Biomedical Engineering Reference
In-Depth Information
the raster-scanning as the probe is not in constant contact with the sample
surface. Therefore, it is no surprise that tapping mode is frequently used for
high-resolution imaging of delicate or loosely bound samples. Fine adjust-
ment on the imaging condition allows the resolution to be comparable to
that of contact mode.
116
Additional development also allows the tracking of
biomolecules at a relatively high speed, at the order of seconds.
117
Force modulation and phase imaging
Force modulation is a form of tapping mode imaging in that the tip oscil-
lates sinusoidally above the sample surface during raster-scanning.
11,118-121
However, the focus of the investigation here is mainly the viscoelastic prop-
erties of the sample. During the tip-sample contact, the loading force from
the tip would cause a small indentation to the sample surface. Since a soft
material can absorb more energy from the cantilever than a hard one, more
indentation is found on the former, whereas the opposite applies to the lat-
ter. While the oscillating frequency is basically unchanged, the extent of
indentation is proportional to the tip loading force and the compliance of
the sample contact area, which is manifested as amplitude modulation and
phase shift on the recording unit. The local viscoelastic properties can then
be estimated from the amplitude and phase changes. In amplitude modula-
tion and phase imaging, the frequency and amplitude of the tip oscillation
can be optimized to give a better contrast than height imaging on the sur-
face features.
119,121-125
Force imaging
Force imaging is made possible by taking advantage of the information col-
lected from force plot. As discussed before, the tip interaction with the sample
surface when plotted as tip defl ection with respect to the separation between
the tip and sample leads to information regarding the magnitude of the inter-
action force at a particular point of the surface (X,Y) and local viscoelas-
tic properties (see previous section, 'Force modulation and phase imaging').
When many of these force plots are collected over a two-dimensional (2D)
plane, a distribution of force is assembled which correlates to the topography
of the sample.
126,127
The terms force volume or force mapping are used due to
the 3D nature of this operating mode.
128,129
Electrochemical AFM
Electrochemical AFM (EC-AFM) measures the local electrochemi-
cal states of material at high resolution by raster-scanning a conductive
probe tip across two-dimensional substrates.
130-135
A simple setup involves
the use of a two-electrode confi guration to measure the current changes
