WO2016126208A1

WO2016126208A1 - Defect-free direct dry delamination of cvd graphene using a polarized ferroelectric polymer

Info

Publication number: WO2016126208A1
Application number: PCT/SG2016/050057
Authority: WO
Inventors: Barbaros ÖZYILMAZ; Iñigo MARTIN FERNANDEZ; Chee Tat TOH
Original assignee: National University of Singapore
Current assignee: National University of Singapore
Priority date: 2015-02-03
Filing date: 2016-02-03
Publication date: 2016-08-11
Anticipated expiration: 2017-08-03
Also published as: CN107406258A; JP6749942B2; CN112194120B; KR102256000B1; KR20170110133A; CN112194120A; JP2018512370A

Abstract

A method of peeling a graphene layer from its growth substrate involves using the electrostatic field from a polarized ferroelectric layer to produce a reduced relative adhesion between the graphene layer and the growth substrate with respect to the adhesion between the graphene layer and the polarized ferroelectric polymer layer. Related articles and techniques for delamination of a graphene layer from a growth substrate are provided.

Description

DEFECT-FREE DIRECT DRY DELAMINATION OF CVD GRAPHENE USING A POLARIZED FERROELECTRIC POLYMER

RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No.

62/1 11 ,195, filed on February 3, 2015, the entire teachings of which application are incorporated herein by reference.

BACKGROUND

[0002] Graphene has attracted much interest because of its extraordinary characteristics. However, its processing is still a bottleneck hindering commercialization of graphene applications.

[0003] Graphene needs to be transferred from its growth substrate to the device surface because the growth substrate and/or the growth conditions are not compatible with the final device. The main steps in such transfer processes are:

[0004] (1 ) Fully or partially coating or covering the graphene with a transfer layer. See FIG. 1, which shows a schematic diagram of a substrate/graphene/peeling layer structure, in accordance with the prior art, which includes a peeling layer 100, graphene 1 10 and a substrate 120.

[0005] (2) Releasing the graphene/transfer layer from the growth substrate either by chemically etching the substrate, by (electro-) chemically delaminating or mechanically peeling the graphene from the substrate. Chemical etching and (electro-) chemical delamination are wet processes because the graphene is processed in a solution. Mechanical peeling is a dry process.

[0006] (3) Transfer of the graphene/transfer layer onto the target substrate. This step may be skipped if the transfer layer is to be the target substrate of the graphene

[0007] (4) Removal of the transfer layer. This step may be skipped if the transfer layer is to remain with the graphene. [0008] This multi-step-processing results in contamination on the graphene and in defects to its structure that worsen the characteristics of the final devices and make them non-uniform throughout a sample.

[0009] The method to release the graphene/transfer layer from the growth substrate strongly affects the residual and defective level of the resulting graphene. As above, the methods can be grouped as wet (chemical etching, (electro-) chemical delamination) or dry (mechanical peeling)

[0010] In the case of the wet processes:

[0011] - The chemicals used or the products from the chemical reactions leave residues on graphene that cannot be easily removed by thorough rinsing.

[0012] - These residues may bind to the graphene and cause defects within its crystalline structure. This causes graphene to be more brittle and prone to mechanical defects during handling, like during transfer to the substrate or when removing the transfer layer. These defects will also degrade the electronic properties of graphene when used within an electronic device.

[0013] - Even when the residues do not cause defects, their presence on graphene will still degrade and/or make less uniform the electrical, optical, chemical or mechanical properties of the graphene. For example, within a device, a clean interface between graphene and the adjacent layers is required for high device efficiency, and such residues will reduce the device performance and the uniformity throughout different devices.

[0014] - Once the graphene has been transferred onto the target substrate, the chemical residues at the interface between the graphene and the target will reduce the adhesion of the graphene to the substrate, therefore, limiting the mechanical stability of the device.

[0015] - Wet processes are slow because of the chemical reactions to happen. For example, etching of a 35 micrometer thick copper substrate where the graphene may be grown will typically last more than 2 hours. (Electro-) Chemical delamination processes are faster processes involving any etching but speed is limited to ~1 millimeter per second. These processing speeds are not compatible with high throughput fabrication

[0016] In the case of dry processes:

[0017] - Methods to peel the graphene by ensuring sufficient adhesion of the peeling layer to the graphene so that it will be higher than the adhesion between the graphene and its substrate. Where the substrate is copper, the adhesion of graphene to copper is on the order of 8 J/m².

[0018] - A first mechanical peeling method uses pressure and/or temperature to achieve a conformal contact between the graphene and the peeling layer and/or high voltage to induce direct chemical bonding between them. Then, the chemical adhesion of the peeling layer to the graphene will need to be higher than the adhesion of the graphene to its substrate.

[0019] - A second type of mechanical peeling method requires an adhesive layer as the peeling layer. Then, the adhesion of the adhesive layer to the graphene will need to be higher than the adhesion of the graphene to its substrate.

[0020] - However, it has been shown that transferred graphene is always badly damaged due to non-uniform strain and/or non-uniform peeling force.

[0021] - Non-uniform strain occurs when the peeling layer does not have a uniform coating over the entire graphene surface or when there is a large change in parameters like temperature and pressure.

[0022] - Non-uniform peeling force is due to practical conditions of the graphene on substrate, where the substrate topography consists of grain boundary and terraces (10-100 μιη length dimensions). This changing topography leads to non-uniform peeling force during the peeling.

