  Two-dimensional crystals have emerged as a new class of materials with novel
properties that may impact future technologies. Experimentally identifying and
characterizing new functional two-dimensional materials in the vast material
pool is a tremendous challenge, and at the same time potentially rewarding. In
this work, we succeed in fabricating field-effect transistors based on
few-layer black phosphorus crystals with thickness down to a few nanometers.
Drain current modulation on the order of 10E5 is achieved in samples thinner
than 7.5 nm at room temperature, with well-developed current saturation in the
IV characteristics, both are important for reliable transistor performance of
the device. Sample mobility is also found to be thickness dependent, with the
highest value up to ~ 1000 cm2/Vs obtained at thickness ~ 10 nm. Our results
demonstrate the potential of black phosphorus thin crystal as a new
two-dimensional material for future applications in nano-electronic devices.
