Pacific Nanotechnology Inc.
Polymers
Material Sensing
The AFM makes it possible to measure not only the topography of the sample but also changes in the mechanical properties of the surface. The two most common types of material sensing modes are Lateral Force Microscopy (LFM) and Phase Contrast Mode (PCM).
The torsion of the cantilever is monitored with the AFM scanning head in lateral force microscopy. As the cantilever is scanned across the surface, the amount of torsion changes depending on the types of materials at the surface. It is possible to acquire both an LFM image and a topographic image simultaneously with an AFM.
Figure 6: As the probe is scanned across a surface, any change in chemical composition can be detected by a change in the lateral torsional motion of the cantilever. In this example the yellow area will show different "friction" than the white area.
Although this technique is not quantitative, it provides a contrast mechanism for differentiating chemical materials at a sample surface. LFM is best applied to samples with small topographic features because changes in topography can also cause changes in the cantilever torsion.
Figure 7: Topography (left) and Lateral Force images (right) of a structured polymer material. Both images were measured simultaneously. Only the topography is seen in the top image while the changes in chemical composition are readily observed in the LFM image.
It is possible to measure changes in the mechanical properties at a polymer surface with PCM. Close contact mode operates by maintaining the amplitude of a vibrating cantilever constant while scanning over a polymer surface. At the same time, monitoring changes in the phase of the cantilever gives an image that is related to the localized "hardness" of the surface. A harder section of a sample will give more (or less) phase shift of the vibrating cantilever as it scans across a sample.
Figure 8: Topography, right, and Phase Contrast, left, images of a co-block polymer. In the phase contrast image the contrast is derived from changes in the mechanical properties of the surface. Features as small as 50nm are readily resolved with phase contrast mode.
Figure 9: Phase contrast mode measures the changes in the phase of a vibrating cantilever as the cantilever is scanned across a surface. Areas of varying "hardness" result in changes in the phase of the vibrating cantilever.
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