Copper migration and surface oxidation of ${\text{Cu}}_x{\text{Bi}}_2{\text{Se}}_3$ in ambient pressure environments

Chemical modifications such as intercalation can be used to modify surface properties or to further functionalize the surface states of topological insulators. Using ambient pressure X-ray photoelectron spectroscopy, we report copper migration in ${\text{Cu}}_x{\text{Bi}}_2{\text{Se}}_3$, which occurs on a timescale of hours to days after initial surface cleaving. The increase in near-surface copper proceeds along with the oxidation of the sample surface and large changes in the selenium content. These complex changes are further modelled with core-level spectroscopy simulations, which suggest a composition gradient near the surface which develops with oxygen exposure. Our results shed light on a new phenomenon that must be considered for intercalated topological insulators$\text{unicode}x2014$and intercalated materials in general$\text{unicode}x2014$that surface chemical composition can change when specimens are exposed to ambient conditions.