Graphene production techniques

A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.

Isolated 2D crystals cannot be grown via chemical synthesis beyond small sizes even in principle, because the rapid growth of phonon density with increasing lateral size forces 2D crystallites to bend into the third dimension. However, other routes to 2d materials exist:

Fundamental forces place seemingly insurmountable barriers in the way of creating [2D crystals]... The nascent 2D crystallites try to minimize their surface energy and inevitably morph into one of the rich variety of stable 3D structures that occur in soot. But there is a way around the problem. Interactions with 3D structures stabilize 2D crystals during growth. So one can make 2D crystals sandwiched between or placed on top of the atomic planes of a bulk crystal. In that respect, graphene already exists within graphite... One can then hope to fool Nature and extract single-atom-thick crystallites at a low enough temperature that they remain in the quenched state prescribed by the original higher-temperature 3D growth.

The early approaches of cleaving multi-layer graphite into single layers or growing it epitaxially by depositing a layer of carbon onto another material have been supplemented by numerous alternatives. In all cases, the graphite must bond to some substrate to retain its 2d shape.

Exfoliation

As of 2014 exfoliation produced graphene with the lowest number of defects and highest electron mobility.

Adhesive tape

Andre Geim and Konstantin Novoselov initially used adhesive tape to split graphite into graphene. Achieving single layers typically requires multiple exfoliation steps, each producing a slice with fewer layers, until only one remains. After exfoliation the flakes are deposited on a silicon wafer. Crystallites larger than 1 mm and visible to the naked eye can be obtained.

Wedge-based

In this method, a sharp single-crystal diamond wedge penetrates onto the graphite source to exfoliate layers. This method uses highly ordered pyrolytic graphite (HOPG) as the starting material. The experiments were supported by molecular dynamic simulations.

Graphite oxide reduction

P. Boehm reported producing monolayer flakes of reduced graphene oxide in 1962. Rapid heating of graphite oxide and exfoliation yields highly dispersed carbon powder with a few percent of graphene flakes. Reduction of graphite oxide monolayer films, e.g. by hydrazine with annealing in argon/hydrogen also yielded graphene films. Later the oxidation protocol was enhanced to yield graphene oxide with an almost intact carbon framework that allows efficient removal of functional groups, neither of which was originally possible. The measured charge carrier mobility exceeded 1,000 centimetres (393.70 in)/Vs. Spectroscopic analysis of reduced graphene oxide has been conducted.

Shearing

In 2014 defect-free, unoxidized graphene-containing liquids were made from graphite using mixers that produce local shear rates greater than 10×10⁴. The method was claimed to be applicable to other 2D materials, including boron nitride, Molybdenum disulfide and other layered crystals.

Sonication

Solvent-aided

Dispersing graphite in a proper liquid medium can produce graphene by sonication. Graphene is separated from graphite by centrifugation, producing graphene concentrations initially up to 0.01 mg/mL in N-methylpyrrolidone (NMP) and later to 2.1 mg/mL in NMP,. Using a suitable ionic liquid as the dispersing liquid medium produced concentrations of 5.33 mg/mL. Graphene concentration produced by this method is very low, because nothing prevents the sheets from restacking due to van der Waals forces. The maximum concentrations achieved are the points at which the van der Waals forces overcome the interactive forces between the graphene sheets and the solvent molecules.

Adding a surfactant to a solvent prior to sonication prevents restacking by adsorbing to the graphene's surface. This produces a higher graphene concentration, but removing the surfactant requires chemical treatments.

Immiscible liquids

Sonicating graphite at the interface of two immiscible liquids, most notably heptane and water, produced macro-scale graphene films. The graphene sheets are adsorbed to the high energy interface between the heptane and the water, where they are kept from restacking. The graphene remained at the interface even when exposed to force in excess of 300,000 g. The solvents may then be evaporated. The sheets are up to ~95% transparent and conductive.

Molten salts

Graphite particles can be corroded in molten salts to form a variety of carbon nanostructures including graphene. Hydrogen cations, dissolved in molten Lithium chloride, can be discharged on cathodically polarized graphite rods, which then intercalate into the graphite structure, peeling graphite to produce graphene. The graphene nanosheets produced displayed a single-crystalline structure with a lateral size of several hundred nanometers and a high degree of crystallinity and thermal stability.

Electrochemical synthesis

Electrochemical synthesis can exfoliate graphene. Varying a pulsed voltage controls thickness, flake area, number of defects and affects its properties. The process begins by bathing the graphite in a solvent for intercalation. The process can be tracked by monitoring the solution’s transparency with an LED and photodiode.

Hydrothermal self-assembly

Graphene has been prepared by using a sugar (e.g. glucose, fructose, etc.) This substrate-free "bottom-up" synthesis is safer, simpler and more environmentally friendly than exfoliation. The method can control thickness, ranging from monolayer to multilayers.

Chemical vapor deposition

Epitaxy

Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where there is registry between the two. In some cases epitaxial graphene layers are coupled to surfaces weakly enough (by Van der Waals forces) to retain the two dimensional electronic band structure of isolated graphene. An example of this weak coupling is epitaxial graphene on SiC.

