Structure,performance And Application Exploration Of Rare-earth Ions Doped Single Upconversion Micro/Nanoparticle

Rare earth（RE）ion doping upconversion（UC）luminescence is a photoluminescence（PL）process that relies on the rich energy level characteristics of RE ions,in which the sequential absorption of two or more low-energy photons leads to high-energy luminescent emission.Back in the mid-20th century,the UC luminescence from typical RE ion couple and the physical mechanism of UC have been studied widely.In recent years,duo to the achievement of controllable synthesis of UC micro/nanomaterials,the rapid development of micro/nanotechnology has further pushed the UC research to a new climax.RE ions doped UC micro/nanoparticles with irreplaceable advantages,including various emission bands and lifetime,absence of on-off blinking and photo-bleaching,deep excitation penetration,no overlay with cellular autofluorescence,as well as toxicity,have gained wide attention in biomedicine,3D displays,sensing and photovoltaics.One will easily find that nearly all of the prevalent research on UC micro/nanoparticles are focused on the optical performance of the ensemble spectroscopy techniques.Little effort has been contributed to single particle UC luminescence behavior.In fact,seemingly identical particles have different characteristics,which are important for the optimization of the material’s macroscopic performance,and even have significant guidance for the development and application of new performance.And when considering micro/nano-devices,the precise acknowledgement of single micro/nanoparticle is particularly important.Based on above,we have prepared a series of well monodispersed RE ion single-or co-dopedβ-Na YF₄microcrystals（MCs）,which can be used to investigate the properties at single particle level,and the optical as well as structure performance of these single MC were investigated in detail,especially the anisotropy and UC light polarization characteristic.Finally,based on the research of these single MC,the applications of RE ion doped single UC micro/nanoparticle in optical waveguide,white light modulation,white light-emitting micro/nanolaser,microscopic multi-information transportation and security anti-counterfeiting were explored.Our detailed research includes the following four parts:（1）Base on the Yb³⁺/Pr³⁺co-doped singleβ-Na YF₄MC,we have investigated the optical anisotropy and structure anisotropy.Different PL spectra variation dependence on excitation polarization orientation at different excitation directions indicate singleβ-Na YF₄:Yb³⁺,Pr³⁺MC has obvious optical anisotropy.While the detected different Raman spectra at different excitation directions demonstrate the structure anisotropy of singleβ-Na YF₄:Yb³⁺,Pr³⁺MC.According to the analysis of local site symmetry and electron cloud distribution of REs in hexagonal structure by density functional theory（DFT）calculation,it has revealed that the optical anisotropy of single MC originates from the structure performance,and established the relationship between microstructure and optical properties for the first time,which provides a powerful theoretical basis for the subsequent study about the light polarization characteristic of different RE ions in anisotropic structures.Besides,the structure anisotropy of singleβ-Na YF₄:Yb³⁺,Pr³⁺MC well explained the difference in Raman spectra among theβ-Na YF₄:Yb³⁺,Pr³⁺MC powders with different doping concentration of Pr³⁺,indicating the necessary of investigation about single particle.（2）The UC light polarization characteristics of different RE ions inβ-Na YF₄anisotropic structure were investigated in detail:when the RE ion single-doping ion into theβ-Na YF₄MC,with the increase of ion radius,the UC light polarization characteristic decreases gradually;while under the sensitization of Yb³⁺,the UC light polarization characteristic of active ion enhanced gradually with the increase of ion radius.After the detail analysis of crystal structure,local symmetry and properties of RE ions themselves,as well as the calculation of electron cloud distribution and bonding structure,it is proposed that the light polarization characteristic of RE ions not only depends on the crystal structure and site symmetry,but also on the radius and polarizability of the RE ions themselves,which provides a theoretical basis for the subsequent achievement of tunable UC light polarization characteristics.（3）Choosing Er³⁺as emission center,its tunable UC light polarization characteristic were realized successfully through co-doping with other RE ions:under the excitation of 980nm,based on the sensitization of Yb³⁺,changing Yb³⁺concentration can adjust the UC light polarization characteristic of Er³⁺,and even at a suitable doping level,the UC light polarization characteristic of Er³⁺could be eliminated;while co-doping with Gd³⁺,the larger polarizability will make the electron cloud distribution of adjacent Er³⁺more anisotropic,as a result,enhanced light polarization could be obtained;increasing the doping concentration of Er³⁺can also effectively improve its UC light polarization characteristics.These methods are universal and applicable to other emission centers.As an inherent property of anisotropic materials,realizing tunable light polarization not only makes UC materials own better performance in difference application fields,but also makes the UC light polarization characteristic from RE ions own wider application prospect.（4）Based on the precise understanding of RE ions doped singleβ-Na YF₄MC,we have explored their application prospects in different fields:first,the refractive index ofβ-Na YF₄higher than that of air can lead to total internal reflection of the emitting light at the crystal-air interface and directional propagation along the c axis,therefore,RE ion doped singleβ-Na YF₄MC can be used in a near-infrared excited optical waveguide system.Second,under the excitation of 980 nm,a convenient white light-emitting at single particle level was obtained by Yb³⁺/Pr³⁺couple,providing a more efficient solution for miniaturized and integrated white light systems.Meanwhile,taking advantage of the stable white light-emitting polarization characteristic from singleβ-Na YF₄:Yb³⁺,Pr³⁺MC or MC array can realize microscopic multi-information transportation.Combined with the optical waveguide effect,it is expected to output white light laser through the whispering wall mode in a singleβ-Na YF₄:Yb³⁺,Pr³⁺MC,providing a new solution for the realization of white light-emitting micro/nanolaser.Finally,different fluorescence colors and light polarization characteristics from different RE ion doping combinations make RE ion doped singleβ-Na YF₄MC carry unique optical information,which can be widely used in security fields,such as optical anti-counterfeiting,information storage,and information encryption.