Influence of Ho3+ and Yb3+ Concentration Distributions on Ce3+-Fine-Tuned Upconversion Luminescence in Highly Doped Core–Multi-Shell Upconversion Nanoparticles

Although widely used for Ho-based upconversion luminescence (UCL) fine-tuning of upconversion nanoparticles (UCNPs), Ce3+ doping has lacked comprehensive investigation in core–multi-shell and highly doped structures. Herein, a series of highly doped core–multi-shell UCNPs with Ce3+ doping were synthesised via oleate-based methods. The structures and morphologies were characterised by X-ray diffraction (XRD), transmission emission microscopy (TEM) and elemental mapping, while UCL properties were assessed using emission spectra, decay curves and pump power measurements. The distributions of Ho3+ and Yb3+ concentrations were systematically varied to explore, for the first time, their distinct effects on Ce3+-mediated UCL fine-tuning. These effects arose from different energy mechanisms and varied with the number of shell layers. A deficiency in phonon energy was found to reduce the efficiency of both Yb3+–Ho3+ energy transfer and Ho3+–Ce3+ cross-relaxation while also exacerbating surface energy quenching. Beyond mitigating the phonon energy shortage and providing greater shielding for surrounding Ho3+ ions, a low Ho3+ concentration in the core and a high concentration in the first shell (S1) were found to reduce the overall distance between Yb3+ and activated Ho3+ ions, enhancing energy transfer but increasing surface quenching. A high Yb3+ concentration in the second shell (S2) and a low concentration in the third shell helped to promote inward sensitising energy transfer and suppress surface quenching, though this configuration partially intensified the phonon energy shortage. UCNPs with the composition NaHoF4:10%Ce3+,40%Gd3+@NaHoF4:10%Ce3+@NaGdF4:10%Ce3+,60%Yb3+@NaGdF4:20%Yb3+ demonstrated optimal performance, exhibiting a red UCL decay time of 164.97 μs, a green-to-red intensity ratio of 0.43 and a quantum yield of 1.23%. Both metrics declined when Ce3+ dopants were confined only to S2. Multi-layer Ce3+ doping outperformed configurations with doping limited to the core or S1. Moreover, the emission colour of Ce3+-doped UCNPs remained tunable with variations in pump power. These findings provided valuable theoretical guidance for the design of Ce3+-doped and highly doped UCNPs and enhanced the multifunction potential of NaHoF4-based materials.