Optical and Electronic Interactions at the Nanoscale

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Doctor of Philosophy (PhD)
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Physics & Astronomy
field effect transistor
quantum dots
time-resolved absorption
time-resolved photoluminescence
ultrafast spectroscopy
Condensed Matter Physics
Nanoscience and Nanotechnology
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In this dissertation, we discuss optical and electronic interactions in three nanometer scale semiconductor systems in a broadly defined sense. These studies are performed using time-integrated and time-resolved optical spectroscopies and temperature- and field-dependent electrical transport measurements. We first discuss the construction and optimization of an optical apparatus for performing broadband, time-integrated and sub-picosecond fluorescence and absorption measurements. Using this apparatus, we then characterize the impact on the optically-excited carrier relaxation dynamics of cadmium selenide quantum dots due to a surface treatment previously shown to increase interparticle coupling, namely the solution exchange of native, aliphatic ligands for thiocyanate followed by subsequent sample annealing. We find that this ligand treatment leads to faster surface state electron trapping, a greater proportion of surface photoluminescence, and an increased rate of nonradiative decay due to enhanced interparticle coupling. In contrast to trends previously observed at room temperature, we also show that at 10 K the band-edge absorptive bleach is dominated by 1Sh hole occupation in the quantum dot interior. In the second study detailed here, we use this time-resolved photoluminescence apparatus to demonstrate an enhancement of radiative rates in cadmium sulfide nanowires due to plasmonic enhancement from interactions of hot excitons with a concentric electrically conductive silver coating. In the final experiment we return to cadmium selenide quantum dots to investigate the electronic interactions among quantum dots in high-mobility indium-doped field effect transistors at low temperature. We show that application of a gate bias to the transistor to accumulate electrons in the quantum dot channel increases the "localization product" (localization length times dielectric constant) describing transport at the Fermi level, as expected for Fermi level changes near a mobility edge. Our measurements suggest that the localization length increases to significantly greater than the quantum dot diameter and further that application of gate bias decreases the mobility gap separating localized and extended states.

James M. Kikkawa
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