Noble metal colloids optical properties

An intense femtosecond laser spectroscopy-based research focusing on the fast relaxation processes of excited electrons in nanoparticles has started in the past decade. The electron dynamics and non-linear optical properties of nanoparticles in colloidal solutions [1], thin films [2] and glasses [3] have been studied in the femto- and picosecond time scales. Most work has been done with noble metal nanoparticles Au, Ag and Cu, providing information about the electron-electron and electron-phonon coupling [4] or coherent phenomenon [5], A large surface-to-volume ratio of the particle gives a possibility to investigate the surface/interface processes. 

New routes to high quality colloidal NCs of varying compositions and shapes appear in the literature almost on a weekly basis. The most successful of these approaches to crystalline semiconductor NCs with excellent optical properties share the same basic premise of pyrolysis of metal-organic precursors in solutions of hot coordinating solvents. In contrast, the syntheses of noble metal NCs typically rely on the reduction of metal salts in the presence of citrate ions (e.g. An or Ag), in two-phase systems (e.g. An or Ag), or under high-temperature conditions in the presence of stabilizing... 

Whereas for small clusters the field attenuation inside the cluster is small, in larger clusters (> 30 nm) only surface regions are excited by the field. Thus, the electron mean free path plays a key role for the optical properties of large metal colloids. The skin depth at 2 eV is smallest in alumimun (13 nm) and larger for most noble metals such as gold or silver (Ag 24 nm, Au 31 nm). 

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