperature range of 300oC in an oxygen pressure range of 20 mTorr. Film thickness ranged fro m 200 nm and typical growth time was 2 h. The deposited films were characterized using X-ray diffraction, atomic force microscopy and field emission microscopy. Four-point van der Pauw Hall measurements were performed to determine transport properties. A Perkin-Elm er Lambda 800 UV/Vis double-beam spectrometer was used for optical absorption measurements. 7.3 Results and Discussion 7.3.1 Properties of undoped SnO2 thin films Epitaxial SnO2 films on (0001) Al2O3 were realized for deposition at temperature as low as 400 oC in an oxygen pressure of 50 mTorr. Figur e 7-1 shows the X-ray diffraction (XRD) -2 patterns for SnO2 thin film prepared by PLD at different temperatures. The (200) and (400) SnO2 are the dominant peaks in all scans, wh ich indicates that the films are highly a-axis oriented; i.e. the principal out-of plane orientation is SnO2 (100) // Al2O3 (0001). The weak peaks for SnO2 (101) grains are also observed at higher temperatures. The re sults clearly show that epitaxial growth of a-axis oriented SnO2 grains on the (0001) Al2O3 substrate is favored, placing the film c-axis in the plane of the surface. As seen in Figure 7-2, the a-axis lattice para meter shows an increase with increasing growth temperature. However, the a-axis spacing is consistently less than that seen in bulk SnO2.

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155 The epitaxial crystallinity of the films was confirmed by looking at both out-of-plane rocking curves and in-plane -scans. The rocking curve thr ough the (200) plane for SnO2 film grown at 700oC is shown in Figure 7-3, yields a fu ll width half maximum (FWHM) of 0.0156o, which confirms that the film is highly oriented with the a-axis perpendicular to the surface. With increasing growth temperature, the crystal linity significantly improved as reflected in corresponding smaller FWHM values. Overall, th e films exhibit good out-of-plane alignment of the (100) planes. The in-plane alignment of the SnO2 film is seen from the -scan in Figure 7-4. The film inplane mosaic ( ~10o) is much larger than the out-of-plane mosaic. The in-plane alignment can be described as SnO2 [010] // Al2O3 < 0211> or equivalently SnO2 [001] // Al2O3 <01 10> and 60o rotations. The SnO2 films grow epitaxially on the (0001) Al2O3 substrate with three orientations rotated 60o in plane with respect to each other due to the six-fold symmetry of the c-plane surface of Al2O3. This epitaxial structure is consis tent with the matching of the oxygen octahedral arrangements existing on the SnO2 (100) surface and on the Al2O3 (0001) surface as illustrated in Figure 7-5, resulting in the epitaxial growth of (100) oriented SnO2. This in-plane variant structure with three diffe rent symmetry-equivale nt orientations has also been observed on films synthesized by metal-organic chemical vapor deposition (MOCVD)223 and sputtering approaches.235 A likely explanation as to why the in-plane mosaic is much larger than the out-ofplane is that the rotational mosaic is due to lattice matching between the film (tetragonal symmetry) and the substrate (rhombohedral symme try). On average the film (010) and (001) planes are aligned with low-inde x sapphire directions. But then ot her low-index film planes are not aligned. For example, the film (011) planes are 56.3 from (010) planes and 33.7 from (001) planes. Thus, if these planes tend to align with low-index Al2O3 planes, they would be frustrated

