THE INFLUENCE OF HUMIDITY ON THE SHEAR FORCE BETWEEN TIP AND SAMPLE IN NSOM USING PIEZOELECTRIC FORK

The distance between tip and sample can be regulated using piezoelectric quartz fork glued with micro optic fiber probe. A biquadrate vibration equation for the fork–probe–sample system is established to theoretically analyze the relations of the electric current flow through fork versus tip–sample (T–S) distance (I–d). The I–d curve and the action distance for shear force are influenced by environmental humidity. The results reinforce the opinions in the earlier works by other researches that the physical origin of the shear force is due to a material filling the tip–sample gap. Furthermore, the intrinsic reasons for shear force damping between tip and sample have been confirmed and developed, i.e. the water and hydrocarbon coupled between optic fiber probe and sample due to the capillary cohesion force.