Noncontact Atomic Force Microscopy - Yale School of Engineering ...

Relation between lateral forces and dissipation in FM-AFM
Michael Klocke, Dietrich E. Wolf
Department of Physics, University of Duisburg-Essen, Germany.
P.I-32
We study the coupling of torsional and normal cantilever oscillations and their effect on
the imaging process of a frequency-modulated atomic force microscope by means of
molecular dynamics simulations. We show that the bending and torsional modes of the
cantilever are coupled if the tip is near the surface and connect this coupling to the
damping of the cantilever oscillation. The strength of the coupling is determined roughly
by the strength of the lateral forces on the closest approach of the tip. Energy is
transferred from the normal to the torsional excitation which can be detected as damping
of the cantilever oscillation. Energy is actually dissipated by the usually uncontrolled
mechanical damping of the torsional excitation. For high Q factors, the transferred energy
is not completely dissipated during one cycle. The question, what happens to the
remaining energy of the lateral degree of freedom in the long run, is addressed by
studying a simplified two-dimensional point-mass model. We show that in succeeding
cycles, energy is transferred back into the normal degree of freedom. The observation of
the energy swapping process (amplitude and frequency) can therefore give additional
information of the surface structure, especially on lateral forces.
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