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156 by ~4 on average. All crystal directions can not be aligned when a tetragonal film grows on a rhombohedral (pseudo-hexagonal) c-axis substrate. Figure 7-6 s hows the growth rate of SnO2 thin films at different temperatur es. The growth rate is measured to be approximately 0.3.4 /s. For all growth temperatures, the growth rate shows a weakly linear relation to growth temperature, suggesting that an increase in growth temperature enhances the SnO2 phase formation. The surface morphology of the SnO2 film was measured usin g atomic force microscopy (AFM) measurements. AFM measurements were pe rformed in air using a Veeco Nanoscope III. All samples were scanned over a 5 m5 m area. In Figure 7-7, the AFM images for SnO2 grown on sapphire at 700oC are shown. The surfaces showed a dense columnar structure with an rms roughness of 16.76 With increasing temp erature, the surface roughness increases, reflected in the formation of large columns. The resistivity and carrier concentration of the films were determined at room temperature using Hall measurements (Lakeshore 7507). The Hall data is shown in Table 7-2. Hall measurements showed that the epitaxial SnO2 films were n-type semiconductors with carrier concentration varying from 417 cm-3 to 4.219 cm-3. Figure 7-8 shows the resistivity and carrier concentration of SnO2 grown on sapphire as a function of deposition temperature. It has been postulated that the conductivity is related to the existence of shallow donor levels near the conduction band, formed by a large concentratio n of oxygen vacancies. The electrical conduction in undoped SnO2 is associated with nonstoichiomet ry and with oxygen-related intrinsic defects.189 The resistivity increase with growth te mperature suggests that fewer oxygen vacancies formed during high temperature deposition, resulting in lower carrier concentrations as well.

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157 Optical absorption measurement was used to de termine the band-gap of films. Figure 7-9(a) shows transmission data for SnO2 film grew at 400oC in 50 mTorr of oxygen, showing a maximum transmission of 70%. The bandgap of the film was calculated by (h )2h plot in Figure 7-9(b). The band-gap was approximately 3.89 eV, which is in the range of the SnO2 thin film reported else where. 7.3.1 Properties of gallium-doped SnO2 thin films In order to optimize the growth conditions and examine the effects of Ga doping on the films, different growth temperature and oxygen pressure were used. The EDX (Figure 7-10) was used to examine the Ga dopant in the film, the de tected elements are Al, Sn, O and Ga. No other impurity or contamination was detected. The X-ray diffraction was used to examine the crystallinity of the films grew at temperature from 400 to 700oC in 50 mTorr of oxygen. All the SnO2 films showed (200) and (400) SnO2 peaks, which indicates that the films are highly a-axis oriented, same as described in undoped SnO2 films previously. The results show that dopants do not effect the epitaxial growth of SnO2 on the (0001) Al2O3 substrate. A comparison of a-axis lattice parameter between undoped and Ga-doped SnO2 films shows an increase with increasing growth temperature in both cases. However, the a-axis spacing is consistently less than that seen in bulk SnO2. Note that the Ga-doped SnO2 films have slightly smaller a-axis constant in all growth temperature. This might indicate that the Ga3+ does not substitute the Sn4+ sites, which leads to the decrease of the lattice constants due to the larger size of Ga3+ (0.76) compared to Sn4+ (0.71 ). The effects of oxygen pressure on th e films were further examined. Figure 7-12 shows the X-ray diffrac tion patterns of SnO2 films deposited at diffe rent oxygen pressure at 400oC. The results show that the a-axis parameter (Figure 7-13) decreased with increasing oxygen pressure, while the crystallinity increased with oxygen pressure. The a-axis parameter

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158 was close to bulk at 10 mTorr of oxygen, suggests a loose structure which is consistent with a larger full width half maximum (FWHM) value of (200) peak. Optical absorption measurement was used to determine the band-gap of films. Figure 714(a) shows transmission data for Ga-doped SnO2 film grew at 400oC in 50 mTorr of oxygen, showing a maximum transmission of 80%. The band-gap of the film was calculated by ( h )2h plot in Figure 7-14(b). The band-gap was approxi mately 3.94 eV, which is close to value of undoped SnO2 reported previously. The re sults show that no remarkable changes were found for the band-gap of Ga-doped films. The resistivity and carrier con centration of the Ga-doped films were determined at room temperature using Hall measurements (Lakeshore 7507). The Hall data is shown in Table 7-3 and 7-4. Hall measurements showed that the Ga-doped SnO2 films in most case were n-type semiconductors with carrier c oncentration varying from 4.81015 cm-3 to 4.219 cm-3. In general, the Hall data can be explained by consid ering the donor (intrinsic defects) and acceptors (substation of Sn by Ga) in the films. At low te mperature, the gallium atoms were not activated as acceptors and the films were n-type because of large numbers of intrinsic defects. At high temperature, the number of intrinsic defects de creased and results in high resistivity and low carrier concentration. However, the gallium atoms may compensate with donor due to high temperature and do not behave as acceptors. Interestingly, the p-type SnO2 film can be realized at specific growth parameter (400oC and 50 mTorr O2) and was reproduced again in same condition. The optimum temperature and oxygen pr essure was realized and Ga atoms were activated and the film showed p-type characteristic with carrier concentration approximately 1019 cm-3.

