Electrochemical formation of quantum-conductance Cu-metal nanobridges

Abstract

The electrodeposition of copper with formation of narrow nanobridge contacts was investigated. Stable quantum nanobridges (QNB) with ballistic conductance have been obtained by pulsed Cu deposition/electroetching on Cr interdigitated microelectrodes nanolithographed on a quartz wafer. The minimum quantum conductance associated with monatomic-wide constriction in the nanobridge was observed. Stepped differential conductance G = ng 0 (where g 0 is the minimum quantum conductance and n = 1, 2, 3, ...) was generally observed although fractional multiples of go were also found, likely due to the deviations of the nanowire junction geometry from linear configuration and reflection losses. In the layer-by-layer Cu deposition/etching technique employed for junction formation, the current amplification associated with interdigitated electrode pattern was utilized, which allowed us for easy electrochemical control with no need for special instrumentation. Quantum mechanical calculations of the electronic structure of Cu-QNB have been carried out using small-cluster approach with configuration 14-m-14, where m denotes the number of Cu atoms in the monatomic-wide bridge, and the base electrodes are simulated by clusters with 14 Cu atoms each. The calculations performed for m = 1 to 4, indicate that corrugations of electron density along the conductance channel virtually disappear for m = 3, whereas for even number of Cu atoms in the bridge, a distinct, very narrow constriction is formed in the center of the bridge. These results corroborate earlier findings for alkali metals, for which considerable differences between odd and even number of atoms in a monatomic-wide conductance channel have been reported.