Near-infrared second-region excited single rare earth ion doped up-conversion luminescent material and preparation method thereof

Abstract

The invention discloses a near-infrared second-region excited single rare earth ion doped up-conversion luminescent material and a preparation method thereof.The near-infrared second-region excited single rare earth ion doped up-conversion luminescent material comprises a hexagonal-phase NaYF4 main body structure with low phonon energy and Er < 3 + > ions serving as a sensitizing agent and an activating agent, the molar content of the Er < 3 + > ions is 12.5%, the molar content of the NaYF4 main body structure is 12.5%, and the molar content of the Er < 3 + > ions is 12.5%. According to the invention, the redistribution of energy transition in a luminescence mechanism is caused by the design of a simple core-shell structure and the change of doped ion concentration, so that multicolor luminescence is realized, and the defects of doping of multiple ions and complex structural design in the prior art are overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of fluorescent luminescent materials, and in particular to an up-conversion luminescent material doped with a single rare earth ion excited in the second region of near-infrared and a preparation method thereof. Background Art

[0002] Rare earth element compounds are the most widely reported type of upconversion materials. In recent years, researchers have reported upconversion systems of various sizes, morphologies, and doped ions. Among the reported compound systems, NaYF4:Er is the most widely studied system and has been used in biomedicine. [7-9,72] Such as cancer diagnosis, drug delivery, and photochemical reactions such as photocatalysis. [33-38] , photoisomerization

[39] The research progress of Er has also been made in practical application. Most of these studies were conducted under 980nm laser light source excitation, which limits the further research and application of this system. 3+ Doped rare earth upconversion materials have strong excited state absorption 4 I 15 / 2 → 4 I 13 / 2 (~0.8eV, ~1550nm), and this absorption is just below the silicon band gap of solar cells (~1.1eV). This matched excitation transition will produce strong emission energy higher than the silicon band gap, making it have good application prospects in the field of single-crystal silicon solar cells; the same is true in biological applications. Excitation in the second near-infrared region often has a deeper penetration depth and lower biological autofluorescence interference; in addition, the excitation wavelength at about 1500nm is also within the telecommunication wavelength range (1300-1550nm), so the conversion of infrared light in this range into visible light is of great significance to optical communications and night vision, solar cells and medical applications.

[0003] In existing technologies, multicolor luminescence is primarily achieved by doping the same material with two or more luminescent ions or by simultaneously irradiating the material with multiple excitation light sources. These methods often require the simultaneous doping of multiple ions and the design of a multilayer structure to disperse the different ions in different sensitizing layers. This results in complex nanomaterial structures and cumbersome preparation processes. Simultaneous irradiation with multiple excitation light sources also leads to high experimental costs and technical difficulties. Summary of the Invention

[0004] The purpose of the present invention is to provide an up-conversion luminescent material doped with a single rare earth ion excited in the second region of the near-infrared and a preparation method thereof. By designing a simple core-shell structure and changing the concentration of the doped ions, the energy transition in the luminescence mechanism is redistributed to achieve multi-color luminescence, overcoming the shortcomings of multiple ion doping and complex structural design in the prior art. The nanomaterial designed by the present invention has the advantages of simpler structure and easier preparation.

[0005] The object of the present invention is achieved like this:

[0006] A near-infrared second-zone excited single rare earth ion doped upconversion luminescent material, comprising a hexagonal NaYF4 main structure with low phonon energy, Er as a sensitizer and activator 3+ ion.

[0007] The Er 3+ The molar content of ions is 12.5%.

[0008] The general formula of the up-conversion luminescent material is NaY(1-x)F4:xEr@NaYF4, wherein x is any value of 0.05, 0.1, 0.125, 0.15, 0.20, 0.40, 0.60, 0.80 and 1.

[0009] The up-conversion luminescent material exhibits up-conversion luminescence at wavelengths of 525 nm, 545 nm, 655 nm and 810 nm under excitation at a near-infrared second region wavelength of 1550 nm.

[0010] The up-conversion luminescent material is uniform spherical nanoparticles with a particle size of 10-14 nm.

