  We study the environmental instability of mechanically exfoliated few-layer
black phosphorus (BP). From continuous measurements of flake topography over
several days, we observe an increase of over 200% in volume due to the
condensation of moisture from air. We find that long term exposure to ambient
conditions results in a layer-by-layer etching process of BP flakes.
Interestingly, flakes can be etched down to single layer (phosphorene)
thicknesses. BP's strong affinity for water greatly modifies the performance of
fabricated field-effect transistors (FETs) measured in ambient conditions. Upon
exposure to air, we differentiate between two timescales for doping of BP FET
transfer characterisitcs: a short timescale (minutes) in which a shift in the
threshold voltage occurs due to physisorbed oxygen and nitrogen, and a long
timescale (hours) in which p-type doping occurs from water absorption.
Continuous measurements of BP FETs in air reveal eventual degradation and
break-down of the channel material after several days due to the layer-by-layer
etching process.
