The interaction of charge carriers with excitons and plasmons within colloidal and epitaxial semiconductor nanostructures.

摘要

This dissertation describes the influence of the presence of charges--both bound electron and hole charge pairs (excitons) as well as free carriers--on the optical properties of semiconductor nanostructures. In a characterization of the evolution of the electronic structure of semiconductor nanostructures with increasing shape anisotropy, transient absorption (TA) and photoluminescence excitation (PLE) anisotropy measurements revealed that the band-edge absorptive and emissive transitions of CdSe nanorods contain both linear (z) and planar (xy) polarization character. The degree of planar character at the band-edge, modulated by classical local field effects arising from the dielectric contrast between the nanorod and the solvent, limits the degree of photoselection at this wavelength, while variation in the magnitude of the xy projection of the absorptive transitions within states above the band-edge is responsible for the observed wavelength-dependence of the absorption and emission anisotropies. The interaction of excess charge with the excitonic states of CdSe quantum dots (QDs) was probed with steady-state photoluminescence (PL) and ultrafast transient absorption measurements. The results of these experiments were utilized to construct a model for photobrightening (PB) - an increase in PL quantum yield with photoexcitation - based on migration of photoexcited electrons within the film. The basis for this model is that PB is limited by the rate of migration of electrons among surface-localized energetically shallow traps in the film, and not by the rate of creation of surface-trapped charge carriers. Finally, TA and transient third harmonic generation probe (THG-probe) spectroscopies were used to investigate the response of the localized surface plasmon resonances (LSPRs) of vertically aligned ITO nanowires (NWs) upon UV excitation. Both TA and THG-probe experiments show an increase in the frequency of the LSPR upon population of the conduction band of the ITO. The LSPR shifts back to its original energy on the single-picosecond lifetime of the photoexcited electrons. These results indicate that UV excitation modulates the plasma frequency of ITO on the ultrafast timescale by the injection of electrons into, and their subsequent decay from, the conduction band of the NWs, with ~13% changes to the electron concentration in the conduction band achievable in these experiments.