Raman scattering by acoustic phonons in wurtzite ZnO prismatic nanoparticles

We report an observation by low-frequency Raman spectrometry of acoustic phonons in wurtzite prismatic zinc oxide nanoparticles of sizes varying from 2.3 to 6.6 nm. Several acoustic phonons confined in the nanoparticles are exhibited, shifting toward higher frequencies with a decrease in the nanoparticle size. We provide a comparison between experimental frequencies and those given by linear elasticity theory including both structural anisotropy and the specific shape of nanoparticles. We found that in addition to the usual breathing and fundamental extensional modes, spectra exhibit a peak attributed to the first harmonic of the extensional mode of nanoparticles.