HK1234721B - Method for the fabrication and transfer of graphene - Google Patents

Description

Graphene production and transfer methods

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Application No. 62/015,116, filed June 20, 2014, which is incorporated herein by reference in its entirety.

Federally funded research or development

This invention was made with government support under Grant No. FA9550-13-1-0156 awarded by the Air Force Office of Scientific Research and under a National Science Foundation Graduate Research Fellowship awarded under DGE-1144086. The government has certain rights in this invention.

Technical Field

The present invention relates to the field of graphene, and more particularly to a method for transferring a graphene layer onto a substrate.

Background Art

Graphene is an allotropic form of carbon consisting of a two-dimensional hexagonal arrangement of carbon atoms. Each graphene layer is essentially a single-atom-thick planar layer of carbon atoms bonded in a honeycomb lattice. Graphene can be in the form of one, two, hundreds, or thousands of graphene layers. The electrical, mechanical, optical, and chemical properties of graphene make it attractive for applications in high-performance electronics and optical devices, and it is expected to play an important role in future technologies spanning from consumer electronics to devices for energy conversion and storage, to biomedical devices for suitable health care. However, in order to achieve these applications, a low-cost method for synthesizing large-area, high-quality graphene is needed. Current methods for growing high-quality, large-area, single-layer graphene compatible with roll-to-roll manufacturing are highly wasteful, with each 1g of graphene produced destroying approximately 300kg of copper foil (thickness=25μm). The effort to reduce this waste is driven by two goals. The first goal is to reduce the cost and environmental impact of relatively high-end applications (i.e., nanoelectronics and transparent electrodes), for which graphene is currently considered an important component. The second goal is to enable potential applications, namely disposable electronics, textiles, suitable biomedical devices, and thin-film photovoltaic modules, which are difficult to achieve using graphene at current costs. In a well-known roll-to-roll compatible method originally described by Bae et al. (Bae, S. et al., S. Nat. Nanotechnol. 2010, 5, 574), a single layer of graphene is grown on a large area of copper foil by chemical vapor deposition (CVD) and stripped to a carrier substrate by chemical etching of the copper. This method is important for its ability to produce films over large areas, but the cost of a single-atom-thick graphene layer includes the destruction of an equivalent area of 70,000-atom-thick copper foil, as well as the economic costs and environmental externalities associated with disposing of large amounts of corrosive waste. Therefore, there is a need for improved methods for synthesizing high-quality graphene over large areas.

Summary of the Invention

The present invention is based, at least in part, on the development of an environmentally friendly and scalable method for transferring graphene to substrates (e.g., flexible, strong, or rigid substrates). The method is based on the preferential adhesion of certain thin metal films to graphene; the graphene is separated from a catalyzed metal foil (e.g., copper or nickel foil) and then laminated to a flexible target substrate in a process compatible with roll-to-roll manufacturing. The metal foil (e.g., copper or nickel foil) substrate is reusable indefinitely, and the method is substantially greener than current processes that use corrosive iron (III) chloride to etch metals (e.g., copper).

In one aspect, the present disclosure provides a method for fabricating a graphene layer on a substrate, comprising providing a graphene layer disposed on a first substrate, applying a metal layer to the graphene layer to form a metallized graphene layer, removing (e.g., peeling, flaking) the metallized graphene layer from the first substrate, and applying (e.g., laminating) the metallized graphene layer to a second substrate. In some aspects, the method further comprises removing a thermal release adhesive tape from the metal layer.

Providing a graphene layer disposed on a first substrate may include providing a first substrate and subsequently growing the graphene layer on the first substrate layer using a chemical vapor deposition method. The first substrate may be a metal foil (e.g., a catalytic metal foil) selected from copper foil, nickel foil, or any other metal foil material capable of supporting graphene deposition via chemical vapor deposition. In some embodiments, the first substrate comprises copper, nickel, or alloys thereof.

In some embodiments, the graphene layer is a single layer. In other embodiments, the graphene layer includes two or more graphene layers.

In some aspects, the method disclosed herein includes a process for applying a metal layer to the graphene layer. The process for applying the metal layer to the graphene layer can be completed by a vacuum metallization method or an electrochemical metallization method. In some aspects, the vacuum metallization method is selected from the group consisting of electron beam evaporation, thermal evaporation and sputtering. In some aspects, the electrochemical metallization method is selected from the group consisting of electroplating process, electroless deposition and atomic layer deposition. In some embodiments, the metal layer applied to the graphene layer includes gold, nickel, cobalt, iron, silver, copper, tin, palladium, platinum, its alloy or its combination. In some other embodiments, the metal layer applied to the graphene includes a transition metal or its alloy (such as scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, lanthanum, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold and mercury). For example, at least one metal or alloy selected from cobalt and nickel can be used. In an exemplary embodiment, the metal layer includes nickel, cobalt, or gold. The metal layer applied to the graphene layer can have a thickness of about 1 to about 1000 nanometers (nm), about 20 nm to about 1000 nm, about 50 nm to about 750 nm, about 100 nm to about 500 nm, about 125 nm to about 250 nm, or about 150 nm to about 200 nm. For example, the metal layer can be applied to the graphene with a thickness of about 20 nm to about 1000 nm, about 50 nm to about 750 nm, about 100 nm to about 500 nm, about 125 nm to about 250 nm, about 150 nm to about 200 nm, about 75 nm, about 100 nm, about 125 nm, about 150 nm, about 175 nm, or about 200 nm.

In some embodiments, the metal layer applied to the graphene layer includes two or more sequentially deposited metal layers. For example, the metal layer applied to the graphene layer may include two or more sequentially deposited metal layers, the metal layers including transition metals or alloys (e.g., scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, lanthanum, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, and mercury). For example, at least one metal or alloy selected from cobalt and nickel may be used. In an exemplary embodiment, the metal layer includes nickel, cobalt, or gold. The two or more sequentially deposited metal layers may have a thickness of about 1 to about 1000 nanometers (nm), about 20 nm to about 1000 nm, about 50 nm to about 750 nm, about 100 nm to about 500 nm, about 125 nm to about 250 nm, or about 150 nm to about 200 nm. For example, each metal layer can have a thickness of about 20 nm to about 1000 nm, about 50 nm to about 750 nm, about 100 nm to about 500 nm, about 125 nm to about 250 nm, about 150 nm to about 200 nm, about 75 nm, about 100 nm, about 125 nm, about 150 nm, about 175 nm, or about 200 nm.