[0023] - During peeling, the non-uniformities cause cracks and/or non-transferred graphene regions. As a result of these cracks the quality of the transferred graphene is degraded irreversibly. These cracks form on the 0.1 mm scale and will have significant impact on the graphene quality when measured at the 1 mm length scale or longer. When using perfect crystalline graphene on the growth substrate as the starting material, such measurements show a drop in mobility to less than 200 cm^"2/Vs and its unsuitability for transparent electrodes with the lowest sheet resistance even at high doping to be >1 ,000 ohm/sq. In comparison, if such graphene remains defect-free, the mobility is usually around 5,000 cm^" /Vs and lowest sheet resistance of 150 ohm/sq.

[0024] - Both types of peeling method strive to achieve a high enough and uniform enough adhesion energy between graphene and the peeling layer to yield a defect-free peeling. These methods do not act on the adhesion energy between the graphene and the substrate. Therefore the minimum energy required to delaminate the graphene from the substrate remains the same. To further improve the delaminated graphene quality, a peeling layer with a stronger adhesion to the graphene, and a technique that minimizes the non- uniformities throughout the whole graphene surface, is still required.

[0025] Therefore a commercially viable technique for graphene delamination from growth substrate is still needed, and this should preferably be a defect-free mechanical peeling method.

SUMMARY OF THE INVENTION

[0026] In accordance with a version of the invention, there is provided a method to peel the graphene layer from its growth substrate. Previous methods rely on achieving an adhesion between the graphene and the peeling layer that will be stronger than that of the graphene to its substrate by putting the graphene in contact with a surface that will bond it stronger than the graphene bonds to its substrate. A method in accordance with a version of the invention, instead, uses the polarization of a ferroelectric polymer layer to induce stronger adhesion between the graphene and the ferroelectric layer compared to the adhesion between the graphene and its substrate.

[0027] In accordance with a version of the invention, there is provided an article for delamination of a graphene layer from a growth substrate. The article comprises a graphene layer on a growth substrate, and a polarized ferroelectric polymer layer on the graphene layer. The graphene layer is adhered to and sandwiched between the polarized ferroelectric polymer layer and the growth substrate. The polarized ferroelectric polymer layer is arranged and polarized to produce a reduced relative adhesion between the graphene layer and the growth substrate with respect to adhesion between the graphene layer and the polarized ferroelectric polymer layer. The polarized ferroelectric polymer layer may be arranged and polarized to strengthen the adhesion of the ferroelectric polymer layer to the graphene, and the adhesion between the ferroelectric polymer and graphene composite to the substrate.

[0028] In further, related versions of the invention, the graphene layer may comprise single layer or multilayer graphene (such as between 2 and 10 layers) grown by a chemical vapor deposition-like process on a catalytic substrate such as copper. The catalytic substrate may be other metals, including nickel, platinum or cobalt, or other materials known to catalyze graphene, including germanium. The catalyst may comprise a metal foil or a metal thin film on a further substrate. Graphene may be graphene by other epitaxial methods, such as by heating of silicon carbide. The polarized ferroelectric polymer layer may comprise a fluoropolymer, such as polyvinylidene fluoride or a copolymer of polyvinylidene fluoride. The polarized ferroelectric polymer layer may comprise a thickness of between about 1 nanometer and about 1 millimeter, such as a thickness of between about 100 nanometers and about 2000 nanometers. The polarized ferroelectric polymer layer may comprise a remanent

9 9 9 polarization of between about 5 μθ/αη and about 10 μθαη , such as about 7.5 μθ/αη .

[0029] In accordance with another version of the invention, there is provided a method of separating a composite from a growth substrate, the composite including a ferroelectric polymer layer and a graphene layer, and the graphene layer being adhered to and sandwiched between the ferroelectric polymer layer and the growth substrate. The method comprises (i) polarizing the ferroelectric polymer to produce a reduced relative adhesion between the graphene layer and the growth substrate with respect to adhesion between the graphene layer and the polarized ferroelectric polymer layer; and (ii) peeling the composite to separate the graphene layer from the growth substrate. The ferroelectric polymer may be polarized to generate an attractive force to strengthen the adhesion of the ferroelectric polymer layer to the graphene and the adhesion between the ferroelectric polymer and graphene composite to the substrate.

[0030] In further, related versions of the invention, the method may further comprise applying the ferroelectric polymer layer to the graphene layer to form the composite. The method may further comprise transferring the peeled composite to a target substrate by adhering the graphene layer to the target substrate; and may further comprise removing the ferroelectric polymer layer from the graphene layer to leave the graphene layer adhered to the target substrate. The continuity of the graphene layer on the ferroelectric polymer layer after the peeling may be 90% or more of an initial coverage of the graphene layer on the growth substrate, such as 95% or more or such as 99% or more of the initial coverage of the graphene layer on the growth substrate. The composite may further include a secondary substrate adhered to the ferroelectric polymer layer, and the ferroelectric polymer layer may be sandwiched between the secondary substrate and the graphene layer. The method may further comprise transferring the peeled composite to a target substrate by adhering the graphene layer to the target substrate; and releasing the secondary substrate from the ferroelectric polymer layer to leave the ferroelectric polymer layer and the graphene layer adhered to the target substrate; and the ferroelectric polymer layer may be removed from the graphene layer to leave the graphene layer adhered to the target substrate. [0031] In other, related versions of the invention, the polarizing may include applying an external electric field to the polymer layer. The method may further comprise peeling the composite with a peeling force of at least about 85 J/m to separate the composite from the growth substrate. Polarizing the ferroelectric polymer may result in a remanent polarization of the ferroelectric polymer of between about 5 μΟ/αη²3ΐκΙ about 10 μθ/αη², such as about

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

[0033] FIG. 1 is a schematic diagram of a substrate/graphene/peeling layer structure, in accordance with the prior art.

[0034] FIG. 2 is a schematic diagram of a graphene peeling method in accordance with a version of the invention: the left panel shows graphene on the growth substrate, the center panel shows coating the graphene with a ferroelectric polymer such as polyvinylidene fluoride (herein, "PVDF"); and the right panel shows graphene-PVDF peeling from the graphene growth substrate.

