Graphene - History and Experimental Discovery

History and Experimental Discovery

The term graphene first appeared in 1987 to describe single sheets of graphite as one of the constituents of graphite intercalation compounds (GICs); conceptually a GIC is a crystalline salt of the intercalant and graphene. The term was also used in early descriptions of carbon nanotubes, as well as for epitaxial graphene, and polycyclic aromatic hydrocarbons.

Larger graphene molecules or sheets (so that they can be considered as true isolated 2D crystals) cannot be grown even in principle. An article in Physics Today reads:

Fundamental forces place seemingly insurmountable barriers in the way of creating ... 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.

Single layers of graphite were previously (starting from the 1970s) grown epitaxially on top of other materials. This "epitaxial graphene" consists of a single-atom-thick hexagonal lattice of sp2-bonded carbon atoms, as in free-standing graphene. However, there is significant charge transfer from the substrate to the epitaxial graphene, and, in some cases, hybridization between the d orbitals of the substrate atoms and π orbitals of graphene, which significantly alters the electronic structure of the epitaxial graphene.

Single layers of graphite were also observed by transmission electron microscopy within bulk materials (see section Occurrence), in particular inside soot obtained by chemical exfoliation. There have also been a number of efforts to make very thin films of graphite by mechanical exfoliation (starting from 1990 and continuing until after 2004) but nothing thinner than 50 to 100 layers was produced during these years.

A key advance in the science of graphene came when Andre Geim and Kostya Novoselov at Manchester University managed to extract single-atom-thick crystallites (graphene) from bulk graphite in 2004. The Manchester researchers pulled out graphene layers from graphite and transferred them onto thin SiO₂ on a silicon wafer in a process sometimes called micromechanical cleavage or, simply, the Scotch® tape technique. The SiO₂ electrically isolated the graphene, and was weakly interacting with the graphene, providing nearly charge-neutral graphene layers. The silicon beneath the SiO₂ could be used as a "back gate" electrode to vary the charge density in the graphene layer over a wide range.

The micromechanical cleavage technique led directly to the first observation of the anomalous quantum Hall effect in graphene, which provided direct evidence of the theoretically predicted pi Berry's phase of massless Dirac fermions in graphene. The anomalous quantum Hall effect in graphene was reported around the same time by Geim and Novoselov and by Philip Kim and Yuanbo Zhang in 2005. These simple experiments started after the researchers watched colleagues who were looking for quantum Hall effect and Dirac fermions in bulk graphite.

Geim has received several awards for his pioneering research on graphene, including the 2007 Mott medal for the "discovery of a new class of materials – free-standing two-dimensional crystals – in particular graphene", the 2008 EuroPhysics Prize (together with Novoselov) "for discovering and isolating a single free-standing atomic layer of carbon (graphene) and elucidating its remarkable electronic properties", and the 2009 Körber Prize for "develop the first two-dimensional crystals made of carbon atoms". In 2008 and 2009, Reuters (which also runs a bibliometric service Web of Science) tipped him as one of the front-runners for a Nobel prize in Physics. On October 5, 2010, the Nobel Prize in Physics for the year was awarded to Andre Geim and Konstantin Novoselov from the University of Manchester for their work on graphene.

The theory of graphene was first explored by P. R. Wallace in 1947 as a starting point for understanding the electronic properties of more complex, 3D graphite. The emergent massless Dirac equation was first pointed out by Gordon Walter Semenoff and David P. DeVincenzo and Eugene J. Mele. Semenoff emphasized the occurrence in a magnetic field of an electronic Landau level precisely at the Dirac point. This level is responsible for the anomalous integer quantum Hall effect. Later, single graphene layers were also observed directly by electron microscopy.

More recently, graphene samples prepared on nickel films, and on both the silicon face and carbon face of silicon carbide, have shown the anomalous quantum Hall effect directly in electrical measurements. Graphitic layers on the carbon face of silicon carbide show a clear Dirac spectrum in angle-resolved photoemission experiments, and the anomalous quantum Hall effect is observed in cyclotron resonance and tunneling experiments. Even though graphene on nickel and on silicon carbide have both existed in the laboratory for decades, it was graphene mechanically exfoliated on SiO₂ that provided the first proof of the Dirac fermion nature of electrons in graphene.