In some aspects, the method disclosed herein includes a method for peeling a metallized graphene layer from a first substrate. In some embodiments, the process for peeling a metallized graphene layer from a first substrate includes: adhering an intermediate substrate to the metal, and applying a force sufficient to overcome the interaction between the first substrate and the graphene layer to the intermediate substrate to remove the metallized graphene from the first substrate. The intermediate layer can be a thermal release adhesive tape, another metal (magnetic) layer, or include a conductive or insulating layer. The thermal release adhesive tape is applied to the metal layer using a roller to apply the thermal release adhesive tape from one edge of the metal layer to the opposite edge of the metal layer. In some embodiments, the peeling process includes removing the intermediate layer from the metal layer (e.g., removing the thermal release adhesive tape, metal (magnetic) layer, conductive layer, or insulating layer). In some aspects, the method disclosed herein includes a step of applying a thermal release adhesive tape layer directly to a metal layer disposed on the graphene layer. The heat release adhesive tape can be applied to the metal layer by hand or using a roller (eg, a roll-to-roll transfer process) that applies the heat release adhesive tape from one edge of the heat release adhesive tape to an opposite edge of the heat release adhesive tape.

In some aspects, the methods disclosed herein further comprise removing the metal layer from the graphene layer after laminating the graphene layer to the second substrate. In one embodiment, the methods disclosed herein comprise etching the metal layer to remove the metal layer from the graphene layer.

In some aspects, the present disclosure provides a method for transferring a graphene layer to a flexible substrate, the method comprising providing a graphene layer, applying a metal layer to the graphene layer to form a metallized graphene layer, applying a heat-release adhesive tape to the metal layer, and laminating the metallized graphene layer to the flexible substrate. According to some aspects, the method further comprises removing the metal layer from the graphene layer after laminating the metallized layer to the flexible substrate.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Methods and materials for use in the present invention are described herein; other suitable methods and materials known in the art may also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, sequences, database entries, and other references mentioned herein are incorporated by reference in their entirety. In the event of a conflict, the present specification (including definitions) will control.

Other features and advantages of the invention will be apparent from the following detailed description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing printed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG1 is a schematic diagram illustrating an exemplary metal-assisted exfoliation (MAE) method for large-area transfer of single-layer graphene from a catalyzed copper substrate to a flexible sheet, which includes preferential adhesion of a metal (e.g., nickel or cobalt) layer to the graphene, exfoliation of the metallized graphene, and lamination mediated by a tape with a heat-deactivated adhesive.

Figures 2a-h are photographs showing the sequential steps of an exemplary MAE process for large-area transfer of single-layer graphene. A single-layer graphene on copper foil after: (a) nickel metallization, (b) application of hot-peel tape, (c) peeling of the metallized graphene from the copper foil, (d) lamination of a polyethylene terephthalate (PET) sheet to the metallized graphene (hot-peel tape deactivated), (e) removal of the hot-peel tape from the PET/graphene/nickel sheet, and (f) immersion of the PET/graphene/nickel sheet in an iron(III) chloride solution (3-5 seconds). The PET sheet coated with a single-layer graphene (g) is shown against a postcard with the UCSD Geisel Library (the outline of the PET/graphene sheet is indicated by a box) and (h) illuminated with low-angle incident light. The insets in (c) and (h) depict anisotropic cracks formed in the metal film during peeling of the hot-peel tape/metal film/graphene sheet from the copper foil.

Figure 3 shows Raman spectra of (a) graphene grown on copper foil, (b) copper foil after metal-assisted graphene exfoliation (the absence of graphene peaks indicates complete removal of graphene from the copper foil), (c, e, g) graphene on metal films transferred from copper via metal-assisted exfoliation, and (d, f, h) pure metal films, respectively (gold and copper substrates significantly enhance Raman scattering and produce strong, clear graphene peaks - gray highlights (e.g., D, G, 2D) - compared to cobalt and nickel).

FIG4 is a graph showing the Raman spectra of graphene transferred to Si/ _SiO2 by conventional wet transfer method (upper spectrum) and metal assisted (Ni) method (lower spectrum).

Figure 5a-e shows an optical micrograph of graphene transferred from the same copper foil substrate to a Si/SiO2 _wafer after the first (a), second (b) and third (c) synthesis using a conventional wet transfer method. After graphene was transferred via nickel evaporation, each successive synthesis produced cleaner, better quality graphene. That is, the number of multilayer regions appearing as darker spots in the image was significantly reduced from (a) to (c). The white contaminants visible in (ac) are residual PMMA that cannot be removed in a boiling acetone bath. Picture (d) shows the residual graphene grains (previous multilayer regions) on the copper substrate after graphene peeling in the MAE method. The Raman spectrum of the transferred graphene (e) also shows the improvement in quality when graphene was continuously synthesized on the same substrate (the D/G peak ratio was reduced from 0.08 in the first growth to 0.04 in the third growth).

FIG. 6 is a schematic diagram illustrating a method of patterning metallized graphene.

DETAILED DESCRIPTION

The present disclosure is based in part on the discovery of a new environmentally friendly and scalable method for transferring graphene to flexible substrates. The method is based on the preferential adhesion of certain thin metal films to graphene, the peeling of graphene from a metal foil substrate used as a substrate for growing graphene, and the subsequent lamination of the graphene layer to a flexible target substrate using a heat-deactivated adhesive in a method compatible with roll-to-roll manufacturing. The metal foil substrate (e.g., a copper foil substrate) can be reused indefinitely, and the method is substantially greener than the current process of etching the metal foil using corrosive chemical solutions. The quality of graphene produced by this new method is similar to that of graphene produced by standard methods, given the defects observable by Raman spectroscopy. The synthesis of green and inexpensive high-quality single-layer graphene will enable applications in flexible, stretchable and disposable electronics, low-profile and lightweight barrier materials, and large-area displays and photovoltaic modules, which are inaccessible to current expensive and environmentally harmful methods for producing this versatile material.

