Noncontact Atomic Force Microscopy - Yale School of Engineering ...

Noncontact observation in liquid with van der Pol-type FM-AFM
M. Kuroda 1 , H. Yabuno 2 , T. Someya 3 , R. Kokawa 4 , and M. Ohta 4
1 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
2 Department of Mechanical Engineering, Keio University, Yokohama, Japan
3 Mitsubishi Heavy Industries Ltd., Hiroshima, Japan
4 Analytical & Measuring Instruments Division, Shimadzu Corp., Kyoto, Japan
Atomic force microscopy (AFM) is crucial for nanobiotechnology
studies. Observing bio-related samples
in liquids using AFM is important, but deformable,
uneven, and easily damaged surfaces of such
specimens require non-contact AFM observation.
For probe-cantilever excitation, the eigenfrequency
must be estimated based on frequency response
characteristics for the external excitation method. It
cannot estimate the probe cantilever’s eigenfrequency
precisely because of many spurious peaks (Fig. 1). But,
the self-excitation method needs fine adjustment of the
linear feedback gain near the oscillation limit by an
automatic gain controller (AGC) to prevent the probe
cantilever from touching the sample surface:
oscillation can easily halt, disabling the observation.
In solving those problems, van der Pol-type (vdP)
self-excited oscillation represents a positive use of
nonlinearity [1–2]. Along with conventional positive
linear velocity feedback for self-excited oscillation,
vdP self-excited oscillation is realized by adding
nonlinear feedback proportional to the squared
deflection times the velocity. With the eigenfrequency
component alone (Fig. 2), vdP self-excited oscillation
guarantees existence of a stable stationary amplitude.
Increased nonlinear feedback gain reduces the response
amplitude but retains the high gain linear feedback.
A 19-nm-high SiN4 surface pattern with 3 μm pitch
was observed in pure water using vdP-AFM (Fig. 3).
The smaller probe-cantilever vibration amplitude than
the probe-cantilever – sample gap verifies noncontact
observation. The vdP-AFM takes sample surface
images with equal accuracy to that of contact-mode
AFM. Even in liquid, oscillation continues.
[1] H. Yabuno et al. Nonlinear Dynamics 54, 137 (2008).
[2] M. Kuroda et al. Journal of System Design and Dynamics 2-3, 886 (2008).
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P.II-17
Figure 1: Frequency response
curve in pure water (External
excitation method)
f=54.4 kHz
Figure 2: Power spectrum in pure
water (vdP self-excited oscillation
method)
Figure 3: Sample observed in pure
water by vdP FM-AFM