Research: Graphene grows better on certain copper crystals

Oct 27, 2011
An illustration of rendered experimental data showing the polycrystalline copper surface and the differing graphene coverages. Graphene grows in a single layer on the (111) copper surface and in islands and multilayers elsewhere. Credit: Joshua D. Wood, University of Illinois

New observations could improve industrial production of high-quality graphene, hastening the era of graphene-based consumer electronics, thanks to University of Illinois engineers.

By combining data from several imaging techniques, the team found that the quality of depends on the crystal structure of the substrate it grows on. Led by electrical and professors Joseph Lyding and Eric Pop, the researchers published their findings in the journal .

"Graphene is a very important material," Lyding said. "The future of electronics may depend on it. The quality of its production is one of the key unsolved problems in nanotechnology. This is a step in the direction of solving that problem."

To produce large sheets of graphene, is piped into a furnace containing a sheet of . When the methane strikes the copper, the carbon-hydrogen bonds crack. Hydrogen escapes as gas, while the carbon sticks to the copper surface. The move around until they find each other and bond to make graphene. Copper is an appealing substrate because it is relatively cheap and promotes single-layer graphene growth, which is important for .

"It's a very cost-effective, straightforward way to make graphene on a large scale," said Joshua Wood, a graduate student and the lead author of the paper.

"However, this does not take into consideration the subtleties of growing graphene," he said. "Understanding these subtleties is important for making high-quality, high-performance electronics."

While graphene grown on copper tends to be better than graphene grown on other substrates, it remains riddled with defects and multi-layer sections, precluding high-performance applications. Researchers have speculated that the roughness of the may affect graphene growth, but the Illinois group found that the copper's is more important.

Copper foils are a patchwork of different crystal structures. As the methane falls onto the foil surface, the shapes of the copper crystals it encounters affect how well the carbon atoms form graphene.

Different crystal shapes are assigned index numbers. Using several advanced imaging techniques, the Illinois team found that patches of copper with higher index numbers tend to have lower-quality graphene growth. They also found that two common crystal structures, numbered (100) and (111), have the worst and the best growth, respectively. The (100) crystals have a cubic shape, with wide gaps between atoms. Meanwhile, (111) has a densely packed hexagonal structure.

"In the (100) configuration the carbon atoms are more likely to stick in the holes in the copper on the atomic level, and then they stack vertically rather than diffusing out and growing laterally," Wood said. "The (111) surface is hexagonal, and graphene is also hexagonal. It's not to say there's a perfect match, but that there's a preferred match between the surfaces."

Researchers now are faced with balancing the cost of all (111) copper and the value of high-quality, defect-free graphene. It is possible to produce single-crystal copper, but it is difficult and prohibitively expensive.

The U. of I. team speculates that it may be possible to improve copper foil manufacturing so that it has a higher percentage of (111) crystals. Graphene grown on such foil would not be ideal, but may be "good enough" for most applications.

"The question is, how do you optimize it while still maintaining cost effectiveness for technological applications?" said Pop, a co-author of the paper. "As a community, we're still writing the cookbook for graphene. We're constantly refining our techniques, trying out new recipes. As with any technology in its infancy, we are still exploring what works and what doesn't."

Next, the researchers hope to use their methodology to study the growth of other two-dimensional materials, including insulators to improve graphene device performance. They also plan to follow up on their observations by growing graphene on single-crystal copper.

"There's a lot of confusion in the graphene business right now," Lyding said. "The fact that there is a clear observational difference between these different growth indices helps steer the research and will probably lead to more quantitative experiments as well as better modeling. This paper is funneling things in that direction."

