The incorporation of dye molecules into solid matrixes is an attractive and widely investigated method to prepare dye-doped solid-state devices [1][2][3][4][5]. Large quantum yield of organic dye molecules combined with the advantages offered by the solid host matrix with respect to liquid solutions (bulky volume, flammable, toxic solvents, and difficult to be manageable) indicate that these materials can be good candidates, for solid-state dye laser applications [5,6]. It has been shown that the lifetime and thermal stability of dyes are enhanced when they are entrapped in solid matrices [5]. Solid matrix offers a larger mechanical and thermal stability, reduces the risks of environmental and operator hazards, and allows to increase achievement of larger concentrations of the dye, reducing the formation of H-aggregates responsible for the quenching of the luminescence. Among the investigated possibilities, the embedding of dye molecules into silica materials prepared via sol-gel methods can offer the highest physical and chemical performances.Two different approaches [2] can be used to prepare dye-doped silica materials: post-doping method (the impregnation), where the selected dye is incorporated into the sol-gel prepared porous silica and the pre-doping method, where the dye molecule is introduced at the sol stage in situ of solgel procedure. Post-doping method has disadvantage and some limits: difficult to determine the exact location of the dye molecules, dimension pores, presence of sorption center or defects on the surface, what induce untimely aggregation of the dye molecules. The main
Electron micrographs of discontinuous Au films deposited on NaCl substrates have been obtained. Specimens include both films which have been subjected to externally applied voltages as well as films which have not had an external voltage applied. The micrographs offer direct photographic evidence of island distribution, size, and morphology which are highly supportive of injected charges affecting the film morphology in the vicinity of the electrodes.
In studies 011 excitonic reflectivity spectra up to now tarious modifications of the symmetric curve R(hw ) corresponding to the c l a s s i a l harmonic oscillator have been reported for a number of direct-gap pure 11-VI cryslals (see, for example, /1/ for references). In this paper we report the first experimental observation of the Lion-classical excitonic reflectivity f o r mixeJ crystals, namely, ZnxCdl -xS and ZnxCdl -xSe. ductivity measurements indicated 1 0' to 1 J g X d mors/cm3. These and other impurities manifest themselves also in luminescence /2, . Due to the rather poor qkality of our crystals it was possible to observe excitonic structures only in spectra taken from cleaved faces and not from as-grown ones. The presence of impurities and imperfections caused the excitmic b m d s to be 3 to 5 times wider than those of "pure" melt-grown CdS and (cubic) ZnS, though the lineshapes generally remained classical. For a few samples, however, some non-classical features occurred: the "inverse" excitonic reflectivity band (Fig. 1 , 2), the band without minimurn (Fig. 1 to 3 ) . It should be noted that at higher temperaturesup to T 25 80 Kthese features remained pretty the same whereas at T i % 90 K no structure was seen at all.We studied melt-grown hexagonal ZnxCd S samples with 0 6 x 20.8. Con-Our ZnxCdl -xSe bulk crystals grown from vapour phase appeared of better quality than ZnxCdl-xS samples for we d e r e able to observe excitonic structures in spectra taken from as-grown faces, a o . A s well as in /3/ we lound that in general the excitonic reflectivity spectra of ZnxCdl -xSe followed the wurtzitezincblende transition that took place with growing x. As for non-classical features we succeeded in observing them only for some hexagonal samples comparatively 1) Prospekt Nauki 144, 252650 Uev-28, USSR.
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