  Black phosphorus consists of stacked layers of phosphorene, a two-dimensional
semiconductor with promising device characteristics. We report the realization
of a widely tunable bandgap in few-layer black phosphorus doped with potassium
using an in-situ surface doping technique. Through band-structure measurements
and calculations, we demonstrate that a vertical electric field from dopants
modulates the bandgap owing to the giant Stark effect and tunes the material
from a moderate-gap semiconductor to a band-inverted semimetal. At the critical
field of this band inversion, the material becomes a Dirac semimetal with
anisotropic dispersion, linear in armchair and quadratic in zigzag directions.
The tunable band structure of black phosphorus may allow great flexibility in
design and optimization of electronic and optoelectronic devices.
