Date of Award

8-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Committee Chair/Advisor

Dr. Apparao M. Rao

Committee Member

Dr. Catalina Marinescu

Committee Member

Dr. Endre Takacs

Committee Member

Dr. Sriparna Bhattacharya

Abstract

The emergence of two-dimensional (2D) layered materials provides unprecedented opportunities for studying excitonic physics due to the strong Coulomb interaction between the electron-hole pair. Because of the reduced dimensionality and weak dielectric screening, the exciton is stable at room temperature, unlike bulk semiconductors. The evolution from low to high carrier density for optical gain in 2D semiconductors involves insulating exciton gas, exciton condensation, co-existence of various excitonic complexes, electron-hole plasmas (EHPs), or electron-hole liquids (EHLs), leading to the Mott transition. Strong interaction among the excitons, such as exciton-exciton annihilation (EEA), serves as a hot-carrier generation. A bound exciton dissociates into a hot electron and a hole - the latter serves as an alternate blueprint for hot-carrier harvesting in excitonic rich nanostructured materials.

Transition metal dichalcogenides (TMDCs) are the emerging layered materials of the form MX2, where each layer constitutes a hexagonal lattice of transition metal atoms (M = Mo, W, etc.) sandwiched between two hexagonal layers of chalcogen atoms (X = S, Se, etc.). Here we use CVD-grown WS2 monolayers to elucidate the excitonic interactions. Using ultrafast photocurrent spectroscopy (UPCS), we measure the hot carrier photocurrent in monolayer WS2. The hot carriers are induced by Auger recombination, including trion Auger recombination in the low exciton density region transitioning to biexciton Auger recombination in the high exciton density region. The enhanced quantum confinement in 2D materials gives rise to the strong dipole-dipole interaction between excitons leading to the generation of hot carriers via the Auger mechanism.

The organic-inorganic hybrid perovskites (OIHP) are at the center of attraction because their excellent optical and electronic properties are favorable for applications such as photodetectors, field-effect transistors (FETs), solar cells, x-ray scintillators, and other light-emitting optoelectronic devices. Two-dimensional halide perovskites are especially interesting because of their excellent photoluminescence quantum yield, extended carrier lifetime, significant carrier diffusion length, strong light absorption, and easy fabrication. The 2D halide perovskites can form organic-inorganic hybrid quantum wells (QWs), the confinement of which keeps electrons and holes closer together and hence increases the radiative recombination probability favoring light-emitting applications. Halide perovskites are also popular because of their outstanding structural and compositional tunability.

In this study. Specifically, we explore the optical and electrical properties of various OIHP using UPCS, photoluminescence (PL), and time-resolved PL (TRPL) spectroscopy. These materials serve as an excellent testbed for excitons in QWs at room temperature because of their high binding energies (typically exceeding 150 meV) arising from the quantum confinement and weak dielectric screening. The dielectric constants of the organic ligand and inorganic octahedral layers differ very much. As a result, the presence of the organic barrier strongly screens the Coulomb interaction in these QWs. It is essential to understand the charge transport properties in 2D OIHP to translate these materials into various optoelectronic devices. Here, we investigated their comprehensive photophysical processes such as photogeneration, recombination, and charge transport in Pb and Sn-based n = 1 and n = 2 layered perovskites.

Author ORCID Identifier

https://orcid.org/0000-0002-2515-6314

Available for download on Friday, July 21, 2023

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