共掺杂Ce~(3+)调控β-NaLuF_4:Yb~(3+)/Ho~(3+)纳米晶体的上转换荧光发射

在980 nm近红外光激发下,通过共掺杂Ce~(3+)离子调控六方相NaLuF_4:Yb~(3+)/Ho~(3+)纳米晶体的上转换荧光发射.实验结果表明,当掺杂Ce~(3+)离子浓度从0增加到12.0%时,Ho~(3+)离子的上转换荧光发射实现了由绿光向红光的转变,其红绿比提高了近24倍.根据Ho~(3+)离子的能级结构发现,Ho~(3+)离子的红光发射源自~5F_5能级到5I8能级的辐射跃迁,因此要增强红光发射,必须提高该能级粒子数布居.Ho~(3+)与Ce~(3+)离子之间相近的能级差促使它们之间产生了共振交叉弛豫,从而有效地提高了Ho~(3+)离子~5F_5能级的粒子数布居,增强了红光发射.同时对Ho~(3+)离子的上转换调控机理进行讨论,并借助不同的激发策略,进一步证实了Ho~(3+)与Ce~(3+)离子之间相互作用的发生.

Tuning upconversion fluorescence emission of β-NaLuF4:Yb3+/Ho3+ nanocrystals through codoping Ce3+ ions

Rare-earth-doped up-conversion (UC) fluoride materials have been widely used in phosphors, color displays, optical storages, solid-state lasers, solar cells and biomedical imaging, due to the fact that their low phonon energy can effectively suppress the nonradiative multiphonon relaxation process. In this work, the NaLuF₄:Yb³⁺/Ho³⁺ nanocrystals are successfully synthesized by a facile solvothermal method. The crystal structure and morphology of the NaLuF₄ nanocrystals are characterized by the X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) respectively. The diffraction peaks are well consistent with those of high-purity hexagonal NaLuF₄ (JCPDS No. 77-2042, P6₃/m space group). The TEM image reveals that the product is composed of monodisperse hexagonal rods with an average length of about 170 nm and an average diameter of 30 nm. The crystal structure and morphology do not present obvious change with the increasing Ce³⁺ ion concentration, which is due to the similarity in ion radius between Ce³⁺ and Lu³⁺. Under 980 nm excitation, the UC emissions of β-NaLuF₄:Yb³⁺/Ho³⁺ nanocrystals with different Ce³⁺ codoping concentrations are carefully studied. The strong green and red UC emissions of Ho³⁺ ions are observed in β-NaLuF₄ nanocrystals. It can be found that the UC emission of Ho³⁺ ions is tuned from green to red in β-NaLuF₄ nanocrystals through increasing Ce³⁺ ion concentrations from 0 to 12%, and the red-to-green (R/G) ratio is enhanced from 0.34 to 8.44. According to the level structure of Ho³⁺ ions, the red UC emission originates from the excited state ⁵F₅. However, the population of the ⁵F₅ excited state mainly depends on the two nonradiative relaxation processes of ⁵S₂/⁵F₄→5F₅ and ⁵I₆→5I₇ transitions. In fact, the two nonradiative relaxation processes are very difficult to occur according to multiphonon nonradiative relaxation rate. When Ce³⁺ ion is introduced into the system, the red UC emission intensity and R/G ratio of Ho³⁺ are increased, because the energy gap from the excited state ⁵F_7/2 to the ground state ²F_5/2 is about 3000 cm^-1 for Ce³⁺ ions, which is similar to the gaps of ⁵S₂/⁵F₄→5F₅ and ⁵I₆→⁵I₇ transitions of Ho³⁺ ions. According to the energy conservation law, the two inefficient nonradiative processes from the ⁵S₂/⁵F₄ and ⁵I₆ states of Ho³⁺ ions are substituted in order by resonant cross relaxation (CR) processes ⁵S₂ (⁵F₄) (Ho³⁺) + ²F_5/2 (Ce³⁺→⁵F₅ (Ho³⁺) + ²F_7/2 (Ce³⁺) and ⁵I₆ (Ho³⁺) + ²F_5/2 (Ce³⁺)→⁵I₇ (Ho³⁺) +²F_7/2 (Ce³⁺) between Ho³⁺ and Ce³⁺ ions. These two resonant CR processes can transfer populations from the ⁵S₂/⁵F₄ state and ⁵I₆ state to the ⁵F₅ state and its intermediate ⁵I₇ state, respectively. The resonant modality and the strong interaction between Ho³⁺ and Ce³⁺ ions are employed to enhance the red emission and suppress the green emission. The occurrence of CR process between Ho³⁺ and Ce³⁺ ions is further proved by the down-conversion emission spectra of Ho³⁺ ions under 532 and 980 nm laser excitation, respectively. We demonstrate that the highly efficient red UC emission of β-NaLuF₄:Yb³⁺/Ho³⁺/Ce³⁺ nanocrystals offers opportunities as desired optical materials for color displays, anticounterfeiting techniques and multiplexed labeling applications.