Making graphene and other 2D materials on liquid metal
Graphene can be made by the Chemical Vapour Deposition (CVD) method on metals. Get the conditions right and graphene forms on the surface then, when the metal is covered, the reaction stops. This means you automatically get a one atom thick layer of graphene.
The trouble with metals is that the surface always contains defects caused by boundaries between crystal domains that create flaws in the graphene layer that grows on the surface. These defects remain no matter how much polishing you do.
Can graphene be grown on liquid metals?
As we mentioned in previous column entries, in 2010 a team in California had a smart idea, could graphene be grown on molten metals? The answer turned out to be yes, and this creates a metal surface free from defects. Since then further work in 2013, 2014 and 2016 has grown graphene on a variety of metal melts including Tin, Copper, Silver, Indium, Nickel and Gallium.
Does liquid metal create perfect graphene?
Having a defect free surface doesn’t guarantee defect free graphene because it can form at multiple points on the liquid surface. These islands or snowflakes grow and eventually collide to form defects where they meet. In a previous column entry we proposed a solution to this problem by using a sliding mask to control the growth.
Now the only problem is to get the graphene sheet off the surface. At the moment the current state of the art seems to be to cool the metal then peel off the graphene layer.
Could graphene be pulled directly from the surface of liquid metal?
No one seems to be actively considering pulling the graphene layer directly from the melt. So in January and March of this year I published a set of ideas at InvestorIntel designed to do just that using an invention called a freezewall.
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As I have made these ideas free for the public, (Any new thinking published in this column is free to use by anyone) I’m having interesting discussions with many people and this is creating new ideas.
One example is a conversation with a colleague, Karla Robertson of Shifting Gears. We discussed alternative ways of getting the graphene sheet off the metal surface. Karla pointed out that graphene was impermeable to gases. (She is perfectly correct, even Helium cannot pass through graphene) “Why not use suction to lift the graphene sheet from the surface?” she said. Why not indeed? That’s a smart idea Karla, a long thin suction nozzle just might be able to overcome the surface tension of the metal to pull the graphene layer from the surface. While most CVD is carried out at very low pressures, recent work in China has grown graphene at ambient pressure. This means a vacuum transfer process has a better chance of pulling the graphene sheet from the surface of the metal melt.
Pulling the graphene layer directly from the surface of the melt is important because if it can be done this forms the basis of a continuous manufacturing process for making sheet graphene. Dear InvestorIntel reader you’ll remember we discussed this manufacturing process in a previous column entry.
All this work is in the realm of ideas at the moment, however spin off thinking is already getting more interesting. Graphene has a sister two dimensional (2D) product called hexagonal Boron Nitride (hBN – more of this in a future column). A similar CVD process can make it. So I can see no reason why hBN should not be made in continuous sheets too.
It is worth restating that until a team actually does the experiment to prove the idea this, manufacturing process for continuous sheet graphene and other 2D materials from liquid metal, is still a hypothesis. Making all this thinking public has enabled me to bring this to the attention of many scientists around the world over the past few months. The purpose of a hypothesis is firstly to try to break or disprove the idea, then run the experiment to find out if it works for real.
In all the conversations I’m having, no one has yet disproved the hypothesis.
In summary, making continuous sheets of 2D materials by this process will not be easy. It will be industrial-strength-hard-to-do, but it is not impossible…
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>