<%BANNER%>

Synthesis and Characterization of Nanophosphors by Flame Spray Pyrolysis

Permanent Link: http://ufdc.ufl.edu/UFE0024401/00001

Material Information

Title: Synthesis and Characterization of Nanophosphors by Flame Spray Pyrolysis
Physical Description: 1 online resource (131 p.)
Language: english
Creator: Lee, Jae
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

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

Notes

Abstract: Three different types of nanophosphor materials were prepared by flame spray pyrolysis (FSP) from nitrate-based liquid precursor. Rare-earth doped binary oxide nanophosphor particles were obtained from urea-added liquid precursor using FSP without post-heat treatment. Also, rare-earth doped ternary oxide nanophosphors were prepared from urea-added using FSP with less annealing temperature and reduced time. In addition, metal-ion doped ternary oxide nanophosphors could be easily generatred by FSP from nitrate-based liquid precursor. The prepared phosphor particles properties such as crystallinity, luminescence properties, surface morphology and particle size were investigated by many techniques in order to obtain the optimum condition for high quality nanophosphor particles. Firstly, Y2O3:Eu3+ nanophosphor was synthesized by FSP from nitrate-based liquid precursor. Urea was added to the liquid precursor to enhance the crystallinity of Y2O3:Eu3+ phosphor particles. The resultant phosphor particles showed Y2O3:Eu3+ cubic phase in the XRD pattern without the need for post-heat treatment. The influence of synthesis conditions such as different molar concentrations of urea, overall molar concentration of liquid precursors, and doping concentration on luminescent properties was investigated. The particle size of product was found to be in the range of 20-30 nm as determined from transmission electron microscopy (TEM) images. Photoluminescence (PL) measurement of the synthesized Y2O3:Eu3+ nanophosphor show peak in red region of visible spectrum with a maximum peak at wavelength of 609 nm when excited with 398 nm wavelength photons. Secondly, YAG:Ce3+ nanophosphors were synthesized by FSP from nitrate-liquid precursor. As-prepared nanoparticles were annealed in the temperature range of 800? to 1100? for 1 hour. The influence of addition of urea and the molar ratio of yttrium to aluminum in the liquid precursor on crystallinity and luminescence properties of YAG:Ce3+ nanophosphor particles were studied. The heat-treated phosphor particles were spherical in shape with an average size blow 50 nm. The crystallinity of YAG:Ce3+ nanophosphors improved with addition of urea and overloaded aluminum in starting liquid precursor. In addition, high PL intensity with pure YAG:Ce3+ phase was observed for phosphor particles prepared with addition of urea and excess of aluminum in liquid precursor with less annealing temperature and reduced time compared to other methods. Green emitting Zn2SiO4:Mn2+ phosphor particles were synthesized by FSP from a function of different liquid precursors. Luminescence and crystalline properties were investigated as the different Zn-source materials in aqueous precursor. Also, the influence of post-heat treatment temperatures on the crystal structure and PL intensity of Zn2SiO4:Mn2+ nanophosphors was investigated. Mn-doped zinc silicate crystalline structures were obtained when annealed at 1000? for 1hour. The emission peak was found at 525 nm. Furthermore, the effect of the flame temperatures by varying methane flow rate on the crystallinity and luminescence properties of Zn2SiO4:Mn2+ nanophosphors was investigated. The phosphor particles prepared from high flame temperature showed good crystallinity with pure zinc silicate phase and the maximum PL intensity. We conclude that different experimental conditions such as liquid precursor prepared from different Zn-source and annealing temperature influence both crystallinity and the luminescence properties of Zn2SiO4:Mn2+ nanophosphors.
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 Jae Lee.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Singh, Rajiv K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024401:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024401/00001

Material Information

Title: Synthesis and Characterization of Nanophosphors by Flame Spray Pyrolysis
Physical Description: 1 online resource (131 p.)
Language: english
Creator: Lee, Jae
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

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

Notes

Abstract: Three different types of nanophosphor materials were prepared by flame spray pyrolysis (FSP) from nitrate-based liquid precursor. Rare-earth doped binary oxide nanophosphor particles were obtained from urea-added liquid precursor using FSP without post-heat treatment. Also, rare-earth doped ternary oxide nanophosphors were prepared from urea-added using FSP with less annealing temperature and reduced time. In addition, metal-ion doped ternary oxide nanophosphors could be easily generatred by FSP from nitrate-based liquid precursor. The prepared phosphor particles properties such as crystallinity, luminescence properties, surface morphology and particle size were investigated by many techniques in order to obtain the optimum condition for high quality nanophosphor particles. Firstly, Y2O3:Eu3+ nanophosphor was synthesized by FSP from nitrate-based liquid precursor. Urea was added to the liquid precursor to enhance the crystallinity of Y2O3:Eu3+ phosphor particles. The resultant phosphor particles showed Y2O3:Eu3+ cubic phase in the XRD pattern without the need for post-heat treatment. The influence of synthesis conditions such as different molar concentrations of urea, overall molar concentration of liquid precursors, and doping concentration on luminescent properties was investigated. The particle size of product was found to be in the range of 20-30 nm as determined from transmission electron microscopy (TEM) images. Photoluminescence (PL) measurement of the synthesized Y2O3:Eu3+ nanophosphor show peak in red region of visible spectrum with a maximum peak at wavelength of 609 nm when excited with 398 nm wavelength photons. Secondly, YAG:Ce3+ nanophosphors were synthesized by FSP from nitrate-liquid precursor. As-prepared nanoparticles were annealed in the temperature range of 800? to 1100? for 1 hour. The influence of addition of urea and the molar ratio of yttrium to aluminum in the liquid precursor on crystallinity and luminescence properties of YAG:Ce3+ nanophosphor particles were studied. The heat-treated phosphor particles were spherical in shape with an average size blow 50 nm. The crystallinity of YAG:Ce3+ nanophosphors improved with addition of urea and overloaded aluminum in starting liquid precursor. In addition, high PL intensity with pure YAG:Ce3+ phase was observed for phosphor particles prepared with addition of urea and excess of aluminum in liquid precursor with less annealing temperature and reduced time compared to other methods. Green emitting Zn2SiO4:Mn2+ phosphor particles were synthesized by FSP from a function of different liquid precursors. Luminescence and crystalline properties were investigated as the different Zn-source materials in aqueous precursor. Also, the influence of post-heat treatment temperatures on the crystal structure and PL intensity of Zn2SiO4:Mn2+ nanophosphors was investigated. Mn-doped zinc silicate crystalline structures were obtained when annealed at 1000? for 1hour. The emission peak was found at 525 nm. Furthermore, the effect of the flame temperatures by varying methane flow rate on the crystallinity and luminescence properties of Zn2SiO4:Mn2+ nanophosphors was investigated. The phosphor particles prepared from high flame temperature showed good crystallinity with pure zinc silicate phase and the maximum PL intensity. We conclude that different experimental conditions such as liquid precursor prepared from different Zn-source and annealing temperature influence both crystallinity and the luminescence properties of Zn2SiO4:Mn2+ nanophosphors.
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 Jae Lee.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Singh, Rajiv K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024401:00001


This item has the following downloads:


Full Text

PAGE 1

SYNTHESIS AND CHARACTERIZATION OF NANOPHOSPHORS BY FLAME SPRAY PYROLYSIS By JAE SEOK LEE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009 1

PAGE 2

2009 Jae Seok Lee 2

PAGE 3

To my parents, my colleagues, and other family members Their endless support made it possible for me to complete this work. 3

PAGE 4

ACKNOWLEDGMENTS First, I would like to express my heartfelt thanks to my adviso r, Dr. Rajiv K. Singh, for his continuous support. I was al so honored to meet and wo rk for him. I shall ne ver forget his endless advice and help. I would like to express my tha nks to my other committ ee members (Dr. Madhav B. Ranade, Dr. Stephen J. Pearton, Dr. David Norton, Dr. Brent Gila, and Dr. Fan Ren) for their helpful advice. Also, I would like to express my special thanks to all of my fellow group members: Myoung Hwan Oh, Tae Kon Kim, Se Jin Kim, Feng-chi Chang, Prushottam Kumar, Sushant Gupta and Anirrddh Khanna. I also thank the sta ff of MAIC and PERC, and my friends for their help and sincere discussion. Finally, I would like to thank all of my family members, my parents and brother, for their c ontinuous trust and supports. 4

PAGE 5

TABLE OF CONTENTS page ACKNOWLEDGMENTS ..............................................................................................................4 LIST OF TABLES ..........................................................................................................................8 LIST OF FIGURES ........................................................................................................................9 ABSTRACT ...................................................................................................................................12 CHAPTER 1 INTRODUCTION................................................................................................................. .15 2 LITERATURE REVIEW.......................................................................................................20 2.1Basic Concept and Applicati ons of Phosphor Materials .............................................20 2.1.1Mechanism of Light Emissi on in Phosphor Particles .......................................21 2.2Nanophosphor Particles ..............................................................................................22 2.2.1Advantages of Na nophosphor Particles ............................................................22 2.2.2Potential Applications of Nanophosphor Particles in Solid State Lighting Devices ........................................................................................................................23 2.2.3Issues on Nanopho sphor Particles ....................................................................25 2.3Preparation Techniques for Nanophosphor Particles..................................................26 2.3.1Sol-Gel (SG) .....................................................................................................26 2.3.2Combustion Synthesis (CS) ..............................................................................28 2.3.3Spray Pyrolysis (SP) .........................................................................................29 2.4Flame Spray Pyrolysis (FSP) ......................................................................................30 2.4.1What is FSP? .....................................................................................................30 2.4.2Mechanism of Nanophosphor Pa rticle Formation by FSP ...............................31 2.4.3Advantages of FSP for Nanophosphor Particles ...............................................32 2.4.4Selection of Precursor Solvents, Additives and Source Materials ....................33 3 EXPERIMENTAL PROCEDURE.........................................................................................46 3.1 Apparatus of Flame Spray Pyrolysis ...............................................................................46 3.2 The Preparation of Liquid Precursors ..............................................................................46 3.2.1 Y2O3:Eu3+ Red Phosphor .......................................................................................46 3.2.2 Y3Al5O12:Ce3+ Yellow Phosphor ...........................................................................47 3.2.3 Zn2SiO4:Mn2+ Green Phosphor .............................................................................47 3.3 Characterization of Nanophosphor Particles ...................................................................48 4 ENHANCED LUMINE SCENCE PROPERTIES OF CUBIC Y2O3:EU3+ NANOPHOSPHORS BY FSP...............................................................................................52 4.1 Introduction ......................................................................................................................52 5

PAGE 6

4.2 Experimental ....................................................................................................................53 4.3 Results and Discussion ....................................................................................................55 4.4 Conclusion .......................................................................................................................57 5 LUMINESCENCE PROPERTIES OF Y2O3:EU3+ NANOPHOSPHOR PARTICLES PREPARED FROM UREA A DDED PRECURSOR USING FSP.......................................61 5.1 Introduction ......................................................................................................................61 5.2 Experimental ....................................................................................................................62 5.3 Results and Discussion ....................................................................................................63 5.4 Conclusion .......................................................................................................................66 6 LUMINESCENCE PROPERTIES OF YAG:CE3+ NANOPHOSPHORS PREPARED FROM UREA ADDED LIQU ID PRECURSOR BY FSP.....................................................75 6.1 Introduction ......................................................................................................................75 6.2 Experimental ....................................................................................................................76 6.3 Results and Discussion ....................................................................................................77 6.4 Conclusion .......................................................................................................................80 7 THE INFLUENCE OF DIFFERENT CO NDITIONS ON THE LUMINESCENCE PROPERTIES OF YAG:CE3+ NANOPHOSPHORS USING FSP.......................................85 7.1 Introduction ......................................................................................................................85 7.2 Experimental ....................................................................................................................86 7.3 Results and Discussion ....................................................................................................87 7.4 Conclusion .......................................................................................................................90 8 SYNTHESIS AND CHARACTERIZATION OF ZN2SIO4:MN2+ NANOPHOSPHORS PREPARED FROM DIFFEREN T ZN SOURCE IN LIQUID PRECURSOR BY FSP.......96 8.1 Introduction ......................................................................................................................96 8.2 Experimental ....................................................................................................................97 8.3 Results and Discussion ....................................................................................................99 8.4 Conclusion .....................................................................................................................102 9 LUMINESCENCE PROPERTIES OF ZN2SIO4:MN2+ NANOPHOSPHORS BY FSP.....109 9.1 Introduction ....................................................................................................................109 9.2 Experimental ..................................................................................................................110 9.3 Results and Discussion ..................................................................................................111 9.4 Conclusion .....................................................................................................................115 10 CONCLUSION.................................................................................................................. ...122 10.1 Single Step Processing For Nanophosphor Particles ...................................................122 10.2 The Effect of Additi on of Urea to liquid Precursors on the Properties of Nanophosphor Particles ...................................................................................................122 6

PAGE 7

10.3 The inflence of Different Zn-source to Liquid Precursors on Luminescence Properties of Na nophosphor Particles ..............................................................................123 LIST OF REFERENCES ............................................................................................................124 BIOGRAPHICAL SKETCH ......................................................................................................131 7

PAGE 8

LIST OF TABLES Table page Table 2-1. Most important phosphors for practical uses ...............................................................45 Table 3-1. Experimental Condition of Flame Spray Pyrolysis .....................................................49 8

PAGE 9

LIST OF FIGURES Figure page Figure 1-1. Nanotechnol ogy; Nanoscale Particles. ......................................................................19 Figure 2-1. General luminescent materials ..................................................................................34 Figure 2-2. Phosphorescence proces s in phosphors .....................................................................35 Figure 2-3. Energy transfer mechanis m for phosphorescence .....................................................36 Figure 2-4. Phosphorescence .......................................................................................................37 Figure 2-5. Blue LED with yellow phosphor coating for white light generation ........................38 Figure 2-6. Principle of white light generati on form the mixing of blue luminescence and yellow phosphorescence ...................................................................................................39 Figure 2-7. Sequen ce of events during sol-gel process ................................................................40 Figure 2-8. Sequence of events during combustion synthesis. ....................................................41 Figure 2-9. Morphology of particle prepared by spray pyro lysis method ...................................42 Figure 2-10. Spra y pyrolysis system ............................................................................................43 Figure 2-11. Nanoparticl e formation from precur sor droplets by the FSP ..................................44 Figure 3-1. FSP system for nanophosphor particles ....................................................................50 Figure 3-2. Flame nozzle .............................................................................................................51 Figure 4-1. X-ray Diffraction (XRD) patte rns of as-prepared 5 mol% Eu-doped Y2O3 nanophosphor by FSP with different mole of urea. Inset is a Scanning Electron Microscopy (SEM) image of phosphor produced in pr esence of 2 M of urea. ................58 Figure 4-2. Transmission El ectron Microscopy (TEM) images of as-prepared 5 mol% Eudoped Y2O3 nanophosphor by FSP with 2 mole of urea in liquid precursor. ...................59 Figure 4-3. Photoluminescence (PL) spec tra of as-prepared 5 mol% Eu-doped Y2O3 nanophosphor with diffe rent mole of urea in liquid precursor. ........................................60 Figure 5-1. XRD patterns of as-prepared 5% Eu-doped Y2O3 nanophosphor by FSP with different overall concentration of liqu id precursor with 2 M of urea addition. ................68 Figure 5-2. SEM image of as-prepared 10% Eu-doped Y2O3 nanophosphor by FSP with 2 M of urea addition in 0.9 M loading concentration of liquid precursor ...........................69 9

PAGE 10

Figure 5-3. TEM image of as-prepared 5% Eu-doped Y2O3 nanophosphor by FSP with 2 M of urea in the liquid precursor ...........................................................................................70 Figure 5-4. PL exc itation and emission spectra of as-prepared 5% Eu-doped Y2O3 nanophosphor at 2 M of urea addition: PL emissi on of corresponding Y2O3:Eu3+ nanophosphor excited with wavelength of 393 nm ..........................................................71 Figure 5-5. PL intensity of as-prepared 5% Eu-doped Y2O3 nanophosphor at different mole of addition of urea to the liquid precursor ........................................................................72 Figure 5-6. PL intensity of Y2O3:Eu3+ nanophosphor with different doping concentrations. .....73 Figure 5-7. PL spectra of as-prepared 10% Eu-doped Y2O3 nanophosphor w ith different overall concentration at fixed 2 M of urea addition. .........................................................74 Figure 6-1. XRD patterns of YAG:Ce3+ prepared with different Y:Al mo lar ratios in the precursors with and without urea. .....................................................................................81 Figure 6-2. SEM images of YAG:Ce3+ particles prepared from liquid precursor without and with the addition of urea ...................................................................................................82 Figure 6-3. PL spectra for 4 mol% Ce-doped YAG afte r annealing at 1100 for 1hr. ..............83 Figure 6-4. Emission spectra of YAG:Ce3+ (4 mol%) nanophos phors prepared from different liquid precursors after annealing at 1100 for 1hr ...........................................84 Figure 7-1. XRD patterns of 4 mol% Ce-doped YAG at different annealing temperatures ........91 Figure 7-3. PL spectra of 4 mol% Ce-doped YAG at differe nt annealing temperatures.. ...........93 Figure 7-4. XRD patterns of 4 mol% Ce-doped YAG at different flame temperatures by controlling the methan e flow rate and the molar ratio of yttrium to aluminum in liquid precursor .................................................................................................................94 Figure 7-5. PL spectr a of 4 mol% Ce-doped YAG after annealing at 1100 for 1hr. ...............95 Figure 8-1. XRD patterns of 4 mol% Mn-doped Zn2SiO4 prepared using different liquid precursors by FSP ...........................................................................................................104 Figure 8-2. SEM images of 4 mol% Mn-doped Zn2SiO4 nanophosphor prep ared by FSP using different liquid precur sors after an nealed at 1000 .............................................105 Figure 8-3. PL spectr a of 4 mol% Mn-doped Zn2SiO4 prepared using different liquid precursors by FSP. ..........................................................................................................106 Figure 8-4. XRD patterns obtai ned from 4 mol% Mn-doped Zn2SiO4 prepared from nitratebased liquid precursor at different annealing temperatures ............................................107 10

PAGE 11

Figure 8-5. PL spectr a of 4 mol% Mn-doped Zn2SiO4:Mn2+ phosphor ob tained at different annealing temperatures. ..................................................................................................108 Figure 9-1. XRD patterns of 4 mol% Mn-doped Zn2SiO4 at different annealing temperatures 116 Figure 9-2. SEM images of 4 mol% Mn-doped Zn2SiO4 nanophosphors pr epared from nitrate-based liquid precursor. ........................................................................................117 Figure 9-3. Excitation and emission spectra of 4 mol% Mn-doped Zn2SiO4 annealed at 1000 for 1hr. The inset in Figure 9-3 shows the relative PL intensity as a function of annealing temperature.................................................................................................118 Figure 9-4. XRD patterns for Zn2SiO4:Mn2+ phosphor particles at different flame temperature as functions of methane flow rate. ..............................................................119 Figure 9-5. FE-SEM images of Zn2SiO4:Mn2+ phosphor particles at different flame temperature as functions of methane flow rate. ..............................................................120 Figure 9-6. Emission spect ra of 4 mol% Mn-doped Zn2SiO4:Mn2+ phosphor obtained at different methane flow rate. ............................................................................................121 11

PAGE 12

Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SYNTHESIS AND CHARACTERIZATION OF NANOPHOSPHORS BY FLAME SPRAY PYROLYSIS By Jae Seok Lee May 2009 Chair: Rajiv. K. Singh Major: Materials Scie nce and Engineering Three different types of nanophosphor materials were prepared by flame spray pyrolysis (FSP) from nitrate-based liquid precursor. Rare-earth doped bina ry oxide nanophosphor particles were obtained from urea-added liquid precursor using FSP without post-heat treatment. Also, rare-earth doped ternary oxide nanophosphors were prepared from urea -added using FSP with less annealing temperature and reduced time. In addition, me tal-ion doped ternary oxide nanophosphors could be easily generatred by FSP from nitrate-base d liquid precursor. The prepared phosphor particles properties such as crystallinity, luminescence properties, surface morphology and particle size were investigated by many techniques in order to obtain the optimum condition fo r high quality nanopho sphor particles. Firstly, Y 2 O 3 :Eu 3+ nanophosphor was synthesized by FSP from nitrate-based liquid precursor. Urea was added to the liquid precurso r to enhance the crystallinity of Y 2 O 3 :Eu 3+ phosphor particles. The result ant phosphor partic les showed Y 2 O 3 :Eu 3+ cubic phase in the XRD pattern without the need for post-h eat treatment. The infl uence of synthesis conditions such as different molar concentrations of urea, overall molar concentra tion of liquid precursors, and doping concentration on luminescent properties was investigated. The pa rticle size of product was found to be in the range of 20-30 nm as determined from tr ansmission electr on microscopy 12

