  Very recently, field-effect transistors based on few-layer phosphorene
crystals with thickness down to a few nanometres have been successfully
fabricated, triggering interest in this new functional two-dimensional
material. In this work, we apply the first-principles calculations to studying
the evolution of electronic structures and lattice dynamics with vertical
strain for monolayer and bilayer phosphorenes. It is found that, by changing
the thickness of phosphorene or the strain applied on it, its band gap width
can be well tuned, and there will appear a transition from semiconductor to
metal. In particular, the bilayer phosphorene may become a good BCS
superconductor by adjusting the interlayer distance, in which the interlayer
van der Waals coupling is favorable to the dynamical stability against strain.
