Large Ultraviolet Photoresponsivity of Few-layer Black Phosphorus

Black phosphorus has recently gained much attention in the scientific community. Black phosphorus can be seen as a crystal generated by periodic repetition of tetraphosphorus (P$_{4})$ molecules. It is known that tetraphosphorus P$_{4}$ can be transformed temporarily to diphosphorus P$_{2}$ upon ultraviolet (UV) irradiation. Thus, it is expected that the P$_{4}$ structured black phosphorus also has strong interaction with light especially in the UV range. Here we report on the optoelectronic characteristics of few-layer black phosphorus field effect transistors (FETs) ranging from the UV to the near infrared (NIR). We demonstrate that black phosphorus is an excellent ultraviolet (UV) photodetector with a specific detectivity \textasciitilde 3x10$^{13}$ Jones. We report also an exceptional photo responsivity of 10$^{7}$ times higher than previously reported values for black phosphorus visible light photodetectors. We attribute such a colossal UV photo responsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for high-efficient UV absorption due to their singularity nature.