I've been wanting to write about graphene for a little while now, both because it's what I work with in my real-life job and because there seems to be some appetite for it among the general public. It always makes me proud to think that what I'm researching may be of interest beyond my co-workers and myself. But graphene is already pretty well covered in the mainstream press, and I don't want to bore anyone by rehashing what's already been said. So I thought I'd choose a few lesser known aspects of the material and see how that goes. The flip side, of course, is that the less well-known areas may be that way because they're just mind-numbingly boring to the average reader. We will find out.
So, back to the question in my title. Have you ever thought about this? Graphene is one atomic layer thick: that is almost unfathomably thin in relation to everyday things (think like 200,000x thinner than a coat of paint). And yet I can see it (the image at the top there is one of mine) with just a standard light microscope. How can this be?
Let's take another step back, to the Nobel Prize ceremony in 2010, where Andre Geim and Konstantin Novoselov won the Physics prize "for groundbreaking experiments regarding the two-dimensional material graphene". While we tend to think of this groundbreaking work as being the discovery that graphene can be made with regular adhesive tape, or that it displays the electric field effect, there was an additional component to their work: they discovered how to see it.
Graphene, in tiny portions, is not all that hard to make. You probably make minute quantities of it every time you write with a pencil, or when you hook one way left into the woods and break your graphite-shafted driver over your knee in a fit of rage. But these pieces are so small and hidden within so many other thicker pieces of graphite, you'll never find them.
If you foolishly decided to give it a shot though, prior to Geim and Novoselov's work in 2004, you'd be looking for them with an Atomic Force Microscope (AFM). An AFM measures topography by, essentially, dragging a very fine tip over the surface of a sample and monitoring as it goes up and down (AFM is of course way more complex and interesting than this, but I don't want to go into it here: wikipedia can fill you in). This process, as you might imagine, is really slow. You don't see an image all at once as in a microscope — you generate it pixel by pixel. So now you're looking for your impossibly small, buried pieces of graphene using an instrument that can maybe scan a 100 micron area in an hour. The graphene might as well not be there.
So what was the solution? It turned out to be the substrate. When the graphene pieces were distributed on a silicon chip with a very specific thickness of silicon dioxide (SiO2) on the surface, they were visible in a regular light microscope. How does this work? The trick is to take advantage of interference effects. When light hits an interface (air-graphene, graphene-SiO2, SiO2-Si), some of that light is reflected back and some is transmitted through, with the ratio of each dependent on the optical properties of the material. In this particular system, there are a lot of interfaces and thus many possible pathways for the light, some of which are shown in Figure 1.
The actual observed reflected beam is the combination of all of these pathways and is not trivial to predict. However, computational physicists have put together models that allow for the selection of the SiO2 thickness to optimize the contrast between the light that hits the graphene and the light that hits the surrounding oxide surface. Having wasted a good 6 months of research being unable to find any graphene before discovering that our SiO2 layer was 10 nm thicker than expected, I can assure you that this stuff is very important and very sensitive.
So to conclude with an answer to my original question: graphene is invisible most of the time, and it took Nobel Prize-winning research to reveal it.
The information above is gathered from a number of sources. If you want to read further, or are strangely interested in actually seeing the models used to predict oxide thickness, they are available here:
Jung, I. et al. Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets. Nano Lett., 2007, 7, 3569.
Roddaro, S. et al. The Optical Visibility of Graphene: Interference Colors of Ultrathin Graphite on SiO2. Nano Lett., 2007,7,2 707.
Blake, P. et al. Making graphene visible. Appl. Phys. Lett., 2007, 91, 063124.
Novoselov, K.S. et al. Electric Field Effect in Atomically Thin Carbon Films. Science, 2004, 22, 666.