CN103449418A - Method for transferring graphene with atomic cleanness - Google Patents

Abstract

The invention discloses a method for efficiently transferring CVD graphene and obtaining an atomic-level clean surface, which comprises the following steps: spin-coat a layer of PMMA solution on the surface of graphene/metal substrate, and pass through (NH ₄ ) ₂ S after it is solidified ₂ O ₈ solution was used to remove the metal substrate by etching, the graphene/PMMA was cleaned, picked up and dried with the target substrate, and then an organic acid solvent was used to dissolve the PMMA layer, and finally the residual PMMA on the graphene surface was further removed by high-temperature annealing, so as to obtain atomically clean graphene surface. The graphene provided by the invention has a large surface area, a complete structure, and few defects and PMMA residues. The method provided by the invention has very little PMMA residue, and it is easier to obtain an atomically clean surface. The graphene obtained by this method is suitable for the construction of electronic devices such as field effect transistors, light-emitting diodes, transparent electrodes, etc., and can also be used for theoretical research on surfaces and interfaces.

Description

A method for atomically clean transferred graphene

technical field

The invention relates to a method for atomically clean transfer of graphene, in particular to a method for efficiently transferring CVD graphene and obtaining an atomically clean surface, belonging to the field of nanomaterials and technologies.

Background technique

Graphene has a series of excellent properties, such as ultra-high carrier mobility, thermal conductivity, mechanical strength, etc. Since its discovery in 2004, it has been a research hotspot in many frontier fields such as physics, materials science, and surface and interface science. . The main ways to obtain graphene are mechanical/solution exfoliation method, SiC epitaxial growth method, reduced graphene oxide method and chemical vapor deposition method (CVD method). Among them, the CVD method has shown potential application prospects in the fields of touch screen devices and transparent electrodes due to its advantages such as controllable layer number and easy enlargement of growth size.

Although the growth of CVD graphene has reached a mature and controllable level in terms of morphology and defect control, applications based on the properties of graphene carrier mobility and light transmittance, such as ultra-high frequency field effect transistors, contactors, etc. Screen devices, flexible electrodes, etc. all have special requirements for graphene loading substrates (Nature Nanotechnology., 2010, 5, 722-726), which means that graphene grown by CVD method can be transferred to the corresponding substrate efficiently and controllably. At present, the most important method for CVD graphene transfer is polymethyl methacrylate (PMMA) assisted transfer method. In this method, the residual problem caused by the easy repolymerization of PMMA has always been the bottleneck that limits the further development of this method. Ruoff et al. found that the residue of PMMA on the surface of graphene greatly reduced the electrical and optical properties of graphene (Nano Letters., 2013, 13, 1462-1467). A transfer method that reduces PMMA residues and does not introduce additional defects is particularly important.

Contents of the invention

The object of the present invention is to provide a kind of efficient transfer of CVD graphene and the transfer method of obtaining atomic level clean surface. The main feature of the invention is to introduce a simple organic acid solvent treatment method to obtain atomically clean CVD graphene. The graphene provided by the invention has large area, complete structure, and few defects and PMMA residues. Compared with the traditional method of using acetone as a solvent to remove PMMA, the method provided by the present invention has very little PMMA residue and can obtain an atomically clean surface. The graphene obtained by this method is suitable for the construction of electronic devices such as field effect transistors, light-emitting diodes, transparent electrodes, etc., and can also be used for theoretical research on surfaces and interfaces.

The present invention realizes through following technical scheme:

An atomic-level clean CVD graphene transfer method is characterized in that an organic acid is selected as a solvent to remove the PMMA coating.

According to the present invention, the organic acid is at least one selected from acetic acid, trifluoroacetic acid, formic acid and oxalic acid. Acetic acid is preferred, and acetic acid has the best solubility and degradation degree for PMMA.

The present invention uses an organic acid solvent that has both good degradability and solubility for PMMA. Compared with the traditional method of removing PMMA solely relying on the solubility of acetone, the method provided by the present invention removes PMMA more thoroughly and can obtain atomically clean CVD graphene surface.

According to the present invention, the transfer method includes: immersing the target substrate spread with graphene/PMMA in an organic acid solution, and dissolving the PMMA to obtain the target substrate spread with graphene.

