CN113622024A

CN113622024A - Single crystal graphene and preparation method thereof

Info

Publication number: CN113622024A
Application number: CN202010380747.XA
Authority: CN
Inventors: 刘忠范; 刘海洋; 孙禄钊; 李杨立志; 王悦晨
Original assignee: Peking University; Beijing Graphene Institute BGI
Current assignee: Peking University; Beijing Graphene Institute BGI
Priority date: 2020-05-08
Filing date: 2020-05-08
Publication date: 2021-11-09
Anticipated expiration: 2040-05-08
Also published as: CN113622024B

Abstract

The invention provides a preparation method of single crystal graphene, which comprises the steps of preparing the single crystal graphene through a chemical vapor deposition process; wherein, the chemical vapor deposition process comprises the following steps: s1: forming a graphene core on a substrate in a reaction system not containing a reducing gas; and S2: forming the single crystal graphene on the basis of the graphene nuclei under the action of a reducing gas. According to the preparation method of the single crystal graphene, provided by the embodiment of the invention, reducing gas is not used in the graphene nucleation stage, so that the orientation of the graphene nucleus is completely controlled by the single crystal substrate, and the consistent orientation is realized.

Description

Single crystal graphene and preparation method thereof

Technical Field

The invention relates to graphene, in particular to single crystal graphene and a preparation method thereof.

Background

Graphene is a carbon atom sp-bonded structure²The hybridized monolayer or few-layer two-dimensional crystal material has excellent electrical, optical and mechanical properties. Has been particularly appreciated by the scientific and industrial community since its discovery. In the graphene growth process, if lattice orientations of different domains are not perfectly consistent but have a certain angle deviation, linear defects, namely, what we call grain boundaries, are generated after splicing. The presence of grain boundaries can adversely affect the properties of the graphene, including reduced electrical mobility, reduced mechanical strength, reduced thermal conductivity, and the like. Therefore, the preparation of the large-size and grain boundary-free single crystal graphene has important significance for improving the quality of the graphene.

Chemical Vapor Deposition (CVD) is currently the most potential method of achieving high quality graphene among the various fabrication methods. At present, the CVD method for growing single crystal graphene mainly includes the following two methods: 1) the nucleation density of the graphene is reduced as much as possible, the nucleation speed of the graphene is reduced, a single crystal seed is controlled, and the single crystal seed gradually grows up to obtain large-size single crystal graphene; 2) by using the single crystal substrate, the graphene seed crystals are induced to adopt consistent orientation, and then the grown and seamless splicing is carried out, so that the large-size single crystal graphene is obtained.

At present, in the growth methods developed by adopting the growth strategy of the graphene, the first method is often slow in growth speed, and the efficiency of the first method limits the amplification of the graphene preparation process; in the second method, although the speed of growing and preparing graphene is greatly improved, the condition that a small part of domain regions are not completely oriented uniformly so as to introduce part of grain boundaries occurs, and the performance and quality of graphene are reduced. Therefore, it is meaningful to further search for a preparation technology method of single crystal graphene and find a growth method of the single crystal graphene which is completely aligned and perfectly spliced.

Disclosure of Invention

A main object of the present invention is to provide a method for preparing single crystal graphene, comprising preparing the single crystal graphene by a chemical vapor deposition process; wherein, the chemical vapor deposition process comprises the following steps:

s1: forming a graphene core on a substrate in a reaction system not containing a reducing gas; and

s2: forming the single crystal graphene on the basis of the graphene nuclei under the action of a reducing gas.

The embodiment of the invention also provides single crystal graphene prepared by the method.

According to the preparation method of the single crystal graphene, provided by the embodiment of the invention, reducing gas is not used in the graphene nucleation stage, so that the orientation of the graphene nucleus is completely controlled by the single crystal substrate, and the consistent orientation is realized.

