Graphene, a nanomaterial whose properties were discovered some 15 years ago, offers many opportunities for innovation in a wide range of industrial sectors. Europe has made it one of its strategic axes for the future by launching a major initiative in 2013, the Graphene Flagship. Although some applications have already emerged, many difficulties remain to reach all the promises that were made a few years ago. The provision of reliable measurement tools and methods to support quality control that is still lacking is one of the main ones.
Graphene is the most emblematic of two-dimensional or 2D materials. These materials consist of one to a few atomic layers, each made up of atoms strongly bonded to each other, and therefore have one dimension, their thickness, nanometric or smaller and the other two dimensions generally at larger scales. As such, graphene is an emerging nanomaterial, the subject of much research and promises decisive advantages for many applications.
The first evidence of its properties, in 2004, earned the 2010 Nobel Prize in Physics for A. Geim and K. Novoselov. This ultimate material ideally consists of a single layer of carbon atoms where each atom is bonded to three neighbours in a honeycomb structure. This honeycomb structure gives graphene exceptional properties in terms of electrical and thermal conductivity, optical transparency and mechanical strength while remaining extremely light and flexible.
Its success in research laboratories is due to the richness and originality of its physico-chemical properties, which open up a field of fundamental investigations, but also to its historical manufacturing technique, by simple mechanical exfoliation of graphite using an adhesive tape. Other production techniques have since been considered and developed, such as chemical vapour deposition growth, thermal decomposition of a silicon carbide substrate or liquid phase exfoliation.
However, behind the word "graphene" lie different realities, as production processes can lead to the presence of a variable number of carbon layers, specific defects, or even oxidation. The materials obtained can therefore have properties that are very often degraded compared to the ideal graphene. It has thus been established, in practice, that a stack of more than 10 layers of carbon can be treated like the corresponding solid material, namely graphite. In order to help the various actors involved in graphene-based developments to find their way around and to avoid confusion between the different materials of this vast family, the International Organization for Standardization (ISO) has recently established an internationally harmonized classification and terminology. The Technical Specification ISO/TS 80004-13:2017 distinguishes, for example, between graphene, multilayer graphene, graphene oxide and reduced graphene oxide.
Many of the properties of graphene, whether mechanical, electrical, thermal or optical, are exceptional ("The rise of graphene", A.K. Geim and K.S. Novoselov, Nature Materials 6, 183 (2007)). With an ultimate thickness of only one atom, it is among the thinnest materials. Graphene is also extremely light, 200 times stronger than steel, gas-impermeable, flexible and 97.7% transparent to visible radiation. It is also an excellent electrical conductor, capable of carrying current densities 106 times higher than those permissible with copper, and the material has a record thermal conductivity. The exceptional nature of this material also lies in the fact that it combines so many extraordinary properties.
The main fields of application concerned today are composite materials (functional coatings, structural materials, or electrodes for water treatment), energy (fast batteries, supercapacitors, solar cells, etc.), telecommunications (long-distance optical communications), wireless networks, ...), electronics (HF electronics, printable, flexible, spintronics), sensors and imaging (photodetectors and physico-chemical sensors, ...), as well as biomedical technologies (sensors, neuronal interfaces, drug delivery).
Faced with the promise of graphene, various research and innovation programmes have been launched by Europe to preserve its advantage in a context of international competition. The Graphene Flagship, launched in 2013 for a period of 10 years, is the most remarkable. Financed to the tune of 1 billion euros, this large-scale project brings together around 150 academic and industrial partners, including LNE, and its main objective is to support the development of the most innovative applications of graphene and related materials.
Two of the essential prerequisites for the development of graphene-based applications concern process control (production and integration), as well as quality control of graphene-based materials. The latter, in particular, must often be obtained at lower cost and on a large scale with a view to industrialization, which is key to supporting the adoption of these innovative materials by manufacturers and consumers. This concerns both the structural properties, defects and impurities, the physico-chemical properties directly responsible for the expected performance and the behaviour of the material itself during the life of the product, which is a source of potential risks.
The reliable and traceable characterization of various properties of graphene-based materials and products is therefore necessary to support and even accelerate their development, industrialization and marketing. In view of the variety of graphene-related materials and their properties, their characterization is, however, a vast multidisciplinary field and is a very complex exercise, even though reliable instrumentation and measurement protocols are not necessarily available to date.
Several initiatives exist around the world to address the need for metrology in this area. In Europe, the Graphene Flagship has set up the "Validation service", of which LNE is a partner, while the UK supports the Graphene Characterization Service between the National Graphene Metrology Center at the NPL and the NGI - National Graphene Institute. In the United States, the Graphene Council has launched the Verified Graphene Producer programme, while the National Graphene Association promotes and defends developments based on graphene-based materials, particularly with regard to standardization, the environment, health and safety (SHE) and regulations. For quality control of graphene-based materials, China can rely on the International Graphene Product Certificate Center (IGCC) and the "National Graphene Product Quality Supervision and Inspection Centre (NGSIC)."
