Graphene Literature Reviews

Open Access and the House of Representatives

2009-03-06T20:20:00.003-05:00

Sorry, no research summaries this week. If you are in the research world, you probably know how important access to information is to science. You probably know, at least from this blog, that many of the leading researchers in the graphene field voluntarily post their work to the fabulous open access physics journal/website arXiv.org. American readers might know that the National Institutes of Health (NIH) has a policy which states that any articles written from research they've funded have to be freely available after 12 months.

Apparently, a representative from the U.S. House has decided that's a pretty bad policy, and is trying to pass a bill (not for the first time) which bars federal agencies from requiring open access to work they sponsor. Let me see if I can follow the logic of the sponsor, John Conyers:

1. My constituents pay taxes.
2. Some of that tax money goes to pay for research.
3. Leading scientists think more research can get done if everyone can access the initial research, which was paid for with tax money.
4. ....
5. Let's make sure that my constituents or any other scientists have to pay a lot of money to access the results of the research they paid for in the first place.

Bad idea. If you're into the U.S. politics side of things, feel free to contact John Conyers and ask him what in the world he's thinking.

This will be the last political post on this blog. We'll get back to real business next week.

Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition

2009-02-26T12:23:00.000-05:00

Alfonso Reina, Xiaoting Jia, John Ho, Daniel Nezich, Hyungbin Son, Vladimir Bulovic, Mildred S. Dresselhaus, Jing Kong (2009). Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition Nano Letters, 9 (1), 30-35 DOI: 10.1021/nl801827v

A very nice CVD paper where the graphene is transferred to a device friendly substrate (SiO2). The authors used chemical vapor deposition (CVD, also discussed here) on a polycrystalline nickel film, pumping hydrogen and methane gases at ambient pressure and 900-1000 C temperatures. The graphene grown was varied in the number of layers, with the larger number of layers occuring at the step edges of the Ni substrate. The graphene was transfered to SiO2 (although it could have been any other substrate) by protecting graphene with the polymer PMMA, etching the nickel away, and then placing it on the SiO2 surface. The transfer doesn't introduce many more defects, which is quite nice. They mention that some of the multilayers they saw on the nickel might have been electronically disconnected, meaning that they will still act like single-layer graphene (this is also seen sometimes with epitaxial graphene). Here's the stats on their graphene:

Sheet resistance: 770-1000 ohms/cm2
Optical transmittance: around 90% in the 500-1000nm range
On/off ratio at high voltage: between 1.3 and 2
Mobility for e's and holes: between 100-2000 cm2/V (huge range)

Moral of the story: This paper makes decent-quality graphene by CVD at ambient pressure, and they come up with a very nifty way to transfer their graphene to a useable substrate, an approach that overcomes a large disadvantage of using CVD to make graphene.

Structure of Graphite Oxide Revisited

2009-02-25T19:35:00.000-05:00

Anton Lerf, Heyong He, Michael Forster, Jacek Klinowski (1998). Structure of Graphite Oxide Revisited The Journal of Physical Chemistry B, 102 (23), 4477-4482 DOI: 10.1021/jp9731821

A pre-graphene paper exploring what graphite oxide actually looks like. The authors make graphite oxide from Hummer's method (KMnO4/H2SO4), then fool about with it by pumping it full of things like water, KI, thiourea, NaOH, NaOEt, dioxane, DMSO, and a number of other compounds. They then used a fancy diffraction measurement called a Debey-Sherrer photograph to determine interlayer distances and 13C and 1H magic angle spinning (MAS) NMR to determine structure. They found that water is an intricate part of graphite oxide, and that most of the oxygen that is covalently bound to graphite oxide is in the form of alcohols and epoxides. Although this isn't in the paper, I believe that most of the defects in graphene formed from this method come from the epoxide centers, not the hydroxide centers. The original paper also found that, instead of oxidation happening uniformly over the area of the graphite, there are oxidation-heavy regions and then regions where hardly anything is oxidized.

Moral of the story: Graphite (and graphene) oxide has a lot of water in it, and most of the oxygen is in the form of hydroxyl and epoxide groups.

