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Synthesis of Metal Selenide Semiconductor Nanocrystals Using Selenium Dioxide as Precursor

Permanent Link: http://ufdc.ufl.edu/UFE0021483/00001

Material Information

Title: Synthesis of Metal Selenide Semiconductor Nanocrystals Using Selenium Dioxide as Precursor
Physical Description: 1 online resource (56 p.)
Language: english
Creator: Chen, Xian
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: dioxide, nanocrystal, selenide, selenium, synthesis
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Nanotechnology has been one of the most popular research areas in these two decades. The semiconductor nanocrystals, which are also called quantum dots, are of the great interest because of their unique size-dependent properties. The nanomaterials have wide applications, including light emitting diodes, solar cells, biological labeling, and so on. The critical part in the use of quantum dots is to prepare monodispersed nanocrystals. The methods to synthesize high-qulity nanocrystals have been well developed. Selenium element was used in most method for synthesizing high quality metal selenide nanocrystals. However selenium element is toxic and unstable in the air, thus requires complicated operations. Herein, we developed a new approach for using selenium dioxide to replace the selenium element. Selenium dioxide is very stable and nontoxic. It is found that when adding of 1,2-hexadecanediol the quality of nanocrystals can be improved. Experiments were carried out to test the results of using different amount of 1,2-hexadecanediol. It turns out that adding more 1,2-hexadecanediol can result in smaller size nanocrystals, higher nuclei number, and slower growth rate. Different cadmium precursors were used and the results show that with longer carbon chains in cadmium precursor, smaller size CdSe nanocrystals can be obtained. Cadmiun selenide nanocrystals with diameters of 4.5 nm by carrying out multiple-addition experiment were generated. Selenium dioxide was also employed to prepare other metal selenide nanocrystals. Gallium selenide nanocrystals with diameters of 2.0 nm were formed. Silver selenide nanocrystals with diameters of around 7.4 nm and a lattice spacing of 0.21 nm were obtained. The absorption spectrum shows that during the formation of AgSe nanocrystals, silver nanocrystals were formed first and then gradually reacted with SeO2 to form AgSe nanoparticles. Lead selenide aggregates consisting of uniform nanocubes were observed.
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.
Statement of Responsibility: by Xian Chen.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Cao, Yun Wei.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-08-31

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Source Institution: UFRGP
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Classification: lcc - LD1780 2007
System ID: UFE0021483:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021483/00001

Material Information

Title: Synthesis of Metal Selenide Semiconductor Nanocrystals Using Selenium Dioxide as Precursor
Physical Description: 1 online resource (56 p.)
Language: english
Creator: Chen, Xian
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: dioxide, nanocrystal, selenide, selenium, synthesis
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Nanotechnology has been one of the most popular research areas in these two decades. The semiconductor nanocrystals, which are also called quantum dots, are of the great interest because of their unique size-dependent properties. The nanomaterials have wide applications, including light emitting diodes, solar cells, biological labeling, and so on. The critical part in the use of quantum dots is to prepare monodispersed nanocrystals. The methods to synthesize high-qulity nanocrystals have been well developed. Selenium element was used in most method for synthesizing high quality metal selenide nanocrystals. However selenium element is toxic and unstable in the air, thus requires complicated operations. Herein, we developed a new approach for using selenium dioxide to replace the selenium element. Selenium dioxide is very stable and nontoxic. It is found that when adding of 1,2-hexadecanediol the quality of nanocrystals can be improved. Experiments were carried out to test the results of using different amount of 1,2-hexadecanediol. It turns out that adding more 1,2-hexadecanediol can result in smaller size nanocrystals, higher nuclei number, and slower growth rate. Different cadmium precursors were used and the results show that with longer carbon chains in cadmium precursor, smaller size CdSe nanocrystals can be obtained. Cadmiun selenide nanocrystals with diameters of 4.5 nm by carrying out multiple-addition experiment were generated. Selenium dioxide was also employed to prepare other metal selenide nanocrystals. Gallium selenide nanocrystals with diameters of 2.0 nm were formed. Silver selenide nanocrystals with diameters of around 7.4 nm and a lattice spacing of 0.21 nm were obtained. The absorption spectrum shows that during the formation of AgSe nanocrystals, silver nanocrystals were formed first and then gradually reacted with SeO2 to form AgSe nanoparticles. Lead selenide aggregates consisting of uniform nanocubes were observed.
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.
Statement of Responsibility: by Xian Chen.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Cao, Yun Wei.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021483:00001


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SYNTHESIS OF METAL SELENIDE SEMICONDUCTOR NANOCRYSTALS USING
SELENIUM DIOXIDE AS PRECURSOR




















By

XIAN CHEN


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2007

































2007 Xian Chen




































To my parents









ACKNOWLEDGMENTS

Above all, I would like to thank my parents for what they have done for me through these

years. I would not have been able to get to where I am today without their love and support.

I would like to thank my advisor, Dr. Charles Cao, for his advice on my research and life

and for the valuable help during my difficult times. I also would like to thank Dr. Yongan Yang

for his kindness and helpful discussion. I learned experiment techniques, knowledge, how to do

research and so on from him. I also appreciate the help and friendship that the whole Cao group

gave me.

Finally, I would like to express my gratitude to Dr. Ben Smith for his guidance and help.









TABLE OF CONTENTS



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

LIST OF FIGURES ...................................................... .7

A B S T R A C T .................................................................................................................... ......... .. 9

CHAPTER

1 SEMICONDUCTOR NANOCRYSTALS ......................................................................... 11

1 .1 In tro d u c tio n .................................................................................................................. 1 1
1.2 General Synthetic Methods for Nanocrystals ......................................................... 11
1.2.1 Injection-B ased Synthetic M ethod................................................ ............... 13
1.2.2 O ne-Pot Synthetic M ethod............................................................ ............... 14
1.3 Applications of Semiconductor Nanocrystals........................................................ 16
1.3.1 B biological D election .. .. .... .......... ......... ............................................. 16
1.3.2 Hybrid Electroluminenscent Device............................................................ 17
1.3.3 Photovoltaic Device ........................................................................ 18

2 SYNTHESIS OF CADMIUN SELENIDE NANOCRYSTALS USING SELENIUM
DIOXIDE AS PRECURSOR ..................................................................................... 19

2 .1 In tro d u ctio n ................................................................................................................ .. 1 9
2 .2 E xperim ental Section ... ........................................................................ ................ 20
2 .2 .1 M materials ............................................................................................................ 2 0
2 .2 .2 Instrum entation .............. .................. ................................. ...............20
2.2.3 Preparation of Cd-Precursors....................................................................22
2.2.3.1 Cadmium myristate (CdC14).................................................... 22
2.2.3.2 C adm ium stearate (C dC is) ................................................. ................ 22
2.2.3.3 Cadmium docosanate (CdC22)............................................................22
2.2.4 Preparation of CdSe Nanocrystals ................................................................23
2 .3 R results and D iscu ssion .. ...................................................................... ................ 23
2 .3 .1 D io l E ffect.......................................................................................................... 2 3
2 .3.2 P recursor E effect ... .................................................................... ...............32
2.3.3 M ultiple-A addition M ethod ............................................................ ................ 36
2 .4 C o n c lu sio n ................................................................................................................. .. 3 7

3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM
DIOXIDE AS PRECURSOR ......................................................................................39

3 .1 In tro d u ctio n ................................................................................................................ .. 3 9
3.2 E xperim ental Section ... ........................................................................ ................ 40
3 .2 .1 M materials ............................................................................................................ 4 0
3.2.2 Instrumentation .............. ................................. 41









3.2.3 P reparation of P recursors.............................................................. ................ 4 1
3.2 .3.1 G allium m yristate ...................................... ...................... ................ 4 1
3.2.3.2 Silver oleate .......................................................................................4 1
3.2.3.3 Copper oleate ..................................................................................... 41
3.2 .3.4 N ickel oleate ........................................... .. ....................... .... ......... 42
3.2.4 Preparation of Nanocrystals......................................................................42
3.2.4.1 G allium selenide nanocrystals............................................ ................ 42
3.2.4.2 Lead selenide nanocrystals....................................................42
3.2.4.3 Silver selenide nanocrystals ............................................... ................ 43
3.2.4.4 Copper selenide nanocrystals .............................................................43
3.2.4.5 N ickel selenide nanocrystals .............................................. ................ 43
3.2.5 Purification of N anocrystals ......................................................... ................ 43
3.3 R results and D iscu ssion .. ...................................................................... ................ 44
3 .4 C o n c lu sio n ...................................................................................................................4 8

4 SUMMARY AND FUTURE WORK ........................................................50

4 .1 S u m m a ry ......................................................................................................................5 0
4 .2 F u tu re w o rk ..................................................................................................................5 0
4.2.1 Injection-Synthetic Method for CdSe........................................................... 50
4.2.2 Improvement of Other Metal Selenide Nanocrystals.................................... 51
4.2.3 Mechanism Study ................................................................................... 51

L IST O F R EFE R E N C E S ............................................................................................. 52

B IO G R A PH IC A L SK E T C H .............. ...................................................................... 56


























6









LIST OF FIGURES


Figure page

1-1 Schem e of the form ation of nanocrystals ..................................................... ................ 12

1-2 LaM er Curve .......................................................................... ...... .... ..................... 13

1-3 Representation of the synthetic apparatus employed in the injection-based method ........14

1-4 Absorption spectrum of CdS nanocrystals (d = 3.5 nm). ..............................................15

1-5 Representation of the synthetic apparatus employed in the one-pot synthetic method...... 16

2-1 Sizing curve of C dSe nanocrystals ...................................... ....................... ................ 19

2-2 HWHF and peak sharpness used for size distribution determination...............................20

2-3 Schem atic diagram of a U V-Vis m icroscope ................................................ ................ 21

2-4 Schematic diagram of a Fluorolog-3 Model FL3-12 spectrofluorometer. .......................21

2-5 M olecular structures of organic solvents used. .................................................. 23

2-6 Temporal evolution of the absorption spectra during the CdSe synthesis ......................24

2-7 Characterization of CdSe nanocrystals synthesized in ODE with reaction time of 40
m in u te s............................................................................................................. ........ .. 2 5

2-8 Temporal evolution of CdSe nanocrystal concentration synthesized in ODE with
different C 16-diol/SeO 2 ratios........................................... ......................... ................ 26

2-9 CdSe particle growth rate in the synthesis with different C 16-diol/SeO2 ratios .............27

2-10 CdSe particle size and normalized nuclei number in the synthesis with different
C 16-diol/SeO 2 ratios ................................................................ ............... .. .... .. .... 28

2-11 Temporal evolution of the absorption spectra during the CdSe synthesis with
d iffe re n t d io ls .................................................................................................................. ... 2 9

2-12 HWHM of CdSe during synthesis with different diols. ................................................29

2-13 Temporal evolution of CdSe nanocrystal concentration with different diols..................30

2-14 CdSe particle size in the synthesis with different numbers of carbon atom per diol. ........30

2-15 Temporal evolution of the absorption spectra during the CdSe synthesis with
d ifferen t alco h o ls. .............................................................................................................. 3 1









2-16 Temporal evolution of the absorption spectra during the CdSe synthesis with
different Cd precursors ...................................... ........................... 32

2-17 M multiple exiton peak s......................................................................................................... 33

2-18 Effect of Cd precursor on the nuclei concentration during the CdSe synthesis ..............34

2-19 Effect of Cd precursor on the CdSe particle size in the synthesis.................................34

2-20 CdSe particle growth rate in the synthesis with different Cd precursors. ........................35

2-21 Characterization of CdSe nanocrystals during the multiple-addtion synthesis ...............36

2-22 Temporal evolution of the absorption spectra of the as-prepared CdSe nanocrystals........38

3-1 Evolution of absorption spectrum of GaSe nanocrystals ..............................................44

3-2 TE M im age of G aSe nanocrystals ......................................... ....................... ............... 45

3-3 TE M im age of PbSe nanocrystals .......................................... ....................... ............... 45

3-4 Evolution of absorption spectrum of AgSe nanocrystals. .............................................46

3-5 H R -TEM im ages of A gSe nanocrystals ........................................................... ............... 47

3-6 TE M im age of C uSe nanocrystals ......................................... ....................... ............... 47

3-7 TE M im age of N iSe nanocrystals........................................... ....................... ............... 48









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

SYNTHESIS OF METAL SELENIDE SEMICONDUCTOR NANOCRYSTALS USING
SELENIUM DIOXIDE AS PRECURSOR

By

Xian Chen

August 2007

Chair: Y. Charles Cao
Major: Chemistry

Nanotechnology has been one of the most popular research areas in these two decades. The

semiconductor nanocrystals, which are also called quantum dots, are of the great interest because

of their unique size-dependent properties. The nanomaterials have wide applications, including

light emitting diodes, solar cells, biological labeling, and so on. The critical part in the use of

quantum dots is to prepare monodispersed nanocrystals. The methods to synthesize high-qulity

nanocrystals have been well developed.

Selenium element was used in most method for synthesizing high quality metal selenide

nanocrystals. However selenium element is toxic and unstable in the air, thus requires

complicated operations. Herein, we developed a new approach for using selenium dioxide to

replace the selenium element. Selenium dioxide is very stable and nontoxic. It is found that when

adding of 1,2-hexadecanediol (C16-diol) the quality of nanocrystals can be improved.

Experiments were carried out to test the results of using different amount of C16-diol. It turns out

that adding more C16-diol can result in smaller size nanocrystals, higher nuclei number, and

slower growth rate. Different cadmium precursors were used and the results show that with

longer carbon chains in cadmium precursor, smaller size CdSe nanocrystals can be obtained.









CdSe nanocrystals with diameters of 4.5 nm by carrying out multiple-addition experiment were

generated.

SeO2 was also employed to prepare other metal selenide nanocrystals. GaSe nanocrystals

with diameters of 2.0 nm were formed. AgSe nanocrystals with diameters of around 7.4 nm and

a lattice spacing of 0.21 nm were obtained. The absorption spectrum shows that during the

formation of AgSe nanocrystals, Ag nanocrystals were formed first and then gradually reacted

with SeO2 to form AgSe nanoparticles. PbSe aggregates consisting of uniform nanocubes were

observed.









CHAPTER 1
SEMICONDUCTOR NANOCRYSTALS

1.1 Introduction

In the last two decades, there has been an increasing progress in the synthesis and

characterization of semiconductor nanocrystals. They are of great interest for both fundamental

research and industrial development because of their unique properties. In the nanometer range,

the properties of semiconductor nanocrystals are strongly dependent upon their size, shape, and

crystal structure, which make them differ substantially from the corresponding molecular and

bulk materials. 1,2 Thus controlling the physical size of materials can be used to tune materials

properties. These novel properties lead to many applications such as light emitting diodes

(LEDs), biological fluorescent labels, lasers and solar cells.3-17

Efforts to explore structures on the nanometer scale combine the material science,

chemistry, physics and engineering. Studying size-dependent materials properties requires

synthetic routes to prepare homologous size series of monodisperse nanometer size crystals.18

1.2 General Synthetic Methods for Nanocrystals

Synthesis of high-quality semiconductor nanocrystals is the key element for studying the

size-dependent properties in the nanometer scale. This has been a very active area of research.

