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-2p and N-2p orbits, and the conduction band minimum mainly derives from B-2p orbits. The electron transitions from occupied N-2p states to empty B-2p 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.