Pulsed Force Kelvin Probe Force Microscopy (PF-KPFM)

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The underlying principle of KPFM is to use a DC voltage bias to compensate for the difference in surface potential to prevent electron migration upon electrical contact. KPFM has its conceptual origin in the Kelvin method, by Lord Kelvin. Consider a pair of plates of different metal types close in space: if there is an electrical connection between them, e.g., through a wire, then the Fermi level alignment occurs and electrons move from low work function metal to the high work function metal.

A net result is the emergence of charges between two metal plates, and Coulombic forces.

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The peak force tapping mode of AFM has a deterministic contact region and detachment region. When the tip is just detached from the sample surface, the tip-sample distance is separated but still very close (< 2 nm), which is a good spot for inducing the Fermi-level alignment. The Coulombic force is mainly generated between the AFM tip apex and the sample, which is a requirement for high spatial resolution.

The peak force tapping mode has a characteristic trait. The resonant profile of the AFM cantilever dynamically switches between two sets: one is when the tip is detached from the sample surface; the other is when the tip is in momentary contact with the sample. The former is dominated by the cantilever free-space oscillation; the latter contains contact resonance and a shift of the free space oscillation frequency.

PFIR operates when the tip and sample are in contact; the PF-KPFM operates when they are detached. The two signal read-out channels are in parallel without interference. Integrating PFIR with PF-KPFM can be done without compromising the capability of each modality.