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New characterisation techniques for ‘wonder material’ graphene provided by EMPIR project now ready for adoption by industry
Graphene is a two-dimensional lattice of carbon only one atom thick. Graphene’s structure gives it unique properties including a high electrical and thermal conductivity. These attributes open up the possibility for its use as a new material in semiconductors, electronics and composite materials. However, the uptake of graphene by European industry has been restricted by the inability to grow large areas of graphene with sufficient purity. Accurate and reproducible characterisation methods – required for test samples and production lines – have not been validated for use.
The EMPIR project ’Developing electrical characterisation methods for future graphene electronics’ (GRACE, 16NRM01) addressed this and, as part of the work, for the first time, validated an Electrical Resistance Tomography (ERT) technique for Graphene analysis.
The ERT technique applied to graphene
ERT is a non-invasive contact technique where a number of electrical contacts are positioned around the sample; these allow for performing a number of so-called “boundary (electrical) measurements” from which mapping of the sample’s conductivity is made possible. In simple terms the way the current flows through the sample, which depends on the local electrical conductivity, can be determined with no need for sampling the entire sample’s surface (a non-invasive method). This approach allows for a scaling towards larger samples, of industrial interest, without increasing the measurement time (as it happens for the scanning techniques that map the sample point by point). The conductivity map can resolve areas of imperfections in the graphene and such measurements now have direct metrological traceability to the International System of Units, the SI.
Graphene used in the GRACE project was grown as a monolayer on copper or silicon carbide by Graphenea (a project partner) a Spanish firm in graphene synthesis and by NIST, the National Metrology Institute (NMI) of the United States of America (a project stakeholder) respectively. The GRACE consortium performed a series of interlaboratory comparisons on a number of contact and non-contact methods (ERT included), which the project has reviewed in two good practice guides for graphene characterisation.
Dr Alessandro Cultrera of INRiM, the NMI of Italy, who developed the technique for graphene analysis and led the inter-laboratory comparison, said about the new technique:
“ERT, which was initially developed for physiological measurements, is proving itself to be a very reliable tool for materials science. We are now successfully applying ERT even to the characterisation of novel nanowire networks (the topic of another EMPIR project, 20FUN06 MEMQuD, lead by my colleague Gianluca Milano). I am intrigued by ERT because it merges both accurate electrical measurements and advanced computational methods capable of returning a two-dimensional map from a one dimensional set of data as input.”
The improved ERT technique is anticipated to be used in another EMPIR project, ‘Two dimensional lattices of covalent- and metal-organic frameworks for the Quantum Hall resistance standard’ (20FUN03, COMET), which is looking at other materials potentially even more interesting than graphene for metrology applications.
This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.
Figure reprinted from “Mapping the conductivity of graphene with Electrical Resistance Tomography”. Author: Alessandro Cultrera et al. Publication: Scientific Reports. Publisher: Springer Nature. Date: Jul 23, 2019. Copyright © 2019, The Author(s).
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