Large-area, few-layer two-dimensional material exfoliation and electronic device preparation method

Abstract

The present invention proposes a method for stripping a large-area, few-layer two-dimensional material and preparing an electronic device. In the present invention, when preparing a two-dimensional material, a PVA layer is prepared on the surface of a substrate; an adhesive is used to stick a single crystal block to obtain an adhesive with a two-dimensional material attached; the adhesive is attached to the substrate so that the two-dimensional material is in contact with the PVA layer; the substrate is heated, and the adhesive is torn off to obtain a substrate with a large-area, few-layer two-dimensional material attached. In the present invention, when preparing a two-dimensional material, a PVA layer is spin-coated on the substrate as an auxiliary layer, and the flat surface and strong adhesiveness of the PVA layer are utilized to strip off a large-area, nanometer-level, few-layer two-dimensional material on the substrate with the PVA layer, and the stripping process is clean without damaging the two-dimensional material.

Description

Technical Field

[0001] The present invention relates to the field of two-dimensional materials, and in particular to a method for stripping large-area, few-layer two-dimensional materials and preparing electronic devices. Background Art

[0002] Since graphene was successfully prepared, the unique electronic and optoelectronic properties and excellent mechanical properties of two-dimensional materials have attracted widespread attention from researchers. Compared with silicon-based materials, two-dimensional materials have atomic-level thickness and no dangling bonds on the surface, and almost no short channel effect, and are expected to be used in the next generation of integrated circuits. Therefore, how to prepare high-quality and large-area two-dimensional materials will determine the industrialization process of two-dimensional materials.

[0003] At present, there are three common methods for preparing two-dimensional materials: liquid phase ultrasound, chemical vapor deposition, and mechanical exfoliation. It is difficult to control the number of layers when preparing two-dimensional materials by liquid phase ultrasound, and the material surface is very small. Long-term ultrasound also leads to poor quality of single crystals. Chemical vapor deposition will introduce some defects when growing two-dimensional materials, and the quality of single crystals is not high. The two-dimensional materials prepared by mechanical exfoliation method are of high quality, but the yield is low and the size is small, as shown in Figure 1 (a) (b).

[0004] In recent years, the gold-assisted stripping method has emerged. It deposits gold on the surface of a single crystal block, and then uses an adhesive to peel off the surface gold film, that is, the gold film with two-dimensional materials is peeled off from the single crystal block by adhesive adhesion. A large area of ​​two-dimensional material attached to the surface below the gold film can be obtained, but the deposited gold will introduce defects, strain and Fermi pinning on the surface of the two-dimensional material, and the quality of the two-dimensional material obtained by stripping is poor. The improved gold-assisted stripping method requires gold as a substrate, which cannot achieve the characterization of electrical properties, and requires the use of KI / I2 solution to remove the gold film, which will inevitably cause the surface of the two-dimensional material to be contaminated or damaged, which is contrary to the goal of preparing high-quality two-dimensional materials by mechanical cleavage. In addition, the gold-assisted method has many steps and is difficult to operate. For example, the gold surface must be smooth and cannot be in the air for more than 15 minutes.

[0005] It can be seen that there is currently a lack of mechanical exfoliation solutions that can effectively exfoliate large-area two-dimensional materials and facilitate transfer. Summary of the invention

[0006] In order to solve the defects of high difficulty and low yield in the above-mentioned prior art in the peeling and transfer of two-dimensional materials, the present invention proposes a peeling preparation method for large-area few-layer two-dimensional materials, which can quickly prepare large-area two-dimensional materials.

[0007] The present invention provides a method for preparing a large-area, few-layer two-dimensional material by exfoliation, comprising the following steps:

[0008] S1, preparing a PVA layer on the surface of the substrate; using an adhesive to adhere the single crystal block to obtain an adhesive with a two-dimensional material attached;

[0009] S2, attaching the adhesive to the substrate so that the two-dimensional material is in contact with the PVA layer;

[0010] S3. The substrate is subjected to a heating treatment, and then the adhesive is removed to obtain a substrate with a large area of ​​a few-layer two-dimensional material attached thereto.

[0011] Preferably, the method for preparing the PVA layer in S1 is: spin coating the PVA solution on the surface of the substrate with a coating machine and then heating it; the spin coating parameters are 2000-5000r / min; the heating process is baking at 60-130°C for 1-5 minutes.

