Controllable preparation and upconversion luminescence properties of Tm³⁺/Ho³⁺ doped NaErF₄@NaYF₄ core-shell nanocrystals with bright red emission

In order to obtain upconversion nanomaterials with bright red emission for deep bioimaging applications, a series of Tm³⁺, Ho³⁺ doped NaErF₄@NaYF₄ core-shell upconversion nanocrystals were prepared by thermal decomposition method, and their morphology, structure and luminescence properties were characterized. The results show that NaErF₄, NaErF₄∶Tm³⁺ and NaErF₄∶Ho³⁺ bare core all exhibit hexagonal phase structure and good spherical morphology, with average sizes of 28.01 nm 23.19 nm, and 27.89 nm, respectively. After coating NaYF₄ inert shell, the crystal phase remains unchanged, but the morphology became short rod-like, and the average length increases to 37.82 nm, 38.51 nm, 42.65 nm. Under the excitation of 980 nm near-infrared light, NaErF₄, NaErF₄∶Tm³⁺ and NaErF₄∶Ho³⁺ show obvious red emission due to the ⁴F_9/2→⁴I_15/2 transition of Er³⁺, and the luminescence intensity and lifetime increase significantly after coating NaYF₄ inert shell. In particular, the luminescence intensity of NaErF₄@NaYF₄ core-shell sample is about 1787 times that of NaErF₄ bare core, and the lifetime is 2.04 ms. In addition, compared with NaErF₄@NaYF₄, the Tm³⁺, Ho³⁺ ions in the NaErF₄∶(Tm³⁺, Ho³⁺)@NaYF₄ system act as energy trapping centers and generate energy transfer with Er³⁺, resulting in a larger red-green emission peak ratio (R/G), and the luminescence color is closer to red, which is consistent with the chromaticity coordinate (CIE) color region. Finally, according to the relationship between luminescence intensity and excitation power, the upconversion luminescence enhanced mechanism and the possible energy transfer process are analyzed in detail.