Preparation And Time/Frequency Domain Spectral Design Of Rare Earth Fluoride Up Conversion Fluorescent Materials

Lanthanide-dopedβ-NaYF₄ upconversion（UC）micro/nanocrystals,that can convert several near-infrared（NIR）photons into one high energy photon,have received intensive attentions due to their promising applications in varying areas,e.g.,fluorescent probes,three-dimensional display,cell imaging,solar photovoltaic and anti-counterfeiting.However,rare earth’s luminescence emission derived from the 4f electron transition that is the parity forbidden,which results in low UC luminescence efficiency,difficult to distinguish single-particle fluorescence patterns,and lack of ultraviolet upconversion fluorescence.UC luminescence efficiency further was suppressed by surface effects,which restricts UC luminescence material’s practical application in various fields.Therefore,achieving high-efficiency up-conversion fluorescence emission,especially ultraviolet light emission and obtaining high-resolution single-particle spatial fluorescence pattern recognition,are still basic challenges in the field of luminescence and materials.The common strategy to control the upconversion（UC）fluorescence efficiency and color is to control the concentration of Re³⁺,but the quenching effect caused by high concentration seriously limits the further improvement of the fluorescence efficiency and its application.Aiming at the purpose of improving the fluorescence intensity and obtaining ultraviolet up-conversion fluorescence,this paper is based on the idea of enhancing the absorption of incident light and transient energy storage capacity,and designed a two-dimensional array structure to enhance the incident light absorption flux and suppress the absorption energy dissipation.The micro-rod cavity waveguide array on the top/bottom surface of the recess is used to manipulate the energy flux and amplify the UC fluorescence,thereby obtaining a clear fluorescence pattern of the array.Compared with the disordered stacking of microrods,the directional emission amplification of polychromatic fluorescence and the chrysanthemum shaped fluorescence pattern of array organization can be realized by enhancing the collection of incident light and changing the concentration of Yb³⁺.The design describes an energy flux control strategy,which converges the low-power incident beam into the photon hot spot of high field intensity region,and produces directional light amplification and clear three-dimensional light pattern.In the design framework of upconversion UV,a simple two-level system is used to couple Tm ions to suppress concentration quenching,and high concentration Yb sensitized upconversion fluorescence emission from Tm across infrared to UV is obtained,which is applied to anti-counterfeiting.The main contents of this paper are as follows:The first part:Focuses on reviewing the research progress and related theories and preparation schemes of rare earth luminescent materials,and analyzes the main problems currently existing and puts forward the design ideas and objectives of this article.The second part:Successfully synthesized three kinds of rare earth-doped NaYF₄ nano-arrays by solvothermal method.The regulation of the up-conversion fluorescence intensity of the three arrays was studied in detail.It was found that the array with recessed top/bottom surfaces had a significant enhancement effect on up-conversion fluorescence.By changing the concentration of dopant ions and Yb ions,a clear two-dimensional array of fluorescent patterns of various colors can be obtained.Based on the enhanced incident light and waveguide guided directional luminescence amplification fluorescence emission,the fluorescence enhancement mechanism is explained,and the theoretical and experimental basis and ideas are provided for the design and synthesis of efficient fluorescent materials.The third part:The NaYF₄:Yb/Tm microtubes with uniform size distribution were synthesized by the hydrothermal method assisted by sodium citrate,and the main factors affecting the fluorescence intensity and lifetime were studied by laser spectroscopy.Abnormally strong deep ultraviolet UC emission was obtained at 290 nm and 345 nm,and near ultraviolet emission was obtained at 361 nm.Under the excitation of near-infrared diode laser,NaYF₄:Yb/Tm microtubes have visible light emission at 452 nm and 478 nm and 803 nm.The near-infrared emission.With the increase of Yb³⁺doping from 10%to 80%,the fluorescence of different luminescent bands is enhanced,the luminous intensity of UV and blue light reaches about 56 times and 8 times,respectively,and the infrared fluorescence lifetime at 803 nm decreases from 1507μs to 43μs.The combination of sensitizer and activator based on Tm³⁺with ultra-long mid-level lifetime and Yb³⁺with simple two-level structure to overcome concentration quenching explains the mechanism of fluorescence enhancement of samples with high Yb concentration.The excellent up-conversion fluorescence characteristics of the NaYF₄:Yb/Tm microtubes here provide opportunities for anti-counterfeiting applications.