Shear force distance control in near-field optical microscopy: experimental evidence of the frictional probe-sample interaction

The properties of the probe-surface contact for a near-field optical micros cope driven in the shear force mode have been studied applying lateral ampl itudes of the probing fiber tip larger than 15 nm. Electric current measure ments between a conductive tip and a conductive sample reveal a pulsed curr ent behavior at the very beginning of the approach curve. Tn the upper part of the approach curve it turns to the quasiconstant current. From this obs ervation a conclusion is drawn about the presence of permanent mechanical c ontact between the probe and the surface in the shear force mode. A shift o f the approach curve along the z-axis as a function of dither amplitude was discovered. These results are in contradiction to the established concepti on of possible physical mechanisms of shear force interaction. To settle th is issue the friction model is proposed according to which the damping of t he probe vibrations is caused by the friction between the tip and the surfa ce. (C) 2000 Elsevier Science B.V. All rights reserved.