CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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39 distribution. To achieve good size distribution it is essential to separate nucleation and growth [1.51]. This is achieved by using optimal concentration of precursors and capping agents during synthesis. Function of capping agent is multi fold (i) It binds to the surface strongly therefore controls growth, (ii) It enables dissolution of nanoparticle in a solvent (iii) Very often it caps the surface electronically thereby eliminating trap states on the surface of nanoparticle. One of the most familiar approaches to colloidal synthesis of quantum dots is arrested precipitation technique. In this technique one of the precursors in solution is maintained at high temperature and the other precursor is injected rapidly into the solution leading to formation of nuclei and subsequent growth takes place over these nuclei forming monodisperse nanoparticles at a lower temperature to reduce further nucleation. To understand how nucleation and growth is separated it is necessary to recognize different phases of reaction. Let us take the case of CdSe where cadmium precursor used is CdO dissolved in Oleic acid and selenium precursor is freshly prepared TOPSe at reaction temperature of 300˚C. TOPSe is added to cadmium precursor maintained at 300˚C swiftly leading to very high super-saturation. This leads to formation of nuclei instantly also called burst nucleation. Formation of nuclei is a high barrier process however due to high supersaturation and temperature, nucleation commences. On addition of TOPSe temperature decreases by ~20-30˚C and additionally precursor concentration also decreases due to formation of nuclei leading to lower probability of formation of further nuclei. Once nucleation is achieved, growth commences. Therefore this method offers a way to separate nucleation and growth thereby enabling formation of monodisperse nanoparticles. The growth stage is not simple as growth of different size particles takes place at different rates. The precursor concentration versus time is depicted in figure 1.16 below.
40 Figure 1. 16. Nucleation and growth stages for preparation of monodisperse nanoparticles in La Mer Growth model. The schematic in figure 1.16 represent growth of monodisperse nanoparticles in LaMer model which shows a distinct temporal separation between nucleation and growth. This model also tells us why nucleation is separated from growth. As soon as precursors are added a burst nucleation which takes place till the concentration is above nucleation threshold below nucleation threshold, no new nuclei are formed and remaining precursors leads to growth. Therefore synthetic conditions can be achieved where the nucleation threshold is tuned to optimize monodipersity of nanoparticles. Within this model monodispersity is achieved when consumption of precursor does not exceed precursor addition rate. Moreover it can be seen from the schematic that if during nucleation, growth is much less then one can achieve a good size distribution. While nucleation is one of the factors governing monodispersity in initial times, growth stage also can lead to loss of
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Charge Transfer Dynamics in Quantum
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DECLARATION I, hereby declare that
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ACKNOWLEDGEMENTS It is my privilege
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1.3.5. Defect Mediated Relaxation 2
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2. 8. 4. White Light Generation- 45
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6.2. Experimental 6.2.1.Synthesis o
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devices based on QDs it has been sh
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Furthermore generation of pump (~40
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shell). This clearly indicated that
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12. V. I. Klimov, J. Phys. Chem. B,
Page 28 and 29: 1.13 Reactant and Product Potential
Page 30 and 31: light. Inset: Kinetic traces monito
Page 32 and 33: 6.5 Transient decay kinetics of gra
Page 34 and 35: at 670 nm after exciting at 400 nm
Page 37 and 38: ABBREVIATIONS BET BQ CB CCD CdS CdT
Page 39: 1 Chapter 1
Page 42 and 43: 3 size dependent optical properties
Page 44 and 45: 5 1.2. Physics of Semiconductors 1.
