Phonon Properties, Thermal Expansion, and Thermomechanics of Silicene and Germanene

TL;DR

This study uses first-principles calculations to explore how lattice dimensionality and bond properties influence phonon behavior, thermal expansion, and thermomechanics in silicene and germanene, revealing the roles of bond bending and stretching.

Contribution

It provides a detailed analysis of lattice dynamics in silicene and germanene, highlighting the effects of chemical functionalization on phonon properties and thermal behavior.

Findings

Bond bending negatively affects the Grüneisen constant.

Chemical functionalization alters bond strength and phonon modes.

Negative thermal expansion arises from specific phonon mode excitation.

Abstract

We report a hierarchical first-principles investigation on the entangled effects of lattice dimensionality and bond characteristics in the lattice dynamics of silicene and germanene. It is found that bond bending (stretching) negatively (positively) contributes to Gr\"uneisen constant, which results in the negative acoustic (positive optical) Gr\"uneisen constant. The layer thickening (bond weakening) caused by chemical functionalization tends to increase (decrease) the acoustic (optical) Gr\"uneisen constant, due to the increased (decreased) bond-stretching effect. The excitation of the negative-$\gamma$ modes results in negative thermal expansion, while mode excitation and thermal expansion compete with each other in thermomechanics. The sensitive structural and electronic responses of silicene and germanene to functionalization help us to derive a generic physical picture for two-dimensional lattice dynamics.