  Recently, black phosphorus quantum dots were fabricated experimentally.
Motivated by these experiments, we theoretically investigate the electronic and
optical properties of rectangular phosphorene quantum dots (RPQDs) in the
presence of an in-plane electric field and a perpendicular magnetic field. The
energy spectra and wave functions of RPQDs are obtained numerically using the
tight-binding (TB) approach. We find edge states within the band gap of the
RPQD which are well separated from the bulk states. In an undoped RPQD and for
in-plane polarized light, due to the presence of well-defined edge states, we
find three types of optical transitions which are between the bulk states,
between the edge and bulk states, and between the edge states. The electric and
magnetic fields influence the bulk-to-bulk, edge-to-bulk, and edge-to-edge
transitions differently due to the different responses of bulk and edge states
to these fields.
