  Few-layer black phosphorus (BP), also known as phosphorene, is poised to be
the most attractive graphene analogue owing to its high mobility approaching
that of graphene, and its thickness- tunable band gap that can be as large as
that of molybdenum disulfide. In essence, phosphorene represents the much
sought after high-mobility, large direct band gap two-dimensional layered
crystal that is ideal for optoelectronics and flexible devices. However, its
instability in air is of paramount concern for practical applications. Here, we
demonstrate air-stable BP devices with dielectric and hydrophobic
encapsulation. Microscopy, spectroscopy, and transport techniques were employed
to elucidate the aging mechanism, which can initiate from the BP surface for
bare samples, or edges for samples with thin dielectric coating highlighting
the ineffectiveness of conventional scaled dielectrics. Our pioneering
months-long studies indicate that a double layer of Al2O3 and hydrophobic
fluoropolymer affords BP devices and transistors with indefinite air-stability
for the first time, overcoming a critical material challenge for applied
research and development.
