Study Of The Synthesis And Upconversion Luminescence Properities Of Heterogeneous Ions Doping In NaYF₄

Lanthanide ions doped upconversion nanoparticles?UCNPs?have a wide range of potential applications as a new generation of functional nanomaterials.However,the low luminescent efficiency,mainly attributing to the weak and narrow absorptions,has greatly hampered the transformation of UCNPs-based technologies from experimental research to real world applications.In recent years,many methods,including core-shell structure,high concentration activator doping,dye sensitization,noble metal plasmon resonance,and heterogeneous ion doping,have been tried in terms of improving upconversion?UC?luminescence intensity and efficiency.The heterogeneous ion doping can tune the local symmetry of the substrate and adjust the UC luminescence performance without changing the performance of the substrate.In this paper,NaYF₄ is used as the matrix,and four kinds of divalent metal ions,Cu²⁺,Ni²⁺,Ba²⁺and Pb²⁺,which are less studied at present,are selected for heterogeneous ions,a series of NaYF₄:Yb³⁺,Er³⁺,M(M=Cu²⁺,Ni²⁺,Ba²⁺and Pb²⁺)samples were synthesized via hydrothermal method using sodium citrate as a complexing agent.Characterization methods as X-ray diffraction?XRD?,Scanning electron microscope?SEM?,Energy dispersive Spectrometer?EDS?and Fluorescence spectrophotometer were used to investigate their crystal phase,morphology,UC performance and elemental composition.This work mainly studies the influence of heterogeneous ionic radius,type and doping concentration on the UC performance,and discusses the regularity of the hetero-valence doping on the crystal change of the UC material and the principle of UC performance enhancement.The experimental results showed that the four heterogeneous ions doping can increase the UC luminescence intensity of NaYF₄:Yb³⁺,Er³⁺to different extents.The luminescence of the green and red emission generated by the(²H_11/2,⁴S_3/2)?⁴I_15/25/2 and ⁴F_9/2?⁴I_15/2 energy level transitions of the activated ions Er³⁺can be observed in the visible light region by the fluorescence spectrophotometer.Under the comparative analysis of various characterizations,the effects of different ion doping on NaYF₄:Yb³⁺,Er³⁺crystals are different.Moreover,the regularity of the hetero-valence doping on the crystal change of the UC material is not obvious,which needs to be further studied.?1?Cu²⁺ion doping?0¹0mol%?induced the shrinkage of NaYF₄ crystal lattice,and also changed the crystal morphology of NaYF₄.After doped,the NaYF₄ crystal showed a flower-like structure.Compared with NaYF₄:Yb³⁺,Er³⁺microcrystals without doping of Cu²⁺ions,the green and red emission intensities of the microcrystals co-doped with 7.5%Cu²⁺ions were boosted by 6.31 and 4.71 times,respectively.The power dependency the UC luminescence confirmed both the red and green UC emission of NaYF₄:Yb³⁺,Er³⁺,Cu²⁺are governed by three photon absorption process.?2?In the case of Ni²⁺ion doping?0²5mol%?,the low doping concentration resulted in lattice expansion,but as the concentration increases,the crystal size became smaller.The doping of Ni²⁺caused a change in the morphology of the NaYF₄ crystal,and the lattice distortion caused by doping also led to defects on the surface of the NaYF₄ crystal and generated a flower-like structure.A significant enhancement of the green and red UC emissions of 13.04-and 11.75-fold,respectively,was achieved with 15%Ni²⁺co-doping.At this point,UC luminescence is a two-photon absorption process.?3?Ba²⁺ion doping?0²5mol%?led to the lattice expansion of NaYF₄ and generated other impurities.The crystal morphology of NaYF₄ also changed irregularly,and the particle size was not uniform,which indicated that the doping of Ba²⁺ions could not control the morphology of NaYF₄ crystal.It was found that the UC intensity reached a maximum at the doping concentration of 15%Ba²⁺,and the UC intensities of the green and red emission were increased up to 11.00 and 3.98 times,respectively.At this point,UC luminescence is a two-photon absorption process.?4?In the case of Pb²⁺ion doping?0²0mol%?,the lattice of NaYF₄ was expanded with Ba²⁺ion doping,but Pb²⁺ion doping brought about a phase transition of NaYF₄ crystal.In the images of SEM,the cubic phase NaYF₄ was adsorbed on the hexagonal NaYF₄ surface at a low concentration doping?0¹0%?.The hexagonal phase NaYF₄ crystal were completely covered with the cubic phase NaYF₄ when the doping concentration is 20%.It was found that,when incorporating 5%Pb²⁺ions,the UC red and green emission intensities were drastically enhanced by a factor of 15.72 and 17.33,respectively.After calculation,it was also found that green and red emission is a three-photon absorption luminescence process.