[0035] FIG. 3 is a schematic diagram illustrating mechanisms for a strong graphene- ferroelectric polymer binding (such as a graphene-PVDF binding), in accordance with a version of the invention: the left panel shows PVDF-induced stronger electrostatic strengthening of the graphene to the ferroelectric film with respect to the strengthening of the graphene to the substrate; and the right panel shows PVDF-graphene atomic scale binding.

[0036] FIG. 4 is a set of atomic force microscopy images and cross sections of the roughness of the interfaces of the substrate-graphene-ferroelectric polymer system, in accordance with a version of the invention. The left panel is the height scan of the graphene on the substrate before coating the ferroelectric polymer. The central panel is the height scan of the graphene-polarized ferroelectric polymer after the graphene-ferroelectric polymer has been peeled from the substrate. The right panel shows the cross sections of the substrate- graphene and the graphene-polarized ferroelectric polymer according to the sections on the left and central panels. [0037] FIG. 5 is a plot of the adhesion energies between the substrate, graphene and the ferroelectric polymer layer after processing according to a particular example of the invention where the substrate is a copper foil, graphene is one layer of graphene grown by chemical vapor deposition on the copper, and the ferroelectric polymer layer is PVDF. Dotted lines represent the critical adhesion between the polarized ferroelectric polymer layer and graphene composite and the composite, the critical adhesion energy between graphene and the copper substrate when an adhesive with sufficient adhesive strength has been applied onto graphene, and the critical adhesion energy between the ferroelectric polymer layer and graphene when the ferroelectric polymer is not polarized. Experimental data relates to the peeling conditions of the polarized ferroelectric polymer layer and graphene composite from the substrate at different loads.

[0038] FIG. 6 is a set of photographs showing a comparison of graphene peeling yield using three different graphene peeling techniques. The left panel is a photograph showing results after peeling with a polymer that does not result in a strong enough binding and/or electric field perpendicular to the graphene; the center panel is a photograph showing results after peeling with a ferroelectric polymer which structure has not been processed to form ferroelectric grains and/or these grains have not been aligned according to a field

perpendicular to graphene; and the right panel is a photograph showing results after peeling using a method according to a version of the invention. It can be seen that the technique used for the left panel leaves areas of no graphene 61 and graphene flakes 62; the technique for the center panel leaves areas of no graphene 63 and graphene patches 64; whereas the technique of the right panel, in accordance with a version of the invention, obtains coverage with both both graphene 65 and multilayer graphene 66.

[0039] FIG. 7 is a set of charts showing a statistical comparison of defects (cracks) after peeling of graphene using a method in accordance with a version of the invention and using a standard state-of-the-art graphene transfer process: the left chart shows the statistical distribution of the cracks according to their areas, and the right chart is a bar graph of the statistics on the total cracked area.

[0040] FIG. 8 is a schematic diagram showing uses of a direct peeling transfer method in accordance with a version of the invention. Panel (a) shows a graphene/ferroelectric polymer; panel (b) shows a graphene-ferroelectric polymer on a substrate, with the graphene facing up; panel (c) shows a graphene-ferroelectric polymer on a substrate, with the graphene facing down; and panel (d) shows graphene on a surface.

DETAILED DESCRIPTION OF THE INVENTION

[0041] A description of example embodiments of the invention follows.

[0042] A version according to the invention provides a method of peeling a graphene layer from its growth substrate. Previous methods rely on achieving an adhesion between the graphene and the peeling layer that will be stronger than that of the graphene to its substrate by putting the graphene in contact with a surface that will bond it stronger than the graphene bonds to its substrate. A method in accordance with a version of the invention, instead, uses the polarization of a ferroelectric polymer layer to induce stronger adhesion between the graphene and the ferroelectric layer compared to the adhesion between the graphene and its substrate.

[0043] FIG. 2 is a schematic diagram of a graphene peeling method in accordance with a version of the invention. The left panel shows the initial graphene 210 on the growth substrate 220. The center panel shows coating the graphene 210 with a ferroelectric polymer 230, such as polyvinylidene fluoride (here, "PVDF"), with the characteristics that are described below. The right panel shows peeling of the graphene/ferroelectric polymer layer 210/230 from the graphene growth substrate 220.

[0044] A) Peeling of graphene by polarized ferroelectric layer (mechanisms)

[0045] FIG. 3 is a schematic diagram illustrating mechanisms for a strong graphene- ferroelectric polymer binding (such as a graphene-PVDF binding), in accordance with a version of the invention, without wishing to be bound by theory. A method according to a version of the invention uses the polarization of a ferroelectric polymer layer on the graphene to increase its adhesion to the graphene. Polarizing the ferroelectric polymer layer also increases the adhesion of the polarized ferroelectric polymer layer and graphene composite to the substrate. Polarizing the ferroelectric polymer layer also weakens the adhesion of the graphene to the substrate with respect to the adhesion of the graphene to the polarized ferroelectric polymer layer.

[0046] On the one side (see FIG. 3, left panel), an attraction force between the graphene 310 and the polarized ferroelectric polymer 330 and between the polarized ferroelectric polymer layer 330 and the graphene 310 composite and its substrate 320 is created. A method according to a version of the invention enables this mechanism by using a polarized ferroelectric polymer layer 330 on the graphene/substrate 310/320.

[0047] -The alignment of ferroelectric dipoles within the polarized ferroelectric polymer layer 330 in the direction perpendicular to the graphene 310 generates a strong electric field perpendicular to the graphene 310 regardless of what the electric orientation of the dipoles will be.

