CN107406258A - Defect-free direct dry exfoliation of CVD graphene using polarized ferroelectric polymers - Google Patents

Abstract

A method for exfoliating a graphene layer from its growth substrate includes using an electrostatic field from a polarized ferroelectric layer to generate a relative adhesion force between the graphene layer and the growth substrate that is reduced relative to the adhesion force between the graphene layer and the polarized ferroelectric polymer layer. Related articles and techniques for exfoliating the graphene layer from the growth substrate are provided.

Description

Defect-free direct dry exfoliation of CVD graphene using polarized ferroelectric polymers

related application

This application claims the benefit of US Provisional Application No. 62/111,195, filed February 3, 2015, the entire teachings of which are incorporated herein by reference.

Background technique

Graphene has attracted great interest due to its excellent properties. However, its process is still a bottleneck hindering the commercialization of graphene applications.

Since the growth substrate and/or growth conditions do not match the final device, graphene needs to be transferred from its growth substrate to the device surface. The main steps in this transfer process are:

(1) Coating or covering the graphene fully or partially with the transfer layer. Referring to FIG. 1 , FIG. 1 shows a schematic diagram of a substrate/graphene/exfoliation layer structure according to the prior art, which structure includes an exfoliation layer 100 , graphene 110 and a substrate 120 .

(2) Release of the graphene/transferred layer from the growth substrate by chemical etching of the substrate, by (electro)chemical delamination or mechanical exfoliation of graphene from the substrate. Since graphene is processed in solution, chemical etching and (electro)chemical delamination are wet processes. Mechanical stripping is a dry process.

(3) Transfer the graphene/transfer layer onto the target substrate. This step can be skipped if the transfer layer is the target substrate for graphene.

(4) Remove the transfer layer. This step can be skipped if the transfer layer is kept on graphene.

This multi-step process leads to contamination on the graphene and induces structural defects that degrade the properties of the final device and make it inhomogeneous throughout the sample.

The method of releasing the graphene/transferred layer from the growth substrate strongly affects the residual and defect levels of the resulting graphene. As mentioned above, methods can be grouped as wet (chemical etching, (electro-)chemical delamination) or dry (mechanical stripping).

In case of wet process:

- Chemicals used or products from chemical reactions leave residues on the graphene that cannot be easily removed by thorough rinsing.

- These residues can bind to graphene and create defects in its crystal structure. This makes the graphene more brittle and prone to mechanical defects during handling, for example during transfer to the substrate or when removing the transferred layer. These defects would also degrade graphene's electronic properties when used in electronic devices.

- Even when residues do not cause defects, the presence of residues on graphene can degrade and/or become inhomogeneous in electrical, optical, chemical or mechanical properties of graphene. For example, within devices, a clean interface between graphene and adjacent layers is required to achieve high device efficiency, and such residues will degrade device performance and uniformity across devices.

- Once the graphene is transferred onto the target substrate, chemical residues at the interface between the graphene and the target substrate reduce the adhesion of the graphene to the substrate and thus limit the mechanical stability of the device.

- The wet process is slow due to the chemical reactions that occur. For example, etching a 35-micron thick copper substrate on which graphene can grow would typically take more than 2 hours. The (electro-)chemical delamination process is a faster process including any etching, but its speed is limited to about 1 millimeter per second. These processing speeds are no match for high-throughput manufacturing.

In case of dry process:

- A method of exfoliating graphene by ensuring sufficient adhesion of the exfoliated layer to the graphene such that the adhesion will be higher than the adhesion between graphene and its substrate. In the case where the substrate is copper, the adhesion of graphene to copper is about 8 J/m ² .

- The first mechanical exfoliation method utilizes pressure and/or temperature to achieve a conformal contact between the graphene and the exfoliated layer, and/or utilizes high voltage to induce direct chemical bonding between them. Then, the chemical adhesion of the exfoliated layer to the graphene needs to be higher than the adhesion of graphene to its substrate.

- The second type of mechanical release method requires an adhesive layer as release layer. Then, the adhesion of the adhesive layer to the graphene needs to be higher than the adhesion of the graphene to its substrate.

-However, it has been demonstrated that the transferred graphene is always severely damaged due to non-uniform strain and/or non-uniform exfoliation force.

- Inhomogeneous strain occurs when the exfoliated layer does not have a uniform coating over the entire graphene surface or when parameters such as temperature and pressure vary greatly.

- The inhomogeneity of the peeling force is caused by the actual conditions of the graphene on the substrate, where the substrate morphology consists of grain boundaries and terraces (length dimension 10-100 μm ² ). This changing topography leads to non-uniform peel force during peeling.

- During exfoliation, inhomogeneities cause cracks and/or areas of untransferred graphene. Due to these cracks, the quality of the transferred graphene is irreversibly reduced. These cracks form on the order of 0.1 mm, and when measured as cracks with a length of the order of 1 mm or longer, will have a significant impact on graphene quality. When using ideal crystalline graphene as the starting material on the growth substrate, such measurements show a drop in mobility to less than 200 cm ^-2 /Vs and exhibit incompatibility for transparent electrodes with the lowest sheet resistance, even at > 1,000ohm/sq high doping case. In contrast, if such graphene remains defect-free, the mobility is typically around 5,000 cm ⁻² /Vs, with a minimum sheet resistance of 150 ohm/sq.