The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons

2009-02-24T14:42:00.004-05:00

Kyle A. Ritter, Joseph W. Lyding (2009). The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons Nature Materials, 8 (3), 235-242 DOI: 10.1038/nmat2378

Characterization/electronic properties testing paper. These authors did mechanical exfoliation of graphite, but used a silicon wafer (Si(100)-2X1:H) instead of SiO2 and made sure to pulverize their sample as much as possible. They were able to find both graphene quantum dots (GQD, basically small square or round pieces of graphene) and graphene nanoribbons (GNR's). They then used Scanning Tunneling Microscopy (STM) and it's cousin Scanning Tunneling Spectroscopy (STS) to determine different pieces morphology, edge state (zigzag vs. armchair), and band gap (HOMO-LUMO gap, for us chemists; a band gap of 0 means the material is metallic). Their results suggest that zigzag GQD's are metallic, while armchair GQD's are semiconducting. The nanoribbons, it turned out, were semiconducting no matter what the edge state, but zigzag ribbons had a lower band gap (0.14eV, meaning they were more conductive) than armchair ribbons (0.38eV), as expected.

The large disparity in conductivity between edge states in carbon nanotubes is a big reason why they didn't pan out to be the wonder materials everyone thought they would, since most synthetic procedures gave a mixture of zigzag and armchair (metallic and semiconducting) nanotubes. This paper shows us that GQD's would have exactly the same problems. Although the difference is not as big in graphene nanoribbons, it would still be very difficult to build a device where some of your connecting elements would be twice as conductive as others.

Moral of the story: Zigzag-edged graphene structures are more metallic than armchair structures, with zigzag GQD's being metallic, zigzag GNR's semiconducting, and armchair GQD's and GNR's semiconducting.

This article also got some press, which (in my humble opinion) gets the quantum dot and nanoribbon results a little bit mixed up:
EE Times
Science Daily

EN#7

Journal of Unpublished Research

2009-02-23T13:11:00.002-05:00

Although they haven't yet published a paper on graphene, I would recommend the latest edition of this esteemed journal for your perusal.

Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups

2009-02-22T10:17:00.004-05:00

Elena Bekyarova, Mikhail E. Itkis, Palanisamy Ramesh, Claire Berger, Michael Sprinkle, Walt A. de Heer, Robert C. Haddon (2009). Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups Journal of the American Chemical Society, 131 (4), 1336-1337 DOI: 10.1021/ja8057327

This paper detailed attempts to modify the surface of epitaxial graphene with a diazonium salt, shown below in the paper's graphical abstract:

This reaction has been used before to modify carbon nanotubes, and is a very logical next step to tuning the electronic properties of graphene (since the reaction essentially creates defects which lower the conductivity). The authors proved they actually functionalized their graphene by IR (which clearly showed the N02 group) and XPS (giving a different C1s region and a brand new peak in the N1s region). The resulting material was about half as conductive as the starting material and the conductivity showed a much greater dependence on temperature.

Moral of the story: if you want semiconducting graphene (which would be very helpful in making actual devices), one way to get it is to modify the surface of the graphene with plain old organic chemistry.

Flexible Graphene Films via the Filtration of Water-Soluble Noncovalent Functionalized Graphene Sheets

2009-02-20T18:55:00.002-05:00

Yuxi Xu, Hua Bai, Gewu Lu, Chun Li, Gaoquan Shi (2008). Flexible Graphene Films via the Filtration of Water-Soluble Noncovalent Functionalized Graphene Sheets Journal of the American Chemical Society, 130 (18), 5856-5857 DOI: 10.1021/ja800745y

This quick communication features a standard Hummers-style graphene oxide (GO) reduction, but uses a water-soluble pyrene derivative (1-pyrenebutyrate, PB-) as a stabilizer to keep the GO soluble in water after hydrazine reduction. They find that depositing films from this solution gives a PB-/reduced graphene complex that's one layer (1.7nm) thick in some regions. Filtering the solutions instead of depositing them gives a black, flexible film; one 30 micrometer thick film, for example, had a tensile strength of 8.4 MPa with a modulus of 4.2 GPa. The conductivity of these films was around 2X10^2 S/m, similar to our previous GO paper and about 7 orders of magnitude higher than the pre-reduced GO. The authors also use this material in some TiO2 solar cells and claim significant improvement.

