CN108069416B - Ultra-clean graphene and preparation method thereof - Google Patents

Abstract

The invention discloses an ultra-clean graphene and a preparation method thereof. The method for preparing ultra-clean graphene provided by the present invention includes the following steps: after placing foam copper on the copper substrate and abutting it, introducing carbon source gas and hydrogen to carry out chemical vapor deposition, and after the deposition is completed, the copper substrate and the copper substrate are placed on the copper substrate. The ultra-clean graphene is obtained from the contact side of the copper foam. The preparation method is simple, can be produced on a large scale, has a continuous clean area of sub-centimeter level, and is suitable for applications in electronics, optics and the like.

Description

Ultra-clean graphene and preparation method thereof

technical field

The invention belongs to the field of materials, in particular to an ultra-clean graphene and a preparation method thereof.

Background technique

Graphene is a two-dimensional thin film material composed of a single layer of carbon atoms arranged in a hexagonally symmetrical honeycomb structure. Due to the excellent properties of graphene in electrical, optical, thermal and mechanical aspects, it has attracted extensive attention in the fields of physics, chemistry, biology and materials since its discovery. For example, single-layer graphene has a Dirac tapered band structure, and at the Fermi level, energy and momentum have a linear dispersion relationship. This unique energy band structure determines the extremely high carrier mobility of graphene, so graphene has gradually become a favorable substitute for traditional silicon-based electronic materials. Since graphene is a single-atom-layer thin film material, its absorbance is only 2.3%. Combined with its excellent conductivity and flexibility, graphene becomes a possible material for the next generation of flexible transparent conductive materials. At the same time, because of graphene's perfect hexagonal symmetrical structure, it has extremely high electron transmittance, extremely high electrical and thermal conductivities, so as a transmission carrier, graphene has extremely high imaging resolution and radiation resistance. , has been widely used.

At present, the methods for preparing graphene mainly include the method of mechanical exfoliation, the reduction of graphene oxide, and the method of chemical vapor deposition. Among them, the size of continuous graphene domains obtained by mechanical exfoliation methods is usually in the micrometer level, and is not suitable for large-scale preparation. The graphene prepared by the redox method has many defects caused by the chemical reduction reaction process, and the oxide groups are difficult to completely reduce, resulting in serious doping, which seriously limits its application in the field of electronics. The chemical vapor deposition method is suitable for the large-scale preparation of graphene thin film materials, but there are a large number of amorphous carbon pollutants on the surface of the prepared graphene, and the existence of these pollutants leads to a significant decrease in the light transmittance and conductivity of graphene. At the same time, the continuous clean size under the transmission electron microscope is only at the nanometer level, which severely limits the imaging and observation range of graphene as a transmission carrier substrate. Therefore, using the chemical vapor deposition method, how to prepare clean and contaminant-free ultra-clean graphene in a large area becomes particularly important.

SUMMARY OF THE INVENTION

The purpose of this invention is to provide a kind of ultra-clean graphene and preparation method thereof.

The method for preparing ultra-clean graphene provided by the present invention comprises the following steps:

After the copper foam is placed on the copper base and abutted, carbon source gas and hydrogen gas are introduced to carry out chemical vapor deposition, and the ultra-clean graphene is obtained on the side of the copper base in contact with the foamed copper after the deposition is completed.

In the above method, the pore size of the foamed copper is 0.1-2.0 mm;

The distance between the foamed copper and the copper base is not more than 0.1mm;

The copper substrate is a single crystal copper sheet, a polycrystalline copper sheet or a copper foil; the copper substrate can catalyze the cracking of the carbon source gas.

The thickness of the copper substrate is 2 μm-100 μm.

The carbon source gas is methane, ethane or ethylene; the purity of the carbon source gas is not less than 99.999%.

In the chemical vapor deposition step, the flow rate of the carbon source gas is 0.05sccm-7sccm (flow unit standard-state cubic centimeter per minute, standard condition milliliter per minute), specifically 0.36sccm, 1sccm or 7sccm;

The flow rate of the hydrogen gas is 10-1000 sccm, specifically 11 sccm or 500 sccm; the ratio of the hydrogen gas to the carbon source gas determines the graphene domain region, and the domain region size is in the order of micrometers to millimeters. In addition, in the process of chemical vapor deposition, hydrogen can dilute the precursor carbon source, and the hydrogen-rich environment plays a role in activating the carbon-hydrogen bond on the microscopic chemical kinetics and regulating the growth of the monolayer.

The deposition temperature is 980-1040°C, specifically 1020°C;

The deposition time is not less than 30s, specifically 30s, 300s or 24h;

The chemical vapor deposition step is performed in an inert atmosphere (eg, an argon atmosphere);

The flow rate of the inert gas is 100sccm-200sccm;

The deposition pressure is 20Pa-700Pa, specifically 48Pa, 50Pa or 500Pa.

