Ultrahigh-order nonlinear photon avalanche fluorescent nano material based on low-dose alkali metal ion doping

Abstract

The invention discloses an ultrahigh-order nonlinear photon avalanche fluorescent nano material based on low-dose alkali metal ion doping, and belongs to the field of nonlinear nano photonics. According to the material, rare earth fluoride nanoparticles are taken as a main body, and alkali metal ions are introduced for gap doping in a synthesis process, so that a crystal field environment, local symmetry and rare earth ion space distribution are effectively regulated and controlled, and the photon avalanche fluorescent nano material which is stable in structure, uniform in morphology and particle size and good in dispersity is obtained. Specifically, the method comprises the steps of preparation of a precursor solution, introduction of a low-dose alkali metal source, control of reaction temperature and time and aftertreatment of a product, and predictable regulation and control of the symmetry of a crystal field are achieved. The used raw materials are easy to obtain, the process conditions are loose and high in controllability, and the method has good large-scale preparation potential. The obtained nano material has wide application prospects in the aspects of super-resolution imaging, ultra-sensitive sensing and nonlinear optical device integration.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of nonlinear nanophotonics, and particularly relates to a super-high-order nonlinear photon avalanche fluorescence nanomaterial based on low-dose alkali metal ion doping. BACKGROUND

[0002] Photon avalanche upconversion is a special nonlinear optical process, which realizes high-order nonlinear upconversion luminescence through a chain reaction mechanism of ground state absorption, excited state absorption and cross relaxation. The photon avalanche process can make the material produce a super-high-order nonlinear response (usually, the nonlinear order N > 20) when the excitation power exceeds a certain threshold, thereby realizing huge optical gain and signal amplification. Compared with traditional upconversion luminescence, the photon avalanche material shows a broad application prospect in the fields of super-resolution imaging, single molecule detection, multi-physical field sensing and optical computing.

[0003] Although the photon avalanche process is highly concerned due to its super-high-order nonlinearity, its actual performance still has significant limitations. Before 2025, the reported nonlinear order has long hovered around 30, which is difficult to meet the demand for extremely high nonlinear order in the field of super-resolution imaging. To break this bottleneck, traditional researches mostly rely on the design of complex core-shell structures. However, this method usually needs multiple coatings, and the synthesis process is complicated and has poor process repeatability, which often leads to the decline of crystal quality and luminescent performance, and seriously hinders the large-scale preparation and practical application of the material. At the same time, the research and development of high-performance materials still mainly focuses on a few rare earth ions such as thulium, holmium and praseodymium. This dependence on specific ion systems, combined with the lack of universal performance enhancement strategies, greatly limits the flexibility of material design and the expansion of application range. Therefore, there is an urgent need for a universal and efficient strategy that can realize structure simplification and synthesis controllability while maintaining the high nonlinearity of the photon avalanche, thereby promoting the development and application of new upconversion nanomaterials. SUMMARY

[0004] The application aims to solve the problems of limited luminescent efficiency and insufficient nonlinear response order of the photon avalanche nanomaterial in the prior art, and proposes a super-high-order nonlinear photon avalanche fluorescence nanomaterial based on low-dose alkali metal ion doping. By introducing small-radius and high-charge-density alkali metal cations into the matrix material, the photon avalanche luminescence process is effectively regulated, thereby overcoming the shortcomings of the prior art.

[0005] To achieve the above object, the application adopts the following technical scheme:

[0006] The application provides a super-high-order nonlinear photon avalanche fluorescence nanomaterial based on low-dose alkali metal ion doping, which comprises a substrate NaLnF4 doped with rare earth luminescent active ions R 3+and alkali metal ions A in a mole percentage of x + with a general chemical formula of NaLnF4:R 3+ / x A + wherein, the Ln is an inert rare earth matrix ion, which can be selected from one or more of Y, Lu; the R is an active rare earth luminescent ion, which can be selected from one or more of Tm, Eu, Tb, Ho, Er, Pr; the A is an alkali metal ion, which can be selected from one or more of Li or K; x represents the mole percentage of the A element in the total alkali metal element, and satisfies 0 < x < 100%.

[0007] In a preferred technical solution, the Ln is a rare earth matrix ion, which can be selected from one or more of Y, Lu; the R is a rare earth luminescent active ion, which can be selected from one or more of Tm, Eu, Tb, Ho, Er, Pr; the A is an alkali metal ion, which can be selected from one or more of Li or K; x represents the mole percentage of the A element in the total alkali metal element, and satisfies 0 < x < 100%. The NaLnF4 matrix is preferably NaYF4, which has good chemical stability and low phonon energy; the R is preferably Tm, which can realize high-order photon avalanche fluorescence under 1064 nm continuous laser excitation; the A is preferably Li, and the doping amount x is preferably 0.1-1%.

[0008] Further, the mole ratio of the R and Ln is x:(1-x), wherein 0 < x < 100%, preferably 5%-20%, which ensures that the avalanche active ions R 3+ and the matrix ions Ln 3+ are uniformly distributed in the crystal lattice, forming a high-efficiency positive feedback network population, avoiding low cross-relaxation efficiency caused by a small amount of doping or concentration quenching caused by excessive doping.