In one aspect, the present disclosure provides a metal-assisted exfoliation (MAE) method for large-area transfer of single-layer graphene from a catalyzed metal substrate to a flexible sheet, which includes preferentially adhering a metal layer to the graphene, exfoliation, and lamination mediated by a tape with a heat-deactivated adhesive.

The terms "heat-deactivated adhesive" and "heat-peel adhesive" are used interchangeably and refer to an adhesive that can be deactivated when exposed to heat, for example, at least 80°C, at least 85°C, at least 90°C, at least 95°C, at least 100°C, at least 105°C, at least 110°C, at least 115°C, at least 120°C, at least 130°C, at least 140°C, at least 150°C, or at a temperature of about 80°C to about 150°C, a temperature of about 90°C to about 140°C, a temperature of about 100°C to about 130°C, or a temperature of about 110°C to about 120°C. Thus, the selective application of heat will modify the heat-deactivated adhesive, eliminating or substantially reducing the adhesion between the tape and the substrate. The term "heat-peel tape" refers to a tape that includes at least one adhesive layer containing a heat-deactivated adhesive.

FIG1 provides exemplary steps of a metal-assisted exfoliation (MAE) method according to the present invention for transferring a grown single-layer graphene to a polyethylene terephthalate (PET) sheet over a large area via a chemical vapor deposition (CVD) process, the method comprising preferential adhesion of nickel (or cobalt) to graphene, exfoliation, and lamination mediated by a tape having a thermally deactivated adhesive. Briefly, a single-layer graphene layer is grown on a metal foil substrate by ambient pressure CVD ( FIG1 , step 1). Although the process described herein includes forming a single-layer graphene layer (e.g., a graphene monolayer), the method includes forming multiple layers of graphene (e.g., more than one layer, more than two layers, more than fifty layers, more than one hundred layers, more than one thousand layers, or more than one hundred thousand layers of graphene) on a substrate.

The growth of the graphene layer on the metal foil can be achieved using chemical vapor deposition (CVD) methods known to those skilled in the art. For example, graphene can be deposited using rapid thermal chemical vapor deposition (RTCVD), inductively coupled plasma chemical vapor deposition (ICP-CVD), low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), metal organic chemical vapor deposition (MOCVD), plasma enhanced chemical vapor deposition (PECVD), etc.

CVD has become the preferred method for large-scale production of single-layer graphene. In the CVD method, graphene is deposited on a metal substrate (i.e., metal foil) at a relatively high temperature between 600°C and 1100°C. Exemplary metals used as metal substrates include copper (Cu), nickel (Ni), platinum (Pt) and iridium (Ir), although any metal is suitable for supporting the formation of graphene films. The combination of CVD and copper catalysts enables relatively large-scale production of single-layer graphene. Different types of CVD equipment can be used to carry out CVD reactions, such as cold wall and hot wall reactors. During the deposition method, a carbon source solid, liquid or gas is inserted into the reaction chamber. At high temperatures between 600°C and 1100°C, graphene is formed on the surface of the metal catalyst. Graphene deposition can be carried out at atmospheric pressure or under vacuum. The advantages of using copper foil are their very low cost, flexibility and easy processing. The copper catalyst can be in the form of a thin film on the top of the silicon substrate or a thicker film in the form of foil. Graphene can be deposited on copper foil of varying thicknesses, ranging from 10 μm to 1000 μm.

The first substrate (e.g., a metal substrate) can be a metal foil (e.g., a catalytic metal foil) selected from copper foil, nickel foil, or any other metal foil material capable of supporting graphene deposition via a chemical vapor deposition method. The first substrate (e.g., a metal substrate) can include at least one metal or alloy selected from copper (Cu), nickel (Ni), cobalt (Co), iron (Fe), platinum (Pt), gold (Au), ruthenium (Ru), and aluminum (Al). For example, at least one metal or alloy selected from Cu and Ni can be used.

In some aspects, the methods disclosed herein include a method for metallizing a graphene layer by depositing (e.g., applying) a metal layer (e.g., a film) on the graphene by physical vapor deposition ( FIG. 1 , step 2). The method of applying the metal layer to the graphene layer can be accomplished by a vacuum metallization method or an electrochemical metallization method. The metallization of graphene can be accomplished using methods familiar to those skilled in the art, including vacuum metallization techniques (e.g., electron beam evaporation, thermal evaporation, or sputtering) or electrochemical metallization techniques (electroplating (applying a potential between graphene on copper (cathode) and a metal anode in an electrolyte solution containing anode metal ions); electroless deposition (precipitating a metal from a solution due to a chemical reduction reaction on the cathode (graphene on copper)); or atomic layer deposition (controlled deposition of a single atomic layer of material by a sequentially controlled chemical reaction)). The metal layer deposited on the graphene can be, for example, nickel, cobalt, gold, iron, aluminum, silver, copper, tin, palladium, platinum, or a combination thereof. For example, at least one metal or alloy selected from Co and Ni can be used. In an exemplary embodiment, the metal layer includes nickel or cobalt. In some embodiments, the metal layer deposited on the graphene can be about 20 nm to about 1000 nm thick (e.g., about 50 nm to about 750 nm, about 100 nm to about 500 nm, about 125 nm to about 250 nm, about 150 nm to about 200 nm, or about 75 nm, about 100 nm, about 125 nm, about 150 nm, about 175 nm, or about 200 nm thick).

