Decoupling of the Crystal Structure and Upconversion in HfO₂ Nanocrystals: Emission Chromaticity to Determine the Nanoscale Dopant Distribution

Abstract

The study of rare-earth (RE)-based upconversion in HfO₂ nanocrystals must consider the dual role of aliovalent doping. In fact, in this oxide, RE³⁺ doping not only induces luminescence but also perturbs the host material structure, leading to higher-symmetry polymorphs. Such a change of the crystal structure further affects the optical properties, making equivocal the interpretation of the upconversion luminescence (UCL) dependence on rare-earth sensitizer (Yb³⁺) and activator (Er³⁺) concentrations. We propose the introduction of an additional optically inactive dopant (Lu³⁺), enabling the investigation of the separate influence of the crystal structure and dopant concentrations on upconversion. For identical concentrations of Er³⁺ and Yb³⁺, the comparison of the cubic and monoclinic polymorphs of HfO₂ shows the essential role of the crystal structure alone on UCL. In cubic HfO₂, the emission intensity and chromaticity vary by more than 2 orders of magnitude depending on Er³⁺ and Yb³⁺ concentrations. Furthermore, the labeling of upconversion spectral features in phase-pure polymorphs and their correlation with the composition allow a clearer interpretation of UCL in phase mixtures, where the use of UCL chromaticity to determine the local rare-earth distribution reveals its intrinsic inhomogeneity at the nanoscale between different polymorphs. Hence, these results represent an important step toward highly efficient upconverting hafnia nanocrystals with tunable optical properties and provide a deeper understanding of doping distribution during the formation of mixed-phase particle systems.