Frank Loncke

The Growth of Co:ZnO/ZnO Core/Shell Colloidal Quantum Dots: Changes in Nanocrystal Size, Concentration and Dopant Coordination

We report a synthesis route for the growth of Co:ZnO/ZnO core/shell quantum dots. This procedure consists of successive steps, comprising the addition of diluted precursor salt solutions, and heat treatment at 50 degrees C. By deriving a relation between the extinction coefficient at 250 nm and the nanocrystal diameter, we are able to monitor changes in quantum dot concentration during shell growth. We found that a mechanism based on the nucleation of new particles after salt addition and subsequent Ostwald ripening during the heat treatment is responsible for the shell growth. Based on ligand-field absorption spectroscopy, we demonstrate that the Co(2+) ions adsorbed at the surface of Co:ZnO quantum dots are incorporated inside the ZnO shells. Finally, EPR spectroscopy indicates that the surface-adsorbed Co(2+) ions can be incorporated as substitutional as well as interstitial Co(2+) ions.

Paramagnetic Eu2+ center as a probe for the sensitivity of CsBr x-ray needle image plates

Needlelike CsBr:Eu x-ray storage phosphor plates exhibit an excellent sensitivity as expressed by their photostimulated luminescence (PSL) intensity and have ever growing applications in medical radiography. An earlier electron paramagnetic resonance (EPR) study revealed spectra due to a Eu2+ center with axial ⟨100⟩ symmetry. In this work, its EPR intensity is shown to exhibit a linear correlation with the PSL intensity and the sensitivity of the plates. It can thus be expected that the structural information retrieved from EPR and related techniques may substantially contribute to the understanding of the physics behind the writing and reading processes in the phosphor.

K{sub 2}YF{sub 5} crystal symmetry determined by using rare-earth ions as paramagnetic probes

The electron paramagnetic resonance angular dependences for Gd{sup 3+} and Ce{sup 3+} centers in K{sub 2}YF{sub 5} crystals show that the Y{sup 3+} site has monoclinic C{sub h} symmetry in these crystals. This site symmetry is compatible with the crystal structure having the Pnam space group. From the zero-field splitting parameters of the Gd{sup 3+} center, it is deduced that the symmetry of the Y{sup 3+} sites is close to trigonal around the b axis, distorted by the overall orthorhombic symmetry of the crystal structure. This information is required for the identification of radiation-induced centers in this material, which shows favorable properties for applications as thermoluminescent dosimeter.