Synthesis of Graphene

Graphene is essentially an isolated atomic plane of graphite. Therefore, from this perspective, graphene has been known since the invention of X-ray crystallography. Graphene planes become even better separated in intercalated graphite compounds. In 2004 physicists from University of Manchester and Institute for Microelectronics Technology, Chernogolovka, Russia, found a way to isolate individual graphene planes by using Scotch tape and they also measured electronic properties of the obtained flakes and showed their fantastic quality. In 2005 the same Manchester group together with researchers from the Columbia University (see the History chapter below) demonstrated that quasiparticles in graphene were massless Dirac fermions. These discoveries led to the explosion of interest in graphene.

Since then, hundreds of researchers have entered the area and, naturally, they carried out the extensive search for relevant earlier papers. The first literature review was given by the Manchester pioneers themselves. They cite several papers in which graphene or ultra-thin graphitic layers were epitaxially grown on various substrates. Also, they point out at a number of pre-2004 reports in which intercalated graphite compounds were studied in a transmission electron microscope. In the latter case, researchers occasionally observed extremely thin graphitic flakes (“few-layer graphene” and possibly even individual layers). An early detailed study on few-layer graphene dates back to 1962. The earliest TEM images of few-layer graphene were published by G. Ruess and F. Vogt in 1948. However, already D.C. Brodie was aware of the highly lamellar structure of thermally reduced graphite oxide in 1859. It was studied in detail by V. Kohlschütter and P. Haenni in 1918, who also described the propertites of graphite oxide paper.

It is now well known that tiny fragments of graphene sheets are produced (along with quantities of other debris) whenever graphite is abraded, such as when drawing a line with a pencil. There was little interest in this graphitic residue before 2004/05 and, therefore, the discovery of graphene is often attributed to Andre Geim and colleagues  who introduced graphene in its modern incarnation, although it may be argued that this is as accurate as attributing the discovery of America to Columbus.

In 2008 graphene produced by exfoliation was one of the most expensive materials on Earth, with a sample that can be placed at the cross section of a human hair costing more than $1,000 as of April 2008 (about $100,000,000/cm2). Since then, exfoliation procedures were scaled up, and now companies sell graphene by ton. On the other hand, the price of epitaxial graphene on silicon carbide is dominated by the substrate price, which is approximately $100/cm2 as of 2009. Even cheaper graphene has been produced by transfer from nickel by Korean researchers, with wafer sizes up to 30″ reported.

In the literature, specifically that of the surface science community, graphene has also been commonly referred to as monolayer graphite. This community has intensely studied epitaxial graphene on various surfaces (over 300 articles prior to 2004). In some cases, these graphene layers are coupled to the surfaces weakly enough (by Van der Waals forces) to retain the two dimensional electronic band structure of isolated graphene, as also happens with exfoliated graphene flakes with regard to silicon dioxide. An example of weakly coupled epitaxial graphene is the one grown on silicon carbide (see below).

Drawing method

In 2004, the British researchers obtained graphene by mechanical exfoliation of graphite. They used Scotch tape to repeatedly split graphite crystals into increasingly thinner pieces. The tape with attached optically transparent flakes was dissolved in acetone and, after a few further steps, the flakes including monolayers were sedimented on a Si wafer. Individual atomic planes were then hunted in an optical microscope. A year later, the researchers simplified the technique and started using dry deposition, avoiding the stage when graphene floated in a liquid. Relatively large crystallites (first, only a few microns in size but, eventually, larger than 1 mm and visible by a naked eye) were obtained by the technique. It is often referred to as a scotch tape or drawing method. The latter name appeared because the dry deposition resembles drawing with a piece of graphite. The key for the success probably was the use of high throughput visual recognition of graphene on a proper chosen substrate, which provides a small but noticeable optical contrast. For an example of what graphene looks like, see its photograph below.

The isolation of graphene led to the current research boom. Previously, free-standing atomic planes were often “presumed not to exist” because they are thermodynamically unstable on a nm scale and, if unsupported, have a tendency to scroll and buckle. It is currently believed that intrinsic microscopic roughening on the scale of 1 nm could be important for the stability of purely 2D crystals.

