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MgO crystals have been exposed to vacuum ultraviolet (VUV) radiation from a synchrotron, with energies up to 9 eV, and the emitted light, at wavelengths above 200 nm, was observed. It is concluded that bulk excitons, play an important role in the diffusion of energy inside MgO crystals, resulting in 5.85 eV (212 nm) emission from the MgO terraces of large (0.2-2 μm) MgO: F crystals. In the case of aliovalent impurity doping, then the bulk exciton energy is also transferred to the Vk centres and 5.3 eV (235 nm) light is emitted. Both fluorine and silicon doping appear to promote UV surface emission, acting similarly to an ns2 ion inside MgO, while strong scandium doping is killing the surface emission completely. The 212 nm surface UV emission and the 235 nm bulk UV emission can be excited only at the bandgap edge. Broadband visible light, centred around 400 nm, is also emitted. Contrary to the UV emission, this is not generated when excited at the bandgap edge; instead, we find that it is only excited at sub-bandgap energies, with a maximum at the 5C surface excitation energy of 5.71 eV (217 nm) for the MgO terraces. [ABSTRACT FROM AUTHOR]

We investigated the UV optical properties of 2.2 to 2.5 nm colloidal ZnS quantum dots (QDs) that were synthesized using a one-step solution-processed method. X-ray diffraction patterns and transmission electron microscopy images show clear cubic zinc-blende ZnS structure. Photoluminescence results show defect-related light emission with a peak wavelength that can be tuned from 400 to 389 nm by changing the particle size of the ZnS QDs. As judged from absorption spectroscopy, the optical bandgaps of the QD suspensions then change from 4.0 to 4.2 eV and for the QD films from 3.9 to 4.0 eV, showing clear quantum confinement. The excitation spectra show excitation maxima at the exciton energy, changing from 3.89 eV (319 nm) down to 4.07 eV (305 nm). [ABSTRACT FROM AUTHOR]