Drawings

Fig. 1 is a schematic flow chart of a method for preparing single crystal graphene according to an embodiment of the present invention;

fig. 2 is an optical photograph of single-crystal graphene prepared in example 1 of the present invention;

fig. 3 is an optical photograph of single-crystal graphene prepared in example 2 of the present invention;

fig. 4 is an optical photograph of polycrystalline graphene prepared in comparative example 1 of the present invention;

fig. 5a is a selected area electron diffraction image of single crystal graphene prepared in example 1 of the present invention;

FIG. 5b is a diffraction peak intensity curve extracted from FIG. 5 a;

fig. 6a is a histogram of statistical distribution of orientation angles of different domains of single crystal graphene prepared in example 1 of the present invention;

fig. 6b is a representative selected area electron diffraction image of different domains of the single crystalline graphene prepared in example 1 of the present invention.

Detailed Description

Exemplary embodiments that embody features and advantages of the invention are described in detail below in the specification. It is to be understood that the invention is capable of other embodiments and that various changes in form and details may be made therein without departing from the scope of the invention and the description and drawings are to be regarded as illustrative in nature and not as restrictive.

Referring to fig. 1, an embodiment of the present invention provides a method for preparing single crystal graphene, including preparing single crystal graphene through a chemical vapor deposition process; the chemical vapor deposition process comprises the following steps:

s1: forming uniformly oriented graphene nuclei on a substrate in a system not containing a reducing gas; and

s2: and continuing to grow under the action of reducing gas to form the single crystal graphene.

In the present invention, "single-crystal graphene" refers to a single layer of graphene having no grain boundary.

In the present invention, in step S1, nucleation is performed on the substrate by chemical vapor deposition; wherein, the term "nucleation" refers to the beginning of the formation of small islands of graphene by the cracked carbon species after the introduction of the carbon source.

In one embodiment, the reducing gas is hydrogen.

In one embodiment, hydrogen is introduced into the system of step S2 to continue growing graphene, and the graphene cores formed in step S1 are seamlessly spliced to form single-crystal graphene.

In one embodiment, the gases of step S1 include a first carbon source gas and a first auxiliary gas; the gases of the system of step S2 include a second carbon source gas and a reducing gas.

In one embodiment, the gases of the system of step S2 include a second carbon source gas, a reducing gas, and a second auxiliary gas.

In one embodiment, the first auxiliary gas and the second auxiliary gas may both be argon gas, the reducing gas is hydrogen gas, and the first carbon source gas and the second carbon source gas are respectively and independently selected from one or more of methane, ethane, ethylene, acetylene, ethanol, and propane.

In one embodiment, the pressure in the system of step S1 is the same as the pressure in the system of step S2.

In one embodiment, the partial pressure of the first carbon source gas in the step S1 system is the same as the partial pressure of the second carbon source gas in the step S2 system.

In one embodiment, the sum of the partial pressure of the first auxiliary gas in the step S1 system and the partial pressure of the second auxiliary gas and the reducing gas in the step S2 system is the same.

In one embodiment, in the step S1, a partial pressure ratio of the first auxiliary gas to the first carbon source gas is 50 to 5000, such as 100, 200, 500, 800, 1000, 2000, 2500, 3000, 4000, etc.

In one embodiment, the gases of the step S2 include a second carbon source gas and a reducing gas, and a ratio of a partial pressure of the reducing gas to a partial pressure of the second carbon source gas is 50 to 5000, for example, 100, 200, 500, 800, 1000, 2000, 2500, 3000, 4000, etc.

In one embodiment, the gas of the step S2 includes a second carbon source gas, a reducing gas and a second auxiliary gas, and a ratio of a sum of partial pressures of the second auxiliary gas and the reducing gas to a partial pressure of the second carbon source gas is 50 to 5000, for example, 100, 200, 500, 800, 1000, 2000, 2500, 3000, 4000, etc.

In one embodiment, in the step S2, a ratio of a partial pressure of the reducing gas to a partial pressure of the second carbon source gas is 100 to 2000; wherein the partial pressure of the first carbon source gas in the system of step S1 is the same as the partial pressure of the second carbon source gas in the system of step S2, and the sum of the partial pressure of the first auxiliary gas in the system of step S1 and the partial pressure of the second auxiliary gas and the reducing gas in the system of step S2 is the same.

In one embodiment, the duration of step S1 or the growth time of graphene in step S1 is 1 second to 2 minutes, for example, 5 seconds, 10 seconds, 20 seconds, 30 seconds, 45 seconds, 1 minute, 1.5 minutes, and the like.