Silicon carbide

Main article: Carbide-derived Carbon

Heating silicon carbide (SiC) to high temperatures (>1100 °C) under low pressures (~10⁻⁶ torr) reduces it to graphene. This process produces epitaxial graphene with dimensions dependent upon the size of the wafer. The face of the SiC used for graphene formation, silicon- or carbon-terminated, highly influences the thickness, mobility and carrier density.

Graphene's electronic band-structure (so-called Dirac cone structure) was first visualized in this material. Weak anti-localization is observed in this material, but not in exfoliated graphene produced by the drawing method. Large, temperature-independent mobilities approach those in exfoliated graphene placed on silicon oxide, but lower than mobilities in suspended graphene produced by the drawing method. Even without transfer, graphene on SiC exhibits massless Dirac fermions. The graphene–substrate interaction can be further passivated.

The weak van der Waals force that coheres multilayer stacks does not always affect the individual layers' electronic properties. That is, while the electronic properties of certain multilayered epitaxial graphenes are identical to that of a single layer, other properties are affected, as they are in bulk graphite. This effect is well understood theoretically and is related to the symmetry of the interlayer interactions.

Epitaxial graphene on SiC can be patterned using standard microelectronics methods. A band gap can be created and tuned by laser irradiation.

Silicon/germanium/hydrogen

A normal silicon wafer coated with a layer of germanium (Ge) dipped in dilute hydrofluoric acid strips the naturally forming germanium oxide groups, creating hydrogen-terminated germanium. Chemical vapor deposition deposits a layer of graphene on top. The graphene can be peeled from the wafer using a dry process and is then ready for use. The wafer can be reused. The graphene is wrinkle-free, high quality and low in defects.

Metal substrates

The atomic structure of metal substrates can seed the growth of graphene.

Ruthenium

Graphene grown on ruthenium does not typically produce uniform layer thickness. Bonding between the bottom graphene layer and the substrate may affect layer properties.

Iridium

Graphene grown on iridium is weakly bonded, uniform in thickness, albeit rippled, and can be highly ordered. Due to the long-range order of these ripples, minigaps in the electronic band-structure (Dirac cone) become visible.

Nickel

High-quality sheets of few-layer graphene exceeding in area have been synthesized via CVD on thin nickel films using multiple techniques. First the film is exposed to Argon gas at 900–1000 degrees Celsius. Methane is then mixed into the gas, and the methane's disassociated carbon is absorbed into the film. The solution is then cooled and the carbon diffuses out of the nickel to form graphene films.

Another method used temperatures compatible with conventional CMOS processing, using a nickel-based alloy with a gold catalyst. This process dissolves carbon atoms inside a transition metal melt at a certain temperature and then precipitates the dissolved carbon at lower temperatures as single layer graphene (SLG).

The metal is first melted in contact with a carbon source, possibly a graphite crucible inside which the melt is carried out or graphite powder/chunks that are placed in the melt. Keeping the melt in contact with the carbon at a specific temperature dissolves the carbon atoms, saturating the melt based on the metal–carbon binary phase diagram. Lowering the temperature decreases carbon's solubility and the excess carbon precipitates onto the melt. The floating layer can be either skimmed or frozen for later removal.

Using different morphology, including thick graphite, few layer graphene (FLG) and SLG were observed on metal substrate. Raman spectroscopy proved that SLG had grown on nickel substrate. The SLG Raman spectrum featured no D and D′ band, indicating its pristine nature. Since nickel is not Raman active, direct Raman spectroscopy of graphene layers on top of the nickel is achievable.

Another approach covered a sheet of silicon dioxide glass (the substrate) on one side with a nickel film. Graphene deposited via chemical vapor deposition formed into layers on both sides of the film, one on the exposed top side, and one on the underside, sandwiched between nickel and glass. Peeling the nickel and the top layer of graphene left an intervening layer of graphene on the glass. While the top graphene layer could be harvested from the foil as in earlier methods, the bottom layer was already in place on the glass. The quality and purity of the attached layer was not assessed.

Copper

Copper foil, at room temperature and very low pressure and in the presence of small amounts of methane produces high quality graphene. The growth automatically stops after a single layer forms. Arbitrarily large films can be created. The single layer growth is due to the low concentration of carbon in methane. The process is surface-based rather than relying on absorption into the metal and then diffusion of carbon into graphene layers on the surface. The room temperature process eliminates the need for postproduction steps and reduces production from a ten-hour/nine- to ten-step procedure to a single step that takes five minutes. A chemical reaction between the hydrogen plasma formed from the methane and ordinary air molecules in the chamber generates cyano radicals—carbon–nitrogen molecules without electrons. These charged molecules scour away surface imperfections, providing a pristine substrate. The graphene deposits form lines that merge into each other, forming a seamless sheet that contributes to mechanical and electrical integrity.

Larger hydrocarbons such as ethane and propane produce bilayer coatings. Atmospheric pressure CVD growth produces multilayer graphene on copper (similar to nickel).

