Flat out for the future

Two inches thick, five feet square and 380 feet above sea level, the Walk of Faith is a plate of glass embedded in the floor of the observation gallery atop Blackpool Tower. It provokes a range of reactions. Some eye the dare from 'a safe distance' and step gingerly around and across it; others stride straight onto the glass and gave it some trampoline treatment. Still, it's all perfectly safe because, the Tower's owners assert, the glass can support the weight of five baby elephants, assuming you could get them into the lift in the first place.

Now, consider a hypothetical alternative. We have stretched a membrane across the length and breadth of the Grand Canyon that is, across an area about 275 miles long, one mile deep and an average of 10 miles wide. There is some good news: the material itself is opaque. However, here's the not-so-good news: this membrane undulates in fact, it is only one atom in thickness that is to say, it is two-dimensional. So, fancy a walk?

It's time for a safer promenade through the world of graphene, the basic structural element for all those black graphitic materials and also one that, according to accumulating research, could be strong enough to provide a greater test of your faith in structural materials, albeit without the visual thrill.

In principle, graphene has no end of attractive commercial properties, above and beyond the scientific significance of having a 2D material to study. Beyond its strength, for example, there are the material's electron mobility and sensing qualities that immediately recommend it to the world of electrical and electronic engineering. In short, all the constituents are in place for a good old-fashioned hype cycle.

So, it is worth taking note that one of graphene's research parents is warning against placing too much faith in one material. Professor Andre Geim, of the University of Manchester's Mesoscopic Physics Laboratory, is blunt when it comes to proposals that talk up graphene as a replacement for silicon in mainstream integrated circuits particularly microprocessors and the new standard bearer for Moore's Law.

"This kind of 'Graphenium Inside' thing that you hear about is unrealistic," he says. "It is not going to happen in the next 20 years you could even say that it is out there with CERN creating a black hole or the sun exploding."

However, his more fundamental objection concerns the apparent rush-to-applications.

"We are at a very early stage. In fact, you could even say that we 'know' less today about graphene than we did two years ago. New research has changed our perception of the material and reversed some of our original thinking," Geim says.

"The other factor here is that there is still a lack of people working in the area and therefore an enormous amount of research that needs to be done. It is not because we lack money, I must say, but the problem remains.

"We are not even studying potential applications. We are just trying to appreciate the material, and then we are looking at what insight it can give us into fundamental physics. And we need to go through that process before we start talking about applications. Where they have emerged, it has largely been from speculation or by accident."

Geim and his Manchester colleagues are credited with discovering graphene in 2004. The material had been talked about for years. In a rolled-up form it gives us carbon nanotubes and it is the bedrock of a traditional graphite pencil indeed, it's nigh-on on a certainty that you produce some every time scribble away with an HB. But the real challenge that the Manchester team overcame was to isolate the material in its single-layer form.

How they did that initially appears to have owed more to art and craft than high-end physics. First, a flake of graphite was placed on some sticky-backed plastic. Then, the plastic was folded over itself and pulled apart repeatedly so that the flake became thinner and thinner as it was worked on by the adhesive. Then, the plastic was attached to a silicon wafer and rubbed and it is at this point that the really important discoveries occurred.

Researchers had tried to seek out graphene using the most powerful and sophisticated microscopes, but Geim's group found that their graphite residue interacted with the silicon oxide on the wafer's surface, leading to a change of colour according to the thickness of that part of the sample. A part that was around 100 layers thick would yield yellow, but a monolayer of constituent graphene would yield an almost invisible pink. Moreover, these and other thickness-based colour variations could be detected with a traditional light microscope.

Inspection of the monolayer showed it to be a chicken-wire-like structure of carbon atoms in where there are no impurities hexagonal form. It is this 2D substance that is now exciting the scientific community. "It has great potential in helping us to explore various properties," says Geim.

Commercial applications

Yet though such a remarkably straightforward process can produce graphene for laboratory research, that same process also shows how far there is to go in terms of bringing graphene to the commercial world.

"Micromechanical cleavage or what more people call the 'Scotch Tape' technique will last forever for fundamental research. The flakes are very high quality, very good for study and very good for proof of concept work. It's very cheap per atom," says Geim. "But using the same technique for nanograms or picograms starts to take you into the thousands of dollars. On this basis, it is the most expensive material you can buy."

There are alternative fabrication techniques. Graphene can be grown epitaxially and also ground out as a powder from graphite on much larger scales than the tape-based approach.

"But the problem is that the material from the epitaxial technique is not as good as that gained using the tape," says Geim. "Also, while it is a breakthrough that you can now get industrial-scale quantities of graphene as small flakes en masse, and that they are uncoagulated, there are limits there too. The powder is best used as a filler for making very strong devices, strong membranes."