Colloidal methods are of most interested because the optical and electrical properties of

semiconductor nanocrystals made by these methods can be tuned by changing the physical size

of the nanocrystals. Synthesis of high-quality colloidal nanocrystals have been reported by

several groups. The research group of Alivisatos and Bawendi developed methods of using

molecular precursors.6'19 In early 1990s, Cd(CH3)2 as precursor and technical-grade

trioctylphosphine oxide (Tech TOPO) as the reaction solvent were used to synthesize

high-quality CdSe nanocrystals.6 But Cd(CH3)2 is extremely toxic, expensive and unstable, and








this synthesis is not very reproducible. Since 2001, CdO, CdCO3 and Cd(OOCCH3)2 precursors

with functionalized organic ligands have been used to replace Cd(CH3)2 precursor, for a

"greener" approach and noncoordinating solvents, such as 1-octadecene, were used to replace

TOPO.20-23 This thermal decomposition method has also been extended to the synthesis of ZnS

and ZnSe nanocrystals.24

In a typical colloidal synthesis there are three components: precursors, surfactants and

solvents. In some cases, solvents also serve as surfactants. When the system is heated to a

sufficiently high temperature, the precursors chemically transfer to active atoms or molecules,

which are called monomers. The monomers then form nanocrystals whose subsequent growth is

greatly affected by the presence of surfactants. The formation of the nanocrystals involves two

steps: nucleation of an initial "seed" and growth. In the nucleation step, precursors decompose at

a high temperature to form a supersaturation of monomers followed by a burst of nucleation of

nanocrystals. These nuclei then grow by incorporating additional monomers still present in the

reaction solution.25 The scheme of the formation of nanocrytals is shown in Figure 1-1.


A + 0 Reaction + m
Precursors Monomers Organic by-products



Nucleation Growth
*A A
Monomers Nucleus Larger nanocrystal

Figure 1-1. Scheme of the formation of nanocrystals.









1.2.1 Injection-based Synthetic Method

In colloidal synthesis, chemists developed a method to separate the nucleation stage from

the nanocrystal's growth stage6-8 as described by LaMer Curve (Figure 1-2). Rapid injection of

metal-organic precursors into a vigorously stirred flask containing a hot coordinating solvent can

form the supersaturation and subsequent nucleation. A short nucleation burst partially relieves

the supersaturation. As long as the consumption of feedstock by the growing colloidal

nanocrystals is faster than the rate of precursor addition to solution, no new nuclei form.18

Growth rate can be controlled by diffusion rate and/or reaction rate. Finally, the growth will be

balanced by the solubility.


critical limiting supersaturation
0 C max
"S Cmn -- ^------------
C min
i Growth by diffusion or reaction
C solubility

I Il

Time

Rapid self nucleation

Figure 1-2. LaMer Curve.

Figurel-3 illustrates a synthetic apparatus employed in the injection-based method. This

method has led to synthesis of a variety of high-quality nanocrystals ranging from II-VI (e.g.,

CdS and CdSe) and III-V (e.g., InP and InAs) to IV-VI (e.g., PbS and PbSe)

semiconductors.6,7,20,26-30









Ar--



Thermal Precursors
couple




-- Heating mental

Stirring

Figure 1-3. Representation of the synthetic apparatus employed in the injection-based method.

However, the injection-based synthetic method is not suitable for large-scale, industrial

preparation. It is very difficult to inject precursors rapidly because industrial preparation may use

hundreds of kilograms of precursors. 27 In the laboratory, nucleation time is determined by the

rate of the injection and the mass transfer in the reaction systems, and the temperature is very

hard to control. So this injection based synthesis method is not ideal for mechanistic

mechanism studies that require a highly reproducible system for quantitative measurement.

Therefore, methods that do not require the injection of precursors have to be developed.

1.2.2 One-Pot Synthetic Method

Several groups have reported the one-pot synthesis of semiconductor nanocrystals without

the injection of precursors. However, the quality of their nanocrystals was not comparable to that

of the nanocrystals made by the injection-based method. Typically, they do not exhibit as many

multiple-exciton absorption peaks, 31-34 while high-quality nanocrystals with multiple

exciton-absorption peaks are critical for the applications in advanced optical and electronic

devices.35 Recently, the Cao group has developed a new non-injection synthesis for making CdS,

CdSe and CdTe nanocrystals.27'28 The quality of the nanocrystals made by this new synthesis is









at least comparable to the best particles made by injection-based methods.Without size-selective

separation, the nanocrystals formed by this new synthesis exhibit up to four exciton-absorption

peaks, indicating their very narrow size distribution and excellent optical properties (Figure 1-4).

The set-up for the one-pot synthesis is shown in Figure 1-5.


Figure 1-4.


300 350 400 450 500
Wavelength (nm)

Absorption spectrum of CdS nanocrystals (d = 3.5 nm).


The one-pot synthetic method is based on a new concept of controlling the

thermodynamics and kinetics of the nanocrystal nucleation stage.27 The precursors are thermal

decomposed when heat up to a sufficiently high temperature, so more and more monomers are

produced as time passes, when the concentration of monomers increased to supersaturation,

nucleation happens. As the monomer concentration drops lower than the nucleation

concentration, the nucleation stops, followed by the nuclei formed growth. When the

concentration of monomer drops to saturation concentration, the particles stop growing.













Thermal
couple




Heating mental

7Stirring

Figure 1-5. Representation of the synthetic apparatus employed in the one-pot synthetic
method.

Although compared to the injection method, one-pot synthesis has the advantages of

reproducibility, capability of large-scale and industrial preparation, when dealing with different

materials core-shell nanocrystals and doped nanocrystals, the injection method is the only choice.

The one-pot synthesis can only be employed when same material core-shell nanocrystals are

desired.

1.3 Applications of Semiconductor Nanocrystals

1.3.1 Biological Detection

In 1998, both Alivisatos group16 and Nie17 group first reported the use of colloidal quantum

dots for biological labeling. They suggested that due to the photochemical stability and the

tubable luminescence of the quantum dots, they may make these materials extremely useful for

biolabeling. Compared to regular organic dyes, quantum dots have the advantages of tunable

luminescence, high quantum yield, broad light absorption, narrow emission spectra and high

stability. Since 2002, there has been development of a wide range of methods for bio-conjugating









colloidal quantum dots36-40 in diverse areas of application: cell labeling41, cell tracking42, in vivo

imaging43, DNA detection44, and multiplexed beads45.

Colloidal quantum dots with a wide range of bio-conjugation and with high quantum yields

are now available commercially, so that it is no longer necessary for each experimenter to grow

their own or to become lost in the myriad discussions concerning the best way to render colloidal

dots water soluble and bio-compatible.46

1.3.2 Hybrid Electroluminenscent Device

A light-emitting diode (LED) is a semiconductor device that emits incoherent

narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This

effect is a form of electroluminescence. An LED is a small extended source with extra optics

added to the chip that makes it emit a complex radiation pattern.46 Since the first observation of

light emission from organic materials by Tang et al.48, continuous and rapid improvement in

device performance have enabled organic light emitting devices (OLEDs) to compete with

existing technologies. However, there are still many problems to be overcome, such as

improving device stability and color purity. Full width at half maximum (FWHM) of

photoluminescence of colloidal semiconductor nanocrystals is about 30nm which is narrower

than those from organic materials. Moreover, these inorganic nanocrystals are much more stable

and robust than organic molecules. So hybrid OLEDs using semiconductor nanocrystals as an

emission layer have to been to have good stability and efficiency. The first demonstration of a

hybrid OLED was reported by Colvin et al in 1994.49 In order to enhance the quantum efficiency

of hybrid OLED devices, several problems must be solved including more efficient charge

transfer between the organic layer and nanocrystals, the imbalance of injected conduction

through nanocrystals, a high density of pinhole defects in the nanocrystal layer, uniformity of

nanocrystals in the deposited layer, and optimization of interlayer structure of device.3'50









1.3.3 Photovoltaic Device

Inorganic solar cells that have limitations due to the high costs of fabrication have power

conversion efficiencies of 10%. While organic solar cells that use polymers which can be

processed from solution have been investigated as a low-cost alternative have solar power

efficiencies of up to 2.5%.51 One way to overcome these limitations is to combine polymers with

inorganic semiconductors Because of the nanoscale nature of light absorption and photocurrent

generation in solar energy conversion, the advent of methods for controlling inorganic materials

on the nanometer scale opens new opportunities for the development of future generation solar

cells. Alivisatos group used colloidal semiconductor nanorods as the inorganic phase in the

construction of these solar cells via solution-phase nano-assembly. By varying the radius of the

rods, the quantum size effect can be used to control the band gap; furthermore, quantum

confinement leads to an enhancement of the absorption coefficient compared with the bulk, such

that devise can be made thinner. One-dimensional (ID) nanorods are preferable to quantum dots

or sintered nanocrystals in solar energy conversion, because they naturally provide a directed

path for electrical transport. The length of the nanorods can be adjusted to the device thickness

required for optimal absorption of incident light.5









CHAPTER 2
SYNTHESIS OF CADMIUM SELENIDE NANOCRYSTALS USING SELENIUM
DIOXIDE AS PRECURSOR

2.1 Introduction

In chapter 1, we have mentioned that CdO was used to replace Cd(CH3)2 in the molecular

precursor synthesis because Cd(CH3)2 is extremely toxic, unstable and expensive. Actually,

selenium element has the same problem. Se is unstable in the air, and it is toxic. Here, we present

a method of synthesizing CdSe nanocrystals using Se02 to replace Se element. We found that

with the presence of C16-diol and using cadmium precursors with longer carbon chain, the

quality of the CdSe quantum dots formed by this method is comparable to that of the best CdSe

nanocrystals reported in the literature, and some nanocrystals are even better.

To characterize the nanocrystals, the diameters of the nanocrystals were calculated from

the wavelength of first exciton peak using the CdSe sizing curve (Figure 2-1). The size

distribution of the particle was evaluated by measuring the HWHM of first exciton peak, which

is a well accepted method to estimate nanocrystals' size distribution.52 In our previous work, we

also found that peak sharpness is also a way to evaluate the size distribution (Figure 2-2).

Fluorometer and TEM was also employed.

9.50
8.50-
E
7.50-
6.50 /
E 5.50 -
4.50 -
) 3.50-
O 2.50
1.50
450 470 490 510 530 550 570 590 610 630 650 670
UV Peak Position (nm)
Figure 2-1. Sizing curve of CdSe nanocrystals













Peak sharpness = H|IH,
0.4




0- H.
H2
0.1 --




250 300 350 400 450 500 550
Wavelength (nm)
Figure 2-2. HWHM and peak sharpness used for size distribution determination.

2.2 Experimental Section

2.2.1 Materials

1-octadecene (ODE, 90%), 1-tetradecene, (TDE, 92%), squalene (98%), dioctyl ether

(99%), docosanoic acid (CH3(CH2)20COOH, 99%), selenium dioxide (SeO2, 99.9+%) and

1,2-hexadecanediol (C16-diol, 90%), 1,2-decanediol (Cio-diol, 98%), 1,2-octanediol (C8-diol,

98%), 1-octadecanol (C18-OH, 99%), and phenol were purchased from Sigma-Alrich. Methanol

(99.9%), toluene (99.9%), acetone (99.8%) were purchased from Fisher. Sodium myristate

(CH3(CH2)12COONa), sodium stearate (CH3(CH2)16COONa) were purchased from TCI.

Cadmium nitrate tetrahydrate (Cd(N03)2-4H20) was purchased from Alfa Aesar.

Tetrabutylammonium hydroxide (IM in methanol) was purchased from Acros. All chemicals

were used without further purification.

2.2.2 Instrumentation

Absorption spectra of aliquots were collected by a Shimadzu UV-1700 UV-Visible

Spectrophotometer (Figure 2-3). The wavelength, absorption and the half width at half maximum









(HWHM) of first exciton peak for each aliquot were recorded. This method also was compared

with our previous one-pot synthesis method and other current injection method.

Toroidal Grating D2 lamp (UV)


"R Mirror 3 U'L I____ Lens 1

Figure 2-3. Schematic diagram of a UV-Vis microscope.

Photoluminescence (PL) was measured at room temperature from nanocrystals suspended

in toluene using a JOBIN YVON HORIBA Fluorolog-3 Model FL3-12 spectrofluorometer

(Figure 2-4).


Figure 2-4. Schematic diagram of a Fluorolog-3 Model FL3-12 spectrofluorometer.









High resolution transmission electron microscopy (HR-TEM) images were obtained using

a JEOL 2010F microscope for lattice imaging and crystal size determination. TEM samples were

prepared by dispersing the nanocrystals in toluene and depositing them onto formvar-coated

copper grids.

2.2.3 Preparation of Cd-Precursors

2.2.3.1 Cadmium myristate (CdC14)

1.5g cadmium nitrate was dissolved in 75ml methanol, while sodium myristate solution

was prepared by dissolving 3.8g sodium myristate in 550ml methanol. Then the cadmium

nitrate solution was dropped slowly into sodium myristate solution under magnetic stirring

conditions. The observed white precipitate was washed with methanol 4-5 times to get rid of

impurities and dried under vacuum overnight to remove all solvents.

2.2.3.2 Cadmium stearate (CdC18)

0.3g Cadmium nitrate was dissolved in 40ml methanol, Sodium stearate solution was

prepared by dissolving 0.90g sodium stearate in 600ml methanol. Then the cadmium nitrate

solution was dropped slowly with into sodium stearate solution under magnetic stirring

conditions. The observed white precipitate was washed with methanol 2-3 times get rid of

impurities then put back in flask with 600 ml methanol and ultrasonicated. Wash the product

again and dried under vacuum overnight to remove all solvents.

2.2.3.3 Cadmium docosanate (CdC22)

0.3g Cadmium nitrate was dissolved in 40ml methanol. Sodium docosanate solution was

prepared by dissolving 0.90g docosanoic acid in 600ml methanol and slowly adding 1.1 ml

tetrabutylammonium hydroxide (IM in methanol) dropwise. Then the cadmium nitrate solution

was dropped slowly into sodium docosanoicate solution under magnetic stirring conditions. The

observed white precipitate was washed with methanol 2-3 times get rid of impurities then put









back in flask with 600 ml methanol and ultrasonicated. Wash the product again and dried under

vacuum overnight to remove all solvents.

2.2.4 Preparation of CdSe Nanocrystals

Cadmium precursor (0.1mmol), SeO2 (0.05mmol), C16-diol (0.05mmol) and non

coordinating solvent (5g) were mixed in a three-neck flask equipped with condenser, magnetic

stirrer, thermocouple, and heating mantle (as shown in Figure 1-5), degassed before heated to

265 C with gentle stirring under vacuum to synthesize CdSe nanocrstals. Aliquots of the

solution for each reaction were taken quantitatively with a syringe at different time intervals, and

quickly cooled and diluted in toluene to stop further growth. These aliquots were employed to

monitor the reaction via UV-Vis and photoluminescence measurement.

2.3 Results and Discussion

2.3.1 Diol Effect

C16-diol Effect on the Quality and Size of CdSe Nanocrystals


CH3(CH2)1sCH=CH2 CH3(CH2)1iCH=CH2

ODE TDE N




Squalene




Octyl ether

Figure 2-5. Molecular structures of organic solvents used.