It is interesting to note (see Talk:Graphene) that there were a number of previous attempts to make atomically thin graphitic films by using exfoliation techniques similar to the drawing method. Multilayer samples down to 10  nm in thickness were obtained. These efforts were reviewed in. Furthermore, a couple of very old papers were recently unearthed, in which researchers tried to isolate graphene, starting with intercalated compounds (see History and experimental discovery). These papers reported the observation of very thin graphitic fragments (possibly, monolayers) by transmission electron microscopy. Neither of the earlier observations was sufficient to “spark the graphene gold rush”, until the Science paper did so by reporting not only macroscopic samples of extracted atomic planes but, importantly, their unusual properties such as the bipolar transistor effect, ballistic transport of charges, large quantum oscillations, etc. The discovery of such interesting qualities intrinsic to graphene gave an immediate boost to further research, and several groups quickly repeated the initial result and moved further. These breakthroughs also helped to attract attention to other production techniques such as epitaxial growth of ultra-thin graphitic films. In particular, it has later been found that graphene monolayers grown on SiC and Ir are weakly coupled to these substrates (how weakly remains debated) and the graphene-substrate interaction can be passivated further.

Not only graphene but also free-standing atomic planes of boron nitride, mica, dichalcogenides and complex oxides were obtained by using the drawing method. Unlike graphene, the other 2D materials have so far attracted surprisingly little attention.

Epitaxial growth on silicon carbide

Yet another method is to heat silicon carbide to high temperatures (>1100 °C) to reduce it to graphene. This process produces a sample size that is dependent upon the size of the SiC substrate used. The face of the silicon carbide used for graphene creation, the silicon-terminated or carbon-terminated, highly influences the thickness, mobility and carrier density of the graphene.

Many important graphene properties have been identified in graphene produced by this method. For example, the electronic band-structure (so-called Dirac cone structure) has been first visualized in this material. Weak anti-localization is observed in this material and not in exfoliated graphene produced by the pencil trace method. Extremely large, temperature independent mobilities have been observed in SiC epitaxial graphene. They approach those in exfoliated graphene placed on silicon oxide but still much lower than mobilities in suspended graphene produced by the drawing method. It was recently shown that even without being transferred graphene on SiC exhibits the properties of massless Dirac fermions such as the anomalous quantum Hall effect.

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

Epitaxial graphene on silicon carbide can be patterned using standard microelectronics methods. The possibility of large integrated electronics on SiC epitaxial graphene was first proposed in 2004 by researchers at the Georgia Institute of Technology, only a couple of months after the discovery of isolated graphene made the drawing method. (A patent for graphene based electronics was applied for in 2003 and issued in 2006). Since then, important advances have been made. In 2008, researchers at MIT Lincoln Lab have produced hundreds of transistors on a single chip and in 2009, very high frequency transistors have been produced at the Hughes Research Laboratories on monolayer graphene on silicon carbide.

Epitaxial growth on metal substrates

This method uses the atomic structure of a metal substrate to seed the growth of the graphene (epitaxial growth). Graphene grown on ruthenium doesn’t typically yield a sample with a uniform thickness of graphene layers, and bonding between the bottom graphene layer and the substrate may affect the properties of the carbon layers. Graphene grown on iridium on the other hand is very weakly bonded, uniform in thickness, and can be made highly ordered. Like on many other substrates, graphene on iridium is slightly rippled. Due to the long-range order of these ripples generation of minigaps in the electronic band-structure (Dirac cone) becomes visible. High-quality sheets of few layer graphene exceeding 1 cm2 (0.2 sq in) in area have been synthesized via chemical vapor deposition on thin nickel films. These sheets have been successfully transferred to various substrates, demonstrating viability for numerous electronic applications. An improvement of this technique has been found in copper foil where the growth automatically stops after a single graphene layer, and arbitrarily large graphene films can be created.

Hydrazine reduction

Researchers have developed a method of placing graphene oxide paper in a solution of pure hydrazine (a chemical compound of nitrogen and hydrogen), which reduces the graphene oxide paper into single-layer graphene.

Sodium reduction of ethanol

A recent publication has described a process for producing gram-quantities of graphene, by the reduction of ethanol by sodium metal, followed by pyrolysis of the ethoxide product, and washing with water to remove sodium salts.

From nanotubes

Experimental methods for the production of graphene ribbons are reported consisting of cutting open 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 are produced by plasma etching of nanotubes partly embedded in a polymer film.