Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of ‘relativistic’ condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
by A. K. GEIM AND K. S. NOVOSELOV from Nature (truncated)
Graphene is the name given to a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite. Theoretically, graphene (or ‘2D graphite’) has been studied for sixty years, and is widely used for describing properties of variouscarbon-based materials. Forty years later, it was realized that graphene also provides an excellent condensed-matter analogue of (2+1)-dimensional quantum electrodynamics, which propelled graphene into a thriving theoretical toy model. On the other hand, although known as an integral part of 3D materials, graphene was presumed not to exist in the free state, being described as an ‘academic’ material and was believed to be unstable with respect to the formation of curved structures such as soot, fullerenes and nanotubes. Suddenly, the vintage model turned into reality, when free-standing graphene was unexpectedly found three years ago — and especially when the follow-up experiments confirmed that its charge carriers were indeed massless Dirac fermions. So, the graphene ‘gold rush’ has begun.
In the absence of quality graphene wafers, most experimental groups are currently using samples obtained by micromechanical cleavage of bulk graphite, the same technique that allowed the isolation of graphene for the first time. After fine-tuning, the technique now provides high-quality graphene crystallites up to 100 μm in size, which is sufficient for most research purposes. Superficially, the technique looks no more sophisticated than drawing with a piece of graphite8 or its repeated peeling with adhesive tape until the thinnest flakes are found. A similar approach was tried by other groups but only graphite flakes 20 to 100 layers thick were found.
At low temperatures, all metallic systems with high resistivity should inevitably exhibit large quantum-interference (localization) magnetoresistance, eventually leading to the metal–insulator transition at σ ≈ e2/h. Such behaviour was thought to be universal,
but it was found missing in graphene.
Despite the reigning optimism about graphene-based electronics, ‘graphenium’ microprocessors are unlikely to appear for the next 20 years. In the meantime, many other graphene-based applications are likely to come of age. In this respect, clear parallels with nanotubes allow a highly educated guess of what to expect soon. The most immediate application for graphene is probably its use in composite materials. Indeed, it has been demonstrated that a graphene powder of uncoagulated micrometre-size crystallites can be produced in a way scaleable to mass production. This allows conductive plastics at less than one volume percent filling, which in combination with low production costs makes graphene-based composite materials attractive for a variety of uses. However, it seems doubtful that such composites can match the mechanical strength of their nanotube counterparts because of much stronger entanglement in the latter case. Another enticing possibility is the use of graphene powder in electric batteries that are already one of the main markets for graphite. An ultimately large surface-to-volume ratio and high conductivity provided by graphene powder can lead to improvements in the efficiency of batteries, taking over from the carbon nanofibres used
in modern batteries. Carbon nanotubes have also been considered for this application but graphene powder has an important advantage of being cheap to produce. One of the most promising applications for nanotubes is field emitters, and although there have been no reports yet about such use of graphene, thin graphite flakes were used in plasma displays (commercial prototypes) long before graphene was isolated, and many patents were filed on this subject. It is likely that graphene powder can offer even more superior emitting properties. Carbon nanotubes have been reported to be an excellent material for solid-state gas sensors but graphene offers clear advantages in this particular direction. Spin-valve and superconducting fieldeffect transistors are also obvious research targets, and recent reports describing a hysteretic magnetoresistance and substantial bipolar supercurrents prove graphene’s major potential for these applications. An extremely weak spin-orbit coupling and the absence of hyperfine interaction in 12C-graphene make it an excellent if not ideal material for making spin qubits. This guarantees graphene-based quantum computation to become an active research area. Finally, we cannot omit mentioning hydrogen storage, which has been an active but controversial subject for nanotubes. It has already been suggested that graphene is capable of absorbing a large amount of hydrogen, and experimental efforts in this direction are duly expected.
It has been just a few years since graphene was first reported, and despite remarkably rapid progress, only the very tip of the iceberg has been uncovered so far. Because of the short timescale, most experimental groups working now on graphene have not published even a single paper on the subject, which has been a truly frustrating experience for theorists. This is to say that, at this time, no review can possibly be complete. Nevertheless, the research directions explained here should persuade even die-hard
sceptics that graphene is not a fleeting fashion but is here to stay, bringing up both more exciting physics, and perhaps even wideranging applications.
