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

Fr-0940
Determination of the Optimum Spring Constant and Oscillation
Amplitude for Atomic/Molecular-Resolution FM-AFM
Yoshihiro Hosokawa 1 , Kei Kobayashi 2 , Hirofumi Yamada 1 and Kazumi Matsushige 1
1 Department of Electronic Science and Engineering, Kyoto University, Kyoto, Japan
2 Innovative Collaboration Center, Kyoto University, Kyoto, Japan.
Several studies have shown that the lateral resolution of FM-AFM on Si(111)-7x7 surface
can be improved by oscillating a force sensor with a very high spring constant (> 1,000
N/m) at a very small amplitude (< 1 nm) [1,2]. However, the resolution of FM-AFM on
an organic thin film was not improved by the use of a cantilever with a spring constant of
about 700 N/m [3]. Here we propose a general procedure to determine the optimum
imaging parameters (spring constant and oscillation amplitude) for obtaining atomic or
molecular resolution by FM-AFM.
We first determine the minimum distance between the tip and the sample, which is
limited by the instability caused by jump-into-contact the sample or by the dissipative tipsample
interaction forces. Then we defined effective signal intensity for atom/molecularresolution
FM-AFM as the difference between the frequency shift on top of the
atom/molecule and on the gap between the atoms/molecules at the minimum distance.
Figure 1 shows a plot of the calculated signal-to-noise ratio on lead-phthalocyanine thin
films on MoS2. The optimum spring constant and oscillation amplitude were 10 N/m and
25 nm, respectively. Figure 2(a) is a preliminary experimental result using 7.4 N/m
cantilever. Line profiles obtained from Fig. 2(a) (red, bold) and from the image (black,
thin) using 40 N/m cantilever [3] are shown in Fig. 2(b). From Fig.2(b), the corrugation
was larger and consequently signal-to-noise ratio was improved when we used a
cantilever with a smaller spring constant and a larger oscillation amplitude.
Figure 1: Signal-to-noise ratio for
molecular-resolution FM-AFM on PbPc with
various spring constant and oscillation
amplitude. The optimum spring constant and
oscillation amplitude is 10 N/m and 25 nm.
[1] F. J. Giessibl et al., Appl. Surf. Sci., 140 (199) 352.
[2] S. Kawai et al., Appl. Phys. Let., 86 (2005) 193107.
[3] Y. Hosokawa et al., Jpn. J. Appl. Phys., 47(2008) 6125.
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Figure 2: (a) Topographic
image of PbPc
obtained with using small spring constant
cantilever (7.4 N/m) at large amplitude (15
nm). (b) Line profiles obtained from (a)
(red, bold) and from the image obtained
using 40 N/m cantilever