UC Davis

Bi2Se3 is an ideal three-dimensional topological insulator in which chemical modifications such as doping and intercalation can be used to engineer surface properties or to further functionalize the topological surface state. In this work, we study the Bi2Se3 system of materials with three experimental techniques: 1) ultrafast optical pump-probe spectroscopy (OPP), 2) angle-resolved photoemission spectroscopy (ARPES), and 3) X-ray photoelectron spectroscopy (XPS). These spectroscopic techniques are utilized to study the out-of- equilibrium carrier properties, surface chemistry, and band structure of Bi2Se3, Bi2–xSbxSe3, and CuxBi2Se3.

The photoexcited carrier decay in Bi2–xSbxSe3 nanoplatelets was studied with ultrafast optical pump-probe spectroscopy, demonstrating a substantial slowing of the bulk carrier relaxation time in bulk-insulating Bi2–xSbxSe3 as compared to n-type bulk-metallic Bi2Se3 at low temperatures, which approaches 3.3 ns in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in n-type Bi2Se3, namely the slow bimolecular recombination of bulk carriers.

Using ambient pressure X-ray photoelectron spectroscopy, copper migration in interca- lated CuxBi2Se3 is demonstrated, occurring 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, with the development of copper and selenium gradients. These complex changes are further modelled with core level spectroscopy simulations (SESSA), which suggest a composition gradient near the surface which develops with oxygen exposure. These results shed light on a phenomenon that must be considered in intercalated topological insulators and intercalated materials in general.