[0048] -This ferroelectric layer generated electric field highly dopes graphene such that it induces charges within the graphene sheet. Resulting from this electrostatic interaction graphene gets attracted to the ferroelectric layer. This attraction improves the adhesion between graphene and the ferroelectric layer.

[0049] -As a result of the electrostatic field from the polarized ferroelectric layer, attractive interactions are also induced between the substrate and the polarized ferroelectric polymer layer and the graphene composite.

[0050] As a result of this ferroelectric polymer/graphene/substrate structure 330/310/320, the adhesion energy between the ferroelectric polymer 330 and the graphene 310 is strengthened above the adhesion energy required to peel graphene 310 from the substrate 320. Resulting from these interactions, the graphene can be peeled from the substrate defect- free, i.e, without any induced mechanical defects.

[0051]

[0052] On the other side, in accordance with a version of the invention (see FIG. 4), the coating of the graphene 310 with the ferroelectric polymer 330 is made to ensure a uniform interface that results in an atomically precise strong binding that limits the non-uniformities in the peeling process that may prevent the graphene from suffering stresses during the process. Therefore, the graphene may be prevented from mechanical damage.

[0053] The interface between the graphene 310 and the ferroelectric layer 330 is also engineered to reverse their low adhesion energies in order to achieve a binding energy that enables the peeling of graphene.

[0054] -Graphene has inherently weak van der Waals adhesion strength to most materials

[0055] -Fluoropolymers have inherently weak attraction force, with Teflon being used for non-stick surface as an example.

[0056] -However, the interaction of the graphene 310 and a fluoropolymer 330 becomes strong after a proper engineering of the coating and polarization of the polymer on the graphene, as a result of the atomic van der Waals interaction of the highly electronegative Fluorine atoms at the polymer with the pi-orbitals from the graphene.

[0057] -The van der Waals interaction between the graphene and the fluoropolymer does not induce any defect to the graphene.

[0058] -Ferroelectric polymers being fluoropolymers, they are the ideal material for strong van der Waals adhesion strength based on the Fluorine-Pi bond interactions with graphene, once the molecules are oriented accordingly.

[0059] -Coating the ferroelectric polymer on the graphene surface results in a uniform interaction over the entire graphene surface and without inducing any defects in graphene.

[0060] -In the ferroelectric/graphene/substrate structure in accordance with a version of the invention, overall, the Fluorine-Pi intermolecular bond has a stronger attraction force between graphene/ferroelectric, enabling the mechanical peeling of graphene from the growth substrate.

[0061] -However, the mechanical strength between the ferroelectric polymer and the graphene may not be strong and/or uniform enough at sites such as grain boundaries or copper terraces and, therefore cracks may happen during the transfer even though graphene is peeled.

[0062] Each of the individual mechanisms contributes towards defect- free graphene and the combination of both effects results in pristine condition, defect- free graphene, even across sites such as grain boundaries or copper terraces over large areas of graphene.

[0063] B) Modes of practice in accordance with versions of the invention

[0064] 1) Graphene formation on growth substrate

[0065] In an example of the invention, graphene may be grown by a chemical vapor deposition (CVD) on a copper foil substrate. The copper foil may be cleaned by, but not limited to, solvents to remove residues from its surface before placing it inside the growth chamber. The growth process may include an annealing step where a gas such as hydrogen will be flown at a temperature around the growth temperature, that is, around 1000°C. Next, a hydrocarbon such as methane will be flown, maybe together with hydrogen to promote the growth of the graphene. Finally, the chamber will be cooled down and the copper foil with the graphene on its surfaces will be taken out. [0066] The graphene may be formed as a single layer or also as multiple layers according to types of growth substrate or conditions inside the synthesizing chamber.

[0067] Alternatively, SiC can be used to grow graphene. In this case, the substrate will be annealed at a temperature that will sublimate the Si atoms at the surface of the SiC and will promote the recrystallization of the C atoms to form one layer or multilayers of graphene.

[0068] 2) Coating the graphene with a polarized ferroelectric polymer film

[0069] The graphene is coated with a solution of the ferroelectric polymer in a dry environment. Processes such as, but not limited to spin-coating, Langmuir Blodgett, dip coating, slot die, bar coating, doctor blade or wire coating, may be used to form such coating.

[0070] In an example of the invention a PVDF may be dissolved in dimethyl formamide (DMF) and this solution can be later coated on the graphene.

[0071] In an example of the invention, the graphene substrate/graphene can be coated with a PVDF thin film by spin coating. Spin coating of a 10% solution of PVDF in DMF at 2000 rpm may result in the coating of a 500 nm thick film. The substrate may need to be annealed previously to evaporate the water molecules on the graphene in order to achieve a graphene polymer interface to be free of water residues. The coating may need to be completed in a dry environment for the polymer layer to be free of defects from water molecule trapping between its molecules.

[0072] After the coating, the film may be annealed to evaporate the solvent and to recrystallize the polymer chains into grains. The annealing temperature should be below the melting temperature of the polymer to promote the formation of the ferroelectric phase. In an example of the invention, a 500 nm thick layer of PVDF film may be annealed at 135°C between 1 minute and 24 hours.

[0073] The resulting thin film polymer layer may be 1 nanometer to 1 millimeter thick, such as a thickness of between about 100 nanometers and about 2000 nanometers.

[0074] After annealing, the dipoles in the ferroelectric polymer film may be aligned perpendicular to the graphene by applying an electric field across the film. The field can be applied by a method such as, but not limited to, using external electrodes to apply a voltage across them or by ionizing the surface of the polymer. Depending on the setup, annealing and polarization may be done in a single process. In accordance with a version of the invention, polarizing a ferroelectric polymer may include applying an external electric field to the polymer layer, such as an external electric field comprising an electric field strength of between about 50 V/μηι and about 500 V/μηι; and electrically polarizing a ferroelectric polymer may include ionizing the polymer's surface, such as ionizing at a voltage of between about 1 kV/cm and about 10 kV/cm.

