Enhanced Light Output from Luminescent Oxide Nanoparticles

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Enhanced Light Output from Luminescent Oxide Nanoparticles Synthesis, Characterization and Scintillation Application
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english
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Choi,Jihun
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University of Florida
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Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Materials Science and Engineering
Committee Chair:
Holloway, Paul H
Committee Co-Chair:
Davidson, Mark R
Committee Members:
Jones, Jacob L.
Hummel, R. E
Tanner, David B

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Subjects / Keywords:
core -- luminescence -- nanoparticles -- scintillator -- shell -- synthesis
Materials Science and Engineering -- Dissertations, Academic -- UF
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Materials Science and Engineering thesis, Ph.D.
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Abstract:
Scintillator materials have been grown in the past as single crystal phase which generally lead to high cost and small size of radiation detector crystals. In this work, highly efficient scintillator nanoparticles have been synthesized using non-hydrolytic hot-solution growth, sol-gel, and aqueous precipitation methods. Gd2O3:Eu3+ nanoparticles with sizes and shapes of 20 nm clovers, 10 nm squares and 15 nm rounds were prepared by a non-hydrolytic high temperature (320 ?C) solution growth method. Synthesis parameters such as reaction procedure, time and temperature were varied to investigate their effects on nanocrystal shape and luminescent properties. The variation of nanoparticle shapes was explained by nucleation and growth of oxide nanocrystals. The photoluminescence properties and quantum yields of Gd2O3:Eu3+ nanoparticles could be controlled by the synthesis parameters. Round Gd2O3:Eu3+ nanoparticles exhibited the highest quantum yield of 67%, whereas clover and square Gd2O3:Eu3+ nanoparticles showed lower quantum yields of 24% and 48%, respectively. The luminescent properties were discussed in terms of doping concentration, host environment and dopant location. Nanoparticles of Gd2O3:Eu3+ ~20 nm in diameter were synthesized at ~180? C using a facile high boiling-point alcohol (polyol) method. The Gd2O3 nanoparticles, doped with 5 mol% Eu, were crystalline cubic phase and exhibited intense 5D0-7F2 photoluminescence (PL) from Eu3+ after calcination at 600 ?C for 2h in air. Photoluminescence excitation (PLE) data showed that while a small fraction of the emission resulted from direct excitation of Eu3+, most of the excitation resulted from adsorption in the Oxygen to Europium charge-transfer band between 225 and 275 nm. Transmission electron microscopy (TEM) images showed that the Gd2O3:Eu3+ cores were slightly agglomerated, but the thin Y2O3 shell could not be detected by TEM. X-ray photoelectron spectroscopy (XPS) was used to detect the thin Y2O3 shell around the Gd2O3:Eu3+ core. Drop-cast thin films of the Gd2O3:Eu3+/Y2O3 core/shell nanoparticles exhibited PL intensities up to 40% larger than from bare core Gd2O3:Eu3+ nanoparticles. Increased PL was attributed to reduced non-radiative recombination based on longer luminescence decay times. Spherical SiO2 cores have been coated with oxide dual-shells of Gd2O3:Eu3+ and Y2O3 by a solution precipitation method. Based on transmission electron microscopy (TEM) data, luminescent Gd2O3:Eu3+ shells with ~4.5 nm thickness were successfully coated on mono-dispersed SiO2 nanocores ~210 ? 15 nm in radius. A continuous Y2O3 shell ?45 nm thick was then grown on the SiO2/Gd2O3:Eu3+ nanoparticles. The SiO2/Gd2O3:Eu3+ core/shell nanoparticles exhibited 5D0-7F2 photoluminescence (PL) from Eu3+ transitions after calcination at 600?C for 2h in air. Photoluminescence excitation (PLE) data showed that most of the excitation resulted from absorption in the oxygen to europium charge-transfer band between 225 and 275 nm. Drop-cast thin films of core/dual-shell samples exhibited quantum yields (QYs) up to 2 times larger than that of core/single-shell nanoparticles, which was attributed to reduced non-radiative recombination, consistent with longer luminescence decay lifetimes. Nanophosphor Gd2SiO5:Ce3+ (GSO) with sizes between 2 nm - 5 nm was synthesized by sol-gel and hot-solution methods. X-Ray luminescence, photoluminescence (PL) and structural properties of GSO:Ce3+ nanoparticles were compared for the two different synthesis methods. PL and X-ray luminescence were both from the Ce3+ 5d-4f transitions at 390~430 ?C. The Ce dopant concentration was varied between 0.1% - 10% and concentration quenching was observed at 0.5% under UV excitation. Hot-solution synthesized GSO doped with ?0.5% Ce exhibited PL after calcination at either 600 ?C or 1000 ?C for 2 h in air, while GSO:Ce3+ nanoparticles prepared by the sol-gel method required calcination at 1000 ?C to observe PL.
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In the series University of Florida Digital Collections.
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Statement of Responsibility:
by Jihun Choi.
Thesis:
Thesis (Ph.D.)--University of Florida, 2011.
Local:
Adviser: Holloway, Paul H.
Local:
Co-adviser: Davidson, Mark R.

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1 ENHANCED LIGHT OUTPUT FROM LUMINSECENT OXIDE NANOPARTICLES: SYNTHESIS, CHARACTERIZATION AND SCINTILLATION APPLICATION By JIHUN CHOI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011

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2 2011 J ihun C hoi

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3 T o my wife, my son and my parents

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4 ACKNOWLEDGMENTS I would never have been able to finish my dissertation without the guidance of my committee members, help from our group members and support from my family I would like to express my deepest gratitude to my advisor, Dr. Paul H. Holloway for his excellent guidance, caring, patience, and providing me with an excellent a tmosphere for research. His countless passion and endless effort to pur sue knowledge always made me refreshed and motivated, and his logical attitude toward science ha s reinforced my perception about research. I also would like to thank my supervisory comm ittee; Dr. Mark Davidson, Dr. Rolf Hummel, Dr. Jacob Jones and Dr. David Tanner for their advice and support. I sincerely thank Ludie Harmon for her hospitality and support. I also would like to acknowledge members in our group for all their support and valuable discussion, particularly Jason Rowl and, Evan Law, Marc Plaisant Te ng Kuan Tseng and Kathryn O Bri en. In addition, I want to thank the staff of MAIC and PERC for their assis tance, especially Kerry Siebein for her support with TEM analysis and Eric Lambers for his assistance with XPS analysis. I would also like to thank my parents parents in law and three sisters. The ir belief and encouragement with their best wishes could make me endure and complete my work. Finally, I am thoroughly grateful to my lovely wife, Naree Yoon. She has been always there cheering me up and standing by me through the good and bad times I deeply thank my son, Nathan for motivating me to complete my long journey. You taught me about responsibility and patience. I love you all.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 8 LIST OF FIGURES .......................................................................................................... 9 ABSTRACT ................................................................................................................... 14 C HAPTER 1 INTRODUCTION .................................................................................................... 17 2 LITERATURE REVIEW .......................................................................................... 19 2.1 Fundamental of Colloidal Nanoparticles ........................................................... 19 2.1.1 Synthesis Process for Colloidal Nanoparticles ........................................ 19 2.1.2 Shape Control of Colloidal Nanoparticles ................................................ 24 2.1.2.1 Shapes of n anoparticles ................................................................. 24 2.1.2.2 Growth m echanism of shapecontrolled n anoparticles ................... 27 2.2 Inorganic Luminescent M aterials ...................................................................... 32 2.2.1 Fundamentals of Luminescent Materials ................................................. 32 2.2.2 Application of Luminescent Materials ...................................................... 36 2.3 Scintillation Materials ........................................................................................ 39 2.3.1 Fundamentals of Scintillation Materials ................................................... 40 2.3.2 Application of Scintillation Materials ........................................................ 44 2.4 Luminescent Nanoparticles ............................................................................... 46 2.4.1 Fundamental and Application of Luminescent Nanoparticles .................. 46 2.4.2 Surface Modification of Luminescent Nanoparticles ................................ 49 3 SHAPE CONTROLLED GADOLINIUM OXIDE DOPED WITH EUROPIUM COLLOIDAL NANOCRYSTALS GROWN BY HOT SOLUTION METHOD ............ 52 3. 1 Introduction ....................................................................................................... 52 3.2 Experimental ..................................................................................................... 53 3. 2.1 Materials .................................................................................................. 5 3 3. 2.2 Gd2O3:Eu3+ N anocrystal S ynthesis .......................................................... 53 3. 2.3 Characterization ...................................................................................... 54 3. 3 Result s and Discussion ..................................................................................... 55 3. 3.1 S hape C ontrol of Gd2O3:Eu3 + N anocrystals ............................................. 55 3 .3 .2 Crystal S tructure of Gd2O3:Eu3+ N anoparticles ........................................ 58 3.3 .3 Photoluminescent P roperties of N anoparticles ........................................ 59 3.3.4 X ray Luminescence P roperties of N anoparticles .................................... 63 3. 4 Conclusion ........................................................................................................ 64

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6 4 ENHANCED PHOTOLUMINESCENCE FROM GADOLINIUM OXIDE DOPED WITH EUROPIUM NANOCORES WITH YTTRIUM OIXDE THIN SHELL .............. 65 4.1 Introduction ....................................................................................................... 65 4.2 Experimental ..................................................................................................... 66 4.2.1 Synthesis of Gd2O3:Eu3+ N anocores ....................................................... 66 4. 2. 2 Synthesis of Y2O3 in the P resence of Gd2O3:Eu3+ Nanoc ores ................. 66 4. 2. 3 Characterization of N anoparticles ........................................................... 67 4. 3 Result s and D iscussion ..................................................................................... 68 4. 3.1 Crystal St ructures and M orphologies of C ore and C ore/ S hell N anopar ticles ................................................................................................ 68 4. 3.2 Determination of S hell T hickness by XPS A nalysis ................................. 72 4. 3.3 Photoluminescence P roperties of C ore and C ore/ S hell N anoparticles .... 74 4. 3.4 Luminescence D ecay T imes of C ore and C ore/ S hell N anoparticles ....... 76 4. 4 Conclusion ........................................................................................................ 78 5 ENHANCED PHOTOLUMINESCENCE FROM EUROPIUM DOPED GADOLINIUM OXIDE BASED CORE/DUAL SHELL NANOPARTICLES .............. 79 5. 1 Introduction ....................................................................................................... 79 5 2 Experimental ..................................................................................................... 80 5. 2.1 Materials .................................................................................................. 80 5. 2.2 S ynthesis of Silica C ore s ......................................................................... 80 5. 2.3 Synthesis of Gd2O3:Eu3+ S hell on SiO2 C ores ......................................... 80 5. 2.4 Synthesis of Y2O3 O uter S hell on SiO2/Gd2O3:Eu3+ N anoparticles .......... 81 5. 2.5 Characterization of N anoparticles ........................................................... 81 5. 3 Result s and D iscussion ..................................................................................... 82 5. 3.1 Morphology and C rystal St ructure of C ore/ S hell N anoparticles ............... 82 5. 3.2 Surface A nalysis of C ore/ S hell N anoparticles ......................................... 84 5. 3.3 Luminescence P roperties of C ore/ S hell N anoparticles ............................ 86 5.3.4 Thin Film Quantum Yield Measurement .................................................. 88 5. 3. 5 Luminescent D ecay T ime of Core/Shell N anoparticles ............................ 89 5. 4 Conclusion ........................................................................................................ 90 6 X RAY AND PHOTO LUMINESCENCE FROM GADOLINIUM SILICATE DOPED WITH CERIUM NANOPARTICLES SYNTHESIZED BY SOLUTION BASED METHODS ................................................................................................. 92 6. 1 Introduction ....................................................................................................... 92 6.2 Experimental ..................................................................................................... 93 6. 2.1 Hot S olution S ynthesis ............................................................................ 93 6. 2.2 Sol G el S ynthesis .................................................................................... 94 6. 2.2 Characterization ...................................................................................... 94 6. 3 Result s and D iscussion ..................................................................................... 95 6.3.1 Thermogravimetric Analysis (TGA) of GSO Nanoparticles ...................... 95 6.3.2 Morphology and Structure of GSO Nanoparticles ................................... 96 6.3.3 Luminescent Properties of GSO Nanoparticles ....................................... 99

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7 6.3.4 X ray Luminescence of GSO Nanoparticles .......................................... 102 6. 4 Conclusion ...................................................................................................... 103 7 CONCLUSIONS ................................................................................................... 105 7.1 Shape Controlled Two Dimensional Gd2O3:Eu3+ Colloidal Nanocrystals Grown by Hot Solution Method .......................................................................... 105 7.2 Enhanced Photoluminescence from Gd2O3:Eu3+ Nanocores with Y2O3 Thin Shell ................................................................................................................... 105 7.3 Enhanced Photoluminescence from Eu Doped Gd2O3 Based Core/Dual Shell Nanoparticles ............................................................................................ 106 7.4 X RAY and PhotoLuminescence from Gd2SiO5:Ce3+ Nanoparticles Synthesized by Solution Based Methods ........................................................... 106 8 FUTURE WORK ................................................................................................... 108 L IST OF REFERENCES ............................................................................................. 109 BIOGRAPHICAL SKETCH .......................................................................................... 119

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8 LIST OF TABLES Table page 2 1 Crystal structures and morphologies of the as obtained rareearth oxides synthesized by thermolysis of Ln(BA)3(H2O)2 (Ln=La Y) or Ce(BA)4 in oleic acid (OA)/oleylamine (OM) at 250 330C for 20 60 min. .................................. 27 2 2 Applications of luminescent materials cl assified by different excitation sources. .............................................................................................................. 37 2 3 Scintillator requirements in various applications. ................................................ 44 3 1 Experimental parameters of clover, square and round Gd2O3:Eu3+ nanoparticles (H1H4). ....................................................................................... 57 3 2 The quantum yield and molar concentration of Eu ions for clover, square and round Gd2O3:Eu3+ nanoparticles (H1H4). .......................................................... 62 5 1 Quantum yield, radius, volume ratio and weight ratio of SiO2, SiO2/Gd2O3:Eu3+ core/singleshell and SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles with density of each material. ....................................................... 88

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9 LIST OF FIGURE S Figure page 2 1 Transmission electron microscopic photographs of Fe3O4 nanoparticles at different preparation temperatures at 303K and 323K ....................................... 20 2 2 Representative TEM micrographs of as synthesized anatase nanoparticles, obtained at 40 C. ............................................................................................... 21 2 3 Schulmans model of a reverse micelle colloidal particle, as published in 1943. .................................................................................................................. 22 2 4 Transmission electron micrograph of CTAB coated cerium oxide nanoparticles annealed at 200 C and 500 C for 2 h. ....................................... 22 2 5 HRTEM images of Gd2O3:Eu3+ nanoparticles synthesized from thermal decomposition of Gd(acac)3 precursor using either HDD or TOPO with a Gd(acac)3/surfactant. .......................................................................................... 23 2 6 length/diameter ratio. .......................................................................................... 24 2 7 Variation of the relative lattice parameter as the function of the diameters of Pd nanoparticles ................................................................................................ 25 2 8 HRTEM micrographs of CoFe2O4 nanocrystal s. ................................................. 25 2 9 CdSe nanorods with different sizes and aspect ratios in different concentrations of HPA /TOPO surfactants. ........................................................ 27 2 10 Homogeneous nucleation and the free energy change associated with homogeneous nucleation of a sphere of radius r. ............................................... 29 2 11 Lamer plot with illustration of nucleation and growth diagram. S=Sc: critical saturation of monomer concentration to induce nucleation. S=1: equilibrium monomer concentration below which growth stops. ........................................... 29 2 1 2 A schematic of the nucleation and growth process, in which silver continuously deposits onto the ( 100 ) facet to eventually result in a complete octahedron with (111) facet. ............................................................................... 30 2 13 Monomer concentrationdependent growth path of CdSe n anocrystal. .............. 31 2 14 Formation of rareearth oxide nanopolyhedra, nanoplates, and nanodisks. ....... 32

