Ultrafast Dynamics of Metal Nanospheres and Nanorods

Abstract

The optical properties of metal nanoparticles have played a key role in the development of physical chemistry and nanotechnology. The existence of metal particles in solution was first recognized by Faraday in 1857, and a quantitative explanation of their colour was given by Mie in 1908. Mie’s theory allows the extinction spectra of spherical particles to be calculated from the dielectric constants of the metal and the surrounding medium. These calculations show that the distinctive colours of metal particle solutions arise from a collective dipolar oscillation of the conduction electrons, which is called the surface plasmon band. The position of the plasmon band depends on the identity of the metal and the surroundings and is also sensitive to the distance between particles. For example, the characteristic purple colour of flocculated Au sols arises from dipolar coupling between the plasmon oscillations of neighbouring particles. For Au, Ag and Cu the plasmon band falls in the visible region of the spectrum, and is responsible for the brilliant colours of solutions of these particles. For most other metals the plasmon band occurs in the UV, yielding solutions with a drab grey or brown colour. Mie’s theory is extremely successful, so much so that deviations from it are automatically assigned to changes in the dielectric constant of the material. For example, the broadening of the plasmon band for small Au or Ag particles is attributed to electronic scattering at the particle surface, which becomes significant when the particle diameter is less than the mean free path of the electrons in the metal. Recent optical experiments with metal nanoparticles have concentrated on using time-resolved techniques to study the electron dynamics. The majority of these experiments (ours included) have been performed with ca. 100 fs laser pulses, which provide sufficient time resolution to study coupling between the excited electron distribution and the phonon modes of the particles. Our view of the photophysics of these materials is that the ultrafast laser pulse excites single electrons, that rapidly redistribute their energy over the entire electron distribution, causing an increase in the electronic temperature. This broadens the plasmon band, causing a strong