Upconversion Emission And Electrical Property Of Rare Earth Doped Molydbate

Upconversion（UC） phosphors emit high-energy photons when they are excited by low-energy photons. The UC properties have been investigated for several decades, and remain a continuous research interest in various application field including in vivo imaging, biological labelling, solar cells, displays, sensor technology and so on. Inorganic molybdates have attracted a great amount of interest because of their significant technological importance and novel structure-property relationship. They have been widely used as electrode, catalysts, oxide ion-conducting materials, negative thermal expansion materials, high-performance luminescent materials and other functional materials on the basis of their promising optical, electronic, thermal properties. Tailoring UC emission of lanthanide ions doped in molybdates is of great importance and imperative for exploring the possible applications of these materials in multifunctional devices. Normally, there are several approaches to tailor the UC behavior, e.g. via varying chemical compositions of host materials, constructing core-shell nanostructure, employing sensitizers like dyes, noble metals, transition metal ions, etc. In this dissertation, we focus our attention on the other means of tailoring UC behavior, like electric field component of laser and migration of oxide-ion. The main research results of this dissertation are:（1） The Yb³⁺/Eu³⁺ codoped double perovskite molybdate and tungstate solid solution Sr2Ca（W,Mo）O6:Yb³⁺,Eu³⁺ serial samples are synthesized by traditional solid state reaction. Anomalous UC emission of the electric dipole transition ⁵D₀ â†' ⁷F₄ at ⁶90 nm of Eu³⁺ ions can be tailored by a 976 nm laser beam in Sr₂Ca（W,Mo）O₆:Yb³⁺,Eu³⁺ samples at various pumping power or temperatures, while the intensity ratios of the magnetic transition ⁵D₀ â†' ⁷F₁ at ⁵94 nm to the electric dipole transition ⁵D₀ â†' ⁷F₂ at ⁶12 nm change little. This could be most probably attributed to the electronic polarization effect of host materials with Mo on the electric dipole transition ⁵D₀ â†' ⁷F₄, which is proved by the larger dielectric permittivity of Sr₂Ca MoO₆ than that of Sr₂CaWO₆（108 Hz） according to the alternating current（AC） impedance spectroscopy measurement and further evidenced by the theoretical calculations of the dielectric function at high frequency（¹014 Hz of the 976 nm laser light）. The research gives an unusual perspective to tailor the UC behavior of lanthanide ions through the electric field of laser beam itself besides external electric field, varying chemical compositions and different codopants, etc, which might be inspiring in the design of opto-electric multifunctional devices.（2） The Yb³⁺/Er³⁺ codoped oxide ion conductors lanthanum molybdate La₂Mo₂O₉:Yb³⁺,Eu³⁺ samples are synthesized by traditional solid state reaction. Interesting UC emission of Eu³⁺ ions are detected when excited by a 976 nm laser beam in La2Mo2O9:Yb³⁺,Eu³⁺ at various temperatures. According to the results, the UC luminescence intensity ratio of 2H11/2â†'4I15/2 to ⁴F_9/2â†'⁴I_15/2 of Er³⁺ can reveal the activation process of oxide-ion and oxygen vacancy swapping at 150-200 °C in α-La₂Mo₂O₉, which has a break point. It is also evidenced and convinced by the temperature-dependent UC decay curves, temperature-dependent alternating current（AC） impedance data, temperature-dependent internal friction data and temperature-dependent Raman data. Also, single Er³⁺ ions doped La₂Mo₂O₉ is synthesized and temperature dependent Stokes photoluminescence are measured to exclude the influence of the energy transfer efficiency of Yb³⁺â†' Er³⁺ on the strange UC of Er3+ in La₂Mo₂O₉:Yb³⁺,Eu³⁺. The analysis according to Judd-Ofelt theory supports this point as well. The research may open a new perspective to study oxide-ion conductors and promote new application of luminescence.