Nov 7, 2011
The story of graphene
In our sister publication Journal of Physics D: Applied Physics, Yvette Hancock documents the remarkable properties of graphene and rounds up some of the potential applications that have grabbed the attention of scientists worldwide. Here are some of the highlights.
In October 2004, Science magazine published a paper entitled, "Electric Field Effect in Atomically Thin Carbon Films" by Novoselov et al, which contained optical and atomic force microscopy observations of a few-layer and single-layer, one atom-layer-thick, two-dimensional carbon material, called graphene. Also reported in this paper were transport measurements of few-layer graphene, which were soon followed by the publication of transport results on graphene itself.
Only six years after the publication of their first paper on this topic, Andre Geim and Konstantin Novoselov, were jointly awarded the Nobel Prize in Physics, "for ... producing, isolating, identifying and characterizing graphene" and for their "ground-breaking experiments" on this material system. What Geim and Novoselov had succeeded to do was to demonstrate a very simple method (see How to make graphene) of extracting and identifying large, high-quality samples of few- and single-layer graphene from bulk graphite, and to then show how these samples could be used to make devices.
Origins as a model structure
Graphene, a single atomic layer of sp2-hybridized carbon arranged in an honeycomb structure, is the two-dimensional allotrope of carbon, which forms the basic building block of buckyballs (quasi-zero-dimensional), carbon nanotubes (quasi-one-dimensional) and graphite (three-dimensional). Long before graphene was experimentally identified, its structure was used in theoretical models as a means of approximating the electronic properties of graphite. The earliest theoretical reference of this system is Wallace’s paper in 1947, which demonstrates the use of a nearest neighbour, tight-binding model to determine an analytical expression for the electronic band structure pertaining to the pi-bonds of a monolayer of graphite.
Geim had been impressed by the remarkable advances that took place in the 1990s in carbon nanotube research, and namely in the demonstration of carbon nanotube transistors and transport properties. "Would it be possible to do the same thing for an unrolled carbon nanotube? How can I do something similar, but something different?", he wondered. It was the synthesis of these ideas that led Geim to consider thin-film graphite as one possible candidate for realizing two-dimensional, carbon-based transistors.
Graphene has a current density, which is a measure of the density of flow of charged carrier particles, that is a million times that of copper, and boasts a micrometre range mean-free path, which is the longest measured for any material. Graphene’s impressive electronic properties can be explained by its remarkable relativistic characteristics, namely, its massless Dirac-like charge carriers, which are able to penetrate high and wide potential barriers and exhibit Klein tunnelling.
Graphene has a very high optical transmission with only 2.3% absorption of white light. Optical properties, such as the optical transmission and infrared reflectivity can be tuned as a function of the applied voltage. Hence, graphene also lends itself nicely to photonic applications, such as liquid crystal displays and touch-screens, which have recently become a possibility with the advent of CVD-grown graphene sheets that are approaching a metre in size. Graphene's optical properties have also found application as transparent, conductive electrodes, with application in liquid crystals and solar cell systems.
Graphene has also been used in light emitting diodes and can now be transferred and placed onto TEM supports to enable the visualization of atoms, molecules and biological materials.
Continuing work on graphene has also revealed its impressive structural properties, namely, that it is the strongest material ever measured (∼42 N m–1), it is also the stiffest known material (Young's modulus ∼1.0 TPa) and is structurally stable, perhaps even down to a single benzene ring therefore opening the door to the design of future, ultra-small devices by top-down, lithography patterned approaches. On the nanoscale, graphene is a semiconductor, and when made into nanometre width ribbons demonstrates a tunable band-gap, which varies inversely as a function of the ribbon-width. The band-gap properties of graphene nanoribbons are being exploited in the fabrication of nanosized field effect transistors. Single electron transistors have also been demonstrated.
Speeds of between 100 and 300 GHz have been measured for graphene-based field effect transistors operating at room temperature under ambient conditions. This has been followed by a recent report of the first room temperature, fully integrated graphene circuit. Investigations into magnetic device applications of graphene are also underway, with the demonstration of graphene spin-valves, flash memory and current-induced magnetism in graphene, which may have application in spintronics. Top-down fabrication processes often result in structural disorder, which can radically alter the properties of these systems, with edge quality in nanoscale graphene being seen as a limiting factor in the development of devices. Bottom-up synthesis methods such as templated epitaxial growth or chemical synthesis may therefore provide future solutions to producing structurally pristine, nanoscale graphene.
Despite being one atom thick, graphene has been demonstrated to be enormously flexible. Graphene can be stretched elastically on a silicone substrate up to 6%, with a failure strain of up to 12%. When graphene is transferred to a pre-strained silicone substrate it can be stretched even further, up to 25%. The measured resistance in graphene was found to remain stable up to 11% stretching, with an order of magnitude change at approximately 25% stretching. Many of graphene’s properties arising from its band structure, namely its electronic and optical properties remain stable under stretched conditions, therefore lending graphene to application in the new field of "flexible" electronics.
As well as its structural and electronic properties, graphene’s thermal properties have also been measured recently and are equally impressive. Graphene has been reported to have a record thermal conductivity, which is higher than diamond at ∼5300 W m–1 K–1, thus it has the potential to be used as a thermal control material in electronic devices. Graphene’s chemical properties have also been exploited, with one of graphene’s first applications being as a gas sensor. New spin-off materials have been made using graphene’s chemistry, namely hydrogenated graphene, or graphane and also more recently flurographene (graphene + fluorine), the two-dimensional version of Teflon, with both materials demonstrating insulating properties. Graphene can not only be grown on a variety of substrates, it can also be transferred to any type of substrate, therefore opening up further possibilities of realizing new graphene-based effects.
Many more details, including an extract from Novoselov’s laboratory notebook, can be found in Journal of Physics D: Applied Physics.