A B₂N 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 B₂N, B₃N, and B₄N using a combination of crystal structure searches and first-principles calculations. Among them, B₂N has an ultraflat surface and consists of eight-membered B₆N₂ and pentagonal B₃N₂ rings. The eight-membered B₆N₂ rings are linked to each other through both edge-sharing (in the y direction) and bridging B₃N₂ pentagons (in the x direction). B₂N 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 B₂N. The hole mobility of B₂N is as high as 0.6 × 10³ cm² V^-1 s^-1 along the y direction, 7.5 times that in the x direction. These combined novel properties render the B₂N monolayer as a natural example in the field of two-dimensional functional materials with broad application potential for use in field-effect transistors.