Exploration of Structures of Two-Dimensional Boron–Silicon Compounds with sp<sup>2</sup> Silicon

The most stable structures of two-dimensional (2D) boron–silicon (B–Si) compounds containing planar sp<sup>2</sup>-bonding silicon (sp<sup>2</sup>-Si) are explored using the first-principles calculation-based particle-swarm optimization method. Among 10 B–Si compounds considered, we find that for BSi<sub>4</sub>, BSi<sub>3</sub>, BSi, B<sub>2</sub>Si, B<sub>3</sub>Si, B<sub>5</sub>Si, and B<sub>6</sub>Si, each Si atom is bonded with three B or Si atoms within the same plane, representing a preference of planar sp<sup>2</sup>-Si structure in B–Si compounds. For BSi<sub>2</sub> and B<sub>4</sub>Si, the predicted lowest-energy structures entail a small out-of-plane buckling. Furthermore, a planar-tetracoordinated Si (ptSi) atom bonded with four B atoms within the same plane is observed in the lowest-energy structure of B<sub>7</sub>Si compound. Dynamical stabilities of the predicted 10 2D B–Si compounds are confirmed via phonon-spectrum calculation. The lowest-energy 2D B–Si compounds are all metals, regardless of the B–Si stoichiometry considered in this study.