  The vibrational spectroscopy of semiconductor nanostructures can provide
important information on their structure. In this work, experimental Raman and
infrared spectra are compared with vibrational spectra of CdSe/CdS core/shell
nanoplatelets calculated from first principles using the density functional
theory. The calculations confirm the two-mode behavior of phonon spectra of
nanostructures. An analysis of the experimental spectra reveals the absence of
modes with a high amplitude of vibrations of surface atoms, which indicates
their strong damping. Taking into account the difference in the damping of
different modes and their calculated intensities, all bands in the spectra are
unambiguously identified. It is found that the frequencies of longitudinal
optical modes in heterostructures are close to the frequencies of LO phonons in
bulk strained constituents, whereas the frequencies of transverse modes can
differ significantly from those of the corresponding TO phonons. It is shown
that an anomalous thickness dependence of CdS TO mode is due to a noticeable
surface relaxation of the outer Cd layer in the nanostructure.
