Metallic behavior in low-dimensional honeycomb SiB crystals: A first-principles prediction of atomic structure and electronic properties

We present a detailed analysis of the atomic and electronic structure of a two-dimensional monolayer of boron and silicon elements within periodic density functional theory. The proposed h-SiB sheet is a structural analog of hexagonal boron nitride (h-BN) and exhibits a good structural stability, compared to the structure of silicene. The calculated cohesive energy of an infinite sheet of h-SiB is of 4.71 eV/atom, whereas the corresponding value for silicene is 4.09 eV/atom. However, h-SiB sheets are not able to be stacked into a three-dimensional graphitelike structure, leading to a new hexagonal phase. On the other hand, h-SiB is predicted to roll up into single-walled silicon boron nanotubes (SWSiBNTs) of which we examine the electronic properties of some zigzag and armchair tubes. The strain energy of the SWSiBNTs are four to five times lower than the strain energy of the corresponding carbon nanotubes. In contrast to more polar honeycomb monolayers, the h-SiB sheet is not semiconducting or semimetallic. It has a delocalized charge density like graphene, but the pi band and the two highest occupied sigma bands are only partly filled. This results in a high density of states around the Fermi level and a metallic behavior of the h-SiB sheet. Interestingly, all the low-dimensional h-SiB-based structures, including the smallest to the largest stable tubes studied here, are predicted to form metallic systems.