Method for inducing conversion of self-trapping exciton luminescence mode of zinc oxide

Abstract

The invention provides a method for inducing conversion of a self-trapping exciton luminescence mode of zinc oxide, which comprises the following steps of: pressurizing nano zinc oxide to form deviating stress, so that the zinc oxide is converted from an original self-trapping exciton state to a new self-trapping exciton state, the fluorescence color is converted from yellow to dark blue, and the fluorescence quantum yield is improved; wherein the magnitude of the deviating stress is controlled by selecting different zinc oxide crystal morphologies and pressure transmitting media. The method for inducing conversion of the zinc oxide self-trapping exciton luminescence mode provides a new method and mechanism, and is easy to implement.

Description

Technical Field

[0001] The present invention belongs to the field of material luminescence, and particularly relates to a method for inducing the transformation of the self-trapped exciton luminescence mode of zinc oxide by bias stress. This method can convert the fluorescence of the self-trapped exciton of zinc oxide from yellow to dark blue and improve the fluorescence quantum yield. Background Art

[0002] Zinc oxide has high chemical stability, a large exciton binding energy (60 meV), and excellent optical and electrical properties. The self-trapped exciton (STE) luminescence in zinc oxide has the advantages of a wide spectral range, small self-absorption, and high efficiency, and has potential application prospects in the field of advanced lighting and display. However, current research reports on the self-trapped exciton luminescence of zinc oxide usually lack means for regulating the emission band, and there are limitations in the fixed spectral range and non-tunable color.

[0003] In order to solve the above problems, people have been seeking an ideal technical solution. Summary of the Invention

[0004] The purpose of the present invention is to address the deficiencies of the prior art and provide a method for inducing the transformation of the self-trapped exciton luminescence mode of zinc oxide.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: A method for inducing the transformation of the self-trapped exciton luminescence mode of zinc oxide, which performs a pressurization operation on nano-zinc oxide to form a bias stress, causing the transformation of zinc oxide from the original self-trapped exciton state to a new self-trapped exciton state.

[0006] The particle size of the nano-zinc oxide is 4 - 140 nm; the pressurization pressure is 0.2 - 8 GPa; non-liquid medium pressure transmission or liquid medium pressure transmission is used.

[0007] The nano-zinc oxide is pyramid-shaped zinc oxide nanocrystals or quasi-spherical zinc oxide nanocrystals.

[0008] During the process of using liquid medium pressure transmission, the liquid medium is one of silicone oil or isopropanol.

[0009] A diamond anvil press is used to pressurize the nano-zinc oxide.

[0010] The pressurization steps of the diamond anvil press include: first pre-pressing a steel gasket with the diamond anvil press, then drilling a hole at the indentation center of the steel gasket as the sample cavity, then loading the nano-zinc oxide into the sample cavity, and using ruby as the pressure standard substance to apply pressure to the nano-zinc oxide in a hydrostatic or non-hydrostatic pressurization manner.

[0011] The steel gasket is a T301 steel gasket with a thickness of 40 - 60 μm.

[0012] The present invention has prominent substantive features and significant progress compared with the prior art. Specifically, the present invention provides a method for inducing the transformation of the self-trapped exciton luminescence mode of zinc oxide, which promotes the fluorescence conversion of self-trapped excitons through a bias stress. Specifically, by coordinating the morphology of zinc oxide with pressure and the pressure transmission method, a bias stress is formed. The bias stress causes local yield strain of the grains, and then new self-trapped exciton STE-2 states are induced in the nanometer zinc oxide under pressure and finally completely transformed into the STE-2 state. Moreover, the fluorescence intensity, quantum yield, and fluorescence lifetime are significantly increased. The intensity is 10 times the initial value, the fluorescence quantum yield is 4 times the initial value, and the fluorescence lifetime increases from about 100 ns to 2.8 μs. By regulating the emission band and intensity of self-trapped excitons under high pressure by the bias stress, a strategy is provided for realizing a new single matrix white light emitting material. Further, the morphology of zinc oxide being relatively sharp and not using a pressure transmission medium is conducive to the formation of a bias stress during the pressurization process, and new self-trapped exciton STE-2 states can be induced in the nanometer zinc oxide at 0.2 GPa. Brief Description of the Drawings

[0013] Figure 1 is the transmission electron microscope photograph of pyramid-shaped zinc oxide nanocrystals.

[0014] Figure 2 is the transmission electron microscope photograph of quasi-spherical zinc oxide nanocrystals.