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10 2 1 5 The excitation curve of CaYBO4:Ce3+ monitored at 435 nm and t he emission curves of CaYBO4:Ce3+ excited at 199 nm (dotted line, marked at curve A) and 295 nm (thin solid line). ............................................................................... 33 2 1 6 Emission transitions in a semiconductor. The band gap Eg separates the valence band (VB) and the conduction band (CB). Excitation over the band gap creates electrons in CB and holes in VB. ..................................................... 35 2 17 Absorption (solid lines) and photoluminescence (dotted lines) spectra of CdS nanorod. ............................................................................................................ 36 2 18 Color diagram of the CIE (CIE=Commission Internationale d'Eclaira ge) and CIE with Color Temperature Line. ...................................................................... 37 2 19 Fundamental construction of the fluorescent lamp. ............................................ 38 2 20 Scheme of relaxation of electronic excitations general scheme, and rareearth containing scintillators .............................................................................. 42 2 2 1 Schematic diagram of a scintillation detector ..................................................... 43 2 22 Schematic diagram of positron emission tomography (PET) with the inset illustrating BGO detectors coupled with PMT and a PET scan image of a brain. .................................................................................................................. 45 2 23 Pulse height spectrum of a 137Cs source measured with a 100cm2 area and 2mm depth with LSO crystal coupled with a Hamamatsu R878 PMT. ................ 46 2 24 Schematic illustration of the density of states in metal and semiconductor clusters. .............................................................................................................. 47 2 25 Absorbance properties of II VI semiconductor nanoparticles ............................. 48 2 26 Schematic of organic and inorganic passivation. ................................................ 49 2 27 The absorption spectra of CdSe/ZnSe nanocrystals during the shell growth. .... 50 2 28 Plot of PL intensity of the 5D07F2 emission at 619 nm versus the YPO4/YVO4:Eu3+ molar ratio. .............................................................................. 51 3 1 Flow chart of nonhydrolytic hot solution synthesis procedure. .......................... 54 3 2 TEM images of Gd2O3:Eu3+ nanoparticles synthesized with different experimental parameters ................................................................................... 56 3 3 Modified LaMer plots with an additional axis of reaction temperature. S=Sc: Critical saturation of monomer concentration to induce nucleation. S=1: Equilibrium monomer concentration below which growth stops. ......................... 57

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11 3 4 XRD patterns of as synthesized samples .......................................................... 58 3 5 PL and PLE spectra of Gd2O3:Eu3+ nanoparticles. T he PL spectra of excitation at 250 nm and the PLE spectra for the emission at 609 nm. .............. 59 3 6 Resolved PL spectra of square nanoparticles with reaction time of 40 min (H2), round nanoparticles with reaction time for 90 min (H3) and for 180 min (H4). ................................................................................................................... 60 3 7 X ray luminescence spectrum of as prepared spherelike Gd2O3:Eu3+ nanocrystals ....................................................................................................... 63 4 1 XRD patterns of calcined samples: Gd2O3:Eu3+ nanocores; Gd2O3:Eu3+/Y2O3 core/shell nanoparticles with the shell grown with a Y to Gd+Eu precursor ratio R = 1:4 and 1:1 and pure Y2O3 nanoparticles ............................................ 68 4 2 Resolved XRD peak of Gd2O3:Eu3+/Y2O3 core/shell nanoparticles with shell grown with a Y to Gd+Eu precursor ratio R =1:1 ................................................ 69 4 3 TEM images of Gd2O3:Eu3+,Y2O3 and Gd2O3: Eu3+/Y2O3 core/shell nanoparticles: Gd2O3:Eu3+core, Gd2O3:Eu3+/Y2O3 (R=1:8), Gd2O3:Eu3+/Y2O3 (R=1:4), Gd2O3:Eu3+/Y2O3 (R=1:1), High resolution of Gd2O3:Eu3+/Y2O3 (R=1:1), and Y2O3. .............................................................................................. 70 4 4 XPS peaks from Gd2O3:Eu3+ and Gd2O3:Eu3+/Y2O3 core/shell nanoparticles grown with different precursor molar ratios (R) and normalized intensity of the Gd 3d5/2 peak versus Y2O3 shell thickness. ........................................................ 71 4 5 Idealized concentric spherical core/shell structure from which a maximum shell thickne ss for a given R can be calculated. ................................................. 72 4 6 PL and PLE spectra from Gd2O3:Eu3+ and Gd2O3:Eu3+/Y2O3 core/shell nanoparticles. The PLE spectra are for emission at 612 nm and the PL spectra are for excitation at 250 nm. The inset shows the uncorrected PL peak intensity versus the value of R. .................................................................. 73 4 7 Integrated area of PL peak at 612 nm versus R; Solid squares are uncorrected data. Solid diamonds are uncorrected normalized data. Open circles are normalized and corrected for the fraction of nonluminescent Y2O3 based on the value of R. ..................................................................................... 75 4 8 Luminescence decay data for the Eu3+ 5D0-7F2 transition (612 nm peak). The data are well fit by the solid line which is a single exponential function with a time constant ...................................................................................................... 77 5 1 TEM photomicrographs of dual shell ................................................................. 83

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12 5 2 X Ray diffraction spectra obtained from pure amorphous silica nanoparticles or calcined SiO2/Gd2O3:Eu3+ core/single shell nanoparticles and c alcined SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles. ......................................... 84 5 3 X ray photoelectron peaks from the SiO2 nanocores (solid line), from SiO2/Gd2O3:Eu3+ core/singleshell (dashed line) and SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles (dotted line). ......................................................... 85 5 4 The Si 2p XPS peaks from SiO2, SiO2/Gd2O3:Eu3+ core/single shell nanoparticles and normalized intensity of the Si 2p peak versus Gd2O3:Eu3+ shell thickness. ................................................................................................... 85 5 5 Photoluminescence (PL) and photoluminescence excitation (PLE) spectra from SiO2/Gd2O3:Eu3+ core/singleshell and SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles. The PLE spectra are for emission at 612 nm and the PL spectra are for excitation at 250 nm. .................................................................. 86 5 6 Model of a spherical core/concentric dual shell structure from which the weight of nanoparticles necessary for a constant volume of luminescent Gd2O3:Eu3+ can be determined. .......................................................................... 87 5 7 Schematic diagram of thin film quantum yield (QY) measurement. .................... 88 5 8 Luminescence decay data for the Eu3+ 5D0 7F2 transition (612 nm peak). The data fit very well a single exponential decay (solid line) with a time .......................................................................................................... 89 6 1 Flow chart of twopot hot solution growth of GSO nanoparticles ........................ 94 6 2 Thermogravimetric analysis (TGA) data for HSG Gd2SiO5:Ce3+ 0 .5% nanoparticles. Heating rate is 10 Cmin1. .......................................................... 96 6 3 XRD patterns from Gd2SiO5:Ce3+ nanoparticles compared to the JCDPS pattern for monoclinic Gd2SiO5 (JCPDS # : 40 0282) : GSO:Ce3+ n anoparticles after calcination at 600 C and 1000 C for 2hr in air. .................. 97 6 4 TEM images of Gd2SiO5:Ce3+ nanoparticles prepared by the sol gel (SG) method and the hot solution (HSG) method. ...................................................... 98 6 5 PL spectra and intensity for various Ce concentrations showing quenching for Gd2SiO5:Ce3+ nanoparticles synthesized by the hot solution method (excitation wavelength = 344 nm). ..................................................................... 99 6 6 PL and PLE spectra from Gd2SiO5:Ce3+ nanoparticles calcined at 600 C (dashed line) and 1000 C (solid line). .............................................................. 100 6 7 Configuration coordinate model energy levels for C e3+ ions in a Gd2SiO5 host. ......................................................................................................................... 101

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13 6 8 X ray luminescence spectrum of calcined Gd2SiO5:Ce3+ nanoparticles. .......... 102 6 9 Differential pulse height spectrum of calcined GSO:Ce3+ nanoparticles by an 241Am source showing a broad scintillation response centered at approximat ely channel 300. .............................................................................. 103 8 1 SiO2/Gd2O3:Eu3+/Y2O3/SiO2 core/multi shell nanoparticles ............................... 108

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14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ENHANCED LIGHT OUTPUT FROM LUMINSECENT OXIDE NANOPARTICLES: SYNTHESIS, CHARACTERIZATION AND SCINTILLATION APPLICATION By Jihun Choi August 2011 Chair: Paul. H. Holloway Cochair: Mark R. Davidson Major: Material s Science and Engineering Scintillator materials have been grown in the past as single crystal phase which generally lead to high cost and small size of radiation detector crystals. In this work, high ly efficient scintillator nanoparticles have been synthesized using nonhydrolytic hot solution growth, sol gel and aqueous precipitation methods. Gd2O3: Eu3+ n ano particles with sizes and shapes of 20 nm clover s, 10 nm squares and 15 nm rounds w ere prepared by a nonhydrolytic high temperature (320 C) solution growth method. Synthesis parameters such as reaction procedure, time and temperature were varied to investigate their effects on nanocrystal shape and luminescent properties T he variation of nanoparticle shapes was explained by nucleation and growth of oxide nanocrystals. The photoluminescence properties and quantum yields of Gd2O3:Eu3+ nanoparticles could be controlled by the synthe sis parameters. Round Gd2O3:Eu3+ nanoparticles exhibited the highest quantum yield of 67%, whereas clover and square Gd2O3:Eu3+ nanoparticles show ed lower quantum

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15 yield s of 24% and 48%, respectively. The luminescent properties were discussed in terms of doping concentration, host environment and dopant location Nanoparticles of Gd2O3:Eu3+ ~20 nm in diameter were synthesized at ~180 C using a facile high boiling point alcohol (polyol) method. The Gd2O3 nanoparticles, doped with 5 mol% Eu, were crystalline cubic phase and exhibited intense 5D0-7F2 photoluminescence (PL) from Eu3+ after calc ination at 600 Photoluminescence excitation (PLE) data showed that while a small fraction of the emission resulted from direct excitation of Eu3+, most of the excitation resulted from adsorption in the Oxygen to Europium chargetransfer b and between 225 and 275 nm. Transmission electron microscopy (TEM) images showed that the Gd2O3:Eu3+ core s were slightly agglomerated but the thin Y2O3 shell could not be detect ed by TEM X ray photoelectron spectroscopy (XPS) was used to detect the thin Y2O3 shell around the Gd2O3:Eu3+ core. Drop cast thin films of the Gd2O3:Eu3+/ Y2O3 core/shell nanoparticles exhibited PL intensities up to 40 % larger than from bare core Gd2O3:Eu3+ nanoparticles Increased PL w as attributed to reduced nonradiative recombination based on longer luminescence decay times. Spherical SiO2 cores have been coated with oxide dual shells of Gd2O3:Eu3+ and Y2O3 by a solution precipitation method. Based on transmission electron microscopy ( TEM) data, luminescent Gd2O3:Eu3+ shells with ~ 4. 5 nm thickness were successfully coated on monodispersed SiO2 nanocores ~ 210 15 nm in radius A continuous Y2O3 shell 45 nm thick was then grown on the SiO2/Gd2O3:Eu3+ nanoparticles. The SiO2/Gd2O3:Eu3+ core/shell nanoparticles exhibited 5D0-7F2 photoluminescence (PL) from Eu3+ transitions after calcination at 600C for 2h in air. Photoluminescence

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16 excitation (PLE) data showed that most of the excitation resulted from absorption in the oxygen to europium charge transfer band between 225 and 275 nm. Dropcast thin films of core/dual shell samples exhibited quantum yields (QYs) up to 2 times larger than that of core/singleshell nanoparticles, which was attributed to reduced nonradiative recombination, consistent with longer luminescence decay lifetimes. Nanophosphor Gd2SiO5:Ce3+ (GSO) with sizes between 2 nm 5 nm was synthesized by sol gel and hot solution methods. X Ray luminescence, photoluminescence (PL) and structural properties of GSO:Ce3+ nanoparti cles were compared for the two different synthesis methods. PL and X ray luminescence were both from the Ce3+ 5d4f transitions at 390~430 C The Ce dopant concentration was varied between 0.1% 10% and concentration quenching was observed at 0.5% under UV excitation. Hot solution synthesized GSO doped with calcination at either 600 or 1 0 00 3+ nanoparticles prepared by the sol gel method required calcination at 10 00

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17 CHAPTER 1 INTRODUCTION Nano is one of the most frequently cite d words in recent science and engineering fields. Micro and nano fabrication is a representative approach of top down application of the nano world, which has been led by the Si based electronic devices. T he typical example of bottom up technology in the nanoworld is a synthesis of nanoparticles, often used as nanocrystals and nanoclusters. Nanoparticles mean nanometer scale particles that are neither small molecules nor bulk solids. In the nanometer scale regime, the physical and chemical properties of materials become unique and novel compared to their bulk count erpart. Among various characteristics of nanoparticles, luminescent propert ies will be the main focus of my dissertation. The aim of the work described in this dissertation is to synthesize highly luminescent oxide nanoparticles for scintillat ion (i.e. lum inescence due to energetic particle radiation) To achieve this goal the following approach was use d 1. Synthesize cores and/or core/shell luminescent oxide nanoparticles using solution based synthesis methods. 2. Characterize the structure, morphology, surface, photoluminescence and X ray luminescence of the oxide nanoparticles 3. Evaluate the mechanism s to enhance the light output from luminescent oxide nanoparticles. In the dissertation, Chapter 2 reviews the fundamentals of nanoparticles, their various synthesis methods and applications in different fields. T he f undamentals of luminescent materials and several approaches to achieve enhanced luminescence are also reviewed. In C hapter 3, the preparat ion of shape controlled gadolinium oxide

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18 doped with europium nanoparticles with a non hydrolytic synthesis is discussed. T he mechanism of nucleation and growth of oxide nanoparticles were also reviewed. In C hapter 4, the synthesis and characterization of gadolinium oxide nanocores doped with europium/yttrium oxide core/shell hetero structures is discussed. Luminescent cores could be passivated by a nonluminescent shell with a facile polyol synthesis method so that enhanced phot oluminescence was achieved. I n C hapter 5, silica core/dual shell structures are created and characterized. Silica cores have several advantages such as uniformly spherical shape, ease of synthesis and low cost. Luminescent shell with europium doped gadolinium oxide was grown on silica nanocores. In addition, core/singleshell structures could be coated by yttrium oxide outer shell to passivate nonradiative defects by solution precipitation method. The passivation of surface defects was postulated based on a longer luminescent decay lifetime. Chapter 6 describes the synthesis and characterization of gadolinium silicate (GSO) nanoparticles doped with cerium ions. Single crystals of GSO are used in a scintillation detector. GSO nanoparticles were synthesized by both hot solution and sol gel method and their properties were compared. Finally, Conclusions and F uture W ork are summarized in Chapter 7 and 8.

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19 CHAPTER 2 LITERATURE REVIEW 2.1 Fundamental of Colloidal Nanoparticles The development of small size materials is of interests for chemical and material research. The electronic industry with semiconductor devices is a good example of the approach of novel materials, where the size of devices has s teadily decreased to the nanometer dimension for the reduction of power consumption and increased computing speed. The counterpart of a microfabrication is synthesis of nanoparticles. The synthesis and application of nanoparticles have been intensively st udied to understand fundamental phenomena and scientific properties. Nanoparticles exhibit novel properties and functions due to their small size, typically under 100 nm. 2.1.1 Synthesis Process for Colloidal Nanoparticles Nanoparticles can be synthesized using at least two different general methods, i.e. top down and bottom up approaches. A topdown method uses physical methods such as milling and microfabrication, while a bottom up method utilizes liquid phase colloidal chem istry. In our study, the bottom up methods will be used to synthesize nanoparticles due to their advant a ges in controlling size and shape. S everal bottom up synthesis methods will be introduced. Many of the earliest synthesis of nanoparticles were synthesized by the precipitati on method from aqueous solutions followed by thermal decomposition of precursors. For example, Cu, Ag, Pt, Ni, Co and Au nanoparticles were achieved by the precipitation method [1 5] The precipitation of metals from aqueous or nonaqueous solutions typically requires the reduction of metal cations. Oxide nanoparticles also could be synthesized by the precipitation method. The precipitation of oxide

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20 nanoparticles, from both aqueous and nonaqueous solutions can be broken into two categories, i.e. those that produce modified precursors which must undergo further processing such as calcinations, and those that produce oxide nanoparticles directly. Figure 21 shows Fe3O4 nanoparticles synthesized by a precipitation method at different reaction temperatures [6]. Ferric chloride ( FeCl3) and ferrous chloride (FeCl2) were used as iron precursors and ammonia was us ed as a precipitating agent. Without further heat treatment, Fe3O4 nanoparticles could be synthesized [6] Figure 2 1 Transmission electron microscopic photographs of Fe3O4 nanoparticles at different preparation temperatures at 303K and 323K Typically, sol gel synthesis refers to the hydrolysis and condensation of alkali oxide based precursors. In a typical sol gel synthesis for nanoparticles the following steps are frequently used [7] ( 1 ) Formation of stable solutions of precursors (the sol). ( 2 ) Gelation from the formation of oxide or OH linked network (the gel). ( 3 ) Drying of the gel, when volatile liquids are r emoved from the gel network. ( 4 ) Calcination, stabilizing the gel against rehydration.