CdSe nanocrystals were formed using SeO2 compound instead of Se element in the solvent

of ODE, which means that SeO2 is active at high temperature. However, the quality is not as










good. We found that for CdSe nanocrystals synthesized in ODE, addition of equal molar

amounts of C16-dio land SeO2 has several effects on the growth, including growth rates,

HWHMs, sharpness, optical densities, and final sizes. This phenomenon was also observed in

CdSe nanocrystals synthesized in TDE, squalene, and octyl ether. The molecular structures of

these four solvents are shown in Figure 2-5.

Figure 2-6 shows the absorption spectra of CdSe nanocrystals made of 0.1 mmol CdC14,

0.05 mmol SeO2 and 0.05 mmol C16-diol in different solvents.


a) ODE no C16-diol b) squalene, no C16-diol c) octyl ether, no C16-diol d) TDE, no C16-diol

88








-- 6 10- 8-
6- 8- 6


0
4 30min 30min
0 2 L 5 m 4-2


2200C 2200C "0 min
0 0-0-- 0- l
e) ODE, C16-diol adde(b) and (f) squalene, C16-diol added g)octyl ether, C16-diol added h)TDE, C16-diol added
8-
8- 10- 8-

6- 0 8- 66














(a), (b), (c) and (d), C16-diol was not added while in syntheses (e), (f), (g) and (h),
C16-diol was added.

In Figure 2-7, we show the absorption and photoluminescent (PL) spectra and TEM image

of CdSe nanocrystals which were made in ODE and have a reaction time of 40 minutes. It can be
of CdSe nanocrystals which were made in ODE and have a reaction time of 40 minutes. It can be









seen that the sample is nearly monodispersed. The average diameter is 3.3 nm, which is very

close to the calculated diameter of 3.4 nm.

The CdSe nanocrystals formed with the presence of C16-diol have much better quality than

those formed without adding C16-diol. With the presence of C16-diol, the first peak is narrower

and deeper than that without the presence of C16-diol, which indicates that size distribution is

better. Thus one can conclude that adding C16-diol can improve the quality of CdSe nanocrystals.

It was also observed that the final sizes of CdSe nanocrystals are smaller and the ODs are higher

in the case of adding C16-diol. To better understand the role that C16-diol assumes in the

synthesis, three syntheses with different amounts of C16-diol were performed. Nuclei

concentration, nuclei number, growth rate as well as peak sharpness and HWHM were employed

to analyze the data.


a b

1.0

0.8-

(b) image.

0.4

0.2

0.0
500 550 600 650
Wavelength (nm)


Figure 2-7. Characterization of CdSe nanocrystals synthesized in ODE with reaction time of
40 minutes. (a) Absorption (in blue) and photoluminescent (PL) (in red) spectra and
(b) TEM image.










Nuclei concentrations are calculated by using the exciton energy to determine the particle

radius and then the extinction coefficient for each size to determine the particle concentration.52

CdSe : D = (1.6122 x 10 )4 (2.6575 x 106)3 + (1.6242 x 10 3)A2 (0.4277)A +41.57 (2-1)

In the above equation, D (nm) is the diameter of a given nanocrystal sample, and X (nm) is the

wavelength of the first excitonic absorption peak of the corresponding sample.

The extinction coefficient of CdSe is calculated as

e = 5857(D)265 (2-2)


Then using the Lambert-Beer's law,

A = sCL (2-3)

The molar concentration C (mol/L) of the nanocrystals of the sample can be calculated. A is the

absorbance at the peak for a given sample and L is the path length (cm). In our experiments, L

was fixed at 1 cm.


10



S8- \

.0

C
C 5-
O
4-



U 2 b
1- 3
S 2 b---,----- --"^..-.-.-


2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8
size of CdSe nanocrystals (nm)


Figure 2-8. Temporal evolution of CdSe nanocrystal concentration synthesized in ODE with
different C16-diol/SeO2 ratios, (a) C16-diol/SeO2=0, (b) C16-diol/SeO2= 1, and (c)
C16-diol/SeO2=2.









The calculated temporal evolution of concentrations and growth rates of CdSe nanocrystals

made in ODE with different C16-diol/SeO2 ratio is shown in Figure 2-8 and Figure 2-9,

respectively. Figure 2-8 shows that the more C16-diol in the reaction solution, the higher the

CdSe nuclei concentration and the concentration dramatically increases when the C16-diol to

SeO2 ratio changes from 1 to 2. It can be seen in Figure 2-9 that the higher the ratio of C16-diol

to SeO2, the slower the growth rate.






S m-----,
6-
E


4- c



2-



I . .. ..

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
CdSe particle size (nm3)

Figure 2-9. CdSe particle growth rate in the synthesis with different C16-diol/SeO2 ratios (a)
C16-diol/SeO2=0, (b) C16-diol/SeO2=1, and (c) C16-diol/SeO2=2.

It was found that the C16-diol affects not only the quality and nuclei number of CdSe

nanocrystals, but also the final CdSe particle sizes, as shown in Figure 2-10. The more the

C16-diol in the reaction solution, the smaller the CdSe nanocrystals will be obtained.

Addition of C16-diol can slow the CdSe particle growth remarkably with the nuclei

number increasing at the same time. It can be concluded that nuclei number is related the particle











growth rate. The faster the particles growth, the lower the nulclei number. This leads us to a

hypothesis that C16-diol acts as a reducing agent in the reaction, helping reduce the selenium in

SeO2 from Se+4 to Se -2, helping increase the concentration of selenium monomers. This leads to

easier nucleation and a higher nuclei number and thus, a smaller size.



3.8-

3.7-
E
3.6-

3.5-
o
S3.4-
cU
"0

0 3.2-
C
o 3.1-

3.0
3 .0 --- --- --- --- --- --- --- --- --- --
0.0 0.5 1.0 1.5 2.0
Ratio of C 16-diol/SeO2


Figure 2-10. CdSe particle size and normalized nuclei number in the synthesis with different
C16-diol/SeO2 ratios.

Effect of Numbers of Carbon Atoms per Diol

Effect of different carbon chain length of diols (Cs-diol, C10-diol and C16-diol) was

studied. The temporal evolution of the absorption spectra is shown in Figure 2-11. As shown in

Figure 2-12, the HWHM of CdSe nanocrystals made with C16-diol is the best and this is

equivalent to a tighter size distribution. Figure 2-13 shows CdSe nanocrystal concentration with

different diols. The nuclei concentration with C10-diol is very close to but slightly higher than

that with C16-diol, while the concentration with C16-diol is higher than that of Cs-diol. Figure

2-14 illustrates that with C10-diol and C16-diol, the final particle sizes are very similar and with

Cs-diol slightly larger size particles can be obtained.


























400 500 600


wavelength (nm)


400 500 600
wavelength (nm)


400 500 600
wavelength (nm)


Figure 2-11. Temporal evolution of the absorption spectra during the CdSe synthesis with
different diols: (a) C16-diol, (b) Co0-diol and (c) C8-diol.




18- C


17 -






S15-






2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
Size of CdSe nanocrystals (nm)


Figure 2-12. HWHM of CdSe during synthesis with different diols: (a) C16-diol, (b) C10-diol
and (c) Cs-diol.















Eb
4 ,, ----------------

io \ '

2-

U a






2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
Size of CdSe nanocrystals (nm)


Figure 2-13. Temporal evolution of CdSe nanocrystal concentration with different diols: (a)
C16-diol, (b) Co10-diol and (c) Cs-diol.


8 9 10 11 12 13 14 15 16 17
number of carbon atoms per diol


Figure 2-14. CdSe particle size in the synthesis with different numbers of carbon atom per diol.









Based on all the information, one can conclude synthesis with C16-diol has the lowest

growth, smallest size. The possible explanation for this is that the longer the carbon chain in the

diol molecule, the hydrogen bonding will be weaker, and thus the more active the diol would be.

So among all three, C16-diol is the most active.

Comparison of Alcohols and Diols


a) b) c)
8 8 8


6 30min 6 6
30min 30min

10min 10min
4- 4- 4
5mmin 5 min 5 min
Omin 0 min 0 min
S 2 2- 2-
2400C 240C 2400C

2200C 220C 2200C
0 0 0
I I I I I I I I '-
400 500 600 700 400 500 600 700 400 500 600 700

wavelength (nm) wavelength (nm) wavelength (nm)


Figure 2-15. Temporal evolution of the absorption spectra during the CdSe synthesis with
different alcohols. (a) C16-diol, (b) C18-OH and (c) phenol.

Synthesis with alcohols was performed. The results are compared with diols. C18-OH and

phenol were used. The nanocrystals were formed by cadmium myristate (0.1 mmol) reacted with

Se02 (0.05 mmol) and C18-OH or phenol (0.05 mmol). The absorption spectra are shown in

Figure 2-15. Compare HWHM and sharpness, one can conclude that the qualities CdSe

nanocrystals with C18-OH are better than that without alcohol, but not are not comparable to

those with C16-diol. The CdSe nanocrystals made with phenol did not get improved.










The possible reaction happened is proposed below. The SeO2 got reduced by the alcohol

and selenium was formed. The active selenium then reacted with cadmium precursor to form

CdSe nanocrystals.
O 0
II II
2CH3(CH2)16CH2-OH + Se Se + 2H20 + 2CH3(CH2)16CH2C-OH
II2.3.2 Precursor Effect


2.3.2 Precursor Effect


400 500 600 700
wavelength (nm)

c)


400 500 600 700
wavelength (nm)


400 500 600 700
wavelength (nm)


220 C
210C

400 500 600 700
wavelength (nm)


Figure 2-16. Temporal evolution of the absorption spectra during the CdSe synthesis with
different Cd precursors. (a) CdC14, (b) CdC18, (c) CdC22 and (d) CdCio.


o 6
0

4
02










Besides the diol effect, it was found that using precursors that have longer carbon chains

can also improve the quality of CdSe nanocrystals. Figure 2-16 shows the absorption spectra of

CdSe nanocrystals made of four different cadmium precursors (0.1 mmol) reacted with SeO2

(0.05 mmol) and C16-diol (0.05 mmol) in ODE. The particles made of CdC22 have the best

quality, followed by those made of CdC18, and particles formed by CdC10 have poor spectra. So

we can conclude that the longer carbon chain in the cadmium precursors, the higher-quality

nanocrystals can be obtained. The absorption spectrum of CdSe nanocrystals made of CdC1i and

CdC22 exhibit multiple exiton peaks (Figure 2-17).

a b


1.5


R

S1.0
C

0
0/.
0.
< 0.5


300 400 500 600
wavelength (nm)


1.5


(0
R
S1.0
C
0
0/.
0.
<0.5


300 400 500 600
wavelength (nm)


Figure 2-17. Multiple exiton peaks. (a) CdC18, (b) CdC22.
















S20-


0
15-


0
' 10-



) 5-


a --------
I I I I I I I I I I
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
Diameter of CdSe nanocrystals (nm)


Figure 2-18. Effect of Cd precursor on the nuclei concentration during the CdSe synthesis.
(a) CdC14, (b) CdCi8 and (c) CdC22.


J..t


10 12 14 16 18 20
carbon atom number in precursor


22 24


Figure 2-19. Effect of Cd precursor on the CdSe particle size in the synthesis.


.A










The calculated nuclei concentrations of syntheses with different cadmium precursors are

shown in Figure 2-18. The more carbon atoms in the cadmium precursors, the higher nuclei

concentration will be achieved. Figure 2-19 illustrates the relationship between the final particle

size and the number of carbon atoms per cadmium precursor. When a cadmium precursor with

longer carbon chains is used, CdSe nanocrystals with smaller size will be generated. Growth

rates are shown in Figure 2-20. The longer the carbon chains in the precursor, the slower the

growth rate. The result is that cadmium precursor and selenium precursor have more comparable

reactivity and this may cause high-quality nanocrystals.

The relationship between the nuclei number and the particle growth rate is also shown

here. As has been discussed, the ratio of diol to SeO2 effect, the faster the particles growth, the

lower the nuclei number. Precursor with longer carbon chains causes slower growth rate and

higher nuclei number probably because its molecular size is larger and thus it is harder to get

activated. It takes a longer time to transfer to monomers and results in higher nuclei number.



14-1

12a
C U
(b) CdC and (c) C10C22.



6- U

4-
:2

0VC. 0on

4 6 8 10 12 14 16 18 20 22
CdSe particle size (nm3)



Figure 2-20. CdSe particle growth rate in the synthesis with different Cd precursors: a) CdC14,
(b) CdC18 and (c) CdC22.









2.3.3 Multiple-Addition Method

With the above method, the acceptable nanocrystals with the largest size we can get is 3.4

nm. It was found that with the multiple-addition method CdSe nanocrystals with a size of 4.5 nm

were generated, which has an absorption peak at 582 nm. In the experiment, cadmium myristate

(0.1 mmol), SeO2 (0.05 mmol), and C16-diol (0.05 mmol) were added into a three-neck flask with

5 g ODE. The mixture solution was degassed for 10 min under vacuum (-16 mTorr) at room

temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to

2650C (250C /min) with gentle stirring and let to react for 1 hour. The reaction solution was

cooled down to room temperature, and then cadmium myristate (0.025 mmol), SeO2 (0.0125

mmol), and C16-diol (0.0125 mmol) were added. After degassing, the solution was heated to

2650C again and let react for 1 hour. Lastly, the second addition step and condition was repeated

twice. Aliquots were taken out for UV and photoluminescence (PL) measurement.


a b
12

10




S6-


,-




400 500 600 700
wavelength (nm)

Figure 2-21. Characterization of CdSe nanocrystals during the multiple-addtion synthesis (a)
Temporal evolution of the absorption spectra multiple-addition synthesis. (b) TEM
image.









The temporal evolution of the absorption spectra during the CdSe synthesis is shown in

Figure 2-21(a). The TEM image of the final CdSe nanocrystals made by this method is shown in

Figure 2-21(b). It can be seen that the nanocrystals are uniformly spherical, and the size

distribution is good. The average size is around 4.5 nm.

2.4 Conclusion

In this part of work, it is showed that SeO2 can be used to replace selenium powder to

synthesize CdSe nanocrystals. With the presence of C16-diol, the quality of CdSe nanocrystals

can be improved. The effects of the C16-diol to SeO2 ratio were studied. It was found that the

more diol added, the higher-quality and smaller-size CdSe nanocrystals will be obtained. The

quality of our product is comparable to the best results published and nanoparticles smaller than

3 nm are even better. C16-diol can make the CdSe particle growth slower with the nuclei number

increasing at the same time. Compared the quality of CdSe nanocrystals made with C8-diol,

C10-diol and C16-diol, these diols have similar effect. Results with C18-OH and phenol were

worse than those with diols. Cadmium precursor effect was studied. The results show that the

longer the carbon chains in cadmium precursor, the smaller size particles will be synthesized.

Cadmium precursors with longer carbon chains can retard the growth rate and increase the nuclei

number. Multiple-addition method was performed and obtained CdSe nanocrystals with size of

around 4.5 nm. The best spectrum of different peak position were chosen from several

experiments and shown in Figure 2-22.













