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How does PiFM improve AFM topography?

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Background

PiFM (photo-induced force microscopy) is a spectro-nanoscopy technique (also called AFM-IR or nano-IR type technique) based on an AFM (atomic force microscope). An AFM uses an extremely sharp probe to scan the surface of a sample and collect topographic information with a resolution of only a few nanometers. While fantastic, this resolution alone isn’t always helpful for understanding a surface. Without chemical data, many surface features on a sample are missed because the topography alone can not fully describe a sample’s surface features. Here, this idea will be illustrated with a simple example, showing how much more information a PiFM data can add to AFM.

Sample description

The sample in question is an ultra-thin cross-section of spruce wood. Under an optical microscope, the cell walls are easily visible.

Figure 1. The spruce wood cells under an optical microscope. This is an ultra-thin cross section without any resin. Notice the back of the cantilever out-of-focus above it.

Sample analysis with PiFM

The goal is to investigate the architecture and chemistry of the cell walls. Therefore, the first AFM image is a 4-micron square region of the cell walls at a junction where three cells meet:

Figure 2. AFM topography of the cell walls in an ultra-thin cross-section of spruce wood.

This AFM image provides a great view of the cell walls, but by itself it cannot provide any more info about what that surface is made of.

To get PiFM data, an IR QCL laser is tuned to a wavenumber that corresponds with a chemical absorption peak in the relevant materials. Then, the PiFM instrument scans the surface and simultaneously collects the AFM topography as well as the PiFM chemical absorption map. The specific wavenumbers chosen for scanning are determined by examining PiF-IR spectra taken on the surface. For more context, the PiF-IR spectra may be cross-referenced with a database of FTIR spectra. More information about how PiF-IR spectra compare to FTIR can be found in this comparison article.

If we add the PiFM data, it becomes clear that the cell walls show a combination of lignin and cellulose, where the lignin is concentrated in the center. At this scale, PiFM offers important chemical data, but the chemical separation matches the topographic features.

Figure 3. Two fixed-wavenumber PiFM images show the chemical distributions of cellulose (green) and lignin (magenta) on the surface of the sample. The cell wall appears to be mainly composed of cellulose with lignin in the central section.

Now, see what happens as smaller and smaller regions of this sample are scanned.

Dropping the scan region down to 1 micron shows that there are some patches of cellulose in what looked like a lignin-only part of the cell wall.

Figure 4. An AFM topography image and a combined PiFM image taken on a 1-micron square region of the spruce wood cell walls. It appears there is some mixing of the materials not easily seen in the larger image.
Figure 5. This animated image shows the topography visualized in 3D. The PiFM composite image is overlaid to demonstrate how PiFM can show new details in the chemical structure.

This mixing of the cellulose in the lignin seems interesting, so to investigate further, a much smaller region is scanned.

At only 150 nm square, the AFM topography shows nothing remarkable. However, the PiFM images reveal much more!

Figure 6. A 150 nm square region shows almost no detail in the topography. However, the PiFM chemical data reveals that the surface chemistry is surprisingly complex!

Because of the tip-enhanced electric field that makes PiFM possible, PiFM images will always be higher resolution than the AFM topography taken with the same tip (tip-enhanced field profile is smaller than the physical size of the tip apex). In fact, this sample demonstrates a chemical spatial resolution of only 2.5 nm! However, resolution alone is not what makes PiFM such a powerful technique.

Figure 7. The 3D topography is overlaid with two PiFM images. A line trace shows that the spatial resolution of the chemical data is less than 5 nm!

PiFM images show chemical information that can reveal hidden secrets on the surface of any sample. Surface chemistry and surface topography are sometimes correlated. However, on more complicated surfaces, like this spruce wood cell, the topography provides no information about what the surface is. In fact, this topography is noticeably unremarkable. Yet, the PiFM images show a fascinating and complex distribution of cellulose in this lignin matrix. No other analytical technique can provide insights like this.

Summary

PiFM chemical imaging is an extremely powerful technique. While AFM images can show surface topography, they provide no information about what that surface is. On some samples, the topography may be nearly featureless while the surface chemistry is complex and nuanced. PiFM chemical mapping is the only technique that can reveal surface chemistry at such high resolutions, making it ideal for a wide variety of applications demanding quantitative results at the nanoscale.

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