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Graphene-based hybrid film touts transparency, flexibility

Posted: 24 Aug 2011 ?? ?Print Version ?Bookmark and Share

Keywords:hybrid electrode? graphene? metal nanowire?

A hybrid electrode developed by researchers from Rice University could be a viable replacement for indium tin oxide (ITO), which is popularly used as a transparent, conductive coating.

The thin film combines a single-layer sheet of highly conductive graphene with a fine grid of metal nanowires, resulting in better performance and transparency and lower resistance to electric current than ITO, the researchers claimed.

Although ITO is widely used as a primary element in virtually all flat-panel displays, including touchscreens on smartphones and iPads, and is part of organic LEDs and solar cells, it has several disadvantages. The element indium is becoming rare and expensive. It is also brittle, which heightens the risk of a screen cracking when a smartphone is dropped. It further rules ITO out as the basis for flexible displays.

Hybrid graphene

A hybrid material that combines a fine aluminum mesh with a single-atom-thick layer of graphene outperforms materials common to current touch screens and solar cells. The transparent, flexible electrodes were developed in the lab James Tour.

"Many people are working on ITO replacements, especially as it relates to flexible substrates," said James Tour, professor of mechanical engineering and materials science at Rice University. "Other labs have looked at using pure graphene. It might work theoretically, but when you put it on a substrate, it doesn't have high enough conductivity at a high enough transparency. It has to be assisted in some way," stated Yu Zhu, postdoctoral researcher and lead author of the paper. Fine metal meshes show good conductivity, but gaps in the nanowires to keep them transparent make them unsuitable as standalone components in conductive electrodes.

Microscope image of the hybrid electrode

An electron microscope image of a hybrid electrodeshows solid connections after 500 bends. The transparent material combines single-atom-thick sheets of graphene and a fine mesh of aluminum nanowire on a flexible substrate.
Source: Rice University.

However, combining the materials works superbly, Zhu said. The metal grid strengthens the graphene, and the graphene fills all the empty spaces between the grid. The researchers found a grid of five?m nanowires made of inexpensive, lightweight aluminum did not detract from the material's transparency. "Five?m grid lines are about a 10th the size of a human hair, and a human hair is hard to see," Tour said. He continued that metal grids could be easily produced on a flexible substrate via standard techniques, including roll-to-roll and ink-jet printing. Techniques for making large sheets of graphene are also improving rapidly. In fact, some commercial labs have developed a roll-to-roll graphene production technique. "This material is ready to scale right now," he noted.

The flexibility is almost a bonus due to the potential savings of using carbon and aluminum instead of expensive ITO, Zhu added. In tests, he found the hybrid film's conductivity decreases by 20C30 percent with the initial 50 bends, but after that, the material stabilizes. "There were no significant variations up to 500 bending cycles," Zhu said. More rigorous bending tests will be left to commercial users, he said.

The film also proved environmentally stable. When the research paper was submitted in late 2010, test films had been exposed to the environment in the lab for six months without deterioration. After a year, they remain so.

- Julien Happich
??EE Times

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