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

Tuning upconversion fluorescence emission of β-NaLuF_4:Yb^(3+)/Ho^(3+)nanocrystals through codoping Ce^(3+) ions

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.

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：Yb^3＋/Ho^3＋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,P 63/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 Ce^3＋ ion concentration, which is due to the similarity in ion radius between Ce^3＋ and Lu^3＋. Under 980 nm excitation, the UC emissions of β-Na Lu F^3＋4：Yb/Ho^3＋ nanocrystals with different Ce^3＋ codoping concentrations are carefully studied. The strong green and red UC emissions of Ho^3＋ions are observed in β-NaLuF4 nanocrystals. It can be found that the UC emission of Ho^3＋ ions is tuned from green to red in β-NaLuF4 nanocrystals through increasing Ce^3＋ ion concentrations from 0 to 12%, and the red-to-green（R/G） ratio is enhanced from 0.34 to8.44. According to the level structure of Ho^3＋ 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 Ce^3＋ ion is introduced into the system, the red UC emission intensity and R/G ratio of Ho^3＋ are increased, because the energy gap from the excited state ^5F7/2 to the ground state ^2F5/2 is about 3000 cm^-1 for Ce^3＋ ions, which is similar to the gaps of ^5S2/^5F4→^5F5 and ^5I6→^5I7 transitions of Ho^3＋ ions.According to the energy conservation law, the two inefficient nonradiative processes from the ^5 S2/^5 F4 and5 I6 states of Ho^3＋ ions are substituted in order by resonant cross relaxation（CR） processes ^5S2（^5F4）（Ho^3＋） ＋^2 F5/2（Ce^3＋） →^5F5（Ho^3＋） ＋^2F7/2（Ce^3＋） and ^5I6（Ho^3＋） ＋^2F（5/2）（Ce^3＋） →^5I7（Ho^3＋） ＋^2 F7/2（Ce^3＋） between Ho^3＋ and Ce^3＋ ions.These two resonant CR processes can transfer populations from the ^5S2/5 F4 state and ^5I6 state to the ^5F5 state and its intermediate ^5I7 state, respectively. The resonant modality and the strong interaction between Ho^3＋ and Ce^3＋ ions are employed to enhance the red emission and suppress the green emission. The occurrence of CR process between Ho^3＋ and Ce^3＋ ions is further proved by the down-conversion emission spectra of Ho^3＋ ions under 532 and 980 nm laser excitation,respectively. We demonstrate that the highly efficient red UC emission of β-NaLuF4：Yb^3＋/Ho^3＋/Ce^3＋nanocrystals offers opportunities as desired optical materials for color displays, anticounterfeiting techniques and multiplexed labeling applications.