In some aspects, the method disclosed herein includes the step of removing (e.g., peeling) a metallized graphene layer from a first substrate. The metallized graphene layer can be removed from the first substrate by peeling the metallized graphene layer from the first substrate. The peeling can be performed by adhering the metallized graphene to an intermediate substrate (via: van der Waals forces, magnetic forces, pressure differentials, electrostatic forces, or any combination thereof, etc.), applying a force sufficient to overcome the first substrate (e.g., copper)/graphene interaction to the intermediate substrate, thereby effectively peeling the metallized graphene from the first substrate. In addition, the peeled graphene layer can be pressed onto a final receiving substrate (e.g., a second substrate), and the intermediate substrate can be detached from the metallized graphene by terminating van der Waals bonds, electrostatic forces, magnetic forces, or pressure differentials.

Examples of using the van der Waals force include, for example, using a heat-peelable tape, a transparent tape, polydimethylsiloxane, and the like.

Examples of using electrostatic forces include, for example, metallized graphene on a first substrate is pressed onto an intermediate substrate consisting of a conductive layer and an insulating layer. An electrical bias is applied across the insulating layer between the metallized graphene and the conductive layer of the intermediate substrate. The bias generates an attractive electrostatic force between the metallized graphene and the conductive layer that is large enough to overcome the potential of the first substrate/graphene interaction. After the metallized graphene is peeled off from the first substrate, the intermediate substrate that houses the metallized graphene is laminated with a final receiving substrate (e.g., a second substrate), and the electrical bias is interrupted to release the laminated metallized graphene from the intermediate substrate.

Examples of using a pressure differential include, for example, pressing a metallized graphene on copper onto a porous intermediate substrate (e.g., a nanoporous alumina plate, etc.). A pressure differential is applied across the plate, thereby generating a vacuum suction force on the metallized graphene on the first substrate sufficient to overcome the first substrate/graphene interaction during exfoliation. When exfoliating, the intermediate substrate housing the metallized graphene is laminated with the final receiving substrate (e.g., a second substrate), and the vacuum suction is interrupted, thereby releasing the intermediate substrate.

Examples of using magnetic forces include, for example, pressing a magnetized intermediate substrate onto metallized graphene on a first substrate (e.g., copper). In cases where the metallized material is ferromagnetic in nature, the magnetic field causes the metal film to magnetize and results in a magnetic force between the metallized graphene and the intermediate substrate sufficient to overcome the first substrate/graphene interaction during exfoliation. During exfoliation, the metallized graphene on the intermediate substrate is laminated to the final receiving substrate (e.g., a second substrate), and the magnetic field is interrupted, resulting in the release of the intermediate substrate.

The intermediate substrate can be planar or cylindrical and can be adapted for batch or continuous roll-to-roll processing.

In some aspects, the method disclosed herein includes applying (e.g., laminating) the metallized graphene layer to a step of a second substrate. The second substrate can be a flexible, sturdy, rigid, or brittle substrate. In some embodiments, the second substrate is a flexible substrate selected from the group consisting of polyethylene terephthalate (PET), polyimide, polyethylene naphthalate (PEN), polycarbonate (PC), elastomeric polymers, chemical vapor deposition (CVD) deposited polymers, and combinations thereof. The polymer deposited by CVD can be, for example, parylene-C, parylene D, or parylene-N. In some embodiments, the second substrate is transparent.

The metallized graphene layer can be applied to the second substrate by hand or using a roller (e.g., roll-to-roll transfer). The lamination process is performed at a temperature that deactivates the adhesive of the adhesive heat release tape (e.g., between about 100° C. and 120° C.). In some embodiments, the lamination step and the removal of the heat release adhesive tape are performed simultaneously. Thus, when the graphene surface is laminated to the second substrate, the metallized graphene bilayer separates from the adhesive heat release tape and is simultaneously attached to the second substrate.

In some aspects, the methods disclosed herein include a method of laminating an adhesive hot release tape to a metal layer (e.g., nickel, cobalt, gold, iron, aluminum, silver, copper, tin, palladium, platinum, or a combination of these layers) formed on graphene ( FIG. 1 , step 3). In some embodiments, the adhesive hot release tape is applied by hand. The adhesive hot release tape can be applied using a specific machine, such as a roller, applying controlled pressure and speed from one edge of the adhesive hot release tape to the opposite edge, thereby avoiding the formation of bubbles between the metal layer and the adhesive tape and avoiding the need to apply the tape by hand. The application of the adhesive hot release tape can be carried out at room temperature without the use of a controlled atmosphere. In addition, the method does not require complex equipment or vacuum conditions to implement. Bulk standard equipment that can apply controlled pressure at a controlled speed is sufficient. The equipment cost is low; even a manual process is sufficient. These adhesive tapes can be heat-sensitive or pressure-sensitive tapes, but hot release tapes are preferred. The adhesive hot release tape can be polymeric. The composition of the adhesive polymer may be based on polyester polymers such as polyvinyl acetate, polyethylene vinyl acetate, polyacrylates (polymethyl acrylate, polyethyl acrylate, polypropyl acrylate, polybutyl acrylate, etc.), polymethacrylates (polymethyl methacrylate, polyethyl methacrylate, polypropyl methacrylate, polybutyl methacrylate, polyhydroxyethyl methacrylate, etc.), etc. In some embodiments, the adhesive hot-peel tape is peeled at a temperature of about 70° C. to about 140° C., about 80° C. to about 130° C., about 90° C. to about 120° C., about 100° C. to about 110° C., about 70° C., about 80° C., about 100° C., about 110° C., about 120° C., about 130° C., or about 140° C.

After applying the adhesive thermal release tape to the metal layer, the metal/graphene bilayer film is removed (e.g., peeled off) from the first substrate (e.g., metal substrate) by peeling off the adhesive thermal release tape (step 4). After removing the metal/graphene bilayer film, the metal substrate (e.g., copper substrate) can be reused for CVD without further processing.

In some aspects, the methods disclosed herein include methods of applying (e.g., laminating) the metallized graphene layer to a second substrate. The second substrate can be a flexible, strong, rigid, or brittle substrate.

