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Doped and Undoped Nanoparticles

Permanent Link: http://ufdc.ufl.edu/UFE0041505/00001

Material Information

Title: Doped and Undoped Nanoparticles Synthesis and Characterization
Physical Description: 1 online resource (173 p.)
Language: english
Creator: Chen, Ou
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: doping, dpph, mechanism, nanocrystal, nanoparticle, semiconductor, synthesis
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: DOPED AND UNDOPED NANOPARTICLES: SYNTHESIS AND CHARACTERIZATION Nanocrystals are very small particles, which contain from a few hundred to thousands of atoms, depending on the size of nanocrystal. Because of their unique properties compared with the bulk materials, nanocrystals have found many promising applications in areas such as biological labeling, catalysis, light-emitting diodes (LED), solar cells, and spintronics. This dissertation presents studies on the mechanism of nanocrystal growth, the synthesis of high-quality metal-selenide nanocrystals using SeO2 as the selenium precursor, the synthesis and properties of radial-position-controlled doping of CdS/ZnS core/shell nanocrystals, and the synthesis of water-soluble DPPH (2,2?-diphenyl-1-picrylhydrazyl) nanoparticles as a standard electron paramagnetic resonance (EPR) label. First, the growth stage of nanocrystals has been studied. It has been found that the number of nanocrystals decreases during the nanocrystal growth stage. Mechanistic studies show that only the solvent and crystal structure can affect the amount of decomposed nanocrystals. A model was proposed relating the nanocrystal growth patterns to stacking faults. The XRD measurements and computer XRD-simulation results show that fewer decomposed nanocrystals are associated with fewer stacking faults in the final nanocrystals, and vice versa. These results strongly support our model. Second, a non-injection synthesis (NIS) method for the synthesis of high-quality metal-selenide nanocrystals (e.g., CdSe, PbSe, PdSe, CuSe, etc.) with SeO2 as the selenium precursor has been developed. Mechanistic studies show that octadecene (ODE) acts as a reducing agent for SeO2 in this synthesis. Moreover, this synthesis exhibits controllable kinetics in both the nucleation and growth stages, and thus allows detailed control of the numbers of nuclei and the final sizes of the resulting nanocrystals. Importantly, this synthesis can be conducted in air eliminating the need for air-free manipulations using a glove box or a Schlenk line. Third, a three-step synthesis doping method has been developed. This new approach allows precise control of the Mn radial position and doping level in the CdS/ZnS core/shell nanocrystals. Based on this new synthesis, the detailed mechanism of this doping process has been studied. Nanocrystals doping is determined by the chemical kinetics of three activation-controlled processes: dopant adsorption, replacement, and ZnS-shell growth. We also studied the photoluminescence (PL) properies of our Mn doped nanocrystals in two excitation intensity regimes: weak excitation regime and strong excitation regime. Under weak excitation, the efficiency of the emission from one Mn ion (?Mn) exhibits a radial-position-dependent change that nearly perfectly corresponds to that of the Mn EPR linewidth of the nanocrystals: the higher the ?Mn, the narrower the Mn EPR linewidth. Under strong excitation, these Mn-doped nanocrystals exhibit an excitation-intensity-dependent, color-tunable dual emission property. Fourth, a size-controlled synthesis of water soluble DPPH (1,1-diphenyl-2-picrylhydrazyl) nanoparticles has been developed. Importantly, these nanoparticles exhibit size-dependent absorption spectra and fast-exchange-narrowed single-line EPR spectra with linewidths of ~1.5-1.8 G. They are also stable over a wide pH range (from pH 3.0 to pH 10.0). Furthermore, the EPR linewidth can be controlled by partially reducing the DPPH radical. These water-soluble DPPH nanoparticles are a perfect standard EPR labels for biological and biomedical systems
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Ou Chen.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Cao, Yun Wei.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041505:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041505/00001

Material Information

Title: Doped and Undoped Nanoparticles Synthesis and Characterization
Physical Description: 1 online resource (173 p.)
Language: english
Creator: Chen, Ou
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: doping, dpph, mechanism, nanocrystal, nanoparticle, semiconductor, synthesis
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: DOPED AND UNDOPED NANOPARTICLES: SYNTHESIS AND CHARACTERIZATION Nanocrystals are very small particles, which contain from a few hundred to thousands of atoms, depending on the size of nanocrystal. Because of their unique properties compared with the bulk materials, nanocrystals have found many promising applications in areas such as biological labeling, catalysis, light-emitting diodes (LED), solar cells, and spintronics. This dissertation presents studies on the mechanism of nanocrystal growth, the synthesis of high-quality metal-selenide nanocrystals using SeO2 as the selenium precursor, the synthesis and properties of radial-position-controlled doping of CdS/ZnS core/shell nanocrystals, and the synthesis of water-soluble DPPH (2,2?-diphenyl-1-picrylhydrazyl) nanoparticles as a standard electron paramagnetic resonance (EPR) label. First, the growth stage of nanocrystals has been studied. It has been found that the number of nanocrystals decreases during the nanocrystal growth stage. Mechanistic studies show that only the solvent and crystal structure can affect the amount of decomposed nanocrystals. A model was proposed relating the nanocrystal growth patterns to stacking faults. The XRD measurements and computer XRD-simulation results show that fewer decomposed nanocrystals are associated with fewer stacking faults in the final nanocrystals, and vice versa. These results strongly support our model. Second, a non-injection synthesis (NIS) method for the synthesis of high-quality metal-selenide nanocrystals (e.g., CdSe, PbSe, PdSe, CuSe, etc.) with SeO2 as the selenium precursor has been developed. Mechanistic studies show that octadecene (ODE) acts as a reducing agent for SeO2 in this synthesis. Moreover, this synthesis exhibits controllable kinetics in both the nucleation and growth stages, and thus allows detailed control of the numbers of nuclei and the final sizes of the resulting nanocrystals. Importantly, this synthesis can be conducted in air eliminating the need for air-free manipulations using a glove box or a Schlenk line. Third, a three-step synthesis doping method has been developed. This new approach allows precise control of the Mn radial position and doping level in the CdS/ZnS core/shell nanocrystals. Based on this new synthesis, the detailed mechanism of this doping process has been studied. Nanocrystals doping is determined by the chemical kinetics of three activation-controlled processes: dopant adsorption, replacement, and ZnS-shell growth. We also studied the photoluminescence (PL) properies of our Mn doped nanocrystals in two excitation intensity regimes: weak excitation regime and strong excitation regime. Under weak excitation, the efficiency of the emission from one Mn ion (?Mn) exhibits a radial-position-dependent change that nearly perfectly corresponds to that of the Mn EPR linewidth of the nanocrystals: the higher the ?Mn, the narrower the Mn EPR linewidth. Under strong excitation, these Mn-doped nanocrystals exhibit an excitation-intensity-dependent, color-tunable dual emission property. Fourth, a size-controlled synthesis of water soluble DPPH (1,1-diphenyl-2-picrylhydrazyl) nanoparticles has been developed. Importantly, these nanoparticles exhibit size-dependent absorption spectra and fast-exchange-narrowed single-line EPR spectra with linewidths of ~1.5-1.8 G. They are also stable over a wide pH range (from pH 3.0 to pH 10.0). Furthermore, the EPR linewidth can be controlled by partially reducing the DPPH radical. These water-soluble DPPH nanoparticles are a perfect standard EPR labels for biological and biomedical systems
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Ou Chen.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Cao, Yun Wei.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041505:00001


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1 DOPED AND UNDOPED NANOPARTICLES : SYNTHESIS AND CHARACTERIZATION By OU CHEN 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 2010

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2 2010 Ou Chen

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3 To my loved parents Congye Chen and Kaihong Shi ; To my loved wife Shuang Zhao

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4 ACKNOWLEDGMENTS F irst and foremost, I would like to express my sincere gra titude to my supervisor Dr. Y. Charles Cao who has supported me throughout my research work at UF. Without his support and encouragement, I could never ha ve accomplished what I have His broad knowledge and serious attitude to science have benefited me i mmensely and will influence me through out my future career. My thankfulness also goes to my Ph.D. committee members, Dr. Charles Martin, Dr. Alexander Angerhofer, Dr. David Micha and Dr. Paul Holloway. Your ambition for science and passion for research ha ve intrigued my enthusiasm for scientific exploration. For my experiments, I would like to thank Dr. Kathryn Williams for the DSC measurement and help revising my dissertation Dr. Alexander Angerhofer for the EPR measurements Dr. Nicolo Omenetto and Dani el Shelby for helping me with the laser experiments Dr. Oleg Matveev for teaching me how to operate ICP instrumentation and Ms. Kerry Siebein in the Major Analytical Instrumentation Center (MAIC) for TEM measurements For my colleague I would like to th ank Dr. Yongan Yang and Dr. Jiaqi Zhuang for teaching me the experimental skills and valuable discussions I would also thank all my great colleagues for their supports and sharing their wisdom and experience with me I also acknowledge all my friends in Gainesville for their support, understanding and encouragement. Without these great people my life in the past years would never be this enjoyable and memorable Finally, I would like to express my special appreciation to my wife Shuang Zhao and my paren ts Their support care and love are endless, so is my gratitude.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................................... 4 LIST OF TABLES .............................................................................................................................. 10 LIST OF FIGURES ............................................................................................................................ 11 ABSTRACT ........................................................................................................................................ 15 CHAPTER 1 INTRODUCTION ....................................................................................................................... 18 1.1 Semicondutor Nanocrystals .................................................................................................. 18 1.1.1 Quantum Size Effect .................................................................................................. 18 1.1.2 General Synthetic Strategies ...................................................................................... 22 1.1.2.1 Synthesis of semiconductor nanocrystals in aqueous solution ..................... 22 1.1.2.2 Synthesis of semiconductor nanocrystals in organic solution ...................... 24 1.2 Core/Shell Structure Semiconductor Nanocrystals ............................................................. 29 1.2.1 Type -I Core/Shell Nanocrystals ................................................................................ 31 1.2.2 Type -II Core/Shell Nanocrystals ............................................................................... 33 1.2.3 Reverse Type I Core/Shell Nanocrystals .................................................................. 34 1.3 Doping of Semiconductor Nanocrystals .............................................................................. 36 1.3.1 Difficulty of Synthesis of Doped Semiconductor Nanocrystals .............................. 37 1.3.2 Synthetic Development .............................................................................................. 37 1.3.3 Theoretical Model ...................................................................................................... 40 1.4 Summary of the Present Research ....................................................................................... 42 2 MECHANISM STUDY OF SEMICONDUCTOR NANOCRYSTAL SYNTHESIS: PARTICLE NUMBER DECREASES IN NANOCRYSTAL GROWTH STAGE ............... 44 2.1 Introduction ........................................................................................................................... 44 2.2 Experimental Section ............................................................................................................ 45 2.2.1 Chemicals .................................................................................................................... 45 2.2.2 Nanocrystal Synthesis ................................................................................................ 45 2.2.2.1 Synthesis of wurtzite CdSe (W CdSe) nanocrystals ..................................... 45 2.2.2.2 Synthesis of zinc blende CdSe (ZB -CdSe) nanocrystals .............................. 46 2.2.3 Particle Number Decrease Study ............................................................................... 47 2.2.3.1 Precursor preparation....................................................................................... 47 2.2.3.2 CdSe/CdS e core/shell nanocrystal growth ..................................................... 47 2.2.4 InductivelyCoupled Plasma Atomic Emission Spectroscopy (ICP) Measurements ................................................................................................................... 47 2.2.5 Pho toluminescence Measurements ........................................................................... 48 2.2.6 TEM Measurements ................................................................................................... 48 2.2.7 XRD Measurements ................................................................................................... 48

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6 2.2.8 Determination of the Extinction Coefficient of CdSe Nanocrystals ....................... 48 2.3 Results and Discussion ......................................................................................................... 49 2.3.1 Confirmation of Particle Number Decrease During Growth ................................... 49 2.3.1.1 Sizing curve ...................................................................................................... 50 2.3.1.2 Extinction coefficient curve ............................................................................ 51 2.3.1.3 CdSe/CdSe core/shell growth ......................................................................... 52 2.3.2 Mechanistic Study ...................................................................................................... 57 2.3 .2.1 Concentration effect ........................................................................................ 58 2.3.2.2 Temperature effect ........................................................................................... 59 2.3.2.3 Precursor effect ................................................................................................ 59 2.3.2.4 Solvent effect ................................................................................................... 60 2.3.2.5 Stacking faults (defects) associated with the nanocrystal growth model ..... 61 2.4 Summary................................................................................................................................ 70 3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS THE SELENIUM PRECURSOR ...................................................................... 71 3.1 Introductio n ........................................................................................................................... 71 3.2 Experimental Section ............................................................................................................ 72 3.2.1 Chemicals .................................................................................................................... 72 3.2.2 Prepar ation of Precursors ........................................................................................... 73 3.2.2.1 Preparation of cadmium myristate .................................................................. 73 3.2.2.2 Preparation of cadmium docosanate ............................................................... 73 3.2.2.3 Preparation of copper oleate ........................................................................... 73 3.2.3 Metal Selenide Nanocrystal Synthesis ...................................................................... 74 3.2.3.1 CdSe nanocrystal synthesis under air ............................................................. 74 3.2.3.2 CdSe nanocrystal synthesis under Ar using a Schlenk line .......................... 74 3.2.3. 3 CdSe nanocrystal synthesis with various final sizes ..................................... 75 3.2.3.4 PbSe nanocube synthesis ................................................................................. 76 3.2.3.5 CuSe nanocrystal synthesis ............................................................................. 76 3.2.3.6 Pd4.5Se nanocrystal synthesis .......................................................................... 76 3.2.4 Mechanistic Study ...................................................................................................... 77 3.2.4.1 The reaction with cadmium myristate ............................................................ 77 3.2.4.2 The reaction without cadmium myristate ....................................................... 77 3.2.5 Characterization of Metal -Selenide Nanocrystals .................................................... 78 3.2.5.1 Absorption measurements ............................................................................... 78 3.2.5.2 Photoluminescence measurements ................................................................. 78 3.2.5.3 TEM and EDS measurements ......................................................................... 78 3.2.5.4 XRD measurements ......................................................................................... 78 3.2.5.5 NMR measure ments ........................................................................................ 78 3.2.5.6 FT IR measurements ....................................................................................... 79 3.2.5.7 Differential scanning calorimetric (DSC) measurements ............................. 79 3.3 Results and Discussion ......................................................................................................... 79 3.3.1 Synthesis and Characterization of CdSe Nanocrystals ............................................ 79 3.3.2 Oxygen Effect ............................................................................................................. 80 3.3.3 Size Control of CdSe Nanocrystals ........................................................................... 81 3.3.4 Mechanistic Study ...................................................................................................... 84

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7 3.3.5 Generalization of SeO2Based Metal Selenide Synthesis ....................................... 89 3.3.5.1 PbSe nanocube synthesis ................................................................................. 89 3.3.5.2 CuSe nanocrystal synthesis ............................................................................. 90 3.3.5.3 Pd4.5Se nanocrystal synthesis .......................................................................... 91 3.4 Summary................................................................................................................................ 92 4 RADIAL -POSITION CONTROLLED DOPING IN CDS/ZNS CORE/SHELL NANOCRYSTALS: SYNTHESIS AND DOPANT GROWTH STUDY .............................. 93 4.1 Introduction ........................................................................................................................... 93 4.2 Experimental Section ............................................................................................................ 94 4.2.1 Chemicals .................................................................................................................... 94 4.2.2 Three Step Synthesis of Mn Doped CdS/ZnS C ore/Shell Nanocrystals ................ 94 4.2.2.1 Preparation of precursors ................................................................................ 94 4.2.2.2 Synthesis of Mn-doped CdS/ZnS core/shell nanocrystals ............................ 95 4.2.3 Kinetic Study of Mn Adsorption ............................................................................... 98 4.2.3.1 Formation of weakly bound Mn ..................................................................... 98 4.2.3.2 Formation of strongly bound Mn .................................................................... 98 4.2.4 Characterization of Mn Doped CdS/ZnS Core/Shell Nanocrystals ........................ 99 4.2.4.1 Absorption measurements ............................................................................... 99 4.2.4.2 Photoluminescence measurements ................................................................. 99 4.2.4.3 TEM and electron diffraction (ED) measureme nts ....................................... 99 4.2.4.4 X ray powder diffraction (XRD) measurements ......................................... 100 4.2.4.5 Electron paramagnetic resonance (EPR) measurements ............................. 100 4.2.4.6 Inductively-coupled plasma atomic emission spectroscopy (ICP) measurements ......................................................................................................... 100 4.3 Results and Discussion ....................................................................................................... 101 4.3.1 Synthesis and Characterization of Mn-Doped CdS/ZnS Nanocrystals ................. 101 4.3.2 Radial -Position -Controlled Doping ......................................................................... 105 4.3.3 Mechanistic Study .................................................................................................... 107 4.3.3.1 Surface bound Mn.......................................................................................... 107 4.3.3.2 Weakly and strongly b ound Mn .................................................................... 110 4.3.3.3 Kinetics of Mn-dopant adsorption ................................................................ 112 4.3.3.4 Replacement of Mn dopants ......................................................................... 115 4.3.3.5 Replaced Mn species ..................................................................................... 118 4.4 Conclusion ........................................................................................................................... 119 5 RADIAL -POSITION CONTROLLED DOPING OF CDS/ZNS CORE/SHELL NANOCRYSTALS: POSITION DEPENDENT AND EXCITATION INTENSITY DEPENDENT PROPERTIES .................................................................................................. 121 5.1 Introduction ......................................................................................................................... 121 5.2 Experime ntal Section .......................................................................................................... 122 5.2.1 Chemicals .................................................................................................................. 122 5.2.2 Three Step Synthesis of Mn Doped CdS/ZnS Core/Shell Nanocrystals .............. 122 5.2.2.1 Preparation of precursors .............................................................................. 122 5 .2.2.2 Synthesis of Mn-doped CdS/ZnS core/shell nanocrystals .......................... 123

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8 5.2.3 Characterization of Mn Doped CdS/ZnS Core/Shell Nanocrystals ...................... 125 5.2.3.1 Absorption measurements ............................................................................. 125 5.2.3.2 Steady-state photoluminescence measurements .......................................... 125 5.2.3.3 TEM measurements ....................................................................................... 126 5.2.3.4 Electron paramagnetic resonance (EPR) measurements ............................. 126 5.2.3.5 Inductively-coupled plasma atomic emission spectroscopy (ICP) measurements ......................................................................................................... 126 5.2.4 Photolumines cence Measurements Using a XeCl Excimer Laser as Excitation Source ............................................................................................................................. 127 5.2.4.1 Solution samples ............................................................................................ 127 5.2.4.2 Film samples .................................................................................................. 127 5.2.5 Lifetime Measurements ............................................................................................ 128 5.2.5.1 Band-edge (BE) lifetime measurements ....................................................... 128 5.2.5.2 Mn lifetime measurements ............................................................................ 128 5.3 Results and Discussion ....................................................................................................... 128 5.3.1 Weak Excitation Intensity Regime .......................................................................... 128 5.3.1.1 Dynamics of carrier relaxiation .................................................................... 128 5.3.1.2 Radial -position -dependent PL and EPR properties ..................................... 132 5.3.2 Strong Excitation Intensity Regime: ExcitationIntensity -Dependent PL Properties ........................................................................................................................ 136 5.4 Conclusion ........................................................................................................................... 142 6 SYNTHESIS OF WATER -SOLUBLE 2,2 DIPHENYL 1 -PICRYLHYDRAZYL NANOPARTICLES: A NEW STANDARD FOR ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY .......................................................................................... 143 6.1 Introduction ......................................................................................................................... 143 6.2 Experimental Section .......................................................................................................... 144 6.2.1 Chemicals .................................................................................................................. 144 6.2.2 Synthesis of DPPH Nanoparticles ........................................................................... 144 6.2.3 Synthesis of 2,2 -diphenyl 1 picrylhydrazine (DPPH H) Doped DPPH Nanoparticles .................................................................................................................. 145 6.2.4 Synthesis of DP PH/DPPH H Core/Shell Nanoparticles ........................................ 145 6.2.5 Absorption Measurements ....................................................................................... 145 6.2.6 TEM and Electron Diffraction Measurements ....................................................... 146 6.2.7 EPR Measurements .................................................................................................. 146 6.2.8 EPR Simulations ....................................................................................................... 146 6.3 Results and Discussion ....................................................................................................... 147 6.3.1 Synthesis and Characterization of DPPH Nanoparticles ....................................... 147 6.3.2 Understanding of the SpinExchange Interac tion .................................................. 152 6.3.3 Stability Test ............................................................................................................. 154 6.4 Conclusion ........................................................................................................................... 155 7 CONCLUSIO NS ....................................................................................................................... 157 7.1 Summary of Current Research ........................................................................................... 157 7.2 Perspectives ......................................................................................................................... 158

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9 APPENDIX CALCULATION OF SATURATION FLUENCE .............................................. 160 LIST OF REFERENCES ................................................................................................................. 162 BIOGRAPHICAL SKETCH ........................................................................................................... 173

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10 LIST OF TABLES Table page 1 1 Material parameters and properties of selected bulk semiconductors. ............................... 19 1 2 Precursors surfactants and solvents used in the synthesis of various semiconductor nanocrystals ............................................................................................................................ 28 2 1 Parameters used in XRD simulations for wurtzite crystal structures of CdSe nanoparticles. .......................................................................................................................... 66 2 2 Parameters used in XRD simulations for zinc -blende crystal structures of CdSe nanoparticles. .......................................................................................................................... 69 3 1 The EDS results of Pd4.5Se sa mple measured from randomly selected three different areas on the TEM grid.. ......................................................................................................... 92

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11 LIST OF FIGURES Figure page 1 1 The change of CdSe nanocrystal emissi on color with the increase of nanocrystal size ... 21 1 2 LaMer curve ........................................................................................................................... 24 1 3 Electronic energy levels of selected semiconductor mat erials and scheme of different types of core/shell structure .................................................................................................. 30 1 4 TEM images and UV -Vis and PL spectra of CdSe and CdSe/ZnS nanocrystals ............... 32 1 5 UV-Vis and PL spectra and TEM images of ZnSe and ZnSe/CdSe nanocrystals ............. 35 1 6 EPR spectra of Mn doped CdSe nanocrystals ...................................................................... 38 1 7 Absorption and ligand -field electronic absorption spectra of Co-doped CdS nanocrystals ............................................................................................................................ 39 1 8 Schemes of theoretical doping modes ................................................................................... 41 2 1 Sizing curve and the extinction coefficient curve of CdSe nanocrystal ............................ 50 2 2 CdSe nanocrystal sizes as a function of first exciton absorption peak. .............................. 51 2 3 Size -dependent extinction coefficient of CdSe nanocrystals ............................................. 52 2 4 The evo lution of the absorption and PL spectra during typical CdSe/Cd Se core/shell nanocrystal growth. ................................................................................................................ 52 2 5 TEM image of CdSe nanocrystals synthesized by the CdSe/CdSe core/shell growth. ..... 53 2 6 D iameters and extinction coefficients of CdSe nanocrystals synthesized by Cd Se/CdSe core/shell growth ............................................................................................... 54 2 7 CdSe nanocrystal amount as a function of diameter during particle growth .................... 54 2 8 Amount of Cd not in nanoparticles as a function of CdSe nanocrystal diameter ............. 55 2 9 Evolution of the PL spectrum and large partic le number during the reaction ................... 56 2 10 Particle amount as a function of diameter during the particle growth with different precursor s and core concentration s ...................................................................................... 58 2 11 Particle amount as a function of diameter during the particle growth with different growth temperature s .............................................................................................................. 59

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12 2 12 Particle amount as a function of diam eter during particle growth with different cadmium precursors .............................................................................................................. 60 2 13 Particle amount as a function of diameter during particle growth with different ODE/OAm ratios and in different solvents .......................................................................... 61 2 14 Schemes of the stacking sequence of wurtzite and zinc -blende crystal structures ... ......... 61 2 15 Evolution of the absorption and PL spectra and size distribution of W CdSe nanocrystals grown in ODE/OAm ....................................................................................... 63 2 16 Evolution of the absorption and PL spectra and size distribution of W CdSe nanocrystals grown in TOPO ............................................................................................... 63 2 17 W CdSe nanocrystal amount as a function of diameter during particle growth in different solvents .................................................................................................................... 64 2 18 TEM image and XRD of final CdSe nanocrystals grown in the ODE/OAm and TOPO ...................................................................................................................................... 65 2 19 XRD patterns simulations of W CdSe nanocrystals ........................................................... 66 2 20 Evolution of the absorption and PL spectra and size distribution of ZB CdSe nanocrystals grown in ODE/OAm ....................................................................................... 67 2 21 Evolution of the absorption and PL spectra and size distribution of ZB CdSe nanocrystals grown in TOPO. ............................................................................................... 68 2 22 ZB CdSe nanocrystal amount as a function of diameter during particle growth in different solvents .................................................................................................................... 68 2 23 XRD pattern simulations of ZB -CdSe nanocrystals ........................................................... 69 3 1 Characterization of CdSe nanocrystals in SeO2-based synthesis ........................................ 79 3 2 Temporal evolution of the absorption spectrum of CdSe nanocrystal growth in air and under Ar .......................................................................................................................... 81 3 3 The concentration and size of the final CdSe nanocrystals in the SeO2-based synthesis u nder the different conditions .............................................................................. 82 3 4 Absorption and PL spectra and TEM images of the CdSe nanocrystals ........................... 83 3 5 Mechanistic stud y of formation of CdSe nanocrystals ....................................................... 85 3 6 13C NMR spectra of the product mixture, myristic acid and 2 octadeanone .................... 86 3 7 1H N MR spectra of the product mixture, ODE and 2 -octadecanone and proposed reaction ................................................................................................................................... 87

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13 3 8 13C NMR spectra of the product mixture, 2 -octadecanone and ODE ............................... 88 3 9 DSC thermogram of the black precipitates .......................................................................... 89 3 10 TEM and HR TEM images of PbSe nanocubes .................................................................. 90 3 1 1 TEM image of CuSe nanocrystals ........................................................................................ 90 3 12 Characterization of Pd4.5Se nanocrystals ............................................................................. 91 4 1 Scheme of three -step synthesis of Mn doped CdS/ZnS core/shell nanocrystals ............ 102 4 2 Characterization of Mn -doped CdS/ZnS nanocrystals ..................................................... 102 4 3 XRD pattern and ED pat tern of Mn -doped CdS/ZnS nanocrystals ................................. 103 4 4 PL of Mn doped CdS/ZnS core/shell nanocrystals ........................................................... 104 4 5 PL spectra of Mn -doped Cd S/ZnS core/shell nanocrystals with different CdS nanocrystals sizes ................................................................................................................ 105 4 6 Position -controlled Mn -doped CdS/ZnS nanocrystals ..................................................... 106 4 7 EPR and ICP data of Mn -doped CdS/ZnS core/shell nanocrystals ................................. 108 4 8 Mn -growth yield measurements ......................................................................................... 110 4 9 Kinetic stud y of Mn adsorption. .......................................................................................... 113 4 10 Proposed mechanism for the formation of the weakly and strongly bound Mn. ............. 115 4 11 Study of Mn repl acement. .................................................................................................... 117 4 12 Study of the replaced Mn species ....................................................................................... 119 5 1 Scheme of energy levels and carrier relaxation pathways in a Mn -dope d nanocrystal .. 129 5 2 PL spectra of Mn -doped CdS/ZnS core/shell nanocrystals excited with three excitation -light intensities .................................................................................................... 131 5 3 Plots of QYMn and EPR linewidth as a function of Mn radial position ............................ 133 5 4 Plots of QYMn, QYBEETMn Mn NR as a function of Mn radial position ............ 135 5 5 Steady -state photoluminescence of Mn -doped CdS/ZnS nanocrystals ............................ 137 5 6 Intensity decay of BE and Mn emissions as a function of time ........................................ 137

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14 5 7 Mn emission intensity decay of CdS/ZnS core/shell nanocrystals under different laser fluences ........................................................................................................................ 138 5 8 Excitation intensity -dependent PL property of Mn -doped CdS/ZnS nanocrystals ........ 139 5 9 IMn as a function of laser fluence for the CdS/ZnS nan ocrystals doped with different doping levels ......................................................................................................................... 140 5 10 Color tunable property of Mn -doped CdS/ZnS nanocrystals ............................................ 141 6 1 TEM images of DPPH nanoparticles ................................................................................. 148 6 2 An electron diffraction (ED) pattern of DPPH nanoparticles .......................................... 1 48 6 3 Absorption spectrum of 250-n m DPPH nanoparticles ...................................................... 149 6 4 Absorption spectra of different sizes of DPPH nanoparticles .......................................... 150 6 5 The peak position of DPPH nanoparticles plotted as a function of their diameter s ....... 150 6 6 An EPR spectrum of 250-nm DPPH nanoparticles .......................................................... 151 6 7 EPR spectra o f DPPH nanoparticles, DPPH/DPPH H core/shell nanoparticles and DPPH H doped DPPH nanoparticles ................................................................................. 152 6 8 TEM images of DPPH nanoparticles, DPPH/DPPH H core/shell nanoparticles and DPPH H doped DPPH nanoparticles ................................................................................. 153 6 9 The absorption spectra of DPPH nanoparticles, DPPH/DPPH H core/shell nanoparticles and DPPH H doped DPPH nanoparticles ................................................... 154 6 10 Stability test of DPPH nanoparticles .................................................................................. 154 6 11 The integrated intensity of EPR absorption and g-factor of DPPH nanoparticles as a function of pH ..................................................................................................................... 155

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15 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 DOPED AND UNDOPED NANOPARTICLES: SYNTHESIS AND CHAR ACTERIZATION By Ou Chen May 2010 Chair: Y. Charles Cao Major: Chemistry Nanocrystal s are very small particles, which contain from a few hundred to thousands of atoms d epending on the size of nanocrystal Because of their unique properties compared wit h the bulk materials, nanocrystal s have found many promising applications in areas such as biological labeling catalysis, light -emitting diodes (LED), solar cells, and spintronics This dissertation presents studies o n the mechanism of nanocrystal growth the synthesis of high quality metal -selenide nanocrystal s using SeO2 as the selenium precursor, the synthesis and properties of radial position-controlled doping of CdS/ZnS core/shell nanocrystal s, and the synthesis of water -soluble DPPH (2,2 -diphenyl 1 -picrylhydrazyl) nanoparticles as a standard electron paramagnetic resonance (EPR) label First, t he growth stage of nanocrystal s has been studied. It has been found that the number of nanocrystals decrease s during the nanocrystal growth stage. Mechanistic studies show that only the solvent and crystal structure can affect the amount of decomposed nanocrystals. A model was proposed relating the nanocrystal growth patterns to stacking faults. The XRD measurements and computer XRD -simulation results show that fewer decomposed nanocrystals are associated with fewer stacking faults in the final nanocrystals, and vice versa. These results strongly support our model.

