Transparent Sol-Gel Matrices Doped with Quantum Sized PbS Particles
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T. GACOIN*, J.P. BOILOT*, M. GANDAIS**, C. RICOLLEAU** AND M. CHAMARRO*** * Laboratoire de Physique de la Mati~re Condensde, CNRS URA D1254, Ecole Polytechnique, 91128 Palaiseau Cedex, France. **Laboratoire de Min6ralogie-Cristallographie, Universit6s Paris VI et Paris VII, CNRS URA 009, 4 place Jussieu 75252 Paris Cedex 05, France. ***Groupe de Physique des Solides, Universit6s Paris VI et Paris VII, CNRS URA 17, 4 place Jussieu 75251 Paris Cedex 05, France.
ABSTRACT Sol-gel matrices doped with lead sulfide nanoparticles showing a structured absorption spectrum were synthesized by using the procedure previously reported for CdS doped matrices. The morphology of the PbS aggregates was investigated by high resolution electron microscopy, and the observations are correlated with the absorption spectra. Finally, preliminary results concerning some linear optical properties of the materials are presented.
INTRODUCTION Since the pioneering works of Efros1 and Brus2 about ten years ago, it is well known that semiconductor particles in the size range 2-20 nm (quantum dots) exhibit interesting electronic and optical properties, resulting from the spatial confinement of photogenerated carriers. The
intense activity which is now observed around those systems is motivated first by fundamental questions. How can the quantum size effect be theoretically described? What is the influence of the surface state of the particles? How can chemical treatments modify this influence? But a second source of motivation is given by the technological applications which could result from the non-linear optical properties of composite materials consisting of the dispersion of semiconductor nanoparticles in a transparent dielectric matrix. It is now clear that the of this latter point is strongly dependent on the answers given to the previous development 3 questions. So far, most studies have concerned II-VI semiconductors, and especially in the system CdSxSel-x. Nanoparticles can be precipitated in a colloidal state, 4 or dispersed in different solid media such as glasses, 5 porous glasses, 6 zeolites, 7 or silica gels. 8 The choice for this system is explained first by the relative facility of obtaining those compounds as nanoparticles, their low sensitivity towards oxidation, but also because the manifestations of the quantum size effect are easily detectable (blue shift of the absorption spectra and appearance of discrete transition levels). A fewer number of studies have concerned other semiconductors such as Bi 2 S3, 9 Cd 3 P2 , 10 Pbi 2 1I. In the present work, we focused our attention on the case of lead sulfide nanoparticles. Bulk PbS is a IV-VI semiconductor with a cubic rock salt structure. Its gap is direct at the L the point of the Brillouin zone, with a value of 0.41 eV. 12 The starting point of our study was 13 synthesis of PbS nanoparticles as described in previous works published by Gallardo et a1 247 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
and Nenadovic. 14 These authors synthesized colloidal
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