Tuning nanorod surface plasmon resonances

Size reduction in metallic materials has lead to the discovery of novel and exciting optical properties, chiefly related to the localized surface plasmon resonance of conduction electrons. The preparation of such reduced-size metals no longer relies on traditional top-down (lithographic) methods. For instance, bottom-up processes, mostly based on the wet-chemical methods of colloid science, have become increasingly popular over the past decade. Colloidal techniques have the advantage that they can be tailored to obtain particles with different shapes while controlling particle size, which in turn affects the optical response of the material. Nanoparticle shape control often depends on the presence of a specific surfactant or polymer, but the specific role of these organic molecules is not necessarily the same in all cases.

For example, while uniform gold spheres can be grown using sodium citrate as a reducing and stabilizing agent, the one-dimensional growth of nanorods typically also requires the presence of preformed seeds and that of a cationic surfactant, with the aspect ratio depending on synthesis conditions. If, on the other hand, small Au seeds are grown in N, N-dimethylformamide both under sonication, and in the presence of a standard polymer such as poly-vinylpyrrolidone, monodisperse pentagonal bipyramids can be obtained, with tightly controlled particle sizes. Representative transmission electron microscopy images of nanoparticles with the above mentioned morphologies are shown in Figure 1 with their corresponding ultraviolet-visible spectra in Figure 2.

Figure 1. Transmission electron microscope images of metal nanoparticles with various geometries.

Figure 2. Ultraviolet-visible spectra of gold nanoparticle colloids with various geometries.

Among the various available shapes, anisometric nanoparticles have generated significant interest, since they can display various resonance conditions as a function of orientation. This results in an anisotropic response towards incoming light, which allows the further modulation of optical effects through alignment. Such anisotropy is extreme in the case of nanorods, which typically display longitudinal and transverse plasmon resonances (electron oscillations along or across their long axis, respectively). When the rods are aligned in the same direction, such resonances can be independently excited, as shown in Figure 3.

Figure 3. Photograph (top) and absorption spectra (bottom) of polymer films containing oriented gold nanorods, illuminated with non-polarized light (black line), and with parallel (blue) and perpendicular polarisation (red).

In this example, the rods were aligned by stretching a polyvinyl alcohol film. Upon illumination under polarized light, the color of the composite film could easily be tuned through rotation with respect to the polarization direction, resulting in complete damping of the non-excited resonance mode. Plasmon resonances can also be tuned by preparing larger particles. This initially results in retardation, but leads to a red-shift and broadening of the dipolar plasmon band that eventually allows the observation of higher order modes, such as quadrupolar resonances. Apart from the spectral effects, the quadrupolar modes can be observed through calculation of maps for the near-field enhancement in the vicinity of the particles, which is one of the most striking features of localized surface plasmons (see Figure 4).

Figure 4. Near-field enhancement of spherical gold nanoparticles in water (diameters 12 and 180nm) when illuminated at their corresponding plasmon wavelengths. The polarization and illumination directions are shown by the arrows.

In summary, colloid chemistry methods represent a powerful and versatile tool for the fabrication of gold nanoparticles with well-defined sizes and shapes, as they allow the simple manipulation of their localized surface-plasmon resonances.

Contributions by Dr. Isabel Pastoriza-Santos, Dr. Jorge Pérez-Juste, Prof. Paul Mulvaney, Dr. F. Javier García de Abajo, Jessica Rodríguez-Fernández and Ana Sánchez-Iglesias are gratefully acknowledged. This work was mainly funded by the Spanish Ministerio de Educación y Ciencia (project MAT2004-02991).