According to the present invention, before the target substrate spread with graphene/PMMA is soaked in the organic acid solution and soaked in the organic acid solution, the following steps are also included:

(1) Spin-coat a layer of PMMA solution on the surface of the substrate with graphene, and solidify it;

(2) Soak the PMMA layer of the sample obtained in step (1) facing up in the corrosion solution to corrode the substrate, and then transfer the PMMA with graphene to deionized water to clean the residual ions;

(3) Select a suitable target substrate to pick up the graphene/PMMA layer and dry it.

According to the present invention, the substrate on which graphene grows is a metal substrate such as copper foil, Ni, Pt, etc.

According to the present invention, the method specifically includes the following steps:

(1) Spin-coat a layer of PMMA solution on the surface of the substrate with graphene, and solidify it;

(2) Soak the PMMA layer of the sample obtained in step (1) facing up in the corrosion solution to corrode the substrate, and then transfer the PMMA with graphene to deionized water to clean the residual ions;

(3) Select a suitable target substrate to pick up the graphene/PMMA layer and dry it;

(4) The target substrate spread with graphene/PMMA is soaked in an organic acid solution, and the PMMA is dissolved to obtain the target substrate spread with graphene;

(5) Finally, the graphene-coated substrate is annealed in H ₂ /Ar mixed gas to obtain an atomically clean graphene surface.

In the above method, the PMMA solution in step (1) is spin-coated to form a film on the graphene surface. The adhesion between PMMA and graphene is then increased by heating and curing.

According to the present invention, the preparation method of graphene in step (1) can refer to Nanotechnology, 2012, 23, 0957-4484.

According to the present invention, the spin-coating speed of a layer of PMMA solution is spin-coated on the surface of the substrate (such as copper foil, Ni, Pt and other metal substrates) grown with graphene at 3000rpm. After the spin coating, the metal substrate coated with PMMA was placed on a heating plate to be heated and cured. Preferably, the heating temperature of the heating plate is 80-200°C, preferably 100-180°C.

In the above method, the etching solution in step (2) can be selected according to the substrate material of CVD-grown graphene. The commonly used CVD graphene growth substrates are Ni and Cu. Among them, single-layer graphene is easy to form on Cu surface, and multi-layer graphene is easy to form on Ni surface. This patent prefers Cu substrates. Cu corrosion solutions mainly include commercial copper corrosion solution (Transene, CE-100), FeCl ₃ solution and (NH ₄ ) ₂ S ₂ O ₈ , among which (NH ₄ ) ₂ S ₂ O ₈ solution will not cause metal ion Residues help to build some devices such as graphene field effect transistors. Therefore, the preferred etching solution in the present invention is (NH ₄ ) ₂ S ₂ O ₈ solution.

According to the present invention, the solubility of the (NH ₄ ) ₂ S ₂ O ₈ solution is 0.05-2 mol/L, preferably 0.1-1 mol/L.

According to the present invention, the treatment time of the etching solution is 1-5 hours, preferably 2-3 hours. According to the present invention, after the copper foil is fully dissolved, it is transferred to clean water for soaking, and the soaking time is 30-90 minutes, preferably 40-60 minutes.

In the above method, the drying treatment temperature of PMMA/graphene/target substrate in step (3) is 80-180°C.

According to the present invention, the target substrate in step (3) is selected from: SiO ₂ , mica and the like. In addition, the method provided by the present invention can theoretically transfer CVD graphene to any substrate that does not react to organic acids.

According to the present invention, after cleaning the residual ions on the graphene surface in step (3), a clean target substrate is selected to pick up the graphene and PMMA thin layer.

After the graphene is transferred to the target substrate in step (3), the drying treatment can promote the interaction and adsorption of the graphene and the substrate.

According to the present invention, in step (3), the picked up sample is transferred to a heating platform for heating, and the heating temperature is preferably 100-150°C. The heating time is 1-3h, so that the graphene is in full contact with the target substrate.

In the above method, the organic acid solution in step (4) is selected from at least one of acetic acid, trifluoroacetic acid, formic acid and oxalic acid. Among them, acetic acid has the best solubility and degradation degree for PMMA.

