Graphene in the real world
Andrew J Pollard* and Charles A Clifford** of the National Physical Laboratory, UK, talk on the characterisation of graphene as a vehicle towards wider commercialisation.
Graphene has been labelled a wonder material due to its many exciting properties, such as electrical and thermal conductivity and strength. While a source of intense scientific curiosity over the last 14 years, not to mention being the
subject of the 2010 Nobel Prize in Physics, there
are many different application areas where this one-atom-thick material has disruptive potential, including the multi-billion industries of sensors, composites and energy storage.
There is an extraordinary interest in graphene from companies both SMEs and multinationals worldwide. This fascination has also led to research into other two-dimensional materials – new materials with similar exciting properties but are fundamentally different, providing an atomic-scale toolbox of conductors, semiconductors, insulators and superconductors.
The potential of graphene has been demonstrated in prototypes developed by different companies for products such as anti-corrosion coatings, supercapacitors, heat management devices for electronics, biomedical sensors, lightning strike protection, and printed electronics. Furthermore, graphene has been incorporated into niche products, including sports equipment, performance bikes and supercars. However, with Ford’s October announcement that by the end of 2018 two of its cars will incorporate graphene-enhanced foams for improved noise-reduction, graphene is starting to enter real-world products used in everyday life. This increase in commercial activity is, in part, due to the viability of industrial-scale production of graphene realised by producers in the last couple of years. There is, therefore, optimism that the range and availability of graphene products will continue to grow.
Graphene is described as a single layer of carbon atoms with each atom bound to three neighbours in a honeycomb structure. The following points are important to note:
- It is an important building block of many carbon nano-objects
- As graphene is a single layer, it is also sometimes called monolayer graphene or single layer graphene and abbreviated as 1LG to distinguish it from bilayer graphene (2LG) and few-layered graphene (FLG)
- Graphene has edges and can have defects and grain boundaries where the bonding is disrupted.
- In comparison, two-dimensional material consists of one or several layers, with the atoms in each layer strongly bonded to neighbouring atoms in the same layer, which has one dimension, its thickness, in the nanoscale or smaller, and the other two dimensions generally at larger scales. Note the following:
- The number of layers when a two-dimensional material becomes a bulk material varies depending on both the material being measured and its properties. In the case of graphene layers, it is a two-dimensional material up to 10 layers thick for electrical measurements, beyond which the electrical properties of the material are not distinct from those for the bulk – also known as graphite
- Interlayer bonding is distinct from and weaker than intralayer bonding
- Each layer may contain more than one element
- A two-dimensional material can be a nanoplate
Barriers to commercialisation
While it is widely applicable, there are still issues for the graphene supply chain to overcome. One such observed consistently over the last few years is that application developers do not know what kind of material they are actually using in their development process (see Materials World, December 2018, page 30).
Graphene can be produced through many different routes, leading to fundamentally different material properties, depending on the source. The capability to provide either powders on the tonne-per-year scale or substrates covered with a graphene sheet at thousands of square metres per year, although required, also leads to problems in reproducibility. Quality and authenticity are also crucial considerations, as it has emerged that some material suppliers may not actually be selling graphene at all, but powders that contain solely graphite flakes, even if these flakes do have a nanoscale (1–100nm) thickness. The scale of the problem was indicated when the UK Financial Conduct Authority warned investors off companies that are actively providing material that is not genuine graphene. This is an unregulated area, and problems like these are exacerbated by the fact that there are now hundreds of companies selling so-called graphene worldwide.
However, even if the material supplied does indeed contain graphene, there are still many different types of graphene available, so it may only be beneficial for certain applications and not others. Thus, reliable material characterisation is extremely important. For example, for graphene flakes sold as a powder or liquid dispersion, the lateral size and thickness of the flakes will impact whether the material is better for applications such as composites or energy storage. Furthermore, for commercially produced graphene, physical measurands such as the lateral flake size distribution can have a broad range that must be fully understood, and not just quoted as an average value.
There are many ways to measure the physical and chemical properties of graphene, including Raman spectroscopy and scanning probe and electron microscopies. However, the same methods used to understand the properties of small amounts of material produced in the laboratory are not necessarily transferrable to material produced at a large industrial scale. Even if the measurement itself is performed correctly, the methods of sample preparation and data analysis may not be valid, or may require extremely detailed knowledge of the measurement method.
Accurate characterisation required
Without the ability to fully describe the material properties in a quantitative way with defined measurement uncertainties and agreed terminology, there will be an absence of confidence in the graphene supply chain, which will limit commercialisation. Also, SMEs in this area need to be able to secure investments and customers, which is difficult without independent assessment. These obstacles will ultimately hinder the development of real-world applications for graphenes.
Typically, companies want to understand the physical and chemical properties of graphene flakes – in a powder or liquid dispersion form – or as a graphene sheet on the surface of a substrate. They need to be able to compare different technical data sheets of measurands that will impact the performance of any end-product, and at the same time can be measured in a reproducible, comparable and accurate way.
