Type of Document	Dissertation
Author	Lovingood, Derek Dean
URN	etd-03212010-145527
Title	Synthesis And Analysis Of The Quantum Dot Core and Surface Structures Probed By Solid State Nuclear Magnetic Resonance
Degree	Doctor of Philosophy
Department	Chemistry and Biochemistry, Department of
Advisory Committee	Advisor Name Title Geoffrey F. Strouse Committee Chair Albert E. Stiegman Committee Member Gregory B. Dudley Committee Member Peng Xiong University Representative
Keywords	Quantum Dots CdSe QD Suface Reconstruction ssNMR InP
Date of Defense	2010-02-22
Availability	unrestricted
Abstract This dissertation outlines how the optical properties of InP quantum dots were improved by microwave synthetic techniques, how CdSSe alloy quantum dots were determined from a single source precursor, and how specific regions of CdSe quantum dots were identified by solid state nuclear magnetic resonance determining size dependent reconstruction. High quantum yield (47%) InP nanocrystals can be prepared without the need for post HF treatment by combining microwave methodologies with the presence of a fluorinated ionic liquid. Growing the InP nanocrystals in the presence of the ionic liquid 1-hexyl-3-methyl-imidazolium tetrafluoroborate (hmim BF4) allows in-situ etching to be achieved. The optimization of the PL QY is achieved by balancing growth and etching rates in the reaction. The development of ternary nanoscale materials with controlled cross-sectional doping is an important step for the use of chemically prepared quantum dots for nanoscale engineering applications. We report cross-sectional, elemental doping with the formation of an alloyed CdSSe nanocrystal from the thermal decomposition of Li4[Cd10Se4(SPh)16]. The sulfur incorporation arises from surface mediated phenylthiolate degradation on the growing quantum dot surface. In the alloy, we identify a pure CdSe nucleus of ~ 1.5 nm consistent with the predictions of nucleation theory. As the particle grows, S2- incorporation increases until the CdSSe reaches ~4 nm where a marked reduction in phenylthiolate content on the nanocrystal is observed in CP-MAS NMR spectroscopy implying rapid decomposition of the phenylthiolate arises with subsequent enhanced S2- incorporation at the level of the stoichiometry of the reaction (namely ~60%). The use of molecular clusters to allow controlled defect ion incorporation can open new pathways to more complex nanomaterials. Evidence of size dependent reconstruction in quantum dots leading to changes in bonding is observed through analysis of the 77Se{1H} cross polarization magic angle spinning and 77Se spin-echo solid state NMR for Cd77Se quantum dots. Insight into structural and electronic perturbations experienced within Cd77Se is achieved by contact time dependent 77Se{1H} CP-MAS measurements providing NMR features assignable to the surface, comprised of multiple layers, coupled to spin diffusion into the core. Due to the nearly 100% enrichment level for 77Se, efficient coupling arises between the surface 77Se and sub-layer 77Se sites due to spin diffusion resulting in long T2 relaxation times in the Cd77Se quantum dots. The observed chemical shift for the discrete 77Se sites can be correlated to the effective mass approximation via the Ramsey expression indicating a 1/r2 size dependence for the change in chemical shift with size, while a plot of chemical shift versus the inverse band gap is linear. The correlation of NMR shift for the discrete sites allows a valence bond theory interpretation of the size dependent changes in bonding character within the reconstructed QD. Based on the NMR results, discrete surface and core 77Se sites exist within Cd77Se QDs where global reconstruction occurs below 4 nm in diameter, while an apparent self-limiting reconstruction process occurs above 4 nm.
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