Yeung, Nelson (2019) Computational modelling of lateral and vertical heterostructures of transition metal dichalcogenides. PhD thesis, University of Warwick.

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Abstract

When certain materials in their bulk phase are reduced to a two-dimensional (2D) form, their electronic behaviour alters drastically, which has led to intense research into low-dimensional semiconductors within the condensed matter physics community. This thesis focuses on two relatively unknown heterostructures, lat- eral heterostructures (LHSs) and vertical heterostructures (VHSs), formed by one of the most studied and promising 2 Dmaterials, transition metal dichalcogenides (TMDC). LHSs comprises of two different 2 Dmaterials connected by covalent bond in-plane. VHSs are composed of different stacked layers that are weakly interacting. The linear-scaling density functional theory (DFT) package ONETEP was used to study defects in small lattice mismatch and large lattice mismatch lateral heterostructures.

We first examine the single sulphurvacancy formation energies across aLHS, then we present an approximation scheme to compute the critical thickness in which a dislocation will be formed for large lattice mismatch LHSs. The construction of a LHS for first-principles calculations is also presented. We then move on to the work in collaboration with experimentalists, where angle resolved photoemission spectroscopy (ARPES) measurements with electro- static gating was made for the first time on 2D materials to observe the band structure with the location of the conduction band edge. We show that the state of the art GW approximation is able to produce high quality band structure that is comparable to experiment. Here we have studied monolayer, bilayer and trilayer tungsten diselenide and tungsten disulfide. We also explore the possibility of using the virtual crystal approximation with GW to study the Mo1–xWxS2 alloy.

Finally, we describe a novel scissor operator method for largescale DFT calculations on VHSs. Due to the band gap problem within DFT, the DFT band alignment of certain TMDCs will not be realistic, and thus can predict incorrect band structures for certain VHSs. The present method corrects the band structure of a VHS using GW band alignment. We first describe the scissor operator imple- mentation into ONETEP, then we show validation of this method with titanium disulphide–molybdenum disulfide VHS. xi

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry T Technology > TK Electrical engineering. Electronics Nuclear engineering
Library of Congress Subject Headings (LCSH):	Heterostructures, Transition metals, Transition metals -- Electric properties, Transition metal compounds, Two-dimensional materials
Official Date:	September 2019
Dates:	Date Event September 2019 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Hine, Nicholas
Format of File:	pdf
Extent:	xiii, 127 leaves : illustrations
Language:	eng

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