Published on August 27th, 2012 | by Sara Callori0
The skinny on graphene: a quick introduction
Graphene, a material made of single layers of carbon atoms, is arguably one of the solid state physics’ hottest topics. Many people have heard of it, especially since the first scientists to isolate it were awarded the 2010 Nobel Prize in physics. The media often touts graphene as the future of computing or a wonder material destined to change technology, but rather than get caught up in the hype, let’s look at some of graphene’s basic physics.
What is it?
Graphene is a form of carbon where the atoms are arranged in a repeating hexagonal pattern formally called a two dimensional honeycomb lattice. You are probably familiar with its cousin, graphite, which you observe when you write with a pencil; but the big difference is that graphite is made of layers of thin honeycomb lattice stacked on top of each other, whereas graphene is a single layer of this material. This means that a piece of graphene is like a piece of paper, wide in width and length, but very thin, 0.335 nm thin to be exact.
How do you get some graphene?
Since the field x-ray crystallography began, physicists knew about graphene. But it always occurred in other materials and never as isolated layers. Single layers of graphene weren’t produced until 2004. You might think that was because scientists needed higher-tech developments to obtain graphene, but actually the solution to getting single layers depended on a common, every-day product: adhesive tape.
The carbon molecules in graphite have bonds of different strength depending on their direction. So bonds between carbons in the in-plane graphene sheets are very strong, but the bonds connecting the layers are relatively week. So physicists Andrei Geim, Konstantin Novoselov, and their colleagues tried using tape to lift up layers of graphene from a larger piece of graphite. (Fun fact: Geim actually signed a tape dispenser and donated it to the Nobel Museum). This process involves repeatedly “peeling off” thinner and thinner layers from graphite until you are left with single-layer graphene, which can then be deposited onto a substrate.
There are currently other techniques to make graphene (enough to fill an entire article) but the “scotch tape” method is noteworthy as both the first to isolate single layer graphene as well as being a very accessible laboratory technique.
If graphene is so thin, how do you know it’s there?
Different research groups have different methods of producing and identifying graphene, but one common way is simply by looking at the sample under and optical microscope. While a single layer of graphene may be too small to see with the naked eye, under certain wavelengths it can produce an optical contract different than that of the substrate it is deposited on or other materials, such as bi-layer graphene, graphite, or any contaminants. Once physicists see a plausible graphene signal optically, they can then use other microscopy techniques, such as scanning electron microscopy or atomic force microscopy, to check that they are indeed looking at a single layer of the material.
Ok…I get that graphene is thin, but what kind of properties does it have?
Graphene has a lot of interesting properties, and the more research done, the more physicists find they can do with this material. But for now, let’s take a look one of the material’s signature properties that may lead to it being a contender for a material capable of changing technology.
A very important feature of graphene is the interactions between neighboring atoms. When this interaction is modeled it produces electronic levels shown below.
Briefly, the curve above the plane at zero energy (red in the right blow-up) is called the conduction band and the one below the plane (blue in the right blow-up) is the valence band. The zero energy plane is the Fermi level, which shows where electrons sit when they are in graphene’s ground state. In normal metals, the Fermi level usually crosses the conduction band, which means the electrons are free to “move around” and therefore the material is conducting. In non-metals, the Fermi level crosses the valence band, which means those electrons aren’t going anywhere and the material is insulating. But for graphene, not only do the conduction and valence bands touch, but the Fermi level is right between them. This means that, with a little added energy (say by applying an electric field via a voltage), the electrons in graphene can easily become conducting.)
In addition, the speed of the electrons in a material is related to the curvature of the electronic bands. Since in graphene the bands are very sharp where they meet, they have a very high curvature and the electrons should be able to move very fast. They are calculated to move at around 10^6 m/s, or only one hundred times slower than the speed of light. If this were used in electronics, it would mean ultra-fast signals and data processing. Unfortunately, there are still a lot of experimental hurdles to cross to achieve these electron mobilities. They include dealing with graphene-substrate interactions, finding reliable ways to make ultra-pure graphene, and eliminating contaminants, all of which can cause electron mobilities to plummet. But don’t let this get you discouraged. Many physicists are out there trying to resolve these exact issues.
That’s not all this wonder material can do. Some potential uses are as a component in gas sensors, alcohol/ethanol filters (seriously), and energy storage. And there’s still a lot of basic physics work to be done, too. So keep an eye out because graphene will be making scientific headlines for a long time to come.
Header image: Workie (Flickr)