  Since the discovery of large, non-saturating magnetoresistance in bulk WTe2
which allows microexfoliation, single- and few-layer WTe2 crystals have
attracted increasing interests. However, as it mentioned in existing studies,
WTe2 flakes appear to degrade in ambient conditions. Here we report
experimental observations of saturating degradation in few-layer WTe2 through
Raman spectroscopy characterization and careful monitoring of the degradation
of single-, bi- and tri-layer (1L, 2L & 3L) WTe2 over long time. Raman peak
intensity decreases during WTe2 degradation and 1L flakes degrade faster than
2L and 3L flakes. The relatively faster degradation in 1L WTe2 could be
attributed to low energy barrier of oxygen reaction with WTe2. We further
investigate the degradation mechanisms of WTe2 using XPS and AES and find that
oxidation of Te and W atoms is the main reason of WTe2 degradation. In
addition, we observe oxidation occurs only in the depth of 0.5nm near the
surface, and the oxidized WTe2 surface could help prevent inner layers from
further degradation.
