Brightening heavily doped upconversion nanoparticles by tuning characteristics of core-shell structures

Heavily doped upconversion nanoparticles (UCNPs) potentially have exceptional photon upconversion abilities that are promising for diverse applications, such as lasing and super-resolution microscopy. However, heavily doped UCNPs typically can only offer mediocre upconversion luminescence intensity, and there still lacks general guidelines for the design and synthesis of heavily doped UCNPs. Herein, in order to boost the upconversion luminescence of heavily doped UCNPs, we studied the influence of characteristics of the core–shell structure on heavily doped UCNPs’ upconversion luminescence. We find that some empirical guidelines derived from conventional UCNPs are not suitable for heavily doped UCNPs. Using NaYbF₄:Tm@NaYF₄ core–shell UCNPs with a high concentration of Yb³⁺ as a representative, our studies reveal that a rather thick inert NaYF₄ shell is needed to protect the UCNPs from surface quenching, and the upconversion luminescence may undergo the cooperative sensitization process, which should be due to the highly concentrated Yb³⁺ dopant. In addition, the upconversion luminescence of heavily doped NaYbF₄:Tm UCNPs exhibits no obvious dependence on the type of inert shell. Furthermore, our results show that confining both Yb³⁺ and Tm³⁺ dopants in a thin layer (known as the δ-doping strategy) does not work well in the heavily doped UCNPs. Accordingly, we propose a NaYbF₄:Tm@NaYbF₄@NaYF₄ core–shell–shell structure to enhance the luminescence of heavily doped UCNPs, by weakening the dissipation of excitation energy and strengthening the absorption. These findings should be helpful to establish general design principles for developing the brightest possible UCNPs that can meet the requirements of various applications.