[0075] The direction of polarization is preferred such that the Fluorine atoms of the ferroelectric polymer are aligned towards the graphene surface.

[0076] In the case of using PVDF as the ferroelectric polymer, a field on the order of 100 ν/μηι may be required to align the dipoles. In an example of the invention, the dipoles in a PVDF film around 500 nm thick may be aligned by ionizing the surface of the polymer at a voltage of 6 kV/cm. The polarized ferroelectric polymer layer may comprise a remanent

9 9 polarization of between about 5 μ αη and about 10 μθαη , such as about 7.5 μθατι .

[0077] 3) Peeling the graphene/ ferroelectric polymer composite

[0078] In accordance with a version of the invention, peeling of graphene/ferroelectric polymer from the growth substrate can be completed by applying a peeling force

perpendicular to the growth substrate.

[0079] In accordance with a version of the invention, peeling forces of at least about 85 J/m² result in a reliable defect-free peeling (see FIG. 5). Higher peeling forces may also result in a reliable peeling. In accordance with a version of the invention, the critical force for the peeling of graphene, that is, the lowest force for the peeling of graphene to occur, is equal to or below 85 J/m².

[0080] Peeling forces below the critical force may result in cracks in graphene. However, a reliable defect free peeling at smaller forces may still be possible with sufficient control of the force applied or a sufficiently high peeling energy.

[0081] Peeling may be completed by a process such as, but not limited to, manual peeling and rolling a material that will attach to the PVDF (or the graphene substrate) stronger than the critical adhesion energies at the substrate-graphene-polymer interfaces.

[0082] In an example of the invention, the ferroelectric polymer may be thick enough to enable direct manual fast peeling of the copper foil from the ferroelectric

polymer/graphene/substrate stack by using some tool to hold the copper or the

graphene/ferroelectric polymer composite such as metallic tweezers. [0083] In other examples of the invention additional supports such as polymer foils, epoxies or tapes may be attached either to the graphene substrate, to the ferroelectric polymer or to both of them to ease the peeling. In these cases the binding of the additional supports to either of the surfaces will have to be stronger than the adhesion of the graphene to its substrate. In an example of the invention where the substrate is a copper foil, the support may be a thermal release tape with an adhesive strength of 3.7 N/20 mm (see FIG.5). The adhesive strength between the tape and the polarized ferroelectric layer increases with peeling velocity and this needs to be at least 0.15 m/s to peel graphene from the substrate with adhesion strength of 85 J/m². Graphene peeling is unsuccessful at insufficient peeling velocity since the thermal release tape does not adhere sufficiently to the polarized ferroelectric layer to induce graphene peeling, i.e. the ferroelectric layer and the graphene remain on the copper foil.

[0084] In the case of graphene growing on both surfaces of the substrate, the transfer process may be completed simultaneously on both sides of the substrate to peel the graphene on each of the surfaces.

[0085] A method in accordance with a version of the invention is compatible with the patterning of the graphene and/or the ferroelectric layer prior to the peeling of the composite from the substrate. Depending on the patterning methods, the benefits of the peeling method may only apply to the unpattemed graphene/ferroelectric polymer composite. For example, if an area of the initial composite was removed from the substrate prior to the peeling, that area will not be peeled.

[0086] The continuity of the graphene after peeling is a parameter that can be used to validate a method of peeling of graphene, since any mechanical defect (crack) in the graphene layer will degrade irreversibly the properties of the peeled graphene. FIG. 6 compares the yield in the peeling of graphene by various techniques.

[0087] In the left panel of FIG. 6, when a poly-methyl-methacrylate film was used to peel graphene from a copper foil following the peeling parameters in accordance with a version of the invention except without using a ferroelectric polymer (in which previously described steps 1 and 3 were followed, but step 2 was substituted with the coating of graphene with PMMA), only graphene flakes were peeled. This is an unsuccessful transfer. It results from the adhesion of the non- ferroelectric polymer with the graphene being weaker than the adhesion of the graphene to its substrate. [0088] In the center panel of FIG. 6, when a method in accordance with a version of the invention is not followed fully (previous steps 1 -3 were followed but some of the steps of the PVDF coating or polarization were omitted), peeling of the graphene happens but the yield of the transfer is not maximal because the graphene does not peel uniformly and mechanical defects appear in the graphene layer. It is a non-continuous transfer of graphene. In this case a ferroelectric polymer was used but it was not processed so that its molecules would form grains and/or the dipoles in these grains were not aligned perpendicular to the graphene.

[0089] These results are considered here to be non-successful because of the non- continuity of the graphene. However, these results could be claimed as successful if the peeling would be shown at a few micrometer square range. In this scale length the graphene could appear continuous. However, such conditions should not validate a method for large area peeling of graphene if the validation does not include areas such as grain boundaries on the catalyst substrate, graphene on different adjacent grains and areas that are relevant to large area/wafer scale fabrication.

[0090] In the right panel of FIG. 6, when a method in accordance with a version of the invention is used, full and defect-free transfer of the graphene is achieved. A statistical analysis of the continuity of the graphene after peeling (minimum detectable defect size is 0.5 μηι ) concludes that the amount of defects is significantly improved with respect to standard transfer conditions and that the peeled graphene covers more than 99.5% of the sample surface over square millimeter areas. FIG. 7 is a set of charts showing a statistical comparison of defects (cracks) after peeling of graphene using a method in accordance with a version of the invention and using a standard graphene transfer process: the left chart shows the statistical distribution of the cracks according to their areas, and the right chart is a bar graph of the statistics on the total cracked area.

