Increased downconversion efficiency and improved near infrared emission by different charge compensations in CaMoO4:Yb3+ powders

All of the samples were synthesized by sol-gel methods. Two approaches to charge compensation, (i) 2Ca2+→Yb3++M+, where M+ is an alkali ion like Li+, Na+ and K+, and (ii) indirect charge compensation: 3Ca2+→2Yb3++vacancy, were studied in detail. It was found that charge compensation would be very beneficial for the growth of the grains, especially in Li+ ions added samples. All the grains were homogeneously spherical with less boundaries; in addition, a great variety of the absorption ability in different charge compensation samples were observed: in comparison with the phosphors without charge compensation, indirectly charge compensated and Li+ ions added phosphors showed much stronger absorption strength in the ultraviolet (UV) region whereas that of Na+ and K+ ions added samples was much weaker; moreover, measurements of the emission intensities showed that: in comparison with the phosphors without charge compensation, the visible emission intensity from MoO42− decreased a lot in indirectly charge compensated and Li+ ions added phosphors, whereas there was a remarkable increase of the near infrared (NIR) emission intensity from Yb3+ ions in the two types of samples under 266 nm excitation, implying more efficient energy transfer (ET) from MoO42− to Yb3+ ions; at last, measurements and analysis of the decay curves of the visible 495 nm emission were carried out, and it was found that the energy transfer from MoO42− to Yb3+ ions were more efficient in the two above types of phosphors. The theoretical quantum cutting (QC) efficiency was also improved greatly. Overall, the addition of Li+ ions would be very beneficial for the morphology of the powders in addition to the growth of the grains. It was advantageous to increase the downconversion (DC) quantum efficiency; however, indirect charge compensation would enhance the NIR emission intensity to the most for its strongest absorption ability in the UV region.