How local is the Phantom Force?

TL;DR

This study investigates the phantom force in atomic force microscopy, demonstrating its presence on surfaces without metallic states and highlighting the significant impact of local resistance on its magnitude.

Contribution

It shows that the phantom force is not suppressed by metallic surface-states and emphasizes the role of local resistance in its strength, challenging previous assumptions.

Findings

Phantom force observed on H/Si(100) without metallic surface-states.

Local resistance Rs strongly influences the phantom force magnitude.

Surface-state metallicity does not suppress the phantom force.

Abstract

The phantom force is an apparently repulsive force, which can dominate the atomic contrast of an AFM image when a tunneling current is present. We described this effect with a simple resistive model, in which the tunneling current causes a voltage drop at the sample area underneath the probe tip. Because tunneling is a highly local process, the areal current density is quite high, which leads to an appreciable local voltage drop that in turn changes the electrostatic attraction between tip and sample. However, Si(111)-7\times7 has a metallic surface-state and it might be proposed that electrons should instead propagate along the surface-state, as through a thin metal film on a semiconducting surface, before propagating into the bulk. In this article, we investigate the role of the metallic surface-state on the phantom force. First, we show that the phantom force can be observed on H/Si(100), a surface without a metallic surface-state. Furthermore, we investigate the influence of the surface-state on our phantom force observations of Si(111)-7\times7 by analyzing the influence of the macroscopic tip radius R on the strength of the phantom force, where a noticeable effect would be expected if the local voltage drop would reach extensions comparable to the tip radius. We conclude that a metallic surface-state does not suppress the phantom force, but that the local resistance Rs has a strong effect on the magnitude of the phantom force.