PAGE 13

(TEM) images. Photoluminescence (PL) measurement of the synthesized Y 2 O 3 :Eu 3+ nanophosphor show peak in red region of visible spectrum with a ma ximum peak at wavelength of 609 nm when excited with 398 nm wa velength photons. Secondly, YAG:Ce 3+ nanophosphors were synthesized by FSP from nitrate-liquid precursor. As-prepared nanoparticles were annealed in the temp erature range of 800 to 1100 for 1 hour. The influence of addition of urea and the molar ratio of yttrium to aluminum in the liquid precursor on crystallinity and luminescence properties of YAG:Ce 3+ nanophosphor particles were studied. The heat-treated phosphor particles were spheri cal in shap e with an average size blow 50 nm. The crystallinity of YAG:Ce 3+ nanophosphors improv ed with addition of urea and overloaded aluminum in starting liquid precur sor. In addition, high PL intensity with pure YAG:Ce 3+ phase was observed for phosphor partic les prepared with addition of urea and excess of aluminum in liquid precursor with less annealing temperat ure and reduced time compared to other methods. Green emitting Zn 2 SiO 4 :Mn 2+ phosphor particles were synthesized by FS P from a function of different liquid precursors. Luminescence and cr ystalline properties were investigated as the different Zn-source materials in aqueous precursor. Al so, the influe nce of post-heat treatment temperatures on the crystal st ructure and PL intensity of Zn 2 SiO 4 :Mn 2+ nanophosphors was investigated. Mn-dop ed zinc silicate crystalline structures were obtained when annealed at 1000 for 1hour. The emission peak was found at 525 nm. Furthermore, th e effect of the flame temperatures by varying methane flow rate on the crystallinity and luminescen ce properties of Zn 2 SiO 4 :Mn 2+ nanophosphors was investigated The phosphor particles pr epared from high flame temperature showed good crysta llinity with pure zi nc silicate phase and the maximum PL intensity. We conclude that different experiment al conditions such as liquid precursor prepared 13

PAGE 14

from different Zn-source and annealing temperature influence both crystallinity and the luminescence properties of Zn 2 SiO 4 :Mn 2+ nanophosphors. 14

PAGE 15

CHAPTER 1 INTRODUCTION Presently, significant research in the field is focu sed on narrower an d smaller universe called nanotechnology. Genera lly, nanotechnology can be defined as rese arch and technology development at th e atomic, molecular or macro-molecu lar level, in th e scale range of approximately 1-100 nm [1 ]. The nanotechnology re presents the most active discipline all over world and is considered as the fastest growing technology-revolution the human history has ever seen. Thus, nanomaterials have been extensively st udied in next-generation technologies such as displays, devices, biotec hnology, aerospace and ener gy, etc because of th eir unique properties and great application potentials [2 ]. For example, as th e size of a material is decreased to the nanoscale, their properties may e xhibit quantum size effect which leads to increase band gap as a result of strong re duction of quantum mechani cal allowed states in a small particle and also surface and interface effects ca used by much larger su rface/volume ratio [3]. Nanophosphors, typically solid inorganic materials, have received consid erable attention during the past few ye ars due to their unique chemical and physical ch aracteristics apart from their bulk phosphor mate rials. Many studies have focused on their fundamental and practical applications for various novel high-performance and novel displays and devices. Typically, nanosize phosphor particles, havi ng a spherical shap e, are strongly de sired extend their application to high resolution display devices [4 ]. Unlike semiconductor qu antum dots, rare earth doped inorganic luminescent materials, also calle d nanophosphors, have different optical properties compare to those obs erved in bulk phosphor s due to non-radiat ive relaxation and spatial confinement in nanophosphors [5]. Howeve r, as the particle si ze of phosphor materials was reduced to the nanos cale, the luminescence e fficiency of nanophospho rs decreased due to large surface area with many de fects as compared to bulk phosphor s. Thus, phosphors with 15

PAGE 16

several micron diameters have been mostly applied to displa ys and lamps [6]. In order to get high photoluminescence in nanophosphor particles, large amount of excitin g energy s hould be absorbed by th e activator and simultaneousl y the excitons re turn to the ground state by the radiative process. The luminesc ence efficiency of pho sphors used for lighting applications is mainly depends on the characterist ics of the prepared pho sphors such as partic le size [7], surface morphology [8], con centration quenching [9] and crystallinity [10]. Methods for improving the efficiency of phosphor materials have cen tered on improving the physical properties of phosphor materials by controlling surf ace morphology and size of phosphor materials and reducing concentration qu enching, etc. These properties can be co ntrolled by preparation technique and processing temperatures. The conventional method for sy nthesis of phosphor materials is the solid-state reaction process which requires repeated milling and anneal ing of precursor materials. The phosphor materials prepared by solid-state reaction pro cess/method usually contai n impurities caused by poor mixing of chemical compounds and require further annealing at high temperat ure for a long time. Moreover, this convention al reaction method is not appr opriate to obta in nanophosphor particles as it requires cumbersome and time-cons uming process such as ball milling is needed [11]. Thus, several synthesis tech niques such as as sol-gel [12] combustion synt hesis [13], and co-precipitation [14] have been used to overcom e above mentioned limitati ons for preparation of nanophosphor particles. However, the current synthesis methods for nanophosphor materials ha ve some limitations such as high cost, low production rate, expensive prec ursor materials and inhomogeneous composition. Therefore, to overcome the main drawback of the preparation technique for 16

PAGE 17

commercial use of nanophosphor part icles, it is required to develop a new sy nthesis method for nanophosphor particles. FSP is one of the most powerful methods for synthesis of nanophosphor particless because of its advantages such as less contamination as it is a simple method, good precursor mixing using nitrate aqueous precursor and high purity of phase due to high flame temperature. Also, the composition of phosphor particle s generated by FSP can be easil y controlled. The FSP process will be used to systematically co ntrol the surface area, the particle size, and the crys tallite size of phosphor materials like YAG:Ce 3+ and Y 2 O 3 :Eu 3+ which are one of the most popular yellow and red phosphors respectively, bein g used for lamp an d display pane l. The single isolated nanophosphor particle ge nerated by FSP will show high puri ty, controlled stoichiometry and crystallinity because the flame can be maintained at high temperatures, required for complete thermal decompositions through intense oxidation. To better control the properties of nanophosphor particles such as particle size, si ze distribution and morphol ogy, an understanding of the effects of main factor s during the FSP is required. Since phosphor materials have become a practical important in for modern engineering applications in the area of opti cal, electrical, biologi cal, mechanical, it is really important to investigate the full po tential of nanopho sphor material s such as Y 2 O 3 :Eu 3+ YAG:Ce 3+ and Zn 2 SiO 4 :Mn 2+ [5]. The main goal of this work is the fabrication of hi gh quality nanophosphor particles using FSP for ge tting high light emitting efficiency Moreover, experimental conditions such as addition of additives, overall concentration an d different starting materials for liquid precursor for synthesis of nanophosphor particles by FSP having high luminescence efficiency were investigated. With FSP meth od, we can easily control the crystallinity, crystal phase, size and morphology of the na nophosphor particles by us ing different experime ntal conditions and 17

PAGE 18

starting precursors. As I ment ioned above, our study suggests that high quality nanophosphor particles have some a dvantages over powders for solid st ate lighting and flat panel display application and can be obtained through the FSP process. 18

PAGE 19

Figure 1-1. Nanotechnol ogy; Nanoscale Particles. 19

PAGE 20

CHAPTER 2 LITERATURE REVIEW 2.1 Basic Concept and Applicatio ns of Phosphor Materials Phosphor materials play a key role in manuf acturing high quality fluorescent lamps and emissive displays [15]. Rare ear th doped oxide phosphor materials are wi dely used in optical devices such as cathode ray tubes (CRTs) and fiel d emission displays (F EDs) [16]. Almost all the luminescent materials or pho sphors are solid inorga nic materials which consist of a host lattice and luminescent center as intentionally doped small amounts of certain impurties [17-19]. Small amount of intentionally added impurities, so-called activators, are primarily responsible for the luminescence. When externa lly provided excitation light hits the host the absorp tion of the energy takes place by either the host or on impurities. The ex cited photoelectrons by absorption of energy on impurities are generated which then re turn to a lower energy state by the emission of a photon or heating of the host matrix (nonr adiative return). Almo st all the emissions occur on intentionally doped impurity at oms (such as rare-earth elements). Phosphor materials are widely applied to applic ations such as fluore scent lamps, emissive displays but currently th ey are also used in some X-ray detector systems [19]. The BaMgAl 10 O 17 dopoed with Eu 2+ is a typical blue phosphor material for high qual ity fluorescent lamp and plasma display panel (PDP) applications because of its high luminescen ce properties under UV or VUV excitation. In CRT application, Y 2 O 3 :Eu 3+ phosphor is the mo st popular phosphor material for red color because it shows good saturation behavior compared to other red phosphor materials. In addition, Zn 2 SiO 4 :Mn 2+ as green emitting phosphors is most widely applied for commercial PDPs because of its high luminescence efficiency an d chemical stab ility under UV or VUV excitation. 20

PAGE 21

2.1.1 Mechanism of Light Emissi on in Phosphor Particles Typically, phosphor materials are solid inorganic materials cons isting of a host lattice and intentionally doping with a small amount of rare-e arth ion impurities, as shown in Fig 2-1. When the external photon sour ces are used for exc itation of the phosphor, a phosphorescence process (light absorption, excitati on, relaxation, and emissi on) takes places in the material, as depicted in Fig. 2-2. The photon energy is absorbed by either the host material or is directly ab sorbed on impurity atom; the absorbed energy on host atom is tran sferred to the impurity atom (so-called activators). As a result of that, the activators emit visible light by relaxation of photo excited electron to lower energy state. In most cases, the em ission of photon from phosphors is generated from impurities. The energy transf er mechanism in phosphor materials is descri bed in Fig. 2-3. The luminescence process in pho sphor materials further explai ned in detail below [15, 20, 21], as depicted in Fig. 2-4. When the excitation radiation h it the phosphor material, it is absorbed by the activator. Typically, the meta stable state is a triplet state, and the ground state is a singlet state. The ground state of the activator energy level represents the singlet ground state (S 0 ). Upon excitation by an incident radiation, the activator energy level is raised to the singletexcited state (higher than S 1 ) by absorbing energy fr om the host. After the absorption, the initial excited states of energy leve l relax to an energy level S 1 through radiationl ess transition by emitting phonons or lattice vibrations. The intersystem cr ossing from a singlet state (S 1 ) to a triplet state (higher than T 1 ) takes place by spin-orb it coupling. In a triplet state, it decays down relatively quickly to the lowest energy level and once gets to the lowest triplet state, and stuck there, at least for a while. The excited triplet state (T 1 ) of the lowest energy level returns to the ground state (S 0 ) by emission of a photon, which is emitted luminescent radiation. In the light emission (phosphorescence) proces s of the rare-earth doped phosphor materials, the color of the emitting light from phosphor materi als depends on the type of doping impurities in the host 21

PAGE 22

matrix. For example, red pho sphor could be generated by adding europium to Y 2 O 3 cubic structure when UV or VUV is used as excitation source [2224]. In additi on, Mn-doped Zn 2 SiO4 (Green) [25-27], Eu-doped BaMgAl 10 O 17 (Blue) [28-30], and Ce-doped Y 3 Al 5 O 12 (Yellow) [3134] are used as phosphor material s for practical applications. 2.2 Nanophosphor Particles Nanophosphors are phosphor particles with size close to or below 100 nm. Since early 1990s, a novel concept of nanophos phor has been used and extens ively investig ated due to unique chemical and phy sical properties compared to thei r bulk materials. The significant differences are expected fr om the confinement effects on nanophosphor which affect luminescence efficien cy and photodynamics [35]. For na nophosphors, the phonon density of states (PDOS), which are significantly different from that of bulk ma terials, are reduced by dimensionality of particles. In addition, nanophosphor phonon levels are also discrete [36, 37]. Further, 4f electronic states originated from rare -earth ions contained in the nanophosphor particles is highly local ized and is not altere d by reduced size of phosphors. These unique properties can extend nanophosphors use to many potential applications such as optical, electrical, medical and bi ological technologies. 2.2.1 Advantages of Nanophosphor Particles Recently, many na nophosphor materials have been investigated for display applications such as Y 2 O 3 :Eu 3+ and white ligh t LEDs with Y 3 Al 5 O 12 :Ce 3+ Nanosize phosphor particles are highly desirable for their use in high resolution imagin g applications. One of the advantages of nanophosphors is that when particle size of phosphor materials is redu ced to nanometer range, the surface/volume ratio is much larger than that of micron-sized powders. This can extenuate volume effects over surface and interface effect s [38, 39]. In addition, the doping materials in nanophosphors can be we ll distributed and high ly uniformed compare to bulk phos phor during 22

PAGE 23

their synthesis [38]. Thus the high surface/volume ratio can reduce the energy tr ansfer between the activators as luminescence center due to well separated and located in each nanophosphors and, as a result of that, thes e interface effects of the nanosize material s would suppress the concentration quenching [40, 41]. According to these effects, photolumi nescence characteristics can be enhanced by the use of nanophosphor part icles. Furthermore, it ha s been reporte d that the efficiency of phosphors for white light applica tion could be increased by nanophosphor because nano-sized YAG:Ce 3+ phosphor shows higher lu minescent efficiency than that of larger size particles [38]. This enha ncement comes from reduction of inte rnal light scatte ring in phosphor layer when coated onto a bare LED surface. When the nanophosphor particles ar e applied to coat onto the surface of LEDs, internal scatteri ng within the materials ca n be reduced because the particle size of na nophosphor is much smaller than the wavelength of th e visible light [38]. So, using nanophosphors for ligh ting applications would improve th e emission effici ency of white LEDs [42, 43]. With these advantages nanophosphor materials can be exte nded in many areas for display and solid-state lighting fields. Therefore, the preparation of the nanophosphor particles with narrow particle size distribution and good crystallinity is very important pa rameter to enhance the photoluminescence properties fo r commercial applications. 2.2.2 Potential Applications of Nanophosphor Particles in Solid State Lighting Devices Among those applicatio ns, nanophosphor particles are promising in the application of solid state lighting devices. Since the blue light emitting di ode (LED) based on GaN was invented, it has become a promising solid stat lighting (SSL ) device, which has great potential in lighting applications due to hi gh lighting efficiency and low consumption of electrical energy [44]. Especially, white light can be generated by combining a blue LED with yellow phosphor materials which convert part of the blue LED emission. 23

PAGE 24

For lighting applications white light LEDs are ve ry comparable to s unlight spectrum. The most straightforward met hod to generate white light is by combining the lights with three fundamental colors, namely, red, green and blue (RGB). White LEDs may be produced in this way as well [45-47]. However, th is method requires a more compli cated electrical design for the control of light intensity and uniformity. A white LED may also be produced by coating downconverting phosphor layers onto the surface of a LED. The principle is ba sed on the absorption and re-emission of light [48]. One example is to coat the yellow phosphor layer (YAG:Ce 3+ ) on a blue LED chip [49, 50]. Phospho r materials are deposited on top of an LED chip in order to convert part of the blue light in such a way that by additive color mixing, white light is generated (see Fig. 2-5). With the excitation by blue light the phosphor layers can emit yellow light. The un-absorbed blue light, combined with the exc ited yellow light, can re sult in white light, as illustrated in Fig. 2-5 and 2-6, to give and i llumination of white ligh t [51-55]. Currently, the phosphor coating methods are commonl y used because of the lower co st and this approach yields a higher efficien cy and much better light-distribution characteristics compared to, for instance, a combination of separate LED chips emitting blue, green, and red light. Si milarly, an UV-LED chip coated wi th red, green and blue phos phor layers can also generate white light [56, 57]. However, these methods require a precise weighting of each kind of phosphor laye r in order to give desired white light. For white light applica tion, coating on blue LEDs is more popular be cause of the higher energy conversion efficiency Therefore, in order to get the high efficiency light extrac tion, high quality of yellow or gr een and red phosphors with strong blue absorption is ne eded. Specially, for both white light UV LEDs and white light blue LEDs, the efficiency of the devices is largely dependent on the efficien cy of the phosphors. Color rendering of the phosphors is also critical to 24

PAGE 25

obtain a spectrum close to su nlight. Highly effici ent light emitting phosphors are important components for white light LEDs [58]. In ta ble I., most important pho sphors for practical uses are showed [59]. 2.2.3 Issues on Nanophosphor Particles In recent years, nano-s ized phosphor materials have been of interest in di splay fields because of its improved properties such as high luminescent efficiency quantum effect, and higher doping concentration without concentration qu enching [60]. Goldburt et al [61]. reported that the photoluminescen ce efficiency could be improved by decreasing the particle size of doped nanocrystal line phosphors. Furthermore, decrease in phosphor part icles to nano-size lead to a large surface-to-volume ratio, which makes it a more promising material in display field [62]. Crystallinity of phosphor pa rticles depends on the reaction temperature and sintering time during the particle formation and also on subsequent annealing treatments. In order to obtain better quality of phosphor particles, it is necessary to have good precursor mixing for ho mogeneity, controlled morphology an d narrow size distribut ion in phosphor particle s, and less impurity contents [22, 62]. In case of cerium-doped Y 3 Al 5 O 12 (YAG:Ce 3+ ) particle as yellow emission phosphor, it is one of the most important phospho rs in solid state li ghting fields [63-65 ] because it can be applied to generate the white lig ht by mixing of blue LEDs. One of important parameters in white LED is the qu ality of phosphor part icles. In order to improve the light emitting efficiency from white LEDs, it is necessary to have highly efficient light emitting phosphors on blue LEDs [66]. Mn-doped zinc silicate (Zn 2 SiO 4 :Mn 2+ ) is widely used as a green-emitting phosphor materials for plasma display panels (PDPs) and cathode ray tubes because of its chemical stability, high luminescence efficiency and semi-conducting propert ies [67-70]. For these 25

PAGE 26

application, it is important to synthesize the pho sphors with high luminescence efficiency, small particle sizes and cont rolled morphology and good crystallinity with well -doped impurities. The luminescence properties of phosphor particle s are strongly dependent upon the synt hesis method. Therefore, it is important to develop a process which can cont rol the characteristics of the prepared nanophosphor pa rticles such as surf ace morphology, size, comp osition, and purity. Becoming a commerciali zed method to generate nanophosphor particles, the technique requires to be low cost, simple processi ng and high production rate and to be capable of producing high purity materials at high yields. 2.3 Preparation Techniques for Nanophosphor Particles Phosphor particles can be fabricated using vari ous techniques such as solid-state reaction method [71, 72], sol-ge l processing [73, 74], combustion method [75, 76], and spray pyrolysis [77]. Traditionally, phos phor particles are pr epared by a solid-state re action method with high reaction temperature for long time period followed by repeated ball milling process. It is difficult to prepare phosphor particles with controlled shape and fine size by this method. Another drawback of the solid-state reactio n method is the introduction of impurities in pho sphors during milling process or washing with chemicals. Thes e limitations strongly affect the luminescence properties of the phosphor particles. Another major problem with solid-state reaction method is very high cost particularly to obtain nano-sized particle s due to multi step and long processing time. This makes it industri ally non-reliable method. Thus, many techniques have been researched to prepare nanophosphors or nanoparticles for about 20 years.. 2.3.1 Sol-Gel (SG) The sol-gel method is one of the trad itional wet chemical tech niques for producing metal oxide nanoparticles through chemical processes; hydrolysis, gelation, fo llowed by drying, and finally thermal treatment, as shown in Fig. 2-7. In general, the sol is defined as colloidal particles 26

