Persistence of the Topological Surface States in Bi₂Se₃ against Ag Intercalation at Room Temperature

The electronic and atomic structures of topological insulator Bi2Se3, upon Ag atom deposition, have been investigated by combined experimental methods of scanning tunneling microscopy (STM), photoelectron spectroscopy, and first-principles calculations. We show from the results of STM that the deposited Ag atoms are stabilized beneath the surface instead of being adsorbed on the topmost surface. We further reveal from the angle-resolved photoemission spectroscopy that the Bi2Se3(0001) topological surface states stay uninterrupted after a large amount of absorption of Ag atoms. Our analysis of the photoelectron intensity of Ag core states excited by soft X-ray suggests that a large amount of deposited Ag atoms diffused into a deeper place, which is beyond the probing depth of X-ray photoelectron spectroscopy. The first-principles calculations identify the octahedral site in the van der Waals gaps between quintuple layers to be the most favorable locations of Ag atoms beneath the surface, which yields good agreement between the simulated and experimental STM images. These findings pave an efficient way to tailor the local lattice structures of topological insulators without disturbing the topologically nontrivial surface states. This work was partially supported by National Key R&D Program of China (2017YFA0305400). We also acknowledge the financial support by KAKENHI (Grant No. 17H06138, No. 18H03683). The photoemission and STM/STS experiment were performed with the approval of the Proposal Assessing Committee of HSRC (Proposal Nos.11-B-40, 12-B-12), Hiroshima University, Japan. The XPD measurement was conducted at SPring-8 with the approval of the Proposal Assessing Committee of Institute for Solid State Physics, the University of Tokyo (JASRI Proposals Nos. 2012A7427, 2010B7411, 2011A7412, 2011B7421). We thank Dr. Hiroyuki Matsuda for the technical support and instruction during the XPD experiments. The supports by the Diputación Foral de Gipuzkoa (Project No. 2018-CIEN-000025-01), Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-103910GB-I00) and the Saint Petersburg State University grant for scientific investigations (ID No. 51126254) are also acknowledged. DFT calculations of atomic structure carried out by A.G.R. within the Russian Science Foundation Grant No.19-72-30023. A.E. acknowledges the support by the German Research Foundation (DFG) in the framework of the Special Priority Program (SPP 1666) ”Topological Insulators”. Peer reviewed