Korean researchers have found a way to make large graphene films that are both strong and stretchy and have the best electrical properties yet. These atom-thick sheets of carbon are a promising material for making flexible, see-through electrodes and transistors for flat-panel displays. Graphene could also lead to foldable organic light-emitting diode (OLED) displays and organic solar cells. However, it has not been easy finding a way make large, high-quality sheets of graphene.
Researchers from the Sungkyunkwan University and the Samsung Advanced Institute of Technology, in Suwon, Korea, have made centimeters-wide graphene films that are 80 percent transparent and can be bent and stretched without breaking or losing their electrical properties. Others have made large graphene films using simpler techniques, but the new films are 30 times more conductive. In addition, it is easy to transfer the new films onto different substrates. “We have demonstrated that graphene is one of the best materials for stretchable transparent electronics,” says Byung Hee Hong, who led the work, which is published in Nature.
Graphene is an excellent conductor, and it transports electrons tens of times faster than silicon does. It could replace the brittle indium tin oxide (ITO) electrodes that are currently used in displays, organic solar cells, and touch screens. Graphene transistors could also replace silicon thin-film transistors, which are not transparent and are hard to fabricate on plastic.
The easiest way to make tiny flakes of high-quality graphene is to peel off graphene layers from graphite (which is, essentially, just a stack of graphene sheets). Last year, a group led by Rutgers University materials-science and engineering professor Manish Chhowalla devised a method for making centimeters-scale pieces for practical applications. The researchers dissolved graphite oxide in water, creating a suspension of individual graphene-oxide sheets, which they deposited on top of a flexible substrate.
The Korean researchers use a method called chemical vapor deposition. First, they deposit a 300-nanometer-thick layer of nickel on top of a silicon substrate. Next, they heat this substrate to 1,000 Cº in the presence of methane, and then cool it quickly down to room temperature. This leaves behind graphene films containing six to ten graphene layers on top of the nickel. By patterning the nickel layer, the researchers can create patterned graphene films.
Others, such as MIT electrical-engineering professor Jing Kong, are working on similar approaches to making large graphene pieces. But the Korean researchers have taken the work a step further, transferring the films to flexible substrates while maintaining high quality. The transfer is done in one of two ways. One is to etch away the nickel in a solution so that the graphene film floats on its surface, ready to be deposited on any substrate. A simpler trick is to use a rubber stamp to transfer the film.