ABSTRACT
On a theoretical point of view, the pioneering experiments by Faraday have been interpreted later by Mie (1908) who solved Maxwell’s equations of a metallic sphere in a homogeneous
24.1 Introduction ...........................................................................................................................24-1 24.2 ›eoretical Description ........................................................................................................24-2
Nanoparticle Synthesis • Techniques of Optical Spectroscopy • Experimental Results 24.4 Conclusion and Outlooks ...................................................................................................24-21 Acknowledgments ..........................................................................................................................24-21 References .........................................................................................................................................24-22
medium submitted to an external electromagnetic œeld (plane wave). Mie showed that the original properties of metallic nanoparticles are a consequence of the “dielectric conœnement” (limited volume of the material) of particles whose sizes are smaller or of the same order as the wavelength of the excitation œeld. At the same time, Maxwell-Garnett developed an e˜ective œeld model (Maxwell-Garnett 1904) in order to describe the optical properties of a medium containing “minute metal spheres.” ›e main feature in the optical extinction spectra of small metallic clusters is the emergence of a giant resonance in the near UV-visible range, called surface plasmon resonance (SPR) that is related to the collective motion of the conduction electrons induced by the applied œeld. ›is resonance is clearly noticeable only in the case of simple metal (alkali, trivalent) and noble metal (gold, silver, and copper) clusters. Its spectral position and width depend on the morphology of the particles (size, shape, and internal structure for alloyed systems), but also on their dielectric environment (medium in which the particles are embedded, local neighborhood) (Kreibig and Vollmer 1995). ›e development of numerous cluster sources (Sattler et al. 1980, Smalley 1983, Milani and de Heer 1990, Siekmann et al. 1991) in the 1980s enabled to probe the intrinsic properties of the clusters of very small size for which quantum size e˜ects were expected. From a fundamental point of view, alkali clusters constitute a perfect model owing to their simple electronic structure and have been widely studied in the gas phase (Pedersen et al. 1991, Blanc et al. 1992, Bréchignac et al. 1992, de Heer 1993). However, they are immediately oxidized in contact with air once deposited on a surface, and thus are not suitable for applications. In spite of their more complex electronic structure, noble metal clusters in solution or embedded in a transparent matrix are more promising for potential applications because they are more robust toward oxidization. By varying the morphology, structure, or environment of these clusters, their optical response may be more or less controlled, making them attractive in several areas (linear and nonlinear optics [Kreibig and Genzel 1985], nanomaterials, nano-photonics, plasmonics, biosensors [Raschke et al. 2003]). Conversely, as the optical response is closely linked with the electronic structure, the SPR can also be used as a probe of the structure of metallic clusters, especially bimetallic systems or nanoalloys. Furthermore, the exaltation of the electromagnetic œeld in the vicinity of the particle can be exploited to increase the coupling of molecules with light, for developing biological markers for instance (McFarland and Duyne 2003).
