Quantum links with Oxford
The National Physical Laboratory (NPL) is using the unique properties of graphene to develop quantum electrical standards with higher accuracies than ever before. However, the precise reasons why graphene performs so well in these devices remain unknown.
Graphene is an incredibly thin material, only one carbon atom thick, and has potential commercial applications ranging from ultrafast analogue transistors to touch-screen displays. To investigate the exhibited behaviour of graphene, NPL quantum scientists have joined forces with the Clarendon Laboratory at the University of Oxford. Here NPL and Oxford scientists use a device called a variable temperature cryostat, which provides a low temperature environment, a fraction above absolute zero, ideally suited to studying quantum effects. The cryostat at the Clarendon laboratory is special because it also provides a very large magnetic field of up to 20 tesla, around half a million times larger than the Earth's magnetic field.
This work has already resulted in two papers published in the journal Physical Review B which begin to explain some of the unique properties of graphene by studying the material produced via three different methods:
- Exfoliated - pulling graphite apart, for example with sticky tape, until you are left with a carbon layer one atom thick
- Epitaxial - growing graphene in layers on Silicon Carbide (SiC)
- Chemical Vapour Deposition (CVD) - growing graphene on top of a thin film of metal such as copper
The exfoliated method is the most well-known and the easiest way to produce graphene. Andre Geim and Kostya Novoselov used Scotch tape to exfoliate graphene flakes for their Nobel Prize-winning experiments at the University of Manchester. Although the quality of exfoliated graphene is very high, the Scotch tape method is not suitable for the mass production of graphene needed for commercial applications. The epitaxial and CVD methods can more easily be scaled up and could potentially produce larger sheets of graphene, however achieving the same quality as exfoliated graphene remains a challenge.
Despite this, the majority of graphene research is carried out on exfoliated graphene, because in practice this method still provides the best samples. So there is still a need to compare the properties of all three graphene types to ensure that both the graphene used in research and the graphene eventually used in commercial devices perform in a similar way.
The papers published in Physical Review B look at factors vital for electronic applications. The first paper investigates 'carrier energy loss rates' in the different graphene types, which influence how heat is dissipated and are important to understand for heat management in electronic devices. The second paper looks at an effect called 'weak localization', which affects the motion of electrons, changing electrical resistance.
The results show that the three types of graphene perform in broadly the same manner but highlight certain areas where characteristics may be different. For example, it was found that the energy loss rate is slightly enhanced in graphene produced by the CVD method, which could make the use of this type of graphene beneficial for high-power electronic applications.
Read the papers:
- Energy loss rates of hot Dirac fermions in epitaxial, exfoliated, and CVD graphene
- Weak localization scattering lengths in epitaxial, and CVD graphene
More on NPL's Graphene research
More on NPL's work on Electrical Quantum Standards
For more information, please contact JT Janssen
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