Date of Award

Spring 2018

Project Type

Thesis

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Karsten Pohl

Second Advisor

Olof Echt

Third Advisor

Kai Germaschewski

Abstract

Two-dimensional (2D) materials have attracted much attention as an emerging category of materials over the past decade due to their novel mechanical, optical and electronic properties with many potential applications in photovoltaics, photo-catalysts, and modern electronics. However, the detailed atomic structural information has been rarely experimentally investigated due to the following difficulties: (i) the limited sample size of 2D materials prepared through mechanical exfoliation of a few µm, and (ii) the easy oxidation and surface instability of various 2D materials under high energy probing techniques. Selected area low-energy electron diffraction analysis (µLEED-IV performed in a low-energy electron microscopy (LEEM) system, is a highly surface sensitive and non-intrusive surface characterization technique, which has the advantage of µm sampling size selectivity. Here I present the first detailed experimental characterizations of atomic crystal structures of a series of technologically promising 2D materials: MoS2, black phosphorus (BP), the topological crystalline insulator (TCI) SnSe, 1T phase SnSe2 and tungsten doped MoTe2. Furthermore, the LEED-IV calculation package was rewritten and parallelized to improve calculation efficiency and enabling high performance computing on super computers. µLEED-IV in a high spatial resolution LEEM system is shown to be a powerful tool for study of atomic crystal structure of 2D materials. We believe the detailed surface structural information is of fundamental importance and provides crucial input for better understanding the intriguing electronic properties of various 2D materials and a more solid guidance for engineering 2D materials based devices.

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