Referring to Figure 1 , graphene is laminated to a substrate bearing a thermoplastic adhesive coating (i.e., the second substrate) at 100°C, deactivating the adhesive on the thermal release tape and retaining the graphene on the flexible substrate (Figure 1 , step 5). The flexible substrate/graphene/metal film is then immersed in a bath containing a metal etchant solution for 3-5 seconds (Figure 1 , step 6) and rinsed with deionized water to obtain a substrate covered with a single layer of graphene (Figure 1 , step 7).

In some embodiments, the second substrate is a flexible substrate. Flexible substrates include, for example, polyethylene terephthalate (PET), polyimide, polyethylene naphthalate (PEN), polycarbonate (PC), elastomeric polymers, and combinations thereof. For example, the elastomeric polymer can be, but is not limited to, transparent. For example, the elastomeric polymer can include, but is not limited to, polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA), or silicone rubber. In some embodiments, the flexible substrate can be, for example, a transparent flexible substrate.

In some embodiments, the second substrate is a rigid substrate, including, for example, an oxide substrate selected from the group consisting of a glass substrate, a Si substrate, a SiO ₂ substrate, an ITO substrate, etc.; a metal substrate; and combinations thereof. For non-limiting examples, the metal substrate may include at least one metal or alloy selected from copper (Cu), nickel (Ni), cobalt (Co), iron (Fe), platinum (Pt), gold (Au), ruthenium (Ru), and aluminum (Al). For non-limiting examples, the oxide substrate may include, for example, an oxide substrate of a metal having insulating, conductive, or semiconductor properties. The oxide substrate may include, for example, a SiO ₂ substrate, an ITO substrate, a SnO ₂ substrate, a TiO ₂ substrate, and an Al ₂ O ₃ substrate.

Advantageously, removal of the adhesive heat release tape and lamination of the graphene substrate can be achieved in a simultaneous process as described below.

The graphene transfer method according to the present disclosure can be used on flexible substrates, rigid substrates, and brittle substrates.

The method disclosed herein can be expanded to supply large graphene films. In principle, there is no maximum limit on the size of graphene, except for the graphene size given by the equipment for attaching adhesive tape and the equipment for producing graphene. The equipment can be defined as being able to process meter-scale graphene films. In addition, the method can be easily integrated into online, continuous or batch production processes, making industrial production of graphene feasible. Therefore, opportunities for commercializing graphene-based products are opened up.

Once the graphene is on the desired substrate, the quality and uniformity can be assessed. When the graphene has been transferred to a silicon substrate containing a 300nm thermal oxide layer (Si/ _SiO2 ), optical microscopy can be used to evaluate the uniformity and consistency of the graphene. Alternatively, Raman spectroscopy can be used to determine the quality of the graphene on the same substrate (Si/ _SiO2 ).

On the other hand, the method disclosed herein includes a method for patterning metallized graphene, which includes a step of metal etching in an acid bath, via various patterning methods (soft lithography, nanoimprint lithography, photolithography, screen printing, etc.), which can be patterned with a water-insoluble resist (polymer or another metal or ceramic, etc.). This protective patterning of the metal film can result in selective etching of the metal film in an acid bath, resulting in a residual metal pattern on top of the graphene (wherein the metal is protected by the resist). In addition, the resist can be removed by an appropriate solvent. The result of this step is a PET sheet covered with continuous graphene with a metal pattern (which can be used for solar cells or OLEDs, where the graphene is a continuous flattened electrode and the metal pattern can be used as a low-resistance electrode).

Alternatively, an additional step of exposing the metallized graphene to an oxygen plasma (reactive ion etching, etc.) to remove the exposed (unpatterned) graphene can be used to produce an all-graphene transparent flexible circuit element. In addition, the metal pattern can be dissolved in an acidic bath to produce a graphene pattern on PET. As shown in Figure 6, the patterning method of metallized graphene includes (1) depositing a protective polymer pattern on the substrate/graphene/metal layer via imprint lithography, photolithography; (2) etching the metal in an acid solution; (3) removing the polymer from the etched substrate/graphene/metal layer in a solvent bath; (4) removing the graphene from the substrate/graphene/metal layer via plasma etching (e.g., reactive ion etching, etc.); and (5) removing the metal in an acid solution, leaving a substrate/graphene pattern sandwich.

Example

The present invention is further described in the following examples, which do not limit the scope of the invention described in the claims.

Clean the copper foil.

Graphene was synthesized on 25 μm thick copper foil (Alpha Aesar, 13382, 99.8%) with dimensions of 10 cm × 11 cm or 18 cm × 20 cm. Prior to graphene growth, the copper foil was cleaned by soaking it in a shallow acetone bath and wiping it with a Kimwipe ^™ tissue (while in acetone). The foil was then rinsed with acetone and transferred to a similar bath containing isopropyl alcohol (IPA), and the mechanical cleaning was repeated in this solvent. The inventors noted that mechanical cleaning produced purer graphene than after cleaning the foil ¹ via ultrasonic treatment in acetone and IPA; this method also saved a large amount of both solvents (considering the large volume required to ultrasonically treat large areas of copper foil). After mechanical cleaning in IPA, the foil was rinsed in IPA and dried in a stream of compressed air.

Electrolytically polished copper foil.

To primarily produce single-layer graphene, copper foil was electropolished prior to graphene synthesis. ^1,2 A clean, dry copper foil was placed in a 250 mL beaker, following the contours of the beaker's sidewalls, and used as the anode. A copper tube (d = 2.54 cm, l = 15 cm) was inserted into the beaker along the cylindrical axis and used as the cathode. The cylindrical shape of the cathode and the curved surface of the anode generated a uniform electric field during electropolishing. Concentrated phosphoric acid (H ₃ PO ₄ , 15 M) was used as the electrolyte and poured into the beaker after securing the cathode and anode with a clamp and alligator clips, respectively. A 20 W DC power supply was used to generate the necessary current and voltage. The voltage was set to 1.6 V, and electropolishing was performed until the current dropped 50% from its initial value and reached a stable level (typically between 5 and 10 minutes). After electropolishing, the cathode and electrolyte were removed from the beaker, and the copper foil was rinsed extensively with deionized water for 3 minutes. The copper foil was then rinsed with IPA, blown dry under a stream of compressed air, and immediately loaded into the middle of a quartz tube of a chemical vapor deposition (CVD) reactor.

