Force sensing in scanning tunnelling microscopy: observation of adhesion forces on clean metal surfaces

Tip sample interaction forces were investigated during normal tunnelling operation of the STM using an Ir tip and a polycrystalline Ir sample. Metallic adhesion interaction was observed for tunnel conductivity ranging from 10−6 to 19−9Ω‐1 implying that the actual gap width was of the order of 1–4 Å. Similar experiments performed on a polycrystalline Al sample exposed to 1 Langmuir O2 showed that tip sample interaction changed from attractive to repulsive on well‐defined areas extending over ∼100 Å2 which we identified with the oxidized Al surface. 1988 Blackwell Science Ltd