According to the present invention, after the sample is cooled to room temperature, it is soaked in the acetic acid solution, and the soaking time is 1-5 hours, preferably 2-4 hours. The soaking temperature is a constant temperature, preferably 40-80°C, moreover 55°C-70°C constant temperature treatment.

In the above method, the annealing atmosphere of graphene in step (5) is 5-15% H ₂ /Ar mixed gas, the annealing time is 1-4h, the annealing temperature is 350-500°C, and the processing pressure is one standard atmospheric pressure.

The present invention also provides the atomically clean graphene surface obtained by the above method.

According to the present invention, the atomically clean graphene has a complete surface structure. Preferably, the surface undulation is less than 0.2nm, which is an atomically flat surface.

According to the present invention, additional defects other than CVD growth are not introduced, and PMMA remains little.

The present invention also provides an application of the atomically clean graphene surface obtained by the above method, characterized in that the graphene can be used to construct electronic devices such as field effect transistors, light-emitting diodes, transparent electrodes, etc., or as an atomically smooth surface substrate to study microscopic processes at the surface and interface.

According to the present invention, the self-assembled structure of 8-octyloxyphthalocyanine copper on the graphene surface can be clearly observed on the graphene substrate provided by the present invention through a scanning tunneling microscope.

The advantage of the present invention is that, using an organic acid solution to dissolve the PMMA coating can greatly improve the residual problem of polymerized PMMA in the conventional acetone treatment method; another advantage of the present invention is that the graphene surface can be removed at the atomic level after annealing. PMMA remains on the graphene surface to obtain an atomically clean graphene surface.

Compared with the prior art, the present invention has the following characteristics:

(1) The organic acid solvent used in the present invention is non-toxic, cheap, and has no residue or adsorption effect on the surface of graphene.

(2) After the annealing treatment, the surface of the graphene is clean at the atomic level, and a large-scale clean surface with no PMMA residue at the micron level can be obtained.

(3) The graphene obtained by the present invention fully reduces PMMA residues, does not introduce additional defects, and greatly retains the initial properties of CVD graphene.

Description of drawings

Fig. 1 is the transfer and processing flowchart of CVD graphene provided by the present invention.

Figure 2 is a scanning electron microscope image of the initial morphology of graphene on the copper foil surface.

Figure 3 is an optical micrograph of Example 1 transferred to SiO ₂ substrate graphene.

Figure 4 is an optical micrograph of comparative example 1 transferred to SiO ₂ substrate graphene.

Fig. 5 is the atomic force microscope image of the graphene transferred to the _SiO2 substrate surface in Example 1.

Figure 6 is an atomic force microscope image of graphene transferred to _SiO2 substrate surface in Comparative Example 1.

7 is a large-scale scanning tunneling microscope image of graphene transferred to the mica substrate surface in Example 2.

Fig. 8 is a high-resolution scanning tunneling microscope atomic image of graphene transferred to the surface of mica substrate in Example 2.

Fig. 9 is a scanning tunneling microscope image of 8-octyloxyphthalocyanine copper molecules dropped on the surface of graphene in Example 3.

Fig. 10 is the Raman spectrum of graphene transferred to the surface of _SiO2 substrate in Example 1, wherein the inserted picture is the initial Raman spectrum of graphene on the surface of copper foil before transfer.

Fig. 11 is a surface scan image obtained by mapping the Raman intensity ratio of the D peak and the G peak of the graphene transferred to the SiO ₂ substrate surface in Example 1.

Wherein the flowchart of Fig. 1 records the following steps: first spin-coat a layer of PMMA solution on the surface of graphene/copper foil, after it solidifies, remove copper foil by (NH ₄ ) ₂ S ₂ O ₈ solution corrosion, graphene/PMMA cleaning Finally, the target substrate is picked up and dried, and then an organic acid solvent is used to dissolve the PMMA layer. Finally, the residual PMMA on the graphene surface is further removed by high-temperature annealing, thereby obtaining an atomically clean graphene surface.

The SEM results in Figure 2 show that the surface of graphene grown by CVD method is uniform and flat, without impurities and obvious damage.

