  Several sensing concepts using nanostructures prepared by focused electron
beam induced deposition have been developed over the last years. Following work
on highly miniaturized Hall sensors for magnetic sensing with soft magnetic Co,
strain and force sensing based on nano-granular platinum-carbon structures
(Pt(C)) was introduced. Very recently the capability of nano-granular Pt(C)
structures to detect the presence of adsorbate water layers by conductance
modulations was demonstrated. For magnetic and strain sensing the underlying
physical mechanisms of the sensing effect have been analyzed in detail and are
now quite well understood. This is not the case for the adsorbate layer induced
conductance modulation effect. Here we provide a theoretical framework that
allows for a semi-quantitative understanding of the observed water-sensing
effect. We show how the near-interface renormalization of the Coulomb charging
energy in the nano-granular metal caused by the dielectric screening of the
polarizable adsorbate layer leads to a conductance modulation. The model can
account for the conductance modulation observed in the water adsorbate
experiments and can also be applied to understand similar effects caused by
near-interface dielectric anomalies of ferroelectric thin films on top of
nano-granular Pt(C). Our findings provide a pathway towards optimized
nano-granular layer structures suitable for a wide range of dielectric or local
potential sensing applications.
