CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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135 indicating a confinement induced shift. Particle size of as prepared CdSe QD calculated by using sizing curve given by Peng [5.15] and coworkers is around 2 nm. The figure indicates an evident red shift of 1S exciton peak with increasing thickness of ZnTe (Figure 3). From our optical absorption studies, we can clearly see that the formation of ZnTe shell on CdSe core, where exciton peak shows red shift with increasing thickness of ZnTe. It has been observed that exciton peak of core-shell CdSe/ZnTe1 (thinnest shell) shifted to ~470 nm, for CdSe/ZnTe2 it has been shifted to ~482 nm and for CdSe/ZnTe3 the exciton peak shifted to ~ 492 nm from exciton peak of CdSe which has been observed at ~455nm. Absorbance (a.u.) 0.6 0.4 0.2 a CdSe b CdSe/ZnTe 1 c CdSe/ZnTe 2 d CdSe/ZnTe 3 e CdSe/ZnTe 4 0.0 400 500 600 Wavelength (nm) Figure 5.3: Steady state absorption spectra of a) CdSe (thick solid line), b) CdSe/ZnTe1 (short dash), c) CdSe/ZnTe2 (dash dot), d) CdSe/ZnTe3 (dash-dot-dot) and e) CdSe/ZnTe4 (thin solid line), In the present case the band offsets are in such a way that on photoexcitation the hole can be transferred to ZnTe shell. Therefore the dielectric constant and confinements are considerably different leading to wave function manipulation enabling a redshift [5.3]. UV-
136 vis absorption spectra of the thickest sample show a featureless absorption from Visible- NIR region. This absorption is related to direct transition from ZnTe valence band to CdSe conduction band. It is clear that in the NIR region it is considerably weak due to forbidden nature of the transition [5.3, reference therein]. The distinguishing feature of the core shell samples is an increase in the absorbance with an increase shell thickness in the blue side of the exciton transition indicating a clear increase in the DOS contribution from the ZnSe also towards the optical absorption. To corroborate optical absorption properties in the above systems we have also carried out steady state emission spectroscopy for CdSe QD and CdSe/ZnTe core-shell material of different thickness and are shown in Figure 5.4. Figure 5.4a shows broad emission band from 525 -770 nm with a peak 650 nm for CdSe QD samples. The emission with a large stoke shift clearly indicate the emission originate from surface states. Earlier Wuister et al [5.16] reported emission from surface states from thiol capped CdSe where the inter band gap states are formed via interaction of thiolate group with the metal ion. However on formation shell the emission spectra of CdSe/ZnTe core-shell systems changes dramatically. Figure 4b shows broad emission spectra for CdSe/ZnTe1 core-shell (thinnest shell) with two emission peaks at 524 and 633 nm. The band at 523 nm region can be attributed as exciton emission and the emission band at 633 nm shows large stoke shift which can be attributed to surface (defect) state emission. This clearly indicates that the shell formation is not complete for the CdSe/ZnTe1 core-shell sample. However it is interesting to see that on formation of ZnTe shell surface state emission is drastically reduced. In CdSe/ZnTe2 core-shell sample (Figure 4c) we can observe only one peak at 600 nm, which can be attributed to exciton emission of CdSe/ZnTe2 core-shell. We have observed in optical
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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
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63 The electrical signal is then ch
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65 delayed and inverted. The two si
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67 Pump-probe technique is one of t
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69 Amplifier Jade Stretcher fs Osci
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71 Figure 2.6. Optical layout of Ti
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73 Grating Convex Mirror Concave Mi
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75 changes the polarization from ho
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77 Grating Output Input Mirror Grat
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79 μJ has a very high peak power.
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81 2.6. Dynamical Theory of X-Ray D
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83 Chapter 3
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85 Therefore the study of interfaci
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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 151: 107 Chapter 4
Page 154 and 155: 109 of the QDs. As a result surface
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 173: 127 Chapter 5
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 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
Page 205 and 206: 157 Studies by Wang et. al. [6.15]
Page 207 and 208: 159 graphene in a 1M NaOH solution.
Page 209 and 210: 161 Intensity (a.u.) 5 4 3 2 1 CdTe
Page 211 and 212: 163 dynamics of graphene oxide and
Page 213 and 214: 165 GO as shown in the Figure 3 rev
Page 215 and 216: 167 electron acceptor for Graphene.
Page 217 and 218: 169 governs the recombination dynam
Page 219 and 220: 171 Sample τ 1 (ps) τ 2 (ps) τ 3
Page 221 and 222: 173 A (mO.D) 10.0 0.0 -10.0 -20.0 0
Page 223 and 224: 175 involved in the relaxation may
Page 225 and 226: 177 Electron quencher (benzoquinone
Page 227 and 228: 179 Scheme 6.1. Schematic of Electr
Page 229 and 230: 181 6.6. Reina, A.; Jia, X.; Ho, J.
Page 231: 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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CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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