Elastic, electronic and optical properties of new 2D and 3D boron nitrides

The current work investigates a novel three-dimensional boron nitride called bulk B 4 N 4 and its corresponding two-dimensional monolayer B 4 N 4 based on the first-principles of density functional theory. The phonon spectra prove that bulk B 4 N 4 and monolayer B 4 N 4 are dynamically stable. The molecular dynamics simulations verify that bulk B 4 N 4 and monolayer B 4 N 4 have excellent thermal stability of withstanding temperature up to 1000 K. The calculated elastic constants state that bulk B 4 N 4 and monolayer B 4 N 4 are mechanically stable, and bulk B 4 N 4 has strong anisotropy. The theoretically obtained electronic structures reveal that bulk B 4 N 4 is an indirect band-gap semiconductor with a band gap of 5.4 eV, while monolayer B 4 N 4 has a direct band gap of 6.1 eV. The valence band maximum is mainly contributed from B-2 p and N-2 p orbits, and the conduction band minimum mainly derives from B-2 p orbits. The electron transitions from occupied N-2 p states to empty B-2 p states play important roles in the dielectric functions of bulk B 4 N 4 and monolayer B 4 N 4 . The newly proposed monolayer B 4 N 4 is a potential candidate for designing optoelectronic devices such as transparent electrodes due to its high transmissivity.