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More information: The paper, "Effects of Polycrystalline Cu Substrate on Graphene Growth by Chemical Vapor Deposition," is available online at pubs.acs.org/doi/abs/10.1021/nl201566c

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eachus
5 / 5 (1) Oct 27, 2011
The solution, of course, is to find an even better (111) surface that can be grown economically. Obviously, silicon is one candidate, but it may be necessary to grow an epitaxial layer of some other element on top to allow the graphene to form. Copper is possible, but as indicated in the article, difficult. Aluminum? GaAs on sapphire?
RealScience
not rated yet Oct 27, 2011
Silicon is already grown in near-perfect single crystal. The same process (Czochralski) also works for crystals of gold and silver, so it should work for copper.
It is not an inexpensive process, but I would expect the copper substrate to be reusable, and that the cost of single-crystal copper could thus be amortized over many sheets of graphene.
eachus
5 / 5 (1) Oct 27, 2011
Silicon is already grown in near-perfect single crystal. The same process (Czochralski) also works for crystals of gold and silver, so it should work for copper.


It does, however there are no commercial sources.

It is not an inexpensive process, but I would expect the copper substrate to be reusable, and that the cost of single-crystal copper could thus be amortized over many sheets of graphene.


Many, many sheets of graphene. But I have a suspicion that the electronics industry would either keep the graphene on the copper substrate while making chips, or prefer copper grown epitaxially on silicon to fit with current technology.

If you want the graphene for some other purpose, and there are lots of potential uses outside the micro-electronics area, then the copper substrate cost should be minimal.
antialias_physorg
5 / 5 (1) Oct 28, 2011
(111) surface that can be grown economically. Obviously, silicon is one candidate, but it may be necessary to grow an epitaxial layer of some other element on top to allow the graphene to form. Copper is possible, but as indicated in the article, difficult. Aluminum? GaAs on sapphire?

The problem is not so much the correct orientation but the correct spacing of the atoms of the substrate. While we can manufacture high quality Si or GaAs crystals they don't have the right spacing for growing graphene on top. If the spacing of the substrate and the graphene don't match then you will get stresses in the graphene which will induce defects.
plasma_jock
not rated yet Oct 28, 2011
It is not an inexpensive process, but I would expect the copper substrate to be reusable, and that the cost of single-crystal copper could thus be amortized over many sheets of graphene.

Unfortunately this is not the case. Graphene on copper is fairly useless from a device manufacturing standpoint. The current method used for removing it from the copper involves dissolving the copper in a bath of strong acids. This process destroys the copper and often leads to defects in the graphene. Unless a better method to transfer the graphene is found I'm afraid graphene on copper is non-starter from a manufacturing perspective.
Callippo
not rated yet Oct 28, 2011
This process destroys the copper and often leads to defects in the graphene
The article clearly says, these defects are related to orientation of copper grains, not the acid treatment.
RealScience
not rated yet Oct 28, 2011
Plasma - right now the cost of the copper is so low that there is no incentive to preserve it, and the graphene is weaker because of the interfaces between regions that grew on different copper regions. The SP2 orbital bonds in single sheet of monocrystaline graphene should be enough stronger than the P bonds to the copper that lift-off should be practical. Some lift-off methods should allow copper reuse; if the copper becomes expensive those would be preferred.
(Of course 'should' still means a few years of R&D with no guarantee of success.)
plasma_jock
5 / 5 (1) Oct 29, 2011
My understanding is that dissolving the copper takes a very long time (a few hours), which is why I think it would be cost prohibitive to remove graphene that way. I agree with the need to find a way to lift the graphene off the copper using some kind of adhesion process. I think that's going to be one of the silver bullet that makes building devices economical.
eachus
not rated yet Oct 30, 2011
It seems crazy, but one way of lifting the graphene would be to deposit some other material between the graphene sheet and the copper. A properly tuned ion deposition of helium or neon should create a gas layer between the copper and graphene. Some of the gas would "leak" through the chicken wire holes in the graphene, but that's fine, as long as you can construct an adsorbed gas layer.

An alternative would be to oxidize the graphene surface, then use low heat to move the oxygen under the graphene and binding to the copper. Lots of potential games to play in a fab. But as I indicated, a fab would tend to use the graphene layer in place (well, cut into billions of little pieces to make individual transistors).

To make a long story short, cut the graphene into ribbons, or grow it that way. Etch away the copper under the middle of a very short ribbon, now grow the gate there or on the surface. The two copper connections at the ends of the ribbon are the source and drain.