Compare Figure 3 and Figure 4. When acetone is used as the solvent, PMMA (gray flocs in Figure 4) tends to remain on the graphene surface, and a uniform and clean graphene surface can be obtained with organic acids as the solvent.

In Figure 5, the bright bars are the intrinsic folds formed during the growth of graphene, and there are few PMMA residues.

In Figure 6, in addition to the intrinsic growth folds of graphene on the graphene surface, PMMA residues are clearly visible, indicating that the effect of traditional acetone removal of PMMA is not ideal, and organic solvent removal of PMMA is more thorough.

In Fig. 7, the surface of graphene is uniform and free of impurities, and the surface fluctuation is less than 0.2nm, which is an atomically flat surface.

The single packet parameter of the honeycomb high-resolution atomic image in Figure 8 is 0.25nm, which is consistent with the theoretical size of graphene, indicating that the graphene structure is complete.

In Fig. 9, the 8-octyloxyphthalocyanine copper molecule is clearly visible, indicating that the graphene surface is flat and clean, and also shows that the treatment method provided by the present invention can provide an atomically flat graphene substrate, which can be used for the microscopic theory of the surface interface Research.

After the transfer of graphene to the SiO2/Si substrate in Figure 10, the D peak (~1350cm ^- 1) in the Raman spectrogram has no obvious enhancement, and the peak shape is consistent with the graphene signal on the surface of the copper foil before transfer, indicating that the treatment provided by the present invention The method introduces no additional defects beyond graphene growth.

In Figure 11, the area where the ratio of D peak intensity to G peak intensity on the sample surface is less than 0.1 is greater than 90%, indicating that graphene has excellent properties and few defects.

Detailed ways

The present invention is illustrated by specific examples below, but those skilled in the art understand that the present invention is not limited thereto, and any improvements and inventions made on the basis of the present invention are within the protection scope of the present invention.

The experimental methods described in the following examples, unless otherwise specified, are conventional methods; the reagents and materials, unless otherwise specified, can be obtained from commercial sources.

Embodiment 1, CVD graphene is transferred to SiO ₂ substrate surface with acetic acid as organic acid solution

The preparation steps are as follows:

(1) Spin-coat a layer of PMMA solution (960PMMA A4, MicroChem) on the surface of copper foil with graphene (for the specific preparation method, refer to Nanotechnology,.2012, 23, 0957-4484.), and the spin-coating speed is 3000rpm. After the spin coating, the copper foil coated with PMMA was placed on a heating plate at 180°C for heating and curing.

(2) The sample PMMA was floated on the surface of 0.1mol/L (NH ₄ ) ₂ S ₂ O ₈ solution with its face up, and was corroded for 3 hours. After the copper foil is fully dissolved, transfer it to clean water and soak for 60 minutes.

(3) After cleaning the residual ions on the graphene surface, select a clean _SiO2 substrate to pick up the graphene and PMMA thin layer. Transfer the sample to a heating platform and heat at 180 °C for 1 h to make the graphene fully contact with the SiO ₂ substrate.

(4) After the sample is cooled to room temperature, it is soaked in acetic acid solution, and treated at a constant temperature of 55° C. for 1 hour.

(5) Transfer the cleaned graphene/SiO ₂ substrate to a tube furnace for annealing treatment at 350°C under normal pressure for 1 h, and the treatment atmosphere is 5% H ₂ /Ar mixed gas.

Comparative Example 1, using acetone as a solvent to transfer CVD graphene to SiO ₂ substrate surface

The traditional acetone solvent was used to transfer CVD graphene to the _SiO2 substrate surface, except that the sample was transferred to the acetone solution in step 4, all the other operations were the same as in Example 1.

Embodiment 2, CVD graphene is transferred mica substrate surface with acetic acid as organic acid solution

Using an organic acid as a solvent to transfer CVD graphene to the surface of the mica substrate, except that the target substrate was changed from SiO ₂ to mica in step (3), the rest of the operations were the same as in Example 1.

Embodiment 3, using the graphene obtained in Embodiment 2 as a substrate, constructing a self-assembled layer of 8-octyloxyphthalocyanine copper molecules on its surface.