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159 In order to further confirm the p-type behavior, a BRHB plot was used to determine the Hall coefficient. In Hall measurement, the m easured Hall voltage (Vmeasured) is given by: Vmeasured = VH + Voffset + Vnoise For Hall system, the recorded Hall voltage (Vmeasured) that calculates Hall coefficient (RH) for single magnetic field polarity, includes noise (Vnoise) and offset voltages (Vnoise). However, the noise and offset voltages are both magnetic field independent. If we plot the Hall coefficient times magnetic field (BRH) versus magnetic field (B), we can extract the actual Hall coefficient (VH) by the slope of the linear f it line. The carrier concentration is given by the equation: nq RH1 Figure 7-15(a) shows the BRH B plot for Ga-doped SnO2 film (312 nm) grew at 400oC in 50 mTorr of oxygen, clearly showing a positiv e slope with Hall coefficient of 0.2291 cm3C-1. The carrier concentration of film is 2.719 cm-3, which is close to the results given by Hall system. A similar approach has been used on another sample (184 nm) to confirm p-type characteristics as shown in Figure 7-16(a). Alt hough the data points were more scattered in this case, the slope was also positive with carrier concentration of 318 cm-3. The results show that the Ga-doped SnO2 films were p-type at specific growth condition. However, the p-type behavior was not stable and degraded as time proceeds. The Hall measurements were preformed again on the same samples after one month. The carri er type was found convert from p-type to n-type after one month (Figure 7-15(b) and 7-16(b) ). Similar behavior was also observed on thermal annealed Ga-doped SnO2 film shown in Figure 7-17(a), showing p-type with carrier concentration of 717 cm-3 after annealing at 800oC in oxygen for 1 h. The carrier type converted back to ntype after 2 weeks (Figure 7-17(b )). The results suggest the instab ility of Ga dopants in the SnO2 films, similar results were found in other oxide materials.

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160 7.4 Summary and Conclusions In conclusion, epitaxial SnO2 thin films were realized on (0001) Al2O3 substrates using pulsed laser deposition. X-ray diffr action shows that the films have a phase-pure rutile structure and grow along the (100) plane. The epitaxi al relationship can be described as SnO2 [010] // Al2O3 < 0211> or equivalently SnO2 [001] // Al2O3 <01 10> and 60o rotations. The undoped SnO2 films were n-type semiconductor with carrier concentration varied from 417 cm-3 to 4.219 cm-3. The electrical transport properties are strongly dependent on the growth temperature. The effects of Ga doping on SnO2 films were studied. The Ga-doped SnO2 films were epitaxially grown on (0001) Al2O3 substrate with slightly smaller a-axis parameters. No remarkable changes were found for the band-ga p of Ga-doped films. The Hall data showed ptype behavior occurs only at specific growth condition, but converted back to n-type and degraded as time proceeds. More work is needed to study the p-type instability on Ga-doped SnO2 films.

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161 Table 7-1. Candidate dopant atoms for SnO2 Atom Valence Radius () Sn O Li In Ga +4 -2 +1 +3 +3 0.71 1.38 0.68 0.94 0.76 Table 7-2. Hall data of SnO2 thin films grown at different temperature. Temp (oC) Thickness (nm) Resistivity (ohm cm) Hall Coefficient (cm/C) Carrier Density (1/cm) Hall Mobility (cm/VS) 300 205 0.047 -0.149 4.219 3.1 400 239 0.296 -0.728 8.618 2.4 500 223 0.244 -0.703 8.918 2.9 600 237 15.193 -10.3 6.117 0.67 700 279 28.534 -16.417 4.017 0.58 Table 7-3. Hall data of Ga-doped SnO2 films grown at different temperature. Temp (oC) Thickness (nm) Resistivity (ohm cm) TypeCarrier Density (1/cm) Hall Mobility (cm/VS) 350 400 0.762 n 6.918 1.19 400 312 0.039 p 2.919 6.75 500 328 0.316 n 7.118 2.8 600 227 0.437 n 7.518 1.92 750 806 5130 n 4.815 0.265 Table 7-4. Hall data of Ga-doped SnO2 films grown at different oxygen pressure. Pressure (oC) Thickness (nm) Resistivity (ohm cm) TypeCarrier Density (1/cm) Hall Mobility (cm/VS) 20 260 0.011 n 4.219 6.09 50 184 0.337 p 2.718 8.36 100 163 6.175 n 8.918 1.04 150 113 36.568 n 6.117 2.32

