A new 2D Janus family with multiple properties: auxetic behavior, strain-tunable photocatalyst, high Curie temperature ferromagnets, and piezoelectric quantum anomalous Hall insulator

Discovering new two-dimensional (2D) materials, exploring their unique properties and potentials in various applications are of paramount importance to condensed matter physics and materials science. Here based on the diverse properties of the novel square lattice S-XS2 materials found in experiments and computations, we identified 7 novel 2D Janus S-XSSe (X = Si, Sn, V, Cr, Mo, Re, Os) monolayers by means of density functional theory computations. Remarkably, both S-SiSSe and S-SnSSe monolayers possess the auxetic behavior. In addition, they can act as potential photocatalysts, and their photocatalytic performance can be enhanced by changing the pH and applying biaxial strains. Without spin-orbit coupling (SOC), the S-VSSe, S-CrSSe, and S-MoSSe are ferromagnetic half-metals, and have high Curie temperatures T-C (210, 810, and 390 K, respectively). When SOC is included, the S-VSSe becomes a quantum anomalous Hall insulator with a sizable gap (45.4 meV) and one chiral edge state (Chern number C = -1). By symmetry analysis of semiconducting S-XSSe (X = Si, Sn, V) monolayers, only out-of-plane piezoelectric response can be induced by a uniaxial strain in the basal plane, and among them, S-VSSe has both the largest out-of-plane piezoelectric coefficients d(31) and d(32), with values of -0.013 and 0.025 pm V-1, respectively. The concurrence of ferromagnetism, topology, and piezoelectricity empowers the S-VSSe monolayer as a potential platform for multi-functional spintronics applications with a large gap and high T-C. This theoretical work brings new members, also manifoldness in the properties and functions to the renown 2D materials family.