Panchromatic upconversion single nanocrystal excited by single wavelength as well as preparation and application of panchromatic upconversion single nanocrystal

Abstract

The invention discloses a panchromatic up-conversion single nanocrystal excited by single wavelength as well as preparation and application of the panchromatic up-conversion single nanocrystal. And the up-conversion single nanocrystalline rare earth is doped with up-conversion luminescent nanoparticles UCNPs. The structural composition of the UCNPs is NaYbF4: Er NaYbF4 NaYbF4: Yb, Tm, the UCNPs is realized by constructing a core / shell / shell structure, and the reasonably designed architecture modulates the excitation power density by utilizing a photon-level dependent up-conversion process, so that the emission of green light, red light and blue light is realized. Along with the increase of the power density of the 980nm continuous wave laser, the emission color is systematically changed from green to red and finally to blue. The invention provides a simplified but highly universal excitation scheme for complete RGB adjustability, and has wide application prospects in the fields of advanced high-resolution color displays, anti-counterfeiting, multifunctional biological probes, drug controlled release and the like.

Description

Technical Field

[0001] The present invention belongs to the fields of color display, anti-counterfeiting, multifunctional biological probe and controlled drug release, and particularly relates to a full-color up-conversion single nanocrystal with power density tuned by single-wavelength excitation, and its preparation and application. Background Art

[0002] Lanthanide ion (Ln³⁺)-doped nanocrystals with fixed composition and tunable upconversion colors have attracted considerable attention due to their promising applications in 3D color display devices, information storage, and advanced anti-counterfeiting technologies. Emission color can be modified by adjusting dopant concentration, doping with different lanthanide ions, introducing luminescence quenchers, or varying the spacing of emitting ions in multilayer particles. However, these strategies typically result in specific emission colors that may only be suitable for certain applications. Achieving tunable luminescence in nanocrystals with fixed ion composition is challenging because these nanocrystals are minimally responsive to environmental changes. Consequently, changes in ambient temperature, humidity, electric field, or pressure typically result in only slight shifts in emission color. The most effective strategy for achieving color tunability in a single particle involves different excitation wavelengths, each stimulating a different lanthanide ion to emit a different color. However, these wavelength-dependent strategies require complex designs with multiple nanocrystal layers. To prevent color interference, inert shells are often introduced between the layers, resulting in complex multilayer structures requiring four to six layers. This structural complexity translates into lengthy and challenging synthesis processes and high production costs, limiting practical applications. These strategies typically require multiple excitation sources, significantly increasing the complexity and cost of the luminescence tunability setup. Temporal modulation of excitation can produce tunable colors within a single nanocrystal, offering a promising path to achieving high-quality color tunability with fewer excitation sources. However, this approach requires additional equipment to control pulse width and frequency, complicating the setup and yielding only two tunable components in the RGB spectrum.

[0003] Varying the excitation power density provides a simple, low-cost method for color tuning using external stimuli. A single excitation source with varying power or pulse width and frequency can only induce two RGB components at most. Combining these two strategies could potentially yield tunable RGB emission and simplify the excitation setup. By combining wavelength- and time-domain excitation principles, tunable RGB emission was demonstrated using two excitation sources (800 and 980 nm). Achieving RGB upconversion emission with a single excitation wavelength would significantly simplify the operational complexity and setup cost of these luminescent nanocrystals for practical applications. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention provides a single nanocrystal that can upconvert full-color light using a single wavelength, as well as its preparation and application. As the power density of a monochromatic 980nm continuous-wave laser increases, the emission color systematically changes from green to red and eventually to blue.

[0005] The technical solutions adopted by the present invention to solve the technical problems are as follows: A full-color upconversion single nanocrystal with power density tunable by single-wavelength excitation has a structure consisting of a core, an inert intermediate shell, and an active outer shell from the inside out, and is composed of NaYbF4:2%Er@NaYF4@NaYF4: 18%Yb, 2%Tm. The full-color upconversion nanocrystal utilizes a photon-dependent upconversion process to modulate the excitation power density, thereby achieving green, red, and blue light emission.