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16 Second, a non injection synthesis (NIS) method for the synthesis of high-quality metal selenide nan ocrystals (e.g., CdSe, PbSe, PdSe, CuSe, etc.) with SeO2 as the selenium precursor has been developed. Mechanistic studies show that octadecene (ODE) acts as a reducing agent for SeO2 in this synthesis. Moreover, this synthesis exhibits controllable kineti cs in both the nucleation and growth stages, and thus allows detailed control of the numbers of nuclei and the final size s of the resulting nanocrystals. Importantly, this synthesis can be conducted in air eliminat ing the need for air -free manipulations us ing a glove box or a Schlenk line. Third, a three -step synthesis doping method has been developed. This new approach allows precise control of the Mn radial position and doping level in the CdS/ZnS core/shell nanocrystals. Based on this new synthesis, the detailed mechanism of this doping process has been studied. N anocrystals doping is determined by the chemical kinetics of three activationcontrolled processes: dopant adsorption, replacement, and ZnS -shell growth We also studied the photoluminescence (PL ) properies of our Mn doped nanocrystals in two excitation intensity regimes: weak excitation regime and strong excitation regime U nder weak excitation, the efficiency of the emission from one Mn ion ( Mn) exhibits a radial -position-dependent change that nearly perfectly corresponds to that of the Mn EPR linewidth of the nanocrystals: the higher the Mn, the narrower the Mn EPR linewidth. Under strong excitation, these Mn -doped nanocrystals exhibit an excitationintensity -depend e nt, color tunable dual emis sion property. Fourth, a size -controlled synthesis of water soluble DPPH (1,1 -diphenyl 2 picrylhydrazyl) nanoparticles has been developed Importantly, t hese nanoparticles exhibit size -dependent absorption spectra and fast -exchange -narrowed single line EPR spectra with linewidths of ~1.51.8 G T hey are also stable over a wide pH range (from p H 3.0 to p H 10.0) Furthermore, the

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17 EPR linewidth can be controlled by partially reducing the DPPH radical. Th ese water -soluble DPPH nanoparticle s are a perfect standa rd EPR label s for biological and biomedical systems

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18 CHAPTER 1 INTRODUCTION Nanostructured materials research has been recognized as one of the most exciting areas in modern science Over the l ast three decades, the synthesis of nanocrystals crystalline particles ranging in size from a few nanometer s to one hundred nanometers has been of great interest not only for fundamental research but also for many novel technological applications.1 Nanocrystals possess very interesting size -dependent optical, electr ical, magnetic, physical and chemical properties which cannot be achieved by their bulk counterparts.1 To fully exploit the potential of nanocrystals, the synthesis of monodisperse nanocrystals is very important for many future applications of nanocrystal s because of their strong size depend e nt properties. Since t his research focuses on the synthesis and doping of semiconductor nanocrystals this introduction will focus on the advances in the synthesis of different types of nanoparticles T he introduction provides a brief description of semiconductor nanocrystals including the quantum confinement effect and general synthetic strategies ; synthesis of core/shell structure semiconductor nanocrystals ; doping of semiconductor nanocrystals ; and a short summary of the present research work. 1.1 Semicondutor Nanocrystals 1.1.1 Quantum Size Effect S emiconductor nanocrystals so -called quantum dots (QDs) have generated a great deal of interest in the past two decades because of their involvement in both fundament al research and technical applications .116 An exciton is a bound state of an electron and an imaginary particle called a hole in a semiconductor and is often called a n electron hole pair In the nanometer r ange the crystal size i s smaller than the Bohr radius of the exciton, which is given by

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19 2 22**11 () 4exciton ehh R emm (1 1) where represents the dielectric coefficient, e is the elementary charge, 346.62610h JS is P lanck s constant, me and mh are the effective masses of the electron and hole, respectively. T he band gap of the semiconductor increases and at the edges of the valence and conduction bands discrete energy levels occur. This phenomenon the so -called quantum size effect ,13 cannot be explained by classical th eor y.17 Table 1 1 Material parameters and properties of selected bulk semiconductors.18 Group Material Band gap (eV) m e m h S tructure (at 300K) Lattice Parameter () II VI CdS CdSe CdTe ZnS ZnSe ZnTe 2.53 1.74 1.50 3.61 2.58 2.28 0.20 0.13 0.11 0.39 0.17 0.15 0.90 0.80 0.35 Wu rtzite Wurtzite Zinc blende Zinc blende Zinc blende Zinc blende a: 4.136 c: 6.714 a: 4.299 c: 7.010 6.477 5.406 5.667 6.101 III V GaP GaAs GaSb InP InAs InSb 2.25 1.43 0.69 1.28 0.36 0.17 0.13 0.07 0.045 0.07 0.28 0.013 0.67 0.5 0.39 0.40 0.33 0.18 Z inc blende Zinc blende Zinc blende Zinc blende Zinc blende Zinc blende 5.450 5.653 6.095 5.869 6.058 6.479 IV VI PbS PbSe PbTe 0.37 0.26 0.29 0.1 0.07 (ml) 0.039 (mt) 0.24 (ml) 0.02 (mt) 0.1 0.06 (ml) 0.03 (mt) 0.3 (ml) 0.02 (mt) Rocksalt Rocksalt Ro cksalt 5.936 6.124 6.460 O n the basis of th e effective mass approximation and particle in a box model, the band gap shift with respect to the bulk material value can be approximately calculated by 22 22**111.8 () 8eh he E RmmR 1 2

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20 where R is the radius of the spherical semiconductor nanocrystal. Equation 1 2 is an analytical approximation for the first electronic transition of an exciton, which can be described by a hydrogenic Hamiltonian, 222 22 2*2* 88eh e hehhhe H mmrr 1 3 T he total band gap of the semiconductor nanocrystal could be calculated by Equation 1 4, ngEEE 22 22**111.8 () 8g ehhe E RmmR 1 4 Here the first term Eg, is the band gap of a bulk semiconductor (Table 1 1) with the same composition as the nanocrystal, and it is a constant for a certain material. The second term is the quantum localization term which shifts the first excited electronic state to higher energy by decreasing the radius of the nanoparticle The third term is the Coulombic interaction term, which is a small term but can be added as a first order energy correction. E quation 1 4 has been experimentally confir med for a wide range of semiconductor nanocrystals.1,1 9,20 Although this calculation is not valid for all kinds of semiconductors 17, this model does provide a useful qualitative understanding of quantum size effects that observed in semiconductor nanocrys tals. These quantum size effects cause semiconductor nanocrystals to differ dramatically from the corresponding molecules and bulk materials and the optical properties of QDs are strongly size dependent.13 As a result of these effects, the band gap of th e semiconductor increases as the nanocrystal decreas es in size leading to tunable blue shifts in both optical absorption and emission spectra. The most popular example of this effect is the continuous absorption and fluorescent emission of CdSe nanocrysta ls (Figure 1 1). By changing the diameter of CdSe

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21 nanocrystals from 2.3 nm to 5.5 nm, the energy gap of this material varies from 2.7 eV to 1.9 eV and covers almost the entire visible region of the optical spectrum .21 Figure 1 1. The color of CdSe nanocrystals depends on nanocrystal size. (a) Photo image of CdSe nanocrystal s under UV irradiation; (b) absorption spectra (left) and emission spectra (right) of CdSe nanocrystal s. This phenomenon creates the possibility of tuning the band gap of the semiconductor nanocrystals to satisfy specific needs such as biological labeling and electroluminescent device.8,9, 22, 23 Because of t heir novel properties and flexible chemical processibility, semiconductor nanocrystals might be used as biological fluorescent labels and l ight emitting diodes as well as in lasers solar cells and other optical and electroni c devices.6 16 (a) (b)

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22 1.1 .2 General Synthetic Strategies In general, there are two different approaches for synthesizing nanocrystals. The f irst approach, called the top -down approach, utilizes physical methods. Top -down physical processes such as metal evaporation, ball milling, and electrodeposition24,25 can produce large quantities of nanocrystals with high purity. However, the synthesis of monodispers e uniform nanocrystals with a contolled particle size is very difficult with these physical methods. Second approach, the collo idal chemical synthetic method, is called the bottom up approach. This method is based on solution-phase colloida l chemistry. Based on the reaction media, this solution -phase synthetic method can be classified as either aqueous -phase synthesis or organic phase synthesis. 1.1.2.1 Synthesis of semiconductor nanocrystals in aqueous solution Aqueous -based techniques are always considered to be safe and environmentally friendly techniques because water is a natural media for all lifes. Historically, the first successfull synthesis of colloi dal semiconductor nanocrystals wa s in aqueous solution. Initially, nanocrystals were formed in homogenous aqueous solution containing appropriate reagents and surfactant type or polymer type stabilizers .26 27 The charged stabilizers can bind to the surfaces of the nanocrystals and stabilize the particles by steric hinderance and/or electr ostatic repulsion. Besides this monophase synthesis, a bi phase technique has also been developed This method is based on the arrested precipitation of nanocrystals within inverse micelles.28 30 In this technique, nanometer -sized water droplets (dispersed phase) are stabilized in an organic (continuous phase) solvent by an amphiphilic surfactant. The nanocrystals are generated inside the water droplets (nanoreactor) whose dimensions determine the size of the nanocrystals. At the same time, because these w ater droplets are dispersed in a continuous phase, they can prevent the newly formed nanocrystals from aggregation. CdS nanocrystals were first obtaind by this strategy.28 30

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23 Early in stabilizer development, colloidal silica sol30,31 and short chelating pe ptides32 were used. Later Weller and coworkers pioneered the use of short -chain water soluble thiols such as 1 thioglycerol, 2 -mercaptoethanol, 1,2 -dimercapto 3 propanol, thioglycolic acid and cysteamine as stabilizing agents.33 The use of these short -c hain thiols as capping agents has led to successful aqueous syntheses group II -VI semiconductor nanocrystals like CdS33, CdSe34, CdTe35 ,3 6, HgTe37, and ZnSe.38 A queous -based synthesis normally is simple and highly reproducible. T he reaction temperature is r elatively low ( 100 oC) compare d to high temperature organic -solution synthesis (> 200 oC). Moreover, especially for nanocrystals intended for bioapplications, the particles produced in aqueous media are naturally hydrophilic and therefore more likely to be biocompatible. In addition, the aqueous approach is the only existing reliable method reported today for the synthesis of mercury chalcogenide nanocrystals.37,39,40 However, nanocrystals synthesized by the aqueous approach do not possess the degree of c rystallinity of the nanocrystals prepared in organic solvents Aqueous -phase synthesis usually results in particles having more defects and poor surface passivation which lead to a rather low exciton photoluminescence quantum yield (PL QY), sometimes even dominated by defect -related emission.35,38,4143 Furthermore, the size distribution of these nanocrystals is relatively broad, which is at least on the order of 15%. Therefore, the post -preparative approaches need to be applied to obtain a narrow size dist ribution. Currently, t he most important and widely used technique is size -selective precipitation from solvent -non-solvent mixtures. This technique is based on the solubility differences of small and large particles and was firstly introduced for CdS and C dSe nanocrystals.4 44 This size -selection process is very laborious and tedious.

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24 Rapid nucleation Time Concentration I II III Growth by diffusion or reaction Solubility Critical limiting supersaturation C s C min C max 1.1.2.2 Synthesis of semiconductor nanocrystals in organic solution Recently, g reat progress has been made in o rganic -phase synthesis for colloidal semiconductor nanocrystals .45 Numerous monodisperse, size and shape controllable semiconductor nanocrystals have been synthesized in organic solution .5,46 Since most of the present research has utilized organic -phase syntheses, this method will be described in more detail. Organic -phase nanocrystal synthesis involves three important steps: nucleation, growth, and passivation. LaMer and coworkers extensively studied nucleation and growth in sulfur sols, from which they developed an understanding of the mechanism for the formation of colloids or nanocrystals from a homogeneous, supersaturated medium.47 Their results showed that the synthesis of monodisperse colloids via homogeneous nucleation requires a temporal separation of nucleation and growth. 47 Figure 1 2. LaM er plot : Change of the degree of supersaturation as a function of reaction time. Stage I, no nucleation or growth occurs; Stage II, rapid nucleation occurs and this is followed by particle growth; Stage III growth of nuclei

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25 In nanocrystals synthesis process, nuclei appear in a homogeneous solution without any seed. Because the system must change from a homogeneous phase to the heterogeneous phase, there must exist a high energy barrier for the nucleation. The LaMer plot, as shown in Figure 1 2, is well k nown and very helpful to illustrate how the burst nucleation occurs in the formation of nanocrystals. In a typical synthesis, initially, the monomer, the minimum subunit of the crystals, is generated from the precursor. The concentration of monomer const antly increases with time as shown in Figure 1 2, stage I. At this stage, no nucleation occurs even under supersaturated conditions (C > Cs), due to the extremely high energy barrier for spontaneous homogeneous nucleatio n. At stage II, w hen the degree of supersaturation is high enough to overcome the energy barrier (C > C* Min), nucleation occurs, thus resulting in formation and accumulation of stable nuclei. Then, t he nuclei start to grow and consume the monomers. When the rate of monomer consumption induc ed by both the nucleation and growth processes equals the rate of monomer supply, the monomer concentration reaches the highest point (C = C* max). Subsequently, the concentration of monomer start s to de crease until it falls below C* Min, where the net nucleation rate is zero and stage II ends It is essential that the time range of stage II be short in order to have an effectiv e separa tion of nucleation from growth. When C < C* Min, the system enters the pure growth stage (stage III), in which no further nuc leation occurs and the existing particles grow as long as the solution is in the supersaturated regime The growth rate is controlled by both diffusion and reaction kinetics. Finally, the nanocrystals are passivated with organic ligands which prevent them from agglomeration and fusing. These organic ligands also make the particles soluble in certain solvents. It is possible that crystal growth in solution could involve a second distinct growth process, which is refered to as Ostwald ripening.48,49 In Ostwal d

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26 ripening the smaller nanocrystals dissolve to free monomers that are incorporated onto larger nanocrystals This process decreases the total concentration of the nanocrystals, and usually broadens the size distribution to the order of 1520%.50 Therefor e, for preparation of monodisperse nanoparticles, the reaction should be terminated before Ostwald ripening happens In this process, all the nuclei are generated with in a short period ( nucleation time period is short ), and then start to grow without fur ther nucleation. In this case, they should have n e arly identical growth histories. This is the essence of the separation of nucleation and growth process which makes the cont rol of the size distribution of ensembled nanocrystals possible. Otherwise, if n ucleation process occurred throughout the particle growth process, the growth history of each particle would be differ ent, and the control of the size distribution would be very difficult. Experimentally, the first successful synthesis of monodisperse semi conductor nanocrystals was reported by Murray and Bawendi in 1993.4 The separation of nucleation and growth was achieved by rapid injection of a cold TOP ( tri -n -octylphosphine) solution containing the organometallic reagents (dimethyl cadmium) and chalcoge nide precursors into hot trioctylphosphine oxide (TOPO) solution .4 This method is so called hot -injection method. Nuclei of CdE (E = S, Se, Te) formed immediately after injection, and nucleation was quenched by the rapid cooling of the reaction solution and by the decreased supersaturation after the nucleation burst. Th is was followed by growth at lower temperature without further nucleation. After size selection process, monodisperse CdSe nanocrystals with a narrow size distribution of 5% were produced b y this hot injection method .4 However, dimethyl cadmium is highly toxic, expensive, and unstable at room temperature and it can be explosive at high temperature. For these reasons, Peng et al. have proposed the use of greener and safer cadmium reagents s uch as

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27 cadmium oxide,51 cadmiu m carbonate, or cadmium acetate52,53 instead of the pyrophoric dimethylcadmium. They also suggested that the size distribution of the nanocrystals could be improved by the use of hexadecylamine, a long-chain phosphonic acid or a carboxylic acid as the surfactants Without any post -size sorting, the typical nanocrystals from these synthese s possess a size distribution of about 5%. At the same time, the non -coordinating solvent 1 octadecene (ODE) was introduced in the synthesis t o replace the coordinating solvent (e.g., TOPO) .54 ODE has a low melting point and high boiling point, and it is inexpensive and less toxic. More importantly, controlling the reactivity of the precursors by changing the concentration of the coordinating li gands became possible by using this non-coordinating solvent.54 In this hot -injection method, the nucleation time is determined by the rate of the injection and the mass transfer in the reaction system. Thus, injection based syntheses have poor reproducibi lity and are not suitable for large -scale industrial preparation. Consequently, a heating up method also called non injection synthesis (NIS) or one -pot synthesis method, which does not require the injection of precursors needs to be developed. Af ter experiencing some low quality nanocrystal syntheses ,55,56 in recent years, the Caos group reported a noninjection synthesis method which is based on the concept of controlling the thermodynamics and kinetics in the nanocrystal nucleation stage .57,58 In this method, the precursors are mixed with solvent at room temperature and then gradually heated to the reaction temperature. By fine control of the reactivities of the precursors, the nucleation and growth can be automatically separated in a homogeneou s reaction solution and the size and shape of the resulting particles can be controlled The quality of these nanocrystals (e.g., CdS, CdSe and CdTe) synthesized using this NIS method is at least as good as that of the best particles made by the injection based method.57,58

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28 Based upon these synthetic developments, a variety of high -quality colloidal semiconductor nanocrystals which are from groups II -VI (e.g., Cd S Cd Se and CdTe ), IV -VI (e.g., PbS and PbSe), to III -V (e.g., InAs and GaP) have been synthes ized in organic solution with uniform size and shape, high crystallinity, and well surface passivation. Reports of syntheses of monodisperse semiconductor nanocrystals are summarized in Table 1 2 Table 1 2 Precursors, surfactants and solvents used in the synthesis of various semiconductor nanocrystals Group Material Precursors S urfactant S olvent R eference II VI CdS, CdSe,CdTe CdSe, CdTe CdSe CdS CdSe CdSe CdSe CdS e CdS CdSe,CdTe CdTe CdTe CdS, ZnS CdS, ZnS ZnS ZnS ZnSe ZnTe Cd(Me) 2 /TOP, (TMS)2Se, (TMS)2S, (BDMS)2Te CdO, Se/TOP, Te/TOP CdO, Cd(Ac)2, CdCO3 Se/TOP CdO, S/ODE, Se/TBP Cd(Me)2/TOP, Se/TOP CdO, Se/ODE Cd(St)2, Se/TBP Cd(Ac)2, S Cd(My)2, CdO, Se, Te/TBP Cd(Me)2, Te/TOP CdO, Te/TBP, Te/TOP Cd(Hdx)2, Cd(Ex)2, Cd(Dx)2, Zn(Hdx)2 CdCl2, ZnCl2, S/OAm Zn(St)2, S/ODE Zn(Et)2, S/ODE Zn(Et)2, Se/TOP Zn(Et)2, Te/TOP TOP TOPO TDPA TDPA, SA, LA OLA HDA,TOPO OLA SA, D D A MA OLA, ODPA DDA OLA HDA TOPO,OAm SA HDA HDA ODA TOPO TOPO TOPO ODE TOPO ODE ODE ODE ODE DDA ODE HDA OAm ODE ODE HDA ODE [ 4 ] [51] [52] [54] [59, 60] [61] [62] [57] [58] [63] [64, 65] [66, 67] [68] [69] [70] [71] [72] III V InP InP, InAs InP InAs GaP GaP GaN, AlN, InN InCl 3 (TMS) 3 P In(Ac)3, (TMS)3P, (TMS)3As In(Me)3, (TMS)3P InCl3, (TMS)3As [Cl2GaP(SiMe3)2]2 GaCl3, (TMS)3P [M(H2NCONH2)6]Cl3 (M=Ga, Al, In) TOPO MA MA TOP TOPO,TOP TOPO TOA TOPO ODE MM, DBS TOP TOPO TOPO TOA [73,74] [75] [76] [77] [78] [79] [80]

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29 Table 1 2. Continued Group Material Precursors Surfactant Solvent Reference IV VI PbSe PbSe PbS e PbS PbS PbTe PbTe Pb(Ac) 2 Se/TOP PbO, Se/TOP Pb(Chbt)2, Se/TBP PbO, (TMS)2S PbCl2, S/OAm Pb(Ac)2, Te/TOP PbO, Te/TOP OLA OLA TOPO OLA OAm OLA OLA DPE ODE TOPO ODE OAm DPE ODE [81] [82] [83] [84] [68] [85] [86] Me methyl; TMS trimethylsilyl; BDMS bis(te rt -butyldimethylsilyl); TDPA tetradecylphosphonic acid; ODPA octadecylphosphonic acid; SA stearic acid; LA lauric acid; OLA oleic acid; MA myristic acid; Ac acetate; My myristate; St stearate; Hdx hexadecylxanthate; Ex ethylxanthate; Dx decylxanthate ; Chbt cyclohexylbutyrate; MM methyl myristate; DBS dibutyl sebacate; TOPO trioctylphosphine oxide; TOP trioctylphosphine; TBP tributylphosphine; HDA hexadecylamine; OAm oleylamine; DDA dodecylamine; ODA octadecylamine; TOA trioctylamine; ODE 1 octadecene ; DPE d iphenylether. 1. 2 Core/ Shell Structure Semiconductor Nanocrystals After s uccess fully improving the syntheses and properties of semiconductor nanocrystals, many efforts shift ed from these simple composition nanocrystals to more complex nanocrystals. One of the most developed class of hetero -material nanocrystal s is the so -called core/shell semiconductor nanocrystal s (core/shell QD s ). Core/shell semiconductor nanocrystals are the semiconductor nanocrystals that are epitaxially overcoated by another semiconduc tor shell material. This inorganic outer layer shell p rov ides an efficient passivation for the bare core and a physical barrier between the core and the surrounding medium. The growth of the shell layer should be uniform in order to make the core/shell na nocrystals monodisperse. A reasonable lattice mismatch (less than 15%) is very important for epitaxial growth O therwise, there will be either no growth or a huge number of defects will form at the interface between core and shell materials mak ing the she ll ineffective.87 Typically, in organic solution, the core/shell nanocrystals are synthesized by a two -step procedure: first, the synthesis of initial core nanocrystals, foll owed by a purification process; second, the appropriate

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30 shell precursors are slowl y introduced into a hot dispersion of the purified core nanocrystals.88,89 The shell growth can be achieved by the reaction of all the necessary precursors or by alternating deposition of each atomic species of shell material. The shell thickness can be co ntrolled by the amount of shell precursor add ed .89 In many cases a few monolayers of shell materials can be deposited on the cores. The most direct characterization technique for proving successful shell growth is considered to be transmission electron mi croscopy (TEM) and highresolution TEM. Some other techniques, such as UV -Vis and photoluminescence (PL) spectroscopy, powder X ray diffraction (XRD), energy -dispersive X -ray spectroscopy (EDX) and X -ray photoelectron spectroscopy (XPS) etc, can also provi de some indirect information about the shell overcoating.8892 Figure 1 3. (a) Electronic energy levels of selected semiconductor materials. (VB: Valence band, CB: conduction band) (b) Scheme of the energy level alignment in different core/shell semicon ductor nanocrystals. The upper and lower edges of the rectangles correspond to the positions of the conduction band and valence band of the core (center) and shell materials, respectively. (Reprinted with permission from ref 93)

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31 Based on the relative posit ions of valence band and conduction band of the semiconductor materials, the core/shell QDs can be classified into three different types: type I, type II and reversed type I.94 Figure 1 3 shows an overview of the band alignment of the semiconductor bulk ma terials used in nanocrystals synthesis and the scheme of the energy -level alignment in different types of core/shell semiconductor nanocrystals. 1. 2 .1 Type -I Core/Shell Nanocrystals In t he type I core/shell structure the valence band (conduction band) of the shell semiconductor is lower (higher) than that of the core semiconductor (e.g., CdSe/CdS, CdSe/ZnS and CdS/ZnS) as shown in Figure 1 3b. This type of core/shell nanocrystal confines the electron and the hole predominantly in the core region.59,88,89, 95 Consequently, such core/shell nanocrystals typically exhibit much higher photoluminescence quantum yield s (PL QY), and are more stable both chemically and physically, compared to the corresponding plain -core nanocrystals.88 There fore the type I system is the most intensively studied system in core/shell nanocrystals. In terms of PL QY, t he first high quality type I core/shell nanocrystals were CdSe/ZnS nanocrystals synthesized by Hines and Guyot Sionnest ,96 who reported making CdSe/ZnS nanocrystals wit h room temperature PL QY of 50%. Shortly after, Dabbousi and coworkers described the synthesis and characterization of a series of high PL QY (30%50%) CdSe/ZnS core/shell nanocrystals with narrow band-edge luminescence from 470nm to 625nm.97 Typically, du ring the ZnS shell growth, a small red shift (5~10 nm) in both absorption and PL peaks was observed attributed to partial leakage of the e x citon into the shell material ( Figure 1 4c ).59, 97 Based on these advance s other modified synthesis methods for maki ng CdSe/ZnS nanocrystals have been reported to improve the monodispersity PL QY and stability of the core/shell nanocrytals, as well as coverage of the whole visible region of the spectum.59,9 8100 It is

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32 important that these CdSe/ZnS core/shell nanocrysta ls have been used for practical applications such as fluorescent biological labels.9,22, 101 Figure 1 4 TEM images of (a) CdSe nanocrystals (b) CdSe nanocrystals covered with 1.6 monolayers of ZnS shell. (c) Room -temperature absorption and emission spec tra of CdSe nanocrystals before and after deposition of ZnS shells of different thickness (ML: monolayers). (Reprinted with permission from ref 59) Other than the ZnS, CdS and ZnSe are two other common shell materials for CdSe core nanocrystals. CdSe/CdS a nd CdSe/ZnSe both have band alignment s different from that of the CdSe/ZnS system. In CdSe/CdS case, the band offset is large in the valence band and small in the conduction band (Figure 1 3a), result ing in confinement of the holes in the core, while the e lectrons are delocalized in the whole particles This phenomenon was observed experimentally by continuous shifting of the absorption and PL peaks throughout the shell growth.88,89 In contrast to CdSe/CdS, the CdSe/ZnSe system exhibits a strong confinement for the electrons and a relatively weak confinement for the holes due to the larger conduction band offset and smaller valence band offset as shown in Figure 1 3a. However, in this case, no large red shift was

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33 observed in either the absorption spectrum o r the PL spectrum.95 This is probably because of lower energy and mobility of the holes compared to the electrons. Both CdSe/CdS and CdSe/ZnSe nanocrystals possess a relatively small lattice mismatch (CdSe/CdS: 3.9%, CdSe/ZnSe: 6.3%) compare d to that of CdSe/ZnS (10.6%). Furthermore, they also contain either a common cation (Cd2+) or anion (Se2-). These properties make these two systems more favorable for eptaxial shell growth.94 After modifying the synthesis method, many high quality CdSe/CdS a nd CdSe/ZnS e core/shell nanocrystals have been synthesized in recent years with the room temperature PL QY as high as ~80%.95 ,102 With the goal of covering an even larger spectral region, some types of core semiconductor nanocrystals other than CdSe have been employe d For example, to obtain the emission wavelength in the range of blue and near -UV, la r ger bandgap material s such as CdS, ZnSe have been used as cores ,7, 103 105 and emission wavelength s in the 400nm range have been achieved.104 On the other hand, CdTe, InP InAs and PbSe have been exp l ored to push the emission into the near -infrared (near IR) region.6,1 06118 It is noteworthy that InP/ZnS core/shell nanocrystal s are efficient emitter s covering the blue to near -IR regions, and are considered as a greener ca dmium free system to replace CdSe -based nanocrystals.110 1. 2 .2 Type -II Core/Shell Nanocrystals As shown in Figure 1 3b, type -II core/shell QD s ha ve both the valence band and conduction band in the core lower (or higher) than those in the shell. As a resu lt, one carrier (electron or hole) is mostly confined to the core, while the other is predominantly confined to the shell. This spatial separation of electron and hole gives rise to some novel properties that are different from these of the type -I structur e QDs.119 First, type II QDs can emit wavelenghts which can not be achieved by either their co re or shell isolated counterparts For example, CdS/ZnSe can give orange and red colour emission,120 and CdTe/CdSe nanocrystals can emit at wavelength s greater

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34 th an 1000nm ,119 providing a new route to obtain near IR emitters. In addition, the thickness of the shell and the core size also play a role in determining the effective band gap through quantum confinement effects. Thus by changing the core size and shell thickness, the emission behavior of these type II QDs can be tuned.119 122 Moreover this sepa ration can also slow the exciton recombination process by reducing the wavefunction overlap between electron and hole T his creates that one of the photocarriers from the nanocrystal may be injected into another matrix before recombination happens.119 121 122 This novel property makes these type II nanocrystals more suitable for photovoltaic or photoconduction applications. T here are many combinations of semicondu ctor materials can be used for synthesizing different type -II core/shell nanocrysta l s To date, many systems have been investigated including CdTe/CdSe, CdSe/ZnTe, ZnTe/CdS, ZnTe/CdSe, ZnTe/CdTe and CdS/ZnSe.119124 In general the PL QYs of these type II core/shell nanocrystals are much lower than th ose of type I structure .119 124 These low QYs have been labeled as an intrinsic limitation of type II QDs,119 since the slower radiative recombination of indirect excitons can facilitate the dominance of nonra diative recombination at defects. Therefore, t o improve the photoluminescence QYs of type -II QDs minimization of both surface and interfacial defects needs to be paid more attention.121 1. 2 .3 Reverse Type -I Core/Shell Nanocrystals In the r everse type I core/shell structure the valence band (conduction band) of the shell semiconductor is higher (lower) than that of the core semiconductor As shown in Figure 1 3b this constitutes an inver sion of the type I structure The electrons and holes are delocalized in the shell and generally a significant red -shift of the bandgap with increasing the shell thickness is observed.(Figure 1 5)125,126 The absorption and emission properties of this structure strongly

