Surface Plasmon Resonance Spectra of 2.8 ± 0.5 nm Diameter Copper Nanoparticles in Both Near and Far Fields

Cu nanoparticles of 2.8 ± 0.5 nm diameter were synthesized in the gas phase using a sputtering−aggregation-type source. High-density films of nanoparticles were deposited onto NaCl substrates, and strong interparticle coupling was observed. In the near-field spectrum, the surface plasmon absorption was more intense than, and shifted 20 nm to longer wavelengths from, that observed in the far-field spectrum. These are the first near-field spectra of such small Cu nanoparticles, and the results agree well with theoretical predictions.