Electronic Properties of Few-layer Black Phosphorus

Black phosphorus is a layered allotropy of phosphorus that closely resembles graphite. In a single atomic layer, phosphorus atoms are covalently bonded into a puckered honey comb structure. All five valence electrons are localized, so unlike graphene monolayer black phosphorus is a semiconductor with a band gap of $\sim$ 2 eV. In a bulk crystal the interlayer coupling reduces the band gap to $\sim$ 0.3 eV. Using mechanical exfoliation method, we have successfully fabricated few layer black phosphorus field effect transistors. Our samples exhibit bipolar behavior with on-off ratio up to 10$^6$, and a low off-state current. Electronic mobilities up to $\sim$ 1000 cm$^2$V$^{-1}$s$^{-1}$ are currently achieved, with possibilities for further improvement. Such characteristics make black phosphorus a potential candidate for future nanoelectronic applications.