Two-dimensional Boron Monosulfides: Semiconducting and Metallic Polymorphs.

The typical two-dimensional semiconductors, group \uppercase\expandafter{\romannumeral3A} chalcogenides, have garnered tremendous interest for their outstanding electronic, mechanical, and chemical properties. However, so far, there have been almost no reports on boron monosulfides (BS) binary material. Here, four two-dimensional BS sheets, namely the $\alpha$-, $\beta$-, $\gamma$-, and $\delta$-BS sheets, are proposed and discussed from $\emph{ab initio}$ calculations. State-of-the-art first-principles calculations reveal all these structures are thermally and dynamically stable, indicating the potential for successful experimental synthesis. Especially, for $\alpha$-BS, it has a calculated exfoliation energy of 0.96 J m$^{-2}$, suggesting the preparation of $\alpha$-BS is feasible by the exfoliation of bulk rhombohedral-BS. Our results show that $\alpha$-, $\beta$-, $\gamma$-BS are semiconductors, whereas $\delta$-BS is a metallic system. Remarkably, our calculations indicate that $\delta$-BS is a superconductor with a large electron-phonon coupling ($\lambda$ = 1.51) leading a high superconducting critical temperature ($T_c$ $\approx $ 21.56 K), which is the first report of intrinsic superconducting property among all two-dimensional group \uppercase\expandafter{\romannumeral3A} chalcogenides. The desired mechanical and electronic properties render the BS sheets as the promising two-dimensional materials for future applications in nanoelectronics.