Wednesday, March 12, 2008

Few-layer graphene on SiC, pyrolytic graphite, and graphene: A Raman scattering study

Few-layer graphene on SiC, pyrolytic graphite, and graphene: A Raman scattering study

C. Faugeras, A. Nerrière, M. Potemski, A. Mahmood, E. Dujardin, C. Berger and W. A. de Heer

Applied Physics Letters 92, 011914 (2008)

DOI: 10.1063/1.2828975 (EDIT: commentator Clement pointed out this paper is available free from here)

And now, a characterization paper. This paper is partially written by Walter de Heer, who pioneered the method to make epitaxial graphene. His method is to heat SiC to stupid temperatures to make it grow graphene; however, one ends up with multi-layers of graphene (5-10 at least, and sometimes more). Since graphene (a single sheet of sp2 carbons) has very different properties than graphite (stacks of these single sheets), a multiple layer "graphene" might in fact act more like graphite than graphene. The purpose of this paper is to use Raman scattering to probe the differences between highly ordered graphite, few layer graphene (FLG), and single layer graphene made with the Scotch tape exfoliation method.

I'm not an analytical chemist, so we'll take the black box approach to Raman spectroscopy. According to the paper, the raman spectrum of graphite looks like this:

See the shoulder on the D' band, around 2600 cm-1? That multiple component peak can roughly be traced back to a multilayer electronic structure; a peak there without a shoulder implies a graphene monolayer. The other two peaks don't give us much to compare with.

Well, I guess you see where this is going; they're about to give us a graph that shows us that their beautifully made products give a D' band with a single component, somehow implying that their multilayer structure acts, at least electronically (where it counts) like a single layer of graphene. Congratulations, you guessed right!

In this figure, we're not worried about the broadness of the peaks at 2650, but rather whether or not they have a shoulder on them. Plots b and c are different samples of 5-10 layer thick graphene; plot d is 70-90 layers thick. You'll notice that the peaks look about the same, and all three have a single component. The authors think this implies that their epitaxial FLG has very similar electronic properties to the exfoliated single layer, which would neutralize a very large problem with their preferred method of making graphene. How can you have multilayer graphene that acts like single layer graphene? They rationalize this by saying that their multilayers of graphene are not stacked flat on top of each other as in graphite, but are instead touching at a variety of angles in a manner analgous to something called turbostatic graphite. This eliminates pi-pi stacking and therefore electronic communication between the sheets, approximately giving us a large pile of independent graphene sheets.

One small problem with the figure is that the peaks are slightly offset. The single layer exfoliated graphene peak is at 2641 cm-1, while the epitaxial peaks vary between 2655 and 2665 cm-1. The authors don't seem too concerned with this, but say that it has something to do with a difference in the Fermi velocity of Dirac cones in the system. I have no idea what a Fermi velocity or a Dirac cone is, but since those words come up all the time in those physics papers I was griping about, it seems like that could be a serious problem.

Here's the quick summary: a muddled peak in the Raman spectrum means you have graphite, while a straight peak means you have graphene. Epitaxial graphene has a straight peak, so it must really be graphene. This paper seems to be very careful about saying that the two materials' electronic properties might be similar based on this data, but that when taken together with other papers (which I haven't read), they say that there's a good chance that you can use their graphene product to make graphene devices.

By the way, does anyone know if I can get sued for using the images from a paper? To me it seems to be fair use, since I'm using only a small part of their work in order to discuss it academically (and maybe journalisticly).

Faugeras, C., Nerrière, A., Potemski, M., Mahmood, A., Dujardin, E., Berger, C., de Heer, W.A. (2008). Few-layer graphene on SiC, pyrolitic graphite, and graphene: A Raman scattering study. Applied Physics Letters, 92(1), 011914. DOI: 10.1063/1.2828975


clement said...

for this paper in particular, you can find all the figures free of copyright on ARXIV

Rob W said...

Thanks- that looks like a quite handy site. Open source is always nice.