PAGE 27

suspended in a liquid from which a gel can be formed. On the other hand, the gel is an interconnected, rigid network, having submicro meter pores and a polymer ic chain whose average length is of the order of microns. In sol-gel processing, metal alkoxides are used as a reactive metal precursor and hydrolyzed wi th water. By adding of the appropriate reag ents, homogeneous gels from the mixture of alkoxid es can be produced th rough the processes of hydrolysis and gelation. After gelation, the precipitate is subse quently washed, dr ied, and then sintered at an elevated temperature to obta in crystalline metal oxide nanoparticles [17, 78]. The sol-gel process for preparation of phos phor particles has many ad vantages over otherl method. The advantages of the solgel method include ea sier composition control, better homogeneity and a lower re action temperature which is better suited to sy nthesizing high purity, fine powders. In addition, the pa rticle size of the final product can be easily controlled by changing the initial concen tration of the starting sols and interm ediate processing conditions such as gelation, drying, calcination, and rates of cooling. It has been reported that na nophosphors or metal-oxide nanopa rticles can be produced by sol-gel method [79, 80]. Howeve r, even though sol-gel method has a lot of advantages such as high homogeneity of the chemical composition of the materials, processi ng temperature can be very low, and high unifo rmity of doping ion distri bution, it has the follow ing disadvantages. In sol-gel processing, the drying and annealing proce sses have to be slow and deliberate; otherwise, cracks and striations w ill appear in the samples, and it is difficult to comple tely remove the residual hydroxyls from the sol-gel materials. In order to get rid of these orga nic groups, samples have to be an nealed above 1000 and this may produce unde sirable side effects. 27

PAGE 28

2.3.2 Combustion Synthesis (CS) In recent years, combustion synthesis has been resear ched to produce homogeneous, crystalline, oxide nanoparticles ov er time-consuming techniques su ch as solid-state reaction and sol-gel processing [17, 81, 82]. Combusti on synthesis is a self-pro pagating high temperature synthetic method an d an effective, low-cost method for the production of homo geneous, very fine agglomerated multicomponen t oxide ceramic powd ers without intermed iate decomposition and/or calcination steps [83]. Fo r phosphor particle preparation, combustion synthe sis method is promising technique due to its ab ility to produce fine size of part icles without high temperature annealing and extra steps such as grinding or milling [84]. In combustion sy nthesis, a starting aq ueous precursor corres ponding metal nitrates (oxidizers) and a suitable organic fuel (reducer), predetermined in stoichiometric ratio, is induced to boil. The mixture is ignited and a self-sustaining fast combustion reaction resulting from the appropriate combination of oxidizers and re ducers produces fine crysta lline oxide po wders. In the above process, the energy released from highly exothermic ( H < -170 kJ/mol) reaction between the nitrates and the fuel, which is usuall y ignited at a temperatur e much lower than the actual phase transformation temperature, can rapidl y heat the system to a high temperature and sustain it long enough, even in th e absence of an external heat so urce. Sequence of events during combustion synthesis for the preparation of pho sphor particles is sh own in Fig. 2-8. The advantages of combustion synthesis method are inexpens ive sources, easy to set-up the equipment and simple process to produce th e powder samples, unlike multi-step needed in conventional method. Cu rrently, it has been reported that nanopho sphors or metal-oxide nanoparticles can be produced by combustion synthesis technique [85, 86, 87]. However, combustion synthesis has some problems when appl ied for preparation of nano-sized particles. The process is inherently difficult to control because of high temperature experimental condition 28

PAGE 29

and also requires high energy consumption. Indeed, the product requi res additional processing like grinding or milling to obtain nano-sized phos phor particles. The post-processing may introduce the contaminat ion to final product. 2.3.3 Spray Pyrolysis (SP) Recently, spray pyrolysis has attracted attention because it is easy to produce fine size and spherical morphology of phosphor particles. In addition, it has some advantages such as inexpensive precursor ma terials and high production rate. To prepare phos phor particles by spray pyrolysis, liquid precurso rs as a starting solution, usually in expensive source materials such as nitrate, acetate and chloride are prepared by diss olving in water or alcoho l as precursor solvent. Figure 2-9 shows a general stage of spray pyro lysis process [88]. The droplets, which are atomized from a starting solution are supplied to a series of furnaces. And then the aerosol droplets experience evaporation of the solvent, di ffusion of solute, dryi ng and precipitation, reaction between precursor and surrounding gas. The precipitate und ergoes pyrolysis, or sintering inside furnace at higher temperature to form final product [88]. For phosphor particles, two furnace systems in series are required to enhance the quality such as crystallinity and morphology of final product. The first furnace promotes formation of micro-porous particle, and the second furnace further densifies the micro-porous particle and increases the cr ystallinity. In spray pyrolysis, the final compos ition of phosphor partic les would be determined by the starting precursor solution composition. Furthermore, with a starting solu tion, the particle size can be easily controlled by adjusting overall concentration of liquid precursors at fixed a sprayed droplet size. The schematic of spray pyrolysis system for production of fine particles is shown in Fig. 2-10 [88]. Although spray pyro lysis method is relati vely simple and low co st method to produce phosphor particles with high production rate in a single continuous process, it also has 29

PAGE 30

limitations to obtain nanophosphor particles wi th high luminescence efficiency. In spray pyrolysis process, hollow and hi ghly porous phosphor particles are generated. In case of phosphors porosity act as structural defects and can lead to decr ease in luminesc ence efficiency of the prepared pho sphor particles. Mo reover, in order to produce nano-sized particles by spray pyrolysis, it is require d to have lower overall concentration of starting precurso r solution or by introducing smaller droplet size into the reactors, as a result, the production rate in decrease. 2.4 Flame Spray Pyrolysis (FSP) Today, flame synthesis is the most widely us ed process to produce oxi de nanoparticles at commercial level. Carbon black made by Cabot, Comlombia, Degussa., etc ., fumed silica (Cabot, Degussa., Wacker, etc.), pigmen tary titania (Dupont, Ishihara, Milenium Kerr-McGee) and optical fibers (Corning, Heraeu s, Lucent, Sumitomo) [89-91] ar e some of the commercially available material synthesized by FSP. Also, industrial flame reactors are well-established technology for the synthesis of particles. FSP is promising tech nique because various types of precursors can be applied for production of oxide nanoparticles. 2.4.1 What is FSP? Flame process is the most wide ly used synthesis technique for productio n of nanoparticles at industrial scale due to its cost-eff ectiveness and process versatil ity for controlled production of nanopaticles, especially with sphe rical shape and fine size. More recently, classical flame aerosol method based on vapor-feed has evolved into a more versatile liquid-fed tec hnique so-called flame spray pyrolysis (FSP). FSP is originated fro m combination of flame and spray techniques and was based on modification of conventional spra y pyrolysis to overcome its drawback such as the formation of hollow particle. Electrical furnace was replaced to flame to give high temperature near the melting po int of the preferre d materials during flame synthesis. 30

PAGE 31

FSP is a flame method that utilizes a liquid mixture of source salt and solvent as a precursor [89]. By utilization of a liquid precurso r, mixed metal oxide particles can be generated easily. The use of a liquid pr ecursor is also dr iven by the low cost of me tal salts (such as nitrates and acetates) and the high availability and also high solubility of metal sa lts in water or organic solvent. As the name sugg est, a liquid spray instead of vapors is fed into the flame, undergoing subsequent evaporation and combustion. In flame reactors, the energy of the flame is used to drive chemical reactions of prec ursors producing clusters which further gr ow to nanoparticles by surface growth and/or coagulation and coales cence at high temper ature [92, 93]. Synthesis of nanoparticles by FSP is a process where the precursor is released to the gas phase by combustion of fuel-droplets. These droplets consist of precursor and liquid fuel. This process is different from conven tional synthesis of pa rticles in flames, where the precursor already is present in the gas-phase from the beginning. The advantage of FSP compared to conventional flame-synthesis processes is that it is possible to use a wide range of precursors and liquid fuels. Furthermore, it is also possible to includ e a carrier-material in the liquid, which can be useful in the manufacture of heterogene ous catalyst. 2.4.2 Mechanism of Nanophosphor Particle Formation by FSP The mechanism of particle generation and gr owth from aqueou s droplets sp rayed in the diffusion flame is as follows (F igure 2-11): first, large precur sor droplets ge nerated by the ultrasonic atomizer disintegrate into much smaller droplets as they enter the hi gh temperature flame, and then the small dr oplets successively experien ce evaporation of solvent (H 2 O or Xylene), precipitation of solute and drying, th ermal decomposition and oxidation, and finally coalescence, and coagulation at th e end of high temperat ure flame, as illustrat ed in Fig. 2-11 [94]. The disintegration of large drople ts of the precursor into smalle r droplets slightly above the burner exit would occur due to hi gh temperature gradient betwee n the aqueous droplets and the 31

PAGE 32

high-temperature flame, probably causing an abru pt change in surface en ergy of the droplets. The disintegration of large droplets plays a very crucial role in the generation of nanoparticles by the FSP. The disintegration of large droplets plays a very cruc ial role in the generation of nanoparticles by FSP method. 2.4.3 Advantages of FSP for Nanophosphor Particles FSP, also called liquid flame sp ray (LFS), is a promising partic le synthesis method because it can use a wide range of precursors fo r synthesis of a broad spectrum of functional nanoparticles. The heat released from the comb ustion of a ga seous or liquid fuel and the precursor itself ca n provide the high-temperature environm ent, which is favo rable to phosphor synthesis. The flame temperatur e and particle residence time, which are the most important parameters determining the character istics of the particles, can be easily controlled by varying fuel and oxidizer flow rates. Mo reover, the particle size can be controlled by varying precursor solution concentration. Multi-component particles can also be obtained by adding different salts into the solution. This technique can be easily scaled up with high production rates for the manufacturing of commercial quantities of nanopar ticles. Also, the advantages of FSP include the ability to dissolve the precur sor directly in the fuel, simplicit y of introduction of the precursor into the hot reaction zone (e.g. a flame), and the fl exibility in using the high-velocity spray jet for rapid quenching of aerosol. Finally, in general, most nanop article synthesis processes re quire a post production step for crystallizat ion of the final products and elimination of undesir able byproducts, such as chlorine and water [95]. However, flame synthesis of nanoparticles eliminates this post-heat treatment step be cause calcinations take plac e within the process, resu lting in dense (spherical shape) and crystal line final product. 32

PAGE 33

2.4.4 Selection of Precursor Solven ts, Additives and Source Materials First of all, the optimi zation of an efficient phosphor requires proper selection of the host and the activator mate rial. The host is fo r the absorption of energy (e xcitation), chem ical stability, thermal quenching and shou ld be available as powde r or thin film. The activ ators should be red, green and blue line emitters preferably with loca lized transition leading to efficient trapping center for high efficiency The rare-earth (RE) ac tivators satisfy the above requirements with the exception that the excited carriers in the host transfer rath er slowly to the ra re-earth activators. For example, Cerium(III)-doped Yt trium aluminum garnet (YAG:Ce 3+ ) is a phosphor, or a scintillator when in pure single-crystal form, with wide range of uses. Because phase-pure YAG is optically transparent, the dod ecahedral site can be partially doped or complete ly substituted with other rare-earth ca tions for applications such as soli d-state lighting an d phosphor powders for cathode ray tubes [ 96]. It emits yellow light when subjected to blue or ultravio let light, or to x-ray light. It is used in white light-emitting diodes as a coating on a hi gh-brightness blue InGaN diode, converting part of th e blue light into yellow, which then appears as white. In this study, nitrate-based liqui d precursors are used as a starting mate rial. These nitrate source materials are widely used in flame synthesis process due to low cost, simple and very good solubility in water. However, the meta l-nitrates are well-known as the endothermic species, which promotes form ation of hollow or fragme nted phosphor part icles [97, 98, 99]. The final product prepared using these precu rsors experienced th rough insufficient en ergy environment. Thus, in order to get enough energy to react in flame zone, it is required to give external energy using fuel solvents such as alcohol or xlyene an d additives like urea in th e precursor solutions. 33

PAGE 34

Figure 2-1. General luminescent materials 34

PAGE 35

Figure 2-2. Phosphorescen ce process in phosphors 35

PAGE 36

Figure 2-3. Energy transfer mechanis m for phosphorescence 36

PAGE 37

Ground singlet state Excited singlet state Photon Absorbtion Excitation T1S1S0Spin-orbit coupling Excited triplet state Metastable triplet state Vibrational relaxation Figure 2-4. Phosphorescence: light absorption, excitati on, nonradiative dacay and light emission, and return to the ground state S 0 37

PAGE 38

White Light White Light Figure 2-5. Blue LED with yellow phosphor coating for white light generation 38

PAGE 39

Figure 2-6. Principle of white light generation form the mixing of blue luminescence and yellow phosphorescence [57] 39

PAGE 40

Figure 2-7. Sequence of events during sol-gel process [100] 40

PAGE 41

Figure 2-8. Sequence of events during combus tion synthesis. A) self-proragating combustion mode, B) thermal explosion mode [101] 41

PAGE 42

Figure 2-9. Morphology of particle prepar ed by spra y pyrolysis method 42

PAGE 43

Figure 2-10. Spray pyrolysis system 43

PAGE 44

Figure 2-11. Nanoparticle formation fr om precursor droplets by the FSP 44

PAGE 45

Table 2-1. Most important phosphors for practical uses [102] 45

PAGE 46

CHAPTER 3 EXPERIMENTAL PROCEDURE 3.1 Apparatus of Flame Spray Pyrolysis The schematic of the FSP system is shown in Figure 3-1. The system consisted of spray burner, quartz tube, spray generator, particle collection bag filters and vacuum pump. The fine droplets were supplied to the hi gh temperature diffusi on flame zone above spray burner using syringe pump with cons tant liquid feed rate, in which the phosphor particles we re generated, resulting from evaporation, decomposition and melting of the droplets. Figure 3-2 shows the schematic of flame nozzle which consisted of th ree concentric pipes. An oxygen and methane gas through the outer pipe s in flame nozzle was us ed to generate the diffu sion flame. The flame temperature and the residence time in the diffusion flame zone ca n be controlled by varying the amounts of the methane and oxygen gases and th e flow rate of the li quid precursor. The experiment condition for nanophosphor particle s by FSP is illustrated in Table 3-1. 3.2 The Preparation of Liquid Precursors 3.2.1 Y 2 O 3 :Eu 3+ Red Phosphor For the preparation of liquid precursor, yttr ium nitrate(Y(NO 3 ) 3 6H 2 O, 99.99% Alfa Aesar) was used as the source of Y 3+ and europium nitrate (Eu(NO 3 ) 3 6H 2 O, 99.99% Alfa Aeser) was used as doping material of Eu 3+ These nitrate raw materials were dissolved in et hanol and then urea was added into the starting liquid precursor. Addition of urea into liquid precursor serves as fuel for flame synthesis. Urea addition was varied from 1 M to 4 M into the precursor. The mixture was stirred for 30 min at room temperature until a hom ogenous solution was achieved. The doping concentration wa s varied from 5 mol% to 25 mol% with respect to y ttrium. To study the effect of overall loadin g concentration of liquid precursor on crystallinity and photoluminescent spectra, it was varied from 0.3 M to 0.9 M. Methane gas was used as the fuel 46

PAGE 47

source gas with a constant flow rate. Oxygen gas was used for both complete combustion reaction and also for spraying of droplets. 3.2.2 Y 3 Al 5 O 12 :Ce 3+ Yellow Phosphor The starting materi als for the YAG:Ce 3+ phosphors were used yttrium nitrate (Y(NO 3 ) 3 H 2 O, 99.99%) and aluminum nitrate (Al(NO 3 ) 3 H 2 O, 99.99%) as the source of Y 3+ and Al 3+ Also, cerium nitrate (Ce(NO 3 ) 3 H 2 O, 99.99%) was used as doping material of Ce 3+ These nitrate raw materials were dissolved in ethanol and then urea was added into the starting liquid precursor. Addition of u rea into the liquid precursor serves as fuel for flame synthesis. The different molar ratios of yttrium to aluminum were prepared by 3:5 and 3:7 in order to find the effect of excess of aluminum in liquid precur sor on luminescence pr operties. The overall concentration and th e mole of urea were fixe d at 0.1 M and 2 M into the precursor. The doping concentration was fixed at 4 mol% with respect to yttrium. The mixture was stirred for 30 min at room temperature until a homoge nous solution was achiev ed. Methane ga s was used as the fuel source gas while oxygen was used as an oxidant and as a carrier gas to spray precursor. Asprepared phosphor pa rticles were placed in an electric furnace fo r post heat treat ment, which was carried out under atmospheric conditions. The particles were annealed at different temperature varying from 800 to 1100 for 1hr to confirm th e phase transformation temperature of asprepared phosphors. The he ated-particles were cooled to room temperature by natural convention and were used as it is for further characterization. 3.2.3 Zn 2 SiO 4 :Mn 2+ Green Phosphor Zn 2 SiO 4 :Mn 2+ green phosphor particles were prepared from th ree different liquid precursors. Three different types of starting materials for the preparation of Zn 2 SiO 4 :Mn 2+ nanophosphors were prepared using zinc nitrate (Zn(NO 3 ) 2 6H 2 O), zinc 2-ethylhexanoate 47

PAGE 48

(Zn(OOCCH(C 2 H 5 )C 4 H 9 ) 2 ) and zinc a cetate (Zn(C 2 H 3 O 2 ) 2 2H 2 O) as the zinc source; and tetraethylorthosilicate (TEOS, Si(OC 2 H 5 ) 4 ) was used as the silica so urce; and manganese acetate tetrahydrate (Mn(CH 3 COO) 2 4H 2 O) was used as the manganese doping source. All the source materials were dissolved in ethanol or etha nol and water with high purity. It is well known that the zinc acetate is not fa vorable to dissolve in etha nol. Thus, in case of th e zinc acetate to liquid precursor, distilled water and ethanol were used to prepare the starti ng precursor. The total overall concentration of the liquid precursor wa s fixed at 0.1 M, while the manganese doping concentration was at 4 mol% with respect to zinc. The mo lar ratio of zinc-source:TEOS:Mn = 1.96:1:0.4 was used for the preparation of liquid precurso rs. The mixture was stirred for 30 min at room temperature un til a homogenous solution was obtain ed. Methane gas wa s used as the fuel source gas with a constant flow rate. Oxygen gas was used for both complete combustion reaction and also for sp raying of droplets. As-prepared phos phor particles were placed in an electric furnace for post heat treatment, which was carried out under atmosphere condition. The particles were annealed from 800 to 1000 for 1hr to confirm th e phase transformation temperature of as-prepared phos phors. The heated-parti cles were cooled down natura lly after completely annealed. 3.3 Characterization of Nanophosphor Particles The crystallinity of nanophosphor particles is analyzed by using powder X-ray diffraction (XRD). The morphology and pa rticle size of na nophosphors were exam ined using field emission-scanning electron micros copy (FE-SEM) and transmission electron microscopy (TEM). Photoluminescent (PL) properties were determined using Xe-ramp as the excitation source. All the luminescence characterizat ion of the phosphors was carried out at room temperature. 48

PAGE 49

Table 3-1. Experimental Condition of Flame Spray Pyrolysis Nanophosphor Pa ritlces Y 2 O 3: Eu 3+ Y 3 Al 5 O 12 :Ce 3+ Zn 2 SiO 4 :Mn 2+ Spray Oxygen 2.1 l/min Combustoin Oxygen 0.75 l/min Methane Gas 1-3 l/min Feed Rate of Liquid Pr ecusor 1.91 cc/min Flame Temperature > 1500 Additives Urea Source Materials Nitrate, Actetate, 2-ethlyhexanoate Precursor Solvent Acohol, Water, Xylene 49

PAGE 50

M Spray Flame CH4 O2 O2 Knob Flow Meter Gas Cylinder Hood Glasfiber Filter Baghouse Filter Liquid Precursor from syringe or nebulizer Figure 3-1. FSP system for nanophosphor particles 50

PAGE 51

Combustion Oxygen Figure 3-2. Flame nozzle Ox yg en Methane Dro Methane Liquid Spray Ox p let p recursor yg en 51

PAGE 52

CHAPTER 4 ENHANCED LUMINE SCENCE PROPERTIES OF CUBIC Y 2 O 3 :EU 3+ NANOPHOSPHORS BY FSP 4.1 Introduction Phosphors are used in manufacturing high quali ty florescent lamps and emissive displays [15]. Rare earth doped oxide pho sphor materials are widely used in optical devices such as cathode ray tubes (CRTs) and fi eld emission displays (FEDs) [16]. Among these phosphors, europium doped yttrium oxide (Y 2 O 3 :Eu 3+ ) is the most popular phospho r material for red color applications. In recent years, na no-sized phosphor materials have been of intere st in display fields because of its improved pr operties such as hi gh luminescent efficiency, quantum effect, and higher doping concentration without concentration quenching [60]. Goldburt et al [61]. reported that the pho toluminescence efficiency could be impr oved by decreasing the particle size of doped nanocr ystalline phosphors. Furtherm ore, decrease in phosphor particles to nano-size lead to a large surface-to -volume ratio, whic h makes it a more promising material in display field [62]. Crystallinity of phosphor pa rticles depends on th e reaction temperature and sintering time during the particle formation an d also on subsequent annealing treatments. In order to obtain better quality of phosphor particles, it is necessary to have good precursor mixing for homogeneity, controlled mor phology and narrow size distribution in phosphor particles, and less impurity contents [8, 62]. Various types of techniques, such as sol-gel method [12], hydrothermal synthesis [103], co -precipitation [14], and combustion [13] have been used to generate phosphor particles. Most phosphor materi als are currently be ing produced by the conventional solid-state techni ques. The conventiona l way to produce pho sphors is by ball milling and calcination at high temperatures. In order to overcome multi-step process for generation of phosphors, flame spra y pyrolysis (FSP) met hod as direct synt hesis technique is attractive, particularly for fabricating nano-si zed, phosphors due to their possib ility of high 52