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35 depend on the shell thickness but not the core size, indicat ing quantum confinement only occurs along the radial direction.125 Thus, this is also called the quantum shell structure .125 Figure 1 5. TEM images of plain ZnSe core nanocrystals and the corresponding ZnSe/CdSe core/shell nanocrystals with di fferent shell thickness es but identical cores: (a) 2.8nm Zn Se core nanocrystals; (b ) -(e ) ZnSe/CdSe nanocrystals with 1,2,4 and 6 ML of CdSe shell, respectively (thickness of 1 ML CdSe = 0.7 nm in diameter); (f) HR TEM of ZnSe/CdSe nanocrystals with 6 ML of CdSe shell. Normalized PL (g) and corresponding absorption (h) spectra of plain ZnSe core nanocrystals (a ) and ZnSe/CdSe core/shell nanocrystals with different numbers of monolayers of CdSe shell: b, 0.1; c, 0.2; d, 0.5; e, 1; f, 2; g, 4; h, 6. (Reprinte d with permission from ref 126) Although, in principle any material combination for type -I structure can be employed for synthesizing reverse type I core/shell nanocrystals, only a few have been studied including CdS/HgS, CdS/CdSe ZnS/CdS and ZnSe/CdSe.125130

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36 1. 3 Doping of Semiconductor Nanocrystals Besides core/shell nanocrystals, doped semiconductor nanocrystals (doped QDs) constitute another important class of semiconductor nanocrystals which may have an even brighter future in the field of nanocrsy tal -based applications. Doping refers to the intentional introduc tion of impurities (dopants) into a pure semiconductor. It is well known that the incorporation of impurities into semiconductor lattice affect s the optical, magnetic, electrical conductivity and other physica l properties of semiconductors.131134 For instance doping of a semiconductor with m agnetic impurities produces a d ilute m agnetic s emiconductors (DMS) one of the exciting developments in semiconductor science and technology.135 The m agn etic impurity can generate a huge Zeeman splitting over two orders of magnitude la rger than the splitting of normal semicondutors giving rise to potential applications in optical gating and spin -based electronics (i.e., spintronics).132 F urthermore, d op ing with conventional impurities allows control of the number of carriers (electrons and holes) in semiconductors. For example, an impurity with one fewer valence electron than the host atom can provide its extra hole to the semiconductor resulting in p type doping. I n contrast, an impurity with one more valence electron can donate an electron to give n type doping. These types of doping have built the foundation for p n junction -based semiconductor devices, such as computer chips.136 S t imulated by the success of doping in bulk materials, doping of semiconductor nanocrystals has attracted considerable attention in recent years. Doped QDs are likely to display properties which have not been observed in either doped bulk material s or undoped QDs because of the strong confinement of electron and hole in the latter .137 The promise of nanocrystals for applications including wavelength tunable lasers, bioimaging and solar cells may ultimately depend on tailoring their behavior through doping.138

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37 1.3.1 Difficu lty of Synthesis of Doped Semiconductor Nanocrystals Unforunately, the introduction of impurities is much more difficult in nanometer range particles than in bulk materials For example, although Mn atoms have very high solubility (tens of percent) in bulk II -VI semiconductors it is not eas ily incorporated into CdS and CdSe nanocrystals using the standard conditions at either low or high temperatures.139141 In many cases, the impurity atoms exist only at the surface of the nanocrystals but not ins ide.139141 In a nother study several synthetic techniques at various temperatures varying from room temperature to as high as 800oC were used to embed lanthanides ( Eu, Tb and Er) into nanocrystalline CdS and ZnS.143 Although lan thanide emissions were observed, excitation spectra showed that there was no energy transfer from host nanocrystals to the lanthanide ions. Based on these data, authers concluded that none of the synthesis methods employed was able to introduce lanthanides into II -VI semiconductor n ano crystals but instead all the lanthanides were bounded to the particle surface.143 This incompatibility was initially attributed to several reasons such as large surface to volume ratio of nanocrystals, inherent inhomogeneities of the surface and inner core impurity segregation, self annealing of the host particle and charge and size differences between the impurities and the host material .135,139 142,143 The effects of the impurity on the nanocrystals properties are minimized when the dopant is at t he surface of the nanocrystals and not inside the core. 1.3.2 Synthetic Development To overcome these problems and encourage dopant incorporation inside the nanocrsytal two successful methods have been explored In the first method, a single -source precursor having both the impurity cation and the host anion was introduced as the impurity source. Since the impurity and host are initially bonded, it is easier to embed impurities inside the nanocrystals than when a typical dopant precursor containing the i mpurity alone is used For example, in one

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38 of the earliest studies, two types of Mn doped CdSe nanocrystals were produced by the hot injection method using a Mn only precursor ( e.g., Mn(CO)5Me ) or Mn -Se single precursor (e.g., Mn2 -SeMe)2(CO)8).139 Electr on paramagnetic resonance (EPR) spectroscopy was used to moniter the location of Mn impurit ies and pyridine exchange treatment was carried out to remove the surface Mn. The EPR measurements showed that only the sample made using the Mn -Se single precursor showed a six -line Mn hyperfine splitting pattern after pyridine treatment.(Figure 1 6) Although both the large hyperfine splitting constant ( ~ 83 G ) and the etching experiments showed that the Mn impuriti es were located near the nanocrystal surface, dopants were indeed inside the nanocrystals and not loosely b ound to the surface.139 On the basis of this advance, some other doped nanocrystals have been successfully prepared using modified single -source precursor method s .144 147 Figure 1 6. Low temperatur e (5K) EPR spectra of 4.0 nm diameter CdSe n anocrystals using a Mn -only precursor (a, b) and Mn2 SeMe)2(CO)8 precursor (c, d). Before pyridine exchange (a, c), both samples display the 6 line Mn hyperfine splitti ng pattern. After pyridine exchange (b, d) only the sample prepared with Mn2 SeMe)2(CO)8 precursor show the Mn signal. (Reprinted with permission from ref 139)

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39 The second method called the isocrystalline core/shell growth method was developed by Gamelin et al .143 This procedure involves the isolation and purification of nanocrystals prepared with dopants at the surface, followed by isoc rystalline growth of additional layers of the pure host material to incorporate surface impurities inside the nanocrystal Co -doped CdS and ZnS were initially synthesized using this method.143 Ligand -field electronic absorption spectroscopy was used as a d irect and sensitive probe of Co dopant inside CdS and ZnS nanocrystals. It was verified that surface -bound Co dopants could be converted to internal dopants by overgrowth of additional shell layers (Figure 1 7). This approach ha s been generalized to synthe size some other doped nanocrystals such as Co -doped ZnO Mn -doped ZnS and Cu -doped ZnSe nanocrystals.148150 Beside s using these pure nanocrystals, this core/shell growth method has also been applied to heterocrystalline systems.149, 151155 Figure 1 7. (a) Absorption spectra (300K) of 3.0 nm diameter 2.3% Co-doped CdS QDs in pyridine colloid solution, showing the CdS band gap transitions (left panels) and Co2+ -field transitions (right panels) The solid line was collected 2 hours and dashed l ine 23 hours after synthesis. (b) Absorption spectra (300K) of 3.7 nm diameter 0.9% Co doped CdS QDs prepared by the isocrystalline core/shell method 2 (solid line) and 28 hours (dashed line) after synthesis. (Reprinted with permission from ref 143)

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40 One im portant advantage of this core/shell growth method is the enhancement of photoluminescence especially when the dopant acts as an emitter. For example, the growth of ZnS shell around Mn -doped CdS nanocrystals led to an approximately 9 -fold increase in Mn (4T1-6A1) PL QY compare to the QY before shell growth.151,152 Also by shell growth, the PL QY of Mn -doped ZnSe nanocrystals can reach as high as ~50%.149,155 This improvement in PL QY makes these doped QDs more adaptable to applications such as nanocrystal -b ased biomedical sensors and electroluminescence devices.8,156,157 1.3.3 T heoretical Model Besides the progress in synthetic methodologies, t wo important models have been proposed to explain the effects of doping of semiconductor nanocrystals.158161 Norris et al. proposed a Mn doping model called trapped dopant, in which dopant incorporation is governed by dopant -growth kinetics (Figure 1 8a) .158 T hey suggested that the surface morphology, nano crystal shape and surfactant are three major factors that affe ct doping.158 D ensity f unctional theory (DFT ) calculation results show that Mn impurities are more easily incorporated in nanocrystals with the zinc -blende structure by adsorption on (001) facets In addition, they suggest ed that surfactant s must not bind too strongly to the nanocrystal surface or the impurity .158 T his model can quantitative ly predict the dopant -growth yield (or doping efficiency) for a colloidal synthesis.158 For example, for Mn -doped ZnSe nanocrystal s Mn growth yield is predicted to be i n the 0 and 30% range which is consistent with the observation that Mn growth yield is often 10% in doping synthesis.137, 158,162 However, th is model cannot explain the Mn -growth yield of 40% in Mn -dop ed wurtzite ZnO nanocrystals.163 Chelikowsky et al. suggested a doping -growth model in which self -purification is an intrinsic property of defects in semiconductor nanocrystals (Figure 1 8b) .159,160 They argu ed that

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41 doping is determined by the formation energy of impurities inside semiconductor nanocrystals. DFT calculation s show that the formation energy of defects in nanocrystals increases as the n anocrystal size decreases. In this model, the size of nanocrystals is important for incorporating dopants into the nanocrystal lattice. This model is based on an assumption that Mn dopant incorporation is under a thermodynamic equilibrium. However, such a thermodynamic equilibrium is normally not established, because the diffusion of Mn is negligible at typical growth temperatures which lower than 350 C.138,158 Figure 1 8 (a) A scheme of trapped dopant model, in which kinetic fators govern the doping p rocess. (b) A sch eme of self purification model, in which dopant solubility is lower in the nanocrystals than in the bulk semiconductor.138 Both theoretical doping models have their own limitation s and they are inconsistent with each other. In addition, al l the experimental results were obtained from a conventional one -pot doping synthesis in which dopant precursors and host growth precursors were mixed together to produc e doped nanocrystals.137,138 In this type of synthesis, t he dopant adsorption process a nd host lattice growth occur concurrently in the synthesis. Thus it is extremely difficult to obtain quantitative kinetic data on the dopant adsorption process, which is important for establishing a

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42 general doping model. To fully understand the dopant growth mechanism and establish a general doping model more mechanistic studies need to be carried out with better controllable synthetic methodologies. 1.4 Summary of the P resent R esearch T his research was untaken to study the growth and doping mechanism s of nanoparticles and to synthesize different types of nanomaterial s with interesting and promising properties. In C hapter 2 the studies of growth stage of CdSe nanocrystal s are presented It has been found that the number of nanocrystal s decreases during t he nanocrystal growth stage. Mechanistic studies show that only the solvent and crystal structure can affect the amount of decomposed nanocrystals. A model based on stacking faults associate d with growth is proposed. Chapter 3 describes a non -injection sy nthesis (NIS) method for the preparation of highquality metal -selenide nanocrystals with SeO2 as the selenium precursor. Mechanistic studies show that octadecene (ODE) acts as a reducing agent for SeO2 in this synthesis. Importantly, this synthesis can be conducted in air, and eliminates the need for air -free manipulations using a glove box or a Schlenk line. Chapter 4 describes a three-step synthesis doping method. This new approach allows precise control of the Mn radial position and doping level in the CdS/ZnS core/shell nanocrystals. Based on this new synthesis, we have studied the detailed mechanism of this doping process. In C hapter 5, studies of the photoluminescence (PL) properies of the Mn doped nanocrystals are reported for two excitation intensi ty regimes: weak and strong. U nder weak excitation, the efficiency of the emission from one Mn ion ( Mn) exhibits a radial -position-dependent change that nearly perfectly corresponds to that of the Mn EPR linewidth of the nanocrystals: the higher

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43 the Mn, the narrower the Mn EPR linewidth. Under strong excitation, these Mn-doped nanocrystals exhibit an excitationintensity -depend ent, color tunable dual -emission property. Chapter 6 describes a size -controlled synthesis of water soluble DPPH (1,1 -diphenyl 2 p icrylhydrazyl) nanoparticles T hese nanoparticles exhibit size dependent absorption spectra and fast -exchange -narrowed single line EPR spectra with linewidths of ~1.5 1.8 G. They are stable over a wide pH range (from p H 3.0 to p H 10.0) Furthermore, the EP R linewidth can be controlled by partially reducing the DPPH radical. Chapter 7 presents conclusion s of the current research and suggesions for future studies

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44 CHAPTER 2 MECHANISM STUDY OF SEMICONDUCTOR NANOCR YSTAL SYNTHESIS: PAR TICLE NUMBER DE CREASE S IN NANOCRYSTAL GROWT H STAGE 2 .1 Introduction Considerable effort ha s been devoted to optim ize the synthesis of semiconductor nanocrystal s because of many promising applications of these materials A variety of highquality colloidal semiconductor nanocrystals from groups II -VI (e.g., Cd S Cd Se and CdTe ), IV VI (e.g., PbS and PbSe), to III -V (e.g., InAs and GaP) have been synthesized with uniform size, shape, crystallinity, and surface passivation.( 4, 515 4, 57 86) Thus far, cadmium chalcogenide nanocr ystals are the most developed system in terms of synthesis. In particular, CdSe nanocrystals have been utilized as a model system, n ot only for developing synthetic methods but also for studying the mechanism of nanocrystals nucleation and growth.( 4, 51 54, 5661, 164 168) Peng and colleagues have carried out a systematic investigation of the extinction coefficient an d sizing curve of II-VI ( CdSe, CdS, and CdTe ) nanocrystals and have correlated the optical absorption spect rum with the size of the nanocrystal and the ir concentration s in solution.166 The resul ts are helpful in quantitative investigations of the dependence of nanocr ytsal nucleation and growth on particle size and nuclei number.( 57,58, 164,165,169 ) D espite th is progress s in the synthesis of semico nductor nanocrystals, there are still numerous unanswered fundamental questions. For instance, during the synthesis process, calculation s have show n that the number of nanocrystals in the reaction solution decreases after the nucleation stage.58 This occur ence can not be explained by Ostwald ripening theory i n which the larger particles grow at the expense of the smaller particles. Ostwald ripening occurs at low precursor concentration s subsequent ly leading s ize defocusing of the nanocrystals.3 However, high precursor concentration and size fo cusing are concurrent with the calculated number decrease. For nanoparticles, does this number truly decrease or is it attributed to uncertainties of

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45 the sizing and coefficient curve s ? If so, what factors affect this nuc lei number decrease and why does it happen ? To answer these questions, in this chapter, a same material core/shell growth method was designed to study the mechanism of nanocrystal growth. F irst this number decrease during nanocrystals growth was confirmed Different experimental parameters were investigated, but only the solvent and crystal structure can affect the amount of decomposed nanocrystals. A growth model based on stacking faults was proposed to explain these phenomena The XRD measurements and com puter simulation results show ed that a smaller amount of decomposed nanocrystals are associated with fewer average stacking faults in the undecomposed nanocrystals, and vice versa. These experimental results strongly support our model. 2 .2 Experimental Sec tion 2 .2.1 Chemicals 1 octadecene (ODE, 90%), trioctylphosphine o xide (TOPO, 99%), tributylphosphine (TBP, 97%), trioctylphosphine (TOP, 90%), and oleylamine (OAm, 70%) were purchased from Sigma Alrich. Oleic acid (OLA, 90%) was obtained from Av o cado Cadm ium o xide (CdO, 99.98%), c admium nitrate tetrahydrate ( Cd(NO3)24H2O, 99.99%) myristic acid (MA, 99%) and s elenium powder (Se, 99.9 99%) w ere purchased from Alfa Aesar. Dimethylcadmium (97 % ) was purchased from Strem. Nitric acid (HNO3, >69.5%) and perchlo ric acid (HClO4) were obtained from Fluka. All chemicals were used directly without any further purification. Sodium hydroxide (NaOH) and a ll the other solvents were purchased from Fisher Scientific International Inc. 2 .2.2 Nano crystal Synthesis 2 .2.2.1 Sy nthesis of w urtzite CdSe (W -CdSe) nanocrystals CdSe nanocrystals were synthesized based on a modified literature method.3 Briefly, CdO powder (0. 0 256 g, 0. 2 mmol) and stearic acid ( 0. 2280 g, 0. 8 mmol) were mixed in a 25 -m L

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46 three neck flask. The mixture was heated to 240oC under argon flow and kept at that temperature until a colorless solution was formed. After the solution was cooled to room temperature, octadecylamine ( 1.5 g) and trioctylphosphine oxide ( 0.5 g) were added to the reaction flask. The react i on system was re heated to 280oC under argon flow, and a Se solution containing 0. 158 g (2 mmol) of Se dissolved in 0. 474 g ( 2.36 mmol ) of TBP and 1. 37 g of ODE was swiftly injected into the system. The temperature was naturally dropped to 25 0oC and mainta ined at that point. The particle size was m onitored by UV -Vis spectroscopy and the reaction was stopped when the nanoparticles reached a desired size. Typically, CdSe nanocrystal with the size smaller than 4.0nm can be generated by this procedure. For making larger particles, a decreas e the amount of i njected and an increas e in both ODA and TOPO to 2 g may be need. The resulting nanoparticles were extracted from the reaction solution using a hexane/methanol (1:1) mixture, and the extraction was performed t hree more times. The purified W CdSe nanoparticles were dispersed in hexane as a stock solution. 2 .2.2.2 Synthesis of z inc blende CdSe (ZB -CdSe) nanocrystals ZB CdSe nanocrystals w ere prepared according to a modified literature method .58 In general, seleni um powder (0.05 mmol, 100 mesh from Aldr ich) and cadmium myristate (0.1 mmol) were added to a three -neck flask with ODE (5.0 g). The mixture was degassed for 10 min under vacuum at room temperature. Under Ar flow, the solution was stirred and heated to 240oC at a rate of 25Kmin1. The time was counted as zero when the temperature reached 240oC. After the 3 minutes growth, 0.1 mL oleic acid was added dropwise into the reaction solution to stabilize the growth of the nanocrystals. The reaction was monitored by UV-Vis spectroscopy and was stopped by removing the heat when the nanoparticles reached a desired size. The resulting particles were precipitated by adding acetone, and then redispersed in hexane. T he particles were further purified by precip itation redi spersion three more times. The purified ZB -

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47 CdSe nanoparticles were dispersed in hexane as a stock solution. 2.2.3 Particle Number Decrease Study 2.2.3 .1 Precursor preparation (1) Cd -Oleate solution CdO powder (25.7 mg 0.2 mmol ) was added into a flask wi th OLA (0.57 mL 1.6 mmol ) and ODE (0.43 mL). After degassing at room temperature for 10 min and 60 oC for 10 min, the mixture was heated to 120 oC and kept for 10 min to dissolve CdO. After CdO was totally dissolved, 4 mL ODE was injected into the flask t o make the final precursor concentration 0.04 M. The final colorless solution was cool ed to room temperature for use as a stock solution (2) Se /TBP solution Se powder (0.0158 g, 0.2 mmol) was dissolved in TBP (0.643g, 3.2mmol), then diluted by ODE to tot al volume of 5mL with a Se concentration of 0.04M. The final solution was protected by Ar and kept for use as a stock solution for use 2.2. 3 2 CdSe/CdSe core/shell nanocrystal growth In a typical experiment, a hexane solution of CdSe nano particles ( e.g. 52.510 m mol) was added ito a mixture solution of ODE and OAm ( e.g. 6 mL, ODE/OAm: 1 :1), and then hexane was removed under vacuum. Under Ar flow, the nanocrystal solution was heated to a growth temperature ( e.g. 240 C), and Cd -oleate so lution and Se/TPB were introduced into the solution by dropwise addition. Growth time was 10 min after each addition. Serial quantitative aliquots were taken after each growth. Aliquots were measured by UV -Vis spectroscopy for calculati on of the diameter and number of the particles. The resulting Cd S e/Cd S e core/shell nanocrystals were precipitated by adding acetone, redispersed in hexane and further purified by precipitation -redispersion three more times 2.2.4 Inductively -Coupled Plasma Atomic Emission Spectroscopy (ICP) Measurements The ICP measurements were performed on a Vista RL CCD Simultaneous ICP -AES

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48 (Varian, Inc.). The purified nanocrystal samples were further purifie d by precipitation redispersion using acetone/hexane three more times. A nd then a solution of 1 mL purified nanocrystal sample with known absorbance was carefully dried by gentle heating and digested with nitric acid (69.5%). The digestion was performed at about 100oC until the solution became colorless. For digestion of the crude reaction solution, a combination of nitric acid (69.5%) and perchloric acid was used with molar ratio of 1:1. The digestion temperature was about 200 oC. The digest ed solutions were further diluted with a nitric acid solution to obtain a fina l nitric acid concentration of about 1 2%. The concentrations of Cd in solutions were determined by data from ICP measurements as compared with the corresponding working curve. 2.2. 5 Photoluminescence Measurements Photoluminescence (PL) experiments were p erformed on a fluorometer (Fluorolog3, Horiba Jobin Yvon, Irvine, CA). Nanocrystals were dissolved in toluene for the measurement. 2.2. 6 TEM Measurements TEM measurements were performed on a JEOL 200X operated at 200 kV, or a JEOL 2010F TEM operated at 20 was dropped onto a 200 mesh copper grid, and was dried overnight at ambient conditions. 2.2. 7 XRD Measurements XRD measurements were performed on a Philips XRD 3720 spectrometer. Th e specimens were prepared as follows: about 15 mg of the purified nanocrystals w ere dissolved in about 0.5 mL of toluene and then dropped onto a low -scattering quartz sample holde r, dried in air and kept overnight in a vacuum dess icator 2.2.8 Determin at ion of the Extinction Coefficient of CdSe Nanocrystals The cadmium concentration was determined via the ICP measurement. The concentration of the particle solution was obtained by dividing the Cd concentration by the number of Cd atoms

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49 in one dot And the number of Cd atoms in one dot is calculated from the density of bulk CdSe material, the molecular weight and the average diameter of the particle measured by TEM. Finally, the extinction coefficient per mole of CdSe nanocrystals at the first exciton absorption peak was calculated from the Lambert Beer law 2 .3 Results and Discussion 2 .3.1 Confirmation of Particle Number Decrease During Growth The concentration ( or number) of nanocrystals in solution can be determined by Lambert Beers law. ACL (2 1 ) Where, A is the absorbance at the peak position of the first exciton absorption peak for a given sample; is the extinction coefficient p er mole of nanocrystals (L/mol cm); C is the molar concentration (mol/L) of the nanocrystals of the same sample; and L is the path length (cm) of the radiation beam used for recording the absorption spectrum. In our experiments, L is fixed at 1cm and A is obtained from absorption spectrum. Particle concentration is obtained by dividing absorbance by extinction coefficient. Yu and his colleagues have studied this extinction coefficient per mole of nanocrystals at the first excitoni c absorption peak.166 They gave the sizing curve and the extinction coefficient curve of the high-quality spherically -shaped CdSe nanocrystals,166 which are shown in Figure 2 1.

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50 Figure 2 1. Sizing curve (a) and size -dependent extinction coefficient curv e (b) of CdSe spherical nano crystals. Based on the sizing curve the average diameter of the CdSe nanocrystals in solution can be easily calculated by measuring the peak position of the first absorption peak. Then, the extinction coefficient of that sample can be extracted from the extinction coefficient curve. Thus, the particle concentration ( or number) can be easily calculated by Equation 2 1. To eliminate any possible error s which may be caused by these two curves in particle number calculation, the accuracy of these two curves was checked using our high -qu a lity spherical CdSe nanocrystals. 2 .3.1.1 Sizing curve To check the sizing curve (Figure 2 1a), t en CdSe nanocrystals samples with the first exciton absorption peak at 524nm, 543nm, 563nm, 570nm, 584n m, 593nm, 604nm, 614nm, 618nm, and 635nm were synthesized Figure 2 2 shows that all these samples with narrow size distributions (~5%) fit the sizing curve very well. So this sizing curve will be used for later calculation. The fitting function of sizing curve is provided by :166 94 63 321.6122102.6575101.6242100.427741.57 D (2 2 ) where D (nm) is the diameter of a given nanocrystal sample, first excitonic absorption peak of the corresponding sample.

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51 Figure 2 2. CdSe nanocrystal sizes as a function of first exciton absorption peak. Red line is the sizing curve for CdSe nanocrystals shown in Figure 2 1. 2.3.1.2 Extinction coefficient curve Thirty samples with size range from 2.2nm to 7.0nm were used to check the extinction coefficient per mole of CdSe nanocrystals at the first excitonic absorption peak. The result (Figure 2 3 ) indicates that the extinction coefficients from our measurements are larger than the values calculated from the extinction coefficient curve given by Pengs group (th e blue line in Figure 2 3 ), especially when the particle sizes are smaller than 6nm. One possible reason could be the different purification process es Typically, Pengs group purified their samples by a double extraction and a single precipitation .166 In contrast, triple extractions and three cycles of precipitation/redispersion were performed in our purification process. The nanocrystals with some unreacted Cd precursors in the solution will increase the particle concentration in calculation resulting in a decreas e in the extinction coefficient. In order to maintain consistency, our extinction coefficient curve (red line in Figure 23) was used for CdSe nanocrystal number calculations with the fitting function: 2.0518595(D) (2 3) where, D (nm) is the average diameter of a given sample.

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52 Figure 2 3. Size for CdSe nanocrystals sizes as a function of first exciton absorption peak. Red line is the fitting curve, green line is the extinction coefficient curve shown in figure 2 -1 2.3.1.3 CdSe/CdSe core/shell growth Typical CdSe/CdSe core/shell growth reaction s w ere carried out. The reaction was monitored by measuring the UV-Vis and photoluminescence ( PL ) spectra of the aliquots taken from the reaction solution. The absorption and PL spectra are shown in Figure 2 4. Figure 2 4. (a) The evolution of the absorption (a) and PL (b) spectra of typical CdSe/CdSe core/shell nanocrystal growth.

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53 The high -quality UV -Vis absorption spectra with multiple exciton peaks and narrow p hotoluminescence (PL) peaks indicate that the quality of CdSe nanocrystals generated by the core/shell growth method is at least comparable to that of the CdSe nanocrystals produced by the typical synthesis method. This conclusion is confirmed by the TEM measurement shown in Figure 2 5 F igure 2 5. TEM image of CdSe nanocrystals synthesized by the CdSe/CdSe core/shell growth method. The average diameter of particles is 6.1nm with a relative standard deviation of ~5%. To further demonstrate that the CdSe nanocrys tals synthesized by this growth method are identical with the CdSe nanocrystals synthesized by typical method, the sizes and extinction coefficients of six grown CdSe nanocrystal samples with first absorption peak at 577nm, 585nm, 599nm, 610nm, 619nm and 626nm were determined by TEM and ICP measurements. Figure 2 6 demonstrates that the sizes and extinction coefficients of these grown CdSe nanocrystals fit the sizing curve and our extinction coefficient curve very well. On the basis of all of these results, the number evolution of the CdSe nanocrystals during the particle growth is shown in Figure 2 7. Dilution effects and peak width calibration were considered during these number calculations.166 The results in Figure 2 7 indicate that, when the CdSe nanoc rytstals grew from 3.5nm to 6 .1 nm, 47% of the original number of CdSe nanocrystals was lost. The interesting question is:

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54 Where did this material go? If the nanocrystals decomposed and returned to the ionic or molecular forms (e.g. Cd2+, Se2-, CdSe), then the reaction solution should contain the amount of Cd and Se atoms not in the form of the remaining CdSe nanocrystals. To test this hypothesis and quantitate the amount of Cd not in particles, four parallel reactions were performed. F igure 2 6 Diameters (D) and of CdSe nanocrystal s synthesized by CdSe/CdSe core/shell growth method. F igure 2 7 CdSe nanocrystal amount as a function of diameter during the p article growth. Starting from the same amount ( 52.510 mmol ) of CdSe cores (3.5nm in diameter) the reactions were terminated at four different final particle sizes (3.8nm, 4.1nm, 4.6nm and 5.6nm in diameter). Then, each of these crude reaction solutions was separated to two portions. One was purified by purificat ion process described above, and the purified particles were redissolved

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55 in hexane to make the solution with exactly the same first peak absorbance as that of the crude reaction solution. A 1mL sample of each purified (CdSe only as nanoparticles) and unpur ified (all forms of Cd&Se) solution was digested and the numbers of Cd were obtained by ICP measurements. The amount of Cd not in nanoparticles in 1mL of the crude reaction solution was obtained by subtracting the Cd amount in the purified sample from that of unpurified one. So, t he total amount of Cd not in particles can be easily calculated by multiplying the amount in 1mL crude solution by the final volume of the reaction solution. Figure 2 8 Amount of Cd not in nanoparticles as a function of CdSe n anocrystals diameter: the red dots are the amounts obtained from ICP measurements; the blue squares are the amounts calculated based on Equation 2 4 ICP measurements show that the re are indeed some non -particle -formed Cd atoms in all these four reaction s olutions (red dots in Figure 2 8) In addition, this amount keeps increasing when the particle s grow larger (Figure 2 8) consistent with the decrease in CdSe nanocrystals left in the reaction solution Furthermore, the amount of Cd in the starting core Cd Se nanocrystals ( CdI ni) is known, and the total amount of Cd precursor injected into the reaction solution (CdP re) is also known. The average diameter and amount of the resulting CdSe nanocrystals can be obtained from the sizing curve (Figure 2 2) and exti nction coefficient curve (Figure 2 3). Thus, the amount of Cd in the final CdSe nanocrystals ( CdFin) can be obtained by

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56 the number of Cd atom s per dot times the number particle s Therefore, this amount Cd not in nanoparticles (CdNon) in the final reaction solution is described by: Fin pre IniNonCd Cd Cd Cd (2 4) The calculat ed results (blue squares in Figure 2 8) are nearly identical to those measured from ICP ( red dots in Figure 2 8 ). This result further confirms the accuracy of our extinction coefficient and sizing curve s and support s our conclusion that the particle number decreas e s during particle growth. Another interesting question is that whether the nonparticulate Cd and Se left in the reaction solution are still active, mean ing that they are still active precursors and can grow onto the particles. To answer this question another experiment was conducted. Figure 2 9. (a) E volution of the PL spectrum during the reaction. (b) A mount of larger CdSe nanocryst als as a function of reaction time ( with zero corresponding to the first precursor addition time ). L ine (I) and line (II) show the amount of larger particle s and corresponding PL spectra obtained from the aliquot s withdrawn 10min af ter the last precursors addition and 1min after the injection of the smaller particle s A 52.510 mmol sample of 3.5nm CdSe nanocrystals was grown to 5.5nm. The nanocrystal growth was terminated by ceasing to add the precursors. Then, 52.510 mmol of 2.4nm in CdSe nanocrystals dissolved in 0.5mL ODE and 0.5mL OAm were quickly injected

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57 into the reaction solution and a liquots were taken to monitor the reaction. Figure 2 9 a shows that after injected the smaller CdSe nanocrystals, the PL peak of smaller particles red shifted from 512nm to 580nm, whereas, the PL peak of larger nanocrystals had almost no change. This result indicates that the original larger CdSe nanocrystals did not grow after precursors were no longer added H owever, the later in jected smaller particles quickly grew even though no further precursors were introduced into the system This experiment again proves that copious amount s of precursors accumulated in the reaction system during the nanocrystal growth Moreover, it clearly answered the question that these accumulated precurso rs are active and can still grow onto the CdSe nanocrystals Since the absorbance of the lat er injected samller particles overlapped that of the exsting larger particles in the reaction solution, it wa s not easy to calculate the number of the smaller particles. However, the first absorption peak of the larger particles was not affected by the smaller particles, so the number calculation for larger particles was performed. Calculation results showed that particle number decreased about 40% when the original CdSe nanocrystals grew from 3.5nm to 5.5nm in diameter (Figure 2 9b), which is consistent with our previous conclusion. Then the number remained nearly constant which is consistent with the observation that the larger CdSe nanocrystals did not grow 2 .3.2 Mechanistic Study The decrease in nanocrystal number ha s been confirmed in section 2.3.1. This phenomenon cannot be simply explained by the Ostwald ripening theory, b ecause Ostwald ripening happens onl y under the condition of very low precursor concentration and subsequently leads to size defocusing of nanocrystals. In contrast, high precursor concentration and size focusing are concurrent with the number decrease in our case So, what are the possible reason (s) caus ing these nanocrystals to decompose and what parameter(s) can affect the amount of particle

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58 decrease during nanocrystal growth ? To answer these questions, different experiments have been carried out 2 .3.2 .1 Concentration e ffect First, to te st the effect of shell precursor (Cd -oleate and Se/TBP) concentration, all other experimental parameters were held constant at th ose for typical CdSe/CdSe core/shell growth, and three different concentrations of shell precursors (0.02M, 0.04M, 0.2M) were u sed Figure 2 10a show s that there is no noticeable effect on particle number decrease by changing the concentrations of growth shell precursors Figure 2 10. (a) Particle amount s as a function of diameter during the particle growth with different shel l precursor concentrations (red circle: 0.02M, blue triangle: 0.04M, green diamond: 0.2M). (b) Normalized particle amounts as a function of diameter during particle growth with different core concentrations (red dot : 610 8 0 mM, blue triang le: 610 0 4 m M, green diamond: 610 0 8 m M). Next, t he concentration of shell precursors was held constant as 0.04M and three d ifferent concentrations of starting core ( 610 8 0 mM, 610 0 4 m M, 610 0 8 m M ) w ere studied. The results show that th e core concentration can not affect the amount of decomposed CdSe nanocrystals during growth (Figure 2 10b) These two series experiments indicated that in the high precursor concentration reg ion, the precursor per particle amount is not the determi ning parameter for this rapid decompos ition of nanocrystals.

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59 2 .3.2.2 Temperature effect Figure 2 11. Particle amounts as a function of diameter during the particle growth with different growth tem perature (red dot : 200 oC, blue square : 2 4 0 oC). The effect of t emperature was also examined. Because when the temperature is lower than 200oC, the activity of shell precursors is insufficient for core/shell growth, result ing in no growth reaction. On the other hand, when the temperature is higher than 240 oC, for example 260 oC or above, it is difficult to prevent Ostwald ripening of the core nanocrystals. So, the reactions were run under two different temperatures (200 oC and 240 oC), and results are show n in Figure 2 11. It is safe to conclude that, within this temperature range (between 200 oC and 240 oC), different temperatures will not affect this number decrease. 2 .3.2.3 P recursor effect Cadmium oleate (Cd oleate), cadmium myristate (Cd -myristate) and dimethyl cadmium (dimethyl Cd) were used as cationic precursors, respectively. These three precursors gave almost identical amount s of nanocrystal loss (shown in Figure 2 12). This result illustrates that the number decrease is not related to the type of the precursors. In addition, this result also proved that particle decomposition is not due to the excess amount of oleic acid in the cadmium oleate precursor.

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60 Figure 2 12. Particle amount as a function of diameter during particle growth with different cadmium precursors (red dot : Cd oleate, blue triangle: Cd -myristate, green diamond: dimethyl Cd ). 2 .3.2.4 Solvent effect In a typical reaction, a mixture of ODE/OAm with a volume ratio of 1:1 was used as the solvent. Initially d ifferent ratios of ODE and OAm varying from pure ODE to pure OAm were investigated Because ODE is noncoordinating solvent, there were not enough ligands to stabilize the particle s in the solution and a ll the nanocrystals precipitated out during the growth in pure ODE. Other result s are shown in Figure 2 13a In addition, d ifferent kinds of solvents were also screened. However, in most cases, the solvents could not provide a suitable environment for nanocrystal growth. They either caused Ostwald ripening of the core nanocrystal (trioctylphosphine Oxide ( TOPO 90%) or oleic acid (OLA) ), or led core particles to grow to some irregular shapes (trioctylphosphine ( TOP )). Pure TOPO (99%) is another good solvent for nanocrystals growth. The numbers of particles grown in different solvents ( ODE/OAm and TOPO) are shown in Figure 2 1 3b. Figure 2 13a shows that different percentages of OAm in the reaction solution still has no effect on this particle number decrease, but the particle grown in TOPO did show a difference (Figure 2 13b). The identi cal 3.5nm CdSe nanocrystals were grown to 5.0nm. In ODE/OAm (1:1

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61 volume ratio) solution, 35% of the starting cores were lost, while only 22% of the starting cores decomposed in TOPO solution. Figure 2 13. (a) Particle amounts as a function of diameter d uring particle growth with different ODE/OAm ratios (red circle: 1:1, blue triangle: 10:1, green diamond: pure OAm). (b) Particle amounts as a function of diameter during particle growth with different solvents (red dot : TOPO, blue square: ODE/OAm with a v olume ratio 1:1) 2 .3.2.5 Stacking faults (defects) associate d with the nanocrystal growth model It is well known that most II -VI semiconductors nanocrystals (e.g., CdSe, CdS) have two stable crystal structures which are the w urtzite (W) structure and z inc -b lende (ZB) structure.58 Based on the close -packing model, these two structures have different stacking sequences. The w urtzite structure can be described as an ABABAB packing sequence ( Figure 2 1 4 a), while the z inc Blende structure corresponds to an ABCABCABC packing sequence (Figure 2 1 4 b). Figure 2 14. (a) Wurtzite crystal structure with a stacking sequence of ABABAB... (b) Zinc blende crystal structure with a stacking sequence of ABCABC...

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62 It has been proven that, during the nanocrystal growth, th e stacking seq uence is not restricted to the wurtzite structure ( ABABAB ) or zinc -blende structure ( ABCABC ), which means stacking fault s (defect s ) (e.g., ABABCB ) will be present ,(4, 170) mak ing the nanocrystal less stable. When nanocrystals grow larger, a particle could involve more than one stacking fault.(4, 6, 170 ) These stacking defect s could account for the particle decomposing during growth. Here, we propose a model to explain this number decrease phenomenon. In our model, each type of nanocrystal has a series of critical number of stacking faults corresponding to different size ranges. When nanocrystal grows to certain size range, if the number of stacking faults in the nanocrystal exceeds its critical number in that size range, it will quickly and totally decompose ; otherwise, it will survive and continue growing. If this model is correct, growth resulting in fewer decomposed nanocrysta ls (e.g. growth in 99% TOPO) should be associated with fewer average stacking faults in the final nanocrystals In co ntrast, growth resulting in more decomposed nanocrystals (e.g. growth in ODE/OAm) should be associated with more average stacking faults in the final product. To verify this hypothesis, two X -ray powder diffraction (XRD) samples were synthesized by growin g the same batch of W CdSe nanocrystals (3.2nm in diameter) to 5.0nm in diameter. One was synthesized in ODE/OAm ( volume ratio : 1:1 ), while the other was synthesized in TOPO (99%) All the other conditions were identical. The reactions were monitored by UV -Vis and P L spectroscopy and t he evolutions of sizes and size distributions were confirmed by TEM measurements.

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63 Figure 2 1 5 Evolution of the absorption (a) and PL (b) spectra of W CdSe nanocrystals gr o w n in ODE/OAm (volume ratio of 1:1) solu tion. (c) Histogram of size and size distribution of the CdSe nanocrystals during the growth. Figure 2 1 6 Evolution of the absorption (a) and PL (b) spectra of W CdSe nanocrystals gro w n in TOPO (99%) solution. (c) Histogram of size and size distribution of the CdSe nanocrystals during the growth. High quality absorption and PL spectra show that particles keep size focusing through the entire growth stage in these two solvent systems N either secondary nucleation nor size broadening w as observed during t he growth (Figure 2 1 5 a,b, Figure 2 1 6 a,b) These results are

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64 consistent with the evolutions of size and size distribution measured by TEM. The histograms show th at the average diameters of particles shift from 3.2nm to 5.0nm with a nearly constant relativ e standard deviation of ~6% through the entire growth stage (Figure 2 1 5 c, Figure 2 1 6 c) However, the decomposed particle amount again shows a dramatic differen ce in these two system s (Figure 2 1 7 ). In the ODE/OAm system, about 40% startin g cores decompos ed, while in TOPO system, only 25% starting cores decomposed. Figure 2 1 7 W CdSe nanocrystals (NC) amounts as a function of diameter during particle growth with different solvents (blue square: ODE/OAm with a volume ratio of 1:1 red dot : TOPO, 99% ) TE M measurements show that the two final CdSe nanocrystals (synthesized in ODE/OAm and in TOPO) have nearly identical size of 5.0nm diameter with the same relative standard deviation of ~6% (Figure 2 18a,c). This result is consistent with the UV -Vis measurem ents (Figure 2 15a, Figure 2 16a). The first exicton peaks of these two samples are narrow (HMHW ~13.5nm) and at the same position of 610nm. The XRD patterns of both resulting CdSe nanocrystals clearly show (102) and (103) diffractions at 35. 1o and 45.8o (Figure 2 18b,d). These two peaks indicate that both samples have the wurtzite crystal structure which is because of (1) wurtzite crystal structure of the starting core particles and (2) epitaxial growth. However, the

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65 (102) and (103) peaks of the sample s y nthesized in TOPO solution are more pronounce d than those of the sample made in ODE/OAm solution. S tronger (102) and (103) peaks indicate a more perfect wurtzite crystal structure with fewer average stacking faults.4 Figure 2 1 8 (a) TEM images of the final CdSe nanocrystals grown in the ODE/OAm system. (b) XRD pattern of CdSe nanocrystals measured from the sample shown in panel (a). (c) TEM images of the final CdSe nanocrystals grown in the TOPO system. (d) XRD pattern of CdSe nanocrystals measured f rom the sample shown in panel (c). The stick patterns in panel s (b) and (d) show the positions of standard XRD peaks for bulk W -CdSe. To quantita te the average number of stacking faults inside the resulting nanocrystals, computer simulation processes were carried out. The nanocrystals were built by the stacking plane along the (002) axis of the wurtzite crystal structure. Five n m diameter spherical ly shaped CdSe nanocrystals and bulk lattice constants were applied for these XRD simulations. The simulation r esults are shown in Figure 2 1 9 and the parameters applied in the simulatio n are listed in T able 2 1.

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66 Figure 2 1 9 Experimental XRD patterns (blue lines) compared with computer simulations (red lines) of the W CdSe nanocrystals grown in (a) ODE/OAm an d (b) TOPO. Table 2 1. Parameters used in XRD simulations for wurtzite crystal structures of CdSe nanoparticles. System Lattice parameter () shape Diameter (nm) Stacking sequences along (002) # of stacking faults ODE/OAm (1:1) a = 4.299 c = 7.010 Sph ere 5.0 ABABA CBCBCBC AB CBC ABAB CBCB CACACA BAB 3.0 TOPO (99%) a = 4.299 c = 7.010 Sphere 5.0 ABAB CBCBCBCBC ABAB ABABA CBCBC AB CBCBC 2.2 r epresents the location of a stacking fault The simulation spectra fit the experimental data almost perfectly (Figure 2 1 9 ). Different number s of stacking faults were put during the simulation. For ODE/OAm system, 3 stacking faults per particle were used, while only 2.2 stacking faults on average were put for TOPO system. Because of the close agreement with t he experimental data, t hese simulations confirm that the particles which are gro w n in TOPO system indeed possess fewer stacking faults than the particles gr o w n in ODE/OAm system. The XRD and simulation results combine d with the numb er calculation (Figure 2 1 7 ) prove our hypothesis that fewer decomposed nanocrystals are associated with fewer average stacking faults in the final nanocrystals, and vice versa Taken together, a ll these results support our stacking faults associated growth model

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67 To further t est this model, zinc -blende structured CdSe nanocrystals (3.2nm diameter) were produced by the one -pot synthesis method and purified.58 As in the wurtzite case, two growth reactions of growing ZB CdSe nanocrystals from 3.2nm t o 5.0nm in diameter were con du cted in ODE/OAm ( volume ratio : 1:1 ) and the other in TOPO (99%) system. All the other conditions were identical. Figure 2 20. Evolution of the absorption (a) and PL (b) spectra of the ZB CdSe nanocrystals grown in ODE/OAm (volume ratio of 1:1) solution. (c) Histogram of size and size distribution of the CdSe nanocrystals during growth. High quality absorption and PL spectra again indicate that the nanoparticle sizes are focused through the entire growth stage in these two systems. Neither secondary nucl eation nor size broadening was observed during the growth process (Figure 2 20a, b and Figure 221a, b). These results are consistent with the evolutions of sizes and size distributions measured by TEM. The histograms show that the average diameters of par ticles shift from 3.2nm to 5.0nm with a nearly constant relative standard deviation of ~6% through the entire growth stage (Figure 2 20c, Figure 2 21c). Interestingly, in this case, the decomposed particle amounts are nearly the same (Figure 2 22). In both ODE/OAm and TOPO systems, about 30% of starting cores decomposed when the particles grew from 3.2nm to 5.0nm in diameter (Figure 2 22).

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68 Figure 2 21. Evolution of the absorption (a) and PL (b) spectra of the ZB CdSe nanocrystals grown in TOPO (99%) sol ution. (c) Histogram of size and size distribution of the CdSe nanocrystals during growth. Figure 2 2 2 ZB CdSe nanocrystals (NC) amounts as a function of diameter during particle growth in different solvents (blue square s : ODE/OAm with a volume ratio o f 1:1 red circle s : TOPO, 99% ) The XRD measurements and the computer simulation processes were also conducted for these two resulting CdSe nanocrystals. The results are shown in Figure 2 2 3 and the parameters applied in the simulation are listed in T able 2 2

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69 Figure 2 2 3 Experimental XRD patterns (blue lines) compared with computer simulations (red lines) of the ZB CdSe nanocrystals grown in (a) ODE/OAm and (b) TOPO. The stick patterns in panel (a) and (b) show the positions of standard XRD peaks for bulk ZB CdSe. Table 2 2. Parameters used in XRD simulations for zinc -blende crystal structures of CdSe nanoparticles. System Lattice parameter () shape Diameter (nm) Stacking sequences along (111) # of stacking faults ODE/OAm (1:1) a = 6.077 Sphere 5.0 ABCABC BCABCAB ACBA AB ABCABCA CBA B CABC 2.5 TOPO (99%) a = 6.077 Sphere 5.0 ABC ABC BCABCAB ACBA AB ABCABCA CBA BCABC 2.5 represents the location of a stacking fault The XRD results clearly show that both samples have the zinc -blende crystals str ucture because of the deep valley between the (220) and (311) peaks (Figure 2 2 3 ). This is consistent with the zinc -blende structure of initialing CdSe cores. To obtain the best fitted simulation spectra, for each case, the same number of stacking faults ( an average of 2.5 stacking faults per particle) w as used during the simulation for both systems This simulation result is consistent with the same amount of decomposed na nocrystals in these two systems. Moreover, we conside r ed the crystal structure effec t for these two systems. F or the ODE/OAm (volume ratio: 1:1) system, when particle s grow from 3.2nm to 5.0nm, more decomposed nanocrystals for W -CdSe particles (40% of initial amount compared to 30% for ZB -

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70 CdSe ) corr esponds to more stacking faults in the r esulting nanocrystals (3.0 stacking faults per particle for W CdSe compared to 2.5 stacking faults per particle for ZB -CdSe ). In contrast, for TOPO (99%) system, when particles grow from 3.2nm to 5.0nm, fewer decomposed nanocrystals for W CdSe particles (2 5% of initial amount compared to 30% for ZB CdSe) corresponds to fewer stacking faults in the resulting nanocrystals (2.2 stacking faults per particle for W CdSe compared to 2.5 stacking faults per particle for ZB CdSe). This is again consistent with our m ode l and further indicates that the critical number of stacking fault s for decomposing CdSe nanocrystals is crystal structure in dependent. Taken together, all these XRD and computer simulation results strongly support our stacking fault associated growth m odel 2 .4 Summary In summary, we have studied CdSe nanocrystal growth. T he number of particles decreases in the nanocrystal growth stage. This decrease cannot be explained by Ostwald ripening theory. The effects of d ifferent experiment al parameters were in vestigated O nly the solvent and crystal structure affect the amount of decomposed nanocrystals. A stacking faults associate d with nanocrystals growth model has been proposed. In this model, w hen each nanocrystal grows to a certain size range, if it contai ns more stacking faults than its critical number in that size range, it will quickly and totally decompose ; otherwise, it will survive and continue growing. The XRD measurements and computer simulation results show that fewer decomposed nanocrystals are as sociated with fewer average stacking faults in the final nanocrystals, and vice versa. All these results strongly support our model.

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71 CHAPTER 3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM DIOXI DE AS THE SELENIUM PRECURS OR 3.1 Introduction This chapter describes a one pot synthesis method for making high -quality metal -selenide (CdSe, PbSe CuSe and PdSe ) nanocrystals using the selenium dioxide as selenium precursor Colloidal metal -selenide nanocrystals (e.g., CdSe and PbSe) comprise a class of s emiconductor nanocrystals that are important to both fundamental science and technological applications.11,14, 45,156, 171174 In the fundamental science area, the ability to synthesize high quality metal -selenide nanocrystals with well -controlled size, sha pe and surface passivation has led to systematic elucidation of size -dependent scaling laws and photophysical processes in quantum -confined systems.45 On the application side, metal -selenide nanocrystals have been used as major building blocks for applica tions such as biomedical diagnosis, solar cells, and light -emitting diodes (LEDs).11,14,171 174 To date, two general synthesis schemes have been developed for producing high -quality metal -selenide nanocrystals.4,51 54 5862,71,8183 The first synthesis sch eme relies on rapid precursor injection to achieve a separation of the nanocrystals nucleation and growth stages.4,45,51 54 This injection based synthesis has led to the preparation of nearly all types of highquality metal selenide nanocrystals.4,45,51 54 However, this injection -based synthesis is not suitable for large scale, industrial preparation of high -quality nanocrystals (e.g., tens to hundreds of kilograms) because of the limitation of the rates of precursor injection and mass transfer in the app aratus for industrial synthesis.57,58 The second scheme is a non -injection synthesis (NIS), which is based on a concept of controlling the thermodynamics and kinetics in the nanocrystal nucleation stage.57,58 In previous studies, we have demonstrated tha t the separation between nucleation and growth can be automatically achieved in a homogeneous reaction system by controlling the

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72 chemical reactivity of precursors.57 The quality of the nanocrystals made by this NIS method is at least comparable to the bes t particles made by injection -based methods.57,58 In addition, this NIS method is suitable for industrial preparation of high quality nanocrystals.57,58 However, to produce high-quality nanocrystals, both synthesis schemes normally require air -free oper ations using a glove box because of the use of the following air -sensitive compounds as selenium precursors: bis(trimethylsilyl)selenium, organophosphine selenide, and selenium powder.4,51 54,5862,71,8183 Such air -free operations substantially increase t he complexity as well as the cost of the synthesis of metal -selenide nanocrystals. Herein, we report a simple NIS method for making high -quality CdSe nanocrystals using SeO2 as the selenium precursor. Because of the high chemical stability of SeO2, this new synthesis does not require the use of a glove box. In addition, the synthesis can be conducted in air, without the use of a Schlenk line. Moreover, this new synthesis scheme can be generalized for making high -quality metal -seleni de nanocrystals of other compositions such as PbSe, CuSe and PdSe. 3 .2 Experimental Section 3 .2 1 Chemicals Selenium dioxide (SeO2, 99.9+%) copper(II) nitrate trihydrate (Cu(NO3)2 3 H2O 99% ), palladium(II) acetylactonate ( Pd(C5H7O2)2, 99%), oleic acid (O L A, 90%) 1 -o ctadecene (ODE, 90%) octadecane (ODA, 99 %), t rioctylphosphine (TOP, 90 %), oleyl amine ( OAm, 70%), docosanoic acid (99%), 1,2 hexadecanediol (9 0 %), phenyl ether (PE, 99%), methanol anhydrous (98%) were purchased from Aldrich. Cadmium nitrate tetrahydrate (Cd(NO3)24H2O, 99.99%) cadmium acetate (Cd(C2H3O2)22H2O, 99.999%) myristic acid (MA, 99%) were purchased from Alfa Aesar. S odium oleate (95%) was purchased from T CL. 2 octadecanone (99%) was purchased from Fluke. Tetrabutylammonium hydroxide (1M in methanol) was p urchased from

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73 Acros. Lead acetate trihydrate (Pb(C2H3O2)2 3 H2O ACS ), sodium hydroxide (NaOH) and a ll the other solvents were purchased from Fisher Scientific International Inc. 3.2.2 Preparation of Precursors 3 2 .2 .1 Preparation of cadmium myristate Cadm ium nitrate (5 mmol) was dissolved in anhydrous methanol (50mL). A sodium myristate solution was prepared by dissolving sodium hydroxide (15 mmol) and myristic acid (15 mmol) in anhydrous methanol (500 mL). Then the cadmium -nitrate solution was added dro pwise (one drop per second) into the sodium -myristate solution with vigorous stirring. The resulting white precipitate was washed with methanol three times, and then dried at ~60oC under vacuum overnight. 3.2.2.2 Preparation of cadmium docosanate Cadmium nitrate (5 mmol) was dissolved in anhydrous methanol (50mL). Tetrabutylammonium docosanate solution was prepared by slowly adding a tetrabutylammonium hydroxide solution (5 mL, 1M in methanol) into a docosanoic acid methanol solution (0.03M, 2000m L). Then the cadmium -nitrate solution was added dropwise (one drop per second) into the tetrabutylammonium docosanate solution with vigorous stirring. The resulting white precipitate was washed with methanol three times, and then dried at ~60oC under vacu um overnight. 3.2.2.3 Preparation of copper oleate Five mmol copper nitrate was dissolved in 50m L anhydrous methanol. Sodium oleate solution was prepared by dissolving 8 mmol sodium oleate in 500mL anhydrous methanol. Then the cadmium nitrate solution was slowly dropped (~1 drop/second) into the sodium oleate solution with vigorous magnetic stirring. The resulting light blue precipitate was washed three

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74 times with methanol and dried at room temperature (RT) under vacuum overnight to remove all solvents. 3. 2 .3 Metal -Selenide Nanocrystal Synthesis 3.2.3.1 CdSe nanocrystal synthesis under air In a typical synthesis for making CdSe nanocrystals, SeO2 (0.1 mmol) and cadmium myristate (0.1 mmol) were added into a three neck flask with 1 -octadecene (ODE, 6.3 mL). Under stirring, the resulting mixture was heated to 240 oC at a rate of 25 oC/min. After the temperature reached 240 oC, serial aliquots were taken for kinetic studies. As the nanocrystals grew, their size distribution continued to decrease. After the si ze of nanocrystals had reached 3.0 nm, oleic acid (0.1 mL) was added dropwise (1 drop per 10 s) into the reaction solution, to provide extra ligands for stabilizing the nanocrystals during further growth. The size distribution of nanocrystals further narr owed with particle growth until a final size of about 4.0 nm in diameter was reached 3.2.3.2 CdSe nanocr ystal synthesis under Ar using a Schlenk line The synthesis procedure is similar to the synthesis described in section 3.2.3.1. Cadmium myristate (0.1 m mol), SeO2 (0.1 mmol) and ODE (6.3 mL) were mixed in a 25mL three neck flask. The resulting mixture was degassed under vacuum ( ~50 mTorr, 10 min ) at room temperature. Under argon flow and with stirring, the mixture was heated to the reaction temperature (240 oC) at a rate of 25 oC/min. The time was counted as zero when the temperature reached 240 oC. Serial quantitative aliquots ( 50 L) were taken for monitoring the kinetics of nanocrystal formation. When the size of nanocrystals reached 3.0 nm, oleic acid (0.1 mL) was added dropwise (one drop per ten seconds) into the reaction solution to further stabilize the nanocrystals.

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75 3.2.3.3 CdSe nanocrystal synthesis with various final sizes (1 ) Synthesis of CdSe nanocrystals of 3.0 -3.5 nm in diameter. T he s ynthesis procedure is similar to the ty pical synthesis described in section 3.2.3.1. Cadmium myristate (0.1 mmol), SeO2 (0.1 mmol), 1,2 -hexadecanediol (0.1 mmol ) and ODE (6.3 mL) were mixed in a 25-mL three neck flask. The resulting mixture was then heat ed to the reaction temperature (240 oC) at a rate of 25 oC/min. After 15 min at the reaction temperature, oleic acid (0.1 mL) was added dropwise (one drop per ten seconds) into the reaction solution to further stabilize the nanocrystals. With particle gr owth, the size distribution of nanocrystals further narrowed. The reaction temperature was maintained for an additional 25 45 min, and then the reaction solution was cooled to room temperature. The resulting nanocrystals were precipitated from the reacti on solution using acetone, and were redispersed in toluene. Serial quantitative aliquots ( 50 L) were taken for monitoring the kinetics of nanocrystal formation. The number of nanocrystals formed during the synthesis was measured using UV -Vis spectroscop y according to Beers law. (2 ) Synthesis of CdSe nanocrystals of 2.8 -3.0 nm in diameter T he synthesis procedure is similar to the synthesis described in section 3.2.3.1, except that cadmium myristate was replaced by cadmium docosanate. (3 ) For making Cd Se nanocrystals of 2. 0 -2.8 nm in diameter. T he synthesis procedure is similar to the synthesis described above in ( 2 ), except that additional 1,2 -hexadecanediol (0.1 mmol ) was added to the reaction system. (4 ) For making CdSe nanocrystals of 4.0-6.2 nm in diameter. T he synthesis procedure is similar to the typical synthesis described in section 3.2.3.1, except that additional cadmium acetate (0.0037 0.017 mmol (1.0mg4.7mg)) was added into the reaction system.

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76 3.2.3.4 Pb Se nano cube synthesis First, lead oleate was prepared as follows: lead acetate trihydrate (1 mmol) was mixed with ODE (4 mL) and oleic acid (1.3 mL) in a 50 -mL three -neck flask. The mixture was degassed under vacuum ( ~50 mTorr ) at RT for 10 minutes and then gradually heated to 85 oC and kept for 1 hour to further remove the water and acetic acid. After the mixture turned colorless, the vacuum was removed and the lead -oleate solution was cooled to room temperature. Then, TOP (8 mL) and SeO2 (1 mmol) were added to the lead -oleate solution. U nder argon flow and with stirring, the resulting mixture was heated to 180 oC. After 5 min at th at temperature, the reaction was terminated by cooling the reaction solution to room temperature. The resulting PbSe nanocrystals were precipitated from the r eaction solution using acetone and redispersed in toluene. 3.2.3.5 CuSe nanocrystal synthesis P remade copper oleate powder (0.1 mmol) and 0.1 mmol SeO2 powder were mixed with 2 mL ODE and 8 mL TOP in a 25 -mL three -neck flask. After degassing under vacuum (~ 2510 Torr ) at RT for 10 minutes, the mixture was stirred and gradually heated to 280 oC under argon flow. After 5 min at th at temperature, the reaction was terminated by cooling the reaction solution to room temperature. The resulting CuSe nanocrystals were precipitated from the reaction solution using acetone and redispersed in toluene. 3.2.3. 6 Pd4.5Se nano crystal synthesis P alladium (II) acetylactonate (100 mg, 0.33 mmol) and TOP (1 mL) were added to a 25-mL three neck flask. T he mixture was gently stirred for 10 minutes, and an orange solution was formed. Then SeO2 (36.6 mg, 0.33 mmol), ODE (2 mL) and OAm (8 mL) were added to the orange solution. At room temperature, the resulting solution was degassed for 5 min, and then the solution was heated to 250 oC. After 10 min at th at temperature, the synthesis was

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77 terminated by cooling the reaction solution to room temperature. The resulting p alladium selenium nanocrystals were precipitated from the reaction solution using acetone and redispersed in toluene. 3.2.4 Mechanistic Study 3.2.4.1 The reaction with c admium myristate T he reaction of cadmium myristate, SeO2, and ODE was conducted as follows: cadmium myristate (2 mmol), SeO2 (2 mmol), and ODE (4 mmol ) were mixed in a 10 -mL flask. A t room temperature, t he mixture was degassed under vacuum (~ 50 mTorr) for 10 min Under argon flow and with stirring, the mixture was heated to 240 oC. After 5 min at th at temperature, the reaction solution was cooled to room temperature. A bout 10 mg of the final yellow -brown mixture was loaded onto a NaCl window for FT IR measurement and about 10 mg of the mixture was dissolved in CD2Cl2 (~0.8 mL) for the 13C NMR measurement. 3.2.4.2 The reaction without c admium myristate The reaction withou t cadmium myristate was conducted using a procedure similar to that above: SeO2 (2 mmol) and ODE (4 mmol ) were mixed in a 10 -mL flask. A t room temperature, t he mixture was degassed under vacuum (~ 50 mTorr) for 10 min Under argon flow and with stirring, t he mixture was heated to 240 oC. After 5 min at th at temperature, the reaction solution was cooled to room temperature. A bout 10 mg of the final yellow -brown mixture was loaded onto a NaCl window for FT IR measurement and about 10 mg of the mixture was dissolved in CDCl3, (~0.8 mL) for 1H NMR or was dissolved in CD2Cl2 (~0.8 mL) for 13C NMR measurements.