[0006] The upconversion nanocrystal has the following composition: core NaYbF4: 2%Er, comprising 2% erbium (Er) and 98% ytterbium (Yb) in a molar ratio; and active shell NaYF4: 18%Yb, 2%Tm, comprising 2% thulium (Tm) and 18%Yb in a molar ratio.

[0007] The diameter of the upconversion nanocrystal is 50 nm.

[0008] The upconversion nanocrystals use a three-primary color tunable upconversion luminescence with a single excitation wavelength of 980 nm; as the power density of the 980 nm continuous wave laser increases, the emission color systematically changes from green to red and finally to blue.

[0009] The method for preparing a full-color up-conversion single nanocrystal with power density tunable by single-wavelength excitation comprises the following steps: Step (1), synthesis of NaYbF4: 2%Er; Step (2) uses thermal decomposition method to prepare NaYbF4: 2%Er 3 A NaYF4 shell is grown on the core; Step (3), synthesize NaYbF4: 2%Er@NaYF4@NaYF4: 18%Yb, 2%Tm.

[0010] The step (1) comprises the following steps: stirring 1 mmol RECl3·xH2O and 3 mmol sodium oleate in a mixture of ethanol and hexane in a volume ratio of 1:1 at 70°C for 3 hours; separating the precipitated rare earth oleate by centrifugation, washing with ethanol and hexane, and then drying in vacuo at 60°C to obtain rare earth oleate, i.e., RE-oleate; mixing 1 mmol RE-oleate in a Yb: Er molar ratio of 0.98:0.02 with 5 mL 1-octadecene and 10 mL oleic acid; heating the mixture to 60°C to completely evaporate the ethanol; subsequently, raising the temperature to 160°C and maintaining it under a steady argon flow for 30 minutes; then, allowing the solution to cool to room temperature, during which time the residual gas is removed; dissolving 4 mmol ammonium fluoride and 2.5 mmol sodium hydroxide in 10 mL methanol respectively; adding the solution to the cooled reaction mixture in a three-necked flask; and then heating the mixed solution to 300°C at a controlled rate of 20°C per minute and maintaining it at this temperature for 90 minutes. minutes; after the reaction, the mixture was allowed to cool naturally to ambient temperature, the precipitate was collected by centrifugation, washed with ethanol to remove impurities, and then centrifuged again; finally, the purified sample was dispersed in hexane for storage and further analysis.

[0011] In the step (2), the precursor solution of sodium, yttrium and fluoride source prepared in the step (1) is used as the shell material; the precursor solution is mixed with the pre-synthesized NaYbF4: 2%Er 3+ The core nanoparticles were carefully mixed to ensure uniform dispersion; the mixture was then transferred to a three-necked flask equipped with a condenser, thermometer, and argon inlet to maintain an inert atmosphere; and gradually heated to a temperature of 300 °C at a controlled rate. Once the reaction temperature reached 300 °C, it was maintained for 1.5 hours to allow the controlled deposition of the NaYF4 shell onto the NaYbF4: 2%Er 3+ on the core particles; after the reaction, naturally cool to room temperature; the resulting core-shell nanoparticles are precipitated by adding acetone or ethanol and collected by centrifugation; the precipitate is washed with ethanol several times and then centrifuged again; finally, the purified core-shell nanoparticles are dispersed in hexane for storage and further characterization.

[0012] In step (3), a shell is grown on the NaYbF4: 2%Er@NaYF4 core by heating a precursor solution containing standard concentrations of sodium, yttrium and fluoride sources to 300 °C. The precursor solution is mixed with the pre-synthesized NaYbF4: 2%Er@NaYF4 core to ensure uniform dispersion in the reaction medium. The mixture is kept at 300 °C for 1.5 hours under an inert argon atmosphere to decompose the precursor and uniformly deposit the shell material on the core. After the reaction is completed, the mixture is cooled to room temperature, and the resulting core-shell nanoparticles are collected by precipitation, thoroughly washed with ethanol, and finally dispersed in hexane for further use and characterization. 0012. The application of the full-color upconversion single nanocrystal tuned by power density excitation at a single wavelength is applied in the fields of color display, anti-counterfeiting, multifunctional biological probes and controlled drug release.