PAGE 53

production rate, simple method, high purity of phase, and low production cost By use of mixed liquid precursors, mixed metal oxide particle s can be easily generated by FSP. Also, the composition of phosphor particle generated by FSP can be easily c ontrolled. In the usual method for production of Y 2 O 3 :Eu 3+ by FSP, salts of yttrium and eu ropium e.g. sulfates, nitrates dissolved in water or ethanol are used as precursor [8, 104, 105]. The europium doping concentration is controlled by varying its salt concentration in the precursor. Y 2 O 3 :Eu 3+ produced by this method needs some postproduction annealing treatments to improve the crystallinity and luminescence properties of phosphor particles. One way to form particles with better properties, hence avoiding any post-treatment, is to increas e the reaction temperatur e during formation of these particles. Urea serv es as fuel for th e flame reaction. Varying th e amount of urea in liquid precursor, the flame temperature can be controlled. Th e addition of urea in liquid precursor for generation of phosphor particles provides additional heat in flame zone, which contributes to complete burning duri ng flame synthesis, and lead s to nano-size particle formation [106]. In this study, we have investigated the influence of urea addition on properties of Y 2 O 3 :Eu 3+ nanophosphors using FSP. The optical properties and crystallinity of Y 2 O 3 :Eu 3+ nanophosphors without post-heat treatment were investigated. The st rongly red light emitting Y 2 O 3 :Eu 3+ nanophosphors were produce d using urea-assisted fl ame synthesis techniqu e without any further treatment. 4.2 Experimental In this study, 5 mol% Eu-doped Y 2 O 3 (Y 2 O 3 :Eu 3+ ) nanophosphor powders were prepared by FSP without any post heat-treat ment. For starting liquid precursor the nitrates of yttrium and europium were dissolved in ethanol. The overall concentration of liquid precursor was fixed at 0.3 M. The addition of urea into liquid precurso r was varied from 1 M to 4 M. In order to achieve the homogeneity of aqueou s solution, this mixt ure was stirred by ultr asonic agitation for 53

PAGE 54

30 min at room temperature. The precursors were then placed in syringe pump fo r applying to burner. Methane gas was us ed as the fuel source gas with constant flow rate. Oxygen gas was used for both complete combusti on reaction and for spraying of precursor droplets. The FSP system consisted of spray burner, quartz tube, spra y generator, particle co llection bag filters and vacuum pump. The fine droplets were supplied to the high temperatur e diffusion flame zone above spray burner usi ng syringe pump with cons tant feed rate. The pho sphor particles were generated by evaporation, decomposition and melti ng of the droplets. The flame nozzle consisted of three concentric pipes. Oxygen and methane gas, through the outer pipes in flame nozzle, was used to generate the diffusion flame. The flame temperature and the residence time in the diffusion flame zone could be controlled by varying the amounts of the methane and oxygen gases and the flow rate of the liquid precursor. The phosphor partic les produced we re collected on filter paper. The filt er paper was kept over 200 C using heating tape to prevent the condensation of water vapor insi de the metal cylinder wall. The crystallographic phase purity of Eu-doped Y 2 O 3 (Y 2 O 3 :Eu 3+ ) nanophosphors was examined by XRD (Philips, APD-3720). The pho sphors prepared by FSP without further heattreatment showed the cu bic phase crystalline pa tterns. The morphology and size of phosphor particles were analyz ed by a FE-SEM (JEO L-6335F). TEM(JEOL-2010F) wa s used to determine particle size and shape at the accelerating voltage of 100 kV. Optical pr operties were measured using fluorescence spectrophotom eter. A xenon lamp was us ed for excitation of Y 2 O 3 :Eu 3+ nanophosphors in the UV region For comparison, all luminesc ence characteriza tion of the phosphors was measured at room temperatur e under the same measurement condition. 54

PAGE 55

4.3 Results and Discussion The crystallographic phase purity of Y 2 O 3 :Eu 3+ nanophosphors was an alyzed by XRD. Yttrium oxide has the two types of crystal structure: cubic ( ) and monoclinic ( ). It is well known that photoluminesc ence intensity of th e cubic phase of Y 2 O 3 :Eu 3+ phosphors is higher than that of monoclinic phase [104]. Thus, on ly pure cubic phase Y 2 O 3 :Eu 3+ phosphors are favorable for commercial luminescence. XRD patterns of as-prepared Y 2 O 3 :Eu 3+ nanophosphors with different mole of urea addition without he at-treatment are shown in Figure 4-1. The XRD patterns of a ll synthesized products, Y 2 O 3 :Eu 3+ phosphor, well matched with Joint Committee on Powder Diffraction Standards ca rd No. 41-1105 for cubic Y 2 O 3 Figure 4-1 shows no additional peaks of other phases, indicating that Eu 3+ dopant has been well doped in the host materials. Mixtures of cubic and monoclinic Y 2 O 3 were obtained by convention al FSP in previous studies [104, 105]. The pr esence of both phases necessitated a post-ann ealing treatment around 1000 C for monoclinic to cubic phase transfor mation. The XRD patt erns (Fig. 4-1) show the possibility of single step fabrication of cubic nanocrystalline Y 2 O 3 :Eu 3+ phosphor withou t any post-heat treatment by flame spray pyrolysis method. In FSP method, flame temp erature is an important factor to produce the cubic phase Y 2 O 3 :Eu 3+ phosphor with dense and sphe rical shape. It can be controlled by optimum methane/oxy gen gas ratio and precursor com position. In previous study, Purwnato et al [106]. mentioned that the additi on of urea into nitrate based liquid precursor promotes the formation of nanopar ticle and supplies additional heat in the flame zone, during the formation of YAG:Ce nanoparticles. Inse t in Figure 4-1 show s SEM image of Y 2 O 3 :Eu 3+ phosphor particles with 2 M of urea, shows a fi ne size and dense morpho logy in a nanometer dimension. Figure 4-2 shows TEM images of Y 2 O 3 :Eu 3+ nanophosphors prepar ed with 2 M of urea in the liquid precurso r. The particle size can be observed to be in 20-30 nm range. 55

PAGE 56

Figure 4-3 shows the luminescen ce properties of as-prepared Y 2 O 3 :Eu 3+ nanophosphor with different concentration of urea. Photolumin escence excitation spect ra were measured for emission at 609 nm, with excitation radiation from a xenon la mp varying from 350 nm to 450 nm. Figure 4-3 A shows the emi ssion spectra of as-prepared phosphors with different mo le of urea, at fixed 5 mol% Eu dopant in Y 2 O 3 The relative photoluminescence emission spectra are presented in Figure 4-3 B. We can observe that Y 2 O 3 :Eu 3+ nanophosphors exhibit a st rong red emission at 609 nm. The luminescent sp ectra of the as-prepared particles shows typical Y 2 O 3 :Eu 3+ emission spectrum, which is a transitions from 5 D 0 7 F J (J=0,1,2,3,4) levels of the Eu 3+ ion [107, 108]. The strongest emissi on peak around 609 nm arises from the 5 D 0 7 F 2 transition while less emission peak around 590 nm is due to the 5 D 0 7 F 1 transition. Luminescence intensity increased with urea concentr ation in the precur sor till around 2 M, after which the photoluminescence intensity shows a decreased in intensity. Photoluminescence inte nsity strongly depends on crystallinity as well as morphology of phosphors. Luminescent properties were observed to be dependen t on the urea c oncentration in the liquid precursor. Maximum photoluminescence intensity was observed when 2M of urea was present in the liquid precursor. The improvem ent of relative photoluminescence intensity is related with flame temper ature and complete burning step under proper experiment condition. Qin et al [105]. report ed that higher flame te mperature improves the br ightness and results in better crystallinity on as-prepared Y 2 O 3 :Eu 3+ phosphor particles. However, over 2 M of urea, low photoluminescence intensity results from impurit y caused by further a ddition of urea on phosphors. 56

PAGE 57

4.4 Conclusion In conclusion, we have demonstrated that nanocrystalli ne cubic phase Y 2 O 3 :Eu 3+ phosphor can be prepared by single step route, which i nvolves direct flame synthe sis from starting liquid precursor with addition of urea by FSP method. The e ffect of addition of urea in nitrate based liquid precursor on crystallinity, mor phology, and photoluminesc ence characteristics of Y 2 O 3 :Eu 3+ nanophosphor was invest igated. The addition of urea in starti ng liquid precursor provides additional he at in flame zone to produ ce phosphors with high crystallinity, contributes to complete burning du ring flame synthesis, and leads to nano-size particle formation. The phosphor particles produced we re in nanometer size range with dense morphology. Y 2 O 3 :Eu 3+ produced in the presen ce of 2 M of urea in the liquid precursor showed the maximum photoluminescence intensity. 57

PAGE 58

20304050607080 JCPDS No. 41-1105 2-Theta Intensity (a.u.)4 M 3 M 2 M 1 M Figure 4-1. X-ray Diffraction (XRD) patte rns of as-prepared 5 mol% Eu-doped Y 2 O 3 nanophosphor by FSP with different mole of urea. Inset is a Scanning Electron Microscopy (SEM) image of phosphor produced in pres ence of 2 M of urea. 58

PAGE 59

Figure 4-2. Transmission Elect ron Microscopy (TEM) images of as-prepared 5 mol% Eu-doped Y 2 O 3 nanophosphor by FSP with 2 mole of urea in liquid precursor. 59

PAGE 60

560580600620640660 4 M of Urea 0 M of Urea 1 M of Urea 3 M of Urea 2 M of UreaIntensity (a.u.)Wavelength (nm) A 0 M1 M2 M3 M4 M Relative PL intensity (a.u.)Mole of Urea B Figure 4-3. Photoluminescence (PL) spec tra of as-prepared 5 mol% Eu-doped Y 2 O 3 nanophosphor with different mole of urea in li quid precursor. A) em ission intensity of Y 2 O 3 :Eu 3+ nanophosphor, B) rela tive PL intensity of corresponding Y 2 O 3 :Eu 3+ nanophosphor 60

PAGE 61

CHAPTER 5 LUMINESCENCE PROPERTIES OF Y 2 O 3 :EU 3+ NANOPHOSPHOR PARTICLES PREPARED FROM UREA A DDED PRECURSOR USING FSP 5.1 Introduction Phosphor materials are wi dely used in hi gh quality optical displa y and lighting devices. Especially, europium doped yttr ium oxide is one of the most popular phosphor materials in red light emitting applicatio ns [15]. Recently, us e of nanometer size pho sphors has become an attractive idea in high resolution displays because it has great po tential in lighting applications with high effici ency [61, 109]. Many st udies were focused on cr ystallinity, morphology, luminescent properties and concentration quen ching of nanoscale phos phors with different synthesis methods and conditio ns. Decrease in pho sphor particle size in nanometer range increases surface-to-volume ratio and also the qua ntum efficiency [110]. Moreover, a smaller phosphor size could result in a smaller screen pixel size and a higher degree of transparency for phosphors dispersed in other substrates. Th e luminescent efficiency of phosphors for lighting applications is usually related to emission efficien cy, concentration quenching, and size and morphology of phosphors [8 111]. The requirements for impr oving the efficien cy of phosphor materials are based on hi gh quality of ho st materials, and reducing concentration quenching. Also spherical and dens e morphology of phosphors are required to get better photoluminescent characteristics [8]. Ther efore, it is necessary to make the phosphors with highe r luminescence efficiency and in na nometer size range. Many different types of techniques such as sol-gel met hod [12], hydrothe rmal synthesis [103], co-precipitation [14], and combustion [13] have already been applied to generate the phosphor particles. Among these methods, flame spray pyrolysis (FSP) is a powerful method and has a simple process for making particles in nanometer range. The advantage of FSP compared to conventional methods is the possibility to use a wide range of precursors and liquid fuels such 61

PAGE 62

as ethanol or xylene. Furthermore, FSP is a con tinuous single step system to make nanophosphor particles without post-he at treatment because fl ame temperature could be easily controlled by varying liquid fuel and oxidiz er flow rates, which is hi gh enough to make cubic Y 2 O 3 :Eu 3+ phosphors. In this research, we report the preparation of Y 2 O 3 :Eu 3+ nanophosphors using flame spray pyrolysis under different synthesis conditions. The crystal structure, morphology, size, and photoluminescent properties of the phosphor particle s are presented. The effect of urea addition in starting liquid precursor and overall concentr ation of aqueous solution on formation of asprepared phosphors wa s investigated. The influence of Eu-doping conc entration on luminescent properties of nanophos phors was studied. 5.2 Experimental For the preparation of liquid precursor, yttr ium nitrate(Y(NO 3 ) 3 6H 2 O, 99.99% Alfa Aesar) was used as the source of Y 3+ and europium nitrate (Eu(NO 3 ) 3 6H 2 O, 99.99% Alfa Aeser) was used as doping material of Eu 3+ These nitrate raw materials were dissolved in et hanol and then urea was added into the starting liquid precursor. Addition of urea into liquid precursor serves as fuel for flame synthesis. Urea addition was varied from 1 M to 4 M into the precursor. The mixture was stirred for 30 min at room temperature until a hom ogenous solution was achieved. The doping concentration wa s varied from 5 mol% to 25 mol% with respect to y ttrium. To study the effect of overall loadin g concentration of liquid precursor on crystallinity and photoluminescent spectra, it was varied from 0.3 M to 0.9 M. Methane gas was used as the fuel source gas with a constant flow rate. Oxygen gas was used for both complete combustion reaction and also for sp raying of droplets. 62

PAGE 63

The FSP system consisted of spray burner, quartz tube, spray generator, particle collection bag filters and vacuum pump. The fine droplets we re supplied to the high temper ature diffusion flame zone above spray burner us ing syringe pump with constant liquid feed rate, in which the phosphor particles were generated, resulting from evaporatio n, decomposition and melting of the droplets. An oxygen and methan e gas through the out er pipes in flame nozzle was used to generate the diffusion flame. Th e flame temperature and the reside nce time in the diffusion flame zone can be controlled by varying the amounts of the methane and oxygen gases and the flow rate of the liqu id precursor. The phosphor particles pr oduced were collected by filter paper. The f ilter paper was kept above 200 C using heating tape to prevent the cond ensation of water vapo r inside the metal cylinder wall. All samples were directly prepared by FSP method without further heat treatment. The crystallinity of Y 2 O 3 :Eu 3+ nanophosphors is anal yzed by using po wder XRD. The morphology and part icle size of Y 2 O 3 :Eu 3+ nanophosphors were examin ed using FE-SEM and TEM. Photoluminescent properties were determined using Xe-ramp as the excitation source. All the luminescence charact erization of the phosphor s was carried out at room temperature. 5.3 Results and Discussion Figure 5-1 shows the XRD spectra of as-prepared Y 2 O 3 :Eu 3+ nanophosphor particles with different overall concentration of liquid precursor at fixed 2M of urea addition: (a) 0.3 M; (b) 0.5 M; (c) 0.7 M; (d) 0. 9 M; (e) JCPDS No. 411105 Cubic phase of Y 2 O 3 The XRD patterns of all samples are in good agre ement with JCPDS No.411105 as cubic phase. Th e XRD patter ns show the possibility of single step fa brication of nanocrystalline Y 2 O 3 :Eu 3+ phosphor without any postheat treatment by FSP method. During the format ion of phosphor particles in FSP method, flame temperature is the most important parameter to de termine the properties of the phosphor particles. 63

PAGE 64

Flame temperature can be controlle d by varying oxidizer gas flow and adding fuel such as urea or carbohydrazide. In previo us study, Lu et al [12]. mentioned th at urea addition in starting liquid precursor provides additi onal heat in flame zone for particle formation and subsequently, the flame temperature increased. By increasing the flame temperatur e, it will support to evaporate the precursor droplet and generate smaller particle s. At high flame temper ature, the crystallinity of phosphor particle improved [111]. It impl ies that cubic phase Y 2 O 3 :Eu 3+ phosphor can be generated by urea assisted FSP method without further post-annealing step. The morphology of the Y 2 O 3 :Eu 3+ nanophosphors obse rved by SEM is show n in Figure 5-2. It shows a dense surface of the phosphor particles with slight agglomeration between particles. Y 2 O 3 :Eu 3+ phosphor particles have spherical shape. From analys is of TEM images, shown in Figure 5-3, the particle size is around 20-30nm. TEM micrograph also reveals th at as-prepared Y 2 O 3 :Eu 3+ nanophosphors are slightly agglomerated. Figure 5-4 shows the lumine scence properties of as-pre pared 5 mol% Eu-doped Y 2 O 3 nanophosphors with 2 M of urea addition in the precursor, while the overall concentration of liquid precursor was fixed at 0.3 M. The PL spectra of Y 2 O 3 :Eu 3+ nanoparticles was measured with excitation radiation from a xenon lamp at the wa velength of 393nm. Re d light emission was observed from Y 2 O 3 :Eu 3+ phosphor particles. Th e strongest emission pe ak was found at 609 nm, showing the typical lumi nescence spectrum of Y 2 O 3 :Eu 3+ phosphors, which is a transitions from 5 D 0 7 F J (J=0,1,2,3,4) levels of the Eu 3+ ions [107, 108]. The luminescent spectra around 609 nm is due to the cubic structur e of the as-prepared phosphor particles [110 ], which is in good agreement with XRD measurements The main emission peak at 609 nm is generated from the 5 D 0 7 F 2 transfer in Eu 3+ whereas the secondary em ission at shorter wave length, 590 nm, is due to the 5 D 0 7 F 1 transition [112]. 64

PAGE 65

The relative PL in tensity of the corre sponding phosphor partic les with different concentration of urea is shown in Figure 5-5. Lu minescent properties were dependent on the urea concentration in the liquid pr ecursor. PL intensity of Y 2 O 3 :Eu 3+ nanophosphors in creased with urea concentration in the precursor till 2 M, further excess of urea resulted in decr ease in the PL intensity. The most intense PL intensity was observed when 2 M of urea was added into the precursor. It is well known that crystal stru cture of phosphor particles has influence on luminescence propertie s. The addition of ur ea in the liquid precur sor can promote the luminescence intensity because it supplies additio nal heat in the flame zone and generates high flame temperature, consequently, improving the crystallinity of Y 2 O 3 :Eu 3+ phosphor particles. The improvement of relative PL intensity is related to complete bu rning at high flame temperature from urea added liquid precursor. Qin et al [105]. mentioned that the crystallinity and the brightness of th e phosphor particles coul d be improved at high flame temperature. However, over 2 M of urea, PL intensity decrea sed due to the presence of impurity from excess urea in the phos phor particles. A higher doping concentration without conc entration quenching on phosphor particles would show higher luminescence efficiency. It is very important and challenging to develop phosphor materials with le ss quenching effect in solid state lighting fiel ds. The effect of Eu doping concentration on the PL in tensity of phosphors wa s investigated at fi xed 2 M of urea in liquid precursor. Figure 5-6 shows the PL intensity of as-prepared phosphor particles with different Eu doping concentrations while overall concentration in precursor is fixed at 0.3 M. Figure 5-6 A shows the emissi on spectra of as-prepared phosphors with di fferent doping concentration. The relative photol uminescence emission spectra are presented in Figure 5-6 B. From the luminescence spectrum, we can obse rve the strong red em ission at 609 nm from 65

