Researchers from RWTH Aachen University and the Graphene Flagship Standardization Committee have pushed through a new IEC standard for assessing the strain uniformity of single-layer graphene using Raman spectroscopy.
Research has shown that the electrical and the structural quality of graphene are intimately connected, and that nanoscale lattice deformations caused by surface corrugations limit the mobility of electrons in graphene. Therefore, controlling the flatness of a graphene sheet is fundamental for the fabrication of high-quality graphene layers for electronic devices – and the possibility of measuring this parameter with a simple and fast method is a major technological advantage. Furthermore, the new standard for detecting graphene flatness, pioneered by the Graphene Flagship and published by the International Electrotechnical Commission (IEC), could expedite the manufacture and implementation of single-layer graphene.
The method used to measure graphene’s flatness is Raman spectroscopy, a standard tool of graphene research. This technique is fast, non-destructive and well established, especially if the sample under evaluation consists of single-layer graphene. It allows to distinguish between single and few-layer graphene, and it helps to determine the doping of a graphene, the amount of mechanical strain and defects in the lattice.
In 2015, Graphene Flagship researcher Christoph Stampfer at RWTH Aachen University, Germany, showed that Raman spectroscopy contains unambiguous information on the amount of nanometer-scale strain variations in a graphene sheet, directly correlated to flatness.
This discovery came at a very appropriate time, as nanometer-scale strain variation had been pinpointed as the main source of electron scattering in defect-free graphene just the year before. “It was clear that this result had the potential for setting the basis of an internationally recognized standard for the strain uniformity of graphene”, says Norbert Fabricius, from International Standards Consulting GmbH & Co. KG.
At that time, Fabricius led the Graphene Flagship Standardization Committee, a team of experts dealing with the long process between scientific discovery and the establishment of an international standard. The results provide a well-defined way to use scanning confocal Raman mappings to produce a “strain uniformity parameter”. This measurement is based on statistical interpretations of the linewidth of the characteristic 2D-peak generated by single-layer graphene,
“The strain uniformity parameter is a figure of merit that quantify the influence of nanometer-scale strain variations on the electronic properties of the layer. It gives an upper limit on the electronic performance of the characterized graphene. It can therefore help manufacturers to classify their material and decide whether or not it is potentially suitable for various applications”, explains Fabricius.
The new standard was published by the EIC in October 2021, and represents a good example of the work supported by the Graphene Flagship Standardization Committee, currently chaired by Thurid Gspann from Graphene Flagship partner Karlsruhe Institute of Technology, Germany. “A standard must meet the different needs of different stakeholders: researchers, manufacturers, and buyers”, says Gspann. “In the Committee we mediate the discussion between different actors to find consensus on the specifications of the standard, we support the scientific work that forms the backbone of the standard, and we link to international organizations such as the International Organization for Standardization (ISO) or, in this case, the IEC.”