Probe damage evaluation in frequency-modulation shear-force acoustic near-field microscopy

Shear-force acoustic near-field microscopy (SANM) and Whispering Gallery Acoustic Sensing have recently been introduced as a tandem system to characterize the viscoelastic response of fluids confined between two solid-boundaries in relative oscillatory lateral motion. SANM uses a) a laterally oscillating tapered probe (attached to a quartz tuning fork QTF) as one of the trapping boundaries, and b) an acoustic sensor (attached to the other flat-substrate boundary) that independently monitors the fluid's acoustic emission. On the other and, WGAS is another technique that uses an acoustic transducer (attached to the frame holding the probe) to monitor the probe's lateral motion amplitude. Steps towards improving the standing of the SANM/WGAS system as a nanometrology tool include i) to integrate frequency modulation methods, to thus be able to discriminate the elastic from the inelastic component of the probe-fluid interactions, and ii) evaluate the integrity and robustness of the probe to endure such surface interactions, since eventual deformation of the probe affect the reproducibility of the measurements. Here we report using the WGAS signal (or alternatively the QTF electrical response) to control the probe's motion in frequency modulation modality. In addition, systematic evaluation of eventual probe damage (acquiring scanning electron micrograph before and after the probe exerts one approach/retraction trip to/from the substrate) are also presented. The non-monotonic behavior of the fluid's acoustic emission (first increasing and then decreasing) as the probe approaches the sample—which occurs before the probe touches the surface, as revealed by the lack of damaged observed on the probe—constitutes one of the main findings reported here. The acoustic signal provides clues on the fluid's dynamic response when subjected to nanoscale confinement. (paper)