[0011] A method for preparing a near-infrared second region excited single rare earth ion doped upconversion luminescent material comprises the following steps:

[0012] S1, add the set amount of YCl3·6H20, ErCl3·6H20, Na + After mixing the source, NH4F, oleic acid and octadecene, the core NaY(1-x)F4:xEr nanocrystal core was obtained after high temperature treatment under an inert gas atmosphere and then cooled to room temperature; S2, another set amount of YCl3·6H20, Na + After mixing the source, NH4F, oleic acid and octadecene, the core of S1 NaY(1-x)F4:xEr nanocrystal core is added, and high-temperature treatment is carried out under the protection of inert gas. After the reaction is completed, it is naturally cooled to room temperature, and an inert layer is epitaxially grown on the crystal core. After centrifugal washing, rare earth upconversion nanoparticles with high fluorescence intensity are obtained; rare earth upconversion nanoparticles are upconversion luminescent materials doped with single rare earth ions excited in the near-infrared second region.

[0013] The specific operating conditions of the high temperature treatment in S1 and S2 are a temperature of 300° C. and a holding time of 60 min.

[0014] The Na + The source is NaOA.

[0015] The specific operation of S1 is as follows: 15 mL of octadecene and 6 mL of oleic acid are measured in a 100 mL round-bottom flask, the set molar amounts of YCl3·6H2O and ErCl3·6H2O are measured and added to the system, and the mixture is stirred and heated in an electric heating mantle to 156°C under N2 protection, kept warm for 30 minutes, and then cooled to room temperature; a methanol solution containing 2.5 mmol of NaOA and 4 mmol of NH4F is added, stirred at room temperature for 30 minutes, then heated to 100°C to remove methanol, and then heated to 300°C and kept warm for 60 minutes. After the end, it is naturally cooled to room temperature; it is precipitated with ethanol and cyclohexane solution, centrifuged, and washed three times, and then all dissolved in 6 mL of cyclohexane to obtain a core NaY(1-x)F4:xEr solution.

[0016] The specific operation of S2 is as follows: 15 mL of octadecene and 6 mL of oleic acid are measured and placed in a 100 mL round-bottom flask, 1 mmol of YCl3·6H2O is measured and added to the round-bottom flask, and the mixture is stirred and heated in an electric heating mantle under N2 protection to 156°C, kept warm for 30 minutes, and when the temperature drops to 80°C, a core NaYF4:Er solution dissolved in 6 mL of cyclohexane is added; after cooling to room temperature, a methanol solution containing 2.5 mmol of NaOA and 4 mmol of NH4F is added, and the mixture is stirred at room temperature for 30 minutes and heated to 100°C to remove methanol and cyclohexane, and then heated to 300°C and kept warm for 90 minutes. After the reaction, the mixture is naturally cooled to room temperature; the mixture is precipitated with a mixed solution of ethanol and cyclohexane, centrifuged, and washed three times, and then dissolved in 6 mL of cyclohexane to obtain upconversion luminescent material nanoparticles doped with a single rare earth ion excited in the second region of the near-infrared.

[0017] The beneficial effects of the present invention are as follows: the technical solution provided by the present invention has the following advantages over the reported technologies: (1) the rare earth up-conversion material provided by the present invention can realize different colors of luminescence under the condition of only one emitting photoion; (2) the rare earth up-conversion material provided by the present invention can be excited by a near-infrared second-zone laser with better penetration; (3) the present invention adopts a solvent thermal method to precisely control the synthesis temperature, reaction time, etc., so that the prepared nanoparticles have a small particle size and a uniform size distribution, which broadens the application range of rare earth materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a flow chart of the present invention;

[0019] Figure 2 The present invention is different under 1550nm excitation Er 3+Content of NaYF4:Er upconversion luminescence spectrum (a), the integrated intensity distribution histogram of each band (b) and the total luminescence intensity of each content sample and the green / red light intensity ratio (c);

[0020] Figure 3 Figure 1 shows the upconversion luminescence spectra of NaYF4:Er at different holding times under 1550nm excitation, the integrated intensity distribution of each emission band, and the total intensity and green / red light intensity ratio of the NaYF4:Er system of the present invention.