[0009] Further, the alkali metal ion has a small radius, occupies the vacancy in the rare earth fluoride matrix crystal lattice in a way of interstitial doping, reduces the symmetry of the original local coordination environment, thereby weakening the prohibition of the 4f-4f transition of the avalanche active ion, significantly improving the probability of the electric dipole transition. The distortion of the local crystal field further enhances the cross-relaxation process between the avalanche active ions, improves the transition rate and amplifies the nonlinear optical response; the crystal field distortion makes the cross-relaxation rate between the avalanche active ions increase by 10%-200%, so that the optical nonlinear order of the material is higher than 60 orders;

[0010] Further, the low-dose alkali metal ion doped super-high-order nonlinear photonic avalanche fluorescence nanomaterial has a high-order nonlinear photonic avalanche effect, and is superior to a conventional photonic avalanche nanoparticle, preferably N≥60; and the photonic avalanche fluorescence nanomaterial can be used in frontier fields such as biological imaging, single molecule detection, and ultra-sensitive sensing.

[0011] The application provides a preparation method of the low-dose alkali metal ion doped super-high-order nonlinear photonic avalanche fluorescence nanomaterial.

[0012] S1: Dissolving a rare earth metal salt in a mixed solvent of oleic acid and octadecene, stirring uniformly, and then heating by an oil bath to coordinate the rare earth ions with the oleic acid to form a stable lanthanide oleate complex;

[0013] S2: Separately weighing alkali metal sources, sodium sources and fluorine sources, dissolving in a polar organic solvent, and treating by ultrasonic to obtain a transparent and clear pre-configuration solution for standby;

[0014] S3: Slowly adding or injecting the pre-configuration solution containing the alkali metal, sodium source and fluorine source prepared in step S2 into the mixed solution obtained in step S1 after cooling to room temperature, fully stirring to mix the ions, heating to increase the reaction temperature, fully reacting for a certain time to accelerate the precursor complex reaction, and obtaining a uniform mixed solution;

[0015] S4: Heating and increasing the temperature of the mixed solution obtained in step S3 under vacuum conditions for a certain reaction time to remove low-boiling impurity solvents and enhance the stability of the complex;

[0016] S5: Continuing to heat and increase the temperature of the mixed solution obtained in step S4 to a high temperature under inert gas protection for a certain time to promote the generation of crystal nucleus and the growth of crystal;

[0017] S6: Naturally cooling the system after the reaction to room temperature, centrifugally separating, repeatedly washing the solvent to remove unreacted substances and by-products, and finally dispersing the obtained solid in cyclohexane to obtain the low-dose alkali metal ion doped super-high-order nonlinear photonic avalanche fluorescence nanocrystal.

[0018] In step S1, the rare earth metal salt can be selected from one of a chloride salt, a nitrate salt and an acetate salt, and preferably is an acetate salt; when the salt solution uses an acetate salt, the acetate salt includes an inert rare earth metal acetate and an active rare earth metal acetate; the inert rare earth metal acetate can be selected from one or more of yttrium acetate and lutetium acetate, and preferably is yttrium acetate; the active rare earth metal acetate can be selected from one or more of thulium acetate, neodymium acetate, erbium acetate, praseodymium acetate, europium acetate, holmium acetate and terbium acetate, and preferably is thulium acetate;

[0019] In step S1, the concentration of the rare earth metal salt is 0.2 M, the solvent is a mixture of oleic acid and octadecene in a volume ratio of (1-10):(1-20), the oil bath heating temperature is 110-150°C, and the heating time is 0.5-1.5 h, preferably 150°C for 1 h.

[0020] In step S2, the alkali metal source can be selected from a potassium source or a lithium source, preferably a lithium source; when a lithium source is used, the alkali metal source can be selected from one or more of lithium chloride, lithium nitrate, lithium trifluoroacetate, or lithium hydroxide, preferably lithium hydroxide; the sodium source can be selected from one or more of sodium hydroxide, sodium carbonate, sodium trifluoroacetate, sodium nitrate, or sodium fluoride, preferably sodium hydroxide; the fluorine source can be selected from one or more of ammonium fluoride, sodium fluoride, and ammonium trifluoroacetate, preferably ammonium fluoride; and the polar organic solvent can be selected from one or more of methanol, ethanol, dichloromethane, cyclohexane, and n-hexane, preferably methanol.

[0021] In step S2, in the pre-configuration solution containing the alkali metal source, the sodium source, and the fluorine source, the molar ratio of the alkali metal ion A solvent to the sodium-containing solvent is x:(1-x), where 0

[0022] In step S3, the heating temperature is 40-70°C, and the reaction time is 30-60 min, preferably 50°C for 30 min.

[0023] In step S4, the heating temperature is 100-130°C, the reaction time is 20-40 min, the vacuum degree is 10-100 Pa, and the heating process can significantly reduce the residual of low-boiling-point solvents such as methanol, ethanol, and water, thereby avoiding interference with subsequent crystal growth.

