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.