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Title:Scanning tunneling microscopy and spectroscopy study of layered transition metal chalcogenide thin films tuned by thickness, structural phase, and heterostructure
Author(s):Chen, Guannan
Director of Research:Madhavan, Vidya
Doctoral Committee Chair(s):Eckstein, James N.
Doctoral Committee Member(s):Wagner, Lucas K.; DeMarco, Brian Leeds
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Transition metal chalcogenide
Abstract:Layered transition metal chalcogenides (TMCs) have a long and fruitful history, exhibiting diverse properties and interesting quantum phenomena, such as superconductivity, charge density wave (CDW), and non-trivial topological states. Since the discovery that layered materials can be exfoliated to two-dimensional (2D) sheets, such as graphene, 2D TMCs have attracted vast interest for both fundamental study and technological development. More often than not, the thin film samples behave differently from the corresponding bulk single crystals. For example, monolayer WTe2 is a quantum spin Hall insulator, a novel phase of quantum matter, while bulk WTe2 is a Weyl semimetal. Moreover, the electronic and magnetic properties of thin films are able to be tuned by various methods, including chemically doping, electrically gating, varying the structural phase, changing the sample thickness, constructing heterostructures with other 2D materials, and mechanically manipulation. This thesis presents experimental results on the effect of the number of layers, lattice structure, and heterostructure in several thin-film systems. To synthesize the high-quality samples, a home-built molecular beam epitaxy system was utilized to precisely control the thickness and structural phase. The films were characterized by scanning tunneling microscopy and spectroscopy. Our finds are the following. The 1T-VSe2 samples, grown at low temperatures, display different CDW patterns on the first and second layers. In contrast, a novel phase, i.e. distorted 1T, is obtained in the second layer by elevating the growth temperature by 250 C. No CDW is observed in this phase. The same phase engineering strategy was adopted for NbSe2 growth. While monolayer 1H-NbSe2 is demonstrated to host unconventional superconductivity, the Mott insulating ground state and strong evidence for the presence of spinon Fermi surface are disclosed in metastable 1T-NbSe2 monolayers. Besides, thin films of FeSexTe(1-x) grown on single-crystal Bi2Te3$ were systematically investigated under a low-temperature scanning tunneling microscope. Three different heterostructures coexist in the sample. At 0.3 K, the superconductivity appears in all three kinds of FeSexTe(1-x) islands, but with strikingly different gap sizes and depths. The data analysis from the phase-sensitive quasiparticle interference technique implies an odd-parity s+- pairing in superconducting states.
Issue Date:2021-03-23
Rights Information:Copyright 2021 Guannan Chen
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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