The material has fewer defects, which in higher temperature processes result from thermal expansion/contraction. Ballistic transport was observed in the resulting material.

Sodium ethoxide pyrolysis

Gram-quantities were produced by the reduction of ethanol by sodium metal, followed by pyrolysis of the ethoxide product and washing with water to remove sodium salts.

Roll-to-roll

Large scale roll-to-roll production of graphene based on chemical vapor deposition, was first demonstrated in 2010. In 2014 a two-step roll-to-roll manufacturing process was announced. The first roll-to-roll step produces the graphene via chemical vapor deposition, and the second step binds the graphene to a substrate.

Cold wall

Growing graphene in an industrial resistive-heating cold wall CVD system was claimed to produce graphene 100 times faster than conventional CVD systems, cuts costs by 99 percent and produce material with enhanced electronic qualities.

Nanotube slicing

Graphene can be created by cutting open carbon nanotubes. In one such method multi-walled carbon nanotubes are cut open in solution by action of potassium permanganate and sulfuric acid. In another method graphene nanoribbons were produced by plasma etching of nanotubes partly embedded in a polymer film.

Carbon dioxide reduction

A highly exothermic reaction combusts magnesium in an oxidation–reduction reaction with carbon dioxide, producing a variety of carbon nanoparticles including graphene and fullerenes. The carbon dioxide reactant may be either solid (dry-ice) or gaseous. The products of this reaction are carbon and magnesium oxide. US patent 8377408  was issued for this process.

Spin coating

In 2014, carbon nanotube-reinforced graphene was made via spin coating and annealing functionalized carbon nanotubes. The resulting material was stronger, flexible and more conductive than conventional graphene.

Supersonic spray

Supersonic acceleration of droplets through a Laval nozzle was used to deposit small droplets of reduced graphene-oxide in suspension on a substrate. The droplets disperse evenly, evaporate rapidly and display reduced flake aggregations. In addition, the topological defects (Stone-Wales defect and C
₂ vacancies) originally in the flakes disappeared. The result was a higher quality graphene layer. The energy of the impact stretches the graphene and rearranges its carbon atoms into flawless hexagonal graphene with no need for post-treatment. The high amount of energy also allows the graphene droplets to heal any defects in the graphene layer that occur during this process.

Another approach sprays buckyballs at supersonic speeds onto a substrate. The balls cracked open upon impact, and the resulting unzipped cages then bond together to form a graphene film. The buckyballs are released into a helium or hydrogen gas, which expands at supersonic speeds, carrying the carbon balls with it. The buckyballs achieve energies of around 40 keV without changing their internal dynamics. This material contains hexagons and pentagons that come from the original structures. The pentagons could introduce a band gap.

Intercalation

Producing graphene via intercalation splits graphite into single layer graphene by inserting guest molecules/ions between the graphene layers. Graphite was first intercalated in 1841 using a strong oxidizing or reducing agent that damaged the material's desirable properties. Kovtyukhova developed a widely used oxidative intercalation method in 1999. In 2014, she was able to achieve intercalation using non-oxidizing Brønsted acids (phosphoric, sulfuric, dichloroacetic and alkylsulfonic acids), but without oxidizing agents. The new method has yet to achieve output sufficient for commercialization.

Laser

In 2014 a laser-based single-step, scalable approach to graphene production was announced. The technique produced and patterned porous three-dimensional graphene film networks from commercial polymer films. The system used a CO2 infrared laser. The sp³-carbon atoms were photothermally converted to sp²-carbon atoms by pulsed laser irradiation. The result exhibits high electrical conductivity. The material can produce interdigitated electrodes for in-plane microsupercapacitors with specific capacitances of >4 mF cm⁻² and power densities of ~9 mW cm⁻². Laser-induced production appeared to allow roll-to-roll manufacturing processes and provides a route to electronic and energy storage devices.

Applying a layer of graphite oxide film to a DVD and burning it in a DVD writer produced a thin graphene film with high electrical conductivity (1738 siemens per meter) and specific surface area (1520 square meters per gram) that was highly resistant and malleable.

Microwave-assisted oxidation

In 2012, a microwave-assisted, scalable approach was reported to directly synthesize graphene with different size from graphite in one step. The resulting graphene does not need any post reduction treatment as it contains very little oxygen. This approach avoids use of potassium permanganate in the reaction mixture. It was also reported that by microwave radiation assistance, graphene oxide with or without holes can be synthesized by controlling microwave time. This method uses a recipe similar to Hummer's method, but uses microwave heating instead of traditional heating. Microwave heating can dramatically shorten the reaction time from days to seconds.

Ion implementation

Accelerating carbon ions under an electrical field into a semiconductor made of thin Ni films on a substrate of SiO2/Si, creates a wafer-scale (4 inches (100 mm)) wrinkle/tear/residue-free graphene layer that changes the semiconductor's physical, chemical and electrical properties. The process uses 20 keV and a dose of 1 × 10¹⁵ cm⁻² at a relatively low temperature of 500 °C. This was followed by high-temperature activation annealing (600–900 °C) to form an sp²-bonded structure.

See also

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