- Both types of exfoliation methods aim to achieve a sufficiently high and uniform enough adhesion energy between the graphene and the exfoliated layer to yield defect-free exfoliation. These methods do not affect the adhesion energy between graphene and substrate. Therefore, the minimum energy required to delaminate graphene from the substrate remains unchanged. In order to further improve the quality of exfoliated graphene, exfoliated layers with strong adhesion to graphene and techniques to minimize heterogeneity across the graphene surface are still required.

Therefore, there is still a need for a commercially viable technique for exfoliating graphene from growth substrates, and this technique should preferably be a defect-free mechanical exfoliation method.

Contents of the invention

According to one version of the invention there is provided a method of exfoliating a graphene layer from its growth substrate. Previous methods relied on achieving stronger adhesion between graphene and the exfoliated layer than between graphene and its substrate by bringing the graphene into contact with a surface that binds graphene more strongly than graphene binds to its substrate. Attachment. Instead, according to one version of the method of the invention, the polarization of the ferroelectric polymer layer is used to induce a stronger bond between the graphene and the ferroelectric layer than between the graphene and its substrate. Strong adhesion.

According to one version of the invention, an article for delamination of a graphene layer from a growth substrate is provided. The article comprises a graphene layer on a growth substrate and a poled ferroelectric polymer layer on the graphene layer. A graphene layer is adhered to and sandwiched between the polarized ferroelectric polymer layer and the growth substrate. Arranging and polarizing the polarized ferroelectric polymer layer to create a reduced relative adhesion between the graphene layer and the growth substrate relative to adhesion between the graphene layer and the polarized ferroelectric polymer layer Adhesion. The polarized ferroelectric polymer layer can be arranged and polarized to enhance the adhesion of the ferroelectric polymer layer to the graphene, and the adhesion of the ferroelectric polymer and graphene composite to the substrate.

Further, in other related versions of the present invention, the graphene layer may comprise single or multilayer graphene (for example, 2 to 10 layers) grown on a catalytic substrate (for example, copper) by a method similar to chemical vapor deposition . The catalytic substrate can 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 metal film on another substrate. The graphene may be graphene obtained by other epitaxy methods, for example by heating 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 about 1 nanometer to about 1 millimeter, for example a thickness of about 100 nanometers to about 2000 nanometers. The polarized ferroelectric polymer layer may comprise a remanent polarization of about 5 μC/cm ² to about 10 μC/cm ² , for example about 7.5 μC/cm ² .

According to another version of the invention, there is provided a method of separating a composite material comprising a ferroelectric polymer layer and a graphene layer from a growth substrate, the graphene layer being adhered to and sandwiched between a ferroelectric polymer between the 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 relative to the adhesion between the graphene layer and the poled ferroelectric polymer layer adhesion; and (ii) exfoliation of the composite to separate the graphene layer from the growth substrate. The ferroelectric polymer layer can be polarized to create an attractive force, thereby enhancing the adhesion of the ferroelectric polymer layer to the graphene and the adhesion of the ferroelectric polymer and graphene composite to the substrate.

Further, in other related versions of the present invention, the method may further include applying a ferroelectric polymer layer to the graphene layer to form a composite material. The method may also include transferring the exfoliated composite material to the target substrate by adhering the graphene layer to the target substrate; and may also include removing the ferroelectric polymer layer from the graphene layer to leave the graphene layer. After exfoliation, the continuity of the graphene layer on the ferroelectric polymer layer may be 90% or higher of the initial coverage of the graphene layer on the growth substrate, such as the initial coverage of the graphene layer on the growth substrate 95% or higher, or for example 99% or higher. The composite material may also 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 also include transferring the exfoliated composite to the target substrate by adhering the graphene layer to the target substrate; and releasing the secondary substrate from the ferroelectric polymer layer, leaving the ferroelectric polymer adhered to the target substrate layer and the graphene layer; and the ferroelectric polymer layer can be removed from the graphene layer, leaving the graphene layer adhered to the target substrate.

Additionally, in other related versions of the invention, poling may include applying an external electric field to the polymer layer. The method may also include peeling the composite material with a peel force of at least about 85 J/ ^m2 to separate the composite material from the growth substrate. Polarizing the ferroelectric polymer may result in a remanent polarization of the ferroelectric polymer of about 5 μC/cm ² to about 10 μC/cm ² , for example about 7.5 μC/cm ² .

Description of drawings

The foregoing will be apparent from the following more particular description of exemplary embodiments of the invention illustrated in the accompanying drawings, wherein like reference numerals designate like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the invention.

Figure 1 is a schematic diagram of a substrate/graphene/exfoliated layer structure according to the prior art.

Figure 2 is a schematic diagram of a version of the graphene exfoliation method according to the present invention: the left figure shows graphene on a growth substrate, the center figure shows the use of a material such as polyvinylidene fluoride (herein "PVDF") Ferroelectric polymer coated graphene; and right panel showing exfoliation of graphene-PVDF from graphene growth substrate.

