Graphene : materials of the future ?

The last two lessons were devoted to a particularly topical research theme in materials science: graphene. Indeed, over the course of 2016, this subject has given rise to more than 25,000 publications and patents. This sharp increase in the scientific communities' interest in graphene in recent years is linked to several factors: its remarkable properties, the Nobel Prize in Physics awarded to André Geim and Konstantin Novoselov in 2010, its significant potential for applications, and the strong economic support from funding agencies and Europe in particular. To better understand the structure of graphene, we need only describe that of graphite, the black carbonaceous material used in pencil leads. Incidentally, the name "graphite" is derived from the Greek word graphein, meaning "to write". The structure of graphite is similar to that of a mille-feuille, with each layer made up of an infinite network of polycyclic aromatic hydrocarbons. Graphene corresponds to one of these sheets when isolated. It can therefore be defined either as a two-dimensional layer composed solely of carbon atoms arranged in a hexagonal, honeycomb-like pattern, or as the infinite-sized end member of the well-known aromatic cyclic hydrocarbon series: benzene (1 ring), naphthalene (2 rings), anthracene (3 rings), tetracene (4 rings), coronene (7 rings), ovalene (10 rings)... graphene (very large number of rings). By extension, the term "graphene" is often used to designate a variety of compositions ranging from monolayers of carbon atoms to a few layers (2 to 5). The number of graphene layers and their nature are often specified in the literature, and depending on the materials obtained, the terms "single-layer graphene", "multi-layer graphene", "graphene oxide" or "reduced graphene oxide" are used. All these designations define the main members of the graphene family.

The scientific history of graphene dates back to the 1840s. Since then, numerous authors have explored processes for the oxidative intercalation of small chemical entities in graphite. Oxidation and exfoliation of graphite lead to the formation of graphite oxide, then graphene oxide, which can be reduced, in a final step, to obtain a graphene that often remains defective. As early as the 1940s, a series of theoretical analyses by P. R. Wallace suggested that these monolayers - if isolated and pure - could display extraordinary electronic characteristics. Graphene-like materials derived from graphite oxide were reported in the late 1960s, and articles from the surface science community between 1968 and 1975 described deposits of polycyclic aromatic hydrocarbons deposited on metals or graphitic monosheets (i.e. graphene), or obtained by silicon sublimation from silicon carbide films. The micromechanical exfoliation of graphite to produce high-grade graphene sheets began in 1999 and took shape in 2004 with the so-called "scotch tape" methodology developed by Andre Geim and Konstantin Novoselov. Indeed, when a pyrolitic graphite surface is pressed against a scotch tape surface and then removed, thin graphene flakes can be isolated and observed by optical microscopy. These can then be selected to enable the study of the physical properties of graphene monosheets that are largely free from the significant presence of functional groups and therefore of very high purity. It is the study of the properties of this graphene that has laid the foundations for exciting research raising important challenges with great technological relevance.