Clearing a fabrication hurdle with graphene

April 15, 2010 – Researchers from Lawrence Berkeley National Labs say they have taken a big step forward in addressing one of the major challenges in graphene: figuring out an economical, high-quality and production-worthy way of making it.

Graphene’s unique properties are well known: excellent electron mobility (100× faster than silicon) and an atomic structure with great flexibility and mechanical strength. But manufacturability is a problem — current fabrication methods based on mechanical cleavage or ultrahigh vacuum annealing aren’t compatible with volume production, notes Yuegang Zhang, materials scientist at Berkeley Labs. "Before we can fully utilize the superior electronic properties of graphene in devices, we must first develop a method of forming uniform single-layer graphene films on nonconducting substrates on a large scale," he says.

In their work, published in Nano Letters, Zhang and colleagues used direct chemical vapor deposition (CVD) to synthesize single-layer films of graphene on a dielectric substrate (they evaluated single-crystal quartz, sapphire, fused silica, and silicon oxide). Hydrocarbon precursors were catalytically decomposed over thin copper films (100-450nm thickness) which were predeposited via e-beam evaporation on the dielectric substrate. Dewetting and evaporating the Cu films yielded single-layer graphene film on a bare dielectric. Scanning Raman mapping, spectroscopy, and SEM and AFM confirmed continuous single-layer graphene films coating metal-free areas of dielectric substrate, measuring "tens of square micrometers."

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Figure 1. To make a graphene thin film, Berkeley researchers (a) evaporated a thin layer of copper on a dielectric surface; (b) then used CVD to lay down a graphene film over the copper. (c) The copper dewets and evaporates leaving (d) the graphene film directly on the dielectric substrate.

"This is exciting news for electronic applications because chemical vapor deposition is a technique already widely used in the semiconductor industry," Zhang notes.

Improved control of the dewetting and evaporation could lead to direct deposition of patterned graphene for large-scale electronic device fabrication, and could be used to deposit other 2D materials such as boron nitride, according to Zhang. And although wrinkles in the graphene film following the dewetting shape of the copper introduce mobility-slowing strains, "if we can learn to control the formation of wrinkles in our films, we should be able to modulate the resulting strain and thereby tailor electronic properties," he said.

Moreover, observing the films after Cu evaporation will help the researchers learn more about growth of graphene on metal catalyst surfaces, which will help inform better control of the process, leading to ways to tailor the film properties or produce different morphologies, such as graphene nanoribbons, he added.

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Figure 2. (a) Optical image of a CVD graphene film on a copper layer showing the finger morphology of the metal; (b) Raman 2D band map of the graphene film between the copper fingers over the area marked by the red square on left. (image from Yuegang Zhang)



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