Quantized electronic structure and growth of Pb films on highly oriented pyrolytic graphite

We have measured the electronic structure of thin Pb films grown on highly oriented pyrolytic graphite (HOPG) by angle-resolved photoemission spectroscopy. Quantum well states (QWSs) corresponding to confined Pb valence electrons are observed. Their energy positions are fixed, but their intensities evolve for increasing Pb coverages. The results indicate that the films are rough, consisting of multiple thicknesses. Nevertheless, the thickness distribution is sufficiently narrow to allow a unique assignment for each QWS peak in terms of a quantum number and the exact film thickness in atomic layers. For increasing Pb coverages of up to 10 monolayers (ML), the even film thicknesses of 2, 4, 6, 8, and 10 ML are much more prevalent than the odd film thicknesses of 1, 3, 5, 7, and 9 ML, thus suggesting significant differences in surface energy between the even and odd thicknesses. These results are consistent with an available first-principles calculation of the surface energies of freestanding films; an implication is that the interaction between the Pb film and the HOPG substrate is weak. The in-plane dispersion relations of the QWSs are measured. The effective masses at the surface zone center agree well with the results calculated from the bulk Pb band structure, in sharp contrast to the strongly enhanced or anomalous effective masses in Pb films grown on Si(111) as reported previously. This finding indicates that the anomalous effective masses in Pb/Si(111) are not caused by increased electron correlation effects in a confined geometry, but are rather attributable to a strong interfacial interaction between the QWSs and the substrate electronic structure.