[0091] Also, when a method in accordance with a version of the invention is used, a stack of multilayers up to 10 multilayers, or more, can be peeled together with the main graphene layer. These graphene multilayers are inherent to the graphene growth method. The darker spots in the optical image of the right panel of FIG. 6 correspond to the multilayers of graphene that peeled together with the continuous graphene layer.

[0092] A method in accordance with a version of the invention provides a method for the peeling of graphene that results in continuous and defect free graphene [0093] Further exemplary versions are provided below. FIG. 8 is a schematic diagram showing uses of a direct peeling transfer method in accordance with a version of the invention. As described further below, panel (a) of FIG. 8 shows a graphene/ferroelectric polymer 810/830; panel (b) of FIG. 8 shows a graphene-ferroelectric polymer 810/830 on a transfer substrate 840, with the graphene 810 facing up; panel (c) of FIG. 8 shows a graphene-ferroelectric polymer 810/830 on a target substrate 850, with the graphene 810 facing down; and panel (d) of FIG. 8 shows graphene 810 on a target surface 850.

[0094] A) Graphene/polarized ferroelectric polymer- see panel (a) of FIG. 8

[0095] The exemplary method of panel (a) of FIG. 8 may result in a continuous composite material.

[0096] - The graphene 810 has a low sheet resistance because of the doping from the PVDF film 830. Electrostatic doping (by PVDF) is stable overtime, in contrast with other doping methodologies

[0097] - The graphene 810 is exposed and therefore it can be post-processed

[0098] - The PVDF graphene 810 is free of residues, its continuity is >99%

[0099] - The graphene 810 is clean from contamination

[00100] Processing for this example follows the description above.

[00101] B) Graphene/polarized ferroelectric polymer with a transfer substrate - see panel (b) of FIG. 8

[00102] The exemplary method of panel (b) of FIG. 8 may result in a continuous composite material on top of a substrate, the graphene 810 facing up.

[00103] - The graphene 810 has a low sheet resistance because of the doping from the PVDF film 830. Electrostatic doping (by PVDF) is stable overtime, in contrast with other doping methodologies

[00104] - The graphene 810 is exposed and therefore it can be post-processed

[00105] - The PVDF graphene 810 is free of residues, its continuity is >99%

[00106] - The graphene 810 is clean from contamination

[00107] Processing for this may be:

[00108] -Forming the graphene 810 on the first substrate (graphene substrate) as described in the previous examples. [00109] -Forming the ferroelectric polymer layer 830 on the graphene as described in the previous examples.

[00110] -Attaching a second substrate (also called a transfer substrate) 840 on the polarized ferroelectric layer 830 (second substrate/ferroelectric/graphene/substrate), for example, but not limited to, a polyethylene terephthalate (PET) foil. The second substrate 840 preferably is in intimate contact with the ferroelectric film 830. An adhesive strength between the substrate 840 and the ferroelectric film 830 of 85 J/m² or higher enables the peeling of the graphene 810 without the second substrate 840 releasing from the ferroelectric film 830. When the adhesive strength of the second substrate to the polarized will be 85 J/m² or higher, peeling velocity may be unrestricted. When the adhesive strength of the second substrate to the polarized will be lower than 85 J/m², the critical adhesion strength may be achieved by peeling at a high velocity because adhesion between at the interface will depend on this parameter.

[00111] -Peeling the second substrate/ferroelectric/graphene from the graphene substrate as described in the previous examples.

[00112] C) Polarized ferroelectric polymer/graphene on a target substrate - see panel ( c) of FIG. 8

[00113] The exemplary method of panel (c) of FIG. 8 may result in a continuous composite material on top of a substrate, the graphene 810 facing the target substrate.

[00114] - The graphene 810 has a low sheet resistance because of the doping from the PVDF film 830. Electrostatic doping (by PVDF) is stable overtime, in contrast with other doping methodologies

[00115] - After removal of one or more of the surrounding layers, a surface of the graphene 810 can be exposed and therefore it can be post-processed

[00116] - The PVDF graphene 810 is free of residues, its continuity is >99%

[00117] - The graphene 810 is clean from contamination

[00118] Processing for this may be:

[00119] -Forming the graphene 810 on the first substrate (graphene substrate 320 of FIG. 3) as described in the previous examples.

[00120] -Forming the ferroelectric polymer layer 830 on the graphene 810 as described in the previous examples. [00121] -Attaching a second substrate 840 (not shown in panel (c)) on the polarized ferroelectric polymer layer 830 (second substrate/ferroelectric/graphene/substrate). In this example, the second substrate 840 is compatible with the peeling of the graphene 810 in the next step of the processing and may be released later on. An example of the second substrate 840 is thermal release tape. The second substrate 840 has to be in intimate contact with the ferroelectric film 830. An adhesive strength between the substrate 840 and the ferroelectric film 830 of 85 J/m or higher enables the peeling of the graphene 810 without the second substrate 840 releasing from the ferroelectric film 830. When the adhesive strength of the second substrate to the polarized will be lower than 85 J/m , that adhesion strength may be achieved by peeling at a high velocity. In an example of the invention where the substrate is a copper foil, the support may be a thermal release tape with an adhesive strength of 3.7 N/20 mm (see FIG.5). The adhesive strength between the tape and the polarized ferroelectric layer increases with peeling velocity and this needs to be at least 0.15 m/s to peel graphene from the substrate with adhesion strength of 85 J/m . Graphene peeling is unsuccessful at insufficient peeling velocity since the thermal release tape does not adhere sufficiently to the polarized ferroelectric layer to induce graphene peeling, i.e. the ferroelectric layer and the graphene remain on the copper foil

[00122] -Peeling the second substrate/ferroelectric/graphene from the substrate as described in the previous examples.

