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Date of Award
Doctor of Philosophy (PhD)
The large absorption cross sections and the tunability of the energetic spacings between the states in the conduction (CB) and valence band (VB) within a semiconductor nanoparticle (NP) make them promising media for capturing electromagnetic radiation and converting it into charge carriers, or electricity. In photovoltaic devices that incorporate semiconductor NPs, it would be ideal if every photon could be absorbed by a NP and the carriers could be collected with perfect efficiency and without loss of energy. The relaxation pathways of the carriers within the NPs down to the band edge and their fate at the band edge contribute significantly to this ideal goal. For samples of NPs that are not in a device, but are suspended in solution, the carriers (electrons and holes) would relax to states near the band edge of the CB and VB, and if there are no competing pathways, they would recombine radiatively, and give off light or emission. The specific relaxation mechanisms and their efficiencies depend on many factors including chemical composition, local environment, and the dimensionality of the NP. In this dissertation, I describe the results obtained from a barrage of optical spectroscopy experiments aimed at characterizing the energetics and the relaxation dynamics of photogenerated charge carriers in semiconductor NP samples. Particular focus was placed on samples of one-dimensional (1D) cadmium telluride (CdTe) quantum wires (QWs) as they offer the tunability of their energetics through adjustment of their diameters as well as the length dimensions for efficient charge transport over macroscopic distances. Furthermore, through collaborations with the research group of William E. Buhro, we can also make high quality samples of CdTe QW that enable their quantum-mechanical properties and resultant dynamics to be carefully characterized. In order to probe the role of the densities of the quantum-mechanical states on the relaxation of electrons and holes down to those at the band edge, the dependencies of the efficiency for radiative recombination on the excess energy with which the carriers are prepared was investigated on numerous samples of NPs with varying dimensionality. These samples included cadmium selenide (CdSe) quantum dots (QDs), CdSe quantum platelets (QPs), CdTe QWs, and surface-passivated CdTe QWs. We identified two common trends of the efficiency on excitation energy without distinct differences associated with the dimensionality of the NPs. (1) The overall efficiency of radiative recombination decreases with increasing excitation energy. (2) There are often local minima in the efficiencies when exciting at energies between the spectral features present in the absorption spectra. We were able to conclude that it is not just how high above the band gaps the electrons and holes are prepared, but that at specific energies used to excite these charge carriers, competing, non-radiative relaxation pathways are opened. The relaxation dynamics of the electrons and holes were further investigated in CdTe QWs using time-resolved transient absorption (TA) spectroscopy. In order to untangle the complicated TA signals, we developed a model, termed quantum-state renormalization (QSR), that accounts for the shifting of the quantum-confinement states that occurs due to the change in electron density caused by photoexcitation. The QSR model enabled us to directly probe the population of the electrons and holes through the different states, as well as track their changing energies with time. Several noteworthy results were obtained. The photogenerated holes relax to the band edge on very fast, instrument-limited timescales, and the electrons relax more slowly with a rate of ~0. 6 eV psб. The prominent relaxation mechanism was concluded to be a phonon-coupling mechanism, where the carriers relax by converting the kinetic energy associated with quantum confinement to vibrational or phonon modes of the CdTe QW. Lastly, the emission lifetimes of the CdTe QWs were measured as a function of the emission efficiency, or photoluminescence (PL) quantum yield (_PL). The lifetimes for CdTe QW samples with photoluminescence quantum yields > 4% were nearly an order of magnitude greater than the radiative lifetime of CdTe QDs, 200 ns versus ~25 ns. We propose these long lifetimes are a consequence of the conservation of momentum that must be maintained during radiative recombination. The photogenerated electron-hole pairs relax to the lowest quantum confinement states, and in these high-quality QWS, the pairs remain bound as 1D excitons. These 1D excitons have a thermal distribution of translational kinetic energy along the long, unconfined dimension of the QWs, and thus they contain significant momentum. This momentum cannot be conserved during radiative recombination, and this channel is closed. Only when the 1D excitons become localized can they emit. These results suggest that long charge carrier lifetimes coupled with the dimensionality of these high-quality semiconductor QWs offer distinct advantages for use in photovoltaics.
Chair and Committee
Richard A. Loomis
William E. Buhro, Jacob Schaefer, Bryce F. Sadtler, Li Yang,
Sanderson, William Matthew, "Spectroscopic Investigations of Excited Charge Carriers in II-VI Nanoparticles" (2020). Arts & Sciences Electronic Theses and Dissertations. 2239.
Available for download on Saturday, March 05, 2022