Hybrid graphene films could lead to flexible displays
HOUSTON – Graphene-based electrodes could revolutionize touch-screen displays, LED lighting and solar panels, bringing flexible, transparent electronics closer to reality.
The thin films created in the lab of Rice University chemist James Tour combine a single-layer sheet of highly conductive graphene with a fine metal nanowire grid. The scientists claim that the material outperforms indium tin oxide (ITO), making it a strong candidate to replace the transparent conductive coating, which is rare, expensive and brittle. ITO is used in almost every flat panel display, including touch screens on smartphones, and it is part of organic LEDs and solar cells.
A hybrid material that combines a fine aluminum mesh with a one-atom-thick layer of graphene outperforms materials common to current touch screens and solar cells. Courtesy of Yu Zhu, Rice University.
Although ITO works well in these applications, its disadvantages have proved troublesome as researchers look to develop flexible, transparent consumer electronics. Because it is brittle, the risk of a smartphone screen cracking when dropped is heightened. This rules ITO out as the basis for flexible displays.
“Many people are working on ITO replacement, 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, good conductivity is observed with fine metal meshes, but gaps in the nanowire to keep the meshes transparent make them unsuitable as a stand-alone component in conductive electrodes, the scientists say.
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. Courtesy of Jeff Fitlow, Rice University.
However, they discovered that combining the two materials had the best outcome. The metal grid strengthened the graphene, and the empty spaces between the grid were filled with graphene. The researchers found that a 5-µm nanowire made of lightweight, inexpensive aluminum did not detract from the transparency of the material.
Tour said metal grids could be easily produced on flexible substrates via roll-to-roll or ink-jet printing. Techniques to produce large sheets of graphene also have improved, he said; commercial labs have already developed a roll-to-roll graphene production method.
After testing the hybrid film, the researchers found that its conductivity decreased by 20 to 30 percent with the initial 50 bends, but after that, the material stabilized. No significant variations were observed after up to 500 bending cycles, but more rigorous testing would have to be performed by commercial users to see just how much bending and flexing the material could withstand.
An electron microscope image of a hybrid electrode developed at Rice University shows solid connections after 500 bends. The transparent material combines one-atom-thick sheets of graphene and a fine mesh of aluminum nanowire on a flexible substrate. Courtesy of James Tour Lab, Rice University.
“I don’t know how many times a person would roll up a computer,” Tour said. “Maybe a thousand times? Ten thousand times? It’s hard to see how it would wear out in the lifetime you would normally keep a device.”
The research was reported online July 20 in ACS Nano (doi: 10.1021/nn201696g). It was supported by the Office of Naval Research Graphene MURI program, the US Air Force Research Laboratory through University Technology Corp., the US Air Force Office of Scientific Research and the Lockheed Martin Corp./LANCER IV program.
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