High-Throughput Discovery and Investigation of Auxetic Two-Dimensional Crystals

Auxetic two-dimensional (2D) crystals
that yield a negative Poisson’s
ratio (NPR) exhibit unusual transverse expansion when stretched. Such
materials have great potential for electronic skins, energy harvesting,
mechatronic shape display devices, and strain sensors. However, most
2D crystals have positive Poisson’s ratios, and the number
of existing 2D materials with NPR is limited. Accelerating the discovery
of these rare auxetic materials remains a long-term challenge. Utilizing
high-throughput computations, this work discovers 108 stable 2D crystals
with negative in-plane Poisson’s ratios that could be successfully
fabricated from the 4047 2D crystals in the Computational 2D Materials
Database (C2DB). We find 4 crystal types with the potential for significant
auxetic effect regardless of the orientations, that is, MX2O8, MX2O6, MX3, and MXX′.
The auxetic effects for the former two are simply from crystal geometry,
while further first-principles calculations based on MX3 (M = Sc, Ti, V, Cr, ... and X = Cl, Br, I) and MXX′ (M =
Mn, Fe, Co, Ni; X = Li, Na, Mg, Ca, Sr; and X′ = Si, Ge, P,
As, Sb) reveal that the NPR originates from both the crystal geometry
and electronic structures. Because some databases lack the information
of elasticity and stability, deep learning models are trained using
C2DB and employed to find 92 other stable 2D crystals with NPR. The
band gaps of the 200 stable auxetic 2D crystals discovered in our
work cover a wide range from 0 to 5.7 eV, which provide abundant candidates
for nanoscale electronics with intriguing physicochemical properties.