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78 3.2.5 Characterization of Metal Selenide Nanocrystals 3.2.5.1 Absorption measurements UV-Vis absorption spectra were measured using a Shimadzu UV1701. Nanocrystals were dissolved in toluene for the measurement. 3.2.5.2 Photoluminescence measurements Photoluminescence (PL) experiments were performed on a fluorometer (Fluorolog3, Horiba Jobin Yvon, Irvine, CA). Nanocrystals were dissolved in toluene for the measurement. 3.2.5.3 TEM and EDS measurements TEM measurements were performed on a JEOL 200X operated at 200 kV, or a JEOL 2010F TEM operated at 200 kV. Energy dispersive X ray spectroscopy (EDS ) measurements were acquired by the 2010F TEM and operat ed at 200 kV. The specimens were prepared as follows: a -mesh copper grid, and was dried overnight at ambient conditions. 3.2.5.4 XRD measurements XRD measurements were performed on a Philips XRD 3720 spectro meter. The specimens were prepared as follows: about 15 mg of the purified nanocrystals w ere dissolved in about 0.5 mL of toluene and then dropped onto a low -scattering quartz sample holder and dried in air and kept overnight in a vacuum des i c cator 3.2.5 .5 NMR measurements 1H NMR analysis was used to identify the chemical compositions of the reaction products 1H NMR spectra were recorded using a Varian Mer cury 300 NMR Spectrometer (300 MHz) and 13C NMR spectra were recorded using a Varian Mercury 300BB N MR Spectrometer (75 MHz).

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79 For standard samples, about 5 mg of standard sample (ODE or 2 -octadecanone) was dissolved in CDCl3 (~ 0.8 mL) for 1H -NMR measurement, and about 10 mg of standard sample (ODE or 2 octadecanone) was dissolved in CD2Cl2, (~ 0.8 mL) f or 13C NMR measurement. 3.2.5.6 FT -IR measurements FT IR spectra were recorded using a Bruker Vector 22 FT IR Spectrometer. The specimens were prepared by directly loading about 10 mg of sample onto a NaCl window. 3.2.5 .7 Differential scanning calorimetric (DSC ) measurements DSC diagrams were recorded using a 2910 MDSC instrumentation. About 20 mg of the black precipitate resulting from the reaction between SeO2 and ODE was loaded i nto a DSC pen and gradually heated to 275 oC. The heating rate from 175 oC t o 275 oC was 2 oC/min. 3.3 Results and Discussion 3.3 .1 Synthesis and Characterization of C dSe Nanocrystals Figure 3 1 (a) Temporal evolution of the absorption spectrum (blue) and photoluminescence (PL) spectrum (red) (b) TEM image of CdSe nanocrystal s with a diameter of 4.0 nm and a standard deviation of ~ 5% (c) T he XRD pattern of CdSe nanocrystals measured from the same sample shown in panel (b). The blue bars in panel (c) show the positions of standard XRD peaks for bulk zinc -blende CdSe.175

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80 The sy nthesis of CdSe nanocrystals was monitored by UV -Vis absorption spectrosco py and photoluminescence spectroscopy (Figure 3 1a). No nucleation occurred as the temperature reached 240 oC (Figure 3 1a). After 30 s at this temperature, small nanocrystals appear ed. As the nanocrystals grew, their size distribution continued to decrease. After the size of nanocrystals had reached 3.0 nm, oleic acid (0.1 mL) was added dropwise (1 drop per 10 s) into the reaction solution, to provide extra ligands for stabilizing t he nanocrystals during further growth. Afterward, the kinetics of nanocrystal growth indeed turned out to be very stable. The size distribution of nanocrystals further narrowed with particle growth until a final size of about 4.0 nm in diameter was reach ed (Figure 3 1a). With further annealing at the reaction temperature, the narrow size distribution of the resulting particles was maintained for at least overnight. Neither Ostwald ripening nor secondary nucleation was observed during the synthesis. In a ddition, this SeO2-based synthesis exhibits more stable nanocrystal nucleation kinetics than the synthesis using selenium powder as a precursor. Without size sorting, the typical CdSe nanocrystals from this synthesis exhibit sizes with a relative standard deviation of about 5% (F igure 3 1 b). The X ray powder diffraction (XRD) pattern shows that the resulting CdSe nanocrystals have a zinc -blende structure (Figure 3 1 c). This result is consistent with the absorption spectra of these nanocrystals. The gap be tween the first (1S3/21Se) and second (2S3/21Se) exciton peaks for these CdSe nanocrystals is wider than that of a typical wurtzite particle, but is identical to that for those zinc -blende CdSe nanocrystals made using selenium powder.58 3.3. 2 Oxygen Effe ct To explore the effect of oxygen on the formation of CdSe nanocrystals, we conducted a control synthesis under air -free conditions using a Schlenk line. In this synthesis, small particles appeared when the temperature reached 230 oC (Figure 3 2b ) The subsequent kinetics were

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81 similar to those of the synthesis conducted in air (Figure 3 2a). The resulting nanocrystals from this air -free synthesis exhibit a nearly identical quality to those made by the synthesis in air. These results show that the existen ce of oxygen in the reaction system affects the early nucleation stage in CdSe -nanocry stal synthesis, and delays CdSe nanocrystal nucleation, as compared to the air -free synthesis. However, the existence of oxygen does not substanti ally affect the quality of CdSe -nanocrystal products. These results are likely due to the following reasons: (1) oxygen has an extremely low solubility in ODE, and (2) the trace amount of oxygen in the reaction solution is consumed at a very early stage of the synthesis. In sh ort, these results further confirm that this new synthesis does not have to be conducted under air -free conditions. Figure 3 2. Temporal evolution of the absorption spectrum of CdSe nanocrystal growth in air (a) and under Ar (b). The time was counted as zero w hen the temperature reached 240oC. 3.3.3 Size Control of CdSe Nanocrystals Because of its stable nucleation kinetics, this SeO2-based synthesis allows the control of the number of nuclei and thus the final size of the resulti ng nanocrystals, wh ile the high -quality of resulting nanocrystals remains unchanged. In a previous study, we demonstrated the relationship

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82 between nanocrystal growth rate and the number of nuclei in a synthesis of CdS nanocrystals: the slower the nanocrystal growth rate, the larger the number of nuclei and thus the smaller the final particles, and vice versa.57 Herein, we found that this relationship is also valid in this SeO2-based synthesis. Figure 3 3. The concentration and size of the final CdSe nanocrystals in the Se O2-based synthesis under the following conditions: (a) cadmium docosanate (0.1 mmol), SeO2 (0.1 mmol) and 1,2 hexadecanediol (0.1 mmol); (b) cadmium docosanate (0.1 mmol), and SeO2 (0.1 mmol); (c) cadmium myristate (0.1 mmol), SeO2 (0.1 mmol) and 1,2 -hexad ecanediol (0.1 mmol); (d) cadmium myristate (0.1 mmol) and SeO2 (0.1 mmol); (e) cadmium myristate (0.1 mmol), SeO2 (0.1 mmol) and cadmium acetate ( 2 (0.1 mmol) and The concentration of the resulting nanocrystals is determined using UV -Vis spectroscopy according to Bee rs law. The size of the resulting nanocrystals is determined by TEM. Under these synthesis conductions, the overall nanocrystal growth rate is ~2.7 nm/ h in (a), 3.3 nm/h in (b), ~3.5 nm/h in (c), ~4.0 nm/h in (d), ~4.6 nm/h in (e), and ~5.1 nm/h in (f). In this study, control of the nanocrystal growth rate at the nucleation stage of the formation of CdSe nanocrystals was achieved by tuning the reactivity of cadmium precursors. On one hand, we used two approaches to decrea se the nanocrystal growth rate. First, because of its relatively low reactivity, cadmium docosanate was used to replace cadmium myristate as the cadm ium precursor in the synthesis. Second, 1,2 -hexadecanediol was introduced into the reaction

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83 system because cadmium myristate (or docosanate) can be further stabilized by 1,2 hexadecanediol through chelation. Indeed, these two approaches led to slower nanocrystal growth rates, larger numbers of nuclei and smaller resulting particles (Figure 3 3 ). On the other hand, we introduced a small amount of cadmium acetate into the synthesis system to increase the nanocrystal growth rate at the nucleation stage, because cadmium acetate exhibits a higher reac tivity than cadmium myristate. Our results show that this approach did lead to the synthesis of la rger CdSe nanocrystals than the typical synthesis without cadmium acetate Figure 3 4 (a) Absorption (in blue) and PL (in red) spectra of the as -prepared CdSe nanocrystals in various sizes. TEM image s of CdSe nanocrystals with a diameter of 6.2 nm ( b), 4.5 nm (c) and 3.0 nm (d). The relative standard deviations of these sizes are ~5%, and the corresponding optical spectra are indicated by dashed arrows. Based on these approaches, we have synthesized high -quality, zinc -blende CdSe nanocrystals with s izes ranging from about 2.0 to 6.2 nm (Figure 3 4 ). These nanocrystals exhibit a typical photoluminesc ence quantum yield of about 40% and exhibit up to four absorption peaks, indicating their narrow size distribution s (Figure 3 4a) Indeed, TEM

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84 observat ions show that these nanocrystals have a typical size distribution of ~5% (Figure 3 4bd) 3.3. 4 M echanistic Study In this new synthesis, the selenium precursor (SeO2) needs to be reduced to an active form, such as Se(0), for the formation of CdSe nanocrys tals, and ODE is likely the reducing agent. To test this hypothesis, we conducted the following three experiments, in which ODE was replaced by other solvents, while the amount of cadmium myristate and SeO2 were kept unchanged, as in the typ ical synthesis described above. First, ODE was replaced by its saturated counterpart: octadecane (ODA). The results show that the number of CdSe nuclei formed in this ODA -based synthesis is only about half of that formed in the synthesis with ODE as solvent (Figure 3 5 a) In addition, based on the absorption spectra, the quality of the resulting particles made in ODA is much lower than that of particles made in ODE (Figure 3 5a) These results indicate that the vinyl group of ODE is critical for the synthesis of high -quality CdSe nanocrystals, but the saturated alkyl chain can also reduce SeO2 to Se(0) through a reaction such as dehydrogenation.176, 177 In the second experiment, phenyl ether was chosen as a solvent to replace ODE, because phenyl ethers have only arom atic rings and are not easily oxidized by SeO2. Indeed, the results show that only trace amounts of CdSe particles were formed in this reaction, and the quality of the resulting particles is poor (Figure 3 5 a) Moreover, unreacted SeO2 white powders were observable in the reaction flask, whereas this phenomenon was not observed in syntheses with either ODE or ODA as solvent. To further explore the role of ODE in the CdSe -nanocrystal synthesis, we carried out the third experiment with phenol ether as sol vent and a small amount of ODE (85 L) at a molar

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85 ratio of ODE and SeO2 of 3:1. Amazingly, UV -Vis spectra show that the resulting CdSe nanocrystals from this synthesis exhibit nearly identical quality to those nanocrystals made from the typical synthesis with ODE as solvent (Figure 3 5 a). Taken together, these results demonstrate that ODE is not only a solvent but also a reactant in this SeO2-based CdSe nanocrystal synthesis. Figure 3 5. (a) The absorption spectra of CdSe nanocrystals synthesized in various solvents: ODA (in black), phenyl ether (in red), phenyl ether mixed with ODE (in blue), and ODE (in green). (b) FT -IR spectra of ODE (in black); of a reaction mixture of Cd(My)2, SeO2 and ODE (before reaction, in red and after reaction at 240 oC, in blue ); and of the product of the reaction of SeO2 and ODE at 240 oC (in green). (c) The equation for the reaction between ODE and SeO2. X is the mixture of organic by -products. To investigate the molecular mechanism of precursor evolution in this new CdSe synthesis, we studied a reaction of cadmium myristate (2 mmol), SeO2 (2 mmol) and ODE (4 mmol), which was conducted at 240 oC. Before the reaction, the reagent mixture exhibited the characteristic IR peaks of the vinyl group of ODE (out -of -plane C H b e nd: 909.7 cm1 and 991.2 cm1; C=C stretch: 1641.4 cm1; and C H stretch: 3077.6 cm1).178 The IR peak of the carbonyl group of

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86 cadmium myristate was at 1542 cm1 (Figure 3 5 b ). After the reaction, the FTIR spectrum of the reaction product mixture shows that all the characteristic IR peaks of ODE and cadmium myristate disappear, while a new peak appears at 1713.2 cm1 (Figure 3 5 b ). This result shows that both ODE and cadmium myristate are consumed during the reaction, and further confirms that ODE is i ndeed a reactant in the CdSe nanocrystal synthesis. Moreover, the new peak of 1713.2 cm1 is in the typical frequency range of a carbonyl group. 13C NMR spectroscopy shows the organic by product mixture contains both carboxylic acid and ketone functional ities (Figure 3 6 ). The carboxylic group is assigned to a by -product from cadmium myristate, while the ketone is possibly a by -product of the reaction between ODE and SeO2. Figure 3 6 13C NMR spectra of (a) the product mixture of the reaction of cadm ium myristate, SeO2 and ODE; (b) myristic acid; and (c) 2 octadeanone. The results indicate that the reaction mixture contains both ketone and carboxylic acid functionalities To examine this possibility, we further studied a control reaction (SeO2: 2 mmo l and ODE: 4 mmol) without cadmium myristate. The IR spectrum of the product mixture from this reaction is very close to the spectrum of the products from the reaction with cadmium myristate (Figure 3 5 b ). This result indicates that the ketone is indeed a product of the reaction of ODE and SeO2.

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87 However, thin layer chromatography shows that the product from this reaction is a complicated mixture. 1H NMR and 13C NMR show that the product mixture is composed of methyl ketones, a small amount of alkenes wit h a C=C double bond in the middle of their chains and a trace amount of aldehydes (Figure 3 7 and Figure 3 8 ). Methyl ketones and aldehydes are assigned to the products of SeO2-mediated oxidation of the vinyl group of ODE, while the alkenes are the produc ts of SeO2-mediated dehydrogenation of the hydrocarbon chain of ODE.176,177 Figure 3 7 (a) 1H NMR spectra of (i) the product mixture from the reaction of SeO2 and ODE, (ii) 2 octadecanone, and (iii) ODE (b) The proposed functional groups found in the product mixture, which include methyl ketone, aldehyde, and alkenyl with C=C double bond in the middle of the hydrocarbon chains. In addition, the reaction of SeO2 and ODE also yielded dark gray precipitates. Differential scanning calorimetric measurement shows that the precipitates exhibit a meting point of 221 oC (Figure 3 9 ) indicating that they are selenium crystals .176,177 Taken together, the results from these mechanistic studies suggest that the reduction of Se(IV) to Se(0) is an important step in t his SeO2based, non -injection s ynthesis of CdSe nanocrystals (Figure 3 4c). The formation of Se(0)

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88 requires a high temperature, thus decreasing the chance s for multiple nucleation during the formation of CdSe nanocrystals.58 This can, in part, explain why the SeO2-based synthesis exhibits more stable nucleation kinetics than the noninjection synthesis directly using Se(0) as the selenium precursor. Figure 3 8 13C NMR spectra of (a) the product from the reaction of SeO2 and ODE ; I nsert, zoom in spectrum of the product (top) and 2 octadecanone (bottom); (b) 2octadecanone, and (c) ODE Taken together, these spectra suggest that the product mixture should contain methyl ketones, consistent with the results from 1H -NMR spectra shown in Figure 3 6.

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89 F igure 3 9 DSC (d ifferential scanning calorimetry ) thermogram of the black precipitates resulting from the reaction of SeO2 and ODE. The melting point of 221 oC indicates the black precipitates are selenium crystals (D. R. Lide, Handbook of Chemistry and Physi cs ; 84th ed. New York, 2003). 3.3. 5 Generalization of SeO2-Based Metal -Selenide Synthesis Since this SeO2-based synthesis shows very stable nucleation and growth stages for making high -quality CdSe nanocrystals, it is possible to generalize this method for making other kind of metal -selenide nanocrystals To test this possibility, high quality lead -selenide, palladium selenide and cupper selenide nanocrystals have been synthesized using SeO2 as selenium precursor. 3.3. 5 .1 PbSe nanocube synthesis In the syn thesis of PbSe nanocrystals, we fo und that oleic acid alone is in sufficient to stabilize the resulting nanocrystals during growth, because the reactions often resulted in either particles with poor size distributions or insoluble products. To overcome thi s difficulty, we introduced trioctylphosphine (TOP) into the synthesis system because TOP can stabilize PbSe nanocrystals by binding to both the Pb and Se sides on their surfaces. With TOP, the kinetics of PbSe nanocrystal growt h turned out to be very sta ble.

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90 Figure 3 10. (a) TEM image of PbSe nanocubes with a side length of 15.9 nm and a relative standard deviation of 7%. (b) A high resolution TEM image of PbSe nanocubes: the ordered distance of 3.05 corresponds to the lattice spacing of the (200) fa ces in cubic PbSe. The resulting nanocrystals from a typical synthesis exhibit a cubic shape with edge length of 15.9 nm an d distribution of 7% (Figure 3 10a ). High re solution TEM observation show s that these nanocrystals have (200) faces parallel to two edges o f these PbSe nanocubes (Figure 310b ), which is consistent with the cubic crystal structure of PbSe 3.3. 5 .2 CuSe nanocrystal synthesis Moreover, based on the same idea, using both oleic a cid and TOP as the ligand s high quality CuSe nanocrystals we re sy nthesized. TEM measurement shows that the CuSe nanocr ystals have an diameter of 5.0nm with a relative standard deviation of 6% (Figure 3 1 1 ) Figure 3 1 1 TEM image of Cu Se nanocrystals with a diameter of 5.0nm and a relative standard deviation of 6 %.

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91 3.3. 5 3 Pd4.5Se nanocrystal synthesis In addition, based on a similar concept for stabilizing nanocrystals in the synthesis, we have successfully prepared palladium -selenide nanocrystals with TOP and oleyamine as ligands. The typical resulting nanocryst als exhibit a diameter of 4.6 nm with a relative st andard deviation of 7% (Figure 3 1 2 a ). Interestingly, energy dispersive X -ray spectroscopy (EDS) shows that these nanocrystals exhibit an atomic molar ratio between palladium an d selenium of 4.5:1 but not 1:1 (Figure 3 1 2 c and Table 3 1 ). This EDS result is consistent with the XRD measurement, where the Bragg diffraction peaks of these nanocrystals nearly perfectly match those of bulk Pd4.5Se (Figure 3 1 2 b ).175 T hese palladium -rich selenide nanocrystals may have some extraordinary properties (e.g. superconductivity). Figure 3 1 2 (a) TEM image of Pd4.5Se nanocrystals with a dimeter of 4.6 nm and a relative standard deviation of 7%. (b ) An XRD pattern of Pd4.5Se nanocrystals measured from the same sample shown in (a ). The blue bars in panel (b) indicate the positions of standard XRD peaks for bulk Pd4.5Se.175 (c) EDS spectra of Pd4.5Se nanocrystals measured from the same sample shown in (a ).

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92 Table 3 1. The EDS results of Pd4.5Se sample measured from random ly selected three different areas on the TEM grid. Data are given as atom percentages. Pd Se Spectrum1 83.44 16.56 Spectrum2 82.20 17.80 Spectrum3 79.62 20.38 Average 81.75 18.25 From these data the atomic ratio between Pd and Se was found to be 4.5 0.6. 3.4 Summary In summary, this chapter reports a NIS method for synthesizing high -quality zinc -blende CdSe nanocrystals with SeO2 as the selenium precursor. Mechanistic studies show that ODE is a reducing agent for SeO2 in this synthesis. The synthe sis can be conducted in air, without the need for air -free operations requiri ng a glove box or a Schlenk line. Moreover, this synthesis exhibits controllable kinetics in both the nucleation and growth stages, and thus allows detailed control of the numbers of nuclei and final size s of the resulting nanocrystals. Importantly, such a synthesis is easy for a small -scale laboratory preparation, and it is also suitable for a large -scale industrial synthesis of high-quality nanocrystals at low cost. In addition we have generalized this SeO2based NIS method for making other metal -selenides, such as PbSe CuSe and Pd4.5Se nanocrystals. T o the best of our knowledge, this is the first time that high-quality palladium selenide nanocrystals have been synthesized th rough a colloidal method.

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93 CHAPTER 4 RADIAL -POSITION CONTROLLED DOPING IN CDS/ZNS CORE/SHELL NANOCRYSTALS: SYNTHE SIS AND DOPANT GROWT H STUDY 4.1 Introduction The ability to precisely control the doping of semiconductor nanocrystals can create an opportunity for producing functional materials with new properties of importance to applications such as biomedical diagnosis, solar cells, and spintronics.8,138,156,158 ,179 This opportunity has stimulated research efforts to develop synthetic methods to incorpora te dopants into a variety of colloidal semiconductor nanocrystals.8,137 157 In general, two types of synthetic methods have been developed to produce doped nanocrystals.158,179 The first method is based on aqueous phase coprecipitation, or inverse micelle templates.151,152 This method often leads to nanoparticle products with low crystallinity and broad size distributions.179 The second method is organic phase growth, which can produce monodisperse and highly crystalline colloidal nanocrystals. 137, 139 143 ,149,154,155,158 In many cases, the impurity atoms exist only at the surface of the nanocrystals but not inside the core s therefore minimizing the impuritys effects on the nanocrystals properties.135,139143 Gamelin et al. have introduced a method using isocrystalline shell growth to incorporate surface impurities inside the cores.143 Peng et al. have reported that impurity -doping can be decoupled from the nanocrystal nucleation and growth stages.149 Based on this progress, in this chapter we demonstrat e a colloidal synthesis of high -quality CdS/ZnS core/shell nanocrystals with radial -position -controlled Mn dopants. This approach is based on a three -step synthesis: (1) synthesis of starting host particles, (2) Mn-dopant growth, and (3) host -shell grow th. This approach allows the precise control of Mn radial position and doping level in core/shell nanocrystals. On the basis of this synthesis, we have studied the detail ed mechanism of the Mn -dopant growth process. Our results suggest that nanocrystal

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94 doping is determined by the chemical kinetics of three activation -controlled processes: dopant adsorption, dopant replacement, and ZnS -shell growth. 4.2 Experimental Section 4.2.1 Chemicals Sulfur powder (99.999%), 1 -octadecence (ODE, tech. 90%), oleylamine ( OAm, tech. 70%), 1 octadecylamine (ODA, 97%), t rioctylphosphine oxide (TOPO, 99%) tributylphosphine (TBP, 90%) and p olystyrene -block polybutadiene ( C12H14, 30wt% styrene, 80% diblock ) were purchased from Aldrich. Manganese (II) acetate tetrahydrate (99%) s odium hydroxide (NaOH) and all the solvents were purchased from Fisher Scientific Company. Nitric acid ( 69.5%, TraceSELECT) w as purchased from Fluka. Cadmium nitrate tetrahydrate (Cd(NO3)24H2O, 99.99%), zinc stearate (count as ZnO% %) and myristic acid (MA, 99%) were purchased from Alfa Aesar. Quinine sulfate (99+ %) was purchased from Acros. The chemica ls were used as received without further purification. 4.2.2 Three -Step Synthesis of Mn -Doped CdS/ZnS Core/Shell Nanocrystals 4.2.2.1 Preparation of precursors (1 ) Mn(OAc)2 Solution O leylamine (4 mL) w as added to a 25 mL flask and heated at 120 oC for 10 min under a vacuum of 20 mTorr. After the OAm solvent was cooled to room temperature, Mn(OAc)2 4H2O (4.9 mg, 0.02 mmole) was quickly added to the flask. The mixture was degassed at room temperature and at 120 oC for 10 min for each step. After a clear sol ution was obtained, the solution was cooled to room temperature and was ready for use. Note that the Mn precursor solution should be freshly made before the synthesis. (2 ) Mn(S2CNEt2)2 Solution. An OAm solution of Mn(OAc)2 (2 mL, 10 mM) was prepared accord ing to the protocol above, and then an oleylamine solution of NaS2CNEt2 (2 mL, 22 mM) was made in OAm pretreated to 60 C under Ar flow. Then the NaS2CNEt2 solution was added

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95 to the Mn(OAc)2 solution at 60 C with stirring under Ar flow. After 10 min, a sl ightly yellow solution of Mn(S2CNEt2)2 was obtained and was used directly for dopant growth. Note that the Mn -precursor solution should be freshly made before the synthesis. (3 ) Sulfur solution Sulfur powder (12.8 mg, 0.4 mmol) was added to a flask with O DE (10 mL). After degassing at room temperature for 10 min, the solution was heated to 130 oC under Ar flow. The temperature was maintained for 5 min, and then the resulting sulfur solution was cooled to room temperature for use. Caution should be taken to avoid heating the solution to a higher temperature. (4) Zinc stearate solution Zinc stearate powder (0.4 mmol) was added to a flask with ODE (10 mL). After degassing at room temperature for 10 min, the mixture was heated to 200 oC to dissolve zinc steara te. The solution was c ooled to room temperature and a slurry formed. The slurry was directly used for ZnS -shell growth. 4.2.2.2 Synthesis of Mn-doped CdS/ZnS core/shell nanocrystals (1 ) Synthesis of starting host particles A CdS nanocrystals The synthesi s of CdS nanocrystals was based on a modification of a literature method.57 In a typical synthesis, cadmium myristate ( 1.0 mmol) and S (0. 5 mmol) were loaded into a three -neck flask with 1 -octadecene (ODE, 5 0 g). After degassing under vacuum (~20 mTorr) for 1 0 min, the vacuum was removed. Under argon flow, the temperature was raised to 240 oC. The growth was monitored by taking the absorption spectra of aliquots extracted from the reaction solution. When reaching the desired size, the reaction mixture was allo wed to cool to room temperature, and the nanocrystals were precipitated by adding acetone and redispersed in hexane The as -prepared CdS crystals have a zinc blende crystal structure.

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96 B. CdS /ZnS core/shell nanocrystals First, zinc -blend CdS nanocrystals w ere prepared as descri b ed above. Second, ZnS shells were grown onto the resulting CdS nanocrystals at 220 oC in a solvent mixture of ODE and OAm with a volume ratio of 3:1. ZnS shells were added to the reaction solution with CdS nanocrystals by alternate i njections of a solution of zinc stearate in ODE (40 m M ) and sulfur in ODE (40 m M ). Growth time was 10 min after each injection. When the desired shell thickness was achieved, t he growth solution was cooled to room temperature The resulting Cd S /ZnS core/sh ell nanocrystals were precipitated by using acetone, and redispersed in hexane (2 ) G rowth of dopant In a typical experiment, a hexane solution of host particles (2 mL, 50.4 nmol) was added to a solvent mixture of ODE and OAm (8.0 mL, ODE/OAm: 3:1), and then hexane was removed under vacuum. Under argon flow, the nanocrystal solution (CdS nanocrystals or CdS/ZnS core/shell nanocrystals as starting host particles) was heated to the growth temperature (220oC or 280oC) and then the Mn(S2CNEt2)2 solution or th e Mn( O Ac)2 solution and the s ulfur solution at a molar ratio of 1:1 were introduced into the hot solution by dropwise addition. After a further 20 min reaction, the synthesis was stopped, and the nanocrystals were prec ipitated by adding acetone and re dispe rsed in hexane as a high -concentration solution for further use (3 ) G rowth of ZnS shell A Mn dopant inside the CdS core A hexane solution of Mn-doped CdS nanocrystals was added to a solvent mixture of ODE and OAm (4 .0 mL, ODE/OAm: 3:1 ), and then hexane was removed by vacuum. Under argon flow, the nanocrystal solution was heated to 240 oC, and cadmium myristate solution (0.04M in ODE) and sulfur solution (0.04 M in ODE) were alternatively introduced by dropwise addition. The shell growth was monitored by using UV -Vis

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97 spectroscopy. After two monolayer s (increasing the diameter to 3.8 nm from 2.4nm), the reaction solution was cooled to 220 oC. Then zinc -stearate solution (0.04M in ODE) and sulfur solution (0.04M in ODE) were alternatively introduced into th e hot solution by dropwise addition. When ZnS -shell thickness reached about 1.6 monolayers, the reaction solution was heated to 280 oC for further ZnS shell growth. After the desired shell thickness was achieved, a zinc -stearate solution (0.12 mmol, 0.04M in ODE) was added to the reaction system. Then the synthesis was stopped by cooling the reaction solution to room temperature, and the nanocrystals were precipitated by adding acetone. The nanocrystals can be re -dispersed into non -polar organic solvents. B. Mn dopant at the interface. A hexane solution of Mn-doped CdS nanocrystals was added into a solvent mixture of ODE and OAm (4 .0 mL, ODE/OAm: 3:1 ), and then hexane was removed under vacuum. Under argon flow, the nanocrystal solution was heated to 220 oC. Then zinc -stearate solution (0.04M in ODE) and sulfur solution (0.04M in ODE) were alternatively introduced into the hot solution by dropwise addition. When ZnS -shell thickness reached about 1.6 monolayers, the reac tion solution was heated to 280 oC for fu rther ZnS shell growth. After the desired shell thickness was achieved, a zinc -stearate solution (0.12 mmol, 0.04M in ODE) was added to the reaction system. Then the synthesis was stopped by cooling the reaction solution to room temperature, and the nanocr ystals were precipitated by adding acetone. The resulting nanocrystals can be redispersed into nonpolar organic solvents. C Mn dopant in the ZnS shell. A hexane solution of Mndoped CdS/ZnS nanocrystals was dissolved in a solvent mixture of ODE and OAm (4.0 mL, ODE/OAm: 3:1 ), and then hexane was removed under vacuum. Under argon flow, the nanocrystal solution was heated to 280 oC. Zinc -stearate solution (0.04M in ODE) and sulfur solution (0.04M in ODE) were alternatively introduced into the hot solution b y dropwise addition. After the desired shell thickness was

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98 achieved, a zinc -stearate solution (0.12 mmol, 0.04M in ODE) was added to the reaction system. Then the synthesis was stopped by cooling the reaction solution to room temperature, and the nanocryst als were precipitated by adding acetone. The nanocrystals can be re -dispersed into non polar organic solvents. The resulting nanocrystals were further purified by three precipitationredispersion cycles using acetone and hexane For EPR or ICP measurements the samples were further purified twice by a pyridine -exchange treatment according to a literature method.139, 158 4.2.3 Kinetic Study of Mn Adsorption 4.2.3.1 Formation of weakly bound Mn In a typical experiment, 4.1 nm CdS/ZnS core/shell nanocrystals wi th 3.1 -nm CdS cores (150 nmol) were loaded into a solvent mixture of ODE and OAm (12.0 mL, ODE/OAm: 3:1). Under Ar flow, the nanocrystal solution was heated to the growth temperatures (180, 220, or 240C), and doping precursors Mn(OAc)2 and sulfur at a mol ar ratio of 1:1 ( 7.2 mol, an amount equivalent to 48 atoms per nanocrystal) were added. After the addition, aliquots (1.5 mL) were taken periodically during dopant growth. The nanocrystals in the aliquots were purified by three precipitation/redispersion cycles for EPR and ICP measurements. 4.2.3.2 Formation of strongly bound Mn In a typical experiment, 4.1 nm CdS/ZnS core/shell nanocrystals with 3.1 -nm cores (500nmol) were loaded into a solvent mixture of ODE and OAm (40 mL, ODE/OAm : 3:1). Under Ar flow, the nanocrystal solution was heated to the growth temperatures (250, 260, 270, or 280 C), and doping precursors (Mn(OAc)2 and sulfur at a molar ratio of 1:1 (24 mol, an amount equivalent to 48 atoms per nanocrystal) were added. After the addition, aliquots ( 5 mL) were taken periodically during dopant growth. The nanocrystals in the aliquots were purified by three precipitation/redispersion cycles. To remove the weakly bound Mn species from the

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99 purified particles, ZnS -shell growth was conducted at 220 C with zinc stearate and sulfur as precursors. The thickness of the ZnS shell was approximately 1.5 monolayers. After the ZnS shell growth, the resulting particles were purified according to the procedures described above. The Mn -doping levels of these particles obtained by ICP were used to determine the amount of strongly bound Mn formed in the dopant -growth step. 4.2. 4 Characterization of Mn -Doped CdS/ZnS Core/Shel l Nanocrystals 4.2. 4 .1 Absorption measurements UV-Vis absorption spectra were measured using a Shimadzu UV1701. Nanocrystals were dissolved in toluene for the measurement. 4.2. 4 .2 P hotoluminescence measurements Nanocrystals were dissolved in toluene for th e measurement. Photoluminescence (PL) and photoluminescence excitation (PLE) experiments were performed on a fluorometer (Fluorolog 3, Horiba Jobin Yvon, Irvine, CA). Room temperature fluorescence quantum yields of Mndoped nanocrystals were determined usi ng literature methods.189 Briefly, quinine sulfate in 0.5 m H2SO4 was used as a reference standard, of which the concentration was adjusted to have an absorbance between 0.05 and 0.1 at its peak position of 348 nm. A toluene solution of Mn-doped CdS/ZnS nan ocrystals was also adjusted to a concentration with absorbance equal to that of quinine sulfate. 4.2. 4 .3 TEM and electron diffraction ( ED ) measurements TEM measurements were performed on a JEOL 200X operated at 200 kV, or a JEOL 2010F TEM operated at 200 kV. ED measurements were acquired by the 2010F TEM operated at 200 200mesh copper grid, and was dried overnight at ambient conditions.

