Dynamics of Atomic Scale Metallic Junctions and Adsorbed Lewis Base Adsorbates Studied by Fluctuation Spectroscopy

Atomic-scale junctions (ASJs) are the ultimate thin nanowires that exhibit great potential as chemical (Lewis bases) sensors through adsorbate-induced conductance changes. The goal of this work was to fabricate robust ASJs for sensing applications that fully utilize this potential and detect chemical noise. Au-Ag-Au bimetallic ASJs were formed with high yield using a novel fabrication scheme that creates initially overgrown junctions followed by controllable thinning processes. Ag deposition was galvanically triggered to initiate junction formation across specially prepared Au electrodes in the presence of aqueous Ag(I). The process was then self-terminated through the agency of an external resistor. Junction thinning could be performed via three different approaches: self-dissolution at low Ag(I) concentration, current-induced electromigration, or potential-induced oxidation. The fabricated ASJs showed robustness, enabling pyridine sensing studies. The fluctuation of surface population at equilibrium was reflected in the noise in steady-state electrical signals which cannot be observed directly. Fluctuation spectroscopy was used to isolate and study this chemical noise. The combination of ASJs with fluctuation spectroscopy constitutes a powerful method of studying adsorption-desorption kinetics at high sensitivity and may lead to quantification of single molecules on atomic surfaces by purely electrical measurements.