Effective quantum-well width of confined electrons in ultrathin Ag(111) films on Si(111)7 x 7 substrates

Ag(111) islands nucleate on a one-monolayer-thick wetting layer with an aperiodic arrangement of Ag atoms on a Si(111)7 x 7 substrate. Recent X-ray diffraction studies have revealed that the Ag wetting layer is crystallized under the Ag islands. We examined whether this wetting layer contributes to the formation of the quantum-well states (QWSs) in the Ag(111) islands by measuring the relation between the energy E of the QWSs and the height of the Ag islands w from the wetting layers using scanning tunneling microscopy and dz/dV spectroscopy. The E-w relation on the Si(111)7 x 7 substrate was found to be shifted by plus one monolayer relative to that for Ag islands on the Si(111)root 3 x root-B substrate, for which nucleation occurs directly on the substrate without the involvement of a wetting layer. The subsurface B atoms on the Si(111)root 3 x root-B substrate were confirmed not to be responsible for the shift in the E-w relation through density functional theory calculations. These results indicate that the wetting layer was incorporated into the Ag(111) islands and contributed to the formation of the QWSs as the bottom part of the crystalline Ag(111) layers.