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. (Credit: Tour Lab/Rice University) |
Flexible, transparent electronics are closer to reality with the creation of graphene-based electrodes at Rice University.
The
lab of Rice chemist James Tour lab has created thin films that could
revolutionize touch-screen displays, solar panels and LED lighting. The
research was reported in the online edition of ACS Nano.
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, Tour said. Combined with
other flexible, transparent electronic components being 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.
The
lab’s hybrid graphene film is a strong candidate to replace indium tin
oxide (ITO), a commercial product widely used as a transparent,
conductive coating. It’s the essential element in virtually all
flat-panel displays, including touch screens on smart phones and iPads,
and is part of organic light-emitting diodes (OLEDs) and solar cells.
ITO
works well in all of these applications, but has several disadvantages.
The element indium is increasingly rare and expensive. It’s also
brittle, which heightens the risk of a screen cracking when a smart
phone is dropped and further rules ITO out as the basis for flexible
displays.
The
Tour Lab’s thin film combines a single-layer sheet of highly conductive
graphene with a fine grid of metal nanowire. The researchers claim the
material easily outperforms ITO and other competing materials, with
better transparency and lower resistance to electric current.
“Many
people are working on ITO replacements, especially as it relates to
flexible substrates,” said Tour, Rice’s T.T. and W.F. Chao Chair in
Chemistry as well as a professor of mechanical engineering and materials
science and of computer science. “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.”
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 of Rice University chemist James Tour. (Credit: Yu Zhu/Rice University) |
Conversely,
said postdoctoral researcher Yu Zhu, lead author of the new paper, 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.
But
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-micron nanowires
made of inexpensive, lightweight aluminum did not detract from the
material’s transparency.
“Five-micron grid lines are about a 10th the size of a human hair, and a human hair is hard to see,” Tour said.
Tour
said 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, he said; commercial labs have already developed a roll-to-roll
graphene production technique.
“This material is ready to scale right now,” he said.
The
flexibility is almost a bonus, Zhu said, due to the potential savings
of using carbon and aluminum instead of expensive ITO. “Right now, ITO
is the only commercial electrode we have, but it’s brittle,” he said.
“Our transparent electrode has better conductivity than ITO and it’s
flexible. I think flexible electronics will benefit a lot.”
In
tests, he found the hybrid film’s conductivity decreases by 20 to 30
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 test will be left to commercial
users, he said.
“I
don’t know how many times a person would roll up a computer,” Tour
added. “Maybe 1,000 times? Ten thousand times? It’s hard to see how it
would wear out in the lifetime you would normally keep a device.”
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.
“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.
Rice graduate students Zhengzong Sun and Zheng Yan and former postdoctoral researcher Zhong Jin are co-authors of the paper.
The
Office of Naval Research Graphene MURI program, the Air Force Research
Laboratory through the University Technology Corporation, the Air Force
Office of Scientific Research and the Lockheed Martin Corp./LANCER IV
program supported the research.
Rational Design of Hybrid Graphene Films for High-Performance Transparent Electrodes
Roll-to-roll production of 30-inch graphene films for transparent electrodes