Synthesis of graphene.

Atmospheric pressure CVD graphene synthesis was performed in a tubular quartz furnace (MTI OTF-1200X-HVC-UL) with the following tube dimensions: d = 7.6 cm, l = 100 cm. The CVD chamber and reactor gas supply lines were purged with air for 5 minutes by flowing a mixture of all synthesis gases (hydrogen, methane, and argon) at maximum flow rate while evacuating the CVD chamber with a diaphragm vacuum pump. After 5 minutes, the gas flow was stopped and the chamber was evacuated to approximately ^10-4 Torr with a turbomolecular vacuum pump to remove methane and hydrogen from the gas mixing and reaction chamber, as well as to desorb possible organic contaminants from the surface of the copper foil. The chamber was then re-pressurized to atmospheric pressure with ultra-high purity argon (700 SCCM), which flowed continuously throughout the graphene synthesis process. The copper foil was heated to 1050°C (30 minutes) in an argon stream. When this temperature was reached, additional hydrogen (60 SCCM) flowed for 30 minutes to anneal and activate the copper substrate. After annealing for 30 minutes, the flow rate of hydrogen was reduced to 5 SCCM, and 0.7 SCCM of methane flowed for 20 minutes for the synthesis of graphene (total gas flow rate: 700 SCCM argon + 5 SCCM hydrogen + 0.7 SCCM methane = 705.7 SCCM). After 20 minutes of graphene growth, the furnace was turned off and opened by 5 cm (continuous same gas flow). When the furnace cooled to 700 ° C (about 5 minutes), it was opened to 10 cm. At 350 ° C (about 30 minutes), the furnace was fully opened. At 200 ° C, the hydrogen and methane flows were cut off, and the reactor chamber was cooled to room temperature with an argon flow (total cooling time was about 1 hour).

Electron beam evaporation.

A Temescal BJD-1800 electron beam evaporator was used to metallize graphene on copper foil with a 150 nm film of nickel, cobalt, gold, iron, or aluminum. The metal evaporation rate was 200 Å and the pressure of the chamber was maintained at 7×10 ⁻⁷ Torr.

Application of heat release tape .

Nitto Denko Revalpa 3196 thermal release tape was manually laminated to the metallized (e.g., nickel) graphene on the copper foil. The bubbles formed between the adhesive and the metal film were removed using the flat side of a wafer tweezer (the presence of bubbles resulted in incomplete graphene exfoliation and cracks in the metal film due to the high strain they exerted on the film). In addition, the copper foil was peeled off from the thermal release tape and immediately placed in the chamber of a CVD reactor under ultrahigh vacuum until the next graphene synthesis. The peeling of the copper foil resulted in the exfoliation of the metallized graphene and its attachment to the thermal release tape.

Simultaneously remove the heat release tape and laminate to PET.

The exposed graphene surface was laminated using a commercial office laminator (used for laminating paper between plastic sheets) and polyethylene terephthalate (PET) bags (Office Depot, 125-μm) with a heat-activated coating exposed on its inner surface. The laminator was operated at temperatures between 100°C and 120°C (the range where the adhesive of the heat-release tape is deactivated). As a result, when the graphene surface was laminated, the graphene/metal bilayer film detached from the heat-release tape while simultaneously adhering to the PET.

The sacrificial metal film is etched.

A solution of iron (III) chloride ( ₁ M FeCl₃) was used to remove the metal film from the graphene surface. A 3-5 second immersion in the _FeCl₃ bath was sufficient to completely remove a 150 nm nickel film (compared to the 20-30 minutes required to etch a 25 μm copper foil using a conventional wet transfer method). After etching the metal, the PET/graphene film was rinsed with running deionized water for 5 minutes to remove the etchant.

Graphene was transferred using a conventional wet transfer method ³ .

For graphene to Si/SiO ₂ on the conventional wet transfer, with 4000rpm with poly (methyl methacrylate) (PMMA) 2.5% w / w solution in toluene spin coating carrier CVD growth of the graphene film of the copper foil 60 seconds. After spin coating, use oxygen plasma cleaner (30s, 30W, 200 millitorr oxygen pressure) to etch the exposed graphene on the copper foil side relative to PMMA. Next, the PMMA/ graphene coated copper foil is floated in 1M iron (III) chloride (FeCl ₃ ) bath for 30 minutes, to etch copper. In order to remove residual etching agent, the free-floating PMMA-supported graphene is transferred three times in a deionized water bath. Subsequently, the PMMA-supported graphene is transferred to a 2.5cm × 2.5cm sheet of a silicon wafer carrying a 90nm thermally grown oxide. After drying for 5 hours at room temperature, the silicon wafer chip is placed in a boiling acetone bath for 30 minutes to remove PMMA.

Raman spectroscopy.

Raman spectra of graphene on all substrates were obtained using a Renishaw micro-spectrometer (532 nm laser) (no graphene spectra were obtained on PET substrate).

Sheet resistance measurement.

The sheet resistance (Rs) of the PET/graphene sheet was measured using a Keithley 2400 SourceMeter instrument equipped with a 4-point probe (0.5 mm probe tip radius, 1 mm spacing between tips). Since the size of the graphene sheet is at least an order of magnitude larger than the spacing between the probes, we approximated the size of the sheet to be infinite, and used a standard multiple of 4.53 to convert the measured resistance to the sheet resistance,

The quality of graphene produced by the methods described herein is similar to that of graphene produced by standard methods, given the defects observable by Raman spectroscopy. The methods disclosed herein for synthesizing high-quality single-layer graphene enable applications in flexible, stretchable, and disposable electronics, thin and lightweight barrier materials, and large-area displays and photovoltaic modules that are inaccessible to expensive and environmentally harmful methods for producing this versatile material.

