Research On The Surface Defect State Of Rare Earth-doped Up-conversion Nanoparticles And Exploration Of Defect Treatment Technology

Rare earth doped up-conversion nanoparticles have been extensively studied for their unique optical properties.However,the agglomeration and unsastified luminescence efficiency caused by the high-energy surface states of nanoparticles has seriously hindered their practical application.Although many successes on surface passivation of upconversion nanoparticles have been achieved,the intrinsic properties and formation mechanism of surface defect states remains to be elucidated and there is room for improvement in the up-conversion efficiency.In this study,?-NaGdF₄ and LuF₃ nanoparticles doped with different rare earth were synthesized by a thermal decomposition method.Moreover,the surface defects and their formation mechanism were studied by the application of aberration-corrected high-angle annular dark field scanning transmission electron microscopy?HAADF-STEM?for the first time.Based on the understanding of the formation mechanism of surface defects,two novel post-treatment strategies were designed to crystallize the surface layers of nanoparticles with different morphologies.The up-conversion emission intensity of post-treated nanoparticles was improved significantly by eliminating surface defect states effectively.The main contents of this paper are as follows:Firstly,the intrinsic properties and formation mechanism of surface defect states of?-NaGdF₄ nanoparticles was elucidated in this study.The surface defect state of the?-NaGdF₄ nanoparticles,which is namely the surface amorphous layer,is detected by transmission electron microscopy?TEM?.Further study showed that the amorphous precursors and organic ligand molecules in the solution could attach to their surface when large particles grow.If the reaction is terminated before the amorphous precursors are exhausted,the amorphous layer on the surface of nanoparticles would inevitably appear.The crystallization of amorphous layer on the surface of nanoparticles could be realized without particle agglomeration by prolonging the reaction time or applying shell-coating method,which could improve the surface defect state of nanoparticles.Unfortunately,the pro-longed reaction time results in uncontrollable increase of particle size,while shell-coating approach requires harsh experimental condition.Secondly,a novel wet chemical annealing process to modify the surface defect of nanoparticles were successfully explored,which is aiming to reconstruct the amorphous precursors on the surface of?-NaGdF₄ nanoparticles.In the study,it is found that both the degree of surface crystallization and morphology of the obtained nanoparticles is highly dependent on the annealing temperature.After annealing at 250°C,the intensity of the blue emission originated from NaGdF₄-250 nanoparticles achieves an order of magnitude improvement under 980 nm laser excitation.Meanwhile,the range of emission color of NaGdF₄-250 nanoparticles has been further expanded during the unsteady up-conversion process.All results show that the novel wet-chemical annealing method is a feasible method which could effectively improve the optical properties of the up-converting nanoparticles by eliminating their surface defect states.Finally,the wet-chemical annealing treatment will inevitably cause artificial damage to the morphology of special morphology?rod,sheet,etc?nanoparticles.In this study,we have successfully explored a simple and fast method for the post-annealing treatment of monodisperse LuF₃ nanoparticles via laser treatment.Spherical aberration correctedtransmission electron microscopy showed that the surface layer of synthesized sheet-like LuF₃ nanoparticle is filled with disordered atoms and organic ligand molecules.After the high-power near-infrared laser treatment,the morphology and phase of nanoparticles is not destroyed.However,the thermal effect caused by laser irradiationcauses the disordered atoms on the surface of nanoparticles turn into an ordered structure,along with the success desorption of some organic ligands.The experimental results show that the red light emission intensity of Er³⁺ions at 665 nm is nearly 103 times higher after laser annealing.Further study showed that the novel method could effectively eliminates the surface defect of nanoparticles by the crystallization of their surface layer.Meanwhile,the nanoparticles after the laser annealing treatment have lower phonon energy,which greatly reduces the probability of no-radiation transition and ultimately realizes the efficient up-conversion luminescence of LuF₃:Yb³⁺,Er³⁺nanoparticles.Moreover,the LuF₃ nanoparticles doped with different upconversion systems(Yb³⁺/Ho³⁺,Yb³⁺/Tm³⁺)have been significantly improved in up-conversion luminescence performance after laser treatment.All results demonstrate that the fluorescence efficiency of up-conversion nanoparticles could be effectively and greatly improved by laser annealing treatment,which will pave the way for further development and practical application of up-conversion single-particle luminescence nanoparticles.