  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.