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21 It is reported that oxide nanoparticles have been synthesized by a sol gel method. Titania nanoparticles were easily prepared by controlled hydrolysis and condensation of Ti(OiPr)( titanium isopropoxide) in an alcohol solution acidified by HCl after drying [8, 9] A modified sol gel method proposed by Neiderberger et al. produced nanocrystalline particles without a calcination. The p article size was controlled in the range of 48 nm by various temperatures and reactant concentrations, as shown in Figure 22 [10] Figure 2 2. Representative TEM micrographs of as synthesized anatase nanoparticles, obtained at 40 C. Hoar and Schulman reported that combinations of water, oil, surfactant, and an alcohol or aminebased co surfactant produced clear and homogeneous solutions which is called as microemulsions [11] A model of a microemulsion colloidal particle is shown in Figure 23, where the surfactant forms spherical aggregates through iondipole interactions with the polar cosurfactant. The cosurfactant acts as an

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22 electronegative s pacer that minimizes repulsions between the positively charged surfactant heads [12] Figure 23. Schulman s model of a reverse micelle colloidal particle as published in 1943 [12] Figure 2 4. Transmission electron micrograph of CTAB coated cerium oxide nanoparticles annealed at 200 C and 500 C for 2 h. Various oxide nanoparticles, such as Al2O3, TiO2 and CeO2, have been synthesized by this microemulsion method [13 16] Figure 24 shows CeO2 synthesized

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23 by a microemulsion method, in which cet yltrimethylammonium bromide (CTAB) was used as a surfactant. Most of synthesis methods with aqueous solutions have disadvantages of relatively poor crystallinity and/or polydispersity in their size and shape, in which the pH value of the mixture should be adjusted in both synthesis and washing steps [17, 18] Figure 2 5. HRTEM image s of Gd2O3:Eu3+ nanoparticles synthesized from thermal decomposition of Gd(acac)3 precursor using either HDD or TOPO with a Gd(acac)3/surfactant. On the contrary, nonaqueous high temperature thermal reaction methods with organic surfactants can reduce those problems. Nanoparticles synthesized by a nonaqueous colloidal route often exhibit crystallinity and monodispersity [19 23] Furthermore, it gives controllable shapes of nanoparticles through easier controls of growth parameters by changing variables such as the types of surfactants, precursor concent rations and reaction temperatures. It was reported that CdSe quantum dots were prepared by using dimethyl cadmium (Me2Cd) and trioctyl phosphine selenium (TOP Se) as precursors [24] S. Seo et. al. used nonaqueous high temperature thermal reaction to synthesize shapecontrolled Gd2O3:Eu3+ nanoparticles, as shown in Figure

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24 2 5 where the dimensions of nano plates was an edge length and a thickness of 5 nm and 1 nm, respectively [25] 2.1.2 Shape Control of Colloidal Nanoparticles 2.1.2.1 Shapes of n anoparticles Figu re 2 6. Change in bandgap energy, versus d (thickness or diameter) or length/diameter ratio [26] It is well known that quantum effects become increasingly important as the size of structures is reduced b ut the influence o f shape on quantum confinement has been less studied. Buhro and Colvin proposed that shape matters as much as size for nanoparticles [27] They cited experimental work by Kan et al. which show ed that the electronic structure and optical properties of rodlike semiconductor nanocrystals depend ed sensitively on the ratio of their length and diameter, as illustrated in Figure 2 6 [26] W.H. Qi and M.P. Wang [28] mod eled the effects of shape and size on the lattice parameters of a metallic nanoparticle formed from an ideal bulk crystal. In order to predict shape effects on lattice parameters, they introduced a shape factor. Their data show that the lattice parameter s trongly depends on nanoparticle size and shape, as illustrated by Figure 2 7

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25 Figure 2 7. Variation of the relative lattice parameter as the function of the diameters of Pd nanoparticles [28] Figure 2 8. HRTEM micrographs of CoFe2O4 nanocrystal s. Nanoparticles of spheres, cubes and polyhedrons can be classified as zerodimensional (0 D) structures. Song and Zhang synthesized shapecontrolled 0D nano-

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26 spheres and cubes of CoFe2O4 [29] By varying the temperature and heating rate, the shape of CoFe2O4 nanoparticles could be controlled, as shown in Figure 28. Other shapes of nanoparticles have been reported such as nanorods, nanocylinders and nanodisks. Rods, cylinders and wires are onedimensional (1D) nanoparticles. Many studies on the synthesis of 1D nanoparticles have been reported [30 32] 1 D nanoparticles exhibit novel optical and magnetic properties due to their anisotropic shapes. N ano rods based on semiconduct or mater ials have been investigated for the application as energy harvesting and light emitting devices. In 2006, Ramanathan et. al. demonstrated electric field mediated tunable photoluminescence from ZnO nanorods, with potential application s as novel sources of n ear ultraviolet radiation [33] Non hydrolytic synthesis can be used to produce high quality nanorods. With increasing hexylphosphonic acid (HPA), CdSe nanorods with the different aspect ratios ( the ratio of the diameter/length of the nanorod ) could be synthesized, where 1D rod shaped nanoparticles results from the preferred growth along the [001] direction of wurtzite CdSe (Figure 29) [34] Discs and plates of polygons are classified as two dimensional (2D) nanoparticles. When a specific axis is inhibited during the synthesis, 2D nanoparticles can be formed. Various 2D structured nanoparticles were prepared by R Si et al, in which a family of rare earth oxide nanoparticles were synthesized by thermolysis [ 35] Due to the selective adsorption of the capping ligands on certain cubic facet s during growth, nanoparticles with different morphologies, such as nanopolyhedra, nanoplates, and

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27 nanodisks were created, which exhibit an ability to self assemble into l arge area nanoarrays as summarized in Table 21. Figure 2 9. CdSe nanorods with different sizes and aspect ratios in different concentrations of HPA /TOPO surfactants. Table 21. Crystal structures and morphologies of the as obtained rareearth oxides synthesized by thermolysis of Ln(BA)3(H2O)2 (Ln=La Y) or Ce(BA)4 in oleic acid (OA)/oleylamine (OM) at 250 330C for 20 60 min. 2.1.2.2 Growth m echanism of s hape controlled n anopart icles The precipitation of nanoparticles has often been discussed in numerous books and review articles [36 38] N ucleation is a key step in the precipitation process which

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28 can be explained by thermodynamics Assuming that some of atoms of the liquid cluster together to form a small sphere of solid, the free energy of the systems can be express by [39] : G= ( V+ V) G (2 1) G= VG + VG + A (2 2) W here G1 and G2 are free energy of two systems, VS is the volume of the so l id sphere, VL the volume of liquid, ASL is the solid/liquid interfacial area, GV S and GV L are the free energies per unit volume of solid and liquid respectively, and SL the solid/liquid interfacial free energy. The change of free energy ( GV) from G1 to G2,with a sphere of radius r, is given by Equation 23 as G= r G+ 4 r ( 2 3) It can be seen from Figure 2 1 0 that for a given undercooling c ooling a material below the transformation temperature without obtaining the transformation, there is a certain radius, r*, which is associated with a maximum excess free energy. When r > r*, the free energy of the system decreases if the solid either grows or shrinks The critical radius of the solid nuclei can be expressed by: r= = (2 4) where H is the enthalpy of fusion and T is the degree of undercooling. The initial stage of the growth of nuclei can be explained by heterogeneous nucleation limited by either diffusion or reaction rates For diffusionlimited growth concentration gradient s and temperature control the growth rate. Ostwald ripening is a secondar y growth process in which smaller particles are consumed by larger particles [40, 41] The

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29 size, morphology and properties of nanoparticles are dramatically affected by the Ostwald ripening. Figure 210. Homogeneous nucleation and t he free energy change associated with homogeneous nucleation of a sphere of radius r. Figure 2 1 1. Lamer plot with illustration of nucleation and growth diagram. S=Sc: critical saturation of monomer concentration to induce nucleation. S=1: equilibrium monomer concentrat ion below which growth stops A Lamer plot is used to describe the formation of clusters and colloids in homogeneous and supersaturated solutions [42, 43] As illustrated in Figure 211, in the

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30 first stage, the concentration of monomer (precursor) is increased with reaction time. When the monomer (precursor) reaches a sufficiently high concentration, nucleation can occur (stage 2). The growth of nuclei causes the monomer (precursor) concentration to fall, stopping nucleation. Until the monomer (precursor) concentration falls to the S=1 level the nanoparticles continue to grow (stage 3) [43] The most frequently reported model for the mechanism of shape control is the Wulff facets argument or Gibbs Curie Wulff theorem, which states that the shape of nanoparticles is determined by the specific surface energy of each face of the nanocrystal [44] However, many other reports revealed that the theories based on thermodynamics c annot explain most of the cases. Figure 2 1 2 A schematic of the nucleation and growth process, in which silver continuously deposits onto the ( 100 ) facet to eventually result in a complete octahedron with (111) facet. Tao et al reported on shapecontrolle d Ag nanocrystals synthesi zed using the polyol method and controlling reaction kinetics [45] Fast nucleation and fast growth produced nanowires whereas fast nucleation and slow growth formed polyhedral nanocrystals. Shape evolution of Ag nanocrystals was also investigated by extending

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31 the polyol reaction for a given time period. As shown in Figure 212, the cubic Ag has evolved to octahedrons to form completely (111) bound symmetry which is a low surface energy facet [45] Figure 2 1 3 Monomer concentrationdependent growth path of CdSe nanocrystal T he anisotropic growth of CdSe nanoparticles was studied by X. Peng [46] by modulating the reaction kinetics var i ed with the monomer (precursor) concentrations T he concentration of monomers (precursors) after the nucleation process is a critical factor in the control of the nanoparticles shape. For a high monomer concentration, the solution could supply sufficient monomers to the nucle i to grow arms on the (111) faces of the zinc blende structure of the tetrahedral nuclei, which results in tetrapo d s. A moderately high monomer (precursor) concentration results in the growth of a single arm to produce the onedimensional nanorods or nanowires Medi um and low monomer (precursor) concentrations produce oval and spherical nanoparticles, respectively due to the low chemical potential s. Figure 213 illustrates the correlation between shape and monomer (precursor) concentrations for CdSe nanoparticles.

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32 Surfactants also play a huge role in shape control of nanoparticles. Rui Si et. al reported the effect of surfactants on the s hape of CeO2 and Ln2O3 ( Ln=La Y) nanoparticles [35] T heir experiments showed that CeO2 nuclei grew in a threedimensional mode to generate nanopolyhed ral shapes enclosed by both (111) and (200) facets with oleylamine, since oleylamine is characterized by its nonselecti ve adsorption on faces of nanocrystals. On the contrary, the growth with oleic acid is anisotropic. The denser faces such as (111) plane of FCC structures are selectively capped by oleic acid, which means that the nuclei grow in a twodimensional mode to p roduce nanoplates with confined (001) planes or nanodisks with restricted (111) planes, as shown in Figure 214. Figure 2 1 4 Formation of rare earth oxide nanopolyhedra, nanoplates, and nanodisks. 2. 2 Inorganic L uminescent M aterials 2.2 .1 Fundamentals of Luminescent M aterials The definition of luminescent materials is a solid which converts certain types of energy into electromagnetic radiation. Luminescent materials can be excited by

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33 electromagnetic waves, electron beams, voltage, X rays rays, particles neutrons, and so on. The important physical processes which play a role in lumines cent material can be described by [47] : 1. Absorption of energy (excitation) which may occur in the luminescent center (activator) itself, in another ion (the sensitizer), or in the host lattice. 2. Energy transfer to luminescent centers. 3. Emission from the activator. 4. Non radiative return to the ground state, which decrease the luminescent efficiency. Figure 2 1 5 The excitation curve of CaYBO4:Ce3+ monitored at 435 nm and t he emission curves of CaYBO4:Ce3+ excited at 199 nm (dotted line, marked at curve A) and 295 nm (thin solid line). A luminescent material only emits light after absorption of the excitation energy. The absorption will occur at activators, sensitizers and by the host lattice, as noted above. The rareearth ions are commonly used as activators in luminescent materials.

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34 They are characterized by an incompletely filled 4f shell. For exampl e, Eu3+, Yb3+, Er3+ and Nb3+ ions show chargetransfer absorption bands and Ce3+, Pr3+, Tb3+ show 4f 5d absorption bands i n the ultraviolet [47] A typical photoluminescence excitation (PLE) spectrum from Ce3+ doped CaYBO4 is shown in Figure 215 [48] T he absorption of energy does not necessarily take place at the activator, but may also occur in the host lattice. There are two different classes of optical absorption transitions. O ne results in free electrons and holes and another does not make free charge carriers [47] ZnS is an example of the former class. I t is a compound semiconductor. Optical absorption occurs for energies larger than the width of the band gap (Eg) and it creates electrons in the conduction band and holes in the valence band. However, not every host lattice makes free charge carriers by optical excitation. For example, in CaWO4, energy is absorbed in the WO4 2 complex In the excited state of the tungsten group, the hole and the electron form an exciton. Once luminescent materials absorb enough energy to make excited carriers or excitons (electronhole pair) light is emitted by relaxation of excited carriers to lower energy state, and ultimately to the ground state. In terms of the emission from activators, rare earth ions could be used for a typical example. The emission of Eu3+ ions co nsists usually of peaks with a small FWHM because they show parallel parabolas ( R = 0) in a configurational coordinate diagram [47] The line emission s corresponds to transitions from the excited 5D0 to the 7FJ (J = 0, 1, 2, 3, 4, 5, 6) levels of the 4f6 configuration [49] The transition 5D0 7F2,4 of Eu3+ is a forbidden electric dipole transition (parity selection rule), whereas the 5D0 7F1,3 of Eu3+ is an allowed magnetic dipole transition [47]

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35 However, this selection rule can be relaxed for Eu3+ in a host lattice lacking inversion symmetry, such as Gd2O3, Y2O3 and LaPO4 [50 52] Figure 2 1 6 Emission transitions in a semiconductor. The band gap Eg separates the valence band (VB) and the conduction band (CB). Excitation over the band gap creates electrons in CB and holes in VB. In semiconductors, there are several paths to explain a radiative process. As illustrated in Figure 2 1 6 emission from a semiconductor can be achieved by [47] : ( 1 ) recombination of free electrons and holes ( 2 ) a free hole recombines with an electron trapped in a shallow trap level ( 3 ) the same with a deep electrontrapping level (4 ) a free electron recombines with a trapped hole (5 ) doner acceptor pair emission (v i ) electronhole recombination at a donor accept or complex ( e.g. at coupled defects such as vacancy and substitution of atoms) Emission due to recombination of free electrons and holes is exceptional. U sually recombination occurs close to or at defects in the crystal lattice so that the emitted

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36 wavelength should be increased (lower energy). T his energy shift is called the S tokes shift and is sh own for a CdS nanorods in Figure 2 1 7 [53] Figure 2 1 7 Absorption (solid lines) and photoluminescence (dotted lines) spectra of CdS nanorod. 2.2.2 Application of Luminescent M aterials The application of luminescent materials can be classified by the excitation sources for the luminescent materials as shown in Table 22 [54] Luminescent materials can be found in a broad range of everyday applications such as cathode ray tubes (CRTs), projection televisions, fluorescent lamps, X ray detectors, solid state lighting, sensors, and displays. Generally research and development on luminescent materials has resulted in synthesis and testing of thousands of phosphors [54] However, only about 50 materials exhibit properties that are suitable for appropriate technological applications in terms of efficiency, emission color, decay time, physical stability, availability of raw materials, environmental aspects, cost, reproducibility and ease of materials preparation [55 57]

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37 Table 22. Applications of luminescent materials classified by different excitation sources. Figure 218. Color diagram of the CIE (CIE=Commission Internationale d'Eclairage) and CIE with Color Temperature Line. Recently, an important application of luminescent material is to create white light emitting diodes (LEDs). W hite light can be generated by combining the output from blue, green and redemitting diodes to yield white light. There are three different techni cal approaches to realiz e inorganic white LEDs [58] The first successful device combined a blue LED covered with a classical yellow phosphor, such as yttrium aluminum garnet Y3Al5O12 doped with Ce3+ (YAG:Ce3+) [59] These LEDs have been wide ly used as simple long life white light sources in traffic lights, cycle lamps, car headlights, outdoor

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38 lighting or flashlights. The white light looks cold due to its high color temperature (CCT=>5000K) More specific applications like indoor lighting require warm white light and an excellent color rendering. The white light should contain red wavelength regions and thus exhibit a lower color temperature. These targets are achieved by a combination of a blue (Ga,In)N LED with two phosphors emitting red and green light [60] In the third approach suitable phosphors for (Ga,Al)N UV LEDs are dev eloped. These are covered with three different phosphors, which emit red, green, and blue. The advantage of these white LEDs is that they enable variation of three broadband emitters, giving access to a larger color area in the CIE diagram (Figure 2 1 8 ) a nd better color rendering [61] Figure 219. Fundamental construction of the fluorescent lamp A fluorescent lamp is a classic application of luminescent materials. T he fluorescent lamp or fluorescent tube is a gas discharge lamp that uses electricity to excite mer cury vapors. The main emission from a mercury discharge can be tuned from 185 to 254 to 365 nm when the mercury gas pressure is increased. In fluorescent lamps, a combination of phosphors emitting at the appropriate color s are coated on the inside of the d ischa rge tube, as shown in Figure 219 Selection of the appropriate luminescent

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39 materials enables special application of fluorescent lamps such as security marking, sun tanning, photocopying, display backlight and advertising billboards [62] 2 .3 Scintillation M aterials An important application of luminescent nanoparticles is a scintillator. Scintillators are substances that absorb high energy electromagnetic or charged particle radiation then, in response, emit photons at a desirable wavelength, releasing the previously absorbed energy. Scintillators are used in several physics research and military applications to detect electromagnetic waves or particles. S cintillators are usually made of bulk single crystalline materials, which are very expensive and hard to make. Key factors for good scintillator materials are [63] High atomic number and density (Improve photoabsorption) Short decay time (Improve ti me resolution) High efficiency of emitting UV or visible light (Large light output) Low afterglow Low cost Bulk is very expensive Scintillator materials are characterized by high stopping power, which makes them well suited for detecting highenergy r adiation so that high atomic number materials have to be used. S horter decay time result s in higher count rates and time resolution. Afterglow is the phenomenon that luminescence can still be observed a long time after the end of the excitation pulse. A long time here is defined as a time much longer ( 5X) than the decay time of the luminescence. So, for the improvement of detection, low afterglow is a necessary behavior of good scintillator materials.

