Single-Layer Ag2S: A Two-Dimensional Bidirectional Auxetic Semiconductor.

Two-dimensional auxetic materials have attracted considerable attention due to their potential applications in medicine, tougher composites, defense, and so on. However, they are scare especially at low dimension, as auxetic materials are mainly realized in engineered materials and structures. Here, using first-principles calculations, we identify a compelling two-dimensional auxetic material, single-layer Ag2S, which possesses large negative Poisson's ratios in both in-plane and out-of-plane directions, but anisotropic ultralow Young's modulus. Such a coexistence of simultaneous negative Poisson's ratios in two directions is extremely rare, which is mainly originated from its particular zigzag-shaped buckling structure. In addition, contrary to the previously known metal-shrouded single-layer M2X (M = metal, X = nonmetal), single-layer Ag2S is the first nonmetal-shrouded M2X. Electronic calculations show that it is an indirect-gap semiconductor with gap value of 2.83 eV, and it can be turned to be direct with strain. These intriguing properties make single-layer Ag2S a promising auxetic material in electronics and mechanics.