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O: Fachverband Oberflächenphysik

O 35: Poster Session I (Metal, semiconductor and oxide substrates: structure and adsorbates; Graphene)

O 35.23: Poster

Dienstag, 12. März 2013, 18:15–21:45, Poster B1

A measurement of the hysteresis loop in force-spectroscopy curves using a tuning-fork atomic force microscope — •Merlin Schmuck, Manfred Lange, Dennis van Voerden, and Rolf Möller — Faculty of Physics, Center of Nanointegration Duisburg-Essen, University of Duisburg- Essen, 47048 Duisburg, Germany

The Analysis of the frequency shift versus distance in noncontact atomic force microscopy (NC-AFM) allows to measure the force gradient between the oscillating tip and a surface (force-spectroscopy measurements). When nonconservative forces act between the tip apex and the surface the oscillation amplitude is damped. The dissipation is caused by bistabilities in the potential energy surface of the tip-sample system, and the process can often be understood as a hysteresis of forces between approach and retraction of the tip. We present the direct measurement of the whole hysteresis loop in force-spectroscopy curves at 77 K on the PTCDA/Ag/Si(111) sqrt(3) x sqrt(3) surface by means of a tuning-fork-based NC-AFM with an oscillation amplitude smaller than the distance range of the hysteresis loop. The hysteresis effect is caused by the making and breaking of a bond between PTCDA molecules on the surface and a PTCDA molecule at the tip. The corresponding energy loss was determined to 0.57 eV by evaluation of the force-distance curves upon approach and retraction. Furthermore, a second dissipation process was identified through the damping of the oscillation while the molecule on the tip is almost in contact with the surface. It reaches a maximum value of about 0.22 eV/cycle.