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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.

Two-dimensional materials with a planar lattice, suitable direct band gap, and high and highly anisotropic carrier mobility are desirable for the development of advanced field-effect transistors. Here we predict three thermodynamically stable B-rich 2D B-N compounds with the stoichiometries of B 2 N, B 3 N, and B 4 N using a combination of crystal structure searches and first-principles calculations. Among them, B 2 N has an ultraflat surface and consists of eight-membered B 6 N 2 and pentagonal B 3 N 2 rings. The eight-membered B 6 N 2 rings are linked to each other through both edge-sharing (in the y direction) and bridging B 3 N 2 pentagons (in the x direction). B 2 N is a semiconductor with a direct band gap of 1.96 eV, and the nature of the direct band gap is well preserved in bilayer B 2 N. The hole mobility of B 2 N is as high as 0.6 × 10 3 cm 2 V -1 s -1 along the y direction, 7.5 times that in the x direction. These combined novel properties render the B 2 N monolayer as a natural example in the field of two-dimensional functional materials with broad application potential for use in field-effect transistors.