Ten years ago, the discovery of the wonder material – Graphene – was announced. Graphene is thin, stronger than steel, flexible, non-metallic, yet electrically conductive. For all these reasons, graphene promises to transform electronics, as well as other technologies. Because of its potential in industry, researchers have been looking for ways to make defect-free graphene in large amounts.
If graphene sounds exotic, the atomic element that makes it really isn’t.
Nothing, but Carbon
Carbon is an almighty element. Familiar, but intriguing.
The 15th most abundant element in the Earth’s crust, and the 4th most abundant element in the Universe by mass after hydrogen, helium and oxygen, carbon is also present in all known life forms, including the human body where it is the 2nd most abundant element by mass (about 18.5%) after oxygen.
Together with the unique diversity of organic compounds and their unusual polymer-forming ability at temperatures commonly encountered on Earth, the abundance of carbon makes it the chemical basis for all known life.
The physical properties of carbon vary widely according to its allotropic form, which means it can look like radically different stuff, depending on its molecular structure. Carbon can be brittle, or it can be immensely strong…
Graphite vs Diamonds
Graphite is mixed with clay to produce the lead in pencils. Diamonds ARE a girl’s best friend… 😉 And their exceptional durability has come to symbolise eternity (or is it, “an” eternity?) in a relationship.
Graphite and diamond are both allotropes of carbon. So, why do they look so different?
Graphite is opaque and black, while diamond is highly transparent. Graphite is soft enough to leave a streak on paper. Diamond is the hardest naturally-occurring material.
To understand why those allotropes behave so differently, we can take a look at the molecular arrangement of the carbon atoms in both materials. Diamond has a crystalline molecular structure, whereas graphite looks like a stack of seemingly unconnected smaller molecules. Effectively, graphite is a material made up of many layers of graphene stacked on top of one another.
Actually, it’s not just the appearance that differs. While diamond has a very low electrical conductivity, graphite is an excellent electrical conductor.
But graphene is special.
The theory of graphene was first explored by P.R. Wallace in 1947, as a starting point for understanding the electronic properties of 3D graphite.
Graphene research has gone a long way since the substance was first isolated in 2004. The initial findings were reported in the academic journal Science.
Research was informed by theoretical descriptions of graphene’s composition, structure and properties, which had all been calculated decades earlier. High-quality graphene also proved to be surprisingly easy to isolate, making more research possible.
In 2010, Manchester University researchers Andre Geim and Konstantin Novoselov shared the Nobel Prize in Physics for their discovery of graphene.
Geim and Novoselov famously used sticky tape to peel off the layers of graphene from graphite. They pulled graphene layers from graphite and transferred them onto thin SiO 2 on a silicon wafer in a process called either micromechanical cleavage or the ‘Scotch tape’ technique.
Graphene can be described a one-atom-thick sheet of graphite arranged in a honeycomb structure. It is the basic structural element of other carbon allotropes, including graphite, charcoal, carbon nanotubes, and fullerenes. It can also be considered as an indefinitely large aromatic molecule.
Currently, graphene can be grown atom-by-atom via chemical vapour deposition. However, while this process can produce metre-scale sheets of graphene, they also contain defects which can inhibit their physical properties.
Most recently, Graphene Nano-Ribbons (GNRs) have been prepared by the oxidative treatment of carbon nanotubes and by plasma etching of nanotubes embedded in polymer films.
Properties of Graphene and Nano-ribbons
The atomic structure of isolated, single-layer graphene was studied by transmission electron microscopy (TEM) on sheets of graphene suspended between bars of a metallic grid. Electron diffraction patterns showed the expected honeycomb lattice. Suspended graphene also showed rippling of the flat sheet, with amplitude of about one nanometre.
Graphene can self-repair holes in its sheets, when exposed to molecules containing carbon, such as hydrocarbons. Bombarded with pure carbon atoms, the atoms perfectly align into hexagons, completely filling the holes.
Chemically, graphene is the most reactive form of carbon, owing to the lateral availability of carbon atoms. Graphene burns at very low temperature (e.g. 350 °C).
The electronic properties of graphene also have some similarity with carbon nanotubes. Graphene is a semi-metal or zero-gap semiconductor.