[0024] In step S5, the temperature of the heating is 250-320 DEG C, the time of the reaction is 1.5-3 h, preferably 290 DEG C for 2 h, the inert gas can be selected from one of nitrogen, helium, argon, preferably nitrogen, the average particle size of the nanocrystal obtained after the high-temperature reaction is 10-40 nm, and the particle size distribution standard deviation is less than 10%, and the crystal morphology generated can be spherical, rod-shaped, prismatic, flaky or polyhedral, wherein low temperature (250-280 DEG C) is easy to form a spherical shape, high temperature (290-320 DEG C) is easy to form a rod-shaped or prismatic shape, and the morphology is jointly controlled by the reaction temperature and the reaction time. The vacuum degree is 10-100 Pa, preferably 30-50 Pa, which can effectively remove residual low-boiling-point solvents and dissolved gases, and can avoid the system from boiling or nucleating prematurely due to excessively high vacuum, thereby ensuring the stability of the complex and providing protection for the uniform nucleation and controllable growth of the subsequent crystal;

[0025] In step S6, the washing solvent can be selected from one or more of anhydrous ethanol, cyclohexane, acetone or isopropanol, and the washing number is 2-6 times, preferably anhydrous ethanol and cyclohexane mixed solution.

[0026] Compared with the prior art, the technical effects and advantages of the present application are specifically as follows:

[0027] 1. The present application introduces small-radius alkali metal ions for interstitial doping, induces local crystal field distortion, effectively reduces the forbidden nature of rare earth luminescent ion 4f-4f transition, and significantly improves the probability of its electric dipole transition. More importantly, this lattice distortion significantly enhances the cross-relaxation process between avalanche active ions, significantly improving the nonlinear optical response performance. After optimization of the doping concentration, the material prepared by the present application can exhibit an optical nonlinear effect better than 60 orders;

[0028] 2. The preparation method of the present application is optimized, the process is simple, the parameters are clear and controllable. By accurately adjusting the precursor ratio (such as the molar ratio of rare earth active ions / matrix ions, alkali metal / sodium), the reaction temperature and time, the nucleation and growth process of the nanocrystal can be effectively controlled. The obtained nanocrystal (including single core and core-shell structure) has uniform size (core average particle size 10-40 nm, distribution standard deviation <10%), controllable morphology (controllable for spherical, rod-shaped, prismatic, flaky or polyhedral), stable composition, and good batch-to-batch reproducibility. This high controllability lays a solid foundation for large-scale stable preparation and application of nanomaterials;

[0029] 3、Based on its ultra-high non-linear order, low excitation threshold and composition stability, the nanomaterial prepared by the application shows great application potential in multiple frontier fields: in super-resolution imaging, by virtue of its ultra-high non-linear response, the traditional optical diffraction limit can be broken through, and nanoscale spatial resolution can be realized; in super-sensitive sensing, the extreme amplification effect of photon avalanche on weak light signals can realize high-precision detection of single molecules or even single ions; in addition, in the field of infrared signal detection and new photoelectric devices, the material also shows important potential application value. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 It is a principle schematic diagram of a typical rare earth photon avalanche non-linear system in embodiment 1 of the application.

[0031] Figure 2 It is a transmission electron microscope morphology and size distribution diagram of NaYF4:Tm / x% Li (x=0, 0.25, 0.5, 0.75, 1, 5) photon avalanche nanocrystals prepared in embodiment 1 of the application, wherein a is NaYF4:Tm photon avalanche nanocrystals; b is NaYF4:Tm / 0.25% Li photon avalanche nanocrystals; c is NaYF4:Tm / 0.5% Li photon avalanche nanocrystals; d is NaYF4:Tm / 0.75% Li photon avalanche nanocrystals; e is NaYF4:Tm / 1% Li photon avalanche nanocrystals; and f is NaYF4:Tm / 5% Li photon avalanche nanocrystals.

[0032] Figure 3 It is an X-ray powder diffraction diagram of NaYF4:Tm / x% Li (x=0, 0.25, 0.5, 0.75, 1, 5) photon avalanche nanocrystals prepared in embodiment 1 of the application.

[0033] Figure 4 It is a power curve test of NaYF4:Tm / x% Li (x=0, 0.25, 0.5, 0.75, 1, 5) photon avalanche nanocrystals prepared in embodiment 2 of the application under 1064 nm excitation.

[0034] Figure 5 It is a time-resolved dynamics test of 800 nm emission of NaYF4:Tm / Li photon avalanche nanocrystals prepared in embodiment 3 of the application under pulsed 1064 nm excitation light excitation, wherein a is the excitation power 129 KW cm -2 , and the NaYF4:Tm / Li photon avalanche nanocrystals have a rise time of 1.3 ms; b is the excitation power 144 KW cm -2NaYF4: Tm / Li photonic avalanche nanocrystals have a rise time of 2.4 ms; c is the excitation power 215 KW cm -2 NaYF4: Tm / Li photonic avalanche nanocrystals have a rise time of 19.5 ms; d is the excitation power 244 KW cm -2 NaYF4: Tm / Li photonic avalanche nanocrystals have a rise time of 16.5 ms; e is the excitation power 275 KW cm -2 NaYF4: Tm / Li photonic avalanche nanocrystals have a rise time of 9.7 ms; f is the excitation power 306 KW cm -2 NaYF4: Tm / Li photonic avalanche nanocrystals have a rise time of 8.1 ms.

