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

Abstract

We have measured the electronic structure of thin Pb films grown on highly oriented pyrolytic graphite ͑HOPG͒ by angle-resolved photoemission spectroscopy. Quantum well states ͑QWSs͒ 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 ͑ML͒, 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 Si͑111͒ as reported previously. This finding indicates that the anomalous effective masses in Pb/Si͑111͒ 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.

Selective growth of Pb islands on graphene/SiC buffer layers

“…This even-number growth mode of Pb islands or thin films is also evidenced on epitaxial graphite. 20,33 But it is different from that on Si(111) 7 Â 7 substrate in which Friedel-typed oscillation with a small 2 ML period and a big 9 ML period is usually observed. 26 The difference may be due to distinctive adsorption energy of Pb on graphene and Si(111) 7 Â 7 substrates.…”

“…It is well known that the electronic states of Pb islands are extremely sensitive to the parity of layer number due to quantum confinement effect. 24,32,33 In experiments, if the Pb islands have the same parity of layer numbers, the STS peaks will appear at almost the same bias voltages. Accordingly, as indicated by nearly the same positions of STS peaks, the two Pb islands in Fig.…”

“…Moreover, the stronger Pb/Si interaction should change the surface energy and lead to a larger phase shift of layer number. 20,33 In such a case, the even-number growth mode rather than the Friedeltyped oscillation mode is preferred for Pb islands on graphene, which is different from that on Si(111) 7 Â 7 substrates.…”

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO2

“…This implies that DFT-based approaches can be reliably employed to design IrO 2 -based quantum well systems with engineered, sub-band-dependent electronic properties. To our knowledge, this strong dependence of the sub-band effective mass with band filling is unprecedented, in the sense that this effect can be explained without the need for invoking either strong electron correlations [37] or interactions with the substrate [38][39][40].…”

Consonant diminution of lattice and electronic coupling between a film and a substrate: Pb on Ge(100)

“…For this reason, Pb films grow in direction (111) on a noninteracting substrate. 21,22 In our case, the large lattice mismatch between Pb and Ge causes the growth of film (100) on substrate surface (100) to be unfavorable, whereas hybridization favors it. Nature apparently has a subtle way of attaining minimum energy for the growth of a (111) film simply by making the substrate less interacting.…”