Unexplored territory in the AFM force curve contains nanomechanics information

Abstract

We demonstrate the existence of a previously unknown damped oscillating signal just after the point when an atomic force microscope tip hits a sample surface. This oscillating signal is below the noise in a single force-displacement measurement. Autocorrelating 20 measurements using the snap to contact feature as the reference mark allows the oscillation to be clearly visible above the noise. We show that the amplitude of the signal’s oscillation is largely insensitive to the speed with which the sample is brought toward the tip proving that the impulse that generates the signal comes primarily from the snap-to-contact event. This speed-independence sets a lower limit on how softly a sample may be interrogated when measuring mechanical properties in the surface region. Collection and analysis of this damped oscillating signal eliminates the need for standard low bandwidth lock-in based techniques to determine time dependent surface mechanical properties. This allows conventional atomic force microscopes to make a single pass of force collection over a surface and, after post-processing, yield the full time dependent mechanical behavior of the surface. To demonstrate a practical use of the oscillations, we produce images of a polystyrene/polyethylene sample where the contrast mechanisms are stiffness and viscosity.

An Inquiry-Based Introduction to Atomic Force Microscopy Techniques through Optical Storage Disc Surface Imaging

“…This is the second segment of the retrace line. The cantilever may oscillate after leaving the sample surface (inset 6 in Figure A and Figure B) that shows up as sinusoidal (ringing) in the F–t and F–d curves (segment 6 in green curve in Figure B) …”

“…The cantilever may oscillate after leaving the sample surface (inset 6 in Figure 2A and Figure 2B) that shows up as sinusoidal (ringing) in the F−t and F−d curves (segment 6 in green curve in Figure 2B). 37 During the generation of force curves, snap-in occurred as the tip transitioned from the air to the surface of the sample and snap-out occurred as the tip transitioned from the surface of the sample to the air. 38,39 The snap-in contact is observed from segment 1 to segment 2, whereas snap-out is observed in segment 5 to segment 6 (Figure 2A and Figure 2B).…”