Figure 3 is a schematic diagram illustrating the mechanism of a strong graphene-ferroelectric polymer bond (e.g., graphene-PVDF bond) according to a version of the invention: Strong electrostatic enhancement (relative to graphene-to-substrate reinforcement); right panel shows PVDF-graphene atomic-scale bonding.

4 is a set of atomic force microscope images and cross-sectional views of the roughness of the interface of a substrate-graphene-ferroelectric polymer system, according to a version of the invention. The image on the left is a height scan of graphene on a substrate before coating with a ferroelectric polymer. The center panel is a height scan of the graphene-polarized ferroelectric polymer after the graphene-ferroelectric polymer has been exfoliated from the substrate. The right figure shows a cross-sectional view of a substrate-graphene and a graphene-polarized ferroelectric polymer according to parts of the left and center figures.

Figure 5 is a graph of the adhesion energy between a treated substrate, graphene and a ferroelectric polymer layer according to certain embodiments of the present invention, wherein the substrate is copper foil and the graphene is deposited on copper by chemical vapor deposition A layer of graphene is grown, while the ferroelectric polymer layer is PVDF. The dotted line indicates the critical adhesion between the polarized ferroelectric polymer layer and the graphene composite and the composite when an adhesive with sufficient adhesive strength is applied to the graphene, graphene and Critical adhesion energy between copper substrates, and between ferroelectric polymer layers and graphene when the ferroelectric polymer is not polarized. Experimental data relate to conditions under which polarized ferroelectric polymer layers and graphene composites are exfoliated from substrates under different loads.

Figure 6 is a set of photographs showing a comparison of graphene exfoliation yields using three different graphene exfoliation techniques. The left panel is a photo showing the results after exfoliation using a polymer that does not result in sufficiently strong bonding and/or an electric field perpendicular to the graphene; the center panel is a photo showing the results after exfoliation using a ferroelectric polymer, not The ferroelectric polymer structure is processed to form ferroelectric grains and/or these grains are not aligned according to the field perpendicular to the graphene; and the figure on the right is showing exfoliation using a version of the method according to the present invention Photos of the results. It can be seen that the technique used in the left figure leaves the regions without graphene 61 and graphene flakes 62; the technique used in the center figure leaves the regions without graphene 63 and graphene fragments (patches) 64; One version, the technique on the right obtains an overlay with both graphene 65 and multilayer graphene 66 .

Figure 7 is a set of graphs showing statistical comparisons of defects (cracks) after exfoliation of graphene using a version of the method according to the invention and using a standard prior art graphene transfer process: The statistical distribution of the cracks, and the right figure is the bar graph of the total crack area statistics.

Figure 8 is a schematic diagram illustrating the use of a direct peel transfer method according to a version of the present invention. Figure (a) shows graphene/ferroelectric polymer; Figure (b) shows graphene-ferroelectric polymer on a substrate with graphene facing up; Figure (c) shows graphite on a substrate ene-ferroelectric polymer, wherein the graphene faces down; and (d) shows the graphene on the surface.

detailed description

The following is a description of exemplary embodiments of the present invention.

According to one version of the invention there is provided a method of exfoliating a graphene layer from its growth substrate. Previous methods relied on achieving stronger adhesion between graphene and the exfoliated layer than between graphene and its substrate by bringing the graphene into contact with a surface that binds graphene more strongly than graphene binds to its substrate. Attachment. Instead, according to one version of the method of the invention, the polarization of the ferroelectric polymer layer is used to induce a stronger bond between the graphene and the ferroelectric layer than between the graphene and its substrate. Strong adhesion.

Figure 2 is a schematic diagram of a version of the graphene exfoliation method according to the present invention. The left figure shows initial graphene 210 on a growth substrate 220 . The center panel shows the coating of graphene 210 with a ferroelectric polymer 230 such as polyvinylidene fluoride (herein "PVDF"), which has the characteristics described below. The right figure shows the graphene/ferroelectric polymer layer 210 / 230 exfoliated from the graphene growth substrate 220 .

A) Exfoliation of graphene via a polarized ferroelectric layer (mechanism)

3 is a schematic diagram illustrating the mechanism of a strong graphene-ferroelectric polymer bond (eg, graphene-PVF bond) according to one version of the invention, without wishing to be bound by theory. According to one version of the method of the invention, the polarization of the ferroelectric polymer layer on the graphene is used to increase its adhesion to the graphene. Poling the ferroelectric polymer layer also increases the adhesion of the polarized ferroelectric polymer layer and graphene composite to the substrate. The polarized ferroelectric polymer layer also reduces the adhesion of graphene to the substrate relative to the adhesion of graphene to the polarized ferroelectric polymer layer.

On the one hand (see FIG. 3 , left panel), there is an attraction between the graphene 310 and the poled ferroelectric polymer 330 and between the poled ferroelectric polymer layer 330 and the graphene 310 composite and its substrate 320. force. According to one version of the method of the present invention, this mechanism can be achieved by using a polarized ferroelectric polymer layer 330 on the graphene/substrate 310/320.

- The alignment of the ferroelectric dipoles in the polarized ferroelectric polymer layer 330 in a direction perpendicular to the graphene 310 produces a strong electric field perpendicular to the graphene 310, regardless of the dipole's electrical orientation (electric orientation) how.

