Radiofrequency impedance variation of characterized tip–sample nanocontacts in shear force microscopy with vertically oriented cantilevers connected to a vector network analyser

The authors have recently presented in a note the technical working principles of a new method for the simultaneous acquisition of sample topography, shear forces and RF spectra at the nanoscale. The novel method, based on a shear force microscope (ShFM) that works with a vertically oriented cantilever (VOC) connected to a vector network analyser, uses the nano-apex of the VOC at its free end as a sensing element for both the shear forces and the radiofrequency signals. Here, in the present work, we report on a detailed investigation of the tip-sample shear nanocontact by measuring and characterizing the variation of the RF complex impedance at the tip-to-sample interface in the RF range 100 kHz to 8.5 GHz. The experiments were performed on atomically flat graphite substrates to avoid topographical effects. The different types of nanocontact conditions were: (i) measurement of RF complex impedance in static conditions (constant probe-to-sample distance), indicating that during ShFM observations the probe behaves like a capacitive non-contact probe, at least for frequency <100 MHz; and (ii) dynamic measurement of impedance during cyclic approach and withdrawal of the probe at different cycling frequencies. In this case, a switch-like behaviour related to a drastic change of the impedance at extremely short probe-to-sample distances was observed. The RF behaviour of different types of cantilevers is also reported.