3 novembre 2020

[CVD] – Controlling the Orientation, Edge Geometry and Thickness of CVD Graphene

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2 D graphen CVD - Codex International

Creating defect-free sheets of Graphene is said to be one of the main ‘hurdles’ in the search for wide scale commercialization of the material… but why is it so important to form large defect free sheets?
Graphene has received an astonishing amount of interest these past few years, so much so that now it is commonly referred to as the miracle material that’s predicted to revolutionize the 21st century. However, all of this interest, and by extension, potential for commercialization, stems from graphene’s unique properties; it’s one-atom thick, incredibly strong, flexible, and a great electrical and thermal conductor, to name but a few. The problem is that there is currently no method available to produce large-area sheets of high-quality graphene that actually exhibit the optimum properties for which it is famous! In practice, even the best large-area graphene grown by CVD will be a polycrystalline sheet with many grain boundaries and defects. These defects detrimentally influence the sheet’s properties, including the mechanical strength and electrical transport, thereby reducing the suitability of graphene for a number of applications. The goal then, of creating defect-free graphene, is to produce the highest quality material that can then be readily used in as many applications as possible.

What are the most common types of defects found in Graphene?
Firstly, if considering defects in a perfect graphene lattice, there are a number of scenarios that can produce defects, including atomic vacancies (single or multiple carbon atoms missing), restructuring of the graphene network (e.g. Stone-Wales defects, with pentagons and heptagons introduced to the hexagonal lattice), adatoms, heteroatoms, and edge states. Of course, it is worth mentioning that in some scenarios these defects could be beneficial, for example, the addition of heteroatoms into the graphene network could lead towards electrical doping of the graphene sheet, and will also provide localised sites for increased chemical reactivity.

If however, we consider a large-area CVD graphene sheet, while the above defects will be present at grain boundaries, there may also be defects due to inconsistent coverage of the graphene sheet across the copper surface, and the presence of multilayers.

What are the main reasons for the occurrence of these defects?
The answer is relatively straight forward, the solution to overcoming these defects is the tricky bit, but let me introduce Adrian T Murdock, my DPhil student, who played a key role in this project and who has found several of the answers to solving these issues.

The defects arise due to the way the CVD formation occurs. While it’s commonly said that large-area sheets of CVD graphene can be produced on copper foils, such sheets do not form all at once. Instead, many small graphene flakes, called domains, nucleate and grow as a one-atom thick layer on top of the copper surface. Eventually these domains merge together so that the entire copper foil is completely covered by a polycrystalline graphene film. Grain boundaries will form where adjacent domains merge, and it is at these sites that the majority of defects will arise. Of course, the growth process is not perfect, and so in addition there may be some defects present within individual domains.

How does the Renishaw inVia Raman system help you to identify and characterise defects?
The Renishaw inVia Raman provides the opportunity for large-area mapping of Raman spectra of the graphene the copper surfaces. Graphene’s Raman spectrum is well known, with the position and intensity of characteristic peaks providing information about the sample quality, including the number of layers, the presence of defects and strain. It’s fair to say that the InVia system is a state-of-the-art, primarily due to its flexibility; it allows Raman mapping from square inch regions to sub-micron, and so can provide a general overview of sample quality as well detailed analysis of individual domains.What other information does Raman analysis give you about the Graphene formation process and the properties of the Graphene film?
By correlating the Raman mapping with a map of the copper surface’s crystallographic orientation (obtained by electron backscatter diffraction) we were also able to observe a variation of graphene thickness on different copper grains for samples grown under low-pressure conditions.

What are the main techniques for producing Graphene?
There are a number of different techniques including micromechanical exfoliation (Scotch tape method), chemical processing of graphite, epitaxial growth on SiC, and the growth on metal surfaces, commonly through chemical vapour deposition. Each of these techniques shows advantages and disadvantages. For example, micromechanical exfoliation produces high-quality flakes of graphene, but is inherently restricted to micron-sized samples. Alternatively, chemical treatments of graphite by sonication in purposely selected solvents, or production of reduced graphene oxide, produces large quantities of solution or powders of graphene, suitable for use as inks and dyes, or incorporation in a composite, but shows limited opportunity for producing large-area, single-layer sheets.

Your research is currently focused on growing Graphene using CVD – what are the main benefits of this method over the other techniques mentioned above?
Chemical vapour deposition on metal surfaces shows the greatest potential for industrial-scale production of large-area sheets of single-layer graphene suitable for use as a transparent, conducting layer, surface coating, or integration into electronic devices. The technique uses a relatively cheap metal foil, often copper, as the substrate, and by control of the reactant gas mixture the quality and type of graphene produced can be carefully controlled. The size of the graphene sheet produced is therefore restricted only by the size of the metal foil used, and a suitable reaction chamber to conduct the CVD growth in.

How does the use of a copper substrate aid the growth of Graphene? And what other substrates could be used?
The copper substrate acts as a catalyst and substrate for the decomposition of methane and growth of the graphene adlayer. Growth of graphene has also been demonstrated on many other types of transition metal substrates- nickel, platinum, iridium, ruthenium etc. The key benefit of copper, apart from it’s availability and low cost, is the low solubility of carbon in the metal, which means control of the thickness of graphene layer is significantly easier compared to other metals where a large amount of carbon can dissolve and then segregate to the surface during cooling (for example nickel).

