  Orthorhombic arsenene was recently predicted as an indirect bandgap
semiconductor. Here, we demonstrate that nanostructuring arsenene into
nanoribbons can successfully transform the bandgap to be direct. It is found
that direct bandgaps hold for narrow armchair but wide zigzag nanoribbons,
which is dominated by the competition between the in-plane and out-of-plane
bondings. Moreover, straining the nanoribbons also induces a direct bandgap and
simultaneously modulates effectively the transport property. The gap energy is
largely enhanced by applying tensile strains to the armchair structures. In the
zigzag ones, a tensile strain makes the effective mass of holes much higher
while a compressive strain cause it much lower than that of electrons. Our
results are crutial to understand and engineer the electronic properties of two
dimensional materials beyond the planar ones like graphene.
