Local work function Moiré pattern on ultrathin ionic films: NaCl on Ag(100)

In the submonolayer island growth of NaCl on Ag(100) two orientations, 0^∘{} and 45^∘{}, of the NaCl unit cell with respect to the substrate are observed by scanning tunneling microscopy. For the latter we observe a pronounced Moir\'e pattern, which we assign to a modulation of the local work function, as deduced by tunneling spectroscopy of image-potential states. First-principles calculations elucidate the charge-transfer mechanisms responsible for the local work function corrugation and give indications on the local relative position of the NaCl molecules with respect to the substrate.