Synthesis And Luminescence Properties Of NaGdF₄:Nd³⁺Core-shell Nanocrystals

As a biological fluorescent probes applied in biological imaging, the near-infrared to near-infrared lanthanide (Ln3+) ions-doped fluorescence nanomaterials have a great many advantages.In recent years, sodium gadolinium tetrafluoride (NaGdF4), as a kind of excellent lanthanide doped host material, has good chemical stability and low phonon energy, and has been widely studied. Nd3+ions have high application value in near-infrared to near-infrared emitting materials, and have the large absorption cross section and high quantum yield. The surface defects of nanomaterials reduce the luminous efficiency. And surface modification can reduce the surface defects of nanocrystals and improve the luminous efficiency.In this paper, we have synthesized NaGdF4:Nd3+nanocrystals and different core-shell ratio core/shell NaGdF4:3%Nd3+@NaGdF4nanocrystals using solvothermal. The main research includes the following three aspects:(1) The preparation of hexagonal NaGdF4:Nd3+nanocrystals. NaGdF4:Nd3+nanocrystals was synthesized using solvothermal. The prepared nanocrystals characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The phase of all nanocrystals is hexagonal,and dispersivity is good, uniform particles and narrow diameter division. NaGdF4:Nd3+core nanocrystals have an average size of6nm. Characterize the prepared nanocrystals luminescent properties by photoluminescence spectrum. The NIR emission was observed. The spectrum band is sharp and large stokes shift. The decay curves of NaGdF4:Nd3+nanocrystals had been studied. The surface defects of nanomaterials reduce the luminous efficiency.(2) Improving the efficiency of NaGdF4.-Nd3+nanocrystals. The luminous efficiency of the sample was improved successively through the study of optimum doping concentration of Nd3+and coated homogeneous shell NaGdF4method.①Characterize luminescent properties of NaGdF4:Nd3+nanocrystals doped with1,3,6, and10%Nd3+, the luminescent property of NaGdF4:3%Nd3+is best. With the concentration of Nd3+further increasing, the concentration quenching effect is clearly observed.②In order to reduce the surface defects of nano materials, homogeneous shell NaGdF4was coated. The luminous intensity of NaGdF4:Nd3+@NaGdF4is stronger, and luminescence lifetime of Nd3+emission increased. The results show that the quenching at or near the surface of the nanocrystals was reduced.(3) NaGdF4:Nd3+nanocrystals and NaGdF4:Nd3+@NaGdF4nanocrystals with different shell thicknesses were successfully prepared. The NaGdF4:Nd3+nanocrystals and NaGdF4:Nd3+@NaGdF4were obtained with average sizes of6nm,10nm and11nm,13nm, respectively. The prepared nanocrystals’luminescent properties and lifetime were characterized by photoluminescence spectrum and decay curve. The core/shell nanocrystals’luminous intensity is stronger than that of the core nanocrystals. Compared to the core nanocrystals, the fluorescence lifetime of core/shell nanocrystals increases significantly. With increasing of the shell thickness, the fluorescence lifetime of core/shell nanocrystals becomes much longer. The shell NaGdF4improves the luminous efficiency by reducing the surface defects. Within a certain range of shell thickness, thicker shell can protect the emitting Nd3+ions on the surface of core nanocrystals more effectively.