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40 2. 3.1 Fundamentals of S cintillation M aterials The mechanisms of excitation of the luminescent centers (activators) in a scintillator are st r ongly influenced by the surrounding medium. The coupling between the lattice and activators is essential in the way the energy is transferred between them. Electrons and holes created by absorption of high energy radiation have several possible pathways for the scintillation process [64] : ( 1 ) e lectron + h ole ( 2 ) e lectron + h ole ( 3 ) e lectron + h ole + A ( 4 ) e lectron + h ole + A 1+ + e ( 5 ) e lectron + h ole + A 1 -)* + h ole h (6 ) A + h where A mean s activat or ions and A* represent s their excited states. The simplest scintillation process ( 1 ) is the result of the direct radiative recombination of free electrons in the conduction band with holes in the valence band. In most cases, the recombination takes place when the energy of electrons and holes decreases because they form bonding states called excitons (ex) with energy smaller than the bandgap. T he excited carriers can also be bound in the lattice, for example, in the vicinity of a specific structural defec t ( 2 ). They are called autolocalized excitons and their radius depends on the electr ostatic field in the configuration. L uminescence f rom free or bound excitons has been observed so far only in simple oxides [65] Sometimes, in the presence of impurity centers or activat or ions ( A ) the exciton luminescence is efficiently quenched, causing a sensitization of the luminescence of the

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41 activating ions A. In this case, the excitation of radiative centers results from an energy transfer from excited matrix states The process competing to the formation of excitons is the direct capture of free thermalized carriers, electrons ( 4 ) or holes ( 5 ) by activating ions (A) with t he subsequent formation of their excited state (A*). The cross section for electron or hole captu re depends on the nature of the activat or ion and on the structure of the local electrostatic field in its vicinity. Finally, the direct excitation of activated centers by ionizing radiation ( 6 ) provides a crucial contribution to the scintillation in the case of self activated or heavy doped scintillators. CeF3 shows a typical example of the direct excitation. After excitation from high energy particles or high energy electromagnetic waves, relaxation of electronic excitations involves complex mechanisms. A description of multiplication and thermalization processes has been proposed by different authors using various models [66 68] Vasil ev proposed a schematic model that use s simple schemes of relaxation of electronic excitations deduced from simulation and which account qualitatively for the energy distribution and space correlation of excitations. Figure 220 shows t wo different schemes of relaxation, one is the general case and another is that of rareearth containing scintillators. As shown in Figure 220 (a), the first stage starts with the production of primary excitations by interaction of ionizing particles with the material. For extremely high energy particles, the excitations are essentially deep holes and ho t electrons which are created in inner core bands and in the conduction band, respectively.

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42 Figure 2 20. Scheme of relaxation of electronic excitations general scheme, and rareearth containing scintillators

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43 I n a very short time (~1015 s), a large number of secondary electronic excitations are produced through inelastic electronelectron scattering and Auger processes with creation of electrons in the conduction band and holes in shallow core and valence bands. At the end of this stage, the multiplication of excitations stops T he second state is thermalization of electronic excitations with production of phonons, leading to low kinetic energy electrons in the bottom of the conduction band and of holes in the top of the valence band. The third stage is characterized by the localization of the excitations through their interaction with stable defects and impurities of the material The two last steps are related with migration of relaxed excitations and radiative and/or nonradiative recom bination. Rareearth doped scintillators exhibit a more complicated mechanism Since the doping level is located between the forbidden bandgap, they have additional pathway of excitation and would be expected to have a high light yield (Figure 220 (b)). Figure 22 1 Schematic diagram of a scintillation detector Figure 221 helps the reader to understand the process of a scintillation detector. Briefly summarizing, as soon as an incoming radiation is absorbed by a scintillation

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44 material, the ray will excit e the scintillation material then detectable light will be emitted and detected by photomultipliers 2. 3.2 Application of S cintillation M aterials Van Eijk et al. have summarized the most important requirements for a number of applications of scintillating materials as presented in Table 2.3 including the following characteristics: relative light yield LR, decay time density, atomic number Z, emission spe r), ruggedness (Rug.) and radiation hardness (Rad H) [69] Table 23. Scintillator requirements in various applications. Calorimeters are typical examples of high energy physics (HEP) appli cations of scintillator materials. The purpose of calorimeters is the measurement of energy of particles. For HEP, an ideal calorimeter would require a fast, dense and radiatively hard scintillator with a reasonably low light output. In terms of high densi ty to reduce the total volume, low melting point and easy and cheap production method, leadbased compounds, particularly PbWO4, can be promising scintillators for HEP [70]

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45 Positron emission tomography (PET) is widely used in medical radiology. S cintillators for PET applications should meet the following requirements: (1) high density, (2) light output > 5~10 % of that of NaI:Tl and (3) decay time <3~5 ns. Figure 222. Schematic diagram of positron emission tomography (PET) with the inset illustr ating BGO detectors coupled with PMT and a PET scan image of a brain. A Bi4Ge3O12 (BGO) crystal has properties that best meet these requirements. In general, PET system s consist of many rings with a number of scintillation detectors, as illustrated in Figure 2 2 2 (a) so that a 3D image of processes in the body can be reconstructed, as shown in Figure 2 2 2 (b) [71 73] Inorganic scintillators are often used in gamma spec troscopy. Figure 223 shows a typical pulse height spectrum of 662 keV gamma rays originated from a 137Cs source by using the Lu2SiO5:Ce (LSO) scintillator crystal [74] The dominant peak at the right side is the full energy peak containing events in which the total 662 keV energy was absorbed in the crystal. In the low energy regime, the Ba K peak of X ray emission at 32 keV is observed as a result of 173Cs decay.

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46 Figure 22 3 Pulse height spectrum of a 137Cs source measured with a 100cm2 area and 2mm depth with LSO crystal coupled with a Hamamatsu R878 PMT 2. 4 Luminescent N anoparticles Low dimensional systems reviewed in this section are zerodimensional (0D) systems, one dimensional systems (1D) and twodimensional systems (2D). 2. 4 .1 Fundamental and A pplication of L uminescent N anoparticles O ne of the most remarkable characteristics of luminescent nanoparticles with low dimensions is their p hysical and optical modification through different distributions of energy levels and densities of states. The origin of different quantum states is the spatial confinement of electrons and holes inside the nanoparticles. The density of stated of each low dimensional nanostructure can be describe by [75] : ( E ) = ( E E E) (2 5) ( E ) =( ) / ( ) / (2 6)

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47 ( E ) = 2 ( E E E E E) (2 7) delta function and the Esub refer to the three directions of spatial confinement. Figure 2 2 4 describes the density of states for three low dimensional systems using a particlein a box model [7 5, 76] Figure 2 2 4 Schematic illustration of the density of states in metal and semiconductor clusters. Luminescent nanoparticles can be divided by two different categories, undoped and doped nanoparticles. Semiconduct or nanoparticles are representative for undoped nanoparticles. II VI (CdSe, CdTe, CdS and ZnSe) and III V (InP, InAs) nanoparticles have been synthesized and studied due to their tunable emission in the visible range [77 81] Among these semiconductor nanoparticles CdSe nanocrystals are the most widely studied due to their tunable emission wavelength in the visible range. In 1993, Bawaendi et al developed a unique synthesis of CdS, CdSe and CdTe nanoparticles using the high temperature organometallic procedure (TOPO technique), which result ed in high quality nanoparticles with a narrow size distribution and high quantum yield. F igure 225 (a) shows the absorbance spectra of 20~30 diameter CdS, CdSe and CdTe nanocrystals. All the samples show the effect of quantum confinement, where the

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48 absor bance edges are shifted to shorter wavelength than those of bulk band gaps with 512, 716 and 827 nm, respectively. T he evolution of absorbance spectra can be shown in Figure 225 (b) with size tuned CdSe nanoparticles. Figure 22 5 Absorbance properties of IIVI semiconductor nanoparticles Luminescence caused by intentionally incorporated impurities (activators) is classified as extrinsic luminescence in contrast to intrinsic (band edge) luminescence. When a dopant with quantum states remote from the val ence and conduction band edges are added to the semiconductor host, another radiative mechanism is involved. This mechanism results in localized luminescence, not band edge recombinations, since the luminescence excitation and emission processes are confined in a localized luminescence center. These localized transitions are classified as allowed (intershell) or forbidden (intrashell) based on the parity selection rule [82] Some of examples for allowed transitions include s p and f d transition, while forbidden transitions include d d and f f transitions. ZnS nanoparticles doped with Mn, Tb and Eu have

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49 been reported for the reduction of luminescent decay time [83, 84] Rare earth ions doped oxide nanoparticles also have been widely studied. For example, Eu3+ or Nd3+ doped Gd2O3 nanoparticles have been used for cathodoluminescence and laser applications [85, 86] 2. 4 2 Surface Modification of Luminescent N anoparticles Figure 2 26. Schematic of organic and inorganic passivation. The electronic and optical properties of all inorganic crystals depend on the three dimensional periodicity of the potential wells that exist in the materials T he discontinuity of periodicity at the surface layer will cause changes to these properties. T he lack of atoms on one side of the crystal leads to dangling chemical bonds that generate structural modification such as reconstruction and relaxation [87] To achieve stable photoluminescence with a higher efficiency, passivation of the surface is crucial and it has been successfully obtained by capping layers on the nanocores, as shown in Figure 2 26 [88]

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50 Coating nanoparticles with higher band gap inorganic materials has been shown to enhance photoluminescent quantum yields by passivating surface nonradiative recombination paths [19, 89 91] In addition, these core/shell heterostructures are more robust than those passivated organic ally. Moreover, wider band gap of the surface materials results in the confinement of charge carriers inside the core material. Figure 2 27. The absorption spectra of CdSe/ZnSe nanocrystals during the shell growth. Some important examples of the core/shell structured nanoparticles include CdSe nanoparticles coated by ZnSe, ZnS or CdS [19, 89, 90, 92, 93] P. Reiss, et al reported that highly luminescent CdSe/ZnSe core/shell nanoparticles were synthesized [94] As the shell thickness is getting thicker, the absorption spectrum of CdSe/ZnSe has shifted to the lower energy, which is the result of partial leakage of the excitons into the shell (Figure 2 27(a)). T he evolution of the luminescence quantum yield of CdSe/ZnSe core/shell nanoparticles was described in Figure 227(b). The luminescent quantum yield of CdSe/ZnSe first rose because of the passivation of the surface defects of

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51 nanocores, then decreased as a result of larger density of structural defects created in the thick shells. While the surface modification of semiconductor nanocrystals plays a significant role, enhanced photoluminescence has been reported from rareeart h ion dope oxide nanoparticles. For example, some heterostructures based on YVO4:Eu3+ phosphor such as YVO4:Eu3+/YBO3:Eu3+, Y2O3:Eu3+/ SiO2/ YVO4:Eu3+, SiO2/ YVO4:Eu3+, Y(OH)3Eu3+/YVO4Eu3+ composite, and YV(0.7)P(0.3)O4:Eu3+,Bi3+/ SiO2 have been proposed [ 95 99] Figure 228 shows the enhancement of photoluminescence versus shell thickness for YVO4:Eu3+/Y P O4 heterostructures [100] Figure 228. Plot of PL intensity of the 5D07F2 emission at 619 nm versus the YPO4/YVO4:Eu3+ molar ratio

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52 CHAPTER 3 SHAPE CONTROLLED GADOLINIUM OXIDE DOP ED WITH EUROPIUM COLLOIDAL NANOCRYST AL S GROWN BY HOT SOLUTION METHOD 3. 1 Introduction Rare earth doped luminescent nanocrystals are being intensively studied due to their potential applications in displays, lightings and biologic diagnostics [101, 102] It is well established that the optical, electrical and chemical properties of nanoparticles depend strongly on their shape, morphology and crystal structure [103 105] Zero and o nedimensional (1D) nanostructures, such as quantum dots, nanorods, nanowires and nanotubes have been widely studied [106, 107] Specific organic surfactants, which can modulate the growth kinetics, have been employed to grow anisotropic luminescent nanostructures using nonhydrolytic procedures with and without templates [108110] For example, rare earth doped zero and one dimensional ( 0D and 1D) structures such as LaPO4: RE ( RE= Eu3+, Tb3+) nanowires, Y2O3:RE nanotubes and Gd2O3: Eu3+ nanoplates have been reported [25, 111114] Colloidal growth in nonhydrolytic liquid media can give reproducible shapes and sizes of nanoparticles This synthesis route results i n nanonarticles (NPs) with sizes tuned over the 1 10 nm range. These NPs can be dispersed in organic media for numerous potential applications [115, 116] Rare earth doped gadolinium oxide (Gd2O3) generally exhibit photoluminescen ce (PL) with high quantum yields For example, Eu3+ doped gadolinium oxide exhibits red luminescence due to the electric dipole 5D07F2 transitions on the trivalent europium ion (Eu3+, 4f6) [47] Gd2O3:Eu3+ nanocrystals produced by nonhydrolytic thermal reactions in the presence of organic surfactants exhibit intense luminescence, and is crystallin e and mo nodispers ed. Furthermore, this synthetic route allows control of the shape of

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53 nanoparticles through the use of processing parameters such as precursor types precursor concentration, surfactant solvent and reaction temperature and time [16] In this chap ter the luminescent properties of Gd2O3:Eu3+ will be reported and discussed with respect to the crystal structure, shapes of nanoparticles and the Eu3+ dopant location in the Gd2O3 host. 3.2 Experimental 3. 2.1 Materials The following precursor compounds and solvents were purchased from Aldrich Chemical Co. : Gd(III) acetylacetonate hydrate, Eu(III) acetate hydrate, oleic acid (90% tech.), Eu (III) acetate hydrate (90% tech.), oleic acid (90% tech.), oleyamine (70%) benzyl ether (99%) and 1,2 hexadecanediol (HDD, 97%) All chemicals were used without further purification. Absolute ethanol, benzyl ether and hexane were analytical grade and used as received. 3. 2.2 Gd2O3:Eu3+ N anocrystal S ynthesis In a one pot hot solution synthesis method for the growth of Gd2O3:Eu3+ nanocrystals, Gdac e tylacetonate (3 mmol) and Euacetate (0.6 mmol) were mixed and dehydrated at 120 C for 6 hours, followed by addition of oleic acid (9 mmol), oleylamine (9 mmol), benzyl ether (15 mmol) and hexadecanediol (3mm ol ) The precursor solution was transfe r r ed and vigorously stirred under UHP N2 gas ambient in a threeneck flask surrounded by a heating mantle. The mixture was held at 120 C for 30 min, resulting in a transparent yellowish solution. In some cases, samples were preheat ed to 200 C for 30 min to change the shape of the nanocrystals. After preheating, the temperature was increased to 290~320 C with a heating rate of 5~25 C/min, and the steady state condition was maintained for 30 to 1 8 0 min. The color of the mixture changed to deep

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54 brown after 40 min. Aliquots of the solution were extracted at selected intervals to determine the time dependence of growth. After the desired growth times, samples were cooled and dispersed in ethanol Dispersed NPs were washed and centrifuged several (at least three) time in order to purify the nanoparticles and remove residual surfactants and precursors. Washed Gd2O3:Eu3+ nanocrystals were easily redispersed in nonpolar solvents such as hexane and/or toluene. Figure 3 1. Flow chart of nonhydrolytic hot solution synthesis procedure. 3. 2.3 Characterization The crystal structure of as grown and calcined nanoparticles were characterized by X ray diffraction (XRD radiation ). The XRD patterns were collected from dried powder samples with a 0.02 determined with a JEOL 2010F high resolution transmission electron microscope (HR TEM) o perated at an accelerating voltage of 200 kV. The TEM samples were prepared

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55 by dropcasting NPs dispersed in hexane onto a carbon coated holey copper grids, followed by drying at room temperature. Photoluminescence (PL) and photoluminescence excitation (P LE) spectra w ere measured at room temperature using a JASCO FP6500/6600 research grade fluorescence spectrometer with a 150 W Xenon lamp. The quantum yields were determined by comparing the integrated emission from colloidal NP solutions with those from a n ethanol Rhodamine 6G solution growth the same optical density and excited at the same wavelength of 280 nm. The concentrations of Eu3+ dopant in as prepared Gd2O3:Eu3+ nanoparticles w ere measured using an inductively coupled plasma (ICP) spectrometer (Perkin Elmer Plasma 3200). X ray luminescence was measured with a 40 kV BulletTM X ray tube combined with a Ocean Optics USB 2000 miniature fiber optic spectrometer. The distance from the x ray target to the sample was ~3 cm and the x ray tube was operated at 100 A. For each measurement, a crucible with 57 mm2 area and 2 mm thickness was filled with nanopowder such that each sample had the same area exposed to the x rays. 3. 3 Results and Discussion 3. 3.1 S hape C ontrol of Gd2O3:Eu3 + N anocrystals T hree distinct shapes, clover plate and round of nanoparticles were observed for the present precursors and one pot hot solution method Typical NPs are shown in the high resolution TEM images of Fig ure 32 The preheat and r eaction times and temperatures, and observed shapes are summarized in Table 1. Fig ure 32 ( a ) shows ~20 nm clover Gd2O3:Eu3+ nanoparticles with e ach nanoparticle being composed of several petals that self assembles into the clover NPs which were grown with a preheating step. S quare and round nanoparticles with characteristic dimensions of 1015 nm are shown in Fig ure 32 ( b d )

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56 Fig ure 32 TEM images of Gd2O3:Eu3+ nanoparticles synthesized with different experimental parameters The shape of the nanoparticles was a function of the pre heat condition, synthesis temperature and reaction time, as shown in T able 1 Clover nanoparticles (sample H1) were produced using preheating at 200 C for 30 min, followed by a 90 minute synthesis at 290 C as mentioned before. For synthesis at 320 C without the preheat step, platelet (H2) and round (H3 and H4) nanoparticles were produced. The particle shape can be explained by a LaMer plot which depicts the nanocrystal nucleation and growth versus time [42] A proposed LaMer plot for Gd2O3:Eu3+ nucle ation and growth by the hot solution method is shown in F igure 3 3 and precursor concentration (solid black line) and temperature ( solid blue line) are shown versus time.

