A B 2 N 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 B 2 N, B 3 N, and B 4 N using a combination of crystal structure searches and first-principles calculations. Among them, B 2 N has an ultraflat surface and consists of eight-membered B 6 N 2 and pentagonal B 3 N 2 rings. The eight-membered B 6 N 2 rings are linked to each other through both edge-sharing (in the y direction) and bridging B 3 N 2 pentagons (in the x direction). B 2 N 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 B 2 N. The hole mobility of B 2 N is as high as 0.6 × 10 3 cm 2 V -1 s -1 along the y direction, 7.5 times that in the x direction. These combined novel properties render the B 2 N monolayer as a natural example in the field of two-dimensional functional materials with broad application potential for use in field-effect transistors.