Image Contrast in Lateral Force Mode (LFM)

       In LFM images, the brighter regions correspond to a higher frictional coefficient (for the forward scan direction). That is the first rule of thumb. One can often intuitively interpret LFM results based on this first rule. For the razor blade images, Example 1, we know that teflon is the ultimate non-stick material. Thus, it makes sense that the teflon is imaged darker in the LFM image because it is less "sticky" than the stainless steel, even though it is softer than metal.

       Conversely, in Example 2, the polymer coating on metal tubing has a higher coefficient of friciton and so is brighter in the LFM image. Given that these examples make sense, we can have confidence that LFM will give us information on unknown samples. In Example 3, a hair fiber is shown to have some hair-care products as residual deposits. There is no reason to know which component has a higher coefficient of friction - the hair or the hair product residue. The LFM image shows that the residue has a lower frictional response than the hair fiber, because the residue is darker.

       The main source of confusion in interpreting LFM images is separating topographic artifacts from real frictional information. The following schematic diagrams should help clarify how one can understand the source of topographic artifact in LFM images and how to distinguish this effect from useful frictional information. Figure (a) is a schematic of the photodiode detector and indicates lateral motion of the laser beam across the detector in response to cantilever torque, and Figure (b) shows the corresponding probe torque. This measurement of cantilever torque is the mechanism for sensing a change in lateral forces using a scanning probe in contact with the sample surface.

       Thus, it should be understood that image contrast is created with LFM whether the cantilever undergoes torque as the probe tip abruptly hits a topographic edge, or whether the cantilever torques in response to a relative change in surface frictional properties (which would make the tip undergo differential stick-slip motion with the surface).

       Figure (c) shows the relative probe tip orientation on different regions of a sample that has both a topographic up-step and a compositional (and frictional) change within the step, relative to outside the step. Figure (d) indicates the LFM line intensity profile created by the scanning probe in relation to the features in (c). Note, that the topographic step produces an asymmetric pattern of very high and very low signal intensity, whereas, the surface area inside the up-step has a uniformly higher intensity relative to the region outside the step. It is those regions of uniform contrast that represent useful frictional information. Figure (e) shows a topographic downstep and (f) shows the corresponding LFM line intensity pattern. In both cases (c) & (e) the probe response to the area within the topographic step is one of greater cantilever torque, and thus indicates a higher degree of surface friction between the tip and sample, relative to the area outside the step. In the line intensity profiles of (d) and (f) the topographic edge effects have a reversed intensity profile because the probe tip first hits an up-slope in (d), whereas, a downslope is first hit in (f).

       Figures (g) and (h) illustrate what is observed when a surface particle is imaged with LFM. This example shows very well how an asymmetric pattern dominates the image contrast. It is quite futile to try and extract any useful frictional information about this particle in relation to its surroundings, because the image contract results almost solely from the effects of topography. Similarly, any rough surface will be quite difficult to interpret with respect to frictional information because of the dominance of topographic edge-effects in the LFM image. Thus, the second rule of thumb: The flatter the sample the better the LFM data.

The images at right are of a polymer film on metal tubing (as seen on Example 2). There is a particle in the center of a pore, resting on the bare metal surface. The particle is a perfect example of LFM contrast that is dominated solely by topography. It is strikingly similar in appearance to the schematic diagrams in (g) and (h).