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

P.I-04
Improved atomic-scale contrast via bimodal dynamic force microscopy
S. Kawai, Th. Glatzel, S. Koch, B. Such, A. Baratoff, and E. Meyer
Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel Switzerland
We extend multi-frequency AFM to atomically resolved frequency-modulation
dynamic force microscopy and demonstrate a further improvement of spatial resolution in
ultra-high vacuum [1]. The first and second flexural resonance modes of a commercially
available Si cantilever are simultaneously self-excited with given amplitudes, and the
resonance frequency shifts (Δf1st and Δf2nd) are demodulated by two phase-locked loop
circuits (Nanonis: Dual-OC4). The combination of sub-angstrom amplitude oscillation
A2nd at the second resonance with the commonly used large amplitude oscillation A1st at
the first resonance enables higher resolution imaging at closer tip-sample distances while
avoiding atomic jump-to-contact instabilities, which are more likely at such distances
during small amplitude operation with a single mode [2-4].
Figure 1 shows a series of simultaneously recorded Δf1st and Δf2nd maps of KBr(001),
obtained with A1st=16 nm and A2nd=50 pm at decreasing tip-sample distances in the
attractive region, recorded in the quasi-constant height mode. Being more sensitive to the
short-range interaction, Δf2nd exhibits a stronger distance dependence and contrast than
Δf1st at short distances. Although A2nd was much smaller then the width of the measured
force minimum, the signal-to-noise in the Δf2nd map was higher than in the Δf1st map at
close tip-sample distances [(e) and (f)]. The enhanced sensitivity to the short-range
interaction could even reveal tip and/or sample deformations induced by the interaction
force.
Figure 1: A series of simultaneously recorded Δf1st and Δf2nd maps at decreasing tip-sample
distances (a – f). The first and second resonance frequencies are 154021 Hz and 960874 Hz, and
the Q factors and amplitudes are 31059 and 6246, and A1st=16 nm and A2nd=50 pm, respectively.
[1] S. Kawai et al., submitted.
[2] F. J. Giessibl et al., Science 289, 422 (2000).
[3] S. Kawai et al., Appl. Phys. Lett. 86, 193107 (2005).
[4] S. Kawai and H. Kawakatsu, Appl. Phys. Lett. 88, 133103 (2006).
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