Wednesday, April 2, 2008

Highly oriented monolayer graphite formation on Pt(111) by a supersonic methane beam

Highly oriented monolayer graphite formation on Pt(111) by a supersonic methane beam.

Hirokazu Ueta, Morihiko Saida, Chikara Nakai, Yoichi Yamada, Masahiro Sasaki, and Shigehiko Yamamoto

Surface Science, 2004, 560(1-3), 183-190.

Ok, so really I just wanted to post this because the authors use a supersonic methane beam. I mean, are you kidding me? Supersonic? Methane? Beam? I want one, even if all it does is leave a thin layer of graphite upon mine enemies.

So let's cut to the chase- the background portion of this paper mentions that "single layer graphite" is made by thermal decomposition of hydrocarbons on a metal surface (usually using CVD)- I posted on this before, and there are supposedly papers going back to 1975 that have done similar things. This is how one makes the fancy graphite used for "Scotch tape" graphene exfoliation, and closely related techniques are used to make carbon nanotubes. The authors of this paper mention that the graphene layers made by this method are often quite heterogeneous and that other carbon structures are present, making characterization difficult. Their solution is to use kinetic instead of thermal energy to convert hydrocarbons (methane) into graphene.

So, a quick word on their setup: they shoot methane onto a substrate that's connected to a mass spec, then take the substrate out and take some STM images. Here's their schematic of the device:

So now you can go home and build one yourself. After blasting their platinum substrate with methane, they look at it with STM (using the local tunneling barrier height technique, LBH, which I'll skip for now) and, as expected, find a Moire pattern, suggesting few- or single-layer graphene. They make two major finds:

First, their domain size is much larger than in contemporary (2004) thermal graphitization techniques, meaning they get a more homogeneous film. For this to happen, the graphene layers must be reasonably mobile on the surface. Graphene made with higher kinetic energy methane was the most homogeneous.

Second, they imply that their graphene layers actually grow over the step (like a blanket on a pillow; a similar effect was found in the previously mentioned post) instead of stopping and starting on steps.

What does these two things have in common? The authors suggest that the mobility of the graphene layers and their independence from surface features mean that the film doesn't interact very much with the metal surface, both during graphitization and afterwards. This is a good thing, since surface defects won't be translated into the final product, and the smaller interaction may also give different electronic (or catalytic) properties. The authors finish with a discussion of how the kinetic energy they use effects the film, and also make a comment about how the graphene must be distorted to give a lower-energy conformation (this seems a bit dated), but we'll wrap it up knowing that our graphene just has a lower interaction with the metal surface.

All in all, the graphene layers made here are not very uniform (compared to current 2008 techniques) and probably won't be used for industrial purposes. It's still a nice paper that uses a technique equal in coolness to sharks with laser beams on their heads.

UETA, H. (2004). Highly oriented monolayer graphite formation on Pt(111) by a supersonic methane beam. Surface Science, 560(1-3), 183-190. DOI: 10.1016/j.susc.2004.04.039

Tuesday, April 1, 2008

I Need a Physicist

I have a confession to make- I'd like to know some graphene physics.

I can't tell a Dirac fermion from a Klein paradox.

I don't know about the ambipolar field effect or the K' point of the graphene band structure.

I'm not sure why one would need a low interface trap density, and if I was trapped in the Brilllouin zone, I'd never get out.

Can anyone recommend any good resources for this information, written at the level of a knuckle-dragging organic chemist? I've been unable to find any good primers on the subject, and every introduction I read names some effect I have no idea about. If anyone with some knowledge in the field would like to guest post (or open their own blog), that would also be great. You can leave feedback in the comments, or email me if you have any information as to the whereabouts of my physics blindspots.

A study of graphenes prepared by different methods: characterization, properties and solubilization.

A study of graphenes prepared by different methods: characterization, properties and solubilization.

K. S. Subrahmanyam, S. R. C. Vivekchand, A. Govindaraj and C. N. R. Rao

J. Mater. Chem., 2008, 18, 1517 - 1523

This paper is from a special theme issue in Journal of Materials Chemistry on carbon nanostructures; if you're in to that sort of thing, check it out here. There's also a feature article by organic "graphenes" guru Klaus Mullen in there that I'll be reviewing soon, but you should go ahead and read it yourself since it's pretty nifty. I love the concept of the paper we're reviewing today; it's a head-to-head comparison of graphene made by different methods. Unfortunately, two of the three methods they use are pretty obscure, and the authors do not test "Scotch-tape" style graphene. Fortunately (and accidentally), I've already reviewed the relavent methods.

The paper evaluates graphene made by the dreaded camphor method (they call this CD), the well-respected method of oxidizing graphite to graphene oxide (EG, since they make a big deal of exfoliating it once it's oxidized; to me that's too easy to confuse with our Scotch tape method), and the circa-2003 method of thermally converting nanodiamond (they call this DG). I'd like to point out that that last sentence is more self-referential than most Wikipedia articles, and I don't care who knows it. The authors also mention graphene made from arc evaporation of SiC, but only take TEM images of it.

Anyway, this is an amazing amount of data, so I'll just break this down by characterization method and skip the barely-mentioned TEM.

