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Thoughts on C&E News graphene cover story March 5, 2009

Posted by fetzthechemist in Musings, PAHs.

I just read the cover story on graphenes in Chemical and Engineering News, March 2, 2009 issue. Lots of thoughts about graphene making and isoletion versus graphene properties.

The graphene everyone is looking at as the basic material is the infinite hexagonal-fused sheet, that infinite peri-fused PAH structure. That is the one with the various electrical and optical properties. Graphite is thought of as a layered material made just of these.

That is the theory. Real graphite has some of that structure, but there also are crosslinks between layers. Impurities, especially sulfur and nitrogen, can be incorporated into the structure. Five- and seven-carbon rings occur in the sheets that cause bulges – up or dow if you define a perspective.

So, first of all you need ultrapure, uniformly formed graphite to make great graphene. Dislocations are bad as they do not extend the aromaticity, just shunting it around the dislocation. This is always true for the carbon dislocations of five- and seven-carbon rings. That is why fluoranthene behaves as it does and pyrene is the more fully aromatic isomer – or any other similar pair, like indeno[1,2,3-cd]pyrene and benzo[ghi]perylene or anthranthrene.

Sulfur and nitrogen places in a sheet are the weakest chemically, sensitive to high temperatures or chemical reactions. My theory is that these are the spots where strong acids and other agents form the graphene oxide. (I studied the stability of many carbonaceous material in my dissertation work in grad school, and sulfur and nitrogen bits get removed pretty readily from graphites.)

Making graphene through these chemical means of attack probably chews up bits, thermal condensation probably sews a lot of them back up, but some are in the non-alternant forms, those five- and seven-carbon rings. Lots of those would alter the electrical properties – but that could be good or bad. Variations might be useful versions.

The mechanical methods do not in any way alter the structure, but this also means that those heteroatoms are still there. Ultra-ultra-ultra high purity graphite is needed, no matter what. So heteroatom specs are a key to defined and reproducible behavior.

The vapor deposition on a metal substrate, which is later removed, ought to produce the most regular hexagonal array form. (But once again, the purity of the carbon starting material have to be super high).

How to tell if your graphene have impurity or dislocation issues? Bulk methods, which seem to be the material scientists way, are not going to tell. Solid-sample NMR might. X-ray photoelectron techniques, like XANES, ought to see the different matrix forms of the carbons, alternant, non-alternant, heteroatom bound to S, N, O, etc.

I might muse some more, but these are the first impressions on one reading of the article.



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