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162 304050607080 SnO2(101)SnO2(400)700oC 600oC 500oC 400oC Log Intensity (a.u.)2 (degrees)300oCAl2O3(0006) SnO2(200) Figure 7-1. X-ray diffr action patterns of SnO2 films deposited on (0001) Al2O3 at different temperature. 4005006007004.64 4.66 4.68 4.70 4.72 4.74 a-axis constant () Growth Temperature (oC) Bulk Figure 7-2. a-axis constant as a function of growth temperature.

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163 -0.2-0.10.00.10.2 0.0 1.0x1062.0x1063.0x1064.0x106 FWHM = 0.0156o Intensity (counts/s)Omega (deg) Figure 7-3. Rocking curve of (200) reflection of SnO2 films grown at 700oC.

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164 0 1000 2000 3000 4000 5000 -180-120-60060120180Counts (deg) -scan through SnO2 (110) 0 1000 2000 3000 -180-120-60060120180Counts (deg) -scan through SnO2 (101) Figure 7-4. -scans (a) through the SnO2(110) and (b) through the SnO2(101) reflections.

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165 Figure 7-5. The (a) SnO2 crystal structure projection on th e (100) plane, a nd (b) in-plane epitaxial growth orientations by SnO2 (100) on sapphire (0001) plane. 300400500600700 0.28 0.32 0.36 0.40 Growth Rate (/s)Growth Temperature (oC) Figure 7-6. Growth rate of SnO2 films on (0001) Al2O3 as a function of temperature.

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166 Figure 7-7. Atomic force microscope images of SnO2 thin film grown at 700oC. 300400500600700 10-1100101102 1017101810191020 Carrier Density [1/cm3] Resistivity [ohm-cm]Growth Temperature (oC) Figure 7-8. Resistivity a nd carrier density of SnO2 films grown at different temperatures. 0 2.5 5.0 Scan Length (m) 5.0 2.5 0 20 m 10 m 0 m

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167 234560 20 40 60 80 100 Transmittance (%)Energy (eV)3.03.54.04.55.0 0 10 ( h 2 10-10Energy (eV) E g =3.89 eV Figure 7-9. (a) Transmi ssion spectra of the SnO2 film grew at 400oC in 50 mTorr oxygen. (b) ( h )2h plot shows the band-gap was approximately 3.89 eV.

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168 0358100 1k 2k 3k 4k 5k 6k Ga Ga Sn Al Sn Sn Sn Sn Sn O CountsKinetic Energy (keV) Figure 7-10. Energy-Dispersive X-ray Spectroscopy analysis of Ga-doped SnO2 thin film grew at 400oC in 50 mTorr O2 4005006007004.64 4.66 4.68 4.70 4.72 4.74 a-axis constant ()Growth temperature (oC)undopedGa-doped Bulk Figure 7-11. Comparison of a-axis constant as a functi on of growth temperature.

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169 30405060708090 log Intensity (arb.)2 (degree)SnO2(200) SnO2(400) (101) 10 mTorr 20 mTorr 30 mTorr 40 mTorr Al2O3(0006) SnO2 Figure 7-12. X-ray diffracti on patterns of Ga-doped SnO2 films deposited on (0001) Al2O3 at different oxygen pressure. 10203040504.64 4.66 4.68 4.70 4.72 4.74 a-axis constant ()Oxygen pressure (mTorr) Bulk Figure 7-13. a-aixs constant of Ga-doped SnO2 films as a function of growth pressure.