25-


20-




c15 -
C

0

10 -




5-




0-

400 500 600
wavelength (nm)

Figure 2-22. Temporal evolution of the absorption spectra of the as-prepared CdSe
nanocrystals. Black: made by CdC22+SeO2+C16-diol (0.1:0.05:0.05); Red:: made by
CdC18 +SeO2+C16-diol (0.1:0.05:0.05); Blue: made by CdC14 +SeO2+C16-diol
(0.1:0.1:0.05); Purple: CdC14 +SeO2+C16-diol (0.1:0.05:0.05); Green: motile-addtion
reaction, CdC14 +SeO2+C16-diol (0.1:0.05:0.05).









CHAPTER 3
SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM
DIOXIDE AS PRECURSOR

3.1 Introduction

In Chapter 2 it was that using Se02 to replace selenium element, CdSe nanocrystals can be

obtained. Experiments where Se02 was employed with gallium, lead, silver, copper and nickel

precursors were performed to synthesize metal selenide nanocrystals.

The Kelly group has done a lot of research on the GaSe nanoparticles, from synthesis to

physical properties.53-56 GaSe has a hexagonal layered structure57 consisting of Se-Ga-Ga-Se

sheets. GaSe is a semiconductor with indirect band gap58'59 having a 2.11 eV direct band gap.

Their GaSe synthesis is based on the reaction of an organometallic (GaMe3) with TOPSe in a

high-temperature solution of TOP and TOPO. The absorption spectra of GaSe nanoparticles have

an onset in the 400-500 nm region. The nanoparticle diameters range from 2 to 6 nm, with an

average size of about 4 nm. After chromatographic purification the average size is about 2.5

53
nm.

Lead selenide nanocrystals have been widely studied. The popular method is colloidal

synthesis method. In the Murray group, PbSe nanocrystals is synthesized by rapidly injecting a

lead oleate and TOPSe dissolved in trioctylphosphine into a well-stirred solution of dioctylether

at 150C.60 Temperature is tuned to control the size of the nanocrystals.

Ag2Se has two phases. The low-temperature phase (a-Ag2Se) is a narrow band-gap

semiconductor, and has been widely used as a photosensitizer in photographic flims to

thermochromic materials. P-Ag2Se is the high-temperature phase, and it is a superionic conductor

that is used a solid electrolyte in photochargable secondary batteries.61 These two phases are

reversible. There are only a few reports on the preparation of Ag2Se nanocrystals. Yi Xie et .al

synthesize Ag2Se nanocrystals at room temperature through the reaction of AgNO3, Se, and









KBH4 in pyridine.62 The Vittal group synthesized Ag2Se nanoparticles by thermolysis of silver

selenocarboxlyate in TOPO/TOP.61

CuSe is used in solar cells.63 The methods to synthesize CuSe nanocrystals include

thermolysis of Cu and Se powder mixtures64, the mechanical alloying of Se and Cu in a

high-energy ball mill, and the reaction of Se with Cu element in liquid ammonia65. The Vittal

group synthesized CuSe nanoparticles by thermolysis of copper selenocarboxlyate in

TOPO/TOP.66

NiSe is one of the typical Pauli paramagnets with metallic conductivity.67 These

stoichiometric compounds and the solid solutions between them now have been regarded as

typical materials for studies of the physical characteristics associated with a narrow band

electron system.68-70 Meanwhile, transition metal dichalcogenides have extensive applications in

energy areas such as electrochemistry and catalysis.71'72 The large surface areas and high activity

of nanomaterials will enhance their applications in these fields.73 Several methods have been

used to prepare NiSe nanocrystals, including elemental reactions74, organnometallic precursor

method65 and solvothermal processes76

3.2 Experimental Section

3.2.1 Materials

Gallium nitrate hydrate (Ga(N03)3-xH20, 99.999%), lead oxide (PbO, 99%), silver nitrate

(AgNO3, 99%), copper nitrate trihydrate (Cu(N03)2-3H20, 99 %), nickel nitrate hexahydrate

(Ni(N03)2-3H20, 99.999%), selenium dioxide (Se02, 99.9+%) ,1-octadecene (ODE, 90%),

trioctylphosphine oxide (TOPO, 90%), trioctylphosphine (TOP), 1,2-hexadecanediol (C16-diol,

90%), oleyamine (OAm, 70%), octadecylphosphonic acid (ODPA), tributylphosphine (TBP,

97%) were purchased form Aldrich. Sodium myristate (CH3(CH2)12COONa), sodium oleate

(CH3(CH2)7CH=CH(CH2)7COONa) were purchased from TCI. All chemicals except oleyamine









and TOPO were used without further purification. Methanol (99.9%), toluene (99.9%), acetone

(99.8%) were purchased from Fisher.

3.2.2 Instrumentation

Absorption spectra of aliquots were collected by a Shimadzu UV-1700 UV-Visible

Spectrophotometer. High resolution transmission electron microscopy (HR-TEM) images were

obtained using a JEOL 2010F microscope for lattice imaging and crystal size determination.

TEM samples were prepared by dispersing the nanocrystals in toluene and depositing them onto

formvar-coated copper grids.

3.2.3 Preparation of Precursors

3.2.3.1 Gallium myristate

0.6 g gallium nitrate was dissolved in 20 ml methanol, while sodium myristate solution

was prepared by dissolving 1.5 g sodium myristate in 100 ml methanol. Then the gallium

nitrate solution was dropped slowly into sodium myristate solution under magnetic stirring

conditions. The observed white precipitate was washed with methanol 4-5 times and dried under

vacuum overnight to remove all solvents.

3.2.3.2 Silver oleate

0.8 g silver nitrate was dissolved in 40 ml methanol, while sodium myristate solution was

prepared by dissolving 0.6 g sodium oleate in 100 ml methanol. Then the silver nitrate solution

was dropped slowly into sodium oleate solution under magnetic stirring conditions. The

observed white precipitate was washed with methanol 4-5 times and dried under vacuum

overnight to remove all solvents.

3.2.3.3 Copper oleate

1.2 g copper nitrate was dissolved in 30 ml methanol, while sodium myristate solution was

prepared by dissolving 1.2g sodium oleate in 150 ml methanol. Then the copper nitrate









solution was dropped slowly into sodium oleate solution under magnetic stirring conditions. The

observed blue precipitate was washed with methanol 4-5 times and dried under vacuum

overnight to remove all solvents.

3.2.3.4 Nickel oleate

0.6 g nickel nitrate was dissolved in 30 ml methanol, while sodium myristate solution was

prepared by dissolving 1.2g sodium oleate in 200 ml methanol. Then the nickel nitrate solution

was dropped slowly into sodium oleate solution under magnetic stirring conditions. Hexane was

added into extract the nickel oleate and collected into a flask. The nickel oleate hexane solution

was evaporated by a rotary evaporator. The green product was dried under vacuum overnight.

3.2.4 Preparation of Nanocrystals

3.2.4.1 Gallium selenide nanocrystals

Gallium myristate (0.1 mmol), SeO2 (0.05 mmol), and C16-diol (0.05 mmol) were added

into a three-neck flask with 5 g ODE. The mixture solution was degassed for 10 min under

vacuum (-16 mTorr) at room temperature, and then the vacuum was removed. Under an argon

flow, the solution was heated to 2850C with gentle stirring and let to react for 2 hours. The color

of the reaction solution changed from colorless to light yellow.

3.2.4.2 Lead selenide nanocrystals

Lead oxide (0.1 mmol), ODPA (0.2 mmol) were added into a three-neck flask with 3 mL

ODE. The mixture solution was degassed for 10 min under vacuum (-16 mTorr) at room

temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to

1600C with gentle stirring for 1 hour until PbO dissolved. The solution was cooled to 1200C and

degassed at this temperature to get rid of water. When the solution was cooled down to room

temperature, SeO2 (0.05 mmol), C16-diol (0.05 mmol) and 3.3 mL ODE was added. The mixture

was degassed for 10 min and then heat up to 180 C under argon flow.









3.2.4.3 Silver selenide nanocrystals

SeO2/TOP was obtained by dissolving SeO2 (0.1 mmol) in TOP (1.2 mmol, 0.54 mL).

Silver oleate (0.1 mmol) was added into a three-neck flask with 5 g purified TOPO. The

mixture solution was degassed for 10 min under vacuum (-16 mTorr) at room temperature, and

then the vacuum was removed. Under an argon flow, the solution was heated with gentle stirring.

When the temperature reached 1200C, 0.27 mL SeO2/TOP (0.05 mmol / 0.6 mmol) was quickly

injected to the solution and let react for 40 min.

3.2.4.4 Copper selenide nanocrystals

SeO2/TOP was obtained by dissolving SeO2 (0.1 mmol) in TOP (1.2 mmol, 0.54 mL).

Copper oleate (0.1 mmol) and 0.27 mL SeO2/TOP (0.05 mmol / 0.6 mmol) were added into a

three-neck flask with 5 g purified OAm. The mixture solution was degassed for 10 min under

vacuum (-16 mTorr) at room temperature, and then the vacuum was removed. Under an argon

flow, the solution was heated to 2200C with gentle stirring and allowed react for 90 min.

3.2.4.5 Nickel selenide nanocrystals

Nickel oleate (0.1 mmol) and SeO2 (0.1 mmol) were added into a three-neck flask with 5 g

purified OAm. The mixture solution was degassed for 10 min under vacuum (-16 mTorr) at

room temperature, and then the vacuum was removed. Under an argon flow, the solution was

heated with gentle stirring. TBP (1.2 mmol) was quickly injected to the solution when

temperature reached 1700C. The solution was kept at this temperature to react for 30 min.

3.2.5 Purification of Nanocrystals

Nanocrystals were purified by precipitation in excess acetone followed by centrifugation.

The supernatant contains molecular reaction byproducts and was discarded. The nanocrystals

were redispersed in toluene and centrifuged again. The well-capped nanocrystals remained

dispersed while poorly-capped nanocrystals settled. The precipitate was discarded. The










toluene-dispersed nanoparticles were reprecipitated in excess acetone. The supernatant was

discarded after centrifugation. The purified nanocrystals were redispersed in toluene.

3.3 Results and Discussion

3.3.1 Gallium Selenide

The time evolution of the absorption spectrum is shown in Figure 3-1. The spectrum of

GaSe nanocrystal at 30 minutes starts to have an onset at around 340 nm, which means that the

GaSe nanocrystals made by our method are smaller in size than those of Kelly's. This is proved

by the TEM image (Figure 3-2). The average size of our GaSe nanocrystals is around 2.0 nm. It

was found that the as-prepared GaSe nanocrystals have blue emission. We didn't collect the

emission spectra of the nanoparticles and this will be done later. The size is small probably

because there are too many nuclei. To get larger size particles, one should try to decrease the

nuclei number.



0.25


0.20- 120 min


0.15


o
a 0.10-
\30 min


0.05-
0 min

0.00 _
300 400 500 600
wavelength (nm)


Figure 3-1. Evolution of absorption spectrum of GaSe nanocrystals. Black: 0 min; pink: 30 min
and red: 2 h.




























Figure 3-2. TEM image of GaSe nanocrystals.

3.3.2 Lead Selenide

The PbSe nanocrystals cannot disperse in toluene, hexane, chloroform or other organic

solvents, which means that what was formed is aggregated PbSe nanocrystals. The TEM image

of PbSe nanocrystal is shown in Figure 3-3. It can be seen that the aggregated nanocrystals

consists of nanocubes whose edge is around 74 nm.


















50 nm

Figure 3-3. TEM image of PbSe nanocrystals.










3.3.3 Silver Selenide

The absorption spectrum of AgSe nanocrystals during the synthesis is shown in Figure 3-4.

At the beginning, there is an absorption peak at around 407 nm, which belongs to Ag

nanoparticles. The peak got weaker and weaker and disappeared at the reaction time of 40

minutes. This means that Ag nanoparticle were formed first, and then gradually reacted with

SeO2 to form AgSe. To prove this, X-ray diffraction patterns of samples taken at different

occasions should be obtained.


0.6





0.4



0
e-

o
0.2





0.0


10 min


400 500 600 700
wavelength (nm)


Figure 3-4. Evolution of absorption spectrum of AgSe nanocrystals.

The high resolution TEM images of AgSe nanocrystals made by our method are shown in

Figure 3-5. The shape of the nanocrystals is uniform, and they are all spherical. But the size is

not uniform, ranging from 4.3 nm to 12.2 nm, with an average size of 7.4 nm and the lattice

spacing is 0.21 nm. Size distribution should be improved.









b


U


HR-TEM images of AgSe nanocrystals. The lattice spacing in (b) is 0.21 nm.


TEM image of CuSe nanocrystals.


The TEM image of CuSe nanocrystals is shown in Figure 3-5. Interestingly, some hollow

nanocrystals were found but the size distribution is large and the shape is not uniform. To get


10~


Figure 3-5.


3.3.4 Copper Selenide


S50 nn'-


Figure 3-6.


4C

AL
"far


-W


U









more uniform nanocrystals, one may try to anneal the precursors at 160C for longer time first to

generate more active monomers.

3.3.5 Nickel Selenide

Needle-shape NiSe nanocrystals were obtained. The TEM image of NiSe nanocrystals is

shown in Figure 3-6. Similar to AgSe and CuSe, the problem is that non-uniform shape and size

NiSe nanocrystals were formed.






















Figure 3-7. TEM image of NiSe nanocrystals.

3.4 Conclusion

In this part of the work, experiments to prepare GaSe, PbSe, AgSe, CuSe and NiSe

nanocrystal were performed. For GaSe, spherical nanocrystals were obtained and the average

size is around 2.0 nm. The product has blue emission. To get larger size GaSe nanocrystals, one

should try to decrease the nuclei number. For PbSe, the nanocrystals aggregated. But the

aggregates consist of nanocubes that are uniformly in size and shape. To avoid aggregation, one

might try to anneal the precursors for a longer time to get more active monomers before heating









to the reaction temperature. For AgSe, spherical nanocrystals were obtained but the size

distribution is poor. The diameters range from 4 nm to 12 nm. The high resolution TEM image

showed that the crystalline AgSe nanoparticles have a lattice spacing of 0.21 nm. The absorption

spectrum of the nanocrystals shows that Ag nanoparticles were formed at the beginning and then

gradually reacted with SeO2 to form AgSe nanocrystals. As for CuSe and NiSe, hollow

nanocrystals and needle-shape nanocrystals were got, respectively. But their size and shape

distribution are still poor. To improve the quality, one can try to generate active monomers first

before the reaction.









CHAPTER 4
SUMMARY AND FUTURE WORK

4.1 Summary

It has been demonstrated that SeO2 can be used to replace selenium element to synthesize

metal selenide semiconductor nanocrystals. For CdSe, one-pot synthetic method was used. It is

found that when equal amount of C16-diol as SeO2 was added, the quality of CdSe nanocrystals

can be improved. Effect of the ratios of C16-diol to SeO2 was studied and the result shows that

the higher the ratio of C16-diol as SeO2, the better nanocrystals can be obtained, and the higher

nuclei number and the slower growth. Experiments using different cadmium precursors were

performed and it was found that the longer the carbon chains in cadmium precursor, the better

the quality of CdSe nanocrystals were got, and the higher the nuclei number and the slower the

growth. Multiple-addition reaction was employed to prepare larger size nanocrystals.

