Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots

The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E-2 Raman active optical phonon at 436 cm(-1) is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260-340 attributed to surface vibrations. The E-1-LO mode at 585 cm(-1) is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 angstrom) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode.