Surface Chemistry and Charge Transfer Dynamics in Cadmium Chalcogenide Quantum Dots Tethered to Linker Functionalized Metal Oxides and Redox-Active Molecules
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In this dissertation, I report on the assembly and excited-state charge-transfer reactivity of heterostructures consisting of cadmium chalcogenide quantum dots (QDs) interfaced with molecular or materials-based acceptors via amine-bearing ligands. Chapters 2 and 3 report on the synthesis and characterization of QD-molecule-metal oxide heterostructures using amine-bearing ligands prepared by linker-assisted assembly (LAA), via both in-situ and ex-situ approaches. Chapter 4 reports on heterostructures of QDs and a redox-active ferrocene derivative as a molecular acceptor. For the various heterostructures, we studied charge-transfer dynamics using steady-state emission and time-resolved emission. In LAA, a bifunctional linker molecule is used as the bridge to bind QDs to the metal oxide. The nature and strength of the interaction between the ligands and the surface of QDs determined the stability of the QD-ligand complexes. Thiolate linkers such as 3-mercaptopropionic acid (MPA) have been widely used for this purpose and have shown efficient electron transfer (ET). However, adsorbed thiolates accept valence band holes which reduces (1) the oxidizing potentials of the photogenerated holes and (2) the distance between electrons and holes in the charge-separated state. Amines, on the contrary, can bind to cadmium chalcogenide QDs without accepting holes, as long as they have sufficiently positive oxidation potentials. Amines have shown enhancement of band-edge emission and shift of trap states to higher energies, which can promote an increase in driving force and elimination of electron-hole recombination pathways. Amines are thus attractive alternatives to thiol-bearing ligands for LAA and for tethering redox-active molecular acceptors to surfaces.In in-situ LAA, linear aminoalkanoic acids (AAAs) and linear mercaptoalkanoic acids (MAAs) were used to tether CdSe QDs to nanocrystalline TiO2 thin films. The adsorption of CdSe QDs to linker-functionalized films to followed the Langmuir adsorption isotherm and kinetics. Steady-state and time-resolved spectroscopy measurements showed evidence of electrons being transferred from band-edge and trap states of the CdSe QDs to TiO2 with rate constant on the order of 107 s-1. Thiol-bearing ligands showed more favorable ET dynamics when compared to amine-bearing ligands; however, amine-bearing ligands did not scavenge photogenerated holes from CdSe QDs. Amine-bearing ligands also shifted electron trap-states to higher energies, which minimized the loss of potential energy of electrons before ET.In ex-situ LAA, a two step-ligand exchange was developed to tether CdSe QDs to TiO2 using 4-aminobenzoic acid (PABA). 1H NMR and ATR-FTIR were used to characterized the attachment of the ligands to the surfaces of QDs. Pyridine was used as intermediate ligand to remove introduce PABA onto the surfaces of QDs via successive L-type promoted Z-type displacement and L-type ligand-exchange reactions. Steady-state and time-resolved spectroscopic measurements revealed that electrons were transferred from band-edge states of the CdSe QDs to TiO2 with a rate constant on the order of 108 s-1. Sensitized photocurrent was observed in the CdSe QD-PABA-TiO2 heterostructures through IPCE measurements. Amine-bearing ligands can thus be potential candidates for tethering QDs to TiO2 via both in-situ and ex-situ LAA.The influence of bound molecular acceptors on the dynamics of excited-state deactivation and charge transfer was evaluated for cadmium chalcogenide QDs functionalized with redox-active molecules. Aminoferrocene (Amfc) was the molecular acceptor used for this study, and hexylamine (HXA) was used as a control. 1H NMR was used to determine the number of bound acceptors per QD. Spectroscopic measurements revealed that holes were transferred from band-edge and trap states of the CdS QDs to the amine group of Amfc, whereas for CdSe QDs, holes were transferred to the Fe(II) center of Amfc. Dynamic quenching of the emission from QDs provided evidence for excited-state hole transfer, and hole-transfer dynamics were extracted from time-resolved emission measurements. Observed rate constants were on the order of 107 s-1 and 108 s-1 for CdS QDs and CdSe QDs, respectively. The intrinsic rate constant for CdS QDs systems was on the order of 105 s-1, whereas for CdSe QDs a non-linear relation between rate constant and the number of bound acceptors was observed. This study provided insight into the mechanism and dynamics of charge-transfer processes in QDs and the influence of surface molecular species, which can contribute for building QD-photocatalysts systems.