PAGE 66

Y 2 O 3 :Eu 3+ nanophosphor. The emi ssion intensity increased wi th increasing Eu doping concentration till around 20 mol%, after which it s hows quenching phenomena in PL intensity. It is well known that luminescence be gins to quench when the activator concentration reaches a limit. In bulk Y 2 O 3 :Eu 3+ phosphor usually prepared by conve ntional methods, the concentration quenching was observ ed around 6 mol% wh ereas Qin et al [105]. re ported the co ncentration quenching limit of as-pre pared nanophosphor pa rticles prepared by flame synthesis to be 18 mol% Eu, which was high er than other studies. However, as-prepared Y 2 O 3 :Eu 3+ nanophosphor from urea assisted liquid precursor shows higher doping concentration compared to previous studies [8, 105, 113]. Th e increase of quenching concentrat ion in nanophosphor materials could be explained by interfer ence of energy tr ansfer because of the inte rface effects of nanoscale materials [114]. From this resu lt, it implies the possibility of sy nthesis of na nophosphor using FSP, which has higher doping concentration th an previously reported quenching limits. Figure 5-7 A and B shows the in fluence of overall concentration of the precursor on PL intensity of as-prepared phosphor by FSP. As the overall concentr ation increased from 0.3 M to 0.9 M at fixed 2 M of urea addition and 10 mol% Eu, PL intensit y of as-prepared phosphor increased. It can be seen from Figure 5-7 B that the emission intens ity reaches the maximum value when the overall concentrat ion of precursor is 0.9 M. The spectrum peak position does not exhibit any shift with increase in loading concentrat ion. The rate of increase of PL intensity is decreasing as the loading concentration increa ses from 0.3 M to 0.9 M. It indicates that saturation concentration will be achieved with no further incr ease in PL intensity. This saturation loading concentration appears to be closer to 0.9 M. 5.4 Conclusion In this study, we have investigat ed the effect of sy nthesis conditions on the properties such as crystal structure, particle size, photoluminescent prop erties of the Y 2 O 3 :Eu 3+ nanophosphor by 66

PAGE 67

FSP method. Nanocrystalline Y 2 O 3 :Eu 3+ phosphor can be pr epared via flame spray synthesis with different quantity of urea into nitrate base d liquid precursor. XRD patterns show that the cubic phase phosphors prepared by FSP was synt hesized using urea additive in the liquid precursor without anneal ing step. The emission peaks in photoluminesce nt measurement show the red light emission and the intensity chan ges as the ratio variation of urea to Y 3+ is varied. Urea assisted FSP methods can improve the relative PL intensity and achieve higher doping concentration in phosphor materials compar ed to other methods. 67

PAGE 68

20304050607080 0.9 M 0.7 M 0.5 M 0.3 M JCPDS No. 41-1105 (Y2O3) Intensity (a.u.)2-Theta Figure 5-1. XRD patterns of as-prepared 5% Eu-doped Y 2 O 3 nanophosphor by FSP with different overall concentrat ion of liquid precursor with 2 M of urea addition. 68

PAGE 69

Figure 5-2. SEM image of as-prepared 10% Eu-doped Y 2 O 3 nanophosphor by FSP with 2 M of urea addition in 0.9 M loading concentration of liquid precursor 69

PAGE 70

Figure 5-3. TEM image of as-prepared 5% Eu-doped Y 2 O 3 nanophosphor by FSP with 2 M of urea in the liquid precursor 70

PAGE 71

350400550600650 EmissionIntensity (a.u.)Wavelength (nm) Excitation Figure 5-4. PL exc itation and emission spectra of as-prepared 5% Eu-doped Y 2 O 3 nanophosphor at 2 M of urea addi tion: PL emission of corresponding Y 2 O 3 :Eu 3+ nanophosphor excited with wavelength of 393 nm 71

PAGE 72

0 M1 M2 M3 M4 M Relative PL intensity (a.u.)Mole of Urea Figure 5-5. PL intensity of as-prepared 5% Eu-doped Y 2 O 3 nanophosphor at di fferent mole of addition of urea to th e liquid precursor 72

PAGE 73

560580600620640660 20% Eu 25% Eu 15% Eu 10% Eu 5% EuIntensity (a.u.)Wavelength (nm) A 510152025 Relative PL Intensity (a.u.)Eu concentration (mol%) in liquid precursor B Figure 5-6. PL intensity of Y 2 O 3 :Eu 3+ nanophosphor with di fferent doping con centrations. A) emission intensity of Y 2 O 3 :Eu 3+ nanophosphor, B) relati ve PL inte nsity of corresponding Y 2 O 3 :Eu 3+ nanophosphor 73

PAGE 74

340360380400420440 0.7 M 0.9 M 0.5 M 0.3 MPL Intensity (a.u.)Wavelength (nm) A 560580600620640660 0.5 M 0.9 M 0.7 MPL Intensity (a.u.)Wavelength (nm) 0.3 M B Figure 5-7. PL spectra of as-prepared 10% Eu-doped Y 2 O 3 nanophosphor with different overall concentration at fixed 2 M of urea addition. A) PL exciation of Y 2 O 3 :Eu 3+ nanophosphor, B) PL emi ssion of co rresponding Y 2 O 3 :Eu 3+ nanophosphor excited with wavelength of 393 nm 74

PAGE 75

CHAPTER 6 LUMINESCENCE PROPERTIES OF YAG:CE 3+ NANOPHOSPHORS PREPARED FROM UREA ADDED LIQUID PRECURSOR BY FSP 6.1 Introduction Cerium-doped Y 3 Al 5 O 12 (YAG:Ce 3+ ) particles are yellow emi ssion phosphors that have been widely studied in solid stat e lighting applications [14, 60, 63 -65, 115]. Particularly, in white lighting application, YAG:Ce 3+ phosphor is a promising material because YAG:Ce 3+ phosphors can be combined with blue li ght emitting diodes (LED) for the generation of white LED. The efficiency of white LED is dependent upon the quality of phosphor particles. For the point of view of enhancement in light output effici ency, nano-sized YAG:Ce 3+ phosphors are of current interest. Due to th eir novel properties such as low intern al scattering when nano-sized phosphors are coated onto a ba re LED surface [60, 66, 116], they are extensively be ing researched. Also, it has been report ed that higher doping concentration in nanophosphors could be achieved without concentration quenching because the energy transf er between luminescent cen ters is confined by decreasing particle size s [9, 41]. This increase in dopant concentratio n in phosphor particles would increase luminescence efficiency. Generally, YAG:Ce 3+ phosphor particles can be fabricated by the conventional solid state method [117], sol-gel [12], combus tion synthesis [13], and co-preci pitation [118]. In order to get nano-sized phosphor particles, most of methods require mult i-steps and are time-consuming processes. In addition, many t echniques require post-heat treatment with annealing temperature above 1000 Among these processing techniques, FSP is one of the most powerful methods for synthesis of YAG:Ce 3+ nanophosphors on large s cale. This technique ha s various a dvantages over other methods such as low co ntamination, good precursor mixi ng and high pur ity of phase 75

PAGE 76

in synthesized material. Also, a wide range of precur sors and liquid fuels such as ethanol or xylene can be used to easily to control the compos ition of phospho r particles. In this study, YAG:Ce 3+ nanophosphors were synthesize d via FSP method without long processing time and high annealin g temperatures current ly required for post treatments. Urea was added to liquid precursor which can raise flam e temperature and as a re sult of that, it leads to enhancement in emissi on efficiency from getting better crys tallinity by complete combustion of droplets in flame zone w ith high flame temperature. 6.2 Experimental The starting materi als for the YAG:Ce 3+ phosphors were used yttrium nitrate (Y(NO 3 ) 3 H 2 O, 99.99%) and aluminum nitrate (Al(NO 3 ) 3 H 2 O, 99.99%) were used as the source of Y 3+ and Al 3+ Cerium nitrate (Ce(NO 3 ) 3 H 2 O, 99.99%) was used as the dopant source material. Two molar ratios of yttrium to aluminum were prepared (3:5 and 3:7) in order to study the effect of excess alum inum in the liquid precursor on the luminescence properties. The overall concentration and the number of mole of urea we re fixed at 0.1 M and 2 M, respectively. The doping concentration was fixed at 4 mol% with re spect to yttrium. Thes e nitrate raw materials were dissolved in ethanol and urea was added into the liquid precursor subsequently. Addition of urea into the liquid precursor serv es as fuel for flame synthesis. The mixture was stirred for 30 min at room temperature until a homogenous solution was achiev ed. Methane gas was used as the fuel source gas with a consta nt flow rate. Oxygen gas was used for both oxidants as the combustion reaction and as the carrier gas. FSP system consists of spray bu rner, quartz tube, spray genera tor, bag filters for particle collection and a vacuum pump. Fi ne droplets were supplied to th e high temperature diffusion flame zone above the spray burne r using syringe pump with a constant liqu id feed rate. The flame nozzle consists of three concentric pipes. Outer pipes were used for supplying the oxygen 76

PAGE 77

and methane gas while central mo st feed the liquid precursor. The flame temperature and the residence time of the synt hesized material in the diffusion flame zone can be controlled by varying the amounts of the methane and oxygen gase s and the flow rate of the liquid precursor. The phosphor particles produc ed were collected by fi lter paper. The filter paper was kept above 200 C using heating tape to prevent the condens ation of water vapor in side the metal cylinder wall. All as-prepared YAG:Ce 3+ nanophosphors were annealed at 1100 for 1 hour to get transformed cubic phase YAG. The size and the morphology of phosphor particles we re observed by a field emission scanning electron micr oscopy (FE-SEM). The crystallinity of YAG:Ce 3+ nanophosphor particles was analyzed by powder X-ray diffraction (XRD). The excitation and emi ssion spectrum were determined using Xenon lamp as the excitation source. All the luminescence characterization of the phosphors was car ried out at room temperature. 6.3 Results and Discussion Figure 6-1 shows the XRD patterns of the final products, which were prep ared from different liquid prec ursors. The as-prepared phosphor particles were annealed at 1100 for 1hr as a result of which they were transformed to pure cubic YAG structur e. The XRD results of heat-treated phosphors are in good agr eement with JCPDS No. 33 -0040 for YAG. The XRD pattern of YAG:Ce 3+ particles from the liquid precursor w ith different amount of urea at the molar ratio of yttrium to aluminum 3:5 is shown in Fig. 6-1 A. From XRD results, the addition of urea in liquid precursor do es not affect the crystal structure of cubic YAG. However, it can be seen that the intensities of characteristic peaks increase on addition of urea in the liquid precursor. Previous studies have shown that as-prepare d phosphor particles by FSP show hexagonal YAlO 3 phase and with increasing annea ling temperature, it begins to s how pure YAG stru cture [64]. In 77

PAGE 78

this study, the ann ealed samples at 1100 are almost cubic YAG phas e with some intermediate peaks of hexagonal YAlO 3 When YAG:Ce 3+ phosphor particles were generated from liquid precursor with the Y:Al molar ratio of 3:5, the in termediate phases appear which are identified as hexagonal YAlO 3 phase. This small am ount of hexagonal YAlO 3 in XRD is due to non-uniform composition (aluminum deficient) of the final product by. It is well known that pure YAG crystalline structure is form ed when reaction be tween diffused alumin um ions with YAlO 3 during the annealing process [119]. Thus, minor amounts of the hexagonal YA lO3 phase was obtained due to insufficient diffu sion of aluminum in YAlO3 phase during post he at treatment. Pure YAG phase particles were obtained from slight excess of aluminum in the liquid precursor as shown in Figure 6-1 B. It can be seen from the XRD results that the excess aluminum in starting liquid precursor favors the formation of YAG ph ase during post-heat treatment. Furthermore, addition of urea has similar effect on the intensity of the XRD peak i.e. intensity increases with urea addition (see Fig. 61 B). Terashi et al. [120] reported that both particle morphology and crystallinity were influenced by addi tion of urea to the liquid precursor. The addition of urea in the flame z one delivered additional heat to the particles during synthesis, which produced a large am ount of decomposed gases. These gases combined with oxygen and methane contributes to to wards overall energy prov ided to the particles during synt hesis. The presence of urea in flame zone is useful to form nano-si zed particles and improve the crystallinity [106, 120]. Figure 6-2 shows the FE-SEM photographs of the YAG:Ce 3+ phosphor particles prepared from the nitrate-ba sed liquid precursor with and without addition of ur ea. As-prepared phosphor particles by FSP were annealed at 1100 for 1hr. It can be seen from FE-SEM imag es that the addition of urea to liqui d precursor affects the morphology and size of the prepared -phosphor 78

PAGE 79

particles by FSP. Using the liquid precursor w ithout added urea, the heat-treated phosphor particles exhibited small amount of droplets and nano-sized particles, as shown in Fig. 6-2 A. When 2 M of urea was added to the liquid precur sor, well-dispersed nanoparticles were easily obtained using FSP, as shown in Fig. 6-2 B. The excitation and emission spectra of YAG:Ce 3+ prepared from nitrate-based liquid precursor with 2 M urea by FSP are presented in Figure 6-3, while the dop ing concentration and the overall concentration of li quid precursor were 4 mo% and 0. 1 M, respectively. As-prepared YAG:Ce 3+ phosphor particles we re annealed at 1100 for 1hr. The exc itation peaks of YAG:Ce 3+ phosphor particles in excitation spectra (sho wn in Fig. 6-3) were found at 344 nm and 465 nm. These excitation peaks were originated by electron transitions from 4f ground state to 5d split energy level of Ce 3+ by crystal field [60, 116]. There is a broad emi ssion band in range 480650 nm corresponding to the yellow light. The strongest emis sion peak was f ound at 524 nm, showing the typical lumine scence spectrum of YAG:Ce 3+ phosphors, which due to electron transitions from 5d to 4f of Ce 3+ ion in the YAG lattice [121]. Al so, the emission peak at 524 nm can be combined with the emitted blue light from GaN-LEDs to yield white light. The emission spectra for the YAG:Ce 3+ phosphor particles prepared from differe nt liquid precursors after annealing at 1100 for 1hr are show n in Figure 6-4. The figure shows the effect of urea and the molar ratio of yttrium to alum inum in liquid precursor on the luminescence intensity. The luminescence properties of the prepared-phosphor partic les by FSP are dependent on the addition of urea to liquid precurso r. It was found that the em ission intens ity of YAG:Ce 3+ phosphor particles pr epared from the li quid precursor with urea was higher than that of the phosphor particles prep ared without added. No shift in the peak position of the emission spectra was observed und er different conditions of liquid precursor. Generally it is well known that the 79

PAGE 80

luminescence properties of phosphor particle s are strongly dependent upon the char acteristics of the prepared phosphors su ch as particle size [7], surfac e morphology [8], concentration quenching [9] and crystallinity [10]. The crystallin ity is one of the most important parameters to obtain the phosphor pa rticles with high lu minescence efficien cy. Pan et al. [122 ] reported that the emission intensity increases with increasing the sintering temperature due to improved crystallinity of YAG particles. Thus, the luminesc ence properties, as show n in Fig. 6-4, are in good agreement with the XR D results. Furthermore, we studied the effect of molar ratios yttrium to aluminum on lu minescence properti es of the YAG:Ce 3+ phosphor particles. In case of Y:Al molar ratios of 3:7, the higher luminescence intensity of YAG:Ce 3+ phosphor partic les could be obtained by the FSP method. The improvement in the emission intensity of YAG:Ce 3+ nanophosphors prepar ed with slight ex cess aluminum to li quid precursor is pr obably du e to the improved the crystallinity with pr esence of pure cubic phase YAG by better incorporation of Alions in YAG lattice. Similar results have been reported by Kinsman et al. [119] that luminescence intensity was enhanced with slight ex cess of aluminum. 6.4 Conclusion YAG:Ce 3+ nanophosphor particles were synthesized using FSP from the nitrate-based liquid precursor with urea addition and slight excess of al uminum. The effect of different liquid precursor on prope rties of YAG:Ce 3+ nanophosphors prepared by FSP method was investigated. YAG:Ce 3+ nanophosphors exhibit higher em ission intensity wh en liquid precursor were prepared by the addition of urea. Addition of urea to liquid precursor provides the additional heat by decomposition and burning of urea in flame zone which promotes well-dispersed particles in nanometer range and incr ease the crystallinity of the preparedphosphor particles. Particles prepared with higher yttrium to aluminum molar ratio (3:7) show higher luminescence intensity as compared to one with Y:Al ratio of 3:5. 80

PAGE 81

1020304050607080 0 M of urea, Y:Al = 3:5 2 M of urea, Y:Al = 3:5 JCPDS No. 33-0040 (YAG) Intensity (a.u.)2-ThetaA) A 1020304050607080 2 M of urea, Y:Al = 3:7 0 M of urea, Y:Al = 3:7 JCPDS No. 33-0040 (YAG) Intensity (a.u.)2-ThetaB) B Figure 6-1. XRD patterns of YAG:Ce 3+ prepared with different Y:Al mo lar ratios in the precursors with and without urea. A) Y:Al = 3 : 5, B) Y:Al = 3 :7 81

PAGE 82

A B Figure 6-2. SEM images of YAG:Ce 3+ particles prepared from liquid precurso r without and with the addition of urea. A) 0 M, B) 2 M 82

PAGE 83

300400500600700 Emission Intensity (a.u.)Wavelength (nm) Excitation Figure 6-3. PL spectra for 4 mol% Ce-doped YAG afte r annealing at 1100 for 1hr. A) excitation spectra for the YAG:Ce 3+ nanophosphor, B) emissi on spectr a for the YAG:Ce 3+ nanophosphor excited with wavelength of 465 nm 83

PAGE 84

500550600650700 0 M of urea, Y : Al = 3 : 5 2 M of urea, Y : Al = 3 : 5 0 M of urea, Y : Al = 3 : 7 2 M of urea, Y : Al = 3 : 7Intensity (a.u.)Wavelength (nm) Figure 6-4. Emission spectra of YAG:Ce 3+ (4 mol%) nanophosphors pr epared from different liquid precursors after annealing at 1100 for 1hr 84

PAGE 85

CHAPTER 7 THE INFLUENCE OF DIFFERENT COND ITIONS ON THE LUMINESCENCE PROPERTIES OF YAG:CE 3+ NANOPHOSPHORS USING FSP 7.1 Introduction Cerium-doped Y 3 Al 5 O 12 (YAG:Ce 3+ ) particle as yellow emissi on phosphor is one of the most important phosphors in solid state lighting fields [14-16, 60 63-65, 115] b ecause it can be applied to generate the white light by mixing of blue light emitting diodes (LEDs). One of important parameters in white LED is the qu ality of phosphor pa rticles. In orde r to improve the light emitting efficiency from wh ite LEDs, it is necessary to have highly efficien t light emitting phosphors on blue LED s [66]. It has been repor ted that the efficiency of phosphors for white light application could be achieved by using nanophosphor because nanosized YAG:Ce 3+ phosphor shows higher luminescent effici ency than that of larger size particles [60]. This enhancement comes from reductio n of internal light scattering in phosphor layer when coated onto a bare LED surface. Traditionally, YAG particles have been prepared by the conven tional solid-state reaction method which requires annealing at temper atures higher than 1000 The phosphor particles prepared by the so lid-state reaction method us ually contain impurities caused by poor mixing of chemical compounds and requires annealing for a long time at hi gh temperature. Moreover, in order to get fine phosphor partic les, a cumbersome and time-con suming process such as ball milling is needed. In recent years, several synt hesis techniques such as sol-gel [12], combustion synthesis [13], and co-precipitati on [14] were used to overcome the above mentioned limitations for preparation of the YAG particles. Among the processing techniques, F SP is one of the most powerful methods for synthesis of YAG:Ce 3+ nanophosphors because of it s advantages of less contaminations from being a si mple method, good precursor mi xing using ni trate aqueous 85

PAGE 86

precursor, and high purity of phase from high flame temperature. Al so, the composition of phosphor particle generated by FSP can be easily controlled. In this paper, YAG:Ce 3+ nanophosphors are prepar ed from nitrate based li quid precur sor at different molar ratios Y:Al of 3:5 and 3:7 by fl ame synthesis with different flame temperature. The effect of different methane flow rate and the ratio of yt trium to slight excess aluminum in starting aqueous precur sor on luminescence prope rties were comparatively investigated. The heat-treated phosphor particles were examined by X-ra y diffraction (XRD) for the phase formation and field-emission sc anning electron micr oscopy (FE-SEM) for the morphology and size of phosphors. 7.2 Experimental FSP system consists of a spray burner, quartz t ube, spray generator, bag filter for particle collection and a vacuum pump. The spray burner is the most important component of the system. It consists of three concentric pipes, outer two for fuel and oxygen gas while inner is for spraying liquid precursor. Th e fuel i.e. methane in this case and ox idant i.e. oxygen flow rate are adjusted using flow meter to cont rol the flame temperature. The liquid precur sor is introduced in the high temperature flame zone in form of fine droplets using a syringe. The syringe needle diameter and the syringe plunger velocity control the droplet si ze and hence, the partic le size of synthesized YAG phosphor particles. The phosphor particles pr oduced were collected by filter paper. The filter paper was kept above 200C using heating tape to pr event the condensatio n of water vapor inside the metal cylinder wall. For the preparation of liquid pr ecursor, yttrium nitrate (Y(NO 3 ) 3 H 2 O, 99.99%) and aluminum nitrate (Al(NO 3 ) 3 H 2 O, 99.99%) were used as Y 3+ and Al 3+ source, while cerium nitrate (Ce(NO 3 ) 3 H 2 O, 99.99%) was used as doping material of Ce 3+ Yttrium and aluminum nitrates were dissolved in ethano l at different molar ratios to vary yttrium to alum inum ratio from 86

