Tools for the Characterization of Biomass at the Nanometer Scale - The Nanoscience and Technology of Renewable Biomaterials - page 72

Interpretation of high-resolution SPM images requires a thorough understanding of
probe/specimen interactions. Quantitative modeling of the contrast mechanism is encour-
aged. The use of other microscopy techniques along with SPM is beneficial in two ways:
(1) examination at larger scale will establish a context for high-resolution studies and
help to select representative fields and (2) other imaging methods create different types
of artifacts and so can be used to confirm observations.
Recent reviews of scanning probe microscopies abound (85, 86). One excellent
and comprehensive review by an SPM pioneer offers a good place to begin learning
of the promise and pitfalls of SPM (87). This review begins with scanning tunnel-
ing microscopy ( STM ), follows its evolution, and concludes with the measurement of
mechanical properties utilizing nanoindentation methods.

3.4.2.1 Atomic Force Microscopy

Atomic force microscopy ( AFM ) is easy to use and is the most frequently used SPM
method for describing molecular solids or biological specimens. In its original mode,
contact AFM, the stylus tip was maintained in contact (or near contact) with the specimen
as in a miniature profilometer. To minimize specimen damage, most current work uses
tapping mode, where the stylus and supporting cantilever are set into vibration near their
resonant bending frequency (nominally 100 kHz).

In tapping mode, the AFM tip makes only intermittent contact with the specimen
surface, but the tip/specimen interactions alter the amplitude, resonance frequency, and
phase angle of the oscillating cantilever. Amplitude modulation mode ( AM-AFM )isan
excellent mode for specimens in air or liquids. In AM-AFM, the oscillation frequency
of the tip is kept constant, while the amplitude of vibration reveals topography and the
phase shift between driving force and oscillation reveals interaction forces dependent on
specimen viscoelastic properties (primarily stiffness) and adhesion between the tip and
specimen. The phase information in AFM has been used to measure material properties
of specimens such as elastic modulus, hardness, and material boundaries (88). A good
understanding of the mechanisms involved is important because the interpretation of
phase information is not always clear (87, 89). An exhaustive review of dynamic AFM
including theory and operation is available (90).
Interactions between the AFM tip and specimen arise from many different mechanisms,
including van der Waals attraction, electrostatic, friction, viscoelastic, and wetting forces.
Which of these forces are dominant is not always clear. One frequent effect that is
not always anticipated is the condensation of water on the specimen surface about the
tip. This results in a capillary force large enough to dominate the probe/specimen
interaction. Purging the sample chamber with nitrogen gas diminishes this condensation
effect enough to image most samples but does not eliminate it. The most frequent practice
for high-resolution studies is to work in ultrahigh vacuum (usual for atomic solids) or
under fluids (usual for biological specimens). Static charges on tip or specimen can be
avoided by placing an ionization source (an alpha particle emitter, for example) near the
specimen.
A review of the 20-year history of developments in atomic force microscopy along
with the physics involved is recommended (91). This work also views scanning tunneling
microscopy, subatomic imaging, atom manipulation, and future projections. An excellent

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