Eur. Phys. J. B 18, 31-38 (2000)
https://doi.org/10.1007/s100510070074

The origin of the redshift in Brillouin spectra of silica films containing tin nanoparticles

¹ INFM-Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy
² Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
³ INFM-Dipartimento di Fisica "A. Volta", Università degli Studi di Pavia, Via A. Bassi 6, 27100 Pavia, Italy
⁴ Laboratoire de Physique de la Matière Condensée, URA 190, Université de Nice-Sophia Antipolis, 06108 Nice Cedex, France

Received: 30 March 2000
Revised: 24 July 2000
Published online: 15 November 2000

Abstract

The abrupt change of velocity in surface acoustic waves in thin films of amorphous SiO_x containing nanometre scale β-Sn crystals is shown to be directly associated with the size-dependent melting of the nanoparticles, confirming preliminary experiments. High resolution thin film powder diffraction using synchrotron radiation shows that the abrupt redshift in the Brillouin spectra satellites occurs at the same temperature as the melting of the nanoparticles, evident for the loss of the Bragg peaks. Effective medium theory is used to explain the origin of the anomaly. A central peak in the Brillouin spectrum, the intensity of which shows a maximum at the melting temperature, can be interpreted in terms of overdamped fluctuations in the dielectric function. The melting temperature as a function of particle size is in agreement with theoretical predictions. No evidence for strain could be found on the X-ray diffraction profiles; the a- and c-axis thermal expansion coefficients are the same as those in bulk tin.