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57 Table 3 1. Experimental parameters of clover square and round Gd2O3:Eu3+ nanoparticles (H1H4). Fig ure 33 M odified LaMer plots with an additional axis of reaction temperature. S=Sc: Critical saturation of monomer concentration to induce nucleation. S=1: Equilibrium monomer concentration below which growth stops. After the solution is mixed at 80C for 40 minutes, a pre heat at 200 C for 30 min results in the first nucleation stage, as shown in F igure 33 (a). Continued reaction at the higher temperature of 290 C produces secondary nucleation of the clove r petals and subsequent growth. The final size is larger due to Oswald ripening effect s. To produce platelet and round nanoparticles the temperature was increased directly from the mixing temperature of 80 C to the reaction temperature of 32 0 C without a preheat step ( F igure 3 3 b). This procedure would result in a high density of nuclei with simultaneous decrease in monomer concentration to stop nucleation and minimize

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58 growth. While growth was slow, it was sufficient to change the shape of t he nanoparticles from platelet to round with increasing reaction which can be explained by surface tension effects [117] H igh growth temperature (320 C) of Gd2O3:Eu3+ nanoparticles allows the morphology to change from initial square shape to round pseudosphere nanoparticles which will minimize their surface tension. Fig ure 3 4. XRD patterns of as synthesized samples 3 .3 .2 Crystal S tructure of Gd2O3:Eu3+ N anoparticles The XRD spectr um from a clover sample ( H1 ) show s very broad peaks which match those from cubic Gd2O3 (JCPDS #43 1014) ( Fig ure 3 4 ) The broad peaks indicate small grain size, presumably due to the size of each petal of the Gd2O3:Eu3+ nanoparticles. The XRD spectra of platelet (H2) and for round (H 3 and 4) samples

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59 consisted of much sharper peaks, but they indicate the presence of both a monoclinic (JCPDS #43 10 15) and a cubic Gd2O3 phase. A phase transformation from cubic to monoclinic Gd2O3 has been reported at temperatures above 1300 1400 C with the stable high temperature phase being monoclinic [118] Apparently the monoclinic phase is stabilized in nanometer particles, as suggested by the strength of the main monoclinic 32.47 ) versus that from cubic Gd2O3 27) The data suggest all three samples, H2 to H4, have mixed crystal structures, which is consistent with earlier reports of mixed structures for rare earth oxide na n o particles [25, 119] In addition, change of microstructures for increasing reaction time. 3.3 .3 Ph otoluminescent P roperties of N anoparticles Fig ure 35. PL and PLE spectra of Gd2O3:Eu3+ nanoparticles. T he PL spectra of excitation at 250 nm and the PLE spectra for the emission at 609 nm.

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60 Fig ure 36. Resolved PL spectra of square nanoparticles with reaction time of 40 min (H2), round nanoparticles with reaction time for 90 min (H3) and for 180 min (H4). Photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra of Gd2O3:Eu3+ nanoparticles are shown in Fig ure 3 5 The PLE spectra for the emission at 609 nm from Gd2O3:Eu3+ nanoparticles with different shapes are composed of a broad excitation band extended from 225 ~ 300 nm, which is considered to be both host and chargetransfer band (CTB) excitation The broad excitation peak ca n be divided into three different regions, consisting of the cubic host s absorption (~244 nm), the charge transfer band (CTB) excitation of EuO (~260 nm) and Gd3+ ion direct absorption (~275 nm). A charge transfer between Eu and O results from an electron transferring from the 2p orbital of O2 to the 4f orbital of

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61 Eu3+. The strength of the charge transfer is related to the covalency of the O2 Eu3+ bond and the coordination of the Eu3+ and Gd3+ ion s [120] The small, sharp peak s between 360 and 53 0 nm and F ig ure 35 b are from direct exci tation of the f f shell transitions on Eu3+ ions [121] As dis cussed above, PL excitation spectr a of Gd2O3:Eu3+ nanoparticles have three different components, such as host absorption, CTB and Gd3+ direct absorption, so that it is possible to decompose the broad PL excitation peak from 225 to 300 nm into three Lorentzian curves Their different ratio show that H3 and H4 exhibit higher host and Gd3+ absorption whereas H1 and H2 has a dominant CTB excitation [122, 123] Growth at low temperature for short reaction times p resumably result in more def ected crystalline and low host and Gd3+ absorption. The PL emission spectra from Gd2O3:Eu3+ nanoparticles with various shapes is dominated by the 5D0-7F2 transition at 609 nm with minor peaks from 5D0-7FJ (J = 0, 1, 3 and 4) transitions, which is a characteristic of the Gd2O3 host lattice [47] It was previously reported that several split levels give rise to several PL peaks between 600 and 630 nm in monoclinic Gd2O3:Eu3+, while the 5D0-7F2 peak at 610 nm is dominant from cubic Gd2O3 [124, 125] As shown in F ig ure 3 6 the PL spectra between 600625 nm from Gd2O3:Eu3+ nanoparticles without pre heating (H2, H3 and H4) can be resolved into three different peaks (A, B and C centered at 610, 615 and 621 nm ) where A is from the cubic phase (610 nm) and B and C are from the monoclinic phase (615 and 621 nm). The ratio of the three peak intensi ties normalized to the intensity of peak C is shown in F igure 3 6 T he photoluminescence properties is dependent on the structure of host material [47] Generally, cubic Gd2O3:Eu3+ exhibits brighter luminescence than does monoclinic

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62 Gd2O3 [50] However, in nanoparticles regime, it is not easy to correlate the shape of PL spectra with crystal structures due to complicated and similar structures of nanoparticles based on XRD spectra, as described above. In other words, the crystal linity cannot be the only factor to determine the luminescent efficiency for nanoparticles. Table 3 2. The quantum yield and molar concentration of Eu ions for clover square and round Gd2O3:Eu3+ na noparticles (H1H4). The quantum yields (QY) and Eu3+ doping concentrations (molar %) from H1 H4 nanoparticles are reported in T able 3 2 Even though the concentration of Eu3+ dop ant was constant, the quantum yield varied by more than a factor of seven between H1 to H4, with H4 having the lowest and H3 having the highest quantum yield. P ossible reasons for the different QYs from Gd2O3:Eu3+ nanoparticles inc lude changes in the dop ant concentration and/or location, and differences in the host environment and dopant location. Sometimes the real doping concentration is different from the precursor concentration. However the induc tively coupled plasma (ICP) data show that the Eu doping concentration in Gd2O3:Eu3+ nanoparticles is constant within experimental error and is in dependent o f particle shape XRD data suggest that the low er QY of H1 (QY = 24%) as compared to H2 and H3 (QY = 48 and 67 % respectively ) may be explained by defective crystallin e of H1 due to its small size and low reaction temperature. However the difference in QYs of H2, H3 and H4 cannot be so easily explained. A similar variation of QY has been reported for ZnGa2O4:Eu where a low quantum yield resulted

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63 from migration of Eu ions to the surface of nanoparticles. In our work, nanoparticles have high doping concentration with over 15 %. Assuming that Eu activators can diffuse out and accumulate in the surface region, the local environment of Gd2O3:Eu3+ nanoparticles will be changed due to differen t atomic size between Eu and Gd, which will be discussed below Figure 37 X ray luminescence spectrum of as prepared spherelike Gd2O3:Eu3+ nanocrystals 3.3. 4 X ray Luminescence P roperties of N anoparticles Figure 3 7 shows the X ray lu minescence (XL) spectrum of as synthesized spherical Gd2O3:Eu3+ n anoparticles (H3) irradiated by 40 k V Xray. The spectrum shows a dominating peak at 612 nm from the 5D0 7F2 transition which is similar to the photoluminescence spectrum shown in Figure 3 5 (a). Several X ray and radioluminescence materials have been already developed, such as NaI, but may have

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64 undesirable properties, such as poor high temperature drift effects In this study, X ray luminescence was observed from Gd2O3:Eu3+ nanoparticles synthesized by nonhydrolytic hot solution synthesis method, consistent with its potential application as scintillation detectors. 3. 4 Conclusion Eu3+ doped Gd2O3 nanoparticles were synthesized by a hot solution method. The shape of Gd2O3:Eu3+ nanoparticles that were preheated and then reacted 290 C was clover Without the preheat step and for react ion at 320 C short times produced platelet nanoparticles but became more circular at longer reaction times. Potential mechanisms leading to various shapes of Gd2O3:Eu3+ nanoparticles were discussed. By controlling reaction parameters, the quantum yield of Gd2O3:Eu3+ nanoparticles also varied from 9 % to 67 %. C lover and plate Gd2O3:Eu3+ nanoparticles exhibited relatively low quantum yield, 24% and 48%, respectively, because of the defective crystalline Round Gd2O3:Eu3+ nanoparticles with 1.5 hrs reaction time showed the highest quantum yield of 67 %, which results from the combination of t wo factors, a host environment and a dopant location. For Eu3+ doped nanoparticles with a prolonged reaction time with 3 hrs, the quantum yield decreased to 9% presumably due to the migration of dopant ions and subsequent segregation to the nanoparticle surface region.

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65 CHAPTER 4 ENHANCED PHOTOLUMINE SCENCE FROM GADOLINIUM OXIDE DOP ED WITH EUROPIUM NANOCORES WITH YTTRIUM OIXDE THIN SHELL 4.1 Introduction O xide and sulphide nanophosphors are popular research topics due to their unique physical properties. T he size, shape, structure and composition of nanoparticles can be controlled so that their optical, electrical or mechanical properties can be tuned to des i red values [76, 126] For example, oxide nanophosphors, such as Y2O3, GdVO4 and Gd2O3 doped with rare earth ions have been studied for plasma display panels (PDPs), cathode ray tubes (CRTs) and field emission displays (FEDs) [127129] Europium doped Gd2O3 e xhibits red luminescence from the electric dipole 5D0-7F2 transitions on the trivalent europium ions (Eu3+ 4f6) [47] Gd2O3:Eu3+ nanoparticles synthesized by hydrothermal or nonhydrolytic thermal reactions with organic surfactants exhibit excellent crystallinity and good photoluminescence (PL) efficiency (QY = ~50%) [25] Moreover, the rate of growth and the shape of nanoparticles can be controlled by the type and concentration of precursors and organic surfactant s [25, 130] However, it is well known that states at the surface of phosphor powders can lead to nonradiative relaxation and low luminescent efficiencies [87] especially for nanophosphors with a large surface to volume ratio. C apping of the nanoparticles core with the proper shell materials could passiva te surface defects and eliminate the non radiative pathways [131] T his strategy has been successfully used in core/shell nanoparticles such as CdSe/ZnS and CdS/ZnS as well as oxide nanoparticles like SiO2/Gd2O3:Eu3+ [132 134] The precipitation of nanoparticles in a high boiling point alcoh ol generally called the polyol synthesis method, has been used to produce various materials including elemental metals and oxides [135, 136] Recently, Wang, et al reported the synthesis of

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66 CeF3 core/shell nanoparticles using the polyol method [137] In this stu dy, the polyol method was used to synthesi ze Gd2O3:Eu3+ cores followed by nucleation and growth of Y2O3. It will be shown that only a fraction of the Y2O3 grew as a shell on the Gd2O3:Eu3+ cores, but the shell did increase the PL intensity. The consequences of growth of yttria both as a shell and as a separate phase on the properties of the nanophosphor were investigated. 4.2 Experimental The following precursor compounds and solvents were purchased from Aldrich Chemical Co. : Gd(III) nitrate hy drate (99.9%) Eu(III) nitrate hexahydrate (99.9%) Y (III) nitrate (99.9%) and ethylene glycol (99%) All chemicals were analytical grade and were used without further puri 4.2.1 Synthesis of Gd2O3:Eu3+ N an ocores The doping concentration of Eu3+ in the Gd2O3 host was 5 mol% based on precursor concentrations For synthesis, 1.9mmol of Gd(NO3)3 6H2O and 0.1 mmol Eu(NO3)3 6H2O were dissolved in 25ml of ethylene glycol (EG) in a stirred roundbottomed flask at 100 C. The clear solution was then heated to 180 C for 1hr with vigorous stirring. The resulting suspension was cooled to room temperature, diluted with ethanol and the nanoparticles separated by centrifugation at 7000 rpm. T o remove unreacted precursor and EG, the nanoparticles were redispersed in ethanol and centrifuged at least three times. The nanoparticles were calcined at 600 C for 2 h in air. 4. 2. 2 Synthesis of Y2O3 in the P resence of Gd2O3:Eu3+ Nanoc ore s Growth of Gd1.9O3:Eu0.1 3+/Y2O3 core/shell nanoparticles was attempted with a two step process. F irst, Gd2O3:Eu3+ core nanoparticles were prepared using the procedure described above, then washed with ethanol at least three times. The quantity of Y2O3,

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67 and therefore the shell thickness es, was varied i n the second s tep by changing the precursor molar ratio, R, of Y(NO3)3 6H2O to ( Gd +Eu) (NO3) 3 6H2O from R= 1:16 to 1:8, 1:4 and 1:1. As an example, for R = 1:1 Gd2O3:Eu3+ nan ocores were obtained from 2mmol of ( Gd +Eu) (NO3) 3 6H2O in a first step then 2 mmol of Y(NO3)3 6H2O were added to 25 mL of EG at 100 C and Y2O3 was grown in a second step. Th is solution was heated to 180 C for 2h with vigorous stirring cooled to room temperature, washed and centrifuged as discussed above. 4. 2. 3 Characterization of N anoparticles The crystal structure of as grown and calcined nanoparticles were characterized by X ray diffraction (XRD) with a Phili ps APS 3720 diffractometer using Cu K radiation ( =1.54178 ). The XRD patterns were collected from dried powder samples with a 0.02 st ep scan mode over a 2 range of 20~70 Morphology and size of nanoparticles were obtained by using a JEOL 2010F high resolution transmission electron microscope (HR TEM) operated at an accelerating voltage of 200 kV. Specimens for the TEM were prepared by dropcasting particles dispersed in ethanol on to holey carboncoated copper grids. X ray photoelectron spectroscopy (XPS) was used to analyze the nanoparticle s shell thickness with a Perkin Elmer PHI 510 0 ESCA system and Al K X rays (E=1.486keV) All the XPS peaks were referenced to the C1s peak at 284.6 eV. T o make XPS samples, the same weight of nanoparticles were dispersed in ethanol and 50 l of solution was dropcast onto 1cm2 Si wafers. Photoluminescence (PL) and photoluminescence excitatio n (PLE) spectra and luminescent relaxation lifetimes were measured at room temperature using a JASCO FP 6500/6600 research grade fluorescence spectrometer with a 150 W Xenon lamp using films on quartz slides drop

