WO2018133053A1

WO2018133053A1 - Graphene film and direct method for transfering graphene film onto flexible and transparent substrates

Info

Publication number: WO2018133053A1
Application number: PCT/CN2017/071999
Authority: WO
Inventors: Chun CHENG; Bananakere Nanjegowda CHANDRASHEKAR; Jingwei Wang; Run SHI; Linfei ZHANG; Shiyuan Liu; Wenkai OUYANG; Yi Zhang; Weiguang KONG; Manman HU
Original assignee: Southern University Of Science And Technology
Priority date: 2017-01-21
Filing date: 2017-01-21
Publication date: 2018-07-26

Abstract

A method for transferring graphene, comprising the step of: forming CVD-grown graphene on copper substrate; transferring the CVD-grown graphene from copper substrate to a target substrate by: depositing a thin film on the CVD-grown graphene to form a thin film-graphene and binding the thin film-graphene onto the target substrate; delaminating of the thin film-graphene from copper substrate using hydrogen bubble method. The objective of the present application is to provide a reliable method for transferring continuous CVD-grown monolayer of grapheme from copper substrate to the target substrates in particularly flexible and transparent plastics.

Description

Title of Invention: Graphene film and direct Method for

Transfering Graphene film onto flexible and transparent substrates Technical Field

[0001] The present application relates to the field of transferring graphene, in particular, it relates to a direct method for transferring chemical vapor deposition-grown monolayer of graphene from metal onto different flexible and transparent polymer substrates. Background Art

[0002] Graphene, a single atomic monolayer of sp2-bonded hexagonal carbon, has been the focus of intense research due to its unique electrical, mechanical, optical, and thermal properties. The applications of graphene include photonics, optoelectronics, and organic electronics such as in solar cells, light-emitting diodes, touch screen

technology, photodetector devices, and membranes for molecular separation in gases or liquids. The synthesis of large area single layer graphene on poly crystalline copper foil by chemical vapor deposition (CVD) has shown to be a scalable method for industrial applications. CVD grown graphene is of high quality and has three important characteristic properties, flexibility, transparency and conductivity, which bring about promising application in flexible electronics. The utilization of the CVD-grown graphene requires the transfer of graphene from the metal substrates to dielectric materials.

[0003] Many techniques such as dry transfer and wet transfer have been successfully

demonstrated. Nevertheless, at present, there are only limited reports for graphene transferred onto flexible substrates, and most of the transfer process involves polymethyl methacrylate (PMMA) as an intermediate membrane. As grown graphene transfer from copper onto the flexible substrate such as plastics via wet etching and H ₂ bubbling method is the road map for the application of graphene in flexible electronics. Such approaches, however, lack control of graphene film continuity and deteriorate the electrical properties thus made it unsuitable for use in flexible electronics. Many challenges remained to transfer graphene onto flexible substrates for potential applications of graphene in wearable electronics and optoelectronics.

[0004] Recently, less expensive and more accessible methods such as roll-to-roll process for large area graphene were demonstrated using hydrogen bubbling method. Importantly, roll-to-roll process for graphene delamination onto ethylene vinyl acetate/ polyethylene terephthalate (EVA/PET) is achieved in two methods. First approach was

demonstrated using hydrogen bubbling method to the as grown graphene on copper. Second approach was demonstrated by using hot water (green transfer). The merit of the green transfer is most apparent in that chemical etchant is avoided. However, neither of this green transfer method is applicable to as immediate graphene grown on copper. Irrespective of the delayed process, green transfer method seems to be attractive for the industrial scale graphene. Unless the continuity of the graphene film on EVA/PET is solved, the potential applications of graphene couldn't be realized.

Technical Problem

[0005] A first aspect of the present application comprises a method comprising forming

CVD-grown graphene on copper; transferring the CVD-grown graphene from copper to a target substrate by: depositing a thin film on the CVD-grown graphene to form a thin film-graphene and binding the thin film-graphene onto the target substrate; de- laminating the thin film-graphene from copper substrate using hydrogen bubble method, the target substrate is a flexible polymer substrate.

Solution to Problem

Technical Solution

[0006] In some embodiments, the target substrate is modified before transferring.

[0007] In some embodiments, the target substrate is rubbed by a sand paper before

transferring.

[0008] In some embodiments, the graphene is grown on copper foil.

[0009] In some embodiments, the target substrate is one or more selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), and Cyclic olefin copolymer (COC).

[0010] In some embodiments, the thin film is ethylene vinyl acetate (EVA) deposition.

