Auxetic Black Phosphorus: A 2D Material with Negative Poisson's Ratio

TL;DR

This paper demonstrates that black phosphorus exhibits a negative Poisson's ratio, a rare property where the material expands laterally when stretched, confirmed through experiments and first principles simulations.

Contribution

It is the first to experimentally observe negative Poisson's ratio in a natural 2D material, black phosphorus, and explains its origin from its puckered structure.

Findings

Black phosphorus exhibits negative Poisson's ratio under uniaxial strain.

Both interlayer and intralayer negative Poisson's ratios are observed.

The negative Poisson's ratio arises from its puckered lattice structure.

Abstract

The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus, and that our experimental observations are consistent with first principles simulations. Through application of uniaxial strain along zigzag and armchair directions, we find that both interlayer and intralayer negative Poisson's ratios can be obtained in black phosphorus. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations.