Capillary condensation is a phenomenon that refers to the spontaneous condensation of a liquid from a vapour into a highly constricted space. For example, into porous media or into the tip–sample gap of an STM and thereby forming a cylindrical meniscus, bridging the tip and sample.

Researchers from Queen's University Belfast have shown that the light emission spectrum from a scanning tunnelling microscope acts as a highly sensitive means of characterizing the dielectric properties of the medium in the tip–sample gap, on a volume scale ranging from sub-zepto to atto-litre (<10–21 to 10–18 l). By measuring the intensity decay and spectral shift in the emitted light with increasing relative humidity, the group has been able to track the capillary condensation of water in the gap on the (sub-)nanometre scale.

The team explains its results through the development of an empirical model that successfully describes the changes in emission as due to progressive water filling of the gap or, more pertinently, in optical terms, of the localized surface plasmon modal volume. Asymmetry in the results with respect to polarity of the applied bias is due to the polar nature of the water molecules. This observation is corroborated by the absence of such asymmetry when a medium of non-polar molecules occupies the gap.

While water bridging of the gap is of fundamental importance in all forms of ambient scanning probe microscopy (SPM), the results presented by the group are of particular relevance to apertureless near-field scanning probe microscopy and tip-enhanced Raman scattering. The distinctive nature of this approach, relative to other SPM studies of the capillary condensation of water in the tip–sample gap, lies in the fact that the physical entity (the emitted light) used to probe and characterize the gap medium is not used as the feedback parameter for instrument control.

The researchers presented their work in Nanotechnology.