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Dielectric sensing by charging energy modulation in a nano-granular metal

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.