  Interfaces between a bulk-insulating topological insulator (TI) and metallic
adatoms have been studied using high-resolution, angle-resolved and core-level
photoemission. Fe, Nb and Ag were evaporated onto Bi1.5Sb0.5Te1.7Se1.3 (BSTS)
surfaces both at room temperature and 38K. The coverage- and
temperature-dependence of the adsorption and interfacial formation process have
been investigated, highlighting the effects of the overlayer growth on the
occupied electronic structure of the TI. For all coverages at room temperature
and for those equivalent to less than 0.1 monolayer at low temperature all
three metals lead to a downward shift of the TI's bands with respect to the
Fermi level. At room temperature Ag appears to intercalate efficiently into the
van der Waals gap of BSTS, accompanied by low-level substitution of the Te/Se
atoms of the termination layer of the crystal. This Te/Se substitution with
silver increases significantly for low temperature adsorption, and can even
dominate the electrostatic environment of the Bi/Sb atoms in the BSTS
near-surface region. On the other hand, Fe and Nb evaporants remain close to
the termination layer of the crystal. On room temperature deposition, they
initially substitute isoelectronically for Bi as a function of coverage, before
substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too
immobile for substitution processes and show a behaviour consistent with
clustering on the surface. For both Ag and Fe/Nb, these differing adsorption
pathways leads to the qualitatively similar and remarkable behavior for low
temperature deposition that the chemical potential first moves upward (n-type
dopant behavior) and then downward (p-type behavior) on increasing coverage.
