Gating the charge state of single Fe dopants in the topological insulator Bi 2 Se 3 with a scanning tunneling microscope

Low-temperature scanning tunneling microscope (STM) is exploited to study directly the atomic and electronic

structures of single Fe dopants in topological insulator Bi2Se3 thin films. Fe atoms predominantly and isovalently

substitute for two distinct Bi sites at the subsurface with charge neutrality. In the vicinity of Fe substitutions,

circular depressions in STM topography and sharp rings in spatially resolved conductance map, whose diameters

apparently depend on the tunneling conditions, are observed. We show that the phenomena correlate well with

the switching between Fe3+ and Fe2+ charge states due to the gate effect of the tunneling tip, as further evidenced

by a multidopant system.