Scanning probe microscopy of the quantum Hall effect and charge pumping in graphene for meterological applications

The bulk graphite which one finds at the core of a pencil is composed of many hundreds of layers of carbon atoms stacked on top of one another. It is this simple atomic architecture which makes graphite so easy to deposit when gently rubbed against another surface because the layers are free to slide over one another. It was discovered recently that this process even produces single atomic layers, i.e., tiny flakes of carbon which are only one atom thick. This flat allotrope of carbon is called graphene and has created enormous excitement since its discovery. It exhibits a remarkable number of new electronic, mechanical, and optical properties relevant to a wide range of device applications and fundamental research questions. The electronics community is particularly attracted to graphene because it combines high mobility, high transparency, and the ability to carry very high current densities. Recently the UK's meterological standards agency, the National Physical Laboratory (NPL), has shown that graphene can be used at low temperatures and at high magnetic fields as resistance standard as it shows the quantum Hall effect with very accurate plateau in the Hall resistance. Graphene is not yet competitive with the semiconducting material currently used to calibrate resistors, however, probably due to the level of disorder. The first objective of this project is to use low temperature scanning probe microscopy and chemical functionalisation to characterise and then reduce the disorder in these layers, thus improving the precision of the quantisation. In addition, the results of our characterisation should help those who grow the graphene layers to develop techniques for producing better quality material. Graphene's ability to conduct electricity cannot be switched on and off unless it is patterned so as to have widths less than 5 nm, so at the moment it is unsuitable for applications such as transistors in digital electronics. However, bilayer graphene, which consists of two layers one above the other, can be made insulating using a vertical electric field. The second part of our project aims to exploit this behaviour to control the path taken by electrons as they travel through graphene. In particular our aim is to channel electrons down small conducting pathways and into electron traps, known as quantum dots , where they are localised. Then, using high frequencies we will clock single electrons through the dot one at a time. The effect is to produce a current that is equal to the charge on the electron times the frequency that we clock them through the dot. This opens up the possibility of producing a well defined current that could be used as a standard for calibrating scientific instruments and for making very precise measurements of the fundamental constants of nature. In addition, because we are defining our quantum dots using the electric field from metal electrodes, the confinement potential should be very smooth and the scattering of the charge carriers off this potential should be specular. As a result, electrons will go through narrow channels without back scattering. This behaviour has not been seen in graphene yet, probably because devices designed so far have rough edges and a great deal of disorder with complex scattering properties. By using the bilayer gated devices we should be able to get rid of this scattering and increase the spin lifetime in graphene quantum dots, thereby opening up the tantalising prospect of using pencil lead as the basis for a quantum computer.

The semiconductor industry is the cornerstone of today's high-tech economy, with European semiconductor companies currently supporting more than 100,000 direct and a multitude of indirect jobs in Europe. With worldwide sales valued at US$ 340 billion in 2006, the sector supported a global market ~ US$ 1.3 trillion in terms of electronic systems representing ~ 16% of the world market. As a result, even if graphene only enters a small fraction of this market the number of beneficiaries will be significant. One of the overarching aims of this project is to deepen our understanding of the disorder present in epitaxial graphene, which is the most likely form to reach marketable devices. Hence our research will mainly benefit those wanting to use graphene in commercial applications. There are a number of possible applications for graphene that could have economic impact. Polycrystalline films of few-layer graphene are beginning to be used for transparent electrodes for displays and solar cells. Because it is thin and strong it could also be used on flexible substrates for flexible versions of these products. Graphene can handle very high current densities which means it could also be used in interconnects in CMOS devices, possibly integrated with carbon nanotube vias to make all-carbon architectures. Bilayer graphene has been predicted to have a bandgap up to 400 meV under a vertical electric field, which, combined with the high mobility, make it a serious candidate for high-power radio-frequency electronics. This project has the most impact to the latter area of exploitation as we will also be using high frequency signals to probe the behaviour of sub-micron bilayer gated material. With an ever increasing demand for high bandwidth mobile communication, driven largely through the popularity of smart phones, there is an increasing need for high frequency materials that can help increase the bandwidth of these devices. There is also a growing demand for a skilled scientists and engineers who understand how high frequency measurements are made and how high frequency systems work. One of the other key areas where graphene could have commercial value is in the area of biological sensors. This is because the bulk, which usually dominates charge transport, and the surface, which is the most sensitive to the surrounding environment, are identical in graphene. Thus with appropriate chemistry, specific molecules could be found that bind to graphene and also bind to some unique biological marker. Although this area is further from the general goals of the project, the project will be generating sub-micron electronic devices which will be required for making sensors, so the knowledge we develop will be of value to those working in graphene sensor research. These sensors could be very important for an ageing population, where cheap low cost testing for proteins will become increasingly important in ensuring a high quality of life for the elderly. One of the key pathways to impact will be the generation of valuable patents. We will have regular meetings between researchers in Cambridge and NPL to identify valuable IP and ensure that if it looks commercially exploitable we will submit a patent. The IP will either be licensed to existing firms or if appropriate further funding will be sort to build demonstrator devices to show potential investors the value of the IP. The PI has a strong track record of finding funding for IP developed from University research and he will use that experience to help ensure that any results are exploited to their full potential.