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Potential distribution from the surface influences the physical and chemical properties. Dynamic force microscopy (DFM) has been demonstrated detections of the interaction force gradients in the vertical direction and represented surface topography with atomic resolution.[1] Although the force gradient field also distributes in the plane, detections of the lateral force have been limited at contact region. Recently, attempts to detect the lateral force gradient between the single atoms at non-contact region were carried out.[2, 3] However, the large peak-to-peak dithering amplitudes of 6 Å or 2 nm, and intently or accidentally given tilt angles from the parallel to the surface confused us to interpret of the observed images.
Lateral force gradient field down to 0.01 N/m over the Si(111)-(7x7) reconstructed surface will be precisely represented with our homebuilt DFM, operating at room temperature.[4] We used the torsional resonance of the silicon cantilever to detect lateral force gradients. Its high mechanical quality factor above 10,000 and high resonance frequency of 2.2 MHz improved the detection limit, and its high stiffness enabled ultra-small amplitude operation down to 70 pm for direct force gradient detection. The tip-sample distance was regulated with a constant time-averaged tunneling current. On the adatom site, repulsive lateral force gradient was detected, and observed as oval shapes with the long diagonal perpendicular to the dithering direction of the tip. At the corner hole, the most attractive lateral force was found to be caused. The experimentally detected distribution of the lateral force gradient field shows the origin of the stick-slip motion. The contribution of the vertical interaction force due to the misalignment of the dithering tip was found to be large. When the vertical contribution was negligible, dissipative interaction was not observed at the distance with the bias voltage of 1.8 V and the tunneling current of 1.0 nA. The theoretical calculations were in good agreement with the experiment.
[1] F. J. Giessibl, Science 267, 68 (1995).
[2] F. J. Giessibl, et al., PNAS. 99, 12006 (2002).
[3]S. Kawai, et al., APL. 88, 133103 (2005).
[4]S. Kawai, et al., RSI. 76, 083703 (2005).
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