Modulation of Step Heights of Thin Pb Films by the Quantum Size Effect Observed by Non-Contact Atomic Force Microscopy

Using a home-made Q-plus sensor, simultaneous scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements were performed on the wedge-shaped Pb islands grown on Si(111)-7 × 7. Atomic resolved AFM images were observed. The contrast of AFM topography shows no dependence on the sample bias (tip is grounded), while the simultaneously obtained tunneling current image exhibits strong bias dependence due to quantum well states (QWS). Furthermore, In the AFM mode, neighboring Pb films with one monolayer (ML) thickness difference within the same Pb island show the same apparent height, which means that the apparent step heights of Pb films oscillate with a bilayer periodicity, being consistent with previous observations by helium atom scattering, x-ray diffraction, and STM. The possible reasons underlying the oscillation of apparent step heights in AFM topography are discussed. (condensed matter: structure, mechanical and thermal properties)