The Characterisation and Standardisation of Graphene and Other 2D Materials

Interview conducted by Alexander ChiltonJul 16 2015

Dr. Andrew Pollard, Graphene Research Scientist at NPL, talks to AZoNano about the characterisation and standardisation of graphene and other 2D materials.

Could you provide our readers with an overview of the history of the National Physical Laboratory (NPL) and explain how the organisation has changed since it was founded in 1900?

The National Physical Laboratory (NPL) is the UK's National Measurement Institute and is a world-leading centre of excellence in developing and applying the most accurate measurement standards, science and technology available.

For more than a century, NPL has developed and maintained the nation's primary measurement standards. These standards underpin an infrastructure of traceability throughout the UK and the world that ensures accuracy and consistency of measurement.

Since its establishment, NPL has contributed scientific and technological leadership in the physical sciences, as well as in materials science, computing, and bioscience. Some of the most notable achievements carried out at NPL include the invention of the Automatic Computer Engine (ACE), packet switching, radar and the atomic clock.

NPL, and over 500 scientists currently working here across a large range of disciplines, can be viewed as a bridge between academia and industry, as metrology is an essential area that must be addressed to bridge the “valley of death” that faces new technologies before they can be commercialised.

Why is metrology important in graphene research?

There are currently many concerns in the emerging graphene industry about whether material being sold as ‘graphene’, is really graphene. As graphene is a nanomaterial and atomically-thin in one dimension, there are also many issues facing the large-scale production, processing and application of this material when you consider the world outside of the laboratory.

The accurate measurement of the material is therefore key; is the material you have actually the type of material you require? Are the properties adequate for the targeted application? Without the development of the metrology underpinning these questions the vast amount of different ‘graphene’ material now available in the market cannot be accurately compared.

Metrology research will lead to standardised measurement protocols and address this barrier to commercialisation, as well as form the foundation for future high-throughput and validated quality control (QC) processes.

What are your current research interests at NPL?

At NPL, we are interested in applying our metrology capability and expertise across a range of disciplines to graphene and related 2D materials, studying the measurement issues for a range of properties (such as the electrical, thermal and mechanical) of both the materials themselves and when they are in real-world applications.

My research specifically focusses on the structural and chemical characterisation of the materials themselves, as well as the end applications, such as nanocomposites, flexible electronics, sensors and energy storage.

Image Courtesy of the National Physical Laboratory

One of the pressing problems facing industrial end-users who want to benefit from using graphene, has been the lack of reliable material. The measurement of properties such as the number of layers, lateral dimensions, the level of disorder, the chemical composition, these all need to be determined before using the material or the results obtained (whether good or bad) won’t be possible to reproduce or be systematically improved upon.

The standardisation of this area is also extremely important and something I have been trying to address with my colleagues in the international standardisation community over the last few years.

Could you explain the main surface characterisation techniques which can be used in order to provide actual measurements of graphene and other 2D materials?

There are many techniques that may provide insight into the structural and chemical properties of graphene and other 2D materials - a big part of what we want to develop at NPL and with our external collaborators is the optimal way to use these techniques so that the results can be compared. It is important to look at how a set of complementary techniques can be used to determine the many different properties required, whilst minimising the cost for industry.

Typical techniques used to determine the structural properties are optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and Raman spectroscopy. Meanwhile X-ray photoelectron microscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) are common techniques used for chemical characterisation.

However, these methods are typically what you would call ‘off-line’ measurements, not the sort of thing that could be used for high-throughput quality control methods. Although arguably optical and Raman spectroscopy methods could be developed for this role.

What are graphene defects and how do they affect the properties of graphene?

The term ‘defect’ tends to be specific to the application or material being discussed - a hole in your shirt that you can poke your finger through is a type of defect. Graphene defects are nanoscale in dimension, typically referring to a missing atom or atoms (vacancy defects) or when atoms are not in a continuous honeycomb lattice (dislocation defects).

