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Copper Migrates to Surface of Topological Insulator in Air

Quantum materials such as topological insulators have attracted significant interest for their unique surface electronic structures and potential use in next-generation electronics. The insertion of atoms or molecules between layers (chemical intercalation) can further modify the material properties or create additional electronic phenomena, such as superconductivity.

Controlling the dopant content and its variation during and after synthesis is thus important for realizing ideal superconducting properties. Moreover, in prospective real-world use, such materials must operate in ambient conditions.

“To answer questions about the stability and robustness of the properties of such a system, we used x-ray spectroscopies available at the ALS,” said Adam Gross, a doctoral student at UC Davis and first author of this study. “We studied CuxBi2Se3, an intercalated topological insulator. This material is notable for having topological surface states and can also be made into a superconductor at very low temperatures.”

The study combined ambient-pressure x-ray photoelectron spectroscopy (APXPS) at Beamline 9.3.2 and angle-resolved photoelectron spectroscopy (ARPES) at Beamline 4.0.3 (MERLIN). The samples were grown by collaborators at UC Davis, where additional XPS measurements were performed.

The results showed that the intercalated copper atoms are not stationary after synthesis—they physically diffuse upward to the surface of the material. Interestingly, this process only happens when the samples are exposed to a simulated atmosphere and it accelerates when the sample is exposed to higher pressures in air.

This is significant in two ways. First, it’s a novel way of modifying the surface composition of the material, which can confer it with new properties such as superconductivity. Second, it reveals mechanisms that can lead to changes in the surface properties of these quantum materials, including their degradation.

Greater control over the composition of topological materials can help scientists realize the long-term goal of making those topological states useful in electronic device applications, such as making superconducting qubits for quantum computers. Intercalated two-dimensional materials could also exhibit similar atomic migration, another phenomenon to look out for in those materials and their applications.

Two types of spectroscopic data: electronic structure shows cone-shaped Dirac point indicative of a topological surface state, and XPS spectra showing change in binding energies, indicating presence of more copper at the surface.
Left panel: Cu0.15Bi2Se3 ARPES spectrum of the topological surface state taken at Beamline 4.0.3. Right panels: XPS spectra showing the Cu 3p core levels in Bi2Se3 and Cu0.3Bi2Se3 taken at Beamline 9.3.2. The black arrow indicates the presence of additional Cu at the surface of Cu0.3Bi2Se3. (Credit: A.L. Gross et al., UCD/LBNL)

A.L. Gross, L. Falling, M.C. Staab, M.I. Montero, R.R. Ullah, D.M. Nisson, P. Klavins, K.J. Koski, N.J. Curro, V. Taufour, S. Nemsak, I.M. Vishik, “Copper migration and surface oxidation of CuxBi2Se3 in ambient pressure environments,” J. Phys.: Mater. 5, 044005 (2022), doi:10.1088/2515-7639/ac93b5.