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

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Atomic scale elasticity mapping of Ge(001) surface by multifrequency
FM-AFM
Yoshitaka Naitoh, Zongmin Ma, Yanjun Li, Masami Kageshima and Yasuhiro Sugawara
Department of Applied Physics, Osaka University, Suita 565-0871, JAPAN
Surface elasticity is quite important to study atomic scale properties of the surface
such as bounding strength of surface atoms and surface phonons. The conventional FM-
AFM provides topographic information of various surfaces at atomic scale. However, it is
hardly accessible to the elasticity and the adhesion on surfaces. In this study, we propose
a new technique, multifrequency FM-AFM, on Ge(001) surface using a commercial
cantilever in order to investigate the surface elasticity at atomic scale with the
topographic image.
The cantilever of the multifrequency FM-AFM was simultaneously excited at the
by two sets of automatic gain
1st and the 2nd flexural resonant frequencies (f1st, f2nd)
controller and phase locked loop electronics. Their oscillating amplitudes are set constant
at A1st, A2nd. The cantilever oscillating signal, detected by an optical interferometer
system, is divided into the 1st and the 2nd components through band pass filters. The
surface topography is obtained from feedback signal maintaining the frequency shift of
the 1st component (Δf1st) constant. From the theoretical consideration, we found that the
surface elasticity is obtained as a mapping of the frequency shift of the 2nd component
(Δf2nd).
The cleaned Ge(001) surface showing buckled dimer structure was adopted as a
sample with the elastic distribution at atomic scale. Figures 1(a) and (b) show the
simultaneously obtained topography and the Δf2nd mapping of the surface. The dimer
structure of the surface was clearly resolved in both images. We found Δf2nd of the fig.
1(b) was high at the dimer atom position in comparison with the topography. This result
suggests multifrequency FM-AFM is a nicer tool for exploring the surface elasticity at
atomic scale.
Figure 1: (a) Topographic image of Ge(001) surface with Δf1st=-70Hz, f1st=160kHz and
A1st=4nm. Scan area is 7x7 nm 2 . (b) Simultaneously obtained Δf2nd mapping of the surface area
with Δf2nd=-21Hz, f2nd=1.0MHz and A2nd=0.3nm.
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