Influence of Alkaline-Metals Co-Doping on Luminescence Intensity of Er-Doped Up-Conversion Oxide Phosphors

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Up-conversion (UC) phosphors, which can convert lower energy photons to higher energy ones, have been widely investigated owing to a wide range of applications, including bioimaging to visualize information in living organisms [1], solar cells to improve power conversion efficiency [2], and security inks to prevent counterfeiting [3]. However, in device applications, there are two big problems to overcome, those are 1) UC efficiency and 2) lifetime for long term use. Currently materials containing fluoride are often used for UC phosphors, but several types of oxides with rare-earth ions such as Y2O3:Er, Yb [4] and CaWO4:Er [5] have been reported as efficient UC phosphors with long lifetime. UC phosphors using organic solvents have also been reported to improve emission intensity, but they are difficult to apply to devices and are virtually unusable. To improve UC efficiency, it was reported that UC luminescence intensity can be enhanced by a co-doping of alkaline-metals in CaTiO3:Er [6].In this study, we have investigated the influence of alkaline-metal co-dopings on UC luminescence intensity of various types of oxides doped with Er ions. All the samples were prepared by a conventional solid state reaction method. The systematic analysis were carried out for the synthesized samples, i.e., the crystal structures and the phase analysis were examined by X-ray diffraction, the elemental analysis by X-ray fluorescence, energy levels and electronic structure analysis by diffuse reflectance with UV-vis spectrometer equipped with integration sphere, surface morphology and grain size observation by scanning electron microscope. In addition to such experimental analysis, first-principles calculations within a density functional theory have also been conducted to interpret the influence of alkaline metal co-dopings on the geometrical and electronic structures.As an example of our current UC measurements, UC luminescence spectra of CaMoO4:Er with and without alkaline-metal co-dopings are shown in Fig. 1, in which significant enhancement of UC luminescence has been achieved by alkaline-metal co-dopings in CaMoO4:Er.