Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets

Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets

Inhwa Jung, Matthew Pelton, Richard Piner, Dmitriy A. Dikin, Sasha Stankovich, Supinda Watcharotone, Martina Hausner, and Rodney S. Ruoff

Nano Letters, 2007, 7(12), 3569-3575; Free (but not as pretty) at arXiv.

Being able to correlate film thickness with film color is a pretty big deal in the microelectronics field. Conventional lithographic/microfab techniques make use of the pretty colors that SiO2 gives at different thicknesses; although you can roughly tell the thickness of SiO2 films within 25nm using naked eye and a color chart, an ellipsometer is usually used to give a more precise measurement. Likewise, one can sometimes tell the thickness of other films on top of SiO2 through the same optical method.
Since the graphene field is very interested in the number of layers (or thickness) in a film, this approach has seen wide use since 2004. It's definitely a lot quicker and cheaper than AFM or SEM; unfortunately, for single layers of graphene, the color change for such a thin film is very small. This paper attempts to optimize the properties of the underlying SiO2 (and silicon nitride) to give the highest contrast for thin graphene oxide and graphene films.

The authors start by doing some math, which you can read the paper for; basically they try to optimize the amplitude of a reflected beam of light (at a certain thickness) by fooling with the wavelength of light used, it's incident angle, refractive indices of everything, and absorption coefficients. After they got these factors all dialed in to get a good angle and three good wavelengths to try, they put some graphene oxide layers on SiO2 (using methods described before) and took nice pictures (second column, below); then they applied heat to make the films more graphene-ish and took nice pictures of those (third column, below), using three different wavelengths (a, d, g):


CBC-style pretty pictures, although the colors are actually fake since these are just contrast maps. Don't let that distract you from the beauty of this work; they also did this on silicon nitride to give similar looking contrast maps, then tried the same approach with graphene (they don't mention which graphene they use; I'm guessing scotch-tape graphene).

What does all this mean? I think it means that they've tweaked the parameters for silicon oxide and silicon nitride layers to visualize overlying layers of graphene oxide and graphene, and they've also shown instances where the contrast will change after thermal treatment (implying that this technique can be used to quantify the reduction of graphene oxide to graphene).




Jung, I., Pelton, M., Piner, R., Dikin, D., Stankovich, S., Watcharotone, S., Hausner, M., Ruoff, R. (2007). Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets. Nano Letters, 7(12), 3569-3575. DOI: 10.1021/nl0714177

Great Place for Free Papers

One of the few commentators on this blog (Clement, much thanks) pointed out that many papers on graphene, particularly if they have to do with the physics side, are available free of charge on arXiv, a database originally set up in Los Alamos and now hosted by Cornell. Not all papers from major journals are in there, and not all papers in there have been submitted to a journal, but it's a good place to check if you're interested in the literature but don't have a major research university backing your expensive hobby. Not sure how it's legal, but I'll take it.

To demonstrate how nice this database is, Dr. Geim submitted his and Novoselov's landmark paper, as well as many of their other papers. I was planning to do an actual post on that paper, but I've already written a (dumbed down for my sake) summary of the synthetic method (scotch tape) and the rest of the paper is device physics and electronics, which I know very little about. I'll just add that this paper started off the graphene field not because it found the very first single-layer (or few-layer) graphene, but because it was the first to measure the field effect response in them. It finally gave a method by which all of these amazing predicted properties of graphene could be probed, even if it didn't give films good enough for commercial use. The other breakthrough that makes this paper great is that it gives a way to roughly correlate the color of the graphene film on SiO2 to the number of graphene layers present.

Experimental evidence of a single nano-graphene

Experimental evidence of a single nano-graphene

A. M. Affounea, B. L. V. Prasada, Hirohiko Satoa, Toshiaki Enoki, Yutaka Kaburagib and Yoshihiro Hishiyama

Chemical Physics Letters, 2001, 348(1-2), 17-20.

Although the cornerstone paper in the graphene field came out in 2004, scientists and engineers have been trying to make graphene for a while. This 2001 paper summarizes a nifty trick to (possibly) make some low-quality graphene islands on top of graphite, but fails to measure any of the properties of graphene.

