Method for regulating and enhancing ultraviolet fluorescence of rare earth up-conversion nanocrystal through local structure and local structure measuring method

Abstract

The invention discloses a method for regulating and enhancing ultraviolet fluorescence of rare earth up-conversion nanocrystals through a local structure and a method for measuring the local structure, and belongs to the field of rare earth luminescent nanomaterials. According to the method, in the process of synthesizing the LiREF4: coated LiREF4 core-shell structure, the stoichiometric ratio of LiOH and NH4F in a core layer and / or the use amount of lithium trifluoroacetate in a shell layer are precisely regulated and controlled, and the localized crystal field environment around activated ions is changed, so that the selective regulation and control on the luminescence transition path of the activated ions are realized, and the emission intensity of multiphoton ultraviolet up-conversion luminescence is improved. According to the method, the high-order multi-photon up-conversion process of rare earth ions can be remarkably enhanced, meanwhile, low-order emission is inhibited, the luminescence selectivity of an up-conversion nano material is improved, emission light with the corresponding wavelength can be synthesized from the same matrix material according to the requirements of synthesizers, and the method can be suitable for various rare earth active ions and matrix materials and has a wide application prospect. The method has application potential in the fields of nano photonics and biomedicine.

Description

Technical Field

[0001] This invention belongs to the field of rare earth luminescent nanomaterials, specifically relating to a method for enhancing multiphoton upconversion luminescence of rare earth core-shell structured nanocrystals. Background Technology

[0002] Rare-earth-doped upconversion nanoparticles (UCNPs) are a class of functional materials capable of converting low-energy near-infrared light into high-energy visible or ultraviolet light through multiphoton absorption processes, showing broad application prospects in cutting-edge fields such as bioimaging, drug delivery, photodynamic therapy, 3D displays, and micro / nano lasers. Among these, multiphoton ultraviolet upconversion luminescence (UV-U) possesses unique advantages in photolithography, spectral analysis, and photocatalysis due to its short emission wavelength and high photon energy. However, UV upconversion typically involves high-order multiphoton processes (n≥4), resulting in significant energy loss and extremely low luminescence efficiency compared to common low-order visible upconversion (n≤3). Furthermore, the abundant electronic energy level structure of rare-earth ions often leads to the coexistence of multiple emission peaks during upconversion luminescence, which not only reduces the emission intensity at the target wavelength but also limits the application of materials in devices requiring monochromatic or multicolor selective light emission. Therefore, effectively controlling the selectivity of upconversion luminescence, particularly enhancing the luminescence intensity in the ultraviolet band, has become a current research challenge in this field.

[0003] Currently, common strategies for enhancing upconversion luminescence mainly include optimizing matrix materials, constructing core-shell structures, and controlling the type and concentration of dopant ions. Among matrix materials, the NaREF4 (RE=Y, Lu, Gd, etc.) series is widely recognized as the preferred matrix for upconversion luminescence due to its low phonon energy characteristics. NaYF4, in particular, has become the most widely studied classic system due to its excellent photon conduction efficiency and structural stability. Numerous studies have confirmed that in the NaYF4 matrix, enhancing Na... + The relative content of ions (i.e., Na-rich ions deviating from the ideal stoichiometric ratio) + Design is a key means to enhance upconversion luminescence efficiency: for example, Xiao et al., by modulating Na + Rare earth ions (Yb) 3+ The feed ratio is increased to 2.0:1 (higher than the ideal stoichiometric ratio of 1:1), so that Er 3+ The red-green visible upconversion luminescence intensity of the ions increased by 2.5 times and 3.2 times, respectively, and the core mechanism lies in the excess Na. + It can alleviate lattice strain, reduce vacancy defects, optimize the local coordination environment of activated ions, and suppress nonradiative transition losses; Weitzel et al. further confirmed that Na-rich... + The designed NaYF4 core-shell structure can reduce cation mixing at the core-shell interface and promote Yb 3+ Sensitized ions and Tm 3+ Activating energy transfer between ions increases the five-photon ultraviolet upconversion luminescence intensity at 345 nm by 210 times, directly supporting the "Na" in the NaYF4 system. + The traditional understanding is that "the higher the content, the better the high-order multiphoton luminescence performance." In addition, methods such as suppressing the surface quenching effect through core-shell structure design can mostly only improve the overall luminescence intensity of the material, and it is difficult to achieve selective enhancement of specific wavelengths (especially in the ultraviolet band). Moreover, the synthesis process is complex and the control methods are limited. In particular, achieving precise control of the local structure of activated ions through simple chemical synthesis parameter adjustment, and thus selectively enhancing high-order multiphoton ultraviolet upconversion luminescence, remains a huge challenge. In recent years, Li-based matrices (such as LiREF4 (RE=Y,Gd,Lu,Yb, etc.), or simply Li-based matrices) have gradually become a novel research system for upconversion materials due to their unique crystal structure and low phonon energy characteristics, showing potential advantages in ultraviolet luminescence modulation. However, existing studies directly follow the traditional design logic of the NaYF4 system, assuming that "increasing the alkali metal ion (Li) energy..." + The content of luminescent material can optimize luminescence performance, but related attempts have not achieved ideal results, and even the phenomenon of decreased luminescence efficiency has occurred.

