Tb3+ ions in presence of ZnS:Mn2+ nanocrystals immobilized on silica: Energy transfer ZnS→Tb3+ and coordination state of Mn2+ ions

Three-component luminescent material consisting of silica xerogel as a support with immobilized ZnS:Mn2+ nanocrystals and Tb3+ ions was compared with such two-component materials as the silica support with ZnS:Mn2+ as well as the support with Tb3+. In each case the nanocrystals and the lanthanide ions were immobilized at silica surface by impregnation procedure. Size of the ZnS quantum dots doped with Mn2+ were estimated by Scherrer method from the X-ray diffraction (XRD) pattern. The materials have been characterized by EPR and optical spectroscopy techniques. EPR spectra allow to distinguish two different Mn2+ sites: the first is assigned to isolated Mn2+ substitutionally and incorporated into cubic ZnS lattice and the second is ascribed to the Mn2+ situated near the nanocrystal surface. From the optical spectra we have found that in the three-component material, energy transfer from excited ZnS:Mn2+ nanocrystals to Tb3+ ions takes place. The different mechanisms of such transfer are discussed.