Synthesis And Luminescence Performance Of Yb³⁺-doped Vanadate And Molybdate

Rare-earth(RE) is an enormous fortune for the development of luminescence materials, phosphors doped with different kinds of RE ions have attracted much attention from scholars at home and abroad. The transition between different energy levels of 4f electrons in REs can produce a variety of luminescence performance, and then RE luminescent materials have been widely applied in LED, display, laser, biomedical and opto-electronics. Trivalent ytterbium ion Yb3+ has very simple energy levels of the ground state 2F7/2 and excited state 2F5/2, then Yb3+ is often used as sensitizer in up-conversion luminescence because of this special property, and cooperative luminescence of Yb3+ can be realized under the irradiation of the pump laser. Besides,Yb3+ can emit the infrared emission around 1000 nm with the excitation of UV-visible light, which is attributed to the 2F5/2→2F7/2 transition of Yb3+ ions. And this down-conversion luminescence has potential application for spectral convertor to enhance the efficiency of silicon based solar cells.In general, the composition, structure, physical and optical properties of host are critical factors for improving the luminescence efficiency of RE-doped phosphors.Among various compounds, vanadate and molybdate are ideal matrix for their special crystal structure, excellent physical and chemical stability, as well as high optical damage threshold.In this work, vanadate and molybdate were selected as hosts, Yb3+-doped vanadate and molybdate were prepared by high temperature solid-state reaction or Pechini method to investigate their up- and down-conversion luminescence. Their composition and phase purity were verified by X-ray powder diffraction(XRD). The morphology and size distribution of particles were characterized using scanning electron microscope(SEM).Meanwhile, the diffuse reflectance spectrum, photoluminescence excitation(PLE) andemission(PL) spectra were measured to investigate the influence of Yb3+ doping concentration and structure of hosts on the luminescence performance. Besides, the luminescence decay curves can help to discuss the energy transfer(ET) process and mechanisms of up- and down-conversion. And photoluminescence quantum efficiency(PL-QE) of the down-conversion material can be compared with related materials to evaluate its potential application.In chapter 3, the cooperative luminescence material of Ca9Yb(VO4)7:Yb3+ was synthesized by high temperature solid-state reaction, its composition and structural properties were characterized by XRD, which indicated that the prepared samples were well crystalized with a pure phase, and no impurity lines were observed. Besides, the luminescence property were investigated with PLE and PL spectra, Ca9Yb(VO4)7:Yb3+can emit the infrared emission with multi peaks in 960, 980, 997 and 1025 nm under an266 nm excitation, which is corresponding to transition from the lowest Stark level 2F7/2to the excited state 2F5/2. What’s more, the cooperative up-conversion luminescence of Yb3+ in Ca9Yb(VO4)7 were investigated under the excitation of a 976 nm pump laser, the dependence of emission intensity on the pump power and luminescence decay curves were also studied to deduce the possible cooperative luminescence mechanism.In chapter 4, Ba Gd2(Mo O4)4 doped with different concentrations of Yb3+ ions were prepared using high temperature solid-state reaction. The diffraction peaks in XRD were compared with the standard PDF card to analyze their composition, space group and detailed cell parameters. Meanwhile, the morphology, crystallinity and size distribution of particles were verified by SEM. Besides, the infrared emission of Yb3+ in Ba Gd2(Mo O4)4 was characterized using PLE and PL spectra, as well as the diffuse reflectance spectrum. The luminescence quenching was investigated from the dependence of luminescence intensity on Yb3+ doping concentration. At last, the QEs of samples with various doping contents were measured to evaluate their potential application as converters to increase the efficiency of conventional Si solar cells.In chapter 5, Yb3+ doped MZn V2O7(M=Ca,Sr,Ba) and Zn2V2O7 were prepared byhigh temperature solid-state reaction and Pechini method, respectively, and their structural features and luminescence properties were investigated. The phase composition,structure and morphology were studied by XRD and SEM at first. Then the luminescence property was characterized with PLE and PL spectra, an intense infrared emission in900-1100 nm attributed to the 2F5/2→2F7/2 transition of Yb3+ under the excitation of UV light was observed in Yb3+doped MZn V2O7(M=Zn, Ca, Sr, Ba), and the absorption bands of pyrovanadates is located in the region of 300-450 nm. Besides, the absolute QEs and luminescence decay curves were measured to discuss the mechanism of near infrared emission and the efficient energy transfer from VO4 group in vanadate to Yb3+ ions.What’s more, the connection between infrared emission efficiency and the structural difference in Yb3+ doped Ca Zn V2O7, Sr Zn V2O7 and Ba Zn V2O7 was mainly researched,and a critical conclusion was obtained that the PL quantum efficiency of Yb3+ infrared emission had a dependence on the distortion of VO4 in the lattices.In this work, the preparation processes, structural features and typical luminescence properties of Yb3+ ions doped vanadate and molybdate were systematically studied. The cooperative luminescence of Yb3+ in Ca9Yb(VO4)7 and UV to infrared converter of single Yb3+ doped Ba Gd2(Mo O4)4 were firstly investigated. Besides, the dependence of Yb3+infrared emission efficiency on the distortion of MZn V2O7(M=Zn, Ca, Sr, Ba) was discussed, and Yb3+ doped MZn V2O7 were expected to be an efficient converter to increase the efficiency of Si solar cells, which could also provide valuable information for improving efficiency in the use of solar energy.