Using surface x-ray diffraction and scanning tunneling microscopy in combination with first-principles calculations, we have studied the geometric and electronic structure of Cs-deposited Bi2Se3(0001) surface kept at room temperature. Two samples were investigated: a single Bi2Se3 crystal, whose surface was Ar sputtered and then annealed at ∼500 ◦C for several minutes prior to Cs deposition, and a 13-nm-thick epitaxial Bi2Se3 film that was not subject to sputtering and was annealed only at ∼350 ◦C. In the first case, a considerable fraction of Cs atoms occupy top layer Se atoms sites both on the terraces and along the upper step edges where they form one-dimensional-like structures parallel to the step. In the second case, Cs atoms occupy the f cc hollow site positions. First-principles calculations reveal that Cs atoms prefer to occupy Se positions on the Bi2Se3(0001) surface only if vacancies are present, which might be created during the crystal growth or during the surface preparation process. Otherwise, Cs atoms prefer to be located in f cc hollow sites in agreement with the experimental finding for the MBE-grown sample.

We acknowledge financial support from DFG through priority program SPP1666 (Topological Insulators), as well as by the University of the Basque Country (Grant Nos. GIC07IT36607 and IT-756-13), the Spanish Ministry of Science and Innovation (Grant Nos. FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P, and FIS2016-75862-P) and Tomsk State University Academic D.I. Mendeleev Fund Program in 2015 (research Grant No. 8.1.05.2015). Partial support by the Saint Petersburg State University project No. 15.61.202.2015 is also acknowledged. The study has also been supported by the Russian Science Foundation (project No. 17-12-01047) in part of the single crystal growth and structural characterization. Technical support by F. Weiss is gratefully acknowledged.

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