Important measurands are the lateral flake/grain size, thickness/number of layers, level of disorder (defects), chemical species, functional groups and trace metals. There is no measurement technique that can determine all of these material properties and so complementary techniques are required for either physical or chemical characterisation.
In 2017, the National Physical Laboratory (NPL), in partnership with the National Graphene Institute (NGI) at the University of Manchester, UK, took the first step in providing characterisation methodologies for graphene in the freely available paper, NPL good practice guide (GPG) 145: Characterisation of the structure of graphene. Not only does this GPG provide the decision-making processes and techniques required to determine the physical properties of graphene, but it includes detailed, step-by-step protocols for each type of measurement.
Following the publication of the GPG in July 2018, NPL and NGI launched a joint graphene characterisation service at the House of Commons, in the UK, for companies requiring an independent report on the properties of their graphene material, and an understanding of the application areas where their material may increase performance. This applies to both graphene flakes and sheets, and the measurement of physical and chemical properties, as well as the electrical properties of graphene sheets. NPL and NGI hope this work will accelerate the industrialisation of graphene in the UK – forging the missing link between graphene research and development, and its application in next-generation products.
International standardisation of graphene
For any industry to thrive, all parts of the supply chain must be able to rely on reported values from different test-houses from around the world. Measurement standards mean that large-scale purchasers of graphene material, who already understand the material properties they need, can easily compare and choose between different grades of material from many different global suppliers. This is because the properties reported in each of the technical data sheets will have been measured in the same way. International standards play an essential part in lowering barriers to commercialisation and trade, and can form the basis of regulation, provide a common language and enable comparability between products. This is true in all industries and is especially required at this stage in the commercialisation of graphene and related 2D materials, but also for nanomaterials in general.
In the commercialisation of these nanomaterials there are two paradigms. Firstly the low-cost, high-volume manufacturer that can result in a race to the bottom in terms of price and quality. Here, standards are required especially in measurement and characterisation and material specification to confirm what is being sold is what it says on the tin. On the other side is the highly specified nano-object with specific and tailored properties such as size, shape and particularly surface chemistry. These materials are highly specified within tight tolerances and hence need standards for verification.
In nanotechnologies standards, the main international committee is ISO/TC 229 (nanotechnologies). This is a large horizontal industry crosscutting committee with 33 members and 19 observer countries. For electrical applications, there also exists the IEC TC113 electro-technical nanotechnologies committee. In addition, member organisations, such as ASTM, also develop their own standards. Here, we will focus on the activities of ISO/TC 229 and, in particular, the growing graphene standardisation activity.
In order to avoid confusion and misunderstandings, clear, agreed and standardised terminology is a must. This facilitates conversations between people from different fields and interests through all parts of the commercial supply chain can use. Consequently, one of the first standards in graphene to be published was the ISO terminology standard ISO/TS 80004-13:2017 on Vocabulary-Graphene and related two-dimensional (2D) materials, led by NPL.
There are 99 terms defined in this standard, covering the types of materials, material production, material characterisation and material properties. These, along with all ISO terms, are freely available via the ISO Online Browsing Platform (OBP).
The publication of this terminology standard is just the first step, as measurement and characterisation standards are much needed and called for by the industry. However, these take time as the required techniques are developed, validated through the use of international interlaboratory studies and then international consensus obtained. International interlaboratory studies are now underway within the Versailles Project on Advanced Materials and Standards committee, which contains a technical working area specifically for graphene and related two-dimensional materials. Several projects are active, on the measurement of physical and chemical properties using different measurement techniques, to feed directly into the standards being developed in ISO.
Currently, five main measurement standards are being developed in ISO TC229. Nearing publication is an ISO technical report linking measurement techniques to key characteristics via a measurement matrix.
For graphene in flakes and liquid dispersion form, structural characterisation is a key first step. Here, a standard providing an information-effective and time-effective way of characterisation is under development. For chemical characterisation, a standard is in development that will help users determine the amount of oxygen and functional groups in their samples using four complementary techniques. Very recently, a proposal was accepted to start development of a standard on the structural characterisation of graphene oxide.
For graphene grown in sheet form using chemical vapour deposition techniques, a structural characterisation standard is being developed. In a similar way to the flake structural characterisation, this document will consist of a flow chart providing the optimum characterisation route as well as providing sample preparation, measurement and data analysis protocols.
Ultimately, the efforts of the international graphene community to develop the measurement science, protocols and standards required by industry, will enable the material properties of graphene and related two-dimensional materials to be measured accurately and reliably. This will enable graphene producers to provide comparable technical data sheets with known uncertainties, which other parts of the supply chain can use to hasten the development and adoption of real-world graphene products.
* Andrew Pollard is the Science Area Leader for the Surface Technology Group at the National Physical Laboratory (NPL) and leads NPL’s research in the material characterisation of graphene and related 2D materials.
** Dr Charles Clifford is a senior research scientist in the Surface Technology group at the National Physical Laboratory (NPL).