Phonon confinement and substitutional disorder in Cd1-xZnxS Nanocrystals

1LO optical phonons in free-standing mixed Cd1-xZnxS nanocrystals, synthesized using chemical precipitation, are investigated using Raman spectroscopy. As expected for the nanocrystals, the 1-LO modes are found to appear at slightly lower wavenumbers than those in the bulk mixed crystals and exhibit one mode behavior. On the other hand, the line broadening is found to be much more than that can be accounted on the basis of phonon confinement. From the detailed line shape analysis it turns out that the substitutional disorder in the mixed crystals contributes much more to the line broadening than the phonon confinement. The linewidth arising from these mechanisms are also extracted from the analysis.Comment: 15 Pages,8 Figures, Accepted in J. Raman Spectroscop

Resonant raman scattering in CdSxSe1-x nanocrystals : effects of phonon confinement, composition and elastic strain

Optical phonon modes confined in CdSxSe1-x nanocrystal (NC) quantum dots (≈ 2 nm in radius) grown in a glass matrix by the melting-nucleation method, were studied by means of resonant Raman scattering (RRS)spectroscopy and theoretical modelling. The formation of nanocrystalline quantum dots (QDs) is evidenced by the observation of absorption peaks and theoretically expected resonance bands in the RRS excitation spectra. Since the underlying material is a ternary alloy, this system offers the possibility to investigate the nterplay between the effects of phonon localization by disorder and phonon confinement by the NC/matrix interface. Based on the concept of propagating optical phonons, accepted for two-mode pseudo-binary alloys in their bulk form, we extended the continuous lattice dynamics model that has successfully been used for nearly-spherical NCs of binary materials, to the present case. After determining the alloy composition for NCs (that can be evaluated with only 2-3% uncertainty using the bulk longitudinal optical phonon frequencies) and the NC size (using atomic force microscopy and optical absorption data), the experimental RRS spectra were described rather well by this theory, including the lineshape and polarization dependence of the scattering intensity. Even though the presence of a compressive strain in the NCs, introduced by the matrix masks the expected downward shift owing to the phonons’ spatial quantization, the asymmetric broadening of both Raman peaks is similar to that characteristic of NCs of pure binary materials. Although with some caution, we suggest that both CdSe-like and CdS-like optical phonon modes indeed are propagating within the NC size (and not localized by disorder at a shorter length scale) unless the alloy is considerably heterogeneous.Portuguese Foundation for Science and Technology (FCT

Stability of CdS Nanocrystals in Glass

We have measured and analyzed optical absorption and x-ray absorption spectra of CdS nanocrystals grown by solid-state reaction in a borosilicate glass, with average crystallite radii from 1.4 to 5 nm. The first (Cd-S) and second (Cd-Cd) neighbor distances are within 0.2% of the bulk values in all cases. We establish an upper bound for the interfacial tension of 0.15 N/m. Such low interface tension suggests that glass-embedded nanocrystals are inherently more stable than bare or capped nanocrystals of the same size

Electronic and optical properties of Cd 1-x Zn xS nanocrystals

We report a numerical simulation of the conduction and valence band edges of Cd 1-x Zn xS nanocrystallites using a one — dimensional potential model. Electron — hole pairs are assumed to be confined in nanospheres of finite barrier heights. Optical absorption measurements are used to fit the bandgap of the Cd 1-x Zn xS nanocrystal material. A theoretical analysis is also made to calculate the energy location of bound excitons and the oscillator strength of interband transitions as a function of zinc composition. The aim of the latter study is to investigate the optical behavior of Cd 1-x Zn xS nanocrystals. An attempt to explain all the results is presented. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200673.21.La Quantum dots, 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals, 71.55.Gs II-VI semiconductors,