[0004] Therefore, developing a simple, effective, and universally applicable method for local structure modulation of Li-based UCNPs, breaking through the traditional limitations of the NaYF4 system, and achieving efficient modulation of ultraviolet multiphoton upconversion luminescence has significant scientific research value and practical application value. Summary of the Invention

[0005] Based on the above, the inventors unexpectedly discovered in their preliminary research that: [the substance] is related to "Na" in the NaYF4 matrix. + The traditional rule that "higher content equals better luminescence performance" is contradicted in Li-based matrices. + The relative content of ions is not necessarily better the higher it is, but there is an optimal ratio range—when Li + When the concentration deviates from this range (too high or too low), the local crystal field environment (bond length, bond angle, coordination number, and local symmetry) of the activated ions undergoes abnormal distortion, leading to enhanced nonradiative transitions and a significant decrease in ultraviolet upconversion luminescence efficiency; however, by precisely controlling the Li content... + By maximizing the content to the optimal range, the localized structure of activated ions can be ordered, thereby selectively enhancing higher-order multiphoton ultraviolet upconversion luminescence.

[0006] Therefore, this invention first provides a method for enhancing the ultraviolet fluorescence of upconversion nanocrystals by adjusting the proportion of Li-based matrix to achieve local structure regulation.

[0007] In this method, the upconversion nanocrystals are LiREF4@LiREF4 core-shell structures. The local structure of the activating ions is altered by controlling the precursor stoichiometry during the synthesis process. This control is achieved through at least one of the following methods: (a) In core layer synthesis, the regulation of rare earth ion RE 3+ The molar ratio of RE to LiOH is 1:1.25 to 1:3.75. (b) In core layer synthesis, the regulation of rare earth ions (RE) 3+ The molar ratio of RE to NH4F is 1:3 to 1:5. (c) In the shell synthesis, the molar ratio of rare earth trifluoroacetate salt to lithium trifluoroacetate CF3COORE:CF3COOLi was controlled to be 1:0.5 to 1:2.5.

[0008] As some embodiments of the present invention, the control methods (a), (b), and (c) can be used individually or in any combination.

[0009] As some embodiments of the present invention, the RE in the core matrix is ​​selected from one or more of Y, Gd, Lu, and Yb, and is doped with rare earth activating ions and sensitizing ions.

[0010] As a preferred embodiment of some embodiments of the present invention, the activating ions are selected from Tm 3+ Er 3+ Ho 3+ At least one of them.

[0011] As a preferred embodiment of the present invention, the sensitizing ion is selected from Yb. 3+ 、Nd 3+ At least one of them.

[0012] As some embodiments of the present invention, in the LiREF4@LiREF4 core-shell structure, the RE in the shell matrix is ​​selected from one or more of Y, Gd, and Lu.

[0013] As a preferred embodiment of the present invention, changing the local structure of the activating ion refers to regulating the bond length, bond angle, coordination status and / or local symmetry of the local lattice around the rare earth ion.

[0014] This invention also provides a method for measuring the local structure of enhanced upconversion nanocrystals by regulating the local structure. The method measures the local crystal field environment (bond length, bond angle, coordination, local symmetry, etc.) around rare earth ions and obtains the above-mentioned lattice parameters by combining XRD structure refinement or X-ray absorption fine structure spectroscopy. This method is used to measure the influence of the Li-based matrix ratio on the local structure.

[0015] This invention also provides a method for measuring the local structure of upconversion nanocrystals by regulating the local structure to enhance their ultraviolet fluorescence. This involves first synthesizing a series of LiYF4:Eu@LiYF4 molecules using the same stoichiometric regulation method, and then utilizing the optical probe Eu... 3+ The red-orange ratio is sensitive to local symmetry, thereby measuring the local symmetry of the local lattice around rare earth ions, realizing the measurement of local structure, and can be used to reveal the photoluminescence behavior that is relatively sensitive to local symmetry.

[0016] This invention also provides a method for measuring the local structure of upconversion nanocrystals by regulating the local structure to enhance ultraviolet fluorescence. A series of LiYF4:Ce@LiYF4 molecules are first synthesized using the same stoichiometric regulation method, and then the optical probe Ce is used to measure the local structure. 3+ The sensitivity of the 4f→5d transition to local symmetry allows for the measurement of the local symmetry of the local lattice around rare earth ions, enabling the measurement of local structures. It can also be used to reveal photoluminescence behavior that is relatively sensitive to local symmetry.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Breaking through traditional limitations, establishing a new mechanism for the regulation of luminescence in Li-based matrices: This breaks the traditional understanding that "the higher the alkali metal ion content, the better the luminescence performance" in the NaYF4 system, revealing the new mechanism for the regulation of luminescence in Li-based matrices. + The core principle is the existence of an optimal ratio range. This can be achieved through precise control of the kernel layer RE. 3+ By adjusting the molar ratios of LiOH and NH4F, and the molar ratios of the shell CF3COORE and CF3COOLi, the local crystal field environment (bond length, bond angle, coordination number, and local symmetry) of the activated ions can be directionally optimized. This effectively suppresses nonradiative transition losses and achieves selective modulation of the luminescence transition path—significantly enhancing the intensity of high-order multiphoton (n≥4) ultraviolet upconversion luminescence while suppressing low-order visible emission. This solves the technical bottleneck of low ultraviolet luminescence efficiency and difficulty in achieving specific wavelength selective enhancement in existing Li-based upconversion materials. Furthermore, this modulation method is applicable to various rare-earth activated ions and matrix materials, and can be flexibly adapted to different wavelength ultraviolet luminescence requirements.

