Size-dependent lattice contraction inCdS1−xSe

Abstract

%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. ...

Raman scattering of InSb quantum dots grown on InP substrates

“…In this system a blue-shift of the phonon frequencies of the nanocrystals has also been observed and attributed to a compressive strain imposed on nanocrystals by the host medium. 18 If we assume that the buried InSb dots are under a compressive strain, from the shift of the phonon frequencies we obtain a decrease of the InSb lattice parameter in the dots of the sample shown in Fig. 4 of about 0.8%.…”

Optical phonons in colloidal CdSe nanorods

“…The Grüneisen parameter for longitudinal optical phonons in CdSe was determined by experiments and is approximately 1.1 [99]. The relative shift of the LO frequency is related to strain by [100] ∆ω…”

Size dependence of nanostructures: Impact of bond order deficiency

“…The blueshift of LFR mode has also been attributed to the effect of lattice contraction induced stain. CdS x Se 1ÿx nanocrystals embedded in a borosilicate (B 2 O 3 e SiO 2 ) glass matrix [450] have been found to experience size-dependent compressive strain. The lattice strain causes the surface tension to increase when the crystal size is reduced.…”