Author ORCID Identifier

https://orcid.org/0009-0003-5753-840X

Semester

Spring

Date of Graduation

2024

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Lian Li

Committee Member

Matthew Johnson

Committee Member

John Stewart

Committee Member

Wathiq Abdul-Razzak

Committee Member

Stinespring Charter

Abstract

Crystal polymorphism is a phenomenon in which compounds with the same chemical formula can be crystallized into different crystal structures. This phenomenon can be observed in elemental materials, such as diamond and graphite, as well as in compounds, such as the trigonal (1H) or octahedral (1T) prismatic MoS2. Crystals can also exhibit polytypism by stacking different polymorphs in a certain order, with the stacking sequence determining the variation between polytypes. Although all polymorphs and polytypes have the same chemical composition, each polymorph and polytype possesses unique electronic and physical properties.

This study explores the additive-assisted chemical vapor deposition (CVD) of transition metal chalcogenides and oxides. First, using MoO2 and S as the transition metal and chalcogen sources and BiOCl and water vapor as the promotors, we synthesized monoclinic and hexagonal phases of MoO2 on SiO2/Si(100) substrates. By varying the BiOCl:MoO3 ratio, we demonstrated controlled growth of the MoO2 polymorphs. Specifically, with a 1:8 ratio, the hexagonal phase was observed predominantly for MoO2, as evidenced by the hexagon-shaped structures. With a 1:4 ratio, the growth was dominated by the monoclinic phase of MoO2, as evidenced by the rhombus-shaped structures. Second, we further applied the same additive-assisted CVD growth to the synthesis of twisted MoS2 multilayers. Here, the monoclinic MoO2 was used as a seed, providing the necessary constraint for the growth of twisted MoS2 multilayers. Without such a constraint, the growth of the semiconducting twisted MoS2 multilayers would not have been possible. Finally, we extended the same technique to grow hexagonal-FeSe multilayer films to investigate magnetism. Using BiOCl as the promotor and Fe and Se powder as the precursors, we synthesized hexagonal-FeSe multilayers on C-plane sapphire substrates, which exhibit ferromagnetic order with a Curie temperature above room temperature. The samples were characterized using optical microscopy, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, photoluminescence, and vibrating sample magnetometry to analyze their crystal structure, chemical composition, optical properties, and magnetic properties.

The ability to grow polymorph and polytype heterostructures at the atomic scale via additive-assisted CVD opens the possibility of designing and synthesizing materials with tailored properties using a low-cost high-throughput growth technique. This enables the exploration of quantum phenomena such as non-linear optical response and topological superconductivity.

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