Strain-engineered graphene: growth, modification and electronic properties

We have recently demonstrated that crystalline layers of graphene can be grown on a solid surface using a newly installed high temperature growth system based on a technique called molecular beam epitaxy (MBE). This system was purchased in 2013 using equipment funding from the EPSRC Graphene Engineering Call and was successfully installed in 2014 and has since been used to demonstrate, for the first time in the world, that graphene which is strained, i.e. stretched, can be grown. It is thought that the stretching arises from the high temperatures used during growth - as the graphene cools after growth it tries to contract but cannot do so since it is pinned at several anchoring points on the surface on which it grows. The presence of strain was completely unexpected and results in many novel properties, for example the graphene can be punctured by a nanoscale mechanical stylus and snap back into a relaxed form - rather like a burst balloon. In addition, it is known that stretching graphene can modify strongly its electrical properties making it more compatible with technological applications such as the fabrication of transistors. In this proposal we are requesting support to build on our initial success so that we can explore the promise of this new type of graphene, to gain a much better understanding of how it grows, to investigate its novel physical properties and also to try and exploit strained graphene to make simple prototype devices.

Historically, the discovery of graphene and its remarkable electronic properties by Geim, Novoselov and colleagues in 2004 has provided scientists and engineers with a material system for revolutionising electronics and opto-electronics. Graphene has many remarkable properties - it is highly flexible, very strong and is an excellent electrical and thermal conductor. However, there are some limitations of current graphene research. Firstly, it cannot be used directly in many electronic applications because the flow of electrical current cannot be switched off in graphene, an essential requirement for the fabrication of a transistor, the central component of modern electronics. The reason for this may be traced back to the quantum mechanical properties of electrons within graphene, in particular the fact that for all energies there are available quantum mechanical states which electrons can occupy - in other words the material lacks an energy gap which is present in semiconductors. Since 2004 there has been an enormous effort worldwide to develop methods to control the electronic properties of graphene with a particular focus on introducing a band-gap to provide a semiconducting analogue material in which many of the other, highly desirable qualities of graphene, are retained. One of the most promising routes towards this goal is through the introduction of strain which occurs spontaneously in the MBE grown graphene.

In addition, a second drawback of the original graphene work was the reliance on exfoliation, or peeling off layers of graphene from a block of material. Although this has been extraordinarily successful in terms of investigating the fundamental properties of graphene, exfoliation has significant limitations in the technological exploitation of graphene. In particular, it is desirable to form layers over large areas. The approach adopted by the Nottingham group, to use MBE to grow graphene, makes use of a technique which is used widely in industry to grow other materials. However, before the work of the Nottingham group, attempts to grow graphene by MBE, in which growth is achieved by firing carbon atoms at a suitable surface, had been largely unsuccessful. Our system, which is unique worldwide, allows growth of graphene at much higher temperatures than have been used previously and we have already demonstrated that growth of high quality graphene is possible using this technique and offers exciting opportunities for new scientific and technological directions.

Who will benefit from the proposed research?

1) The UK academic and industrial community who are engaged in graphene engineering research will benefit by the provision of revolutionary, MBE (molecular beam epitaxy) grown strained graphene and its incorporation into graphene/boron nitride heterostructure materials and devices.
2) Manufacturers of graphene based electronic and opto-electronic devices.
3) Innovators in any field where graphene or graphene/boron nitride heterostructures are required.
4) Members of the wider society, who can benefit from the novel device structures resulting from this new engineering project.
5) Specifically our five Project Partners drawn from industry, governmental organisations and academia will directly benefit from the availability of the new material; our ambition during the project is to significantly expand our network of commercial and academic partners through the dissemination activities described below and in the proposal.

We will build on our recent results to improve the technology of MBE growth of graphene working with a commercial manufacturer of MBE equipment. Specifically we will explore new designs for carbon deposition sources which can be adopted commercially. This knowledge can then be used to develop production MBE systems for graphene based devices as a route to translation of our activities.

Industrial and academic users will benefit from access to a wide range of high quality graphene-based heterostructure materials and devices produced in the unique, world-class, MBE facility. They will also benefit from the expertise and knowledge-base developed by the investigators, PDRAs and PhD students involved in this project. In order to publicise our results we will, in addition to the standard forms of research dissemination in high profile journals, introduce a number of additional activities to highlight our research and the availability of our materials to potential third party collaborators. These will include the organisation of workshop to disseminate the results of this project to other researchers on graphene, nitride and the wider academic and industrial semiconductor community within the UK. The availability of material for collaborators will be publicised at Open Days and also through e-newsletters and national and international conferences.

We will also visit UK companies to discuss results and to give seminars where requested. Researchers from Nottingham are active participants in the UK Nitride Consortium (UKNC), we are a partner institution of the EPSRC National Centre for III-V Technologies and also a member of the EU Graphene Flagship Programme, and we will exploit these links to ensure the maximum number of beneficiaries in the semiconductor community are aware of this research.

In addition to the impact arising from research outputs, we highlight the output of trained researchers, who will provide a new generation of talent for the developing industry related to graphene based electronic and optoelectronic materials.

The establishment of MBE growth facilities for graphene/boron nitride at Nottingham has provided a new source of material for graphene engineering in UK Universities and Industry. Through our proposed programme of research we will further underpin UK graphene research through the availability of this new material to UK researchers.