Understanding Graphene – Part 3
The thinnest material in the world is also one of the strongest and it conducts electricity too. This is graphene and everyone would like to use it to make their products better – and to create new ones. The race is on to make graphene in useful amounts. One method is to build these sheets atom by atom using self assembly chemistry, this is called the bottom up approach and is the subject of this column so, dear Investorintel reader, please read on…
In a previous column we looked at the top down method of making graphene from graphite. This can be scaled to produce graphene by the tonne. However this product is composed of countless tiny plates of graphene rather than the large-scale sheets that many expect.
The bottom up method holds the promise of creating larger scale sheets and our analysis of worldwide graphene supply has found 24 organisations that are actively developing this method of graphene manufacture.
So, how is it done? There are several possible methods:
Direct chemical synthesis
This would be the obvious place to start. The largest commercially available molecule is called coronene, imagine six hexagons surrounding a central one and you’ll get the idea. Larger sheets of graphene have been made in lab conditions but no further work seems to have been done to scale up this process. It is worth noting that ‘larger sheets’ means a size around 500 nanometres which is about 200 times smaller than the full stop (period) at the end of this sentence.
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Making graphene from silicon carbide (SiC)
Take a piece of silicon carbide and heat it to around 800 degrees centigrade in a chlorine atmosphere. The silicon atoms leave the surface and the carbon stays behind to form a graphene layer on the surface. The graphene produced by this method is dependent on the structure of the original SiC crystal. To scale up this method to make graphene would need highly ordered SiC material and at the moment this does not seem to be available. So this method stays more in the R&D lab than the factory for the moment.
Chemical Vapour Deposition (CVD)
This is the most common method for making graphene by the bottom up route.
The graphene sheet is grown on a metal surface, usually copper (Cu) or nickel (Ni) by passing a hot gas (usually methane (CH4) over the surface.
The high temperatures weaken the C-H bonds in the methane allowing the metal surface to attract the carbon, which forms graphene on the surface. Once the surface is covered it is harder for other layers to form and the sheet of graphene is formed.
As you’ll have realised dear reader, I have simplified the production process. In practise the copper surface needs careful preparation, it needs to be electropolished and then annealed with hot hydrogen gas to smooth the surface further. This is all carried out at very low pressures and high temperatures around 500 degrees centigrade. Working with very hot methane and hydrogen gases falls in the category of ‘Don’t try this at home’
Researchers from Korea, Singapore and Japan have realised that this basic process can be scaled up to a production method using a roll-to-roll process.
The idea is that graphene is grown on a long sheet of copper as it is pulled through a methane furnace from a supply roll. This is then passed into a bath where the copper is etched away and transferred to a support sheet (called a target substrate where it is washed and rolled up ready for use.
Samples of CVD graphene are available commercially. Expect to pay around $1000 USD for a 10x20cm copper foil coated in graphene and $500 for a 5x5cm piece of glass coated with graphene.
By now you may have the impression that large-scale sheets of graphene are going to be available in the near future. I thought so too, then I read the small print in the technical data for these products.
It turns out that these large-scale coatings of CVD graphene on metals and glasses are not quite the continuous sheets of graphene that the marketing would lead you to believe. The analysis data shows that these coatings are in fact made up of grains, or crystal domains of graphene. Large grain sizes of graphene can be 10,000 nanometres wide – about a tenth the size of the dot on this character in the brackets ( i )
It turns out that there are a few problems to solve yet in making that elusive continuous connected graphene sheet. The first problem is that the surface of the copper or other metal has surface defects at the nanometre scale that polishing and annealing cannot remove. When the graphene starts to grow on the surface it follows these defects and this stops the continuity of the layer. The second problem is called snowflake deposition. You might think that graphene grows on the surface all at once, when in fact it forms at many specific points that grow into one another (think of frost growing on your window in winter) when these areas meet they form a discontinuity.
Don’t give up just yet, there is progress. A clever team from Leeds and Oxford in the UK and New York in the USA has solved the surface defect problem by growing graphene on molten copper. Using this method they have produced continuous graphene crystal domains of 200,000 nanometres, – just a bit bigger than the dot on this character ( i )
So, progress is being made in making graphene, just make sure you read the small print behind the marketing messages and this will improve your understanding of the art of the possible.
In the meantime we will keep reading the small print for you and will return with part 4 of this series.
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>