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Graphene: miracle material?

There have been a lot of posts on the mainpage recently extolling the various amazing properties of graphene. And on each of them, there's always at least one astute reader saying, "if graphene is so amazing, why hasn't it had any impact on my day-to-day life so far?" Which is a completely valid question that is rarely addressed by science journalists because it's not nearly as exciting as the next big discovery. However, in the interest of completeness, someone ought to write about graphene's problems. And that someone is me.


There are two major obstacles that need to be overcome in order to realize graphene electronics on the scale that many journalists (and lets be honest, graphene researchers) like to envision, and I'll discuss them both a bit here: (a) lack of a band gap, and (b) lack of a reliable large area production technique for high quality graphene.

(a) Lack of a band gap

A band gap in a material is an energy range where no electrons can exist. It is a major determining factor in a material's properties. Large band gap materials are insulators, small band gap materials are semiconductors and materials without a band gap are conductors. But graphene is none of these. Graphene has a vanishingly small gap at what is called the Dirac point. So all this talk about super-fast, super small graphene transistors and it turns out it's not even a semiconductor! It's actually classified as a semi-metal.

A band gap is crucial in digital logic applications because it's the energy region in which the "off" or "0" state occurs. There have been a number of attempts to open a band gap in graphene with varying degrees of success. One of the most successful strategies has been to cut the graphene sheets into very narrow ribbons, but the opening of the band gap leads to a simultaneous decrease in device speed such that by the time a reasonable size gap has been opened, the graphene isn't really performing any better than silicon. Other ideas have been to add other molecules to the graphene surface or to strain it, but so far no one has achieved single layer graphene with a band gap that doesn't suffer major performance losses.


(b) Producing large area graphene

One of the great things about graphene as compared with carbon nanotubes (which have almost equally impressive electronic properties) is its structure. It's a nice uniform flat sheet which means that if we could make it big enough, it could be processed using all the technology we've already developed and built for silicon electronics. Unfortunately, that has turned out to be a pretty significant "if".


The best way to get really high quality graphene is still the Scotch tape method developed for the very first graphene experiments a decade ago. Unfortunately, it yields tiny, randomly located pieces of graphene amidst a field of thicker chunks of graphite. Fine for lab experiments, but useless for commercialization.

Instead, researchers have turned to catalyzed growth of graphene from carbon gases. This method can indeed yield graphene over large areas and on any surface. It undeniably has a lot of potential, but the graphene grown so far still turns out to be much lower quality than that produced by the tape method. (Figure reprinted by permission from Macmillan Publishers Ltd: Nature, 483, S32–S33, copyright 2012)


Other research has looked at solution-based processes using graphene oxide or other modified graphenes, or that soapy blended nonsense everyone got so excited about last week. But the modifications required to get graphene to dissolve in solution are generally the same ones that lead to decreases in its quality.


Now, I don't want to give the impression that graphene is a lost cause. It is easy to forget with the constant media coverage of graphene breakthroughs and the (in my opinion premature) award of the Nobel Prize for its discovery, that graphene as a field is only 10 years old! In that light, what has been achieved so far is actually quite remarkable and the absence of a lot of commerical applications is not all that surprising. Someone's going to find a way to make it on a large scale, and someone is going to open a band gap, or design a new type of transistor perfectly suited to graphene's bizarre conical bandstructure. We just need to be a little patient.


For the record, as a graphene researcher, my money is on transparent electrodes as the first really practical graphene application. They'll find use in things like flexible solar cells or foldable smart phones.

If you want to read a bit more on these topics, this review by Frank Schwierz has a lot of information about the challenges associated with graphene transistors and this Nature Outlook covers some alternative production techniques.


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