Page 46 and 47: 7 As seen from the schematic, poten
Page 48 and 49: 9 Substituting this in Schrödinger
Page 50 and 51: 11 ( r, r ) ( r ) ( r ) (1.13) e
Page 52 and 53: 13 E E E 2 2 2 EX ne, le nh,
Page 54 and 55: 15 spherical symmetry of field. The
Page 56 and 57: 17 gE ( ) 2Em 2 3 3 (1.25) For a
Page 58 and 59: 19 1.3.2. Electron-Hole energy tran
Page 60 and 61: 21 Impact Ionization Figure 1.7. Sc
Page 62 and 63: 23 understanding on mechanistic asp
Page 64 and 65: 25 carriers are unable to sample th
Page 66 and 67: 27 CB VB QD Metal Figure. 1. 10. Sc
Page 68 and 69: 29 energy barrier. Therefore it is
Page 70 and 71: 31 1 f q 2 A qB (1.30) 2 In equa
Page 72 and 73: 33 Vibrational contribution can be
Page 74 and 75: 35 1. 7. 1. Electron Injection ET i
Page 76 and 77: 37 Under assumption of invariance o
Page 80 and 81: 41 initially achieved monodispersit
Page 82 and 83: 43 and rate of charge transfer acro
Page 84 and 85: 45 1.32 P. V. Kamat, J. Phys. Chem.
Page 90 and 91: 49 chemical species. Since a partic
Page 92 and 93: 51 fluorescence forms an important
Page 94 and 95: 53 eliminated by use of standard wh
Page 96 and 97: 55 sample. Raman spectroscopy is ba
Page 98 and 99: 57 will appear bright and region wi
Page 100 and 101: 59 volatile solvent is drop casted
Page 102 and 103: 61 spots arise from diffraction fro
Page 104 and 105: 63 The electrical signal is then ch
Page 106 and 107: 65 delayed and inverted. The two si
Page 108 and 109: 67 Pump-probe technique is one of t
Page 110 and 111: 69 Amplifier Jade Stretcher fs Osci
Page 112 and 113: 71 Figure 2.6. Optical layout of Ti
Page 114 and 115: 73 Grating Convex Mirror Concave Mi
Page 116 and 117: 75 changes the polarization from ho
Page 118 and 119: 77 Grating Output Input Mirror Grat
Page 120 and 121: 79 μJ has a very high peak power.
Page 122 and 123: 81 2.6. Dynamical Theory of X-Ray D
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85 Therefore the study of interfaci
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87 3. 2. Experimental Section 3.2.1
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89 and intensity show absence of ot
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91 which is also neutral; therefore
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93 electron transfer times. This re
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95 r B a * 0 3.1 me / me Now the
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97 nonadiabatic case the electron t
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99 drastically reduced leading to a
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101 While the study of injection dy
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103 study. Based on the injection d
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105 17. L. E. Brus, J. Phys. Chem.
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107 Chapter 4
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109 of the QDs. As a result surface
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111 4.2.2. Synthesis: The CdTe QDs
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113 lower hole state. In CdSe the l
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115 4.2 inset. The kinetic traces a
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117 growth of the bleach kinetics (
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119 previous section we have discus
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121 lived charge transfer complex w
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123 100 times concentration of the
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125 4.5. References 4.1. Efros, Al.
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127 Chapter 5
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129 been made in the synthesis of t
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131 metallic synthesis by arrested
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133 5.3. Result and discussion: 5.3
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135 indicating a confinement induce
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137 absorption studies that on form
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139 1000 c b PL Intensity 100 10 a
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141 samples due to a very weak exci
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143 dynamical spectrum is negligibl
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145 Earlier authors [5.4] reported
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147 Sample t r(ps) t 1(ps) t 2(ps)
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149 In addition to the cooling dyna
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151 pulse-width time scale followed
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153 5.20. Sreejith Kaniyankandy, Sa
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155 CHAPTER 6 Charge Separation in
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157 Studies by Wang et. al. [6.15]
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159 graphene in a 1M NaOH solution.
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161 Intensity (a.u.) 5 4 3 2 1 CdTe
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163 dynamics of graphene oxide and
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165 GO as shown in the Figure 3 rev
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167 electron acceptor for Graphene.
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169 governs the recombination dynam
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171 Sample τ 1 (ps) τ 2 (ps) τ 3
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173 A (mO.D) 10.0 0.0 -10.0 -20.0 0
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175 involved in the relaxation may
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177 Electron quencher (benzoquinone
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179 Scheme 6.1. Schematic of Electr
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181 6.6. Reina, A.; Jia, X.; Ho, J.
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183 6.26. Kaiser, A. B.; Navarro, C
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185 assigned to injection into diff
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187 transfer from CdSe core to ZnTe
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189 However the position of bands i
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