Electrons propagating through graphene’s honeycomb lattice effectively lose their mass, producing quasi-particles that are described by a 2D analogue of the Dirac equation, rather than the Schrödinger equation for spin-1/2 particles.
Electron waves in graphene propagate within a single-atom layer, making them sensitive to the proximity of other materials such as high-κ dielectrics, superconductors and ferromagnetics.
Graphene electrons can move over tens or hundreds of microns without scattering, even at room temperature. Electrical resistance in 40-nanometre-wide nanoribbons of graphene changes in discrete steps following quantum mechanical principles.
Electrons travel ballistically, similar to those observed in cylindrical carbon nanotubes. The ribbons’ theoretical conductance exceeds predictions by a factor of 10. The ribbons can act more like optical waveguides or quantum dots, allowing electrons to flow smoothly along the ribbons’ edges. By contrast, in conductors such as copper, electrical resistance increases in proportion to the length as electrons encounter impurities while moving through the conductor.
Anomalous Quantum Hall Effect and Berry Phase
Even though graphene on nickel Ni and silicon carbide SiC have both existed in the laboratory for several decades, the cleavage technique led directly to the first observation of the anomalous quantum Hall effect in graphene, providing direct evidence of graphene’s theoretically predicted Berry’s phase of massless Dirac fermions. The effect was reported soon after by Philip Kim and Yuanbo Zhang in 2005.
Mechanically exfoliated graphene on SiO 2 (silicon dioxide) provided the first proof of the Dirac fermion nature of electrons.
As of 2009, graphene was reported to be one of the strongest materials known with a breaking strength over 100 times greater than a hypothetical steel film of the same (thin) thickness, with a Young’s modulus (stiffness index) of 1 TPa and intrinsic strength of 130 GP, similar to Single Walled carbon NanoTubes (SWNTs).
The Nobel announcement illustrated this fact by saying that a 1 square-metre hammock made of graphene would support a 4 kg cat, but only weigh as much as one of the cat’s whiskers!
Electron mobility in graphene is extraordinarily high (15,000 cm2V-1s-1 at room temperature) and ballistic electron transport is reported to be on length scales comparable to that of SWNTs.
One of the most promising aspects of graphene involves the use of GNRs. Cutting an individual graphene layer into a long strip can yield semiconducting materials where the band gap is tuned by the width of the ribbon.
While graphene’s novel electronic and physical properties guarantee this material will be studied for years to come, there are some fundamental obstacles yet to overcome before graphene based materials can be fully utilised. Although the aforementioned methods of graphene preparation are effective, they have proved impractical for large-scale manufacturing.
The most plentiful and inexpensive source of graphene is bulk graphite. Chemical methods for exfoliation of graphene from graphite provide the most realistic and scalable approach to graphene materials.
Graphene layers are held together in graphite by enormous van der Waals forces.
Overcoming these van der Waals forces is the major obstacle to graphite exfoliation. Until now, chemical efforts at graphite exfoliation have been focused mainly on intercalation, chemical derivatization, thermal expansion, oxidation-reduction, the use of surfactants, or a combination of these.
Culinary Physics: Ready, Steady, Mix
An Irish-UK team from Trinity College in Dublin tested out a variety of laboratory mixers, as well as kitchen blenders, as potential tools for manufacturing the wonder material.
Jonathan Coleman and colleagues poured graphite powder (the stuff of pencil leads) into a blender, then added water and dishwashing liquid, mixing at high speed.
They showed that the shearing force generated by a rapidly rotating tool in solution was sufficiently intense to separate the layers of graphene that make up graphite flakes without damaging their two-dimensional structure.
The precise amount of dishwashing fluid that’s required is dependent on a number of different factors. The black solution containing graphene needs to be separated afterwards. But the researchers said their work “provides a significant step” towards deploying graphene in a variety of commercial applications.
The results are reported in the journal Nature Materials.
The scientists have been working with UK-based firm Thomas Swan to scale up the process, with the aim of building a pilot plant that could produce a kilo of graphene per day by the end of the year.
We could soon be finding graphene everywhere. And I mean EVERYWHERE.
In addition to its potential uses in electronics, graphene might have applications in water treatment, oil spill clean-up, and even in the production of anything from stronger hosiery to thinner condoms.
Hey! I just thought… Do you reckon graphene is a girl’s new best friend? 😀