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100 4.2. 4 .4 X -ray powder diffrac tion ( XRD) measurements XRD measurements were performed on a Philips XRD 3720 spectrometer. The specimens were prepared as follows: about 15 mg of the purified nanocrystals w ere dissolved in about 0.5 mL of toluene and then dropped onto a low -scattering qu artz sample holder The sample was dried in air and kept overnight in a vacuum des i c cator 4.2. 4 .5 Electron paramagnetic resonance (EPR) measurements The EPR measurements were performed in CW mode on an X -band Bruker Elexsys 580 spectrometer (9.5 GHz) usi ng an Oxford ESR900 cryostat (all the experiments were performed at a temperature of 6 K). The purified nanocrystals were dissolved in a toluene solution with 10% polystyrene to form a glass upon freezing. 4.2. 4 .6 Inductively -coupled plasma atomic emissio n spectroscopy (ICP) measurements The ICP measurements were performed on a Vista RL CCD Simultaneous ICP -AES (Varian, Inc.). The purified nanocrystal samples were digested with nitric acid (69.5%). The digestion was performed at about 100oC until the solution became colorless. The digestion solutions were further diluted with a nitric acid solution to obtain a final nitric acid concentration of about 1 2%. The concentrations of Mn, Cd, and Zn in solutions were determined by data from ICP measurements as com pared with the corresponding working curve s The Mn -doping level (DLMn) is defined as : ] [ ] [ ] [ ] [ Mn Zn Cd Mn DLMn (4 1) In addition, Mn -doping level is also described as the number of Mn per nanocrystal. The n umber was calculated using the size of the CdS core determined by TEM and the ratio of [Mn]/[Cd]

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101 determined by ICP. The Mn -growth yield (Mn GY) is defined as Added ICPMn Mn GY Mn (4 2) where MnICP is t he total amount (mol) of Mn in nanocrystal samples, which is determined by ICP measurements, and MnAdded is the amount (mol) of Mn added as doping precursors at the dopant growth step. The Mn -replacement yield (Mn RY) is defined as 1 21 GY GY RY Mn (4 3) where GY1 and GY2 are measured after the dopant growth and ZnS shell growth steps, respectively. 4.3 Results and Discussion 4.3.1 Synthesis and Characterization of Mn -Doped CdS/ZnS Nanocrystals Our approach to the radial -position -controlled doping of semiconductor nanocrystals is based on a three -step colloidal synthesis: (1) synthesis of starting host particles, (2) Mn-dopant growth, and (3) host -shell growth (Figure 4 1 ). T he radial positions of Mn dopants inside the host core/shell nanocrystals are determined by the diameter of the starting host particles and the thickness of host shells. Mn-doping levels (i.e., concentration of the dopants) of the nanocryst als are controlled in the second step in direct proportion to the amount of MnS -growth precursors added (e.g., Mn( O Ac)2 and S). To achieve a more precise control of the Mn position inside the nanocrystals, a separation is carried out to remove the un reac ted Mn -species from the growth solution before host -shell growth in the third step.

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102 Figure 4 1. Scheme of t hree -step synthesis for synthesizing Mn -doped CdS/ZnS core/shell nanocrystals. (1) synthesis of host particles, (2) Mn-dopant growth, and (3) ho st shell growth. Figure 4 2 (a ) A TEM image of a typical sample of Mn -doped CdS/ZnS nanocrystals (diameter of 6.2 nm with a relative standard deviation of 6 %) with Mn at d = 1.6 ML and doping level (DL) of 0.12%. Inset is a typical highresolution TEM im age. (b ) The PL spectrum of the Mn-doped CdS/ZnS nanocrystals shown in (a ). (c ) An EPR spectra of Mn -doped CdS/ZnS nanocrystals with diameter of 6.2 nm with Mn at d = 1.6 ML and DL of 0.36% A transmission electron microscopy (TEM) image of a typical sample shows that the Mn doped nanocrystals have a diameter of 6.2 nm with a relative standard deviation of 6 %, and remain single crystalline (Figure 4 2 a ). These Mn doped core/shell nanocrystals exhibit two PL -

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103 bands (Figure 4 2 b ): the blue band is assigned to th e emission from the recombination of the quantum -confined excitons at the band edge (BE ) of the core/shell nanocrystals, and the red band is assigned to the emission from Mn dopants (4T1 to 6A1).138 The Mn doping levels of these core/shell nanocrystals wer e determined by a combination of electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma atomic emission (ICP). EPR was used to determine whether Mn dopants are incorporated inside the core/shell nanocrystals, and ICP was used to quantitate the Mn -doping level of these nanocrystals. EPR measurements show that these Mn -doped core/shell nanocrystals exhibit a sixline spectrum with a hyperfine coup ling constant of 69.4 G ( Figure 4 2 c ). This hyperfine coupling constant indicates that the Mn are in cubic ZnS lattice sites, this result is consistant with the X -ray powder diffraction ( XRD ) and electron diffraction ( ED ) patterns (Figure 4 3 ). Both of them show that these core/shell nanocrystals have a zinc blende (i.e., cubic) crystal stru cture and confirm that the dopants are indeed located inside the core/shell nanocrystals.180 182 Figure 4 3 (a ) XRD pattern of Mn -doped CdS/ZnS nanocrystals with Mn at 1.6 monolayer ( ML ) and doping level (DL) of 0.36%. The blue bars show the positions of standard XRD peaks for bulk zinc blende ZnS (b ) An electron diffraction (ED) pattern of the same sample of Mn -doped CdS/ZnS nanocrystals The three -step synthesis allows a systematic study of the effects of Mn -doping level of CdS/ZnS core/shell nanocr ystals. Nine types of core/shell nanocrystals were synthesized with

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104 Mn -doping levels from 0.013% to 1.8%. These nanocrystals have a 3.1 nm CdS core coated with a ZnS shell of 4.8 ML, and Mn dopants are at 1.6 ML in the shell. The Mn-doping levels were dete rmined by ICP measurements. With the increase of the Mn -doping level, the QY of the Mn emission increases, while the QY of blue -exciton emission decreases (Figure 4 4 a). The exciton emission is totally quenched when the doping level reaches 0.44%. In addit ion, the QY of Mn emission reaches a maximum of around 44% when the doping level is 0.36% (Figure 4 4 b ). With a further increase of the Mn-doping level, the QY of the Mn emission decreases. This further increase in Mn -doping level could cause stronger MnM n interactions and/or create greater crystal -field strain in ZnS shells. Both cases can increase nonradiative decay of the Mn excited state, and thus lead to a decreased Mn -emission QY. Figure 4 4 (a ) Normalized PL spectra of Mn -doped CdS/ZnS core/shel l nanocrystals with different doping levels. These core/shell nanocrystals have a CdS core diameter of 3.1 nm ( ~ 6%), ZnS -shell thickness is 1.55 nm (~ 4.8 ML), and the Mn dopants are located at 1.6 ML in the shell. (b ) A plot of Mn QY as a function of dop ing level for these nanocrystals shown in (a). Furthermore, by choosing different sizes of CdS nanocrystals, the distance between BE emission and Mn emission can be easily controlled (Figure 4 5 ). Taken together these resultes show that our three -step synt hesis method can be used to make Mn -doped core/shell nanocrystals with any peak intensity ratio and different peak positions for these two emissions

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105 Figure 4 5 Normalized PL spectra of Mn -doped CdS/ZnS core/shell nanocrysta ls with different CdS nanocrystals sizes (2.3nm, 3.1nm, 3.8nm, 4.5nm) These core/shell nanocrystals have a ZnS -shell thickness of 1.5 5 nm (~ 4.8 ML), and the Mn dopants are located at 1.6 ML in the shell and an average of 4Mn per particle 4.3.2 Radial -Pos ition Controlled Doping To explore the effects of position -controlled Mn doping, we synthesized three types of CdS/ZnS core/shell nanocrystals with Mn dopant at different positions: inside the CdS core (Figure 4 6 IIIa), at the core/shell interface ( Figure 4 6 IIIb), and in the ZnS shell ( Figure 4 6 IIIc). The data from TEM and IPC show that these three types of Mn-doped core/shell nanocrystals have a nearly identical CdS -core size, ZnS -shell thickness, and Mn-doping level (0.10%, ~4 Mn atoms per particle). The PL excitation spectra of these CdS/ZnS nanocrystals indicate that energy transfer from the exciton in the nanocrystals to the Mn dopants gives rise to the red emission.3 In addition, these nanocrystals exhibit a nearly identical absorption peak positio n for their first exciton band. This result is consistent with TEM measurements demonstrating that the CdS core size is nearly identical for these Mn -doped core/shell nanocrystals. Importantly, these Mn -doped nanocrystals exhibit dopant position-dependent optical properties (Figure 4 6d, e and f). The QY of the BE emission is similar for these nanocrystals, but the QY of the Mn emission is substantially different for the nanocrystals with Mn dopants inside a CdS core (8%), at the core/shell interface (16% ), and in the ZnS shell (24%). These

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106 results indicate that the nonradiative decay of the Mn excited state (4T1)not the overlap between the Mn and exciton wave -functions (i.e., energy transfer from the exciton to the Mn) plays the dominant role in controlling the Mn -emission QY. Therefore, the position -dependent Mn -emission QY would be caused by the following two factors: (1) the Mn -Mn interactions inside a doped core/shell nanocrystal, and (2) the local crystal -field strain on the Mn dopants. Figure 4 6 A scheme of Mn -doped CdS/ZnS core/shell nanocrystals with different Mn positions: (a) inside the CdS core, (b) at the core -shell interface, and (c) in ZnS shell. The final core/shell particles (i.e., IIIa, IIIb, and IIIc) have CdS core diameter of 3. 8 nm (with a relative core/shell nanocrystals: (d) IIIa, (e) IIIb, and (f) IIIc. The corresponding EPR spectra (taken at 9.5 G Hz and 6 K) are shown in (g) as the black (IIIa), blue (IIIb) and red (IIIc) lines. Inset is a zoom -in plot of the third peak of these EPR spectra. The Mn positions inside the core/shell nanocrystals were identified using electron paramagnetic resonance ( EPR) spectroscopy. These three types of core/shell nanocrystals

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107 exhibit a six line spectrum with a similar hyperfine coupling constant of 69.7 G (Figure 4 6 g). Importantly the line -width of the EPR peaks is different for the nanocrystals with Mn dopants in the CdS core (12 G), at the core/shell interface (12 G), and in the Zn S shell (7.4G, inset of Figure 4 6 g). The narrower EPR -peak line -width indicates weaker Mn -Mn interactions and less local strain on the Mn dopants in the ZnS shell. Both effects can lead to less non radiative decay of the Mn excited state, and therefore, a higher Mn -emission QY for the core/shell particles with Mn in the shell. It is consistent with the results from o ptical measurements (Figure 4 6 d -f). Taken together, the results f rom both optical and EPR measurements suggest that radial -position controlled Mn -doping of CdS/ZnS nanocrystals is achieved by our three -step synthesis. 4.3. 3 Mechanis tic Study After successfully synthesizing the radial -position -controlled Mn doped nanocr ystals, a detail ed mechanistic study is needed to further understand the dopant growth process. 4.3. 3 .1 Surface bound Mn In the dopant -growth step, Mn atoms are adsorbed onto the surface of 4.1-nm CdS/ZnS core/shell nanocrystals (3.1nm CdS core with 1.6ML ZnS shell) To determine whether Mn atoms are indeed on the surface of nanocrystals, we carried out four sets of experiments. The first two sets of experiments were conducted at 220 and 280 C. In these experiments, dopant precursor (Mn acetate and S at a molar ratio of 1:1 with 4 Mn atoms per nanocrystal) was added into growth solutions (4 mL, ODE and OAm at a ratio of 3:1) with CdS/ZnS nanocrystals (25.2 nmol). After growth for 20 min, the growth solutions were cooled to room temperature. The nanocrystals were isolated from the growth solutions, and further purified by three precipitation redispersion cycles. Resulting nanocrystals were redispersed in toluene.

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108 Figure 4 7 EPR spectrum of 4.1 nm CdS/ZnS core/shell nanocrystals with surface -bound Mn (in red) and that of the control (in blue): synthesized at 220 C (a), and 280 C (b). (c) ICP data for the Mn doping level (MnDL) of the two nanocrystal samples (in red) and their corresponding controls (in blue): the Mn DL is equi valent to an average of 2.5 Mn per particle for the sample made at 220 C, and 2.8 Mn for the sample made at 280 C. (d) EPR spectrum of 6 2 -nm CdS/ZnS core/shell nanocrystals with Mn incorporated inside ZnS shell before (in blue) and after (in red) pyridi ne -exchange treatment. (e) ICP data for the corresponding nanocrystal samples: the DL is equivalent to an average of 4 Mn atoms per particle. EPR results show that typical samples from these two sets of experiments exhibit a broad six line spectrum with a similar hyperfine coupling constant of 92.8 G (red lines in Figure 4 7 a b ). Such a large hyperfine coupling constant is close to that of Mn in octahedral sites.13 9, 180 183 This result suggests that Mn atoms are bound on the surface of nanocrystals. However these EPR data cannot rule out the possibility that Mn atoms are in molecular complexes dispersed in the nanocrystal toluene solutions. To examine this possibility, we carried out two sets of corresponding control experiments, in which only Mn precursors were added t o the reaction solutions at 220C and 280 C without host particles. After heating for 20 min at the

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109 corresponding temperatures, the solutions were cooled to room temperature, and 4.1 nm CdS/ZnS nanocrystals were added. Then nanocrystals were i solated from the solutions, and further purified by three precipitationredispersion cycles as before. The resulting particles were redispersed in toluene and used as control samples. Both EPR and ICP measurements show no measurable Mn signals from these c ontrol samples (blue lines in Figure 4 7 a -c ). These results unambiguously demonstrate that the observed Mn signals do not arise from any Mn molecular complexes dispersed in the nanocrystal toluene solutions, but rather come from the Mn atoms bound to the s urface of the CdS/ZnS nanocrystals. More importantly, these results also suggest that the incorporation of Mn atoms onto the nanocrystal surface was achieved through dopant growth at the reaction temperatures, but not due to the subsequent purification tre atment at room temperature. Therefore, the quantitative data from ICP measurements should represent the actual Mn -doping levels of the resulting nanocrystals. Accordingly, the Mn-growth yield for each reaction was calculated as the ratio of [Mn]ICP to [Mn]Added. In the ZnS -shell -growth step, surface -bound Mn atoms can be incorporated into the lattice of the ZnS shell in the resulting particles ( Figure 4 1). This is confirmed by the EPR measurements (Figure 4 2c Figure 4 6g and Figure 4 7 d ). Interestingly, the sample before pyridine treatment shows a nearly identical EPR spectrum (blue line in Figure 4 7 d ) to that of the sample after pyridine treatment (red line in Figure 4 7 d ). In addition, ICP measurements show that these two samples have a nearly identic al doping leve l (Figure 4 7 e ). These results indicate that these Mn -doped nanocrystals have a very clean surface, and the amount of surface bound Mn is not detectable by either EPR or ICP. In other words, the Mn species that are not incorporated into the lattice of CdS/ZnS core/shell nanocrystals should exist in reaction solutions, and they can be easily removed by the purification procedure using three

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110 precipitation/redispersion cycles. On the basis of the doping levels determined by ICP, the net Mn -grow th yield after ZnS -shell growth was calculated as described above. 4.3. 3 .2 Weakly and strongly bound Mn In the mechanistic study, 4.1 -nm CdS/ZnS core/shell nanocrystals were chosen as host particles and a mixture of ODE and OAm (3:1) as growth solutions, w hile keeping the particle concentration (6.3 M) and dopant -precursor concentration (0.3 mM) unchanged in the doping experiments. With manganese acetate and sulfur as precursors, doping growth leads to a Mn growth yield of about 63% in the experiment at a growth temperature of 220 C, and a yield of 69% at 280 C (Figure 4 8 a -A, b -A ). Figure 4 8 Mn -growth yield (Mn GY) measured by ICP after the dopant growth and ZnS -shell growth steps: ( a ) T = 220 C and ( b ) T = 280 C; Column a : taken after dopant gro wth step, and column b : taken after the growth of 0.5 nm ZnS shell. ( c ) Schematic for Mn adsorption and replacement. (i) Host particle, (ii) particle with Mn at surface, (iii) final particle. ( d ) Schematic for the proposed structures of the weakly and stro ngly bound Mn species on the surface of CdS/ZnS core/shell nanocrystals. After ZnS -shell growth, however, nearly all the surface bound Mn atoms were removed from the host particles prepared in the experiment with the dopant growth at 220 C, which

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111 resulted in a net Mn growth yield of nearly zero (Figure 4 8 a B). This result indicates that these Mn dopants are only weakly adsorbed on the surface of host particles at this low temperature growth condition (called weakly bound Mn). In contrast, for the experim ent with Mn dopant growth at 280 C, the ZnS -shell growth removes only about three -fourths of Mn atoms from the host particles, and results in a net Mn growth yield of about 17% for the doping experiment (Figure 4 8 b B). Therefore, at least onefourth of M n atoms are strongly adsorbed on the surface of host particles in this high temperature dopant growth condit ion (called strongly bound Mn). Interestingly, before ZnS -shell growth, the core/shell nanocrystals with strongly bound Mn do not exhibit a distinguishable EPR spectrum as compared to those without strongly bound Mn (red lines in Figure 4 7 a and b ). These EPR data show that the formation of strongly bound Mn does not substantially change the Mn coordinating environment on the surface of CdS/ZnS core/ shell nanocrystals. F urther more, these data indicate that both weakly bound and strongly bound Mn exist as molecular complexes with a six -coordination environment around Mn(II) centers (e.g., acetate and/or OAm as chelating ligands).180, 182 Altogether, the se results suggest that the weakly bound Mn complexes likely serve as Lewis acid ligands which bind onto the Lewis -base sites (i.e., sulfur sites) on the surface of CdS/ZnS core/shell nanocrystals via weak coordination bonds, whereas the strongly bound Mn complexes are attached onto the nanocrystal surface through one or more strong MnS bonds (Figure 4 8 d ). In addition, the results show that the Mndoping level is not detectable for the CdS/ZnS core/shell nanocrystals prepared in the experiments with the dopant growth and ZnS -shell growth temperature at 240 C, but the doping level is detectable for the nanocrystals in the experiments conducted at higher temperatures (Figure 4 8 c ). This temperature -dependent

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112 phenomenon strongly suggests that the adsorption of Mn dopants is determined by the kinetics of chemical reactions between Mn precursors and nanocrystal surface atoms. 4.3. 3 .3 Kinetics of Mn dopant adsorption The kinetics of Mn adsorption were studied by monitoring the Mn -growth yield as a function of t ime during dopant growth at different temperatures. For the formation of the weakly bound Mn, the growth kinetics were studied at three temperatures (180, 220 and 240 C) because only weakly bound Mn can be formed at these conditions. Aliquots were taken p eriodically during dopant growth, and the resulting particles were purified by three precipitation/redispersion cycles for ICP measurements. ICP data show that Mn-growth yields reach about 63% at 1 min after the addition of dopant precursors (Mnacetate an d S at a molar ratio of 1:1 and 48 Mn per nanocrystal) (Figure 4 9 a ). The Mn -growth yields remain nearly constant during further growth at the three growth temperatures (Figure 4 9 a ). These ICP data indicate that Mn adsorption reaches a steady state after only 1 min. This result is consistent with the conclusion that weakly bound Mn complexes act as a type of ligand on the nanocrystal surface. T he formation of weakly bound Mn includes ligand adsorption and desorption processes, and the two processes can rapidly reach a chemical equilibrium in the growth solutions at these reaction temperatures ( Figure 4 10).184 For the formation of the strongly bound Mn, the growth kinetics were studied at four temperatures (250, 260, 270 and 280C). Aliquots were taken periodically during dopant growth, and the resulting particles were purified for ICP measurements. Because both weakly bound and strongly bound Mn form under these conditions, the Mn-doping level obtained from the direct ICP measurements of the resulting p articles includes the contributions from these two types of Mn. Surprisingly, ICP measurements show that Mn-growth yields also remain nearly constant during dopant growth in these experiments (Figure 4 9b ). The growth yields do not exhibit

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113 strong temperature dependence, but the yields are just slightly higher than those yields obtained from the experiments at lower temperatures (Figure 4 9a b ). Figure 4 9 Kinetic study of Mn adsorption. The formation of weakly bound Mn: ( a ) Mn growth yield (Mn -GY) as a function of time at three different temperatures (180, 220, and 240C, indicated by green, blue, and red lines, respectively). The formation of strongly bound Mn: ( b ) Mn GY before ZnS growth as a function of time at four different tempe ratures (250, 260, 270, and 280C, indicated by purple, blue, green, and red lines, respectively). ( c ) A plot of ln{[Mn]a/([Mn]a[Mn]s)} versus time (s). [Mn]a is the concentration of total adsorbed Mn on the surface of nanocrystals determined by ICP before ZnS growth, and [Mn]s is the concentration of strongly bound Mn on the surface of nanocrystals determined by ICP after ZnS growth. The first order rate constants ( kT) are extracted from the plots. (D) Arrhenius plot of ln(k ) verses 1/ T Activation energy ( Ea) and the entropy of activation ( S#) are extracted from the plot. These results suggest that there is also an equilibrium between the adsorption and desorption of Mn complexes in growth solutions, and the formation of strongly bound Mn does not substantially affect the equilibrium. This suggestion is consistent with the fact that the

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114 formation of strongly bound Mn does not increase the number of Lewis -base sites on the surface of nanocrystals. Thus the overall equilibrium achieved in these experiments is between the concentration of the total adsorbed Mn on nanocrystals and the concentration of free Mn complexes in the growth solution at these temperatures. To quantitat e the concentration of strongly bound Mn on the resulting particles in these four experiments (i.e., at 250, 260, 270 and 280C), a ZnS -shell growth at 220C was conducted to remove the weakly bound Mn species from the surface of CdS/ZnS core/shell nanocrystals. After the ZnS -shell growth, the core/shell nanocrystals were purified by three precipitation/redispersion cycles, and then followed by pyridine treatment twice. Then the Mn concentrations measured from these nanocrystals were used to determine the concentration of the strongly bound Mn formed during dopant growth. After removing the weakly bound Mn, ICP measurements s how that the concentrations of strongly bound Mn increase with dopant growth time (Figure 4 9 c ). The formation of strongly bound Mn follows first -order kinetics, and the rate constants ( k2, Figure 4 10a ) at the four different temperatures were extracted from linear curve fitting (Figure 4 9 c ).185,186 In addition, the temperature dependence of these rate constants exhibits Arrhenius behavior (Figure 4 9 d ).185,186 On the basis of the Arrhenius plot,185,186 the activation energy Ea s, for the formation of stro ngly bound Mn is 211 13 kJ/mol (Figure 4 9d ). This large activation -energy barrier indicates that the transition state of the formation of strongly bound Mn should be associated with the cleavage or formation of chemical bonds ( Figure 4 10). Furthermore, based on the Eyring equation,185,186 we also extracted the entropy of activation ( S# = 76.1 J mol1 K1) of the transition state from the Arrhenius plot (Figure 4 9d ). The positive entropy of activation suggests that the rate determining step in the formation of strongly bound Mn is a unimolecular

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115 decomposition reaction.185,186 The cleavage of the Mn O bond via the detachment of an acetate group in the weakly bound Mn complexes acts as the rate -determining step, because the Mn O bond is the strongest bond in the complexes. Then the rate determining step is accompanied by the formatio n of the strong Mn-S bond, resulting in the strongly bound Mn on the surface of nanocrystals ( Figure 4 10). Figure 4 10. (a) Proposed mechanism for the formation of the weakly and strongly bound Mn NC (nanocrystal). Manganese acetate (the dopant precurs or) may exist as a coordination polymer in reaction solutions.58 (b ) Proposed reaction profile for the formation of the weakly and strongly bound Mn. Gw is the formation energy of weakly bound Mn, and Ea s is the activation energy for the formation of strongly bound Mn. 4.3. 3 .4 Replacement of Mn dopants The results from the previous sections show that weakly bound Mn on the surface of host particles can be removed by ZnS -shell growth, and specifically that the Mn dopants are replaced by the incorporation o f Zn atoms onto ZnS lattice sites. Here, a fundamental question is whether strongly bound Mn can be removed by ZnS -shell growth. This question is important for

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116 understanding the detailed nanocrystal doping mechanism. If the answer is no, the Mn doping leve l of the final particles is determined solely by the reactions for Mn adsorption. If yes, the Mn -doping level of final particles should also be determined by the reaction kinetics of Mn replacement. To answer this question, s even sets of experiments were c onducted to examine Mn replacement yield as a function of the temperature of ZnS -shell growth (Figure 4 11a ,b ). To guarantee consistency in all these experiments, the CdS/ZnS core/shell nanocrystals with surface-bound Mn were prepared from the same dopant -growth synthesis that was conducted at 280C with Mn acetate and sulfur as precursor (1:13, 48 atoms per nanocrystal). The Mngrowth yield of nearly 100% was found in the doping-growth step and resulted in nanocrystals with an average of 48 Mn atoms on the surface. After ZnS shell growth, the net Mn -growth yield was measured for each experiment. The Mn replacement yield was determined as a ratio between the Mn -growth yields measured after and before ZnS shell growth. Experimental data show that ZnS -shell gr owth at 220, 230, and 240 C resulted in a nearly identical Mn -replacement yield of about 20% (Figure 4 10b ). These results suggest that weakly bound Mn on the surface of the nanocrystals is 20% before ZnS -shell growth. At higher temperatures, additional a mounts of Mn are removed from the nanocrystal surface, and the Mnreplacement yield exhibits strong dependence on the temperature of ZnS -shell growth. The higher the growth temperature, the higher the replacement yield obtained and vice versa. These result s further confirm the conclusion that two types of Mn spe cies (i.e., weakly and strongly bound Mn) are formed in the dopant -growth step ( Figure 4 10). More importantly, these results demonstrate that ZnS -shell growth indeed removes some of the strongly bou nd Mn atoms from the surface of host particles at higher temperatures. The replacement reaction likely follows a

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117 cation -exchange reaction mechanism Mn dopants are replaced from the surface of nanocrystals by the Zn atoms from zinc stearate. Therefore, the Mn -doping level of final particles is determined by the reaction kinetics of both the Mn adsorption and replacement processes. Figure 4 1 1 Study of Mn replacement. The concentration of host particles (4.1 nm CdS/ZnS nanocrystals) is 6.3 M. ( a ): Schem e of experimental design. ( b ): Mn replacement yield (Mn -RY, red) and Mn -growth yield (Mn -GY, blue) as a function of the temperature ( Tr) for the growth of a 0.5 -nm ZnS shell. ( c ): Photoluminescence excitation spectrum (blue) and photoluminescence spectrum (red) of the Mn -doped nanocrystals, which were synthesized with a d opant -growth temperature of 280C and a ZnS -shellgrowth temperature of 280 C. The Mn -precursor concentration is 0.09 mM (i.e., 15 Mn atoms per nanocrystal) and the ZnS -shell thickness is 4. 8 monolayers. The final Mn -doping level is 0.11% (i.e., an average of 3.6 Mn atoms per nanocrystal), as measured by ICP. ( d ): Photoluminescence excitation spectrum (blue) and photoluminescence spectrum (red) of the Mn -doped nanocrystals, which were synthes ized with a dopant -growth temperature of 280C and a ZnS -shell growth temperature of 220C. The Mn -precursor concentration is 0.09 mM The final Mn doping level is 0.35% (i.e., an average of 11 Mn atoms per nanocrystal), as measured by ICP. In addition, th e temperature -dependence of the replacement yield suggests that there is an activation -energy barrier required to start the Mn -replacement reaction. In the ZnS -shell growth step, the Mn replacement reaction and ZnS growth occur concurrently. ZnS growth can embed surface-bound Mn atoms into the ZnS lattice, and thus terminate the Mnreplacement reaction.