- The electric field generated by the ferroelectric layer is highly doped with graphene such that it induces charges within the graphene sheet. As a result of this electrostatic interaction, graphene is attracted to the ferroelectric layer. This attraction improves the adhesion between the graphene and the ferroelectric layer.

- Attractive interactions are also induced between the substrate and the polarized ferroelectric polymer layer and the graphene composite due to the electrostatic field from the polarized ferroelectric layer.

Due to the ferroelectric polymer/graphene/substrate structure 330/310/320, the adhesion energy between the ferroelectric polymer 330 and the graphene 310 is enhanced above that required to peel the graphene 310 from the substrate 320 . As a result of these interactions, graphene can be exfoliated from the substrate defect-free, ie without any induced mechanical defects.

On the other hand, according to a version of the invention (see FIG. 4 ), a coating of graphene 310 with a ferroelectric polymer 330 is prepared to ensure a homogeneous interface, which produces a strong bond precisely at the atomic level, Inhomogeneities in the lift-off process are thereby limited, and the graphene can be prevented from being stressed during the process. Therefore, graphene can be protected from mechanical damage.

The interface between graphene 310 and ferroelectric layer 330 is also designed to invert their low adhesion energy to obtain a binding energy capable of exfoliating the graphene.

- Graphene has inherently weak van der Waals adhesion strength to most materials.

- As an example, fluoropolymers have an inherent weak attraction, where Teflon is used for non-stick surfaces.

-However, due to the highly electronegative fluorine atoms on the polymer interacting with atoms from pi-orbitals (pi-orbitals) of graphene van der Waals, the interaction of graphene 310 and fluoropolymer 330 is critical for proper design of the coating And after polarizing the polymer on the graphene, it becomes stronger.

- Van der Waals interactions between graphene and fluoropolymers do not induce any defects in graphene.

- Ferroelectric polymers are fluoropolymers which are ideal materials for producing strong van der Waals adhesion strength based on Fluorine-Pi bond interactions with graphene once the molecules are oriented accordingly.

- Coating ferroelectric polymers on the graphene surface makes the interaction uniform across the graphene surface and does not induce any defects in the graphene.

- In a version of the ferroelectric/graphene/substrate structure according to the invention, in general the fluorine-π intermolecular bond has a stronger attraction between the graphene/ferroelectric, making Graphene can be mechanically exfoliated from the growth substrate.

-However, the mechanical strength between the ferroelectric polymer and graphene may not be strong and/or uniform at sites such as grain boundaries or copper terraces, so even if the graphene is exfoliated, cracks may still occur during transfer.

Each of the individual mechanisms contributes to defect-free graphene, and the combination of the two effects produces pristine, defect-free graphene, even across large areas of graphene across sites such as grain boundaries or copper terraces. ).

B) Embodiments according to a version of the invention

1) Form graphene on the growth substrate

In one example of the present invention, graphene may be grown by chemical vapor deposition (CVD) on a copper foil substrate. The copper foil can be cleaned with (but not limited to) a solvent to remove residue from its surface before placing it in the growth chamber. The growth process may include an annealing step in which a gas, such as hydrogen, will flow at a temperature around the growth temperature (ie, about 1000° C.). Next, a hydrocarbon such as methane will flow with the hydrogen to promote graphene growth. Finally, the growth chamber was cooled, and the copper foil with graphene on the surface was removed.

Graphene can be formed as a single layer or a multilayer depending on the type of growth substrate or the conditions in the synthesis chamber.

Alternatively, SiC can be used to grow graphene. In this case, the substrate is annealed at a temperature that will sublimate the Si atoms on the SiC surface and promote the recrystallization of the C atoms to form one or more layers of graphene.

2) Coating graphene with a polarized ferroelectric polymer film

In a dry environment, the graphene is coated with a solution of a ferroelectric polymer. Such coatings can be formed using methods such as, but not limited to, spin coating, Langmuir Blodgett, dip coating, slot die coating, bar coating, doctor blade or wire coating.

In an example of the present invention, PVDF can be dissolved in dimethylformamide (DMF), and this solution can be coated on graphene later.

In an example of the present invention, the graphene substrate/graphene can be coated with a PVDF film by spin coating. Spin coating of 10% PVDF in DMF solution at 2000 rpm can produce a coating of a 500 nm thick film. It may be necessary to pre-anneal the substrate to evaporate the water molecules on the graphene to obtain a graphene-polymer interface free of aqueous residues. The coating may need to be done in a dry environment so that the polymer layer is free from defects resulting from trapped water molecules between its molecules.

After coating, the film can be annealed to evaporate the solvent and recrystallize the polymer chains into grains. The annealing temperature should be lower than the melting temperature of the polymer to promote the formation of the ferroelectric phase. In an example of the present invention, the PVDF film layer with a thickness of 500 nm can be annealed at 135° C. for 1 minute to 24 hours.

The resulting thin film polymer layer can be from 1 nanometer to 1 millimeter thick, for example from about 100 nanometers to about 2000 nanometers thick.