[0035] Figure 6 Power curve test of NaLuF4: Tm / 0.5% Li photonic avalanche nanocrystals prepared for Example 4 of the present application under 1064 nm excitation.

[0036] Figure 7 Transmission electron microscope morphology and size distribution diagrams of NaYF4:Eu / x% Li (x = 0, 0.5, 1) photonic avalanche nanocrystals prepared for Example 5 of the present application, wherein a is NaYF4:Eu photonic avalanche nanocrystals; b is NaYF4:Eu / 0.5% Li photonic avalanche nanocrystals; c is NaYF4:Eu / 1% Li photonic avalanche nanocrystals.

[0037] Figure 8 Transmission electron microscope morphology and size distribution diagrams of NaYF4:Ho / x% Li (x = 0, 0.5, 1) photonic avalanche nanocrystals prepared for Example 5 of the present application, wherein a is NaYF4:Ho photonic avalanche nanocrystals; b is NaYF4:Ho / 0.5% Li photonic avalanche nanocrystals; c is NaYF4:Ho / 1% Li photonic avalanche nanocrystals.

[0038] Figure 9 Transmission electron microscope morphology and size distribution diagrams of NaYF4:Er / x% Li (x = 0, 0.5, 1) photonic avalanche nanocrystals prepared for Example 5 of the present application, wherein a is NaYF4:Er photonic avalanche nanocrystals; b is NaYF4:Er / 0.5% Li photonic avalanche nanocrystals; c is NaYF4:Er / 1% Li photonic avalanche nanocrystals. DETAILED DESCRIPTION

[0039] The application will be further described below. It should be noted that the following examples are based on the technical solution, and give detailed implementation and specific operation process, but the protection scope of the application is not limited to the following examples.

[0040] Example 1

[0041] In this embodiment, alkali metal ion doped ultra-high-order nonlinear photonic avalanche fluorescence nanomaterials are prepared by co-precipitation method, and the specific steps are as follows:

[0042] 1、Different Tm 3+ Preparation of NaYF4: Tm nanocrystals with different doping concentrations

[0043] Generally, an aqueous solution containing (2 mL, 0.2 M) Ln(CH3COO)3 (Ln = Y / Tm) is added to a 50 ml flask containing oleic acid (3 mL) and 1-octadecene (7 mL). The mixture is heated to 150°C, magnetically stirred for 90 min to form lanthanide oleate complexes. After cooling to room temperature, a methanolic solution of NH4F (4 mL, 0.5 M) and NaOH (2 mL, 0.4 M) is added dropwise at 50°C and stirred for 60 min. The reaction mixture is heated to 120°C under 50 Pa vacuum for 30 min to remove residual methanol, and then heated to 290°C under nitrogen for 120 min to promote crystal growth. After cooling, the reaction mixture is treated with excess ethanol, centrifuged at 6000 r.p.m for 5 min to collect the resulting product, washed thoroughly with cyclohexane and ethanol to remove residual impurities, and finally dispersed in 4.0 mL of cyclohexane for storage and subsequent characterization.

[0044] 2、Different Li + Preparation of NaYF4: Tm / Li nanocrystals with different doping concentrations

[0045] Typically, an aqueous solution containing (2 mL, 0.2 M) Ln(CH3COO)3 (Ln = Y / Tm) was added to a 50 ml flask containing oleic acid (3 mL) and 1-octadecene (7 mL). The mixture was heated to 150 °C with magnetic stirring for 90 min to form the lanthanide oleate complex. After cooling to room temperature, a methanolic solution of NH4F (4 mL, 0.5 M), 0.8 mmol (1-x) NaOH, and 0.8x mmol LiOH was added dropwise at 50 °C with stirring for 60 min. The reaction mixture was heated to 120 °C under 50 Pa vacuum for 30 min to remove residual methanol, and then heated to 290 °C under nitrogen for 120 min to promote crystal growth. After cooling, the reaction mixture was treated with an excess of ethanol, and the resulting product was collected by centrifugation, washed thoroughly with cyclohexane and ethanol to remove residual impurities, and finally redispersed in 4.0 mL cyclohexane for storage and subsequent characterization.

[0046] Figure 1 In a typical rare-earth photonic avalanche nonlinear system described in the embodiments of the present application, the photonic avalanche upconversion is formed by a positive feedback loop of excited state absorption and energy transfer cross-relaxation, so that after the material is above a certain threshold light intensity, it can produce a dramatic nonlinear optical response. Specifically, an ion is excited to a higher energy level by excited state absorption, and can transfer its energy to another ion in the ground state through an efficient cross-relaxation process, so that it jumps to an intermediate state, and itself returns to the intermediate state. This newly excited ion can absorb another photon for excited state absorption, thereby triggering a new round of energy transfer. This chain reaction makes the absorption of one photon ultimately lead to the excitation of multiple ions to the luminescent energy level, resulting in significant optical gain.

