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

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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.