It was also proved that using SeO2 instead of selenium element, GaSe, PbSe, AgSe, CuSe

and NiSe nanocrystals were obtained. GaSe nanocrystals were uniform in size and shape, but the

size is small. PbSe nanocube aggregates were obtained, and each nanocube is uniform in size and

shape. Crystalline AgSe nanoparticles were obtained with an average size of 7.4 nm and a lattice

spacing of 0.21 nm. Uniform CuSe and NiSe nanoparticles have not been formed yet.

4.2 Future work

4.2.1 Injection-Synthetic Method for CdSe

So far the largest acceptable CdSe nanocrystals obtained by our method have a diameter of

around 4.5 nm. To get larger high-quality CdSe nanocrystals, injection method can be employed.

By quickly injecting precursors at high temperature, fewer nuclei will be formed in a very short

time, resulting in larger, more uniform CdSe nanocrystals.









Cadmium myristate (0.1 mmol) will be added into a three-neck flask with 4.3 g ODE.

SeO2 (0.1 mmol) and C16-diol (0.1 mmol) will be added in 2 mL ODE. The two mixture solutions

will be degassed for 10 min under vacuum (-16 mTorr) at room temperature, and then the vacuum

will be removed. Under an argon flow, the SeO2 and C16-diol solution will be heated to 100C

and SeO2 and C16-diol will dissolve and form yellow solution. Under an argon flow, the cadmium

solution will be heated with gentle stirring. When the temperature reaches 2650C, 1 mL of SeO2

(0.05 mmol) and C16-diol (0.05 mmol) ODE solution will be quickly injected to the cadmium

solution. The temperature will keep at 2650C.

4.2.2 Improvement of Other Metal Selenide Nanocrystals

For GaSe, PbSe, AgSe, NiSe and CuSe, acceptable results haven't been obtained yet. One

can try to generate active monomers before heating to the reaction temperatures for PbSe, NiSe

and CuSe. Injection method and lower reaction temperature may be used for GaSe to get larger

size nanoparticles. X-ray diffraction patterns should be got to identify the crystal structures of

these nanocrystals.

4.2.3 Mechanism Study

The mechanism of SeO2 reacting with C16-diol can also be studied to understand how

C16-diol improve the quality of CdSe nanocrystals using 1H, 13C, and 31P NMR spectroscopy and

mass spectrometry to confirm our hypothesis.









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BIOGRAPHICAL SKETCH

Xian Chen was born in Xiamen, a beautiful city on the southeast coast of China. In 1999,

she started her college life at the University of Science and Technology of China (USTC). After

5 years of study in the Department of Polymer Science and Engineering, she received her

bachelor's degree in engineering in 2004. Then, she joined the Department of Chemistry at the

University of Florida. She would like to pursue a Ph.D. degree after graduation.





PAGE 1

1 SYNTHESIS OF METAL SELENIDE SE MICONDUCTOR NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR By XIAN CHEN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007

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2 2007 Xian Chen

PAGE 3

3 To my parents

PAGE 4

4 ACKNOWLEDGMENTS Above all, I would like to thank my parents for what they have done for me through these years. I would not have been ab le to get to where I am today without their love and support. I would like to thank my advisor, Dr. Charle s Cao, for his advice on my research and life and for the valuable help during my difficult ti mes. I also would like to thank Dr. Yongan Yang for his kindness and helpful discussion. I learne d experiment techniques, knowledge, how to do research and so on from him. I also appreciate the help and friendship that the whole Cao group gave me. Finally, I would like to express my gratitude to Dr. Ben Smit h for his guidance and help.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF FIGURES................................................................................................................ .........7 ABSTRACT....................................................................................................................... ..............9 CHAPTER 1 SEMICONDUCTOR NANOCRYSTALS............................................................................11 1.1 Introduction..................................................................................................................11 1.2 General Synthetic Methods for Nanocrystals..............................................................11 1.2.1 Injection-Based Synthetic Method.....................................................................13 1.2.2 One-Pot Synthetic Method.................................................................................14 1.3 Applications of Semi conductor Nanocrystals..............................................................16 1.3.1 Biological Detection..........................................................................................16 1.3.2 Hybrid Electroluminenscent Device..................................................................17 1.3.3 Photovoltaic Device...........................................................................................18 2 SYNTHESIS OF CADMIUN SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR.................................................................................................19 2.1 Introduction..................................................................................................................19 2.2 Experimental Section...................................................................................................20 2.2.1 Materials............................................................................................................20 2.2.2 Instrumentation..................................................................................................20 2.2.3 Preparation of Cd-Precursors.............................................................................22 2.2.3.1 Cadmium myristate (CdC14)....................................................................22 2.2.3.2 Cadmium stearate (CdC18)......................................................................22 2.2.3.3 Cadmium docosanate (CdC22).................................................................22 2.2.4 Preparation of CdSe Nanocrystals.....................................................................23 2.3 Results and Discussion................................................................................................23 2.3.1 Diol Effect..........................................................................................................23 2.3.2 Precursor Effect.................................................................................................32 2.3.3 Multiple-Addition Method.................................................................................36 2.4 Conclusion...................................................................................................................37 3 SYNTHESIS OF METAL SELENIDE NANOCRY STALS USING SELENIUM DIOXIDE AS PRECURSOR.................................................................................................39 3.1 Introduction..................................................................................................................39 3.2 Experimental Section...................................................................................................40 3.2.1 Materials............................................................................................................40 3.2.2 Instrumentation..................................................................................................41

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6 3.2.3 Preparation of Precursors...................................................................................41 3.2.3.1 Gallium myristate....................................................................................41 3.2.3.2 Silver oleate.............................................................................................41 3.2.3.3 Copper oleate...........................................................................................41 3.2.3.4 Nickel oleate............................................................................................42 3.2.4 Preparation of Nanocrystals...............................................................................42 3.2.4.1 Gallium selenide nanocrystals.................................................................42 3.2.4.2 Lead selenide nanocrystals......................................................................42 3.2.4.3 Silver selenide nanocrystals....................................................................43 3.2.4.4 Copper selenide nanocrystals..................................................................43 3.2.4.5 Nickel selenide nanocrystals...................................................................43 3.2.5 Purification of Nanocrystals..............................................................................43 3.3 Results and Discussion................................................................................................44 3.4 Conclusion...................................................................................................................48 4 SUMMARY AND FUTURE WORK...................................................................................50 4.1 Summary......................................................................................................................50 4.2 Future work..................................................................................................................50 4.2.1 Injection-Synthetic Method for CdSe................................................................50 4.2.2 Improvement of Other Metal Selenide Nanocrystals.........................................51 4.2.3 Mechanism Study...............................................................................................51 LIST OF REFERENCES............................................................................................................. ..52 BIOGRAPHICAL SKETCH.........................................................................................................56

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7 LIST OF FIGURES Figure page 1-1 Scheme of the formation of nanocrystals...........................................................................12 1-2 LaMer Curve.............................................................................................................. .........13 1-3 Representation of the synthetic appara tus employed in the injection-based method.........14 1-4 Absorption spectrum of CdS nanocrystals (d = 3.5 nm)....................................................15 1-5 Representation of the synthetic appara tus employed in the one-pot synthetic method......16 2-1 Sizing curve of CdSe nanocrystals.....................................................................................19 2-2 HWHF and peak sharpness used for size distribution determination.................................20 2-3 Schematic diagram of a UV-Vis microscope.....................................................................21 2-4 Schematic diagram of a Fluorol og-3 Model FL3-12 spectrofluorometer..........................21 2-5 Molecular structures of organic solvents used...................................................................23 2-6 Temporal evolution of the absorp tion spectra during the CdSe synthesis.........................24 2-7 Characterization of CdSe nanocrystals synthesized in ODE w ith reaction time of 40 minutes........................................................................................................................ .......25 2-8 Temporal evolution of CdSe nanocryst al concentration synthesized in ODE with different C16-diol/SeO2 ratios............................................................................................26 2-9 CdSe particle growth rate in the synthesis with different C16-diol/SeO2 ratios................27 2-10 CdSe particle size and normalized nucle i number in the synthesis with different C16-diol/SeO2 ratios..........................................................................................................28 2-11 Temporal evolution of the absorpti on spectra during the CdSe synthesis with different diols................................................................................................................ .....29 2-12 HWHM of CdSe during synt hesis with different diols......................................................29 2-13 Temporal evolution of CdSe nanocryst al concentration with different diols.....................30 2-14 CdSe particle size in the synthesis with different numbers of carbon atom per diol.........30 2-15 Temporal evolution of the absorpti on spectra during the CdSe synthesis with different alcohols............................................................................................................. ..31

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8 2-16 Temporal evolution of the absorpti on spectra during the CdSe synthesis with different Cd precursors......................................................................................................32 2-17 Multiple exiton peaks.................................................................................................... .....33 2-18 Effect of Cd precursor on the nuclei concentration during the CdSe synthesis.................34 2-19 Effect of Cd precursor on the Cd Se particle size in the synthesis......................................34 2-20 CdSe particle growth rate in the synthesis with different Cd precursors...........................35 2-21 Characterization of CdSe nanocrysta ls during the multiple-addtion synthesis..................36 2-22 Temporal evolution of the absorption spec tra of the as-prepared CdSe nanocrystals........38 3-1 Evolution of absorption sp ectrum of GaSe nanocrystals....................................................44 3-2 TEM image of GaSe nanocrystals......................................................................................45 3-3 TEM image of PbSe nanocrystals......................................................................................45 3-4 Evolution of absorption sp ectrum of AgSe nanocrystals...................................................46 3-5 HR-TEM images of AgSe nanocrystals.............................................................................47 3-6 TEM image of CuSe nanocrystals......................................................................................47 3-7 TEM image of NiSe nanocrystals.......................................................................................48

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9 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science SYNTHESIS OF METAL SELENIDE SE MICONDUCTOR NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR By Xian Chen August 2007 Chair: Y. Charles Cao Major: Chemistry Nanotechnology has been one of the most popular research areas in th ese two decades. The semiconductor nanocrystals, which are also called quantum dots, are of the great interest because of their unique size-dependent properties. The na nomaterials have wide applications, including light emitting diodes, solar cells, biological labeling, and so on. The critical part in the use of quantum dots is to prepare monodispersed nanocr ystals. The methods to synthesize high-qulity nanocrystals have been well developed. Selenium element was used in most method for synthesizing high quality metal selenide nanocrystals. However selenium element is t oxic and unstable in the air, thus requires complicated operations. Herein, we developed a new approach fo r using selenium dioxide to replace the selenium element. Selenium dioxide is very stable and nontoxic. It is found that when adding of 1,2-hexadecanediol (C16-diol) the quality of na nocrystals can be improved. Experiments were carried out to test th e results of using different amount of C16-diol. It turns out that adding more C16-diol can result in smaller size nanocrystals, higher nuclei number, and slower growth rate. Different cadmium precursor s were used and the results show that with longer carbon chains in cadmium precursor, sm aller size CdSe nanocrystals can be obtained.

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10 CdSe nanocrystals with diameters of 4.5 nm by carrying out multiple-addition experiment were generated. SeO2 was also employed to prepare other metal selenide nanocrystals. GaSe nanocrystals with diameters of 2.0 nm were formed. AgSe na nocrystals with diameters of around 7.4 nm and a lattice spacing of 0.21 nm were obtained. Th e absorption spectrum s hows that during the formation of AgSe nanocrystals, Ag nanocrystals were formed first and then gradually reacted with SeO2 to form AgSe nanoparticles. PbSe aggreg ates consisting of uniform nanocubes were observed.

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11 CHAPTER 1 SEMIDONDUCTOR NANOCRYSTALS 1.1 Introduction In the last two decades, there has been an increasing progress in the synthesis and characterization of semiconductor nanocrystals. Th ey are of great interest for both fundamental research and industrial development because of th eir unique properties. In the nanometer range, the properties of semiconductor na nocrystals are strongly dependent upon their size, shape, and crystal structure, which make them differ subs tantially from the corr esponding molecular and bulk materials.1,2 Thus controlling the physic al size of materials can be used to tune materials properties. These novel properties lead to many applications such as light emitting diodes (LEDs), biological fluorescent la bels, lasers and solar cells.3-17 Efforts to explore structures on the nanometer scale combine the material science, chemistry, physics and engineering. Studying size-dependent mate rials properties requires synthetic routes to prepare homologous size seri es of monodisperse nanometer size crystals.18 1.2 General Synthetic Methods for Nanocrystals Synthesis of high-quality semiconductor nanocr ystals is the key element for studying the size-dependent properties in the na nometer scale. This has been a very active area of research. Colloidal methods are of most interested beca use the optical and elec trical properties of semiconductor nanocrystals made by these methods can be tuned by changing the physical size of the nanocrystals. Synthesis of high-quality colloi dal nanocrystals have been reported by several groups. The research group of Alivisat os and Bawendi develo ped methods of using molecular precursors.6,19 In early 1990s, Cd(CH3)2 as precursor and technical-grade trioctylphosphine oxide (Tech TOPO) as the reaction solvent were used to synthesize high-quality CdSe nanocrystals.6 But Cd(CH3)2 is extremely toxic, expensive and unstable, and

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12 this synthesis is not very re producible. Since 2001, CdO, CdCO3 and Cd(OOCCH3)2 precursors with functionalized organic ligands have been used to replace Cd(CH3)2 precursor, for a greener approach and noncoordina ting solvents, such as 1-octade cene, were used to replace TOPO.20-23 This thermal decomposition method has also been extended to the synthesis of ZnS and ZnSe nanocrystals.24 In a typical colloidal synthesis there are th ree components: precur sors, surfactants and solvents. In some cases, solvents also serve as surfactants. When the system is heated to a sufficiently high temperature, the precursors chemi cally transfer to active atoms or molecules, which are called monomers. The monomers then fo rm nanocrystals whose subsequent growth is greatly affected by the presence of surfactants. The formation of the nanocrystals involves two steps: nucleation of an initial seed and growth In the nucleation step, precursors decompose at a high temperature to form a supersaturation of monomers followed by a burst of nucleation of nanocrystals. These nuclei then grow by incorporating additional monomers still present in the reaction solution.25 The scheme of the formation of na nocrytals is shown in Figure 1-1. Figure 1-1. Scheme of the formation of nanocrystals.