[0015] Figure 3 is the scanning electron microscope photograph of hexagonal prism-shaped zinc oxide microcrystals.

[0016] Figure 4 is the fluorescence spectrum, fluorescence quantum yield, and color coordinates of pyramid-shaped zinc oxide nanocrystals under high pressure without a pressure transmission medium.

[0017] Figure 5 is the fluorescence spectrum, fluorescence quantum yield, and color coordinates of pyramid-shaped zinc oxide nanocrystals under high pressure with silicone oil as the pressure transmission medium.

[0018] Figure 6 is the fluorescence spectrum, fluorescence quantum yield, and color coordinates of quasi-spherical zinc oxide nanocrystals under high pressure without a pressure transmission medium.

[0019] Figure 7 is the fluorescence spectrum, fluorescence quantum yield, and color coordinates of quasi-spherical zinc oxide nanocrystals under high pressure with silicone oil as the pressure transmission medium.

[0020] Figure 8 is the fluorescence spectrum, fluorescence quantum yield, and color coordinates of hexagonal prism-shaped zinc oxide microcrystals under high pressure with silicone oil as the pressure transmission medium.

[0021] Figure 9 It is a comparison of the pressure points at which new self-trapped exciton states (STE-2) appear under different crystal morphologies of zinc oxide crystals and different pressure transmission methods.

[0022] Figure 10 It is the test result of the fluorescence lifetime of pyramid-shaped zinc oxide nanocrystals under high pressure when silicone oil is used as the pressure transmission medium.

[0023] Figure 11 It is the fluorescence spectrum, fluorescence quantum yield, and color coordinates of pyramid-shaped zinc oxide nanocrystals after annealing at 300 °C for 30 min under high pressure when silicone oil is used as the medium.

[0024] Figure 12 It is the fluorescence spectrum, fluorescence quantum yield, and color coordinates of pyramid-shaped zinc oxide nanocrystals under high pressure when isopropyl alcohol is used as the medium.

[0025] Figure 13 It is the variation of the XRD pattern of pyramid-shaped zinc oxide nanocrystals with pressure.

[0026] Figure 14 It is the particle size analysis diagram of pyramid-shaped zinc oxide nanocrystals.

[0027] Figure 15 It is the particle size analysis diagram of quasi-spherical zinc oxide nanocrystals.

[0028] Figure 16 It is the comparison of transmission electron microscopes of pyramid-shaped zinc oxide nanocrystals before and after pressure treatment, the comparison of electron diffraction photos of pyramid-shaped zinc oxide nanocrystals before and after pressure treatment, and the comparison of transmission electron microscopes of hexagonal prism-shaped zinc oxide microcrystals before and after pressure treatment.

[0029] Figure 17 It is an analysis diagram of the finite element simulation of the stress-strain behavior of hexagonal prism-shaped zinc oxide microcrystals and pyramid-shaped zinc oxide nanocrystals under high-pressure bias stress. It respectively compares the uniform shrinkage of hexagonal prism-shaped zinc oxide microcrystals under hydrostatic pressure and the tip plastic deformation of pyramid-shaped zinc oxide nanocrystals with and without a pressure transmission medium. Specific Embodiments

[0030] The technical solution of the present invention will be further described in detail through specific embodiments below. The diamond anvil press used in the present invention is a commercially available device; the nano-zinc oxide used in the present invention can be directly purchased as a commercially available product or prepared by itself according to the existing technology. The characterization methods used in the present invention are as follows: 1. Morphology characterization of zinc oxide crystals with different morphologies: The morphology of zinc oxide crystals is characterized by scanning electron microscopy and transmission electron microscopy.

[0031] 2. High-pressure fluorescence spectroscopy test of zinc oxide crystals with different morphologies: Place the sample mounted on the high-pressure press onto the fluorescence collection system of the spectrometer to conduct fluorescence tests under different pressure transmission methods and pressures.

[0032] 3. High-pressure fluorescence lifetime test of pyramid-shaped zinc oxide nanocrystals: Place the sample mounted on the high-pressure press onto the fluorescence scanning imaging microscopy system built in the laboratory by ourselves to conduct lifetime tests under different pressures.

[0033] 4. Calculation of fluorescence quantum yield: Combine the fluorescence spectra under different pressures obtained in (2) and the fluorescence lifetimes under different pressures obtained in (3), and calculate the fluorescence quantum yield under high pressure by combining with the reference fluorescence quantum yield at atmospheric pressure.

[0034] 5. Calculation of fluorescence spectrum chromaticity coordinates: Put the fluorescence spectra under different pressures obtained in (2) into the CIE calculation software to calculate the chromaticity coordinates.