[0013] The application is a technology for adjusting color in real time by changing the power density of a single excitation wavelength, thereby improving the spatial resolution of a color display.

[0014] The beneficial effects of the present invention are as follows: This paper provides a method for dynamically controlling the color of nanocrystal upconversion emission by precisely modulating the excitation power density of a single continuous-wave laser. Using a single 980 nm continuous-wave laser, tunable RGB color is achieved, providing a simple, low-cost method for color adjustment using external stimuli.

[0015] The unique composition of 2% Er / 98% Yb in the core and 2% Tm / 18% Yb in the outermost layer of the UCNPs used in this invention enables dominant green and red emission at low and moderate power densities, with switchable blue emission emerging at higher power densities. The photon-order-dependent upconversion process plays a key role in regulating the emission pathway, ensuring precise emission control without interference.

[0016] The present invention provides a simplified yet highly versatile excitation scheme for full RGB tunability, which has broad application prospects in advanced high-resolution color displays, anti-counterfeiting, multifunctional biological probes and controlled drug release. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the full-color up-conversion single nanocrystal structure tuned by power density of single wavelength excitation in the present invention.

[0018] Figure 2 This is the X-ray diffraction pattern of NaYbF4: 2%Er@NaYF4@NaYF4: 18%Yb, 2%Tm obtained in Example 1 of the present invention.

[0019] Figure 3 This is a transmission electron microscope image of NaYbF4: 2%Er obtained in Example 1 of the present invention.

[0020] Figure 4 This is a transmission electron microscope image of NaYbF4: 2%Er@NaYF4 obtained in Example 1 of the present invention.

[0021] Figure 5 This is a transmission electron micrograph of NaYbF4: 2%Er@NaYF4@NaYF4: 18%Yb, 2%Tm obtained in Example 1 of the present invention.

[0022] Figure 6 This is the fluorescence spectrum of the upconversion nanocrystal obtained in Example 1 of the present invention under 980nm excitation at different powers.

[0023] Figure 7 The upconversion nanocrystals obtained in Example 1 of the present invention are excited at 980nm with different powers. 3+ (green / red) and Tm 3+ (blue) Energy level transition diagram.

[0024] Figure 8 The green (Er) of the upconversion nanocrystals obtained in Example 1 of the present invention observed under weak, medium and strong excitation at 980nm with different powers 3+ ), Red (Er 3+ ) and blue (Tm 3+ ) Slope values ​​obtained from the linear fit of the emission. The inset shows the corresponding emission spectrum.

[0025] Figure 9 The upconversion nanocrystals obtained in Example 1 of the present invention achieve dynamic control of the upconversion emission color under different powers of 980nm excitation.

[0026] Figure 10 The corresponding shifts of the chromaticity coordinates of the emission colors of the up-conversion nanocrystals obtained in Example 1 of the present invention under 980nm excitation at different powers in the standard CIE diagram. DETAILED DESCRIPTION