[0011] In some embodiments, the EVA film is formed by coating with 1-4% (l-4g/100ml)

EVA solution, and in some embodiments, a concentration of 1% is preferably used.

[0012] In some embodiments, the EVA solution is prepared by dissolving EVA in cy- clohexane.

[0013] In some embodiments, the coating comprises spray coating, spin coating, blade

coating and dip coating.

[0014] In some embodiments, the thin film-graphene is bound to the target substrate using hot lamination method.

[0015] In some embodiments, the thin film acts as binding agent between graphene and polymers.

[0016] In some embodiments, the thin film-graphene is delaminated in NaOH solution.

[0017] In some embodiments, a step of cleaning the thin film-graphene after delamination is included.

[0018] In some embodiments, the cleaning comprises one or more of the following:

[0019] a. Cleaning with deionized water; [0020] b. Dry with nitrogen gas.

[0021] In some embodiments, the CVD-grown graphene is grown by low pressure chemical vapor deposition technique.

[0022] In some embodiments, the CVD-grown graphene is grown on either electro- chemically polished copper or unpolished copper.

[0023] A second aspect comprises the products formed by the methods described herein.

[0024] The present invention provides a reliable method for transferring continuous CVD- grown monolayer graphene from copper to a target substrate, particularly to a flexible substrate.

Advantageous Effects of Invention

Advantageous Effects

[0025] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as in the drawings.

[0026] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework for understanding.

Brief Description of Drawings

Description of Drawings

[0027] Figure 1 shows the schematic of a coating method: thin layer of EVA is deposited onto graphene grown on copper, (a) by spray coating and (b) by spin coating.

[0028] Figure 2 shows the lamination of thin film graphene on copper with the target

flexible substrates.

[0029] Figure 3 shows the schematic process of delamination of graphene onto target

substrates; (a) EVA coated on Gr/Cu; (b) electrochemical set up for delamination; (c) H 2 bubbling to delaminate Gr from Cu and (d) graphene on polymer.

[0030] Figure 4 shows SEM images of continuous graphene on target substrates; (a) PET ;(b) PI; (c)COC; scale bar: 20μιη.

[0031] Figure 5 shows SEM images of continuous graphene on target substrates; (a) PET;

(b) higher magnification of Gr on PET; (c) PI; (d) higher magnification of Gr on PI; Both PET and PI are rubbed with sand paper before the transfer process; scale bar: 20μιη.

[0032] Figure 6 shows ED AX of the Gr on Different polymers to reveal the clean transfer;

(a) PET; (b) PI; and (c) COC.

[0033] Figure7 shows Raman spectrum of graphene grown on polished copper foil at first time (1 ^st growth) and second time graphene growth on copper foil after the graphene transfer (2 ^nd growth).

Mode for the Invention

Mode for Invention

[0034] In order to make the purposes, technical solutions and advantages of the present application more clear, the present application will be further described in detail hereafter with reference to the following specific embodiments.

[0035] The present application provides a method for transferring graphene, comprising the following steps:

[0036] forming CVD-grown graphene on copper substrate;

[0037] transferring the CVD-grown graphene from copper substrate to a target substrate by:

[0038] i. depositing a thin film on the CVD-grown graphene to form a thin film-graphene;

[0039] ii. binding the thin film-graphene onto the target substrate;

[0040] delaminating the thin film-graphene from copper substrate using hydrogen bubble method.

[0041] In some embodiments, the method further comprises the step of flattening the CVD- grown graphene on copper foil. As grown CVD graphene on copper is flat enough, however while it is taken out from CVD furnace, copper foil gets more wrinkles. To make uniform contact of deposition layer of graphene/copper, the graphene/copper is flattened by being kept between two PET, and the PET/graphene/copper/PET sandwich is rubbed with glass rod until no wrinkle exists.

[0042] A binding strength of the thin film layer with the smooth polymers target substrates such as PET and PI is an important issue. Usually thick smooth polymers do not adhered well with the thin film, in some embodiments, the polymer surface is modified before transferring in order to contact tighter with the thin film graphene layer. PET and PI are rubbed, such as with sand paper which makes polymer substrates rough, and a thin layer of polymer solution is spin coated on it. Coated thin film on to the rubbed polymer makes good contact with the thin film of graphene on copper (Figure. 5).