With the graphene obtained in Example 2 as the substrate, a toluene solution with a concentration of about 10 ^-5 mol/L 8-octyloxyphthalocyanine copper was added dropwise on its surface, and 8-octyloxyphthalocyanine copper could be obtained after the solvent evaporated. Self-assembled layers of copper cyanine on graphene surfaces.

Comparing Figure 3 and Figure 4, it can be found that although the conventional solvent acetone can dissolve most of the PMMA coating, the residue of partially overpolymerized PMMA is still very serious, and the use of acetic acid as a solvent can greatly improve the residual problem of PMMA on the graphene surface. In the atomic force microscope scanning image, the PMMA residue on the graphene surface treated with organic acid as solvent is significantly less than that on the graphene surface treated with acetone as solvent, and the corresponding results are shown in Figure 5 and Figure 6.

In addition, for the graphene on the surface of mica in Example 2, a scanning tunneling microscope imaging can be performed on an optional area to obtain a large-scale atomic-level flattened image, and the corresponding results are shown in FIG. 7 .

The high-resolution atomic image shown in Figure 8 also shows that the unit cell parameter of the honeycomb hexagonal structure on the sample surface is 0.25 nm, which is consistent with the theoretical graphene unit cell parameter, further verifying the existence and structural integrity of graphene.

In Fig. 9, the 8-octyloxyphthalocyanine copper molecules in Example 3 are clearly visible, there is no PMMA residue on the surface, and the substrate undulation is within the range of the molecular level. This shows that the transfer method provided by the present invention can obtain an atomically clean graphene surface.

Raman spectrogram was done on the processed sample, the result is shown in Figure 10, wherein the ratio of 2D peak intensity to G peak intensity is about 2, and the D peak intensity is weak, this result is consistent with the CVD graphene before transfer, which proves that, After the transfer operation, the graphene basically preserves the characteristics of the original CVD graphene, and the transfer method provided by the present invention does not produce obvious defects on the graphene. Raman surface scanning was performed on the selected area of the graphene sample surface with the ratio of D peak intensity and G peak intensity. The experimental results are shown in Figure 11, which also shows that the transferred graphene has fewer defects and is of good quality.

Embodiment 4, CVD graphene is transferred to SiO ₂ substrate surface with acetic acid as organic acid solution

The preparation steps are as follows:

(1) Spin-coat a layer of PMMA solution (960PMMA A4, MicroChem) on the surface of the Pt substrate with graphene (for the specific preparation method, refer to Nanotechnology,.2012, 23, 0957-4484.), and the spin-coating speed is 3000rpm. After the spin coating, the PMMA-coated Pt substrate was placed on a heating plate at 160°C for heating and curing.

(2) The sample PMMA was floated on the surface of 0.15mol/L (NH ₄ ) ₂ S ₂ O ₈ solution with its face up, and was corroded for 2.5 hours. After the copper foil is fully dissolved, transfer to clean water and soak for 40 minutes.

(3) After cleaning the residual ions on the graphene surface, select a clean _SiO2 substrate to pick up the graphene and PMMA thin layer. Transfer the sample to a heating platform and heat at 180 °C for 2 h to make the graphene fully contact with the SiO ₂ substrate.

(4) After the sample is cooled to room temperature, it is soaked in acetic acid solution, and treated at a constant temperature of 55° C. for 1 hour.

(5) Transfer the cleaned graphene/SiO ₂ substrate to a tube furnace for annealing treatment at 350°C under normal pressure for 1.5 hours, and the treatment atmosphere is 8% H ₂ /Ar mixed gas.

Claims (10)

1. the transfer method of an Atomically clean CVD Graphene, is characterized in that, selects organic acid to make removal of solvents PMMA coating.

Preferably, described organic acid is selected from least one in acetic acid, trifluoroacetic acid, formic acid, oxalic acid.Preferred acetic acid.

2. according to the transfer method of claim 1, it is characterized in that, described transfer method comprises: the target substrate that will sprawl Graphene/PMMA is soaked in organic acid soln, dissolves PMMA and can obtain the target substrate of sprawling Graphene.

