Dream Screens from Graphene
HOUSTON, Aug. 4, 2011 — Graphene-based electrodes developed by a team at Rice University could revolutionize touch-screen displays, solar panels and LED lighting, bringing flexible, transparent electronics closer to reality.
Flexible, see-through video screens may be the “killer app” that finally puts graphene — the highly touted single-atom-thick form of carbon — into the commercial spotlight once and for all, said Rice chemist James Tour. When combined with other flexible, transparent electronic components developed at Rice and elsewhere, the breakthrough could lead to computers that wrap around the wrist and solar cells that wrap around just about anything.
An electron microscope image of a hybrid electrode developed at Rice University shows 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. (Image: Tour Lab/Rice University)
The hybrid graphene film is a candidate to replace indium tin oxide (ITO), a commercial product widely used as a transparent, conductive coating. An essential element in flat-panel displays, including touch screens on smartphones and iPads, ITO also is part of organic LEDs and solar cells.
Although ITO works well in several applications, it has several disadvantages. The element indium is increasingly rare and expensive. In addition, it is brittle, heightening the risk of a screen cracking when a smartphone is dropped, and further rules ITO out as the basis for flexible displays.
The new thin film combines a single-layer sheet of highly conductive graphene with a fine grid of metal nanowire. The researchers claim that the material easily outperforms ITO and other competing materials, with better transparency and lower resistance to electric current.
Postdoctoral researcher Yu Zhu holds a sample of a transparent electrode that merges graphene and a fine aluminum grid. It could become a key component of flexible displays, solar cells and LED lighting. Clockwise from top right: Rice professor James Tour, Zhu and graduate students Zheng Yan and Zhengzong Sung. (Image: Jeff Fitlow/Rice University)
“Many people are working on ITO replacements, especially as it relates to flexible substrates,” Tour said. “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.”
Conversely, fine metal meshes show good conductivity, but gaps in the nanowires to keep them transparent make them unsuitable as stand-alone components in conductive electrodes, said postdoctoral researcher Yu Zhu.
The researchers discovered that combining the materials work superbly. The metal grid strengthens the graphene, and the graphene fills all the empty spaces between the grid. The researcher found that a grid of 5-µm nanowires made of inexpensive, lightweight aluminum did not detract from the material’s transparency.
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. (Image: Yu Zhu/Rice University)
The metal grids easily could be produced on a flexible substrate via standard techniques, including roll-to-roll and ink-jet printing, they say. Techniques for making large sheets of graphene also are improving rapidly; commercial labs have already developed a roll-to-roll graphene production technique, according to the Rice scientists.
Its flexibility offers a bonus because of its potential savings of using carbon and aluminum instead of the expensive ITO.
In tests, the scientists found that the hybrid film’s conductivity decreased by 20 to 30 percent with the initial 50 bends, but after that, the material stabilized. After 500 bending cycles, the scientists observed no significant variations. More rigorous bending tests will be left to commercial users, Zhu said.
In addition, the film has proved environmentally stable. When the research paper was submitted in late 2010, the films had been exposed to the environment in the lab for six months without deterioration. After a year, they remain so.
“Now that we know it works fine on flexible substrates, this brings the efficacy of graphene a step up to its potential utility,” Tour said.
The research was presented in the online edition of ACS Nano.
For more information, visit: www.rice.edu
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