Photophysics of Nanometer Sized Metal Particles: Electron-Phonon Coupling and Coherent Excitation of Breathing Vibrational Modes

JOSE H. HODAK; ARNIM HENGLEIN; GREGORY V. HARTLAND

JOURNAL OF PHYSICAL CHEMISTRY B

Año: 2000 vol. 104 p. 9954 - 9965

1089-5647

The wide variety of applications of metal nanoparticles has motivated many studies of their properties. Someimportant practical issues are how the size, composition and structure of these materials affect their catalyticand optical properties. In this article we review our recent work on the photophysics of metal nanoparticles.The systems that have been investigated include Au particles with sizes ranging from 2 nm diameter (severalhundred atoms) to 120 nm diameter, and bimetallic core-shell particles composed of Au, Ag, Pt and/or Pb.These particles, which have a rather narrow size distribution, are prepared by radiolytic techniques. Byperforming time-resolved laser measurements we have been able to investigate the coupling between theelectrons and phonons in the particles, and their low frequency “breathing” modes. These experiments showthat for Au the time scale for electron-phonon coupling does not depend on size, in contrast to metals suchas Ga and Ag. On the other hand, the frequency of the acoustic breathing modes strongly depends on the sizeof the particles, as well as their composition. These modes are impulsively excited by the rapid lattice heatingthat accompanies ultrafast laser excitation. The subsequent coherent nuclear motion modulates the transmittedprobe laser intensity, giving a “beat” signal in our experiments. Unlike quantum-beats in molecules orsemiconductors, this signal can be completely understood by classical mechanics.