[0027] The technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0028] Example 1: In this embodiment, a full-color up-conversion single nanocrystal with power density tunable by single wavelength excitation and a preparation method thereof are provided. The structure of the up-conversion nanocrystal is as follows: Figure 1 As shown, the preparation method comprises the following steps: Step (1), synthesis of NaYbF4: 2%Er. Purchased RECl3·xH2O (1 mmol) and sodium oleate (3 mmol) were stirred in a mixture of ethanol and hexane (volume ratio 1:1) at 70°C for 3 hours. The precipitated rare earth oleate was separated by centrifugation, washed with ethanol and hexane, and then dried in vacuo at 60°C to obtain rare earth oleate (RE-oleate). 1 mmol of RE-oleate with a molar ratio of Yb: Er of 0.98:0.02 was used. RE-oleate was mixed with 5 mL of 1-octadecene and 10 mL of oleic acid. The mixture was heated to 60°C to completely evaporate the ethanol. Subsequently, the temperature was increased to 160°C and maintained under a steady argon flow for 30 minutes. Afterwards, the solution was allowed to cool to room temperature, during which time the residual gas was removed. 4 mmol of ammonium fluoride (NH4F) and 2.5 mmol of sodium hydroxide (NaOH) were dissolved in 10 mL of methanol, respectively. This solution was added to the cooled reaction mixture in a three-necked flask. The mixed solution was then heated to 300°C at a controlled rate of 20°C per minute and maintained at this temperature for 90 minutes. After the reaction was complete, the mixture was allowed to cool naturally to ambient temperature, resulting in precipitation of the product in acetone. The precipitate was collected by centrifugation, washed several times with ethanol to remove impurities, and then centrifuged again.

[0029] Step (2) using thermal decomposition method, in NaYbF4: 2%Er 3+ The NaYbF4 shell is grown on the core. The precursor solution of sodium, yttrium and fluoride source with standard concentrations prepared in step (1) is used as the shell material. The precursor solution is mixed with the pre-synthesized NaYbF4: 2%Er 3+ The core nanoparticles were carefully mixed to ensure uniform dispersion of the core particles in the reaction medium. The mixture was then transferred to a three-necked flask equipped with a condenser, thermometer, and argon inlet to maintain an inert atmosphere. The system was gradually heated to a temperature of 300 ° C at a controlled rate to avoid sudden thermal shock, which could impair the quality of the shell growth. Once the reaction temperature reached 300 ° C, it was maintained for 1.5 hours to allow the NaYbF4 shell to be deposited in a controlled manner onto the NaYbF4: 2%Er 3+The core particles are then deposited on the surface of the nanoparticles. During this time, the high temperature promotes the thermal decomposition of the precursor solution, allowing the NaYbF4 shell to form and grow uniformly around the core nanoparticles. After the reaction, the system is allowed to cool naturally to room temperature. The resulting core-shell nanoparticles are precipitated by adding a suitable solvent (such as acetone or ethanol) and collected by centrifugation. To ensure removal of unreacted precursors and byproducts, the precipitate is washed several times with ethanol and then centrifuged again.

[0030] Step (3) Synthesis of NaYbF4: 2%Er@ NaYbF4@ NaYbF4: 18%Yb, 2%Tm. The shell was grown on the NaYbF4: 2%Er@ NaYbF4 core by heating a precursor solution containing standard concentrations of sodium, yttrium, and fluoride sources to 300 °C. The precursor solution was mixed with the pre-synthesized NaYbF4: 2%Er@ NaYbF4 core to ensure uniform dispersion in the reaction medium. The mixture was kept at 300 °C for 1.5 hours under an inert argon atmosphere to decompose the precursor and uniformly deposit the shell material on the core. After the reaction was completed, the mixture was cooled to room temperature, and the resulting core-shell nanoparticles were collected by precipitation and thoroughly washed with ethanol.

[0031] Test Example 1: The phase purity of the synthesized nanocrystals was confirmed by X-ray powder diffraction (XRD) analysis, e.g. Figure 2 As shown. The XRD pattern of the nanocrystals synthesized in step 1 confirmed the formation of a pure hexagonal NaYbF4: Er structure. The core NaYbF4: Er nanocrystals showed sharp and clear diffraction peaks, consistent with the standard hexagonal NaYbF4 phase (JCPDS no. 27-1427). In step 2, when growing the NaYbF4 inert shell, the XRD peaks remained unchanged, indicating that the hexagonal phase was maintained and the epitaxial shell growth was successful. Step 3 Further coating of NaYbF4: Yb, Tm 3+ layers to form a core / shell / shell structure (NaYF4: Er / NaYF4 / NaYF4: Tm 3+ ) show no additional diffraction peaks, indicating that the shell is crystalline and structurally coherent with the underlying core. The consistent diffraction peak positions and intensities across the core, core / shell, and core / shell / shell samples verify the integrity of the hexagonal phase throughout the layered structure.