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170 23456 0 20 40 60 80 100 Transmittance (%)Energy (eV) 3.03.54.04.55.00 10 E g =3.94 eV ( h 2 10-10Energy (eV) Figure 7-14. (a) Transmission spectra of the Ga-doped SnO2 film grew at 400oC in 50 mTorr oxygen. (b) (h )2h plot shows the band-gap was approximately 3.94 eV.

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171 -10000-50000500010000 -20.0k -18.0k -16.0k -14.0k -12.0k -10.0k RH = 0.2291 n = 2.7x1019 cm-3R = 0.039 cm = 6.75 (cm2/Vs) 1st measurement on 08-15-06 BRH(G-cm3-C-1)B(G)(a)-10000-50000500010000 500.0 1.0k 1.5k 2.0k 2.5k 3.0k 3.5k (b)RH = -0.1258 n = 4.97x1019 cm-3R = 0.043 cm = 2.49 (cm2/Vs) 2nd measurement on 09-18-06 BRH(G-cm3-C-1)B(G) Figure 7-15. Hall plot (BRHB) of Ga-doped SnO2 film grew at 400oC in 50 mTorr oxygen, (a) measured immediately after grown, (b) measured after 1 month.

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172 -10000-50000500010000 -90k -80k -70k -60k -50k -40k -30k -20k -10k RH = 2.065 n = 3x1018 cm-3R = 0.34 cm = 8.36 (cm2/Vs)(a)1st measurement on 08-03-06 BRH(G-cm3-C-1)B(G) -10000-50000500010000 -55.0k -50.0k -45.0k -40.0k -35.0k -30.0k -25.0k -20.0k RH = -0.9955 n = -6.28x1018 cm-3R = 0.192 cm = 3.39 (cm2/Vs)(b)2nd measurement on 09-18-06 BRH(G-cm3-C-1)B(G) Figure 7-16. Hall plot (BRHB) of Ga-doped SnO2 film grew at 400oC in 50 mTorr oxygen, (a) measured immediately after grown, (b) measured after 1 month.

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173 -10000-50000500010000 -550.0k -500.0k -450.0k -400.0k -350.0k RH = 8.7591 n = 7.14x1017 cm-3R = 8.84 cm = 1.13 (cm2/Vs)(a) BRH(G-cm 3 -C1 )B(G)1st measurement on 09-26-06 -10000-50000500010000 0.0 2.0k 4.0k 6.0k 8.0k 10.0k 12.0k (b)RH = -0.42 n = -1.489x1019 cm-3R = 8.84 cm = 0.0717 (cm2/Vs) BRH(G-cm3-C-1)B(G)2nd measurement on 10-11-06 Figure 7-17. Hall plot (BRHB) of Ga-doped SnO2 film grew at 400oC in 20 mTorr oxygen and annealed at 800oC in oxygen for 1h, (a) measured immediately after annealing, (b) measured after 0.5 month.

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174 CHAPTER 8 CONCLUSION This work focused on the synthesis of one-dim ensional metal oxide nanowires and hydrogen sensing applications. In th e synthesis part, the control of initial Ag film thickness and subsequent annealing conditions is shown to provide an effective method for controlling the size and density of nucleation sites for catalyst-driven growth of ZnO nanorods. The completely selective growth is possible on dielectric and silicon substrates. High density cross-linked ZnO nanowires can be synthesized by selecting pr oper metal catalyst size and lattice matched substrate. A high-pressure assisted pulsed laser deposition has been applied to fabricate a variety of metal oxide nanowires (ZnO, ZnMgO, SnO2 and VO2) without catalysts. Vertically wellaligned ZnO and ZnxMg1-xO arrays were grown on c-sapphire substrates at 600oC. The nanowires growth proceeds w ithout employing catalysts for nuc leation, although an epitaxial ZnO thin film template is necessary in order to achieve uniform alignmen t. This study provides a relative convenient approach to synthesize a wide range of metal oxide nanowires. For hydrogen sensing applications it is found that the sensi tivity for detecting hydrogen is greatly enhanced by sputter-depos iting metal catalysts (Pt and Pt ) on ZnO nanowires surface. Ptcoated ZnO nanowires can detect hydrogen down to 100 ppm with relative response of 4%. Pdcoated ZnO nanowires can detect hydrogen dow n to 10 ppm with a relative smaller response than Pt-coated devices. Approximately 95% of the initial conducta nce after exposure to hydrogen was recovered within 20 s by exposing th e device to air. The sensors are shown to detect ppm hydrogen at room temperature usi ng <0.4 mW of power when using multiple nanowires. When using a single ZnO nanowire coat ed with Pt as sensing material, the power consumption can further pushing down to W range. The sensors are not sensitive to oxygen, nitrogen, humidity and air at room temperature, suggests high selec tivity for hydrogen sensing

