NaYF4 upconversion crystals with red light emission by low Er3+ concentration doping

In the work, a range of Er3+-sensitized NaYF4 upconversion (UC) particles doped with Tm3+ and Ho3+ are successfully prepared by hydrothermal process. The UC crystals appear two green emission peaks at 529 and 541 nm and a remarkable red emission peak at 655 nm under 980 and 1532 nm irradiations. Here, Tm3+ and Ho3+ serve as trapping centers, which confine the excitation energy efficiently to promote the red UC emission and improve the red to green ratio. UC mechanisms in the Er3+/Tm3+ co-doped and Er3+/Ho3+ co-doped NaYF4 crystals are discussed at length based on the UC luminescence spectroscopy. Unlike the traditional sensitization with high Er3+ concentration doping to obtain red light, the red UC luminescence is acquired by low Er3+ concentration doping, which can effectively avoid concentration quenching caused by high Er3+ concentration doping. Furthermore, NaYF4:Er3+, Tm3+ crystals exhibit stronger red UC emission compared with NaYF4:Er3+, Ho3+ counterparts. Er3+-sensitized NaYF4 crystals doped with Tm3+ and Ho3+ ions generate pure red light more efficaciously under 1532 nm excitation than under 980 nm excitation. The phosphors emitting brilliant red UC emission have the potential applications in solid state lasers and biological imaging.