Mechanical anisotropy and strain-tailored band structures of pentagonal boron nitride monolayers

Abstract

Employing density functional theory calculations, mechanical and electronic properties of stable penta-B2N4 and penta-B3N3 monolayers are investigated. The different mechanical parameters obtained along different tensile directions suggest both the penta-B2N4 and penta-B3N3 demonstrate mechanical anisotropy. Moreover, due to the lower space group symmetry of penta-B3N3, its anisotropy is more prominent than that of the penta-B2N4. It was found that both the penta-B2N4 and penta-B3N3 are fast to fracture along the direction R1 due to the small fracture strain, but hard to be stretched because of the large Young's modulus. Generally, penta-B2N4 shows better mechanical properties than those of penta-B3N3 in terms of Young's modulus and intrinsic strength. Besides, under the tensile strain, penta-B2N4 keeps its metallicity, but the band gap of penta-B3N3 can be effectively tailored, even inducing a transition from the direct to indirect band gap or transition from the semiconductor to metal. Further analysis of partial charge densities indicates breaking of B–N bonds is the main cause for the band gap enlargement, and similarly, formation of B–N bonds is the reason for the semiconductor-to-metal transition of penta-B3N3.