Off resonance ac mode force spectroscopy and imaging with an atomic force microscope

Using an off resonance ac technique in ultrahigh vacuum we have directly me asured the force-gradient interaction characteristics of a gold tip and sam ple and demonstrated a new atomic force microscope imaging mode with the ti p located very close to the surface. The method involves the application of a small sinusoidal oscillating force to the tip via a magnetic field creat ed by a conducting coil which interacts with a magnetic particle glued on t he backside of the cantilever. By measuring the change in amplitude during the approach and retraction of the sample we have a continuous and accurate measure of the force gradient. The interaction potential is thus found wit hout the need for complex analysis as is necessary in the case of the commo nly used technique of measuring frequency shifts. (C) 1999 Elsevier Science B.V. All rights reserved.