Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods

Colloidal gold has fascinated scientist since the Middle Ages because of its intense red color originating from the surface plasmon absorption. Recently, the interest in small metal particles of nanometer size of found tremendous interest because of possible applications of nanotechnology. By using optical absorption spectroscopy, transmission electron microscopy (TEM) and femtosecond time-resolved transient absorption spectroscopy the electron-electron and electron-phonon interactions as well as the structural dynamics of gold and gold-silver alloy nanoparticles of different sizes and shapes were studied. The shape and maximum of the surface plasmon absorption depends on the particle size and composition. It splits into tow bands for nanorods. This was examined experimentally as well as theoretically. Electron-electron and electron-phonon dynamics in gold and gold-silver alloy nanodots and gold nanorods were measured by femtosecond pump-probe spectroscopy. The electrons thermalize by electron-electron scattering in about 500 fs and couple to the lattice phonons with a relaxation time of about 1 ps which strongly depends on the laser pump power. Further relaxation occurs by phonon-phonon interactions with the surrounding medium on the 100 ps time scale. No effect of an enhanced electron-surface scattering causing a decrease in the measured relaxation time could be observed for nanodots or nanorods of different sizes. On the other hand, changing the matrix surrounding the gold nanoparticles from an aqueous or organic solution to an inorganic salt (MgSO₄) caused the electron-phonon relaxation time to increase. At high laser light intensities, it was found that gold nanorods in colloidal solution transform into spherical nanodots with volumes comparable to the original nanorods. The shape transformation depends on the pulse energy as well as on the pulse. It was found to take place within about 35 with a minimum energy of about 60 femtojoule. High resolution TEM studies further showed that the shape transformation occurs via the creation of defects inside the gold nanorods accompanied by surface reconstruction of the unstable {lcub}110{rcub} surface.