[0021] Figure 4 XRD patterns of NaYF4:Er (12.5%) prepared with different holding times of the present invention (a: 0 min, b: 10 min, c: 30 min, d: 60 min, e: 90 min);

[0022] Figure 5 Schematic diagram of the core-shell structure of rare earth upconversion nanoparticles of the present invention;

[0023] Figure 6 Figures 1 and 2 show the test data of the upconversion luminescent material of the present invention under different conditions; (a) shows the upconversion luminescence spectra of NaYF4:Er(12.5%)@NaYF4 at different holding times under 1550nm excitation; (b) shows the total luminescence intensity histogram of the core and core-shell structures; (c) shows the emission intensity histogram of the core-shell structure nanoparticles prepared at different holding times; (d) shows the total luminescence intensity and the green / red light intensity ratio;

[0024] Figure 7 TEM images of the upconversion nanoparticles of the present invention; (a) and (b) are low-magnification TEM images of NaYF4:Er(12.5%) and NaYF4:Er(12.5%)@NaYF4 upconversion nanoparticles; (c) and (d) are high-magnification TEM images of NaYF4:Er(12.5%) and NaYF4:Er(12.5%)@NaYF4 upconversion nanoparticles; (e) and (f) are dark field images of NaYF4:Er(12.5%)@NaYF4 upconversion nanoparticles and EDS line scan images of Er and Y elements;

[0025] Figure 8 XRD patterns of the core structure NaYF4:Er(12.5%) and the core-shell structure NaYF4:Er(12.5%)@NaYF4 of the present invention;

[0026] Figure 9 Figure 1 is the upconversion luminescence spectrum of NaYF4:Er(12.5%)@NaYF4 at different excitation powers at 1550nm of the present invention (a) and the relationship between the luminescence intensity of each emission peak and the excitation power (b);

[0027] Figure 10This is a diagram of the upconversion luminescence mechanism of NaYF4:Er(12.5%)@NaYF4 under 1550nm excitation of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the accompanying drawings and examples.

[0029] A near-infrared second-zone excited single rare earth ion doped upconversion luminescent material, comprising a hexagonal NaYF4 main structure with low phonon energy, Er as a sensitizer and activator 3+ ions. 3+ The molar content of ions is 12.5%. The general formula of the upconversion luminescent material is NaY(1-x)F4:xEr@NaYF4, where x is any value selected from 0.05, 0.1, 0.125, 0.15, 0.20, 0.40, 0.60, 0.80, and 1. The upconversion luminescent material exhibits upconversion luminescence at wavelengths of 525 nm, 545 nm, 655 nm, and 810 nm under excitation at a near-infrared wavelength of 1550 nm in the second region. The upconversion luminescent material is a uniform, spherical nanoparticle with a particle size of 10-14 nm.

[0030] A method for preparing a near-infrared second region excited single rare earth ion doped upconversion luminescent material, such as Figure 1 As shown, the following steps are included:

[0031] S1, add the set amount of YCl3·6H20, ErCl3·6H20, Na + After mixing the source, NH4F, oleic acid and octadecene, the mixture is treated at high temperature in an inert gas atmosphere and then cooled to room temperature to obtain the inner core NaY(1-x)F4:xEr nanocrystal core; the specific operating conditions of the high temperature treatment are a temperature of 300°C and a holding time of 60 minutes. The specific operation of S1 is as follows: 15 mL of octadecene and 6 mL of oleic acid are measured in a 100 mL round-bottom flask, the set molar amounts of YCl3·6H2O and ErCl3·6H2O are measured and added to the system, and the mixture is stirred and heated in an electric heating mantle to 156°C under N2 protection, kept warm for 30 minutes, and then cooled to room temperature; a methanol solution containing 2.5 mmol of NaOA and 4 mmol of NH4F is added, stirred at room temperature for 30 minutes, then heated to 100°C to remove methanol, and then heated to 300°C and kept warm for 60 minutes. After the end, it is naturally cooled to room temperature; it is precipitated with ethanol and cyclohexane solution, centrifuged, and washed three times, and then all dissolved in 6 mL of cyclohexane to obtain a core NaY(1-x)F4:xEr solution.