Moral of the story: this paper is another incremental step in finding a nice way to make reduced GO films by using different stabilizing/solubilizing agents

EN#32

Electronic and magnetic properties of armchair and zigzag graphene nanoribbons

2009-02-20T18:09:00.003-05:00

Frank J. Owens (2008). Electronic and magnetic properties of armchair and zigzag graphene nanoribbons The Journal of Chemical Physics, 128 (19) DOI: 10.1063/1.2905215

Another computational paper, this one exploring the differences between armchair and zigzag nanoribbons. Just like in carbon nanotubes, you can have two different kinds of graphene ribbon edges: armchair and zigzag, illustrated in the picture below from Nanotechweb.org (via Google):

Because you can draw two aromatic resonance structures for zigzag ribbons but only one aromatic resonance structure for armchair ribbons, zigzag ribbons are expected to be much more conductive (if you've taken sophomore organic chemistry, you can draw this out yourself). This paper does a little more math to prove this, using molecular orbital theory and a DFT program to look at how conductivity changes with nanoribbon width and doping with boron and nitrogen.

The author finds that increasing the nanoribbon width decreases the band gap (increases the conductivity) of armchair ribbons, just as expected. However, the zigzag ribbon band gap varies very much on whether the number of carbons used is even or odd, with zigzag ribbons with an even number of carbons having a lower bandgap than those with an odd number of carbons. This difference is attributed to the unpaired electron that comes with having an odd number of carbons.

Both types of ribbons exhibit possible ferromagnetism when doped, making them candidates to be magnetic semiconductors which could act as both storage and switching units.

EN#28

Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide

2009-02-20T17:27:00.004-05:00

Sasha Stankovich, Dmitriy A. Dikin, Richard D. Piner, Kevin A. Kohlhaas, Alfred Kleinhammes, Yuanyuan Jia, Yue Wu, SonBinh T. Nguyen, and Rodney S. Ruoff (2007). Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide Carbon, 45 (7), 1558-1565 DOI: 10.1016/j.carbon.2007.02.034

This is a mostly characterization-based paper looking in more depth at traditional hydrazine-reduced graphene oxide (GO). After oxidizing using Hummer's method (sulfuric acid, potassium permanganate) and reducing with hydrazine, the researchers characterized the before-and-after transformation with:

SEM: showed crumpled up sheets of the reduced GO
Surface area: 466 m2/g for reduced GO, rather high but still way below "real" graphene (2620 m2/g)
Elemental analysis: C/O ratio of 2.7 before reduction and 10.3 after, which still seems like a lot of oxygen after reduction. The C/N ratio after reduction was 16.1, meaning a good bit of the nitrogen from the hydrazine ended up in the final product.
Water content: 25 wt% before reduction; 2.8% after
TGA: for GO before reduction, mass loss started below 100 C, but main mass loss came around 200. Reduced GO was thermally stable up to 800 C.
C13 NMR (MAS): GO showed peaks for epoxides, hydroxyls and carbonyls, as expected; these peaks were absent in the reduced sample.
XPS: GO had 4 peaks for "oxygenated" components. These 4 peaks were smaller in the reduced sample, but a 5th peak for a C-N bond appeared.
Raman: Best shown with the picture below, borrowed under the auspices of fair use:

The first spectrum is pristine graphene showing a sharp "G" peak at 1581 cm-1. Second spectrum is GO, showing a much broader "G" peak in addition to a new "D" peak at 1363 cm-1, showing disorder. Third spectrum is reduced GO, which interestingly shows an even larger "D" peak, implying that the reduced film might have a higher disorder than the GO.
Conductivity: GO was least conductive (on the order of 10-3 S/m), with the reduced GO being much more conductive (around 10^2 S/m), a conductivity close to that of graphite (around 10^3 S/m).
The authors also briefly discuss the mechanism of hydrazine reduction, but essentially say that any explanation they can come up with doesn't fully explain their observations.

Moral of the story: As we knew, "graphene" made from reduced graphite oxide can be useful, but has so many defects that it's properties are very different from pristine graphene.

