Surface Plasmon Spectra of Silver and Gold Nanoparticle Assemblies
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This work is a brief topical review of recent literature reports of the localized surface plasmon spectra of silver and gold nanoparticles (NPs) in various assemblies. The desire for such a review arises from the numerous recent reports of NP assemblies, their optical properties, and the importance of their potential applications. The optical properties can be viewed from the perspective of isolated particles, particles coupled by near-field light interactions (roughly when the interparticle separation is less than particle size), and those coupled by far-field interactions (for well separated particles in arrays or gratings). The NPs reviewed are sized from ca. 20–500 nm in diameter or major axis (except for wires), are of various shapes, e.g., spheres, triangles, cylinders, etc., and are arranged by various means including synthetic molecular linking architectures. The preparation of interesting NP systems builds, in part, on relatively recent NP synthetic strategies that yield NPs of uniform size and often organized 1-, 2- and 3-D NP assemblies. Methods for making NPs fall into two broadly defined categories: those based on physical metal deposition and those based on solution chemistry, the latter including electrodeposition. Physical metal deposition methods encompass electron beam lithography (EBL), nanosphere- and colloidal lithography, metal island films, and cosputtered composite materials. Powerful EBL methods can produce arbitrary NP shapes and arrangements, and sizes of about 50 nm and larger are in the literature. Nanosphere lithography, pioneered by Van Duyne et al. (vide infra), yields arrays of triangular and related particles in the 90–2500 nm range. Simple metal island films, e.g., from 2 to 3 nm mass thickness Au and Ag evaporated directly onto nonwetting oxide substrates, can yield reasonable NP distributions, as can cosputter-deposited composites of metal and oxide matrix (e.g., Ag and SiO2). Chemical NP preparations resulting in uniform size distributions (e.g., < 10% variability) have been the subject of extensive recent development and are predominantly from two methods: 1) solution phase synthesis (often with a fractional precipitation step) and 2) templating methods such as electrodeposition into nanoporous Al2O3 membranes.