Phonon spectra of CdS_1-xSe_x semiconductor nanocrystals, embedded in dielectric media, have been extensively studied due to their broad applications resulting from confinement-related features in the optical spectra. Among the effects, playing an important role in the phonon spectra of nanometer-sized systems, the most significant are phonon confinement, compositional and size dispersion of nanocrystals within the ensemble, as well as a marked contribution of surface phonons due to the high surface-to-volume ratio.In spite of a considerable number of studies, the available data concerning the surface phonons in CdS_1-xSe_x nanocrystals are incomplete and often contradictory. The surface phonon chracteristics have been reported only for a limited interval of x, and the obtained experimental data are insufficient to discuss their consistency with theoretical predictions. There are also contradictory views on the dependence of the surface phonon frequencies in CdS_1-xSe_x on the nanocrystal size. No special studies have been devoted to resonance behaviour of surface phonons in II-VI nanocrystals.

Here we report on the studies of surface phonons in CdS_1-xSe_x nanocrystals embedded in dielectric media (borosilicate glass, gelatin, polyacrylamide, polyvinyl alcohol) for a broad range of the nanocrystal size and compositions by resonant Raman scattering. The measurements were performed using a Dilor XY 800 triple monochromator with a CCD camera with excitation by Ar+ and Kr+ laser light, tuned from 457.9 to 676.4 nm. The measurements were performed at room temperature.

Dependences of surface phonon parameters on the nanocrystal composition and size are analyzed. For CdS and CdSe nanocrystals a special attention is paid to the dependence of the surface phonon characteristics on the host medium type, the experimental results being compared with the relevant calculations. A strong dependence of the surface phonon frequency and halfwidth on the Raman excitation energy is observed.

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