[0047] Figure 2 The transmission electron microscopy morphology and size distribution of the prepared NaYF4:Tm / x% Li (x = 0, 0.25, 0.5, 0.75, 1, 5) photonic avalanche nanocrystals are shown in FIG. 1, where a is NaYF4:Tm photonic avalanche nanocrystals; b is NaYF4:Tm / 0.25% Li photonic avalanche nanocrystals; c is NaYF4:Tm / 0.5% Li photonic avalanche nanocrystals; d is NaYF4:Tm / 0.75% Li photonic avalanche nanocrystals; e is NaYF4:Tm / 1% Li photonic avalanche nanocrystals; and f is NaYF4:Tm / 5% Li photonic avalanche nanocrystals. As shown in the high-resolution transmission electron microscopy images, the obtained NaYF4:Tm / Li nanocrystals have uniform morphology, and with the increase of the doping concentration of lithium ions, the particle size of the nanocrystals increases, and rod-like structures are produced, indicating that the doping of lithium ions can promote the nucleation and growth of the nanocrystals. + With the increase of the doping concentration, the particle size of the nanocrystals increases, and rod-like structures are produced, indicating that the doping of lithium ions can promote the nucleation and growth of the nanocrystals.

[0048] Figure 3 X-ray powder diffraction patterns of the prepared NaYF4:Tm / x% Li (x = 0, 0.25, 0.5, 0.75, 1, 5) photon avalanche nanocrystals, indicating that different Li + doping concentrations of NaYF4: Tm 3+ The avalanche nanocrystals have good crystallinity, and the diffraction peaks correspond to the diffraction peaks of the standard card (PDF: 16-0334) of hexagonal phase crystals, and no impurity phase is observed, indicating that the synthesized nanocrystals are pure phase.

[0049] Example 2

[0050] The excitation power-luminous intensity relationship of the photon avalanche process usually presents a unique "S" type curve, and its formation process is due to the joint action of multiple dynamic processes. A typical S-shaped curve can be divided into three stages: when the excitation power is low and does not reach the avalanche threshold, the luminous intensity mainly comes from ordinary ground state absorption and weak excited state absorption. At this time, the cross relaxation process is not enough to form an effective positive feedback loop, and the luminous intensity slowly and approximately linearly increases with the excitation power, and the curve is flat. When the excitation power reaches and exceeds the threshold, the system enters a positive feedback loop. In this region, a small increase in excitation power will cause a sharp, super-linear increase in luminous intensity, and the curve becomes extremely steep, showing the middle part of the "S" shape. When the power continues to increase, the proportion of ions participating in the avalanche tends to be limited, and the population of the luminous energy level reaches saturation. At this time, the luminous intensity hardly changes with the increase of the excitation power, and the curve becomes flat again.

[0051] Figure 4 The repeatability power curve test results of the series of NaYF4:Tm / x% Li (x = 0, 0.25, 0.5, 0.75, 1, 5) photon avalanche nanocrystals prepared in Example 1 are shown by using the full-automatic power curve test system independently built in the laboratory. The test data clearly shows that the interstitial doping of alkali metal lithium ions (Li) has a very significant enhancement effect on the nonlinear response of the photon avalanche. Specifically, the nonlinear order of the NaYF4:Tm nanoparticles without Li + doping is 28, and after doping with Li + , the nonlinear order of all samples is systematically improved. In particular, when the Li doping concentration is 0.5%, the nonlinear order reaches 63, which is about 2.3 times higher than that of the undoped sample.

[0052] The above results prove that the introduction of alkali metal source Li effectively enhances the Tm 3+The cross-relaxation process between ions significantly improves the photon avalanche efficiency and nonlinear order. Meanwhile, the good repeatability of the power curves indicates that the fabrication method is stable and reliable, providing strong support for the application of materials in super-resolution imaging and ultra-sensitive sensing.

[0053] Example 3

[0054] Time-resolved dynamics measurement of photonic avalanches is a key technique for directly observing and quantifying energy transfer and luminescence dynamics during avalanches. It primarily uses a 1064 nm pulsed laser to measure Tm. 3+ of 3 The curve showing the decay of luminescence intensity over time at the H4 emission level (800 nm emission) after excitation is used to reveal the microscopic relaxation path, energy transfer efficiency, and the rate of key processes. Rise time is defined as the time required to reach 95% of the asymptotic value.

[0055] Figure 5 This demonstrates time-resolved kinetic measurements of the NaYF4:Tm / Li photonic avalanche nanocrystals prepared in Example 1, where a is the excitation power of 129 KW cm⁻¹. -2 Below, the rise time of NaYF4:Tm / Li photonic avalanche nanocrystals is 1.3 ms; b represents the excitation power of 144 KW cm⁻¹. -2 Below, the rise time of NaYF4:Tm / Li photonic avalanche nanocrystals is 2.4 ms; c is the excitation power of 215 KW cm⁻¹. -2 Below, the rise time of NaYF4:Tm / Li photonic avalanche nanocrystals is 19.5 ms; d represents the excitation power of 244 KW cm⁻¹. -2 Below, the rise time of NaYF4:Tm / Li photonic avalanche nanocrystals is 16.5 ms; e represents the excitation power of 275 KW cm⁻¹. -2 Below, the rise time of NaYF4:Tm / Li photonic avalanche nanocrystals is 9.7 ms; f is the excitation power of 306 KW cm⁻¹. -2 At a threshold power of 215 kW·cm⁻¹, the rise time of NaYF₄:Tm / Li photonic avalanche nanocrystals was 8.1 ms. This result indicates that the rise time of the nanocrystals is significantly prolonged near the threshold power. -2 The rise time at the threshold power was 19.5 ms. As the threshold power was further increased, the rise time gradually decreased. This further demonstrates the existence of photon avalanche upconversion luminescence in the nanocrystals based on alkali metal-doped ultra-high-order nonlinear photonic avalanche fluorescence.

