Tip-sample distance dependence of local work function in Kelvin probe force microscopy on Pb/Si(111) islands

To gain a better understanding of the Kelvin probe force microscopy (KPFM) method, it is important to obtain a quantitative understanding of the measured contact potential difference (CPD) values as a function of the tip-sample distance. This will allow a more complete quantitative understanding of the electronic properties and the forces acting on the tip. Here, we chose Pb islands on a Si(111) 7 × 7 surface, metallic islands on a semiconducting surface. Using a scanning force microscope in frequency modulation mode, we perform point bias-approach measurements at 120 K on Pb islands. We investigate how the CPD changes as a function of tip-sample distance and how the electrostatic force changes as a function of the applied bias between the tip and the sample. The measured distance-dependence of the CPD not only allows a connection between the point-of-view of atomic resolution KPFM and of the averaging effects of the body of the tip but also allows the investigation of the shape of charged objects on the surface.