Uncovering Different Responses and Energy Mechanisms of Sensitizer and Activator in Host Manipulation for Upconversion Nanoparticles.

Abstract

Agile and efficient upconversion luminescence (UCL) fine-tuning strategies are the most demanded for in the frontier applications of highly doped upconversion nanoparticles (UCNPs). By doping Zn2+ ions into NaHoF4 and NaGdF4:Yb3+ shells using the oleate method, the separate influences of Zn2+ on Ho3+ and Yb3+ ions in UCL-related processes were analyzed in detail, revealing relevant UCL changes and underlying energy mechanisms from a novel but explicit perspective. Different behaviors of green and red UCL before and after Zn2+-ion doping were attributed to the disparities in the energy pathways and features of the sample structures. Herein, the populations of 5S2/5F4 and 5F5 states, not the usually mentioned decay time, decided the UCL intensities of the NaHoF4@NaYbF4-structured highly doped UCNPs. The advantageous small sizes and intense single-band red UCL of these UCNPs were further developed by combining our previous strategies with introducing Zn2+ ions into the NaHoF4 matrix. Overcoming energy loss by surface quenchers and Zn2+-triggered inner defects is the key factor in maximizing 4f-4f transitions. To the best of our knowledge, the current study is the first attempt to date to experimentally reveal separate impacts of the heteroions on activators and sensitizers in UCL-related processes and can deepen the theoretical investigation of Ho-based UCL for the broadened applications of NaHoF4 UCNPs.