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The surface plasmon resonance peaks of gold nanostructures can be tuned from the visible to the near infrared region by controlling the shape and structure (solid vs. hollow). In this tutorial review we highlight this concept by comparing four typical examples: nanospheres, nanorods, nanoshells, and nanocages. A combination of this optical tunability with the inertness of gold makes gold nanostructures well suited for various biomedical applications.

Silver colloids of particle diameter 40−60 nm have been synthesized using a chemical reduction method in aqueous medium. These nanoclusters are photoactive and exhibit transient bleaching in the 400−500 nm region followed by a strong absorption in the visible−near-infrared region when subjected to 355 nm laser-pulse excitation. The transient bleaching of the surface plasmon absorption band is a monophotonic process, while the absorption growth in the red region is a biphotonic process arising from the photoejection of electrons. The 40−60 nm clusters were observed to break up into smaller clusters (5−20 nm) with 355 nm laser-pulse excitation. The choice of excitation wavelength provides the size selectivity in the fragmentation of the clusters. For example, when the excitation wavelength was switched to 532 nm, only larger (or irregularly shaped) particles were found to break up.

Electron-phonon coupling in 11 ( 2 nm diameter Au particles and 10 ( 3 nm and 50 ( 10 nm Ag particles has been examined by ultrafast pump-probe spectroscopy. The observed relaxation times are strongly dependent on the pump laser power. At the lowest pump powers used, the time constants for relaxation are 0.8 ( 0.1 ps for the 11 nm Au particles, 1.1 ( 0.1 ps for the 10 nm Ag particles, and 1.0 ( 0.1 ps for the 50 nm Ag particles. The measured relaxation times are similar to those for bulk metals, which implies that there are no size-dependent effects in the dynamics for particles in this size region. The transient absorption/ bleach recovery signals for the particles were modeled using the theory developed by Rosei et al. (Surf. Sci. 1973, 37, 689). These calculations yield the transient absorption spectrum as a function of the temperature of the electron distribution. The time dependence of the electronic temperature after pump laser excitation was calculated using the two-temperature model for electron-phonon coupling. The experimental signal versus time traces at selected wavelengths were then simulated by combining the two calculations. The results from the simulations are in semiquantitative agreement with the experimental results. In particular, the low-power relaxation times are correctly predicted by the model calculations. At very high pump laser power (>5 mJ/cm 2 ) the transient bleach signal for Ag shows an unusual 10 ps growth. This growth is attributed to either a change in the dielectric constant of the surrounding medium due to heat transfer from the particles or thermally induced dissociation of adsorbed molecules.

This article provides a review of our recent Rayleigh scattering measurements on single metal nanoparticles. Two different systems will be discussed in detail: gold nanorods with lengths between 30 and 80 nm, and widths between 8 and 30 nm; and hollow gold-silver nanocubes (termed nanoboxes or nanocages depending on their exact morphology) with edge lengths between 100 and 160 nm, and wall thicknesses of the order of 10 nm. The goal of this work is to understand how the linewidth of the localized surface plasmon resonance depends on the size, shape, and environment of the nanoparticles. Specifically, the relative contributions from bulk dephasing, electron-surface scattering, and radiation damping (energy loss via coupling to the radiation field) have been determined by examining particles with different dimensions. This separation is possible because the magnitude of the radiation damping effect is proportional to the particle volume, whereas, the electron-surface scattering contribution is inversely proportional to the dimensions. For the nanorods, radiation damping is the dominant effect for thick rods (widths greater than 20 nm), while electron-surface scattering is dominant for thin rods (widths less than 10 nm). Rods with widths in between these limits have narrow resonances-approaching the value determined by the bulk contribution. For nanoboxes and nanocages, both radiation damping and electron-surface scattering are significant at all sizes. This is because these materials have thin walls, but large edge lengths and, therefore, relatively large volumes. The effect of the environment on the localized surface plasmon resonance has also been studied for nanoboxes. Increasing the dielectric constant of the surroundings causes a red-shift and an increase in the linewidth of the plasmon band. The increase in linewidth is attributed to enhanced radiation damping.