[0018] 2. The control method is simple and efficient, combining low cost and high performance: No complex equipment or special processes are required; precise local structure control can be achieved simply by adjusting the stoichiometry of the precursors during synthesis. The operation process is simple and highly reproducible. The synthesis uses a conventional oleic acid / octadecene system, with readily available raw materials and mild reaction conditions, avoiding dependence on expensive reagents or harsh reaction environments, significantly reducing the cost of large-scale preparation. Simultaneously, this control can shorten the distance between adjacent rare earth ions, accelerate the energy migration rate between sensitizer-activator ions, reduce the number of photons required for high-energy emission level population, and improve energy transfer efficiency. The prepared nanocrystals possess the advantages of uniform particle size, stable crystal form (tetragonal phase), and a uniform 1.25nm thick shell, effectively suppressing the surface quenching effect.

[0019] 3. Supporting characterization methods to expand application scenarios: A supporting Eu-based characterization method is proposed. 3+ Ce 3+ The method of measuring the local structure of optical probes, combined with X-ray diffraction refinement and synchrotron X-ray absorption spectroscopy, can quantitatively and intuitively characterize the structure-activity relationship between changes in local structure and luminescence behavior. This provides reliable technical support for a deeper understanding of the microscopic mechanism by which Li⁺ content affects upconversion luminescence, and also lays a solid experimental foundation for subsequent material design and optimization. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only one embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 A simplified schematic diagram of the synthesis of core-shell upconversion nanocrystals with tunable local structure.

[0022] Figure 2 The images show transmission electron microscopy (TEM) images and particle size distribution diagrams of the LiYbF4:Tm(1%)@LiYF4 upconversion nanocrystals prepared in Example 1 of this invention. Five LiYbF4:Tm(1 mol%) core samples with RE:LiOH ratios of 1:1.25, 1:1.875, 1:2.5, 1:3.125, and 1:3.75 were named C1, C2, C3, C4, and C5, respectively, for comparison. Five core-shell samples coated with an inert LiYF4 shell were named CS1, CS2, CS3, CS4, and CS5, respectively. The naming rules are the same in other examples. In the particle size distribution diagrams, the blue portion represents the particle size distribution of the core samples, and the green portion represents the particle size distribution of the core-shell samples.

[0023] Figure 3 The X-ray diffraction pattern of the LiYbF4:Tm(1 mol%)@LiYF4 upconversion nanocrystals prepared in Example 1 of this invention.

[0024] Figure 4 The upconversion emission spectra (a) of LiYbF4:Tm(1 mol%)@LiYF4 upconversion nanocrystals with different amounts of LiOH prepared in Example 1 of the present invention and the dependence of the integrated emission intensity of the upconversion ultraviolet and blue light emission bands on the amount of LiOH in the material (the inset shows the ultraviolet / blue light emission intensity ratio) (b).

[0025] Figure 5 The upconversion emission spectra of LiYbF4:Tm(1 mol%)@LiYF4 upconversion nanocrystals with different amounts of NH4F prepared in Example 2 of the present invention are shown.

[0026] Figure 6 The particle size distribution of LiYbF4:Tm(1 mol%)@LiYF4 prepared in Example 2 of this invention with different amounts of NH4F is shown.

[0027] Figure 7 The upconversion emission spectra of LiYbF4:Tm(1 mol%)@LiYF4 upconversion nanocrystals with different amounts of CF3COOLi prepared in Example 2 of the present invention are shown.

[0028] Figure 8 The photoluminescence pattern of the LiYF4:Ce(5 mol%)@LiYF4 down-transfer nanocrystals prepared in Example 4 of this invention is shown.

[0029] Figure 9 The fluorescence lifetimes of the series of LiYbF4:Tm(1 mol%)@LiYF4 upconversion nanocrystals prepared in Example 1 of this invention are shown at 347 nm (a) and 447 nm (b).

[0030] Figure 10 The variable power pump dependence plots of LiYbF4:Tm(1mol%)@LiYF4 upconversion nanocrystals prepared in Example 1 of this invention with different amounts of LiOH.

[0031] Figure 11 The diagram shows the changes in cell parameters of the refined LiYbF4:Tm(1 mol%)@LiYF4 upconversion nanocrystals prepared in Example 1 of this invention.

[0032] Figure 12 The upconversion emission spectrum of the LiYbF4:Ho(2 mol%)@LiYF4 upconversion nanocrystals prepared in Example 6 of this invention is shown.

[0033] Figure 13 The upconversion emission spectrum of LiYbF4:Er(2 mol%)@LiYF4 upconversion nanocrystals prepared in Example 7 of this invention is shown in the inset as an enlarged view of a portion of the spectrum.

[0034] Figure 14 The upconversion emission spectrum of LiYbF4:Tm(1 mol%)@LiLuF4 upconversion nanocrystals prepared in Example 8 of this invention is shown in the inset (the inset shows the UV / blue light emission intensity ratio).

[0035] Figure 15 The EXAFS spectra of CS1, CS3, and CS5 in Example 1 are shown. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only a small part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art to all other embodiments obtained without creative effort should be included within the protection scope of the present invention. The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] Existing research on Li in Li-based matrices + The regulatory mechanism of Li+ content on ultraviolet upconversion luminescence has not yet been elucidated, and there is a lack of methods to elucidate it through simple chemical synthesis parameters (such as Li+). + An effective method for precisely controlling the local structure (feed ratio) and thus selectively enhancing ultraviolet luminescence. To this end, the inventors have adopted the following methods... Figure 1 The examples of the synthesis method shown and the measurement results fully demonstrate the effectiveness of the method for enhancing upconversion nanocrystal ultraviolet fluorescence through local structure regulation according to the present invention.

