Zig and Zag: Graphene style

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  • Published: May 1, 2015
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Zig and Zag: Graphene style

Graphene boost

Zigzag and armchair defects in graphene (Credit: Narayanan et al)

Raman spectroscopy has been used in a study of the carbon material graphene to show how one can increase the amount of electric charge that could be stored.

Researchers at the University of California, San Diego and Clemson University, Clemson, South Carolina, have devised a neat trick that allowed them to bump up the amount of electric charge that can be stored in graphene. Writing in the online edition of the American Chemical Society journal Nano Letters, the team suggests that their findings will not only improve our understanding of the electrical properties of graphene itself but might also lead to new capacitors for energy storage in electric vehicles and for trapping energy from wind turbines and solar panels.

Capacitors have been used for decades as rapid charge and discharge units in all kinds of circuitry. They are used because they can quickly provide a large burst of electrical energy and then be recharged again just as quickly, so they are useful for camera flashes, for instance. But, they cannot store as much power as a rechargeable battery.

Threefold increase

UCSD's Prabhakar Bandaru and colleagues have turned to graphene as having the potential to make higher power capacitors that might have some of the characteristics of batteries. The team is exploiting the natural fact that it is impossible to make a carbon nanotube, or indeed graphene, structure without there being some defects in the structure, holes corresponding to missing carbon atoms, essentially. Rather than attempting to eradicated such defects, the researchers in Bandaru’s laboratory have worked out a practical way to use them to their benefit. “I was motivated from the point of view that charged defects may be useful for energy storage,” explains Bandaru.

The team used a method called argon-ion based plasma processing, in which they irradiate few layer graphene (FLG) samples with positively charged argon ions. During this process, carbon atoms are knocked out of the graphene layers and leave behind holes containing net positive charges - these are the charged defects. Exposing the graphene samples to argon plasma increased the capacitance of the materials by three times.

Zigzags and armchairs

"It was exciting to show that we can introduce extra capacitance by introducing charged defects, and that we could control what kind of charged defect it was," adds graduate student Rajaram Narayanan. The team used Raman spectroscopy and electrochemical measurements to characterize the types of defects that argon plasma processing introduced into the graphene lattices. They revealed that the formation of extended defects known as "armchair" and "zigzag" defects, which are named based on the configurations of the missing carbon atoms. Additionally, the team's electrochemical studies helped them reveal a new length scale that measures the distance between charges. "This new length scale will be important for electrical applications, since it can provide a basis for how small we can make charged defect-free electrical devices," says Bandaru. The length scale is smaller than the structural correlation length - the Tuinstra-Koenig correlation length - determined using Raman spectroscopy, and suggests that there is a distinction between electrical and structural length scales, the team reports.

"Ultimately, we would like to propagate and implement the idea of the importance of positive and negative defects for specific charge and energy storage," Bandaru told SpectroscopyNOW.

Related Links

Nano Lett 2015, online: "Modulation of the Electrostatic and Quantum Capacitances of Few Layered Graphenes through Plasma Processing"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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