The Influence of Defects on the Luminescence of Trivalent Terbium in Nanocrystalline Yttrium Orthovanadate.

Terbium-doped YVO4 has been considered a nonluminescent solid since the first classic studies on rare-earth-doped phosphors in the 1960s. However, we demonstrate that defect engineering of YVO4:Tb3+ nanoparticles overcomes the metal-metal charge transfer (MMCT) process which is responsible for the quenching of the Tb3+ luminescence. Tetragonal (Y1-xTbx)VO4 nanoparticles obtained by colloidal precipitation showed expanded unit cells, high defect densities, and intimately mixed carbonates and hydroxides, which contribute to a shift of the MMCT states to higher energies. Consequently, we demonstrate unambiguously for the first time that Tb3+ luminescence can be excited by VO43- → Tb3+ energy transfer and by direct population of the 5D4 state in YVO4. We also discuss how thermal treatment removes these effects and shifts the quenching MMCT state to lower energies, thus highlighting the major consequences of defect density and microstructure in nanosized phosphors. Therefore, our findings ultimately show nanostructured YVO4:Tb3+ can be reclassified as a UV-excitable luminescent material.

High-sensitivity dual UV/NIR-excited luminescence thermometry by rare earth vanadate nanoparticles.

High-crystallinity rare earth (RE) vanadate nanoparticles doped with Tm3+, Er3+ or Ho3+ combine multiple emissions in red, green, and blue under dual UV/NIR excitation, promoting high performance self-referenced luminescence thermometry. Due to their very high chemical and thermal stability, these versatile single-phase compositions allow optical thermal sensing from cryogenic (77 K) to moderately high (673 K) temperatures with high reproducibility and low temperature uncertainty. Hence, these nanomaterials operate as optical thermometers in a very broad temperature range (∼600 K), owing to the availability of twelve emission intensity ratios for thermometry. The (Y,Yb,Tm,Er)VO4 powders showed temperature-dependent emission colours from yellow (77 K) to green (333 K) to red (673 K), with one of the highest thermal relative sensitivities reported so far for both upconversion (7.4% K-1) and downshift (2.7% K-1) nanothermometry with inorganic nanoparticles. The stability of the particles also allowed dual luminescence thermometry in aqueous colloids, which showed sensitive and stable thermometric behaviour. In addition, we also discussed the temperature variability against the NIR excitation power in colloids as an additional figure of merit to quantify the reliability of the thermometric response. In summary, our results confirmed that REVO4 with multiple emissions and UV/NIR excitability provide stable, reproducible, and sensitive temperature measurements, giving rise to an optically versatile tool for luminescence nanothermometry.