Peak Force Kelvin Probe of Polyphase Polymers

In this example we look at polyphase polymers with PeakForce Kelvin probe microscopy. This sample consists of polystyrene (PS) with polydimethylsiloxane (PDMS) inclusions. These inclusions are soft, with an elastic modulus of 2.5-3.5 MPa, while the surround polystyrene is around 3 GPa.

This is a great sample for contrast in mechanical measurement techniques like PeakForce QNM, force volume, and so on. And it is a good representative of a soft sample that is well suited to PeakForce imaging where the peak normal force is known and controlled with minimal shear force. This allows for imaging under different peak normal forces, which can lead to different surface structures just due to deformation of the surface and interaction with subsurface structures.

The top image is a height image taken with a peak normal force of 5 nN using a fairly stiff probe, a silicon SCM-PIT which has a spring constant of around 3 nN/nm. Deflection sensitivity was calibrated on sapphire, and the spring constant was estimated through thermal tuning to be around 2.1 nN/nm. This is stiff enough of a probe that the Sader method is more appropriate.

Since the SCM-PIT is coated with PtIr for EFM and KPFM, it was possible to do KPFM to image the surface potential. Initially this sample was chosen to calibrate the probe radius to  simultaneously do QNM and KPFM, to correlate both nanomechanical properties and surface potential properties of a different sample.

But-- why not KPFM on a soft biphasic polymer?

The bottom image is the raw PeakForce KPFM channel. The lift was 50 nm with a scan-rate of 0.1 Hz. A three hour image! What is immediately obvious is that the PDMS domains have a surface potential some 50-70 mV higher than the PS. There is also a potential "plateau" across the top left of the image, and athese little specs on some of the PS inclusions are clearly hitchhikers, contaminants, and they have a surface potential of 200-400 mV above the neighboring PS background.

KPFM and EFM of Exfoliated HOPG

In this demonstration, a piece of HOPG was grounded on the stage of the Bruker Icon using graphitic paint. An SCM-PIT probe was used to perform both PeakForce KPFM (Kelvin probe force microscopy) and EFM (electric force microscopy) on a region of the sample that presented broken edges of the exfoliated HOPG lamella.

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?

Multiple Scales of Artefacts

In this example, a tip roughness sample is imaged with a well worn Si probe. An image was taken in PeakForce mode with 5 nN of peak normal force. At 1 x 1 um the image is quite unremarkable. One might suspect a little probe wear based on the lack of resolution of some of the small grains. That could be a good thing if one were wishing to do some nanomechanical measurements or QNM mapping to get elastic moduli. But there is no obvious self-similarity to the objects, and no obvious companions. In the top left some quite small grains can be resolved on the larger grain. Structures well below 100 nm-- suggesting a good probe. Working out at 10 x 10 um would would anticipate good performance.

Except-- that is farthest from the truth. At the larger scan size there is nothing but self-similarity of the objects images, companions, preferred orientation-- all the signs that scream ARTEFACT.

The artefacts we see-- and don't see-- is a function of the 3D structure of the end of our probe, as well as the nature of the structures we are imaging. Their size and shape relative to the structures that define our probe. Here, a horribly damaged probe had a little tiny point on its end-- and that allowed quite good imaging at very small length scales. Above that, the blunt shape of the tip convoluted every feature into an array of heart- or bird-shaped artefacts.