[0038] Example 1: Preparation of core-shell nanoparticles LiYbF4:Tm@LiYF4 with different amounts of LiOH in the core layer: Weigh out 0.4 mmol of the corresponding rare earth thulium acetate solution and rare earth ytterbium acetate solution (where the molar ratio of Yb to Tm is 99:1) and add them to a two-necked flask. Then add 5 ml of oleic acid (OA) and 5 ml of octadecene (ODE) to the two-necked flask and stir at 150 rpm at 150 °C for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol and 1.25 mmol of the corresponding NH4F methanol solution and LiOH solution, mix them, and sonicate for 10 s (LiOH, RE) 3+Five samples with molar ratios of 1:1.25, 1:1.875, 1:2.5, 1:3.125, and 1:3.75 were prepared and named CS1, CS2, CS3, CS4, and CS5 for comparison. This mixed solution was then rapidly added to the precursor solution and stirred uniformly at 350 r / min and 50 °C for 40 min. The flask was transferred to a heating mantle and heated to 100 °C at a rate of 4 °C / min (heating and holding for a total of 30 min) to ensure the removal of methanol. A vacuum was then applied for 5 min, followed by purging with argon gas for 1 min; this process was repeated twice. The temperature was then raised to 290 °C and held for 90 min at a rate of 10 °C / min, with a stirring speed of 330 r / min during the heating and holding process. The mixture was then cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to the centrifuge tubes. Place in a centrifuge at 7000 r / min for 5 min. After removing the supernatant, wash twice with ethanol and cyclohexane. Finally, disperse in 4 ml of cyclohexane for later use.

[0039] Weigh out 0.4 mmol of the corresponding yttrium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution and add them to a 50 mL double-necked flask. Then add 5 mL each of OA and ODE, and stir at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, add 2 mL of the prepared LiYbF4:Tm core nanoparticles to the reaction solution, maintain a vacuum state, and heat to 120 °C and hold for 10 min. Then heat to 310 °C in an argon atmosphere and hold for 35 min, with a heating rate of 10 °C / min. Wash the reaction solution with ethanol and cyclohexane according to the procedure for preparing the core particles, and disperse it in 2 mL of cyclohexane. This sample is a series of LiYbF4:Tm@LiYF4 core-shell structured nanocrystals with tunable local structure.

[0040] This invention alters the crystal field environment, or local structure, of rare earth ions by simply changing the stoichiometry of the precursors used in the synthesis of nanomaterials. Non-radiative energy transfer can then occur between rare earth ions, with transitions within the 4f configuration interacting with Ln... 3+ It is closely related to the surrounding local structure, which refers to the local lattice environment around rare earth ions, including bond length, bond angle, coordination number, lattice symmetry, etc.

[0041] Example 2: Preparation of core-shell nanoparticles LiYbF4:Tm@LiYF4 with different amounts of NH4F in the core layer: Weigh out 0.4 mmol of the corresponding rare earth thulium acetate solution and rare earth ytterbium acetate solution (where the molar ratio of Yb to Tm is 99:1) and add them to a two-necked flask. Then add 5 ml of oleic acid (OA) and 5 ml of octadecene (ODE) to the two-necked flask and stir at 150 rpm at 150 °C for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol and 1.25 mmol of the corresponding NH4F methanol solution and LiOH solution, mix them, and sonicate for 10 s (NH4F, RE) 3+ Five samples with molar ratios of 1:0.75, 1:0.875, 1:1, 1:1.125, and 1:1.25 were used as comparisons. This mixed solution was then rapidly added to the precursor solution and stirred uniformly at 350 r / min and 50 °C for 40 min. The flask was transferred to a heating mantle and heated to 100 °C at a rate of 4 °C / min (heating and holding for a total of 30 min) to ensure the removal of methanol. A vacuum was then applied for 5 min, followed by purging with argon gas for 1 min; this process was repeated twice. The temperature was then raised to 290 °C and held for 90 min at a rate of 10 °C / min, with a stirring speed of 330 r / min during the heating and holding process. The mixture was then cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to the centrifuge tubes. Place in a centrifuge at 7000 r / min for 5 min. After removing the supernatant, wash twice with ethanol and cyclohexane. Finally, disperse in 4 ml of cyclohexane for later use.

[0042] Weigh out 0.4 mmol of the corresponding yttrium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution and add them to a 50 mL double-necked flask. Then add 5 mL each of OA and ODE, and stir at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, add 2 mL of the prepared LiYbF4:Tm core nanoparticles to the reaction solution, maintain a vacuum state, and heat to 120 °C and hold for 10 min. Then heat to 310 °C in an argon atmosphere and hold for 35 min, with a heating rate of 10 °C / min. Wash the reaction solution with ethanol and cyclohexane according to the procedure for preparing the core particles, and disperse it in 2 mL of cyclohexane. This sample is a series of LiYbF4:Tm@LiYF4 core-shell structured nanocrystals with tunable local structure.