PAGE 87

3:5 to 3:7. The total overall concentration of the liquid precursor was fixed at 0.1 M and the doping concentration was fixed at 4 mol% with resp ect to yttrium. The mixt ure was stirred for 30 minutes at room temperature unt il a homogenous soluti on was achieved. Methane gas was used as the fuel source gas while oxy gen was used as an oxidant an d as a carrier gas to spray precursor. As-prepa red phosphor particle s were placed in an electri c furnace for post heat treatment, which was ca rried out under atmospheri c conditions. The particles were annealed at different temperature varying from 800 to 1100 for 1hr to confirm th e phase transformation temperature of as-prepare d phosphors. The heated-par ticles were cooled to room temperature by natural convention and were used as it is for further characterization. The size and morphology of phosphor particles were dete rmined using a field emission scanning electron microscopy (FE-SEM). The crystallinti y of the synthesized YAG:Ce 3+ nanophosphors was analyzed by powder X-ray diffr action (XRD). The exc itation and emission spectrums were obtained using Xe-ramp as th e excitation source. All the luminescence characterization of the phosphors was carried out at room temperature. 7.3 Results and Discussion The post heat treatment is required to tran sform the hexagonal YAIO 3 to pure cubic YAG phase. As-prepared phosphor particles were annealed in th e electric furnace at different annealing temperatures varying between 800 to 1100 under atmospheric condition. Figure 7-1 shows the XRD patterns for the synthesized YAG:Ce 3+ nanophosphor particles after postheat treatment at different anne aling temperatures. It can be s een from the XRD re sults that the as-prepared phosphor particles by FSP present a hexagonal YAlO 3 phase. While on increasing the annealing temperature, it begins to transform into the YAG phase and also increase the peak intensity of XRD patt erns. The cubic phase YAG was ob tained after annealing at 1000 These 87

PAGE 88

XRD patterns are in good agr eement with JCPDS No 33-0040 for YAG. On further increasing the temperature, the in tensity of the characteristic peaks increased showing improvement in crystallinity. Figure 7-2 shows the SEM microg raphs of as-prepare d phosphor particles (Fig. 7-2 A) and annealed phosphor partic les (Fig. 7-2 B). It can be observ ed that the part icle size of the synthesized phosphors is less than 100 nm with broad particle size distribution. From the SEM images, we clearly see that the nano-sized particles ca n be easily produced by FSP from nitratebased liquid precursor. The PL emission spectra of h eat-treated nanophosphors are shown in Fig. 7-3 A. It can be seen from the Fig. 7-3 B that re lative PL intensity increases with the annealing te mperature. The phosphor particles were excited by 465 nm wavelength rays from xenon lamp and start to emit broad yellow emission at an annealing temperature of 1000 The strongest em ission peak was found at 524 nm, showing the typical luminescence spectrum of YAG:Ce 3+ phosphors, which is due to a transitions fr om 5d to 4f of Ce 3+ ion in the YAG lattice [ 121]. The phosphor particles annealed below 900 do not show luminesc ence. The maximum PL intensity was obtained when annealing was carried out at 1100 In addition, no shift in peak was observed under different heat treatment conditions. The PL inte nsity is improved with increasing of annealing temperature, which is due to the improvement of crystallinity from high temperature anealing. YAG:Ce 3+ nanophosphors were prepared by different methane flow rate and molar ratios of yttrium and aluminum in liqui d precursor. It should be noted that all samples were post-heat treated at 1100 for 1hr under atmospheric conditions. The effect of methane flow rate with constant oxygen flow rate and the overloaded aluminum in liquid precurso r on the crystallinity of the YAG:Ce 3+ nanophosphor particle s is depicted in Figure 7-4. In the forma tion of phosphor 88

PAGE 89

particles using FSP method, flame temperatur e is the most important parameter which determines the properties of the phosphor particles. Th e increase methane fl ow rate influences the flame temperature and the residence time of droplets in flame zone. At higher flame temperatures, the crysta llinity of phosphor partic les improves [8]. In Fig. 7-4, nanophosphor particles prepared by the flame in lower methan e flow rate at fixed oxy gen flow exhibit better crystallinity than that in higher methane flow rate. The increase in th e intensity of the XRD patterns results from complete burni ng of liquid precurso r at lower methane flow rate. Moreover, it can be seen from Fig. 7-4 that in case of slight excess of aluminum in liquid precursor, it shows better XRD results. In addition, YAG:Ce 3+ nanophosphors pr epared by Y:Al ratio of 3:5 in liquid precursor exhib it some additional peaks in XRD patterns. This is due to the insufficient diffusion of aluminum in the YAlO 3 phase during po st heat treatment. As a result of this no pure YAG phase was obtain ed. In order to form YAG structure, it is necessa ry to react between Al and YAlO 3 by the diffusion mechanism of Al that during the annealing process [119]. From the XRD results, it can be seen th at the environment of the overloaded alum inum in starting materials helps th e hexagonal YAlO 3 to transform the cubic YAG phase. Thus, during the synthesis of YAG:Ce 3+ nanophosphors by FSP method, it is s hown in Fig. 7-4 that the liquid precursor with slight excess of aluminum is favo rable to generate the pu re YAG phase as a final product. The emission spectra of YAG:Ce 3+ nanophosphor particles are shown in Figure 7-5 A. The curves show the effe ct of methane flow rate in flame zone and the stoichiometry in liquid precursor on relative PL intensity. PL measurements show that nanophosphor pa rticles prepared at lower methane flow rate exhib it higher PL intensity than with higher methane fl ow rate at a constant oxygen flow. This can be attributed to the reducing ch aracteristics of the flame. At 89

PAGE 90

higher methane flow rate, the fl ame is oxygen deficient which le ads to reducing flame. Such reducing environment/flame in turn causes oxygen vacancies in the newly synthesized YAG:Ce 3+ nanophosphors due to incomplete burning of the precursor. It ca n be corroborated by the flame temperature which was observed to be higher for lower methane to oxyge n ratio flame as compared to the flam e with higher ra tio. Furthermore, Figure 75 B shows the relative PL intensity of YAG:Ce 3+ nanophosphors at different Y:Al molar ratios and different methane flow rate. The relative PL intensity of na nophosphors with Y:Al mo lar ratios of 3:7 is higher th an that of 3:5, as shown in Figure 7-5 B. The improvement of the relative PL intensity of YAG:Ce 3+ nanophosphors is probab ly due to the good crystallinity from pure cubic phase YAG by incorporation of Al-ions in YAG lattice. Kinsman et al [119]. also demo nstrated that relative intensity in emission spectra is increased wi th slight excess of aluminum. The luminescence properties of phosphors are influenced by vari ous parameters such as pa rticle size [7], surface morphology [8], and crystallinity [122]. Among those, the crysta llinity is the most important parameter to determine the lumi nescence efficiency in pho sphors. To obtain good luminescence properties in yellow light emitting YAG:Ce 3+ phosphors, the phosphors shou ld have pure cubic YAG structure [64]. 7.4 Conclusion YAG:Ce 3+ nanophosphors were successfully prepared by FSP method combined with postheat treatment. Nano-sized phosphor particles can be ea sily generated via si mple process from nitrate aqueous prec ursor. As-prepared particles found to be hexagonal YAlO 3 transformed to cubic YAG phosphor upon heat treatment above 900C. The increase in annealing temperature raises the PL intensity of YAG:Ce 3+ nanophosphors with occurring maximum em ission intensity at annealing temperature of 1100 C. The nanophosphor pa rticles showed the improvement in PL intensity with slight excess of aluminum and lowe r methane flow rate at a constant oxygen flow. 90

PAGE 91

1020304050607080 No annealing 1100C for 1hr 1000C for 1hr 900C for 1hr 800C for 1hr JCPDS No. 33-0040 (YAG)Intensity (a.u.)2-Theta Figure 7-1. XRD patterns of 4 mol% Ce-doped YAG at different annealing temperatures 91

PAGE 92

A B Figure 7-2. SEM images of 4 mol% Ce-doped YAG nanophosphor prepar ed by FSP. A) asprepared phosphor part icles, B) annealed phosphor particles 92

PAGE 93

500550600650700 1100C 1000C 900C 800C Intensity (a.u.)Wavelength (nm) A) A 700800900100011001200 B)Relative PL intensityAnnealing temperature B Figure 7-3. PL spectr a of 4 mol% Ce-doped YAG at differe nt annealing temperatures. A) emission spectra for YAG:Ce 3+ nanophosphor excited with wa velength of 465 nm, B) Relative PL intensity at different annealing temperature. 93

PAGE 94

1020304050607080 JCPDS No. 33-0040 (YAG) CH4 2 l/min, Y : Al = 3 : 5 CH4 1 l/min, Y : Al = 3 : 5 CH4 1 l/min, Y : Al = 3 : 7 CH4 2 l/min, Y : Al = 3 : 7Intensity (a.u.)2-Theta Figure 7-4. XRD patterns of 4 mol% Ce-doped YAG at different flame temperatures by controlling the methane flow rate and the mola r ratio of yttrium to aluminum in liquid precursor 94

PAGE 95

500550600650700 CH4 1 l/min, Y : Al = 3 : 7 CH4 2 l/min, Y : Al = 3 : 7 CH4 2 l/min, Y : Al = 3 : 5 CH4 1 l/min, Y : Al = 3 : 5A)Intensity (a.u.)Wavelength (nm) A B)Relative PL intensity (a.u.)Y:Al (3:7) CH 4 1 l/min CH 4 2 l/minY:Al (3:5) B Figure 7-5. PL spectr a of 4 mol% Ce-doped YAG after annealing at 1100 for 1hr. A) emission spectra for YAG:Ce 3+ nanophosphor excited with 465 nm wavelength, B) Relative PL intensity vers us Y:Al molar ratios with different methane flow rate 95

PAGE 96

CHAPTER 8 SYNTHESIS AND CHARACTERIZATION OF ZN 2 SIO 4 :MN 2+ NANOPHOSPHORS PREPARED FROM DIFFEREN T ZN SOURCE IN LIQU ID PRECURSOR BY FSP 8.1 Introduction Phosphors are generally composed of highly pure host material and a small amount of intentionally added impurity, so-c alled activator. Such phosphor particles ha ve been extensively investigated to obtain hi gh luminescence effi ciency properties for field emission displays (FED), fluorescent lamps and optoelect ronic devices [15, 67, 123, 124]. Especially, Mn -doped zinc silicate (Zn 2 SiO 4 :Mn 2+ ) is widely used as a green-emitting phosphor mate rials for plasma display panels (PDPs) and cathode ray tubes because of its chemical stabi lity, high luminescence efficiency and semi-conducting pr operties [67-70]. For these applications, it is important to synthesize phosphors which have high luminescence efficiency, smaller particle size, controlled morphology, good crystallinity and appropriately doped w ith activators. The luminescence properties of phosphor particle s are dependent upon the synt hesis method. Traditionally, Zn 2 SiO 4 :Mn 2+ phosphor particles are prepared by solidstate reaction method at high reaction temperature for a long period, involving repeat ed ball milling process, which is required to obtain the pure phase of phosphors. It is difficult to pr epare phosphor particle s with controlled shape and smaller size by this me thod. Another drawback of the solid-state reaction method is the contamination of phosphors by impurities during milling pr ocess or while washing with chemicals. These limitations strongly affect th e luminescence properties of phosphor particles. Thus, many synthesis methods have been applied to improve the performance of Zn 2 SiO 4 :Mn 2+ phosphor particles [69, 70, 125, 126]. Rece ntly, FSP has been introduced as a promising method for producing phosphor particles with high luminescence effi ciency, having sp herical shape and small size. FSP is one of the most powerful methods for synthesis of Zn 2 SiO 4 :Mn 2+ nanophosphors because of its advantages such as good precursor mixing using aqueous precursor 96

PAGE 97

and high purity of phase from high flame temper ature of being a simple method it causes less contamination. Also, the composition of phospho r particle generated by FSP can be easily controlled. In this work, we repor t the fabrication of Zn 2 SiO 4 :Mn 2+ phosphor particles from different Zn-source in aqueous medium by FSP. It was inve stigated that proper Zn source in liquid precursor is required to produce high quality Zn 2 SiO 4 :Mn 2+ phosphor particles. In order to study the effect of different zinc source on the size, mo rphology, crystallin ity and luminescence properties of Zn 2 SiO 4 :Mn 2+ phosphor particles. Th ree types of precursors were prepared by dissolving zinc-nit rate, zinc-2-ethylhe xanoate and zincacetate in ethanol. To find optimal condition for obtaining high luminescence efficiency in Zn 2 SiO 4 :Mn 2+ phosphor particles, asprepared phosphor pa rticles were anneal ed in the temper ature range from 800 to 1000 8.2 Experimental Zn 2 SiO 4 :Mn 2+ green phosphor particles were prepared from th ree different liquid precursors. Three different types of precursors for preparation of Zn 2 SiO 4 :Mn 2+ nanophosphors were prepared using zinc-nitrate (Zn(NO 3 ) 2 6H 2 O), zinc 2-ethylhexanoate (Zn(OOCCH(C 2 H 5 )C 4 H 9 ) 2 ) and zinc-ace tate (Zn(C 2 H 3 O 2 ) 2 2H 2 O) as zinc sources, tetraethylorthosilicate (TEOS, Si(OC 2 H 5 ) 4 ) was used as the source of silica and manganese acetate tetrahydrate (Mn(CH 3 COO) 2 4H 2 O) was used as the source of manganese doping. All the source materials were dissolv ed in ethanol or water with high pu rity. It is well known that zincacetate has less solubility in ethanol, so in th e case of using zinc-acetate as zinc source to liquid precursor, distilled water and ethanol were used to prepare the starti ng precursor. The total overall concentration of the liquid precursor wa s fixed at 0.1 M, while the manganese doping concentration was at 4 mol% with respect to zinc. The mo lar ratio of zinc-source:TEOS:Mn = 97

PAGE 98

1.96:1:0.4 was used for the preparation of liquid precursors The mixture was s tirred for 30 mins at room temperature un til a homogenous solution was obtain ed. Methane gas wa s used as the fuel source gas with a constant flow rate. Oxygen gas was used for both complete combustion reaction and spraying of droplets. As-p repared phosphor particles we re placed in an electric furnace for post heat treatment, which wa s carried out under atmosphe ric condition. The particles were annealed from 800 to 1000 for 1hr to determine phase transformation temperature of as-prepared phosphors. The heated -particles were cooled down na turally after completion of annealing. FSP system mainly consisted of spray burner, quartz tube, spray generator, particle collection bag filters and vacuum pump. Fine dr oplets were supplied to the high temperature diffusion flame zone above spray burner using syri nge pump with a constant liquid feed rate. In this phosphor particles were gene rated as a result of evaporatio n, decomposition and melting of the droplets. The flame nozzle c onsisted of three concentric pipes and the outermost pipe had oxygen and methane gas flowing through it for generating diffusion flame. The flame temperature and residenc e time in the diffusion flame zone can be controlled by varying the amounts of methane and oxygen gases and the flow rate of the liquid precursor. The phosphor particles produced were collected by filter paper which was kept above 200 C using heating tape to prevent condensation of water vapor inside th e metal cylinder wall. The effects of precursor types from different zinc sources and annealing temperature on the crystalline structure and luminesc ence properties of the produced Zn 2 SiO 4 :Mn 2+ nanophosphors were investigated. The crystallinity of prepared particles by heat-treat ment was ex amined by powder X-ray diffractome try (XRD) and the mor phology and size of phosphors were observed using field emission scanning electron micros copy (FE-SEM). The excitation and emission 98

PAGE 99

spectrum were determined using Xenon ramp as the excitation source. Ch aracterization of the phosphors for lumine scence is carried out at room temperature. 8.3 Results and Discussion In order to investigate the eff ect of the precursor type from different zi nc sources on the crystallinity of pr epared phosphor part icles, XRD analysis was car ried out. Figure 8-1 shows XRD patterns for Zn 2 SiO 4 :Mn 2+ green phosphor particles prepared by FSP and annealed at 1000 XRD patterns illustrated that typical zinc silica te structure was obtained with liquid precursors prepared by adding Zn-nitrate and acetate. Ho wever, when Zn-2 -ethylhexanoate source was added to liquid precursor, XRD patterns showed presence of inte rmediate phases that were identified as ZnO structure, even for th e same experimental c onditions and annealing procedure. In case of Zn-2-eth lyhexanoate used for li quid precursor, although the particles were prepared from the same experimental condition including post heat-treatment, the final products did not exhibit pure zinc silicate structure. Th is is probably ca used by poor homo genitiy in liquid precursor, which produces incomplete phase forma tion during flame synthe sis. From Fig. 8-1, we observe that the crystallinity of Zn 2 SiO 4 :Mn 2+ phosphor particles prep ared from zinc nitratebased liquid precursor is better compared to that from zinc acetate-based liquid precursor. XRD analysis of the samples s howed that good quality Zn 2 SiO 4 :Mn 2+ phosphors can be synthesized using nitrate-based liquid precur sor in FSP. For synthesis of pho sphor particles by FSP method, precursor type is a ke y parameter for producing preferred products [89]. Moreover, selecting of precursor is critical as the co mbustion of liqui d precursor itself provides significant amount of heat to flame zone [127]. Typi cally, nitrate-based liquid precursor is widely used as the source material in flame method. Usually nitrate source materials are hi ghly soluble in ethanol and so easy to prepare, while ac etate is relatively less soluble in et hanol. For the preparation of liquid 99

PAGE 100

precursor using zinc acetate, distilled wate r and ethanol were used as pr ecursor solvent. Qin et al [105]. reported that the flame temperature fro m ethanol based pr ecursor is higher than the water based precursor as ethanol itself acts as fuel and provides heat to the flame co mpared to the combustion of water based precursor. Thus, increased flame temper ature promotes the crystallinity of particles prepared by FSP [64, 105, 129 ]. As a result of, it can be seen from Fig. 2 that the crystallinity of phosphor particles prep ared using nitrate-based liq uid precursors is higher than that of phosphors from othe r precursors. The morphology and size of thermally treated Zn 2 SiO 4 :Mn 2+ phosphor partic les prepared using different liquid precursors is as shown in Figure 8-2. All synthesized phosphor particles are observed particle size to have in nanometer range with spherical sh ape and were slightly agglomerated. From the SEM imag es in Fig. 8-2, it is observed that the nano-sized phosphor particles can be easily synthesized by FSP using various types of precursors. When Zn 2 SiO 4 :Mn 2+ phosphors were prep ared using zinc nitrate and acetate precursors and 4 mol% Mn-doped, dense particles which were close to stoichiometric of Zn 2 SiO 4 :Mn 2+ molar ratios of Zn:Si:O:Mn as 1.96:1:0.4 were obtained. However, in case of adding Zn-2-ethylhexanoate to precursor, the prepared phosphor particles do not show uniform co mposition, although the preparation of the starti ng chemicals is carried out under exact same proced ure. It indicates that the homogeneity of liquid precursor composition affects the properties of the final product and hence is very important parameter to pr oduce the desired phosphor particles by FSP. Excitation and emission spectra for Zn 2 SiO 4 :Mn 2+ nanophosphors is as shown in Figure 83 A and B and shows the effect of precur sor type on lumines cence properties of Zn 2 SiO 4 :Mn 2+ nanophosphors. All samples were post heat treated at 1000 for 1hr under atmospheric conditions. The prepared phosphor particles em its bright green light when excited with 100