The authors begin by stating that by heating "nano-diamond powder" at 1600 C, one gets graphite. Knowing this, here's their approach:

1. Put a solution of colloidal diamond nano-particles onto a highly-oriented pyrolytic graphite (HOPG, fancy graphite) surface.
2. Apply an electrical charge to one side, pulling some of the particles down to the HOPG/fancy graphite surface and making a thin film of the diamond nano-particles. This is called electrophoretic deposition (here's another site that's a bit more helpful than the Wikipedia page).
3. Heat the heck out of the HOPG substrate.
4. Observe big spots on the surface of your HOPG using STM.
5. Claim that your spots must be single-layer graphene, since the height of particles is .35-.37 nm, as opposed to normal graphite inter-layer distance of .335 nm.

I'm not totally convinced here- they might have made graphene, but why would you want that to sit on top of graphite? The STM images (not worth seeing, believe me) in step 4 do show some very distinct islands, which I guess means they did something, but having a 0.02 nm (4.5%) difference in your step height doesn't inspire a lot of confidence. The authors try in vain to get scanning tunneling spectroscopy (STS) measurements to check out the electronic properties, but alas, they are unsuccessful.

At the end of the day, the authors use only STM and AFM images to show they made single-layer graphene by electrophoretic deposition and thermal decomposition. I've still got my doubts, and either way it ended up being another 3 years until someone could get graphene nice enough to probe the electronic properties.



Affoune, A. (2001). Experimental evidence of a single nano-graphene. Chemical Physics Letters, 348(1-2), 17-20. DOI: 10.1016/S0009-2614(01)01066-1

Planer nano-graphenes from camphor by CVD

Planer nano-graphenes from camphor by CVD

Prakash R. Somani, Savita P. Somania, and Masayoshi Umeno

Chemical Physics Letters, 430(1-3), 56-59

Let me begin by saying that this is the worst paper I've read in a long time. Let me count the ways I hate this paper:

1. Title misspelled.
2. Poor English throughout (I blame the editors, not the Japanese authors, for that).
3. Implying that using camphor in CVD instead of ethylene gas is more environmentally friendly and low cost.
4. Not citing the multitude of papers who have made flat graphenes using these methods (only mentioning the ones that have made nanotubes and buckyballs).
5. This quote: "Controlled, easy and low cost synthesis of graphene/(planer few layer graphenes) is still a challenge and not much efforts have been made in this direction", which is a complete lie.

I could go on and on, but I'm trying not to waste any more time on this paper than I already have. The authors use CVD the way others have (without citing them until the end of the paper, tangentially) but with camphor on a nickel substrate. Then they scrape their newly made layers off of the nickel and study the powder. They make a huge deal about how they get "planer" sheets of graphene, but then gripe about how they get detailed TEM data because the sheets keep folding up on each other. If everything is folded, it's not planar, jerks.

They conclude by saying that camphor naturally gives both six-membered and five-membered carbon rings and that someone should use something that only gives six membered rings. Oh, and also they can only get down to 20-layer graphene, and they don't mention everyone else's efforts using CVD that are so much better than them. Then they brag about how awesome their approach is.

Please, don't read this paper unless you plan to mock and/or retract it.



SOMANI, P., SOMANI, S., UMENO, M. (2006). Planer nano-graphenes from camphor by CVD. Chemical Physics Letters, 430(1-3), 56-59. DOI: 10.1016/j.cplett.2006.06.081

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 thought) 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

Superior Thermal Conductivity of Single-Layer Graphene

Superior Thermal Conductivity of Single-Layer Graphene

Alexander A. Balandin, Suchismita Ghosh, Wenzhong Bao, Irene Calizo, Desalegne Teweldebrhan, Feng Miao, and Chun Ning Lau

Nano Lett., 8 (3), 902–907, 2008

DOI: 10.1021/nl0731872 (EDIT: Free access to this paper on arXiv, so now you don't have an excuse not to read it).

When I read the title, I thought, wonderful! An easy to read, one topic paper that tests a predicted property of graphenes. One of the many predicted properties of graphenes is their high thermal conductivity, higher even than last decade's (or last year's) wonder material, carbon nanotubes. This paper suspends a graphene layer over a silicon dioxide gap and then measures it's thermal conductivity with Raman spectroscopy.

Usual techniques for measuring thermal conductivity don't work because they depend on measuring the temperature change through the thickness of the material; unfortunately, graphene is only one atom thick. This means that the temperature change will have to be measured across the lateral dimension of the graphene, and it also means that the authors will have to keep the graphene from transferring it's heat to anything under (or over) it; any heat sink will have to be at the edges of the graphene.