Example 1

A schematic diagram illustrating a graphene transfer method according to an embodiment of the present invention is provided in FIG1 . The method is based on the differential adhesion of graphene to different metals, subsequent mechanical peeling, and lamination to a flexible substrate using a heat-deactivated adhesive ( FIG1 ). Briefly, a single layer of graphene is grown on a copper foil by ambient pressure CVD ( FIG1 , step 1). A 150 nm nickel (or cobalt) film is deposited on the graphene by physical vapor deposition ( FIG1 , step 2 and FIG2 a). A hot peeling tape is applied ( FIG1 , step 3 and FIG2 b); the hot peeling tape is peeled off to peel the metal/graphene bilayer from the copper substrate ( FIG1 , step 4 and FIG2 c), which can be reused without further processing. Graphene is laminated to a commercial polyethylene terephthalate (PET) substrate bearing a thermoplastic adhesive coating at 100° C., the adhesive on the hot peeling tape is deactivated, and the graphene remains on the plastic substrate ( FIG1 step 5 and FIG2 d, e). The sheet containing the PET/graphene/metal film was then immersed in a bath of metal etchant solution for 3-5 seconds (Figure 1, step 6 and Figure 2f) and rinsed in deionized water to obtain PET covered with a single layer of graphene (Figure 1, step 7 and Figures 2g and 2h). PET was chosen as the receiving substrate due to its widespread use in flexible electronics.

For these studies, graphene was successfully exfoliated from copper foil using films of nickel, cobalt, and gold. Hamada and Otani's comparative density-function study of the binding energy between graphene and various metal surfaces revealed a stronger preference for nickel (141 meV) than for copper (62 meV) ¹² . Nickel's strong adhesion to graphene was also exploited by Kim et al. in a two-step exfoliation of graphene from SiC surfaces, but this method was not compatible with roll-to-roll manufacturing due to the inflexibility of SiC wafers13. In addition to the metals listed above, the inventors also attempted MAE with iron and aluminum, but found that they did not show preferential adhesion to graphene and therefore could not exfoliate graphene from the copper substrate. Of the ^three metals that could be exfoliated, only nickel and cobalt could be etched without damaging the graphene (i.e., by etching it or rendering it non-conductive). For example, etching gold with a standard solution containing iodine and potassium iodide rendered the graphene non-conductive.

In order to determine the quality of the transferred graphene using the method described herein, the sheet resistance (Rs) and the ratio of the D/G peak from the Raman spectrum were measured. The Rs values obtained varied within the order of magnitude between samples. Without being bound by theory, it is believed that the variability is partly due to the manual nature of the transfer of nickel/graphene or cobalt/graphene bilayer films to the thermal peeling tape. Compared to the minimum value of 325Ωsq ^-1 obtained from the transferred graphene using the standard method of etching copper, the minimum value of Rs obtained was 163Ωsq ^-1 . After peeling, some cracks were observed in the nickel film (Figure 2c), which can again be attributed to the manual nature of the peeling step and the inability of the nickel or cobalt film to adapt to the tensile strain applied thereto during the peeling process. These cracks, which likely propagate through the graphene (Figure 2h), produce anisotropic sheet resistance; the average Rs measured parallel to the cracks (850 ± 250 Ωsq ^-1 ) is an order of magnitude lower than the average Rs measured perpendicular to the cracks (8000 ± 2000 Ωsq ^- 1). Automated processes, in which the metallized graphene film is subjected to reduced tensile strain by using rollers with a large radius of curvature or by using a more viscous adhesive, could reduce the occurrence of cracking. It is possible that kinetically controlled transfer via a reusable stamp, as described by Rogers et al., will allow transfer without the use of hot-release tapes ^.

The Raman spectra of graphene grown and peeled off on copper are shown in Figure 3. The spectra show that the graphene is completely removed from the copper in the peeled-off areas. Figure 4 provides a direct comparison of the defects present in graphene produced by the primary method of wet transfer described by Bae et al. with graphene produced by the MAE method ^4. To obtain these spectra, the graphene produced by these two methods was laminated onto a Si/ _SiO2 substrate. For traditional wet transfer, this was achieved by spin-coating poly(methyl methacrylate) (PMMA) on top of the graphene and etching the copper substrate. For the Raman spectra of the MAE samples, epoxy was cured on top of the metallized graphene and then peeled off from the copper foil. PMMA was spin-coated on top of the exposed surface of the graphene, and the underlying nickel film was etched in _FeCl3 . The independent graphene/PMMA was then transferred to deionized water three times before being applied to a Si/ _SiO2 wafer chip. The PMMA was then removed by immersion in a boiling acetone bath.

The quality of the graphene was determined based on the ratio of the D/G peaks (at 1330 cm ^-1 and 1580 cm ^-1 ) in the Raman spectrum. The D/G peak ratio increased twofold in the MAE method compared to the conventional wet transfer method (from 0.23 for wet transfer to 0.50 for MAE). It is possible that the increase in the D-peak in the graphene transferred by the MAE method is due to damage during electron beam evaporation of the nickel film, mechanical damage during metal-assisted exfoliation, and damage during the subsequent wet transfer method.

The environmental goodness of the MAE method is based on the reusability of the copper foil for growing graphene. In order to determine the impact of the identical copper substrate on graphene growth that is reused, the inventors have studied the quality of the graphene grown on copper after cyclic growth and transfer. Significantly, the quality of graphene improves (Fig. 5 a-c, e) after continuous growth cycles. Without being bound by theory, it is believed that the improvement in quality may be due to the additional annealing of the copper substrate during each cycle of graphene synthesis, and the removal of the surface contaminants of each metal-assisted graphene peeling, which produces a cleaner surface for subsequent growth (after each graphene peeling, the copper foil substrate is immediately put into the CVD reactor chamber under ultra-high vacuum, to avoid surface contamination).

