High temperature stability, metallic character and bonding of the Si₂BN planar structure

The family of monolayered Si₂BN structures constitute a new class of 2D materials exhibiting metallic character with remarkable stability. Topologically, these structures are very similar to graphene, forming a slightly distorted honeycomb lattice generated by a union of two basic motifs with AA and AB stacking. In the present work we study in detail the structural and electronic properties of these structures in order to understand the factors which are responsible for their structural differences as well as those which are responsible for their metallic behavior and bonding. Their high temperature stability is demonstrated by the calculations of finite temperature phonon modes which show no negative contributions up to and beyond 1000 K. Presence of the negative thermal expansion coefficient, a common feature of one-atom thick 2D structures, is also seen. Comparison of the two motifs reveal the main structural differences to be the differences in their bond angles, which are affected by the third nearest neighbor interactions ofcis-transtype. On the other hand, the electronic properties of these two structures are very similar, including the charge transfers occurring between orbitals and between atoms. Their metallicity is mainly due to theporbitals of Si with a minor contribution from theporbitals of B, while the contribution from theporbitals of N atoms is negligible. There is almost no contributions from the Npelectrons to the energy states near the Fermi level, and they form a band well below it. I.e., thepelectrons of N are localized mostly at the N atoms and therefore cannot be considered as mobile electrons of thepcloud. Moreover, we show that due to the relative positions in the energy axis of the atomic energies of theporbitals of B, N and Si atoms, the density of states (DOS) of Si₂BN can be considered qualitatively as a combination of the DOS of planar hexagonal BN (h-BN) and hypothetically planar silicene (ph-Si). As a result, the Si₂BN behaves electronically at the Fermi level as slightly perturbed ph-Si, having very similar electronic properties as silicene, but with the advantage of having kinetic stability in planar form. As for the bonding, the Si-Si bonds are covalent, while theπback donation mechanism occurs for the B-N bonding, in accordance with the B-N bonding in h-BN.