In one embodiment, the duration of step S2 or the growth time of graphene in step S2 is 1 minute to 200 minutes, for example, 5 minutes, 10 minutes, 20 minutes, 30 minutes, 50 minutes, 60 minutes, 90 minutes, 100 minutes, 120 minutes, 150 minutes, 180 minutes, and the like.

In one embodiment, the substrate for growing graphene may be one or more of a silica-silicon substrate, glass, plastic, mica, copper foil, carbon film copper mesh, copper nickel alloy foil.

In one embodiment, the substrate for growing graphene may be a copper foil, and the thickness of the copper foil may be 20 μm to 100 μm.

In the method according to an embodiment of the present invention, step S1 is a graphene nucleation stage, and the nucleation process is completely controlled by substrate induction without using a reducing gas, such as hydrogen, so that the nucleation orientations of the graphene are completely consistent, and graphene nuclei with consistent orientations can be obtained.

In the method according to an embodiment of the present invention, a reducing gas, such as hydrogen, is introduced in step S2, so that the cracking of the carbon source is catalyzed by the hydrogen, which effectively accelerates the growth of graphene without changing the orientation of the existing graphene core.

In one embodiment, by keeping the pressure of the system in the step S1 the same as the pressure of the system in the step S2 and the partial pressure of the first carbon source gas the same as the partial pressure of the second carbon source gas, new graphene nuclei are not formed in the step S2, so that the single-crystal graphene can be simply and effectively prepared by splicing in the same orientation, and the production process of the graphene film can be conveniently amplified.

In an embodiment, the growth speed of the graphene in the step S2 can be increased by changing the growth conditions in the step S2, and the edge morphology of the graphene can be regulated, so that the occurrence of grain boundaries is ensured as far as possible in the graphene core splicing process, and the single crystal graphene can be formed quickly and efficiently.

In one embodiment, the changing the growth conditions of step S2 includes: changing the growth temperature, changing the type of carbon source gas, changing the type of reducing gas, changing the flow of carbon source gas, changing the flow of assist gas, changing the partial pressure of carbon source gas, changing the partial pressure of assist gas, changing the partial pressure of reducing gas, introducing one or more of the other gas groups.

In one embodiment, the growth temperature of step S2 may be 1000-1040 ℃.

In one embodiment, the other gas introduced in step S2 is a gas that does not affect graphene formation, does not generate impurities, and does not react with the reaction gas and graphene, such as carbon dioxide, water vapor, and the like.

In one embodiment, the flow parameter of hydrogen can be adjusted to adjust the surface morphology of graphene.

In one embodiment, the partial pressure of the carbon source gas, the reducing gas and the auxiliary gas may be controlled by controlling the flow rates of the carbon source gas, the reducing gas and the auxiliary gas, which may be selected according to different CVD systems, for example, the flow rates of the reducing gas and the carbon source gas may be determined according to the size of the reaction chamber and the pumping speed of the vacuum pump.

In one embodiment, step S1 and step S2 are performed in the same reaction chamber.

In one embodiment, when the volume of the reaction chamber is small or the pumping speed of the vacuum pump is small (such as a small tube furnace), the flow rate of the first auxiliary gas may be controlled to be 500sccm to 2000sccm, the flow rate of the first carbon source gas may be controlled to be 0.2sccm to 1sccm, the partial pressure of the first auxiliary gas may be 500Pa to 2000Pa, and the partial pressure of the second carbon source gas may be 0.2Pa to 1Pa in step S1; in step S2, the flow rate of the reducing gas is controlled to be 40sccm to 2000sccm, the flow rate of the second auxiliary gas is controlled to be 500sccm to 2000sccm, the flow rate of the second carbon source gas is controlled to be 0.2sccm to 1sccm, the partial pressure of the reducing gas is controlled to be 40Pa to 2000Pa, the partial pressure of the second auxiliary gas is controlled to be 500Pa to 2000Pa, and the partial pressure of the second carbon source gas is controlled to be 0.2Pa to 1 Pa.

