Are artefacts in scanning near-field optical microscopy related to the misuse of shear force?

It is becoming clear that a significant proportion of scanning near-field optical microscopy (SNOM) images are artefacts caused by adjusting the tip–sample separation when scanning. An example that occurred with shear-force regulation is given in this work and it is shown that constant-height imaging provides a partial solution to the problem. The problem could be intrinsic to closed-loop imaging or occur through using shear-force regulation incorrectly. An operational protocol is presented which shows that for maximum sensitivity, shear forces should be measured off resonance. Simultaneous tunnelling measurements have been used to demonstrate that with this protocol, shear-force measurements can provide non-contact operation in the case of differential interferometer-based systems. It has been observed that differences in substrate material produce significant phase changes in the probe oscillation. Here, the phase shift would have resulted in the probe–sample separation being approximately 7 Å smaller when the probe resided over aluminium islands on the test specimen. Whether this error is responsible for the observed optical artefacts is still being determined.