Synthesis And Luminescence Properties Of NaGdF₄:Yb,Tm Upconverting Nanocrystalline/Semiconductor/Au Composites

Rare earth fluoride has low non-radiative decay probability and high luminescent quantum yield due to its low phonon energy,so it is considered as one of the promising rare earth up-conversion luminescent materials.However,in the experimental process,the preparation of fluoride has always been a difficult problem,because it is not easy to control its size and morphology in the synthesis process,so explore to solve these problems and design a feasible experimental scheme,so as to prepare a small size,Fluorides up-conversion luminescent materials with uniform distribution and high conversion efficiency have become one of the research hotspots in the field of luminescent materials in recent years.In addition to enhancing up-conversion emission,the core/shell structure can,in principle,be used to enrich the properties of lanthanide doped nuclei by appropriately selecting functional shell materials,such as semiconductors.These materials can give lanthanide doped nanoparticles unprecedented properties that would be difficult to achieve with a single component,thus providing the opportunity to achieve fascinating and innovative luminescent materials,so it is very interesting to study the law of interaction between semiconductor materials and up-conversion materials.Most of the studies on up-conversion luminescence of Au nanoparticles are based on the surface plasmon resonance(SPR)of Au nanoparticles.However,in those reports about Au nanoparticles,surface plasma resonance of Au nanoparticles can affect both fluorescence quenching and fluorescence enhancement,which shows the opposite results.At the same time,there are few reports on the effect of intact Au shells on up-conversion luminescence,and there is a lack of consensus on the law of physical properties of up-conversion luminescence materials/Au composite structures.Therefore,it is of great significance to study the mechanism of surface plasmon influence on up-conversion luminescence.In this paper,NaGdF₄is used as the matrix material,by doping rare earth ions(Yb,Tm),and coating the semiconductors,Surface plasma shell for its surface modification,so as to make it into a multi-functional core-shell structure nano composite material,to study its luminescence properties and mechanism.The specific research contents and results are as follows:(1)The core-shell composite structure NaGdF₄:Yb,Tm@Cu_2-xS@Au has been successfully synthesized,which is small,uniform in size and soluble in water.The thickness of Cu_2-xS shell is about 2 nm,and the thickness of Au shell is about 3 nm.Compared with the nanoparticles with only one Cu_2-xS shell,the up-conversion luminescence intensity was increased by 1.28 times,1.32 times,1.72 times and 2.64 times,respectively,corresponding to the emission of Tm³⁺at 360 nm,450 nm,475 nm and 805nm,respectively,which realized the regulation of the luminescence intensity.The luminescence mechanism of NaGdF₄:Yb,Tm was investigated.The reasons for the decrease of luminescence intensity after covering Cu_2-xS shell and the effective increase of luminescence intensity after covering Au shell were analyzed,which were due to the absorption of Cu_2-xS and the distance between Au and NaGdF₄:Yb,Tm instead of direct contact.(2)A novel core-shell structure composite,NaGdF₄:Yb,Tm@ZnS:Mn@Au,is constructed.It is small in size and uniformly dispersed.And it can be converted both up and down.The thickness of ZnS:Mn shell is about 0.8 nm,and the thickness of Au shell is about 0.4 nm.The up-conversion luminescence intensity of the Au shell is selectively enhanced compared with that of the ZnS:Mn nanomaterials with only one layer.The emission at 360 nm,475 nm and 805 nm is enhanced by 2.56 times,1.55 times and 5.38times,respectively,while the emission at 450 nm is weakened to about 67%of the intensity of the uncoated Au shell.The up-conversion luminescence mechanism of NaGdF₄:Yb,Tm nucleus and down-conversion luminescence principle of NaGdF₄:Yb,Tm@ZnS:Mn core-shell structure are also studied in this experiment.And the effect of Au SPR on up-conversion luminescence intensity is analyzed.There are few reports on the core-shell structure of up-conversion nanocrystalline@semiconductor@Au.Therefore,this paper mainly synthesized NaGdF₄:Yb,Tm@Cu_2-xS@Au and NaGdF₄:Yb,Tm@ZnS:Mn@Au core-shell structure composites,and studied the mechanism of surface plasmas enhancing up-conversion luminescence of up-conversion material/semiconductor/Au composite structure.