CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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178 The study clearly shows that the dynamics in the presence of quencher is more or less same as shown in the table 6.4. Previous studies by our group and others have reported the effect of benzoquinone on quantum dot dynamics. The results of these have unambiguously verified the role of benzoquinone as an electron shuttler on the surface of CdSe and CdTe quantum dot. Benzoquinone accepts the electron within the pulse width and BQ anion situated on the surface donates the electron back to the valence band hole within few ps. However in the present study we observe quenched bleach in the presence of quenchers indicating electron quenching. However the bleach recovery is dominated by the electrons that are not quenched by benzoquinone and they show negligible effect on the dynamics. These are the electrons which have been injected into Graphene. This is clear from the lifetime contributions which show that the lifetime and contribution are approximately the same. This further ascertains the role of electron injection into Graphene in G-CdTe. Furthermore the dynamics of the bleach also does not arises from Graphene as previously we have shown that the reduced Graphene oxide (Pump-400nm) dynamics gives a positive absorption in this wavelength range as compared to a bleach in the present cases. These experimental observations are summed up in the schematic given in Scheme 1. The CdTe E VB and E CB are -5.2eV and -3.5eV with respect to vacuum. We have obtained the position from fits equation obtained from tight binding calculations as reported previously [6.37]. The expressions for the valence and conduction band shift with respect to diameter of the quantum dots are given below, E E VB CB E E Bulk VB Bulk VB 19.03 1.13 d 16.38 0.92 d (6.1)
179 Scheme 6.1. Schematic of Electron Transfer Process The positions of the valence and conduction band are ~-5.8 and -2.6 respectively. The Fermi level of graphene is -4.5eV. Band alignment is such that the Fermi level of graphene is much lower compared to the conduction band of CdTe, leading to a highly negative free energy for electron transfer (ΔG~-1eV). Therefore graphene similar to metal nanoparticles can act as an electron sink. This has been verified by the present study by use of luminescence spectroscopy and transient absorption technique. 6.4. Conclusions In conclusion we state that we have synthesized and studied ultrafast charge carrier dynamics of Graphene-CdTe (G-CdTe) system and compare the dynamics with CdTe of same size synthesized by colloidal methods. TEM studies confirmed formation of CdTe over graphene. UV-Visible absorption indicated the sizes of quantum dots as ~2.2nm. The sizes of CdTe were found to be the same as observed by 1S 3/2 -1S e first exciton band postion at 450nm.
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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
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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
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
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: 177 Electron quencher (benzoquinone
Page 229 and 230: 181 6.6. Reina, A.; Jia, X.; Ho, J.
Page 231: 183 6.26. Kaiser, A. B.; Navarro, C
Page 234 and 235: 185 assigned to injection into diff
Page 236 and 237: 187 transfer from CdSe core to ZnTe
Page 238 and 239: 189 However the position of bands i
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