Graphene : chemistry, processes, properties

The physical properties (electrical, optical, mechanical, etc.) of graphene are exceptional. Indeed, the behavior of electrons in graphene is very different from that of electrons in other conducting solids. In graphene, it's as if electrons had zero mass and were moving at close to the speed of light. Electrons in this material exhibit both quantum and relativistic behavior, moving with gigantic mobilities. Graphene also exhibits a quantum Hall effect at room temperature. Graphene sheets are light (paper is a thousand times heavier), flexible, virtually transparent, and have a tensile strength a hundred times greater than a layer of steel of the same thickness. This material also boasts the best thermal conduction to date, being twenty times better at conducting heat than aluminum foil. The study of this highly original material required the reconstruction of solid state theory in order to measure and interpret most of its properties.

These new materials are highly promising, and already have numerous applications in fields such as computing and communications, displays, energy, photovoltaics, aeronautics, automobiles, the environment, health and safety... In particular, graphenes are very attractive materials for high-frequency fast electronics and resonators. Graphenes are transparent conductors of particular interest for flexible electronic devices (touch screens and flexible displays). A company in Asia is already producing transparent, conductive 30-inch (≈ 75 cm) flexible screens by CVD deposition. In the energy sector, prototype batteries and supercapacitors featuring graphene as one of their components have already been developed and tested. Ultralight, conductive graphene-based composites have been used to produce inks and papers. Other applications in the automotive and aviation sectors are also in the lecture stage. The excellent mechanical properties of these materials have been put to good use in the manufacture of protective vests and armor. Graphenes are also used in the development of selective membranes and molecular sieves, as well as chemical and biochemical sensors. As these materials are biocompatible, hybridized graphenes are being tested for the diagnosis and treatment of serious diseases in nanomedicine, or as components in antibacterial or anti-fungal coatings. Graphene-based transistors for bioelectronics and neural prostheses have recently been developed. They can restore damaged abilities such as hearing and vision, or help find solutions for the treatment of motor disabilities or brain pathologies. However, the exploitation of these numerous properties is still limited by the production of graphene.

Indeed, although micromechanical exfoliation remains an important method for producing high-quality "virgin" graphene for fundamental physics studies, it is characterized by small-scale feasibility and high cost. Preparing graphene in relatively large quantities (a few grams or more), or forming it on large, flat substrates, remains a challenge. The development of commercial graphene-based devices is therefore dominated by the optimization of the chemistry/process/quality/cost quartet. Graphene chemistry is in full development. The aim is to produce graphene with the quality and properties required for a targeted application, with the emphasis on versatile systems that will enable the tailor-made addition of other functionalities. The properties of the graphene obtained depend on the elaboration processes, which have variable costs and production capacities. They need to optimize energy costs and technical transferability. Mastering the transfer and integration of graphene into devices without degradation is also one of the challenges to be met. Against this backdrop, we have analyzed the various chemical and physical approaches to obtaining graphene, and illustrated the properties of the resulting devices on the basis of very recent literature.

Graphene synthesis methods and processes can be divided into two main routes: thetop-down route, in which the material is split to obtain graphene sheets of different compositions (graphene, graphene oxide, reduced graphene oxide), and thebottom-up route, in which graphene is built up by assembling small atomic or molecular entities. Top-down strategies involve micromechanical exfoliation of graphite (scotch), silicon sublimation from SiC with epitaxial control (low yield, high cost, high purity, very expensive substrates, high-temperature process), liquid-phase exfoliation of graphite or graphite oxide, with or without ultrasonic or electrochemical assistance, or by high-shear mixing (good large-scale feasibility, low cost, low purity, high defect density, in some cases a chemical reduction step is required), opening (unzipping) of carbon nanotubes or fullerene (high yield, high purity, potentially low cost, laboratory trials only). The many strategies involving chemically-assisted exfoliation processes require the reduction of the graphene oxides generally obtained to graphene. Bottom-up strategies involve chemical vapor deposition (CVD) on a metal such as nickel, copper or cobalt (recently demonstrated large-scale feasibility, high cost, high purity, high-temperature process), macromolecular synthesis in solution or on a substrate (high yield, high purity, laboratory-scale testing, potentially at reasonable cost).