Bringing graphene research to market
New research into graphene sheets could provide an important role in the commercialisation of the 2D nanomaterial, as Dr Andrew Pollard explains.
Graphene has been touted as the wonder material, a game-changer for various applications within global markets. When it was first isolated in 2004 at the University of Manchester, UK, it was realised that graphene could be used in many industries including electronics, medical technology and energy. It captured the public imagination in a way that few scientific discoveries do, earning Professors Geim and Novoselov the 2010 Nobel Prize in Physics.
Yet graphene’s inability to immediately change our world for the better has drawn criticism from some circles. While one form of graphene – “flakes” that are suspended in a liquid and can be used for investigating future batteries or composite materials – is being used in real-world applications, the other form, graphene “sheets”, has not yielded the results expected of scientists by the general public. Questions have been raised as awareness of the material increases. But, this has also raised unrealistic expectations with the public regarding the speed of its development. A discovery is one thing, but turning that into an applied technology can take time. Silicon, discovered in 1824, and aluminium, discovered in the same decade, didn’t fulfil their potential until well into the 20th Century. Graphene sheets are already being produced on the metre scale – far ahead of what history tells us is the development timeline for such innovations.
So, how can R&D continue this pace of development and fulfil the expectations of industry and the general public? One of the key stumbling blocks is not a scientific one, but an engineering problem. Dealing with materials on the atomic scale and effectively creating them to specification is difficult. Production of a graphene sheet – a structure one atom thick – is an extremely complicated process to manage. To understand this scale, around three million sheets of graphene are required to create a 1mm-thick layer. On top of this, every single atom is at the surface of the material, making the entire sheet susceptible to contaminants. Finally, couple this with the dominant process by which graphene sheets are made – chemical vapour deposition (CVD) – and the production and use of graphene sheets becomes a complex and finely balanced process, plagued by the interference of unwanted materials, which diminish its properties. However, overcome this and it could lead to widespread production of graphene in many industries.
CVD growth of graphene uses a metal, typically copper, at high temperature, upon which gaseous carbon molecules will breakup and connect to each other to form a hexagonal carbon lattice, i.e, the graphene sheet. A polymer film is then deposited onto the graphene that can be handled when it is removed from the copper. The graphene sheet is “stamped” onto the surface of the material or device where the graphene sheet is required for application. A chemical process is then used to remove the sacrificial polymer layer from the graphene. This process works in practice, removing the graphene sheet from the copper and successfully applying it to a substrate of choice. However, polymer residue is always left on the surface of the graphene sheet, often adding more carbon atoms than are present in the graphene itself, affecting its electrical properties. Given one of the key advantages of the material is that it has the highest electrical conductivity known when in its purest state, it could be argued that a graphene sheet with defects, impurities or contamination becomes uncompetitive when there are other conductive materials that are easier to use.
This is the current challenge facing the graphene industry. How do you create a graphene sheet without the polymer contamination inherent to the CVD process after the transfer? The entire polymer needs to be removed, without impacting the structure of the sheet. Using chemical processes, such as solvents, to remove the polymer may be scalable, but doesn’t remove all of it, effectively creating “speed bumps” for the electrons flowing along the material and reducing conductivity significantly. Using oxygen plasma is another way of removing polymer residue. However, it can etch the sheet, affecting its structure and weakening its properties. There are other methods that have been tried but also have issues, either the same as the above or wider ones, such as their inappropriateness for use at mass scales.
To overcome these problems, a team at the National Physical Laboratory, UK, has focused on the existing silicon wafer industry to inspire a solution. The industry has been using a technique whereby clusters of argon gas are fired towards inorganic silicon wafers, cleaning them of organic material without damaging the underlying silicon. This process has become standard in the industry, allowing for the large-scale processing of silicon chips that perform to expectations. Such functionality – removal of organic material while retaining an underlying inorganic structure – can be directly applied to the process of removing polymer material from graphene.
To test this hypothesis, the team targeted pristine graphene, with no defects or polymer residue, with argon gas clusters, varying in ion size, kinetic energy and dosage (amount of clusters), to see the levels of damage the process did to the graphene and detect any differences in such damage caused by changing the variables. By doing this, a range of settings that caused little damage to the graphene structure was achieved – lowering the kinetic energy per atom of the clusters reduced the amount of defects created in the graphene structure. At the same time, the larger, lower energy clusters could be used to controllably remove the polymer residue left after the transfer of the CVD-grown graphene. In addition, the argon gas sputtering caused no damage to the graphene sheet unless the sputtering continues after the polymer contamination was removed. This sets the groundwork for a system that manages the ‘sputtering’ process to ensure polymers can be fully removed with little to no effect on the sheet it sits upon for large-scale graphene production.
The analysis draws conclusions on the structural damage caused by using an argon gas cluster sputtering technique. The next step is to understand how the small levels of damage affect the electrical properties of the sheet, as well as comparing these results to measurements of graphene conductivity taken after other polymer removal processes have been used. This suite of data will allow the industry and individual companies to make decisions on the best method of graphene cleaning and production. It will determine the performance benchmarks for these methods, helping overcome the engineering issue behind graphene sheet production with clear data on the effects of each method.
Applying this knowledge could provide an important role in the step towards the mass introduction of graphene and the use of its properties in commercial applications. At the same time, deliver the high expectations of a general public that has rarely been so engaged in scientific discovery, producing new and innovative consumer and industrial electronics for the benefit of all.
Figure 1 compares the levels of damage to a graphene sheet that did not originally have any polymer present, after a high dose of of argon gas clusters of different kinetic energies and sizes. The top line shows a smaller but higher energy cluster, the middle line a lower energy but larger cluster, the bottom line shows a graphene sheet that was not sputtered. The larger the size of the left-hand peak, the larger the degradation in the lattice structure of the graphene.
Dr Andrew Pollard works in the National Graphene Metrology Centre at the National Physical Laboratory, UK, leading research into the structural and chemical characterisation of graphene and related 2D materials, with a focus on enabling industrial commercialisation.