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:

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.

Robotic pixel assembly of van der Waals solids

Robotic pixel assembly method for manufacturing vdW solids provides high-speed and controllable design (area, geometry, and angle). In this approach, robotic assembly of prepatterned ‘pixels’ made from atomically thin two-dimensional components forms heterojunction devices. In the first implementation of this approach, the process takes place within a high-vacuum environment to allow clean interfaces.

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 /Vs. Spectroscopic analysis of reduced graphene oxide has been conducted.

Liquid phase exfoliation: Shearing

In 2014 defect-free, unoxidized graphene-containing liquids were made from graphite using mixers that produce local shear rates greater than s-1. The method was claimed to be applicable to other 2D materials, including boron nitride, Molybdenum disulfide and other layered crystals. The liquid phase shear technique with the aid of surfactant is more suitable for pristine graphene exfoliation at room temperature and avoiding multi-step preparation.

Liquid Phase Exfoliation: Sonication

Solvent-aided

Dispersing graphite in a proper liquid medium can produce graphene by sonication in a process known as Liquid Phase Exfoliation. Graphene is separated from graphite by centrifugation, producing graphene concentrations initially up to in N-methylpyrrolidone (NMP) and later to in NMP,. Using a suitable ionic liquid as the dispersing liquid medium produced concentrations of . Graphene concentration produced by this method is can be low, probably because of the large energy required to fragment the crystal during sonication.

Adding a surfactant to a solvent prior to sonication prevents restacking by adsorbing to the graphene's surface. This allows the production of aqueous suspensions, 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.

Laser-Induced Graphene (LIG)

In 2014, a laser-based single-step scalable approach to graphene production was published by Professor James M. Tour's Research Group at Rice University. The technique directly converts the surface of commercial polymer films into porous three-dimensional graphene patterns, using a  infrared laser. The sp³-carbon atoms were photothermally converted to sp²-carbon atoms by pulsed laser irradiation. The resulting material exhibits high electrical conductivity, and has been demonstrated in a variety of applications, including 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 of graphene is compatible with roll-to-roll manufacturing processes, and provides a highly-accessible route to flexible electronics, functional nanocomposites, and advanced energy storage devices. Furthermore, the technique has been extended to a wide variety of carbon sources, such as wood, paper, and cloth, and likewise, other wavelengths of lasers were also demonstrated to form graphene.

Laser-Induced Graphene Fibers (LIGF) and Laser-Induced Graphene Scrolls (LIGS)

In 2018, Professor James M. Tour's Research Group at Rice University published the synthesis of Laser-Induced Graphene Fibers and Laser-Induced Graphene Scrolls. The new morphologies, which were accessible through tuning of laser parameters, found applications in areas such as air filtration and functional nanocomposites.

Flash Joule Heating

In 2019, flash Joule heating (transient high-temperature electrothermal heating) was discovered to be a method to synthesize turbostratic graphene in bulk powder form. The method involves electrothermally converting various carbon sources, such as carbon black, coal, and food waste into micron-scale flakes of graphene. More recent works demonstrated the use of mixed plastic waste, waste rubber tires, and pyrolysis ash as carbon feedstocks. The graphenization process is kinetically controlled, and the energy dose is chosen to preserve the carbon in its graphenic state (excessive energy input leads to subsequent graphitization through annealing).

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.

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 and on Pt(111). On the other hand, the epitaxial graphene layer on some metals can be strongly bonded to the surface with covalent bonds. The properties of the covalently bonded graphene can differ from the ones of free-standing graphene. An example of this strong coupling is epitaxial graphene on Ru(0001). However, the coupling is strong only for the first graphene layer on Ru(0001): the second layer is more weakly coupled to the first layer and has already properties very close to the free standing graphene.