[0012] Preferably, the process of heating the substrate in S3 is: heating at 60-130° C. for 1-5 minutes.

[0013] Preferably, the adhesive member is an adhesive tape.

[0014] The present invention also proposes a method for preparing an electronic device, which uses the method for preparing large-area, few-layer two-dimensional materials to obtain a substrate with two-dimensional materials attached, and uses a photolithography process to manufacture an electronic device on the surface of the substrate with the two-dimensional material attached.

[0015] The present invention proposes a method for preparing an electronic device, which adopts the large-area few-layer two-dimensional material stripping preparation method to obtain a substrate with a large-area few-layer two-dimensional material attached; then dissolves the PVA outside the sample coverage area on the substrate, and then uses a photolithography process to make electrodes to form a two-dimensional electronic device.

[0016] Another method for preparing an electronic device proposed by the present invention comprises obtaining a substrate with a two-dimensional material attached thereto, and manufacturing an electronic device on the surface of the substrate with the two-dimensional material attached thereto by a photolithography process; obtaining the substrate with the two-dimensional material attached thereto comprises the following steps:

[0017] S4, using the above-mentioned stripping preparation method of large-area few-layer two-dimensional material to obtain a substrate with large-area few-layer two-dimensional material attached, and preparing a PMMA film on the surface of the substrate with the two-dimensional material attached;

[0018] S5, dissolving the PVA layer on the substrate to separate the PMMA film attached with the two-dimensional material from the substrate;

[0019] S6, attaching the PMMA film to the substrate so that the two-dimensional material on the PMMA film is in contact with the substrate;

[0020] S7. Dissolve the PMMA film to obtain a substrate with the two-dimensional material attached.

[0021] Preferably, the method for preparing the PMMA film in S4 is: spin coating the PMMA solution on the surface of the substrate to which the two-dimensional material is attached, and then heating; the spin coating process is 2000 r / min; and the heating process is heating at 80-120° C. for 3-5 minutes.

[0022] Preferably, S5 specifically includes: immersing the substrate in deionized water to dissolve the PVA layer; S6 specifically includes: using the substrate to fish out the PMMA film from the deionized water so that the two-dimensional material on the PMMA film contacts the substrate; and then performing a heating treatment, the heating process is: heating at 80° C. for 3-5 minutes.

[0023] The present invention also proposes a method for preparing an electronic device, which adopts the large-area few-layer two-dimensional material stripping preparation method to obtain a substrate with a large-area few-layer two-dimensional material attached; then dissolves the PVA outside the sample coverage area on the substrate, and then evaporates the electrode through a photolithography process to form a two-dimensional electronic device.

[0024] The advantages of the present invention are:

[0025] (1) The preparation method provided by the present invention, when preparing a two-dimensional material, spin-coats a PVA layer on a substrate as an auxiliary layer, and utilizes the flat surface and strong adhesiveness of the PVA layer to bring an adhesive portion with the two-dimensional material torn off from a single crystal block into contact with the PVA layer, thereby peeling off a large area of ​​a few layers of two-dimensional material with a thickness of nanometers on the substrate with the PVA layer, and the peeling process is clean and does not damage the two-dimensional material.

[0026] (2) In the present invention, a coating machine is used to prepare the PVA layer on the substrate, which is efficient, fast, and ensures the quality of the PVA layer.

[0027] (3) In the present invention, the adhesive is torn off after heat treatment, which enhances the adhesion of the PVA layer, facilitates peeling off more two-dimensional materials on the PVA layer, and improves the peeling efficiency.

[0028] (4) The PVA layer is an insulating polymer and is suitable as a gate dielectric. The two-dimensional material with a PVA layer prepared by the present invention can be directly subjected to a photolithography process to prepare electronic devices.

[0029] (5) In the present invention, the two-dimensional material on the substrate can be further transferred to the substrate, i.e., a new substrate, through the PMMA film, thereby achieving the purpose of removing the PVA layer, so that the two-dimensional material is directly adhered to the substrate. This transfer process achieves lossless transfer of the two-dimensional material.