[0056] Example 4

[0057] This embodiment uses thermal decomposition method to prepare super high order nonlinear photonic avalanche fluorescent nanomaterial based on alkali metal ion doping, the specific steps are as follows:

[0058] 1. Preparation of NaLuF4: Tm / Li nanocrystals

[0059] Generally, 1 mmol of LuCl3·6H2O and TmCl3·6H2O are simultaneously added to a 100 mL flask containing oleic acid (10 mL) and 1-octadecene (15 mL) at different molar ratios. The mixture is heated to 130°C under 50 Pa vacuum for 60 min to form the corresponding lanthanide oleate complex. Subsequently, different stoichiometric amounts of solid NaOH and methanolic LiOH solution are introduced under a nitrogen atmosphere and dissolved at 130°C for 60 min. Then 0.148 g of NH4F is added and stirring is continued for 60 min. After the solid is dissolved, the mixture is degassed for 20 min to remove residual solvents and by-products, heated to 315°C, and kept at this temperature under nitrogen for 25 min. After cooling to room temperature, the product is centrifuged, washed thoroughly with cyclohexane and ethanol to remove residual impurities, and finally dispersed in 4.0 mL of cyclohexane for storage and subsequent characterization.

[0060] Figure 6 The results of the repetitive power curve test of the NaLuF4: Tm / 0.5% Li photonic avalanche nanocrystals prepared in Example 4 using the full-automatic power curve test system independently built in the laboratory are shown. The test data clearly shows that the nonlinear response of the photonic avalanche nanocrystals doped with alkali metal lithium ions (Li) can reach about 125.

[0061] Example 5

[0062] This embodiment uses co-precipitation method to prepare super high order nonlinear photonic avalanche fluorescent nanomaterial based on alkali metal ion doping, the specific steps are as follows:

[0063] 1. Preparation of NaYF4: Eu / Li nanocrystals with different Li + doping concentrations

[0064] Typically, an aqueous solution containing (2 mL, 0.2 M) Ln(CH3COO)3 (Ln = Y / Eu) was added to a 50 ml flask containing oleic acid (3 mL) and 1-octadecene (7 mL). The mixture was heated to 150 °C, magnetically stirred for 90 min to form lanthanide oleate complexes. After cooling to room temperature, a methanolic solution of NH4F (4 mL, 0.5 M), 0.8 mmol (1-x) NaOH and 0.8x mmol LiOH was added dropwise at 50 °C and stirred for 60 min. The reaction mixture was heated to 120 °C for 30 min under 50 Pa vacuum to remove residual methanol, then heated to 290 °C for 120 min under nitrogen to promote crystal growth. After cooling, the reaction mixture was treated with excess ethanol, centrifuged at 6000 r.p.m for 5 min to collect the resulting product, washed thoroughly with cyclohexane and ethanol to remove residual impurities, and finally redispersed in 4.0 mL cyclohexane for storage and subsequent characterisation.

[0065] Figure 7 Transmission electron microscopy images and size distribution plots of as-prepared NaYF4: Eu / x% Li (x = 0, 0.5, 1) photon avalanche nanocrystals, where a is NaYF4: Eu photon avalanche nanocrystals; b is NaYF4: Eu / 0.5% Li photon avalanche nanocrystals; c is NaYF4: Eu / 1% Li photon avalanche nanocrystals. High resolution transmission electron microscopy images show that the as-prepared NaYF4: Eu / x% Li (x = 0, 0.5, 1) photon avalanche nanocrystals have uniform morphology and size distribution. + The as-prepared NaYF4: Eu / Li nanocrystals have uniform morphology at different doping concentrations.

[0066] 2. NaYF4: Eu / Li nanocrystals with different Li doping concentrations + Preparation of NaYF4: Ho / Li nanocrystals with different Li doping concentrations

[0067] Typically, an aqueous solution containing (2 mL, 0.2 M) Ln(CH3COO)3 (Ln = Y / Ho) was added to a 50 ml flask containing oleic acid (3 mL) and 1-octadecene (7 mL). The mixture was heated to 150 °C with magnetic stirring for 90 min to form lanthanide oleate complexes. After cooling to room temperature, a methanolic solution of NH4F (4 mL, 0.5 M), 0.8 mmol (1-x) NaOH and 0.8x mmol LiOH was added dropwise at 50 °C with stirring for 60 min. The reaction mixture was heated to 120 °C under 50 Pa vacuum for 30 min to remove residual methanol, then heated to 290 °C under nitrogen for 120 min to promote crystal growth. After cooling, the reaction mixture was treated with excess ethanol, centrifuged at 6000 r.p.m for 5 min to collect the resulting product, washed thoroughly with cyclohexane and ethanol to remove residual impurities, and finally redispersed in 4.0 mL cyclohexane for storage and subsequent characterisation.

