  Phosphorene, a two-dimensional (2D) analog of black phosphorous, has been a
subject of immense interest recently, due to its high carrier mobilities and a
tunable bandgap. So far, tunability has been predicted to be obtained with very
high compressive/tensile in-plane strains, and vertical electric field, which
are difficult to achieve experimentally. Here, we show using density functional
theory based calculations the possibility of tuning electronic properties by
applying normal compressive strain in bilayer phosphorene. A complete and fully
reversible semiconductor to metal transition has been observed at $\sim13.35\%$
strain, which can be easily realized experimentally. Furthermore, a direct to
indirect bandgap transition has also been observed at $\sim3\%$ strain, which
is a signature of unique band-gap modulation pattern in this material. The
absence of negative frequencies in phonon spectra as a function of strain
demonstrates the structural integrity of the sheets at relatively higher strain
range. The carrier mobilities and effective masses also do not change
significantly as a function of strain, keeping the transport properties nearly
unchanged. This inherent ease of tunability of electronic properties without
affecting the excellent transport properties of phosphorene sheets is expected
to pave way for further fundamental research leading to phosphorene-based
multi-physics devices.
