CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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109 of the QDs. As a result surface atom or in other word surface states of QD play a major role in the relaxation dynamics. Surface imperfections leads defect states on the surface which leads to additional competing relaxation pathways by trapping into surface defect sites and eventually can accelerate the depopulation of the band edge. Furthermore, due to smaller size, coupling between electron and hole wave function is much higher which leads to faster relaxation rate. For all the above reasons it is very difficult to predict relaxation behavior of QD particle. Therefore ultrafast spectroscopic techniques would enable the study of complex behavior of the carrier relaxation dynamics in QDs, where many early events take place in ultrafast time scale. Transient absorption (TA) experiments using time-resolved pump-probe spectroscopy are a perfect tool to investigate the ultrafast electron-hole dynamics in semiconductor nanoparticles. Many time-resolved investigations on the electron-hole dynamics in colloidal II-VI semiconductor nanoparticles are found in the literature. Previously TA technique has been used in the extensive study of charge carrier dynamics in CdSe quantum dot. Guyot- Sionnest and co-workers [4.6] demonstrated that the dynamic behavior of excited electrons could be monitored by using a mid-IR probe beam, in contrast to the complex relaxation behaviors in the visible probe measurements due to the contributions from both electrons and holes, as has been reported [4.6]. These studies [4.6] have led to observation of novel physics in play in QD systems. Several studies have demonstrated the influence of surface ligands in the dynamics of the charge carriers by trapping. The understanding from these studies has led to achievement of nanosecond cooling times in multiple shells CdSe QD [4.7]. Extensive Studies [4.6, 4.8] have suggested phonon bottleneck effects, nonadiabatic interaction, and Auger mediated process in the relaxation of electron and holes in CdSe QDs which is
110 impossible in bulk systems. Formation of multiexcitons is another novel observation that has been confirmed [4.2, 4.9]. These studies suggest how different these systems are compared to that of the bulk systems. While studies carried out have been extensive, but much of the literature have concentrated on carrier dynamics in CdSe QDs. It is important to study the dynamics in further systems as they may present some novel phenomena like Multi Exciton Generation in Lead Salt QD [4.9]. Prior studies on CdTe QDs by fluorescence upconversion [4.10], pump probe spectroscopy [4.11], Degenerate Four Wave Mixing [4. 12] on the dynamics had provided valuable information on the dynamics of carriers or dephasing, In the present chapter, we investigate charge carrier dynamics in CdTe by ultrafast TA (Transient Absorption) technique. The samples have been synthesized by standard methods as described in experimental section. The samples were characterized by UV-Visible Absorption technique, TEM. The samples synthesized by this method are spherical so they show a 3-D confinement. The TA technique is used to study how size quantization and surface influences the relaxation dynamics in QDs. 4.2. Experimental Section: 4.2.1. Materials: Cadmium Chloride (≥99%), Te Powder (≥98), 1-Mercaptopropionic acid (≥99%), NaBH 4 and Benzoquinone were purchased from Aldrich and used it as received without further purification. Isopropyl alcohol was used as received from S. D. Fine chemicals. N-Butyl Amine and Benzoquinone were from Aldrich.
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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,
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1.13 Reactant and Product Potential
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light. Inset: Kinetic traces monito
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6.5 Transient decay kinetics of gra
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at 670 nm after exciting at 400 nm
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ABBREVIATIONS BET BQ CB CCD CdS CdT
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1 Chapter 1
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3 size dependent optical properties
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5 1.2. Physics of Semiconductors 1.
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7 As seen from the schematic, poten
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9 Substituting this in Schrödinger
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11 ( r, r ) ( r ) ( r ) (1.13) e
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13 E E E 2 2 2 EX ne, le nh,
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15 spherical symmetry of field. The
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17 gE ( ) 2Em 2 3 3 (1.25) For a
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19 1.3.2. Electron-Hole energy tran
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21 Impact Ionization Figure 1.7. Sc
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23 understanding on mechanistic asp
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25 carriers are unable to sample th
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27 CB VB QD Metal Figure. 1. 10. Sc
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29 energy barrier. Therefore it is
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31 1 f q 2 A qB (1.30) 2 In equa
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33 Vibrational contribution can be
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35 1. 7. 1. Electron Injection ET i
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37 Under assumption of invariance o
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39 distribution. To achieve good si
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41 initially achieved monodispersit
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43 and rate of charge transfer acro
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45 1.32 P. V. Kamat, J. Phys. Chem.
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47 Chapter 2
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49 chemical species. Since a partic
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51 fluorescence forms an important
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53 eliminated by use of standard wh
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55 sample. Raman spectroscopy is ba
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57 will appear bright and region wi
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59 volatile solvent is drop casted
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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
Page 125: 83 Chapter 3
Page 128 and 129: 85 Therefore the study of interfaci
Page 130 and 131: 87 3. 2. Experimental Section 3.2.1
Page 132 and 133: 89 and intensity show absence of ot
Page 134 and 135: 91 which is also neutral; therefore
Page 136 and 137: 93 electron transfer times. This re
Page 138 and 139: 95 r B a * 0 3.1 me / me Now the
Page 140 and 141: 97 nonadiabatic case the electron t
Page 142 and 143: 99 drastically reduced leading to a
Page 144 and 145: 101 While the study of injection dy
Page 146 and 147: 103 study. Based on the injection d
Page 148 and 149: 105 17. L. E. Brus, J. Phys. Chem.
Page 156 and 157: 111 4.2.2. Synthesis: The CdTe QDs
Page 158 and 159: 113 lower hole state. In CdSe the l
Page 160 and 161: 115 4.2 inset. The kinetic traces a
Page 162 and 163: 117 growth of the bleach kinetics (
Page 164 and 165: 119 previous section we have discus
Page 166 and 167: 121 lived charge transfer complex w
Page 168 and 169: 123 100 times concentration of the
Page 170 and 171: 125 4.5. References 4.1. Efros, Al.
Page 176 and 177: 129 been made in the synthesis of t
Page 178 and 179: 131 metallic synthesis by arrested
Page 180 and 181: 133 5.3. Result and discussion: 5.3
Page 182 and 183: 135 indicating a confinement induce
Page 184 and 185: 137 absorption studies that on form
Page 186 and 187: 139 1000 c b PL Intensity 100 10 a
Page 188 and 189: 141 samples due to a very weak exci
Page 190 and 191: 143 dynamical spectrum is negligibl
Page 192 and 193: 145 Earlier authors [5.4] reported
Page 194 and 195: 147 Sample t r(ps) t 1(ps) t 2(ps)
Page 196 and 197: 149 In addition to the cooling dyna
Page 198 and 199: 151 pulse-width time scale followed
Page 200 and 201: 153 5.20. Sreejith Kaniyankandy, Sa
Page 203 and 204: 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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