水热合成六方相NaYbF4：Er^3＋／Tm^3＋的上转换白光性质

White Up-conversion Emission of Hydrothermally Synthesized Hexagonal NaYbF_4∶Er^(3+)/Tm^(3+)

在反应温度为220℃,反应时间为48h的温和条件下,利用水热法合成单掺杂或共掺杂Tm3+、Er3+的六方相NaYbF4体系,利用X射线粉末衍射、紫外-可见-近红外漫反射吸收光谱以及荧光光谱等测试手段,分析体系的物相结构和荧光性能。在980nm红外激光的激发下,NaYbF4∶Er3+体系能发出强的绿光和红光,两者分别对应于Er3+离子的(2H11/2,4S3/2)→4I15/2、4F9/2→4I15/2能级跃迁;NaYbF4∶Tm3+体系能发出对应于Tm3+离子1D2→3F4和1G4→3H6能级跃迁的强蓝光;而NaYbF4∶Er3+/Tm3+体系能同时发出红蓝绿三种颜色的光;各发射的归属与单掺杂相同,但由于Er3+和Tm3+离子之间存在能量交叉弛豫,致使各发光强度发生变化。通过控制Er3+和Tm3+离子的浓度及其比例,可以调整NaYbF4∶Er3+/Tm3+体系的上转换蓝光、绿光和红光强度的比例,结果表明,在980nm红外激光的激发下,NaYbF4∶Er3+/Tm3+(0.4%/0.4%)能发出近似白光的上转换发射。因此,NaYbF4∶Er3+/Tm3+有望成为单一基质的上转换白光材料。

Over the past decades, there has been a significant interest in the production of new materials for multicolor displays. A new technique for solid-state color displays has been developed based upon infrared to visible frequency upconversion in lanthanide doped materials. The three primary colors red, green and blue can be obtained from upconversion luminescence of Er^3＋ , Pr^3＋ , Ho^3 ＋ and Tm^3 ＋ etc. However the relative intensities of the three primary colors are still required to eontrol. Thus, it is necessary to develop novel materials capable of muhieolor upconversion and investigate their upconversion mechanisms of rare earth ions employed. Rare earth doped fluoride crystals is one of the most attractive materials for near-infrared to visible upconversion because its low phonon energies. In this work, we present intense multicolor upconversion luminescence in Er^3 ＋/Tm^3＋ codoped NaYbF4. The relation between upconversion intensity of three fundamental colors and the dopant concentration was discussed and the corresponding upconversion mechanism was proposed. The generation and control of the primary colors through upconversion of Tm^3＋ and Er^3＋ in NaYbF4 with single excitation （980 nm）were obtained. The hexagonal NaYbF4 ： Er^3＋ , NaYbFa ： Tm^3＋ , NaYbF4 ： Er^3＋/Tm^3＋ upconverting phosphors have been prepared by hydrothermal method under 220 ℃ for 48 h. The crystal structure and luminescent properties were investigated by using powder X-ray diffraction, diffuse reflectance absorption spectra and fluorescent spectra.The result of powder XRD spectra reveals that Er^3＋ and Tm^3 ＋ ions can NaYbF4 lattice at any concentration by mild hydrothermal method. The shows that the absorption peak band centered at 957 nm corresponds to incorporate completely into hexagonal diffuse reflectance absorption spectra ^2F7/2→^2F5/2 transition of Yb^3＋ ion for NaYbF4, and the absorption peaks of NaErF4 or NaTmF4 could be assigned to the transitions from ground state of Er^3＋ （4115/2） or Tm^3＋ （3 H6 ） to excited states. Under 980 nm excitation, hexagonal NaYbF4： Er^3＋ can emit 4 4 strong green and, red emission, which correspond to （^2H11/2, ^4S3/2 ）→^4I15/2, ^4F9/2→ ^4I15/2 transition of Er^3＋ ,respectively. NaYbF4： Tm^3＋ exhibit intense blue upconversion emission, which is attributed to ^1D2→^3F4 and ^1G4→^3H6 transition of Tm^3＋ The emission intensity of blue transitions （ ^1G4→^3H6, ^1D2→^3F4 ） is enhanced considerably as increasing Yb^3＋ content and reach the optimum at NaYbF4： Tm^3＋. We agree with the view that the excitation is mobile within the NaYbF4 lattice, it could thus readily reach the Tm^3＋ traps, which cause the high relative intensity of the two blue emissions （^1G4→^3H6, ^1D2→^3F4）. Tm^3＋ and Er^3＋ ions can be easily eodoped in NaYbF4 lattice through the mild hydrothermal route, and an energy transfer between Tm^3 ＋ and Er^3 ＋ exists under 980 nm laser excitation ： ^1D2 （Tm^3 ＋ ） ＋ ^4I15/2 （ Er^3 ＋ ） → ^3F4（Tm^3＋ ） ＋ ^4S3/2 or ^2H11/2（Er^3＋ ）,^1G4（Tm^3＋ ） ＋ ^4I15/2（Er^3＋ ）→^3H6（Tm^3＋ ） ＋^4S3/2 or ^2H11/2（Er^3＋ ）.