After metal-assisted exfoliation of graphene from copper foil, the small amount of graphene remaining on the copper foil was transferred to a Si/SiO2 _wafer using a conventional wet transfer method for optical microscopy (Figure 5d). The presence of individual graphene grains remaining on the copper foil after MAE indicates that the method primarily transfers the continuous top (metallized) layer ^of graphene and supports the theory that during the growth of graphene on copper by CVD, small fragments in the form of a second graphene layer beneath the first layer form.16,17 Furthermore, it is possible that these residual graphene grains serve as “seed grains” for subsequent growth cycles.18 It has been shown in the literature that the best quality CVD graphene on copper is obtained by “pre-seeding” graphene particles on the copper surface prior ^to graphene synthesis.19

Other Implementations

It is to be understood that while the invention has been described in conjunction with its detailed description, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages and modifications are within the scope of the appended claims.

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Claims (37)

1. A method for fabricating a graphene layer on a substrate, comprising: A graphene layer is provided on a first substrate; A metal layer is applied to the graphene layer to form a metallized graphene layer; The metallized graphene layer is peeled off from the first substrate; and The metallized graphene layer is laminated onto a second substrate, wherein the first substrate is a copper foil, and the first substrate is reusable. The process of peeling the metallized graphene layer from the first substrate includes: Adhere the intermediate substrate to the metal layer; A force sufficient to overcome the interaction between the first substrate and the graphene layer is applied to the intermediate substrate to remove the metallized graphene layer from the first substrate. 2. The method according to claim 1, wherein the graphene layer is a single layer of graphene. 3. The method according to claim 1, wherein the graphene layer comprises two or more graphene layers. 4. The method according to claim 1, wherein the metal layer comprises gold, nickel, cobalt, iron, silver, copper, tin, palladium, platinum, or alloys thereof. 5. The method of claim 4, wherein the metal layer comprises nickel or cobalt. 6. The method of claim 1, wherein the metal layer comprises a transition metal or an alloy thereof. 7. The method of claim 6, wherein the metal layer comprises two or more sequentially deposited metal layers, wherein each layer comprises a transition metal or an alloy thereof. 8. The method of claim 7, wherein each of the two or more sequentially deposited metal layers has a thickness of 1 to 1000 nm. 9. The method of claim 7, wherein each of the two or more sequentially deposited metal layers has a thickness of 20 nm to 1000 nm. 10. The method of claim 7, wherein each of the two or more sequentially deposited metal layers has a thickness of 50 nm to 750 nm. 11. The method of claim 7, wherein each of the two or more sequentially deposited metal layers has a thickness of 100 nm to 500 nm. 12. The method of claim 7, wherein each of the two or more sequentially deposited metal layers has a thickness of 125 nm to 250 nm. 13. The method of claim 7, wherein each of the two or more sequentially deposited metal layers has a thickness of 150 nm to 200 nm. 14. The method of claim 1, wherein the metal layer is applied to the graphene layer using a vacuum metallization method or an electrochemical metallization method. 15. The method according to claim 14, wherein the vacuum metallization method is selected from the group consisting of electron beam evaporation, thermal evaporation and sputtering. 16. The method according to claim 14, wherein the electrochemical metallization method is selected from the group consisting of electroplating, electroless deposition, and atomic layer deposition. 17. The method of claim 1, wherein the metal layer has a thickness of 1 to 1000 nm. 18. The method of claim 1, wherein the metal layer has a thickness of 20 nm to 1000 nm. 19. The method of claim 1, wherein the metal layer has a thickness of 50 nm to 750 nm. 20. The method of claim 1, wherein the metal layer has a thickness of 100 nm to 500 nm. 21. The method of claim 1, wherein the metal layer has a thickness of 125 nm to 250 nm. 22. The method of claim 1, wherein the metal layer has a thickness of 150 nm to 200 nm. 23. The method of claim 1, wherein the metal layer has a thickness of 75 nm, 100 nm, 125 nm, 150 nm, 175 nm or 200 nm. 24. The method of claim 1, wherein providing the graphene layer disposed on the first substrate comprises: Provide the first substrate; The graphene layer is grown on the first substrate layer using a chemical vapor deposition method. 25. The method of claim 1, further comprising removing the metal layer from the graphene layer after laminating the graphene layer onto the second substrate. 26. The method of claim 1, wherein the second substrate is a flexible, robust, rigid, or brittle substrate. 27. The method of claim 1, wherein the second substrate is a flexible substrate selected from the group consisting of polyethylene terephthalate, polyimide, polyethylene naphthalate, polycarbonate, elastomeric polymers, thermoplastic polymers, polymers deposited by chemical vapor deposition, and combinations thereof. 28. The method of claim 27, wherein the polymer deposited by the chemical vapor deposition is parylene-C, D, or N. 29. The method of claim 1, wherein the second substrate is transparent. 30. The method of claim 27, further comprising removing the metal layer from the graphene layer after laminating the metallized layer onto the flexible substrate. 31. The method of claim 1, wherein the intermediate substrate is a thermally peelable adhesive tape. 32. The method of claim 31, further comprising removing the thermally peelable adhesive tape after removing the metallized graphene from the first substrate. 33. The method of claim 31, wherein a roller is used to apply the thermally peelable adhesive tape to the metal layer to apply the thermally peelable adhesive tape from one edge of the metal layer to the opposite edge of the metal layer. 34. The method of claim 1, wherein the adhesion step and the removal step are performed simultaneously. 35. The method of claim 1, wherein the intermediate substrate is adhered to the metal layer using van der Waals force, magnetic force, pressure difference, electrostatic force, or any combination thereof. 36. The method of claim 1, further comprising etching a pattern on the metallized graphene layer. 37. A method for transferring a graphene layer onto a flexible substrate, comprising: A graphene layer is provided on a substrate, wherein the substrate is a metal foil including copper; A metal layer is applied to the graphene layer to form a metallized graphene layer; The metallized graphene layer is peeled off from the substrate; and Metallized graphene layers are laminated onto the flexible substrate. The process of peeling the metallized graphene layer off the substrate includes: Adhere the intermediate substrate to the metal layer; A force sufficient to overcome the interaction between the substrate and the graphene layer is applied to the intermediate substrate to remove the metallized graphene layer from the substrate.