The method further includes the step of annealing the system prior to the chemical vapor deposition step.

Specifically, the annealing is performed in a reducing atmosphere or a hydrogen atmosphere;

The flow rate of the reducing gas is 100 sccm-300 sccm, specifically 100 sccm.

The pressure of the system is 30Pa-300Pa, specifically 100Pa;

The annealing temperature is 900-1100°C, specifically 1020°C or 1040°C;

The annealing time is 30min-120min, specifically 30min or 50min.

The crystal domains of the annealed copper substrate can reach hundreds of microns.

The method further includes the step of: cooling the system after the chemical vapor deposition step.

Specifically, in the cooling step, the cooling rate is greater than 80°C/min, such as 90°C/min.

The method further includes the following steps: before the annealing step, surface cleaning the copper substrate and the copper foam;

The surface cleaning step is specifically to perform surface cleaning of the copper substrate and the foamed copper sequentially with dilute hydrochloric acid and water with a concentration of 5% by mass.

The ultra-clean graphene is specifically ultra-clean single-crystal graphene or ultra-clean polycrystalline graphene film.

In addition, the ultra-clean graphene prepared according to the above method also belongs to the protection scope of the present invention; wherein, the ultra-clean graphene is specifically an ultra-clean single-crystal graphene or an ultra-clean polycrystalline graphene film.

Compared with the prior art, the beneficial effects of the present invention are: (1) through the introduction of copper foam, ultra-clean graphene with a continuous area in the micron level can be obtained, effectively reducing the amorphous adsorbate introduced in the growth process; (2) The raw materials of the present invention are safe, cheap and easy to obtain, the preparation method is simple and effective, and due to the excellent structure of the graphene, efficient and non-destructive transfer from a copper substrate to a transmission substrate can be realized, and it can be used as a transmission carrier mesh. (3) Sub-centimeter-level single-crystal graphene (ie, a single domain) can be spliced into a single-layer graphene film after further growth. At this time, the sample area of the single-layer graphene film is only related to the size of the copper foil, so that large-area preparation can be achieved. , which can be extended to mass production.

Description of drawings

Figure 1 is a schematic diagram of the structure of a reaction device for growing ultra-clean graphene.

Figure 2 shows the stacking structure of copper foam and copper foil for growing ultra-clean graphene, and the structure of copper foam itself.

Figure 3 is a comparison of the cleanliness and absorbance of samples obtained after ultra-clean graphene and ordinary graphene are transferred to a quartz plate with the assistance of PMMA.

FIG. 4 is a transmission electron microscope photograph with a continuous clean area of 1 micron in Example 1 and Example 2, respectively.

5 is a large-area high-resolution photo of graphene prepared in Example 1 under a high-resolution transmission electron microscope of ultra-clean graphene.

FIG. 6 is the Raman spectroscopy characterization of the suspended graphene transferred to the porous substrate without glue in Example 1 and Example 2, respectively.

Figure 7 is a transmission microscope photograph of conventionally grown graphene without copper foam. The continuous area is only a few to tens of nanometers.

Figure 8 is an atomic force microscope characterization of graphene on a copper substrate normally grown without copper foam.

Detailed ways

The method of the present invention will be described below through specific embodiments, but the present invention is not limited thereto, and any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention, etc., shall be included in the scope of the present invention. within the scope of protection.

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

The schematic diagram of the structure of the reaction device for single-crystal graphene used in the following examples and the schematic diagram of the steps for growing large single-crystal graphene are shown in Figures 1 and 2, respectively. No. 1 in Figure 2 is a copper foil substrate, and No. 2 is copper foam.

Example 1, preparation of ultra-clean graphene

(1) Use dilute hydrochloric acid and deionized water with a mass fraction of 5% to sequentially clean the copper foil (produced by Alfa Aesar, with a purity of 99.8% and a thickness of 25 μm), and place the copper foil and foam copper in close contact with a magnetic control device. In the casing, the casing is then placed in a tube furnace, under a hydrogen atmosphere with a flow rate of 100sccm, the system pressure is 100Pa, the temperature of the furnace body is raised to 1020°C, and maintained for 30min;

(2) keeping the furnace body temperature at 1020 ℃, changing the hydrogen gas with a flow rate of 11sccm, and a methane gas with a flow rate of 7sccm, the system pressure is 50Pa, and kept for 30s;

(3) Use a magnet to pull out the sleeve loaded with copper foil from the high temperature area, quickly reduce the temperature of the sample to room temperature, and the cooling rate is 90 ° C/min, and the growth of the sample is terminated;

(4) Take out the grown copper foil sample, and remove the graphene on the backside of the copper substrate by plasma bombardment from the graphene on the backside of the copper substrate in contact with the foamed copper. Using the traditional PMMA-assisted transfer method, it was transferred to substrates such as silicon wafers and quartz wafers for subsequent characterization. Replacing the traditional polymer-assisted graphene with a transmission carrier mesh to achieve glue-free transfer, and prepare a transmission sample for subsequent characterization.

