  A combined experimental and theoretical study of doping individual Fe atoms
into Bi2Se3 is presented. It is shown through a scanning tunneling microscopy
study that single Fe atoms initially located at hollow sites on top of the
surface (adatoms) can be incorporated into subsurface layers by
thermally-activated diffusion. Angle-resolved photoemission spectroscopy in
combination with ab-initio calculations suggest that the doping behavior
changes from electron donation for the Fe adatom to neutral or electron
acceptance for Fe incorporated into substitutional Bi sites. According to first
principles calculations within density functional theory, these Fe
substitutional impurities retain a large magnetic moment thus presenting an
alternative scheme for magnetically doping the topological surface state. For
both types of Fe doping, we see no indication of a gap at the Dirac point.