[0068] Figure 8 Transmission electron microscopy images and size distribution plots of as-prepared NaYF4: Ho / x% Li (x = 0, 0.5, 1) photon avalanche nanocrystals, where a is NaYF4: Ho photon avalanche nanocrystals; b is NaYF4: Ho / 0.5% Li photon avalanche nanocrystals; c is NaYF4: Ho / 1% Li photon avalanche nanocrystals. High resolution transmission electron microscopy images show that at lower alkali metal Li + The resulting NaYF4: Ho / Li nanocrystals have uniform morphology at different doping concentrations.

[0069] 2, NaYF4: Ho / Li nanocrystals prepared at different Li + Preparation of NaYF4: Er / Li nanocrystals at different doping concentrations

[0070] Typically, an aqueous solution containing (2 mL, 0.2 M) Ln(CH3COO)3 (Ln = Y / Er) was added to a 50 ml flask containing oleic acid (3 mL) and 1-octadecene (7 mL). The mixture was heated to 150 °C with magnetic stirring for 90 min to form lanthanide oleate complexes. After cooling to room temperature, a methanolic solution of NH4F (4 mL, 0.5 M), 0.8 mmol (1-x) NaOH and 0.8x mmol LiOH was added dropwise at 50 °C with stirring for 60 min. The reaction mixture was heated to 120 °C under 50 Pa vacuum for 30 min to remove residual methanol, and then heated to 290 °C under nitrogen for 120 min to promote crystal growth. After cooling, the reaction mixture was treated with excess ethanol and centrifuged at 6000 r.p.m for 5 min to collect the resulting product, which was washed thoroughly with cyclohexane and ethanol to remove residual impurities, and finally redispersed in 4.0 mL cyclohexane for storage and subsequent characterization.

[0071] Figure 9 Transmission electron microscopy (TEM) images and size distribution histograms of as-prepared NaYF4: Er / x% Li (x = 0, 0.5, 1) photon avalanche nanocrystals, where a is NaYF4: Er photon avalanche nanocrystals; b is NaYF4: Er / 0.5% Li photon avalanche nanocrystals; c is NaYF4: Er / 1% Li photon avalanche nanocrystals. High-resolution TEM images show that, at lower alkali metal Li + The obtained NaYF4: Er / Li nanocrystals have uniform morphology at different doping concentrations.

[0072] Experiments show that the present application proposes a new type of alkali metal ion doped photon avalanche nanomaterial. The alkali metal cation with smaller ionic radius is introduced into the NaREF4 matrix, which can occupy the interstitial position in the crystal lattice, actively induce the symmetry reduction of the local crystal field and the lattice distortion, significantly enhance the probability of 4f-4f electric dipole transition of rare earth luminescent ions such as Tm 3+ , and greatly strengthen the cross-relaxation process between ions. The nanomaterial prepared by this strategy not only realizes super-high-order nonlinear fluorescence response, but also has higher luminescent intensity and better composition stability, which provides a new material solution for developing low-power driven, super-sensitive and high-resolution optical probes.

[0073] For those skilled in the art, various corresponding changes and modifications can be given according to the above technical solutions and concepts, and all these changes and modifications should be included in the protection scope of the claims of the present application.

Claims

1. A low-dose alkali metal ion-doped-based ultra-high-order nonlinear photonic avalanche fluorescence nanomaterial, characterized in that The components include a matrix NaLnF4, doped rare earth luminescent active ions R 3+ and alkali metal ions A in a molar percentage of x + The chemical general formula is: NaLnF4: R 3+ / x A + wherein, the Ln is a rare earth matrix ion, selected from one or more of Y, Lu; the R is a rare earth luminescent active ion, selected from one or more of Tm, Eu, Tb, Ho, Er, Pr; the A is an alkali metal ion, selected from one or more of Li or K; x represents the percentage of the number of moles of A elements in the total number of moles of alkali metal elements, and satisfies 0 2. The low dose alkali ion doped based ultra-high order nonlinear photonic avalanche fluorescence nanomaterial of claim 1, wherein The matrix NaLnF4 is NaYF4; the R is Tm, which realizes high-order photon avalanche fluorescence under 1064 nm continuous laser excitation; the A is Li, and the doping amount x of the A is 0.1-1%.

3. A low-dose alkali metal ion-doped, ultra-high order nonlinear photonic avalanche fluorescence nanomaterial according to claim 1, wherein The alkali metal ions occupy the vacancies in the rare earth fluoride matrix lattice in the form of interstitial doping, reduce the symmetry of the original local coordination environment, weaken the inhibition of 4f-4f transition of the avalanche active ions, and improve the probability of electric dipole transition; the distortion of the local crystal field enhances the cross relaxation process between the avalanche active ions, improves the transition rate and amplifies the nonlinear optical response; the distortion of the crystal field makes the cross relaxation rate between the avalanche active ions increase by 10%-200%, and the optical nonlinear order of the material is higher than 60.

