A preparation method based on solvent-induced non-dense hexagonal PS microsphere array conversion to metal crack network

Abstract

The application belongs to the field of micro-nano photoelectronic device preparation, and discloses a preparation method based on solvent-induced non-dense hexagonal PS microsphere array transformation into metal crack network. The method comprises the following steps: 1) substrate cleaning and hydrophilic treatment; 2) self-assembly of PS microsphere suspension on the substrate treated in step 1); 3) plasma etching of the sample in step 2); 4) adding a solvent which does not dissolve PS microspheres and is volatile to the surface of the sample in step 3), and accelerating solvent volatilization to dry the sample; 5) annealing treatment of the sample in step 4); 6) depositing a layer of metal on the surface of the sample in step 5); 7) ultrasonicating the sample in step 6) in a solvent which can dissolve PS microspheres, removing the surface PS microspheres, and cleaning and drying. The size and shape of the non-periodic metal crack network can be adjusted, and the non-periodic metal crack network is combined with a fully connected periodic or non-periodic metal network, so that the application has great application potential in the fields of composite material transparent electrode and solar cell.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of micro-nano optical electronic device preparation, and particularly relates to a preparation method based on solvent-induced non-dense hexagonal PS microsphere array to metal crack network conversion. BACKGROUND

[0002] Traditional fossil energy has been providing power for the rapid development of human society, but fossil energy belongs to non-renewable energy, and mankind will eventually face the predicament of its depletion. Opening up new sources and saving resources is a good response to this problem. Opening up new sources means looking for new energy sources, turning to solar energy, wind energy and nuclear energy and other new clean energy. Because of the advantages of wide distribution and large total amount, solar energy is the most popular among many new energies. At present, there are application fields such as photovoltaic, photo-thermal and photo-chemical, and the transparent electrode involved in the present application is one of the hotspots in the field of solar energy utilization.

[0003] Materials with high electrical conductivity and optical transmittance are essential for various optoelectronic devices, such as touch screen displays, solar cells, organic light emitting diodes. The indium tin oxide (ITO) material commonly used in the above-mentioned touch screen and other applications, in which the indium element is a rare element, the reserves are decreasing year by year, the price is rising, and some experts predict that there will be no indium element available in a few years, and ITO also does not have the characteristics of flexibility. The next generation of optoelectronic devices requires transparent conductive electrodes, which have very high conductivity and transparency, and also have mechanical flexibility and compatibility with large-scale manufacturing. These problems and requirements have prompted people to seek new materials, such as carbon nanotubes, graphene, metal nanowires, and metal mesh composite materials.

[0004] But there are still many deficiencies in the related network structure. First, the problem of nonlinear response of transmittance. The regular periodic network structure will bring the problem of nonlinear response of plasmon, which will result in the large change of transmittance with wavelength and the low overall transmittance. Second, the problem of carrier diffusion length. For example, Han bing et al. [Han Bing, Pei Ke, Huang Yuanlin, et a. Uniform self-forming metallic network as a high-performance transparent conductive electrode. [J]. Advanced materials (Deerfield Beach, Fla.), 2014, 26 (6),] used gel to produce interconnected cracks, and then deposited metal on the cracks to obtain a metal network. Although the metal network has good conductivity and transmittance, the single crack width and the spacing between the cracks are too large to adapt to the semiconductor materials with small carrier diffusion length. Third, compared with the existing commercial ITO and other transparent electrode materials, the cost of production is high due to the scarcity of indium material, and the ITO material does not have flexibility. SUMMARY

[0005] The purpose of the present application is to provide a preparation method based on the transformation of solvent-induced non-dense hexagonal PS microsphere array to metal crack network, which can be used to make high-performance composite material transparent electrode and other optoelectronic devices. Compared with the existing transparent electrode preparation, the present application creatively uses the capillary action of solvent in the gap of PS array to manufacture crack network, and then performs annealing to obtain non-periodic crack network and annealed non-periodic crack network. Finally, metal is evaporated on the network and the PS microsphere array is removed, and finally non-periodic crack metal network and annealed non-periodic crack metal network are obtained. Compared with the existing transparent electrode technology, the present application can control the width and spacing of the cracks by changing the size of the PS microspheres and the plasma etching time, and can be combined with other periodic or periodic networks to form a composite material, which has more possibilities and potential. And the whole preparation process has the advantages of simple preparation process, large-area preparation and low cost.