[00123] -Applying the second substrate/ferroelectric polymer/graphene stack on a third substrate (target substrate) 850. In the case the second substrate 840 was a thermal release tape, the graphene side of the stack is to be put in intimate contact with the third substrate 850 by applying a pressure of 10 MPa.

[00124] -Releasing the second substrate 840 from the polymer layer/graphene/third substrate. In the case the second substrate 840 was a thermal release tape, the stack will be heated to a temperature 5°C above the release temperature of the tape (typically a temperature between 90 and 150°C) while maintaining the previously applied pressure. Upon reaching the release temperature the tape is to be removed slowly to prevent damage to the polarized ferroelectric polymer/graphene. [00125] D) Graphene on a target substrate - see panel (d) of FIG. 8

[00126] The exemplary method of panel (d) of FIG. 8 may result in a continuous graphene

810 on top of a target substrate 850.

[00127] - The graphene 810 is exposed and therefore it can be post-processed

[00128] - The PVDF graphene 810 is free of residues, its continuity is >99%

[00129] - The graphene 810 is clean from contamination

[00130] Processing for this may be:

[00131] -Forming the graphene 810 on the first substrate (graphene substrate 320 of FIG. 3) as described in in the examples above.

[00132] -Forming the ferroelectric polymer layer 830 (see panels (a) through (c)) on the graphene. As described in the examples above.

[00133] -Attaching a second substrate 840 (see panel (b)) on the ferroelectric film (second substrate/ferroelectric/graphene/substrate). As described in the previous example.

[00134] -Peeling the second substrate/ferroelectric/graphene from the graphene substrate 320 (see FIG. 3). As described in the example above.

[00135] -Applying the second substrate/ferroelectric polymer/graphene stack on a third substrate (target substrate) 850. As described in the example above.

[00136] -Releasing the second substrate 840 (see panel (b)) from the polymer

layer/graphene/third substrate. As described in the example above.

[00137] -Removing the ferroelectric polymer layer 830 (see panels (a) through (c)). The polymer may be removed in a solvent to the ferroelectric polymer, for example, acetone or dimethyl formamide may be used to dissolve the polymer if this was PVDF. Additional solvent cleaning may be utilized to remove residues from the solvents used to dissolve the polymer. Additionally, the sample may be annealed at a temperature and atmosphere that will remove the polymer film, its residues or the residues from the solvent cleaning step when the annealing conditions will be compatible with the substrate/graphene stack. For example, when the substrate is a silicon/silicon oxide wafer, the stack may be annealed in an argon and hydrogen atmosphere at 350°C

[00138] In accordance with a version of the invention, many advantages may be provided, for example as below: (00139] A) Compared to general transfer methods

[00140] A method in accordance with a version of the invention involves no chemicals for the release of the graphene from the graphene substrate

[00141] -The graphene at the graphene-substrate interface is not contaminated as when chemically or electrochemically releasing the graphene from the substrate.

[00142] For example, when the substrate of the graphene is copper and ammonium persulfate is used to chemically remove the copper, ions from the solution will be absorbed at

12 2 the graphene which carrier density concentration may change as much as 10 cm^" . This contamination is avoided when peeling the graphene.

[00143] -Therefore, the graphene can be applied onto a substrate without any

contamination between the graphene and the new substrate. This is of great importance in integrated circuit applications for a high throughput on device fabrication and performance.

[00144] Typically, in prior techniques, etching of copper substrate with ammonium persulfate to isolate the graphene will result in residues that uncontrollably dope graphene with ~10¹² cm^"2 dopants. By contrast, in devices that require undoped or doping graphene with precise concentrations, a method in accordance with a version of the invention enables graphene without any dopants on graphene.

[00145] A method in accordance with a version of the invention is a single-step and is performed fast

[00146] -Chemical or electrochemical removal processes involve etching, rinsing and drying steps that take hours, whereas peeling in accordance with a version of the invention is one single step process

[00147] The fastest transfer process reported uses the electrochemical delamination method which transfers at a rate of ~lmm/s. A method in accordance with a version of the invention is not limited in speed. Peeling according to a version of the invention occurs at velocities of 0.15 m/s and above, for example in the range of about 0.5 m/s, thereby enabling truly fast transfer.

[00148] A method in accordance with a version of the invention results in no residues:

[00149] -Since there are no chemical or electrochemical etching steps involved in the processing of the materials, there are no chemical residual products to dispose. A method in accordance with a version of the invention therefore results in a process that is industrially more sustainable. [00150] B) Compared to other peeling methods

[00151] In a method in accordance with a version of the invention, the following advantages can apply:

[00152] The interaction between PVDF and graphene can be driven by van der Waals and polarization induced interactions, not by any chemical absorption/interaction.

[00153] -There is no chemical or physical modification of the graphene and, therefore, its structure and characteristics can remain unchanged.

[00154] The polarized ferroelectric polymer layer is arranged and polarized to produce a reduced relative adhesion between the graphene layer and the growth substrate with respect to adhesion between the graphene layer and the polarized ferroelectric polymer layer. The polarized ferroelectric polymer layer may be arranged and polarized to strengthen the adhesion of the ferroelectric polymer layer to the graphene, and the adhesion between the ferroelectric polymer and graphene composite to the substrate.

[00155] -The method can enable a reliable peeling of the graphene from the substrate, can minimize the possibility of defects and, therefore improve the peeling yield

[00156] A method in accordance with a version of the invention has experimentally demonstrated statistical data over the peeling of graphene over an area that is significant to large area CVD graphene. Other reported mechanical peeling processes have not been able to validate their methods over such CVD graphene areas.