[0032] S2, take another set amount of YCl3·6H20, Na +After mixing the source, NH4F, oleic acid and octadecene, the core of S1 NaY(1-x)F4:xEr nanocrystal core is added and high temperature treatment is carried out under the protection of inert gas. The specific operating conditions of high temperature treatment are temperature of 300°C and holding time of 60min. After the reaction is completed, it is naturally cooled to room temperature, and an inert layer is epitaxially grown to cover the crystal core. Then, it is centrifuged and washed to obtain rare earth upconversion nanoparticles with high fluorescence intensity. The rare earth upconversion nanoparticles are upconversion luminescent materials doped with single rare earth ions excited in the second region of near-infrared. The Na + The source is NaOA. The specific operation of S2 is as follows: 15 mL of octadecene and 6 mL of oleic acid are measured and placed in a 100 mL round-bottom flask. 1 mmol of YCl3·6H2O is added to the round-bottom flask. Under nitrogen protection, the mixture is heated in an electric heating mantle with stirring to 156°C and held for 30 minutes. When the temperature drops to 80°C, a core NaYF4:Er solution dissolved in 6 mL of cyclohexane is added. After cooling to room temperature, a methanol solution containing 2.5 mmol of NaOA and 4 mmol of NH4F is added. The mixture is stirred at room temperature for 30 minutes and heated to 100°C to remove the methanol and cyclohexane. The mixture is then heated to 300°C and held for 90 minutes. After completion, the mixture is naturally cooled to room temperature. The mixture is precipitated with a mixed solution of ethanol and cyclohexane, centrifuged, washed three times, and dissolved in 6 mL of cyclohexane to obtain upconversion luminescent material nanoparticles doped with a single rare earth ion excited in the second region of the near-infrared.

[0033] The present invention adopts thermal decomposition method to prepare up-conversion nanomaterials. Rare earth chloride (YCl3.6H2O, ErCl3.6H2O), NaOA and NH4F are used as precursors to provide Y 3+ 、Er 3+ 、Na + 、F - Oleic acid is synthesized at high temperature in the binary system of oleic acid and octadecene. Oleic acid not only provides a high temperature environment, but also serves as a nanomaterial modifier to prevent aggregation and improve stability. Octadecene is used to cooperate with oleic acid to provide a high temperature environment. The use of sodium oleate not only provides Na + The excess oleate ions can regulate the crystal nucleus growth process to prepare nanomaterials with smaller and more uniform grain sizes, meeting various application requirements of the material. The upconversion nanomaterial synthesis process includes rare earth chloride dissolution, crystal nucleus formation, and grain growth. The purpose of keeping at 156℃ is to obtain a stable rare earth chloride solution. + and F - The purpose of the subsequent room temperature stirring is to fully form the crystal nucleus, and the added Na + is excessive (4 times the F - ), so that F - The reaction is complete and the holding temperature at 300°C is the process of grain growth.

[0034] Different doping contents of Er 3+ Preparation of nanomaterials NaYF4:Er

[0035] In order to study the effect of activator doping content on the performance of upconversion luminescent materials, a series of different Er 3+ In order to study the effect of activator doping content on the performance of upconversion luminescent materials, a series of different Er2O3 upconversion luminescent materials were prepared using the same preparation method with different formulations. 3+ Content, when Er 3+ The content is 10moL% (YCl3·6H2O is 0.9mmoL, ErCl3·6H2O is 0.1mmoL), when Er 3+ The content is 12.5moL% (YCl3·6H2O is 0.875mmoL, ErCl3·6H2O is 0.125mmoL) / when Er 3+ The content is 15moL% (YCl3·6H2O is 0.85mmoL, ErCl3·6H2O is 0.15mmoL), when Er 3+ The content is 20moL% (YCl3·6H2O is 0.8mmoL, ErCl3·6H2O is 0.5mmoL), when Er 3+ The content is 40moL% (YCl3·6H2O is 0.6mmoL, ErCl3·6H2O is 0.4mmoL), when Er 3+ The content is 60moL% (YCl3·6H2O is 0.4mmoL, ErCl3·6H2O is 0.6mmoL), when Er 3+ The content is 80moL% (YCl3·6H2O is 0.2mmoL, ErCl3·6H2O is 0.8mmoL).

[0036] The specific process is as follows: 15 mL of octadecene and 6 mL of oleic acid were accurately measured and placed in a 100 mL round-bottom flask. Different molar amounts of YCl₃.6H₂O and ErCl₃.6H₂O were then added to the system. Under nitrogen protection, the system was heated to 156°C with stirring (500 rpm) in an electric heating mantle. The temperature was maintained for 30 minutes, and then cooled to room temperature. A methanol solution containing 2.5 mmol of NaOA and 4 mmol of NH₄F was added, and the system was stirred at room temperature for 30 minutes. The temperature was raised to approximately 100°C to remove the methanol, and then the temperature was rapidly raised to 300°C and maintained for 60 minutes. After the temperature was reached, the system was naturally cooled to room temperature. The system was precipitated with ethanol and cyclohexane solutions, centrifuged, and washed several times before being dissolved in 6 mL of cyclohexane.

