In the unusually warm winter that bridges the years 2013 and 2014, graphene continues to be a hot topic of investigation - and we mean "hot" in the physics sense.
Mid-December last year, researchers at Rice University reported a novel method of deicing radar domes using a coating based on graphene. The coating, consisting of a mixture of graphene and a polymer, can be spray-painted onto the surface of the dome. Current is then run through the coating, heating the dome to deice it. The coating project was also supported by Lockheed Martin, who aim to use it on real radar devices onboard ships and aircraft. Importantly, the coating was shown to be nearly transparent at radio frequencies, alleviating the worry that powerful radar signals could bounce back off the coating and burn the radar itself.
The graphene in the coating was produced by unzipping carbon nanotubes, a method perfected by the same research group, led by James Tour (we wrote about Professor Tour's work in our recent article "Graphene for oil exploration"). The process is entirely ready for mass-production, and is hence a serious contender for commercial use. The spray-on graphene layer itself is easily removed by touch, however when mixed with a common automotive paint based on polyurethane, the layer sticks very well, while still retaining all the desired heating and transparency properties. Most importantly, the graphene layer is lightweight, putting it ahead of the currently used technology based on metal constructions and alumina.
The ability to heat graphene for a good use came up again this month, in a report of using a graphene oxide balm to treat skin cancer. The balm, consisting of graphene oxide and hyaluronic acid, is heated with a laser after application to the affected area. The graphene oxide absorbs the laser light, generating enough heat to burn the cancer cells.
The treatment, known as photothermal therapy, is nothing new - reports up to five years old show a similar effect with metal nanoparticles. Compared to metal and other kinds of nanoparticles, however, graphene oxide has a higher light absorption efficiency. Another advantage of the graphene balm is that it can be directly applied to the skin, whereas other treatment options involve injecting nanoparticle solutions into the bloodstream. Although this work still has to pass all the clinical trials and health checks associated with new medicine, it is certainly promising and an interesting direction of graphene research to keep an eye on.
Whereas hot graphene can be useful for treating skin cancer and de-icing radar domes, heat can be detrimental to electronic circuits made of the two-dimensional material. As atomic monolayers of graphene are very promising for constructing ultrafast electronic circuits, the heat issue could become a serious challenge in the future. Luckily, researchers at the US Department of Energy's Lawrence Berkeley Lab have solved the problem faster than it could fully emerge, elegantly using another form of nanocarbon – carbon nanotubes (CNTs).
Figure: Artist's representation of CNT cooling of microchips (Source: Berkeley Lab)
The researchers succeeded in cooling standard microchips by attaching a layer of CNTs on top of the chips. The challenge was to make the bond strong, which they overcame by using organic molecules as an “adhesive layer”. The process is suitable for mass manufacture. The same method could be used for cooling graphene microchips.
Low temperatures are also the key benefit of using graphene as a charge collection element in new high-efficiency solar cells. Teams based in Spain and Oxford University have now constructed photovoltaic cells with a 15.6% efficiency, using perovskite as the active (light-absorbing) material, and a combination of titanium oxide and graphene as the charge collector. The temperature used in the production process does not exceed 150 degrees Celsius, leading to lower production costs as well as the option of integrating the cells on flexible plastics.
With all this graphene heat, it is no surprise we have been seeing extra-warm winter weather. We'll have to wait and see whether the trend is followed by an extra-hot summer, or perhaps graphene will surprise us once again with a display of some unexpected cooling properties.