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118 Technically, it is very difficult to decouple these two reactions. Thus, it is very hard to obtain the value of the activation energy barrier for the Mn rep lacement reaction. However, it is still safe to conclude that the temperature dependence of the Mnreplacement yield is due to overall effects from the reaction kinetics of Mn replacement and ZnS growth, in which both processes need to overcome their respe ctive activation -energy barrier s Based on this new understanding, one can easily control the kinetics of Mn replacement through the reaction temperatures, and thus can control the final Mn -growth yield for doped host particles (Figure 4 1 1 b blue curve, and Figure 4 1 1 c ,d ). Indeed, the two Mn-doped CdS/ZnS nanocrystals made with ZnS -shell growth at different temperatures exhibit a clear difference in the intensity ratio of the BE and Mn emission (Figure 4 1 1 c ,d ). In addition, our results have demonstrated the ability to control the net Mn -growth yield between 25% and 80% by tuning the temperature of ZnS -shell growth (blue curve in Figure 4 1 1 b ). 4.3. 3 5 R eplaced Mn species To produce high -quality nanocrystals with Mn dopants at controlled radial positions we need to know the fate of those replaced Mn atoms in the reaction systems. Can those replaced Mn atoms be readsorbed onto the surface of host particles? If yes, it will be difficult to precisely control the Mn position inside host particles because of the incorporation of the replaced Mn during the subsequent ZnS shell growth (Figure 4 12a ii). To examine this possibility, six experiments were conducted allowing the mixture of Mn-doped particles and replaced Mn species to further react with sulfur at di fferent excess amounts (Figure 4 12). In these six experiments, however, the reaction with additional sulfur did not affect the doping levels of the final products even with 13 -fold excess of sulfur (Figure 4 12b). This result demonstrates that the replace d Mn cannot be readsorbed onto the nanocrystal surfaces at these synthesis conditions. It indicates that the replaced Mn atoms form chemical species with very low reactivity, such as

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119 clusters with polymerized (Mn O)n and (Mn S)n. Zinc stearate likely plays a major role in the formation of such Mn species in the cation -exchange reaction. Because the replaced Mn atoms cannot be reincorporated into the lattice of nanocrystals, one should be able to control the Mn radial position inside nanocrystals through suc h a three -step synthesis. Figure 4 12. Mechanistic study to explore whether the replaced Mn species can be readsorbed onto doped nanocrystals. (a ): Scheme of experimental design, (i) replacement reaction, (ii) and (iii): 0.5 nm ZnS shell is further grow n onto the nanocrystals with surface-bound Mn.; ( b ): Mn -doping level (Mn -DL) and Mn -growth yield (Mn GY) as a function of the amount of additional sulfur (S/Mn ratio). The Mnprecursor concentration is 0.3 mM, and the concentration of host particles is 6.3 M. 4.4 Conclusion In conclusion, we have developed a new doping approach using a three -step synthesis (host particle synthesis, Mn -dopant growth, and ZnS -shell growth ) to produce high quality Mn -doped CdS/ZnS core/shell nanocrystals. This approach allows the precise control of Mn radial position and doping level in core/shell nanocrystals. We have shown the first example in which the optical properties of Mn-doped nanocrystals strongly depend on Mn radial positions inside the nanocrystal. Taking the advantages of this new synthesis approach, the mechanism of dopant growth on the Mn-doping of CdS/ZnS core/shell nanocrystals was studied using a combination

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120 of EPR and ICP to monitor Mn doping level and growth yield during doping synthesis at both the dopant -g rowth and ZnS -shell growth steps. The kinetic s study shows that Mn adsorption onto the nanocrystal surface includes the formation of weakly and strongly bound Mn. The formation of weakly bound Mn is associated with a chemical equilibrium between adsorbed M n species on the nanocrystal surface and free Mn species in the growth solution, while the formation of strongly bound Mn exhibits first -order kinetics with an activation -energy barrier of 211 13 kJ/mol. Furthermore the results demonstrate that both weakl y and strongly bound Mn can be removed from the surface of nanocrystals during ZnS -shell growth. The replacement of strongly bound Mn requires a higher temperature than that of weakly bound Mn. The yield of the replacement of strongly bound Mn is strongly dependent on the temperature of ZnS -shell growth. Taking all these results together, we propose a nanocrystal doping mechanism in which dopant incorporation is determined by three activation-controlled processes: dopant adsorption, dopant replacement, and host shell growth. These results could be generalized for doping nanocrystals of other compositions and with other impurities (e.g., indium and cobalt ), but these impurities should have a negligible diffusion coefficient inside the host lattice at typical synthesis temperatures ( < 300 C).

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121 CHAPTER 5 RADIAL -POSITION CONTROLLED DOPING OF CDS/ZNS CORE/SHELL NANOCRYSTALS: POSITI ON DEPENDENT AND EXCITA TION INTENSITY DEPENDENT PROPERTIES 5 .1 Introduction Manganese-doped semiconductor nanocrystals exhibit unique optical and magneto -optical properties, which make them excellent testbeds for both fundamental studies and technological applications in fields ranging from spintronics to biomedical diagnosis.135,138,179 Large Zeeman effects have been observed in Mn-dop ed ZnS, ZnSe and CdSe nanocrystals, and the effects indicate that quantum -confined excitons feel a large effective magnetic field of up to 400 Tesla, induced by the presence of a few Mn2+ ions inside the nanocrystals.135,137,179 Spin -polarizable excitonic photoluminescence (PL) has also been observed in Mn-doped CdSe nanocrystals.187 Additionally, Mn dopants can introduce new luminescence properties to nanocrystals, resulting in particles with dual emission properties.135,137,138,149,151153,155,179,187 Mor eover, recent work has shown that Mn -doped CdS/ZnS core/shell nanocrystals possess dopant -positiondependent PL properties, and Mn dopants can be used as a radial pressure gauge to measure the lattice strains in the nanocrystals.188 In this chapter we stu dy the PL properties of Mn -doped CdS/ZnS core/shell nanocrystals in two excitation intensity regi mes: (1) weak excitation intensity regime using a fluorometer as the excitation source; (2) strong excitation intensity regime using a XeCl excimer laser as th e excitation source. The results show that, under weak excitation, the integrated intensity ratio of two emissions ( IBE/IMn) is not dependent on the excitation intensity. Also, the Mn -PL QY of the core/shell nanocrystals is determined by the product of two factors: the energy -transfer efficiency (ET) and the efficiency of the emission from one Mn ( Mn). Mn exhibits a radial -position dependent change that nearly perfectly corresponds to that of the Mn EPR linewidth of the

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122 nanocrystals: the higher the Mn, the narrower the Mn EPR linewidth. Under strong excitation, Mn -doped CdS/ZnS core/shell nanocrystals exhibit excitationintensity -dependent dual emissions. The color tunable property of these nanocrystals originates from the excitation of multiple Mn dopan ts inside a nanocrystal. We have discovered that the excitation fluence at which Mn -emission intensity reaches its maximum is dependent on the number of Mn dopants in a nanocrystal. In addition, we have demonstrated that the color tunable process in these nanocrystals exhibits extraordinary reversibility. Th ese new finding s provide important insight into the fundamental mechanisms of carrier relaxation dynamics in Mn -doped semiconductor nanocrystals. 5 .2 Experimental S ection 5 .2.1 Chemicals Sulfur powder (99.999%), oleic acid (OLA, 90%), 1 -octadecene (ODE, 90%), octadecane (ODA, 99%), oley l amine (OAm, 70%) and polystyrene block polybutadiene ( C12H14, 30wt% styrene, 80% diblock ) were purchased from Sigma -Aldrich. Cadmium nitrate tetrahydrate (Cd(NO3)24H2O, 99.99%), zinc stearate (count as ZnO% purchased from Alfa Aesar. Nitric acid (69.5%, TraceSELECT) was purchased from Fluka. Manganese acetate tetrahydrate (99%), s odium hydroxide (NaOH) and all the other solvents were pu rchased from Fisher Scientific International Inc. Quinine sulfate (99+ %) was purchased from Acros The chemicals were used as received without further purification. Cadmium myristate was synthesized according to the literature method.58 5 .2.2 Three -Step Sy nthesis of Mn -Doped CdS/ZnS Core/Shell Nanocrystals 5 .2.2.1 Preparation of precursors (1 ) Mn(OAc)2 Solution OAm (4 mL) w as added to a 25 mL flask and heated at 120 oC for 10 min under a vacuum of 20 mTorr. After the OAm solvent was cooled to room temperat ure,

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123 Mn(OAc)2 4H2O (4.9 mg, 0.02 mmole) was quickly added to the flask. The mixture was degassed at room temperature and 120 oC for 10 min for each temperature. After a clear solution was obtained, the solution was cooled to room temperature and ready for use. Note that the Mn precursor solution should be freshly made before the synthesis. (2 ) Mn(S2CNEt2)2 Solution. An OAm solution of Mn(OAc)2 (2 mL, 10 mM) was prepared according to the protocol above, and an oleylamine solution of NaS2CNEt2 (2 mL, 22 mM) w as made in pretreated oleylamine at 60 C under Ar flow. Then the NaS2CNEt2 solution was added into the Mn(OAc)2 solution at 60 C with stirring under Ar flow. After 10 min, a slightly yellow solution of Mn(S2CNEt2)2 was obtained and was used directly for dopant growth. Note that the Mn -precursor solution should be freshly made before the synthesis. (3 ) Sulfur solution Sulfur powder (12.8 mg, 0.4 mmol) was added to a flask with ODE (10 mL). After degassing at room temperature for 10 min, the solution was heated to 130 oC under Ar flow. The temperature was maintained for 5 min, and then the resulting sulfur solution was cooled to room temperature for use. Caution should be taken to avoid heating the solution to a higher temperature. (4) Zinc stearate solutio n Zinc stearate powder (0.4 mmol) was added to a flask with ODE (10 mL). After degassing at room temperature for 10 min, the mixture was heated to 200 oC to dissolve zinc stearate. The solution was c ooled to room temperature and a slurry formed. The slurr y was directly used for ZnS -shell growth. 5 .2.2.2 Synthesis of Mn-doped CdS/ZnS core/shell nanocrystals (1 ) Synthesis of starting host particles A CdS nanocrystals The synthesis of CdS nanocrystals was based on a modification of a literature method.57 In a typical synthesis, cadmium myristate ( 1.0 mmol) and S (0. 5 mmol) were

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124 loaded into a three -neck flask with 1 -octadecene (ODE, 5 0 g). After degassing under vacuum (~20 mTorr) for 10 min, the vacuum was removed. Under argon flow, the temperature was raised to 240 oC. The growth was monitored by taking the absorption spectra of aliquots extracted from the reaction solution. When particles reach ed the desired size, the reaction mixture was allowed to cool to room temperature, and the nanocrystals were precipitat ed by adding acetone and redispersed in hexane The as -prepared CdS nano crystals have a zinc -blende crystal structure. B. CdS /ZnS core/shell nanocrystals First, zinc -blend e CdS nanocrystals were prepared as descriped above. Second, ZnS shells were grown onto the resulting CdS nanocrystals at 220 oC in a solvent mixture of ODE and OAm with a volume ratio of 3:1. ZnS shells were added into the reaction solution with CdS nanocrystals by alternate injections of a solution of zinc stearate in ODE (40 m M ) and su lfur in ODE (40 m M ). Growth time was 10 min after each injection. When the desired shell thickness was achieved, the growth solution was cooled to room temperature The resulting Cd S /ZnS core/shell nanocrystals were precipitated by adding acetone, and redi spersed in hexane (2 ) Growth of dopant In a typical experiment, a hexane solution of host particles (2 mL, 50.4 nmol) was added to a solvent mixture of ODE and OAm (8.0 mL, ODE/OAm: 3:1), and then hexane was removed under vacuum. Under argon flow, the n anocrystal solution (CdS nanocrystals or CdS/ZnS core/shell nanocrystals as starting host particles) was heated to the growth temperature (220oC or 280oC) and then the sulfur solution and either Mn(S2CNEt2)2 solution or Mn( O Ac)2 solution at a molar ratio o f 1:1 were introduced into the hot solution by dropwise addition. After a further 20 min reaction, the synthesis was stopped, and nanocrystals were precipitated by adding acetone and re dispersed in hexane as a high-concentration solution for further use

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125 (3 ) Growth of host shell A hexane solution of Mn-doped CdS nanocrystals or Mn -doped CdS/ZnS nanocrystals was added to a solvent mixture of ODE and oleylamine (4 .0 mL, ODE/OAm: 3:1 ), and then hexane was removed under vacuum. Under argon flow, the nanocrysta l solution was heated to 220 oC. Then zinc -stearate solution (0.04M in ODE) and sulfur solution (0.04M in ODE) were alternatively introduced into the hot solution by dropwise addition. When ZnS -shell thickness reached about 1.6 monolayers, the reaction sol ution was heated to 280 oC for further ZnS shell growth. After the desired shell thickness was achieved, a zinc -stearate solution (0.12 mmol, 0.04M in ODE) was added to the reaction system. After 5 min the synthesis was stopped by cooling the reaction solu tion to room temperature, and the nanocrystals were precipitated by adding acetone. The resulting nanocrystals can be redispersed into non-polar organic solvents. The resulting nanocrystals were further purified by three precipitationredispersion cycles u sing acetone and hexane For EPR or ICP measurements, the samples were further purified twice by a pyridine -exchange treatment according to a literature method.139,158 5 .2.3 Characterization of Mn -Doped CdS/ZnS Core/Shell Nanocrystals 5 .2.3.1 Absorption me asurements UV-Vis absorption spectra were measured using a Shimadzu UV1701. Nanocrystals were dissolved in toluene for the measurement. 5 .2.3.2 Steady -state photoluminescence measurements Nanocrystals were dissolved in toluene for the measurement. Photolum inescence (PL) and photoluminescence excitation (PLE) experiments were performed on a fluorometer (Fluorolog 3, Horiba Jobin Yvon, Irvine, CA). Room temperature fluorescence quantum yields of Mndoped nanocrystals were determined using literature methods.189 Briefly, quinine sulfate in 0.5 m H2SO4 was used as a reference standard, and the concentration was adjusted to have an absorbance

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126 between 0.05 and 0.1 at its peak position of 348 nm. A toluene solution of Mn-doped CdS/ZnS nanocrystals was also adjusted to a concentration giving the same absorbance intensity as the quinine sulfate. 5.2.3.3 TEM measurements TEM measurements were performed on a JEOL 200X operated at 200 kV, or a JEOL 2010F TEM operated at 200 kV. The specimens were prepared as follows: a pa was dropped onto a 200 mesh copper grid, and was dried overnight at ambient conditions. 5.2.3. 4 Electron paramagnetic resonance (EPR) measurements The EPR measurements were performed in the CW mode on an X -band Bruker Elexsys 580 s pectrometer (9.5 GHz) using an Oxford ESR900 cryostat (all the experiments were performed at a temperature of 6 K). The purified nanocrystals were dissolved in a toluene solution with 10% polystyrene to form a glass upon freezing. 5.2.3. 5 Inductively -coup led plasma atomic emission spectroscopy (ICP) measurements The ICP measurements were performed on a Vista RL CCD Simultaneous ICP -AES (Varian, Inc.). The purified nanocrystal samples were digested with nitric acid (69.5%). The digestion was performed at ab out 100oC until the solution became colorless. The digestion solutions were further diluted with a nitric acid solution to obtain a final nitric acid concentration of about 1 2%. The concentrations of Mn, Cd, and Zn in solutions were determined by data fro m ICP measurements as compared with the corresponding working curve. The Mn -doping level (DLMn) is defined as Equation 5 1 : ] [ ] [ ] [ ] [ Mn Zn Cd Mn DLMn (5 1) In addition, Mn -doping level is also des cribed as the number of Mn per nanocrystal. The number

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127 was calculated using the size of the CdS core determined by TEM and the ratio of [Mn]/[Cd] determined by ICP. 5.2.4 Photoluminescence Measurements Using a XeCl Excimer La ser as Excitation Source 5.2.4.1 Solution sa mples Mn doped CdS/ZnS core/shell nanocrystals were dissolved in hexane with a concentration of 2 16 the solution was placed in a quartz cell (4mm path length) Then this cell was mounted and optically excited with a XeCl excimer laser (308nm, pulse width of ~ 30 ns, repetition frequency of 6 Hz), which was focused using a 28cm focal length lens. The excitation fluence was controlled using a set of neutral density filters. Photoluminescence signals were collected perpendicular to the incident beam by an optical fiber (at a fixed position), and detected using a spectrometer (SD2000, Ocean Optics) w ith 1s integration time. 5.2.4.2 Film samples Polystyrene -block polybutadiene (~300 mg) was dissolved in a toluene solution with Mn doped CdS/ZnS core/shell nanocrystals (1 mL, 1.08 M), and aged overnight. After forming a high viscosity homogenous solution, the solution was drop casted onto a premade stencil with the letter s U and F. After the toluene was totally evaporated the nanocrystal/polymer composite thin fi lm was pulled from the stencil, and placed onto a quartz substrate ( 2.5cm 2.5cm ). This substrate was mounted and optically excited with the output of a XeCl excimer laser (308nm, pulse width of ~ 30 ns, repetition frequency of 6 Hz). The excitation fluence was controlled using a set of neutral density filters. Photoluminescence signals were collected by an optical fiber and detected using a spectrometer (SD2000, Ocean Optics) with an integration time of 1s. The photo images were taken by a digital camera (C anon EOS 40D).

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128 5.2.5 Lifetime Measurements 5.2.5.1 Band -edge (BE) lifetime measurements Mn doped CdS/ZnS core/shell nanocrystals were dissolved in hexane to give an absorbance of 0.1 on a quartz cell, at 375nm The band-edge (BE) lifetime measurements were conducted using a time -correlated single -photon -counting spectrometer with a NANOLED (1 ns, 375 nm, Horiba Jobin Yvon, Irvine, CA). 5.2.5.2 Mn lifetime measurements Mn doped CdS/ZnS core/shell nanocrystals were dissolved in hexane with a concentration o f 2 16 the solution was placed in a quartz cell. Then this cell was mounted and optically excited with a XeCl excimer laser (308nm, pulse width of ~ 30 ns, repetition frequency of 6 Hz), which was focused using a 28cm focal length lens. The excitat ion fluence was controlled using a set of neutral density filters. Photoluminescence signals were collected by an optical fiber (at a fixed position). The fiber was coupled to a R955 photomultiplier tube (PMT, Hamamatsu) through a Micro HR monochromator (H oriba JobinYvon). The signal was amplified by a SR570 current amplifier ( Stanford Research ) and was collected by a TDS520A digital oscilloscope (Tektronix) and saved to a computer by Wavestar 2.4 s oftware (Tektronix). The lifetime is determined by fitting data in FluoFit computer software (PicoQuant) 5 .3 Results and Discussion 5 .3.1 Weak Excitation Intensity Regime 5.3.1.1 Dynamics of carrier relaxiation The PL properties of Mn -doped nanocrystals have been widely studied in the past two decades.135,137,138,149,151153,155,179,187 In general, when a photon is absorbed by a Mn-doped nanocrystal, an electron -hole pair (i.e., exciton) is created and confined inside the nanocrystal.138 The excited electron -hole pair can be deactivated via three processes: (1) r adiative recombination

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129 at the nanocrystal band edge with the rate constant kBE R; (2) nonradiative recombination at the nanocrystal band edge; with the rate constant kBE NR; and (3) energy transfer to a Mn inside the nanocrystal, with the rate constant n kET, where n is the number of Mn inside the nanocrystal. After the energy is transferred to a Mn, the excited Mn (at 4T1) relaxes to its ground state (6A1), radiatively with the rate constant kMn R or nonradiatively with the rate constant kMn NR. F igure 5 1 Scheme of energy levels and carrier relaxation pathways in a Mn doped nanocrystal : CB for conduction band, and VB for valence band. On the basis of the steady -state approximation, the carrier relaxation processes can be described by Equations 5 2 and 5 3 :158 ) (ET NR BE R BE EXnk k k P h I (5 2) ) (NR Mn R Mn Mn ET EXk k nP k nP (5 3) where PEX is the occupation probability of the first exciton state; PMn is the occupat ion probability of the excited state of a single Mn; is the absorption cross section of a single nanocrystal at the excitation wavelength; I is the intensity of the excitation light; and h is the energy of the excitation photon. Under weak excitation, t he efficiency of a deactivation process from an exciton or Mn is simply the ratio of the rates of the process of interest to the total deactivation rate of the state. Therefore, the efficiencies of these proces ses are expressed by Equation 5 4 to Equation 5 7 :

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130 ET NR BE R BE R BE BEk n k k k QY (5 4 ) ET NR BE R BE ET ETk n k k k n (5 5 ) NR Mn R Mn R Mn Mnk k k (5 6 ) Mn ET MnQY (5 7 ) w here, QYB E is the QY of the band-edge PL; ET is the efficiency of energy transfer to Mn; Mn is the efficiency of the emission from a Mn; and QYMn is the QY of the Mn PL. T hese equations are valid only under weak excitation conditions. Because the lifetime of a Mn emission is of the order of mill iseconds,190 further excitation of a nanocrystal having one of the Mn already in its excited state is possible under excitation light with a sufficiently high intensity. In this case, Equation 5 2 no longer applies, because n in Equation 5 2 has changed (note that n is the number of Mn in their ground state). Therefore, Equations 5 2 and 5 3 are valid only under weak excitation conditions. In this case, ET Mn BE Mn BEQY I I / / should not depend on the excitation intensity in the weak excitation regime which is the case when using a fluorometer as excitation source To test it the intensity ratio between the band edge and Mn emissions of Mn -doped CdS/ZnS core/shell nanocrystals was measured as a function of the intensity of the excitation light from the fluorometer (Figure 5 2 ). Two samples with doping level of 1 and 3 Mn per particle were used for the study (Figure 5 2 a,b ). The PL measurements sho w that the intensities of the band edge and Mn emissions decrease with a decrease of excitation -light intens ity using a band pass filter (OD = 0.3 or 0.5), while the ratio of their integrated intensity ( IBE/ IMn) remains constant (Figure 5 2 c ). These results indeed demonstrate that Equations 5 2 to 5 7 are valid under the PL measurement conditions in this study.

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131 Figure 5 2 PL spectra of Mn -doped CdS/ZnS core/shell nanocrystals excited with three excitation -light intensities : the 100% intensity corresponding to the light intensity directly from a fluorometer (Fluorolog 3, Horiba Jobin Yvon, Irvine, CA), the 50% intensity was obtained using a 0.3 OD bandpass filter, and the 30% intensity was obtained using a 0.5 OD bandpass filter. (a ) Nanocrystals with Mn at 1.6 ML and an average of 1 Mn per particle (b ) Nanocrystals with Mn at 1.6 ML and an average of 3 Mn per particle (c ) Plots of the ratio of IBE/IMn as a function of excitation intensity: nanocrystals with an average of 1Mn ( red ) and 3 Mn ( blue ). W hen doping level is low ( trap states of Mn -doped CdS/ZnS nanocrystals are nearly identical to that of undoped CdS/ZnS core/shell nanocrystals. In addition, these CdS/ZnS core/shell nanocrystals have an identical core size an d a nearly identical shell thickness, and were synthesized using a similar high -temperature colloidal synthesis method. Although, in principle, the kB E NR in doped particles may also depend on the doping levels, owing to the lattice strain that the dopant ions introduce, this effect is expected to be small for the low doping levels used in this work Therefore, we can make a reasonable assumption that the nonradiative relaxation rate constants ( kB E NR) are identical for these Mn doped CdS/ZnS nanocrystals w ith low doping level Furthermore, we can also assume that the radiative relaxation rate constants ( kB E R) are identical for these Mn -doped CdS/ZnS core/shell nanocrystals because these nanocrystals have an identical core size and nearly identical shell

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132 t hickness. Based on these two assumptions, Equation 5 5 can be rearranged to give ET NR BE R BE NR BE R BE ETnk k k k k 1 NR BE R BE R BE ET NR BE R BE R BEk k k nk k k k 1 UD BEQY QY 1 (5 8) w here QYUD is the PL QY of the undoped CdS/ZnS core/shell nanocrystals. The QYUD was experimentally found as 63% for the core/shell nanocrystals with a core diameter of 3.1 nm and shell thickness of 1.55nm. Thus, Equation 5 6 c an be rearranged to give ) / 1 (UD BE Mn ET Mn MnQY QY QY QY (5 9) Therefore, ET and Mn can be calculated using the QYBE and QYMn of the doped nanocrystals and QYUD of the undoped nanocrystals. 5.3.1.2 Radial -position -dependent PL and EPR properties To study their radial -positiondependent properties, Mn-doped CdS/ZnS core/shell n anocrystals were synthesized with doping levels of 0.03, 0.12 and 0.36%, which is equivalent to an average of 1, 4, and 12Mn per nanocrystal, respectively. All these core/shell nanocrystals have a core size of 3.1 nm and shell thickness of 1.55 nm ( ~ 4.8 ML ). For each doping level, six samples with Mn radial position ( ) at 0, 0.8, 1.6, 2.4, 3.2, and 4.0 ML were prepared The Mn PL QY and EPR measurements were carried out and the results are shown in Figure 5 3

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133 Figure 5 3 Plots of QYMn as a function of Mn radial position ( ) for CdS/ZnS core/shell nanocrystals wi th an average of (a) 12Mn, (c) 4Mn, and (e) 1Mn. Plo t s of EPR linewidth (LW) as a function of for the CdS/ZnS core/shell nanocrystals with an average of (b) 12Mn, (d) 4Mn, and (f) 1Mn. The insets in these panels are a zoom in plot of the third peak in the EPR spectra of the corresponding Mn-doped nanocrystals with = 3.2. PL QY measurements show that the QY of Mn emission depends on the radi al position of Mn dopants, and a maximum value of QYMn (~ 56%) is achieved for the core/shell nanocrystals with an a verage of 12 Mn and a radial position at 3.2 ML inside the ZnS shell (Figure 5 3a ). EPR measurements show that all these nanocrystals exhibit six line spectra. Importantly, the linewidth of the EPR peaks also exhibits a radial position-dependent behavior (Figure 5 3b ,d,f ). Interestingly, for the samples with an average of 12 Mn, t he trend for the change of the EPR linewidth perfectly correlates with that for the QYMn of the nanocrystals: the narrower the linewidth, the higher the QYMn of the nanocrystals, a nd vice versa (Figure 5 -3a b ). However, the QYMn of core/shell nanocrystals with a lower doping level exhibits a radial -position -dependent behavior different from that of the nanocrystal with 12 Mn. The CdS/ZnS core/shell nanocrystals with an average of 1 Mn exhibit a maximum QYMn for Mn at 0.8 ML, while the particles with an

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134 average of 4 Mn have a maximum QYMn for Mn at 1.6 ML (Figure 5 3c e). In contrast, the EPR linewidths of these two types of samples show nearly identical positiondependent changes t o those for the nanocrystals with an average of 12 Mn (Figure 5 3b d, f ). At first glance, the results from the PL measurements appear to be inconsistent with the EPR measurements for these two types of nanocrystals. To investigate the reasons for th is in consistency, the energy transfer efficiency (ET) using QYB E was calculated in accordance with Equation 5 8 The results show that the ET for the nanocrystals with an average of 12 Mn exhibit weak dependency on the radial position of Mn (Figure 5 4a ). However, the ET for the nanocrystals with an average of 1 or 4 Mn strongly depends on the radial position of Mn (Figure 5 4c e ). Consequently, the ET has to play a major role in determining the QYMn of these two types of nanocrystals. All together, these results demonstrate that the QYMn depends on t wo radial -position-dependent factors: ET and Mn. According to Equation 5 9 t he efficiency of the emission from a Mn (Mn) and the efficiency of nonradiative relaxation of Mn ( Mn NR = 1 Mn) were calculated The results suggest that both Mn and Mn NR strongly depend on the Mn radial position inside the core/shell nanocrystals (Figure 5 4b, d, f). T hese three different doping levels all exhibit a maximum Mn and a minimum Mn NR 4b, d, f). Amazingl y, the change of Mn nearly perfectly correlates to the change of the EPR linewidth for all these samples: the higher the Mn, the narrower the EPR linewidth, and vice versa (Figures 5 3 and 5 4 ). Therefore, it is the Mn (not QYMn) that is directly relate d to the linewidth of the Mn EPR peak. This correlation makes more sense because both Mn and the EPR linewidth are characteristic properties of the individual Mn at a particular radial position, while the QYMn also accounts for the energy transfer efficie ncy ( ET).

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135 Figure 5 4 Plots of QYMn, QYBE ET as a function of Mn radial position ( ) for CdS/ZnS core/shell nanocrystals with an average of (a ) 12 Mn, (c ) 4 Mn, and (e ) 1 Mn. Plots of Mn (red dots ) and Mn NR ( black dots ) as a function of for t he core/shell nanocrystals with an average of (b ) 12 Mn, (d ) 4 Mn, and (f ) 1 Mn. Furthermore, the linewidth s of the Mn EPR peaks also depend on the doping level of the CdS/ZnS core/shell nanocrystals: the higher the doping level, the broader the EPR line width (Figure 5 3 b d f insets). This effect is attributed to additional Mn Mn interactions because such interactions decrease the spin -spin relaxation time and thus lead to a broader EPR linewidth.182 However, the efficiency of the Mn emission ( Mn) slightly increases with the doping level of nanocrystals (Figure 5 4b, d, f). These results suggest that the Mn Mn interaction does not decrease the Mn of these Mn -doped nanocrystals. This phenomenon has not been observed previously. In contrast, Mn Mn interactions have long been believed to be a major factor in decreasing Mn.191 Therefore, our results provide an alternative example, which is important for further understanding the relationship between Mn Mn interactions and Mn in quantum confined n anosystems.

