Preparation of Monodispersed, Nanosized Semiconductor Particles

Fluorescence spectra of semiconductor particles originate from the recombination of charge carriers in either a trapped or exdtonic state [578, 579], The former manifests itself in the appearance of a broad and Stokes-shifted band. In contrast, the spectrum, due to exdtonic fluorescence, appears as a sharp band near the absorption onset and is considered to arise from the detrapping of the trapped electrons [579]. The highly environmentally dependent position of the semiconductor emission maxima has been related to semiconductor sizes and size quantization [579]. 

Measurement of non-linear optical properties [580] also provides a means for characterizing size-quantized semiconductor particles. Third-order optical non-linearity of size-quantized semiconductor particles has been discussed in terms of resonant and non-resonant contributions [11]. Resonant non-linearity is expected to increase with decreasing particle size and increasing absorption coefficients. 

Colloidal sulfide, selenide, telluride, phosphide, and arsenide semiconductor particles are prepared by the controlled precipitation of appropriate aqueous metal ions by H2S, H2Se, H2Te, PH3, and AsH3, respectively. Colloids are stabilized, typically, by sodium poly-phosphate. A large number of experimental parameters determine the size, size distribution, morphology, and chemical composition of a semiconductor particles in a given preparation. Concentrations, rates, and the order of addition of the reagents the counterions selected  

Highly monodispersed, size-quantized semiconductor particles have been prepared by size fractionation of the colloidal samples by gel electrophoresis [591 ]. The method is illustrated by quoting the published recipe for CdS particle formation [591]  

The gel was prepared by mixing 66 ml of gel buffer (0.01 M Cd(C104)2, 0.01 M sodium polyphosphate, pH 11) 7.4 ml acrylamide solution (44.4 g of acrylamide, 1.2 g of methylene-bisacrylamide in 100 ml water) and 62.4 ml water in round bottom flask and deaerated for 5 min with a water pump. Then 1.2 ml of freshly prepared ammonium persulfate solution (150 mg in 10 ml water) and 0.1 ml of N,N,N, N tetramethylethylenediamine (TEMED) were added and mixed carefully to avoid the introduction of air. Finally, 100 ml of the resulting solution was poured carefully, to minimize the introduction of air, into a 25 mm-diameter tube. The monomer solution was overlayered with water to remove the meniscus and reduce the entry of air. The polymerization was allowed to proceed overnight at room temperature. The resulting gel, having a polymer concentration of 2.5% and a cross-link density of 2.6%, was used in a home made apparatus.  

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