Core-shell metal fluoride nanoparticles via fluorolytic sol-gel synthesis - a fast and efficient construction kit

An efficient, fast and easy construction kit using the fluorolytic sol-gel synthesis of rare-earth-doped alkaline earth fluoride core-shell nanoparticles at room temperature is presented, capable of synthesizing several hundred grams to kilograms of core-shell particles in one batch. We show ways for an effective design of energy transfer core-shell systems. Undoped metal fluoride shells rigorously shield a luminescent core from the surrounding solvent, resulting in higher quantum yields, longer lifetimes of the excited states, and finally a brighter luminescence. The heavy SrF2 shields a luminescent core from the surrounding solvent three times more effectively than the light CaF2. Energy transfer processes from core to shell are more efficient than vice versa, and hence, absorbing cores are more effective than absorbing shells. The application of these materials in the preparation of transparent tunable luminescent materials showing different luminescence colours upon different excitation wavelengths is demonstrated.