We report on the measurement of the absolute external luminescence quantum efficiency of various ZnO samples using a miniature integrating sphere fitted into a cryostat. Even the absolute luminescence quantum efficiencies per spectral interval are directly accessible. Measurements have been carried out on high quality bulk samples and different commercially available ZnO powders from 8K up to room temperature. Activated processes lead to an overall decrease in the efficiency with temperature. All efficiencies are considerably below unity, making the identification of the luminescence decay time with the radiative life time very questionable.

The absorption spectra of CuBr, CuCl, CuI, and AgI nanocrystals (NC) embedded in a glass matrix have been investigated in a wide temperature range between T=6 and 860K. The change of the absorption in the vicinity of the exciton resonance for CuBr and CuCl reveals a pronounced reduction of the melting temperature of NCs compared to the bulk value and shows a strong hysteresis between melting and solidification temperatures. The melting temperature and the hysteresis width depend on the sizes of NCs. For CuBr NCs a second hysteresis around the bulk melting point is observed. For large NCs the temperature dependence of the exciton peak energies is analogous to its counterpart in bulk crystals. For CuI NCs the shrinkage effect of the gap for both cubic and hexagonal modifications is observed and the temperature coefficients of the energy gaps are deduced. For AgI NCs a rather unusual temperature dependence of energy gap is observed. This dependence is nonmonotonic and shows a change of the energy gap slope from positive to negative.

The temperature dependence of exciton peak energies in large CuI quantum dots embedded in a glass matrix has been measured. The gap shrinkage effect for both cubic and hexagonal CuI phases was observed. The temperature coefficients of the energy gaps and the effective phonon energies are deduced.

We report the linear absorption spectra of relatively large copper iodide nanocrystals embedded in an alumina borosilicate host network structure. The spectra reveal pronounced exciton lines of both the zincblende and the layered hexagonal structures. In the approximation of the weak-confinement regime, the translational masses for the Z(12) and Z(3) excitons, as well as the anisotropy of the Z(12)-exciton band, i.e., the exciton Luttinger parameters, are deduced from the spectral positions of the exciton lines.

We investigate the exciton localization in cubic CdS/ZnSe type-II superlattices using spatially integrated and spatially resolved photoluminescence spectroscopy. Spatially resolved photoluminescence images (intensity landscapes) show local reductions of the emission intensity. This can be attributed to potential profiles arising from fluctuations in layer thicknesses. Furthermore, narrow line emission (FWHM approximate to 300 mu eV) superimposed on a broad photoluminescence background (FWHM around 30 meV) is observed. Temperature-dependendent investigations reveal the contributions of the usual band-gap shift and phonon-assisted hopping processes to the non-monotonous shift of the spatially integrated luminescence maximum. When comparing the behaviour of the superlattices with the photoluminescence properties of single quantum wells we observe strong differences. (C) 2000 Elsevier Science B.V. All rights reserved.

We report the fabrication, transmission electron microscopy investigation and linear optical properties of silver iodide quantum dots in an aluminaborosilicate host network structure. The nearly spherical quantum dots with radii ranging from 1.7 to 2 nm exhibit a pronounced excitonic absorption and intensive intrinsic photoluminescence in the intermediate confinement regime. The selective photoluminescence and photoluminescence excitation measurements revealed the fine structure in the spectra due to the splitting of the lowest exciton state by the electron-hole exchange interaction. A considerable enhancement of the exchange energy with decreasing quantum dot size is observed.