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 graphene 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 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.