  Few-layer black phosphorus, a new elemental 2D material recently isolated by
mechanical exfoliation, is a high-mobility layered semiconductor with a direct
bandgap that is predicted to strongly depend on the number of layers, from 0.35
eV (bulk) to 2.0 eV (single-layer). Therefore, black phosphorus is an appealing
candidate for tunable photodetection from the visible to the infrared part of
the spectrum. We study the photoresponse of field-effect transistors (FETs)
made of few-layer black phosphorus (3 nm to 8 nm thick), as a function of
excitation wavelength, power and frequency. In the dark state, the black
phosphorus FETs can be tuned both in hole and electron doping regimes allowing
for ambipolar operation. We measure mobilities in the order of 100 cm2/V s and
current ON/OFF ratio larger than 103. Upon illumination, the black phosphorus
transistors show response to excitation wavelengths from the visible up to 940
nm and rise time of about 1 ms, demonstrating broadband and fast detection. The
responsivity reaches 4.8 mA/W and it could be drastically enhanced by
engineering a detector based on a PN junction. The ambipolar behavior coupled
to the fast and broadband photodetection make few-layer black phosphorus a
promising 2D material for photodetection across the visible and near-infrared
part of the electromagnetic spectrum.
