Single and multi-exciton dynamics in nanoscale semiconductors

Abstract

Recent advancements in the synthesis and fabrication of nanoscale semiconductors and optoelectronic devices are revolutionizing an array of industries. Nonlinear spectroscopy is a powerful tool for studying the unique optical and electronic properties of these semiconductors. This thesis describes experiments using a unique multi-dimensional spectrometer to study three semiconductors: double-walled J-aggregate nanotubes, cuprous oxide, and monolayer transition metal dichalcogenides. First, we use two-dimensional correlation spectroscopy and cryogenic tunneling electron microscopy to correlate the excitonic and structural properties of J-aggregate nanotubes in solution. We observe weak coupling between inner and outer wall excitons in both isolated and bundled nanotubes. We also use two-dimensional rephasing spectroscopy to measure the homogeneous and inhomogeneous linewidths in solution at 295 K and in a glass at 10 K and determine dynamic and static disorder dominate, respectively. In addition, we observe photo-induced damage and recovery of nanotubes in the solid state. Quantum process tomography is used to unambiguously elucidate the single-exciton dynamics of J-aggregate nanotubes in solution. Inversion of spectroscopic signals from eight transient grating experiments completely characterizes the time evolution of the single-exciton density matrix by determining the process matrix. We confirm the weak coupling of inner and outer wall excitons and observe no contributions from non-secular processes. Second, we use two-quantum spectroscopy to make the first experimental observation of two-exciton correlations (i.e. biexcitons) in cuprous oxide. The direct measurement of two-exciton correlations supports the proposed mechanism of biexciton-Auger recombination for the efficient suppression of the Bose-Einstein condensation of excitons. Third, we use two-dimensional rephasing spectroscopy to observe substantial inhomogeneous broadening due to large static disorder in monolayer transition metal dichalcogenides. We also use two-dimensional correlation spectroscopy to reveal interactions of excitons with in-plane optical and acoustic phonons. Lastly, we use one- and two-quantum spectroscopy to measure unprecedentedly large inorganic, biexciton binding energies due to reduced dielectric screening in the atomically thin limit. The exciton and phonon dynamics revealed in these experiments contribute to our understanding of the elementary excitations in organic and inorganic nanostructured semiconductors. It is our hope these insights will help guide the design of next generation devices utilizing nanoscale semiconductors.