Mechanical manifestations of rare atomic jumps in dynamic force microscopy

Abstract

The resonance frequency and the excitation amplitude of a silicon cantilever havebeen measured as a function of distance to a cleaved KBr(001) surface with alow-temperature scanning force microscope (SFM) in ultrahigh vacuum. We identify tworegimes of tip–sample distances. Above a site-dependent critical tip–sample distancereproducible data with low noise and no interaction-induced energy dissipation aremeasured. In this regime reproducible SFM images can be recorded. At closertip–sample distances, above two distinct atomic sites, the frequency values jumpbetween two limiting curves on a timescale of tens of milliseconds. Furthermore,additional energy dissipation occurs wherever jumps are observed. We attribute bothphenomena to rarely occurring changes in the tip apex configuration which are affectedby short-range interactions with the sample. Their respective magnitudes arerelated to each other. A specific candidate two-level system is also proposed.