We prepare single-unit-cell FeSe films on insulating SrTiO3 substrates by molecular beam epitaxy and
investigate the evolution of their superconducting properties with annealing by in situ scanning tunneling
microscopy and scanning tunneling spectroscopy and ex situ transport measurements. We find that through
an annealing process, the superconductivity of 1-uc FeSe films on SrTiO3 substrates develops with the formation
of stoichiometric FeSe films and is further enhanced by charge transfer from SrTiO3 substrates to FeSe films.
Moreover, the superconductivity is independent of the bulk property of the SrTiO3 substrate, regardless of whether
it is insulating or conductive. Our results reveal that the high-temperature superconductivity of 1-uc FeSe films
on SrTiO3 substrates indeed occurs at the FeSe/SrTiO3 interface, where the electron doping at FeSe films plays
an important role in this interfacial superconductivity.