  Light emission spectrum from a scanning tunnelling microscope (LESTM) is
investigated as a function of relative humidity and shown to be a novel and
sensitive means for probing the growth and properties of a water meniscus in
the nm-scale. An empirical model of the light emission process is formulated
and applied successfully to replicate the decay in light intensity and spectral
changes observed with increasing relative humidity. The modelling indicates a
progressive water filling of the tip-sample junction with increasing humidity
or, more pertinently, of the volume of the localized surface plasmons
responsible for light emission; it also accounts for the effect of asymmetry in
structuring of the water molecules with respect to polarity of the applied
bias. This is juxtaposed with the case of a non-polar liquid in the tip-sample
nano cavity where no polarity dependence of the light emission is observed. In
contrast to the discrete detection of the presence/absence of water bridge in
other scanning probe experiments by measurement of the feedback parameter for
instrument control LESTM offers a means of continuously monitoring the
development of the water bridge with sub-nm sensitivity. The results are
relevant to applications such as dip-pen nanolithography and electrochemical
scanning probe microscopy.
