Energy Loss Mechanism And Means Of Suppression In Core-Shell Upconversion Nanocrystals

Rare-earth ion-doped upconversion nanocrystals have unique luminescence properties,which can absorb two or more low-energy excitation photons and emit high-energy photons,and thus have promising applications in biomedicine,solar cells,optical anti-counterfeiting,etc.However,the high surface-to-volume ratio of upconversion nanocrystals and the strong bursting effect of adsorbed groups or surface defects on the luminescence center lead to the low luminescence efficiency of upconversion nanocrystals.The construction of core-shell structure is a common method to enhance the luminescence intensity of upconversion nanocrystals,and the inert shell layer can physically isolate the surface burst sites from the luminescent nuclei to achieve luminescence enhancement.Nevertheless,the luminescence efficiency of core-shell upconversion nanocrystals is still 1～2 orders of magnitude lower than that of bulk materials,and it is important to systematically investigate the energy loss mechanism and develop effective suppression means to promote the practical use of upconversion nanomaterials.In this paper,we successfully prepared core-shell upconversion nanocrystals with different composition and structural parameters by co-precipitation and successive layer by layer（SLBL）,investigated the influence of the thickness and type of inert shell layer and Yb³⁺doping concentration on the upconversion luminescence performance of core-shell nanocrystals,analyzed the energy loss mechanism of core-shell nanocrystals,and explored the effective means to suppress the energy loss of core-shell upconversion nanocrystals.The details are as follows.Firstly,three sets of core-shell structured nanocrystal（NaYF₄:x Gd/20%Yb/2%Er@NaYF₄,with core sizes of 27.7,14.6,and 9.8 nm;increasing shell thickness from 1 to 12 nm in steps of1～3 nm）with different core sizes were successfully prepared by co-precipitation and SLBL methods to investigate the effect of inert shell thickness on the upconversion luminescence properties of core-shell nanocrystals.The effect of inert shell layer thickness on the conversion luminescence performance on core-shell nanocrystals was investigated.The results show that the upconversion luminescence intensity of the three groups of nanocrystals shows a trend of increasing and then decreasing with the increasing of the shell layer thickness,which is mainly because the increase of the inert shell layer thickness can suppress the surface burst effect on the one hand,and lead to the decrease of the effective luminescence volume fraction at the same time,and the two interact with each other,so there is an optimal shell layer thickness.At the same time,it is found that the maximum luminescence intensity of nanocrystals with different nuclear particle size corresponds to different shell layer thicknesses,which indicates that the nuclear particle size must be considered when designing the shell layer thickness of nanocrystals with core-shell structure in practice.In addition,it is found that the luminescence intensity per effective volume and the energy level lifetime of Yb³⁺excited states of the core-shell nanocrystals with different nuclear particle sizes both saturate at the shell layer thickness of about 10 nm,indicating that the energy loss path of core-shell nanocrystals mainly comes from the action of the surface burst source on Yb³⁺ions,whose maximum action distance is around 10 nm.Secondly,the variation pattern of the luminescence performance of the conversion on nanocrystals with different Yb concentrations of core-shells with respect to the thickness of the shell layer was investigated.It was found that the shell layer thickness corresponding to the maximum luminescence intensity increased with increasing Yb³⁺concentration,indicating that the high Yb³⁺core-shell nanocrystals are more susceptible to the surface burst effect.In addition,it was found that the luminescence intensity per effective volume and Yb³⁺luminescence lifetime of the high Yb³⁺concentration core-shell nanocrystals still did not show saturation at the shell layer thickness greater than 12 nm,which was speculated to be possibly related to the entry of Yb³⁺ions into the inert shell layer through ion exchange during the shell cladding.Finally,the effects of three inert shells,NaLuF₄,NaYF₄ and NaGdF₄,on the luminescence performance of nanocrystal upconversion were comparatively investigated,and it was found that the different inert shell layer components have important effects on the luminescence performance of core-shell nanocrystals.The results show that the NaLuF₄ inert shells exhibit better inhibition of surface bursts,which is attributed to their relatively small dielectric constants,thus weakening the dipole moment interactions between the rare-earth active ions and the surface burst groups.By comparing the luminescence performance of the three groups of inert shells in different dispersion states,it is found that the luminescence performance of NaLuF₄ inert shell nanocrystals is significantly better than the other two types of shell layer nanocrystals in strong burst environments（e.g.aqueous systems）,which is especially suitable for biomedical fields（aqueous environments）.