Date of Award

August 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Michael Weinert

Committee Members

Daniel Agterberg, Ionel Popa, Min Gyu Kim, Peter Schwander

Keywords

defects, graphene, transition metal dichalcogenides, twisted bilayers

Abstract

ABSTRACT

SYNTHESIS, CHARACTERIZATION, AND SIMULATION OF TWO-DIMENSIONAL MATERIALS

by

Lawrence Hudy

The University of Wisconsin-Milwaukee, 2023Under the Supervision of Professor Michael Weinert

This dissertation focuses on my journey through many aspects of surface science leading to the first principles investigation of transition metal dichalcogenides studying the impact of defects, twist, and decreasing interlayer separation to probe their effect on the electronic properties of these materials. My journey started out learning many aspects of material science such as methods for material synthesis and characterization but later ended on simulation of material properties using density functional theory. In the first experiments, we focus on two-dimensional material synthesis, mostly involving graphene, where we see that polymer transferred graphene forms a Schottky junction when interfaced with a semiconductor. From atomic force microscopy and scanning tunneling microscopy we see that polymer transferred graphene is not entirely flat and forms ripples and ridges on the surface. Scanning tunneling spectroscopy and temperature dependent current-voltage measurements help to show that the behavior of these graphene Schottky diodes are not ideal. The observed temperature dependent Schottky barrier height can be explained using a distribution of barriers with varying barrier heights. The theoretical studies focus on various transition metal dichalcogenides, composed of MoSe2 and WSe2, using their monolayer and their homo and hetero bilayer counterparts. The first studies observed that adding defects alters the electronic band structure, and in particular, a copper dopant creates impurity states at the Fermi level and induces a significant magnetic moment in the material. The resulting occupied unpaired spin states are the key contributor to the creation of the magnetic moment in this material. Next, we see that twisted bilayer transition metal dichalcogenides, specifically bilayers composed of MoSe2 and WSe2, where we observe pressure induced flat bands and real space localization. Using a commensurate set of twist angles and varying interlayer spacing led to the discovery of flat bands and real space localization. These flat bands are a result of forcing the bilayers to interact causing a localization in real space. It is only under special conditions where the closest chalcogens, along with the nearest metal atoms, form a hybridized state that contribute to the flat bands in the energy band diagram. These findings help to highlight the impact impurities can have on transition metal dichalcogenides and the role of twist and interlayer separation has on the formation of flat bands as well as real space localization in these materials.

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