Charge Transfer Dynamics in Cadmium Chalcogenide Quantum Dots based Heterostructure

Quantum confinement effects in semiconductor nanocrystals or quantum dots (QDs) give rise to unusual electronic properties such as, size dependent bandgaps, high molar absorptivities and in some cases, hot carrier extraction and multi-exciton generation, which makes them suitable to harvest solar energy. Competition between charge transfer and electron-hole recombination determines the efficiency of charge separation and thus the performance of QDs based electronic devices. Compared to the bulk semiconductors, surface of QDs significantly affects the opto-electronic properties of QDs due to high surface to volume ratios. The difference in the chemical environment of the surface atoms with that of bulk results in mid-gap states which can trap photoexcited charge carriers. This dissertation explores the role of these surface states in charge transfer and draws comparisons with charge transfer from band edge states.;Mesoporous titanium oxide (TiO2) is often used as the electron extracting component in dye sensitized and QDs based solar cells. CdSe QDs were assembled on TiO2 nanoparticles (NPs) using linker assisted assembly (LAA) approach and electron transfer was studied from band edge and surface states of CdSe QDs to TiO2 NPs using steady-state and time-resolved spectroscopy. Electron transfer was found to be 2--3 times faster from band edge states as compared to surface states, consistent with driving force dependent electron transfer. Well passivated core/shell CdSe/ZnS QDs showed improved electron transfer performance to TiO2 despite the presence of a spatial and energetic barrier of ZnS shell.;Ill-defined features of CdSe QDs-TiO2 donor-acceptor systems such as driving force, surface coverage of QDs on TiO2, distance between donor and acceptor renders it difficult to study fundamental aspects of charge transfer. Molecular acceptors, on the other hand, have well defined energy levels and surface coverage of these acceptors can be easily quantified using various analytical techniques. We studied hole transfer dynamics in well passivated chloride treated CdTe QDs tethered to molecular hole acceptors ferrocenylhexanethiol (FcC6SH). 1-H NMR spectroscopy allowed for facile quantification of surface coverage of FcC6SH on CdTe QDs. Chlroide treated CdTe QDs were found to undergo effective ligand exchange with FcC6SH resulting in atleast 10 times higher coverage of FcC6SH as compared to untreated as-synthesized CdTe QDs. Higher coverage of molecular acceptor on chloride treated CdTe QDs provided multiple hole transfer pathways which facilitated highly efficient (?99%) hole transfer.;Oleate capped CdE QDs (E = S, Se and Te) were ligand exchanged with FcC6SH to unravel the hole transfer dynamics from band edge and surface states of CdE QDs to molecular hole acceptors. Trends in rate constant of hole transfer from band edge and surface states were consistent with driving force dependent hole transfer. Hole transfer to surface bound thiolates was found to be much slower for CdSe and CdS QDs, and absent in CdTe QDs, precluding the possibility of sequential hole transfer from QDs to thiolates to ferrocene.;Finally, charge separation in a novel type II heterostructure was studied using ns and ps timescale transient absorption spectroscopy where V2 O5 NWs were interfaced with CdE QDs using successive ionic layer adsorption and reaction (SILAR) and LAA approach. Type II band offsets between the NWs and QDs resulted in localization of electrons in NWs and holes in QDs. All V2O5/CdE heterostructures showed ultrafast charge transfer and long-lived charge separated state, which can be potentially very useful for photocatalysis and solar energy harvesting.