Chemical vapor deposition

Chemical vapor deposition (CVD) is a common form of epitaxy. The process of deposition of solid material onto a heated substrate through decomposition or chemical reaction of compounds contained in the gas passing over the substrate is called chemical vapor deposition. The reactants, generally in the gaseous or vapor phase, react on or near the surface of the substrates, which are at some elevated temperature. The subsequent reaction results in the deposition of atoms or molecules on the entire substrate surface. CVD processes are also widely used for growing epitaxial layers such as a silicon epitaxial layer on a single-crystal silicon substrate (homoepitaxy or commonly referred to as epitaxy) or epitaxial layer deposition on a sapphire (Heteroepitaxy). A special method in CVD, called Epitaxy or Epitaxial Layer Deposition or Vapor-Phase Epitaxy (VPE), has only a single-crystal form as the deposited layer. This process is usually carried out for certain combinations of substrate and layer materials and under special deposition conditions.

Epitaxy of graphene

Epitaxial graphene films can be grown on various crystalline surfaces. The atomic lattice of the substrate facilitate in orientationally registering the carbon atoms of the graphene layer. The chemical interaction of the graphene with the substrate can vary from weak to strong. This also modifies the properties of the graphene layer. The need for epitaxial graphene arises from the challenges of incorporating carbon nanotubes in large-scale integrated electronic architectures. Research on 2D graphene was thus initiated by experiments on epitaxially grown graphene on single crystal silicon carbide. While significant control has been in growing and characterizing epitaxial graphene, challenges remain in being able to fully exploit the potential of these structures. The promise lies in the hope that charge carriers on these graphene structures, like carbon nanotubes, remain ballistic. If so, it could revolutionize the world of electronics.

Silicon carbide

Heating silicon carbide (SiC) to high temperatures (>) 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 polarity of the SiC used for graphene formation, silicon- or carbon-polar, 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 single crystal substrates

Metal single crystals are often used as substrates in graphene growth since they form a smooth and chemically uniform growth platform for graphene. Especially, the chemical uniformity is an important advantage of metal single crystal surfaces: for example in different oxide surfaces the oxidized component and the oxygen forms very different adsorption sites. A typical metal single crystal substrate surface is hexagonal close-packed surface since this geometry is also the geometry of carbon atoms in a graphene layer. Common surfaces that have hexagonal close packed geometry are for example FCC(111) and HCP(0001) surfaces. Of course, the similar surface geometries alone do not ensure perfect graphene adsorption on the surface since the distances between surface metal atoms and carbon atoms can be different, resulting in moiré patterns. Common metal surfaces for graphene growth are Pt(111), Ir(111), Ni(111), Ru(0001), Co(0001) and Cu(111) but also at least Fe(110), Au(111), Pd(111), Re(101͊0) and Rh(111) have been used.

Preparation methods of metal single crystal substrates

There are several methods how good quality metal single crystal substrates can be manufactured. Czochralski and Bridgman–Stockbarger methods are common industrial methods for bulk metal crystal manufacturing. In these methods, the metal is first melted, after which the metal is let to crystallize around a seed crystal. After crystallization, the crystal is cut into wafers. Other commonly used method especially in research is epitaxy, which enables the growth of numerous different metal single crystal surfaces on some commonly available single crystals like monocrystalline silicon. The advantage of epitaxy over the industrial methods is its low material consumption: with epitaxy substrates with thickness in nanometer scale can be manufactured in comparison to complete self-supporting wafers. This is especially important with rare and expensive metals like rhenium and gold.

Ruthenium(0001)

Graphene can be grown on ruthenium(0001) surface with CVD, temperature programmed growth (TPG) or segregation. In CVD, a hot ruthenium surface is exposed for some carbon containing molecule like methane or ethene. This results in graphene formation. It has been observed that the graphene can grow only “downhill” of the ruthenium surface steps, not uphill.
Graphene bonds strongly with covalent bonds to the surface and has only 1.45 Å separation to the surface. This affects the electronic structure of the graphene layer, and the layer behaves differently than a free-standing graphene layer. However, the CVD graphene growth on ruthenium is not totally self-terminating and multilayer graphene formation is possible. The second and higher layers cannot bond to the existing graphene layers as strongly as the first layer bonds to the metal surface, which results in higher 3 Å separation between the graphene layers. The second layer thus has much weaker interaction with the substrate and has very similar electronic properties as free-standing graphene.
Due to the strong bonding of graphene on the ruthenium surface, only R0 orientation is observed for graphene layer. Although, different studies have shown different lengths for the moiré repeat distance, varying around Graphene(11 x 11) and Ru(10 x 10). The moiré pattern also causes strong corrugation for the graphene layer, peak height being as much as 1.5 Å.

