Recently, we published a paper entitled “Copper migration and surface oxidation of CuxBi2Se3 in ambient pressure environments”
This is our first foray into ambient pressure x-ray photoelectron spectroscopy (XPS), and quantitative analysis of XPS data, which would not have been possible without collaboration with Slavo Nemsak and Lorenz Fitting of the Advanced Lightsource. We also collaborated with several UCD faculty and their students for growing (Curro), intercalating (Koski), and characterizing (Taufour) the specimens.
Intercalation–adding new atoms or molecules between the layers of layered materials–allows to introduce new functionality in various ways. In topological insulators such as Bi2Se3, intercalation is a popular way to introduce superconductivity or magnetism, and copper intercalation is one way to achieve superconductivity. It turns out, that intercalated atoms are not always static, and can sometimes move around due to external factors (perhaps the best example of this is the Li-ion batteries in all our devices, where moving intercalated Li in and out is the entire point). This fact has not been appreciated in Cu-intercalated Bi2Se3, which is a potential topological superconductor. We found that when CuxBi2Se3 is exposed to oxygen, there are chemical changes in the near-surface region including Cu migration and depletion of selenium concomitant with the development of a bismuth oxide layer. This is important because 1) specimens and devices constructed from CuxBi2Se3 may be exposed to air during fabrication or storage which can change their near-surface chemistry 2) near-surface chemistry is particularly critical for these materials because the participation of the surface states in superconductivity determines if these are topological superconductors or not.
See also, this ALS science brief: https://als.lbl.gov/copper-migrates-to-surface-of-topological-insulator-in-air/
Congratulations to Adam, Matt, and the entire collaboration team!