%e demonstrate that CdSq~Se nanocrystals embedded in glass suer compressive strain which increases as the crystallite radius decreases. A model based on the dependence of the cluster surface free energy on the surface-to-volume ratio explains very well the observed blueshift of the phonon bands in the Raman spectra.The phonon bands appearing in the first-order Raman spectrum of a macroscopic crystal have a Lorentzian line shape peaked at the u(q = 0) phonon frequencies. If the crystal size is smaller than about 30 nm, the localization of the phonon wave function causes the relaxation of the q = 0 selection rule which, in turn, results in (i) the redshift of the peak position and (ii) the low-frequency asymmetric broadening of the Raman bands for materials with a negative phonon dispersion.The smaller the crystal dimensions" the stronger the finite size effects.Semiconductor crystallites with dimensions in the nm range (nanocrystals) can be produced either in different solid matrices (glass, ionic crystals, gel, polymer) or liquids (semiconductor colloids) or deposited on a substrate by epitaxial techniques.~Particularly, CdS(Se) nanocrystals embedded in glass matrices can be considered the prototypical material for the study of size-dependent physical effects.Both the redshift and the asymmetric broadening of the Raman spectrum, as predicted by the phonon localization model, have been observed in semiconductor colloids as well as in epitaxial samples " for small crystallite sizes. In contrast, although the expected band broadening is observed, no redshift or even a blueshift of the Raman peaks is reported when the semiconductor nanocrystals are embedded in a glass matrix.In the following we describe our Raman spectra of CdSi Se nanocrystals embedded in glass matrices.The observation of a strong size-dependent blueshift of the Raman peaks demonstrates the existence of compressive strain which overcomes quantum confinement in determining the phonon energies. Our results can be explained by considering the dependence of the surface free energy of clusters on the surface-to-volume ratio and, to a lesser extent, on the thermal expansion mismatch between the semiconductor clusters and the host medium.The investigated samples are either commercial filters produced by Corning (series labeled by the letters CS) or experimental samples prepared in German (series labeled by the letters OA) or Russian (series labeled by the letters KS) laboratories. The CdSi Se crystallites are grown by the "heat striking" method in borosilicate glass matrices. The mean values of the nanocrystal radii as well as the crystallinity have been determined by means of transmission electron microscopy (TEM) measurements. Values in the range 25 -100 A are found. The selenium mole fraction z has been determined, according to the method outlined in Ref. 13, from the ratio where I(CdSe) and I(CdS) are the integrated scattering intensities of the CdSe-like and the CdS-like phonons of the CdSi Se alloy, with exciting photon energies far from resonances. ...

No abstract

Nanostructured semiconductor particles are currently under intense investigation because of their interesting photophysical and photochemical properties. Several preparation methods have been exploited for this class of materials; among them, the use of reverse micelles or “water-in-oil” microemulsions is a simple and successful preparative route. In this paper, a novel synthetic medium has been exploited, consisting of a quaternary CTAB/n-pentanol/n-hexane/water microemulsion. The presence of cosurfactant (n-pentanol), in addition to water, allows the simultaneous modulation of the water droplets dimensions and their exchange dynamics. Nanoparticles were characterized by UV−vis and photoluminescence spectroscopy, High-Resolution Transmission Electron Microscopy, and Raman spectroscopy. All measurements showed that the mean size of the nanoparticles is governed by the pentanol content of the system. They also possess a narrow size distribution and a good degree of crystallinity. Moreover, pentanol has an influence on the stability of CdS clusters, behaving as a capping agent at high concentrations. Stopped-flow measurements elucidated the influence of the cosurfactant in the kinetics of nucleation and growth.

No abstract

No abstract