Graphene's unique properties have attracted a great deal of research attention. Teams around the world are working to develop potential applications for the material, ranging from aerospace, lighting and energy to electronic and optical devices. This article discusses four fabrication techniques and three potential applications of graphene.
The fabrication methods covered are chemical vapor deposition (CVD), sublimation on silicon carbide (SiC), liquid phase exfoliation, and mechanical exfoliation. The three potential applications considered are graphene batteries, optical transistors, and photodetectors.
Mechanical Exfoliation
Mechanical exfoliation is the original technique used to fabricate pure single-layer graphene in the Nobel Prize-winning research carried out by Geim and Novoselov at the University of Manchester.
Their technique involved using scotch tape to rip off layers from a sample of graphite, until only a single layer remained on a substrate. Although this method produces high-purity and high-quality graphene, it is difficult to effectively scale up for industrial production, and is not capable of producing graphene sheets larger than about 100 µm across.
Liquid Phase Exfoliation
Liquid phase exfoliation produces a suspension of graphene flakes in a solvent, which can be deposited onto a substrate, or mixed with other chemicals to impart different functionalities.
This type of graphene is typically used for battery applications, but involves various physical and chemical processing steps, including oxidation of graphite and subsequent reduction of graphene oxide to graphene.
These additional processing steps result in a lower-quality graphene compared to other techniques. However, liquid exfoliation scales well for industrial production, and the quality of graphene produced is sufficient for some applications.
Sublimation on SiC and CVD Method
Sublimation of carbon atoms from a silicon carbide (SiC) substrate produces high-quality graphene films, but the high cost and limited size of the substrate are major shortcomings for this approach.
CVD growth provides a good compromise in terms of cost, size, and film quality. This technique is a chemical process which takes place in a vacuum furnace, and produces consistent, thin graphene layers in sheets up to several meters long.
The graphene films are grown on thin metal substrates such as copper, which are then etched away to leave the graphene to be transferred onto any other substrate required by the application.
The CVD process is versatile, and has now become the most widely used fabrication technique for graphene films. Figure 1 shows a CVD grown graphene on a silicon/silicon oxide substrate.
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Figure 1. CVD grown graphene on silicon/silicon oxide substrate
Applications of Graphene
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%, which is attributable to the thinness of the film.
Electrons in graphene which have been excited to a higher energy state by absorbing incoming light have been shown to preferentially transfer their energy to neighbouring electrons, rather than radiating it as photons. This effectively multiplies the 'usability' of each photon - a key capability for future photovoltaic cells and photodetectors.
Another potential advanced application of graphene is in ultra-fast optical transistors using surface plasmons. Surface plasmons are light waves tightly confined to a conductor surface, such as graphene.
The light propagates with its normal velocity across the conducting surface - this means that transistors based on plasmons rather than electrons would be incredibly fast.
Initial studies have shown that graphene can switch propagation of plasmons on its surface on and off with electronic gating, like a standard transistor. Although this is early-stage research, Graphenea's contribution opens the door for graphene to be seriously considered as a potential platform.
Graphene also holds potential for energy storage purposes. A 50-µm thick battery was constructed using a graphene cathode and a lithium anode. This battery is flexible, and has an energy density in the range of supercapacitors, much higher than thin lithium batteries
Conclusion
Graphene certainly shows promise to dramatically transform the future of technology, but requires more efforts to realize commercial success. Graphenea is playing a key role in both fundamental and application-based research.
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