In one embodiment, when the volume of the reaction chamber is medium or the pumping speed of the vacuum pump is medium (such as a medium tube furnace), the flow rate of the first auxiliary gas may be controlled to be 1000sccm to 4000sccm and the flow rate of the first carbon source gas may be controlled to be 0.4sccm to 2sccm in step S1; in step S2, the flow rate of the reducing gas is controlled to be 80sccm to 4000sccm, the flow rate of the second auxiliary gas is controlled to be 1000sccm to 4000sccm, and the flow rate of the second carbon source gas is controlled to be 0.4sccm to 2 sccm.

In one embodiment, when the volume of the reaction chamber is larger or the pumping speed of the vacuum pump is larger (such as a production-type device or a pilot plant), the flow rate of the first auxiliary gas may be controlled to be 2000sccm to 8000sccm and the flow rate of the first carbon source gas may be controlled to be 0.8sccm to 4sccm in step S1; in step S2, the flow rate of the reducing gas is controlled to be 2000sccm to 25000sccm, the flow rate of the second auxiliary gas is controlled to be 2000sccm to 25000sccm, and the flow rate of the second carbon source gas is controlled to be 2sccm to 20 sccm.

In one embodiment, the pumping speed can be controlled by arranging a throttle valve at the front section of the vacuum pump, so that the large equipment and the large pumping speed can be converted into small equipment and small pumping speed.

In one embodiment, when the first carbon source gas and/or the second carbon source gas is selected from two carbon source gases of ethane, ethylene, acetylene, ethanol, etc., the flow rate thereof may be selected to be one half of the flow rate of the methane gas containing one carbon according to specific situations; for the same reason, when the first carbon source gas and/or the second carbon source gas is a carbon source gas having three carbons such as propane, the flow rate thereof may be selected to be one third of the flow rate of the methane gas having one carbon, depending on the case.

In one embodiment, an auxiliary step may be included to facilitate graphene formation, such as subjecting the substrate to a temperature-raising, annealing step to convert the polycrystalline substrate into a single-crystal substrate before performing the graphene nucleation reaction (before step S1).

In an embodiment, after the single crystal graphene is formed, the temperature of the single crystal graphene may be reduced, that is, the temperature of the prepared single crystal graphene is reduced from the growth temperature to room temperature.

In one embodiment, the cooling mode of the cooling process may be natural cooling or rapid cooling.

In one embodiment, the cooling rate is greater than 80 ℃/min during the rapid cooling treatment.

In one embodiment, the rapid cooling process can be performed by various conventional methods, such as pulling the material boat out of the constant temperature zone by a transmission device such as a transmission rod or a magnet, or moving the furnace body to separate the position of the substrate from the heating zone. Before the temperature is reduced to 400 ℃, the mixed gas of the reducing gas and the second carbon source gas in the step S2 is always introduced to ensure that the graphene on the substrate is not oxidized and is not etched by the reducing gas; after the temperature is reduced to below 400 ℃, the second carbon source can be cut off, but a certain amount of reducing gas is still required to be introduced as protective gas before the temperature is reduced to room temperature.

In one embodiment, after the temperature reduction treatment, in order to characterize the obtained single crystal graphene, a single crystal graphene sample deposited on, for example, a copper foil substrate may be transferred to another target substrate according to various conventional methods.

In one embodiment, the graphene formed on the copper foil substrate may be transferred onto a target substrate by:

spinning and coating a layer of polymethyl methacrylate (PMMA) film on the surface of the graphene deposited on the surface of the copper foil;

baking and plasma bombarding the other surface of the copper foil which is not covered with the PMMA film;

placing the copper foil with the graphene into a copper etching agent for copper etching, and washing with water to obtain large single crystal graphene attached to the polymethyl methacrylate film;

transferring the polymethyl methacrylate film/large single crystal graphene to the surface of a target substrate, putting the target substrate on acetone steam for fumigation or putting the target substrate in hot acetone solution, and removing the polymethyl methacrylate film to obtain the single crystal graphene attached to the surface of the target substrate.

In one embodiment, the polymethyl methacrylate solution is spin-coated on the surface of the graphene, and the film is formed by solvent evaporation.

In one embodiment, the solvent of the polymethylmethacrylate solution may be ethyl lactate, such as chemically pure ethyl lactate; the solute polymethylmethacrylate may be a commercially available solid particle whose weight average molecular weight may be, for example, 996K.