[0037] Preparation of NaYF4:Er Nanomaterials with Different Growth Times

[0038] In order to study the effect of grain growth time on material properties, upconversion luminescent materials were prepared using the same formula and the same synthesis steps (refer to the above synthesis steps) at a temperature of 300°C with different holding times (0min, 30min, 60min and 90min).

[0039] Er 3+ Effect of doping content on upconversion luminescence properties of NaYF4:Er

[0040] Figure 2 Different Er under 1550nm excitation 3+ Upconversion luminescence spectrum (a), integrated intensity of each emission band (b), total intensity and green-red intensity ratio (c) of the content-doped NaYF4:Er upconversion luminescent material. 3+ The emission positions of the samples with the same doping content are all at about 525nm, 540nm, 650nm and 810nm. As the doping amount increases, the emission peak intensity first increases and then decreases. 3 + content is 12.5%, the total luminescence intensity reaches the maximum, and then 3 + content continues to increase, the luminescence intensity becomes weaker. This may be because as the concentration of luminescent ions increases, the "concentration quenching" effect increases, more energy disappears through non-radiative transitions, causing the up-conversion luminescence to weaken. Therefore, the Er 3 + content of 12.5% ​​was used as the condition for subsequent optimization experiments.

[0041] Effect of grain growth time

[0042] Upconversion spectrum analysis of NaYF4:Er prepared with different holding times under 1550nm excitation

[0043] The preparation process of upconversion materials includes nucleation, particle growth, size reduction, and aggregation. The duration of high temperature influences the particle's crystal form and size. If the duration is too short, the reaction may terminate before the tetragonal phase has time to transition to the hexagonal phase. The final product is an α-phase nanomaterial with higher phonon energy, resulting in a weaker luminescence intensity than the hexagonal phase.

[0044] Figure 3The upconversion luminescence spectra (a) and the integrated intensity distribution of each emission band (b) of NaYF4:Er upconversion luminescent materials prepared at 300℃ for different holding times under 1550nm excitation are shown in the figure. As can be seen from the figure, the emission positions of the samples at different holding times at 300℃ are the same, with the emission peak positions all located at approximately 525nm, 545nm, 655nm and 810nm. As the holding time increases, the green and near-infrared emission intensities first increase and then decrease. When the holding time is 60min, the luminescence intensity of the prepared nanomaterial is the highest, and the red light intensity reaches its maximum at 90min. This is because when the holding time is relatively short, the particle size of the nanoparticles is too small, the specific surface area is large, and the surface quenching is severe, resulting in weaker luminescence; as the holding time increases, the particle size increases, the specific surface area decreases accordingly, and the surface quenching decreases accordingly; however, when the particle size continues to increase, more luminescent ions are located inside the particles, and the concentration quenching effect comes into play again, and more energy disappears through non-radiative transitions, causing the detected upconversion luminescence to weaken again.

[0045] Phase analysis of NaYF4:Er prepared with different holding times

[0046] Figure 4 The XRD spectra of NaYF4:Er nanomaterials prepared at different holding times are shown. According to the standard cards JCPDS:06-0342 and JCPDS:16-0334, the X-ray diffraction pattern of the cubic phase NaYF4 (α-phase) has obvious diffraction peaks at 2θ=28.4°, 32.8°, 47.1°, 56.0°, and 68.8°, which correspond to the (111), (200), (220), (311), and (400) crystal planes of the standard cubic phase, respectively. The hexagonal NaYF4 (β-phase) has obvious diffraction peaks at 2θ = 17.2°, 30.1°, 30.8°, 39.6°, 43.5°, 46.6°, 53.3°, and 53.8°, which correspond to the (100), (110), (101), (111), (201), (210), (300), and (211) crystal planes of the standard hexagonal phase, respectively, and are consistent with the literature analysis. [1,41] . Compared with the standard spectrum, when the holding time is 0min and 10min, there are obvious cubic phase NaYF4 characteristic peaks (2θ=28.4°) and hexagonal phase diffraction peaks on the XRD spectrum, indicating that the sample is a mixed phase of cubic phase (i.e. α-phase) and hexagonal phase (i.e. β-phase). When the holding time is 30min, 60min and 90min, the α-phase characteristic peak disappears. The hexagonal phase has lower phonon energy than the cubic phase.

