Insight into the Luminescence Alternation of Sub‐30 nm Upconversion Nanoparticles with a Small NaHoF4 Core and Multi‐Gd3+/Yb3+ Coexisting Shells

Abstract It is absolutely imperative for development of material science to adjust upconversion luminescence (UCL) properties of highly doped upconversion nanoparticles (UCNPs) with special optical properties and prominent application prospects. In this work, featuring NaHoF 4 @NaYbF 4 (Ho@Yb) structures, sub‐30 nm core–multishell UCNPs are synthesized with a small NaHoF 4 core and varied Gd 3+ /Yb 3+ coexisting shells. X‐ray diffraction, transmission electron microscopy, UCL spectrum, UCL lifetime, and pump power dependence are adhibited for characterization. Compared with the former work, except for a smaller total size, tunable emission in color from red to yellow to green, and intensity from low to stronger than that of traditional UCNPs is achieved for ≈ 10 nm NaHoF 4 core size by means of changing number of layers and Gd 3+ /Yb 3+ concentration ratios in different layers. Besides, simultaneously doping Ho 3+ into the shells will result in lowered UCL intensity and lifted green/red ratio. Surface energy loss and sensitizing energy supply, which can be modulated with inert shielding of Gd 3+ and sensitization of Yb 3+ , are proved to be the essential determinant. More UCL properties of these peculiar Ho@Yb UCNPs are uncovered and detailedly summarized, and the findings can help to expand the application scope of NaHoF 4 into photoinduced therapy.