  Atomic layers of Black Phosphorus (BP) present unique opto-electronic
properties dominated by a direct tunable bandgap in a wide spectral range from
visible to mid-infrared. In this work, we investigate the infrared
photoluminescence of BP single crystals at very low temperature. Near-bandedge
recombinations are observed at 2 K, including dominant excitonic transitions at
0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is
calculated with an anisotropic Wannier-Mott model and found equal to 9.1 meV.
On the contrary, the PL intensity quenching of the 0.276 eV peak at high
temperature is found with a much smaller activation energy, attributed to the
localization of free excitons on a shallow impurity. This analysis leads us to
attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons and
free excitons in BP. As a result, the value of bulk BP bandgap is refined to
0.287 eV at 2K.
