Effect of light scattering on upconversion photoluminescence quantum yield in microscale-to-nanoscale materials.

Scattering affects excitation power density, penetration depth and upconversion emission self-absorption, resulting in particle size -dependent modifications of the external photoluminescence quantum yield (ePLQY) and net emission. Micron-size NaYF4:Yb3+, Er3+ encapsulated phosphors (∼4.2 µm) showed ePLQY enhancements of >402%, with particle-media refractive index disparity (Δn): 0.4969, and net emission increases of >70%. In sub-micron phosphor encapsulants (∼406 nm), self-absorption limited ePLQY and emission as particle concentration increases, while appearing negligible in nanoparticle dispersions (∼31.8 nm). These dependencies are important for standardising PLQY measurements and optimising UC devices, since the encapsulant can drastically enhance UC emission.

Microwave-Assisted Solvothermal Synthesis of Upconverting and Downshifting Rare-Earth-Doped LiYF4 Microparticles.

Growing attention toward optically active materials has prompted the development of novel synthesis methods for a more reliable and efficient access to these systems. In this regard, microwave-assisted approaches provide unique advantages over traditional solvothermal methods reliant on convectional heating: namely, significantly shorter reaction durations, more rigid reaction conditions, and thus a higher degree of reproducibility. Reported herein for the first time is a rapid synthesis of rare-earth (RE3+)-doped LiYF4 upconverting and downshifting microparticles with well-defined bipyramidal morphology and good size dispersion via a microwave-assisted solvothermal process. The suggested material growth mechanism identifies a suitable Li+ to RE3+ ion ratio, an abundance of pH-sensitive acetate surface-capping ligands, and an appropriate reaction temperature/time profile as crucial for enabling a phase transformation of an intermediary yttrium ammonium fluoride phase into LiYF4 and subsequent particle ripening. The versatility of the reported method is highlighted by its extension toward the synthesis of other state of the art M(RE)F4 (M = alkali metal) optical materials: RE3+-doped LiYbF4 microparticles and ß-NaGdF4 and α-NaYF4 nanoparticles. All of the obtained Yb3+/Er3+- and Yb3+/Tm3+-codoped M(RE)F4 materials exhibited characteristic upconversion emission, while downshifting capabilities were induced through Ce3+/Tb3+ codoping of LiYF4. Further attention was devoted to single-particle optical characterization via hyperspectral imaging of Yb3+/Er3+- and Yb3+/Tm3+-codoped LiYF4 microparticles to explore the spatial variability of upconversion emission within individual particles.