Quadrupolar plasmon resonance in arrays composed of small-sized Ag nanoparticles prepared by a dewetting method

Quadrupolar plasmon resonances that originate from interparticle near-field coupling interaction were observed in arrays composed of small-sized Ag nanoparticles (∼30 nm). The arrays were prepared by a dewetting process, during which hexagonally arranged aluminum pits were used to induce and confine the nucleation and growth of Ag nanoparticles. With the increase in the thickness of the initial Ag film, the particle size increases, and the interparticle spacing decreases to sub-10 nm scale, making adjacent particles strongly coupled. As a result, the quadrupolar resonance mode occurs, and its wavelength remains practically unchanged, while the dipolar mode shifts toward to a longer wavelength by about 80 nm. In comparison, only a dipolar resonance mode was observed in the non-coupled array that has a large interparticle spacing. The coupled array was demonstrated to exhibit much stronger surface enhanced Raman scattering (SERS) activity than that of the non-coupled array, suggesting that the quadrupolar mode can also be viewed as an indication of strong SERS enhancement in arrays composed of small-sized metal nanoparticles.