A Chemical Route to Graphene for Device Applications

A Chemical Route to Graphene for Device Applications

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

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

DOI: 10.1021/nl0717715

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

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

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

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

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

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