Ultra‐Small YPO₄‐YAG:Ce Composite Nanophosphors with a Photoluminescence Quantum Yield Exceeding 50%

Abstract Synthesis of high quality colloidal Cerium(III) doped yttrium aluminum garnet (Y₃Al₅O₁₂:Ce³⁺, “YAG:Ce”) nanoparticles (NPs) meeting simultaneously both ultra‐small size and high photoluminescence (PL) performance is challenging, as generally a particle size/PL trade‐off has been observed for this type of nanomaterials. The glycothermal route is capable to yield ultra‐fine crystalline colloidal YAG:Ce nanoparticles with a particle size as small as 10 nm but with quantum yield (QY) no more than 20%. In this paper, the first ultra‐small YPO₄‐YAG:Ce nanocomposite phosphor particles having an exceptional QY‐to‐size performance with an QY up to 53% while maintaining the particle size ≈10 nm is reported. The NPs are produced via a phosphoric acid‐ and extra yttrium acetate‐assisted glycothermal synthesis route. Localization of phosphate and extra yttrium entities with respect to cerium centers in the YAG host has been determined by fine structural analysis techniques such as X‐ray diffration (XRD), solid state nuclear magnetic resonance (NMR), and high resolution scanning transmission electron microscopy (HR‐STEM), and shows distinct YPO₄ and YAG phases. Finally, a correlation between the additive‐induced physico‐chemical environment change around cerium centers and the increasing PL performance has been suggested based on electron paramagnetic resonance (EPR), X‐ray photoelectron spectrometry (XPS) data, and crystallographic simulation studies.