[0035] Example 1 Reference: Liang et al., J. Phys. Chem. Lett. 2019, 10, 3557 - 3562. Prepare pyramid-shaped zinc oxide nanocrystals. The specific preparation process includes: Add 2 g of zinc acetate dihydrate to 10 ml of anhydrous ethanol solution, stir continuously at room temperature for 30 minutes to obtain a suspension, then transfer the suspension to a hydrothermal reaction kettle, conduct hydrothermal reaction at 200 °C for 30 minutes, and then successively perform centrifugation, washing, and drying at 60 °C for 12 hours for standby.

[0036] See Figure 1 and Figure 14 , The crystal morphology of the zinc oxide nanocrystals prepared by the above method is pyramid-shaped, and the particle size is 20 - 140 nm; Using this zinc oxide nanocrystal as a sample, conduct the following experiments with it as the raw material without a liquid pressure transmission medium.

[0037] Use a diamond anvil press with a 500 μm anvil surface to pre-press a 50 μm thick T301 metal steel sheet, and drill a small hole with a diameter of 120 μm at the center of the indentation as the sample cavity; Fill the sample cavity with pyramid-shaped zinc oxide nanocrystals, and use ruby as the pressure calibration substance.

[0038] See Figure 17 , The sharp edges and corners of the pyramid-shaped crystal and the high-pressure pressure transmission method without a liquid medium are conducive to the generation of deviatoric stress, introducing the maximum high-pressure deviatoric stress. Local plastic deformation occurred in the crystal under the high-pressure deviatoric stress. See Figure 4 and Figure 9, during the pressure application process, a new self-trapped exciton fluorescence peak appears at the 430 nm position in the pyramidal zinc oxide nanocrystals under a pressure of 0.2 GPa, which is named the STE-2 state. As the pressure increases, the original self-trapped exciton state STE-1 at the 540 nm position continuously weakens and gradually transforms into STE-2. The STE-2 state reaches the strongest at 2 GPa, and the fluorescence quantum yield slightly decreases under pressure, and the fluorescence color changes from yellow-green to sky-blue.

[0039] See Figure 13 , the atomic structure of the sample only changes when the pressure is greater than 11.8 GPa. Therefore, the reason for the fluorescence change is not the change in the atomic structure. See Figure 16 , Figure 16 In [reference], ab are the transmission electron microscope and the corresponding selected area electron diffraction photographs of the original pyramidal crystal, and cd are the transmission electron microscope and electron diffraction photographs of the pyramidal crystal after pressure treatment. By comparison, it can be seen that the tip collapses after pressure, and the single crystal of the pyramidal crystal becomes polycrystalline. See Figure 17 , Figure 17 In [reference], c is the stress analysis of the finite element simulation of the pyramidal crystal under pressure without a pressure-transmitting medium, indicating that plastic deformation occurs at the tip position of the pyramidal crystal under extrusion.

[0040] Conclusion: Under the condition of no liquid pressure-transmitting medium, the pyramidal zinc oxide nanocrystals introduce the maximum high-pressure bias stress, and the appearance of the STE-2 fluorescence peak and the transformation between self-trapped exciton states are induced at a pressure of 0.2 GPa.

[0041] Example 2 This example uses the same press, sample, and sample chamber as Example 1, with the only difference being that silicone oil is used as the liquid pressure-transmitting medium for the experiment.

[0042] See Figure 17 In [reference] b, under the pyramidal zinc oxide nanocrystals and the silicone oil pressure-transmitting medium, a moderate bias stress is introduced. Similar to Example 1, local plastic deformation occurs in the crystal under the high-pressure bias stress, but the external pressure value at which the local plastic deformation occurs is smaller. See Figure 5 and Figure 9 , a new self-trapped exciton STE-2 state is induced to appear at a pressure of 1 GPa. As the pressure increases, its fluorescence intensity gradually increases and reaches the maximum value when the pressure rises to 8 GPa (as Figure 5 shown), the peak intensity is 10 times the initial value, the fluorescence quantum yield is 4 times the initial value, and the fluorescence lifetime increases from about 100 ns to 2.8 μs (as Figure 10 shown). It can be clearly seen that the fluorescence intensity of the zinc oxide nanoparticles after pressure application is significantly enhanced, and the color also changes from yellow-green to dark blue.

