“Sviluppo di nuovi materiali inorganici in grado di produrre emissione di luce accordabile nella regione del visibile” (AdR2729/16)

Inorganic materials doped with trivalent lanthanide (Ln3+) ions find numerous applications in important technological fields. Among these materials phosphors have attracted a lot of interest due to their properties, and are widely used in various fields like displays and screens or lighting. In the recent years, the interest of developing white LEDs has led to numerous works in phosphors. One of the current approaches to white emission is to combine UV-excited blue, green and red emitters. However, red phosphors for LEDs are relatively difficult to develop, so new approaches for stable and efficient red emitters are required. Energy transfer processes involving trivalent lanthanide ions have been widely studied over the years. In concentrated lanthanide compounds the absorbed excitation energy can migrate over the Ln3+ sublattice via multi step energy transfer, leading either to concentration quenching or to light emission. Avoiding luminescence quenching is a big issue in most applications, whereas energy transfer in pairs of ions and from the host to the activator is the basis of many luminescence based technologies. As a consequence, the knowledge of the energy transfer processes present in luminescent materials is crucial, including also the role of energy migration. This is particularly true in the case of the Tb3+→Eu3+ energy transfer in Tb3+−based materials containing Eu3+ as a dopant, where different types of behaviour can be observed in different systems due to the structural peculiarities of the host. These differences determine the colour emission of the material, which is strongly related to the presence or absence of energy transfer (of both types: sensitizer-sensitizer and sensitizer-activator). In Tb3+−based materials, the Tb3+→Eu3+ transfer processes depend on the efficiency of the Tb3+→Tb3+ energy migration and of the sensitizer activator transfer. This migration process is affected by the interionic distances in the crystal structure, the symmetry of the sites occupied by the lanthanide ions, and the presence of disorder, that can affect resonant migration.