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Introduction to MFM

Atomic Resolution

Scanning probe methods are nearfield techniques. Therefore, the achieveable resolution is determined by (i) the probe size and (ii) the probe sample distance. Consequently, atomic resolution in force microscopy requires an atomically sharp tip, i.e., a stable atomic configuration at its apex with one atom protruding, and a stable tip sample distance of the order of interatomic distances, i.e. below one nanometer.

Condition (i) is often fullfilled, because for many atomic configurations at a tip apex, one atom protrudes. In the static mode condition (ii) usually cannot be fullfilled, since soft cantilevers have to be used to obtain a reasonable force resolution. Therefore, the tip snaps to the surface before and a suitable distance cannot be adjusted. If tip and sample are in contact, typical contact areas contain several ten atoms. The atomic scale contrast in the static contact mode reflects the translational periodicity of the sample and is governed by friction forces. Point defects or monoatomic step cannot be imaged with atom,ic resolution.

In the dynamic mode the measured quantity is the frequency shift. Soft springs are not a prerequisite to achieve a sufficient force sensitivity. Moreover, the restoring force in the lower turnaround point allows the adjustmant of tip sample distances of the order of some tenth of a nanometer. However, atomic resolution and imaging of individual point defects has only be achieved in vacuum, were the the cantilever is not damped, and therefore enables a higher force sensitivity.

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