X-Ray Diffraction (XRD): Gives number of layers (two of the samples gave two different sets of layered structures) and size of the crystallites:
CG: 51 layers, 6.1 nm crystallites
EG: 3 and 16 layers, 4.7 nm crystallites
DG: 6 and 87 layers, 5.0 nm crystallites

AFM: Also number of layers:
CG: 20
EG: 3-6
DG: 3-6

Raman: Gives multiple sizes of crystallites, in nm:
CG: 7, 10, 12
EG: 4, 6, 7
DG: 3, 4, 5

Magnetic susceptibility, in emu g^-1:
CG: No (publishable) data
EG: -3.5*10^-6
DG:-4.4*10^-6 (both EG and DG show Curie behavior, which I can't find a good summary for)

TGA: supposedly gives the oxidation temperature, but I think in the graphene oxide (EG) case it might better reflect the decarboxylation temperature (in degrees C):
CG: 730
EG: 520
DG: 700

Surface Area (in m^2 g^-1)
CG: 46
EG: 925
DG: 520
Lit value for single-layer graphene: 2600

Hydrogen Uptake (at low pressure and temp, then high pressure and temp, in wt%):
CG: No data
EG: 1.38, 3.1
DG: 0.68, 2.5 (these are comparable with carbon nanotubes)
Department of Energy Target: 6.0

CG: acts like basal plane of graphite
EG, DG: better kinetics than CG

Supercapacitor measurements, in F g^-1:
CG: No data
EG: 117 (supposedly good)
DG: 35

Then they chemically modified these things with the nitric/sulfuric treatment or with an amidation treatment to make them more soluble.

Well, there you have it folks. I'm frankly blown away that they put all of this data into one paper, and although I wish they would have included the epitaxial "Scotch tape" graphene, it was interesting to see that the nanodiamond (DG) sample had properties similar to the graphene oxide. If you're making graphene and want to see how your samples compare- here's a whole battery of tests to match up to. Expect this paper to get a lot of citations in the future.

Also, the camphor graphene pretty much sucked. Told you.

Subrahmanyam, K.S., Vivekchand, S.R., Govindaraj, A., Rao, C.N. (2008). A study of graphenes prepared by different methods: characterization, properties and solubilization. Journal of Materials Chemistry, 18(13), 1517. DOI: 10.1039/b716536f

Defect formation in graphene nanosheets by acid treatment: an x-ray absorption spectroscopy and density functional theory study

Defect formation in graphene nanosheets by acid treatment: an x-ray absorption spectroscopy and density functional theory study.

Coleman, V.A., Knut, R., Karis, O., Grennberg, H., Jansson, U., Quinlan, R., Holloway, B.C., Sanyal, B., Eriksson, O.

Journal of Physics D: Applied Physics, 2008, 41(6), 062001

And now, for something completely different. You might remember when I griped about a theoretical paper called "graphene nanoribbons with chemically modified edges"; apparently, those authors weren't alone. Many physicists (or P-Chemists) were excited enough about broken graphene to do a lot of calculations on the subject, since this may give metallic as opposed to semi-metallic properties (or something). Our current paper details an attempt to actually put holes in graphene sheets and check out a variety of the properties.

Let me begin by saying that the authors use a form of graphene I haven't encountered before, which they call carbon nanosheets (CNS). They use "radio frequency plasma enhanced chemical vapor deposition" to get a structure best described by the picture from the text:

The authors claim (or imply) that this structure will be an adequate analog for actual graphene sheets, and proceed to treat it with HCl for a few hours. They then did the same treatment on another sample using distilled water instead of HCl and took some SEM images to make sure they still had carbon nanosheets. The authors moved on to analyze the sheets with X-ray absorption spectroscopy (XAS, pdf summary from the University of Calgary here) which, in analogy to XPS, gives information on what kind of bonds are present in the sample (pi* C-C, sigma* C-O, pi* C=O, etc.). The authors find that, compared to a control, their acid-treated samples have a significantly higher peak that they ascribe to C-O bonds and a lower peak in the area attributed to C-C pi* and sigma* bonds. They theorize that the acid broke C-C bonds, which were replaced by hydroxyl groups, and back this up by mentioning the higher wettability (read: lower hydrophobicity) of the film and by XPS measurements showing a much higher proportion of C-O bonds (we've seen this before).

After establishing that acid does indeed add oxygen (and probably hydroxyls) to their films, the authors start doing some calculations of broken graphene layers (essentially calculating the situation where some C-C bonds are absent, which they say gives the same results as having pendant hydrogens where the bonds broke). I'll spare myself the details, but the energy states they find are consistent with their XAS measurements, and the calculations predict a metallic state in the graphene. They conclude that although they did not attempt to measure any metallic properties in the graphene, their calculations are probably valid since they correctly predicted their XAS results.

Maybe I'm missing something, but treating graphene/graphite with acid and finding that it oxidized isn't exactly new. The XPS and XAS data presented look very close to the relevant data for graphene oxide, and although it's surprising that graphene can be oxidized without a strong oxidant like HNO3, there's no reason to think oxidized defects introduced in this manner will be different than oxidized defects induced by more conventional acids. And although I'm out of my league here, I don't understand how they would get away with modeling the defects as simply breaking the C-C bonds (or using hydrogens), instead of having electron-donating hydroxyl groups present.

All in all, Coleman et al. did a good job communicating what they did, and I'm always glad when people try to test theory with wet lab research. These defects will have a huge role in modulating the electronic properties of graphene, and more work needs to be done on the subject. It just seems like the end result (and a lot of hard work, I'm sure) went into a study that told us what we already knew.

Coleman, V.A., Knut, R., Karis, O., Grennberg, H., Jansson, U., Quinlan, R., Holloway, B.C., Sanyal, B., Eriksson, O. (2008). Defect formation in graphene nanosheets by acid treatment: an x-ray absorption spectroscopy and density functional theory study. Journal of Physics D: Applied Physics, 41(6), 062001. DOI: 10.1088/0022-3727/41/6/062001