Damped and overdamped acoustic phonons in tin nanoparticles detected by low frequency Raman scattering

Confined acoustic phonons were measured in tin nanoparticles of different sizes, embedded in a SiOx thin film, using surface light scattering measured by a Sandercock interferometer. This kind of scattering is usually called low-frequency Raman. Confinement effects were also detected by the broadening of Raman optical modes of tin in the smallest particles. The eigenfrequencies of the acoustic modes scale as the inverse of the particle size and are in agreement with theoretical calculations of vibrations in a sphere. The vibrational modes of the smallest size particles turned out to be overdamped. This is in agreement with femtosecond pump-and-probe spectroscopy measurements, where oscillations were detected for the big particles (but not for the smallest particles) at frequencies comparable to those observed in our measurements. By increasing temperature, melting of nanoparticles was also observed.