After annealing, the dipoles in the ferroelectric polymer film can be aligned perpendicular to the graphene by applying an electric field across the film. The electric field can be applied by methods such as, but not limited to, applying a voltage across it using external electrodes or by ionizing the polymer surface. Depending on the setup, annealing and poling can be done in a single process. According to one version of the invention, polarizing the ferroelectric polymer may comprise applying an external electric field to the polymer layer, for example comprising an external electric field with an electric field strength of from about 50 V/μm to about 500 V/μm; and electrically polarizing the ferroelectric polymer may A surface comprising a polymer is ionized, for example, at a voltage of about 1 kV/cm to about 10 kV/cm.

The polarization direction is preferably such that the fluorine atoms of the ferroelectric polymer are aligned towards the graphene surface.

In the case of using PVDF as the ferroelectric polymer, a field of about 100 V/μm may be required to align the dipoles. In an example of the present invention, dipoles in an approximately 500 nm thick PVDF film can 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 polarization of about 5 μC/cm ² to about 10 μC/cm ² , for example about 7.5 μC/cm ² .

3) Exfoliated graphene/ferroelectric polymer composites

According to one version of the invention, exfoliation of the graphene/ferroelectric polymer from the growth substrate can be accomplished by applying an exfoliation force perpendicular to the growth substrate.

According to one version of the invention, a peel force of at least about 85 J/m ² results in reliable defect-free peeling (see FIG. 5 ). Higher peel forces also enable reliable peeling. According to one version of the invention, the critical force for exfoliating graphene, ie the minimum force at which graphene exfoliation occurs, is equal to or lower than 85 J/m ² .

A peeling force below the critical force may lead to cracks in graphene. However, reliable defect-free peeling with small forces is still possible by adequately controlling the applied force or sufficiently high peeling energy.

Exfoliation can be accomplished by processes such as (but not limited to) manual exfoliation and rolling of materials that adhere to PVDF (or graphene substrate) stronger than the critical adhesion energy at the substrate-graphene-polymer interface.

In one example of the present invention, the ferroelectric polymer can be thick enough to enable the removal of the ferroelectric polymer/graphene/substrate from the The stack is directly peeled off the copper foil by hand quickly.

In other examples of the invention, an additional support such as a polymer foil, epoxy, or tape can be attached to the graphene substrate, to the ferroelectric polymer, or to both to facilitate release. In these cases, the additional support must bind to either surface more strongly than the graphene adheres to its substrate. In an example of the present invention, wherein the substrate is copper foil, the support may be a thermal release tape with an adhesion strength of 3.7 N/20 mm (see FIG. 5 ). The adhesion strength between the tape and the polarized ferroelectric layer increases with the peeling speed, and at least 0.15 m/s is required to peel graphene from the substrate with an adhesion strength of 85 J/m ² . Graphene exfoliation was unsuccessful in the case of insufficient exfoliation speed because the thermal release tape could not adhere sufficiently to the polarized ferroelectric layer to induce graphene exfoliation, i.e., the ferroelectric layer and graphene remained on the copper foil.

In the case of growing graphene on both surfaces of the substrate, the transfer process can be done simultaneously on both sides of the substrate to exfoliate the graphene on each surface.

A version of the method according to the invention is matched to the patterning of the graphene and/or ferroelectric layers prior to exfoliation of the composite from the substrate. Depending on the patterning method, the benefits of the exfoliation method may only be applicable to unpatterned graphene/ferroelectric polymer composites. For example, if an area of the initial composite material is removed from the substrate prior to stripping, that area will not be stripped.

After exfoliation, the continuity of the graphene is a parameter that can be used to validate the graphene exfoliation method, since any mechanical defects (cracks) in the graphene layer will irreversibly degrade the properties of the exfoliated graphene. Figure 6 compares the yields of graphene exfoliation by various techniques.

In the left panel of Figure 6, when graphene is exfoliated from copper foil according to a version of the invention using a polymethylmethacrylate film according to the exfoliation parameters, except that no ferroelectric polymer is used (in which the steps described above are performed 1 and 3, but using a graphene coating with PMMA instead of step 2), exfoliating only the graphene flakes. This is an unsuccessful transfer. It is caused by the weaker adhesion of non-ferroelectric polymers to graphene than graphene to its substrate.

In the center panel of Fig. 6, when the method according to one version of the invention is not followed exactly (performing the previous steps 1-3, but omitting some steps of PVDF coating or poling), graphene exfoliation occurs, but The yield of the transfer is not maximal because the graphene exfoliation is not uniform and mechanical defects appear in the graphene layer. This is a discontinuous transfer of graphene. In this case, a ferroelectric polymer was used, but it was not treated so that its molecules formed grains and/or the dipoles in these grains were not aligned perpendicular to the graphene.

These results are considered unsuccessful here due to the discontinuity of graphene. However, these results can be considered successful if exfoliation is shown in the range of a few square microns. In lengths of this magnitude, graphene can exhibit continuity. However, such conditions should not be validated for large area if the validation does not include areas such as grain boundaries on catalyst substrates, graphene on distinct adjacent grains, and areas relevant for large area/wafer scale fabrication. Method for exfoliating graphene.

