Date of Graduation

5-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Barraza-Lopez, Salvador

Committee Member

Bellaiche, Laurent

Second Committee Member

Churchill, Hugh O.H.

Third Committee Member

Kumar, Pradeep

Keywords

Graphene; Hexagonal boron nitride; Transition metal dichalcogenide monolayers; Two-dimensional materials

Abstract

The goal of this dissertation is to study the structure of ferroelectric group-IV monochalcogenide monolayers (MMLs) and their material properties. This work also established a new buckled honeycomb phase of these monolayers. Chapter 1 serves as an introduction where the motivation for these studies is established. In chapter 2, I used eight different exchange-correlation functionals (XC) to study the structure of these 2D ferroelectrics and found that their properties are independent of the choice of XC functionals. These findings reveal that the ground-state ferroelectric unit cell can be described by only five independent parameters, whereas their paraelectric (square or rectangular) unit cells can be described by three or two independent variables. It is found that reducing the number of independent variables increases their energy barriers Jc (the energy difference among the ground state structure and the paraelectric ones).

In chapter 3, 2D structural phase transformation of two-dimensional monochalcogenides have been discussed through ab initio molecular dynamics and a 2D discrete clock model on the basis of the SnSe monolayer. Group-IV monochalcogenide monolayers undergo a two- dimensional phase transition from a ground state rectangular unit cell (threefold-coordinated structure) to a square unit cell (fivefold-coordinated structure) at a critical temperature Tc that is well below the melting point. Our DFT calculation shows a transition temperature Tc of about 1.42 Jc, where Jc is given in units of Kelvin per unit cell.

In chapter 4, we established a new phase of MMLs with a buckled honeycomb structure. Its structural stability is discussed on the basis of three independent methods. Also, the out- of-plane intrinsic electric polarization and piezoelectric response of these buckled honeycomb structures are studied for the first time in this work.

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