Quantum size effects in Pb films from first principles: The role of the substrate

Three different Pb films-free standing, on a semiconducting Ge(111) substrate, and on a metallic Cu(111) substrate-are studied with first-principles calculations. Our studies show that the properties of these films-surface energy, work function, and lattice relaxation-oscillate strongly with the film thickness. The oscillation follows a bilayer pattern interrupted by even-odd crossovers. However, the positions of the crossovers and the separation between the crossovers depend on the substrate, showing that the substrate plays an important role in the Pb film properties. In particular, the results for Pb films on Cu(111) substrate challenge the existing physical picture of Pb films.