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 $20nm$⁠. 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 $(∼14cm−1)$ 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.