Upconversion luminescence characteristics of Ho<sup>3+</sup> ion doped single-particle fluoride micron core-chell structure

Constructing core-shell structures can effectively reduce the surface quenching effect of luminescent materials, which becomes an effective method to enhance upconversion luminescence. In this work, a series of NaLnF₄@NaLnF₄ (Ln = Y³⁺, Yb³⁺, Ho³⁺) core-shell microcrystals is successfully synthesized based on epitaxial growth technology, thereby enhancing and regulating the upconversion emission of Ho³⁺ ions. The results of the XRD and SEM indicate that the NaLnF₄@NaLnF₄ core-shell microcrystal possesses a pure hexagonal-phase crystal structure with a rod-like shape. Meanwhile, it is found that the epitaxial growth direction of the micro-shell is not affected by the crystal characteristics in the core, but determined by the crystal characteristics of the shell. Under 980 nm near-infrared laser excitation, the upconversion luminescence properties of single microrods with different core-shell structures are investigated via a confocal microscope spectroscopy. It is found that in the NaLnF₄ micro-crystal, the coated NaYF₄ inert shell can also effectively reduce the quenching effect on the surface of the micro-crystal for enhancing upconversion emission. When the Yb³⁺ ions are introduced into NaYF₄ or NaYbF₄ active shell that is coated, the Yb³⁺ ions in the shell can effectively transfer excitation energy to Yb³⁺ in the core through energy migration, and then establish new energy transfer channels, thereby realizing the Ho³⁺ ion luminescence enhancement. For NaHoF₄@NaYbF₄ core-shell microrods, the Yb³⁺ in the shell can transfer more excitation energy to Ho³⁺ ions at the adjacent interface for enhancing the overall luminescence intensity, and its higher red-green ratio is mainly due to the cross-relaxation process occurring between the Ho³⁺ ions at high doping concentration of Ho³⁺ in the NaHoF₄ core. Meanwhile, the luminescence process of the micron core-shell system is further confirmed based on the luminescence characteristics of different structures and the dynamic luminescence process. It can be seen that constructing different micron core-shell structures and introducing sensitizing ions, can not only effectively enhance the luminous intensity of the micron materials, but also adjust the output color. Therefore, this research is an important experimental reference for enhancing the luminous intensity of the micron system and the precise adjustment of luminescence, and can effectively expand the applications of micron crystals in the fields of displays, micron lasers and anti-counterfeiting.