Electronic Theses and Dissertations

Himal Pokhrel

Date

2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

Committee Chair

Shawn Pollard

Committee Member

Benjamin Keller

Committee Member

Gladius Lewis

Committee Member

Sanjay Mishra

Abstract

Transition metal dichalcogenides (TMDs) are among the most promising groups of two-dimensional (2D) materials for applications in electronics, optoelectronics, electrocatalysis, energy storage, and spintronics. However, large-scale, high-quality growth of 2D TMDs remains a challenge, and research is ongoing to develop controllable fabrication methods for 2D TMDs, with the aim of their potential applications in various domains. In this dissertation, we present a comprehensive investigation into the growth, morphology, optical, electronic, and interfacial properties of 2D transition metal dichalcogenides (TMDs) and their heterostructures, with a primary focus on MoS2 and WS2 synthesized via plasma-assisted and salt-promoted low-pressure chemical vapor deposition (LPCVD) techniques. The research highlights the role of process parameters such as plasma power, NaCl promoter concentration, temperature, and carrier gas flow in tuning crystal size, morphology, and structural uniformity. Key findings of this research include the demonstration that argon plasma enhances molybdenum availability during MoS2 growth without compromising crystal quality, resulting in increased growth extent and an improved degree of edge disorder with increasing plasma power. We also investigated the critical influence of NaCl quantity on the morphology transition and layer thickness in WS2 films, directly affecting their electronic and optical properties. Furthermore, the work enhances our understanding of the interplay between growth conditions and material properties by employing complementary characterization methods, including Raman spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and time-resolved Kerr rotation (TRKR). These analyses reveal how modifications in plasma power, NaCl content, and flow rate and temperature control and defect engineering affect the transition from monolayer to few-layer WS2 with corresponding changes in bandgap emission, carrier dynamics, and valley exchange interactions. Additionally, in MoS2/WS2 heterostructures, plasma power is shown to critically affect interlayer coupling, leading to significant modulation of PL intensity, spectral shifts, chemical states, and carrier recombination lifetimes, thereby highlighting the tunability of interfacial properties for device applications. In fact, this research provides a systematic framework for optimizing LPCVD growth of 2D TMDs and their heterostructures, facilitating scalability and precise control over their functional characteristics. The insights gained into the effects of plasma assistance and salt promoters on atomic-scale growth dynamics and resultant material behavior offer a platform for advancing next-generation optoelectronic, catalytic, and valleytronic devices based on engineered TMDs.

Comments

Data is provided by the student.”

Library Comment

Dissertation or thesis originally submitted to ProQuest.

Notes

Open Access

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