[0030] (6) The present invention directly prepares electronic devices on a PVA substrate, providing a mild device preparation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG1(a) is a photolithography image of boron nitride prepared on a common SiO / Si substrate;

[0032] FIG1( b ) is another photolithography image of boron nitride prepared on a common SiO / Si substrate;

[0033] FIG2(a) is a photoimage of boron nitride prepared in Example 1;

[0034] FIG2( b ) is another photolithography image of boron nitride prepared in Example 1;

[0035] FIG2( c ) is a light image of boron nitride of standard size prepared in Example 1;

[0036] FIG2( d ) is a comparison diagram of the size distribution of the boron nitride prepared in Example 1 and the size distribution of the boron nitride prepared on a common SiO / Si substrate;

[0037] FIG2(e) is a light display image of standard-sized indium zinc sulfide (Znln2S4) prepared in Example 1;

[0038] Figure 3 This is a comparison diagram of graphite materials prepared by two methods in Example 2;

[0039] Figure 4 This is a comparison diagram of the Raman spectra of the graphite material before and after exfoliation in Example 2;

[0040] Figure 5 This is a comparison chart of XPS characterization of the graphite material obtained in Example 2 before and after transfer;

[0041] FIG6( a) is a schematic diagram of the electronic device prepared in Example 4;

[0042] FIG6( b) is a light display image of the electronic device prepared in Example 4;

[0043] FIG6( c ) is an IV (current-voltage) characteristic curve of the electronic device prepared in Example 4;

[0044] FIG6( d ) is a FET (field effect transistor) transfer curve of the electronic device prepared in Example 4;

[0045] FIG7( a) is a characterization of the thickness and roughness of graphene on the silicon oxide substrate prepared in Example 4 under an atomic force microscope;

[0046] Figure 7(b) is the thickness and roughness analysis curve corresponding to Figure 7(a);

[0047] FIG8( a) is a schematic diagram of the electronic device prepared in Example 5;

[0048] Figure 8(b) is a light display image of the electronic device prepared in Example 5

[0049] FIG8( c ) is an IV (current-voltage) characteristic curve of the electronic device prepared in Example 5;

[0050] FIG8( d ) is a FET (field effect transistor) transfer curve of the electronic device prepared in Example 5;

[0051] FIG9( a) is a characterization of the thickness and roughness of graphene on the silicon oxide substrate prepared in Example 5 under an atomic force microscope;

[0052] Figure 9(b) is the thickness and roughness analysis curve corresponding to Figure 9(a);

[0053] FIG10( a ) is a characterization of the thickness and roughness of the PVA layer on the silicon oxide substrate prepared in Example 5 under an atomic force microscope;

[0054] Figure 10(b) is the thickness and roughness analysis curve corresponding to Figure 10(a);

[0055] Fig.11 A flow chart of a method for exfoliating and preparing large-area, few-layer two-dimensional materials;

[0056] Fig.12 Flowchart for transferring large-area few-layer 2D materials prepared for exfoliation onto a substrate as a new substrate. DETAILED DESCRIPTION

[0057] Example 1: A method for preparing boron nitride (BN)

[0058] In this embodiment, Fig.11 The method for preparing boron nitride comprises the following steps:

[0059] The first step is to spin-coat a PVA solution on a clean silicon oxide substrate using a coating machine, and then bake it at 60-130°C for 3 minutes to prepare a PVA layer; and to bond a tape to a boron nitride crystal to obtain a tape with boron nitride attached thereto;

[0060] Step 2: attaching the adhesive tape to the silicon oxide substrate so that the boron nitride is in contact with the PVA layer;

[0061] The third step is to heat the silicon oxide substrate at 60-130°C for 3 minutes, and then remove the tape to obtain the silicon oxide substrate with boron nitride attached.

[0062] In this embodiment, boron nitride is attached to the PVA layer on the surface of the silicon oxide substrate. The distribution of boron nitride on two silicon oxide substrates obtained by this embodiment is shown in Figure 2 (a) and Figure 2 (b)

[0063] In this embodiment, a piece of few-layer boron nitride on the PVA layer is measured, and the obtained plane size is 126.96 μm (as shown in FIG. 2( c )).

