Unraveling the Growth Process of Core–Shell Structured Upconversion Nanoparticles: Implications for Color Tuning of Upconversion Luminescence

The core–shell strategy has been routinely employed to improve emissive properties of upconversion nanoparticles (UCNPs). However, the underlying process of forming the core–shell structure still remains largely unexplored. In this work, we have performed a time-course study on the growth process of NaErF4@NaYF4 core–shell UCNPs to track time-dependent variations in size, phase, elemental distribution, and luminescence properties throughout the whole synthesis process. Our results show that the formation of core–shell UCNPs includes three major stages: formation of α-phase shell precursors, transition of the α-phase to β-phase, and seed-mediated growth. Among these stages, the first two mainly occur in the early stage of the growth process, while the last one dominates the subsequent process. During the seed-mediated growth process, the elemental distribution of the core–shell nanostructure is observed to change after a short time of reaching the reaction temperature and it continues throughout the rest of the growth process. Most importantly, the emissive properties also showed time-dependent variations, which were attributed to the activation of different optical pathways due to different shell thicknesses over time. This study is believed to provide fundamental insights into the growth mechanism of core–shell UCNPs, which can also be potentially applied for tuning the luminescence of other core–shell luminescent nanostructures.