  Recently rediscovered black phosphorus is a layered semiconductor with
promising electronic and photonic properties. Dynamic control of its bandgap
can enable novel device applications and allow for the exploration of new
physical phenomena. However, theoretical investigations and photoemission
spectroscopy experiments performed on doped black phosphorus through potassium
adsorption indicate that in its few-layer form, an exceedingly large electric
field in the order of several volts per nanometer is required to effectively
tune its bandgap, making the direct electrical control unfeasible. Here we
demonstrate the tuning of bandgap in intrinsic black phosphorus using an
electric field directly and reveal the unique thickness-dependent bandgap
tuning properties, arising from the strong interlayer electronic-state
coupling. Furthermore, leveraging a 10-nm-thick black phosphorus in which the
field-induced potential difference across the film dominates over the
interlayer coupling, we continuously tune its bandgap from ~300 to below 50
milli-electron volts, using a moderate displacement field up to 1.1 volts per
nanometer. Such dynamic tuning of bandgap may not only extend the operational
wavelength range of tunable black phosphorus photonic devices, but also pave
the way for the investigation of electrically tunable topological insulators
and topological nodal semimetals.
