Graphene–a flat single layer of carbon atoms–can transport electrons at remarkable speeds, making it a promising material for electronic devices. Until recently, researchers had been able to make only small flakes of the material, and only in small quantities. However, Rutgers University researchers have developed an easy way to make transparent graphene films that are a few centimeters wide and one to five nanometers thick.
Thin films of graphene could provide a cheap replacement for the transparent, conductive indium tin oxide electrodes used in organic solar cells. They could also replace the silicon thin-film transistors common in display screens. Graphene can transport electrons tens of times faster than silicon, so graphene-based transistors could work faster and consume less power. (See “Graphene Transistors” and “Better Graphene Transistors.”)
In fact, Rutgers materials science and engineering professor Manish Chhowalla and his colleagues used their graphene films to make prototype transistors and organic solar calls. In a recent Nature Nanotechnology paper, they showed that they can deposit the transparent films on any substrate, including glass and flexible plastic. Chhowalla says that the method could be adapted to a larger scale to coat “meters and meters of substrates with graphene films,” using roll-to-roll processing, a technique being developed to make large flexible electronic circuits.
By contrast, current techniques for making graphene yield small quantities of the material, fit only for experimental use. One common technique is called the “Scotch tape method,” in which a piece of tape is used to peel graphene flakes off of a chunk of graphite, which is essentially a stack of graphene sheets. This results in micrometer-sized graphene fragments, which are placed between electrodes to make a transistor. “But if you talk about large-scale devices, you want to make macroscopic [sheets],” says Hannes Schniepp, a graphene researcher at Princeton University. For that, you need to guide the assembly of smaller graphene pieces over a large area, Schniepp says, which is exactly what the Rutgers researchers do.
The researchers start by making a suspension of graphene oxide flakes. They oxidize graphite flakes with sulphuric or nitric acid. This inserts oxygen atoms between individual graphene sheets and forces them apart, resulting in graphene oxide sheets, which are suspended in water.
The suspension is filtered through a membrane that has 25-nanometer-wide pores. Water passes through the pores, but the graphene oxide flakes, each of which is a few micrometers wide and about one nanometer thick, cover the pores. This happens in a regulated fashion, Chhowalla says. When a flake covers a pore, water is directed to its uncovered neighbors, which in turn get covered, until flakes are distributed across the entire surface. “The method allows you to deposit single layers of graphene,” Chhowalla says. “[It] results in a nearly uniform film deposited on the membrane.” The researchers place the film-coated side of the membrane on a substrate, such as glass or plastic, and wash away the membrane with acetone. Finally, they expose the film to a chemical called hydrazine, which converts the graphene oxide into graphene.
James Tour, a chemistry professor at Rice University, says that this is “certainly the easiest method I’ve seen for making [graphene thin films] over large areas.” He thinks that the process could easily be converted into a larger, commercial-scale manufacturing technique. “It’s very amenable for rapid production,” he says. “It’s not going to take much to get these things produced … and cover large areas.”
Chhowalla and his colleagues control the thickness of the film by changing the suspension’s volume. A volume of 20 milliliters results in a film that is mostly one to two nanometers in thickness, while an 80-milliliter suspension results in films that are mainly three to five nanometers thick. The thinner films are 95 percent transparent. The researchers have used the films as the transparent electrodes in organic solar cells. They have also made transistors by placing their films on a silicon substrate and depositing gold electrodes on them.
The graphene films need a lot more work. Right now, the transistors do not carry as much current as those made from individual graphene flakes, which, the researchers speculate, is because of overlapping flakes in their films. For high-quality transistors, they will need to make single-layer graphene films with no overlap. They also need to improve the conductivity of their film: indium tin oxide is still hundreds of times more conductive. Organic solar cells with indium tin oxide electrodes are between 3 percent and 5 percent efficient. “With graphene thin-film electrodes, we get 0.1 percent,” Chhowalla says, “but these are proof-of-concept devices and of course will improve with time.”
Tour believes that the film holds more promise for organic solar cells than for transistors. Many researchers are also studying carbon nanotube films as a way to replace indium tin oxide coatings on solar cells. But Tour says that graphene would be “possibly easier than using carbon nanotubes because of the greater availability of the material.” The industry might also find it easier to adopt graphene because of the concerns that some people have about the effects of carbon nanotubes on the environment.