[0043] In some embodiments, in the process of depositing a thin film on the CVD-grown graphene to form a thin film-graphene, EVA is used as the thin film deposition material. The preparation method is as follows: A 1% EVA solution is made by dissolving EVA in cyclohexane at 75°C using magnetic stirrer. Then the 1% EVA (lg/100 ml) solution is made into a thin film on large area flattened graphene/copper using spray coating. The deposition rate is 20mm/s, and the pressure of the spray coating is 0.2Mpa. A substrate holder is used to keep the samples and to evaporate the solvent cyclohexane fast, the temperature of the substrate holder is kept at 60°C.

[0044] In some embodiments, the samples to deposit the solution are kept at room tem- perature to evaporate the solvent. The time needed to evaporate under RT is about one hour.

[0045] A temperature higher than 80 °C will cause the EVA film to melt with the fast

evaporation of solvent cyclohexane. A temperature lower than 60 °C is preferred, at a slow evaporation rate, a uniform thin film layer can be obtained.

[0046] Spin coating and dip coating are as well applicable to this process of coating, but are limited to small area of graphene samples.

[0047] EVA is a good binding agent and has already been used as a covering layer to encapsulate solar cells.

[0048] An EVA solution of 1-4% is used, and a concentration of 1% is more preferably used, due to fast solidification of the EVA solution, a higher concentration may cause the blockage of a spray nozzle. Multilayer of EVA deposition can be employed to control the thickness of an EVA layer.

[0049] A higher temperature might be employed for easier dissolution of EVA, preferably a temperature of 120°C at first and then reduce the temperature to 75°C with constant stirring for 30 minutes to get a homogenous mixture.

[0050] The thin film-graphene can be bound onto various substrates, especially polymer substrates such as PET, PI and COC using hot lamination method. The thin film acts as binding agent between graphene and polymer substrates. Hot lamination is proceeded preferably at a temperature of 120°C to 140°C based on the thickness of the target polymer substrates. For a PET substrate of a thickness of 100 micrometer, a temperature of 120°C is preferred, under a higher temperature, the substrate will be either softened or in a melted state, and will adhere to the lamination machine rollers and make the process inefficient. EVA is softened at a temperature of 120°C, and the EVA will lost its uniform film structure and lead to a discontinuity in graphene transfer under a higher temperature.

[0051] The thin film-graphene is delaminated from copper substrate using hydrogen bubble method which is based on water electrolysis method. An aqueous solution of sodium hydroxide is used as electrolyte in delamination process. A thin layer of polymer spray coated onto the graphene on copper is dipped into NaOH aqueous solution and used as cathode with a constant current supply, a large area of H ₂ bubbles are generated in the confined area between thin film graphene and Cu. The reaction is as follows:

[0052] 2H ₂0(l) + 2e → H ₂(g) + OH

[0053] The copper substrate will not be affected by delamination, and thus can be reused for repeated graphene growth after graphene transfer from the copper substrate, the thin film solution doesn't contain any chemical constituent that would degrade Cu quality.

[0054] When the distance between cathode and anode is kept at 5mm apart, more hydrogen bubbles can be produced and better delamination effect can be achieved. [0055] In some embodiments, the method further comprises a step of cleaning the thin film- graphene after delamination. The polymers/graphene film is rinsed with deionized water and blow dried with nitrogen to ensure it to be free of chemical residues.

[0056] In some embodiments, cyclohexane is used as solvent to dissolve the thin film

precursor. Cyclohexane is preferably used for EVA, it can dissolve EVA better than any of the other solvents.

[0057] In some embodiments, 1M NaOH solution is used as electrolyte, and the voltage is adjusted between 2 to 4 Volts. The NaOH solution will not damage the graphene on the target plastic substrates.

[0058] In some embodiments, thin film graphene/copper and different polymers stack as cathode and platinum as anode, which were placed close together for higher H ₂ bubbles at graphene copper interface. The thin copper/graphene film with different polymers stack is immersed in electrolyte surface to increase the current density at graphene/copper interface with electrolyte solution.

[0059] The copper can be reused to grow graphene by CVD technique. The copper can be used until it becomes too thin for CVD graphene growth and the transferring onto flexible substrate.

[0060] In some embodiments, other metal substrates such as Ni and Cu-Ni alloy are used.

[0061] Figure 1 shows the schematic of coating method: a thin layer of polymer film is deposited onto graphene grown on copper, (a) deposited by spray Coating and (b) deposited by spin coating respectively.

[0062] Spray coating method is well applicable for large area graphene sample for the

continuous uniform thin film. Even polymer film could be tightly made with the spray coating method and same is controlled with the pressure of spraying (Figure la). Spin coating is also popular commonly used to control the thickness of the thin film formation and it is well proved in the transfer of graphene using PMMA solution. High concentration of thin film formation solution couldn't be used in the spray coating method whereas spin coating can be used in deposition of higher concentration of thin film solution onto the Gr/Cu.

