A BN monolayer: a direct band gap semiconductor with high and highly anisotropic carrier mobility.

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 BN, BN, and BN using a combination of crystal structure searches and first-principles calculations. Among them, BN has an ultraflat surface and consists of eight-membered BN and pentagonal BN rings. The eight-membered BN rings are linked to each other through both edge-sharing (in the direction) and bridging BN pentagons (in the direction). BN 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 BN. The hole mobility of BN is as high as 0.6 × 10 cm V s along the direction, 7.5 times that in the direction. These combined novel properties render the BN monolayer as a natural example in the field of two-dimensional functional materials with broad application potential for use in field-effect transistors.