How is the CVD method beneficial for the commercialisation of Graphene relative to the techniques mentioned above?
The CVD technique is the top candidate for producing large-area sheets of graphene. Admittedly, growth on SiC also shows some potential for producing large-area sheets, however, the cost of substrates and reaction conditions required are more expensive. The CVD technique appears to be the most cost effective, and also, due to the nature of the substrate and reaction system, can be easily scaled-up for industrial level production. I should like to mention that production of large quantities of graphene powders through chemical processes will undoubtedly achieve (or have achieved) industrial quantities, with suitability for incorporating in composites or multilayer conducting surface coatings, however this is a very different type of graphene to what we can produce with CVD. Moreover, it is important to stress that the overall ‘appearance’ of graphene changes with the different production methods. While it is still graphene it is not necessarily suitable for all applications. Therefore, each graphene application will require tailored production techniques towards a specific application or type of application, e.g. electronic devices vs. thin film composite materials – a fact that is often overlooked by the media but of fundamental importance, hence it must be pointed out more strongly.

Could you give us a little more insight into how you can control the orientation of Graphene flakes as they are deposited?
The key finding of our recent publication is that the growth of graphene on copper is not entirely random, and in fact, the orientation of graphene domains on the copper surface is actually dependent on the crystallographic orientation of the copper; there appears to be more epitaxial relationship than what researchers had previously believed. We found that hexagonal graphene domains align with zigzag edges parallel to one specific direction, Cu, on Cu(111) and Cu(101), whereas on Cu(001) the domains align to two directions. With this knowledge we can now recommend specific copper surfaces, namely Cu(111) and Cu(101), as beneficial for the production of large-area graphene sheets composed of aligned domains.

What areas will your research group be focusing on next?
The focus of my research is on the synthesis, processing, and characterisation of novel carbon and non-carbon 0D, 1D, and 2D nanomaterials. We use these nanomaterials as building blocks towards the development of novel multifunctional hierarchical nanostructures and investigations are geared towards their implementation in applications found in the health-care sectors and for their use in energy and structural applications. Production routes for the controlled manufacturing of nanomaterials include chemical vapour deposition, template routes, arc discharge, and wet-chemical techniques. State-of-the-art in-situ characterisation plays a crucial role in order to elucidate the importance of individual growth parameters for the controlled formation and the study of structure properties relationships of these novel nanomaterials.

Graphene is a member of these materials and plays an important role in the over all strategy of our research. Therefore, we currently have a number of interesting topics being investigated, both developing on this discovery and also continue some of our other work that is soon to be published. Let me just mention a couple. We are endeavouring to produce large-area sheets of graphene composed of aligned and misaligned domains and investigate the properties of these films. We are also investigating the growth of large domains and have recently produced 0.5 mm single crystal domains. We hope to be able to grow even larger. Another topic is the doping of graphene sheets with heteroatoms to control the electrical properties. We’ve recently achieved atomic-resolution STEM imaging of nitrogen-doped graphene, and aim to investigate the electrical properties of these samples while also attempting doping with boron and other elements.

There has been a surge in interest and commercial patent applications related to potential future applications of Graphene; Why do you think there is so much interest, and do you think this level of investment in the material is justified?
As previously mentioned, the reason for such significant interest is due to the incredible properties of pristine graphene and, by follow through, the potential future applications where it could be integrated. Once you start considering these properties you can quickly envisage many applications in a wide-range of fields. Owing to this, I believe that the level of investment is justified. Taking fundamental research to applications addressing modern society’s needs takes time and money – often it is a lot more than one might think or wishes to spend. While it’s still debatable whether graphene will live up to some of the initial hype, and whether all its incredible properties will be utilised, I believe it won’t be long until graphene of some form is integrated into products and available on the market, and from there I foresee that graphene will find countless applications. In that sense the level of investment is justified and also needed to take graphene to the next level. However, it must also be said that there are many other subjects (‘graphenes’) out there that are as important but just haven’t had the luck to receive the same amount of publicity. I guess there is an element of being at the right place at the right time. It is not always 100% fair.

Graphene has been highlighted by the European Commission, who have pledged €1 billion to develop the material as part of Europe’s first “Future Emerging Technology’’ programme – how will this affect the direction of Graphene research?
The grant provides an amazing opportunity for European researchers to make a concerted effort to impact the worldwide development of graphene. In a field that receives as much interest as graphene does the key to success is well-directed investigations and financial investment. The graphene proposal highlights a number of directions for future investigations and by providing a significant amount of funds I believe you will see a measurable outcome in coming years, placing Europe at the forefront of the graphene race. Europe is an inspiring environment to work in, and I would like to think that ‘graphene’ can act as a ‘role model’ for the Science of Europe and future investments to take blue sky research to market. One must not forget: There is no applied science without investment in fundamental or blue sky research.

What are the next key steps on the commercial timeline of Graphene?
If working alone, neither academia nor industry will make much progress. Graphene is the perfect example where the close collaboration and development with industry are vital to overcome the valley of death.

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