Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Matthias Batzill, Ph.D.

Committee Member

Humberto Rodriguez Gutierrez, Ph.D.

Committee Member

Inna Ponomareva, Ph.D.

Committee Member

Dario Arena, Ph.D.

Committee Member

Stephen E. Saddow, Ph.D.

Keywords

2D materials, doping, monolayer, STM, TMDC

Abstract

Two dimensional (2D) materials present interesting property variation when compared to bulk materials as they are single layer materials with reduced dimensions effects, minimal interlayer interactions and may present changes in lattice symmetry. Van der Waals (vdW) materials are layered materials with strong in-plane covalent bonds and weak out-of-plane vdW forces, which allows crystals to be isolated down to a single molecular layer and these single layers to be classified as 2D materials. The ability of combining these materials independent of lattice matching and modifying electronic properties by using vdW heterostructures makes them promising materials for electronic devices applications. Moreover, these materials can also be grown by molecular beam epitaxy (MBE) and modified by doping, defect engineering, phase transition, compositional change and characterized in situ by UHV compatible techniques.

Among vdW materials, transition metal dichalcogenides (TMDCs) represent a large class with a metal layer sandwiched between two layers of chalcogens and each of these trilayers interacts with another by vdW interactions. The TMDC samples studied here are grown and/or modified by MBE, using high purity material sources under ultra-high vacuum (UHV) environment, also allowing in situ characterization using UHV compatible techniques such as scanning tunneling microscopy (STM), low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and angle resolved photoemission spectroscopy (ARPES).

It is expected that TMDC films grown by MBE might present some intrinsic or induced defects. However, these defects can exhibit exciting properties and by understanding and/or controlling their formation, new functionalities can be induced to the material. Here, we present three different processes of modifying monolayer and multilayer materials by the introduction of extra metal into their crystal structure. We explain for the first time the formation mechanism of previously reported metallic Mirror Twin Grain Boundaries (MTB) networks and controllably modify 2D materials based on the understanding of this mechanism. MoTe2 and MoSe2 were modified with extra atoms of Mo and the controllable formation of MTB was investigated. The same mechanism of incorporation of extra metal atoms into the lattice of TMDCs was used to induce magnetism into nonmagnetic substrates by post synthesis doping of MoTe2 with V atoms. We also discuss a layered dependence compositional change that transforms transition metal ditelluride films into intercalated layered materials, which can also be considered as a modification related to the presence of extra metallic atoms in the vdW gap between the layers of these transition metal ditelluride films. Moreover, the ability of synthesizing isolated monolayers of early transition metal ditellurides enables further investigation of their electronic and magnetic properties. The results of the studies presented here collaborate to a deeper understanding of the fundamental physics behind 2D TMDC systems and possible electronic applications in devices based on these systems.

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