In the right panel of Figure 6, a complete and defect-free transfer of graphene is achieved when using a version of the method according to the invention. From the statistical analysis of the continuity of the exfoliated graphene (minimum detectable defect size of 0.5 μm ² ), it was concluded that the number of defects was significantly improved with respect to the standard transfer conditions, and the exfoliated graphene covered More than 99.5% of the sample surface on a square millimeter area. Figure 7 is a set of graphs showing a statistical comparison of defects (cracks) after exfoliation of graphene using a version of the method according to the invention and using a standard graphene transfer process: the left graph shows the distribution of cracks according to their area Statistical distribution, and the right picture is a bar chart of total crack area statistics.

Furthermore, stacks of up to 10 or more multilayers can be exfoliated together with the main graphene layer when using a version of the method according to the invention. These graphene multilayers are intrinsic to the graphene growth method. The darker spots in the optical image of the right panel of Fig. 6 correspond to multilayer graphene exfoliated together with continuous graphene layers.

The method according to one version of the invention provides a method for exfoliating graphene that produces continuous and defect-free graphene.

Other exemplary versions are provided below. Figure 8 is a schematic diagram illustrating the use of a direct peel transfer method according to a version of the present invention. As described further below, graph (a) in FIG. 8 shows graphene/ferroelectric polymer 810/830; graph (b) in FIG. 8 shows graphene-ferroelectric polymer on transfer substrate 840 Object 810/830, wherein graphene 810 faces up; Figure (c) in Figure 8 shows graphene-ferroelectric polymer 810/830 on target substrate 850, wherein graphene 810 faces down; Figure 8 Panel (d) in FIG. 2 shows graphene 810 on a target surface 850 .

A) Graphene/Polared Ferroelectric Polymer - see panel (a) in Figure 8

The exemplary method of panel (a) in Figure 8 can produce a continuous composite material.

- Graphene 810 has low sheet resistance due to doping from PVDF film 830 . In contrast to other doping methods, electrostatic doping (by PVDF) is stable over time.

- Exposes the graphene 810 so it can be post-processed.

-PVDF Graphene 810 has no residue and its continuity is >99%.

- Graphene 810 is clean and pollution-free.

The process used for this example was as described above.

B) Graphene/polarized ferroelectric polymer with transferred substrate - see panel (b) in Figure 8

The exemplary method of panel (b) in FIG. 8 can produce a continuous composite material on top of the substrate with the graphene 810 facing upwards.

- Graphene 810 has low sheet resistance due to doping from PVDF film 830 . In contrast to other doping methods, electrostatic doping (by PVDF) is stable over time.

- Exposes the graphene 810 so it can be post-processed.

-PVDF Graphene 810 has no residue and its continuity is >99%.

- Graphene 810 is clean and pollution-free.

The process can be:

- Graphene 810 is formed on the first substrate (graphene substrate) as described in the previous examples.

- A ferroelectric polymer layer 830 is formed on the graphene as described in the preceding examples.

- attaching a second substrate (also referred to as transfer substrate) 840 on the polarized ferroelectric layer 830 (second substrate/ferroelectric/graphene/substrate), such as (but not limited to) polyethylene terephthalate Glycol ester (PET) foil. The second substrate 840 is preferably in close contact with the ferroelectric film 830 . The adhesion strength between the substrate 840 and the ferroelectric film 830 was 85 J/m ² or higher, so that the graphene 810 could be peeled off without releasing the second substrate 840 from the ferroelectric film 830 . When the adhesion strength of the second substrate to the polarization is 85 J/m ² or higher, the peeling speed may not be limited. When the adhesion strength of the second substrate with polarization will be lower than 85 J/ ^m2 , the critical adhesion strength can be obtained by high-speed peeling, because the adhesion force at the interface will depend on this parameter.

-Exfoliation of the second substrate/ferroelectric/graphene from the graphene substrate as described in the preceding examples.

C) Polarized ferroelectric polymer/graphene on target substrate - see panel (c) in Figure 8

The exemplary method of panel (c) in FIG. 8 can produce a continuous composite material on top of the substrate, with the graphene 810 facing the target substrate.

- Graphene 810 has low sheet resistance due to doping from PVDF film 830 . In contrast to other doping methods, electrostatic doping (by PVDF) is stable over time.

- After removal of one or more surrounding layers, the surface of the graphene 810 can be exposed so it can be post-processed.

-PVDF Graphene 810 has no residue and its continuity is >99%.

- Graphene 810 is clean and pollution-free.

The process can be:

- Graphene 810 is formed on a first substrate (graphene substrate 320 in FIG. 3 ) as described in the preceding examples.

- A ferroelectric polymer layer 830 is formed on the graphene 810 as described in the previous examples.

- Attaching a second substrate 840 (not shown in figure (c)) on the poled ferroelectric polymer layer 830 (second substrate/ferroelectric/graphene/substrate). In this example, the second substrate 840 is compatible with the exfoliation of the graphene 810 in the next process step and can be released later. An example of the second substrate 840 is heat release tape. The second substrate 840 must be in close contact with the ferroelectric film 830 . The adhesion strength between the substrate 840 and the ferroelectric film 830 was 85 J/m ² or higher, so that the graphene 810 could be peeled off without releasing the second substrate 840 from the ferroelectric film 830 . When the adhesion strength of the second substrate to the polarization is lower than 85 J/m ² , this adhesion strength can be achieved by high-speed peeling. In an example of the present invention, wherein the substrate is copper foil, the support may be a thermal release tape with an adhesion strength of 3.7N/20mm (see FIG. 5 ). The adhesion strength between the tape and the polarized ferroelectric layer increases with the peeling speed, and at least 0.15 m/s is required to peel graphene from the substrate with an adhesion strength of 85 J/m ² . Graphene exfoliation was unsuccessful in the case of insufficient exfoliation speed because the thermal release tape could not adhere sufficiently to the polarized ferroelectric layer to induce graphene exfoliation, i.e., the ferroelectric layer and graphene remained on the copper foil.

