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Abstract
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 h in ambient, followed by a ∼ 10(3) decrease
in FET current on/off ratio and mobility after 48 h. 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 cm(2) V(-1) s(-1) for over 2
weeks in ambient conditions. 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.
[1
]Department of Materials Science and Engineering, ‡Department of Chemistry, and §Graduate
Program in Applied Physics, Northwestern University, Evanston, Illinois 60208, United
States