Two-dimensional Si 2 S with a negative Poisson's ratio and promising optoelectronic properties.

Two-dimensional materials with a negative Poisson's ratio, known as auxetic materials, are of great interest owing to their improved mechanical properties, which enable plenty of advanced nanomechanical devices. Here, by first-principles swarm-intelligence structural search methods, we predict a thermodynamically stable Si 2 S monolayer, which has a puckered 2D lattice in which the S atoms are adsorbed on the top of a distorted tetragonal silicene layer. The puckered 2D lattice makes the Si 2 S monolayer exhibit in-plane negative Poisson's ratios of -0.05 and -0.069 along the x and y directions, respectively. Moreover, electronic structure calculations reveal that the Si 2 S monolayer is a semiconductor with a quasi-direct band gap of 1.81 eV, which can be converted into a direct gap semiconductor of 1.43 eV by applying a low tensile strain (∼2%). The Si 2 S monolayer has a large visible light absorption coefficient of 10 5 cm -1 . The hole (electron) mobility is 200 (81) cm 2 V -1 s -1 along the y direction, 3.4 (1.5) times that along the x direction, comparable to MoS 2 . Moreover, the Si 2 S monolayer has the good ability of oxidation resistance. We provide a possible route to experimentally grow a Si 2 S monolayer on a suitable substrate such as the Cu(100) surface. The versatile properties render the Si 2 S monolayer potential for advanced application in nanodevices.