Hidden layered structures from carbon-analog metastability in metal dichalcogenides

TL;DR

This paper reveals that certain non-layered metal dichalcogenides can transform into layered structures, uncovering hidden 2D materials and linking high-pressure science with 2D materials research.

Contribution

It demonstrates that layered ground states can exist in non-layered metal dichalcogenides, showing spontaneous transformation and hidden 2D phases in these materials.

Findings

Ultrathin pyrite-type ZnSe₂ transforms into a layered phase.

Layered phase has a larger bandgap and strong elastic anisotropy.

A two-valued potential energy surface explains the hidden layered ground state.

Abstract

Carbon exhibits both a layered ground state structure that produces two-dimensional (2D) nanosheets and a non-layered diamond structure created under high pressure conditions. Motivated by this metastability relationship, we revisit the ground state structure of metal dichalcogenides that are known to have non-layered pyrite-type structure. Ultrathin films of pyrite-type ZnSe$_2$ spontaneously transform into a layered phase. This phase is identified as a ground state, and the monolayer exhibits strong elastic anisotropy and a semiconducting bandgap larger than that of the pyrite phase by a factor of two. We demonstrate that a two-valued but directional potential energy surface exists along a Bain-like distortion path, hiding the layered ground state. This work implies that many 2D materials are hidden in non-layered materials and connects 2D materials science with surface and high-pressure science.