Graphene Lithography - Atomic Precision Etching
Kat - Also in the news this week, researchers at Rice university have developed a new way to etch structures into stacked piles of graphene, the marvellous material consisting of a single layer of carbon atoms. This could allow manufacturers to make computer chips from graphene in much the same way they currently do for silicon. But sometimes in science, you make a great discovery entirely by accident - and this was just one of those cases, as Professor Jim Tour explains..
Jim - What we set out to do was to convert graphene to graphein, and what that is, is taking the carbon structure of graphene which is a bunch of 6-member rings in a plane and attaching hydrogen to it. That makes then graphein, and that would make an area that would be non-conductive. We thought if we could pattern zinc upon graphene, we could then use the hydrogen which is generated from the zinc reduction reaction from where zinc is treated with acid to hydrogenate the graphene to graphein. But what happened was, it turned out that wherever the zinc landed, it removed the graphene layer, but left the underlying layer completely intact.
Chris - It's really reminiscent of what people do with silicon and lasers to make microchips, isn't it? But you're doing this using zinc and graphene which is interesting because people are talking about using graphene as a material to make the next generation of microchips.
Jim - Precisely and so, this constitutes lithography. Lithography is the way we make computer chips. You take a big silicon wafer and you chip away at it using chemicals and light to make the small features, namely transistors and wires for example. So one takes a mask and has certain areas as holes from the mask and then shines light through those holes and that will develop what are called "resists" on top of the silicon to develop and build up these structures that we've seen on chips. But now, to be able to do this in graphene brings graphene one step closer. There is a huge difference between a monolayer and a bilayer of graphene. A monolayer of graphene is a metal. It doesn't have a band gap. It's not something that's easily made into a transistor, but if you have two layers of graphene, it opens up a gap and it becomes a little bit more like silicon where you can make it into a transistor. And so, to be able to have one layer or two layers, or three layers which can be more conductive then you can have different devices next to each other, and that's what you want. You want to have heterogeneity in devices. You don't want all devices to be exactly the same. So it's a new tool and a toolbox for making graphene into electronic chips.
Chris - Do you actually know though, Jim what the zinc is doing? Why this actually works to strip away the single layers and leave the one underneath untouched?
Jim - I think we have a reasonable idea. So what happens is the zinc is spattered on the surface so that will cause zinc atoms to fly up from a chunk of zinc metal, and to hit the surface that we're trying to pattern. You make a mask and wherever you want the metal to go, you have holes in the mask and it hits the surface. So what happens is about 0.5% of the zinc atoms, come with enough energy to actually knock out a carbon atom from the graphene and substitute in with the zinc atom. The zinc metal has a very high oxidation potential, so it's really begging to oxidise rather rapidly and that's going to then leave, cause the zinc atom to come out and you'll get oxygen-carbon bonds. So wherever a zinc atom had been, now the carbon atom is knocked out and the surrounding carbon atoms become oxygenated. So what you end up with is, instead of a sheet of graphene now, you have a sheet of graphene with holes in it. Then what's done is we put it into acid and acid then strips away the zinc, and in stripping away the zinc, it generates hydrogen. And that bubbles and helps to wash away the small pieces of graphene that have now been diced up on the surface. But the zinc atoms that hit never had enough energy to go through one layer and affect the second layer below. And so, it turns out to be quite a selective technique that's not only shown now with zinc. We showed we could do it with aluminium as well. So that's a reasonable understanding that we have now of the mechanism.
Chris - And once you cut through by substituting oxygen onto some of these area so they may be removed or floated off, the residual graphene from the layer where you've got - say, a step is the bit that remains behind stable or will that chemically deteriorate with time?
Jim - Everything that we have seen at the step edge, where you have one sheet of graphene that is one step higher than the sheet below it, is stable. We haven't seen that curling up. We haven't seen that undergoing any problem. It undoubtedly has different atoms at the edge. There's going to have to be - they're hydrogen atoms or oxygen atoms at that very edge. But no, there is no delamination that occurs. The other fascinating point about this is lithography is always done in industry but we have hit the ultimate in lithography. It is single atom layer precision. It will never get better than this. So in other words, in a thousand years after doing lithography, they can't do better than this. We are stripping off a single atomic layer. You can't cut an atom in half. That's as thin as you're going to get. This shows that we can have precision that you could never have in silicon - single atom resolution.
Chris - And you think that this will be a practical way to make, if you had to - a microchip of the future using graphene as a base material.
Jim - It's certainly a new wrench in the tool box. There was no way to do this before, so if we are going to make large scale patterns out of graphene, this is definitely a way to do it. And it uses methodologies that are commonly used in silicon manufacture.
Kat - ...which will make ousting silicon from its position at the heart of the microprocessor industry that bit easier. That was Professor Jim Tour, from Rice University - he's published that work in the journal Science this week.