In one embodiment, the mass fraction of the polymethylmethacrylate solution may be 3% to 8%.

In one embodiment, the spin coating may be performed at 2000rpm to 4000rpm for 30s to 60 s.

In one embodiment, the baking temperature may be 150-170 ℃, such as 155 ℃, 160 ℃, 165 ℃, etc.; the baking time can be 1-5 min.

In one embodiment, the plasma bombardment may be performed in a plasma machine, and the power of the plasma machine may be 60-90W, such as 65W, 70W, 75W, 80W, 85W, etc.; the bombardment time of the plasma bombardment can be 3-5 min.

In one embodiment, the copper etchant may be an aqueous solution of ferric trichloride and/or persulfate, wherein the molar concentration of ferric trichloride or persulfate in the aqueous solution of ferric trichloride or persulfate is not less than 0.5 moL/L. Among them, the persulfate may be sodium persulfate.

In one embodiment, the etching time of the copper etching is 5-30 min, such as 10min, 15min, 20min, 25min, and the like.

In one embodiment, the temperature of the acetone vapor may be 57 to 62 ℃, for example, 58 ℃, 59 ℃, 60 ℃, 61 ℃, etc.

An embodiment of the invention provides single crystal graphene prepared by the method.

According to the method provided by the embodiment of the invention, in the step S1, reducing gas is not introduced firstly, so that the nucleation process is completely controlled by the induction of the single crystal substrate, and thus the nucleation orientation of graphene is completely consistent; then, step S2 is performed, and a reducing gas, such as hydrogen, is introduced again to accelerate the growth of graphene, so that the graphene cores in the same orientation grow and are seamlessly spliced to form a single-crystal graphene film.

The single crystal graphene and the preparation method thereof according to an embodiment of the present invention are further described below with reference to the accompanying drawings and specific examples. Wherein, the raw materials are all obtained from the market.

Example 1

The preparation of the single crystal graphene comprises a pretreatment process of a substrate and a growth process of the graphene, wherein the two processes are carried out in the same reactor, namely, the substrate does not move in the preparation process.

Pretreatment of substrates

1) Copper foil (produced by Alfa Aesar Co., Ltd., purity)99.8% by thickness of 25 μm) was electrochemically polished in an ethylene phosphate solution at a phosphoric acid mass concentration of 85% by volume, a phosphoric acid to ethylene glycol volume ratio of 3:1, and a polishing current density of about 100A/m was maintained²Polishing time is about 30 min; the polished copper foil is washed clean with deionized water and dried with nitrogen.

2) The polished copper foil is placed in a sleeve with magnetic control, the sleeve is placed in a large quartz tube of a tube furnace, and the temperature is raised to the annealing temperature of 1020 ℃ in argon of 500 sccm.

3) And after the temperature of the system rises to 1020 ℃, stopping introducing argon, introducing 500sccm hydrogen instead, keeping the hydrogen flow unchanged, and annealing in the atmosphere for 30min to obtain the annealed copper foil.

Growth of graphene

S1: keeping the temperature of the system at 1020 ℃ of the temperature in the step 3), stopping introducing hydrogen, introducing 2000sccm argon and 0.8sccm methane into the system, performing primary growth, and performing reaction for 1min to form discrete graphene islands with consistent orientation.

S2: and introducing hydrogen into the system again, increasing the hydrogen flow from 0 to 1200sccm, reducing the argon flow from 2000sccm to 800sccm, keeping the methane flow unchanged, performing secondary growth, and performing the reaction for 30 min.

Example 2

The raw materials and process conditions used in this example were substantially the same as in example 1, except that: in step S1, the flow rate of argon gas was 1500sccm, the flow rate of methane was 0.5sccm, and the growth time (reaction time) was 30S; in step S2, the flow rate of argon gas is 1000sccm, the flow rate of hydrogen gas is 500sccm, the flow rate of methane is 0.5sccm, and the growth time is 20 min.

Comparative example

In this example, the pretreatment process of the substrate is the same as in example 1, and the graphene growth process includes: keeping the temperature of the system at 1020 ℃, and continuously introducing argon, hydrogen and methane into the system, wherein the flow of the argon is 800sccm, the flow of the hydrogen is 1200sccm, the flow of the methane is 0.8sccm, and the growth time is 31 min.