[0006] To achieve the above purpose, the present application provides the following technical scheme:

[0007] A preparation method based on the transformation of solvent-induced non-dense hexagonal PS microsphere array to metal crack network, comprising the following steps:

[0008] 1) cleaning and hydrophilic treatment of the substrate;

[0009] 2) self-assemble the prepared PS ball suspension to the substrate treated in step 1) to obtain a hexagonal dense arrangement of PS microsphere array;

[0010] 3) plasma etch the dense arrangement of PS microsphere array in step 2) to obtain a non-dense hexagonal PS microsphere array;

[0011] 4) add a solvent which does not dissolve PS microspheres and is volatile to the sample prepared in step 3) and accelerate solvent evaporation to dry the sample;

[0012] 5) anneal the sample prepared in step 4) to seal the triangular holes between the balls;

[0013] 6) deposit a layer of metal on the sample prepared in step 5);

[0014] 7) place the sample prepared in step 6) in an ultrasonic water bath in a solvent which can dissolve PS microspheres, remove the PS microspheres on the surface, and clean and dry the sample;

[0015] As a preferred technical solution of the present application, the substrate in step 1) is a non-flexible or flexible substrate with water stability and good transmittance, including glass sheet, quartz sheet, polyvinyl alcohol (PVA), polyester (PET) or polyimide (PI), the cleaning process uses anhydrous ethanol and deionized water for ultrasonic cleaning, and the hydrophilic treatment uses a plasma cleaning machine or a chemical reagent with strong oxidizing property.

[0016] Preferably, the diameter of the PS ball in step 2) is 200-5000 nm, and the PS ball array preparation method is surface tension self-assembly, air-liquid surface self-assembly or spin coating.

[0017] Preferably, the plasma etching in step 3) is performed at a power of 30-70 W, and the etching time is 100 s-2000 s; the gas used is air or a mixed gas of oxygen and inert gas.

[0018] Preferably, the solvent in step 4) includes one or more of methanol, ethanol and isopropanol.

[0019] Preferably, the annealing in step 5) is performed in an adhesive drying and heating chamber at 100-200℃ for 30-60 min to seal the gaps between the balls.

[0020] Preferably, the metal deposition method in step 6) is one or more of vacuum electron beam evaporation, magnetron sputtering and chemical vapor deposition, the metal is one or more of gold, silver and copper, and the deposition thickness is 0%-45% of the diameter of the PS microspheres.

[0021] Preferably, the solvent in step 7) includes toluene, acetone, normal temperature ultrasonic water bath, and then cleaning with acetone, isopropanol, anhydrous ethanol, deionized water, and drying.

[0022] Compared with the prior art, the present application has the following advantages:

[0023] (1) The non-periodic network structure prepared by the present application avoids the plasmonic nonlinear optical response caused by the periodic array, greatly improving the stability and average value of the network transmittance.

[0024] (2) The present application creatively adopts a solution-induced method, and the interval of the crack network and the width of the crack can be controlled by changing the diameter of the initial PS microspheres and the plasma etching time, which can be used in combination with semiconductor materials with a short carrier diffusion length, achieving higher collection efficiency in practical applications.

[0025] (3) The crack network of the present application can also be combined with other periodic or non-periodic networks to form a composite material, which has higher flexibility and possibility in practical use.

[0026] (4) Compared with the existing commercial ITO transparent electrode material, the present application can avoid the scarcity of indium material in raw material cost, and the manufacturing process is simpler, so that the manufacturing cost can be lower. The substrate material of the present application can be selected from other transparent and flexible water-stable materials. Compared with the brittle ITO material, the present application is expected to achieve flexible characteristics, which is more suitable for the flexible requirements of the next generation electrode. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is the process flow chart of the present application.

[0028] Figure 2 is the SEM image of the PS microsphere array after plasma etching for different lengths of time, solvent induction and annealing in Example 1 and Example 3 and Example 5 (the purple scale is 1 μm, the green scale is 2 μm), and the SEM images surrounded by black frame lines (A, B, C), blue frame lines (D, E, F) and orange frame lines (G, H, I) correspond to plasma etching for 200 s, 400 s and 600 s, respectively.

