Stacking-dependent energetics and electronic structure of ultrathin polymorphic V$_2$VI$_3$ topological insulator nanofilms

TL;DR

This study uses first principles calculations to explore how stacking sequences influence the energetics and electronic properties of ultrathin topological insulator nanofilms, revealing potential for property control via stacking modifications.

Contribution

It provides the first detailed analysis of stacking-dependent effects on the energetics and electronic structure of ultrathin V$_2$VI$_3$ topological insulator nanofilms.

Findings

Band gap is highly sensitive to stacking sequence.

Low energy barrier for layer slippage suggests easy stacking modification.

Stacking affects the critical thickness for topological phase transition.

Abstract

Topological insulators represent a paradigm shift in surface physics. The most extensively studied Bi$_2$Se$_3$-type topological insulators exhibit layered structures, wherein neighboring layers are weakly bonded by van der Waals interactions. Using first principles density-functional theory calculations, we investigate the impact of the stacking sequence on the energetics and band structure properties of three polymorphs of Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$. Considering their ultrathin films up to 6 nm as a function of its layer thickness, the overall dispersion of the band structure is found to be insensitive to the stacking sequence, while the band gap is highly sensitive, which may also affect the critical thickness for the onset of the topologically nontrivial phase. Our calculations are consistent with both experimental and theoretical results, where available. We further investigate tribological layer slippage, where we find a relatively low energy barrier between two of the considered structures. Both the stacking-dependent band gap and low slippage energy barriers, suggest that polymorphic stacking modification may offer an alternative route for controlling the properties of this new state of matter.