Robustness of topological order and formation of quantum well states in topological insulators exposed to ambient environment

Abstract

The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi(2)Se(3) and Bi(2)Te(3), upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications.

Fig. 1.

Fermi surface and band structure of Bi₂(Se_3-xTe_x) (x = 0, 2.6, 3) topological insulators cleaved in situ and measured at 30 K in ultrahigh vacuum. (A–C) Fermi surface of Bi₂Se₃, Bi₂(Se_0.4Te_2.6), and Bi₂Te₃, respectively. The Fermi surface here, and in other figures below, are original data without involving artificial symmetrization. The band structures along two high symmetry lines and are shown in D and E for Bi₂Se₃, in F and G for Bi₂(Se_0.4Te_2.6), and H and I for Bi₂Te₃.

Fig. 2.

Fermi surface and band structure of Bi₂Se₃ cleaved in air and measured in the ultrahigh vacuum (UHV) chamber. (A) Band structure of the fresh Bi₂Se₃ cleaved and measured in the UHV chamber at 30 K along direction. (B) Band structure of Bi₂Se₃ cleaved in air and measured in UHV at 30 K along direction. (C) Band structure of Bi₂Se₃ cleaved in air and measured in UHV at 300 K along direction. (D and E) Fermi surface of Bi₂Se₃ cleaved in air and measured in UHV at 30 K and 300 K, respectively. Black dashed lines in B and C mark the parabolic bands above the Dirac point from the two-dimensional electron gas.

Fig. 3.

Emergence of quantum well states in Bi₂Te_2.6Se_0.4 after exposing to N₂. (A and B) Band structure measured at 30 K along and , respectively. Black dashed lines in B mark the quantum well states formed in the bulk conduction band (BCB) above the Dirac point. (C) The corresponding Fermi surface. It shows threefold symmetry where three corners of M points are strong while the other three are weak. This is also in agreement with the asymmetric band structure in Fig. 3B. (D) Schematic band structure showing the possible formation of the quantum well states near the sample surface in the bulk conduction band. The blue dotted lines between the bulk valence band (BVB) and bulk conduction band (BCB) represent the topological surface states while the blue solid lines represent quantum well states.

Fig. 4.

Persistence of topological surface state and formation of quantum well states in Bi₂Te₃ after exposure to N₂ or air. The sample was first cleaved and measured in UHV at 30 K. (A and B) The corresponding band structure along the and directions. The sample was then pulled out from the UHV chamber and exposed to N₂ at 1 atm for 5 min before transferring back into UHV chamber for the ARPES measurement. (C and D) The band structure of the N₂-exposed sample along the and directions. The black dashed lines in C illustrate the quantum well states formed in the bulk valence band below the Dirac point. The sample was then pulled out again and exposed to air for 5 min before putting back in vacuum for ARPES measurement. (E and F) The band structure of the air-exposed sample at 30 K along the and directions. (G and H) The measurements at 300 K, and I and J show their corresponding second-derivative images in order to highlight the bands. (K) Fermi surface of N₂-exposed sample. (L) First principle calculation of the band structure of Bi₂Te₃ slab with seven quintuple layers.