Optical And Electrical Characterization Of Two Dimensional Transition Metal Dichalcogenides

Author : Mingzu Liu
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (134 download)

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Download or read book MODIFICATION OF TRANSITION METAL DICHALCOGENIDES' OPTICAL, ELECTRICAL, AND MAGNETIC PROPERTIES THROUGH ALLOYING. written by Mingzu Liu and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation focuses on two-dimensional (2D) transition metal dichalcogenides (TMDs), and studies the modification of their physical properties through the method of substitutional doping/alloying. The controlled synthesis and novel properties such as room-temperature ferromagnetism in doped/alloyed 2D TMD systems are investigated. Two notes are addressed here: 1) The general concept of TMDs contains a large family of binary compounds, while we are mainly concentrated on the semiconducting members (sTMDs) inside, including MoS2, WS2, MoSe2, and WSe2. 2) The term of alloying is applied here to denote the situation where impurity atoms are incorporated intentionally into the host 2D lattice, with a concentration greater than 1 at%, while conventionally, the term of doping is only adapted to describe similar cases but with significantly lower concentrations less than 1 at%, and even less than 1000 ppm = 0.1 at% in some literatures. However, a clear quantitative classification between the two concepts does not exist, and we will study cases with a wide range of impurity concentrations in this dissertation. Therefore, we will indiscriminately use both the terms of doping and alloying for convenience, regardless of the exact concentration/amount of impurity atoms. The main context of this dissertation is outlined as follows. In Chapter 1, we will introduce the fundamentals of 2D TMD materials and techniques for 2D surface characterizations. 2D magnetism is one of the most active fields in condensed matter physics, and will be a major topic of our research, thus a brief introduction in principles of magnetic materials will also be given. Controllable substitutional doping/alloying in monolayer 2D materials is a prerequisite to any further characterizations and applications. In Chapter 2, we will first introduce the liquid-phase precursor-assisted chemical vapor deposition (CVD) method we developed for universal substitutional doping with tunable dopant concentrations in sTMDs. Different dopant elements are successfully incoporated in sTMD monolayers, and the defect types and optical properties are studied. It is then discovered that the edge termination in single crystal sTMD monolayers will greatly affect the spatial distribution of dopant atoms, and this effect is elucidated through the synthesis of doped hexagonal sTMD monolayers that have as-grown domains with different dopant concentrations. Combined with the solution-based CVD method we developed, the edge termination can be utilized for engineering in-plane heterojunctions that display fascinating electronic, optoelectronic, and magnetic properties. Long-range ferromagnetic ordering has been considered as hard to achieve for a long time in 2D systems, especially at room temperature. In Chapter 3, we will demonstrate room-temperature ferromagnetism in vanadium-doped (V-doped) sTMD monolayers and its coupling with the thermal, optical, and electrical properties. The magnetization is dependent on both the dopant concentration and temperature, and an abnormal crossover in the hysteresis loop of monolayer V-WSe2 is described, which is attributed to a strong 2D thermally induced spin flip phenomenon. The ferromagnetism is induced by indirect exchange interactions between V moments, which are mediated by hole carriers. It is then revealed that optical excitations creating excess hole carriers can be used to control the magnetization. Finally, we attempt on applying the induced magnetization to break the degeneracy in the valley degrees of freedom through a circularly polarized photoluminescence (PL) experiment, and look into the effect of magnetization on the optical emissions. The 2D magnetic systems based on V-doped sTMD monolayers hold great promise for novel magneto-electronic or magneto-optical devices, as well as offering a platform for fundamental physical studies. Magnetic domain structure is an inherent phenomenon in finite-size ferromagnetic systems. In Chapter 4, we will discuss the direct visulization of magnetic domains in Vdoped sTMD monolayers. Magnetic force microscopy (MFM) is first used in the tentative observation of domain structures experimentally, and a mathematical model is proposed to estimate the signal level. Lorentz transmission electron microscopy (Lorentz-TEM) is then used, and the observation of an field-dependent reversible domain contrast is reported in monolayer V-WS2. The contrast signal is revealed to be highly affected by the sample magnetization orientation and charge doping, which are mainly determined by the dopant concentration, beam-sample interactions and the type of substrates. Atomic-resolution electron microscopy is finally performed to detect potential connections between lattice atomic motions and macroscopic magnetization. The observed contrast signals on V-doped sTMD provide primary knowledge in domain structures of 2D diluted magnetic systems, and is the first reported imaging of a monolayer magnetic material through Lorentz-TEM. Finally, Chapter 5 summarizes the studies discussed in this dissertation, and provides outlooks to potential continuing future works.