Biomedical Engineering Reference
In-Depth Information
Fig. 3.4. Illustration of how the photodetector detects vertical and horizontal bending of the
cantilever.
convention AFM images are presented with the fast scan axis data appearing horizontally,
no matter in which direction the AFM tip was scanned. So there will be forward and
backward height, vertical deflection and lateral deflection images available, meaning up to
six data channels might be recorded. If the instrument is equipped with a
z
axis calibration
sensor, both
z
voltage and
z
sensor channels might be available, raising the total to eight
channels. Typically, while the probe scans over each line in both directions, only one
direction will be saved. This is because the height data in the two directions should be
identical. The vertical deflection images should be the same on parts of equal slope, and
give opposite contrast on regions of changing slope, but the information available from the
data collected in the two directions is effectively the same. So for most channels, there is
rarely any need to save the data collected in both directions, although it is sometimes
useful to observe both forward and backward height data while optimizing scanning
conditions, as discussed in Section 4.2. Lateral deflection data from both directions is
sometimes saved, to help understand frictional properties of the sample, which is discussed
in Section 3.2.3.1.
Applicability
Contact-mode AFM has a really wide range of potential applications and some of these
are described in Chapter 7. However, it is possible to summarize some general cases
where contact mode is likely to be chosen in preference to other techniques. Probably
the best reason to use contact mode is its high resolution. Some dynamic modes can also
achieve extremely high resolution, but compared to, for example, intermittent-contact
mode, the resolution of contact mode is potentially extremely high. What keeps it from
being used more widely is that the applied normal force leads to a high lateral force
applied to the sample as well. In the case of weakly adsorbed samples, or soft, easily
deformed samples, this can lead to problems of sample distortion, damage, or even
removal from the substrate [97, 98]. Because of this, it has been suggested that contact-
mode AFM is no good for soft samples. This is not the case, as there are many reports of
soft biological samples, even in a hydrated state, being successfully imaged by contact-
mode AFM (for examples, see [99-103]). Often, contact mode is chosen to image such
delicate structures when sub-molecular resolution is required [99, 100, 102, 104]. What
