Structural and electronic transformations in Bi₂Se₃/Bi₄Se₅ observed by Scanning Tunneling Microscopy and X-Ray diffraction

Full text: Bismuth selenide (Bi₂Se₃) is a topological insulator compound with a lamellar structure formed by the repetition of stacks of five monolayers, each of them consisting of plane with either Se or Bi atoms. Each ensemble of 5 planes is connected by van der Waals interactions, making this material potentially interesting for building novel devices [1]. Its electronics properties are intimately related to other two-dimensional systems, presenting surface states with an electronic linear dispersion on selected points of the Brillouin zone. The goal of this work is to observe and interpret the transformations that occur upon heating Bi₂Se₃ at temperatures up to 620K. X-ray diffraction and scanning tunneling microscopy (STM) techniques were used to observe these transformations. X-ray diffraction was measured following the 00L and 01L truncation rods. These measurements revealed that upon heating there is a coexistence of a major Bi₂Se₃ phase and another one that presents a structure of five Bi₂Se₃ quintuple-layers followed by a bilayer of Bismuth, leading to an overall Bi₄Se₅ stoichiometry. STM measurements of the surface of this material showed the presence of large (∼200nm) hexagonal Bi₄Se₅ domains embedded in a Bi₂Se₃ matrix. Scanning tunneling spectroscopy using a lock-in amplifier technique revealed that the bilayer termination has a non-conducting behavior, with a corresponding semiconductor-like density of states, and present a well-defined gap [2]. Density functional theory calculations corroborate these findings and show that the topological state survives in this material under specific conditions. References: [1] Paula M. Coelho et al, Nano Lett., 13, 2013, 13, pp 4517-4521; [2] Paengro Lee et al, J. Phys.: Condens. Matter 28, 2016, 085002. (author)