No abstract

ZnO quantum dots ͑QDs͒ of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum confinement effect. From the resonant Raman scattering, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second-to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Fröhlich interaction.

Both band gap engineering and spatial confinement of optical phonon were observed depending upon the size of ZnO quantum dots at room temperature. Size-dependent blueshifts of photoluminescence and absorption spectra reveal the quantum confinement effect. The measured Raman spectral shift and asymmetry for the E2(high) mode caused by localization of optical phonons agree well with that calculated by using the modified spatial correlation model.

Self-assembled secondary ZnO nanoparticles, recognized with the agglomeration of crystalline subcrystals, are successfully synthesized by a simple sol-gel method. TEM images display that one artificial cluster behaves in a single-crystal-like wurtzite structure because subcrystals coagulate as the same crystal orientation. Moreover, from the resonant Raman scattering, the as-grown sample exhibits phonon red shift; meanwhile, the coupling strength between electron and longitudinal optical phonon, determined by the ratio of secondto first-order Raman scattering cross sections, diminishes compared with the samples after postannealing at 350 and 500°C. The size dependence of electron-phonon coupling is principally as a result of the Fröhlich interaction.

Electronic band structures and surface states were investigated for ZnO finite wells or slabs grown along <0001> and <1-100> directions using tight binding representation. The dangling bonds on two end-surfaces caused surface bands for different directions grown slabs, of which the wavefunctions tend to localize at the end surfaces. The increasing splitting of the degenerate surface bands at the Γ point was observed decreasing with the thickness of the nonpolar [1-100] slab. And, the quantum confinement effect is distinctively enhanced by the extra electron-field induced in the <0001> grown finite well with the polar end-surfaces.