Atoms that are not carbon are also a type of defect. This may be due to the chemical substitution of a carbon atom in the sp²-hybridised lattice (each carbon has 3 covalent bonds), or an adatom bound to the graphene layer either above or below, and thus causing the carbon to be sp³-hybridised (4 covalent bonds).

When you say ‘defect’ you instantly think negatively, and this is true for graphene in terms of the electronic properties, as it has been shown for several years that defects reduce the conductivity of graphene.

However, there are other application areas that might require defects to work, such as ionic filtering across a graphene membrane. Defects shouldn’t always be thought of as a hindrance, for some application areas you won’t want a single-layer sheet of graphene the size of a bed-sheet with no disorder in the lattice, but instead you might want small flakes with many defects and a high level of oxygen.

The term ‘quality’ is therefore deceptive as what may be ‘high quality’ material for one application would be ‘low quality’ for another application area. Either way, you want to understand the level of defects in your material, so you need to be able measure it!

How can the size of a graphene defect be determined and why is it important to do so?

There are many ways to determine the size of graphene defects, TEM or scanning tunnelling microscopy (STM) can provide very high-resolution images of defects in graphene. However, these techniques are slow due to sample preparation and image acquisition times. Raman spectroscopy has been shown to be able to rapidly and non-destructively determine the defect density in graphene and is typically the ‘go-to’ technique for many researchers.

A relationship between the intensity ratio of two of the Raman peaks of graphene (the D- and G-peak) and the defect density has previously been shown, however, the defect size is also a variable in this relationship.

This meant that Raman spectroscopy could theoretically be used to determine the defect size as well, although this was not shown experimentally until recently when we investigated graphene bombarded with different ions using a secondary ion mass spectrometry (SIMS) instrument at NPL.

The different ion guns allowed us to create different size defects in the graphene in a controllable way, and with a known defect density so we could investigate the effect on the Raman spectra.

From our investigations we improved the level of uncertainty in these measurements and proved that Raman spectroscopy can be used to quantitatively determine the size of the defects in a fast and non-destructive way - however the caveat is that you need to measure the defect density first.

Conversely, to truly understand the defect density of graphene quantitatively you need to first quantify the size of the defects. This is important for many application areas where either defect density or defect size will affect the final properties of any graphene product.

How can tip-enhanced Raman spectroscopy (TERS) be used to visualise individual defects in graphene?

TERS combines the extensive material characterisation provided via Raman spectroscopy, with the nanoscale resolution of a scanning probe instrument (STM or AFM). This means that rather than recording the material properties that are averaged over an area of about 1 micron (1 millionth of a metre) using confocal Raman spectroscopy, we can investigate the material on the nanometre-scale (1 billionth of a metre).

Image Courtesy of the National Physical Laboratory

This is possible because of the localised enhancement of the Raman scattering signal at the apex of the nanoscale STM or AFM probe. With TERS, you can directly visualise the individual point defects and line defects in graphene using Raman spectroscopy, something that was previously not possible.

How can the defects in other 2D materials, such as the semiconductor molybdenum disulphide (MoS₂), be investigated?

Similar methods and techniques to the ones used for graphene can be used to characterise defects in other related 2D materials such as hexagonal boron nitride (hBN) or transition metal dichalcogenides, which include MoS₂. However, less research has been performed with regards to the measurement science of the other 2D materials, when compared to graphene.

Therefore, we recently investigated single-layer MoS₂ flakes with different defect densities using Raman spectroscopy to systematically understand how the Raman spectra evolves with increasing defect density. Using well-controlled ion bombardment, we found that a Raman peak called the LA(M) peak can be used to quantitatively determine the defect density in MoS₂, in a similar way to the D-peak in graphene.

