Tunable dual-mode photoluminescence from nanocrystalline Eu-doped Li2ZnSiO4 glass ceramic phosphors

We report on tunable photoluminescence from mixed-valence Eu-doped nanocrystalline Li2ZnSiO4 glass ceramics. After preparation of the precursor glass in air, gradual reduction of Eu3+ to Eu2+ occurs intrinsically during thermal annealing and precipitation of crystalline Li2ZnSiO4. Dual-mode photoemission can be generated for exciting at a wavelength of about 360 nm. The resulting colour of luminescence, ranging from orange/red to blue, can be controlled by adjusting the annealing temperature and, hence, the degree of crystallization: with increasing annealing temperature, the ratio of luminescence intensities related to Eu3+ and Eu2+ species, respectively, varies as a result of increasing degree of Eu3+-reduction as well as distinct changes in the optical scattering behaviour of the obtained glass ceramic. At the same time, the bandwidth of Eu2+-related photoemission increases from 87 to 154 nm. The underlying mechanisms of photoemission and energy transfer from Eu2+ to Eu3+ are discussed on the basis of dynamic emission spectroscopy and structural considerations, and a description of the internal reduction process is given.