[0032] Figure 3 、 Figure 4 and Figure 5Three transmission electron microscopy images show that the average diameters of the nanospheres for core-only (C), core / shell (C / S1), and core / shell / shell (C / S1 / S2) nanocrystals are 30, 40, and 50 nm, respectively. The increase in size confirms the successful growth of the encapsulating shell. The nanocrystals are well dispersed and exhibit high uniformity in both size and shape, indicating that there was no phase separation during the epitaxial growth process.

[0033] Test Example 2: The upconversion luminescence spectrum of the synthesized C / S1 / S2 nanocrystals is shown in Figure 6. The excitation power density was controlled by adjusting the output power of the 980 nm laser. Under weak excitation conditions, the nanocrystals mainly emitted green luminescence with peaks at 520 and 540 nm, respectively, attributed to Er 3+ of 2 H 11 / 2 → 4 I 5 / 2 and 4 S 3 / 2 → 4 I 5 / 2 Electronic transition.

[0034] As the excitation power density increases, the main emission color undergoes a significant gradual change, first turning to red and then to blue. The red emission peak at 650 nm comes from Er 3+ of 4 F 9 / 2 → 4 I 15 / 2 The blue emission peaks at 450 nm and 475 nm are attributed to the Tm³⁺ transition, 1 D2→ 3 H5 and 1 G4→ 3 H6 jump.

[0035] This gradual color tuning can be further quantified by examining the relative emission intensity ratios at different excitation power densities. Under weak excitation, the integrated density ratio of green to red emission is 1.6, and the integrated density ratio of green to blue is 8, indicating a dominance of green emission. When the excitation power density is increased to moderate levels, the red-green integrated density ratio rises to 4.4, and the red-blue ratio increases to 10.7, indicating a shift toward red dominance. At high excitation power density, the integrated density ratios of blue to green and blue to red are both 1.6, confirming the emergence of a strong blue emission component. These findings demonstrate the effectiveness of the C-S1-S2 nanocrystal system in achieving precise RGB color tuning using a single continuous-wave 980 nm excitation source. This tunable emission property highlights the potential of these nanocrystals for applications requiring precise color control and high emission purity across the entire RGB spectrum.

[0036] As shown in Figure 7, from Yb 3+ To Er 3+ The continuous energy transfer process, coupled with fast non-radiative relaxation, fills the Er 3+2 H 11 / 2 and 4 S 3 / 2 level, which is the reason for the green radiation. This population pathway is similar to that of 2Er / 18Yb:NaYF4 nanocrystals traditionally used for efficient green upconversion emission, corresponding to the two-photon upconversion process. When the excitation power density is further increased, 4 S 3 / 2 The electrons are raised to a higher energy level 2 G 7 / 2 Energy level. 4 G 11 / 2 After the energy level, Er responsible for red light emission 3+4 F 9 / 2 The energy levels are filled through a reverse energy transfer process: 4 G 11 / 2 (Er 3+ ) + 2 F 7 / 2 (Yb 3+ ) → 4 F 9 / 2 (Er 3+ ) + 2 F 5 / 2 (Yb 3+ ), leading to a three-photon upconversion pathway for red emission. Ultrahigh Yb in synthetic nanocrystals 3+ The doping level ensures the efficiency of this reverse energy transfer process. This mechanism effectively increases the Er 3+ The photon sequence required for red emission enables a clear distinction between the upconversion mechanisms of red and green emission. Therefore, green and red emission can be tuned by varying the excitation power density. Consequently, at moderate excitation power densities, three-photon red upconversion luminescence dominates the emission spectrum of the nanocrystals. Specifically, the luminescence intensity ratio between the 557 nm and 540 nm emission bands gradually increases with increasing excitation power density, confirming the role of the three-photon process in Er upconversion. 3+ The role of upconversion luminescence is enhanced.