[0064] Figures 2(a) and 2(b) are the optical images of the boron nitride stripped by this embodiment, and Figure 2(d) is a statistical comparison of the size and quantity of the boron nitride on the substrate obtained by different preparation methods, wherein SiO / Si-(a) corresponds to the optical image of the boron nitride prepared directly on silicon oxide by a common mechanical stripping method, SiO / Si-(b) corresponds to the optical image of the boron nitride prepared directly on silicon oxide by a common mechanical stripping method, and PVA-SiO / Si-(c) and PVA-SiO / Si-(d) are the optical images of the boron nitride prepared on the PVA layer on the surface of the silicon oxide substrate by the preparation method provided by this embodiment in two experiments. It can be seen from Figure 2(d) that most of the boron nitride obtained by this embodiment has a size in the range of 20 to 100 microns, and a small part has a size in the range of 100 to 300 microns; while most of the boron nitride prepared on the common SiO / Si substrate has a size less than 10 microns, a small part has a size in the range of 1 to 20 microns, and only a very small part has a size in the range of 20 to 50 microns.

[0065] FIG. 2( e ) shows the measured dimensions of indium zinc sulfide (Znln 2 S 4 ) obtained by the steps in this embodiment.

[0066] Example 2: A method for preparing graphite material

[0067] In this embodiment, two preparation methods are used to peel graphite material from the graphite sheet.

[0068] The first preparation method is: sticking a tape to a graphite sheet to obtain a tape with a graphite material attached; then attaching the tape to a silicon oxide substrate prepared with a PVA layer so that the graphite material is in contact with the PVA layer; heating the silicon oxide substrate at 60-130°C for 3 minutes, and then removing the tape to obtain a silicon oxide substrate with a graphite sheet attached. Figure 3 The four pictures on the left are distribution diagrams of graphite materials on the PVA layers of four silicon oxide substrates obtained by the first preparation method. Figure 3 The medium grid size is 1 mm.

[0069] The second preparation method is: sticking the tape to the graphite sheet to obtain the tape with the graphite material attached; then attaching the tape to the silicon oxide substrate so that the graphite material is in direct contact with the silicon oxide substrate; heating the silicon oxide substrate at 60-130°C for 3 minutes, and then removing the tape to obtain the silicon oxide substrate with the graphite sheet attached. Figure 3 The two pictures on the right in the middle are graphite material distribution diagrams on two silicon oxide substrates obtained by the second preparation method.

[0070] It can be seen from this embodiment that the first preparation method using a silicon oxide substrate with a PVA layer attached thereto can prepare a large-area graphite material; the second preparation method directly using a silicon oxide substrate cannot effectively peel off the graphite material.

[0071] Example 3: Another method for preparing graphite material

[0072] Reference Fig.12 This embodiment is used to transfer the graphite material obtained by the first preparation method in Example 2 to another silicon oxide substrate; before the transfer, the graphite material is attached to the PVA layer of the silicon oxide substrate; after the transfer, the graphite material is directly attached to the silicon oxide substrate.

[0073] In this embodiment, the silicon oxide substrate with attached graphite material obtained by the first preparation method in Embodiment 2 is first obtained, and then the following steps are performed.

[0074] The first step is to spin-coat the surface of the silicon oxide substrate with the graphite material attached to the PMMA solution, and then heat it at 80-120°C for 3 minutes to prepare a PMMA film. At this time, the silicon oxide substrate is layered from bottom to top as follows: silicon oxide substrate-PVA layer-graphite material-PMMA film.

[0075] In the second step, the silicon oxide substrate is immersed in deionized water to dissolve the PVA layer on the substrate until the PMMA film is separated from the substrate, and the graphite material is attached to the PMMA film.

[0076] The third step is to use a silicon oxide substrate to fish out the PMMA film from the deionized water, and to attach the PMMA film to the substrate so that the two-dimensional material on the PMMA film is in contact with the substrate; then heat at 80°C for 3-5 minutes to remove excess water, so that the PMMA film is attached to the silicon oxide substrate.

[0077] Step 4: Soak the silicon oxide substrate in acetone to dissolve the PMMA film to obtain a substrate with a two-dimensional material attached.

