Biomedical Engineering Reference
In-Depth Information
Surface approaches the tip
Snap to the
surface
Photodiode
detector
Laser
Adhesion to
the surface
Distance
Surface and tip are out
of the interactive range
Piezo driver
moves the specimen
under computer control
Fig. 3.1.4-16 Schematic diagram illustrating the principle of the atomic force microscope.
The potential of the AFM to explore surface problems
has been greatly expanded by ingenious variants of the
technique. In fact, the term ''atomic force microscopy''
has been generalized to ''scanning probe microscopy
(SPM).'' Table 3.1.4-6 lists many of these creative ap-
plications of the AFM/STM idea.
Since the AFM measures force, it can be used with
both conductive and nonconductive specimens. Force
must be applied to bend a cantilever, so AFM is subject to
artifacts caused by damage to fragile structures on the
surface. Both AFM and STM can function well for
specimens under water, in air, or under vacuum. For
exploring biomolecules or mobile organic surfaces, the
''pushing around'' of structures by the tip is a significant
concern. This surface artifact can be capitalized upon to
write and fabricate surface structures at the nanometer
scale ( Fig. 3.1.4-17 )( Boland et al., 1998; Quate, 1997;
Wilson et al. , 2001 ).
Newer methods
There are many other surface characterization methods
that have the potential to become important in future
years. Some of these are listed in Table 3.1.4-7 . A few of
these evolving techniques that will be specifically men-
tioned here include SFG Raman, and synchrotron
methods.
SFG uses two high-intensity, pulsed laser beams, one in
the visible range (frequency ¼ u visible ) and one in the in-
frared (frequency ¼ u ir ), to illuminate a specimen. The
light emitted from the specimen by a non-linear optical
process, u sum ¼ u visible þ u ir , is detected and quantified
( Fig. 3.1.4-18 ). The intensity of the light at u sum is pro-
portional to the square of the sample's second-order non-
linear susceptibility ( c (2) ). The termsusceptibility refers to
the effect of the light field strength on the molecular po-
larizability. The u sum light
intensity vanishes when
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