Ultrafast Charge Separation and Relaxation in Novel Nanostructures

Abstract

Nowadays, a significant part of the research interest in optoelectronic devices is in making nanostructures such as heterojunctions, nanowires, quantum wells and quantum dots that dramatically change the electron behavior of the material compared to their bulk counterpart. In this dissertation, we applied ultrafast spectroscopy to study specifically the hot carrier separation and recombination dynamics in the CVD-grown monolayer molybdenum disulfide (MoS2) and graphene heterostructure, III-nitride quantum well LED structure and self-organized InGaN nanowires for photocatalysis.

By using ultrafast pump-probe spectroscopy, we observed efficient electron and hole separation within a few hundred femtoseconds by interlayer charge transfer from graphene to MoS2 monolayer following initial excitation and thermalization. The transfer-back process to graphene happens in multiple timescales, the dynamics of which after the thermalization can only be modeled by a Porter-Thomas distribution for the charge transfer rate because the transfer back process after the thermalization is dominated by the material disorder.

The III-nitride quantum well has been the most common LED structure since the 1990s. III-V compounds are also the only known material that can have tunable valence and conduction band levels that cover the chemical potential of a few important photochemical reactions such as water splitting and methane oxidation thus making them ideal for photocatalysis in converting solar energy into electrochemical energy. We studied the time-resolved photoluminescense (TRPL) and the time-resolved differential reflectance (TRDR) of the self-organized InGaN nanowires observed a fast decay component in the PL decay dynamics that were not observed in the DR measurements only when the InGaN is p-doped. We argue that the fast decay component in the PL decay dynamics indicates the charge separation due to the built-in polarization field near the surface of the nanowires. We also studied the temperature-dependent TRPL experiments and room temperature TRDR of the quantum disk-in-wire micro-LED structure that contains tilted quantum wells grown along the semi-polar facets. The TRPL decay curves always show faster dynamics than the TRDR. This is modeled with thermal activated non-radiative process that can be explained with Arrhenius’s Law applied to the lateral charge transport over the local indium composition fluctuation.

Throughout this thesis, we study the ultrafast charge separation and relaxation by directly probing and by combining the PL and DR dynamics in different nanostructures. We have demonstrated a novel approach for detecting carrier spatial separation and the results revealed the important microscopic physics such as material disorder, band bending and local fluctuation that are crucial for future research and development.