Polarized Surface-Enhanced Raman Spectroscopy from Molecules Adsorbed in Nano-Gaps Produced by Electromigration in Silver Nanowires

Uniform nanogaps were produced in silver nanowires previously fabricated by AC electrodeposition in porous anodic alumina templates and released by dissolving the oxide matrix. Configuring the nanowires with electrodes and passing current through them created nanogaps by electromigration. Uniform nanogaps approximately perpendicular to the axes of the nanowires were produced with widths in the range similar to 17 to similar to 40 nm. Gaps as small as 5 nm but with less uniform geometries could also be produced in this manner. A heuristic model, in which electromigrative and diffusional effects are included, is used to relate the size of the nanogap produced to the local resistance of the region in the nanowire where the gap ultimately forms. Intense surface-enhanced Raman spectra were observed from Rhodamine 6G adsorbed on the nanowire in the gap. For gaps that uniformly divided the nanowire across its width, the surface enhanced Raman spectroscopy intensity was maximum when the electric vector of the exciting light was oriented across the gap, as predicted by electromagnetic enhancement calculations.