Fe Adatoms on Bi ₂ Se ₃ and TlBiSe ₂ : Adsorption Geometry and Influence on the Local Density of States

We present a combined scanning tunneling microscopy (STM), atomic force microscopy (AFM), and Kelvin probe force spectroscopy (KPFS) study of Fe adatoms on topological insulators (TIs) Bi2Se3 and TlBiSe2. Utilizing both metallic and CO-functionalized tips at 4.4 K under ultrahigh vacuum conditions, we measure the local density of states (LDOS), adsorption geometry, and charge state of individual Fe adatoms. While Fe atoms on Bi2Se3 induce strong resonances in the LDOS without opening a spectral gap, consistent with previous reports, Fe on TlBiSe2 exhibits a pronounced local suppression of the LDOS at the energy of the Dirac point, indicative of a gap opening. Using high-resolution AFM imaging and probe particle model (PPM) simulations, we determine the adsorption geometries of Fe on both materials with picometer precision. Our KPFS and tunneling spectroscopy measurements confirm that Fe acts as a positively charged electron donor on Bi2Se3. In contrast, we suggest that Fe on TlBiSe2 is close to electrically neutral. Our findings highlight the critical role of the adsorption environment in determining the impact of adatoms on TI surfaces and suggest that a local gap opening can be studied on TlBiSe2.