4. A method for preparing a low-dose alkali metal ion-doped ultra-high-order nonlinear photonic avalanche fluorescence nanomaterial, characterized in that The method comprises the following steps: S1: dissolving a rare earth metal salt in a mixed solvent of oleic acid and octadecene, stirring uniformly, and then coordinating the rare earth ions with the oleic acid through oil bath heating to form a stable lanthanide oleate complex; S2: respectively weighing alkali metal source, sodium source and fluorine source, dissolving in a polar organic solvent, and treating by ultrasonic to obtain a transparent and clear pre-doping solution for standby; S3: after cooling the mixed solution obtained in step S1 to room temperature, adding the pre-doping solution containing alkali metal, sodium source and fluorine source prepared in step S2, and fully stirring to mix the ions; heating to improve the reaction temperature to make the ions fully react to accelerate the precursor complex reaction, and obtaining a uniform mixed solution; S4: heating the mixed solution obtained in step S3 to 100-130 DEG C under a vacuum degree of 10-100 Pa for 20-40 min to remove low-boiling impurity solvents and enhance the stability of the complex; S5: heating the mixed solution obtained in step S4 to 250-320 DEG C under inert gas protection and a vacuum degree of 10-100 Pa for 1.5-3 h to promote the generation of crystal nucleus and the growth of crystal; S6: naturally cooling the system after the reaction to room temperature, repeatedly washing through centrifugal separation and washing solvent to remove unreacted substances and byproducts, and finally dispersing the obtained solid in cyclohexane to obtain the low-dose alkali metal ion-doped super-high-order nonlinear photon avalanche fluorescence nanomaterial.

5. The method for preparing ultra-high-order nonlinear photonic avalanche fluorescent nanomaterials based on low-dose alkali metal ion doping as described in claim 4, characterized in that... In step S1, the rare earth metal salt can be selected from one of chloride, nitrate and acetate, and preferably is acetate; when the rare earth metal salt is acetate, it includes inert rare earth metal acetate and active rare earth metal acetate; the inert rare earth metal acetate can be selected from one or more of yttrium acetate and lutetium acetate, and preferably is yttrium acetate; the active rare earth metal acetate can be selected from one or more of thulium acetate, neodymium acetate, erbium acetate, praseodymium acetate, europium acetate, holmium acetate and terbium acetate, and preferably is thulium acetate; the molar ratio of the active rare earth metal acetate to the inert rare earth metal acetate is x:(1-x), wherein 0 6. The method for preparing ultra-high-order nonlinear photonic avalanche fluorescent nanomaterials based on low-dose alkali metal ion doping as described in claim 4, characterized in that... In step S1, the concentration of the rare earth metal salt is 0.2 M, the solvent is a mixture of oleic acid and octadecene in a volume ratio of (1-10):(1-20), the oil bath heating temperature is 110-150°C, and the oil bath heating time is 0.5-1.5 h, preferably 150°C for 1 h.

7. The method for preparing ultra-high-order nonlinear photonic avalanche fluorescent nanomaterials based on low-dose alkali metal ion doping as described in claim 4, characterized in that... In step S2, the alkali metal source can be selected from a potassium source or a lithium source, preferably a lithium source; when the alkali metal source uses a lithium source, it is selected from one or more of lithium chloride, lithium nitrate, lithium trifluoroacetate or lithium hydroxide, preferably lithium hydroxide; the sodium source can be selected from one or more of sodium hydroxide, sodium carbonate, sodium trifluoroacetate, sodium nitrate or sodium fluoride, preferably sodium hydroxide; the fluoride source can be selected from one or more of ammonium fluoride, sodium fluoride and ammonium trifluoroacetate, preferably ammonium fluoride; the polar organic solvent can be selected from one or more of methanol, ethanol, dichloromethane, cyclohexane and n-hexane, preferably methanol; the molar ratio of the alkali metal source to the sodium source is x:(1-x), where 0 8. The method of claim 4, wherein the low dose alkali ion doped ultra-high order nonlinear photonic avalanche fluorescence nanomaterial is prepared by the method comprising: In step S3, the heating temperature is 40-70°C, and the reaction time is 30-60 min, preferably 50°C for 30 min; in step S4, the mixed solution obtained in step S3 is heated to 120°C under a vacuum of 50 Pa for 30 min; in step S5, the temperature is raised to 290°C, and the reaction is carried out for 2 h; the inert gas can be selected from one of nitrogen, helium and argon, preferably nitrogen; after the high-temperature reaction, the average particle size of the nanocrystals is 10-40 nm, and the particle size distribution standard deviation is less than 10%; the crystal morphology can be spherical, rod-shaped, prismatic, flaky or polyhedral, and the morphology is jointly controlled by the reaction temperature and the reaction time; the vacuum degree is 30-50 Pa. ​ 9. The method for preparing ultra-high-order nonlinear photonic avalanche fluorescent nanomaterials based on low-dose alkali metal ion doping as described in claim 4, characterized in that... In step S6, the washing solvent can be selected from one or more of anhydrous ethanol, cyclohexane, acetone or isopropanol, and the washing frequency is 2-6 times, preferably a mixture of anhydrous ethanol and cyclohexane.

10. The low-dose alkali metal ion doped ultra-high-order nonlinear photonic avalanche fluorescent nanomaterial according to claim 1, wherein the photonic avalanche effect has high-order nonlinear characteristics, N≥60; and the photonic avalanche fluorescent nanomaterial is applied in the fields of biological imaging, single molecule detection and ultra-sensitive sensing.

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