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13 1.2.1 Injection-based Synthetic Method In colloidal synthesis, chemis ts developed a method to separate the nucleation stage from the nanocrystals growth stage6-8 as described by LaMer Curve (F igure 1-2). Rapid injection of metal-organic precursors into a vigorously stirred flas k containing a hot coordinating solvent can form the supersaturation and subsequent nuclea tion. A short nucleation burst partially relieves the supersaturation. As long as the consump tion of feedstock by the growing colloidal nanocrystals is faster than the rate of pr ecursor addition to solution, no new nuclei form.18 Growth rate can be controlled by diffusion rate an d/or reaction rate. Finally, the growth will be balanced by the solubility. Figure 1-2. LaMer Curve. Figure1-3 illustrates a synthetic apparatus employed in the inject ion-based method. This method has led to synthesis of a variety of high-quality nanocrystals ranging from II-VI (e.g., CdS and CdSe) and III-V (e.g., InP and InAs) to IV-VI (e.g., PbS and PbSe) semiconductors.6,7,20,26-30

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14 Figure 1-3. Representation of the synthetic ap paratus employed in the injection-based method. However, the injection-based synthetic method is not suitable for large-scale, industrial preparation. It is very difficult to inject precursors ra pidly because industria l preparation may use hundreds of kilograms of precursors. 27 In the laboratory, nucleation time is determined by the rate of the injection and the mass transfer in th e reaction systems, and the temperature is very hard to control. So this injection based synthesis method is not ideal for mechanistic mechanism studies that require a highly repr oducible system for quantitative measurement. Therefore, methods that do not require the injection of precursors have to be developed. 1.2.2 One-Pot Synthetic Method Several groups have reported the one-pot s ynthesis of semiconductor nanocrystals without the injection of precursors. However, the quality of their nanocrystals was not comparable to that of the nanocrystals made by the injection-base d method. Typically, they do not exhibit as many multiple-exciton absorption peaks, 31-34 while high-quality nanocrystals with multiple exciton-absorption peaks are critical for the ap plications in advanced optical and electronic devices.35 Recently, the Cao group has developed a new non-injection synthesis for making CdS, CdSe and CdTe nanocrystals.27,28 The quality of the nanocrystal s made by this new synthesis is

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15 at least comparable to the best particles made by injection-based methods .Without size-selective separation, the nanocrystals formed by this new synthesis exhibit up to four exciton-absorption peaks, indicating their very narrow size distribu tion and excellent optical properties (Figure 1-4). The set-up for the one-pot synthe sis is shown in Figure 1-5. Figure 1-4. Absorption spectrum of CdS nanocrystals (d = 3.5 nm). The one-pot synthetic method is based on a new concept of controlling the thermodynamics and kinetics of th e nanocrystal nucleation stage.27 The precursors are thermal decomposed when heat up to a sufficiently high temperature, so more and more monomers are produced as time passes, when the concentration of monomers increased to supersaturation, nucleation happens. As the monomer concen tration drops lower than the nucleation concentration, the nucleation stops, followe d by the nuclei formed growth. When the concentration of monomer drops to saturation concentrati on, the particle s stop growing.

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16 Figure 1-5. Representation of the synthetic apparatus employed in the one-pot synthetic method. Although compared to the inje ction method, one-pot synthesi s has the advantages of reproducibility, capability of large-scale and industrial prepar ation, when dealing with different materials core-shell nanocrystals and doped nanocry stals, the injection me thod is the only choice. The one-pot synthesis can only be employed when same material core-shell nanocrystals are desired. 1.3 Applications of Semiconductor Nanocrystals 1.3.1 Biological Detection In 1998, both Alivisatos group16 and Nie17 group first reported the use of colloidal quantum dots for biological labeling. They suggested that due to the photochemi cal stability and the tubable luminescence of the quantum dots, they may make these materials extremely useful for biolabeling. Compared to regular organic dyes, quantum dots have the advantages of tunable luminescence, high quantum yield, broad light absorption, narrow emission spectra and high stability. Since 2002, there has been development of a wide range of methods for bio-conjugating

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17 colloidal quantum dots36-40 in diverse areas of ap plication: cell labeling41, cell tracking42, in vivo imaging43, DNA detection44, and multiplexed beads45. Colloidal quantum dots with a wide range of bio-conjugation and with high quantum yields are now available commercially, so that it is no longer necessary for each experimenter to grow their own or to become lost in the myriad discus sions concerning the best way to render colloidal dots water soluble and bio-compatible.46 1.3.2 Hybrid Electroluminenscent Device A light-emitting diode (LED) is a semic onductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This effect is a form of electroluminescence. An LED is a small extended source with extra optics added to the chip that makes it emit a complex radiation pattern.46 Since the first observation of light emission from organi c materials by Tang et al.48, continuous and rapid improvement in device performance have enabled organic light emitting devices (OLEDs) to compete with existing technologies. However, there are stil l many problems to be overcome, such as improving device stability and color purity. Full width at half maximum (FWHM) of photoluminescence of colloidal semiconductor na nocrystals is about 30nm which is narrower than those from organic materials. Moreover, th ese inorganic nanocrystals are much more stable and robust than organic molecules. So hybrid OLEDs using semiconductor nanocrystals as an emission layer have to been to have good stability and efficienc y. The first demonstration of a hybrid OLED was reported by Colvin et al in 1994.49 In order to enhance the quantum efficiency of hybrid OLED devices, several problems must be solved including more efficient charge transfer between the organic layer and nanocry stals, the imbalance of injected conduction through nanocrystals, a high densit y of pinhole defects in the nanoc rystal layer, uniformity of nanocrystals in the deposited layer, and optimization of in terlayer structure of device.3,50

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18 1.3.3 Photovoltaic Device Inorganic solar cells that have limitations due to the high costs of fabrication have power conversion efficiencies of 10% While organic solar cells th at use polymers which can be processed from solution have been investigated as a low-cost alternative have solar power efficiencies of up to 2.5%.51 One way to overcome these limitai ons is to combine polymers with inorganic semiconductors Because of the nanoscale nature of light absorption and photocurrent generation in solar energy conversion, the adve nt of methods for controlling inorganic materials on the nanometer scale opens new opportunities for the development of future generation solar cells. Alivisatos group used colloidal semiconducto r nanorods as the inorganic phase in the construction of these solar cells via solution-phase nano-assembly. By varying the radius of the rods, the quantum size effect can be used to control the band gap; furthermore, quantum confinement leads to an enhancem ent of the absorption coefficien t compared with the bulk, such that devise can be made thinner. One-dimensional (1D) nanorods are preferable to quantum dots or sintered nanocrystals in solar energy conversion, because they naturally provide a directed path for electrical transport. The length of the nanorods can be adjusted to the device thickness required for optimal absorption of incident light.5

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19 CHAPTER 2 SYNTHESIS OF CADMIUM SELENIDE NANOCRYSTALS US ING SELENIUM DIOXIDE AS PRECURSOR 2.1 Introduction In chapter 1, we have mentioned th at CdO was used to replace Cd(CH3)2 in the molecular precursor synthesis because Cd(CH3)2 is extremely toxic, unstabl e and expensive. Actually, selenium element has the same problem. Se is unstabl e in the air, and it is toxic. Here, we present a method of synthesizing Cd Se nanocrystals using SeO2 to replace Se element. We found that with the presence of C16-diol and using cadmium precurs ors with longer carbon chain, the quality of the CdSe quantum dots formed by this me thod is comparable to that of the best CdSe nanocrystals reported in the literature, and some nanoc rystals are even better. To characterize the nanocrystals, the diameter s of the nanocrystals we re calculated from the wavelength of first exciton peak using the CdSe sizing curve (Figure 2-1). The size distribution of the particle was evaluated by m easuring the HWHM of fi rst exciton peak, which is a well accepted method to estima te nanocrystals size distribution.52 In our previous work, we also found that peak sharpness is also a way to evaluate the size distribution (Figure 2-2). Fluorometer and TEM was also employed. Figure 2-1. Sizing curve of CdSe nanocrystals

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20 Figure 2-2. HWHM and peak sharpness us ed for size distribution determination. 2.2 Experimental Section 2.2.1 Materials 1-octadecene (ODE, 90%), 1-tetradecene, (TDE 92%), squalene (98%), dioctyl ether (99%), docosanoic acid (CH3(CH2)20COOH, 99%), selenium dioxide (SeO2, 99.9+%) and 1,2-hexadecanediol (C16-diol, 90%), 1,2-decanediol (C10-diol, 98%) 1,2-octanediol (C8-diol, 98%), 1-octadecanol (C18-OH, 99%), and phenol were purchas ed from Sigma-Alrich. Methanol (99.9%), toluene (99.9%), acetone (99.8%) were purchased from Fisher Sodium myristate (CH3(CH2)12COONa), sodium stearate (CH3(CH2)16COONa) were purchased from TCI. Cadmium nitrate tetrahydrate (Cd(NO3)2H2O) was purchased from Alfa Aesar. Tetrabutylammonium hydroxide (1M in methanol ) was purchased from Acros. All chemicals were used without further purification. 2.2.2 Instrumentation Absorption spectra of aliquots were co llected by a Shimadzu UV-1700 UV-Visible Spectrophotometer (Figure 2-3). Th e wavelength, absorption and the half width at half maximum

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21 (HWHM) of first exciton peak for each aliquot were recorded. This method also was compared with our previous one-pot synthesis me thod and other current injection method. Figure 2-3. Schematic diagram of a UV-Vis microscope. Photoluminescence (PL) was measured at room temperature from nanocrystals suspended in toluene using a JOBIN YVON HORIBA Fl uorolog-3 Model FL3-12 spectrofluorometer (Figure 2-4). Figure 2-4. Schematic diagram of a Fl uorolog-3 Model FL3-12 spectrofluorometer.

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22 High resolution transmission electron micros copy (HR-TEM) images were obtained using a JEOL 2010F microscope for lattice imaging and crystal size determina tion. TEM samples were prepared by dispersing the nanocrystals in to luene and depositing them onto formvar-coated copper grids. 2.2.3 Preparation of Cd-Precursors 2.2.3.1 Cadmium myristate (CdC14) 1.5g cadmium nitrate was dissolved in 75ml me thanol, while sodium myristate solution was prepared by dissolving 3.8g sodium myrist ate in 550ml methanol. Then the cadmium nitrate solution was dropped slowly into sodi um myristate solution under magnetic stirring conditions. The observed white preci pitate was washed with metha nol 4-5 times to get rid of impurities and dried under vacuum overnight to remove all solvents. 2.2.3.2 Cadmium stearate (CdC18) 0.3g Cadmium nitrate was dissolved in 40ml methanol, Sodium stearate solution was prepared by dissolving 0.90g sodi um stearate in 600ml methanol Then the cadmium nitrate solution was dropped slowly with into sodium stearate solution under magnetic stirring conditions. The observed white pr ecipitate was washed with me thanol 2-3 times get rid of impurities then put back in flask with 600 ml methanol and ultrasonicated. Wash the product again and dried under vacuum overnight to remove all solvents. 2.2.3.3 Cadmium docosanate (CdC22) 0.3g Cadmium nitrate was dissolved in 40ml methanol. Sodium docosanate solution was prepared by dissolving 0.90g docos anoic acid in 600ml methanol and slowly adding 1.1 ml tetrabutylammonium hydroxide (1 M in methanol) dropwise. Then the cadmium nitrate solution was dropped slowly into sodium docosanoicate solution under magnetic s tirring conditions. The observed white precipitate was washed with methanol 2-3 times get rid of impurities then put

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23 back in flask with 600 ml methanol and ultr asonicated. Wash the product again and dried under vacuum overnight to remove all solvents. 2.2.4 Preparation of CdSe Nanocrystals Cadmium precursor (0.1mmol), SeO2 (0.05mmol), C16-diol (0.05mmol) and non coordinating solvent (5g) were mixed in a thr ee-neck flask equipped with condenser, magnetic stirrer, thermocouple, and heating mantle (as s hown in Figure 1-5), degassed before heated to 265 oC with gentle stirring under vacuum to s ynthesize CdSe nanocrstals. Aliquots of the solution for each reaction were taken quantitatively with a syringe at different time intervals, and quickly cooled and diluted in toluene to stop fu rther growth. These aliquots were employed to monitor the reaction via UV-Vis and photoluminescence measurement. 2.3 Results and Discussion 2.3.1 Diol Effect C16-diol Effect on the Quality and Size of CdSe Nanocrystals ODE TDE Squalene Octyl ether Figure 2-5. Molecular structures of organic solvents used. CdSe nanocrystals were formed using SeO2 compound instead of Se element in the solvent of ODE, which means that SeO2 is active at high temperature. However, the quality is not as

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24 good. We found that for CdSe nanocrystals synt hesized in ODE, addition of equal molar amounts of C16-dio land SeO2 has several effects on the grow th, including growth rates, HWHMs, sharpness, optical densit ies, and final sizes. This phenomenon was also observed in CdSe nanocrystals synthesized in TDE, squalene, and octyl ether. The molecular structures of these four solvents ar e shown in Figure 2-5. Figure 2-6 shows the absorption spectra of CdSe nanocrystals made of 0.1 mmol CdC14, 0.05 mmol SeO2 and 0.05 mmol C16-diol in different solvents. Figure 2-6. Temporal evolution of the absorption spectra during the CdSe synthesis in (a) and (e) ODE, (b) and (f) squalene, (c) and (g)octyl ether and (d) and (h)TDE; in syntheses (a), (b), (c) and (d), C16-di ol was not added while in synt heses (e), (f), (g) and (h), C16-diol was added. In Figure 2-7, we show the absorption and photoluminescent (PL) spectra and TEM image of CdSe nanocrystals which were made in ODE a nd have a reaction time of 40 minutes. It can be 400500600700 0 2 4 6 8 0 2 4 6 8 e ) ODE, C16-diol added a ) ODE, no C16-diol30min 10min 5 min 240oC 220oC 0 min Absorbance (a.u.)wavelength (nm)220oC 30min 10min 5 min 0 min 240oC Absorbance (a.u.)400500600700 0 2 4 6 8 0 2 4 6 8 30min 10min 5 min 0 min wavelength (nm)h ) TDE, C16-diol added d ) TDE, no C16-diol0 min 30min 10min 5 min 400500600700 0 2 4 6 8 10 0 2 4 6 8 10 30min 10min 5 min 0 min wavelength (nm)g ) octyl ether, C16-diol added c ) octyl ether, no C16-diol30min 10min 5 min 0 min 400500600700 0 2 4 6 8 10 0 2 4 6 8 10 30min 10min 5 min 0 min 250oC wavelength (nm)f ) squalene, C16-diol added b ) squalene, no C16-diol240oC 220oC 30min 10min 5 min 0 min 220oC

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25 seen that the sample is nearly monodispersed. The average diameter is 3.3 nm, which is very close to the calculated diameter of 3.4 nm. The CdSe nanocrystals formed with the presence of C16-diol have much better quality than those formed without adding C16-diol. With the presence of C16-diol, the first peak is narrower and deeper than that without the presence of C16-diol, which indicates that size distribution is better. Thus one can co nclude that adding C16-diol can improve the qualit y of CdSe nanocrystals. It was also observed that the final sizes of Cd Se nanocrystals are smaller and the ODs are higher in the case of adding C16-diol. To better understand the role that C16-diol assumes in the synthesis, three syntheses with different amounts of C16-diol were performed. Nuclei concentration, nuclei number, growth rate as we ll as peak sharpness and HWHM were employed to analyze the data. 500550600650 0.0 0.2 0.4 0.6 0.8 1.0 a IntensityWavelength (nm) b Figure 2-7. Characterization of CdSe nanocry stals synthesized in ODE with reaction time of 40 minutes. (a) Absorption (in blue) and phot oluminescent (PL) (in red) spectra and (b) TEM image.

