| Lanthanide-doped upconversion luminescent nanoparticles have attracted widespread attention in recent years due to their unique optical properties,such as narrow-band multi-wavelength emission,large anti-Stokes shift,long luminescence lifetime and good photochemical stability,etc..It has a good application prospect in the fields of anti-counterfeiting,biological detection,biological imaging and solar cells,etc..However,its low luminous efficiency and small absorption cross section still greatly restrict the further development of the material.The energy transfer between sensitizer(such as Yb3+and Nd3+,etc.)and activator(such as Tm3+and Er3+,etc.),and between ions and surface defects or ligand molecules plays an important role in upconversion luminescence process.Therefore,research on the energy transfer process in upconversion nanoparticles has important scientific and practical significance.In this work,firstly,we designed a new core-shell structure of NaYF4:Yb/Tm@NaYF4:Yb@NaYF4:Yb/Nd upconversion nanoparticles,and studied the effects of anisotropic growth and multiphoton processes on the luminescence lifetime of upconversion nanoparticles under energy transfer and migration mechanism between rare earth ions.In addition,for NaYF4:Yb/Er upconversion nanoparticles in different aqueous solvents,we quantitatively analyzed the surface quenching effect caused by the energy transfer between rare earth ions and surface defects and ligand molecules.The innovative research results are summarized as follows:1.We designed a new core-shell structure of NaYF4:Yb/Tm@NaYF4:Yb@NaYF4:Yb/Nd upconversion nanoparticles which can be excited by 800 nm and 980nm laser respectively.And we prepared two kinds of upconversion nanoparticles with different shell-core ratios（1:1,3:1 and 7:1,fixed core scale and changing shell thickness）of isotropic growth and anisotropic growth by solvothermal method.The characterization results show that the synthesized nanoparticles have good stability and unity.Since Tm3+has lots of luminescence levels,its upconversion luminescence process has two-photon,three-photon and four-photon processes.2.We studied the effects of anisotropic growth and multiphoton processes on the kinetics of energy transfer upconversion luminescence.We measured the lifetimes of upconversion nanoparticles with isotropic growth and anisotropic growth at different shell-core ratios.The results show that as the shell-core ratio increases,the lifetime of upconversion nanoparticles increases.And since the anisotropic grown upconversion nanoparticles have a longer energy migration distance in the long axis direction,their lifetime changes are longer.In addition,compared with the excitation of 980 nm laser,under the excitation of 800 nm laser,since the light absorption of 800 nm exists only in the outermost shell layer,the distance of energy migration in upconversion nanoparticles is longer,and the lifetime extension is larger.The regulation of the energy transfer distance also makes the anisotropic growth upconversion nanoparticles have higher luminous efficiency.Next,we compared the lifetimes of the two-photon,three-photon and four-photon luminescence processes of anisotropic grown upconversion nanoparticles under different shell-core ratios.The results show that the function of energy transfer is related to the order of upconversion process（n value）.And the larger n is,the narrower half-width of the function curve is,but the peak position is unchanged.Therefore,the change in lifetime decay edge of different multiphoton processes is different,while the change of rising edge is basically same.3.We quantified the surface quenching effect of lanthanide-doped upconversion nanoparticles in four different aqueous solvents（water,methanol,ethanol and N,N-dimethylformamide（DMF））.In this work,a physical model of the relationship between ion-ion interaction and upconversion luminescence is established by using the Monte Carlo simulation method.Furthermore,the quantitative analysis results of the effects of four different aqueous solvents on the luminescence properties of NaYF4:Yb/Er upconversion nanoparticles were given.The steady state and kinetic spectral analysis results of upconversion luminescence of nanoparticles in different solvents show that the surface quenching rate of upconversion nanoparticles in water was the highest,followed by methanol and ethanol,and the lowest in DMF.We attributed this to the effect of hydroxyl groups and their activities on the surface quenching rate of upconversion nanoparticles.The quenching rates of upconversion nanoparticles in different four solvents were as follows:2.5?104 s-1（DMF）,1?105s-1（methanol and ethanol）and 5?105 s-1（water）.In summary,we have studied the energy transfer,migration and surface effects of upconversion nanoparticles.The results show that the anisotropic grown upconversion nanoparticles can achieve larger lifetime regulation range and higher luminous efficiency.The quantitative analysis of the surface quenching effect of upconversion nanoparticles in different aqueous solvents have great scientific significance for deepening the understanding of upconversion luminescence mechanism and for promoting the applications of upconversion nanoparticles in different solvents in different fields such as sensing,labeling and detection,etc.. |