[0043] Conclusion: Under the condition of silicone oil pressure transmitting medium, the pyramid-shaped zinc oxide nanocrystals introduced a moderate deviatoric stress, induced the emergence of a new self-trapped exciton state STE-2 at 1 GPa pressure, and the fluorescence intensity reached the maximum at 8 GPa, the fluorescence peak increased by 10 times, the fluorescence quantum yield increased by 4 times, and the color changed from yellow-green to dark blue.

[0044] Example 3 This example uses the same press and sample chamber as Example 1, the difference is that quasi-spherical zinc oxide nanocrystals are used as the sample.

[0045] The preparation process of the quasi-spherical zinc oxide nanocrystals includes adding 16.62 g of zinc acetate dihydrate to 400 ml of anhydrous ethanol solution and continuously stirring for 30 minutes at room temperature. At the same time, 6.18 g of potassium hydroxide is added to 50 ml of anhydrous ethanol solution and stirred for 30 minutes. Then the potassium hydroxide solution is added to the zinc acetate dihydrate solution and continuously stirred for 30 minutes. After that, the obtained precipitate is washed several times with ethanol and reserved. See Figure 2 for the TEM image of the prepared zinc oxide nanocrystals, and see Figure 15 , the zinc oxide nanocrystals are quasi-spherical with a particle size of 4 - 10 nm.

[0046] See Figure 6 and Figure 9 , because the quasi-spherical zinc oxide nanocrystals have blunter edges and corners and lower packing porosity compared with the pyramid-shaped zinc oxide nanocrystals, it is more difficult to form deviatoric stress, and a slightly smaller deviatoric stress is introduced under high pressure. During the pressurization process, as the pressure increases, a weak STE-2 shoulder peak appears at 430 nm for the quasi-spherical zinc oxide nanocrystals at 5 GPa pressure. With the continuous increase of pressure and the continuous transformation of the two self-trapped exciton states, the relative intensity of the STE-2 peak becomes more obvious. Finally, the change of the fluorescence quantum yield is not obvious, and the color changes from yellow to yellow-green.

[0047] Conclusion: The quasi-spherical zinc oxide nanocrystals introduced a slightly smaller deviatoric stress under high pressure without a pressure transmitting medium, induced the emergence of a new self-trapped exciton state STE-2 at 5 GPa pressure, and realized the transformation between the two self-trapped exciton states.

[0048] Example 4 This example uses the same press and sample chamber as Example 3, the difference is that silicone oil is used as the pressure transmitting medium for the experiment.

[0049] See Figure 7 and Figure 9, since the quasi-spherical zinc oxide nanocrystals have blunter edges and lower packing porosity compared to the pyramid-shaped zinc oxide nanocrystals, it is more difficult to form a deviatoric stress. When silicone oil is used as the pressure-transmitting medium, a weak deviatoric stress is introduced under high pressure. During the pressurization process, as the pressure increases, a weak STE-2 shoulder peak appears at the 430 nm position for the quasi-spherical zinc oxide nanocrystals at a pressure of 6 GPa. With the continuous increase in pressure and the continuous transformation of the two self-trapped exciton states, the relative intensity of the STE-2 peak becomes more obvious. Finally, the fluorescence quantum yield slightly increases, and the fluorescence color changes from yellow to yellow-green.

[0050] Conclusion: Under high pressure with silicone oil as the pressure-transmitting medium, a weak deviatoric stress was introduced for the quasi-spherical zinc oxide nanocrystals, and a new self-trapped exciton state STE-2 was induced to appear at a pressure of 6 GPa, and the transformation between the two self-trapped exciton states was achieved. See Figure 9 , The comparison of the experimental configurations with five different deviatoric stresses shows that the greater the high-pressure deviatoric stress, the earlier the new self-trapped exciton state appears.

[0051] Example 5 This example uses the same press and sample chamber as in Example 1, with the difference that the pyramid-shaped zinc oxide nanocrystals are annealed at 300 °C for 30 minutes and then used as samples for the pressurization test; silicone oil is used as the pressure-transmitting medium for the experiment.

[0052] See Figure 11 , The various organic groups on the grain surface of the annealed pyramid-shaped zinc oxide nanocrystals are removed, and a moderate deviatoric stress is introduced under the silicone oil pressure-transmitting medium. During the pressurization process, a new self-trapped exciton state STE-2 is induced to appear at a pressure of 1 GPa. As the pressure increases, its fluorescence intensity gradually increases and reaches the maximum value when the pressure rises to 8 GPa. The fluorescence intensity and quantum yield increase significantly, and the color changes from yellow-green to dark blue.

