Bi2OS2: a direct-gap two-dimensional semiconductor with high carrier mobility and surface electron states

Two-dimensional (2D) semiconductors with desirable band gaps and high carriermobility are highly sought after for future application in nanoelectronics. Herein, by means of first-principles calculations, we predict that a new 2D material, namely a Bi2OS2 nanosheet, possesses not only a tunable direct band gap, but also ultra-high electron mobility (up to 26570 cm(2) V-1 s(-1)). More interestingly, an anomalous layer-dependent band gap is revealed, derived from the synergetic effect of the quantum confinement and intrinsic surface electron states. 2D Bi2OS2 also exhibits excellent absorption over the entire solar spectrum and the absorption coefficient is comparable to that of inorganic-organic hybrid perovskite solar cells. Moreover, the Bi2OS2 monolayer maintains good structural integrity up to 1000 K and has a relatively small exfoliation energy from its layered bulk. The excellent electronic and optical properties, together with high stability and great experimental possibility, render 2D Bi2OS2 a promising material for future nanoelectronic and optoelectronic applications.