Fig. 2 is a photograph of single crystal graphene grown in example 1 under an optical microscope (scale bar is 100 μm). As can be seen from the figure, the hexagonal domains of all the graphene islands are arranged in parallel, i.e., the orientation is uniform. According to literature reports, if the two graphene domains have different orientations and have a certain angle, an identifiable grain boundary can be formed after long and large splicing, but a similar phenomenon is not observed in fig. 2, and no grain boundary is generated in the spliced domains, so that the graphene domains in fig. 2 have the same orientation, which can indicate that the prepared graphene is single-crystal graphene. In addition, for the convenience of optical microscope characterization, the continuous graphene single crystal film is not prepared in example 1, and based on the growth method in example 1, the growth time of step S2 is only required to be prolonged, so that the graphene domains that are not fused with each other can be further grown and fused, and the continuous single crystal graphene film is formed by seamless splicing.

Fig. 3 is a photograph of the single crystalline graphene grown in example 2 under an optical microscope (scale bar is 100 μm). As can also be seen from the figure, the prepared graphene is single-crystal graphene.

Fig. 4 is a photograph under an optical microscope (scale bar is 100 μm) of graphene obtained by growth of a comparative example. As shown in the figure, the graphene prepared in the comparative example is polycrystalline graphene.

Fig. 5a is a selected area electron diffraction image of the single crystal graphene prepared in example 1 under a transmission electron microscope, and it can be seen that the single crystal graphene has only one set of diffraction spots. FIG. 5b is the extracted diffraction peak, and it can be seen that the first order diffraction peak is higher than the second order diffraction peak. The combination of the two can show that the prepared single crystal graphene is single-layer graphene.

FIGS. 6a and 6b are respectively the histogram of the statistical distribution of the orientation angle of different domains of the single crystal graphene prepared in example 1 and a representative electron diffraction image of selected regions (scale bar 5 nm)^-1) It can be seen that the relative deviation of the orientation angle distribution of the graphene does not exceed 1.5 °, which indicates that the orientation of the graphene is well kept consistent, and the graphene is single-crystal graphene.

Unless otherwise defined, all terms used herein have the meanings commonly understood by those skilled in the art.

The described embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of the present invention, and those skilled in the art may make various other substitutions, alterations, and modifications within the scope of the present invention, and thus, the present invention is not limited to the above-described embodiments but only by the claims.

Claims

1. A preparation method of single crystal graphene comprises the steps of preparing the single crystal graphene through a chemical vapor deposition process; wherein, the chemical vapor deposition process comprises the following steps:

2. The method of claim 1, wherein the pressure of the reaction system of step S1 is the same as the pressure of the reaction system of step S2.

3. The method of claim 1 or 2, wherein the step S1 reaction system comprises a first carbon source gas and a first auxiliary gas; the step S2 reaction system includes a second carbon source gas, a reducing gas, and a second auxiliary gas.

4. The method of claim 3, wherein the partial pressure of the first carbon source gas in the step S1 reaction system is the same as the partial pressure of the second carbon source gas in the step S2 reaction system.

5. The method according to claim 3, wherein, in the step S1 reaction system, the partial pressure ratio of the first auxiliary gas to the first carbon source gas is 50-5000; in the step S2, a ratio of a sum of partial pressures of the second auxiliary gas and the reducing gas to a partial pressure of the second carbon source gas is 50 to 5000.

6. The method according to claim 3, wherein a partial pressure ratio of the reducing gas to the second carbon source gas in the step S2 reaction system is 100 to 2000.

7. The method of claim 3, wherein the first auxiliary gas and the second auxiliary gas are argon, the reducing gas is hydrogen, and the first carbon source gas and the second carbon source gas are respectively and independently selected from one or more of methane, ethane, ethylene, acetylene, ethanol and propane.

8. The method according to claim 1, wherein in the step S1, the growth time of the graphene is 1 second to 2 minutes.

9. The method according to claim 1, wherein in the step S2, the growth time of the graphene is 1 to 200 minutes.

10. A single crystal graphene prepared by the method of any one of claims 1 to 9.