| AVS 60th International Symposium and Exhibition | |
| Graphene and Other 2D Materials Focus Topic | Thursday Sessions |
| Session GR+EM+NS+SS+TF-ThA |
| Session: | Beyond Graphene: Other 2D Electronic Materials and their Heterostructures |
| Presenter: | J.A. Yarmoff, University of California, Riverside |
| Authors: | W. Zhou, University of California, Riverside X. He, University of California, Riverside Z.Y. Wang, University of California, Riverside J. Shi, University of California, Riverside J.A. Yarmoff, University of California, Riverside |
| Correspondent: | Click to Email |
Topological insulator (TI) materials are electrically conductive through unique two-dimensional surface states that are characterized by a Dirac cone, while they are insulators in the bulk. TI’s have garnered great attention because of their potential use in next generation electronic devices. One of the concerns with TI’s, however, is that their surface transport can degrade over time, which has been dubbed the “aging” effect. The transport is dependent on the detailed surface structure and how it changes over time, but this is not completely understood.
Low energy ion scattering (LEIS), which can measure the composition and atomic structure at the surface of a solid, was used to show that the surface termination can change after in situ cleaving, which might be related to aging effect [1]. LEIS spectra were collected from Bi2Se3, the prototypical TI material, after cleaving a bulk single crystal in ultra-high vacuum. Bi2Se3 has a layered structure with a fundamental quintuple layer (QL) unit composed of five atomic layers ordered as Se-Bi-Se-Bi-Se. While the atoms within each QL are covalently bonded, the QLs are attached to each other by relatively weak van der Waals forces. It has thus been assumed that the material cleaves between QLs resulting in a Se-terminated surface. Surprisingly, however, spectra collected at room temperature using 3 keV Na+ ions in a double-alignment orientation, which is sensitive only to the outermost atomic layer, show scattering from Bi and none from Se. It was thus hypothesized that the material initially cleaves between QLs, but a thermally activated process induces a termination change. When cleaved at low temperature (80 K), the process is slowed and LEIS measurements are able to monitor the termination change in real time, consistent with the hypothesis.
We will discuss conditions that lead to the termination change, the stability of the Bi-terminated structure and the mechanisms involved. To quantitatively ascertain the atomic structure of the top few atomic layers, additional LEIS spectra were collected as a function of orientation and compared to the results of simulations.
[1] X. He, W. Zhou, Z. Y. Wang, Y. N. Zhang, J. Shi, R. Q. Wu and J. A. Yarmoff, Phys. Rev. Lett. 110, 156101 (2013).