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68 cast from a mixture of 20 mg of nanoparticl 4 % solution of poly(methy l methacrylate) ( PMMA ) dispersed in chlorobenzene. The drop cast films were dried in laboratory air at 80 12 h. 4. 3 Result s and D iscussion 4. 3.1 Crystal St ructures and M orphologies of C ore and C ore/ S hell N anoparticles As grown nanoparticles were amorphous, but XRD spectra in Figure 4 1 f or (a) Gd2O3:Eu3+, (b, c) Gd2O3:Eu3+/Y2O3 core/shell ( R =1:4 and 1:1, respectively ), and (d) Y2O3 calcined nanoparticles show crystalline peaks that match with (e) JCPDS data for cubic Gd2O3 (Card 43 1014) or cubic Y2O3 (Card 431036) Figure 4 1 XRD patterns of calcined samples: Gd2O3:Eu3+ nanocore s; Gd2O3:Eu3+/ Y2O3 core/shell nanoparticle s with the shell grown with a Y to Gd+Eu precursor ratio R = 1 : 4 and 1 : 1 and pure Y2O3 nanoparticles

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69 Since the cubic phases of Gd2O3 and Y2O3 differ only by the lattice parameter ( 0.106 nm for Y2O3 versus 0.108 nm for Gd2O3), it is difficult to verify by XRD the presence of small amounts of Y2O3 when Gd2O3 is also present, and vice ver sa. For example, f or Gd2O3:Eu3+ and Y2O3 nanoparticles grown with a R of 1:1 ( Figure 4 1c) the diffraction peaks are shift ed to 2O3 and smaller than for pure Y2O3, presumably due to overlapping peaks from the presence of both oxides. Figure 4 2 Resolved XRD peak of Gd2O3:Eu3+/ Y2O3 core/shell nanoparticle s with shell grown with a Y to Gd+Eu precursor ratio R = 1:1 T he broad XRD peak at ~ 29.0o from the R=1:1 sample wa s resolved into three different peaks in Figure 4 2 The sharp peaks 28.6o and 29. 2o correspond to Gd2O3 cores and phase separated Y2O3. The broad peak at 2 9.0o is assigned to Y2O3 in the shell with a shifted 2 due to strain from epitaxy. Cao, et al reported that the diffraction peaks shift to higher angles because of the strain from epitaxial growth of an InP shell on an InAs core [138] In the present case where the shell of Y2O3 is very thin (see below ), the shift in the diffraction peak is consistent with the presence of

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70 nanoparticles of both Gd2O3:Eu3+/ Y2O3 core/shell and undoped Y2O3 nanoparticles for R = 1:1. Using the Scherrer equation, broadening of the ( 222 ) peak indicated an average diameter of 18~20 nm for the Gd2O3:Eu3 and Y2O3 nanoparticles, consistent with TEM data (see below). As mentioned before, t he presence of a mixture of core shell and undoped yttria nanoparticles is consistent with the fact that the diffraction peak full width half max imum (FWHM) was larger for nanoparticles grown with R=1:4 or 1:1 as compared to R=1:16, even though TEM data showed that the average nanoparticle size was constant to within experimental error. Figure 4 3 TEM images of Gd2O3:Eu3+,Y2O3 and Gd2O3:Eu3+/Y2O3 core/shell nanoparticles: Gd2O3:Eu3+core, Gd2O3:Eu3+/Y2O3 (R=1 : 8), Gd2O3:Eu3+/Y2O3 (R=1 : 4), Gd2O3:Eu3+/Y2O3 (R=1 : 1), High resolution of Gd2O3:Eu3+/Y2O3 (R=1 : 1), and Y2O3. Figure 4 3 shows HR TEM images of calcined (a) pure Gd2O3, ( b, c, d and e) Gd2O3:Eu3+and Y2O3 mixtures grown with R = 1:8, 1:4 1:1 and 1:1, respectively (note

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71 the different magnification markers), and (f) pure Y2O3 nanoparticles These images show that the particles are slightly agglomerated after calcination The morphologies of Gd2O3:Eu3+ and Y2O3 nanoparticles are quite similar and t he average size of all nanoparticles is constant at ~ 21 nm, which is in reasonable agreement with the 18~20 nm size from XRD reported above. With higher magnification, it is possible to see lattice fringes ( Figure 4 3 e) which indicate good crystallinity after calcining, again consistent with XRD data. In TEM images, contrast is dependent upon the electron scattering power of the material whic h increases with the average atomic number, Z [19] In principle it should be possible to distinguish between the low Z (lighter) Y2O3 shell and the high Z (darker) Gd2O3:Eu3+ core in Figure 4 3 b e. There are regions in the images that are consistent with the presence of a shell but agglomeration and overlapping particles make it impossible to establish that the shell is present and to determine the shell thickness and uniformity. Figure 4 4. XPS peaks from Gd2O3:Eu3+ and Gd2O3:Eu3+/Y2O3 core/shell nanoparticles grown with different precursor molar ratios (R) and normalized intensity of the Gd 3d5 /2 peak versus Y2O3 shell thickness.

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72 4. 3.2 Determination of S h ell T hickness by XPS A nalysis X ray Photoelectron Spectroscopy (XPS) analysis can be used to show that Y2O3 was present both as a coprecipitated phase and as a shell. Growth of a Y2O3 shell on a Gd2O3:Eu3+ core is indicated by an exponential attenuation of the intensity of the gadolinium core photoelectron peaks. The exponential decrease of the gadolinium peak i ntensities should be complemented by an exponential increase of the intensities of the yttrium photoelectron peaks if Y2O3 only grows as a shell on the Gd2O3:Eu3+ core. If instead, Y2O3 is present as a coprecipitate with Gd2O3:Eu3+ core particles, the i ncrease in the yttrium XPS peak should increase linearly. Figure 4 5. Idealized concentric spherical core/shell structure from which a maximum shell thickness for a given R can be calculated. XPS spectra in Figure 4 4 (a) show that as the Y:Gd precursor ratio was increased the Gd 3d5 /2 (binding energy BE ~1175 eV, kinetic energy KE ~310eV) and Gd 4d peaks (BE ~152eV, KE ~1334eV) decrease d while the Y 3d peak (BE ~167eV, KE ~1319eV) i ncreased. The normalized attenuated intensities of the Gd 3d5 /2 peak fi t an exponential function (solid line in Figure 4 4 (b) ), proving that a Y2O3 shell was formed on the Gd2O3:Eu3+ core [139] Since the attenuation of the Gd XPS peak is exponential with thickness, the thickness determined from Figure 4 4 b is the minimum thickness of

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73 the shell determined by assuming an attenuation distance of 0.8 nm for the 310eV photoelectron in Y2O3 [140] The shell thickness increased with increasing R to a value of ~0.6 nm with an R = 1:1. While a thickness of 0.6 nm is small relative to the unit cell of yttria, the lattice parameters and crystal structures of the two oxides are very simil ar suggesting that epitaxial growth is probable and therefore this small shell thickness is reasonable. As pointed out above, an exponential increase in the Y 4d XPS peak would be expected if the Y2O3 were all present only in a shell. Instead the increas e of the Y 4d was linear with the precursor ratio, consistent with Y2O3 being present both as a coprecipitate and as a shell. Figure 4 6 PL and PLE spectra from Gd2O3:Eu3+ and Gd2O3:Eu3+/Y2O3 core/shell nanoparticles. The PLE spectra are for emission at 612 nm and the PL spectra are for excit ation at 250 nm. The inset shows the uncorrected PL peak intensity versus the value of R.

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74 The presence of both nanoparticles and shell Y2O3 is consistent with the observation by TEM that the average particle size was constant and ~ 21 nm. A fraction of the Y2O3 must be present as nanoparticles based on the following argument. A ssum ing that the shape of the core/shell nanoparticles is spher ical and concentric ( Figure 4 5 ), and that the Gd and Y precursors react completely to produce only a core and a shell respectively, the Y2O3 shell would be ~2. 3 nm thick for 21 nm diameter cores and R = 1:1. This predicted shell thickness is four times larger than that measured by XPS, leading to the conclusion that some of the Y2O3 exi st s as a separate phase, consistent with the XRD peak shift in Figure 4 1 c and Figure 4 2 4. 3.3 Photoluminescence P roperties of C ore and C ore/ S hell N anoparticles Figure 4 6 (a) shows PL and PLE spectra from the bare Gd2O3:Eu3+ and Gd2O3:Eu3+/Y2O3 (core/shell) nanoparticles. PL emission and excitation spectra were measured from thin films dropcast as described above, and were very reproducible as indicated by error bars in Figure 4 7 The PL spectra were dominated by the 5D0-7F2 transition of Eu3+ at 612 nm, while the PL E spectra w ere composed of a broad (FWHM peak centered at ~250 nm and sharp peaks between 360 and 530 nm. The broad peak near 250 nm is from the oxygen to europium charge transfer band (CTB) and the sharp peaks between 360 a nd 530 nm are from europium direct excitation. T h e same characteristic PLE features have been reported for Eu3+doped nanoparticles prepared by combustion, precipitation and the polyol method using chloride precursors [141143] All core/shell nanoparticles, except for th e R = 1 : 1 sample, exhibit ed stronger PL emission than did bare Gd2O3:Eu3+ nanoparticles. The PL intensity was measured from a constant weight (volume) of nanoparticles, therefore it should have been lower at

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75 all R values because a significant fraction of t he sample was nonluminescent Y2O3 (due to the absence of Eu3+ dopants ). Without another effect such as surface passivation, weaker PL intensities would be expected independent of whether the undoped Y2O3 was in a shell or in separate nanoparticles The XRD data could also be consistent with forming a solid solution of GdxY2 xO3:Eu3+ during calcining, but the temperature is too low for alloying. Furthermore, Li, et al report l ower PL from such a solution [144] which they attributed to less efficient charge transfer between oxygen and yttrium versus oxygen to gadolinium Figure 4 7 I ntegrated area of PL peak at 612 nm versus R; Solid squares are uncorrected data. Solid diamonds are uncorrected normalized data. Open circles are normalized and corrected for the fraction of nonluminescent Y2O3 based on the value of R. The enhanced PL shown by the corrected data in Figure 4 7 must result from passivation due to formation of the core/shell structure. Figure 4 7 is a plot of integrated area under the PL peak (560~725 nm and normalized to b a re Gd2O3:Eu3+) versus the

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76 Y2O3 shell thickness from the XPS data in Figure 4 4 b Without correcting for the fraction of nonradiative Y2O3, the normalized PL intensity ha d an apparent maximum (~1.2) at a shell thickness of 0.1 nm, grown with the lowest Y:Gd precursor ratio of R= 1:16. The normalized uncorrected PL intensity gradually decreased for higher values of R (filled squares in Figure 4 7 ) However, after correcting for the fraction of non luminescent Y2O3 (open diamonds in Figure 4 7 ), all core/shell nanoparticles exhibit a larger PL intensity than pure Gd2O3:Eu3+ nanoparticles with the thickest shell of 0. 6 nm grown with the highest R= 1:1 being ~40% brighter. This increased PL intensity is attributed to a reduced concentration of nonradiative recombination surface sites due to the Y2O3 shell on the Gd2O3:Eu3+ core. The larger increase in PL intensity at higher precursor ratios is attribute d to better passivation by the Y2O3 layer at larger values of R. The quantum e fficiency (QE) of the present Gd2O3/Y2O3 core/shell nanoparticles was not measured, However their QEs are expected to be ~ 40% based on our previous results from similar materia ls [145] 4. 3.4 Luminescence D ecay T imes of C ore and C ore/ S hell N anoparticles To evaluate whether the rate of nonradiative decay was reduced by the formation of the Y2O3 shell, the luminescence decay times were measured for bare Gd2O3:Eu3+ and Y2O3/Gd2O3:Eu3+ core/shell nanoparticles ( Figu re 4 8 ) The PL decay of both unpassivated core ( Figure 4 8 a) and core/shell ( Figure 4 8 b with R = 1:1) nanoparticles were very well fit by a single exponential function, i.e. by I=I0exp( t/ ) is the decay time constant As the thickness of the Y2O3 shell increased the decay time ( ) monotonically increased from 1.30 ms to 1.44 ms, as shown in Figure 4 8 ( c). T he decay time for the 5D0 7F2 transition in Eu3+ doped Y2O3 or Gd2O3 has been reported to be 1~2 ms [146, 147] consistent with our data Even though the

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77 only ~11% upon creation of the Y2O3 shell, the error bars in Figure 4 8 ( c) are small, indicating that the increase in decay time is real. The longer luminescence decay time for Eu3+ in Gd2O3 is consistent with a reduced rate of non radiative relaxation due to the Y2O3 shell passivating surface states [87, 148] Figure 4 8 Luminescence d ecay data for the Eu3+ 5D0-7F2 transition (612 nm peak ) The data are well fit by the solid line which is a single exponential function with a time constant Unpassivated surface states are known to trap charge carriers which lead to fast non radiative recombination and therefore shorter luminescent relaxation time constants.

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78 4. 4 Conclusion N anocore s (~21 nm diameter) of Gd2O3:Eu3+ with both a Y2O3 shell and undoped Y2O3 co precipitated nanoparticles were synthesized by a facile twostep high boiling point alcohol ( polyol ) method at 180 C in air for times of a few hours Data from XRD, TEM XPS PL and luminescent decay time measurements f rom calcined ( 600 C, 2 hrs in air) samples showed that crystalline Y2O3 shells up to 0. 6 nm thick were formed on the crystalline Gd2O3:Eu3+ core and that the thickest shell resulted in a 4 0% increase in PL intensity and an 11% longer PL decay time relative to uncoated Gd2O3:Eu3+ nanoparticles These changes were attributed to passivation of surface states that resulted in reduced rates of non radiative relaxation process es.

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79 CHAPTER 5 ENHANCED PHOTOLUMINE SCENCE FROM EUROPIUM DOPED GADOLINIUM OXIDE BASED CORE/DUAL SHELL NANOPARTICLES 5. 1 Introduction Nanophosphors with rareearth materials are of interest due to their potential applications in lightings, displays, biological diagnosis and scintillators [125, 145, 149, 150] Among rare earth materials, gadolinium oxide is a versatile material with potential applications in several fields of technology. When doped with rareearth ions (Eu3+ and Tb3+), it exhibits good luminescent properties [47, 151, 152] For nanophosphors, core/shell heterostructures have been created in two different configuration s, the first of which is a luminescent core and surface passivation shell. Several investigators have reported that the luminescent efficiency of nanophosphors was lower than that of their corresponding bulk counterparts due to the nonradiative recombinat ion at surface defects [87, 131] They frequently reported increased luminescent efficiency upon addition of the passivating shell [122, 123] A second configuration of the core/shell heterostructure is a nonluminescent core and luminescent shell. There are several reports of silica nanocores coated with various phosphor shells [132, 153, 154] These core/shell structures have several advantages, the first being easy control of the size of mono dispersed spherical silica cores from nanometers to micrometers [155, 156] Second, silica is c heap as compared to the more expensive rare earth phosphor materials, therefore the silica core/luminescent shell structures should result in lower costs. However, luminescent shells inherently have very large surface to volume ratio, and frequently exhibi t low quantum yields due to surface states. Here we report a silica core / dual shell with a luminescent Gd2O3:Eu3+

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80 shell and passivating Y2O3 shell which can reduce cost and double the quantum yield these potential scintillator materials. 5 2 Experimental 5. 2.1 Materials Gd(III) nitrate hydrate (99.9%), Eu(III) nitrate hexahydrate (99.9%) and Y(III) nitrate (99.9%) were purchased from Aldrich Chemical Co.. Tetraethyl orthosilicate (TEOS, 98%) and ethanol were purchased from Acros Organics. All chemicals wer e analytical grade and were used without further puri cation. 5. 2. 2 S ynthesis of Silica C ore s Silica nanocores were fabricated via the Stber method [157] where 5 ml of concentrated ammonium hydroxide (NH4OH) was mixed with 20 ml of deionized (DI) water and 100 ml of ethanol and stirred vigorously. Then 10 ml of tetraethyl orthosilicate (TEOS) was added qui ckly and stirred for 2 hrs without heating. After a few minutes, the solution became translucent and then cloudy white as the SiO2 particles grew large enough to scatter visible light. To remove unreacted precursors, the nanoparticles were centrifuged and re dispersed in ethanol at least three times, followed by drying at 70 C for 12 hrs in air. 5. 2. 3 Synthesis of Gd2O3:Eu3+ S hell on SiO2 C ores Growth of Gd1.6O3:Eu0.4 3+ shells was accomplished with a solution precipitation method, in which 500 mg of silica cores were added to 100 ml of DI water and the mixture was sonicated to achieve a homogeneous suspension. This was followed by addition of 4 mmol of gadolinium nitrate and 1 mmol of europium nitrate and 125 mmol of urea which was reacted at 85 C. After 1.5 hrs synthesis, nanoparticles were

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81 collected and washed as discussed above. Some of the core/shell nanoparticles were calcined at 600 C for 2 hrs in air. 5. 2. 4 Synt hesis of Y2O3 O uter S hell on SiO2/Gd2O3:Eu3+ N anoparticles Growth of SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles was accomplished with a two pot solution precipitation method. In the first flask, 250 mg of as prepared SiO2/Gd2O3:Eu3+ nanoparticles a nd 50 mmol of urea were added at RT to 30 ml of DI water and the mixture was stirred vigorously. In the second flask, 2.5 mmol of yttrium nitrate was added to 20 ml of DI water. After mixing for 30 min, the yttrium precursor solution was added to the first flask at 85C. After 1.5 hrs, the nanoparticles were collected and washed as discussed above. A fraction of the core/dual shell nanoparticles were calcined at 600 C for 2 hrs in air. 5. 2. 5 Characterization of N anoparticles Morphology and size of nanoparticles were determined using a JEOL 2010F high resolution transmission electron microscope (HR TEM) operated at 200 kV. The TEM samples were prepared by dropcasting nanoparticles dispersed in ethanol onto a carboncoated holey copper grid. The crys tal structure of as synthesized and calcined nanoparticles was characterized by X ray diffraction (XRD) with a Philips APS 3720 diffractometer using Cu K radiation ( = 1.54178). The XRD patterns were collected from dried powder samples with a 0.02 step 70 X ray photoelectron spectroscopy (XPS) was used to determine the shell thickness of nanoparticles. Data were collected with a PerkinElmer PHI 5100 ESCA system and Al Ka X rays (E=1.486 keV). All the XPS peaks were re ferenced to the C 1s peak at 284.6 eV. XPS samples were prepared by drop casting 50 l of a 2 mg/ml solution of nanoparticles dispersed in ethanol onto 1 cm2 aluminum foil.

