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.

Human Collagen: PeakForce Tapping

The primary advantage of tapping AFM is that it is a non-contact AFM technique. While it "taps" on the sample surface, the tapping is intermittent and as such there is minimum shear force. This is ideal in imaging soft materials, including biological systems, as significant shear forces modify the sample surface while it is being imaged.

That tapping is generally done at the resonant frequency of the probe. A stiff silicon tapping probe has a resonant frequency on the order of 300 kHz, and so a tapping frequency just below resonance is selected for tapping AFM. Constraining the tapping causes the probe frequency to increase, and this will cause the probe frequency to move towards resonance not off resonance.

While this mode of imaging produces minimal shear force, one disadvantage is that it produces an indeterminate maximum normal force. While one can image in soft tapping mode by reducing the tapping amplitude, it is a non-trivial task to estimate the maximum peak normal force in tapping AFM. It is sometimes also desirable to know the maximum force exerted on a sample to quantitate sample deformation.

Bruker's PeakForce tapping combines the best of tapping and contact AFM imaging modes. What it does is perform complete force curves at 1-2 kHz at every image point. The force curve is triggered at the maximum applied normal force according to these force curves, and it is this "PeakForce" that is the set-point or parameter maintained constant during imaging. The second image, borrowed from Bruker, shows force curves as a function of time and position, and the PeakForce set-point is point C on both curves. A further innovation is ScanAsyst technology which dynamically and intelligently monitors and optimizes the scan rate, PeakForce set-point, gains, and Z-limit to produce the best image quality.

In this image human collagen from cadaver skin was imaged using a ScanAsyst Air probe in ScanAsyst mode. This image of a fairly soft biological specimen was imaged with little operator interaction beyond aligning the scanner and focusing on the sample. An additional advantage of the PeakForce tapping imaging mode is that force curves are available at every image point, and these can be used to perform nanomechanical measurements-- what is called PeakForce QNM.