Monday, April 14, 2008

How to Make Graphene

A simple way to deposit thin films of carbon could lead to cheaper solar cells.

By Prachi Patel-Predd

Flexible process: A new fabrication method developed by researchers at Rutgers University can deposit a film of graphene--an atom-thick sheet of carbon--on almost any substrate, including the flexible plastic shown here. The films could be used in thin-film transistors or as conductive electrodes for organic solar cells. Credit: Manish Chhowalla, Rutgers University

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.