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These two sets of images compare phase and friction
contrast. The purpose of this experiment was to establish a correlation
between the material/chemical contrast seen in phase imaging with
the contrast seen in friction imaging, an older technique. In
order to make the comparison, we had to change the probe tip (from
a Si tip used for Tapping Mode/phase imaging to a SiN tip used
for contact mode/friction imaging). So, these images also demonstrate
our capability of re-positioning the probe to the exact same spot
on the sample, within 100 nm. (See "How to relocate&..." below.)
Contaminants on glass.
This material is the residue of fingerprint
contamination on a glass slide. We first wiped the slide with
a clean tissue, applied a fingerprint and wiped it again. Some
residue was visible in the optical microscope, so we scanned the
AFM at that spot.
In contact mode, the friction image (right) showed
two prominent areas of high friction (bright) and one area of
low friction (dark). In the height image (left) there was little
topography corresponding to the high friction areas, but there
was a raised area corresponding to the low friction area.
In Tapping Mode images of the same area, the high friction spots appeared as raised bumps (bright) in the height image (left) and as dark areas (low phase) in the phase image. The low friction area appeared bright in the phase image, with considerable fine structure not seen in contact mode. We interpret these observations as follows:
The high friction/low phase areas are microdroplets of viscous oil. The low friction/high phase areas are solid crystallites, which in some cases are floating on the oil.
In contact mode, the probe tip slides on the solid surface, here the glass substrate.
In Tapping Mode, the probe tip can track the liquid-air
interface of the oil droplets, since the viscoelastic properties
of the oil make it stiffer at high frequencies. Shear forces present
in contact mode, but absent in Tapping Mode, account for the poorer
resolution of the contact mode image.
Wood pulp fiber. The
Tapping Mode phase image (left) shows a bright (high phase) region.
The contact mode friction image (right) shows that the friction
is low (dark) on the same area. Note that the fine structure (cellulose
microfibrils) was more easily seen in the phase image.
How to relocate a given spot:
the key is planning. The general concept of finding
the same spot on a sample is as follows. Create or note landmarks
at a series of length scales. This would include AFM images of
1-10 micron fields of view as well as optical images of 100-1000
micron fields. Using the mechanical stage controls of the Dimension
3000, we note the positions of various optical landmarks within
10 microns. We also note the rotational orientation of the specimen
within 1-2 degrees. In addition, it is necessary to take account
of the shift in probe tip position, by aligning the image relative
to the video crosshair during the 'focus-tip' operation. With
sufficient information, as described above, we have found it possible
to find and re-examine the same spot on a specimen after a 1-month
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