Theoretical Simulation Of The Effect Of Energy Migration Between Sensitizers On Luminescence Efficiency In Upconversion Luminescent Nanoparticles

Lanthanide-doped upconversion nanoparticles have gained significant attention in various fields due to their superior physical and chemical properties such as large anti-Stokes shift,long lifetime,low toxicity,high chemical stability,and low autofluorescence background.However,its low UC luminescence efficiency(typically<1%)limits their widespread use.The related key issue is:what are the main channels/steps responsible for the loss of the excitation energy in the nanomaterials?Because the traditional rate equation theory cannot describe the energy transfer and migration well,the micromechanics of the upconversion process of nanoparticles is still unclear,which hinders the development of effective ways to improve the luminescent efficiency.In this thesis,the micromechanics of upconversion luminescence process of Yb³⁺/Er³⁺co-doped nanoparticles are studied by using Monte Carlo method.1.Upconversion luminescence processes of bare core-structure nanoparticles,NaYF₄:20%Yb³⁺,2%Er³⁺(Yb/Er),were simulate by using Monte Carlo method.When immersed in cyclohexane solution and water,several important factors affect energy migration were studied,including excitation power density,size and doping concentration.The results show that the high excitation power density can reduce the surface quenching effect;the smaller the size of nanoparticles,the more obvious the surface quenching effect.It is also found that the spontaneous composite process of doped ions is competitive with the surface quenching effect;the migration of excitation energy between sensitizers will also change with the change of doping ion concentration.2.Due to the relatively large surface-to-volume ratio of nanoparticles,a large number of lanthanide ions are located on the surface and exposed to the external environment,resulting surface-related quenching effects.It is an effective strategy to suppress the surface-dependent quenching mechanism by epitaxial core-shell structure to achieve high efficiency upconversion luminescence.We simulated core-inert shell nanoparticles NaYF₄:20%Yb³⁺,2%Er³⁺@NaYF₄(Yb/Er@Y)to investigate the effects of excitation power density,size and internal defects on the upconversion luminescence process.The results show that the increase of upconversion luminescence efficiency is limited by the saturation of the ions population in the intermediate excited level at high excitation power density,the energy migration is independent of the size of nanoparticles,and the higher the quality of the samples,the higher the luminescence efficiency of nanoparticles.3.The upconversion luminescence processes of two core-active shell structures,NaYF₄:20%Yb³⁺,2%Er³⁺@NaYF₄:20%Yb³⁺(Yb/Er@Yb)and NaYF₄:20%Yb³⁺,2%Er³⁺@NaYF₄:20%Nd³⁺(Yb/Er@Nd)were simulated.Under 980 nm excitation,the optimal shell thickness was obtained by tuning the migration layer thickness,which is 10 layers(8 nm)in cyclohexane solution and 4 layers(3.2 nm)in water.The upconversion luminescence efficiency increases first and then decreases,indicating that the excitation energy is completely consumed when the number of excitation energy migration steps in the nanoparticle reaches a threshold,which proves that the excitation energy can be transported in a long distance.At the same excitation wavelength of 800 nm,the luminescence efficiency is similar to the upconversion of Yb³⁺-sensitized cases,and the efficiency of energy back-transfer from activator to sensitizer is increased with the increase of migration layer thickness.We have obtained microscopic pictures of luminescence upconversion process of nanoparticles with different core-shell structures based on Monte Carlo method.Through the simulation of the UC curves of core-shell nanoparticles,we confirmed that the energy migration process between sensitizers has a significant effect on the UC process,and is not a negligible process as traditionally viewed.The results of this study will further improve the understanding of the micro-process of luminescence upconversion of nanoparticles,which is of great significance to suppress the energy loss channel,improve the luminescence efficiency,and promote their applications.