[0078] In this embodiment, the graphite sheet obtained by the first preparation method and the graphite sheet transferred to the substrate as a new substrate each took 4 points to characterize the Raman spectrum, as shown in the following figure: Figure 4 shown. Figure 4 The four spectra in the upper middle are the Raman spectra of the graphene flakes before transfer, and the four spectra in the lower middle are the Raman spectra of the graphene flakes transferred to the new substrate. G and 2D represent the characteristic peaks of graphene. Figure 4 It can be seen that the Raman spectra of the graphite sheet obtained by the first preparation method and the graphite sheet transferred to the new substrate are consistent, proving that the preparation method provided by the present invention does not damage the quality of graphite.

[0079] Figure 5The graphite material on the silicon oxide substrate obtained in Example 2 and the graphite material on the silicon oxide substrate obtained in Example 3 are shown in comparison of X-ray photoelectron spectroscopy (XPS) characterizations. Figure 5 The waveform in the upper middle is the XPS characterization of the graphite sheet that has just been peeled off. Figure 5 The waveform in the middle and lower part is the XPS characterization of the graphite sheet transferred to the new substrate. C and O represent the carbon peak and oxygen peak, respectively. Figure 5 The upper and lower waveforms in the middle completely overlap, and it can be seen that this embodiment achieves lossless transfer of the graphite sheet on the PVA auxiliary layer.

[0080] Example 4: A method for preparing an electronic device

[0081] This embodiment includes the following steps:

[0082] The first step is to spin-coat a PVA solution on a clean silicon oxide substrate using a coating machine, and then bake it at 60-130°C for 3 minutes to prepare a PVA layer; and to stick the tape to a graphite (Gr) crystal to obtain a tape with a few layers of graphene attached.

[0083] The second step is to attach the tape to the silicon oxide substrate so that the graphene is in contact with the PVA layer.

[0084] Step 3: Heat the silicon oxide substrate at 60-130°C for 3 minutes, and then remove the tape to obtain a silicon oxide substrate with a few layers of graphene attached. At this time, the silicon oxide substrate is layered from bottom to top as follows: silicon oxide substrate - PVA layer - graphene.

[0085] Step 4: Spin-coat the PMMA solution on the surface of the silicon oxide substrate with graphene attached, and then heat at 80-120°C for 3 minutes to prepare a PMMA film. At this time, the silicon oxide substrate is layered from bottom to top as follows: silicon oxide substrate - PVA layer - graphene - PMMA film.

[0086] Step 5: Soak the silicon oxide substrate in deionized water to dissolve the PVA layer on the substrate until the PMMA film is separated from the substrate, and graphene is attached to the PMMA film.

[0087] Step 6: Use a silicon oxide substrate to fish out the PMMA film from the deionized water, and stick the PMMA film to the substrate so that the graphene on the PMMA film contacts the substrate; then heat at 80°C for 3-5 minutes to remove excess water so that the PMMA film is attached to the silicon oxide substrate.

[0088] Step 7: Immerse the silicon oxide substrate in acetone to dissolve the PMMA film to obtain a silicon oxide substrate with a few layers of graphene attached. The thickness and roughness of the graphene on the silicon oxide substrate are characterized under an atomic force microscope as shown in Figure 7(a), and the thickness and roughness analysis curves are shown in Figure 7(b).

[0089] Step 8: Electrodes are deposited on the surface of the silicon oxide substrate on which graphene is attached to form an electronic device. The schematic diagram is shown in FIG6(a), and the optical display is shown in FIG6(b). ds -V ds The curve and FET transfer curve are shown in Figure 6(c) and Figure 6(d), respectively. ds represents the source-drain current, V ds Represents the voltage between the drain and source of the field effect tube, V gs In FIG6( a ), 5 represents a silicon oxide substrate, 3 represents a graphene layer, and 4 represents an electrode.

[0090] Example 5: Another method for preparing an electronic device

[0091] This embodiment includes the following steps:

[0092] The first step is to use a coating machine to spin-coat a PVA solution on a clean silicon oxide substrate, and then bake it at 60-130°C for 3 minutes to prepare a PVA layer; the tape is adhered to the graphene (Gr) crystal material to obtain a tape with a large area of ​​few-layer graphene attached.

[0093] The second step is to attach the tape to the silicon oxide substrate so that the graphene is in contact with the PVA layer.