PAGE 101

wavelength of 266 nm. The strongest emission pe ak was found at 524 nm, showing the typical luminescence spectrum of Zn 2 SiO 4 :Mn 2+ phosphors, which is a transition from 4 T 1 ( 4 G) to 6 A 1 ( 6 S) for tetrahedral-coordinated Mn 2+ [129, 131]. From Figu re 8-3, we observe th at the luminescence intensity is a maximum for nanophosphor when prepared using Zn-nitra te based liqui d precursor compared to other liquid precursors. However, no shift in the wavelength peak was observed for different precursor types. Zn-nitrate is a better zinc sour ce for formation of Mn-doped zinc silicate phosphor partic les than that of Zn-acetate or Zn-2-ethyl hexanoate. Zn 2 SiO 4 :Mn 2+ nanophosphors prep ared from Zn-nitrate based liquid precursor have higher emi ssion intensity than nanophosphors from other liquid precursors. Lower emissi on intensity of phosphors prepared from Zn-acetate and 2-ethylhexanotate-b ased liquid precursor is due to non-uniformity in composition of the precursor. The crystal linity of phosphor particle s is one of the most important factors for determining the luminescen ce properties as increas e in crystallinity of phosphor particles results in improvement of photoluminescence. The highe r emission intensity in PL measurements can be supported by their XRD resu lts shown in Fig. 81. We can see the presence of good crystall ine structure in th e prepared phosphor particle by FSP from Zn-nitrated based liquid precursor. These results reveal that the Zn-nitrate added liqui d precursor is favorable to produce high quality of Zn 2 SiO 4 :Mn 2+ phosphors by FSP. As a result of that, Zn-nitrate source was used as main precursor for the rest of our study below. XRD pattern of Zn 2 SiO 4 :Mn 2+ phosphor particles prepared from Zn-nitrate based liquid precursor at different annealing temperatures ranging from 800 to 1000 is shown in Figure 8-4. As annealing temperature is increased, it begins to show zinc sili cate phase and produces higher crystalline phosphor particles with increase in peak intensity of XRD patterns. Pure zinc silicate phase wa s obtained at 1000 XRD pattern is in good ag reement with JCPDS No. 37101

PAGE 102

1485 for Zn 2 SiO 4 XRD result suggests that crystallinity can be improved by increasing of the annealing temperature. Figure 8-5 shows the effect of annealing temperatures on luminescence intensity was investigated. It can be se en from Fig. 8-5 that PL intensity fo r zinc silicate vari es with annealing temperature and increases with increasing annealing temperatur es. The phosphor particles were excited by 266 nm wavele ngth rays from xenon lamp and emit green emission peaks after further heat-treatment. The luminescence properties of phosphor s are influenc ed by various parameters such as particle size [7 ], surface morphology [128], and cr ystallinity [122]. Among those, the crystallinity is the most impor tant parameter for determining luminescence efficiency in phosphors. High crystallinity of phosphor particles results in im provement of photoluminescence [131]. All h eat-treated Zn 2 SiO 4 :Mn 2+ nanophosphors show ed increase PL intensity with increasing annealing temperature. From PL spectra it is observed that maximum PL intensity for Zn 2 SiO 4 :Mn 2+ phosphor particle, prepar ed from nitrate ba sed precursor, wa s obtained for phosphor particles that were annealed at 1000 8.4 Conclusion In summary, Zn 2 SiO 4 :Mn 2+ phosphors were synthesized by FSP and comparison of the crystalline properties and lumi nescent efficiencies of Zn 2 SiO 4 :Mn 2+ nanophosphor particles prepared from liquid prec ursors with different zinc sources. XRD result s show that the zinc nitrate-based liquid precursor is better than other precursors for the synthesis of Zn 2 SiO 4 :Mn 2+ nanophosphor particles using FSP. The PL intensity of synthesized Zn 2 SiO 4 :Mn 2+ nanophosphor particles was characterized for each liquid precurso r type of zinc source. The most intense peak in luminescence properti es was obtained for produ ct of synthesized Zn 2 SiO 4 :Mn 2+ nanophosphor particles prepared from nitrate-based precursor. Flame synthesis is a promising method for the 102

PAGE 103

preparation of Zn 2 SiO 4 :Mn 2+ nanophosphor particles annealed at temper atures varying from 800C to 1000 C. The emission intensity was found to incr ease with increas e in annealing temperature. 103

PAGE 104

1020304050607080 Nitrate JCPDS 37-1485 (Zn2SiO4) Intensity (a.u.)2-Theta2-ethylhexanoate Acetate Figure 8-1. XRD patterns of 4 mol% Mn-doped Zn 2 SiO 4 prepared using different liquid precursors by FSP 104

PAGE 105

A B C Figure 8-2. SEM images of 4 mol% Mn-doped Zn 2 SiO 4 nanophosphor prepar ed by FSP using different liquid precursors after annealed at 1000 A) Zn-nitrate, B) Zn-acetate, C) Zn-2-ethlyhexanoate 105

PAGE 106

220240260280300320 A) ExcitationZn-Nitrate Zn-Acetate Zn-2-ethylhexanoateIntensity (a.u.)Wavelength (nm) g A 480500520540560580600 Zn-Acetate B) Emission Zn-2-ethylhexanoateIntensity (a.u.)Wavelength (nm) Zn-Nitrate B Figure 8-3. PL spectr a of 4 mol% Mn-doped Zn 2 SiO 4 prepared using different liquid precursors by FSP. A) excitati on spectra for Zn 2 SiO 4 :Mn 2+ nanophosphors, B) emission spectra for Zn 2 SiO 4 :Mn 2+ nanophosphors. 106

PAGE 107

1020304050607080 Intensity (a.u.)2-Theta JCPDS 37-1485 (Zn2SiO4) 1000C 900C 800C As-prepared Figure 8-4. XRD patterns obtai ned from 4 mol% Mn-doped Zn 2 SiO 4 prepared from nitratebased liquid precursor at different annealing temperatures 107

PAGE 108

220240260280300320 1000C 900C 800C Intensity (a.u.)Wavelength (nm) As-prepared B) Excitation A 480500520540560580600 800C 900C 1000CB) Emission Intensity (a.u.)Wavelength (nm) As-prepared B Figure 8-5. PL spectr a of 4 mol% Mn-doped Zn 2 SiO 4 :Mn 2+ phosphor ob tained at different annealing temperatures. A) excitation spectra for Zn 2 SiO 4 :Mn 2+ nanophosphor, B) emission spectra for Zn 2 SiO 4 :Mn 2+ nanophosphor. 108

PAGE 109

CHAPTER 9 LUMINESCENCE PROPERTIES OF ZN 2 SIO 4 :MN 2+ NANOPHOSPHORS BY FSP 9.1 Introduction Mn-doped zinc silicate (Zn 2 SiO 4 :Mn 2+ ) is the widely used green-emitting phosphor for plasma display panels (PDPs) and cathode ray t ubes because of its chem ical stability, high luminescence effi ciency and semi-c onducting properties [67-70]. Generally, commercial phosphors are synthesized by solid-state reacti on method, which is car ried out at a high temperature of over 1000 and produces phosphors having high luminescence. However, phosphors prepared/synthes ized by solid -state reaction met hod have irregular shape and large diameter in the ra nges of 2 and 20 It is difficult to prepare phosphor partic les with controlled shape and smaller size by this me thod. Another drawback of the solid-state reaction method is the contamination of phosphors with impurities during milling process or while washing with chemicals. For enhancing the e fficiency of phosphors in display devices, smaller size and uniformity in shape is required. Thus, many synthesis methods have be en applied to improve the performance of the phos phor particles [69, 70, 125, 126]. Recently, flam e spray pyrolysis (FSP) has been introduced as a promising method for producing phosphor particle s with high efficiency luminescence, having spherical shape and small size [ 106, 131, 132]. Moreover, FSP is one of the most powerful methods for sy nthesis of nanophosphor particle s because of its advantages such as good precursor mi xing using aqueous precu rsor, high purity of phase from high flame temperature and less contamina tion from being a simple method. Also, the composition of phosphor particle generated by FSP can be easily controlled. In this work, we repor t the synthesis of Zn 2 SiO 4 :Mn 2+ nanophosphor particle s using nitrate precursor by FSP. For synthesis of Zn 2 SiO 4 :Mn 2+ phosphor particles, usin g nitrate precursor as starting chemical has some advant ages such as hi gh solubility in ethanol which acts as precursor 109

PAGE 110

solvent, inexpensive and easy to prepare. We investigate how th e annealing temperature affects the crystal structur e and luminescence properties of Zn 2 SiO 4 :Mn 2+ nanophosphor particles. Moreover, the effect of flame temperature on the crystallinity, luminescence intensity and morphology of Zn 2 SiO 4 :Mn 2+ nanophosphor particles was studied. X-ray diffraction (XRD), field emission scanning el ectron microscopy (FE-SEM) and photoluminescence (PL) measurements have been car ried out to characterize the nanophosphor particles. 9.2 Experimental FSP system consists of spray bu rner, quartz tube, spray genera tor, bag filter for particle collection and a vacuum pump. Spray burner is th e most important compon ent of the system and it consists of three concentric pipes, outer two for fuel and oxygen gas while inner is for spraying liquid precursor. Th e flow rate of fuel i.e. methane in this case and oxida nt i.e. oxygen are adjusted using flow meter for controlling th e flame temperature. Th e liquid precursor is introduced in high temper ature flame zone in th e form of fine droplet s using a syringe. The syringe needle diameter and the syringe plunger veloc ity control the droplet size and hence, the particle size of synthesized Zn 2 SiO 4 :Mn 2+ phosphor particles. The pho sphor particles produced were collected by filter paper which was maintained above 200 using heating tape to prevent condensation of water vapor insi de the metal cylinder wall. For preparation of liquid precursor, zinc nitrate (Zn(NO 3 ) 2 6H 2 O) and tetraethylorthosilicate (TEOS, Si(OC 2 H 5 ) 4 ) were used as the zinc and silica source, respectively; and manganese acetate tetrahydrate (Mn(CH 3 COO) 2 4H 2 O) was used as the source of manganese dopant. The starting materials we re dissolved in etha nol. Overall concentr ation of the liquid precursor was fixed at 0.1 M while the doping concentration was fixed at 4 mol% with respect to zinc. The mixture was stirred fo r 30 minutes at room temperat ure until a homo geneous solution 110

PAGE 111

was obtained. Methane gas was used as the source of fuel while oxygen was used as an oxidant and carrier gas to spray precursor. The particles were annealed at different temperatures varying from 800 to 1000 for 1hr to determine th e phase transformation temp erature of as-prepared phosphors. The heated -particles were cooled to room temperature by na tural convection and used as it is for further characterization. The size and morphology of phosphor particles were de termined using field emission scanning electron microscopy (FE-SEM) while the crystallintiy of the synthesized Zn 2 SiO 4 :Mn 2+ nanophosphors was analyzed by powder X-ray diffr action (XRD). The exc itation and emission spectrums were obtained using Xe-ramp as the ex citation source. Luminescence characterization of the phosphors wa s carried out at room temperature. 9.3 Results and Discussion For increasing the crys tallinity of as-prepare d phosphor particles, it is necessary to carry out post-heat treatment for phase transformation and obtaining the pure phase. As-prepared phosphor particles were annealed in the electric furnace at different temperatures in the ranges from 800 to 1000 under atmospheric conditions. Figure 9-1 shows XRD patterns for synthesized Zn 2 SiO 4 :Mn 2+ nanophosphor particle s after post-heat treatmen t at different annealing temperatures. As the annealing te mperature is incr eased from 800 to 1000 there is an increase in the crystallinity of the heat-treated phosphor particles with the enhanced in peak intensity. When the as-prepared particles are annealed at 800 they exhibit zinc silicate structure with intermed iate phases that are identified as ZnO st ructure (JCPDS 36-1451). However, it can be seen from XRD result in Fig. 9-1 that th e typical zinc silica te structure with pure phase obta ined when annealed at 1000 111

PAGE 112

The morphology and size of as-prepared and annealed Zn 2 SiO 4 :Mn 2+ phosphor particles prepared by FSP from nitrate liq uid precursors are shown in Fi gure 9-2. While the morphology and size of the particles were ch aracterized by FE-SEM, the elemental contents were analyzed by an energy-dispersive X-ray spectro meter (EDS) attached to FE-SEM The nitrate li quid precursor for droplet was prepared at a molar ratio of Zn:Si:Mn as 1.96:1:0.4. EDS confirmed the heattreated nanophosphor particles as Zn 2 SiO 4 :Mn 2+ and having uniform co mposition. The phosphor particles annealed at 1000 depicted in Fig. 9-2 B shows the particle size to be in nanometer ranges, having filled morphology and spherical sh ape with little agglomer ation. However, it can be observed from SEM images in Fig. 9-2 A that as-prepared particles ex hibits irregular shape with agglomeration and broad particle size distri bution. From SEM images in Figure 9-2, it is observed that nano -sized phosphor particles can be easily synthesize d by FSP method using nitrate liquid precursor. Excitation and emission spectra for Zn 2 SiO 4 :Mn 2+ nanophosphors are pr esented in Figure 9-3. Phosphor partic les that were h eat treated at 1000 for 1hr under atmosp heric conditions exhibit bright green emitting light when excited with wavele ngth of 266 nm The strongest emission peak was observed at 524 nm, displaying typical luminescence spectrum of Zn 2 SiO 4 :Mn 2+ phosphors, based on the transition from 4 T 1 ( 4 G) to 6 A 1 ( 6 S) for tetrahedralcoordinated Mn 2+ [129, 130]. The inset in Figure 9-3 shows relative emission intensity as a function of annealing temperatures. It indi cates that the emissi on intensity of Zn 2 SiO 4 :Mn 2+ nanophosphor particles enhances with the increas e of annealing temperatur es and exhibits higher intensity when annealed at 1000 This increase in emission inte nsity is due to the improvement in crystallinity as well as the incorporation of Mn 2+ ions in the zinc sili cate matrix, which results from the annealing treatment. The efficiency of pho sphor materials is strong ly influen ced by the 112

PAGE 113

chemical stability of ho st matrix and well-dis tributed activators in its host system. The crystallinity of phospho r particles is the most important f actor when de termining the luminescence properties. Higher crystallinity of prepared-phosphor par ticles leads improvement in luminescence intensity. Thus, hi gher emission intensity in PL m easurements was attributed to increase crystallinity as shown in Figure 9-1 by XR D measurements. Based on this analysis, annealing temper ature of 1000 was chosen for rest of the study below. The effect of methan e flow rate on the crystallinity of as-prepared and heat-treated nanophosphor particles was investigated using XRD (Figure 9-4) As shown in Fi g. 9-4 A, asprepared particles were identified as mostly unreacted ZnO and sili ca crystal phases. The diffraction peaks of as-prepared particles shows increase in intensity in creased with increasing methane flow rate from 1 l/min to 3 l/min. In order to obtain typical zinc silicate crystal structure, post-heat treatment was carried out. As-prepared nanoparticles by FSP with different methane flow rate from 1 l/min to 3 l/min were annealed at 1000 for 1hr in elect ric furnace under atmospheric conditions. Figure 9-4 B shows XRD patterns fo r heat-treated Zn 2 SiO 4 :Mn 2+ nanophosphor particles prep ared at different methane flow rates. The characteristic peak intensities increased with increasing methane flow rate. For the formation of particles in flame synthesis, flame temperature is the most important parameter which determines the properties of the phosphor particles. The incr eased methane flow rate influe nces flame temperature and the residence time of droplets in fl ame zone. Increasing the methane flow rate raises the flame temperatures in flame zone and as a result of higher flame temperatur e improves crystalline structure in the prepared nanophosphor particle s by FSP [8, 64]. These figures suggest that crystallinity of as-prepared and annealed nanophosphor particle s can be improved with increase of the flame temperature by varying methane flow rate. 113

PAGE 114

The morphology and size of Zn 2 SiO 4 :Mn 2+ phosphor particle s obtained at different flame temperature by varying methane flow rate usi ng FSP are shown in Figure 9-5. It can be seen from FE-SEM images that flame temperature aff ects the morphology and size of the preparedphosphor particles by FSP. At lower flame temperature with less methane flow rate (Figure 9-5 A), it presents irregular shape particles with hi ghly agglomeration. Howeve r, when hi gher flame temperature applied to flame r eaction, it leads to formation of nanophosphor pa rticles with spherical shape and less agglomeration (Figure 9-5 B). The effect of methane flow rate in flame zone on lumine scence intensity is as shown in Figure 9-6. For all samples that were annealed at 1000 for 1hr under atmosp heric conditions. PL measurement shows the improvem ent of emission intensity of Zn 2 SiO 4 :Mn 2+ nanophosphor particles with increasing methane flow rate though no shift in peak position wa s observed. PL measurements show that PL intensity of Zn 2 SiO 4 :Mn 2+ nanophosphor partic les prepared at higher methane flow rate (3 l/min) is higher than that prepared with lo wer methane flow rate. Increasing methane flow rate in flame zone raises the flame temperature whic h introduces to generate the particles with good crystal linity. This is du e to complete co mbustion of the precursor in flame zone as resulting in higher fl ame temperature. As disc ussed, the luminescence properties of phosphors are influenced by vari ous parameters such as pa rticle size [7], surface morphology [8], and crystallinity [122]. Among those, the crysta llinity is the most important parameter to determine the lumine scence efficiency in phosphors. Higher crystallinity of prepared-phosphor particles leads to improve ment in luminescence in tensity [130]. The PL intensity usua lly exhibits en hancement in inte nsity of phospho r particles obtained at high flame temperature. 114

PAGE 115

9.4 Conclusion For FSP method, it was observed that nitrate liquid precursor can supply the droplet with homogeneous composition into spra y burner during flam e synthesis and as a result of that, it can easily produce the preferred phosphor particles as fi nal product. In order to investigate the phase transition of as-prepared particles by FSP, po st-heat treatment was pe rformed at different annealing temperature from 800 to 1000 for 1hr. Pure Zn 2 SiO 4 structure and the most intense peak in PL m easurement were obtained when anneal ed at 1000 There are no significant differences in size and surface mor phology between the phosphor particles with and without post-heat treatment. Furthermore, the effect of flame temperature on the crys tal structure and luminescence properties of Zn 2 SiO 4 :Mn 2+ nanophosphors was studied. High flame temperature by increasing methane flow rate leads to high crystalline structure of partic les as it results in improvement in luminescence properties of prepared phosphor particles by FSP. Thus, FSP can be used for synthesis of high luminescen ce efficiency phos phors, using nitrate ba sed liquid precursor followed by an nealing at 1000 and high flame temperature. 115

PAGE 116

1020304050607080 Intensity (a.u.)2-Theta JCPDS 37-1485 (Zn2SiO4) 1000C 900C 800C As-prepared Figure 9-1. XRD patterns of 4 mol% Mn-doped Zn 2 SiO 4 at different annealing temperatures 116

PAGE 117

A B Figure 9-2. SEM images of 4 mol% Mn-doped Zn 2 SiO 4 nanophosphors prepar ed from nitratebased liquid prec ursor. A) as-prepared, B) annealed at 1000 for 1hr 117

PAGE 118

250300350450500550600 Intensity (a.u.)Wavelength (nm)As-prepared8009001000 Intensity (a.u.)Annealing temperature (C) Figure 9-3. Excitation and emission spectra of 4 mol% Mn-doped Zn 2 SiO 4 annealed at 1000 for 1hr. The inset in Figure 9-3 shows the relative PL intensity as a function of annealing temperature 118

PAGE 119

1020304050607080 JCPDS 36-1451(ZnO) CH 4 2 l/min CH 4 3 l/min JCPDS 37-1485 (Zn2SiO4) Intensity (a.u.)2-ThetaCH 4 1 l/minA) A 1020304050607080 B)CH 4 2 l/min CH 4 3 l/min JCPDS 37-1485 (Zn2SiO4) Intensity (a.u.)2-ThetaCH 4 1 l/min B Figure 9-4. XRD patterns for Zn 2 SiO 4 :Mn 2+ phosphor particles at differe nt flame temperature as functions of methane fl ow rate. A) as-prepare d, B) annealed at 1000 for 1hr 119

PAGE 120

A B C Figure 9-5. FE-SEM images of Zn 2 SiO 4 :Mn 2+ phosphor particles at different flame temperature as functions of meth ane flow rate. A) CH 4 -1 l/min, B) CH 4 -2 l/min, C) CH 4 -3 l/min 120

PAGE 121

480500520540560580600 CH4-3 l/min CH4-2 l/min Emission CH4-1 l/minIntensity (a.u.)Wavelength (nm) Figure 9-6. Emission spect ra of 4 mol% Mn-doped Zn 2 SiO 4 :Mn 2+ phosphor obtained at different methane flow rate. 121

