Synthesis And Upconversion Luminescence Properties Of NaGdF₄, GdF₃and CaSc₂O₄Phosphors Activated By Rare Earth Ions

Considerable attention has been focused on the research for the rare earthion-doped upconversion phosphors ever since the early1950s. The upconversionphosphors take advantage of the level characteristics of rare earth ion, which canabsorb several long-wave radiation with low energy and emit short-wave radiationwith high energy. It means that upconversion phosphors can convert the invisibleinfrared light to visible light. The rare earth ions activated upconversion phosphorshave received extensive attention in the past few decades because of their uniqueproperties and potential applications in the fields of luminescence devices, infrareddetection, high-density optical storage, anti-counterfeiting technology, medicalimaging, etc. This paper studies the rare earth ions activated upconversion phosphorsNaGdF₄, GdF₃and CaSc₂O₄.We have synthesized the NaGdF₄:Yb³⁺,Ho³⁺andGdF₃:Yb³⁺,Ho³⁺upconversion phosphors by a hydrothermal approach and synthesizedthe CaSc₂O₄:Yb³⁺,Er³⁺upconversion phosphors by high temperature solid statemethod. The major achievement obtained is as following:（1） Through to the control of the hydrothermal reaction conditions we haveachieved the transition between NaGdF₄and GdF₃upconversion samples. The crystalstructure can be converted from the mixed phase of GdF₃and hexagonal NaGdF₄tothe single hexagonal NaGdF₄phase by adjusting the molar ratio of F-/Ln³⁺. Then, wefixed the reaction temperature, time and the molar ratio of F-/Ln³⁺in the hydrothermalprocess, only adjusting the value of pH and we can get the single phase samples ofGdF₃and hexagonal NaGdF₄.（2） We have studied the influence of different reaction conditions to themorphology of samples and the dependences of the upconversion luminescence of different samples. We fixed the reaction temperature, time and the molar ratio ofF-/Ln³⁺in the hydrothermal process, then we have obtained diamond GdF₃:Yb³⁺,Ho³⁺samples, hexagonal prism, flake and spherical NaGdF₄:Yb³⁺,Ho³⁺samples byadjusting the value of pH and the molar ratio of Citrate/Ln. It was observed that thehexagonal NaGdF₄:Yb³⁺,Ho³⁺and GdF₃:Yb³⁺,Ho³⁺samples exhibit green（541nm）and red（647nm）emissions under980nm excitation, which are assigned to the5F₄，5S2â†'5I8and5F5â†'5I8transition of Ho³⁺,respectively. It was found that theupconversion efficiency of the hexagonal prism and flake NaGdF₄:Yb³⁺,Ho³⁺samplesare much stronger than that of GdF₃:Yb³⁺,Ho³⁺samples, but the sphericalNaGdF₄:Yb³⁺,Ho³⁺samples are weaker than GdF₃:Yb³⁺,Ho³⁺samples. This suggeststhat the upconversion efficiency of hexagonal NaGdF₄:Yb³⁺,Ho³⁺samples are greatlyimpacted by the different size and crystalline for different morphology.（3） We have synthesized the CaSc₂O₄:Yb³⁺,Er³⁺upconversion phosphors by hightemperature solid state method. The CaSc₂O₄:Yb³⁺,Er³⁺upconversion phosphorsshowed strong green （525⁵65nm） and red （652–674nm） up-conversion luminescencedue to the2H11/2â†'⁴I15/2，4S3/2(Er³⁺)â†'⁴I15/2(Er³⁺) and⁴F9/2(Er³⁺)â†'⁴I15/2(Er³⁺) transitionsof Er³⁺under a980nm semiconductor laser excitation at room temperature. Theoptimized doping concentrations for Yb³⁺and Er³⁺were6.0mol.%and2.0mol.%,respectively.