[0043] Example 3: Preparation of core-shell nanoparticles LiYbF4:Tm@LiYF4 with different amounts of CF3COOLi in the shell: Weigh out 0.4 mmol of the corresponding rare earth thulium acetate solution and rare earth ytterbium acetate solution (where the molar ratio of Yb to Tm is 99:1) and add them to a double-necked flask. Then add 5 ml of oleic acid (OA) and 5 ml of octadecene (ODE) to the double-necked flask and stir at 150 rpm at 150 °C for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol of the corresponding methanol solution and 1.25 mmol of the corresponding LiOH solution, mix them, and sonicate for 10 s. Then quickly add this mixture to the above precursor solution and stir evenly at 350 rpm and 50 °C for 40 min. Transfer the flask to a heating mantle and heat to 100 °C at a rate of 4 °C / min (heating and holding for a total of 30 min) to ensure the removal of methanol solution. At this time, evacuate for 5 min, then purge with argon gas for 1 min. Repeat this operation twice. The temperature was then raised to 290℃ and held for 90 min at a rate of 10℃ / min, with a stirring speed of 330 rpm during the heating and holding process. Afterward, the mixture was cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to centrifuge tubes. The tubes were centrifuged at 7000 rpm for 5 min. After removing the supernatant, the mixture was washed twice more with ethanol and cyclohexane, and finally dispersed in 4 ml of cyclohexane for later use.

[0044] 0.4 mmol of the corresponding yttrium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution (CF3COORE:CF3COOLi ratios of 1:0.25, 1:0.5, 1:0.75, 1:1, and 1:1.25) were weighed and added to a 50 mL double-necked flask. Then, 5 mL each of OA and ODE were added, and the mixture was stirred at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, 2 mL of the prepared LiYbF4:Tm core nanoparticles were added to the reaction solution. The mixture was kept under vacuum and heated to 120 °C for 10 min. Then, the temperature was increased to 310 °C and held for 35 min under an argon atmosphere, with a heating rate of 10 °C / min. The reaction solution was washed with ethanol and cyclohexane following the same procedure as for preparing the core particles, and then dispersed in 2 mL of cyclohexane. This sample represents a series of locally structured LiYbF4:Tm@LiYF4 core-shell nanocrystals.

[0045] Example 4: Preparation of core-shell nanoparticles LiYF4:Ce@LiYF4 with different amounts of LiOH in the core layer: Weigh out 0.4 mmol of the corresponding rare earth yttrium acetate solution and rare earth cerium acetate solution (where the molar ratio of Y to Ce is 98:2) and add them to a two-necked flask. Then add 5 ml of oleic acid (OA) and 5 ml of octadecene (ODE) to the two-necked flask and stir at 150 rpm at 150 °C for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol and 1.25 mmol of the corresponding methanol solution of NH4F and LiOH solution, mix them, and sonicate for 10 s (LiOH, RE) 3+ Five samples with molar ratios of 1:1.25, 1:1.875, 1:2.5, 1:3.125, and 1:3.75 were prepared and named CS1, CS2, CS3, CS4, and CS5 for comparison. This mixed solution was then rapidly added to the precursor solution and stirred uniformly at 350 r / min and 50 °C for 40 min. The flask was transferred to a heating mantle and heated to 100 °C at a rate of 4 °C / min (heating and holding for a total of 30 min) to ensure the removal of methanol. A vacuum was then applied for 5 min, followed by purging with argon gas for 1 min; this process was repeated twice. The temperature was then raised to 290 °C and held for 90 min at a rate of 10 °C / min, with a stirring speed of 330 r / min during the heating and holding process. The mixture was then cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to the centrifuge tubes. Place in a centrifuge at 7000 r / min for 5 min. After removing the supernatant, wash twice with ethanol and cyclohexane. Finally, disperse in 4 ml of cyclohexane for later use.

[0046] Weigh out 0.4 mmol of the corresponding yttrium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution and add them to a 50 mL double-necked flask. Then add 5 mL each of OA and ODE, and stir at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, add 2 mL of the prepared LiYF4:Ce core nanoparticles to the reaction solution, maintain a vacuum state, and heat to 120 °C for 10 min. Then heat to 310 °C in an argon atmosphere and hold for 35 min, with a heating rate of 10 °C / min. Wash the reaction solution with ethanol and cyclohexane according to the steps for preparing the core particles, and disperse it in 2 mL of cyclohexane to finally obtain LiYF4:Ce@LiYF4 core-shell structured nanocrystals.

[0047] Example 5: Preparation of core-shell nanoparticles LiYF4:Eu@LiYF4 with different amounts of LiOH in the core layer: Weigh out 0.4 mmol of the corresponding rare earth yttrium acetate solution and rare earth europium acetate solution (where the molar ratio of Y to Eu is 98:2) and add them to a two-necked flask. Then add 5 ml of oleic acid (OA) and 5 ml of octadecene (ODE) to the two-necked flask and stir at 150 rpm at 150 °C for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol and 1.25 mmol of the corresponding NH4F methanol solution and LiOH solution, mix them, and sonicate for 10 s (LiOH, RE) 3+ Five samples with molar ratios of 1:1.25, 1:1.875, 1:2.5, 1:3.125, and 1:3.75 were prepared and named CS1, CS2, CS3, CS4, and CS5 for comparison. This mixed solution was then rapidly added to the precursor solution and stirred uniformly at 350 r / min and 50 °C for 40 min. The flask was transferred to a heating mantle and heated to 100 °C at a rate of 4 °C / min (heating and holding for a total of 30 min) to ensure the removal of methanol. A vacuum was then applied for 5 min, followed by purging with argon gas for 1 min; this process was repeated twice. The temperature was then raised to 290 °C and held for 90 min at a rate of 10 °C / min, with a stirring speed of 330 r / min during the heating and holding process. The mixture was then cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to the centrifuge tubes. Place in a centrifuge at 7000 r / min for 5 min. After removing the supernatant, wash twice with ethanol and cyclohexane. Finally, disperse in 4 ml of cyclohexane for later use.

