Graphene has emerged as a new material with a very bright future. Applications range from electronics, optoelectronics, and energy, to lighting and aerospace, writes Amaia Zurutuza, Graphenea's Scientific Director, in an article for the E-Nano Newsletter, a publication of the PhantomsNet foundation. The latest issue of the newsletter is focused on applications of graphene.
In her authored article, Amaia describes four methods of fabricating graphene, followed by three examples of potential applications. The methods of fabrication considered are mechanical exfoliation, liquid phase exfoliation, sublimation on silicon carbide, and chemical vapour deposition (CVD). The applications covered are photodetectors, optical transistors, and graphene batteries.
Mechanical exfoliation is the oldest method of obtaining pure single-layer graphene, which was used in the famous experiments at the University of Manchester that earned the Nobel prize for A. Geim and K. Novoselov. The method consists of ripping off layers of graphite using scotch tape, until only a single layer is left on a substrate. While the method yields high-purity and high-quality graphene, a device made with the scotch-tape technique is typically never larger than 100x100 micrometers, which is not suitable for industrial production.
Liquid phase exfoliation yields graphene flakes that float in a solution and can be dispersed onto a substrate or mixed with other chemicals to add various functionalities. This type of graphene is generally used for battery applications, however it involves several chemical or physical processing steps, such as oxidation of graphite and subsequent reduction of graphene oxide to graphene. The quality of graphene made with liquid phase exfoliation is generally not as high as that made with other methods, because of all the processing steps. However, this approach can result in industrially scalable high yield.
Sublimation of carbon atoms on a silicon carbide substrate results in high quality graphene films, however the main drawbacks are the high cost and limited size of the substrate. CVD growth offers a good compromise between film quality, size, and cost. The method is a chemical process that occurs in a well-controlled furnace, yielding layers of graphene up to 100 meters long, and of a solid quality. The films are usually grown on thin metal substrates, for example copper. The copper is later etched away and the graphene transferred onto any other substrate, depending on the application. The CVD method is versatile and the most commonly used method of making graphene films today.
Figure: CVD grown graphene on silicon/silicon oxide substrate
Further, in Amaia's review of the applications of graphene, you will find a description of a recent scientific result on the behaviour of electrons in graphene upon absorption of light. Graphene absorbs light uniformly across the electromagnetic spectrum, from the visible range all the way to microwave and terahertz. The absorption is as low as 2.3%, due to the small thickness of the film. When particles of light, photons, are absorbed by the graphene, electrons in graphene gain energy, becoming "hot", which is an essential ingredient in light harvesting. Scientists, including team Graphenea, found that as the electrons lose energy, they tend to give that energy away to newly promoted "hot" electrons, thus "multiplying" the usability of each photon. Such an effect is highly desirable for future solar cells and photodetectors.
In another application, graphene was used as a platform for supporting surface plasmons, with possible applications to optical transistors. Surface plasmons are light waves tightly confined to the surface of a conductor, in this case graphene. The light propagates with its usual fast speed along the surface, and if we could learn how to control its propagation we could have lightning-fast information and processing on a chip. The work shows that in graphene, propagation of surface plasmons can be tuned and switched off with normal electronic gating, like in a common electronic transistor. The use of plasmonic circuits is in the development stage, however Graphenea's contribution paves the way for graphene to be seriously considered as a viable platform.
Finally, the review describes the use of graphene for energy storage. With a graphene cathode and a lithium anode, a battery only 50 micrometers thin was made. The battery is flexible and holds as much energy density as comparably thin lithium batteries, with the excellent power density of a graphene supercapacitor.
Graphene certainly holds potential to radically change the future of technology. Much work remains to be done on bringing the material from the lab to commercial products, however Graphenea is on the forefront of both research and applications.