PAGE 122

CHAPTER 10 CONCLUSION The objectives of this resear ch focused on enha ncement in the qual ities of nanophosphor particles, which can be determined by controlling in shapes, size and crystallinity of particles. The luminescence properties of pr epared phosphor particles were analyzed to understand the effect of different conditions in liquid precursors. 10.1 Single Step Processing For Nanophosphor Particles Firstly, Y 2 O 3 :Eu 3+ nanophosphor was synthesized by FSP from urea added nitrate based liquid precursor. The end product shows cu bic phase Y 2 O 3 :Eu 3+ nanophosphor successfully prepared by FSP without heat treatment. The infl uence of synthesis conditi ons such as different moles of urea, overall concentration of li quid precursors, and doping concentration on luminescent properties were inve stigated. The particle size of product was f ound to be in the range of 20-30 nm from TEM. In the photoluminescence (P L) properties, Y 2 O 3 :Eu 3+ nanophosphor emitted red light with a peak wavelength of 609 nm when exci ted with 398 nm wavelength photons. 10.2 The Effect of Addition of Urea to liquid Precurs ors on the Properties of Nanophosphor Particles Seconldy, YAG:Ce 3+ nanophosphors were synthesized by FSP from nitrate liquid precursor. As-prepared nanoparticles were annealed in the temp erature range of 800 to 1100 for 1 hour. The influence of the addition of urea and the molar ratio of yttrium to aluminum in the liquid precursor on crystallinity an d luminescence properties of YAG:Ce 3+ nanophosphors prepared by FSP were comparatively studied. Th e heat-treated phosphor particles are spherical shape with an average size blow 50 nm. The crystallinity of YAG:Ce 3+ nanophosphors improved with addition of urea and overloaded aluminum in starting liquid pr ecursor. The nanophosphors 122

PAGE 123

from urea-added and slight excess of aluminum in liquid precursor presen ts higher luminescence intensity than those of from non-urea and less aluminum. 10.3 The inflence of Different Zn-source to Liquid Precursors on Luminescence Properties of Nanophosphor Particles Green emitting Zn 2 SiO 4 :Mn 2+ phosphor particles were synthe sized by FSP from nitrate liquid precursor. Lumine scence and crystalline properties were investigated as the different Znsource materials in aqueous precur sor. Also, the influence of post-heat treatment temperatures on the crystal structure and PL intensity of Zn 2 SiO 4 :Mn 2+ nanophosphors were investigated. Mndoped zinc silicate cr ystalline structures were obtained wh en anneal ed at 1000 for 1hour. The emission peak was found at 525 nm as green emi tting phosphor. Furthermor e, it was investigated the effect of the flame temperat ures by varying methane flow rate on the crystallinity and luminescence properties of Zn 2 SiO 4 :Mn 2+ nanophosphors. The phosphor particles prepared from high flame temperature showed good crystallinity as pure phase and the maxi mum PL intensity. We conclude that the in fluence of different experimental conditions including liquid precursor from different zn-source and annealing temperatur e on both crystallinity and the luminescence properties of Zn 2 SiO 4 :Mn 2+ nanophosphors. 123

PAGE 124

LIST OF REFERENCES 1. K.J. Stout, J. Kor. Soc. of Pre. Eng., 14 29-51 (1997). 2. H. Chander, Mat. Sci. & Eng., R 49 113-155 (2005). 3. R. Schmechel, H. Winkler, L. Zaomao, M. Kennedy M. Kolbe, A. Be nker, M. Winterer, R.A. Fischer, H. Ha hn, H.V. Seggern, Scrip. Mat., 44 1213-1217 (2001). 4. A.M. Pires, O.A. Serra, M.R. Davolos, J. Lumin., 113 174-182 (2005). 5. K.Y. Jung, H.W. Lee, J. Lumin 126 469-474 (2007). 6. M. Hosokawa, Nanoparticle Technology Handbook Elsevier (2007). 7. Y. C. Kang, I. W. Lenngoro S. B. Park, K. Okuyama, Mat. Res. Bul. 35 789-798 (2000). 8. H. Chang, I. W. Lenggoro, K. Okuyama, T. O. Kim, Jpn. J. Appl. Phys., 43 3535 (2004). 9. L.G. Jacobsohn, B.L. Bennett, R.E. Mu enchausen, J.F. Smith, D. W. Cooke, Radi. Measur. 42 675-678 (2007). 10. S. K. Lee, H. H. Yoon, S. J. Park, K. H. Kim, H. W. Choi, J. J. Appl. Phys., 46 79837986 (2007). 11. R.N. Laine, C.R. Bickmore, D.R. Tread well, K.F. Waldner, U.S. Pat. No.5958361. 12. C.H. Lu, W.T. Hsu, J. Dhanaraj, R. Jagannathan, J. European. Ceramic Soc. 24, 37233729 (2004). 13. Z. Yang, X. Li, Y. Yang, X. Li, J. Lumin. 122-123 707-709 (2007). 14. C.C. Chiang, M.S. Tsai, C.S. Hsiao, M.H. Hon, J. Alloys Compd. 416 265 (2006). 15. G. Blasse, B. C. Grabmaier, Limines cent Materials, Ber lin, Springer, 1994. 16. W. W. Zhang, W. P. Zhang, P. B. Xie, M. Yi n, H. T. Chen, L. Jing Y. S. Zhang, L. R. Lou, S. D. Xia, J. Colloid Interface Sci. 262 588 (2003). 17. W.M. Yen, M. J. Weber ( 2004) Inorganic phosphors. C RC Press, Boca Raton. 18. M. Gaft, R. Reisfeld, G. Pa nczer (2005) Luminescence Spec troscopy of Minerals and Materials. Springer, Berlin. 19. C. Ronda (2001) Rare Earth Phosphors: Fundamentals and Applications, En cyclopedia of materials science and te chnology, New York. 20. S. Jung, Y.C. Kang,, J.H, Kim, J. Mater. Sci., 42 9783-9794 (2007). 124

PAGE 125

21. R.C. Ropp, Luminescence and th e sold state., 21 1-711 (2004). 22. C.R. Ronda, T. Ju stel, H. Nikol, J. Alloys Compd., 275-277 669-676 (1998). 23. A. Konrad, T. Fries, A. Gahn, F. Kummer, U. Herr, R. Tidecks, K. Samwer, J. Appl. Phys., 86 3129 (1999). 24. C. He, Y. Guan, L. Yao, W. Cai, X. Li, Zn. Yao, Mater. Res. Bull., 38 973 (2003). 25. R. Morimo, K. Matae, Mater. Res. Bull., 24 175 (1989). 26. T. Miyata, T. Minami, K. Saikai, S. Takata, J. Lumin. 60-61 926-929 (1994). 27. C. Okazaki, M. Shiiki, T. Suzuki, K. Suzuki, J. Lumin. 87-89 1280-1282 (2000). 28. K.Y. Jung, Y. C. Kang, Mat. Lett., 58 ,2161-2165 (2004). 29. H. Chang, I. W. Lenggoro, T. Ogi, K. Okuyama, Mat. Lett., 59 1183-1187 (2005). 30. C. H. Lu, C.T. Chen, B. Bhattacharjee, J. Rar. Ear., 24 706-711 (2006). 31. G. Del Rosario, S. Ohara, L. Mancic, O. Milosevic, App. Surf. Sci., 238 469 (2004). 32. P. Schlotter, R. Schm idt, J. Schneider, Appl. Phys. A, 64 417 (1997). 33. J.H. Yum, S.S. Kim, Y.E. Sung, Coll. Surf. A: Physic ochem. Eng. Aspects, 251 203 (2004). 34. M. Kottaisamy, P. Thiyagarajan, J. Mishra, M.S. Ramachandra Rao, Mat. Res. Bul. 43 1657-1663 (2008). 35. X.Y. Chen, H.Z. Zhuang, G.K. Liu, S. Li, R.S. Niedbala, J. Appl. Phys., 94 5559 (2003). 36. A. Tamura, Phys. Rev. B., 52 2688 (1995). 37. R. S. Meltzer and K. S. Hong, Phys. Rev. B., 61 3396 (2000). 38. D. Jia, Chem. Eng. Comm. 194 1666-1687 (2007). 39. S.J. Hong, J.I. Han, NMDC IEEE., 1, 446-447 (2006). 40. T. Ye, Z. Guiwen, Z. Weiping, X. Shangda, Mater. Res. Bull., 32 501 (1997). 41. D. Jia, Electrochem. Solid State Lett., 9, H93-H95 (2006). 42. N. Narendran, Proc. SPIE, 5941 45 (2005). 43. Y. Huh, Y. Cho, and Y. R. Do, Bull. Korean Chem. Soc., 23 1435 (2002). 125

PAGE 126

44. S. Nakamura, G. Fasol, The Blue Laser Diode, Berlin Springer, 1997. 45. S. Muthu, F.J.P. Schuurmans, M. D. Pashley, 37th Industry Application Conference, 1 327-333 (2002). 46. S. Muthu, F.J.P. Schuurmans, M. D. Pashley, IEEE Journal in Q uantum Electronics, 8 333-338 (2002). 47. S. Muthu, J. Gaines, 38th Industry Application Conference, 1 515-522 (2003). 48. R. Mueller-Mach, IEEE Journal of Q uantum Electronics, 8 339 (2002). 49. H. Wu, IEEE Photonics Tech. Lett. 17 1160-1162 (2005). 50. Y.R. Do, K.-Y. Ko, S. -H. Na,Y.-D. Huh, J. Electrochem. Soc., 153 H142-H146 (2006). 51. J. H. Yum, S.Y. Seo, S. Lee, Y.E. Sung, J. Electrochem. Soc., 150 H47-H52 (2003). 52. J. H. Yum, S.Y. Seo, S. Lee, Y.E. Sung, Proc. SPIE., 4445 60-69 (2001). 53. K. Y. Sasaki, J. B. Talbot, Adv. Mater., 11 91-105 (1999). 54. B. Damilano, N. Grandjean, C. Pernot, J. Massies, Jpn. J. Appl. Phys., 40, L918L920 (2001). 55. S. W. Ricky LEE, C. H. Lau, S. P. Chan, K. Y. Ma, M. H. Ng, Y. W. Ng, K. H. Lee, Jeffery C. C. Lo, Proc. HDP., June, 192-196 (2006). 56. Y. Sato, N. Taka hashi, S. Sato, Jpn. J. Appl. Phys., 35 L838L839 (1996). 57. K.H. Lee, S.W.R. Lee, Electronics Packaging Technol ogy Conference, 2006. EPTC '06. 8th 379-384 (2006). 58. K. Yokota, S.-X. Zhang, K. Kimura, A. Sakamoto, J. Lumin 92 223 (2001). 59. C. Feldmann T. Jstel C.R. Ronda, P.J. Schmidt, Adv. Funct. Mater., 13 No. 7 (2003). 60. D. Jia, Y. Wang, X. Guo, K. Li, Y.K. Zou, W. Jia, J. Electrochem.Soc. 154 J1 (2007). 61. E.T. Goldburt, B. Kulkarni, R. Bhargava, J. Taylor, M. Libera, J. Limin., 72 190 (1997). 62. R. Schmechel, H. Winkler, L. Xaomao, M. Kenndey M. Kolbe, A. Be nker, M. Winterer, R.A. Fisher, H. Ha hn, H. Seggern, Scripta Mater. 44 1213 (2001). 63. O. Milosevic, L. Mancic, M. E. Rabanal, J.M. Torralba, B.R. Yang and P. Townsend, J. Electrochem. Soc. 152 G707 (2005). 64. A. Purwanto, W.N. Wang, I.W. Lenggoro and K. Okuyama, J. Electrochem. Soc. 154 J91 (2007). 126

PAGE 127

65. R. Kasuya, T. Isobe, H. Kuma and J. Katano, J. Phys. Chem. B. 109 22126 (2005). 66. D.A. Steigerwald, J.C. Bhat, D. Co llins, R.M. Fletch er, M.O. Holcomb, M.J. Ludowise, P.S. Martin, S.L. Rudaz, IEEE J. Selected Topics in Quantum Electron. 8 310-320 (2002). 67. A. Morell, N. El Khiati, J. Electrochem. Soc., 140 2019 (1993). 68. S.R. Luki D.M. Petrovi M.D. Drami anin, M. Mitri Lj. a anin, Scrip. Mat., 58 655 (2008). 69. Y.C. Kang, H.D. Park, Appl. Phys. A 77 529-532 (2003). 70. R. Selomulya, S. Ski, K. Pita, C.H. Kam, Q.Y. Zhang, S. Buddhudu, Mat. Sci. & Eng. B, 100 136-141 (2003). 71. P.R. Rao, J. Electrochem. Soc., 152 H115 (2005) 72. K.H. Butler, Fluorescent La mp Phosphors, The Pennsylvan ia State Unversity Press, University Park (USA) 1980, Chap. 3. 73. W.-H. Hsu, M.-H. Sheng, M.-S. Tsai, J. Alloys Compd., 467 491-495 (2009). 74. L. Zhou, B. Yan, J. Phys.Chem. of Sol. 69 2877-2882 (2008). 75. V. Singh, R.P.S. Chakradhar J.L. Rao, D. K. Kim, Mat. Chem. Phy. 110, 43-51 (2008). 76. X. Lou, D. Chen, Mat. Lett. 62 1681-1684 (2008). 77. J. Zhang, Z. Zhang, Z. Tang, Z. Zheng, Y. Lin, Powder Technology, 126 161 165 (2002). 78. Y. Gogotsi (2006) Nanomaterials Handbook. CRC Press, Boca Raton. 79. H.M.H. Fadlalla, C.C. Tang E.M. Elssfah, F. Shi, Mater. Chem. Phys., 109 436-439 (2008). 80. C.C. Lin, K.M. Lin, Y.Y. Li, J. Lumin. 126 795-799 (2007). 81. J.J. Kingsley, K. Su resh, K.C. Patil, J. Mater. Sci ., 25 1305 (1990). 82. S.T. Aruna, A.S. Mukasyan, Curr. Opin. Sol. Stat. & Mat. Sci., In Press, (2009). 83. D.A. Fumo, M.R. Morelli, A.M. Segadaes, Mater. Res. Bull. 31 1243 (1996). 84. L.E. Shea, J. McKittrick, O.A. Lopez, e. Sluzky, J. Am. Ceram. Soc., 79 3257 (1996). 85. S. Liu, Z. Xiu, F. Xu, W. Yu, J. Yu, G. Feng, J. Allo. Comp., 459 407 (2008). 127

PAGE 128

86. Y.-P. Fu, S.-B. Wen, C.-S. Hsu J. Allo. Comp ., 458 318-322 (2008). 87. S. Mukherjee, V. Sudarsan, R.K. Vatsa, A.K. Tyagi, J. Lumin. 129 69-72 (2007). 88. K. Okuyama, I.W. Lenggoro, Chem. Eng. Sci., 58, 537-547 (2003). 89. H.K. Kammler, L. Madler, S.E. Pratsinis, Chem. Eng. Technol ., 24 583-596 (2001). 90. K. Wegener, S.E. Pratsinis, KONA, 18 170-182 (2000). 91. L. Madler, KONA 22 107-120 (2004). 92. R.M. Laine, R. Baranwal, T. Hi nklin, D. Treadwell, A. Sutori k, C. Bickmore, K. Waldner, S.S. Neo, Key. Eng. Mat ., 159 17-21 (1999). 93. C.R. Bickmore, K.F. Waldner, D.R. Treadwell, R.M. Laine, J. Am. Ceram. Soc., 79 1419-1423 (1996). 94. S.E. Pratsinis, Prog. Ener. Comb. Sci., 24 197-219 (1998). 95. B. Xia, L. Duan, and Y. Xie, J. Am. Ceram. Soc ., 83 1077 (2000). 96. F. Yan, T. C. D. Huo, and H. C. Ling, J. Electrochem. Soc. 134 493 (1987). 97. T.J. Gardner, D.W. Sproson, G.L. Messing, MRS Proc. 32 227-237 (1984). 98. T.G. Carreno, M.P. Morales, M. Grasia, G.J. Serna, Mat. Lett 18 151-155 (1993). 99. T.J. Gardner, G.L. Messing, Am. Ceram. Soc. Bull ., 63 1498-1501 (1984). 100. http://www.chemat.co m/html/solgel.html (2009). 101. http://www .uio.no/studier/emner/m atnat/kjemi/KJM5100/h08/undervisningsmateriale/05 kjm5100_2008_shs_a.pdf (2009). 102. C. Feldmann T. Jstel C.R. Ronda, P.J. Schmidt, Adv. Funct. Mater., 13 (2003). 103. P.K. Sharma, M. H. Jilavi R. Nass, H. Schmidt, J. Limin. 82 187 (1999). 104. Y.C. Kang, D. J. Seo, S. B. Park, and H. D. Park, Jpn. J. Appl. Phys 40 4083 (2001). 105. X. Qin, Y. Ju, S. Bernhard, N. Yao, J. Mater. Res., 20 290-2968 (2005). 106. A. Purwanto, W. N. Wang, T. Ogi, W. Lenggoro, E. Tanabe, K. Okuyama J. Alloys. Compd. 463, 350 (2008). 107. B. R. Judd, Phys. Rev. 127 750 (1962). 108. G. S. Ofelt, J. Chem. Phys. 37 511 (1962). 128

PAGE 129

109. R.N. Bhargava, D. Gallaghe r, X. Hong, A. Nurmikko, Phys. Rev. Lett., 72 416 (1994). 110. S.J. Hong, M. G. Kwak, J. I. Han, Appl. Phys., 6S1 e211 (2006). 111. A. Purwanto, I. W. Lenggoro H. Chang, K. Okuyama, J of Chem. Eng. of Jap., 39 68 (2006). 112. G.A. Hirata, J. McKittrick, M. Avalos -Borja, J.M. Siquei ros, D. Devlin, Appl. Surf. Sci., 113/114 509 (1997). 113. M.G. Kwak, J. H. Park, S. Shon Solid State Communi., 130, 199 (2004). 114. Y. Tao, G.W. Zhao, W. P. Zhang, S. D. Xia, Mater. Res. Bull. 32 501 (1997). 115. Y. Narukawa, S. Saijou, Y. Kawakami, S. Fujita, T. Mukai, S. Nakamura, Appl. Phys. Lett., 74 558 (1999). 116. D. Haranath, H. Chander, P. Shama, S. Singh, Appl. Phys. Lett., 89 173118 (2007). 117. W.T. Hsu, W. H. Wu, C. H. Lu, Mater. Sci. Eng. B 104 40 (2003). 118. K. Zhang, H. Liu, Y. Wu, W. Hu, J. Mater. Sci., 42 9200-9204 (2007). 119. K. M. Kinsman, J. McKittrick, J. Am. Ceram. Soc., 77 2866 (1994). 120. Y. Terashi, A. Purwanto, W. N. Wang, F. Isakandr, K. Okyama, J. European. Ceramic Soc., 28, 2573-2580 (2008). 121. J Lin, Q. Su, J. Mater. Chem., 5 1151 (1995). 122. Y. Pan, M. Wu, Q. Su, Mat. Sci. Eng. B 106 251-256 (2004). 123. C.H. Lee, Y.C. Kang, K.Y. Jung, J.G. Choi, Mat. Sci. & Eng. B, 117 210-251 (2005). 124. Q.Y. Zhang, K. Pita, W. Ye, W.X. Que, Chem. Phys. Lett., 351 163 (2002). 125. C. Barthou, J. Benoit, P. Benalloul, A. Morell, J. Electrochem. Soc., 141 524 (1994). 126. T.H. Cho, H.J. Chang, Ceram. Int. 29 611-618 (2003). 127. H. Briesen, A. Fuhrmann, S.E. Pratsinis, Chem. Eng. Sci., 53, 4105-4112 (1998). 128. H. Chang, I.W. Lenggoro, K. Okuyama, T. O. Kim, J. Appl. Phys., 43 3535 (2004). 129. K.S. Sohn, B. Cho, H.D. Park, J. Eur. Ceram. Soc., 20 1043-1051 (2000). 130. A. Patra, G.A. Ba ker, S.N. Baker, J. Lumin., 111 105-111 (2005). 131. Y.C. Kang, D.J. Seo, S.B. Park, H.D. Park, Mat. Res. Bull., 37 263-269 (2002). 129

PAGE 130

132. X. Qin, Y. Ju, S. Bernhard, N. Yao, Mat. Res. Bull., 42 1440-1449 (2007) 130

PAGE 131

BIOGRAPHICAL SKETCH Jae Seok Lee was born in 1977, in Suji, South Korea. In 1997, he entered the Department of Materials Science & Engineering at Korea University. Af ter earning a Bach elor of Science degree, he continued his gradua te study and earned a Master of Scienc e degree in 2005. His research topic was a st udy on growth and characterization of III-N itride thin film by MOCVD for LED applications under the supervision of Dr Dongjin Byun. In August 2005, he enrolled at the University of Florida in the Department of Materials Science and Engineering to pursue his Ph.D under the advisement of Dr. Rajiv.K. Singh. His main research was involved in synthesis and characterization of nanophosphor particles and film by flam e spray pyrolysis (FSP). He also focused on control of phorphor particle size and morpho logy, crystallintiy, and luminescence properties for commercial application using FSP system.