Preparation and Characterization of GdVO4:Tb,Eu Nanoparticles/ Carbon Dots Composites via Hydrothermal Deposition and Physical Mixing for Luminescent Nanothermometry

Carbon dots (CDs) stand out for their facile synthesis, strong photoluminescence, and easy surface modification, making them excellent candidates for integration with other nanostructures to enhance physical and chemical properties.In this work, CDs derived from eco-friendly precursors (xylose and biomass-derived hemicellulose) are combined with GdVO₄:Tb,Eu nanoparticles (NPs) via two distinct approaches: (i) hydrothermal deposition onto the lanthanide-doped particles and (ii) physical mixing of prefabricated components. Notably, the spectroscopic properties of the resulting composites depend on the fabrication route. While method (i) enables competitive energy transfers from the vanadate charge transfer band (CTB) to Eu³⁺ and CDs, method (ii), which involves lower concentrations of the emissive components, promotes a cooperative mechanism wherein CDs sensitize the CTB, enhancing the Eu³⁺ red emission. In both scenarios, Tb³⁺ is believed to serve as an intermediary, aiding the repopulation of the Eu^{3+ 5}D_j energy levels. The dual-emissive nature of the composites results in violet-magenta chromaticity, reflecting intermediate behavior between the blue CDs and red-emitting Ln-NPs, and supporting their use in tunable optical applications. The described mechanisms also influence the composites' performance as ratiometric nanothermometers. Upon evaluating their thermal response from 298 K to 358 K, distinct behaviors emerge, with relative thermal sensitivities ranging from 0.84% K⁻¹ (298 K) to 5.6% K⁻¹ (358 K)—the latter being among the highest reported for similar materials.