The first image shows the height image taken in PeakForce mode. A normal force of about 10 nN was used, and even this is causing some disturbance of the graphitic lamella. Some of the sheets seem to come in multiple copies. This is actually not the result of a tip-artifact, as the effect goes away when switching to a lower peak force set-point, or when switching to tapping AFM in order to subsequently do EFM. The intermittent normal forces the sample sees in tapping AFM are much smaller than the 10 nN peak force used here. A larger peak force set-point was used only to help track the drastic topographic relief in this particular sample. This perturbation of the graphite lamella by 10 nN of peak normal force shows how flexible and loosely coupled they are.
The next image was taken in tapping AFM in lift mode to facilitate EFM imaging. EFM is an imaging mode that applies scanning interleave. There is one pass across the sample to image sample morphology, and then a second pass along the same trace-retrace. One can do anything one likes with the interleave pass. Take a second image with any change of parameters one likes from a different set of gains and set-point to a probe-sample bias. In this case the probe is lifted 100 nm above the surface of the sample and changes in probe amplitude and phase are detected. As in MFM (magnetic force microscopy) the amplitude change says something about the magnitude of an electric field gradient, and a phase change says something about its sign-- whether it is repulsive or attractive as seen by the probe.
The results were are quite curious. HOPG is quite a good conductor, and the sample was well grounded using graphitic paint. It is interesting that any EFM signal is seen whatsoever given that it is not biased. The signal is small. There is only about a 0.5 nm amplitude change across the whole image region, but it clearly correlates with the edges of the lamella. The edges are the source of a gradient.
Like MFM, EFM can be a bit difficult to interpret as one is probing an electric field gradient, and unless the geometry is of planar ferroelectric domains, or a planar fabricated device, it is often not clear what is causing these gradients: geometry, gradient in the potential creating the electric field, dielectric gradients. All that can be determined is that the magnitude of the EFM signal (i.e. the electric field gradient) is larger near the edges of the lamella, and trails off from there.
The EFM phase signal is, like the amplitude signal, very weak. About a half a degree of phase change over the whole image field. The signal is limited to the broken lamella edges and the rest of the lamellae are generally neutral in signal. This would seem to indicate a repulsive electric field gradient-- but it is harder to know what an electric field gradient says about the electric field itself. That is particularly the case in EFM where there are capacitive as well as charge effects that produce the EFM signal.
The last image is the KPFM which was done in PeakForce mode. This is actually measuring the surface potential in the interleave scan. What we see is an accumulation of potential on the broken edges of the lamella. Quite a small signal, a few 10's mV, but definitely consistent, repeatable, and correlating with the edges of the broken lamella. Given that this sample is conducting and well grounded with no applied potential to the probe or stage, this is surplus surface charge from the beaking of bonds?
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