Electron and phonon confinement and surface phonon modes in CdSe-CdS core-shell nanocrystals

The optical and vibrational properties of bare and CdS shelled CdSe nanocrystalline particles are investigated. To confirm the formation of such nanocrystals in our samples we estimate their average particle sizes and size distributions using TEM measurements. From the line profile analysis of the HRTEM images the core-shell structure in the particles has been confirmed. The blue shift in the optical absorption spectra, analysed using theoretical estimates based on the effective bond order model, establishes the electron confinement in the nanoparticles. The main aim of this paper is to show the unique characteristics of the nanocrystals (which are absent in the corresponding bulk material), such as confinement of optical phonons and the appearance of surface phonons. Making use of the dielectric continuum model we are able to match the experimental and theoretical values of the frequencies of the surface phonons. We believe that our studies using optical probes provide further evidence for the existence of core-shell structures in CdSe-CdS type materials.