Interconnected Dimers with Sub-10 nm Nanogaps by Atomic Layer Deposition for Plasmonic Nanojunctions

Plasmonic materials exhibit localized plasmon resonances that collect light and concentrate electric fields around nanostructures. The field enhancements are useful for applications such as spectroscopy, catalysis, and photodetection. Electric field enhancements are dependent on the materials, geometric designs, and positioning of nanostructures. Plasmonic dimers are especially effective to concentrate electric fields between nanostructures, and there is significant interest to develop nanofabrication strategies to control interparticle distances with subnanometer precision for arbitrary designs. In this work, we investigate arrays of interconnected plasmonic dimers with sub-10 nm nanogaps achieved by Cu area-selective atomic layer deposition (ALD). Nanostructures are made by conventional nanofabrication methods and subsequently coated with conformal layers of Cu to control the interparticle distances. Au and Pd layers are used to activate Cu deposition for homodimer and heterodimer combinations with and without interconnects. Optical extinction measurements before and after growth experiments show how plasmon resonances change when Cu layers expand nanostructures and reduce nanogaps formed between dimer pairs. Finite difference time domain simulations are used to model experiments and study how modifications of nanostructure sizes, thicknesses, and shapes affect plasmonic properties. Our findings show that Cu ALD can reduce nanogaps below 10 nm for both interconnected and unconnected nanorods and that interconnected plasmonic dimers have optical properties similar to unconnected dimers. Moreover, the ALD process can scale to large arrays of nanostructures. Results are promising for achieving subnanometer control of interparticle distances for devices that combine electrical and optical functions with intense electric fields generated by localized surface plasmon resonances.