04/16/2008
Speedy Electrons in Thin Carbon
Last week’s column dealt with asthma, nitric oxide and carbon single-walled nanotubes, SWNTs. I had planned to discuss another form of carbon, graphene, but realized that this material deserved a full column to itself after reading an article titled “Carbon Wonderland” by Andre Geim and Philip Kim in the April 2008 issue of scientific American. Geim and Kim are professors at Manchester University in England and Columbia University, respectively, and both have been in the forefront of research on this remarkable material.
What is graphene? Let’s look at some graphite, the stuff in your “lead” pencil. A bit of history – it wasn’t until the 1500s that the English discovered a large deposit of pure graphite, then known as “plumbago”, Latin for “lead ore”. The “lead” ore was quickly recognized as being useful for making marks on things and the lead pencil was born. I find it astonishing that it wasn’t until 1779 that a Swede, Carl Scheele, proved that plumbago was carbon, not lead. Later, a German, Abraham Gottlob Werner, coined the name graphite for the material, after a Greek word meaning “to write”.
But I digress. We’ve discussed before how graphite consists of layers of carbon only loosely held together by a weak force known in the trade as the van der Waals force. We also have talked about how this van der Waals force is involved in the gecko’s ability to walk on ceilings. If we look at the layers of carbon in graphite we see that the carbon atoms are arranged in a hexagonal structure that looks like chicken wire. Last week we noted that carbon nanotubes have this same hexagonal chicken wire structure rolled up into a tube. Unroll a single-walled carbon nanotube and you have a flat layer of hexagonally arranged carbon atoms just like a single layer of carbon in the graphite structure. The ultimate single layer of graphite only one atom thick is called graphene.
So, how do we make graphene? We probably all have made bits of graphene without knowing it whenever we’ve written on a sheet of paper with that “lead” pencil. The graphite layers that slide off the graphite in the pencil may include some bits of layers that are indeed only one atom thick. Being so thin, graphene is transparent and, even with a high-power microscope, bits of it would be difficult to spot amongst the debris of graphite left on your paper.
So, let’s try a bit more sophisticated type of writing – let’s make a “nanopencil”, as did Philip Kim and a graduate student at Columbia. We’ve talked in the past about the atomic force microscope in which a very small sharp tip of a material is more or less dragged across a surface and in the best case actually reveals the structure of the surface atom by atom. In this case, let’s make the tip out of graphite and write on the surface of a silicon wafer. Under the right conditions, what happens when we write lightly on the wafer is that rows of little “pancakes” of graphene are wiped off onto the silicon surface. Actually, when Kim and Zhang did it the pancakes were not really graphene but graphite a few tens of layers thick.
It was deemed highly unlikely that a single atom thick layer of graphene could be achieved and that it would be stable. However, in 2004, Geim, working with postdoc Kostya Novoselov and co-workers at Manchester, went at the problem in a much less sophisticated way. They just took some debris left over after splitting graphite into flakes and sandwiched the flakes between folded plastic adhesive tape and pulled the tape, cleaving the flakes in two. They repeated the process and the flakes got thinner and thinner. Finally, examining their handiwork, they were amazed to find some pieces that were only one atom thick. Not only had they made graphene but the pieces of graphene were chemically stable at room temperature. They found the graphene to be strong and stiff.
It’s amazing that these intrepid scientists were able to make and work with any material only an atom thick. But there’s more. It turns out that electrons travel faster in graphene than any other known material. Whoa! This property makes physicists and electrical engineers drool over the possibilities of making graphene transistors or other devices. We’ve talked about Moore’s law and the fact that transistors are reaching such small sizes that piling more and more transistors on that silicon chip may be reaching a fundamental limit. Could graphene, only an atom thick with its great electrical properties, extend Moore’s law another decade or more?
Warning! Here’s where things get rough and I don’t pretend to understand what follows, namely, the concept of “tunneling”. Actually, I should be ashamed of the preceding statement inasmuch as I once was an expert in making materials related to a device known as the Esaki tunnel diode, named after its inventor, Leo Esaki. …….(Sorry, I just spent an hour reading a paper I published on the subject back in 1960 and was carried away by how much I knew then and how little I know today!)
Tunneling. If you’re driving your car and come to a hill, it takes energy in the form of burning your precious fuel to climb the hill. Going down the hill you get part of that energy back, especially if you coast down the hill. In physics the “hills” are energy barriers. When an electron come to one of these barriers, the weirdness of quantum mechanics comes into play. The electron doesn’t necessarily have to climb over that barrier to get to the other side. Good old Heisenberg’s uncertainty principle says that you can’t pin down the location of an electron precisely and there’s a good chance (more than zero and less than 100 percent) that it’s on the other side of the barrier; that is, it can “tunnel” through the barrier. Obviously, your car can’t tunnel through that hill unless you’ve brought along some sort of borer and extra fuel to power it.
Now take the case of our graphene and the speedy electrons. Apparently, graphene is such a wondrous material that it’s a new ball game when it comes to tunneling. Notice in the preceding paragraph that I said the chances of the electron tunneling to the other side are less than a hundred percent. It seems that in graphene the electrons are traveling so fast that Einstein comes into play and the electron becomes a “relativistic” particle. According to the Scientific American article, it also seems that physicists have for years wanted to find some means of testing the validity of the “Klein paradox”.
Given the right conditions, when one of these relativistic particles encounters a potential energy barrier the Klein paradox is that this particle can tunnel through the barrier no matter how high or how wide. The tunneling probability is 100 percent – “perfect” tunneling. Again according to the article, physicists had thought such quantum effects would only be observable in such places as black holes or very high-energy particle accelerators. Now it seems as though the high electrical conductivity in graphene may be due to this perfect tunneling; when the electron comes to the barrier it just sails right through.
So much for the latest chapter on the wondrous world of carbon. I’m sure there will be more. Meanwhile, it’s back to care giving. For those interested in the latest news on my wife, we’ve just been to a knee replacement surgeon and it looks like a single or double knee replacement is in the offing!
Allen F. Bortrum
|