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26 Nuclei concentrations are calculated by using the exciton energy to determine the particle radius and then the extinction coefficient for ea ch size to determine the particle concentration.52 57 41 4277 0 10 6242 1 10 6575 2 10 6122 1 :2 3 3 6 4 9 D CdSe (2-1) In the above equation, D (nm) is the diameter of a given nanocrystal sample, and (nm) is the wavelength of the first excitonic absorp tion peak of the co rresponding sample. The extinction coefficient of CdSe is calculated as 65 2) ( 5857 D (2-2) Then using the Lambert-Beers law, CL A (2-3) The molar concentration C (mol/L) of the nanocrystals of the sample can be calculated. A is the absorbance at the peak for a given sample and L is the path length (cm). In our experiments, L was fixed at 1 cm. 2.02.22.42.62.83.03.23.43.63.8 1 2 3 4 5 6 7 8 9 10 c a bCdSe nuclei concentration (10-6M)size of CdSe na nocrystals (nm) Figure 2-8. Temporal evolution of CdSe nanocry stal concentration synthesized in ODE with different C16-diol/SeO2 ratios, (a) C16-diol/SeO2=0, (b) C16-diol/SeO2=1, and (c) C16-diol/SeO2=2.

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27 The calculated temporal evoluti on of concentrations and growth rates of CdSe nanocrystals made in ODE with different C16-diol/SeO2 ratio is shown in Fi gure 2-8 and Figure 2-9, respectively. Figure 2-8 shows that the more C1 6-diol in the reaction solution, the higher the CdSe nuclei concentration and the concentration dramatically increases when the C16-diol to SeO2 ratio changes from 1 to 2. It can be seen in Figure 2-9 that the higher the ratio of C16-diol to SeO2, the slower the growth rate. 24681012141618202224262830 0 2 4 6 8 24681012141618202224262830 0 2 4 6 8 24681012141618202224262830 0 2 4 6 8 c b CdSe particle grpwth rate (nm3/min)CdSe particle size (nm3) a Figure 2-9. CdSe particle growth rate in the synthesis with different C16-diol/SeO2 ratios (a) C16-diol/SeO2=0, (b) C16-diol/SeO2=1, and (c) C16-diol/SeO2=2. It was found that the C16-diol affects not only the quality and nuclei number of CdSe nanocrystals, but also the final CdSe particle sizes, as shown in Figure 2-10. The more the C16-diol in the reaction solution, the smalle r the CdSe nanocrystal s will be obtained. Addition of C16-diol can slow the CdSe part icle growth remarkably with the nuclei number increasing at the same time. It can be concluded that nuclei number is related the particle

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28 growth rate. The faster the particles growth, th e lower the nulclei number. This leads us to a hypothesis that C16-diol acts as a reducing agent in the reaction, helping reduce the selenium in SeO2 from Se+4 to Se -2, helping increase the concentration of selenium monomers. This leads to easier nucleation and a higher nuclei number and thus, a smaller size. 0.00.51.01.52.0 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Size of CdSe nanocrystals (nm)Ratio of C16-diol/SeO2 Figure 2-10. CdSe particle si ze and normalized nuclei number in the synthesis with different C16-diol/SeO2 ratios. Effect of Numbers of Carbon Atoms per Diol Effect of different car bon chain length of diols (C8-diol, C10-diol and C16-diol) was studied. The temporal evolution of the absorption spectra is shown in Figu re 2-11. As shown in Figure 2-12, the HWHM of CdSe nanocrystals made with C16-diol is the best and this is equivalent to a tighter size distribution. Figure 2-13 shows Cd Se nanocrystal c oncentration with different diols. The nuclei concentration with C10-diol is very close to but slightly higher than that with C16-diol, while the concentration with C16-diol is higher than that of C8-diol. Figure 2-14 illustrates that with C10-diol and C16-diol, the final particle sizes are very similar and with C8-diol slightly larger size particles can be obtained.

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29 Figure 2-11. Temporal evolution of the abso rption spectra during the CdSe synthesis with different diols: (a) C16-diol, (b) C10-diol and (c) C8-diol. 2.02.22.42.62.83.03.23.43.63.84.0 14 15 16 17 18 c b aHWHM (nm)Size of CdSe nanocrystals (nm) Figure 2-12. HWHM of CdSe during s ynthesis with differe nt diols: (a) C16-diol, (b) C10-diol and (c) C8-diol. 400500600 0 2 4 6 8 30min 10min 5min 0min 240oC 220oCa ) Absorbance (a.u.)wavelength (nm)400500600 0 2 4 6 8 b )230oC 250oC 0 min 30min 5 min 10min wavelength (nm)400500600 0 2 4 6 8 30min 10min 5 min 0 min 240oC 220oCc ) wavelength (nm)

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30 2.02.22.42.62.83.03.23.43.63.84.0 0 1 2 3 4 b a cCdSe nuclei concentration (10-6 M)Size of CdSe nanocrystals (nm) Figure 2-13. Temporal evolution of CdSe nanocry stal concentration with different diols: (a) C16-diol, (b) C10-diol and (c) C8-diol. 891011121314151617 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Size of CdSe nanocrystals (nm)number of carbon atoms per diol Figure 2-14. CdSe particle size in the synthesis with different numbers of carbon atom per diol.

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31 Based on all the information, one can conclude synthesis with C16-diol has the lowest growth, smallest size. The possibl e explanation for this is that the longer the carbon chain in the diol molecule, the hydrogen bonding will be weaker and thus the more active the diol would be. So among all three, C16-diol is the most active. Comparison of Alcohols and Diols Figure 2-15. Temporal evolution of the abso rption spectra during the CdSe synthesis with different alcohols. (a) C16-diol, (b) C18-OH and (c) phenol. Synthesis with alcohols was performed. Th e results are compared with diols. C18-OH and phenol were used. The nanocrystals were formed by cadmium myristate (0.1 mmol) reacted with SeO2 (0.05 mmol) and C18-OH or phenol (0.05 mmol). The ab sorption spectra are shown in Figure 2-15. Compare HWHM and sharpness, on e can conclude that the qualities CdSe nanocrystals with C18-OH are better than that without alc ohol, but not are not comparable to those with C16-diol. The CdSe nanocrystals made with phenol did not get improved. 400500600700 0 2 4 6 8 c )30min 10min 5 min 0 min 240oC 220oCwavelength (nm) 400500600700 0 2 4 6 8 b )30min 10min 5 min 0 min 240oC 220oCwavelength (nm)400500600700 0 2 4 6 8 30min 10min 5min 0min 240oC 220oCa ) Absorbance (a.u.)wavelength (nm)

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32 The possible reaction happened is proposed below. The SeO2 got reduced by the alcohol and selenium was formed. The active selenium th en reacted with cadmium precursor to form CdSe nanocrystals. 2.3.2 Precursor Effect 400500600700 0 2 4 6 8 30min 10min 5min 0min 240oC 220oCa ) Absorbance (a.u.)wavelength (nm) 400500600700 0 2 4 6 8 Absorbance (a.u.)c )30min 10min 5 min 0 min 255oC 240oCwavelength (nm) Figure 2-16. Temporal evolution of the abso rption spectra during the CdSe synthesis with different Cd precursors. (a) CdC14, (b) CdC18, (c) CdC22 and (d) CdC10. 400500600700 0 2 4 6 8 b )30min 10min 5 min 0 min 240oC 220oCwavelength (nm) 400500600700 0 2 4 6 8 10 10min 30min 5min 0min 220oC 210oCd ) wavelength (nm)

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33 Besides the diol effect, it was found that us ing precursors that have longer carbon chains can also improve the quality of CdSe nanocrystal s. Figure 2-16 shows the absorption spectra of CdSe nanocrystals made of four different cad mium precursors (0.1 mmol) reacted with SeO2 (0.05 mmol) and C16-diol (0.05 mmol) in ODE. The particles made of CdC22 have the best quality, followed by those made of CdC18, and particles formed by CdC10 have poor spectra. So we can conclude that the longer carbon chain in the cadmium precursor s, the higher-quality nanocrystals can be obtained. The absorption sp ectrum of CdSe nanocrystals made of CdC18 and CdC22 exhibit multiple exiton peaks (Figure 2-17). Figure 2-17. Multiple exiton peaks. (a) CdC18, (b) CdC22. 300400500600 0.0 0.5 1.0 1.5 2.0 a Absorbance (a.u.)wavelength (nm)300400500600 0.0 0.5 1.0 1.5 2.0 b Absorbance (a.u)wavelength (nm)

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34 2.02.22.42.62.83.03.23.43.6 5 10 15 20 25 c b aCdSe nuclei concentration (10-6M)Diameter of CdSe nanocrystals (nm) Figure 2-18. Effect of Cd precursor on the nu clei concentration during the CdSe synthesis. (a) CdC14, (b) CdC18 and (c) CdC22. 81012141618202224 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Size (nm)carbon atom number in precursor Figure 2-19. Effect of Cd precursor on th e CdSe particle size in the synthesis.

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35 The calculated nuclei concentrations of synt heses with different cadmium precursors are shown in Figure 2-18. The more carbon atoms in the cadmium precursors, the higher nuclei concentration will be achieved. Figure 2-19 illustra tes the relationship between the final particle size and the number of carbon atoms per cadmium precursor. When a cadmium precursor with longer carbon chains is used, CdSe nanocrystals with smaller size will be generated. Growth rates are shown in Figure 2-20. The longer the carbon chains in the precu rsor, the slower the growth rate. The result is that cadmium precursor and selenium precursor have more comparable reactivity and this may cause high-quality nanocrystals. The relationship between the nuclei number and the particle growth rate is also shown here. As has been discusse d, the ratio of diol to SeO2 effect, the faster the particles growth, the lower the nuclei number. Precursor with longer carbon chains causes slower growth rate and higher nuclei number probably because its molecular size is larger and thus it is harder to get activated. It takes a longer time to transfer to monomers and results in higher nuclei number. 46810121416182022 0 2 4 6 8 10 12 14 46810121416182022 0 2 4 6 8 10 12 14 46810121416182022 0 2 4 6 8 10 12 14 c b CdSe particle growth rate (nm3/min)CdSe particle size (nm3) a Figure 2-20. CdSe particle growth rate in the synthesis with different Cd precursors: a) CdC14, (b) CdC18 and (c) CdC22.

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36 2.3.3 Multiple-Addition Method With the above method, the acceptable nanocryst als with the largest size we can get is 3.4 nm. It was found that with the multiple-addition method CdSe nanocrystals with a size of 4.5 nm were generated, which has an absorption peak at 582 nm. In the experiment, cadmium myristate (0.1 mmol), SeO2 (0.05 mmol), and C16-diol (0.05 mmol) were added in to a three-neck flask with 5 g ODE. The mixture solution was degassed fo r 10 min under vacuum (~16 mTorr) at room temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to 265 C (25 C /min) with gentle stirri ng and let to react for 1 hour. The reaction solution was cooled down to room temperature, and then cadmium myristate (0.025 mmol), SeO2 (0.0125 mmol), and C16-diol (0.0125 mmol) were added. After de gassing, the solution was heated to 265 C again and let react for 1 hour Lastly, the second addition st ep and condition was repeated twice. Aliquots were taken out for UV and photoluminescence (PL) measurement. 400500600700 0 2 4 6 8 10 12 b Absorbance (a.u.)wavelength (nm) a Figure 2-21. Characterization of CdSe nanocry stals during the multiple-addtion synthesis (a) Temporal evolution of the absorption spectra multiple-addition synthesis. (b) TEM image.

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37 The temporal evolution of the absorption spec tra during the CdSe synthesis is shown in Figure 2-21(a). The TEM image of the final CdSe nanocrystals made by this method is shown in Figure 2-21(b). It can be seen that the na nocrystals are uniformly spherical, and the size distribution is good. The aver age size is around 4.5 nm. 2.4 Conclusion In this part of work, it is showed that SeO2 can be used to replace selenium powder to synthesize CdSe nanocrystals. With the presence of C16-diol, the quality of CdSe nanocrystals can be improved. The effects of the C16-diol to SeO2 ratio were studied. It was found that the more diol added, the higher-quality and smaller-size CdSe nanocrystals will be obtained. The quality of our product is comparable to the best results published and nanoparticles smaller than 3 nm are even better. C16-diol can make the CdSe particle growth slower with the nuclei number increasing at the same time. Compared the quality of CdSe nanocrystals made with C8-diol, C10-diol and C16-diol, these diols have similar effect. Results with C18-OH and phenol were worse than those with diols. Cadmium precursor effect was studied. The results show that the longer the carbon chains in cadmium precursor, th e smaller size particles will be synthesized. Cadmium precursors with longer carbon chains can re tard the growth rate and increase the nuclei number. Multiple-addition method wa s performed and obtained CdSe nanocrystals with size of around 4.5 nm. The best spectrum of different peak position were chosen from several experiments and shown in Figure 2-22.

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38 400500600 0 5 10 15 20 25 Absorbance (a.u.)wavelength (nm) Figure 2-22. Temporal evolution of the ab sorption spectra of the as-prepared CdSe nanocrystals. Black: made by CdC22+SeO2+C16-diol (0.1:0.05:0.05); Red:: made by CdC18 +SeO2+C16-diol (0.1:0.05:0.05); Blue: made by CdC14 +SeO2+C16-diol (0.1:0.1:0.05); Purple: CdC14 +SeO2+C16-diol (0.1:0.05:0.05); Gr een: motile-addtion reaction, CdC14 +SeO2+C16-diol (0.1:0.05:0.05).