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175 applications. A comparison study of the hydrogen-sens ing characteristics of ZnO thin films with different thickness and ZnO nanowires was descri bed. The Pt-coated single nanowires show a current response of approximately a factor of 3 larger at room temperature upon exposure to 500 ppm of hydrogen. Both types of sensors are s hown to be capable of the detection of ppm hydrogen at room temperature with nW power leve ls, but the nanowires s how different recovery characteristics, consistent with the expected higher surface coverage of adsorbed hydrogen. Finally, SnO2 coated ZnO nanowires were used as ma terials for hydrogen sensors. There was no response to 500 ppm hydrogen at room temperature but showed a 70% response at 400oC. The use of single-crystal ZnO nanowires provide a convenient template for coating with SnO2 and the resulting structure can be used to detect hydrogen at 400oC. The results show that ZnO nanowires have superior propertie s in gas sensing applications. The epitaxial SnO2 thin films were realized on c-sapphire substrates using pulsed laser deposition. X-ray diffraction show s that the films have a phasepure rutile structure and grow along the (100) plane. The epitaxial relationship can be described as SnO2 [010] // Al2O3 < 0211> or equivalently SnO2 [001] // Al2O3 <01 10> and 60o rotations. The undoped SnO2 films were n-type semiconductor with carrier concentration varied from 417 cm-3 to 4.219 cm-3. The electrical transport propertie s are strongly dependent on the gr owth temperature. The effects of Ga doping on SnO2 films were studied. The Ga-doped SnO2 films were epitaxially grown on (0001) Al2O3 substrate with slightly smaller a-axis parameters. No remarkable changes were found for the band-gap of Ga-doped films. The Hall data showed p-type behavior occurs only at specific growth condition, but converted back to n-type and degraded as time proceeds.

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189 BIOGRAPHICAL SKETCH Li-Chia Tie n was born in Taipei, Taiwan, in 1976. He grew up in Taipei city until he finished his high school education. With enthusiasm and great intere st in Chemistry, he enrolled at the Department of Chemistry at National Tsin g Hua University (NTHU), Hsinchu, Taiwan in 1995. He received the B.S in chemistry in 1999, a nd continued graduate study in Department of Materials Science and Engineering at the same institution. During the master program, he spent two years working on surface science with Dr Jenn-Chang Hwang and Dr. Tun-Wen Pi in National Synchrotron Radiation Center (NSRRC) where he lear ned synchrotron photoemission techniques and concepts of scie ntific research. During his MS, he published 4 journal articles in the photoemission study of silicon surface. He recei ved the M.S. degree in materials science and engineering from National Tsing Hua University in 2001. After 20 weeks of military training, mental and physical, he was commissioned second lieutenant in the army. From 2001 to 2003, he served as a platoon leader in th e military police corp. During this period, he learned discipline, management and leadership. From 2003, he began pursuing a doctoral degree in the Department of Materials Science and Engineering at University of Florida. He was fortunate to join Dr. Nortons group in 2004 and begin research on semiconductor oxide materi als. During his PhD, he was able to learn different techniques including synthesis, processing and characterization of semiconductor materials. He was involved in different project s such as, synthesis/ch aracterization of metal oxide nanowires, developing ZnO nanowires devices and synthesis/characterization epitaxial oxide thin films by plused lase r deposition (PLD) and molecular beam epitaxy (MBE). He also developed a new catalyst-free me thod to synthesize metal oxide nanowires by plused laser deposition. The results were published in approxim ately 24 journal articles and were presented in 7 international conferences. His future pl an is to continue research in academia.