Environmental instability of few-layer black phosphorus

TL;DR

This study investigates the environmental instability of few-layer black phosphorus, revealing moisture-induced volume increase, layer-by-layer etching, and performance degradation in FETs due to air exposure.

Contribution

It provides detailed insights into the moisture-driven degradation mechanisms and the effects of ambient conditions on black phosphorus's structural and electronic properties.

Findings

Moisture causes over 200% volume increase in BP flakes.

Long-term air exposure results in layer-by-layer etching down to monolayer.

FET performance deteriorates over days due to environmental interactions.

Abstract

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.