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82 Photoluminescence (PL), photoluminescence excitation (PLE) spectra and luminescent relaxation lifetime were measured at room temperature using a JASCO FP 6500/6600 research grade fluorescence spectrometer with a 150 W Xenon lamp. Quantum yields were measured using an integrating sphere. The samples for PL, PLE and quantum yields were fil ms prepared by adding calcined nanoparticles to 500 L of 4% of poly(methyl methacrylate) (PMMA) dispersed in chlorobenzene and dropcast onto quartz substrates. 5. 3 Result s and D iscussion 5. 3.1 Morphology and C rystal St ructure of C ore/ S hell N anoparticles The morphology and size of as prepared SiO2 nanocores are shown in Figure 5 1(a). Based on TEM images, mono dispersed SiO2 nanocores were spherical and with a diameter of 45 0 nm. A ~5 nm of Gd2O3:Eu3+ layer was coated on SiO2 nanocores by the urea precipitation method, as shown in Figure 5 1(b and c). A nearly uniform SiO2/Gd2O3:Eu3+ core/shell nanostructure was obtained, as indicated by the outer darker and inner lighter regions corresponding to the Gd2O3:Eu3+ shell and SiO2 core, respectively. Figure 5 1(d) shows a SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticle. A dark Gd2O3:Eu3+ ring between two lighter regions is observed, due to differences in the average atomic number. The outer Y2O3 shell is 4 5 nm thick and it unifor mly covers the SiO2/Gd2O3:Eu3+ core/singleshell nanoparticles. According to XRD data, as grown silica nanocores were amorphous with an amorphous scattering peak at ~27 o, as shown in Figure 5 2(a). However, calcined SiO2/Gd2O3:Eu3+ core/shell nanoparticles showed crystalline peaks that matched JCPDS data from cubic Gd2O3 (Card 431014).

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83 Figure 51. TEM photomicrographs of dual shell The broad peaks indicate very small grain size ( 8 nm ) which is consistent with the TEM thickness of the Gd2O3:Eu3+ shell. In Figure 5 2(b), the XRD pattern from calcined core/dual shell nanoparticles with a Y2O3 outer shell show two different peaks at large Bragg angles, one is from cubic Gd2O3 structure and the second is from cubic Y2O3 structure (Card # 431036). Due to the larger volume of the 4 5 nm thick Y2O3 shell compared to the 5 nm thick Gd2O3:Eu3+ shell, the XRD peak intensities are dominated by the cubic Y2O3 and therefore two separate peak s cannot be detected in the 30 o angl e region due to the small intensity of Gd2O3 peaks. However for 50 the weak Y2O3 peaks separated from the Gd2O3 peaks can be seen.

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84 Fig ure 52 X Ray diffraction spectra obtained from pure amorphous silica nanoparticles or calcined SiO2/Gd2O3:Eu3+ core/single shell nanoparticles and c alcined SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles. 5. 3.2 Surface A nalysis of C ore/ S hell N anoparticles X ray photoelectron spectroscopy (XPS) analysis is commonly used to investigate the composition and chemical bonds in surface layers up to depths of a few nm. If SiO2 nanocores are coated with Gd2O3:Eu3+ and Y2O3 shells, the XPS signal from the core should be much less than those from the shells. Figure 5 3 shows XPS data for binding energies between 135170 eV from films of SiO2 core, SiO2/Gd2O3:Eu3+ core/singleshell and SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles. For a SiO2 nanocore sample, only the Si 2s peak (binding energy (BE) ~157 eV) was detected. A core/dual shell sample with a thick Y2O3 outer shell exhibited only the Y 3d peak (BE 163 eV). However, a core/single shell sample with an 5 nm Gd2O3:Eu3+ layers showed a small Si 2s peak at BE ~ 157 eV, as well as a large Gd 4d peaks (BE ~ 14 7 eV). The small 157 eV Si 2s XPS peak results from an exponential attenuation of the intensity of the silica core photoelectron peak [139]

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85 Fig ure 53 X ray photoelectron peaks from the SiO2 nanocores (solid line), from SiO2/Gd2O3:Eu3+ core/singleshell (dashed line) and SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles (dotted line). Fig ure 54 The Si 2p XPS peaks from SiO2, SiO2/Gd2O3:Eu3+ core/singleshe ll nanoparticles and normalized intensity of the Si 2p peak versus Gd2O3:Eu3+ shell thickness. To avoid ambiguity due to overlap with other peaks, attenuation of the Si 2p XPS peak (BE ~ 106 eV) was analyzed, as shown in Figure 5 4(a). Since attenuation of XPS

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86 peak is an exponential function of the overlayer thickness, the thickness of the Gd2O3:Eu3+ shell can be determined from I/I0=exp( t/), where I is an intensity of an XPS peak from the core, I0 is the intensity of this peak f rom a bare core, t is the thickness of the shell, and is the inelastic meanfree path of the photoelectron [139] The inelastic mean free path of a 106 eV BE photoelect ron in Gd2O3 was determined using the NIST Standard Reference Database 71 to be 3.32 nm for an electron with a Kinetic Energy (KE) =1378 eV (BE = 106 eV) [158] The calculated thickness of the Gd2O3:Eu3+ thin shell from I/I0 = 0.26 is 4.78 nm, which is consistent with the TEM value of 5 nm. 5. 3.3 Luminescence P roperties of C ore/ S hell N anoparticles Fig ure 5 5 Photoluminescence (PL) and photoluminescence excitation (PLE) spectra from SiO2/Gd2O3:Eu3+ core/singleshell and SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles. The PLE spectra are for emission at 612 nm and the PL spectra are for excitation at 250 nm. Figure 5 5 shows PL and PLE spectra from SiO2/Gd2O3:Eu3+ and SiO2/Gd2O3:Eu3+/Y2O3 calcined nanoparticles in dropcast films prepared as described

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87 above. In order to measure the photoluminescent properties with the same amount of luminescent Gd2O3:Eu3+, in other words to keep the number of particles constant, different weights of nanoparticles were dispersed i n the PMMA matrix. Assuming that core/dual shell nanoparticles are spherical and concentric (see Figure 5 6) with theoretical densities, a simple calculation with the dimensions reported above gave the weight ratio between core/singleshell and core/dual shell to be 2.3. Fig ure 56 Model of a spherical core/concentric dual shell structure from which the weight of nanoparticles necessary for a constant volume of luminescent Gd2O3:Eu3+ can be determined. As shown in Figure 5 5 the PL spectra were dominated by the 5D0 7F2 transition of Eu3+ ions at 612 nm, while the PLE spectra were composed of a broad peak under 300 nm and sharp peaks between 360 and 550 nm. The broad peak is from the oxygen to europium charge transfer band (CTB) and the sharp peaks are associated with direct excitation of the f f shell transitions of europium, which is the same characteristic PLE features have been reported for a variety of oxide nanoparticle hosts doped with Eu3+ ions and prepared by various methods [114, 142]

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88 5 .3. 4 Thin Film Quantum Yield Measurement Figure 57. Schematic diagram of thin film quantum yield (QY) measurement. The quantum yield of thin film photoluminescence was measured with a spectrophotometer (FP 6500, Jasco, Inc.) equipped with a DC powered 150W Xenon lamp source and a photomultiplier tube (PMT) detector. A quantu m yield measurement system for solid thin film samples collect s the photons with an integrating sphere (60 mm diameter; BaSO4 coating; Spectralon reflectance standards). The quantum yield (QY) of luminescent materials is defined to be the fractional or per centage ratio of the number of emitted photons to the number of absorbed photons, i.e. S2 divided by (S0S1) as illustrated in Figure 5 7 Table 5 1 Quantum yield, radius, volume ratio and weight ratio of SiO2, SiO2/Gd2O3:Eu3+ core/singleshell and SiO2/Gd2O3:Eu3+/Y2O3 core/dual shell nanoparticles with density of each material

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89 Here S0 is the number of incident photons measured using a reflective standard (Alumina block) S1 and S2 are the number of incident photons not absorbed and emitted photons from thin film samples, respectively [159, 160] As shown in Table 5 1, for excitation at 285 nm, the QY was 11.9% and 20.3% from SiO2/Gd2O3:Eu3+ and SiO2/Gd2O3:Eu3+/Y2O3, respectively. The increased PL intensity and QY from coating with the nonluminescent Y2O3 outer shell is attributed to a reduced probability of nonradiative r ecombination on the surface states due to the Y2O3 outer shell on the SiO2/Gd2O3:Eu3+ core/singleshell. 5. 3. 5 Luminescent D ecay T ime of Core/Shell N anoparticles Fig ure 58. Luminescence decay data for the Eu3+ 5D0 7F2 transition (612 nm peak). The data fit very well a single exponential decay (solid line) with a time To test the postulate that the Y2O3 outer shell reduced the rate of nonradiative recombination, the luminescence decay lifetimes were measured for SiO2/Gd2O3:Eu3+ and SiO2/ Gd2O3:Eu3+/Y2O3 nanoparticles, as shown in Figure 5 8 The luminescent decay curves from both core/singleshell and core/dual shell nanoparticles were well fit

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90 by a single exponential curve, i.e. by I=I0exp( The Y2O3 outer shell on SiO2/Gd2O3:Eu3+ core/singleshell nanoparticles increased the decay times from 0.89 ms to 1.53 m s, consistent with a sharp reduction in nonradiative transitions, and therefore more radiative relaxation over longer time constants. The decay time for the 5D0 7F2 transition in Eu3+ doped rareearth oxides has reported to be 1 2 ms [147, 161] The short decay time of core/singleshell nanoparticles of 0.89 ms can we explain by the fact that the SiO2/Gd2O3:Eu3+ core/shell nanoparticles are spherical and concentric, as described above, therefore the percentage of atoms locating on the outer surface (presumably was unsatisfied bonding) is approx imately over 25%. Because of the dangling chemical bonds, there apparently were a large density of surface defects that trapped charge carriers and led to fast nonradiative recombination and therefore shorter luminescent relaxation time constants [87] The Y2O3 outer shell reduced the density of surface states and led to longer relaxation times. 5. 4 Conclusion Nanocores (400 nm diameter) of SiO2 with a 4.5 nm thick Gd2O3:Eu3+ shell, and sometimes with a 43 nm thick Y2O3 shell, were synthesized by the Stber and by solution precipitation methods. The nanostructures were characterized by XRD, TEM, XPS and PL. As synthesized samples were amorphous, but the shells after calcination (600 C, 2 h in air) were crystalline, with both Gd2O3:Eu3+ and Y2O3 shells being cubic while the SiO2 core remained amorphous. Excitation in the UV led to emission at 612 nm from the Eu3+ dopant in the Gd2O3 shell with a quantum yield of 12%. Addition of the Y2O3 shell increased the PL intensity by 72%, raising the QY to 20%. The inc reased QY

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91 was attributed to passivation of surface states that led to reduced surface defect density and lower rate of nonradiative relaxation.

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92 CHAPTER 6 X RAY AND PHOTO LUMINESCENCE FROM GADOLINIUM SILICATE DOPED WITH CERIUM NANOPARTICLES SYNTHE SIZED BY SOLUTION BA SED METHODS 6. 1 Introduction Nanomaterials are under intensive study because they exhibit novel electrical, optical and magnetic properties compared to their bulk counterparts. The physical properties of materials change when the size is reduced from bulk materials to nanocrystals approaching the size of molecules [76, 126] For instance, the color of light from semiconductor nanoparticles (e.g. CdSe) can be tuned by changing the size of nanoparticles and these nanoparticles have been incorporated into light emitting devices [162] Rare earth oxyorthosilicates (R2SiO5) doped with Eu3 +, Ce3+, Tb3+ and Pr3+ have been widely used for luminescent materials due to their scintillation, cathodoluminescent and storage phosphor properties [163, 164] In particular, Ce doped Gd2SiO5 (GSO:Ce3+) is a promising sci ntillator material with high absorption coefficients for high energy particles, high luminescent output (20% of NaI:Tl+), excellent irradiation hardness (>108) and fast decay times (<50 nsec) [165] The excellent scintillation characteristics enable GSO:Ce3+ to be used in medical imaging, radiation detection and high energy physics applications [101, 102, 150] Normally GSO:Ce3+ detectors use a Czochralski grown single crystal, which is a difficult, expensive procedure that frequently denotes only small detector crystals [166] Nano sized rareearth silicates have been suggested as an alternative to overcome the disadvantage. Rareearth silicate nanoparticles doped with rareearth ions have been synthesized by hydrothermal, sol gel and solution combustion techniques, which require high reaction temperature or high temperature post annealing for scintillation [167169] Here we

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93 report a nonhydrolytic colloidal hot solution growth method to produce nanocrystals of GSO. Colloidal growth in nonhydrolyt ic liquid media can produce a variety of shapes and 1 10 nm sizes of highly crystallized nanocrystals that can be dispersed in organic or aqueous media for numerous potential applications [25] This chapter describes the synthesis of GSO:Ce3+ nanoparticles and their structural characteristics and photoluminescence properties by UV and X ray excitations. 6.2 Experimental 6. 2.1 Hot S olution S ynthesis Gd2SiO5 doped with Ce3+ (Ce3+ = 0.1, 0.2, 0.5, 2 and 5 mol %) nanoparticles were prepared by a t wo pot hot solution synthesis for 90 min in oleylamine (Figure 6 1) T he doping concentration is corresponding to the molar concentration of precursors In a typical reaction to produce GSO nanoparticles doped with 5 mol % Ce3+, 1.9 mmol of Gd acetate was mixed with 5 ml of oleylamine in a threeneck reaction flask and heated to 120 C. At the same time, 0.1 mmol of Ce acetate was mixed with 2 ml of oleylamine in a vial and heated to 120 C. After 30 min, the Ce precursor solution was mixed with the Gd prec ursor solution and 2 mmol of (3Amino propyl)triethoxysilane (APTES) was injected into the precursor mixture solution. Finally, the temperature was raised to ~ 320 C with a heating rate of 525 C/min and maintained at that temperature for 90 min with vigo rous stirring. After reflux, the brownish mixture was cooled down to room temperature by removing the heat source. Ethanol was added to the mixture, and GSO:Ce3+ nanoparticles were precipitated and separated by centrifugations. This procedure was repeated at least three times to remove any residue. The purified GSO:Ce3+ nanoparticles, capped with organic species, were well dispersed by organic

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94 solvents such as hexane. After drying at 70 C for 12 h, the nanoparticles were calcined at either 600 or 1000 C for 2 h in laboratory air. Figure 61 Flow chart of twopot hot solution growth of GSO nanoparticles 6. 2.2 Sol G el S ynthesis Nanocrystalline Gd2SiO5:Ce3+ (0.5 mol%) was also prepared by a sol gel method. First, high purity Gd(NO3)3 and Ce(NO3)3 were dissolved in DI water to form GdCe(NO3)3 solution. Next, an adequate amount of TEOS and NH3(OH) solution were added into the GdCe solution. This solution was heated at 80 C, which formed a gel, th en was dried at 80 C for 12 hrs. The resulting white powder was calcined at either 600 or in the 1000 C for 2 h in air. 6. 2.2 Characterization The crystal structure of as grown and calcined GSO:Ce3+ nanoparticles was characterized by X ray diffraction (XRD) with a Pholips APS 3720 diffractometer using Cu ). The XRD patterns were collected from dried powder samples with a 0.02 ogy and size of nanoparticles were determined with a JEOL 2010F high resolution transmission APTES : ( 3 Aminopropyl)triethoxysilane

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95 electron microscope (HR TEM) operated at an accelerating voltage of 200 kV. Specimens for the TEM were prepared by dropcasting particles dispersed in ethanol onto holey carboncoated copper grids. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at room temperature using a JASCO FP6500/6600 research grade fluorescence spectrometer with a 150 W Xenon lamp. X ray luminescence was m easured with a 40 kV BulletTM X ray tube combined with a Ocean Optics USB 2000 miniature fiber optic spectrometer. The distance from the x ray target to the sample was ~3 cm and the x ray tube was operated at 100 A. For each measurement, a crucible with 57 mm2 area was filled with nanopowder such that each sample had the same area exposed to the x rays. Differential pulse height distribution measurements was obtained by a Hidex Triathler scintillation counter with a Hamamatsu R850photomultiplier tube and a 1 Ci 241Am ( E = 60 keV) source. The Trialther was configured with logarithmic amplification to accentuate the difference in the differential pulse height spectra. In our measurements, the relative positions of the samples, detector and source were k ept fixed, with the plated 241Am source being suspended ~1 cm above power contained in a 3.7 m l glass vial. The electronic noise background was determined by measuring an empty vial in identical conditions as the nanopowder sample. 6. 3 Results and D iscussion 6.3.1 Thermogravimetric Analysis (TGA) of GSO Nanoparticles Nanoparticles grown by the hot solution growth (HSG) method were covered by the organic solvent, which often reduces the photoluminescence brightness. Calcining the GSO:Ce3+ nanoparticles at a sufficiently high temperature will remove the oleylamine, based on thermogravimetric analysis (TGA).