However, measurements of MoS₂ using Raman spectroscopy are much slower than for graphene, as the intrinsic properties of graphene leads to a very high Raman scattering signal. This means other techniques are being explored, such as photoluminescence, which provides a very strong signal that is distinctive for MoS₂ and depends on the number of layers.

You are actively involved in the international standardisation of graphene as part of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). Why are standards important in the graphene industry?

Standards are important for any technology, providing confidence in the market, allowing the development of industry regulation and the basic understanding that enables health and safety research. Without terminology standards, companies within the supply chain cannot communicate with each other, without measurement standards material cannot be accurately compared, both these issues hinder the commercialisation of an area.

These issues are particularly poignant for the graphene industry and standardisation is now at the forefront of industry’s concerns for this new technology. For these reasons, NPL has been working with the international standardisation community for the last few years, since my discussions with industry revealed that NPL’s expertise in the international standardisation of other nanomaterials may aid in overcoming this barrier to commercialisation.

How far away are we from seeing a set of standards for graphene terminology and characterisation methods that can be implemented across the industry? What is the biggest obstacle in this challenge?

There are currently several different technical specifications in development within the international standardisation bodies committees concerned with nanotechnology (ISO/TC229 and IEC/TC113). Some of these standards have been developed for longer than others, and at different rates, so there will not suddenly be several standards published in this area, but more a continual release of standards addressing different issues.

Some new technical specifications are only just being proposed now and initiated within the committees, and typically international standards will take several years to go from initiation to publication. This is because an international consensus must be reached on all aspects of the document. To this end, the standardisation body of each interested country will have assigned national experts to work on these documents.

The standardisation process is also a volunteer process, which means that it may be difficult to involve a large number of experts or experts who can devote the time and resource to speed-up the process. As more experts join in, particularly as we see more input from entities such as the European Graphene Flagship, I believe this process will be accelerated. Hopefully we should see the first graphene standards published in the next year or so.

But an important point to bear in mind is that the standardisation process is ongoing and published standards must constantly evolve to continue to be relevant. As an example, we have just had a new version of laser classification standard published in IEC last year, and lasers were first demonstrated 55 years ago!

There has been a huge amount of hype surrounding the commercialisation of graphene since it was first isolated at the University of Manchester in 2004. Why do you think that the commercialisation of graphene has not been fully realised yet 11 years on?

I think it is probably unfair to talk about how graphene commercialisation hasn’t been realised yet, as it is still early days for this material, especially one that is nanoscale in nature and therefore will have many challenges in areas such as large-scale production and processing, to get it into real-world products.

There are a lot of areas that can be explored, and so there is a lot of promise, but these application areas are very varied and so will develop over very different time-frames. There is a lot of exciting stuff happening in industry now though, in areas such as composites and energy storage, as well as membranes, flexible electronics and sensors. The impact of graphene and other 2D materials in these areas could be huge.

Where can our readers find out more about your research into the characterisation and standardisation of graphene?

Information on both my research and other graphene research at NPL can be found on the NPL website npl.co.uk/graphene, as can more details on the standardisation of graphene. More information can also be found at the ISO/TC229 and IEC/TC113 websites.

About Andrew Pollard

Andrew Pollard received his MSci in Physics from the University of Nottingham in 2005 and finished his PhD with Professor Peter Beton from the University of Nottingham's Nanoscience group in 2010. Andrew's previous work includes development of a combined AFM-STM system in UHV, production of graphene monolayers on transition metal surfaces in UHV and subsequent STM surface studies.

After joining NPL in 2009, Andrew is now leading the Surface and Nanoanalysis Group's research into the measurement of graphene, other graphene-like advanced 2-D materials, and associated devices. This metrology research focuses on the actual measurement of the materials with a range of surface characterisation techniques, such as Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS), scanning probe microscopies (SPM), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS) and ellipsometry.

Andrew is also engaged with organisations in the emerging graphene industry, advising on aspects of standards and measurement in this area, and is on the advisory board of the Graphene Stakeholders Association (GSA).