Integrated Strategy for High Luminescence Intensity of Upconversion Nanocrystals

The growing applications of upconversion nanocrystals in bioimaging, therapeutics, and photonics have given rise to a demand of high quality nanocrystals with desirable luminescence intensity. Although the design of optimal nanocrystals such as core-shell nanostructures has improved the intensity, the internal links between dopant concentration balance, epitaxial growth protection, and shell thickness effect encounter a compromised situation that lacks of integrated consideration and comprehensive assessment. Here we propose an integrated strategy based on a core-shell design for the enhancement of upconversion luminescence intensity. Epitaxial protection can enable higher activator accommodation capacity in limited spatial scale, which leads to an Er³⁺ concentration threshold improvement in β-NaYF₄ core-shell nanocrystals from 2 to 6 mol %. We further perform a comprehensive assessment of the nanocrystals with convincing performance improvement in ensemble spectroscopic intensity, upconversion quantum yield, and single nanocrystal intensity. Our findings provide improved understanding of electronic behaviors in multiphoton upconversion and opportunities for diverse applications requiring high quality upconversion nanocrystals.