Figure 3 shows the comparison of the cleanliness and the absorbance of the samples obtained after the ultra-clean graphene and ordinary graphene were transferred to the quartz plate with the assistance of PMMA. It can be seen that the absorbance of the clean samples is significantly lower than that of the ordinary samples, indicating that the ultra-clean samples Fewer contaminants remain on the graphene surface.

Fig. 4 is a transmission electron microscope photograph with a continuous clean area of 1 micron in Example 1, respectively. Under the transmission electron microscope, the darker color is the amorphous carbon adsorbate caused by the growth, and the ultra-clean graphene film does not have this in the continuous range of one micron. The distribution of adsorbate-like species, clean graphene showed uniform contrast, indicating no pollutant adsorption. The continuous clean area reaches the micron scale.

5 is a high-resolution photo of graphene under a high-resolution transmission electron microscope of the ultra-clean graphene prepared in Example 1. The high-resolution imaging of graphene can clearly see the hexagonal symmetrical skeleton structure of graphene, indicating that graphene has no adsorption of pollutants, and the product is a polycrystalline graphene film.

Figure 6 is a Raman spectroscopy characterization of ultra-clean graphene transferred onto a target mesh substrate. The results of Raman analysis and characterization can be seen from Figure 6: the graphene prepared by this method has no D peak, indicating high quality and no doping, and the integral light ratio of 2D peak and G peak is much greater than 2, indicating that it is a perfect monolayer And there is no adsorbate and substrate interference.

Example 2, preparation of ultra-clean graphene

(1) Using a phosphoric acid and ethylene glycol solution with a mass ratio of 3:1 as the electrolyte, a copper foil (produced by Alfa Aesar, with a purity of 99.8% and a thickness of 25 μm) was connected to the positive electrode, and polished at a DC current of 0.5A for 30 minutes. The copper foil and foamed copper are placed in close contact with a sleeve with a magnetic control device, and then the sleeve is placed in a tube furnace. The pressure is 300Pa for 50min;

(2) keeping the temperature of the furnace body at 1040 ℃, changing the hydrogen gas flow rate of 500sccm, and the flow rate of 0.36sccm of methane gas, the system pressure is 500Pa, and kept for 24h;

(3) Use a magnet to drag the sleeve loaded with copper foil out of the high temperature area, and quickly reduce the temperature of the sample to room temperature to end the growth of the sample;

(4) Take out the grown copper foil sample, and remove the graphene on the backside of the copper substrate by plasma bombardment from the graphene on the backside of the copper substrate in contact with the foamed copper. Using the traditional PMMA-assisted transfer method, it was transferred to substrates such as silicon wafers and quartz wafers for subsequent characterization. Replacing the traditional polymer-assisted graphene with a transmission carrier mesh to achieve glue-free transfer, and prepare a transmission sample for subsequent characterization.

Figure 3 shows the comparison of the cleanliness and the absorbance of the samples obtained after the ultra-clean graphene and ordinary graphene were transferred to the quartz plate with the assistance of PMMA. It can be seen that the absorbance of the clean samples is significantly lower than that of the ordinary samples, indicating that the ultra-clean samples Fewer contaminants remain on the graphene surface.

Fig. 4 is a transmission electron microscope photograph with a continuous clean area of 1 micron in Example 1, respectively. Under the transmission electron microscope, the darker color is the amorphous carbon adsorbate caused by the growth, and the ultra-clean graphene film does not have this in the continuous range of one micron. The distribution of adsorbate-like species, clean graphene showed uniform contrast, indicating no pollutant adsorption. The continuous clean area reaches the micron scale.

5 is a high-resolution photo of graphene under a high-resolution transmission electron microscope of the ultra-clean graphene prepared in Example 1. The high-resolution imaging of graphene can clearly see the hexagonal symmetrical skeleton structure of graphene, indicating that graphene has no pollutant adsorption, and the product is an isolated graphene single crystal.

Figure 6 is a Raman spectroscopy characterization of ultra-clean graphene transferred onto a target mesh substrate. The results of Raman analysis and characterization can be seen from Figure 6: the graphene prepared by this method has no D peak, indicating high quality and no doping, and the integral light ratio of 2D peak and G peak is much greater than 2, indicating that it is a perfect monolayer And there is no adsorbate and substrate interference.