3. according to the transfer method of claim 1-2 any one, it is characterized in that, described method comprises the steps:

(1) the surperficial spin coating one deck PMMA solution of metal base (as Copper Foil or other metal base) of Graphene, curing molding are arranged in growth;

(2) the PMMA layer of sample step (1) obtained is soaked in corroding metal substrate in etchant solution upward, and the PMMA that then will be stained with Graphene is transferred in deionized water and cleans survivor ion;

(3) choose suitable target substrate and pick up Graphene/PMMA layer, drying treatment;

(4) target substrate that will sprawl Graphene/PMMA is soaked in organic acid soln, dissolves PMMA and can obtain the target substrate of sprawling Graphene;

(5) finally will be covered with the substrate of Graphene in H ₂anneal in/Ar gas mixture, the Graphene surface that can obtain Atomically clean.

4. according to the method for claim 1-3 any one, it is characterized in that: in step (1), PMMA solution is coated on CVD Graphene surface by the spin coating mode.Preferably by being heating and curing, processing and increase adhesive attraction between PMMA and Graphene.

Preferably, in length, the spin coating rotating speed of copper foil surface spin coating one deck PMMA solution of Graphene being arranged is 3000rpm.

Preferably, the Copper Foil that spin coating will be coated with PMMA after finishing is placed in hot-plate and is heating and curing.Preferably, the Heating temperature of described hot-plate is 80-200 ℃, preferably 100-180 ℃.

5. according to the method for claim 1-4 any one, it is characterized in that: the etchant solution of step (2) is selected from commercialization copper corrosion liquid (Transene, CE-100), FeCl ₃solution or (NH ₄) ₂s ₂o ₈three kinds.Described growth substrate has Ni and Cu.Its

Preferably, described corrosive fluid is (NH ₄) ₂s ₂o ₈solution.Graphene surface survivor ion is cleaned by deionized water.

Preferably, described (NH ₄) ₂s ₂o ₈the solubility of solution is 0.05-2mol/L, preferably 0.1-1mol/L.

Preferably, the treatment time of described etchant solution is 1-5 hour, preferably 2-3 hour.Preferably, after Copper Foil fully dissolves, be transferred in clear water and soak, soak time is 30-90 minute, preferably 40-60 minute.

6. according to the method for claim 1-5 any one, it is characterized in that: the drying treatment temperature of the PMMA/ Graphene/target substrate in step (3) is 80-180 ℃.Preferably, target substrate is selected from: SiO ₂, mica etc.

Preferably, after cleaning the survivor ion on Graphene surface in step (3), choose the SiO of a slice cleaning ₂substrate picks up Graphene and PMMA thin layer.After in step (3), Graphene is transferred to target substrate, drying treatment can promote interaction and the absorption of Graphene and substrate.

Preferably, in step (3), the sample after picking up is transferred to the warm table heating, described Heating temperature is for being preferably 100-150 ℃.Be 1-3h described heat-up time, makes Graphene and SiO ₂substrate fully contacts.

7. according to the method for claim 1-6 any one, it is characterized in that: PMMA removes by organic acid solvent in step (4), wherein the organic acid solvent choosing to following at least one: acetic acid, trifluoroacetic acid, formic acid, oxalic acid.Preferred acetic acid.

Preferably, after sample is cooled to room temperature, it is soaked in acetic acid solution, described soak time is 1-5 hour, preferably 2-4 hour.Described soaking temperature is constant temperature, and preferably 40-80 ℃, more have 55 ℃ of-70 ℃ of constant temperature to process.

8. according to the method for claim 1-7 any one, it is characterized in that: in step (5), the remaining PMMA on Graphene surface passes through H ₂the annealing of/Ar mixed atmosphere is further removed.The annealing atmosphere of described Graphene is 5-15%H ₂/ Ar gas mixture, annealing time is 1-4h, and annealing temperature is 350-500 ℃, and the pressure of processing is a normal atmosphere.

9. the method by claim 1-8 any one is processed the Atomically clean Graphene surface obtained.

Preferably, described Atomically clean Graphene surface tissue is complete.The preferred surface waviness is less than 0.2nm, is the atomically flating surface.

10. the method for claim 1-8 any one is processed the Atomically clean Graphene surface of acquisition for constructing electron device or the application with the microprocess of research surface and interface as the atomically flating substrate.Be preferred in field-effect transistor, photodiode, transparency electrode.

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