Environmental instability of few-layer black phosphorus

Abstract

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