Iridium(111)

Graphene is commonly deposited on iridium(111) by CVD but also with temperature programmed growth (TPG) is possible. In CVD, a hot iridium surface is exposed to ethylene. Ethylene decomposes on the surface due to pyrolysis, and the formed carbon adsorbs to the surface forming a graphene monolayer. Thus, only a monolayer growth is possible. The formed graphene layer is weakly bounded to the iridium substrate and is located about 3.3 Å above the surface. The graphene layer and the Ir(111) substrate also forms a moiré pattern with period around 25 Å, depending on the orientation of the graphene on Ir(111). There are many different possibilities for the graphene layer orientation, the most common ones being R0 and R30. The graphene layer has also corrugation due to the moiré pattern, with height varying from 0.04 Å to 0.3 Å. Due to the long-range order of these ripples, minigaps in the electronic band-structure (Dirac cone) become visible.

Platinum(111)

Graphene sheets have been reported to be grown by dosing ethylene onto the clean, single platinum(111) substrate at temperatures above 1000 °C in ultra-high vacuum (UHV). Graphene monolayer interacts weakly with the Pt(111) surface below it confirmed by the local density of states which is a ‘V’ shape. Kim et al., reported the electronic properties of the graphene nanoislands whose geometry is affected by varying the annealing temperatures and providing a fundamental understanding on graphene growth. The effect of annealing on the average size and density of graphene islands grown on Pt(111) has been widely studied. Sutter et al., reported a thermal-stress driven wrinkle propagation on the graphene sheet as observed from low-energy electron microscopy during cooling after growth. The onset of lattice mismatch precedes the observation of moiré patterns with small (e.g., (3x3)G) and large unit cells (e.g., (8x8)G).

Nickel(111)

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. CVD grown graphene on Ni(111) surface forms (1 x 1) structure, i.e. the lattice constants of Ni and graphene matches and no moiré pattern is formed. There are still different possible adsorption sites for carbon atoms on nickel, at least top, hcp hollow, fcc hollow and bridge sites have been reported 7

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.

Cobalt(0001)

Graphene on cobalt(0001) is grown similarly as on a Ni substrate. A Co(0001) film is first grown on a wolfram(110) substrate, following which chemical vapor deposition of propylene at 450 °C enables graphene growth on Co(0001). This results in a p(1x1) structure along with structures that indicated domains of graphene slightly rotated with respect to the Co lattice. Graphene structures grown on Co(0001) are found to be identical to those grown on Ni(111) upon structural and electronic characterization. Co(0001) is ferromagnetic but the graphene monolayer grown over was found to not diminish the spin polarization. Unlike its Ni(111) counterpart, graphene grown on Co(0001) does not show the Rashba effect.

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.

Tin

Tin has been recently used for synthesis of graphene at 250 °C. Low-temperature as well as the transfer free graphene growth on substrates is the major concern of graphene research for its practical applications. The transfer free graphene growth on SiO2 covered Si (SiO2/Si) substrate at 250 °C based on a solid-liquid-solid reaction has been achieved by tin.

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. In 2018, researchers at MIT refined the roll-to-roll process, creating a promising way to produce large amounts of graphene.

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.

Cold wall CVD technique can be used to study the underlying surface science involved in graphene nucleation and growth as it allows unprecedented control of process parameters like gas flow rates, temperature and pressure as demonstrated in a recent study. The study was carried out in a home-built vertical cold wall system utilizing resistive heating by passing direct current through the substrate. It provided conclusive insight into a typical surface-mediated nucleation and growth mechanism involved in two-dimensional materials grown using catalytic CVD under conditions sought out in the semiconductor industry.

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 nanoribbon