[00157] A method in accordance with a version of the invention has no need of melting any of the polymeric materials in the process.

[00158] -No modification is induced because of a reaction between graphene and the polymer

[00159] -The process can be thermally compatible with applications with thermal, pressure and/or voltage constrictions

[00160] Possible industrial applications:

[00161] Possible industrial applications in accordance with a version of the invention include:

[00162] - Fabrication of graphene based integrated circuits [00163] - Fabrication of devices containing graphene based flexible transparent conductive films

[00164] - Coating of a given surface with a graphene composite for encapsulation in applications including chemical encapsulation, such as a barrier to water vapour or other gases, and magnetic and electric shielding.

[00165] Definitions:

[00166] As used herein, "graphene" is single layer or multilayer graphene (for example, between 2 and 10 layers), preferably grown, for example, by a chemical vapor deposition-like process on a catalytic substrate such as copper. The catalytic substrate may be other metals, including nickel, platinum or cobalt, or other materials known to catalyze graphene, including germanium. The catalyst may comprise a metal foil or a metal thin film on a further substrate. Graphene may be graphene by other epitaxial methods, such as by heating of silicon carbide.

[00167] As used herein, "substrate" refers to the substrate graphene is grown on and may include, for example, a copper foil or film and any other materials known to be catalytic to graphene growth. Substrate may also refer to the secondary substrate used to peel the polarized ferroelectric polymer film and graphene composite from the substrate, as described above, and to enable the transfer of the composite to a target substrate. Target substrate refers to the substrate that the graphene is to be transferred to.

[00168] As used herein, a "ferroelectric polymer" is a polymer that can be processed for it to show ferroelectric characteristics, that is, that it will maintain a permanent electric polarization that can be reversed, or switched, in an external electric field. Ferroelectric polymers are fluoropolymers. Examples of ferroelectric polymers are polyvinylidene fluoride, PVDF, and its co-polymers. One such co-polymer is poly[(vinylidenefluoride-co- trifluoroethylene], P(VDF-TrFE).

[00169] While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

CLAIMS What is claimed is:

1. An article for delamination of a graphene layer from a growth substrate, the article comprising:

a graphene layer on a growth substrate; and

a polarized ferroelectric polymer layer on the graphene layer, the graphene layer being adhered to and sandwiched between the polarized ferroelectric polymer layer and the growth substrate, the polarized ferroelectric polymer layer being arranged and polarized to produce a reduced relative adhesion between the graphene layer and the growth substrate with respect to adhesion between the graphene layer and the polarized ferroelectric polymer layer.

2. The article of Claim 1 , wherein the polarized ferroelectric polymer layer comprises a fluoropolymer.

3. The article of Claim 2, wherein the polarized ferroelectric polymer layer comprises polyvinylidene fluoride or a copolymer of polyvinylidene fluoride.

4. The article of Claim 1 , wherein the polarized ferroelectric polymer layer comprises a thickness of between about 1 nanometer and about 1 millimeter.

5. The article of Claim 4, wherein the polarized ferroelectric polymer layer comprises a thickness of between about 100 nanometers and about 2000 nanometers.

6. The article of Claim 1 , wherein the polarized ferroelectric polymer layer comprises a remanent polarization of between about 5 μΟοχη²είΐΐά about 10 μ^αη².

7. A method of separating a composite from a growth substrate, the composite including a ferroelectric polymer layer and a graphene layer, the graphene layer being adhered to and sandwiched between the ferroelectric polymer layer and the growth substrate, the method comprising: (i) polarizing the ferroelectric polymer to produce a reduced relative adhesion between the graphene layer and the growth substrate with respect to adhesion between the graphene layer and the polarized ferroelectric polymer layer; and

(ii) peeling the composite to separate the graphene layer from the growth substrate.

8. The method of Claim 7, further comprising applying the ferroelectric polymer layer to the graphene layer to form the composite.

9. The method of Claim 7, further comprising transferring the peeled composite to a target substrate by adhering the graphene layer to the target substrate.

10. The method of Claim 9, further comprising removing the ferroelectric polymer layer from the graphene layer to leave the graphene layer adhered to the target substrate.

1 1. The method of Claim 7, wherein a continuity of the graphene layer on the

ferroelectric polymer layer after the peeling is 90% or more of an initial coverage of the graphene layer on the growth substrate.

12. The method of Claim 7, wherein a continuity of the graphene layer on the

ferroelectric polymer layer after the peeling is 95% or more of an initial coverage of the graphene layer on the growth substrate.

13. The method of Claim 7, wherein a continuity of the graphene layer on the

ferroelectric polymer layer after the peeling is 99% or more of an initial coverage of the graphene layer on the growth substrate.

14. The method of Claim 7, wherein the composite further includes a secondary substrate adhered to the ferroelectric polymer layer, and the ferroelectric polymer layer is sandwiched between the secondary substrate and the graphene layer.

15. The method of Claim 14, further comprising transferring the peeled composite to a target substrate by adhering the graphene layer to the target substrate; and

releasing the secondary substrate from the ferroelectric polymer layer to leave the ferroelectric polymer layer and the graphene layer adhered to the target substrate.

16. The method of Claim 15, further comprising removing the ferroelectric polymer layer from the graphene layer to leave the graphene layer adhered to the target substrate.

17. The method of Claim 7, wherein the polarizing includes applying an external electric field to the polymer layer.

18. The method of Claim 7, further comprising peeling the composite with a peeling force of at least about 85 J/m² to separate the composite from the growth substrate.

19. The method of Claim 7, wherein polarizing the ferroelectric polymer results in a remanent polarization of the ferroelectric polymer of between about 5 μ^αη and about 10 μθ/α^η2.