The Electronic and Optical Properties of Close Packed Cadmium Selenide Quantum

The synthesis, structural characterization, optical spectroscopy, and electronic characterization of close packed solids prepared from CdSe QD samples tunable in size from 17 to 150 A in diameter (a<4.5%) are presented. We control the deposition of 3D QD glasses and superlattices by tailoring the solvent composition used to deposit the QD solids. We use high resolution scanning and transmission electron microscopies and small-angle x-ray scattering to develop a well-defined structural model for glassy and ordered solids. Locally the QDs in the solids are close packed with an interdot spacing of 11 ± A maintained by organic ligands coordinating the QD surfaces. We show spectroscopically that electronic energy transfer occurs between proximal QDs in the solids and arises from dipole-dipole interdot interactions. In well-intermixed QD solids of small and large dots, we measure quenching of the luminescence (lifetime) of the small dots accompanied by enhancement of the luminescence (lifetime) of the large dots consistent with electronic energy transfer from the small to the large dots. In QD solids of single size dots, a red shifted and modified emission lineshape is consistent with electronic energy transfer within the sample inhomogeneous distribution. We use F6rster's theory for long-range resonance transfer through dipole-dipole interdot interactions to explain electronic energy transfer in these QD solids. We demonstrate photoconductivity in the QD solids. We measure the photocurrent as a function of excitation energy, voltage, excitation intensity, and temperature to uncover the carrier generation, separation, and transport mechanisms. The spectral response of the photocurrent follows the absorption spectra for the QD solids demonstrating carrier generation in the QDs. The photocurrent is linear with incident intensity consistent with a carrier generation efficiency that is scaled by the photon flux. The photocurrent exhibits an anomalous temperature dependence, reaching a maximum at -75 K, consistent with a thermally activated process that is overcome at higher temperatures by the decreasing exciton lifetime. The I-V curves are nonlinear, independent of excitation energy and photon flux. We present three possible models, describing field-assisted charge separation, collective transport of carriers, and carrier tunneling, to explain the photoconductive properties of QD solids. Thesis Supervisor: Moungi G. Bawendi, Ph.D. Title: Professor of Chemistry