Ultrathin Semiconducting Bi 2 Te 2 S and Bi 2 Te 2 Se with High Electron Mobilities.

High carrier mobility and moderate band gap are two key properties of electronic device applications. Two ultrathin two-dimensional (2D) semiconductors, namely, Bi2 Te2 S and Bi2 Te2 Se nanosheets, with novel electronic and optical properties are predicted based on first-principles calculations. The Bi2 Te2 S and Bi2 Te2 Se monolayers own moderate band gaps (∼0.7 eV) and high electron mobilities (∼20 000 cm2 V-1 s-1 ), and they can absorb sunlight efficiently through the whole incident solar spectrum. Meanwhile, layer-dependent exponential decay band gaps are also unveiled. The relatively low interlayer binding energies suggest that these monolayers can be easily exfoliated from bulk structures. Their high dynamical and thermal stabilities are further verified by phonon dispersion calculations and ab initio molecular dynamics simulations. The exceptional properties render Bi2 Te2 X (X = S, Se) monolayers promising candidates in future high-speed (opto)electronic devices.