Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Humberto R. Gutierrez, Ph.D.

Committee Member

Xiaomei Jiang, Ph.D.

Committee Member

Dmitri Voronine, Ph.D.

Committee Member

John Kuhn, Ph.D.

Keywords

Alloys, Photoluminescence, Raman Spectroscopy, Transition Metal Dichalcogenides

Abstract

Two-dimensional (2D) materials show remarkable physical properties when compared to their bulk form. These are single layer materials where the atoms are covalently bonded to each other. Among the 2D materials, transition metal dichalcogenides (TMDCs) have special consideration due to its unique electrical, optical and mechanical properties. These materials can be combined either vertically or laterally to form heterostructures and can be mixed to form alloys in order to expand the number of members in the 2D family. In turn optoelectrical properties can be tuned for better performances when develop for potential applications.

In the first part of this dissertation, we report the synthesis and characterization of multi-junction lateral heterostructures of the MoS2-WS2 and MoSe2-WSe2 with bilayer thickness in each domain. A phenomenological mechanism, that highlights the role of temperature as well as initial cluster thickness and size, is proposed to explain the self-limited thickness of the heterostructures. Compared to our monolayer system, the bilayer systems show better environmental stability, improved electrical behavior, photovoltaic response and electroluminescence at room temperature. In the second part we discuss synthesis of lateral heterostructures based on ternary alloys, MoSe2(1-x)S2x-WSe2(1-x)S2x. The continuous band gab tuning was achieved by changing the composition of precursors systematically. The composition-dependent band alignment at the interfaces was also studied. The lateral size control and intra-domain homogeneity was also demonstrated in bilayer and trilayer lateral heterostructures of ternary and quaternary alloys.

In the last part of this dissertation, we describe the synthesis and laser-assisted modifications conducted on large-area few-layers MoTe2. The synthesis approach used thermal evaporation and simple CVD process which is differ from the above method. Laser-assisted modifications took place in a home-made stainless steel chamber and in a controlled environment. Real-time Raman spectroscopy was used to monitor the reaction. The parameters (laser power, exposure time) were optimized through a set of experiments. By introducing localized chemical modification in the structure, we were able to generate lateral metal-semiconductor (MoTe2-MoTe2(1-x)S2x ) junction, resulting in reduced contact resistance of related FET devices. Different 2D patterns of heterojunctions were demonstrated using this laser-writing technique.

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