[0048] Weigh out 0.4 mmol of the corresponding yttrium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution and add them to a 50 mL double-necked flask. Then add 5 mL each of OA and ODE, and stir at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, add 2 mL of the prepared LiYF4:Eu core nanoparticles to the reaction solution, maintain a vacuum state, and heat to 120 °C and hold for 10 min. Then heat to 310 °C in an argon atmosphere and hold for 35 min, with a heating rate of 10 °C / min. Wash the reaction solution with ethanol and cyclohexane according to the steps for preparing the core particles, and disperse it in 2 mL of cyclohexane to finally obtain LiYF4:Eu@LiYF4 core-shell structured nanocrystals.

[0049] Example 6: Preparation of core-shell nanoparticles LiYbF4:Ho@LiYF4: Weigh out 0.4 mmol of the corresponding rare earth holmium acetate aqueous solution and rare earth ytterbium acetate solution (where the molar ratio of Yb to Ho is 99:1) and add them to a two-necked flask. Then add 5 ml of OA and 5 ml of ODE to the two-necked flask and stir at 150 r / min at 150℃ for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol and 1.25 mmol of the corresponding NH4F methanol solution and LiOH solution, mix them, and sonicate for 10 s (Li + F - Five samples with ratios of 1:1.25, 1:1.875, 1:2.5, 1:3.125, and 1:3.75 were prepared and named CS1, CS2, CS3, CS4, and CS5 for comparison. This mixed solution was then rapidly added to the precursor solution and stirred uniformly at 350 r / min and 50 °C for 40 min. The flask was transferred to a heating mantle and heated to 100 °C at a rate of 3 °C / min (heating and holding for a total of 30 min) to ensure the removal of methanol. A vacuum was then applied for 5 min, followed by purging with argon gas for 1 min; this process was repeated twice. The temperature was then raised to 290 °C and held for 90 min at a rate of 10 °C / min, with a stirring speed of 330 r / min during the heating and holding process. The mixture was then cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to the centrifuge tubes. Place in a centrifuge at 7000 r / min for 5 min. After removing the supernatant, wash twice with ethanol and cyclohexane. Finally, disperse in 4 ml of cyclohexane for later use.

[0050] Weigh out 0.4 mmol of the corresponding yttrium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution and add them to a 50 mL double-necked flask. Then add 5 mL each of OA and ODE, and stir at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, add 2 mL of the prepared LiYbF4:Ho core nanoparticles to the reaction solution, maintain a vacuum state, and heat to 120 °C and hold for 10 min. Then heat to 310 °C in an argon atmosphere and hold for 35 min, with a heating rate of 10 °C / min. Wash the reaction solution with ethanol and cyclohexane according to the procedure for preparing the core particles, and disperse it in 2 mL of cyclohexane. This sample is the LiYbF4:Ho@LiYF4 core-shell structured nanocrystal.

[0051] Example 7: Preparation of core-shell nanoparticles LiYbF4:Er@LiYF4: Weigh out 0.4 mmol of the corresponding rare earth erbium acetate aqueous solution and rare earth ytterbium acetate solution (where the molar ratio of Yb to Er is 98:2) and add them to a two-necked flask. Then add 5 ml of OA and 5 ml of ODE to the two-necked flask and stir at 150 r / min at 150℃ for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol and 1.25 mmol of the corresponding NH4F methanol solution and LiOH solution, mix them, and sonicate for 10 s (Li + F - Five samples with ratios of 1:1.25, 1:1.875, 1:2.5, 1:3.125, and 1:3.75 were prepared and named CS1, CS2, CS3, CS4, and CS5 for comparison. This mixed solution was then rapidly added to the precursor solution and stirred uniformly at 350 r / min and 50 °C for 40 min. The flask was transferred to a heating mantle and heated to 100 °C at a rate of 5 °C / min (heating and holding for a total of 30 min) to ensure the removal of methanol. A vacuum was then applied for 5 min, followed by purging with argon gas for 1 min; this process was repeated twice. The temperature was then raised to 290 °C and held for 90 min at a rate of 10 °C / min, with a stirring speed of 330 r / min during the heating and holding process. The mixture was then cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to the centrifuge tubes. Place it in a centrifuge at 7000 r / min for 5 min. After removing the supernatant, wash twice with ethanol and cyclohexane. Finally, disperse it in 4 ml of cyclohexane for later use.

[0052] Weigh out 0.4 mmol of the corresponding yttrium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution and add them to a 50 mL double-necked flask. Then add 5 mL each of OA and ODE, and stir at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, add 2 mL of the prepared LiYbF4:Er core nanoparticles to the reaction solution, maintain a vacuum state, and heat to 120 °C and hold for 10 min. Then heat to 310 °C in an argon atmosphere and hold for 35 min, with a heating rate of 10 °C / min. Wash the reaction solution with ethanol and cyclohexane according to the procedure for preparing the core particles, and disperse it in 2 mL of cyclohexane. This sample is the LiYbF4:Er@LiYF4 core-shell structured nanocrystal.

