Thin NaCl films on silver (001): island growth and work function

The surface work function (WF) and substrate temperature dependence of the NaCl thin-film growth on Ag(001) have been studied by noncontact atomic force microscopy and Kelvin probe force microscopy. In the sub-monolayer range, the NaCl film is composed of large crystalline islands, which decrease in density and increase in size with increasing temperature during deposition. Each island is composed of a large base island 2 monolayers (ML) thick (for T > 343 K), which collects impinging NaCl molecules that form ad-islands on top. Kelvin probe force microscopy (KPFM) measurements show a reduction of the silver WF by Delta phi = phi(Ag) - phi(NaCl/Ag) = 0.69 +/- 0.03 eV with no dependence on the film thickness (1-6 ML), in agreement with recent theoretical calculations. The previously observed nanometer-sized moire pattern on islands that are 45 degrees rotated with respect to the silver lattice could be observed in the scanning tunneling microscopy mode. However, no contrast could be obtained in KPFM images.