CdSe Nanocrystals Formation in Silica Sonogels
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INTRODUCTION. The porous and dielectric nature of silica gel is the reason to make this "coherent structure inside a fluid" an attractive material for optical purposes. Inside the porosity exist the possibility of precipitating another compound, i.e., organic and/or inorganic, to provoke a large change in the silica physico-chemical properties [1, 2]. Recently the material research
community focused its attention on the semiconductor nanocrystals-doped gels, due to its high degree of optical non-linearity. A fine dispersion of semiconductor nanocrystals inside the dielectric silica matrix, operate as quantum dots producing the confinement of discrete excitons, and lead to an optical response with intensity dependent refractive index. Moreover silica has a wide band-gap energy and the confinement effect of semiconductor particles embedded is large. The sol-gel processing for the preparation of semiconductor-doped gels is used widely since chalcogenides can be prepared easily by chemical precipitation [3, 4]. On the other hand, chalcogenides are often chosen because they have high values of third-order susceptibility and fast switching times [5, 6]. In order to obtain small crystals it is necessary to produce a great number of nucleation centers and a matrix with fine porosity to prevent the subsequent crystal growth. Previous studies [7, 8] have shown, that the combined effect of the addition of formamide in the sol and the action of high power ultrasound to promote the chemical reactions, permits the tailoring of specific structure [9]. This processing has been applied to obtaining CdSe-doped gels taking advantage of their favorable features. That is homogeneous size distribution and fine porosity. EXPERIMENTAL. Silica sonogels matrices were prepared from TEOS+water (pH=l) +formamide in a molar ratio of 1:10:7. The liquid mixture was submitted to the action of high power ultrasounds. The device processor with 600 W output run at 20 kHz, a standard horn with a 13 mm tip was 673 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
immersed into the liquid solution to a depth of ~3mm. In all cases the ultrasounds were applied in open reactor at room temperature. The ultrasonic energy dose applied to the sol was 22.5 J. cm- 3 . The resulting sample is used as reference and called "blank" (B). Afterwards, Cd+2 ions were added from the Cd(NO 3 )2 dissolution in the hydrolysis water, the concentration of cadmium used corresponds to 1 wt.% of final CdSe in the silica matrix, if a 100% fulfillment is assumed. This sample is called "white" (W). Equal volume of Cd 1 wt.%-doped sols were kept in polyethylene hermetic containers at isothermal conditions (40'C) until the gelation. The gels were aged three days and then the containers were opened to evaporate partially the syneresis liquid, at this step the silica matrix is ready for the CdSe precipitation. The open containers were immersed in formamide and placed in a dissicator under rotatory vacuum. The formamide was then substituted by a solution of acidic
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