Shih et al.
integration into superlattice structures or other devices will almost certainly require control over their surface chemistry. Of all the synthetic approaches that have been developed to date for metal nanostructures, only a few are available that allow for shape control. On the basis of the means employed to achieve shape control, they can be divided roughly into two categories: The first is template synthesis (7), which involves deposition of metal nanostructure in a manner which prevents them from forming the thermodynamically favored spherical geometry. Widely used and less expensive template methods employ a porous host membrane such as anodic alumina (8â€“11) or polycarbonate (12). Metals deposited into the pores of these materials assume the pore geometry and orientation. More expensive and less widely available to most laboratories are the lithographic mask-based methods, which allow for the preparation of 2D structures on surfaces (13). The second method does not employ a preexisting template or mask but relies on the thermodynamically favored structures that result from the interaction of the metal particlesâ€™ surface and some stabilizing reagent. For example, El-Sayed and co-workers have used polyacrylate stabilizers to produce tetrahedral and cubic platinum nanoparticles (14,15). A simple unique electrochemical method for preparing large quantities of Au nanorods suspended in aqueous solution (16,17) has been recently developed. This method utilizes mixed cationic surfactants and offers the advantage of convenient control over the particlesâ€™ dimensions. Several key ingredients and experimental parameters used in this method are discussed. Perhaps the only drawback at the present time is that the particle growth mechanism and its relation to the dynamics of the surfactant micelle structures are not fully understood. Nonetheless, as we intend to demonstrate, this method works well from the standpoint of synthesis. The surface modification of Au nanorods to form silica coatings is attractive from at least two perspectives. First, silica-coated Au nanorods represent a model system for insulated nanowires. Second, coating the Au particle with silica opens up a wide range of surface modification chemistries that were not available for bare gold. In this chapter, we also discuss the experimental scheme for coating Au nanorods with a silica layer of desired thickness. Henceforth, we shall refer to silica-coated Au nanorods as Au nanorod@silica. Before proceeding to the synthesis section, a word about the optical properties of metal nanorods is in order. For particles whose dimensions are small compared to the incident wavelength, the classical electrostatic model has been shown to be reasonably successful in predicting the absorption cross sections of metal nanostructures, where the surface plasmon (SP) resonances are the main spectral features (17). In the case of Au nanorods, the dominant SP band corresponds to the long-axis, or longitudinal, component (henceforth referred to as SPl), while the transverse, or short-axis resonance, SPt, is comparatively weak. We will outline electrostatic models suitable for the two experimental systems considered in
Chemistry of metal nanoparticles