Controlled size of PbS nanocrystals doped ORMOSIL N. de la Rosa-Fox a, * , R. Erce-Montilla a , M. Pi ~ nero b , L. Esquivias a a Dpto. F  ısica de la Materia Condensada, Facultad de Ciencias, Universidad de C adiz, Aptdo. 40, Avda, Rep ublica Saharaui s/n, 11510 Puerto Real (C adiz), Spain b Dpto. F  ısica Aplicada, CASEM, Universidad de C adiz, Aptdo. 40, 11510 Puerto Real (C adiz), Spain Received 31 May 2002; accepted 11 July 2002 Abstract PbS semiconductor nanocrystals were grown inside an organically modified silicate (ORMOSIL) matrix prepared by the sol–gel method assisted by high power ultrasounds. Surface capping agent was used to control the growth and aggregation of PbS nanocrystals. The SANS intensity exhibits a crossover at 0.1 nm 1 . Data analysis using a two- correlation function indicates a medium-range structure built by PbS nanocrystals (6.5 nm mean crystal diameter) as observed by Transmission electron microscopy. A short-range structure accounts for a fine porosity (1 nm mean pore diameter) of the ORMOSIL matrix in agreement with gas isotherm. Band gap blue-shift confirms the exciton quantum confinement inside the PbS nanocrystals. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction PbS is an attractive semiconductor for the fab- rication of quantum dots (QDs) because of its large bulk Bohr exciton radius (a B ¼ 18nm)[1],as a consequence of its small carrier effective masses (m  ¼ 0:1m e ) and large optical dielectric constant (e ¼ 17). The strong quantum confinement regime is thus easily obtained, and the ground-state ab- sorptionedgecanbetunedoverawidewavelength range (from the near infrared to the visible) [2,3], sincethegapinQDsof R radius scales [4] as 1=R 2 . Moreover,duetothesmallersurface/volumeratio, surface states should play a minor role in PbS quantum dots when it is compared to the II–VI semiconductor nanocrystals. These surface trap states are responsible of a decline in the non-linear optical behaviour of the material (e.g., photo- darkening) [5]. Recently, PbS QDs doped glasses have been used as saturable absorbers for solid- state lasers [6]. The performance of these materials is influ- enced by the small size of crystals. Thus, the crystal sizes and their size distribution are critical parameters to improve the quantum confinement effects [7]. In this work we present, the synthesis of PbS/SiO 2 composites based on the sol–gel method assisted by high power ultrasounds (sonogel) [8]. This method consists basically in the promotion of the chemical reactions to form the hybrid organic– inorganic silica network as matrix. The use of these sonogels as matrices takes the advantage of * Corresponding author. Tel.: +34-956-016322/6317; fax: +34-956-016288. E-mail address: nicolas.rosafox@uca.es (N. de la Rosa- Fox). 0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0925-3467(02)00101-5 Optical Materials 22 (2003) 1–6 www.elsevier.com/locate/optmat