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.