News in Brief
10-04 :: April 2010
Graphene Films Clear Major Fabrication Hurdle
Energy (DOE) center for nanoscience, have taken a significant step at clearing the major hurdle with graphene.
“Before we can fully utilize the superior electronic properties of graphene in devices, we must first de- velop a method of forming uniform single-layer gra- phene films on nonconducting substrates on a large scale,” says Yuegang Zhang, a materials scientist with the Lawrence Berkeley National Laboratory (Berkeley Lab). Current fabrication methods based on mecha- nical cleavage or ultrahigh vacuum annealing, he says, are ill-suited for commercial-scale production. Graphene films made via solution-based deposition and chemical reduction have suffered from poor or uneven quality.
The Berkeley researcher have successfully used direct chemical vapor deposition (CVD) to synthesize single-layer films of graphene on a dielectric sub- strate. Zhang and his colleagues made their graphene films by catalytically decomposing hydrocarbon precursors over thin films of copper that had been pre-deposited on the dielectric substrate. The copper films subsequently dewetted (separated into puddles or droplets) and were evaporated. The final product was a single-layer graphene film on a bare dielectric.
“This is exciting news for electronic applications because chemical vapor deposition is a technique
uegang Zhang and colleagues at Berkeley Lab’s Molecular Foundry, a U.S. Department of
already widely used in the semiconductor industry,” Zhang says.
“Also, we can learn more about the growth of gra- phene on metal catalyst surfaces by observing the evolution of the films after the evaporation of the copper. This should lay an important foundation for further control of the process and enable us to tailor the properties of these films or produce desired mor- phologies, such as graphene nanoribbons.”
In their study, Zhang and his colleagues used electron-beam evaporation to deposit copper films ranging in thickness from 100 to 450nm. Copper was chosen because as a low carbon solubility metal cata- lyst it was expected to allow better control over the number of graphene layers produced. Several diffe- rent dielectric substrates were evaluated including single-crystal quartz, sapphire, fused silica and silicon oxide wafers. CVD of the graphene was carried out at 1,000° Celsius (1,832° Fahrenheit) in durations that ranged from 15 minutes up to seven hours.
“This was done to allow us to study the effect of film thickness, substrate type and CVD growth time on the graphene formation,” Zhang says. A combinati- on of scanning Raman mapping and spectroscopy, plus scanning electron and atomic force microscopy confirmed the presence of continuous single-layer graphene films coating metal-free areas of dielectric substrate measuring tens of square micrometers.