[42] , which can avoid energy loss caused by unnecessary non-radiative transitions. The target phase of the present invention is a pure hexagonal phase. Therefore, in order to ensure that the synthesized product is completely converted into the β-phase, the holding time at 300°C is at least 30 minutes.

[0047] Effect of epitaxial inert layer

[0048] Upconversion spectrum analysis of core-shell structure NaYF4:Er@NaYF4 under 1550nm excitation

[0049] The present invention has only one luminescent ion, so the method of growing an inert layer outside the bare core is adopted. The schematic diagram of the prepared core-shell structure is shown in FIG. Figure 5 As shown, the luminescent ion Er 3+ Concentrated in the nuclear structure, Y 3+ The ions are distributed in the core and shell structure.

[0050] Figure 6 The upconversion emission spectrum and the integrated intensity distribution of each emission peak of the core-shell structure NaYF4:Er(12.5%)@NaYF4 nanomaterial prepared with NaYF4:Er(12.5%) prepared at different holding times as the core; As can be seen from the figure, and Figure 3 The trend is consistent. The core-shell structure prepared by keeping the core for 60 minutes has the highest luminescence intensity. And it is calculated that the core-shell nanomaterials prepared by keeping the core for 0 minutes, 30 minutes, 60 minutes, and 90 minutes are 136 times, 58 times, 76 times, and 110 times higher than the total luminescence intensity of the bare core, respectively, as shown in Figure (b). This shows that the core-shell structure can greatly improve the luminescence performance of nanomaterials. It can also be seen from Figure (c) that, compared with the previous bare core luminescence intensity distribution ( Figure 3 Compared to Figure (b), the visible green emission intensity of the core-shell nanomaterial's luminescence spectrum is greater than the near-infrared emission intensity, which is contrary to the test results of the bare core. This is due to the presence of the inert epitaxial layer, which separates the activator from the external environment. This allows the energy absorbed by the luminescence system to be used more for transitions to higher excited states, reducing non-radiative effects and increasing the luminescence intensity. As shown in Figure (d), the nanomaterial prepared with the core heated at 300°C for 60 minutes has the highest total luminescence intensity, which is consistent with the upconversion luminescence spectrum. The green-red light intensity ratio shows an overall downward trend, indicating that the red light accounts for an increasing proportion of the emitted light.

[0051] TEM characterization

[0052] Figure 7 (a) and (b) show transmission electron micrographs of the synthesized upconversion luminescent materials, bare-core NaYF4:12.5%Er and core-shell NaYF4:12.5%Er@NaYF4. The NaYF4:12.5%Er particles range in size from 6.0 to 8.5 nm, while the NaYF4:12.5%Er@NaYF4 core-shell nanoparticles range in size from 10.75 to 13.25 nm, indicating that the thickness of the NaYF4 epitaxial layer is approximately 2.5 nm. The insets are histograms of the nanoparticle size distribution. Figure 7(c) and (d) are high-resolution transmission electron micrographs of bare-core NaYF4:12.5%Er and core-shell NaYF4:12.5%Er@NaYF4 nanoparticles, respectively. High-resolution transmission electron micrographs show that the individual nanoparticles exhibit a single-crystal structure with distinct lattice fringes. Analysis reveals that the interplanar spacings are 0.519 nm and 0.520 nm, respectively, which are similar to the (001) interplanar spacing of standard hexagonal NaYF4. This indicates that both bare-core NaYF4:12.5%Er and core-shell NaYF4:12.5%Er@NaYF4 exhibit hexagonal structures. The insets are electron diffraction patterns of the nanoparticles, where the diffraction rings are clearly visible. Comparison with the standard hexagonal electron diffraction patterns in the literature indicates that these rings correspond to the (100), (110), (101), (111), (201), and (210) planes of the hexagonal phase, respectively, and are generally consistent with the results reported in the literature. Figure 7 (e) is a high-angle dark field image of the core-shell structure. The element distribution of the core-shell structured nanoparticles NaYF4:12.5% ​​Er@NaYF4 was analyzed by EDS line scanning. The results show that the Er element is mainly distributed inside the nanoparticles, while the Y element is distributed not only in the outer layer of the nanoparticles, but also in the inner layer of the nanoparticles, indicating the successful manufacture of the core-shell structure.

