Energy Migration Mediated Upconversion Of Nanoparticles And Their Application

Lanthanide rare earth doped up-conversion nanoparticles（UCNPs）can convert long-wavelength excitation light into short-wavelength emission light,and have excellent physical and chemical properties（such as large anti-Stokes shift,low autofluorescence background,low toxicity and high penetration depth）,showing important applications in solid-state lasers,flat panel displays,optical communications,biological diagnosis,security and anti-counterfeiting.Control of energy process is an important strategy to improve and optimize the upconversion luminescence performance of rare earth.Recent studies revealed that energy migration tuned upconversion（EMU）is also an important upconversion luminescence method besides energy transfer upconversion.EMU can realize the upconversion emission of rare earth ions without intermediate state energy levels.By synthesizing core-shell structure nanoparticles,the sensitizer and the activator ions are spatially separated,and the energy can be effectively transferred from the sensitizer in the core to the activator in the shell to realize the luminescence of activator without long-lived intermediate energy levels(such as Eu³⁺,Tb³⁺,Dy³⁺,Sm³⁺,and Mn²⁺).Although there have been many related studies,the understanding of the EMU mechanism are still overlooked,especially the cross-relaxation,back energy transfer,and concentration quenching at the core-shell structure interfacial region,which have not yet been systematically investigated.Based on the above scientific issues,we propose a new strategy of using migration to enhance the EMU upconversion luminescence.By introducing an NaGdF₄ energy migration nano-layer into the conventional core-shell structure,the sensitizer in the core and the activator in the shell are completely separated but still with the energy migration channel,which allows a fine tuning of the ionic interactions of rare earth ions at the interface on the nanoscale.This design effectively inhibits the cross-relaxation and back energy transfer and successfully enhances the upconversion emission intensity.Utilizing long luminescence lifetime of Eu³⁺and Tb³⁺ions,the latent fingerprint identification and anti-counterfeiting applications of rare earth upconversion luminescent nanoparticles are studied.The research results obtained are as follows:（1）A series of NaGdF₄:Yb/Tm@NaGdF₄@NaGdF₄:A(A=Eu³⁺,Tb³⁺,Dy³⁺,Sm³⁺,Nd³⁺)core-shell-shell structure upconversion nanoparticles were synthesized by co-precipitation method.The thickness of the energy migratory NaGdF₄ layer was adjusted by controlling the reaction temperature,the content of the precursor rare earth ions,and the reaction time.Using this three-layer core-shell-shell structure,the upconversion luminescence enhancement of Dy³⁺,Sm³⁺and Nd³⁺have been realized.In-depth analysis of the crystal phase,particle size,optical properties,lifetime and energy level structure of rare earth ions of the upconversion luminescent nanoparticles confirmed that the design of the NaGdF₄ energy migration layer can reduce the sensitizer and activator ions at the core-shell interface to improve the luminescence efficiency of the activated ions.At the same time the NaGdF₄layer can serve as a"bridge"to facilitate energy transfer from the sensitizer in the core to the activator in the shell.It is further determined experimentally that the thickness of the energy migration layer is mainly limited to the range of 1-2 nm for the optimum upconversion.Finally,the use of Eu³⁺,Tb³⁺-doped upconversion nanoparticles with small particle size and long luminescence lifetime can realize the recognition of latent fingerprints and anti-counterfeiting of light emission.The secondary details of fingerprints can be clearly identified,which provides a new way for the application of upconversion luminescence in forensic medicine.（2）The NaGdF₄:Yb/Tm@Na XF₄:Mn²⁺（X=Gd,Y,Lu）and NaGdF₄:Yb/Tm@Ca F₂:Mn²⁺（X=Gd,Y,Lu）core-shell structured nanoparticles with different shell compositions were synthesizedbythermaldecompositionmethod,andthe NaGdF₄:Yb/Tm@NaGdF₄@Na XF₄:Mn（X=Gd,Y）core-shell-shell nanostructure was designed to further optimize the upconversion luminescence of Mn²⁺.Based on the dependence of the spectral intensity on the excitation power,the 500-600 nm（⁴T₁→⁶A₁）broadband emission of Mn²⁺under 980 nm excitation is determined to be a five-photon upconversion process.The thickness of the NaGdF₄ nano-layer（0-3 nm）was precisely controlled,and the 1.5 nm thick NaGdF₄ nano-layer contributes to the optimum upconversion luminescence of Mn²⁺.This indicates that there exists a strong quenching effect between Mn²⁺and Tm³⁺at the core-shell interface,and the introduction of the NaGdF₄ layer can effectively inhibit the cross relaxation between Tm³⁺and Mn²⁺,thereby enhancing the upconversion luminescence of Mn²⁺.Utilizing the long luminescence lifetime of Mn²⁺up to 34.62 ms as compared to the short luminescence lifetime of Tm³⁺,we have realized the light color anti-counterfeiting of revalent pattern by combining the time-gate technology.