Nanosphere Lithography:  Effect of the External Dielectric Medium on the Surface Plasmon Resonance Spectrum of a Periodic Array of Silver Nanoparticles

In this paper we examine the effect of solvent on the optical extinction spectrum of periodic arrays of surface-confined silver nanoparticles fabricated by nanosphere lithography (NSL). By use of NSL, it is possible to systematically vary the out-of-plane height of the nanoparticles, and by thermal annealing, we can control the nanoparticle shape. We have studied four separate samples of nanoparticle arrays; three samples have nanoparticles that are truncated tetrahedral in shape but that differ in out-of-plane height and one sample has nanoparticles that are oblate ellipsoidal in shape. By performing UV−vis extinction spectroscopy measurements at 12 μm spatial resolution, we show that the defect sites that occur as a byproduct of the NSL fabrication process play a negligible role in the macroscale extinction spectrum. We find that the extinction spectrum of the nanoparticles that are oblate ellipsoidal in shape is least sensitive to the surrounding dielectric medium, and the extinction spectrum of the nanoparticles that are truncated tetrahedral in shape with the smallest out-of-plane height is most sensitive. A 1 nm shift in the extinction maximum corresponds to a 0.005 change in the refractive index of the external medium. Theoretical calculations based on the discrete dipole approximation (DDA) are presented. The DDA is a coupled finite element method capable of calculating the extinction of light for particles of arbitrary shape and size. The discrepancy between the experimental and theoretical results is small for the oblate ellipsoidal-shaped particle but progressively increases for the truncated tetrahedral-shaped particles as they become more oblate. This discrepancy is lessened by including the effect of substrate−particle interactions in the calculation. The DDA theory predicts a significantly larger red shift in the extinction maximum with increasing solvent refractive index than is observed experimentally.