[0053] Example 8: Preparation of core-shell nanoparticles LiYbF4:Tm@LiLuF4: Weigh out 0.4 mmol of the corresponding rare earth thulium acetate aqueous solution and rare earth ytterbium acetate solution (where the molar ratio of Yb to Tm is 99:1) and add them to a two-necked flask. Then add 5 ml of OA and 5 ml of ODE to the two-necked flask and stir at 150 r / min and 150 °C for 50 min to form a yellow transparent solution, which is the precursor solution. Weigh out 4 mmol and 1.25 mmol of the corresponding NH4F methanol solution and LiOH solution, mix them, and sonicate for 10 s (Li + F - Three samples with ratios of 1:1.25, 1:2.5, and 1:3.75, named CS1, CS3, and CS5 respectively, were prepared for comparison. This mixed solution was then rapidly added to the precursor solution and stirred uniformly at 350 r / min and 50 °C for 40 min. The flask was transferred to a heating mantle and heated to 100 °C at a rate of 10 °C / min (heating and holding for a total of 30 min) to ensure removal of the methanol solution. A vacuum was then applied for 5 min, followed by purging with argon gas for 1 min; this process was repeated twice. The temperature was then raised to 290 °C and held for 90 min at a rate of 10 °C / min, with a stirring speed of 330 r / min during the heating and holding process. The mixture was then cooled to room temperature, and the contents of the flask were transferred to 15 ml centrifuge tubes. The flask was rinsed with a small amount of cyclohexane and 4 ml of ethanol, and the washings were transferred to the centrifuge tubes. Place it in a centrifuge at 7000 r / min for 5 min. After removing the supernatant, wash twice with ethanol and cyclohexane. Finally, disperse it in 4 ml of cyclohexane for later use.

[0054] Weigh out 0.4 mmol of the corresponding lutetium trifluoroacetate aqueous solution and lithium trifluoroacetate aqueous solution and add them to a 50 mL double-necked flask. Then add 5 mL each of OA and ODE, and stir at a constant temperature of 120 °C for 35 min to prepare the precursor. After cooling, add 2 mL of the prepared LiYbF4:Tm core nanoparticles to the reaction solution, maintain a vacuum state, and heat to 120 °C and hold for 10 min. Then heat to 310 °C in an argon atmosphere and hold for 35 min, with a heating rate of 10 °C / min. Wash the reaction solution with ethanol and cyclohexane according to the procedure for preparing the core particles, and disperse it in 2 mL of cyclohexane. This sample is the LiYbF4:Tm@LiLuF4 core-shell structured nanocrystal.

[0055] Based on specific implementation cases, to systematically verify the impact of the described modulation method on the material structure and luminescence properties, we conducted systematic characterization and performance testing on a series of prepared samples. The morphology, crystal form, luminescence properties, and local structural changes of the nanoparticles obtained in the examples will be described in detail below with reference to the accompanying drawings: 1. Morphology Characterization. The LiYbF4:Tm@LiYF4 prepared in Example 1 was loaded onto a copper sheet, and its morphology and dimensions were measured using transmission electron microscopy. The results are as follows: Figure 2 As shown. (Through) Figure 2 Transmission electron microscopy (TEM) images and corresponding size distribution histograms show that the synthesized LiYbF4:Tm@LiYF4 samples exhibit excellent particle size uniformity, which increases slightly with increasing LiOH content and has a shell thickness of 1.25 nm.

[0056] 2. Crystal form characterization. The sample synthesized in Example 1 was characterized by X-ray diffraction and lattice distortion. The results are as follows: Figure 3 As shown. By Figure 3 It can be proven that the synthesized nanomaterials are all tetragonal, which also indicates that adjusting the precursor content does not change the macroscopic phase structure of the material itself, but only changes the microscopic local structure. Adjusting the molar concentration of LiOH doping causes lattice expansion or contraction, and as the doping concentration of LiOH increases, the lattice distortion increases. The diffraction peaks gradually shift towards higher angles.

[0057] 3. Upconversion spectroscopy test. The LiYbF4:Tm@LiYF4 prepared in Examples 1-3 were dispersed in cyclohexane and excited by a 980nm near-infrared laser with a laser power density of 1500-2000 mW / cm². 2 Perform fluorescence spectroscopy tests, such as Figure 4 , Figure 5 , Figure 7 As shown, the sample exhibits upconversion fluorescence at 347 nm, 361 nm, 472 nm, 483 nm, and 647 nm. Figure 6 TEM images and corresponding size distribution histograms for varying NH4F dosages. Figure 4 The upconversion emission spectra show that the UV / blue emission intensity ratio decreased from 2.51 in CS1 to 1.02 in CS5 as the amount of LiOH in the nucleus increased. This is because the change in the amount of LiOH led to a change in the local structure around the rare earth ions. The low symmetry of the local structure resulted in stronger electron-phonon interactions, which affected the photoluminescence efficiency and high-order multiphoton UV upconversion luminescence.

[0058] 4. Local structure characterization. The precipitate obtained in Example 4 was dispersed in cyclohexane, and the resulting down-transfer spectrum is shown below. Figure 8As shown, Ce 3+ Because of their 4f-5d transitions, Ce ions can be used to reveal photoluminescence behavior that is relatively sensitive to local symmetry, and thus serve as a good structural probe for inferring Ce. 3+ The local structural environment in which the ion resides. Based on different Li... + The spectral changes of LiYF4:Ce@LiYF4 with varying concentrations show a redshift in both excitation and emission spectra from CS1 to CS5, with a redshift of approximately 5 nm between each concentration gradient.

