Ln-doped rare earth phosphate nanocrystals for luminescence nanothermometry
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In the field of nanomaterials, information about the local temperature in a system can be crucial for the performance of these systems. In literature, several methods of temperature measurements on small length scales, in the order of micrometers, have been reported. Of these techniques, luminescence nanothermometry is a promising non-invasive technique. For some applications, however, the working temperature range of these techniques is not yet sufficient to be able to apply it in processes at elevated temperatures, such as catalytic reactions. In this thesis, different lanthanide-doped rare earth phosphate nanoparticles are investigated for the potential use as nanothermometers. Due to the small size of these particles, in the order of ∼5 nm, very small spatial resolutions can, in principle, be obtained, while thermally coupled energy states in the lanthanide dopants can provide a temperature dependent change in the band shape of the emission. Luminescent dysprosium-cerium co-doped lanthanum phosphate nanoparticles were synthesized and subsequently coated with a pure LaPO4 shell, to enhance the luminescence. In these particles, a temperature dependency of the emission from the 4F9/2 and 4 I15/2 energy levels was found up to a temperature of 250 ◦C. Incorporation of the particles in silica spheres of ∼ 40 nm provided further thermal stability, and temperature dependent emission was observed up to a temperature of 450 ◦C. Additionally, europium-doped LaPO4 nanoparticles with a pure LaPO4 shell were synthesized. In these particles, a temperature dependency of the emission from the 5D1 and 5D0 energy levels was found up to a temperature of ∼ 300 ◦C. Although further studies still have to be performed, these results provide a good indication that lanthanide-doped rare earth phosphate nanoparticles can be a useful addition to the variety of nanothermometry techniques available today.