The first problem, measuring temperature across the width of the sample, is solved by using confocal micro-Raman spectroscopy (a Raman scattering instrument on a microscope). The authors had recently discovered that the G peak in the Raman spectrum of graphene is temperature dependent (an earlier post centered around the D' band), which gave them a convenient handle on measuring thermal conductivity.

The second problem was a bit more interesting. If you're looking for an insulator that won't fool with your spectroscopy, air (or vacuum) is your best bet. How do you get, then measure, a layer of graphene with only air under it? Here's a handy homemade guide:

1. Take a SiO2 surface.

2. Create a trench in the middle of it.

3. Coat the surface with "Scotch tape method" exfoliated graphene.

4. Shine some light (probably a laser) in the middle to heat it up.

5. Check out that G band on the Raman spectrum.

Here's the way the authors depicted it:

After all that, the authors do a whole bunch of math, and I'm pretty much taking their word for it. They give a thermal conductivity value of 4840-5300 W/mK; as a comparison, diamond has a conductivity of about 0.2 W/mK, individual carbon nanotubes have a conductivity of 3500 W/mK, and carbon nanotube bundles have thermal conductivities ranging from 1750-5800 W/mK.

In conclusion, these guys did some really neat engineering to get a result that was really boring. Graphenes, as expected, have a high thermal conductivity.

Balandin, A.A., Ghosh, S., Bao, W., Calizo, I., Teweldebrhan, D., Miao, F., Lau, C.N. (2008). Superior Thermal Conductivity of Single-Layer Graphene. Nano Letters, 8(3), 902-907. DOI: 10.1021/nl0731872

Evaluation of solution-processed reduced graphene oxide films as transparent conductors

Evaluation of solution-processed reduced graphene oxide films as transparent conductors

Héctor A. Becerril, Jie Mao, Zunfeng Liu, Randall M. Stoltenberg, Zhenan Bao, and Yongsheng Chen

ACS Nano 2008; Still ASAP

DOI: 10.1021/nn700375n

Another excellent paper dealing with graphene oxide, it's reduction, and the properties of the films made before and after reduction.

As you might recall from half the posts I've done so far, one method of making graphene layers is to oxidize highly ordered graphite. This oxidation introduces all manner of oxygen-bearing functional groups (phenols, carboxylic acids, epoxides, other carbonyls) to the graphite layers, and in water some of these are deprotonated, giving an electrostatic repulsion between the resulting negative charges on adjacent layers. After sonication, the graphite layers separate to give a graphene oxide solution, which can be put on a substrate and then reduced, or reduced and then put on a substrate. These authors choose the first option.

They begin by making graphene oxide (GO) in the normal manner and putting it on a surface; they find that spin-coating gives them the best quality films (which are about 3nm thick), whereas previous reports have used spray-coating. They found that the conventional reduction with hydrazine solutions pretty much destroyed their precious films, but that they could get some mediocre results putting hydrazine vapor and their substrate together into a petri dish.

Where this paper really gets original is that they decide to play with heating the GO. Previous reports have shown that GO is not thermally stable, and that heating it releases oxygen-containing compounds (this can also be used as an exfoliation method, which I'll post on some day). Releasing oxygen, reasoned the authors, sounds a lot like reducing something; so by heating GO to anywhere between 400 and 1100 C (with or without previous hydrazine reduction), they found that reduction did take place. They proved this with XPS measurements, which show the amount of C-C, C-O, C=O, or C-N bonds in a certain sample. The initial GO films have about 53% C-C bonds, with the rest taken up by C-O or C=O bonds. Treatment with hydrazine increased the proportion of C-C bonds, with C=O and C-N bonds (from incomplete reduction to hydrazone groups) making up the balance. The trend continues after annealing at 400 C, but the most graphitic surfaces were made by heating the GO at 1100 C with no previous hydrazine treatment, and had 88% C-C bonds. Catch that? They reduced most completely when they did not use hydrazine, but instead just heated the material.

After getting the most graphitic layer they could (from heating), the authors turn to examining the transparency and conductivity of their films. They find that conductivity and transparency are inversely proportional to each other, a fact which may be important to people other than me.

