LMCat by Blazon Publishing and Media Ltd

Growing graphene from the bottom up Graphene and other 2-dimensional materials could be used across a wide range of technological applications, yet they remain difficult to produce on large scales with sufficient quality. Researchers in the LMCat project are developing a new method of producing 2-D materials using liquid metal catalysts, as Dr. Irene Groot explains. A lot of attention in research is centred on the development and utilisation of 2-dimensional materials like graphene, which hold rich potential across a wide range of technological applications. However, fabricating these 2-D materials on a large scale with sufficient quality remains a challenge. “When we prepare these materials in an academic setting, we see that there are always some defects. It’s very difficult to upscale this process without producing those defects,” explains Irene Groot, Associate Professor in the Institute of Chemistry at Leiden University. As the Principal Investigator of the LMCat project, Dr. Groot is developing a new approach to the synthesis of 2-D materials, which uses liquid metal catalysts. “The project is about developing a methodology to produce these 2-D materials. We would also like to learn more about the physics and chemistry behind the growth of these materials,” she outlines. The main material of interest here is graphene, which has attracted a lot of interest from the commercial sector due to its interesting electrical properties. However, the project’s research is not limited solely to graphene, with Dr. Groot and her colleagues aiming to identify general principles which could also be applied in the production of other materials. “Some other 2-D materials can also be grown on copper. One of them is hexagonal boron nitride,” she outlines. “In principle, with the reactor and measurement techniques that we have developed, we could use more or less any catalyst material.”

Photograph of the LMCat reactor.

3-D drawing of the LMCat reactor

Chemical vapour deposition Researchers are using a technique called chemical vapour deposition (CVD) to effectively build these 2-D materials from the bottom up, using a reactor that has been developed as part of the project. A highly pure piece of copper is melted into a liquid which is then used as a catalyst, which provides the foundations to then develop the material. “That is the substrate on which the graphene, or another 2-D material, is going to grow. On liquid copper, less defects are expected to form,” says Dr. Groot. Certain

gases are then added, with Dr. Groot and her colleagues using methane, which contains both carbon and hydrogen atoms. “We need a gas that contains carbon atoms, that will then eventually form graphene. This gas will then decompose on the catalyst surface,” she continues. “Methane contains carbon atoms, but also hydrogen atoms. We don’t want this hydrogen, as graphene is formed of an atomthick layer of carbon atoms.” The bonds between hydrogen and carbon are broken at the copper surface, and the

hydrogen atoms disperse while the carbon atoms remain, where they form a layer of graphene. Methods have been developed to then visualise the growth of the graphene, from which researchers can look to gain deeper insights into the process. “We can visualise the growth of the material, and we can see where there are imperfections,” explains Dr. Groot. With CVD, huge numbers of methane molecules hit the catalyst surface simultaneously, which can lead to imperfections. “Graphene does not start

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growing from a single carbon atom, but rather it starts growing on the catalyst surface at multiple places in little graphene sheets,” says Dr. Groot. “Ultimately what people want is one big sheet of what we call single crystalline graphene. So we want it to be a single sheet that does not have what we call domain boundaries.” This is a significant technical challenge, as when two sheets of graphene come together there are typically imperfections of some kind, for example they might be slightly rotated with respect to each other. By modifying the amount of gas that is added to the system, and the ratios in which it is added, Dr. Groot hopes to improve the growth process. “We want to etch away imperfections and ensure that the graphene grows in the optimal way,” she says. Graphene itself is just 1-atom thick, and if additional layers are added on top then the material loses its interesting electronic properties, which is an important consideration in the project. “It is quite difficult to grow graphene so that it is just 1-atom thick,” acknowledges Dr. Groot. “We are modifying the ratio between hydrogen and the carbon-containing gas to try and achieve this, while we are also looking at the importance of pressure.” A lot of progress has been made in this respect, with researchers demonstrating that the growth of graphene can be controlled and tuned through manipulating these gas ratios and the pressure level. The development of the reactor itself represents an important step forward, while Dr. Groot is also keen to highlight the project’s achievements in developing new measurement techniques. “With these measurement techniques we are able to visualise the growth of the graphene while it actually happens, in real time,” she continues. This opens up the possibility of effectively directing the growth of graphene. “If we see that the graphene is not growing in the way that we want it to, then we can change certain parameters, such as the ratios of these gases and the pressure,” explains Dr. Groot. “For example, if we have more than one sheet growing simultaneously, then we can look to manipulate them so that they align well.”

