  We derive within the dielectric-continuum model an integral equation that
defines interface and confined polar optical-phonon modes in nanocrystals with
wurtzite crystal structure. It is demonstrated theoretically, that while the
frequency of confined polar optical phonons in zincblende nanocrystals is equal
to that of the bulk crystal phonons, the confined polar optical phonons in
wurtzite nanocrystals have a discrete spectrum of frequencies different from
those of the bulk crystal. The calculated frequencies of confined polar optical
phonons in wurtzite ZnO nanocrystals are found to be in an excellent agreement
with the experimental resonant Raman scattering spectra of spherical ZnO
quantum dots.
