Date of Graduation

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Salvador Barraza-Lopez

Committee Member

Pradeep Kumar

Second Committee Member

Hugh Churchill

Third Committee Member

Paul Millett

Fourth Committee Member

Rick Wise

Keywords

Electronic Properties, Monochalcogenide, Phase Transition, Piezoelectricity, Two-dimensional Materials

Abstract

Since discovery of graphene in 2004 as a truly one-atom-thick material with extraordinary mechanical and electronic properties, researchers successfully predicted and synthesized many other two-dimensional materials such as transition metal dichalcogenides (TMDCs) and monochalcogenide monolayers (MMs). Graphene has a non-degenerate structural ground state that is key to its stability at room temperature. However, group IV monochalcogenides such as monolayers of SnSe, and GeSe have a fourfold degenerate ground state. This degeneracy in ground state can lead to structural instability, disorder, and phase transition in finite temperature. The energy that is required to overcome from one degenerate ground state to another one is called energy barrier (E¬c). Density Functional Theory (DFT) has been used to calculate energy barriers of many materials in this class such as monolayers of SiSe, GeS, GeSe, GeTe, SnSe SnS, SnTe, PbS, and PbSe along with phosphorene. Degeneracy in the ground state of these materials leads to disorder at finite temperature. This disorder arises in the form of bond reassignment as a result of thermal excitement above a critical temperature (Tc). Tc is proportional to E¬c/KB where KB is Boltzmann’s constant. Any of those materials that have a melting temperature larger than E¬c/KB such as SnSe, SnS, GeSe, and GeS will undergo an order-disorder phase transition before melting point. This order-disorder phase transition will have a significant effect on properties of these 2D materials.

The optical and electronic properties of GeSe and SnSe monolayers and bilayers have been investigated using Car-Parrinello molecular dynamics. These materials undergo phase transition from an average rectangular unit cell below Tc to an average square unit cell above Tc where Tc is well below the melting point. These materials will remain semiconductors below and above Tc. However, the electronic, optical, and piezoelectric properties modify from earlier predicted values. In addition, the X and Y points of the Brillouin zone become equivalent as the materials passes Tc leading to a symmetric electronic structure. The spin polarization at the conduction valley vanishes. The linear optical absorption band edge changes its polarization and makes this structural and electronic transition identifiable by optical means.

Available for download on Wednesday, May 22, 2019

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