Structural, Vibrational and Electronic Properties of Single Layer Hexagonal Crystals of Groups IV and V

TL;DR

This study uses first-principles calculations to explore the structural, vibrational, and electronic properties of novel 2D hexagonal crystals from groups IV and V, revealing stable phases, electronic behaviors, and potential applications.

Contribution

It introduces a comprehensive analysis of $A_2B_2$ 2D crystals with new insights into their stability, electronic structure, and potential uses, including the first report of quartic valence band edges in some materials.

Findings

Both $ ext{P}ar{6} ext{m}2$ and $ ext{P}ar{3} ext{m}1$ phases are dynamically stable.

Most structures are semiconductors, with some metallic; several have direct band gaps.

SnAs and PbAs exhibit purely quartic valence band edges, a novel property.

Abstract

Using first-principles density functional theory calculations, we investigate a family of stable two-dimensional crystals with chemical formula $A_2B_2$, where $A$ and $B$ belong to groups IV and V, respectively ($A$ = C, Si, Ge, Sn, Pb; $B$ = N, P, As, Sb, Bi). Two structural symmetries of hexagonal lattices $P\bar{6}m2$ and $P\bar{3}m1$ are shown to be dynamically stable, named as $\alpha$- and $\beta$-phases correspondingly. Both phases have similar cohesive energies, and the $\alpha$-phase is found to be energetically favorable for structures except CP, CAs, CSb and CBi, for which the $\beta$-phase is favored. The effects of spin-orbit coupling and Hartree-Fock corrections to exchange-correlation are included to elucidate the electronic structures. All structures are semiconductors except CBi and PbN, which have metallic character. SiBi, GeBi and SnBi have direct band gaps, whereas the remaining semiconductor structures have indirect band gaps. All structures have quartic dispersion in their valence bands, some of which make the valence band maximum and resemble a Mexican hat shape. SnAs and PbAs have purely quartic valence band edges, i.e. $E{\sim}{-}\alpha k^4$, a property reported for the first time. The predicted materials are candidates for a variety of applications. Owing to their wide band gaps, CP, SiN, SiP, SiAs, GeN, GeP can find their applications in optoelectronics. The relative band positions qualify a number of the structures as suitable for water splitting, where CN and SiAs are favorable at all pH values. Structures with quartic band edges are expected to be efficient for thermoelectric applications.