Chemistry Reference
In-Depth Information
12.7
Other Type of Imaging
12.7.1
Phase-contrast Imaging
Tapping mode AFM (also known as amplitude-modulation AFM (AM-AFM)) has the
capability of imaging compositional variations in heterogeneous surfaces in addition
to surface topography [31
-
33]. The phase difference between the excitation signal and
the cantilever oscillation is affected by several surface properties. With energy-
conservative tip-sample interaction, the resonant frequency shifts by approximately
0.5kf
c
/k
c
. The frequency shift results in a phase shift as the excitation frequency is
fixed. For a given frequency shift, the phase shift increases withQ. With conventional
cantilevers, the frequency shift is generally around 50Hz. Therefore, phase-contrast
imaging had been possible only with a large Q (hence only at a low imaging-rate).
Because the ratio f
c
/k
c
with themost advanced small cantilevers is
1000 times larger
than that with conventional cantilevers, we can expect a large shift of
50 kHz.
Therefore, even with a small Q, a relatively large phase shift occurs. Thus, we do not
need to detect the phase shift using very sensitive yet slow phase detectors such as
lock-in ampli
ers. In order to explore the possibility of high-speed phase-contrast
imaging, a fast phase detector was developed by Uchihashi et al. [34] based on a
previous design [35]. This can detect phase shifts within a single oscillation cycle and
importantly at any time within a cycle. Images of styrene-butadiene-styrene block
copolymer
lms having different contrasts were obtained in liquids depending on the
detection timing within the oscillation cycle. Remarkably, this phase imaging
was carried out at
80ms/frame (Figure 12.6) [34]. This new technique for
Figure 12.6 High-speed phase-contrast imaging. (a) Topography
image (left) and phase contrast image (right) of a commercial test
sample captured simultaneously at an imaging rate of 78ms/
frame. The nature of the sample is unknown. Scale bar, 100 nm.
(b) Phase contrast image of GroEL attached to amica surface in an
end-up orientation. Scale bar, 10 nm; imaging rate, 0.5 s/frame.
