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

Th-1020
Adhesion-induced energy dissipation and atom-tracked tip changes
S. Kawai, Th. Glatzel, S. Koch, B. Such, A. Baratoff, and E. Meyer
Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel Switzerland
We have simultaneously measured the frequency shift Δf and the interaction-induced
dissipated energy per cycle Ets above a maximum in a topographic image of KBr(001).
The measurements were performed at room temperature in UHV using the 2nd flexural
mode of a silicon cantilever (1039369 Hz) at 11 constant amplitudes A between 12.8 and
0.51 nm. As shown in fig. 1, Ets rises smoothly above the noise level, then almost
saturates as the closest approach distance extends below the attractive force minimum.
Whereas the conservative force vs. distance curves extracted from Δf were smooth and
coincided apart from noise, Ets decreased smoothly by only a factor of 2 and became less
noisy as A was reduced from 12.8 to 0.51 nm. This behavior, and the magnitude of Ets
suggest that the dominant cause of dissipation is a force hysteresis loop narrower than
twice A, but significantly smeared by thermal activation. However, some velocitydependent
damping with a coefficient decaying with increasing distance is also present.
Using A = 0.285 nm, one thousand approach-retraction curves were recorded, separated
by intervals for atom-tracking (Nanonis AT4). In most cases one of three distinct Ets vs.
distance curves are obtained, whereas the conservative force curves are almost the same,
as shown in fig. 1 (a-c). This is consistent with the higher sensitivity of Ets to the tip apex
configuration. In a few cases we observed jumps between curves corresponding to two of
those long-lived configurations, accompanied by significant deviations between approach
and retraction. As shown in fig. 2, the tracked 3D motion of the sample relative to the tip
at Δf = -150 Hz exhibits strong correlations between more frequent jumps (strongly
reduced by averaging over each tracking interval). This suggests that tip changes occur
over a range of time scales. Further implications of those results will be discussed.
Figure 1 Figure 2
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