[0053] Conclusion: After annealing to remove the organic groups on the grain surface, the pyramid-shaped zinc oxide nanocrystals still introduce a moderate deviatoric stress under high pressure, and the fluorescence change is similar to that without annealing, indicating that the surface groups are not the reason affecting the fluorescence change.

[0054] Example 6 This example uses the same press and sample chamber as in Example 1, with the difference that isopropyl alcohol is used as the pressure-transmitting medium for the experiment.

[0055] See Figure 12, under the isopropanol pressure transmission medium, a moderate deviatoric stress was introduced under pressure. A new self-trapped exciton state, the STE-2 state, was induced in the sample at a pressure of 1 GPa. As the pressure increased, its fluorescence intensity gradually increased and reached a maximum when the pressure increased to 8 GPa. The fluorescence intensity and quantum yield increased significantly, and the color changed from yellow-green to dark blue.

[0056] Conclusion: After replacing silicone oil with isopropanol as the pressure transmission medium, the pyramidal zinc oxide nanocrystals still introduced a moderate deviatoric stress under high pressure, and the fluorescence change was similar to that without annealing, indicating that the pressure transmission medium is not the reason for the fluorescence change.

[0057] Comparative Example 1 This implementation used the same press and sample chamber as in Example 1. A single hexagonal prism zinc oxide microcrystal was placed in, and silicone oil was used as the pressure transmission medium for the experiment. The preparation steps of the hexagonal prism zinc oxide microcrystal included: placing a mixture of zinc oxide and graphite powder with a weight ratio of 1:1 in a corundum boat in a tube furnace, covering the corundum boat with a clean silicon wafer, heating to 1120 degrees Celsius, and holding for 45 minutes. During the heating process, argon gas with a constant flow rate of 120 standard cubic centimeters per minute was introduced. Finally, the zinc oxide micro-wire crystals grown on the silicon wafer were taken for standby. See Figure 3 for the TEM pictures of the zinc oxide micro-wire crystals.

[0058] See Figure 17 a in, since there is no mutual extrusion between grains of a single hexagonal prism zinc oxide microcrystal under high pressure, a hydrostatic pressure situation without deviatoric stress is formed. See Figure 8 - 9 , as the pressure increased, no new self-trapped exciton states appeared, the fluorescence quantum yield slightly decreased, and the fluorescence color changed from yellow-green to cyan; see Figure 16 e and f in, after the pressure treatment, the morphology of the hexagonal prism zinc oxide microcrystals did not change.

[0059] Conclusion: Under a single hexagonal prism zinc oxide microcrystal and a silicone oil pressure transmission medium, there is no deviatoric stress, no new self-trapped exciton states and any transformation processes occur. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: it is still possible to modify the specific implementation manners of the present invention or perform equivalent replacements for some technical features; without departing from the spirit of the technical solutions of the present invention, they should all be covered within the scope of the technical solutions claimed by the present invention.

Claims

1. A method for inducing the conversion of the luminescence mode of zinc oxide self-trapped excitons, characterized in that: The nano zinc oxide is pressurized to form a deviatoric stress, so that the zinc oxide is transformed from the original self-trapped exciton state to a new self-trapped exciton state.

2. The method for inducing zinc oxide self-trapped exciton luminescence mode conversion according to claim 1, characterized in that: The particle size of nano zinc oxide is 4-140 nm; the pressurized pressure is 0.2-8 GPa; and the pressure is transmitted by liquid-free medium or liquid medium.

3. The method for inducing zinc oxide self-trapped exciton luminescence mode conversion according to claim 2, characterized in that: The nano zinc oxide is a pyramid-shaped zinc oxide nanocrystal or a quasi-spherical zinc oxide nanocrystal.

4. The method for inducing zinc oxide self-trapped exciton luminescence mode conversion according to claim 3, characterized in that: In the process of transmitting pressure using liquid medium, the liquid medium is silicone oil or isopropyl alcohol.

5. The method for inducing zinc oxide self-trapped exciton luminescence mode conversion according to claim 4, characterized in that: The nano zinc oxide was pressurized using a diamond anvil press.

6. The method for inducing zinc oxide self-trapped exciton luminescence mode conversion according to claim 5, characterized in that: The pressurizing steps of the diamond anvil press include: firstly, pre-pressing a steel sheet gasket using the diamond anvil press, then drilling a hole in the center of the indentation of the steel sheet gasket as the sample cavity, then loading the nano zinc oxide into the sample cavity, and using ruby ​​as a pressure standard material to pressurize the nano zinc oxide.

Images