[0063] Figure 2 shows the lamination of thin film coated graphene on copper with the target flexible substrates using double roller.

[0064] CVD grown thin film graphene on copper is hot-laminated onto the target polymer substrates to from Cu/Gr/thin film polymer/target flexible plastics at the adhesion temperature of EVA onto the polymers (in case of 150μιη PET, 120°C) using two hot rollers and mechanical pressure is applied simultaneously. Thin film polymer layer melts at the specific temperature and adhered with the target substrates. This process enables the binding between graphene and target flexible substrates.

[0065] Figure 3 shows the schematic and delamination of graphene onto target substrates. The thin film deposition on CVD-grown graphene on copper where deposition layer EVA acts as a binding material to stamp graphene onto flexible polymers. EVA of 1% (lg/lOOml) is dissolved using magnetic stirrer at 80°C for 1 hour. CVD-grown graphene on copper is used as deposition substrate, flattened well by placing them between the ΙΟΟμιη PET plastic and roll over using glass rod. After flattened the deposition substrate, EVA is spray coated for about a thickness of 20μιη and dried in room temperature.

[0066] Coated thin layer on graphene are deposited in place over different types of flexible substrates and allow for lamination. Before lamination process thin film deposited graphene on copper is treated with O ₂ plasma to etch the polymer surface adhered to the back of the copper surface. Temperature in the lamination process is adjusted based on the thickness and kept below the melting point of the polymers of choice for the transfer process.

[0067] Delamination process is carried out by hydrogen bubbling method. Coated graphene film stacked with polymer substrate is dipped into 1M NaOH aqueous solution and used as the cathode. For the anode, a platinum electrode is used. Continuous current is supplied to generate hydrogen bubbles and maintained till the graphene detach from copper.

[0068] Reaction in the negative electrode is: 2H ₂0+2e→ H ₂ + 20H; while reaction at positive electrode is 40H + -e→ 2H ₂0 + O ₂. To speed the process, electrodes are placed near to generate H ₂ bubbles at cathode.

[0069] Subsequently, in the process of delamination, graphene adhered to the polymer is separated at the edges using 1 mm needle and hence there would be large area of graphene/copper exposed to the electrolyte which allows easy delamination; electrolyte penetration also affects the delamination process. The delaminated graphene film onto the target substrates are taken out from the electrolysis solution and washed thoroughly in DI water for two to three times. It is either blown dried with N2 or left at room temperature until it gets dry.

[0070] The morphology of graphene transferred onto different polymers is examined by a scanning electron microscope. Fig. 4 and 5 show that the SEM images of transferred graphene film has continuous layer without pinholes, which indicates that the continuous graphene film successfully transfer from copper onto different polymers.

[0071] Energy Dispersive Spectrometer of the Graphene on different polymers shows the clean transfer of graphene as shown in Fig.6, (a) PET, (b) PI, and (c) COC. There is no other metal/impurity interference either in the coating step or in the entire process.

[0072] Switching onto the industrial process of graphene transfer for the low cost graphene production, Fig 7 shows the Raman spectra of high quality CVD-grown graphene on reused copper transferred onto SiO ₂. Repeated use of reused copper for graphene growth via annealing process increased the grain boundary of copper which in turn increased the graphene grain boundary.

[0073] An unique advantage of this process is the reuse of copper to grow graphene until it reaches a critical thickness to grow graphene, which reduces the price of industrial scale monolayer graphene. Optical microscope image shows that the copper surface is not affected with the coated thin film and the coating method. A commonly used hydrogen bubbling method and green transfer methods are complementary to the method of the invention. SEM images of copper before and after the process are taken and show that there are no obvious mechanical damage occurred to the Cu substrate.

[0074] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the materials, articles, and methods described and claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the scope.

[0075] Example 1

[0076] CVD graphene growth:

[0077] A commercially available copper foil (98% purity, 25μιη thick, Alfa Aesar #46565) was electrochemically polished in electrolyte solution composed of phosphoric acid and ethylene glycol(V/V = 3: 1) with a bias of 2V for 30 minutes. The electropolished Cu foils was washed thoroughly in alcohol and water sequentially to make sure the Cu surface was free from electrolytic residues. The monolayer graphene growth on electropolished Cu was carried out in a low-pressure CVD system. Cu was loaded into the 4 inches diameter long tubular quartz at hot center of the furnace and degas the furnace for the initial pressure of 7x10 ² torr. Cu was allowed to be annealed for 60 minutes at 950°C with a passage of 50 seem of H ₂gas maintain a tube pressure of 1 torr. Then 20 seem methane, corresponding to a pressure of 2 torr, was introduced for 30 minutes to grow the continuous monolayer graphene.