[0037] Tm 3+ The 475 nm and 450 nm emissions correspond to 1 G4 and 1The D2 energy level is related to the three-photon and four-photon upconversion processes. As the excitation power density increases, the rapid growth of the high-order upconversion intensity causes the four-photon blue upconversion emission to gradually dominate the emission spectrum under strong excitation power. Tm 3+ of 1 D2 → 3 The appearance of the 507nm emission band generated by the H5 transition further confirms the occurrence of the four-photon process.

[0038] The photon-level dependence of the upconversion emission color is directly supported by its corresponding power dependence. The slope obtained from the logarithmic plot is Er 3+ Green light emission of 1.88, Er 3+ The slope of the red emission is 2.79 and Tm 3+ The blue emission is 3.67, which clearly verifies the proposed two-photon, three-photon and four-photon upconversion processes, as shown in Figure 8, respectively. By adjusting the excitation power density of the 980 nm laser, dynamic control of the upconversion emission color of the nanocrystals is achieved. 2 Increased to 100 W / cm 2 , the main emission color shows a systematic shift from green to red and finally to blue, corresponding to Figure 9 Variations in the relative intensities of the RGB emission bands are shown.

[0039] As shown in Figure 10, the corresponding shifts in chromaticity coordinates in the CIE diagram are plotted, confirming the observed color shifts. The boundaries of the colored horseshoe correspond to the colors of monochromatic light and are marked with wavelengths; the interior of the horseshoe represents the colors produced by mixing monochromatic light. To explain the horizontal and vertical coordinates of the chromaticity diagram: XYZ space can be understood as selecting three imaginary colors as basis vectors in the color space, so that the visible color gamut falls precisely within the first quadrant of XYZ space. The imaginary colors are mixed to form C(X,Y,Z) components, and the sum of these components is normalized to form ratios. The first two ratios are the coordinates on the chromaticity diagram. At low excitation power density, the color point is located in the green region. As the power density increases, the color point shifts to the longer wavelength region, with dominant red emission observed at moderate power density. As the excitation power density increases further, the color point shifts to the blue region, ultimately stabilizing in the strong blue emission region at the highest excitation power.

[0040] Application Example: In conventional color displays, each color is represented by a 2x2 pixel [G, R; B, G] color or color filter, so the resolution is determined by the total size of the 2x2 pixels. However, by utilizing current technology that adjusts color in real time by varying the power density of a single excitation wavelength, we can significantly improve the spatial resolution of color displays (now determined by a single pixel instead of four).

[0041] The embodiments described above may be further combined or replaced, and the embodiments are merely descriptions of preferred embodiments of the present invention and do not limit the concept and scope of the present invention. Various changes and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the design concept of the present invention are within the scope of protection of the present invention. The scope of protection of the present invention is given by the appended claims and any equivalents thereof.

Claims

1. A full-color upconversion single nanocrystal with power density tunable by single wavelength excitation, characterized in that The full-color upconversion nanocrystal structure consists of a core, an inert intermediate shell, and an active outer shell from the inside out, and is composed of NaYbF4:2%Er@NaYF4@NaYF4: 18%Yb, 2%Tm. It uses a photon-level dependent upconversion process to modulate the excitation power density, thereby achieving green, red, and blue light emission.

2. The full-color up-conversion single nanocrystal with power density tunable by single wavelength excitation according to claim 1, characterized in that The upconversion nanocrystal has the following composition: core NaYbF4: 2%Er, comprising 2% erbium (Er) and 98% ytterbium (Yb) in a molar ratio; and active shell NaYF4: 18%Yb, 2%Tm, comprising 2% thulium (Tm) and 18%Yb in a molar ratio.

3. The full-color up-conversion single nanocrystal with power density tunable by single wavelength excitation according to claim 2, characterized in that: The diameter of the upconversion nanocrystal is 50 nm.

4. The full-color up-conversion single nanocrystal with power density tunable by single wavelength excitation according to claim 2, characterized in that: The upconversion nanocrystals use a three-primary color tunable upconversion luminescence with a single excitation wavelength of 980 nm; as the power density of the 980 nm continuous wave laser increases, the emission color systematically changes from green to red and finally to blue.

