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

The current work investigates a novel three-dimensional boron nitride called bulk B₄N₄ and its corresponding two-dimensional monolayer B₄N₄ based on the first-principles of density functional theory. The phonon spectra prove that bulk B₄N₄ and monolayer B₄N₄ are dynamically stable. The molecular dynamics simulations verify that bulk B₄N₄ and monolayer B₄N₄ have excellent thermal stability of withstanding temperature up to 1000 K. The calculated elastic constants state that bulk B₄N₄ and monolayer B₄N₄ are mechanically stable, and bulk B₄N₄ has strong anisotropy. The theoretically obtained electronic structures reveal that bulk B₄N₄ is an indirect band-gap semiconductor with a band gap of 5.4 eV, while monolayer B₄N₄ 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 B₄N₄ and monolayer B₄N₄. The newly proposed monolayer B₄N₄ is a potential candidate for designing optoelectronic devices such as transparent electrodes due to its high transmissivity.