  Unencapsulated, exfoliated black phosphorus (BP) flakes are found to
chemically degrade upon exposure to ambient conditions. Atomic force
microscopy, electrostatic force microscopy, transmission electron microscopy,
X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy
are employed to characterize the structure and chemistry of the degradation
process, suggesting that O2 saturated H2O irreversibly reacts with BP to form
oxidized phosphorus species. This interpretation is further supported by the
observation that BP degradation occurs more rapidly on hydrophobic
octadecyltrichlorosilane self-assembled monolayers and on H-Si(111), versus
hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient
degradation causes large increases in threshold voltage after 6 hours in
ambient, followed by a ~10^3 decrease in FET current on/off ratio and mobility
after 48 hours. Atomic layer deposited AlOx overlayers effectively suppress
ambient degradation, allowing encapsulated BP FETs to maintain high on/off
ratios of ~10^3 and mobilities of ~100 cm2/(V*s) for over two weeks in ambient.
This work shows that the ambient degradation of BP can be managed effectively
when the flakes are sufficiently passivated. In turn, our strategy for
enhancing BP environmental stability will accelerate efforts to implement BP in
electronic and optoelectronic applications.
