  2D materials are well-known to exhibit interesting phenomena due to quantum
confinement. Here, we show that quantum confinement, together with structural
anisotropy, result in an electric-field-tunable Dirac cone in 2D black
phosphorus. Using density functional theory calculations, we find that an
electric field, E_ext, applied normal to a 2D black phosphorus thin film, can
reduce the direct band gap of few-layer black phosphorus, resulting in an
insulator-to-metal transition at a critical field, E_c. Increasing E_ext beyond
E_c can induce a Dirac cone in the system, provided the black phosphorus film
is sufficiently thin. The electric field strength can tune the position of the
Dirac cone and the Dirac-Fermi velocities, the latter being similar in
magnitude to that in graphene. We show that the Dirac cone arises from an
anisotropic interaction term between the frontier orbitals that are spatially
separated due to the applied field, on different halves of the 2D slab. When
this interaction term becomes vanishingly small for thicker films, the Dirac
cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone
at certain electric fields; however, a further increase in field strength
reduces the spin-orbit-induced gap, eventually resulting in a
topological-insulator-to-Dirac-semi-metal transition.