Example 3, preparation of ultra-clean graphene

(1) Use dilute hydrochloric acid and deionized water with a mass fraction of 5% to sequentially clean the copper foil (produced by Alfa Aesar, with a purity of 99.8% and a thickness of 25 μm), and place the copper foil and foam copper in close contact with a magnetic control device. In the casing, the casing is then placed in a tube furnace, under a hydrogen atmosphere with a flow rate of 100sccm, the system pressure is 100Pa, the temperature of the furnace body is raised to 1020°C, and maintained for 30min;

(2) keeping the furnace body temperature at 1020 ℃, changing the hydrogen gas flow rate of 11sccm, and the flow rate of 1sccm methane gas, the system pressure is 48Pa, and kept for 300s;

(3) Use a magnet to pull out the sleeve loaded with copper foil from the high temperature area, quickly reduce the temperature of the sample to room temperature, and the cooling rate is 90 ° C/min, and the growth of the sample is terminated;

(4) Take out the grown copper foil sample, and remove the graphene on the backside of the copper substrate by plasma bombardment from the graphene on the backside of the copper substrate in contact with the foamed copper. Using the traditional PMMA-assisted transfer method, it was transferred to substrates such as silicon wafers and quartz wafers for subsequent characterization. Replacing the traditional polymer-assisted graphene with a transmission carrier mesh to achieve glue-free transfer, and prepare a transmission sample for subsequent characterization. There is no substantial difference between the obtained results and Example 1, and will not be repeated here.

Comparative Example 1. Graphene is prepared on copper substrates grown without foamed copper

The preparation method is the same as that shown in Example 1, the only difference is that no foamed copper is used, and the graphene growth on a separate copper foil is shown in Figure 7. It can be seen from Figure 7 that the foamed copper structure is not used, and the supply of copper vapor is insufficient, and a large amount of copper vapor is not used. Carbon clusters rich in sp ³ bonds are generated in the air atmosphere and then deposited on the graphene surface, contaminating the graphene. Under the transmission electron microscope, these pollutants are aggregated and distributed, so that the continuous area of clean graphene is only a few tens of nanometers. At the same time, the atomic force microscopy analysis shown in Figure 8 also confirmed that such adsorbates exist on the surface of graphene and are directly generated during the growth process.

Claims (10)

1. A method for preparing ultra-clean graphene comprises the following steps:

placing foamy copper above a copper substrate and attaching the foamy copper, introducing carbon source gas and hydrogen to carry out chemical vapor deposition, and obtaining the ultra-clean graphene on one surface of the copper substrate, which is in contact with the foamy copper, after the deposition is finished;

the carbon source gas is methane, ethane or ethylene;

in the chemical vapor deposition step, the flow rate of the carbon source gas is 0.05sccm-7 sccm;

the flow rate of the hydrogen is 10-1000 sccm;

the deposition temperature is 980-1040 ℃;

the deposition time is not less than 30 s;

the chemical vapor deposition step is carried out in an argon atmosphere;

the flow rate of the argon gas is 100sccm-200 sccm;

the pressure of the deposition is 20Pa to 700 Pa.

2. The method of claim 1, wherein: the aperture of the foam copper is 0.1-2.0 mm;

the distance between the foam copper and the copper substrate is not more than 0.1 mm;

the copper substrate is a single crystal copper sheet, a polycrystalline copper sheet or a copper foil;

the thickness of the copper substrate is 2-100 mu m.

3. The method of claim 1, wherein: the method further comprises the steps of: annealing the copper foam and the copper substrate prior to the chemical vapor deposition step.

4. The method of claim 3, wherein: the annealing is carried out in a hydrogen atmosphere;

the flow rate of the hydrogen is 100sccm-300 sccm;

the pressure of the system is 30Pa-300 Pa;

the annealing temperature is 900-1100 ℃;

the annealing time is 30min-120 min.

5. The method of claim 1, wherein: the method further comprises the steps of: after the chemical vapor deposition step, the copper foam and the copper substrate are cooled.

6. The method of claim 5, wherein: in the cooling step, the cooling rate is more than 80 ℃/min.

7. The method of claim 3, wherein: the method further comprises the steps of: before the annealing step, carrying out surface cleaning on the copper substrate and the foam copper;

and the surface cleaning step is to sequentially use dilute hydrochloric acid with the mass percentage concentration of 5% and water to clean the surface of the copper substrate and the foam copper.

8. The method according to any one of claims 1-7, wherein: the ultra-clean graphene is a single crystal graphene or polycrystalline graphene film.

9. Ultra-clean graphene prepared by the method of any one of claims 1 to 8.

10. The ultra clean graphene according to claim 9, wherein: the ultra-clean graphene is a single crystal graphene or polycrystalline graphene film.

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