Biomedical Engineering Reference
In-Depth Information
Fig. 4.1. The major steps involved in measuring AFM images. The oscillation frequency only needs
to be selected for non-contact or intermittent-contact AFM.
lead to further inconsistency in the images, and it is also possible to permanently
contaminate the tip, leading to strange artefacts in the images (see Section 6.1).
• The sample must be clean. Contamination in the form of particles or dried salts will
make the underlying structure very hard to discern. Salt layers in particular are hard to
discern optically, so that to the eyes the sample appears clean, but the salt layer will
prevent imaging of the sample by AFM completely. Most samples imaged in air
typically are coated with what in AFM is known as the 'contamination layer'. This
liquid layer can be a mixture of water and hydrocarbons. Depending on the method
used to image the sample, a light contamination layer (a few nanometres) may not
prevent imaging of the underlying surface (see Sections 3.1 and 3.2). A thick
(
50 nm) contamination layer can cause great difficult in imaging the underlying
sample. Any particulate contamination will be imaged along with the sample, and
complicate analysis. AFM tends to image
everything
on the sample, so it is important
to remove as much contamination as possible.
• The features on the sample surface sample must be small enough to scan. AFM is a
high-resolution technique, and most instruments are designed for small samples. The
very largest scan ranges are on the order of 150
>
m
150
min
x
and
y
, and 28
m
in
z
, but a more typical configuration is a maximum range of 100
m by 100
mor
less in
x
and y, and
z
limited to less than 10
m. This is the size of the largest scan that
