Takaharu OKAJIMA and Shunsuke HIROTSU

Laboratory for Biophysics, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, 226-8501 Japan

(Received July 1, 1998; Accepted July 14, 1998)

The shear force between a glass probe and a mica surface has been investigated as a function of the relative humidity, H, and the lateral spring constant of the probe, K. It was found that the interaction length D₀ decreases with increasing H and exhibits a sharp drop around H= 40%. With increase in K from 5 to 40 N/m, D₀ gradually increases, although this feature was absent when a probe with a softer tip-end was used. The latter result indicates that the shear force in an atmospheric condition is not a remote force but results from some contact between the tip and the surface. Our results that D₀ is independent of the oscillating amplitude and that the resonance curve of the probe is almost symmetric except in close vicinity to the surface are not in accord with the force model proposed recently, i.e., the knocking mechanism. It is proposed that the probe can vibrate even if the probe touches the surface, and that the resonance frequency increases steeply as the contact tightens. Theoretical estimation of the contribution of noncontact forces is also described.

Key words : near-field optics, shear-force microscopy, tip-surface interaction, relative humidity, lateral spring constant

tokajima@bio.titech.ac.jp