Journal of Sol-GelScience and Technology 8,275-283 (1997) 9 1997Kluwer Academic Publishers. Manufactured in The Netherlands. Confinement of CdS Nanocrystals in a Sonogel Matrix R. LITRAN Departamento de Ffsica de la Materia Condensada, Universidad de C6diz, Apdo, 40, Puerto Real (11510) Cddiz, Spain R. ALCANTARA Departamento de Quimica-Fisica, Universidad de Cddiz, Apdo, 40, Puerto Real (11510) C6diz, Spain E. BLANCO AND M. RAMIREZ-DEL-SOLAR Departamento de Ffsica de la Materia Condensada, Universidad de C6diz, Apdo, 40, Puerto Real (11510) C6diz, Spain Abstract. CdS/silica xerogel composites have been prepared using a sonocatalytic method. The confinement effects of CdS semiconductor nanocrystallites have been analyzed through UV-VIS absorption and Raman scatter- ing. The blue shift of the absorption band and the shape of the Low Frequency Inelastic Raman Scattering (LOFIRS) spectra make it possible to evaluate the size of nanocrystallites, which are contrasted with previous results obtained through other techniques. Moreover, in the 200-700 cm -1 region of Raman shift, resonant effects are discussed, through the longitudinal optical mode lines. Keywords: sonogel, nanocomposite, quantum dot, Raman scattering, LOFIRS, UV-VIS absorption, CdS 1. Introduction The future evolution of integrated nonlinear optics de- pends on the performances that nonlinear optical ma- terials can achieve. Nevertheless, nature has not been generous to the optical nonlinearities of bulk optical materials; in the case of integrated nonlinear optics, the situation is aggravated by additional requirements on the materials as regards their processability, adapt- ability and interfacing with other materials [1-3]. One way to enhance the cubic nonlinearities of mate- rials with very delocalized electrons, such as semicon- ductors, is to artificially confine the valence electrons in regions much shorter than their natural delocalization length in the bulk, which extends over many unit cells. These materials are characterized by broad but discrete optical resonances whose position, oscillator strength and dynamics depend on the extension of the artificial confinement and hence can be modified to meet certain requirements. The sol-gel process allows the production of silica matrices with a higher semiconductor concentration at temperatures significantly lower than those required by the classic melting methods [4-6]. Furthermore, sonocatalysis is a good approach for the improvement of the nonlinear properties of the composites, as re- gards the narrower size distribution of the nanocrys- tals induced in matrices presenting a finer porosity. In this sense, our previous papers proved that this method yields CdS-sonogel composites whose particle size dis- tribution and absorption behaviour are consistent with the Efros and Efros model for intermediate degrees of quantum confinement [7]. However, as we will de- scribe in the present paper, the ageing behaviour of these samples reveals a low stability of the nanocrys- tals formed in these conditions. An optimization of the composite processing is therefore required in order to increase the composite performance. For this purpose, diverse experimental parameters were tested and their influence on the sample characteristics evaluated.