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136 5.3.2 Strong Excitation Intensity Regime : Excitation -Intensity -Dependent PL Properties The previour section investigated the PL and EPR properties of Mndoped CdS/ZnS core/shell nanocrystals within the weak excitation intensity regime. U nder weak excitation, the ET) is expr essed by Equation 5 5 and the integrated intensity ratio of the two emissions (IBE/IMn) is expressed by the following equation : NR Mn R Mn R Mn ET R BE ET Mn BE Mn BEk kk k n k QY I I (5 10) According to Equa tion 5 10, IBE/IMn should not depend on the excitation intensity. H owever, as discussed above because the Mn2+ emission lifetime is on the order of milliseconds, additional excitation of a nanocrystal with one of the Mn already in its excited state is pos sible when the excitation source has a sufficiently high intensity. As a consequence, the number of Mn dopants at their ground state (i.e., n) decreases with the increase of excitation ET and IBE/IMn should exhibit excitation intensity dependence. To explore this possibility, a XeCl excimer laser was used as excitation source to study the PL properties of Mn -doped CdS/ZnS core/shell nanocrystals. Three types of Mn -doped core/shell nanocrystals were synthesized according to the three step synthesis method. These nanocrystals have a core diame ter of 3.1 nm and shell thickness of 1.55 nm (~4.8 monolayer, ML) The Mn dopants are at 1.6 ML in the ZnS shell, and the nanocrystals have an average of 2, 6, and 15 Mn per dot, respectively. Under weak excitation using a fluorometer, all these Mn -doped nanocrystals exhibit two emission bands at ~405 nm (band -edge emission) and ~595 nm (Mn2+ emission) (Figure 5 5a,b,c) The nanocrystals doped with 6 Mn have an apparent band -edge emission lifetime of 5 .0 ns. The Mn-emission lifetime can be fitted perfectly by two exponential decays with lifetimes of 0.9ms and 7.1 ms (Figure 5 6) Note that this long lifetime (~7.1ms) is about 2~3 times longer than the Mn emission lifetime

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137 observed by other groups.190,192,193 This long lifetime probably related to the high qua lity of the doped core/shell nanocrystals prepared in this work. Figure 5 5 Steady -state photoluminescence of CdS/ZnS nanocrystals doped with an average of 2 Mn (a), 6 Mn (b), and 15 Mn (c). The corresponding UV -Vis spectra of these particles are shown in (d), (e), and (f), respectively. Figure 5 6 (a) Blue line: band -edge (BE) emission intensity decay as a functi on of time (for CdS/ZnS core/shell nanocrystals doped with an average of 6 Mn). Red line: an exponential fitting line. The major componen t of the emissions has an apparent lifetime of ~5 .0 ns. (b) Blue line : Mn emission intensity decay of CdS/ZnS core/shell nanocrystals doped with an average of 6 Mn under laser fluence of 0. 28 mJ/cm2. Red line: Lifetime fitting of Mn emission. This fitting give s the result of Mn emission lifetime s of ~0.9 ms and ~ 7.1 ms.

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1 38 Figure 5 7 Blue lines: Mn emission intensity decay of CdS/ZnS core/shell nanocrystals doped with an average of 6 Mn under different laser fluences: (a) 2.1 mJ/cm2, (b) 6.1 mJ/cm2, and (c) 17 mJ/cm2. Red lines: F itting of Mn emission. (d) Two lifetime components as a function of fluence. All these fittings give the same result of Mn emission lifetimes of 0.9ms and 7.1ms. In a typical experiment, a hexane solution of the nanocrystals doped wi th 6 Mn ( 2 16 M) was excited by a XeCl excimer laser (30 ns pulses with a repetition rate of 6 Hz at 308 nm). The excitation fluence was fine tuned with a set of neutral density filters. The emissions from this nanocrystal solution were detected utilizing a spectrom eter (Ocean Optics, with an integration time of 1s). In this experiment, the time interval between laser pulses (~166 ms) is much longer than the Mn-emission lifetime, therefore the integrated emissions from the nanocrystals are determined by the pump flu ence of single pulses. At a fluence of 0.1 3 mJ/cm2, the nanocrystals exhibit a PL spectrum with an intensity ratio ( IBE/ IMn) of 0.32, nearly identical to that obtained using the fluorometer (Figure 5 5b ).

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139 Figure 5 8 (a) PL spectra of CdS/ZnS core/she ll nanocrystals doped with 6 Mn under a fluence from 0.13 to 19 mJ/cm2. The Insets illustrate a TEM image of these particles and a cartoon of a Mn -doped particle. (b) IBE/IMn, and (c) IMn as a function of laser fluence. With the increase of fluence, both IBE and IBE/IMn monotonically increase (Figure 5 8a b ). This result demonstrates a characteristic property of the carrier relaxation dynamics of a nanocrystal with multiple Mn dopants in their excited state (4T1). Before all the Mn in this nanocrystal a re excited, the higher the fluence, the more the number of Mn in their excited state, and the fewer the number of Mn in their ground state (n in Equation 5 5 and 510). This results in a decreas e in the energy transfer efficiency ( ET) from exciton to th e Mn Therefore, in such carrier relaxation processes, more photons are generated from the band-edge pathway, and thus both IBE and IBE/IMn increase with increasing exaction fluence. After all the six Mn in the nanocrystal are excited, the increase of ex citation fluence also leads to a larger IBE, and thus a larger IBE/IMn because of the increase in the number of absorption and emission events.194 The creation of a nanocrystal with multiple Mn in their excited state is further confirmed by the result for the change of IMn (Figure 5 8 c ). With the increase of fluence, IMn increases, and

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140 then it reaches a maximum at a fluence of 3.3 mJ/cm2, corresponding to ~10 photon/dot. T he increase of IMn is attributed to the increase of the number of Mn in the 4T1 st ate This maximum Mn -emission intensity is achieved due to the total excitation of all six Mn dopants in the nanocrystals (Figure 5 9 d ). With a further increase of fluence, IMn remains nearly constant T his is because within a single pulse, when all si x Mn are in their excited state (4T1), no more Mn can accept the energy from the exciton. Thus, the energy transfer from exciton to Mn is terminated. As a consequence, IMn has no more increase. If this model is correct, the fluence for Mn emission reaching its maximum should depend on the doping level of the Mn-doped nanocrystals. Figure 5 9 IMn as a function of laser fluence for the CdS/ZnS nanocrystals doped with (a) 2 Mn, and ( b ) 15 Mn. (c) Fm (in blue squares) as a function of the average number (n) of Mn inside a dot. (d) Schematic of energy levels and carrier relaxation pathways in a Mn -doped nanocrystal: CB for conduction band, and VB for valence band. To test this possibility and further investigate the origin of excitation -intensity -dependen t dual emissions, Mn -doped CdS/ZnS core/shell nanocrystals were studied at different doping levels. The nanocrystals doped with 2 or 15 Mn exhibit a similar PL property to that of those

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141 particles doped with 6 Mn (Figure 5 9a b ). However, the Mn emission intensit ies of these particles reach their maxima at different laser fluences. Importantly, the fluence for the saturating Mn emission intensity (Fm) is indeed dependent on the average number of Mn in the nanocrystals. Fm is 1. 3 mJ/cm2 (~4 photon/dot) f or the particles with 2 Mn, and 6 .3 mJ/cm2 (~ 19 photon/dot) for the particles with 15 Mn (Figure 5 10a c) Furthermore, these saturation fluences (Fm) ca n be calculated from the number of Mn per particle and the energy transfer efficiency ( ET) from exciton to each Mn. The calculation result nearly perfectly matches the experimental data ( Figure 5 9c ) (please see Appendix A for detailed calculation ). These results further confirm that the maximum Mn emission intensity is a result of the tota l excitation of all the Mn inside the doped nanocrystals. Figure 5 10. Optical images of a composite film of Mn -doped CdS/ZnS core/shell nanocrystals and polystyrene -block -polybutadiene under a fluence of (a) 0.10, (b) 2.7, and (c) 60 mJ/cm2; (d), (e) and (f) are the corresponding PL spectra with normalized intensities. (g) The color switching cycles (in counts) of the nanocrystal film under two fluences 0.10 and 60.0 mJ/cm2, respectively.

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142 To demonstrate the color tunable property of the Mn -doped nano crystals, a nanocrystal/polymer composite film was prepared using CdS/ZnS core/shell nanocrystals doped with 20 Mn. This film exhibits only Mn emission at 595 nm at a low excitation fluence (0.10 mJ/cm2, Figure 5 10a d). By increasing the fluence to 2.7 mJ/cm2, the color of the nanocrystal film changes from orange to purple because of the onset of emission at the nanocrystal band edge (Fig ure 5 10b e). At an even higher fluence (60 mJ/cm2), the band-edge emission of the nanocrystals becomes the dominan t radiative pathway, and the nanocrystal film turns blue (Figure 5 10c f). This excitation intensity -dependent color change is highly reversible. More than seven thousand cycles of color switch between orange and blue have been recorded in the nanocrysta l film (Fig ure 5 10g ). 5 .4 Conclusion In conclusion, the PL properties of Mn -doped CdS/ZnS core/shell nanocrystals have been studied in two excitation intensity regimes. The results demonstrate that, under weak excitation, Mn -PL QY of the core/shell nanocrystals is determined by the product of two factors: the energy transfer efficiency ( ET) and efficiency of the emission from one Mn ( Mn). Also, Mn exhibits a radial -position -dependent change that nearly perfectly corresponds to that of the Mn EPR linew idth of the nanocrystals: the higher the Mn, the narrower the Mn EPR linewidth. Under strong excitation, we have shown the first evidence that the Mn doped semiconductor nanocrystals exhibit excitation-intensity -dependent dual emissions. The color tunable property of these nanocrystals originates from the excitation of multiple Mn dopants inside a nanocrystal. T he unique excitation fluence for saturating the Mn -emission intensity is dependent on the number of Mn dopants in a nanocrystal. Furthermore, the color tunable process in these nanocrystals exhibits extraordinary reversibility.

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143 CHAPTER 6 SYNTHESIS OF WATER -SOLUBLE 2,2 DIPHENYL 1 -PICRYLHYDRAZYL NANOPARTICLES: A NEW STANDARD FOR ELECTRO N PARAMAGNETIC RESONANCE SPECTROSCO PY 6.1 Introduction The di scovery of size -dependent properties in in organic colloidal nanoparticles has stimulated research efforts to develop synthetic methods for making nanoparticles of small organics.195197 Small -molecule organic nanoparticles are formed through non -covalent i solvophobic interactions.195 197 To date, a variety of organic nanoparticles have been synthesized system.195197 Th ese organic nanoparticles exhibit interesting size -dependent optical properties, and are emerging as a new class of functional materials with potential applications as key components for optoelectronics.195199 In this chapter, we report the synthesis and characterization of paramagnetic nanoparticles made of 2,2 diphenyl 1 -picrylhydrazyl (DPPH). DPPH has historically played an important role in the development of electron paramagnetic resonance (EPR) and was the second organic free radical detected by EPR .200 Because of its single narrow resonance line and its stability, DPPH is commonly used as a standard field marker for g -factor determination and magnetic scan calibration in both low and high -field EPR measurements,201 and as a primary spin -concentrati on standard in quantitative EPR spectrometry for the determination of free radical concentration in various samples.202 However, the low solubility of DPPH in water limits its applications in aqueous solutions. To overcome this limitation and possibly us e DPPH as a spin -probe for aqueous solutions, Tamano, et al. have recently developed an approach in which DPPH is stabilized by encapsulation into aggregates of amphiphilic block copolymers in water.203 The DPPH -containing polymer

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144 aggregates exhibit single -line EPR spectra, with linewidths between 5.0 and 15 G (Gauss),203 which is much broader than the typical linewidth of microcrystallin e DPPH (~1.5 G) observed in the X -band.200202 In this chapter we report a colloidal synthesis approach for making stab le, water -soluble DPPH nanoparticles T he resulting particles exhibit single line EPR spectra with linewidths of ~1.5 1.8 G, which are nearly identical to those commonly observed for microcrystalline DPPH. In addition, these nanoparticles are stable over a wide pH range of 3.0 to 10.0. These properties make these water -soluble DPPH nanoparticles suitable as a new type of EPR standard 6.2 Experimental S ection 6.2.1 Chemicals 2 2 -d iphenyl 1 -picrylhydrazyl (DPPH, free radical), 2,5 -dihydroxy 1,4 -benzoquinone (DHBQ, 98%), tetrahydrofuran (THF, anhydrous, A) were purchased from Aldrich. Nanopure Diamond system 6.2.2 Synthesis of DPPH Nanoparticles In typical exper iments, DPPH (0.01 mmol) was dissolved in t etrahydrofuran (THF 1 mL ) under argon protection to form a deeply purple vigorous stirring. The final DPPH nanoparticle s size was controlled by varying the growth time (0~2 hours) after injecting the DPPH stock solution. To stop the particle growth, 1.8 mL aqueous gelatin solution (2% w/w) was injected into the growth solution w hile keeping the growth solution stirring for another 5 min. For the synthesis of 0 hr growth particles stock solution and 1.8 mL of gelatin aqueous solution (2% w/w) were simultaneously injected

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145 for 5 min. In all these syntheses, the resulting DP PH nanoparticles were isolated from the growth solution by centrifugation, and redispersed in nanopure water. Then this separation procedure was repeated one more time. 6.2.3 Synthesis of 2,2 -diphenyl -1 -picrylhydrazine ( DPPH -H )-Doped DPPH Nanoparticles A yellow DPPH H stock solution was prepared by mixing DPPH (0.01mmol) and DHBQ (0.01 mmol) in 1 mL of THF. Before nanoparticle synthesis, this DPPH H stock solution was mixed with a DPPH stock solution to form a new stock solution with a molar ratio of DPPH H/DPPH of 1 to 4 vigorous stirring. After the nanoparticles grew for about 30 min, a aqueous gelatin solution (1.8 mL, 2% w/w ) was injected into the growth solution which was further stirred for 5 min. The resulting DPPH H -doped DPPH nanoparticles were separated by centrifugation and redispersed in nanopure water, and this separation proce dure was repeated one more ti me. 6.2.4 Synthesis of DPPH/DPPH -H Core / Shell Nanoparticles After the typical synthesis of DPPH nanoparticles with average diameter of 180 nm, 0.03 mmol DHBQ was added to the 6.8 mL original reaction solution. The molar ratio of DPPH and DHBQ of 1:30 is u sed for making core/shell type nanoparticles, which exhibit nearly identical absorption spectrum to that of DPPH -H doped DPPH nanoparticles made according to the procedure above. Then the resulting DPPH/ DPPH H core/shell nanoparticles were purified twice b y centrifugation as described above. 6.2.5 Absorption Measurements UV-Vis absorption spectra were measured using a Shimadzu UV1701. Nanocrystals were dissolved in water (Nanopure) for the measurement.

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146 6.2. 6 TEM and Electron Diffraction Measurements TEM m easurements were performed on a JEOL 200X operated at 200 kV, or a JEOL 2010F TEM operated at 200 kV. Eelectron diffraction ( ED ) measurements were acquired by the 2010F TEM operated at 200 kV. The specimens were prepared as follows: a particle solution ( was dropped onto a 200 mesh copper grid and was dried overnight at ambient conditions. 6.2. 7 EPR Measurements The EPR measurements were performed in CW mode on an X -band Bruker Elexsys 580 spectrometer (9.5 GHz) equipped with a Bruker ER 4123SHQE sup er -high Q cavity. A ll experiments were performed at room temperature. 6.2. 8 EPR Simulations The raw EPR data were imported into the 2D plotting and data analysis tool Grace (http://plasma -gate.weizmann.ac.il/Grace). The spectra were first baseline correc ted and then fitted to a Lorentzian line shape using the nonlinear curve fitting tool with the formula for the differentiated Lorentzian function given by 2 2 2) ) ( ( ) ( 2R RB B B B B B A dB dP (6 1) where A is the amplitude, is the linewidth used throughout this chapter and BR is the resonance field. The electronic g -factor for each transition was determined from its resonance field using the resonance condition: R BB hv g (6 2) where h is Plancks constant and B is the Bohr magneton.

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147 6.3 Results and Discussion 6.3.1 Synthesis and Characterization o f DPPH Nanoparticles The synthesis of water -soluble DPPH nanoparticles is based on a modified reprecipitation method. In a reprecipitation -based synthesis, the nucleation of organic nanoparticles is initiated by a sudden introduction of solvophobic interac tions between molecular building blocks ( i.e ., small -molecule precursors) and their surrounding solvent molecules, which is achieved by the addition of a poor solvent ( e.g. water) for the molecular building blocks.195197, 204207 The subsequent nanoparti cle growth, however, cannot simply be terminated by a temperature quenching process, like those used in advanced hightemperature syntheses of inorganic nanocrystals, because this reprecipitation -based synthesis is normally carried out around room temperat ure. This difficulty has limited the number of approaches available for the size -control of organic nanoparticles In this work we have found that gelatin a common surfactant for organic nanoparticles can rapidly terminate the growth of DPPH nanoparticle s in water. Accordingly, the size control for DPPH nanoparticles can be achieved simply by the injection of a gelatin solution at a chosen particle -growth time. In contrast to the previous reported methods for making small -molecule nanoparticles in which the control of final particle size is achieved by varying the concentrations of precursors and of surfactant molecules,195,196 the synthesis method herein uses only particle growth time to control the final size of organic nanoparticles Figure 6 1 shows that t ransmission electron microscopy (TEM) images of nanoparticles made at different growth times (0~2hrs) The particles have diameters ranging from 90 nm to 310 nm with a r elative standard deviation of ~14%. In addition, these DPPH nanoparticles exhibit high stability in water; and no size ripening was observed for more than six months.

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148 Figure 6 1 TE M images of DPPH nanoparticles : (a) 90 nm, (b) 200 nm, and (c) 310 nm. The scale bars are in 500 nm. (d) DPPH nanoparticle diameter as a function of growth time. Figure 6 2. An electron diffraction (ED) pattern of DPPH nanoparticles (~250 nm in diameter).

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149 Figure 6 3 Absorption spectrum of 250nm DPPH nanoparticles Inset illustrates the peak position of nanoparticles visible absorption band ( II) as a function of their diameter (D). Electron diffraction shows that the as -prepared DPPH nanoparticles possess an amorphous structure (Figure 6 2). The DPPH nanoparticles have two absorption bands in the UV (I) and visible (II) region, respectively (F igure 6 3 DPPH radicals, and the delocalized radical electron makes a major contribution to the visible absorption band (II).208 Both of the nanoparticles absorption bands exhibit a size -dependent red sh ift compared to free DPPH molecules in THF: the bigger the nanoparticles the larger the red shift, and vice versa (Figure 6 3 inset, Figures 6 4 and Figure 6 5 absorption bands is due to the J -type aggregation of DPPH molecul es inside an nanoparticle .195 197,209 T he sizedependent of the red -shift is attributed to an increase of the average intermolecular interactions between DPPH molecules with increasing size of the nanoparticles as proposed by Yao, et al.195197,209

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150 Fig ure 6 4 Absorption spectra of DPPH in THF solution (black line) and of DPPH nanoparticles with diameter s of 9 0 nm (red line), 1 70 nm (blue line), 25 0 nm (orange line), 3 10 nm (green line) in water Figure 6 5 The peak position of band I (UV band) of DPPH nanoparticles plotted as a function of their diameter s A typical EPR spectrum of DPPH nanoparticles consists of a characteristic single narrow Lorentzian line (Figure 6 6 ). The EPR linewidth is weakly dependent on nanoparticle size. With

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151 a decrease in the nanoparticle diameter from 310 nm to 90 nm, the EPR linewidth increases from 1.5 G to 1.8 G. The single Lorentzian EPR line and narrow linewidth of these nanoparticles are due to the spin exchange -narrowing effect in the limit of fast exchange.2 10 In this limit, the EPR lines of individual DPPH radicals, which would otherwise be broadened by dipolar electron electron and electron -nuclear interaction, merge into a single Lorentzian line whose width linearly decreases with the increase of the spin -ex change interaction.201,210 Thus, the slightly broader linewidth observed in 90 nm DPPH nanoparticles is likely caused by a slower average spin exchange rate, as compared with that in the bigger nanoparticles This slower exchange rate may be associated wit h a weaker average intermolecular interaction between DPPH molecules in 90nm nanoparticles as indicated by the peak position of their absorption bands (Figure 6 3 inset, Figures 6 4 and Figure 6 5 ). Figure 6 6 An EPR spectrum of 250-nm DPPH nanopar ticles (taken at 9.5 GHz and 298 K). Inset is nanoparticles EPR linewidth (LW) as a function of their diameter.

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152 6.3.2 Understanding of the Spin-Exchange Interaction Figure 6 7. EPR spectra of DPPH nanoparticles (blue); DPPH/DPPH -H core/shell nanopa rticles (green) and DPPH H doped DPPH nanoparticles (red). To further understand the spinexchange interaction and J aggregation of DPPH molecules inside nanoparticles three types of nanoparticles were synthesized : (1) DPPH ; (2) core/shell particles with DPPH as the core with a shell of 2,2 -diphenyl 1 picrylhydrazine (DPPH -H) a reduced form of DPPH;211 and (3) DPPH particles doped with DPPH H (Figure 6 7 ). All these nanoparticles have a nearly identical size of 180 nm (Figure 6 8 ); the core/shell and doped nanoparticles have a similar DPPH -H concentration of ~20%. Without the radical electron, DPPH H loses the visible band (II) of DPPH, but maintains its UV band (I) at the same wavelength and similar extinction.211 Indeed, because of the similar concentra tion of DPPH -H components, the core/shell and doped nanoparticles exhibit nearly identical reduction in the relative intensity of their visible band (II) as compared to that of their UV band (I) (Figure 6 9 ). The wavelength maxima of the two bands in these two types of particles exhibit no shift from

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153 those in their DPPH counterparts (Figure 6 9 ), indicating that the presence of DPPH H does not substantially perturb the packing of DPPH molecules inside the nanoparticles .195197,209 However, the location of D PPH H significantly affects the EPR linewidth of the nanoparticles (Figure 6 7 ). With DPPH H in the shell, the core/shell particles have an EPR linewidth of 1.7 G, identical to that of pure DPPH particles of the same size. In contrast, the DPPH H doped n anoparticles show an EPR linewidth of 2.2 G, which corresponds to an approximately 30% reduction in spinexchange interaction from that in the pure DPPH nanoparticles (Figure 6 7 ).210 This result is likely due to the fact that the insertion of DPPH H molec ules into the DPPH aggregates blocks the effective exchange interaction between the DPPH radicals.210 In addition, the results of the measurements on the core/shell type nanoparticles further suggest that the surface effects do not play a major role in c ontrolling the optical and paramagnetic properties of DPPH nanoparticles (Figure 6 7 and Figure 6 9) Figure 6 8 TEM images of (a) DPPH nanoparticles; (b) DPPH/DPPH H core/shell nanoparticles; (c) DPPH H doped DPPH nanoparticles. All these three types of nanoparticles have a diameter of 180 nm with a relative standard deviation of 14%.

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154 Figure 6 9 The absorption spectra of DPPH nanoparticles (blue); DPPH/DPPH H core/shell nanoparticles (green) and DPPH H doped DPPH n anoparticles (red). 6.3. 3 Stability Test Figure 6 10. (a) Absorption spectra of DPPH nanoparticles (~3 10 nm in diameter) at various pH s (3.0 to 10.0). (b) The corresponding electron paramagnetic resonance (EPR) spectra. (c) The EPR linewidth in (b) a s a function of pH. In these experiments, the concentration of DPPH nanoparticles was kept unchanged. The pH of nanoparticle solution was adjusted using either standard pH buffers or HCl solutions. The final pH value of the solutions was determined by pH -i ndicator strips (colorpHast).

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155 To verify their suitability as EPR standards, the stability of DPPH nanoparticles was studied as a function of pH. The UV -Vis absorption spectrum (including the position and extinction of the two absorption bands) shows near ly no change in the pH range from 3.0 to 10.0 (Figure 6 10a) From EPR measurements, spectrum shape, EPR linewidth, g -factor and the intensity of integrated EPR absorption of DPPH nanoparticles show no measurable variation in the pH range from 3.0 to 10.0 (Figure 6 10b,c and Figure 6 11). These results show that DPPH nanoparticles are stable under these conditions, demonstrating that these nanoparticles are practically useful as both a standard field marker and a primary spin -concentration standard for aqu eous samples over a wide pH range.201,202 Figure 6 11. The integrated intensity of EPR absorption (I) and g -factor of DPPH nanoparticles (310 nm in diameter) as a function of pH. The uncertainty in g-factor determination is 0.0001, and the relative unc ertainty in determining EPR intensity is ~2.0%. 6. 4 Conclusion In conclusion, in this chapter, we report a new synthesis method to make water -soluble, DPPH (1,1 -diphenyl 2 -picrylhydrazyl) nanoparticles. T he size control of these nanoparticles (90nm~300nm) was achieved by varying the growth time. T hese particles can survive in variable p H conditions (from p H 3 .0 to p H 10.0 ). More importantly, t hese nanoparticles exhibit size dependent absorption spectra and fast exchangenarrowed single -line EPR spectra with linewidths of ~1.5 1.8 G which is quite close to that of the DPPH radical bulk counterpart

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156 (~1.5G). Furthermore, the EPR linewidth can be controlled by partially reducing the DPPH radical. These properties make the DPPH nanoparticles suitable as a new typ e of water -soluble EPR standard, which will be important for many applications in fields such as the food industry and life sciences.200202 Furthermore, the DPPH nanoparticles can be potentially used as a spin probe for biomedical studies.200,203

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157 CHAPTER 7 CONCLUSIONS 7.1 Summary of Current Research This research has led to the design of the new synthetic routes for different types of nanoparticles and the elucidation of the detailed mechanisms of growth and doping processes in semiconductor nanocrystals. An investigation of the nanocrystal growth stage showed that t he number of nanocrystal s decrease s during particle stage. Mechanistic studies further revealed that only the so lvent and crystal structure c an affect the amount of decomposed nanocrystals. A m odel was proposed relating the nanocrystal growth patterns to stacking faults. The XRD measurements and computer XRD -simulation results suppor t this model. A non -injection synthesis (NIS) method for making high -quality metal -selenide (CdSe, PbSe, PdSe, etc .) nanocrystals was designed using SeO2 as the selenium precursor. Mechanistic studies show ed that octadecene (ODE) acts as a reducing agent for SeO2 in this synthesis. Moreover, this synthesis exhibits controllable kinetics in both the nucleation and grow th stages, and thus allows detailed control of the numbers of nuclei and final size s of the resulting nanocrystals. T his synthesis is especially noteworthy because it can be conducted in air, and eliminates the need for air -free manipulations using a glove box or a Schlenk line. The synthesis designed in this work is very suitable for a large -scale industrial preparation of high -quality nanocrystals at low cost A key aspect of semiconductor technology is controlled impurity doping, in this research, a three -step synthesis doping method has been developed for precise control of the Mn radial position and doping level in the CdS/ZnS core/shell nanocrystals. Investigations of t he detailed mechanism of this process showed that nanocrystal doping is determined by the chemical kinetics of three activation -controlled processes: dopant adsorption, replacement, and ZnS -shell

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158 growth Moreover, these Mn -doped nanocrystals exhibit a very interesting excitation intensity dependant, color tunable dual emission property. A s a final project a new synthesis method to make water -soluble, DPPH (1,1diphenyl 2 picrylhydrazyl) nanoparticles was developed in which s ize control (90nm~300nm) was achieved by varying the growth time. These particles can survive in variable pH conditi ons (from pH 3 to p H 10). I mportantly, these DPPH nanoparticles show a single line electron paramagnetic resonance (EPR) signal with very narrow linewidth (~1. 5 1.8 G) which is quite close to that o f the DPPH radical bulk counter part (~1.5G). This water -s oluble DPPH nanoparticle is a perfect standard EPR label for biological and biomedical systems. 7.2 Perspectives Nanocrystals have demonstrated many potential applications, and the synthesis of high quality nanocrystals is always the foremost step toward successful realization of these possibilities Indepth understanding of the mechanism of nanocryst al formation and growth made possible by this research will lead to better synthetic approaches for preparation of monodisperse, shape and size -controll ed na nocrystals. For industrial applications, n ew synthetic methodologies that can produc e large quantities of nanocrytals without sacrificing their quality need to be further explored In addition, the search for new types of nanostructured materials with prom ising properties will still be a hot research area. Considering the toxicity of nanoparticles such as CdSe/ZnS nanocrystals, development of low and non-toxic materials will have priority Doping tailors the properties of nanocrystals by providing new elec tronic, optical, transport and magnetic properties For future direction s of doping in nanocrystals the development of a general synthetic methodology, synthesi s of different types of doped nanocrystals (e.g., n -type or p type) optimiz ation of dopant concentration inside the particle and further investigat ion of

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159 new properties of doped nanocrystals will be exciting and cha llenging tasks for this young research area

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160 APPENDIX CALCULATION OF SATURATION FLUENCE Number of photons per dot injected by sing le pulse is described by : / ) 1 ( N e hc I N N (1) w here N is the total number of the absorbed photons in each pulse; N* is the total number of the particle ; I of light in vaccum; is the absorption cross section of a single nanocrystal at the excitation wavelength. In this experiment, =10.6, so the Equation (1) can be r ewrite d as: / ) 1 ( hcN I N e hc I (2) So, the laser pulse energy for the injection of one photon per dot can be calculated as : 1 hcN I N N (3) L S C hc hcN I 1 1 (4) where, C is the particle concentration; S is the stripe area; L is the path length. In this experiment, M C610 16 2 21 0 cm S cm L 1 0 Plug these values in Equation (4), the laser pulse energy for the injection of one photon per dot can be calculated, the value is 0.033mJ. For the excitation of each Mn inside QDs the energy Es transferred from QD to Mn is equal to the energy fo r the injection of one photon per dot (In our case, Es=0.033mJ), then the total energy (E ) need to be absorbed by QD s is ET sE E (5) w here, ET is the energy t ransfer efficiency from QD to Mn which can be described as:

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161 n K n n k k k n k n k k k nET NR BE R BE ET NR BE R BE ET ET (6) where, n is the number of unexcited Mn ions in the QD. Under strong excitation pulse energy, n de creases when more and more Mn is excited during the increase of the excitation pulse energy. Therefore in this case, the total energy (Etotal) which is needed to excite all Mn in side the nanocrystals is described as follow: 1 1 ) 2 ( ) 2 ( ) 1 ( ) 1 ( K E n K n E n K n E n K n E Es s s s total s n iE i i K 1) ( (7) For doping at 1.6 ML, K equals to 1.5 which is extracted from Figure 5 4. For the number of Mn per particle we take poisson distribution into account. ) ( ) ( j e n n j Pn j (8) where, j is the number of Mn per particle; n is the average number of Mn per particle; ) ( n j P is the percentage of j Mn per particle in the sample with n Mn per particle Then the pulse energy (Em) to just excite all n Mn is expressed by: ) ( ) 033 0 ) 5 1 ( ( ) ( ) ) ( ( ) (30 1 1 max 1 1j e n mJ i i n j P E i i K n En j j j i j j i s m (9) here, w e set the maximum number of Mn in each particle is 30. T he saturation fluence (Fm) is obtained by Equation (10) : 21 0 ) ( ) ( ) ( cm n E S n E n Fm m m (10)

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173 BIOGRAPHICAL SKETCH Ou Chen received his B.S. in chemical physics from University of Science and Technology of China in July, 2004. After that, he moved to Gainesville, Florida and began his Ph.D. study in the chemistry department at the University of Florid a in August, 2004. H e spent f ive y ears of his graduate research working with Dr. Y. Charles Cao to complete his Ph.D. study. His research area in Dr. Cao s g roup includes the synthesis of high -quality nanocrystal s, doping of semiconductor nanocrystal s and also the exploration of the potent ial application s of nanoparticles