Surface Potential Detection

Surface potential detection measures the effective surface voltage of the sample by adjusting the voltage on the tip to match that of the surface, thereby minimizing the electric force from the sample. Samples for surface potential measurements must have a surface voltage in the range –10 to +10 V. Operation is easiest for voltages in the range of –5 to +5 V. The noise level with this technique is typically 10 mV. Samples may include both conducting and nonconducting regions. Samples with regions of different metals will also show contrast due to contact potential differences. Quantitative voltage measurements are made of the relative voltages within a single image.

Surface potential detection 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 Surface Potential measurement system is shown in figure 1.

Figure 1: Block diagram of signals for 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_ACsinωt 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 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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