The authors give more questions and suggestions than hard answers in this paper, which I think is a good thing. They show that the current hydrazine-based method of GO reduction might have to be abandoned, particularly for applications where one is looking for no dopants (nitrogen and phosphorous are dangerously close to each other on the periodic table). They found a way to get nice films, but admit that their method is incomplete, and state that functionalizing graphite with things other than oxygen might be necessary to be able to fully reduce (or de-functionalize) back down to graphene. Kudos to them for asking the right questions and coming up with the first steps to answer them.

As an added bonus- this paper is the result of a collaboration between a chemical engineering group at Stanford and a chemistry group at Nankai University in China. Interdisciplinary work makes for some strange bedfellows.



Becerril, H.A., Mao, J., Liu, Z., Stoltenberg, R.M., Bao, Z., Chen, Y. (2008). Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano DOI: 10.1021/nn700375n

Atomic-layer-deposited nanostructures for graphene-based nanoelectronics

Atomic-layer-deposited nanostructures for graphene-based nanoelectronics

Y. Xuan, Y. Q. Wu, T. Shen, M. Qi, M. A. Capano, J. A. Cooper, and P. D. Ye

Appl. Phys. Lett. 92, 013101 (2008)

DOI: 10.1063/1.2828338

As people have realized that the race to well-made graphene devices is pretty much a gold rush, the literature has seen people try to do what they do best in their own discipline- but do it on graphene (more on this interdisciplinary trend later). In science, the best discoveries often come from an alert scientist realizing they got something different than they expected, and being curious enough not to throw it in the trash. In this paper, the authors try to grow uniform layers of aluminum oxide and hafnium oxide on fancy graphite and stumble upon a way to make some metal nanoribbons in addition to patterning graphite/graphene.

The authors use atomic layer deposition (ALD) to try to make some films of metal oxides on the surface of highly ordered graphite. ALD is a variant of chemical vapor deposition (CVD). In CVD, reactive gasses (such as silane, SiH4, and O2) are exposed to a substrate (such as silica), and the reaction of these two creates a thin layer on the surface of the substrate (for example, SiO2 on silica; the overall reaction is SiH4 + O2 = SiO2 + H2). This is good for thin films; what if you want really thin films? In ALD you expose the substrate to these two gasses one at a time, allowing only one layer of product per cycle to be formed. As you expose the substrate with only one gas, it can only absorb on the surface, since it doesn't have anything to react with; when you put in the second gas, the only thing the second gas has to react with is the molecules absorbed on the surface. At the end of the day, ALD gives you very thin films that are partially controlled by how well your reactants can absorb on the surface.

When ALD was attempted to deposit thin layers of metal oxides on the (fancy ultrasmooth) graphite, the authors found that their reactive gasses did not absorb onto most of the graphite, which was probably quite a bummer. However, they found that they got very thin (1.5 nm) ribbons of their metal oxides on certain sections of the graphite. Further investigations showed that the ribbons only grew on the edges (steps?) of individual graphene sheets. These edges are typically more reactive than the bulk material, since you get C-H or other bonds on the edge instead of the C-C aromatic bonds, which everyone who's taken sophomore organic knows you can't really mess with. They point out that the edges of graphene have absorbed other small molecules in the past, and that the two kinds of edges possible (armchair and zigzag; perhaps I'll write that up some day) might give preferential growth for their nanoribbons, an assertion they say they're looking into for future papers. The lateral size of the ribbons is determined by the number of ALD cycles and temperature, while the vertical size of the ribbons is determined only by the number of ALD cycles.

The authors say that the metal oxide layers could serve as etching/decomposition masks, giving a nice handle on a way to get very thin graphite or graphene ribbons. Unfortunately, you couldn't really pattern with these masks, since the orientation is entirely dependent on where your graphene edges are. Nice paper overall, and a great example of making some delicious lemonade when life gives you lemons (or perhaps realizing that life really gave you lemonade after all).

Like the last post, these guys are electrical engineers that have become interested in jumping on the graphene bandwagon. The field of "graphene science" is quite interdisciplinary; in the few posts I've done so far (which didn't include any theoretical papers), we've had contributions from electrical engineers, physicists, chemical engineers, mechanical engineers, materials scientists, chemists, and three guys who work at the "Grenoble High Magnetic Field Laboratory", which I guess makes them magneticists (or perhaps just magicians). The research world is getting more and more interconnected, and graphene is a perfect example of how a problem can attract a wide array of scientists and engineers, who all try to tackle a problem using their unique skills. For example, the last post had a bunch of nanoimprint guys say, "hey, we know how to do nanoimprint stuff and we've got the equipment; why not use it to contribute to the new gold rush?" The authors of this paper know how to deposit aluminum oxide and hafnium oxide with ALD, and their attempt to use their knowledge in graphene engineering gives us a neat handle on making more nanostructures.

