Monolayer diboron dinitride: Direct band-gap semiconductor with high absorption in the visible range

We predict a two-dimensional monolayer polymorph of boron nitride in an orthorhombic structure (o-${\mathrm{B}}_{2}{\mathrm{N}}_{2})$ using first-principles calculations. Structural optimization, phonon dispersion, and molecular dynamics calculations show that o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ is thermally and dynamically stable. o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ is a semiconductor with a direct band gap of 1.70 eV according to calculations based on hybrid functionals. The structure has high optical absorption in the visible range in the armchair direction while low absorption in the zigzag direction. This anisotropy is also present in electronic and mechanical properties. The in-plane stiffness of o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ is very close to that of hexagonal boron nitride. The diatomic building blocks of this structure hint at its possible synthesis from precursors having B-B and N-N bonds.