Optical phonons in spherical core/shell semiconductor nanoparticles: Effect of hydrostatic pressure

By applying a phenomenological macroscopic approach we have studied polar optical phonons in core-shell semiconductor hybrid nanoparticles with spherical shape. The coupling of electromechanical oscillations is taken into account within the long-wavelength limit. A detailed analysis of the optical-phonon modes is presented with emphasis on the phonon-dispersion laws. Different kinds of II-VI and III-V semiconductor compounds are discussed, showing the differences resulting from their bulk phonon frequency dependence on the wave vector, and on the shell-to-core radius ratio. We make a systematic application of the theory for different nanostructures and report all possible polar optical-phonon modes. The effects of hydrostatic pressure on confined and surface-optical-phonon mode optical vibrations are considered. Also, we provide results for the usual dielectric continuum approach as a function of the applied pressure, where just the electric aspect of the oscillations is addressed. The effects of pressure on confined LO- and TO-like, and on surface-optical-phonon modes in core/shell InP/CdS nanoparticles are explored by Raman scattering. The observed pressure-induced shifts of the Raman spectra are well described by the theoretical calculations, and by applying a fitting procedure to the data we may extract the value for the core diameter and the shell-to-core radius ratio.