[0029] Figure 3 is the dark field microscope image of the PS microsphere array in Example 1 during the solvent induction process.

[0030] Figure 4are SEM comparison figures of the periodic hole metal network prepared by Comparative Examples 1 and 2 and the annealed non-periodic metal crack network prepared by Examples 1 and 2 (scale bar is 2 μm). (A, C) correspond to plasma etching for 200 s, (E, G) correspond to plasma etching for 300 s; (A, E) correspond to the periodic hole metal network, and (C, G) correspond to the annealed non-periodic metal crack network.

[0031] Figure 5 are transmittance curves of different metal networks at different etching times. (A) corresponds to the transmittance curves of the periodic hole metal network of Comparative Examples 1-3. (B) corresponds to the transmittance curves of the annealed non-periodic metal crack network of Examples 1-3. DETAILED DESCRIPTION

[0032] The technical solutions of the present application will be described clearly and completely below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. The preparation is carried out according to the conventional conditions or the conditions recommended by the manufacturer when the specific conditions are not specified in the embodiments of the present application. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be purchased on the market.

[0033] Example 1

[0034] A preparation method based on solvent-induced non-dense hexagonal PS microsphere array to metal crack network conversion, comprising the following steps:

[0035] 1) Cleaning and hydrophilic treatment of the substrate: first cut the glass sheet into 2.5 cm*2.5 cm, and sequentially clean with acetone, isopropanol, anhydrous ethanol, deionized water for 10 minutes, dry the silicon wafer, and then put it into the plasma cleaning machine, adjust the parameters to power 30 w, air environment, real-time flow rate 10 sccm, start the vacuum pump, when the pressure is less than or equal to 38 Pa, open the cleaning switch, 10 min later, close, open the air inlet, open the cavity door and take out the sample, at this time, the cleaning and hydrophilic treatment of the glass substrate is completed.

[0036] 2) PS microsphere array preparation: A 30 cm diameter petri dish was cleaned and filled with deionized water, and left for 10 min. A 784 nm PS microsphere solution was prepared (PS: 1% H2SO4: 1% styrene = 300 uL: 300 uL: 10 uL), where PS was obtained from a supplier with a solid content of about 5 wt.%, and the dispersion medium was water, and H2SO4 and styrene were dispersed in anhydrous ethanol in volume fraction. The prepared solution was ultrasonically oscillated for about 1 min, and then the solution was taken to 1 / 3 of the burette with a special-made curved burette, the burette tip was removed, the burette head was kept upward and slowly put into the middle of the petri dish, when the curved part just contacted the liquid surface, a meniscus was formed at the air-water interface, the PS microsphere array slowly spread onto the liquid surface and formed a circle, and the step was stopped when there was a small circle with a diameter of 1 / 5 of the petri dish in the middle. The PS microsphere array was left for 20 min to 1 h, and then SDS was slowly added along the edge to make the film gather in the middle of the petri dish. Finally, a glass sheet was clamped with a pair of tweezers, and slowly put into the water in the blank area around the film layer at an angle of 45°, and the PS film layer on the liquid surface was slowly pulled up at an angle of 45° in the area with the film layer, and then transferred to the glass sheet. After natural air drying, a neat PS microsphere array was obtained on the glass sheet.

[0037] 3) Plasma etching of the dense hexagonal PS microsphere array: The dense hexagonal PS array obtained in step 2) arranged on the glass substrate was placed in a plasma cleaning machine, the power was set to 64 W, the gas flow rate was 1 sccm of argon and 2 sccm of oxygen, the pressure was 0.14 mba, and the etching time was 200 s.

[0038] 4) Drop the solvent that does not dissolve the polystyrene microspheres onto the surface of the sample in step 3) and wait for it to dry: In this example, 99.8% anhydrous ethanol was selected, 50 uL of anhydrous ethanol was dropped onto the surface of the sample in step 3), and then it was dried. In this step, the gap between the non-dense hexagonal microsphere array obtained in step 3) was dropped, due to the capillary effect, the position of the microspheres changed during the volatilization of anhydrous ethanol, and the transformation from the non-dense hexagonal microsphere array to the crack based on solvent induction was completed.

