Two-dimensional Si₂S 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 Si₂S 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 Si₂S 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 Si₂S 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 Si₂S monolayer has a large visible light absorption coefficient of 10⁵ cm^-1. The hole (electron) mobility is 200 (81) cm² V^-1 s^-1 along the y direction, 3.4 (1.5) times that along the x direction, comparable to MoS₂. Moreover, the Si₂S monolayer has the good ability of oxidation resistance. We provide a possible route to experimentally grow a Si₂S monolayer on a suitable substrate such as the Cu(100) surface. The versatile properties render the Si₂S monolayer potential for advanced application in nanodevices.