Unusual Surface Terminations in Topological Insulators

Topological Insulators are materials for which the quantum states of electrons have a "twist" that must be untwisted in passing an interface to another non-topological material or to vacuum [1]. The untwisting at interfaces requires that surfaces and interfaces of topological materials must be metallic, even though they are insulating in the bulk. (NB: Metallic surface states on insulating surfaces are old news; it the the topological requirement of metallicity that is important).

The twisted electronic structure underpinning topological insulator (TI) character can often arise from spin-orbit coupling in materials containing heavy elements. The modern prototype is Bi2Se3, which was the shining early example of simple TI behavior. Not only was the expected metallic (Dirac-like) surface state observed, spin polarized photoemission proved the expected "spin texture" in this unique state [2].

The Bi2Se3 solid consists of quintuple layer units of (Se-Bi-Se-Bi-Se) held together by van der Waals interactions between the Se layers at each end. These weak interactions define a relatively easy cleavage plane that is expected to leave a Se-terminated surface.

Thus, we were surprised when we found evidence for Bi-terminated surfaces in air-cleaved samples of Bi2Se3. Our X-ray Photoelectron Spectroscopy experiments on such samples (from both commercial and home-grown batches) showed Bi 4f core level spectra with a multiple-peak structure that is a fingerprint of both Bi-Bi and Bi-Se terminations. This data is illustrated in Figure 1 and was published in JVST B [3].

Figure 1. Bi 4f core levels for two different cleavage results.

This study shows that the surfaces of cleaved samples may not be as simple as naively expected and may undergo significant reconstruction. Importantly, this is not a direct effect of contamination. The surfaces are clean according to XPS, though it is possible that the ambient environment during the cleave can change the mechanical details of the process and lead indirectly to surface reconstruction.

This is important because, while topology requires the existence of a metallic surface state, the details of its physical properties depend on the specific surface termination that exists [4]. In our experiments, we found large modification to the surface state band structure in samples with Bi-Bi terminations [3]. For many experiments, air cleavage is an important step and the complexity of surface termination (even in the simple system of Bi2Se3) needs to be considered in interpreting results.

Our group's interest in topological insultor surfaces has continued and focused on efforts to control surface properties using functional molecular films [5,6].

References

[1] M. Fruchart and D. Carpentier, arXiv 1320.0255

[2] Hsieh et al., Nature 460, 1101 (2009).

[3] Hewitt et al., J. Vac. Sci. Tech. B 32, 04E103 (2014).

[4] Zhang et al., Phys. Rev. B 86, 081303 (2012).

[5] Wang et al. J. Phys. Chem C 118, 14860 (2014).

[6] Hewitt et al., Surf. Sci. 662, 87 (2017).