[0053] XRD characterization

[0054] Figure 8 Shown are XRD patterns of the synthesized upconversion luminescent materials, bare-core NaYF4:12.5% ​​Er and core-shell NaYF4:12.5% ​​Er@NaYF4. The results show that the XRD patterns of the two materials are essentially identical, corresponding well to the standard card spectrum peak positions of the hexagonal phase (β-phase), indicating that the growth of the epitaxial inert layer does not affect the crystal structure, and that the core and shell crystal structures are consistent, both being hexagonal.

[0055] Study on the upconversion luminescence mechanism of NaYF4:Er@NaYF4 excited by 1550nm

[0056] Upconversion spectrum analysis of NaYF4:Er@NaYF4 under different excitation powers at 1550nm

[0057] Figure 9 (a) shows the upconversion emission spectra of NaYF4:Er@NaYF4 under different power densities at 1550nm. It can be clearly seen from the figure that the luminescence intensity increases with the increase of excitation power. In addition, for the upconversion luminescence process, there is a certain relationship between the luminescence intensity and the excitation light power, that is, I∝P n, where I is the upconversion emission intensity, P is the excitation light power density, and n is the number of long-wavelength photons absorbed to emit a short-wavelength photon. In other words, the value of n indicates how many photons participate in the upconversion process. By calculating the intensity of each emission peak on the emission spectrum under different laser irradiation, the logarithmic relationship between it and the pump power is studied, and its coordinate diagram is shown in the figure below. Figure 9 As shown in (b), the slopes of the two fitting lines are 2.7, 2.4, 2.3, and 1.8, respectively, indicating that the green emission at 525 nm and 545 nm is a three-photon process, the red emission at 655 nm is a three-photon process, and the short-wave near-infrared emission at 810 nm is a two-photon process.

[0058] Upconversion luminescence mechanism of NaYF4:Er@NaYF4 under 1550nm excitation

[0059] The upconversion luminescent material is excited by 1550nm laser, and the activator Er 3+ The upconversion process is completed by the internal energy level transition of the ion. Figure 10 Shown is Er 3+ In the up-conversion mechanism under 1550nm laser excitation, each emission peak in the up-conversion spectrum corresponds to a photon energy level transition process. After the ion at the ground state energy level absorbs a 1550nm photon, it changes from the ground state to the 4 I 15 / 2 Energy level jump to a higher 4 I 13 / 2 energy level, and then the excited state ion continues to absorb a 1550nm photon and transition to a higher 4 I 9 / 2 Energy level, at 4 I 9 / 2 Some of the ions in the energy level return directly to the ground state 4 I 15 / 2 , emitting an 810nm photon through radiative transition; the other part continues to absorb a 1550nm photon and transitions upward to 2 H 11 / 2 energy level, 2 H 11 / 2 Some of the photons in the energy level return directly to the ground state 4 I 15 / 2 , emitting a 525nm photon, and the other part transitions downward to 4 S 3 / 2 energy level; 4 S 3 / 2 Some of the photons in the energy level radiate back to the ground state 4 I 15 / 2 , a 545nm photon is emitted during this process, and the other part transitions downward without radiation to4 F 9 / 2 energy level, and finally transitions downward to the ground state while emitting a 655nm photon. The above processes may be accompanied by the participation of phonons, which can be simply expressed by the following formula: 4 I 15 / 2 +hν→ 4 I 13 / 2 ; 4 I 13 / 2 +hν→ 4 I 9 / 2 ; 4 I 9 / 2 → 4 I 15 / 2 +810nm photons; 4 I 9 / 2 +hν→ 2 H 11 / 2 ; 2 H 11 / 2 → 4 I 15 / 2 +525nm photons; 4 S 3 / 2 → 4 I 15 / 2 +545nm photons; 4 F 9 / 2 → 4 I 15 / 2 +655nm photons.

[0060] In summary, 3+ The upconversion luminescent material prepared when the doping molar content is 12.5% ​​and the temperature is kept at 300℃ for 60min is in hexagonal phase, and the total upconversion luminescence intensity is the highest; the core-shell structure can significantly improve its upconversion luminescence performance (50-110 times), and compared with the bare core, the proportion of upconversion visible light increases, and the proportion of visible band red light increases. The upconversion luminescence intensity increases with the increase of the excitation light power, and the two are linearly related. 525nm, 545nm, 655nm, and 810nm correspond to 2 H 11 / 2 - 4 I 15 / 2 、 4 S 3 / 2 - 4 I 15 / 2 、 4 F 9 / 2 - 4 I 15 / 2 、 4 I 9 / 2 - 4 I 15 / 2 Energy level transition.

