Gap dependence of the tip-sample capacitance

Abstract

The tip-sample capacitance has been studied in the nontunneling regime and the capacitance-distance characteristics and its dependence on the tip geometry have been determined for the gap distance 1<s<600 nm. Measurements were carried out in ultrahigh vacuum on a capacitor formed between a metal tip (W or Pt–Ir) and a clean Au(111) surface. Tips of different tip radius R=30∼4000 nm were used to investigate the influence of tip geometry on the capacitance. When the gap distance is reduced, the capacitance increases while its gap sensitivity |∂C/∂s| decreases with the gap distance. The capacitance therefore shows no 1/s divergence. The magnitude of the capacitance change is found to depend on the tip geometry: blunt tips (R>1000 nm) show larger capacitance increase than that for sharp tips (R⩽100 nm). The effective tip radius Reff estimated from the C−s characteristics agrees with the real tip radius in a limited distance range which varies with the tip geometry. At small distances (s<30 nm), Reff≃R for sharp tips but Reff<R for blunt tips. On the other hand at large distances (s>200 nm), the relation is reversed, Reff>R for sharp tips while Reff∼R for blunt tips. These results on Reff can be explained by the field concentration to the tip apex and the change of capacitance-contributing tip area with the gap distance. Capacitance calculations indicate that the capacitance of the “truncated cone + half sphere” tip well reproduces the observed C−s characteristics and its dependence on the tip geometry.