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96 Fig ure 6 2. Thermogravimetric analysis (TGA) data for HSG Gd2SiO5:Ce3+ 0.5% nanoparticles. Heating rate is 10 Cmin1. As shown in Figure 6 2 37 % of the total weight was lost by heating up to 800 C, with most of the weight change resulting from elimination of organic compounds by heating to approximately 400 C. On the contrary, GSO:Ce3+ nanoparticles prepared by sol gel (SG) method are not cover ed with organics, since the sol gel synthesis did not use any surfactants 6.3.2 Morphology and Structur e of GSO N anoparticles Rare earth oxyorthosilicates (R2SiO5) are reported to form two monoclinic polymorphs The first has a P21/c space group and is called the X1 phase, while the second has a C2/c space group and has called the X2 phase. If R is Tb or Y, the X1 phase is formed at low temperatures and X2 at high temperatures. From La to Gd, in the

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97 order of the periodic table, the X1 phase is formed, whereas Dy to Lu form s the X2 phase [170] Fig ure 63. XRD patterns from Gd2SiO5:Ce3+ nanoparticles compared to the JCDPS pattern for monoclinic Gd2SiO5 (JCPDS # : 40 0282) : GSO:Ce3+ n anoparticles after calcination at 600 C and 1000 C for 2hr in air. In the present study, as synthesized nanoparticles were amorphous with the typical broad XRD amorphous peak at 27o. Despite of calcination at 600 C, GSO:Ce3+ nanoparticles from both HSG and SG remained amorphous, as shown by XRD data in Figure 6 3 (a). In contrast, the XRD spectra from both HSG and SG GSO:Ce3+ nanoparticles calcined at 1000 C for 2hr were crystalline with peaks that match the JCPDS data for X1 monoclinic Gd2SiO5 (Card 400287). Using the Scherrer equation, broadening of the (321) peak f or both synthesis methods indicated an average nanoparticle diameter of 4050 nm. Figure 6 4 shows HR TEM and Selected Area Diffraction (SAD) images of sol gel prepared GSO:Ce3+ nanoparticles (a and b) and hot solution prepared GSO:Ce3+

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98 nanoparticles (c to f). These images show that the particles are slightly agglomerated after drying and calcinations. Fig ure 6 4. TEM images of Gd2SiO5:Ce3+ nanoparticles prepared by the sol gel (SG) method an d the hot solution (HSG) method. The particle size of as synthesized GSO:Ce3+ nanoparticles was 5 nm for both synthesis methods based on TEM images ( Figure 6 4 ( a and b) ). Figure 6 4 ( c and d) show GSO:Ce3+ HSG nanoparticles after calcination at 600 C. Despite the observation of only a broad amorphous XRD peak, it is possible to find lattice fringes in the high resolution image of a view of nanoparticles. An SAD image and diffraction pattern shown in Figure 6 4 ( d ) also suggests a fine grain size (<20 nm) with faint, broad rings before calcination. The part icle size increased to 45 nm after calcination at 1000 C for 2 hrs, as shown in Figure 6 4 ( f ) which is in reasonable agreement with sizes of 4050 nm based on XRD peak broadening. With the SAD micrograph and diffraction pattern

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99 inset of Figure 6 4 ( f ) incomplete ring patterns indicate larger crystalline grains after calcining at 1000 C, consistent with the XRD data in Figure 6 3 6.3.3 Luminescent Properties of GSO N anoparticles Fig ure 6 5. PL spectra and intensity for various Ce concentrations showing quenching for Gd2SiO5:Ce3+ nanoparticles synthesized by the hot solution method (excitation wavelength = 344 nm). It is well known that the optimum dopant concentration in a phosphor depends on both the host and rareearth dopants. For nanophosphors concentration quenching behavior of photoluminescence has been reported for YSO:Eu3+ [171] and Gd2O3:Eu3+ [122] and the general trend is that the optimum dopant concentration in nanophosphors is much higher than for the bulk materials. As shown in Figure 5 GSO:Ce3+ nanoparticles prepared by HSG exhibited the highest PL intensity for 0.5% Ce, which is higher than that of bulk oxyorthosilicates grown by the CZ method in which the optimum concentration is generally 0.05% [172] Figure 6 6 shows photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra from calcined GSO:Ce3+ nanoparticles prepared by the (a) hot solution and (b) sol gel methods, respectively. PL emission and excitation spectra were

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100 measured at room temperature from nanoparti cles suspended in ethanol with a quartz cuvette. Fig ure 6 6. PL and PLE spectra from Gd2SiO5:Ce3+ nanoparticles calcined at 600 C (dashed line) and 1000 C (solid line). GSO:Ce3+ nanoparticles calcined at 1000 C for both HSG and SG showed PL spectra t hat were very similar to spectrum from bulk single crystal GSO:Ce3+ [173] The PLE spectra consisted of three bands, as shown in Figure 6 6 ( a ) and 67 Band A absorption in the higher energy region at ~240 nm results from a transition between the ground 4f level (2F5/2) and the continuum of the conduction band. The bands labeled B and C at ~285 and ~344 nm result from to transitions from the ground 2F5/2 leve l to excited 5d (2E or 2T2) levels, respectively. The PL spectrum of GSO:Ce3+ nanoparticles calcined at 1000 C for two synthesis methods consists of two bands corresponding to the transitions from the lower 2T2 sublevel (5d) to 2F7/2 and 2F5/2 levels (4f ), which results in a broad and asymmetric emission peak at 424 nm. The crystal structures of both HSG and SG GSO:Ce3+ nanoparticles calcined at 1100 C were reported above to be monoclinic, therefore the match with spectra from bulk

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101 GSO:Ce3+ is expected. GSO:Ce3+ nanoparticles calcined at 600 C, however, show PL and PLE properties different from bulk GSO:Ce3+ due to lack of crystallinity. Fig ure 6 7. Configuration coordinate model energy levels for Ce3+ ions in a Gd2SiO5 host. As shown in Figure 66 GSO:Ce3+ nanoparticles prepared by HSG (dashed lines) and calcined at 600 C versus 1000 C ( Figure 6 6( a) ) exhibit different normalized excitation band intensities and a blue shifted emission peak. In contrast, GSO:Ce3+ nanoparticles prepared by SG did n ot show photoluminesce nce The luminescent properties of Ce3+ ions depend strongly on the structure of the host material through the crystal field splitting of the 5d state. For example, Ce3+ in YBO3 host material show a 390 nm emission peak with 254 nm ex citation, while Ce3+ in a GdPO4 host emits at 350 nm with 280 nm excitation[174] The difference in the PLE spectra and the live shif ted emission

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102 peak of GSO:Ce3+ nanoparticles synthesized by HSG presumably result from changes in the crystal field of the Ce3+ ions in the GSO host. On the other hand, the Ce3+ ions in SG GSO:Ce3+ nanoparticles did not emit after calcining at 600 C calcin ations due to the amorphous host. 6.3.4 X ray Luminescence of GSO Nanoparticles Fig ure 6 8. X ray luminescence spectrum of calcined Gd2SiO5:Ce3+ nanoparticles. Figure 6 8 shows the XL spectrum from HSG GSO:Ce3+ nanoparticles calcined at 1000 C and irradiated by 40 keV X rays. The spectrum is dominated by the same 5d4f peak at 424 nm as for the PL spectrum shown in Figure 5 a. Several X ray and radioluminescence materials have been already developed, such as NaI, but may have undesirable properties, such as poor high temperature drift effects [175] In this study, a good X ray luminance of 2300 Photons/MeV was observed for GSO:Ce3+ nanoparticles synthesized HSG, which compares favorably to a typical value of 7000 Photons/MeV of single crystal detectors [176]

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103 Figure 69. Differential pulse height spectrum of calcined GSO:Ce3+ nanoparticles by an 241Am source showing a broad scintillation response centered at approximately channel 300 The scintillation property of GSO:Ce3+ under ray irradiation was tested using americium (241Am) source. Figure 69 shows the differential pulse height spectra from calcined CSO:Ce3+ nanoparticles. The scintillation response of GSO:Ce3+ nanoparticles under 241Am irradiation determined by means of differential pulse height distribution measurements is shown in Fig ure 6 9 after the subtraction of the electronic noise, where a photopeak centered around channel (a verage pulse amplitude) 30 0 can be observed. 6. 4 Conclusion Gd2SiO5:Ce3+ nanoparticles were synthesized by a sol gel and a hot solution method. As synthesized nanoparticles did not exhibit photoluminescence. After elimination of residual organic compounds from the growth by calcining at 600 C, hot solution grown GSO:Ce3+ nanoparticles exhibited PL and PLE spectra that were different from nanoparticles calcined at 1000C and from a bulk GSO:Ce3+.

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104 Nanoparticles prepared by the sol gel method did not crystallize at 600C, but require calcinati on at 1000C before exhi biting PL and PLE spectra. 40 k V X rays produced at a luminance of 2300 photons/MeV from the same 5d4f transitions at 424 nm as the PL spectra These data show the promise of nanoparticles for scintillation detectors.

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105 CHAPTER 7 CONCLUSIONS 7.1 Shape Controlled Two Dimensional Gd2O3:Eu3+ Colloidal Nanocrystals Grown by Hot Solution Method In this chapter Eu3+ doped Gd2O3 nanoparticles were synthesized by a nonhydrolytic hot solution method. The shape of Gd2O3:Eu3+ nanoparticles were varied from clover to platelet and spherical by adding preheating process and by using different reaction times. Potential mechanisms leading to various shapes of Gd2O3:Eu3+ nanoparticles were discussed and explained by a Lamer pl ot By controlling reaction parameters, the quantum yield of Gd2O3:Eu3+ nanoparticles also varied from 9 % to 67 %. The 40 keV X rays produced an X ray luminance from the same 5D0-7F2 transitions from Eu3+ at 612 nm. These data show the promise of nanoparticles for scintillation detectors. 7.2 Enhanced Photoluminescence f rom Gd2O3:Eu3+ Nanocores with Y2O3 Thin Shell Gd2O3:Eu3+ nanocores of 21 nm diameter and Y2O3 thin shell were synthesized by a facile polyol method. The thickness of Y2O3 shell could be controlled by the different ratio of precursors. The thickness of Y2O3 layers was calculated by using the attenuation length of photoelectrons from X ray photoelectron spectroscopy. Based on XRD and XPS results Y2O3 co precipitated nanoparticles also formed with Gd2O3:Eu3+/Y2O3 core/shell structures. After calcinations at 600 C and 2 hr in air, crystalline Y2O3 shells up to 0.6 nm thick were grown on Gd2O3:Eu3+ cores. As the thickness of Y2O3 shell increased, the PL intensity and luminescence decay lifetime were larger by a portion by 40 % and 11 %, respectively. The enhanced PL intensity

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106 was attributed to passivation of surface defect states that reduced the rates of nonradiative relaxation process. 7.3 E nhanced Photoluminescence from E u D oped G d2O3 B ased C ore /D ual S hell N anoparticles Core/dual shell heterostructures with monodispersed 420 nm SiO2 cores, ~5 nm Gd2O3:Eu3+ inner shell and 43 nm Y2O3 outer shell were prepared by solution precipitation method The uniform coverage of the Gd2O3:Eu3+ shell and it s thickness were characterized by TEM and XPS. Photoluminescence (PL) of SiO2/Gd2O3:Eu3+ core/singleshell structures showed the red emission from Eu3+ by the 5D0-7F2 transition at 612 nm and their quantum yield (QY) was ~12 % after calcination at 600 C for 2 hr in air The core/dual shell nanoparticles with crystalline Y2O3 outer shell increased the QY to 20 % Since a large fraction of the atoms are located in the surface region of nanoparticles it is critical to passivate nanoparticles with an outer shell. Passivation increased the PL intensity as indicated by 71 % l ong er luminescent decay lifetimes. 7.4 X RAY and P hoto L uminescence from G d2S i O5:C e3+ N anoparticles S ynthesized by S olution B ased M ethods Cerium doped gadolinium silicate Gd2SiO5:Ce3+ (GSO:Ce3+) nanoparticles were successfully synthesized by solution based methods such as hot solution growth (HSG) and sol gel method. As synthesized spherical particles were amorphous and their diameter was ~ 5 nm. HSG GSO:Ce3+ nanoparticles after calcination at 600 C exhibited PL and PLE spectra which were different from bulk counterparts. PL spectra of calcined GSO nanocrystals at 1000 C, synthesized by both methods, showed a broad emission band with the peak at 420 nm associated with the Ce3+ ions 5d 4f transition, similar to a GSO :Ce3+ single crystal. Radioluminescence of GSO:Ce3+ nanocrystals excited by 40 kV X rays produc ed 2300 photons/MeV. Excitation by a

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107 241Am 5 9 keV gamma ray source resulted in a scintillation peak centered at channel number 290 with a FWHM of 180 channels.

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108 CHAPTER 8 FUTURE WORK Based on the results of this research, it is evident that highly efficient luminescent nanoparticles for scintillators can be synthesized through controlled growth and/or surface modification of nanoparticles. In terms of light output from scintillation detectors, which is composed of nanocomposit e s with matrix and nanoparticles, reduction of light scattering by nanoparticles is as important as enhancement of yield of nanocomposites. Therefore, novel ideas for reducing scattering for nanocomposites with nanoparticles and a matrix are required. The mechanisms leading to different light intensities from UV versus radioactive particles should be investigated for better detectors. In addition, t o get a better stopping power, it is required to have larger volume of nanoparticles rather than t hin layers. Therefore, Bi2O3 cores, which have high atomic number, should be used as luminescent centers to absorb high energy particles and core/shell structures will be applied for less light scattering. Figure 81. SiO2/Gd2O3:Eu3+/Y2O3/SiO2 core/multi shell nanoparticles

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119 BIOGRAPHICAL SKETCH Jihun Choi was born in Chungcheong do, South Korea in 1977. His family moved to Seoul at an early age whe re he grew up and was educated. His access to the laboratory environment beg a n at Chungdong high school when he was a chief of the Chemistry Club. He entered the Department of Material Science and Engineering at Seoul National University in 1997 and obtained his bachelor s degree in 2002 He continued his study and obtained his M aster of Science degree in 2004. His rese arch topic for the M. S. was E nhanced electrical properties of HfAlO highk dielectric thin films for a gate oxide of Si MOSFET devices After graduation with the M. S. he was hired by LG Electronics Institute of Technology (R&D campus in Seoul) as a research engineer. His research field at LG Electronics was the development of flexible displays with organic transistors. In 2007, he was admitted to the Department of Materials Science and Engineering at the Universi ty of Florida to pursue his Ph. D degree w ith a specialty in electronic materials. His research interest s at Dr. Holloway s group were synthesis and characterization of highly efficient luminescent nanoparticles for an application of scintillator radiation detectors. He received his Ph. D degree in August of 2011 in M aterials S cience and E ngineering