Laser-localized Passivation Of Surface Defect Structures In Lanthanide-doped Upconversion Nanocrystals

Lanthanide ions doped upconversion nanomaterials have attracted attention in the past decade due to their excellent light stability,clear line width,fixed spectral position,and low threshold pump power.They have been widely used in biological tissue imaging.However,knowing subtler biological tissues requires smaller upconversion particles for mark.One of the existing problems is that the size of upconversion particles is inversely proportional to their luminescence intensity.A smaller size means a higher surface-to-volume ratio and more surface defects.Researches shows that the increase of surface defects will increase the number of surface quenching centers,which will cause severe luminescence quenching,and the low luminescence efficiency severely limits its practical application.In order to enhance the fluorescence intensity of upconversion nanocrystals,a new method was explored to enhanced the fluorescence efficiency with 313 folds of Ba₂LaF₇upconversion nanocrystals with a size of 8 nm.The research contents of this article are as follows:(1)The hydrothermal method is used to synthesize Er³⁺doped bio-phase Ba₂Yb F₇ UCNPs.Wet chemical annealing method and change Yb³⁺ions concentration has been employed to obtain pure phase.We found that introducing La³⁺ions instead of Yb³⁺ions,the pure cubic Ba₂LaF₇ can be obtained by reducing the doping concentration of Yb³⁺ions.At this time,the luminescence intensity of cubic Ba₂LaF₇ UCNPs with sub-10 nm is slightly enhanced.In this article,we explore the mechanism of this enhanced luminescence in detail.(2)Because the brightness of the Ba₂LaF₇ upconversion nanocrystals obtained above is still not satisfying,it is not enough to be applied in actual life.Therefore,we have designed a new method to increase the brightness of the upconversion luminescence.We locally heated Ba₂LaF₇ through a focused 980 nm laser beam,so that the amorphous layer of atomic surface of Yb³⁺/Er³⁺co-doped Ba₂LaF₇ UCNPs to restructure without alteration of the dispersion and particle size.The surface defects are reduced and the red emission area is enhanced by 313 times.In addition,multiphoton emission microscopy was used to confirm that the laser treatment was the enhancement of the luminescence of individual particles.(3)At the same time,the laser treatment has also been applied to other fluoride systems(such as KLu₂F₇,Gd F₃,Ba Gd F₅),proving that laser treatment is generally effective for enhancing the luminescence brightness of upconversion nanocrystals.This provides a straightforward,simple,and fast way to achieve the bright luminescence of a single nanoparticle.It has laid a solid foundation for more efficient upconversion nanoparticles for biological imaging applications.