Browsing by Subject "Zinc oxide"
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Item Hot electron dynamics at semiconductor surfaces: implications for quantum dot photovoltaics.(2010-07) Tisdale, William A.Finding a viable supply of clean, renewable energy is one of the most daunting challenges facing the world today. Solar cells have had limited impact in meeting this challenge because of their high cost and low power conversion efficiencies. Semiconductor nanocrystals, or quantum dots, are promising materials for use in novel solar cells because they can be processed with potentially inexpensive solution-based techniques and because they are predicted to have novel optoelectronic properties that could enable the realization of ultra-efficient solar power converters. However, there is a lack of fundamental understanding regarding the behavior of highly-excited, or "hot," charge carriers near quantum-dot and semiconductor interfaces, which is of paramount importance to the rational design of high-efficiency devices. The elucidation of these ultrafast hot electron dynamics is the central aim of this Dissertation. I present a theoretical framework for treating the electronic interactions between quantum dots and bulk semiconductor surfaces and propose a novel experimental technique, time-resolved surface second harmonic generation (TR-SHG), for probing these interactions. I then describe a series of experimental investigations into hot electron dynamics in specific quantum-dot/semiconductor systems. A two-photon photoelectron spectroscopy (2PPE) study of the technologically-relevant ZnO(10-10) surface reveals ultrafast (sub-30fs) cooling of hot electrons in the bulk conduction band, which is due to strong electron-phonon coupling in this highly polar material. The presence of a continuum of defect states near the conduction band edge results in Fermi-level pinning and upward (n-type) band-bending at the (10-10) surface and provides an alternate route for electronic relaxation. In monolayer films of colloidal PbSe quantum dots, chemical treatment with either hydrazine or 1,2-ethanedithiol results in strong and tunable electronic coupling between neighboring quantum dots. A TR-SHG study of these electronically-coupled quantum-dot films reveals temperature-activated cooling of hot charge carriers and coherent excitation of a previously-unidentified surface optical phonon. Finally, I report the first experimental observation of ultrafast electron transfer from the higher excited states of a colloidal quantum dot (PbSe) to delocalized conduction band states of a widely-used electron acceptor (TiO2). The electric field resulting from ultrafast (<50fs) separation of charge carriers across the PbSe/TiO2(110) interface excites coherent vibration of the TiO2 surface atoms, whose collective motions can be followed in real time.Item Synthesis and characterization of ensembles containing zinc oxide nanocrystals and organic or transition metal dyes to probe the early events in a dye-sensitized solar cell.(2011-08) Saunders, Julia ErinThe synthesis of 3',4'-dibutyl-2-phenyl-2,2':5',2"-terthiophene-5"-carboxylic acid, and its behavior with monodispersed ZnO nanocrystals (NCs) having diameters from 2.7 to 3.2 nm are reported. The excited state of the dye (E0* = -1.61 V vs NHE) was quenched upon binding to ZnO Ncs. Adsorption isotherms were measured for the terthiophene dye in ethanol and fit with a Langmuir model, which gave a size-independent Kads of 2.3 ± 1.0 x 105 M-1. The maximum number of attached dyes per nanocrystal depended on the diameter and was consistent with each dye occupying 0.5 ± 0.1 nm2 at maximum coverage. Deviation from the Langmuir model observed at low dye concentrations was attributed to a small amount of free zinc ion present in solution that bind the carboxylate ions more strongly than do ZnO NCs. Incorporation of the equilibrium expression between zinc ion and free carboxylate into the model provided a satisfactory fit for both the adsorption isotherm experiments and the complex shape of the Stern-Volmer graphs. Treatment of the terthiophene dye-nanocrystal dyads with increasing concentrations of sodium acetate in ethanol resulted in gradual displacement of the dye. Time-resolved fluorescence and time- and frequency-resolved pump-probe spectroscopy confirm and characterize electron injection from the dye to the semiconductor nanocrystals in room temperature ethanol dispersions at a series of dye:ZnO NC concentration ratios. The spectrum of the oxidized dye was determined by spectroelectrochemistry. The singlet excited state of the dye (190 ps lifetime in ethanol) is quenched almost exclusively by electron transfer to the ZnO NC, and the electron transfer dynamics exhibit a single time scale of 3.5 ( 0.5 ps at all concentration ratios. In the measured transient responses at different dye:ZnO NC ratios, gain in the amplitude of the electron injection component is anticorrelated with loss of amplitude from unperturbed excited state dye molecules. The dependence of this amplitude on dye:ZnO NC ratio deviates significantly from the prediction of a standard Stern-Volmer model. This observation is in agreement with the static quenching studies. By identifying electron transfer as the quenching mechanism at all ratios, the work presented here helps to exclude concentration quenching as the basis for the complicated quenching results, and supports the model that incorporates competitive binding between ZnO NC s and free Zn2+ cations in solution.