Claims

1. A near-infrared second region excitation single rare earth ion doped upconversion luminescent material, characterized in that: It includes a hexagonal NaYF4 main structure with low phonon energy, Er as a sensitizer and activator 3+ ion.

2. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 1, characterized in that: The Er 3+ The molar content of ions is 12.5%.

3. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 1, characterized in that: The general formula of the up-conversion luminescent material is NaY(1-x)F4:xEr@NaYF4, wherein x is any value of 0.05, 0.1, 0.125, 0.15, 0.20, 0.40, 0.60, 0.80 and 1.

4. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 1, characterized in that: The up-conversion luminescent material exhibits up-conversion luminescence at wavelengths of 525 nm, 545 nm, 655 nm and 810 nm under excitation at a near-infrared second region wavelength of 1550 nm.

5. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 1, characterized in that: The up-conversion luminescent material is uniform spherical nanoparticles with a particle size of 10-14 nm.

6. A method for preparing a near-infrared second region excited single rare earth ion doped upconversion luminescent material according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1, add the set amount of YCl3·6H20, ErCl3·6H20, Na + After mixing the source, NH4F, oleic acid and octadecene, the mixture was treated at high temperature under an inert gas atmosphere and then cooled to room temperature to obtain the core NaY(1-x)F4:xEr nanocrystal core; S2, take another set amount of YCl3·6H20, Na + After mixing the source, NH4F, oleic acid and octadecene, the core of S1 NaY(1-x)F4:xEr nanocrystal core is added, and high-temperature treatment is carried out under the protection of inert gas. After the reaction is completed, it is naturally cooled to room temperature, and an inert layer is epitaxially grown on the crystal core. After centrifugal washing, rare earth upconversion nanoparticles with high fluorescence intensity are obtained; rare earth upconversion nanoparticles are upconversion luminescent materials doped with single rare earth ions excited in the near-infrared second region.

7. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 6, characterized in that: The specific operating conditions of the high temperature treatment in S1 and S2 are a temperature of 300° C. and a holding time of 60 min.

8. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 6, characterized in that: The Na + The source is NaOA.

9. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 6, characterized in that: The specific operation of S1 is as follows: 15 mL of octadecene and 6 mL of oleic acid are measured in a 100 mL round-bottom flask, the set molar amounts of YCl3·6H2O and ErCl3·6H2O are measured and added to the system, and the mixture is stirred and heated in an electric heating mantle to 156°C under N2 protection, kept warm for 30 minutes, and then cooled to room temperature; a methanol solution containing 2.5 mmol of NaOA and 4 mmol of NH4F is added, stirred at room temperature for 30 minutes, then heated to 100°C to remove methanol, and then heated to 300°C and kept warm for 60 minutes. After the end, it is naturally cooled to room temperature; it is precipitated with ethanol and cyclohexane solution, centrifuged, and washed three times, and then all dissolved in 6 mL of cyclohexane to obtain a core NaY(1-x)F4:xEr solution.

10. The near-infrared second region excited single rare earth ion doped upconversion luminescent material according to claim 6, characterized in that: The specific operation of S2 is as follows: 15 mL of octadecene and 6 mL of oleic acid are measured and placed in a 100 mL round-bottom flask, 1 mmol of YCl3·6H2O is measured and added to the round-bottom flask, and the mixture is stirred and heated in an electric heating mantle under N2 protection to 156°C, kept warm for 30 minutes, and when the temperature drops to 80°C, a core NaYF4:Er solution dissolved in 6 mL of cyclohexane is added; after cooling to room temperature, a methanol solution containing 2.5 mmol of NaOA and 4 mmol of NH4F is added, and the mixture is stirred at room temperature for 30 minutes and heated to 100°C to remove methanol and cyclohexane, and then heated to 300°C and kept warm for 90 minutes. After the reaction, the mixture is naturally cooled to room temperature; the mixture is precipitated with a mixed solution of ethanol and cyclohexane, centrifuged, and washed three times, and then dissolved in 6 mL of cyclohexane to obtain upconversion luminescent material nanoparticles doped with a single rare earth ion excited in the second region of the near-infrared.

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