Force-distance studies with piezoelectric tuning forks below 4.2 K

Piezoelectric quartz tuning forks have been employed as the force sensor in a dynamic mode scanning force microscope operating at temperatures down to 1.7 K at He-gas pressures of typically 5 mbar. An electrochemically etched tungsten tip glued to one of the tuning fork prongs acts as the local forc e sensor. Its oscillation amplitude can be tuned between a few angstroms an d tens of nanometers. Quality factors of up to 120 000 allow a very accurat e measurement of small frequency shifts. Three calibration procedures are c ompared which allow the determination of the proportionality constant betwe en frequency shift and local force gradient based on the harmonic oscillato r model and on electrostatic forces. The calibrated sensor is then used for a study of the interaction between the tip and a highly oriented pyrolytic graphite (HOPG) substrate. Force gradient and dissipated power can be reco rded simultaneously. It is found that during approaching the tip to the sam ple considerable power starts to be dissipated although the force gradient is still negative, i.e., the tip is still in the attractive regime. This ob servation concurs with experiments with true atomic resolution, which seem to require the same tip-sample separation. (C) 2000 Elsevier Science B.V. A ll rights reserved.