[0059] 5. Fluorescence lifetime test. Figure 9 The sample tested at 361nm ( 1 D2→ 3 H6) and 447nm ( 1 D2→ 3 The fluorescence decay curve at F4 indicates that the fluorescence lifetime of the sample gradually decreases from CS1 to CS5, which proves that Li + The less dosage used, the higher the efficiency of upconversion energy transfer.

[0060] 6. Figure 10 Pump dependence curves for upconversion luminescence of a series of core-shell structured nanoparticles were obtained. The slope after linear fitting represents the number of photons required for the upconversion process. The slopes of ultraviolet and blue light emission from the core-shell structured nanoparticles were calculated, showing five-photon and three-to-four-photon conversion processes, respectively, following multi-photon upconversion characteristics, while reducing Li... + The n value of the CS5 sample decreased after the dosage was adjusted, indicating that Li + Reducing the dosage can accelerate Yb 3+ -Tm 3+ The energy transfer rate between ion pairs can significantly reduce the number of photons required for high-energy emission level population, thereby enhancing upconversion luminescence intensity.

[0061] 7. Figure 11 Knowing the luminescent center Tm 3+ The local structural symmetry of the location changes with Li + The reduction in dosage causes the Li-F bond length to shrink, resulting in a shorter distance between adjacent rare earth ions. This improves the energy transfer efficiency between sensitizer and activator ions, leading to enhanced high-order multiphoton ultraviolet upconversion luminescence.

[0062] 8. Universality verification. Figure 12 , Figure 13 and Figure 14 This indicates that the regulation method can be applied to Lu 3+ The base shell, and Er 3 + Ho 3+Core layers containing rare earth ions that undergo upconversion emission.

[0063] 9. By Figure 15 This demonstrates that the incorporation of different Li contents can alter the coordination environment of Yb ions.

[0064] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

[0065] Furthermore, it should be understood that although this specification describes various embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for enhancing the ultraviolet fluorescence of rare-earth upconversion nanocrystals through localized structure modulation, wherein the upconversion nanocrystals are LiREF4@LiREF4 core-shell structures, characterized in that... The local structural microenvironment of the activating ions is altered by regulating the precursor stoichiometry during the synthesis process, wherein the regulation is performed in at least one of the following ways: (a) In core layer synthesis, the regulation of rare earth ion RE 3+ The molar ratio of RE to LiOH is 1:1.25 to 1:3.

75. (b) In core layer synthesis, the regulation of rare earth ions (RE) 3+ The molar ratio of RE to NH4F is 1:3 to 1:

5. (c) In the shell synthesis, the molar ratio of rare earth trifluoroacetate salt to lithium trifluoroacetate CF3COORE:CF3COOLi was controlled to be 1:0.5 to 1:2.

5.

2. The method for enhancing the ultraviolet fluorescence of rare-earth upconversion nanocrystals by localized structure regulation according to claim 1, characterized in that, The control methods (a), (b), and (c) can be used individually or in any combination.

3. The method for enhancing the ultraviolet fluorescence of rare-earth upconversion nanocrystals by localized structure regulation according to claim 1, characterized in that, The REs in the core matrix are selected from one or more of Y, Gd, Lu, and Yb, and are doped with rare earth activating ions and sensitizing ions.

4. The method for enhancing the ultraviolet fluorescence of rare-earth upconversion nanocrystals by localized structure regulation according to claim 3, characterized in that, The rare earth activating ions are selected from Tm 3+ Er 3+ Ho 3+ Eu 3+ 、Tb 3+ At least one of them.

5. The method for enhancing the ultraviolet fluorescence of rare-earth upconversion nanocrystals by localized structure regulation according to claim 3, characterized in that, Sensitizing ions are selected from Yb 3+ 、Nd 3+ At least one of them.

6. The method for enhancing the ultraviolet fluorescence of rare-earth upconversion nanocrystals by localized structure regulation according to claim 1, characterized in that, In the LiREF4@LiREF4 core-shell structure, the REs in the shell matrix are selected from one or more of Y, Gd, and Lu.

7. A method for measuring the localized structure of rare-earth upconversion nanocrystals prepared by the method according to any one of claims 1 to 6, characterized in that, The measurement of local structures is achieved by measuring the local crystal field environment around rare earth ions and obtaining the above lattice parameters by combining XRD structure refinement or X-ray absorption fine structure spectroscopy.

8. A method for measuring the localized structure of rare-earth upconversion nanocrystals prepared by the method according to any one of claims 1 to 6, characterized in that, A series of LiYF4:Eu@LiYF4 were first synthesized using the same stoichiometric control method, and then the optical probe Eu was used. 3+ The red-orange ratio is sensitive to local symmetry, thereby measuring the local symmetry of the local lattice around rare earth ions, realizing the measurement of local structure, and can be used to reveal photoluminescence behavior that is relatively sensitive to local symmetry.

9. A method for measuring the localized structure of rare-earth upconversion nanocrystals prepared by the method according to any one of claims 1 to 6, characterized in that, A series of LiYF4:Ce@LiYF4 were first synthesized using the same stoichiometric control method, and then the optical probe Ce was used. 3+ The sensitivity of the 4f→5d transition to local symmetry allows for the measurement of the local symmetry of the local lattice around rare earth ions, enabling the measurement of local structures. It can also be used to reveal photoluminescence behavior that is relatively sensitive to local symmetry.

Images