A Force Limitation for Successful Observation of Atomic Defects: Defect Trappong of the Atomic Force Microscopy Tip

Abstract

Theoretical simulations of atomic force microscopy (AFM) scans while operating in contact mode indicate that there is a natural limit to the maximum nondestructive scan force near atomic defects. This limit is much smaller than the force calculated for nondestructive scans on a defect free surface. The limit is a function of the nature of the sample lattice and imaging medium, and results from a specific force dependence between the AFM tip and sample near the defect, which essentially “traps” the AFM tip apex at constant height in the vicinity of the defect. The AFM feedback system is unable to respond to the trapping, and consequently, the monoatomic apex of the tip collides with the sample surface as the scan continues. The collision effectively produces either a multitip or removes the defect. Further, we find that for a lattice constant less than 0.29–0.3 nm, point-like atomic vacancies cannot be observed, regardless of the scan force used and the medium in which scans are performed.