Formation and disappearance of a nanoscale silver cluster realized by solid electrochemical reaction

We have developed a nanostructuring method using the solid electrochemical reaction induced by a scanning tunneling microscope (STM). This method has some distinctive features that have not previously been obtained by conventional nanostructuring STM methods. The formation and disappearance of the nanostructure are reversible, and the rates can be controlled using STM. These features are realized via a local oxidation/reduction reaction of mobile metal ions in an ionic/electronic mixed conductor. In this study, a crystal of silver sulfide $({\mathrm{Ag}}_2\mathrm{S}\mathrm{),}$ a mixed conductor, was used as the material for the STM tip. A nanoscale Ag cluster was formed at the apex of the ${\mathrm{Ag}}_2\mathrm{S}$ tip when a negative bias voltage was applied to the sample. The Ag ions in the ${\mathrm{Ag}}_2\mathrm{S}$ tip are reduced to Ag atoms by the tunneling electrons from the sample, and the Ag cluster is formed by the precipitation of the Ag atoms at the apex of the tip. The Ag cluster shrank gradually and disappeared when the polarity of the sample bias voltage was switched to positive. Ag atoms in the Ag cluster are oxidized to Ag ions, and the Ag ions redissolve into the ${\mathrm{Ag}}_2\mathrm{S}$ tip. The formation and disappearance rates of the cluster were controlled by regulating the tunneling current.