Both production instances here are unlikely to be satisfactory for more advanced MEMS and other potential electronics applications. At a high level of purity, and as noted earlier, graphene holds the promise of extraordinary electron mobility, speeds have been compared to those for light travelling through optoelectronic systems. However, much the same was said for carbon nanotubes and the purity and structural issues there are still to be overcome for the production of those devices on an industrial scale.

"It's my view that really tough materials will be the first thing," says Geim. Indeed, predicting so much is almost intuitive the kind of speculative application referred to earlier and Geim recalls one colleague's reaction when first introduced to graphene, four years ago: "'What a wonderful material!' he said to me. 'Imagine what sort of condoms we can make out of it.' That was a little shocking."

As reflects a still maturing branch of science, Geim says his own views of graphene's potential can "change by the week". "Right now, I would say that my favourite idea of a near-term application is something like armour. These techniques are giving us graphene powder which can be used as fill to take advantage of the tremendous strength of the material - a bulletproof vest, for example. That suggests itself easily."

"I like to say that the only thing you can predict is the past. So, let's go back two years," says Geim. "We thought that we might be able to make some significant progress in research with graphene because it seemed to behave in a very simple and very predictable way. But during the last year, there have been several observations that have completely turned that confidence on its head."

Use in electronics

One example of these apparent reverses directly concerns graphene's potential use in electronics.

"Among the first questions we asked about graphene was: 'What limits its electronic quality?' For a long time, it was assumed that there were charged impurities or defects
on the surface. It was a simple answer and everybody was agreeing with it. There was a consensus," recalls Geim.

"Yet it has just been shown that this simply is not correct. Other things are happening that have influence. And this is the kind of thing that is pushing us back to the beginning. The material looked very simple, but that is not proving to be the case as we learn more."

The scientist also acknowledges that he has reviewed and changed his own views of some potential graphene applications. He once co-authored an article that proposed graphene for LCDs.

"But again, it turned out that some of the characteristics that had originally been reported in some papers were not good enough. I was swayed by those, but it turns out that there is still a long way to go for LCD," Geim says.

He notes that for all the talk about graphene and it remains one of the hotter areas of the scientific blogosphere there are probably only 20 or so institutions where serious, detailed research is currently taking place.

"And it is not as though there is a money problem. Intel is interested. IBM is interested. Samsung is there. DARPA [the US defence research agency] is there," says Geim. "The issue is really one of people."

Similarly, graphene research does quite often deliver significant results, just not consistently across a whole front. Thus, for example, while Geim is a sceptic when it comes to graphene in microprocessors, he does believe it has potential elsewhere.

"I think MEMS or, more exactly, nano-level devices, NEMS will happen. Some work from the [US] Naval Research Laboratory looked at this in the context of membranes obtained through liquid phase exfoliation," says Geim. "That appeared recently in Nano Letters and I think will prove to be significant."

He also sees a place for graphene as a material for more specialist electronic devices, particularly where its mobility puts it on a par with optoelectronic chips.

"I think you could draw some comparison with gallium arsenide and its use today in very high performance devices. The kind of thing here is an individual, ultra high-frequency device, something that is in the terahertz range. Compared to a Pentium, it is more of a specialist, niche application but with significant markets in mobile devices and satellite equipment. The cost issues here are also important they are why you could see
this being proposed within two or three years, rather than 20 years for a microprocessor. The economics are more attractive, as with gallium arsenide," says Geim.

However, one piece of research needs to be published to lay this path forward.

"I hope that very soon someone will demonstrate the basic circuit you need here, a ring oscillator, because that will draw it all together. Yes, there is a long way to go in our research, but the other important thing is that we have yet to learn anything which suggests that graphene-based electronics are impossible. The mobility is good enough, all the properties are good enough and there are no problems with the size. We really just need this demonstration."

For Geim, the applications debate nevertheless remains largely moot.

"In my career, I've usually liked the opportunity to move to a new area every few years. One of the exciting things with graphene though is that it is still telling us so much at the fundamental level and it has so many aspects that you can explore, that I think it's going to keep my busy for quite a while," he says. "Think about it - we're opening up new possibilities to study relativistic physics in the context of a desktop experiment."

One particular area could, he continues, work in parallel with the intrinsically fascinating aspect of graphene as a 2D material.

"What if you look at it as a molecule without the bulk, and consider how surface chemistry or indeed all chemistry could be applied to it. You have this material that is like an atomic-scale scaffold to which you can apply materials in a prescribed manner," Geim explains.

"For example, there is a hypothetical material called graphane. It is the graphene where a hydrogen atom sits on top of each carbon atom. And it's a completely different material from graphene in one key respect: it's a semiconductor. So, imagine the possibilities that starts to open up. And there is a whole field of opportunities there this kind of chemical alteration which could allow you to achieve properties better then for the graphene itself."