[0039] 5) Annealing treatment: The PS sample containing cracks obtained in step 4) was placed in a tube annealing furnace for annealing, the temperature was 125°C, which was much lower than the melting temperature of polystyrene but sufficient to soften it, so that the triangular pores between the balls were sealed, and the crack network after annealing was obtained.

[0040] 6) Metal deposition: The sample surface obtained in step 5) is deposited with silver to a thickness of about 50 nm to obtain an annealed non-periodic crack metal mesh network. In this example, the metal film method of electron beam evaporation (EBE) is used: first open the chamber, place the sample on the sample disc of the EBE, close the chamber and vacuumize, when the vacuum degree is lower than 7.5x10 -4 Pa, set the evaporation thickness and click start, adjust the beam current of the electron gun to pre-melt silver, after sufficient melting, open the shutter, start evaporation, when the thickness reaches 50 nm, close the shutter, adjust the beam current knob to the minimum and close the electron gun, open the automatic stop, when the molecular pump speed is zero, open the air valve, open the chamber door and take out the sample to complete the metal deposition step.

[0041] 7) Toluene removes the surface PS microspheres and is washed and dried: the sample of step 6) is placed in a beaker containing toluene and is placed in an ultrasonic water bath for about 30 min, after taking out, it is washed with acetone, isopropanol, anhydrous ethanol, deionized water in turn, and is placed in an oven for drying.

[0042] Example 2-5

[0043] Example 2-5 is different from example 1 in that the plasma etching time in step 3) is 300 s, 400 s, 500 s and 600 s respectively, and other preparation steps and conditions are the same as example 1, which will not be repeated here.

[0044] Comparative example 1

[0045] A method for preparing a periodic hole metal network structure, comprising the following steps:

[0046] 1) Cleaning and hydrophilic treatment of the substrate: first cut the glass sheet into a size of 2.5 cm*2.5 cm, sequentially ultrasonic clean with acetone, isopropanol, anhydrous ethanol, deionized water for 10 minutes, dry the silicon wafer and place it in the plasma cleaning machine, adjust the parameters to power 30 w, air environment, real-time flow rate 10 sccm, start the vacuum pump, when the pressure is less than or equal to 38 Pa, open the cleaning switch, 10 min later, close, open the air inlet, open the chamber door and take out the sample, at this time the cleaning and hydrophilic treatment of the glass substrate is completed.

[0047] 2) PS microsphere array preparation: A 30 cm diameter petri dish was cleaned and filled with deionized water, and left for 10 min. A 784 nm PS microsphere solution was prepared (PS: 1% H2SO4: 1% styrene = 300 uL: 300 uL: 10 uL), where PS was obtained from a supplier, with a solid content of about 5 wt.%, and the dispersion medium was water, H2SO4 and styrene were dispersed in anhydrous ethanol in volume fraction. The prepared solution was ultrasonically oscillated for about 1 min, then a special-made curved dropper was used to take the solution to 1 / 3 of the dropper, the dropper tip was removed, the dropper head was kept upward and slowly put into the middle of the petri dish, when the curved part just touched the liquid surface, a meniscus was formed at the air-water interface, the PS microsphere array slowly spread to the liquid surface and formed a circle, and the step was stopped when there was a small circle of 1 / 5 of the petri dish diameter left in the middle. The PS microsphere array was left for 20 min to 1 h, and after it was fully self-assembled, SDS was slowly added along the edge, so that the film gathered in the middle of the petri dish. Finally, a glass sheet was clamped with tweezers at one corner, and slowly put into the water in the blank area around the film layer. The PS film layer on the liquid surface was transferred to the glass sheet at an angle of 45°, and the PS microsphere array neatly arranged on the glass sheet was obtained after natural air drying.

[0048] 3) Plasma etching of the dense hexagonal PS microsphere array: The dense hexagonal PS array arranged on the glass substrate obtained in step 2) was placed in a plasma cleaning machine, with a power of 64 W, an argon gas flow rate of 1 sccm, an oxygen gas flow rate of 2 sccm, a pressure of 0.14 mba, and an etching time of 200 s.

