Shear force interaction in the viscous damping regime studied at 100 pN force resolution

We report a very good force resolution for tuning fork based shear force microscopy as used for feedback regulation in scanning near-field optical microcopy (NSOM). The sensitivity and dynamics of fiber tips attached to 100 kHz tuning forks are investigated both experimentally and theoretically applying a finite element analysis. Operating the tuning fork at vibration amplitudes smaller than 10 nm allows to discriminate between viscous damping due to capillary wetting, and fiber bending upon tip-sample approach to hydrophilic sample surfaces indicating the direct transition from “noncontact operation” (pure viscous damping due to contamination layer) down to tip-sample contact. Viscous damping manifests in frequency shifts of less than 50 mHz, as deduced from resonance curves recorded under feedback control. For relative amplitude changes of less than 0.5% the viscous damping force acting lateral on the fiber tip is calculated to ∼100 pN using the finite element method. This detection limit proves that tuning fork based shear force control is superior to other feedback mechanisms employed in NSOM.