Temperature-Induced Coexistence of a Conducting Bilayer and the Bulk-Terminated Surface of the Topological Insulator Bi 2 Te 3 Paula M. Coelho, † Guilherme A. S. Ribeiro, † Angelo Malachias, † Vinicius L. Pimentel, ‡ Wendell S. Silva, † Diogo D. Reis, † Mario S. C. Mazzoni, † and Rogerio Magalha ̃ es-Paniago* ,† † Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, MG, CEP 30123-970, Brazil ‡ Laborató rio Nacional de Nanotecnologia, CP 6192, Campinas, SP, 13083-970, Brazil ABSTRACT: Topological insulators such as Bi 2 Se 3 and Bi 2 Te 3 have extremely promising transport properties, due to their unique electronic behavior: they are insulators in the bulk and conducting at the surface. Recently, the coexistence of two types of surface conducting channels has been observed for Bi 2 Se 3 , one being Dirac electrons from the topological state and the other electrons from a conventional two-dimensional gas. As an explanation for this effect, a possible structural modification of the surface of these materials has been hypothesized. Using scanning tunneling microscopy we have directly observed the coexistence of a conducting bilayer and the bare surface of bulk-terminated Bi 2 Te 3 . X-ray crystal truncation rod scattering was used to directly show the stabilization of this epitaxial bilayer which is primarily composed of bismuth. Using this information, we have performed density functional theory calculations to determine the electronic properties of the possible surface terminations. They can be used to understand recent angular resolved photoemission data which have revealed this dual surface electronic behavior. KEYWORDS: Topological insulators, 2D crystals, surface structure, electronic properties, scanning tunneling microscopy, X-ray diffraction T opological insulators (TI) have revealed extremely interesting electronic properties. While the bulk has an insulating behavior, Dirac fermions localized at the surface are responsible for the charge conduction, which is protected against back scattering by symmetry properties. 1-3 A subtle mechanism based on the effects of the spin orbit coupling underlies this phenomenology, which is, therefore, observed in compounds formed by heavy elements, such as Bi 2 Te 3 and Bi 2 Se 3 . Novel phenomena, such as the possible observation of Majorana fermions, 4 arises in connection with the interaction of TI’s surface with other materials. The coexistence of Dirac fermions with other states has, therefore, become a topic of increasing interest. In this context, it is worth mentioning the description of a two-dimensional electron gas on the surface of Bi 2 Se 3 due to band-bending effects observed by angular- resolved photoemission electron spectroscopy (ARPES). 5 Also, the appearance of an interfacial Bi bilayer has been reported for both Bi 2 Te 3 and Bi 2 Se 3 TI’s. 6,7 For the former, an epitaxial growth of Bi on top of the Bi 2 Te 3 surface was carried out, while in the latter the surface termination by bismuth was observed by low energy ion scattering (LEIS) after a cleaving procedure at room temperature. In this case, it remained unclear which mechanism was responsible for the bilayer formation; that is, if it was selenium evaporation or Bi diffusion from the bulk. The formation of a bismuth bilayer was hypothesized and shown to be stable by ab initio calculations. 7 In this work, we present the first experimental evidence that a temperature-controlled process leads to a self-assembled Bi bilayer on top of the TI Bi 2 Te 3 . Our data strongly support the evaporation hypothesis in the question raised by ref 7. In contrast with most experiments, where the surface of Bi 2 Te 3 is exposed by cleaving, in our case the surface was prepared by ion sputtering and subsequent annealing at high temperatures (573 K). Scanning tunneling microscopy (STM), X-ray crystal truncation scattering (CTR), and calculations based on the density functional theory (DFT) formalism were employed to Received: July 4, 2013 Revised: August 9, 2013 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A dx.doi.org/10.1021/nl402450b | Nano Lett. XXXX, XXX, XXX-XXX