Does this mean that we should all be getting interdisciplinary degrees, as in "PhD in cancer research"? I would be inclined to say no. Conducting research in an interdisciplinary manner is essential to an area such as graphenes, but most of the good papers I've read in the area are done by people who are (or were) experts in something other than graphene research. Learning to be a great chemist in grad school allows you to do great chemistry in a lot of areas; a degree that teaches you small snippets of everything probably won't make you an expert in any of the disciplines, while leaving your knowledge base less flexible. It warms my heart to see the graphene problem being approached competently from so many angles.

Xuan, Y., Wu, Y.Q., Shen, T., Qi, M., Capano, M.A., Cooper, J.A., Ye, P.D. (2008). Atomic-layer-deposited nanostructures for graphene-based nanoelectronics. Applied Physics Letters, 92(1), 013101. DOI: 10.1063/1.2828338

Graphene transistors fabricated via transfer-printing in device active-areas on large wafer

Graphene transistors fabricated via transfer-printing in device active-areas on large wafer

Xiaogan Liang, Zengli Fu, and Stephen Y. Chou

Nano Lett., 7 (12), 3840 -3844, 2007.

DOI: 10.1021/nl072566s

And now, for a pseudo-device post. As you might have read, all of the current methods in graphene synthesis have some serious shortcomings, particularly for the electronic applications (like FET) that everyone thinks graphene will work great for. This paper introduces a top-down approach to exfoliating and then patterning graphene to make a FET device.

Since I think everyone likes lists and schematics more than my paragraphs worth of text, I'll give the play-by-play and picture of the new technique, which carries the award-winning name "graphene-on-demand by cut-and-choose transfer-printing (DCT)":

a. Lithographically pattern pillars onto a stamp, very similar to a stamp for nanoimprinting (alas, the Wikipedia page doesn't have a pretty picture for it).
(aa). Coat the stamp with a nefariously defined resin.
b. Apply the stamp (with pressure) to some fancy graphite, then lift it off.
c. See if you have graphite/graphene on it.
d. Apply the stamp (with some graphene on it now) to a treated SiO2 surface, then lift it off.
e. Admire the graphene layers, produced and patterned for your amusement.


Does this sound too easy? I thought so too. They claim that the pressure exerted in step 3 actually cleaves the graphene layers and lifts off a few of them (exfoliation). They skip step C altogether, so I'm not quite sure why they include that in the diagram. Once the pillars are applied to the treated SiO2 surface then released, some of the layers stay behind (undergoing more exfoliation). The patterns they get look mostly intact, and they say that on average their films are 3-15 graphene layers thick. This is about the thickness of typical de Heer epitaxial graphene; however, we would expect the graphene layers to be stacked like graphite (since they come from graphite), instead of being blissfully and haphazardly stacked at an angle to each other (as discussed here). They don't do a lot of characterization other than SEM and AFM; how does the FET response measure up?

One of the many save-the-world properties of graphene is that it should have equal electron and hole mobilities; however, measurements made on both epitaxial and exfoliated graphene have shown significantly higher hole mobilities. Chou's graphene-on-demand shows the same trends, with a hole mobility 5 times higher than the electron mobility. In the past this has been attributed to accidental doping in preparation and handling, which seems to be a pretty big problem to me. If your mobilities are can vary 5 fold based on impurities found in the best of techniques, couldn't any change be attributed to just being better (or worse) at purifying your material?

The moral of the story is that purity is a huge huge deal when dealing with electronic applications, which anyone in the field could have told you. We can also take from this that our current methods of graphene synthesis might not give pure enough graphene to realize the amazing properties predicted by the physicists.

Oops- got a little philosophical. Back to Chou's paper: the real cool part of this paper is that they produce and pattern graphene at the same time, which could be great for industry. It's no surprise that Dr. Chou is very involved in nanoimprint lithography (NIL), since this is just NIL using graphite. Neat paper.

