Thursday, February 26, 2009

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

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.

Wednesday, February 25, 2009

Structure of Graphite Oxide Revisited

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.

Tuesday, February 24, 2009

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

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


Sunday, February 22, 2009

Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups

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.

Friday, February 20, 2009

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

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


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

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.


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

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.


Highly conducting graphene sheets and Langmuir–Blodgett films

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.


Thursday, February 19, 2009

Graphene Literature Reviews Express

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.