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Functional Properties in Novel 2D and Layered Materials

Wang, Yaxian
http://orcid.org/0000-0003-4790-2880
http://rave.ohiolink.edu/etdc/view?acc_num=osu1574850743485852

Abstract Details

2019, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Novel two dimensional (2D) materials have been receiving increasing attention due to various exotic properties such as massless fermions, versatile gap engineering, topological edge states and superconductivity. With that, they are currently becoming one of the most import families of quantum materials and feature very high potential to be the basis for a manifold of disruptive technologies within the realm of functional materials. Density Functional Theory (DFT) has been playing an essential role in understanding their fundamental properties and discovering new candidate materials for a wide range of applications. Here, we used DFT to study the structural, electronic, optical and transport properties of a large variety of novel 2D and layered materials. After a high-level overview, we will first discuss structural defects in hydrogen and methyl terminated germanene. There, we will show why random twisting, which is a rotation between neighboring layers, is limited to small angles in germananes and results in turbostratic disorder, which is confirmed by electron diffraction. Twisting has recently been shown to have surprising results on band structures and transport properties, which is rationalized by our results for charge redistribution between Ge atoms and terminating hydrogen atoms in a turbostratically twisted structure. Band structures are also affected by point defects such as vacancies or residual intercalation atoms from the synthesis process, especially in the form of deep level defect states in the band gap, which are validated by cathodoluminescene and surface photovoltage spectroscopy, for which we discuss the role of DFT in analyzing experimental spectra. Such an approach can also be applied to the analysis of experimental Raman spectra of lattice vibrations, as we demonstrate for the example of CrI3, the first reported single layer 2D magnetic semiconductor. In the effort of seeking electric field tuning of band gaps in semiconductors, we propose and study atomically thin InSe two dimensional layers. InSe shows a direct to indirect gap transition going from bulk to confined two dimensional structure. The band gap and effective mass in its condution band minimum shows strong dependence to layer thickness and external electric field perpendicular to the plane. This offers promising application in 2D field effect transistors (FETs). Next, we discovered the concept of axis dependent conduction polarity, or `goniopolarity’, which our theoretical work has helped to establish following directionally dependent sign changes in the measured thermopower of NaSn2As2, an exfoliatable vdW Zintl phase. We introduce a theory that shows that open Fermi surfaces with concave/convex shapes along orthogonal crystallographic axis can produce single-band goniopolarity, which we then expand to a tight-binding model that better identifies the roles of the atomic orbitals for goniopolarity. Furthermore, by a systematic study of prototypical AMX compounds where two mechanisms, e.g. p×n and goniopolar, of axis-dependent polarity are achieved, we further build up reliable chemical design principles for the chemistry community. The concept of goniopolarity challenges the traditional view of charge polarity as an exclusive property in semiconductors as well as the traditional neglect of hole-like conduction in metals and provides an exciting new playing field for electronic device concepts.
Wolfgang Windl (Advisor)
Joseph Heremans (Committee Member)
Joshua Goldberger (Committee Member)
Materials Science
2D materials; First principles; axis-dependent conduction polarity

Recommended Citations

Citations

  • Wang, Y. (2019). Functional Properties in Novel 2D and Layered Materials [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1574850743485852

    APA Style (7th edition)

  • Wang, Yaxian. Functional Properties in Novel 2D and Layered Materials. 2019. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1574850743485852.

    MLA Style (8th edition)

  • Wang, Yaxian. "Functional Properties in Novel 2D and Layered Materials." Doctoral dissertation, Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1574850743485852

    Chicago Manual of Style (17th edition)

osu1574850743485852
188
© 2019, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.