Structural phase transition and material properties of few-layer monochalcogenides

TL;DR

This study investigates how a temperature-induced structural phase transition in few-layer GeSe and SnSe monolayers and bilayers affects their electronic, optical, and piezoelectric properties, revealing significant property changes at the transition point.

Contribution

It provides a detailed theoretical analysis of the temperature-driven phase transition and its impact on material properties in few-layer monochalcogenides, which was previously unexplored.

Findings

Structural transition from rectangular to square lattice at T_c

Electronic structure becomes symmetric at T_c

Optical and piezoelectric properties change markedly at T_c

Abstract

GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a temperature $T_c$ well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above $T_c$. As the in-plane lattice transforms from a rectangle onto a square at $T_c$, the electronic, spin, optical, and piezo-electric properties dramatically depart from earlier predictions. Indeed, the $Y-$ and $X-$points in the Brillouin zone become effectively equivalent at $T_c$, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at $T_c$. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement has been drawn by theoretical predictions of giant piezo-electricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about $3\times 10^{-12}$ $C/K m$ here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides