Two-dimensional square ternary Cu2MX4(M= Mo, W;X= S, Se) monolayers and nanoribbons predicted from density functional theory

Two-dimensional (2D) materials often adopt a hexagonal lattice. We report on a class of 2D materials, Cu${}_{2}$$M{X}_{4}$ ($M$ = Mo, W; $X$ = S, Se), that has a square lattice. Up to three monolayers, the systems are kinetically stable. All of them are semiconductors with band gaps from 2.03 to 2.48 eV. Specifically, the states giving rise to the valence band maximum are confined to the Cu and $X$ atoms, while those giving rise to the conduction band minimum are confined to the $M$ atoms, suggesting that spontaneous charge separation occurs. The semiconductive nature makes the materials promising for transistors, optoelectronics, and solar energy conversion. Moreover, the ferromagnetism on the edges of square Cu${}_{2}$$M{X}_{4}$ nanoribbons opens applications in spintronics.