Applications

A large proportion of current graphene research is device motivated. Its remarkable electronic and mechanical properties make it an ideal candidate for a huge and varied range of applications. I shall summarise some of the most important ones here; please see Refs. [98–100] for a more comprehensive overview.

One aspect, which has received a lot of media coverage, is the use of graphene within the electronics industry. Currently, integrated circuits are made with sil- icon using complementary metal-oxide-semiconductor technology. However, it is well known that in approximately ten years, the reasonable limits of silicon will be reached in terms of minituarising devices, increasing their speed and reducing the cost. Indeed, there is a physical limit imposed by the fact that silicon loses its crystalline structure beyond 10 nm due to increasing thermal fluctuations of the atoms. Thus a new material is required for progress to be made and both car- bon nanotubes and graphene are being seriously investigated for use in MOSFETs. The properties recommending graphene for such a purpose are its high mobility electronic transport, two-dimensional nature, strength and high melting tempera- ture. Consequently there has been a sustained effort towards the fabrication of high performance graphene FETs. Most recently a group at IBM achieved cut-off fre- quencies of 100 GHz [101], which is markedly better than the highest performing

silicon MOSFETs. These have a high chance of being used in analog transistors for communications technology and imaging. However, the fact that graphene has no intrinsic band gap means that it is difficult to turn off the electrical current, so on-off current ratios are much too low [102], ruling out their use in digital circuits for the moment. Since digital transistors make up the bulk of applications, over- coming this problem is now one of the main goals of graphene research. Numerous ways of opening a gap have been suggested. There already exists a gap in bilayer graphene, which is tunable by a gate voltage [103]. Graphene strongly interacting with certain substrates may open a gap, particularly if this induces an asymmetry between the two sublattices [104]. A gap can be opened by constricting the path of electrons, for example in nanoribbons [105], quantum dots [106] or nanoconstric- tions [107]. Chemically modifying graphene by doping it, with hydrogen [108,109] or fluorine [110], for example, will also induce a transition to a gapped insulating state. We have already seen that an excitonic gap should exist in the regime of stronge-e interactions. However, there are problems associated with each of these approaches and it is still not possible to control the band gap of graphene to the degree required for competitive digital transistors. Indeed, the full extent of graphene’s future role in the electronics industry remains unclear.

Graphene is also used in creating nanoelectromechanical systems (NEMs), due to its strength and stiffness [111]. For example, drum resonators have been made from graphene oxide with high quality factors [112]. It is hoped that graphene will be used to realise the ultimate goal of chemical/biological sensors, namely single molecule gas detection [113]. The basic idea is that each adsorbed molecule acts as an acceptor or donor, causing step-like changes in the resistance. The difficulty is that such small signals are easily drowned out by the noise. This is reduced in graphene, since it naturally has a low number of impurities. Other useful properties in this context are that it remains a conductor even at low carrier densities [92] and its 2D nature means its entire volume is available to receive adsorbates.

Another biological application, which has been recently proposed [114], is using graphene to sequence DNA. How to determine the sequence of nucleotide bases on a single molecule of DNA at a reasonable cost and speed, is currently one of the most important and challenging problems in biotechnology research [115]. A previous successful technique, known as shotgun sequencing, involves cutting up the molecule [116]. In another method, which does not modify the molecule, the DNA is drawn through a biological nanopore by an electric field, in an ionic solution [117]. The basic idea is that different bases result in different ionic currents. Researchers are now trying to use holes in graphene instead of biological pores.

Graphene certainly seems to offer several advantages. Its thinness means that only one base will be in the hole at any given time allowing single base resolution. It can be used both as a membrane and electrodes. It is also stable and the size and shape of the hole may be chosen quite precisely.

Graphene is also being investigated for flexible touch screen technology. The material currently used, indium tin oxide, is in short supply. A graphene sheet supported by a polymer is used to create the screen’s transparent electrodes [118]. The final aim is to use this in ultrathin computing devices, which can be folded away and easily transported.

With much of the basic physics underlying graphene now understood, there is a strong emphasis on growing larger, cleaner sheets to be used in technological applications. There has been a lot of progress [75] since the original fabrication of µm size samples via mechanical exfoliation. The current record is a 30 inch sample, which was grown on a flexible copper substrate via chemical vapour decomposition using roll-to-roll production, that allows it to be transfered to a target substrate of choice [119]. There is also a lot of interest in the nature of the impurities in graphene, which limit the mobility. This question is important for improving device performance, but also pertains to the fundamental properties of the material. It is also relevant to the work presented in Chapter 7, where we examine collective excitations of graphene localised on impurities of different types.