[0094] The third step is to heat the silicon oxide substrate at 60-130°C for 3 minutes, and then remove the tape to obtain a silicon oxide substrate with a large area of ​​few-layer graphene attached. At this time, the silicon oxide substrate is layered from bottom to top as follows: silicon oxide substrate-PVA layer-graphene. The thickness and roughness of the graphene on the PVA layer on the surface of the silicon oxide substrate are characterized under an atomic force microscope as shown in Figure 9(a), and the thickness and roughness analysis curve is shown in Figure 9(b); the thickness and roughness of the PVA layer are characterized under an atomic force microscope as shown in Figure 10(a), and the thickness and roughness analysis curve is shown in Figure 10(b).

[0095] The fourth step is to deposit electrodes on the surface of the silicon oxide substrate attached with graphene to form an electronic device, the principle diagram of which is shown in FIG8(a), and the optical display diagram is shown in FIG8(b). ds -V ds The curve and FET transfer curve are shown in Figure 8(c) and Figure 8(d) respectively. In Figure 8(a), 1 represents the silicon oxide substrate, 2 represents the PVA layer, 3 represents the graphene layer, and 4 represents the electrode.

[0096] It can be seen from Examples 4-5 that the large-area, few-layer two-dimensional material prepared by PVA-assisted exfoliation can be used to prepare electronic devices by dissolving excess PVA.

[0097] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing large-area, few-layer two-dimensional materials by exfoliation, characterized in that: The following steps are involved: S1, preparing a PVA layer on the surface of the substrate; using an adhesive to adhere the single crystal block to obtain an adhesive with a two-dimensional material attached; S2, attaching the adhesive to the substrate so that the two-dimensional material is in contact with the PVA layer; S3. The substrate is subjected to a heating treatment, and then the adhesive is removed to obtain a substrate with a large area of ​​a few-layer two-dimensional material attached thereto.

2. The method for preparing large-area and few-layer two-dimensional materials by exfoliation according to claim 1, characterized in that: The method for preparing the PVA layer in S1 is: spin coating the PVA solution on the surface of the substrate with a coating machine and then heating it; the spin coating parameters are 2000-5000r / min; the heating process is baking at 60-130°C for 1-5 minutes.

3. The method for preparing large-area and few-layer two-dimensional materials by exfoliation according to claim 1, characterized in that: The process of heating the substrate in S3 is: heating at 60-130° C. for 1-5 minutes.

4. The method for preparing large-area and few-layer two-dimensional materials by exfoliation according to claim 1, characterized in that: The adhesive member is an adhesive tape.

5. A method for preparing an electronic device, characterized in that: A substrate with a large-area, few-layer two-dimensional material attached is obtained by the stripping preparation method of a large-area, few-layer two-dimensional material as described in any one of claims 1 to 4; the PVA on the substrate is then dissolved, and electrodes are made by a photolithography process to form a two-dimensional electronic device.

6. A method for preparing an electronic device, characterized in that: A substrate with a two-dimensional material attached thereto is obtained, and an electronic device is fabricated on the surface of the substrate with the two-dimensional material attached thereto by a photolithography process; obtaining the substrate with the two-dimensional material attached thereto comprises the following steps: S4, using the stripping preparation method of large-area few-layer two-dimensional material as described in any one of claims 1 to 4 to obtain a substrate with large-area few-layer two-dimensional material attached thereto, and preparing a PMMA film on the surface of the substrate with the two-dimensional material attached thereto; S5, dissolving the PVA layer on the substrate to separate the PMMA film attached with the two-dimensional material from the substrate; S6, attaching the PMMA film to the substrate so that the two-dimensional material on the PMMA film is in contact with the substrate; S7. Dissolve the PMMA film to obtain a substrate with the two-dimensional material attached.

7. The method for preparing an electronic device according to claim 6, characterized in that: The method for preparing the PMMA film in S4 is: spin coating the PMMA solution on the surface of the substrate to which the two-dimensional material is attached, and then heating; the spin coating process is 2000r / min; the heating process is heating at 80-120°C for 3-5 minutes.

8. The method for preparing an electronic device according to claim 6, characterized in that: S5 specifically includes: immersing the substrate in deionized water to dissolve the PVA layer; S6 specifically includes: using the substrate to fish out the PMMA film from the deionized water so that the two-dimensional material on the PMMA film contacts the substrate; and then performing a heating treatment, wherein the heating process is: heating at 80° C. for 3-5 minutes.

Images