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

Abstract

In this paper we examine the effect of solvent on the optical extinction spectrum of periodic arrays of surfaceconfined 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.

Controlled Evaluation of Silver Nanoparticle Dissolution Using Atomic Force Microscopy

“…31,32 These investigations found that AgNP release is largely due to mechanical stress, but the addition of bleaches can also enhance the release of Ag + into the wash water because of greater oxidation potential. Silver released from textiles during washing will typically be transported to a wastewater treatment plant (WWTP).…”

“…Full-scale and bench-scale studies have shown that silver was removed by about 90% in WWTPs, with a small but significant fraction, partially in the form of AgNPs, remaining in the WWTP effluent while the majority partitioned into the sludge. 31,33,34 Although most of the silver was removed, WWTP effluent still had silver concentrations several orders of magnitude higher than typical river water, and it is expected that AgNPs will be relatively concentrated in WWTP effluents. 29,30,34 The results from these studies also suggest that areas where sludge is land applied will have higher than average concentrations of AgNPs in the soil.…”

“…For example, truncated tetrahedral particles have been converted into hemispherical particles by electrochemical and thermal processing. 58,100 Alternatively, the mask itself can be etched to produce a variety of complex shapes and patterns. 94,101 Another advantage of NSL is that it is considered to be a materials-general lithographic technique.…”

“…27 NSL facilitates control over size, shape, and interparticle spacing for a number of different materials and substrates. [28][29][30][31] In this study, the dissolution rates of NSLproduced AgNPs were quantified by atomic force microscopy (AFM), which gave a detailed description of changes in particle size and shape as a function of time and NaCl concentration.…”

O efeito SERS na análise de traços: o papel das superfícies nanoestruturadas

“…Posteriormente, vários autores investigaram a influên-cia das condições de preparo destes filmes sobre os espectros SERS. Um substrato SERS foi desenvolvido por van Duyne et al 70,71 e construído por nanolitografia utilizando mascaramento com esferas poliméricas com diâmetro entre 300 e 800 nm. A formação de uma monocamada auto-organizada destas esferas sobre vidro, seguida da deposição de um filme metálico e posterior remoção do material polimérico por dissolução com tolueno, resultou na formação de ilhas metálicas correspondentes ao material que estava nos interstícios das esferas.…”