UC Riverside

Topological insulator (TI) materials do not conduct electricity in the bulk, but instead conduct along the surface via topological surface states (TSS). To fully utilize these materials, it is critically important to understand their surface atomic and electronic structures, both of which can be sensitively probed by low energy ion scattering (LEIS).

Bi2Se3 is a TI that consists of stacked quintuple layers ordered as Se-Bi-Se-Bi-Se. The surfaces are expected to show a Se-termination after mechanical cleaving, but instead various surface terminations are found. Time-of-flight (TOF) LEIS spectra show that Bi2Se3 surfaces prepared by in situ cleaving or by Ar+ ion bombardment and annealing (IBA) are always Se-terminated. Impact collision ion scattering spectroscopy (ICISS) finds no differences in the geometric structure of the top three atomic layers of IBA-prepared surfaces and in situ cleaved surfaces. Note that the annealing temperature is critical in preparing surfaces with IBA. If it’s too low, then surface will not be well-ordered. If it’s too high, then surface Se vacancies will form. Ex situ cleaved Bi2Se3 surfaces can be either Se-terminated or Bi-rich, which might be caused by contaminants that adsorb at surface defects and induce a chemical reaction. After exposure to air, IBA-prepared surfaces show less surface contamination than cleaved surfaces and maintain their Se-termination.

Cs is adsorbed on Bi2Se3 as a tool to investigate the surface electronic properties of TIs. Scattered alkali ions exchange charge with surfaces through a non-adiabatic resonant charge transfer (RCT) process that depends on the local electrostatic potential (LEP) above the target atom. It is found that the neutralization probability for Na+ scattered from Se sites is unexpectedly larger than from Bi sites. This can be explained by the spatial distribution of the TSS carriers in which the electron density accumulates below the first layer Se atoms and above the second layer Bi. This spatial distribution leads to an upward pointing dipole on Se sites that reduces the LEP and a downward dipole above Bi that increases the LEP.