Optical properties of zinc oxide quantum dots

Size-dependence of efficient UV photoluminescence (PL) and absorption spectra of various sizes of zinc oxide (ZnO) quantum dots (QDs) give evidence for the quantum confinement effect. Bandgap enlargement is in agreement with the theoretical calculation based on the effective mass model for the size of ZnO QDs being comparable to the Bohr radius of bulk exciton. By using the modified spatial correlation model to fit the measured Raman spectra, we reveal that the Raman spectral shift and asymmetry for E₂(high) mode are caused by localization of optical phonons. Furthermore, we present temperature-dependent PL of different sizes of ZnO particles. The unobvious LO-phonon replicas of free exciton (FX) were observed when the ZnO particle sizes were under 12 nm in diameter. The increasing exciton energy (Eb) with the decreasing quantum dot size can be obtained from temperature-dependent PL. From the temperature-dependent change of FX emission energy, we deduce that the exciton-LO phonon coupling strength reduces as the particle size decreases. The reduced exciton Bohr radius aB with particle size obtained from Eb and PL spectrum confirms that the exciton becomes less polar in turn reducing the Fröhlich interaction and the exciton-LO phonon interaction is reduced with decreasing size of the ZnO QDs. In addition, the nearly unchanged spectral shape in power dependent PL of ZnO quantum dots reveals stable exciton states without formation of biexcitons and exciton-exciton scattering.