- Peel off the second substrate/ferroelectric/graphene from the substrate as described in the previous example.

- Applying the second substrate/ferroelectric polymer/graphene stack on the third substrate (target substrate) 850 . In the case that the second substrate 840 is a thermal release tape, the graphene side of the stack was brought into close contact with the third substrate 850 by applying a pressure of 10 MPa.

- releasing the second substrate 840 from the polymer layer/graphene/third substrate. Where the second substrate 840 is a thermal release tape, the stack is heated to a temperature 5°C above the release temperature of the tape (typically 90 to 150°C) while maintaining the previously applied pressure. Once the release temperature is reached, remove the tape slowly to prevent damage to the polarized ferroelectric polymer/graphene.

D) Graphene on target substrate - see panel (d) in Figure 8

The exemplary method of panel (d) in FIG. 8 can produce a continuous graphene 810 on top of a target substrate 850 .

- Exposes the graphene 810 so it can be post-processed.

-PVDF Graphene 810 has no residue and its continuity is >99%.

- Graphene 810 is clean and pollution-free.

The process can be:

- Graphene 810 (graphene substrate 320 of FIG. 3 ) is formed on a first substrate as described in the preceding examples.

- Formation of a ferroelectric polymer layer 830 on the graphene (see Figures (a) to (c)). As described in the previous example.

- Attaching the second substrate 840 on the ferroelectric film (second substrate/ferroelectric/graphene/substrate) (see figure (b)). As described in the previous example.

- Peel off the second substrate/ferroelectric/graphene from the graphene substrate 320 (see Figure 3). As described in the previous example.

- Applying the second substrate/ferroelectric polymer/graphene stack on the third substrate (target substrate) 850 . As described in the previous example.

- Release of the second substrate 840 from the polymer layer/graphene/third substrate (see figure (b)). As described in the previous example.

- Removal of the ferroelectric polymer layer 830 (see Figures (a) to (c)). The polymer can be removed in a solvent for the ferroelectric polymer, for example in the case of PVDF acetone or dimethylformamide can be used to dissolve the polymer. An additional solvent wash can be used to remove residue from the solvent used to dissolve the polymer. Furthermore, when the annealing conditions are matched to the substrate/graphene stack, the samples can be annealed at a temperature and atmosphere that removes the polymer film, its residues, or residues from solvent cleaning steps. For example, when the substrate is a silicon/silicon oxide wafer, the stack can be annealed at 350°C in an atmosphere of argon and hydrogen.

According to a version of the invention, many advantages can be provided, such as the following:

A) Compared to general transfer methods

A version of the method according to the invention does not include the chemicals used to release the graphene from the graphene substrate.

- Graphene at the graphene-substrate interface is not contaminated when the graphene is chemically or electrochemically released from the substrate.

For example, when the substrate of graphene is copper and ammonium persulfate is used to chemically remove the copper, ions from the solution will be absorbed at the graphene with carrier density concentration variations as high as 10 ¹² cm ⁻² . This contamination can be avoided when exfoliating graphene.

- Thus, graphene can be applied to the substrate without any contamination between the graphene and the new substrate. This is very important for high throughput device fabrication and performance in integrated circuit applications.

Typically, in the prior art, etching copper substrates with ammonium persulfate to isolate graphene would result in residues that uncontrollably dope graphene with ~10 ¹² cm ⁻² dopants. In contrast, in devices requiring undoped graphene or doped graphene at precise concentrations, a version of the method according to the invention enables graphene without any dopant thereon.

A version of the method according to the invention is single-step and proceeds quickly.

- The chemical or electrochemical removal process involves etching, rinsing and drying steps which take hours, whereas a version of stripping according to the invention is a one-step process.

The fastest reported transfer process employs an electrochemical delamination method, which transfers at a rate of ~1 mm/s. The speed of the method according to one version of the invention is not limited. The peeling according to one version of the invention takes place at speeds of 0.15 m/s and above, for example in the range of about 0.5 m/s, enabling really fast transfers.

A version of the method according to the invention produces no residue:

- Since no chemical or electrochemical etching steps are included in the processing of the material, there are no chemical residual products to deal with. Thus, a version of the method according to the invention results in an industrially more sustainable process.

B) Comparison with other stripping methods

In a version of the method according to the invention, the following advantages can be applied:

The interaction between PVDF and graphene can be driven by van der Waals forces and polarization-induced interactions rather than by any chemical absorption/interaction.

- There is no chemical or physical modification of graphene, therefore, its structure and properties can remain unchanged.

arranging and polarizing the polarized ferroelectric polymer layer to produce a reduced adhesion between the graphene layer and the growth substrate relative to the adhesion between the graphene layer and the polarized ferroelectric polymer layer relative adhesion. The polarized ferroelectric polymer layer can be arranged and polarized to enhance the adhesion of the ferroelectric polymer layer to the graphene, and the adhesion of the ferroelectric polymer and graphene composite to the substrate.

- The method can achieve reliable exfoliation of graphene from the substrate, which can minimize the possibility of defects, thereby increasing the exfoliation yield.

According to the method of a text of the present invention, the statistics of graphene exfoliation on regions important for large-area CVD graphene are experimentally demonstrated. Other reported mechanical exfoliation processes cannot validate their methods on such CVD graphene domains.

According to one version of the method of the invention, there is no need to melt any polymer material in the method.

- No modification is induced due to the reaction between graphene and polymer.

- The process is thermally compatible with applications with heat, pressure and/or voltage constraints.

Possible industrial applications:

Possible industrial applications according to a version of the invention include:

- Manufacture of graphene-based integrated circuits

- Fabrication of devices containing graphene-based flexible transparent conductive films

- Coating a given surface with a graphene composite for encapsulation in applications including chemical encapsulation, such as barriers to water vapor or other gases and magnetic and electrical shielding.

definition:

As used herein, "graphene" is single or multilayer graphene (eg, 2 to 10 layers), preferably grown on a catalytic substrate such as copper, eg, by methods like chemical vapor deposition. The catalytic substrate can be other metals, including nickel, platinum, or cobalt, or other materials known to catalyze graphene, including germanium. The catalyst may comprise metal foils or metal films on other substrates. Graphene may be graphene obtained by other epitaxy methods, for example by heating silicon carbide.

As used herein, "substrate" refers to a substrate on which graphene is grown, and may include, for example, copper foil or film as well as any other material known to catalyze graphene growth. A substrate may also refer to a secondary substrate as described above for exfoliating the polarized ferroelectric polymer film and graphene composite from the substrate and enabling transfer of the composite to a target substrate. The target substrate refers to the substrate onto which the graphene is to be transferred.

As used herein, a "ferroelectric polymer" is a polymer that can be treated to exhibit ferroelectric properties, ie, it will maintain a permanent electrical 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 copolymers thereof. One such copolymer is poly[(vinylidene fluoride-co-trifluoroethylene], P(VDF-TrFE).

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that modifications may be made without departing from the scope of the invention as encompassed by the appended claims. Various changes were made in form and detail.

Claims (19)

1. An article for delaminating a graphene layer from a growth substrate, the article comprising: a graphene layer on a growth substrate; and a poled ferroelectric polymer layer on the graphene layer, the graphene layer being adhered to and sandwiched between the poled ferroelectric polymer layer and the growth substrate, to the The polarized ferroelectric polymer layer is arranged and polarized to create an adhesion between the graphene layer and the growth substrate relative to the adhesion between the graphene layer and the polarized ferroelectric polymer layer. Decreased relative adhesion in terms of adhesion. 2. The article of claim 1, wherein the poled 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 poled ferroelectric polymer layer comprises a thickness of about 1 nanometer to about 1 millimeter. 5. The article of claim 4, wherein the poled ferroelectric polymer layer comprises a thickness of about 100 nanometers to about 2000 nanometers. 6. The article of claim 1, wherein the polarized ferroelectric polymer layer comprises a remanent polarization of about 5 μC/ ^cm2 to about 10 μC/ ^cm2 . 7. A method of isolating a composite material comprising a ferroelectric polymer layer and a graphene layer adhered to and sandwiched between the ferroelectric polymer layer and the Between growth substrates, the method includes: (i) Polarizing the ferroelectric polymer to generate an adhesive force between the graphene layer and the growth substrate relative to the adhesion between the graphene layer and the polarized ferroelectric polymer layer reduced relative adhesion; and (ii) exfoliating the composite material 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 material. 9. The method of claim 7, further comprising transferring the exfoliated composite material 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. 11. The method of claim 7, wherein after exfoliation, the continuity of the graphene layer on the ferroelectric polymer layer is a fraction of the initial coverage of the graphene layer on the growth substrate 90% or higher. 12. The method of claim 7, wherein after exfoliation, the continuity of the graphene layer on the ferroelectric polymer layer is a fraction of the initial coverage of the graphene layer on the growth substrate 95% or higher. 13. The method of claim 7, wherein after exfoliation, the continuity of the graphene layer on the ferroelectric polymer layer is 1% of the initial coverage of the graphene layer on the growth substrate 99% or higher. 14. The method of claim 7, wherein the composite further comprises a secondary substrate adhered to the ferroelectric polymer layer, and the ferroelectric polymer layer is sandwiched between the secondary substrate and the ferroelectric polymer layer. between the graphene layers. 15. The method of claim 14, further comprising transferring the exfoliated composite material to the target substrate by adhering the graphene layer to the target substrate; and The secondary substrate is released 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 poling includes applying an external electric field to the polymer layer. 18. The method of claim 7, further comprising peeling the composite material with a peel force of at least about 85 J/ ^m2 to separate the composite material 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 about 5 μC/cm ² to 10 μC/cm ² .