# Negative Poisson's Ratio in 1T-Type Crystalline Two-Dimensional Transition Metal Dichalcogenides

## TL;DR

This paper reports the discovery of a new class of 2D materials with intrinsic negative Poisson's ratio, driven by electronic effects, which could enable novel multifunctional applications.

## Contribution

First principles calculations reveal a new class of 1T-type monolayer transition-metal dichalcogenides with intrinsic auxetic behavior due to electronic coupling.

## Key findings

- Exhibit negative in-plane Poisson's ratio
- Distinct crystal structure from known auxetics
- Auxetic behavior driven by electronic effects

## Abstract

Materials with a negative Poisson's ratio, also known as auxetic materials, exhibit unusual and counterintuitive mechanical behavior - becoming fatter in cross-section when stretched. Such behavior is mostly attributed to some special reentrant or hinged geometric structures regardless the chemical composition and electronic structure of a material. Here, using first principles calculations, we report a new class of auxetic single-layer two-dimensional (2D) materials, i.e., the 1T-type monolayer crystals of groups 6-7 transition-metal dichalcogenides, MX$_2$ (M = Mo, W, Tc, Re; X = S, Se, Te). These materials have a crystal structure distinct from all other known auxetic materials. They exhibit an intrinsic in-plane negative Poisson's ratio, which is dominated by the electronic effects. We attribute the occurrence of such auxetic behavior to the strong coupling between the chalcogen p orbitals and the intermetal t$_{2g}$-bonding orbitals within the basic triangular pyramid structure unit. The unusual auxetic behavior in combination with other remarkable properties of monolayer 2D materials could lead to novel multi-functionalities.

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Source: https://tomesphere.com/paper/1701.06529