  Recent synthesis of monolayer borophene (triangle boron monolayer) on the
substrate opens the era of boron nanosheet (Science, 350, 1513,
$\mathbf{2015}$), but the structural stability and novel physical properties
are still open issues. Here we demonstrated borophene can be stabilized with
fully surface hydrogenation, called as borophane, from first-principles
calculations. Most interesting, it shows that borophane has direction-dependent
Dirac cones, which are mainly contributed by in-plane \emph{p$_{x}$} and
\emph{p$_{y}$} orbitals of boron atoms. The Dirac fermions possess an ultrahigh
Fermi velocity up to 3.0$\times$10$^{6}$ m/s, 4 times higher than that of
graphene. The Young's modules are calculated to be 129 and 200 GPa$\cdot$nm
along two different directions, which is comparable with steel. The ultrahigh
Fermi velocity and high mechanical feature render borophane ideal for
nanoelectronics applications.
