Enhancing Upconversion Luminescence In Dye-sensitized NaYF₄:Gd/Yb/Er@NaGdF₄:Yb Nanocrystals

In recent years,dye-sensitized rare earth upconversion nanomaterials are the hotspot of people’s research.Large and wide absorption cross-section of organic dyes can improve the problem that rare earth upconversion nanocrystals have a weak absorption cross-section.These two characteristics above can convert energy with a efficiency through the action of the antenna.Therefore,this material is widely used in various fields such as imaging,display,solar cells and anti-counterfeiting.Here,in this work,IR806 dye is mainly used as the energy donor of the organic antenna,and the small size Na YF₄:Gd,Yb,Er@NaGdF₄:Yb core-shell structure nanocrystal energy acceptor is considered as the luminescent center and explore its luminescence performance and upconversion mechanism.Firstly,the optimal reaction conditions of organic dyes and nanocrystals were discussed experimentally.The effects of different dye solvents,water and oxygen environments,and nanocrystal surface ligands on the optical properties of dye-sensitized nanocrystals were investigated.It was found that the organic dye IR806 had the best effect when it was dissolved in anhydrous chloroform and sensitized with nanocrystals in an anhydrous oxygen environment.On this basis,the experiment used solvothermal method to synthesize small-sized（～ 6 nm）β-phase Na YF₄: Gd,Yb,Er single-core nanocrystals.Due to the heavy atom effect of Gd³⁺,the luminescence performance was explored by changing the ratio of it’s ion doping in the inner core,and then the organic dye sensitization experiments were conducted on nanocrystals with different doping contents to investigate the effect of dye doping concentration on the luminous intensity.The experimental results finally showed that when Gd³⁺ doping content in the inner core is 50%,the reaction concentration of the organic dye IR806 is 1 μg/m L,and the fluorescence intensity at 540 nm is about 1550 times stronger than that of the unsensitized nanocrystals.Based on the absorption spectrum and energy level diagram,the upconversion luminescence mechanism of dye-sensitized upconversion single-core nanocrystals was explored.Finally,Na YF₄:Gd,Yb,Er nanocrystals were coated with active shell of 1～2 nm by injection method.The results showed that changing the concentration of Yb³⁺ in the shell did not change the thickness of the shell,and the XRD detection result was still β phase,indicating that the crystalline phase of core-shell structure nanocrystals synthesized by this method did not change while achieving the controllable synthesis of the shell.After that,dye sensitization experiments were conducted under the same conditions,and it was found that when the Yb³⁺ doping amount is 40% and the optimal concentration of the dye is 5 μg/m L,the luminous intensity of the nanocrystals at 540 nm reaches the strongest,and the sensitivity compared with mononuclear nanocrystals before chemical enhancement are about 26200 times stronger.Subsequently,the luminous mechanism of this system was explored again.Through a series of studies,it was found that water and oxygen have a great influence on the triplet state of the organic dye IR806.At the same time,for a certain concentration of Gd³⁺,the heavy atom effect can induce the triplet state,which is beneficial to the generation of upconversion and improves the upconversion luminescence strength.In addition,the paramagnetic properties of Gd³⁺ are conducive to broadening the application of the material in biological fields such as nuclear magnetic resonance imaging.