5. A method for preparing a full-color upconversion single nanocrystal with power density tunable by single wavelength excitation as claimed in claim 1, characterized in that: The following steps are involved: Step (1), synthesis of NaYbF4: 2%Er; Step (2) uses thermal decomposition method to prepare NaYbF4: 2%Er 3 A NaYF4 shell is grown on the core; Step (3), synthesize NaYbF4: 2%Er@NaYF4@NaYF4: 18%Yb, 2%Tm.

6. The preparation method according to claim 5, wherein The step (1) comprises the following steps: stirring 1 mmol RECl3·xH2O and 3 mmol sodium oleate in a mixture of ethanol and hexane in a volume ratio of 1:1 at 70°C for 3 hours; separating the precipitated rare earth oleate by centrifugation, washing with ethanol and hexane, and then drying in vacuo at 60°C to obtain rare earth oleate, i.e., RE-oleate; mixing 1 mmol RE-oleate in a Yb: Er molar ratio of 0.98:0.02 with 5 mL 1-octadecene and 10 mL oleic acid; heating the mixture to 60°C to completely evaporate the ethanol; subsequently, raising the temperature to 160°C and maintaining it under a steady argon flow for 30 minutes; then, allowing the solution to cool to room temperature, during which time the residual gas is removed; dissolving 4 mmol ammonium fluoride and 2.5 mmol sodium hydroxide in 10 mL methanol respectively; adding the solution to the cooled reaction mixture in a three-necked flask; and then heating the mixed solution to 300°C at a controlled rate of 20°C per minute and maintaining it at this temperature for 90 minutes. minutes; after the reaction, the mixture was allowed to cool naturally to ambient temperature, the precipitate was collected by centrifugation, washed with ethanol to remove impurities, and then centrifuged again; finally, the purified sample was dispersed in hexane for storage and further analysis.

7. The preparation method according to claim 5, wherein In the step (2), the precursor solution of sodium, yttrium and fluoride source prepared in the step (1) is used as the shell material; the precursor solution is mixed with the pre-synthesized NaYbF4: 2%Er 3+ The core nanoparticles were carefully mixed to ensure uniform dispersion; the mixture was then transferred to a three-necked flask equipped with a condenser, thermometer, and argon inlet to maintain an inert atmosphere; and gradually heated to a temperature of 300 °C at a controlled rate. Once the reaction temperature reached 300 °C, it was maintained for 1.5 hours to allow the controlled deposition of the NaYF4 shell onto the NaYbF4: 2%Er 3+ on the core particles; after the reaction, naturally cool to room temperature; the resulting core-shell nanoparticles are precipitated by adding acetone or ethanol and collected by centrifugation; the precipitate is washed with ethanol several times and then centrifuged again; finally, the purified core-shell nanoparticles are dispersed in hexane for storage and further characterization.

8. The preparation method according to claim 5, wherein In step (3), a shell is grown on the NaYbF4: 2%Er@NaYF4 core by heating a precursor solution containing standard concentrations of sodium, yttrium and fluoride sources to 300 °C; the precursor solution is mixed with the pre-synthesized NaYbF4: 2%Er@NaYF4 core to ensure uniform dispersion in the reaction medium; the mixture is maintained at 300 °C for 1.5 hours under an inert argon atmosphere to decompose the precursor and uniformly deposit the shell material on the core; after the reaction is completed, the mixture is cooled to room temperature, and the resulting core-shell nanoparticles are collected by precipitation, thoroughly washed with ethanol, and finally dispersed in hexane for further use and characterization.

9. The use of a full-color up-conversion single nanocrystal tuned by power density under single wavelength excitation according to any one of claims 1 to 4, characterized in that: It is used in the fields of color display, anti-counterfeiting, multifunctional biological probes and controlled drug release.

10. The use according to claim 9, characterized in that The technology is applied to color display and adjusts the color in real time by changing the power density of a single excitation wavelength, thereby improving the spatial resolution of the color display.

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