[0049] 4) Metal deposition: A silver film with a thickness of about 50 nm was deposited on the sample surface obtained in step 3) to obtain a periodic hole metal network structure. In this example, an electron beam evaporation (EBE) metal film method was used: first, open the cavity, place the sample on the sample disc of the EBE, close the cavity and vacuumize, when the vacuum degree is lower than 7.5 x 10 -4 Pa, set the evaporation thickness and click start, adjust the beam current of the electron gun to pre-melt the silver, after sufficient melting, open the shutter, start evaporation, close the shutter when the thickness reaches 50 nm, adjust the beam current knob to the minimum and close the electron gun, open the automatic stop, when the molecular pump speed is zero, open the air valve, open the cavity door and take out the sample, which completes the metal deposition step.

[0050] 5) Toluene removal of the surface PS microspheres and cleaning and drying: The sample of step 4) was placed in a beaker containing toluene and placed in an ultrasonic water bath for about 30 min, then sequentially washed with acetone, isopropanol, anhydrous ethanol and deionized water, and dried in an oven.

[0051] Comparative Example 1 The fabrication process of periodic hole metal network is similar to Example 1, except that the solution drop-induced process in step 4) and the annealing process in step 5) are not performed.

[0052] Comparative Example 2-3

[0053] Comparative Example 2-3 is similar to Comparative Example 1, except that the etching time in step 3) is 300 s and 400 s, respectively. Other fabrication steps and conditions are the same as those in Comparative Example 1, which are not repeated here.

[0054] To further illustrate the technical effects achieved by the preparation method of the present application, the prepared samples of the examples and comparative examples are detected.

[0055] Morphology characterization

[0056] Figure 2 Figure 1 is an SEM image of the array obtained in step 2) in Example 1, Example 3 and Example 5, which is based on a dense hexagonal PS microsphere array with an initial diameter of 784 nm after plasma etching for different lengths, followed by solvent induction and annealing. The purple scale is 1 μm, and the green scale is 2 μm. The SEM images surrounded by black frame lines (A, B, C), blue frame lines (D, E, F) and orange frame lines (G, H, I) correspond to plasma etching for 200 s, 400 s and 600 s, respectively. In the black frame line part (A, B, C), it can be seen that the gap between the PS microspheres increases with the increase of etching time, and the dense hexagonal arrangement changes to a non-dense hexagonal arrangement. The distance and width of the crack network can be controlled by controlling the plasma etching time. The blue frame line part (D, E, F) also undergoes a reorganization step induced by the solvent. The reorganization process is achieved by the lateral capillary force occurring when the template surface is wetted and then evaporated. During the process of wetting the sample surface to dryness, the meniscus formed by the liquid droplets moves on the sample surface. On the boundary of this meniscus, capillary forces act on the microspheres, causing their movement and self-assembly into a network-like pattern, i.e. the cracks appearing in the array. The orange frame line part (G, H, I) also undergoes an annealing step, and it can be seen that the triangular pores between the spheres are closed.

[0057] Figure 3 Figure 2 is a dark field microscope image of the PS microsphere array based on an initial diameter of 784 nm in Example 1 during the solvent induction process. The magnification is 1000 times, (A) is the PS array after adding ethanol and being wetted. The large white clusters are defects caused by particle agglomeration, and the white part represents cracks or other defects. (B-E) show that more and more cracks are formed as the solvent evaporates. (F) After the solvent evaporates, the template passes through the network-like cracks, and it can be seen that the formation of cracks occurs during the process of wetting the sample surface to dryness.

[0058] Figure 4 Figure 6 is SEM comparison of periodic hole metal network prepared after 784 nm initial diameter PS microspheres in Comparative Examples 1 and 2 and non-periodic metal crack network prepared after annealing in Examples 1 and 2, scale bar is 2 pm. (A, C) correspond to plasma etching 200 s, (E, G) correspond to plasma etching 300 s; where (A, E) correspond to periodic hole metal network, (C, G) correspond to non-periodic metal crack network after annealing. For the annealed network, the triangular metal islands caused by the triangular inter-sphere holes can be seen to disappear, replaced by a narrow crack metal network. Comparing the annealed networks etched for 200 s and 300 s, it can be seen that the probability of cracks connecting together increases as the etching time increases.

