Origin of the optical phonon frequency shifts in ZnO quantum dots

We carried out nonresonant and resonant Raman spectroscopy of ZnO quantum dots with diameter of 20 nm . On the basis of our measurements and comparison with a recently developed theory, we were able to clarify the origin of the observed phonon peak shifts in quantum dots as compared to bulk ZnO . It has been found that the spatial confinement of optical phonons in 20 - nm -diameter dots leads to only few cm − 1 peak shifts. At the same time, we have demonstrated, that even a low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy of ZnO , leads to strong local heating of the ZnO quantum dots, which results in very large ( ∼ 14 cm − 1 ) redshifts of the optical phonon peaks. We have estimated from the observed redshift that the local temperature of the quantum dot ensemble is about 700 ° C . The obtained results are important for identification of phonon peaks in the Raman spectra of ZnO nanostructures.