Bridging the Nanogap: Au Electrodeposition at Parallel Electrodes with NM-Scale Spacing

Electrodeposition of gold from a thiosulfate solution was used to bridge the gap between parallel electrodes prepared by etching a metal-insulator-metal (MIM) thin film stack into a pillar, with the separation of the electrodes being equal to the thickness of the insulator layer (typically 50 nm). The current between the two electrodes when different potentials are applied to each provides information on the morphology of the electrodeposited metal as the gap between the electrodes closes. We attribute the observation of clear steps in this current to conductance quantization due to the formation of atomic point contacts. By adding sulfite to the electrolyte, we can prepare nanogaps in which the current increases more smoothly than with thiosulfate alone. Further experiments suggest that the nanogap morphology can be controlled via the applied bias, with nanogaps grown at lower applied bias having significantly smoother edges. We also show that it is possible to use electrochemical cycling to tune the structure of a nanogap.