Improvement Of Luminescent Properties Of NaYF₄:Yb/Tm Nanoparticles By Coating And Codoping Of Ba²⁺

Rare-earth upconversion emissions have a lot of potential applications in many fields such as bioimaging,display devices and photocatalysis due to the unique optical properties,which can absorb near infrared light and emit visible light or ultraviolet light.However,up to now,there are existing serious problems,for example,the intensity and efficiency of upconversion emissions are too weak,which severely limit the application of upconversion emissions in various fields.And in order to resolve these problems,the works we mainly have done are shown as follows:1.Firstly,the hexagonal NaYF4:Yb/Tm(20/2mol%)upconversion nanoparticles(UCNPs)which can mainly emit blue light are synthesized through a solvothermal method.And according to the Transmission Electron Microscope(TEM)images and the X-ray diffraction(XRD)patterns,we prove that these NaYF4:Yb/Tm(20/2mol%)UCNPs distribute uniformly and have uniform particle size and good crystallinity.The crystal phase of NaYF4:Yb/Tm(20/2mol%)UCNPs is proved to be hexagonal.From the luminescence spectrum,we find that the NaYF4:Yb/Tm(20/2mol%)UCNPs can emit ultraviolet light,blue light and red light under the excitation of 980 nm.And the core-shell structured nanoparticles are synthesized by coating NaYF4 on the surface of NaYF4:Yb/Tm(20/2mol%)UCNPs.The intensity of upconversion emissions enhances 5 times after coating NaYF4 under the same measuring condition.2.In order to further improve the luminescent properties and study the influence of coating on luminescent properties,BaF2 is then coated on the surface of hexagonal NaYF4:Yb/Tm(20/2mol%)UCNPs.From the measurements of TEM images,XRD patterns and EDS images of BaF2 coated hexagonal NaYF4:Yb/Tm(20/2mol%)UCNPs,it is found that the BaF2 is successfully coated on the surface of hexagonal NaYF4:Yb/Tm(20/2mol%)UCNPs.And according to the luminescence spectrum,the optical properties of upconversion are improved after coating BaF2,especially the intensity of ultraviolet and red upconversion emission enhance about 20 times and 10 times.The influence of coating BaF2 on mechanism of upconversion is also discussed in detail.3.The influences of Ba ions doping on morphology,crystal structure and optical properties of nanoparticles are studied by doping different concentrations of Ba ions into hexagonal NaYF4:Yb/Tm(20/2mol%)UCNPs.According to the elemental analysis of samples,we prove that Ba ions are successfully doped into hexagonal NaYF4:Yb/Tm(20/2mol%)UCNPs.And from the luminescence spectrum,the intensity and luminescence efficiency of upconversion emissions are highest when the doping concentration of Ba ions is 15mol%and the ultraviolet and blue(450 nm)emissions enhance about 30 times.Besides,the mechanism of upconversion enhancement by Ba ions doping is also discussed by schematic crystal structure,energy diagram and luminescence decay curves.