Understanding Graphene: Part 4 – The Hammock Index
Graphene: Wonder material, a solution looking for a problem, a source of riches. It also a source of immense frustration that we cannot make the stuff in the large-scale sheets needed to realise this potential.
‘Hold on’, I hear you say, ‘surely there are lots of people making graphene?’
There are indeed many organisations making graphene. To be precise there are 63 organisations around the world producing graphene at the moment. We know this because at InvestorIntel we have spent the past year researching the industry and have created an independent report of the global manufacturing market that details who is making graphene by what method and where.
So why the frustration? To understand this I’ll remind you of the properties of graphene. It is 200 times stronger than steel, yet very flexible. It conducts electricity better than any other material and is so thin that it is practically two-dimensional. To illustrate what this all means think of this experiment: A one metre square sheet of the stuff would be one-millionth the width of a human hair yet could easily take the weight of a four-kilogram cat placed in the middle of the sheet without breaking. Amazing isn’t it? Except, that at the moment, no one can make a sheet this big. That is the source of the frustration.
Graphene is being made by the tonne. The top down method currently achieves this large-scale manufacturing. This is where a starting material of graphite is broken down into tiny nanoplatelets of graphene. The product is either a powder or suspension of platelets in a liquid. These platelets are smaller that can be seen with the naked eye. You could try to make the one square metre sheet of graphene from any of these products, perhaps by spreading some of the suspension on a surface and drying it. You might then separate the material away from the surface and have a sheet of graphene. Place the four-kilogram weight, such as a cat, on this material and it would disintegrate immediately as our cat fell to the floor and ran away. Try putting an electrical current through this material (first making sure the cat was no where near) and you would find it conducts electricity 5000 times less than you would expect. So we have graphene but the fabled properties are not there. What is going on?
The reason is that for graphene to have its expected properties it has to have a single continuous layer with no defects. Graphene suspensions and powders are made up of countless billions of very tiny, separate pieces. These do not recombine when you place them next to one another.
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Have you ever seen a car crash? Look at the glass on the road; it has shattered into tiny glittering cubes. Try sticking the cubes back together to form the original sheet of glass. Is it made of the same stuff? Yes. Does it have the same properties? No. It is the same with graphene platelets.
There is another way of making graphene. This is called the bottom-up method. About 30% of graphene manufacture is made this way. Take a sheet of copper foil. Clean it and heat it up to 500 degrees centigrade and pass hot methane and hydrogen gases over the surface. Gradually, carbon from the methane forms a single layer on the copper surface. This layer is graphene. This is called Chemical Vapour Deposition (CVD) A previous column describes this process in more detail.
To the casual observer this CVD process offers the potential to create sheets of graphene of any size. Many manufacturers can supply graphene on copper several of the order of tens and hundreds of square centimetres. At least one manufacturer can supply CVD graphene on copper in 30×30 centimetre sheets.
Again the casual observer may think that to work with a sheet of graphene, just peel it from the surface of the metal. All is not quite as it seems. There are still a few production challenges to overcome that we explored in more detail in a previous column.
These challenges are created by the way graphene grows on the copper surface like frost on a window in winter or disrupted by defects in the copper surface itself. These prevent an even continuous layer forming and instead we get crystal domains or grains of graphene. Where these domains/grains meet there is a discontinuity that could impair the electrical and mechanical properties of graphene that are desired so much. One interesting piece of research at Brown University in Texas has found that if these boundaries can be made to meet at angles of 28.7 or 21.7 degrees then the graphene sheet will connect up across the boundaries and make a very strong sheet. The problem is, I have not seen convincing evidence that anyone has managed to control this in practise.
So, the current state of the art of graphene manufacture by the bottom up and top down methods results in little bits of graphene that do not connect up completely. Look for language that describes nanoplatelets or graphene domains. The largest of these are around 10,000 nanometres wide. If this sounds big then remember that is about a tenth the size of the dot on this character in the brackets ( j ).
The language in the marketplace that describes the state of the art is confusing and evolving. It is time we had a straightforward measure of the ability of manufacturers to make that elusive sheet of graphene, no matter what the production method.
Let’s go back to the thought experiment of a sheet of graphene on which we can place a four-kilogram cat. This example comes from the Nobel Foundation (of Nobel prize fame) so it is a solid foundation.
The sheet of graphene that can support the weight is a hammock made from a one-metre square piece of graphene.
This inspired us to use this as a new measure of sheet graphene production. We can use this index to cut through the hype and get an indication of the true scale of various claims independent of the production method. Using mass as the index for making sheet graphene is misleading. Size needs to be measured in square metres.
The Hammock Index
At a point in the future, someone will have made a 1m2 hammock from graphene and performed the 4kg cat experiment. This requires a free sheet of graphene, which means the graphene layer that is a single continuous domain and not attached to any other surface. So we can establish a benchmark of a free sheet of graphene that is 1 square metre in area.
In the world of graphene areas are currently measured in square micrometres. These are rather small, with lots of zeros before the decimal place when we convert to square metres. Using the base10 logarithm makes the numbers manageable, for example one millionth of a square metre would be 0.000001 the base10 logarithm is -6 and that is a simpler number to read. A thousandth of a square metre would be -3 and 1 square metre would be zero.
We can call this the Hammock Index and use this to measure the ability of any graphene production method to create a sheet of graphene. In essence a single number that shows us the state of the art of graphene manufacturing. Minus numbers are very small. The target of 1 square metre will be 0. So in this race the first to zero wins.
The hand prepared exfoliation of a very high quality piece of flake graphite has made the largest reported free sheet of graphene at 1,500,000 square micrometres.
This is 0.0000015 square metres.
And the hammock index is -5.82
This means we are still at least factor of a million away from making that hammock.
Making that free sheet of continuous graphene is a long way off yet. Smaller pieces of graphene may still be beneficial as an additive and we will look at these in part 5 of this series. In the meantime we will keep a watch on the state of the art of graphene manufacture and report back in further columns.
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>