Energy gap and interband transitions in black phosphorus studied with momentum-resolved EELS*

Black phosphorus has been intensively studied because of its narrow energy gap and large in-plane anisotropy, giving it great potential for micro electronic and optical sensors. However, few studies of the band gap at finite momentum transfer have been reported. Using momentum-resolved electron energy-loss spectroscopy (M-EELS), we measured the interband transitions of black phosphorus as a function of temperature and momentum transfer.

We found that, as the temperature is lowered, the energy gap in bulk black phosphorus decreases for all momenta, from near the optical limit to the Brillouin boundary. We also observed, for T=30K, that a new peak emerges near the band edge, for momenta near the zone boundary. This peak may correspond to a dark exciton from the band structure changes at low temperature.