Nanoparticle spectroscopy: Dipole coupling in two-dimensional arrays of L-shaped silver nanoparticles

The plasmon resonance was measured for two-dimensional arrays of L-shaped Ag nanoparticles fabricated by electron beam lithography. A variety of particle sizes were studied with nominal total edge lengths of ∼150 nm, 63 nm arm widths, and 30 nm height. The single nanoparticle localized surface plasmon resonance (LSPR) of the L particles had two polarized components, which independently coupled in the arrays to create plasmon resonances for the array. The arrays had peak resonance locations and bandwidths that were dependent on grid spacing and particle number in the grid. The array plasmon resonance had a minimum bandwidth of 700-800 cm ^-1 at a grid spacing ∼75 nm smaller than the grid having the largest red shift of the plasmon resonance. This bandwidth is about half the single nanoparticle resonance bandwidth. For arrays with small numbers of nanoparticles, the resonant wavelength and bandwidth had large deviations from the semi-infinite arrays but approached those results as the number of nanoparticles increased to 25 particles on an edge, which defines the range of effective dipole coupling for a 400 nm grid spacing. This observation is consistent with optical changes observed by scanning across a 300 × 300μm ² pad. A solvent effect on these arrays demonstrated a red shift with similar bandwidth effects and some small grating-induced features due to waveguide effects.