PROPERTIES OF SYNTHETIC TWO-DIMENSIONAL MATERIALS AND HETEROSTRUCTURES

Open Access
Author:
Lin, Yu Chuan
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 30, 2017
Committee Members:
  • Joshua Alexander Robinson, Dissertation Advisor
  • Joshua Alexander Robinson, Committee Chair
  • Joan Marie Redwing, Committee Member
  • Md Amanul Haque, Committee Member
  • Mauricio Terrones Maldonado, Outside Member
Keywords:
  • Materials Science and Engineering
  • Thin Film Techniques
  • Material Process
  • Applied Physics
  • Electronic Materials
  • Two-dimensional Materials
  • Graphene
  • Transition Metal Dichalcogendies
  • Chemical Vapor Deposition
  • Surface Characterizations
  • Field-Effect Transistors
  • Diodes
  • Electrical Phenomena
  • Electronic Devices
  • Nanomaterials
  • Growth Mechanism
  • Proximity Effect
  • Atomically Thin Membranes
  • van der Waals Solids
Abstract:
Graphene and other two-dimensional semiconductors have established their own research field, “2D Materials” that covers all of subjects related to fundamental and applied science, engineering, biology, medicine, and etc., where their layered structures and anisotropic properties can help to create interesting results. In particular, they play an important role as electronic materials because that their ultra-thin nature and excellent electrical properties make themselves prominent candidates for the next-generation electronics in the digital industries. However, many researchers using 2D materials are suffering one common issue: limited size of the samples prepared by exfoliating bulk layered crystals. Besides limited size, tape-assisted exfoliation usually leaves the contamination on the surface of 2D materials. Due to the disadvantages of exfoliated samples, the research community has moved on to synthetic 2D materials prepared by thin film deposition techniques, such as chemical vapor deposition (CVD). For material engineers working on synthetic 2D materials, several challenges posed in front of them include: how to make them scalable and large area, how to grow these layers with high-quality and low-defect density, and how to integrate them with other substrates that have dissimilar features in terms of crystal structure and surface chemistry. This thesis is made to address the mentioned challenges thoroughly, or answers the questions associated with them. This thesis covers two main discussions: 1) Vertical integration of 2D layers for van der Waals heterostructures and 2) Investigation on the properties of synthetic semiconducting 2D layers fabricated on insulating substrates. Each research chapter generally has three components in its experiment: 1) material synthesis by thin film deposition techniques, including CVD; 2) property investigation of the synthetic materials using spectroscopic, surface-probing, and electron microscope techniques; and 3) demonstration of optoelectronic devices made of 2D layers. The first two chapters, Chapters 1 and 2, cover fundamental knowledge and short review on 2D semiconductors, heterostructures, and thin film, synthesis techniques; Chapter 3 has two parts covering the properties of synthetic WSe2: The first part is for the first demonstration of the metallic-organic chemical vapor deposition (MOCVD) process for WSe2; the second part covers an improved MOCVD process for WSe2 on insulating substrate and studies the interface of transition metal dichalcogenides (TMDCs)/substrate and their devices. Chapter 4 discusses the direct synthesis of a variety of 2D layers on graphene, and how surface morphology and quality of graphene would impact their nucleation and growth; Chapters 5 and 6 discuss epitaxy relationship between monolayer WSe2 and graphene, the electrical transport through their junction vertically, and modulation of the Fermi level of graphene to improve the performance; In Chapter 7, the resonant tunnel diodes made of TMDC bilayer (MoS2/WSe2 and WSe2/MoSe2) will be thoroughly discussed, including material preparation, property investigation, and device transport; In Chapter 8, 2D TMDCs integrated on VO2, a phase transition material, and the phenomena arising from their interaction are discussed. Finally, Chapter 9 provides a summary for my dissertation.