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Naturally occurring (cellulose)
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What is Phase Imaging?
Phase Imaging is a powerful tool that is sensitive to surface stiffness/softness and adhesion between the tip and surface. It allows chemical mapping of surfaces based on these material differences. It is particularly useful in analyzing polymers, composites, and surface coatings. NanoScope brand AFMs allow capturing both height (topography) and phase data simultaneously allowing surface structure and material domains to be directly compared.
How it works:
A sharp probe is brought into proximity with the specimen
surface. The probe is oscillated vertically near its mechanical resonance
frequency. As the probe lightly taps the surface, the amplitude of oscillation
is reduced and the AFM uses this change in amplitude in order to track
the surface topography. In addition to its amplitude, the probe motion
can be characterized by its phase relative to a driving oscillator. The
phase signal changes when the probe encounters regions of different composition.
Phase shifts are registered as bright and dark regions in phase images,
comparable to the way height changes are indicated in height images.
Phase shift over regions of different composition
Phase signal with cantilever at resonant frequency
Why Are Phase Images Useful?
Phase images often show extraordinary contrast for many
composite surfaces of technological and scientific interest. These include
contamination deposits, discontinuous (i.e. defective) thin films, devices
built of composite materials (e.g. magnetic recording heads), and cross-sectional
specimens of composite materials. Both inorganic and organic materials
can be examined. We have found that phase imaging is more convenient and
gentler than other methods which are based on contact mode operation. It
routinely achieves lateral resolution of 10 nm.
Note the strong contrast between domains in the above
image. While the topographic image shows some corresponding features, surface
roughness hinders the identification of domains. The phase image allows
unambiguous resolution of the different material phases. Field of view
= 1x1 um.
Wood pulp fiber
On this wood fiber, thin patches of lignin cover the cellulose
microfibrils. We have observed similar chemical contrast in a wide range
of material systems, involving both inorganic and organic phases, with
lateral resolution of 10 nm. Field of view = 3x3 um. This wood pulp fiber
was prepared as follows:
[Personal note: with apologies to Helen of Troy ("the face that launched a thousand ships"), DC calls this image "the phase that launched a thousand tips" because it stimulated so much further work, which continues to this day.]
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