Biomedical Engineering Reference
In-Depth Information
Fig. 6.7. Effect of probe or probe side-wall imaging. Top: illustrations of the effect of imaging a
spike - an image of the probe is produced - and imaging a sphere-like feature - only at the top is the
sample topography reproduced, and the rest of the image feature shows the probe's side-walls.
Bottom: examples of probe-side-wall imaging. Left: red blood cells, right:
S. aureus
bacteria. In
both cases, only the upper parts of the cells can be imaged correctly (some examples of probe side-
wall imaging highlighted by arrows).
wall appear instead (see Figure 6.7) [381, 382]. Samples with spike-like features (includ-
ing certain tip-check samples, see Appendix A) lead to repeated copies of the tip in the
resulting images [383].
Typically, any image showing square pyramid-shaped features will be showing images
of the probe rather than true sample features, so these image features can be discounted. In
order to avoid this problem, the user is recommended to use a shaper tip, specifically, one
with a higher aspect ratio. Silicon nitride contact-mode probes are very prone to producing
images of the probe side-wall, as they typically have much wider opening angles (
ca
.
35-40
for most intermittent contact-mode probes). If this artefact causes
real problems, for example in metrology applications, super-high-aspect-ratio probes are
also available (for example, with opening angles
8
versus 15-20
8
) [377]. Example images of such
probes are shown in Figure 2.30. However, for spherical samples such as nanoparticles or
the cocci shown in Figure 6.7, parts of the sample will always be unavailable to most AFM
experiments. Imaging of probe side-walls will tend to increase if there is a mismatch
between the angle of the probe and the sample, as described in the previous section.
<
3
8
As described in Chapter 2, there are a number of different scanner designs available for
commercial AFMs. However, by far the most common design in use is the piezoelectric
tube scanner. This scanner is used because it is easy to integrate into the instrument, cheap
