Exciton Dynamics in Organic and Inorganic Nanoscale Materials

Abstract

The study of the photophysical properties of nanoscale materials has been a substantial research area for several decades due to their unique optoelectronic behaviors and numerous applications ranging from quantum emitters to solar panels and beyond. Any use of these materials, however, that involves their photophysical behavior relies on a careful understanding of the light-matter interactions that occur within the system. Spectroscopic studies provide a powerful tool for probing these interactions and their dependence on the materials’ fundamental properties.

In this thesis I investigate the dynamics of optically excited excitons in these materials and the available pathways for their recombination. I first study the interaction between excitons and the supramolecular lattice found in nanotubular molecular aggregates. The mechanism of photobrightening in these aggregates is explored through fluorescence measurements and wide angle X-ray scattering to demonstrate that the increase in quantum yield is associated with a change of structure. This leads to the development of a population of shielded or trapped excitons due to the formation of large polarons. Furthermore, I demonstrate design handles to control the recovery from the photobrightened state through rigidification of the aggregate structure.

The next section then investigates the dynamics of triexciton recombination pathways in inorganic CdSe/CdS core-shell nanocrystals (NCs). I present the use of time- and spectrally-resolved ensemble emission measurements to determine the emission energy of the P-like above band-edge state in the NCs. This information is used to develop of a state-resolved third order correlation experiment to directly measure the emission pathway of triexciton events under low-flux conditions and observe that the recombination is dominated by band-edge emission. These results are then compared to theory to determine how the relative carrier overlap affects the triexciton recombination dynamics.

In the final section I discuss the nuances of uncertainty estimation that can arise in any spectroscopic measurement. Using two particular case studies of experiments I demonstrate where typical error approximation assumptions begin to fail. This is then followed by a thorough exploration of multiple techniques that can be used to gain more insight into the uncertainty of a measurement even when typical approximations fail.