  Phosphorene is a novel two-dimensional material that can be isolated through
mechanical exfoliation from layered black phosphorus, but unlike graphene and
silicene, monolayer phosphorene has a large band gap. It was thus unsuspected
to exhibit band inversion and the ensuing topological insulator behavior. It
has recently attracted interest because of its proposed application as field
effect transistors. Using first-principles calculations with applied
perpendicular electric field F we predict a continuous transition from the
normal insulator to a topological insulator and eventually to a metal as a
function of F. The continuous tuning of topological behavior with electric
field would lead to spin-separated, gapless edge states, i.e., quantum spins
Hall effect. This finding opens the possibility of converting normal insulating
materials into topological ones via electric field, and making a
multi-functional field effect topological transistor that could manipulate
simultaneously both spins and charge carrier.
