One of the major challenges in scattering-type near-field microscopy is the proper signal assignment. That involves in particular the unique distinction between the true near-field process, the far-field background, and topographic and other effects. The signals emerging from the spatially localized optical tip-sample interaction and the other scattering and emission channels are in most cases indistinguishable a priori . This gives rise to a variety of possible imaging artefacts where the apparent contrast, e.g., in the simplest case, is due to interference effects as a result of topographic height variations. Yet, even for planar surfaces several artefacts are possible, with 3D spatial variations in the dielectric function of the sample material being the most prominent one.

Careful spectroscopic measurements, variations of tip-sample distance,  and control experiments with suitable reference samples can help to identify the signal sources. For details please see our corresponding publications and those of other groups on that subject.

A practical challenge concerns the preparation of suitable tips that "work", i.e., provide sufficient field enhancement and spatial localization for high resolution and sensitive tip-scattered near-field experiments. While a sharp tip apex is of highest importance, a consistent taper and smooth surface are also correlated with sensitivity in near field experiments. Through an electrochemical etching procedure, we can reproducibly fabricate Au tips with suitable features and apex radii of < 20 nm.

This is accomplished by the electrochemical dissolution of Au wire in a 1:1 solution of concentrated HCl:Ethanol. When a potential of ~2.2 V is applied to a gold wire immersed in such a solution with respect to a Pt ring electrode, etching occurs at an increased rate near the meniscus of the solution. The etching will continue in this region until the lower portion detaches. The remaining portion of the wire is then used as a scanning probe tip. However, if the voltage continues to be applied to the Au wire, etching will continue, resulting in blunted tips. In order to prevent this, we use a fast cut-off circuit to remove the applied potential upon the detachment of the lower wire.

The details of the etching process are complex with several parameters influencing the details of the resulting tip geometry. We have been able to refine the technique by monitoring the current during the etching procedure. Periodic current oscillations are observed during this procedure due to the cyclic depletion and replenishing of the local electrolyte concentration at the wire. A good periodicity of the current oscillations is strongly indicative  of a consistent taper as well as smooth surface. Furthermore, this is suitable for the determination of an optimum etching voltage.