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 (∆n): 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.