[0078] Example 2

[0079] Graphene Transfer Procedure:

[0080] The key step of transfer method is to afford uniform contact of the binding agent over graphene grown on copper. To coat the thin film on gr/Cu, 1% (lg/ 100ml) EVA solution was prepared. The solution actually acts as stamping materials onto different materials with the function of hot lamination process. Prior to coating process, copper was flattened using glass rod, edges of the gr/copper were covered with scotch tape to avoid the coating of EVA onto the backside of the cu/Gr. The temperature of the sample holder was kept at 60°C to make fast evaporation of the solvent and leaving behind the EVA film. At the other end, EVA solution was kept at hot water bath to make solution free flow through the nozzle. The deposition of the spray coating is 20mm/s, and the pressure of the spray coating is 0.2 Mpa. After spray coating, coated EVA film on graphene grown on copper was considered as "coated thin film on graphene" kept in dry box for fast thin film formation. Later thin film formation on gr/ cu can be used to transfer onto any of the flexible polymers using hot lamination method. The transfer of a thin film-graphene from cu onto target flexible substrates carried out by two commonly used method such as hydrogen bubbling method and green transfer method. Firstly for the immediate transfer bubbling method is used. For eco-friendly and clean transfer, green transfer method is used. As well this technique is also applicable to chemical etching method.

[0081] Example 3

[0082] Optical microscopy and scanning electron microscopy characterization provide the continuity of graphene on polymers. Optical microscopy shows the characteristic shapes of the gr/cu onto the polymers. To study the continuity transfer of Gr on polymer, a thin film of platinum was deposited. Low and high magnification SEM images shows the continuity of the transferred graphene onto polymer substrates, no pinholes are seen. Sheet resistance measurement was carried out using four probe system, the conductivity of the samples was significantly improved with the method of this invention, compared to the graphene transfer onto the commercial available EVA/ PET. Commercial EVA/PET has too rough surface to contact over the graphene on copper, while lamination of rough EVA with gr doesn't contact well with the gr grown on cu and thereby there exist too much pinholes in the transfer process. Perfect contact with the gr on copper was demonstrated and leaded to a smooth finishing contact of EVA on gr. And hence pin holes and cracks were reduced in this method. As well this method creates the avenue for the graphene based flexible electronics application. The morphology of the transferred gr on polymers remained unchanged after bended many times and as well electrical conductivity also unchanged. Hence it is confirmed that the material used for the thin film formation is flexible enough and is a good transfer agent to transfer graphene onto different flexible polymer substrates.

[0083] The above examples are merely preferred embodiments of the present application.

Any common changes and replacements made within the scope of the technical solution of the present application by one of ordinary skill in the art should be included in the protection scope of the present application.

Claims

[Claim 1] A method for transferring graphene, comprising the step of:

forming CVD-grown graphene on copper;

transferring the CVD-grown graphene from copper substrate to a target substrate by:

i . depositing a thin film on the CVD-grown graphene to form a thin film-graphene;

ii . binding the thin film-graphene onto the target substrate;

delaminating the thin film-graphene from copper substrate using hydrogen bubble method;

wherein, the target substrate is a flexible polymer substrate.

[Claim 2] The method according to claim 1, wherein the target substrate is

modified before transferring, especially the target substrate is rubbed by a sandpaper before transferring.

[Claim 3] The method according to claim 1, wherein the target substrate is one or more selected from the group consisting of PET, PI and COC.

[Claim 4] The method according to claim 1, wherein the thin film is EVA deposition.

[Claim 5] The method according to claim 4, wherein an EVA film is formed by coating with 1-4% (lg-4g/100 ml) EVA solution, preferably by coating with 1% EVA solution.

[Claim 6] The method according to claim 5, wherein the coating comprises spary coating, spin coating, blade coating and dip coating.

[Claim 7] The method according to claim 1, wherein the thin film-graphene is bound to the target substrate using hot lamination method.

[Claim 8] The method according to claim 1 or 7, wherein the thin film acts as binding agent between graphene and polymers.

[Claim 9] The method according to claim 1, further comprising a step of cleaning the thin film-graphene after delamination, wherein cleaning comprises one or more of the following:

a. Cleaning with deionized water;

b. Dry with nitrogen gas.

[Claim 10] A graphene film obtainable by a method according to any one of claims

1-9.