Removing graphene The graphene is grown at very high temperatures inside the reactor, typically in excess of 1000° C, so it’s essential to cool down the system before removing the material. This is where a problem arises, as while the copper shrinks when it is cooled down, the graphene actually expands. “The graphene starts getting bigger while the

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LMCat

Synthesis of 2D-Materials on Liquid Metal Catalysts

Project Objectives

The main objective of the LMCat project is the controlling and tuning of the growth of graphene on liquid copper using real time in situ observations via optical microscopy, X-raybased techniques and Raman spectroscopy. By obtaining a fundamental understanding of the underlying growth processes, perfect, defectfree graphene can be grown.

Project Funding

LMCat: H2020 FET Open project number 736299 / DirectSepa: H2020 FET ProAct project number 9519]\\\43.

Project Partners

Technical University of Munich: Mie Andersen, Postdoc • Hendrik Heenen, Postdoc• Santiago Cingolani, PhD student • Karsten Reuter Principal Investigator. Leiden Probe Microscopy: Gertjan van Baarle, Principal Investigator • Arthur Sjardin, Technician • Marc de Voogd, Application scientist. ESRF: Valentina Belova,Postdoc • Oleg Konovalov, Principal Investigator • Francesco La Porta, PhD student. University of Patras: Marinos Dimitropoulos, PhD student • Costas Galiotis, Principal Investigator • Anastasios Manikas, Postdoc • Christos Tsakonas, PhD student. Leiden University: Irene Groot, Principal Investigator • Mahesh Prabhu, Postdoc • Mehdi Saedi, Postdoc. Alternative Energies Commission: Maciej Jankowski, Postdoc • Gilles Renaud, Principal Investigator.

Contact Details

Project Coordinator, Dr. Irene Groot PO Box 9502 2300 RA Leiden The Netherlands T: +31 (0)71 527 7361 E: i.m.n.groot@lic.leidenuniv.nl W: www.lmcat.eu W: https://www.youtube.com/watch?v=sp6 VujNRDRI&feature=youtu.be Dr. Irene Groot

Irene Groot obtained her PhD degree at Leiden University investigating the dissociation of hydrogen on metals, both experimentally and theoretically. After two postdocs she started her own group at the Leiden Institute of Physics. Currently, Irene Groot is associate professor (tenured) at the Leiden Institute of Chemistry. She investigates the structure-activity relationship of catalysts under industrial conditions focusing on sustainable energy and materials production.

copper gets smaller, and that gives rise to wrinkles in the graphene. So it is already less well oriented on the copper surface,” explains Dr. Groot. Once the system has cooled down to room temperature, the next step is to transfer the graphene and remove the copper catalyst. “Then we add a protective layer to the graphene, and from the bottom, start etching away the copper. So we add acid to the system to etch away the copper, and effectively destroy the copper in that way,” continues Dr. Groot. “The protective layer also needs to be removed eventually, in order to end up with just the graphene.”

There are still many hurdles to overcome before this can be realised, but this project will represent an important step in this respect. One important aim will be to enlarge the reactor so that there is a bigger copper pool available. “It’s not clear whether a scaled-up reactor will still work in the same way. That is something we will need to investigate,” says Dr. Groot. The primary interest for Dr. Groot in research is to learn more about the physics and chemistry behind the growth of graphene and other 2-D materials, yet she is very much aware of the wider interest in this work, from both the commercial and academic sectors.

The project is about developing a methodology to produce 2-D materials like graphene. We would also like to learn more about the physics and chemistry behind the growth of these materials. This research is still at a relatively early stage, with Dr. Groot and her colleagues producing a circular sample of graphene with a diameter of around 1 centimetre. However, funding has been granted for a further project in which researchers will aim to build on the progress achieved in LMCat and move the technology forward. “In this follow-up project, we will look at whether it is possible to remove graphene from the copper while it is still in the liquid phase,” outlines Dr. Groot. This would be a continuous process, with graphene grown on one side of the copper pool, and then pulled off on the other. “In that way, you could get rid of all these difficult transfer steps. That’s the ultimate dream,” says Dr. Groot. “If we can get this to work, then in theory we could grow very large graphene sheets.”

“We are working together with a small company who are looking into a business model for this reactor, to see whether other people, at other labs, might be interested in purchasing it,” she continues.

Optical microscopy image obtained during graphene growth on liquid copper. The light hexagons are graphene sheets, the dark background is liquid copper.

Molecular dynamics simulation of a hexagonal graphene sheet (silver) on a pool of liquid copper (gold).

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