Monday, March 24, 2008

Structural Coherency of Graphene on Ir(111)

Structural Coherency of Graphene on Ir(111)

Johann Coraux, Alpha T. N'Diaye, Carsten Busse, and Thomas Michely

Nano Lett., 8 (2), 565 -570, 2008

This paper is a great lesson in reading papers in your area that you don't really think have much to do with your research. To tell you the truth, I have no interest at all in iridium surfaces, nor in surface morphology of graphene layers. But I'm really glad I read this paper.

It's generally accepted (I thou
ght) that the first useful graphene synthesis was figured out in 2004, with Novoselov and Geim using Scotch tape to obtain a very nice and highly cited paper (621 citations in less than four years, which is more than most of us will get in a lifetime). Other commonly seen methods are the pyrolysis of silicon carbide and the oxidation of graphite.

Apparently, another method to make graphene is to use chemical vapor deposition to deposit, then thermally degrade ethylene (or other carbon-rich) gas onto a metal surface. The idea is that if you put a lot of carbon together in an atomically thin film and heat the hell out of it, the lowest energy conformation might be graphene. If this slightly hair-brained idea could work to make nice quality films, one
would think that it would make a huge impact on the field, and that whoever came up with it would be lauded in the pages of Science and Nature. One would be wrong, however; single-layer graphitic islands (alas, not pretty films) were deposited on platinum in a 1992 paper in Surface Science, and further work has been done on a variety of different metal surfaces in 1997, 1998, 2001, 2004, and onward. Since it blows my mind that someone had figured out this graphene stuff in '92 and got minimal credit (they are cited offhand by Novoselov and Geim), I'm going to do some more investigation before I stick my foot in my mouth. Look for more on these papers in the future.

Anyway, this paper takes a look at the surface morphology of films grown using CVD/thermal decomposition of ethylene on Ir (111). After growing the films, the authors examine them primarily using STM and get some really nice pictures. Here's what graphene grown on iridium looks like under STM:
Image (a) shows the graphene layers built upon the step edges of the iridium; image (b) is a close-up of the graphene layers, where the dark dots are the center of the aromatic rings. The
unusual-looking striped area in image (b) looks like that because of the Moiré effect (dizzying demonstration here), and has a lot to do with matching between the Ir (111) and graphene lattices. In addition to the pretty pictures, the authors find that the step edges seen in image (a) are more rolling hills than steep cliffs. Like a blanket, the graphene curves to fit the steps on the surface of the metal, and the angle of that curvature is mysteriously close to that of carbon nanotubes.

Such a nice looking film must have a dark side, and indeed, there are some small defects. Occasionally the authors see the graphene stop at the step edges (in other words, not acting like the blanket above). There are also a small number of five-membered fullerene-style carbon rings in the structure, which messes up the orientation of the lattice and gives distinct domains.

For me, this paper opened me up to an old synthetic method for graphenes which I should have known about. It's analogous to Mullen-style dehydrogenations, which I'm interested in, but I haven't seen this method mentioned elsewhere. In addition to relieving my ignorance, the authors also show they can make very large, very nice continuous films from this method, in a manner which lends itself well to scaling up. Even the defects they find (misorientation, etc.) are less serious than defects typically found in graphene made from graphite oxide.

Coraux, J., NDiaye, A., Busse, C., Michely, T. (2008). Structural Coherency of Graphene on Ir(111). Nano Letters, 8(2), 565-570. DOI: 10.1021/nl0728874