Disclaimer: I stole that figure from the paper, which is properly cited and can be found at the DOI link above or below.



Liang, X., Fu, Z., Chou, S. (2007). Graphene Transistors Fabricated via Transfer-Printing In Device Active-Areas on Large Wafer. Nano Letters, 7(12), 3840-3844. DOI: 10.1021/nl072566s

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 arXiv.org 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

Overview of Graphene Synthesis

Here's a step-by-step guide to the current synthetic strategies for making graphene:

1. Exfoliated Graphene: Great step-by-step demo from Scientific American
A. Take a piece of scotch tape and some highly ordered graphite.
B. Put the piece of scotch tape on the graphite, then rip it off.
C. Repeat a lot of times.
D. Deposit on a SiO2 substrate.
E. Look for graphene layers.

Pros: Cheap and easy enough you can pay an undergrad to do it. No special equipment needed, and you can find the thickness of the graphene layers based on the color of the SiO2.
Cons: Gives very uneven films, meaning it is very time-consuming to find where the graphene (as opposed to graphite) is. Also labor intensive, which is great for students but bad for industry.

2. Epitaxial Graphene
A. Take a SiC wafer.
B. Heat it to 1100 C.

Pros: Produces the most even films of any method, doesn't require a lot of really complicated steps.
Cons: Gives few-layer graphene (FLG), usually 5-10 layers at best. Also, not everyone has such a fancy (or hot) oven, and the technique isn't very versatile since you can't really functionalize something you grow from thermal decomposition.

3. The Graphene Oxide Approach
A. Oxidize highly-ordered graphite with HNO3 and H2SO4.
B. Sonicate it, then purify via centrifuging.
C1. Reduce to graphene-ish material, then put on substrate, or:
C2. Put on substrate, then reduce to graphene-ish material.

Pros: More versatile than epitaxial methods, less time-consuming and easier to scale up than exfoliation methods.
Cons: Difficult to keep solution from re-aggregating into graphite; after reduction, graphene layers are still partially oxidized, potentially changing electronic, optical, and mechanical properties.

4. CVD Graphene
A. Pump in hydrocarbon gas (usually CH4), sometimes under vacuum
B. Watch as the carbon arranges into graphene on your surface (often a metal surface like nickel)

Pros: Great for making large amounts of film, requires very little labor
Cons: Often makes unpredictably arranged multilayers, with defects being linked to the substrate you're using. Also, the metal surfaces on which this works best are not what you want to build graphene devices on top of.

Well, there you have it. Enjoy.

(Updated Feb. 20, 2009 to include CVD)

Introduction. Carbon-based electronics: fundamentals and device applications

Introduction. Carbon-based electronics: fundamentals and device applications

Robin J. Nicholas, Alison Mainwood, Laurence Eaves

Phil. Trans. R. Soc. A (2008) 366, 189-193.

DOI: 10.1098/rsta.2007.2160

This is a nice little intro into a symposium held about carbon-based electronics. It's 4 and a half pages of conversational prose- go read it yourself for free (at the DOI link above) compliments of the Royal Society.



Nicholas, R.J., Mainwood, A., Eaves, L. (2007). Introduction. Carbon-based electronics: fundamentals and device applications. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 366(1863), 189-193. DOI: 10.1098/rsta.2007.2160

Graphene oxide papers modified by divalent ions- enhancing mechanical properties via chemical cross-linking

Graphene oxide papers modified by divalent ions- enhancing mechanical properties via chemical cross-linking

Sungjin Park, Kyoung-Seok Lee, Gulay Bozoklu, Weiwei Cai, SonBinh T. Nguyen, and Rodney S. Ruoff

ACS Nano, 2008 (still ASAP)

DOI: 10.1021/nn700349a

ACS Nano is a journal that started accepting papers in April of last year, and as far as I can tell, it's complimentary to Nano Letters, carrying mostly papers and reviews on nanostuff. The paper I'm reviewing in this post can be entirely summarized in the graphic from the abstract, which I'd love to post but Blogger won't let me include the picture. Click through for the picture.

So let me first describe how you make graphene oxide paper:
1. Get a graphene oxide dispersion from oxidized graphite using the techniques detailed in the previous posts
2. Filter it

Yeah, that's pretty much it. In this paper, Ruoff (who's kind of a big deal in this graphene oxide stuff) runs a solution of divalent cations (Mg and Ca) through the filtered graphene oxide paper. The paper gets stronger after this treatment, but if you rinse the paper after you run the ions into it, you get a material with mechanical strength somewhere between the treated paper and the untreated paper. Big deal, I guess.