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39 CHAPTER 3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR 3.1 Introduction In Chapter 2 it was that using SeO2 to replace selenium element, CdSe nanocrystals can be obtained. Experiments where SeO2 was employed with gallium, l ead, silver, copper and nickel precursors were performed to synthe size metal selenide nanocrystals. The Kelly group has done a lot of research on the GaSe nanoparticles, from synthesis to physical properties.53-56 GaSe has a hexagonal layered structure57 consisting of Se-Ga-Ga-Se sheets. GaSe is a semiconductor with indirect band gap58,59 having a 2.11 eV direct band gap. Their GaSe synthesis is based on the reaction of an or ganometallic (GaMe3) with TOPSe in a high-temperature solution of TO P and TOPO. The absorption spec tra of GaSe nanoparticles have an onset in the 400-500 nm region. The nanoparticle diameters range from 2 to 6 nm, with an average size of about 4 nm. After chromatogr aphic purification the av erage size is about 2.5 nm.53 Lead selenide nanocrystals have been wide ly studied. The popular method is colloidal synthesis method. In the Murray group, PbSe nanoc rystals is synthesized by rapidly injecting a lead oleate and TOPSe dissolved in trioctylphosphin e into a well-stirred so lution of dioctylether at 150oC.60 Temperature is tuned to contro l the size of the nanocrystals. Ag2Se has two phases. The low-temperature phase ( -Ag2Se) is a narrow band-gap semiconductor, and has been widely used as a photosensitizer in photographic flims to thermochromic materials. -Ag2Se is the high-temperature phase, and it is a superionic conductor that is used a solid electrolyte in photochargable seconndary batteries.61 These two phases are reversible. There are only a few reports on the preparation of Ag2Se nanocrystals. Yi Xie et .al synthesize Ag2Se nanocrystals at room temper ature through the reaction of AgNO3, Se, and

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40 KBH4 in pyridine.62 The Vittal group synthesized Ag2Se nanoparticles by thermolysis of silver selenocarboxlyate in TOPO/TOP.61 CuSe is used in solar cells.63 The methods to synthesize CuSe nanocrystals include thermolysis of Cu and Se powder mixtures64, the mechanical alloying of Se and Cu in a high-energy ball mill, and the reaction of Se with Cu element in liquid ammonia65. The Vittal group synthesized CuSe nanoparticles by ther molysis of copper selenocarboxlyate in TOPO/TOP.66 NiSe is one of the typical Pauli pa ramagnets with me tallic conductivity.67 These stoichiometric compounds and the solid solutions between them now have been regarded as typical materials for studies of the physical characteristics associated with a narrow band electron system.68-70 Meanwhile, transition metal dichalcogeni des have extensive applications in energy areas such as electrochemistry and catalysis.71,72 The large surface areas and high activity of nanomaterials will enhance thei r applications in these fields.73 Several methods have been used to prepare NiSe nanocryst als, including elemental reactions74, organnometallic precursor method65 and solvothermal processes76. 3.2 Experimental Section 3.2.1 Materials Gallium nitrate hydrate (Ga(NO3)3xH2O, 99.999%), lead oxide (PbO, 99%), silver nitrate (AgNO3, 99%), copper nitrate trihydrate (Cu(NO3)2H2O, 99 %) nickel nitrate hexahydrate (Ni(NO3)2H2O, 99.999%), selenium dioxide (SeO2, 99.9+%) ,1-octadecene (ODE, 90%), trioctylphosphine oxide (TOPO, 90%), tr ioctylphosphine (TOP), 1,2-hexadecanediol (C16-diol, 90%), oleyamine (OAm, 70%), oc tadecylphosphonic acid (ODPA), tributylphosphine (TBP, 97%) were purchased form Al drich. Sodium myristate (CH3(CH2)12COONa), sodium oleate (CH3(CH2)7CH=CH(CH2)7COONa) were purchased from TCI. All chemicals except oleyamine

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41 and TOPO were used without fu rther purification. Methanol ( 99.9%), toluene (99.9%), acetone (99.8%) were purchased from Fisher. 3.2.2 Instrumentation Absorption spectra of aliquots were co llected by a Shimadzu UV-1700 UV-Visible Spectrophotometer. High resolution transmissi on electron microscopy (HR-TEM) images were obtained using a JEOL 2010F microscope for latt ice imaging and crystal size determination. TEM samples were prepared by dispersing the nanocrystals in toluene and depositing them onto formvar-coated copper grids. 3.2.3 Preparation of Precursors 3.2.3.1 Gallium myristate 0.6 g gallium nitrate was dissolv ed in 20 ml methanol, while sodium myristate solution was prepared by dissolving 1.5 g sodium myrist ate in 100 ml methanol. Then the gallium nitrate solution was dropped slowly into sodi um myristate solution under magnetic stirring conditions. The observed white preci pitate was washed with meth anol 4-5 times and dried under vacuum overnight to remove all solvents. 3.2.3.2 Silver oleate 0.8 g silver nitrate was dissolved in 40 ml methanol, while sodium myristate solution was prepared by dissolving 0.6 g sodium oleate in 100 ml methanol. Th en the silver nitrate solution was dropped slowly into sodium oleate solution under magnetic stirring conditions. The observed white precipitate was washed with methanol 4-5 times and dried under vacuum overnight to remove all solvents. 3.2.3.3 Copper oleate 1.2 g copper nitrate was dissolved in 30 ml meth anol, while sodium myristate solution was prepared by dissolving 1.2g sodi um oleate in 150 ml methanol Then the copper nitrate

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42 solution was dropped slowly into sodium oleate solution under magnetic s tirring conditions. The observed blue precipitate was washed with methanol 4-5 times and dried under vacuum overnight to remove all solvents. 3.2.3.4 Nickel oleate 0.6 g nickel nitrate was dissolved in 30 ml methanol, while sodium myristate solution was prepared by dissolving 1.2g sodium oleate in 200 ml methanol. Th en the nickel nitrate solution was dropped slowly into sodium oleate solution under magnetic stirring conditions. Hexane was added into extract the nickel ol eate and collected into a flask. Th e nickel oleate hexane solution was evaporated by a rotary evaporator. The gr een product was dried under vacuum overnight. 3.2.4 Preparation of Nanocrystals 3.2.4.1 Gallium selenide nanocrystals Gallium myristate (0.1 mmol), SeO2 (0.05 mmol), and C16-diol (0.05 mmol) were added into a three-neck flask with 5 g ODE. The mixture solution was degassed for 10 min under vacuum (~16 mTorr) at room temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to 285 C with gentle stirring and let to react for 2 hours. The color of the reaction solution changed fr om colorless to light yellow. 3.2.4.2 Lead selenide nanocrystals Lead oxide (0.1 mmol), ODPA ( 0.2 mmol) were added into a three-neck flask with 3 mL ODE. The mixture solution was degassed for 10 min under vacuum (~16 mTorr) at room temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to 160 C with gentle stirring for 1 hour until PbO dissolved. The solution was cooled to 120 C and degassed at this temperature to get rid of wate r. When the solution was cooled down to room temperature, SeO2 (0.05 mmol), C16-diol (0.05 mmol) and 3.3 mL ODE was added. The mixture was degassed for 10 min and then heat up to 180 C under argon flow.

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43 3.2.4.3 Silver selenide nanocrystals SeO2/TOP was obtained by dissolving SeO2 (0.1 mmol) in TOP (1.2 mmol, 0.54 mL). Silver oleate (0.1 mmol) was added into a th ree-neck flask with 5 g purified TOPO. The mixture solution was degassed for 10 min under v acuum (~16 mTorr) at room temperature, and then the vacuum was removed. Under an argon flow the solution was heated with gentle stirring. When the temperature reached 120 C, 0.27 mL SeO2/TOP (0.05 mm ol / 0.6 mmol) was quickly injected to the solution and let react for 40 min. 3.2.4.4 Copper selenide nanocrystals SeO2/TOP was obtained by dissolving SeO2 (0.1 mmol) in TOP (1.2 mmol, 0.54 mL). Copper oleate (0.1 mmol) and 0.27 mL SeO2/TOP (0.05 mmol / 0.6 mmol) were added into a three-neck flask with 5 g purified OAm. The mixture solution was degassed for 10 min under vacuum (~16 mTorr) at room temperature, a nd then the vacuum was removed. Under an argon flow, the solution was heated to 220 C with gentle stirring and allowed react for 90 min. 3.2.4.5 Nickel selenide nanocrystals Nickel oleate (0.1 mmol) and SeO2 (0.1 mmol) were added into a three-neck flask with 5 g purified OAm. The mixture solution was dega ssed for 10 min under vacuum (~16 mTorr) at room temperature, and then the vacuum was removed. Under an argon flow, the solution was heated with gentle stirrin g. TBP (1.2 mmol) was quickly injected to the solution when temperature reached 170 C. The solution was kept at this temperature to react for 30 min. 3.2.5 Purification of Nanocrystals Nanocrystals were purified by precipitation in excess acetone fo llowed by centrifugation. The supernatant contains molecular reaction byproducts and was discarded. The nanocrystals were redispersed in toluene and centrifuged again. The well-capped nanocrystals remained dispersed while poorly-capped nanocrystals settled. The precipitate was discarded. The

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44 toluene-dispersed nanoparticles were reprecip itated in excess acetone. The supernatant was discarded after centrifugation. The purified na nocrystals were redi spersed in toluene. 3.3 Results and Discussion 3.3.1 Gallium Selenide The time evolution of the absorption spectru m is shown in Figure 3-1. The spectrum of GaSe nanocrystal at 30 minutes starts to have an onset at around 340 nm, which means that the GaSe nanocrystals made by our method are smaller in size than those of Kellys. This is proved by the TEM image (Figure 3-2). Th e average size of our GaSe na nocrystals is around 2.0 nm. It was found that the as-prepared GaSe nanocrystals have blue emission. We didnt collect the emission spectra of the nanoparticles and this wi ll be done later. The size is small probably because there are too many nuclei. To get larger size particles, one s hould try to decrease the nuclei number. 300400500600 0.00 0.05 0.10 0.15 0.20 0.25 120 min 30 min 0 minAbsorbancewavelength (nm) Figure 3-1. Evolution of absorption spectrum of GaSe nanocrystals. Black: 0 min; pink: 30 min and red: 2 h.

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45 Figure 3-2. TEM image of GaSe nanocrystals. 3.3.2 Lead Selenide The PbSe nanocrystals cannot disperse in tolu ene, hexane, chloroform or other organic solvents, which means that what was formed is aggregated PbSe nanocrystals. The TEM image of PbSe nanocrystal is shown in Figure 3-3. It can be seen that the aggregated nanocrystals consists of nanocubes whose edge is around 74 nm. Figure 3-3. TEM image of PbSe nanocrystals.

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46 3.3.3 Silver Selenide The absorption spectrum of AgSe nanocrystals during the synthesis is shown in Figure 3-4. At the beginning, there is an absorption peak at around 407 nm, which belongs to Ag nanoparticles. The peak got w eaker and weaker and disappear ed at the reaction time of 40 minutes. This means that Ag nanoparticle were fo rmed first, and then gradually reacted with SeO2 to form AgSe. To prove this, X-ray diffrac tion patterns of samples taken at different occasions should be obtained. 300400500600700 0.0 0.2 0.4 0.6 40 min 10 min 1 minAborbancewavelength (nm) Figure 3-4. Evolution of absorptio n spectrum of AgSe nanocrystals. The high resolution TEM images of AgSe nanocrystals made by our method are shown in Figure 3-5. The shape of the nanocry stals is uniform, and they are all spherical. But the size is not uniform, ranging from 4.3 nm to 12.2 nm, with an average size of 7.4 nm and the lattice spacing is 0.21 nm. Size dist ribution should be improved.

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47 a b Figure 3-5. HR-TEM images of AgSe nanocryst als. The lattice spacing in (b) is 0.21 nm. 3.3.4 Copper Selenide Figure 3-6. TEM image of CuSe nanocrystals. The TEM image of CuSe nanocrystals is show n in Figure 3-5. Interestingly, some hollow nanocrystals were found but the si ze distribution is large and the shape is not uniform. To get

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48 more uniform nanocrystals, one may try to anneal the precursors at 160oC for longer time first to generate more active monomers. 3.3.5 Nickel Selenide Needle-shape NiSe nanocrystal s were obtained. The TEM image of NiSe nanocrystals is shown in Figure 3-6. Similar to AgSe and CuSe, th e problem is that non-uniform shape and size NiSe nanocrystals were formed. Figure 3-7. TEM image of NiSe nanocrystals. 3.4 Conclusion In this part of the work, experiments to prepare GaSe, PbSe, AgSe, CuSe and NiSe nanocrystal were performed. For GaSe, spherica l nanocrystals were obtained and the average size is around 2.0 nm. The product has blue emissi on. To get larger size GaSe nanocrystals, one should try to decrease the nuc lei number. For PbSe, the nano crystals aggregated. But the aggregates consist of nanocubes that are uniformly in size and shape. To avoid aggregation, one might try to anneal the precurs ors for a longer time to get more active monomers before heating

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49 to the reaction temperature. For AgSe, sphe rical nanocrystals were obtained but the size distribution is poor. The diameters range from 4 nm to 12 nm. The high resolution TEM image showed that the crystalline Ag Se nanoparticles have a lattice spacing of 0.21 nm. The absorption spectrum of the nanocrystals shows that Ag nanopart icles were formed at the beginning and then gradually reacted with SeO2 to form AgSe nanocrystals. As for CuSe and NiSe, hollow nanocrystals and needle-shape nanocrystals we re got, respectively. But their size and shape distribution are still poor. To im prove the quality, one can try to generate active monomers first before the reaction.

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50 CHAPTER 4 SUMMARY AND FUTURE WORK 4.1 Summary It has been demonstrated that SeO2 can be used to replace selenium element to synthesize metal selenide semiconductor nanocrystals. For Cd Se, one-pot synthetic me thod was used. It is found that when equal amount of C16-diol as SeO2 was added, the quality of CdSe nanocrystals can be improved. Effect of the ratios of C16-diol to SeO2 was studied and the result shows that the higher the ratio of C16-diol as SeO2, the better nanocrystals can be obtained, and the higher nuclei number and the slower gr owth. Experiments using differ ent cadmium precursors were performed and it was found that the longer the ca rbon chains in cadmium precursor, the better the quality of CdSe nanocrystals were got, a nd the higher the nuclei num ber and the slower the growth. Multiple-addition reaction was employed to prepare larger size nanocrystals. It was also proved that using SeO2 instead of selenium element, GaSe, PbSe, AgSe, CuSe and NiSe nanocrystals were obtained. GaSe nanocry stals were uniform in size and shape, but the size is small. PbSe nanocube aggregates were obta ined, and each nanocube is uniform in size and shape. Crystalline AgSe nanoparticles were obtaine d with an average size of 7.4 nm and a lattice spacing of 0.21 nm. Uniform CuSe and NiSe na noparticles have not been formed yet. 4.2 Future work 4.2.1 Injection-Synthetic Method for CdSe So far the largest acceptable CdSe nanocrystal s obtained by our method have a diameter of around 4.5 nm. To get larger high-quality CdSe nanocrystals, injection method can be employed. By quickly injecting precursors at high temperature, fewer nuclei will be formed in a very short time, resulting in larger, more uniform CdSe nanocrystals.

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51 Cadmium myristate (0.1 mmol) will be added into a three-neck flask with 4.3 g ODE. SeO2 (0.1 mmol) and C16-diol (0.1 mmol) will be added in 2 mL ODE. The two mixture solutions will be degassed for 10 min under v acuum (~16 mTorr) at room temperature, and then the vacuum will be removed. Under an argon flow, the SeO2 and C16-diol solution will be heated to 100 C and SeO2 and C16-diol will dissolve and fo rm yellow solution. Under an argon flow, the cadmium solution will be heated with gentle stirring. When the temperature reaches 265 C, 1 mL of SeO2 (0.05 mmol) and C16-diol (0.05 mmol) ODE solution will be quickly injected to the cadmium solution. The temperature will keep at 265 C. 4.2.2 Improvement of Other Metal Selenide Nanocrystals For GaSe, PbSe, AgSe, NiSe and CuSe, acceptable results havent been obtained yet. One can try to generate active monomers before hea ting to the reaction temper atures for PbSe, NiSe and CuSe. Injection method and lower reaction temp erature may be used for GaSe to get larger size nanoparticles. X-ray diffraction patterns should be got to identi fy the crystal structures of these nanocrystals. 4.2.3 Mechanism Study The mechanism of SeO2 reacting with C16-diol can also be studied to understand how C16-diol improve the quality of CdSe nanocrystals using 1H, 13C, and 31P NMR spectroscopy and mass spectrometry to confirm our hypothesis.

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56 BIOGRAPHICAL SKETCH Xian Chen was born in Xiamen, a beautiful c ity on the southeast coas t of China. In 1999, she started her college life at the University of Science and Technology of China (USTC). After 5 years of study in the Department of Polyme r Science and Engineering, she received her bachelors degree in engineering in 2004. Then, sh e joined the Department of Chemistry at the University of Florida. She would like to pursue a Ph.D. degree after graduation.