Low temperature scanning tunneling microscopy and spectroscopy are used to investigate the
atomic and electronic structure evolution of FeSe films grown on SrTiO3 as a function of postgrowth
annealing. Single unit cell FeSe films are found to bond strongly with the underlying
substrate, and become superconductive with diminishing chemical bond disorders at the
interface via post-annealing. For thicker FeSe films, post-annealing removes excess Se in
the films and leads to a transition from semiconductor into metallic behaviors. In double and
multilayer films, strain-induced complex textures are observed and suggested to be the main
cause for the absent superconductivity.