The interesting part of this paper is that they theorize that the divalent ions are coordinated with the carboxylic acids on seperate graphene sheets, giving a metal-mediated cross linking which increases the mechanical strength. They also theorize that some of the ions get stuck between the sheets and weakly coordinate to the epoxides and phenols on the oxidized graphene. These intercalated ions are removed after a rinse, giving a material that's a bit weaker than before but still has the metal-carboxylic acid interaction to hold the sheets together. Since this is insoluble muck, they use XRD, energy dispersive X-ray (EDX) fluorescent mapping, and XPS to characterize the samples. Using IR, they see a decrease in the C=O peak and in increase in the C-O peak in the treated material, further suggesting that the divalent metals are coordinating to the carbonyls.

In summary, this paper details a way to make stronger graphene oxide paper. You can't use the stuff for electrical applications, which is what I'm interested in, so this paper made me yawn. However, if you think graphene-based materials are going to end up in airplane wings and carbon fiber car hoods, check it out.



Park, S., Lee, K., Bozoklu, G., Cai, W., Nguyen, S.T., Ruoff, R.S. (2008). Graphene Oxide Papers Modified by Divalent Ions—Enhancing Mechanical Properties via Chemical Cross-Linking. ACS Nano DOI: 10.1021/nn700349a

A Chemical Route to Graphene for Device Applications

A Chemical Route to Graphene for Device Applications

Scott Gilje, Song Han, Minsheng Wang, Kang L. Wang, and Richard B. Kaner

Nano Lett., 7 (11), 3394 -3398, 2007

DOI: 10.1021/nl0717715

This paper outlines another attempt at taming the graphite oxide approach to making graphene. As a quick summary, graphene synthesis is hard, and finding a cheap and easy method to make sheets of graphene (particularly on a surface) would make physicists and lazy chemists the world over salivate. One of the main ways, discussed in the previous post (EDIT: that paper and this one are both by Richard Kaner), is to oxidize plain graphite (well, actually very pure graphite, but who's counting) with nitric and sulfuric acid. This functionalizes the graphite with a variety of carboxylic acids, phenols, epoxides, etc. These groups can be partially deprotonated in water, introducing an unfavorable interaction between the negative charges on the different sheets in the graphite and giving, after sonication and purification, individual sheets of graphene. The material can then be either put on a surface and then reduced to a more graphene-like substance or it can be reduced and then deposited on a surface (which is what the previous post, described). This paper describes the former; that is, it describes a method by which a graphene oxide (GO) layer can be reduced in the solid state to build FET devices.

The paper begins with trying to get a high-quality film of GO, necessary for electrical applications, on a silicon dioxide substrate. I thought this problem had already been tackled, but apparently it's not as easy as it seems. Traditional drop-casting and dip-coating techniques were ineffective when using only water as the solvent. After trying an array of organic/water mixtures, they found that no matter what they still got micro-aggregation when using drop-casting and dip-coating. However, they were able to get high quality films by spray-coating their solutions onto a heated substrate, which allowed the solvent to evaporate so quickly there was no time to aggregate.

After getting a nice film of GO, they tried to reduce it using hydrazine (as per the literature). They first used a crude setup where they suspended the wafer above heated anhydrous hydrazine and hoped for the best; while they got reduction, they also unfortunately had hydrazine and water condense on the wafer, resulting in aggregation and unusable films. They altered their setup a bit with a flow cell, and streamed hydrazine vapor and helium gas over the heated GO-coated wafer.

Through this method, they obtained a film that looked different than GO under AFM. They also took Raman measurements (perhaps more on Raman and graphene in a later post) to confirm that at least some of the GO had been reduced. As in the previous post, it's been shown that reduction of GO is often quite incomplete, so it's debatable exactly how much like pure graphene this material is. Further conductivity measurements on FET devices did confirm that the electrical conductivity went up by four or five orders of magnitude after reduction, so they definitely did something. Finally, the authors tested the other FET characteristics, giving the current/voltage plots that show whatever it is that current/voltage plots are supposed to show.