EN#41

Electronic structure and band-gap modulation of graphene via substrate surface chemistry

2009-02-20T17:09:00.005-05:00

Philip Shemella, Saroj K. Nayak (2009). Electronic structure and band-gap modulation of graphene via substrate surface chemistry Applied Physics Letters, 94 (3) DOI: 10.1063/1.3070238

This is a theory paper using DFT to determine the effect of the substrate on the properties of graphene. According to the paper's calculations, O-terminated SiO2 strongly interacts with a graphene layer, making divots in the graphene layer to give a higher surface roughness and resistivity (with a band gap around 0.35eV, similar to armchair graphene nanoribbons). Putting a second layer of graphene on top of this lowers the band gap some, but doesn't eliminate it altogether (0.10 eV, similar to zigzag graphene nanoribbons). If you passivate your SiO2 with some hydrogen, giving hydroxyl groups on the surface instead of dangling oxygen, you get very little interaction with graphene. This graphene would still be metallic (band gap = 0) . The moral of the story is to be mindful of what you put your graphene on, since even different forms of SiO2 can have a big impact on properties.

EN#39

Highly conducting graphene sheets and Langmuir–Blodgett films

2009-02-20T10:19:00.003-05:00

Xiaolin Li, Guangyu Zhang, Xuedong Bai, Xiaoming Sun, Xinran Wang, Enge Wang, Hongjie Dai (2008). Highly conducting graphene sheets and Langmuir–Blodgett films Nature Nanotechnology, 3 (9), 538-542 DOI: 10.1038/nnano.2008.210

Sequel to Hongjie Dai's nanoribbons article. The researchers make "expandable graphite" (basically graphite oxide, GO) by pumping in sulfuric and nitric acid. They expand/partially reduce it by heating to 1000 C, then throw in more sulfuric acid (oleum) and tetrabutyl ammonium hydroxide (TBA) in DMF to further intercalate stuff into the layers. Apparently, the graphite oxide approach works because the oxygen functionalities allow all kinds of stuff to sneak in between the layers.

Dai and friends then make films of their GO-esque material before and after annealing/reducing at 800 C in H2 gas. The films are compared to GO made with a more traditional Hummer's method by AFM, TEM, electron diffraction (ED), IR, and X-ray photoelectron spectroscopy (XPS). They find that their pre-annealed films had less oxygen functionalities, higher conductivity, and were more hydrophobic than the traditional GO. Post-annealing films of graphene (which they could deposit with organic solvents onto surfaces) were shown to have a lower incidence of defects (basically holes) than traditional reduced graphite oxide. Essentially, these authors found a more gentle way to make graphene from the graphene oxide approach.

EN#40

Graphene Literature Reviews Express

2009-02-19T14:14:00.002-05:00

I just wanted to announce that Graphene Literature Reviews is being turned back on, but switching to a more Spartan format. I've still got to read and summarize after all, so I'll be posting much shorter summaries in the future, with minimal links, probably poor grammar, less explanation, and few pretty pictures. I hope you are still able to learn and enjoy.

Gone Fishin (or Synthesizing)

2008-08-05T13:09:00.000-04:00

Hope everyone's been doing well out there. As you might have noticed, there haven't been any posts here for a few months; I'm pretty much done with blogging graphene literature for a while. I learned a lot about graphenes and about reading/understanding literature, and I appreciate everyone coming to check it out. If anyone would like to use my platform to write similar research reviews, please feel free to email me at RobWtzl@gmail.com.

Thanks again to everyone who came and read some stuff, and thanks especially to the kind folks at Carbon-Based Curiosities, Research Blogging, The Chem Blog, and Blogger for providing publicity, inspiration, and technical support.

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

2008-04-02T16:23:00.000-04:00

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

I Need a Physicist

2008-04-01T17:06:00.002-04:00

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 RobWtzl@gmail.com 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.

2008-04-01T15:04:00.003-04:00

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

Electrochem:
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

2008-04-01T14:35:00.000-04:00

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

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

2008-03-26T14:39:00.005-04:00

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

2008-03-26T13:32:00.001-04:00

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

2008-03-26T12:40:00.000-04:00

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

2008-03-24T12:07:00.007-04:00

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)

2008-03-24T12:05:00.003-04:00

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

2008-03-16T15:15:00.002-04:00

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

2008-03-15T03:21:00.004-04:00

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

2008-03-14T09:14:00.005-04:00

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

2008-03-13T14:46:00.006-04:00

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

2008-03-12T15:57:00.007-04:00

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

2008-03-12T15:15:00.006-04:00

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

2008-03-12T14:28:00.005-04:00

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

2008-03-12T13:15:00.005-04:00

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

2008-03-12T10:10:00.002-04:00

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

2008-03-11T16:22:00.001-04:00

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

2008-03-11T13:41:00.001-04:00

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.