  Using the Tight Binding (TB) parameters extracted from Density Functional
Theory (DFT) and Recursive Green's Function method, it is shown that
skewed-zigzag black phosphorous (phosphorene) nanoribbons obtain large and
tuneable bandgap in response to vertical and transverse electric fields.
Depending on the direction of the applied field the mid-gap states could
possess the localized or metallic nature i.e. non-zero mid-gap density of
states. Adjustability of the bandgap and optical dipole transition matrix
elements are explained based on the symmetry of involved band edge states. This
promises new electronic and optical devices based on phosphorene nanoribbons.