Overall, a paper detailed an incremental but good advance in graphene oxide/ graphene chemistry. The authors didn't imply that it would save the world, and Nano Letters seems to be the perfect spot for something that makes a small difference to people in the field but not much to the world at large.

Gilje, S., Han, S., Wang, M., Wang, K., Kaner, R. (2007). A Chemical Route to Graphene for Device Applications. Nano Letters, 7(11), 3394-3398. DOI: 10.1021/nl0717715

Processable aqueous dispersions of graphene nanosheets

On to the first review:
"Processable aqueous dispersions of graphene nanosheets"

Dan Li, Marc B. Müller, Scott Gilje, Richard B. Kaner & Gordon G. Wallace

Nature Nanotechnology 3, 101 - 105 (2007)

DOI: 10.1038/nnano.2007.451

In the quest to find a way to make layers of graphene real cheap and real easy (so that even experimental physicists can make them), Wallace's paper seems a small but good step forward. Wallace takes a technique used before- oxidizing graphite and then breaking it into graphene sheets with sonication. The oxidation works to help break apart the sheets because the "graphene oxide" is coated with phenols and carboxylic acids, giving it a large number of negative charges in water which, naturally, repel each other and overcome the more tame Van der Waals forces that usually hold graphite together. At the end of the day you get an aqueous dispersion of graphene oxide sheets, which can then applied to a substrate in a variety of ways before or after reduction to graphene.

The main problem is that once the graphene oxide is reduced to a more graphene-like product, many of the negative charges go away and so the sheets naturally start to aggregate, sometimes even re-forming graphite. Even an aggregate of 10 graphene sheets gives a substance with more graphitic than graphenic (sp?) properties. Previous attempts (look the papers up yourself) have stabilized the chemically treated graphene with polymers or other surfactants; the triumph of this paper is that the graphene can be stabilized without either of those.

The key to Wallace's approach is realizing that the conventional method of reducing the graphene oxide (with hydrazine) doesn't fully reduce it, leaving at least some carboxylic acid to give a negative charge. Wallace thinks that, as long as those native charges aren't stabilized by some cofactor, they should be enough to give stable colloids in aqueous solutions. Turns out, he's right. He carefully controls the amount of extra hydrazine present and also makes sure no salts are still hanging around to stabilize those charges. He evens adds in some base (ammonia) to ensure that everything is deprotonated. He then shows through light scattering, UV/Vis, IR, and something called a particle analyzer, whatever that means, that he has nice dispersed graphene and not big bad graphite. Adding salt or other electrolytes to the solution (or even lowering the pH) results in aggregation, further confirming that the unstabilized charges are necessary for aggregate prevention. Great. Then, Wallace goes a little crazy.

The authors don't claim that this material will cure cancer, AIDS, poverty, hunger, and global warming; but let's just say they didn't explicitly rule any those out. They made graphene paper (I'll post a paper about that later) which was more flexible and metallic than previous graphene/ graphene oxide paper. Then they discuss putting the graphene-ish material onto surfaces, through drop-casting, air-brushing, and layer-by-layer assembly. To their credit, they get some (supposedly) nice films of graphene in these ways, but they go a little overboard describing the anti-static coatings, FETs, sensors, supercapacitors, and membranes that future generations will make with their materials.

All in all, a decent paper that describes a small but quite important step in furthering graphite oxide-derived graphene science.



Li, D., Müller, M.B., Gilje, S., Kaner, R.B., Wallace, G.G. (2008). Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology, 3(2), 101-105. DOI: 10.1038/nnano.2007.451

Graphene

According to Scifinder, there have been about 120 papers published on graphene between Jan 1, 2008 and March 11, 2008. More than half of them are theoretical papers, with enticing titles like "graphene nanoribbons with chemically modified edges" that don't show you any ways of making graphene nanoribbons with chemically modified edges. Instead, they do some high-brow calculations on what the (electrical, optical, conducting, etc.) properties would look like if some poor schmuck actually made them. This is quite a downer when you're looking to read about the latest in graphene synthesis; a more appropriate title would be "some silly calculations on graphene nanoribbons with theoretically modified edges, but with no chemical modification even attempted."

These reviews/summaries will mostly be on new methods of graphene synthesis and characterization, with some applications work as well. It would be great if someone boiled down all those theoretical papers into a few easy to use principles, but that someone sure isn't me. If you'd like to blog some theoretical, pchem, or physics papers, you're welcome to do so here.