By Will Soutter
Graphene has a wide range of potential applications in industry, but adoption is still limited by the difficulties with large-scale manufacturing of the unique material. A project funded by the EU's ConceptGraphene and the UK National Measurement Office has now overcome a major hurdle to mass production, by developing a rapid, inexpensive and scalable method for quality control of graphene, using optical microscopy.
Graphene device on a silicon carbide substrate, examined by optical microscopy. Image credit: Arseniy Lartsev, Chalmers University
ConceptGraphene is an EU-supported collaborative project between a variety of universities and research institutes, including Chalmers University in Sweden and the National Physical Laboratory (NPL) in the USA. The project's aim is to lay the foundations for a new era in electronics based on graphene, by bringing production of the material up to commercial wafer scale.
NPL's interest in reliable large-scale graphene production was sparked in 2011, when they successfully created graphene devices by growing crystals layer-by-layer, rather than exfoliating graphite. This scalable process was developed in another collaboration with European research teams, including many involved in the current project.
They found that that the graphene they created could be used as a standard for quantum Hall resistance - an electrical property which is used to define the international standards for electrical measurements.
Existing standard measurements involve cooling one of a select few semiconductors to within a few degrees of absolute zero inside a strong magnetic field - a process only a handful of labs in the world can manage.
Graphene's unique properties allow it to show the quantum Hall effect at a much higher temperature, making the measurement standard available to many more labs. However, to meet demand for the graphene standards, NPL had to work out some kinks in their production process to make it viable on a large scale.
Quality control processes for graphene devices has to be able to distinguish between monolayer and bi-layer graphene on a silicon carbide substrate. It was previously thought that straightforward optical microscopy would not provide enough contrast to discern these tiny 0.3nm height differences - making the use of more advanced inspection techniques essential, and limiting the throughput of a manufacturing facility.
In a new experiment, researchers at Chalmers University, based in Gothenburg, and Linkoping University in Switzerland, have demonstrated quality control technique using optical microscopy alone. According to the paper published by the group in Nano Letters, they were able to successfully determine areas of bi-layer and multi-layer graphene, as well as some of the underlying topography of the SiC substrate. The results were verified at NPL using established electrical property measurements like Kelvin force probe microscopy.
The use of optical microscopy to find defects in graphene on SiC, in a non-invasive and scalable way, is a very promising development in the quest to industrialize graphene-based products. It will be fascinating to see what further advances the ConceptGraphene project can generate in their quest for graphene-based electronics.