Preparation of II-VI Semiconductor Nanocrystallites in a Glass Matrix Using Chalcogenizing Agent: Application to CdSe
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ABSTRACT A new route for preparing CdX (X = S, Se, Te, S+Se) nanocrystallites dispersed in a sodium borosilicate glass matrix from a hydrogel is proposed. Chalcogenizing complexing molecules - for instance a mixture of NH 4 SCN + H2 SeO 3 - introduced in the starting solution allowed an in situ crystallite preparation concomitant to gel densification. Prevention of crystallite oxidation is thus obtained. Moreover, coalescence is minimized because of the low gel-glass transition temperature. Low temperature absorption spectra have been interpreted in terms of exciton and electronhole confinements, accounting for both an intrinsic broadening of energy states inside each nanocrystal and a Gaussian size distribution. Crystallite sizes and size dispersion can be adjusted by changing the initial Cd concentration. The crystallinity of the nanoparticles without change in dispersion is strongly improved by thermal treatment above the Tg of the glass matrix.
INTRODUCTION The development of ultrafast devices for optical signal processing and optical data transmission needs the use of materials having non-linear optical (NLO) properties. Among the large choice of such materials, semiconductors play a prominent role. In particular, non-linear optical phenomena are observed in II-VI semiconductors and can be enhanced by quantum confinement effects, when at least one dimension of the material becomes comparable to or smaller than the excitonic Bohr radius. The extreme quantum system is the quantum dot, where the motions of electrons and holes are confined along all three space directions. Materializing quantum dots is equivalent to preparing semiconductor crystallites with radii close to the exciton Bohr radius. Such nanocrystallites will be useful on condition that proper techniques are developed for controlling the size, quantity and distribution of the particles within a matrix. Several attempts were made to address these problems. We briefly remind their principles: - Synthesis in homogeneous liquid medium by a colloidal routeI or in micro-heterogeneous liquid medium 2. In the latter case, the synthesis is not performed in a solvent, but in a microscopically structured medium, by the presence of surfactants or of inverse micelles. - Synthesis in a microporous solid medium. Zeolites 3 or glasses such as porous vycor 4 are well adapted to this kind of preparation. - Glasses doped by nanocrystallites obtained by conventional melting processes'. 901 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
All the above techniques suffer from some inconveniences : lack of physical support for powders obtained via a liquid route, open porosity of microporous solids which do not protect the crystallites from oxidation or any other cause of degradation, partial oxidation and coalescence of crystallites during the fusion of glasses at high temperature. Nevertheless, glasses seem to be the most appropriate supports since they gather the properties of dispersion in space and of long-lived protection of crystallites. This
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