With regard to standardization in the field of graphene, there is significant activity at the international level in the committees IEC/TC 113 Nanotechnology for electrotechnical products and systems and ISO/TC 229 Nanotechnologies, dealing with vocabulary, determination of key parameters for the control of materials, and appropriate measurement methods. A recent example of the work carried out at ISO is that which led to the experimental standard ISO/TR 19733 "Matrix of properties and measurement techniques for graphene and other two-dimensional materials", which provides guidance for the development of international standards in the field. In France, the AFNOR/X457 "Nanotechnologies" standardization commission, in which several experts from the LNE Nanotech Institute participate, provides a relay to the work carried out at the international level. It should be noted, however, that there is still very little interest and activity at the French level at this stage.
Graphene has been one of LNE's research topics since 2007. Graphene was first studied at LNE for an application in quantum electrical metrology, namely the development of the quantum Hall effect electrical resistance standard. In this field, the laboratory has obtained major results, at the global state of the art, which demonstrate the undeniable superiority of graphene over other semiconductors for this application and pave the way for devices that are much simpler to implement (Nature Nanotechnology 10, 965, 2015 "Quantum Hall resistance standard in graphene devices under relaxed experimental conditions"). This success is due to the very high quality of the graphene devices used. However, it has required numerous attempts which have shown how crucial the characterization of the material is to achieve the performances predicted in theory. This research work on graphene conducted at LNE is of major importance for the dissemination of the recently redefined SI by fixing the fundamental constants h, e, k, Na ("On en parle - SI"). They now extend beyond the Quantum Hall effect standard to the development of other integrable quantum electrical standards (ANR GraphMet project) and to the development of electromagnetic quantum detectors (EURAMET/EMPIR/SEQUOIA project).
Graphene and 2D materials are proving to be more generally interesting nanomaterials for the development of quantum technologies. Examples include work on the realization of superconducting circuits for quantum detection or quantum computing (quantum bit) and others for the realization of integrated photonic circuits based on single photon emitters and associated detectors.
With the aim of supporting the development of other applications, the LNE has over the last two years initiated various actions aimed mainly at:
This work is carried out through LNE's participation in the Graphene Flagship Validation Service and the GRAAL research project supported by the French National Metrology Network (in french only).
The Validation service, which brings together two National Metrology Institutes (LNE and NPL) and a Spanish national platform (Instituto de Nanociencia de Aragón - Universidad de Zaragoza), has two ambitions: to develop new measurement methodologies and to offer members of the Graphene Flagship high level metrological characterization services with the aim of facilitating the industrialization and commercialization of graphene-based materials and products. The first part is carried out in conjunction with the standardization bodies and VAMAS (Versailles Project on Advanced Materials and Standards). As proof of the strong interest in this material, TWA (Technical Working Area) No. 41 "Graphene and related 2D materials", co-piloted by Great Britain and China within VAMAS, is currently coordinating several inter-comparison exercises (structural characterization by Raman spectroscopy, impurities by ICP-MS, elemental analysis and O2 content by XPS, thickness by AFM, lateral size by SEM). A survey is also underway to assess the characterization needs in this field. The LNE Nanotech Institute is currently participating in the initiative to characterize the thickness of graphene oxide sheets by atomic force microscopy (AFM).
Image of a graphene bilayer by AFM (LNE)
SEM image of a graphene sample obtained by mechanical exfoliation
Schematic representation of a quantum Hall effect device etched in a graphene layer
The "GRAAL" project - Multidisciplinary metrological characterization of graphene-based materials and products - has two objectives:
1) For advanced metrology: to improve the measurement capabilities of the laboratory by addressing the measurement challenges posed by the outstanding properties of graphene-based materials,
2) In support of industry: the development of measurement methods and the validation of instruments in response to metrological needs in the field, in particular for structural, electrical and thermal properties, in connection with Graphene Flagship, standardisation and certification.
It favours a hybrid metrology approach, based on the combination of complementary measurement methods, to address the measurement challenges posed by these materials.
In order to involve various French players in these efforts to improve the quality of graphene-based materials and thus contribute to the emergence of a graphene sector in France, LNE has also launched various initiatives within the Club nanoMétrologie (CnM), which it co-leads with the C'Nano network. A survey carried out between July and October 2017 among members of the CnM and those of GDR CNRS Graphene & Co, which already structures the academic community interested in graphene and 2D materials, provided a better understanding of metrology issues and needs in this vast and varied field, in terms of materials, properties of interest, applications and players. The results of this survey, discussed during a round table organized during the 7th Annual Meeting on Nanometrology in December 2017, highlighted the importance of setting up inter-technical and inter-laboratory comparisons, in order to raise awareness of metrology issues and at the same time contribute to an improvement in the quality of the materials produced and their reproducibility. A special session at the national congress C'Nano 2018, dedicated to metrology and the characterization of graphene and 2D materials, made it possible to extend this approach by bringing together in December 2018 academic and industrial partners to share the measurement problems they encounter, throughout the value chain associated with these materials.
VAMAS, an international network on pre-standardization for advanced materials
An international network established in 1982, VAMAS' main mission is to support trade in high-technology products through international collaboration on projects that provide the technical basis for the development of codes of practice and specifications for advanced materials. VAMAS activity focuses on pre-normative metrological research, inter-laboratory comparisons of test results, and the unification of views on standardization priorities. As a result of these activities, VAMAS provides an internationally harmonised methodology that can be submitted as recommendations to standards developing organisations, thereby encouraging the development of agreed and applicable standards for advanced materials. Each country may propose up to 3 representatives to contribute to VAMAS actions. For more information, please contact Georges Favre, the French representative in this network.