Lift Mode Surface Potential Imaging (AM-KPFM)

Lift Mode surface potential imaging, also referred to as Amplitude Modulated KPFM (AM-KPFM), is a two-pass procedure where the surface topography is obtained by standard TappingMode AFM in the first pass and the surface potential is measured on the second pass. The two measurements are interleaved: that is, they are each measured one line at a time with both images displayed on the screen simultaneously. A block diagram of the this Surface Potential measurement system is shown in Figure 1.

Figure 1: Block diagram of signals for Lift Mode Surface Potential Detection

On the first pass, in TappingMode, the cantilever is mechanically vibrated near its resonant frequency by a small piezoelectric element. On the second pass, the tapping drive piezo is turned off and an oscillating voltage V_ACsin(ωt) is applied directly to the probe tip. If there is a DC voltage difference between the tip and sample, then there will be an oscillating electric force on the cantilever at the frequency ω. This causes the cantilever to vibrate, and an amplitude can be detected.

1. Cantilever measures surface topography on first (main) scan (trace and retrace).
2. Cantilever ascends to lift scan height.
3. Cantilever follows stored surface topography at the lift height above the sample while responding to electric influences on second (interleave) scan (trace and retrace).

If the tip and sample are at the same DC voltage, there is no force on the cantilever at frequency ω and the cantilever amplitude will go to zero. Local surface potential is determined by adjusting the DC voltage on the tip, V_tip, until the oscillation amplitude becomes zero and the tip voltage is the same as the surface potential. The voltage applied to the probe tip is recorded by the NanoScope Controller to construct a voltage map of the surface.

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