The wavelength corresponding to the extinction maximum, λ max , of the localized surface plasmon resonance (LSPR) of silver nanoparticle arrays fabricated by nanosphere lithography (NSL) can be systematically tuned from ∼400 nm to 6000 nm. Such spectral manipulation was achieved by using (1) precise lithographic control of nanoparticle size, height, and shape, and (2) dielectric encapsulation of the nanoparticles in SiO x . These results demonstrate an unprecedented level of wavelength agility in nanoparticle optical response throughout the visible, near-infrared, and mid-infrared regions of the electromagnetic spectrum. It will also be shown that this level of wavelength tunability is accompanied with the preservation of narrow LSPR bandwidths (fwhm), Γ. Additionally, two other surprising LSPR optical properties were discovered: (1) the extinction maximum shifts by 2-6 nm per 1 nm variation in nanoparticle width or height, and (2) the LSPR oscillator strength is equivalent to that of atomic silver in gas or liquid phases. Furthermore, it will be shown that encapsulation of the nanoparticles in thin films of SiO x causes the LSPR λ max to red shift by 4 nm per nm of SiO x film thickness. The size, shape, and dielectric-dependent nanoparticle optical properties reported here are likely to have significant impact in several applications including but not limited to the following: surfaceenhanced spectroscopy, single-molecule spectroscopy, near-field optical microscopy, nanoscopic object manipulation, chemical/biological sensing, information processing, data storage, and energy transport in integrated optical devices.

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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.

In this paper we measure the optical extinction spectrum of a periodic array of silver nanoparticles fabricated by nanosphere lithography (NSL) and present detailed comparisons of the results with predictions of electrodynamic theory. The silver nanoparticles are small (∼100 nm) compared to the wavelength of light but too large to have their optical properties described adequately with a simple electrostatic model. We make use of the discrete dipole approximation (DDA), which is a coupled finite element method. With the DDA one can calculate the extinction of light as a function of wavelength for particles of arbitrary size and shape. We show that NSL-fabricated Ag nanoparticles can be modeled without adjustable parameters as truncated tetrahedrons, taking their size and shape parameters directly from atomic force microscopy (AFM) measurements and using literature values of the bulk dielectric constants of silver. These AFM measurements are presented as part of this paper, and the resulting theoretical line shapes and peak widths based on the AFM-derived parameters are in good agreement with measured extinction spectra. The peak width measured as the full width at half-maximum (fwhm) is approximately 100 nm, or 0.35 eV, which corresponds to an electron−hole pair lifetime of 2 fs. The combined effects of particle−particle and particle−substrate interactions red-shift the surface plasmon resonance by only about 10 nm versus a single isolated particle. By use of AFM-derived parameters that have been corrected for tip-broadening and by inclusion of an estimate for the effects of particle−particle and particle−substrate interaction, the discrepancy between the theoretical and experimental extinction peak maxima is approximately 25 nm, which is significantly smaller than the plasmon width. This residual difference between theory and experiment is due to shortcomings of the truncated tetrahedron geometry in describing the actual shape of the particles, errors in the literature values of the bulk dielectric constants, and experimental uncertainty due to slight heterogeneities in nanoparticle structure.

A study of the surface-enhanced infrared absorption (SEIRA) spectroscopy of para-nitrobenzoic acid (PNBA) adsorbed on thermally evaporated silver films has been conducted to determine the effect of film architecture on the magnitude of the SEIRA enhancement. Ordered arrays of uniformly sized silver nanoparticles, termed periodic particle arrays (PPAs), were prepared on several different infrared transparent substrates (germanium, silicon, and mica) by nanosphere lithography (NSL). It was found that the ordered arrays deposited by NSL produced well-defined and intense surface plasmon resonance (SPR) bands in the infrared at frequencies between 1500 and 4000 cm−1. The peak frequency of these infrared SPR bands depended on the array architecture and the substrate material. By appropriate design of the nanoparticle array, the infrared SPR band can be made to be coincident with the SEIRA sensitive infrared bands of the PNBA. The trends in the infrared SPR peak frequencies and band shapes were consistent with predictions from electrodynamic theory. The SEIRA responses per unit area of deposited metal obtained with the PPA-type films were at best comparable to results obtained with disordered silver and gold films deposited on the same substrate materials by thermal evaporation (i.e., in the absence of any NSL masking spheres). The results of this study are most consistent with theories and models that attribute SEIRA to the dielectric constant and optical extinction spectrum of the metal film.