[0059] Performance test

[0060] Figure 5 Figure 7 is the transmittance curve of different metal networks at different etching times. (A) corresponds to the transmittance curve of the periodic hole metal network prepared in Comparative Examples 1-3. (C) corresponds to the transmittance curve of the non-periodic annealed metal crack network prepared in Examples 1-3. Where (A) is the typical transmittance curve of the periodic hole metal network, which has the typical spectral characteristics of the surface plasmon active metal film. (C) is the non-periodic crack metal network after annealing, it can be seen that the average transmittance decreases significantly as the plasma etching time increases. The decrease in transmittance can be attributed to the increase in crack gap area caused by annealing, and the uniformity of the spectral response caused by the decrease in light-matter interaction. The present application uses a non-periodic network, which avoids the plasma nonlinear optical response caused by the periodic array, greatly improving the stability of the network transmittance.

[0061] In addition, it can also be seen in the present application that as the plasma etching time increases, the network shows a typical decrease, which is related to the non-negligible contribution of electron scattering on the boundary of the nanometer network. However, the resistance can only be measured in a fully connected network. However, the non-periodic metal crack network after annealing in the present application cannot achieve full connection in structure, so combining other materials for composite material design will achieve better results. A very promising method is to design a fractal network, which can be achieved by creating a second metal network with larger mesh on top using other lithography techniques, which not only inherits the advantages of high transmittance and the ability to adapt to materials with short electron carrier diffusion length in the present application, but also adjusts the resistance of the entire system to the required range. Another method is to combine the metal network with graphene. This method has been proven in related research based on graphene spin-coated silver nanowires and the like. It can be seen that the crack network of the present application combined with other periodic or non-periodic networks to form a composite material has higher flexibility and possibility in actual use.

[0062] The above embodiments of the present application are only examples for clearly illustrating the technical solutions of the present application, and are not intended to limit the specific embodiments of the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the claims of the present application shall be included in the protection scope of the claims of the present application.

Claims

1. A preparation method based on solvent-induced non-dense hexagonal PS microsphere array transformation to metal crack network, characterized by, The method comprises the following steps: 1) washing and hydrophilic treatment of the substrate; 2) self-assembly of the prepared PS ball suspension onto the substrate treated in step 1) to obtain a hexagonal dense arrangement of PS microsphere array; 3) plasma etching of the dense arrangement of PS microsphere array in step 2) to obtain a non-dense hexagonal PS microsphere array; 4) adding a solvent that does not dissolve PS microspheres and is volatile to the sample prepared in step 3) and accelerating solvent volatilization to dry the sample; 5) annealing treatment of the sample prepared in step 4) to seal the triangular holes between the balls; 6) depositing a layer of metal on the sample prepared in step 5); 7) ultrasonic water bath treatment of the sample prepared in step 6) in a solvent that dissolves PS microspheres, removing the surface PS microspheres, and washing and drying. The plasma etching in step 3) is plasma modification at a power of 30-70 W, the etching time is 100-2000 s, and the gas used is air or a mixture of oxygen and inert gas; The annealing in step 5) is template annealing in a binder drying and heating chamber at 100-200℃ for 30-60 min to seal the interstitial pores between the balls.

2. The method according to claim 1, wherein, The substrate in step 1) is a non-flexible or flexible substrate with water stability and good transmittance, including glass sheet, quartz sheet, polyvinyl alcohol, polyester or polyimide, the cleaning process uses anhydrous ethanol and deionized water ultrasonic cleaning, and the hydrophilic treatment uses a plasma cleaning machine or a chemical reagent with strong oxidizing property.

3. The method according to claim 1, wherein, The diameter of the PS balls in step 2) is 200-5000 nm, and the PS ball array is prepared by surface tension self-assembly, air-liquid surface self-assembly or spin coating method.

4. The method according to claim 1, wherein, The solvent in step 4) includes one or more of methanol, ethanol and isopropanol.

5. The method according to claim 1, wherein, The metal deposition method in step 6) is one or more of vacuum electron beam evaporation, magnetron sputtering and chemical vapor deposition, the metal is one or more of gold, silver and copper, and the metal deposition thickness is 0%-45% of the diameter of the PS microspheres.

6. The method according to claim 1, wherein, The solvent in step 7) includes toluene and acetone, the ultrasonic water bath treatment is carried out at room temperature, and then the sample is washed with acetone, isopropanol, anhydrous ethanol and deionized water and dried.

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