UT Austin Researchers Grow Large Graphene Crystals That Have Exceptional Electrical Properties

Nov. 14, 2013

AUSTIN, Texas — When it comes to the growth of graphene — an ultrathin, ultrastrong, all-carbon material — it is survival of the fittest, according to researchers at The University of Texas at Austin.

The team used surface oxygen to grow centimeter-size single graphene crystals on copper. The crystals were about 10,000 times as large as the largest crystals from only four years ago. Very large single crystals have exceptional electrical properties.

“The game we play is that we want nucleation (the growth of tiny ‘crystal seeds’) to occur, but we also want to harness and control how many of these tiny nuclei there are, and which will grow larger,” said Rodney S. Ruoff, professor in the Cockrell School of Engineering. “Oxygen at the right surface concentration means only a few nuclei grow, and winners can grow into very large crystals.”

The team — led by postdoctoral fellow Yufeng Hao and Ruoff of the Department of Mechanical Engineering and the Materials Science and Engineering Program, along with Luigi Colombo, a material scientist with Texas Instruments — worked for three years on the graphene growth method. The team’s paper, “The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper,” is featured on the cover of the Nov. 8, 2013, issue of Science.

One of the world’s strongest materials, graphene is flexible and has high electrical and thermal conductivity that makes it a promising material for flexible electronics, solar cells, batteries and high-speed transistors. The team’s understanding of how graphene growth is influenced by differing amounts of surface oxygen is a major step toward improved high-quality graphene films at industrial scale.

The team’s method “is a fundamental breakthrough, which will lead to growth of high-quality and large area graphene film,” said Sanjay Banerjee, who heads the Cockrell School’s South West Academy of Nanoelectronics (SWAN). “By increasing the single-crystal domain sizes, the electronic transport properties will be dramatically improved and lead to new applications in flexible electronics.”

Graphene has always been grown in a polycrystalline form, that is, it is composed of many crystals that are joined together with irregular chemical bonding at the boundaries between crystals (“grain boundaries”), something like a patch-work quilt. Large single-crystal graphene is of great interest because the grain boundaries in polycrystalline material have defects, and eliminating such defects makes for a better material.

By controlling the concentration of surface oxygen, the researchers could increase the crystal size from a millimeter to a centimeter. Rather than hexagon-shaped and smaller crystals, the addition of the right amount of surface oxygen produced much larger single crystals with multibranched edges, similar to a snowflake.

“In the long run it might be possible to achieve meter-length single crystals,” Ruoff said. “This has been possible with other materials, such as silicon and quartz. Even a centimeter crystal size — if the grain boundaries are not too defective — is extremely significant."

“We can start to think of this material’s potential use in airplanes and in other structural applications — if it proves to be exceptionally strong at length scales like parts of an airplane wing, and so on,” he said.

Another major finding by the team was that the “carrier mobility” of electrons (how fast the electrons move) in graphene films grown in the presence of surface oxygen is exceptionally high. This is important because the speed at which the charge carriers move is important for many electronic devices — the higher the speed, the faster the device can perform.

Yufeng Hao says he thinks the knowledge gained in this study could prove useful to industry.

“The high quality of the graphene grown by our method will likely be developed further by industry, and that will eventually allow devices to be faster and more efficient,” Hao said.

Single-crystal films can also be used for the evaluation and development of new types of devices that call for a larger scale than could be achieved before, added Colombo.

“At this time, there are no other reported techniques that can provide high quality transferrable films,” Colombo said. “The material we were able to grow will be much more uniform in its properties than a polycrystalline film.”

This study was funded at UT Austin by the W.M. Keck Foundation, the Office of Naval Research and the Southwest Area Nanotechnology Center (SWAN), which is supported by the Nanoelectronics Research Initiative (NRI). The paper’s co-authors are from the Cockrell School of Engineering and the Department of Physics. Other co-authors are from Columbia University, A*STAR (in Singapore), Sandia National Laboratories-Livermore, Rice University and Texas Instruments.

Disclosure:

The University of Texas at Austin is committed to transparency and disclosure of all potential conflicts of interest of its researchers. Ruoff has received funding for graphene research from various public and private entities, including the Office of Naval Research and the W.M. Keck Foundation. He is a member of the American Chemical Society, the American Physical Society and the Materials Research Society. He serves as an editor for NANO, Composites Science and Technology, IEEE Transactions on Nanotechnology and the Journal of Nanoengineering and Nanosystems.

Ruoff co-founded nCarbon Inc., which focuses on ultracapacitors. Previously he co-founded Graphene Materials LLC. (now defunct).

For more information, contact: Sandra Zaragoza, Cockrell School of Engineering College of Engineering, (512) 471-2129.

3 Comments to "UT Austin Researchers Grow Large Graphene Crystals That Have Exceptional Electrical Properties"

1.  Tom Billings said on Nov. 14, 2013

On May 31st a Columbia Engineering team reported making polycrystaline graphene up to 90% as strong as exfoliated graphene flakes. They did this by changing the etching solution used to remove the copper foil substrate from the graphene sheet to one that did not damage the boundaries between the graphene crystals.

http://engineering.columbia.edu/even-defects-graphene-strongest-material-world-1

I would be interested to hear if the Texas team's work synchronizes with that of the Columbia team. In particular, the relationship of graphene grain size to Oxygen revealed in the Texas article might interact not only by bringing the sheet strength up past 90%, but it may enhance electronic properties as well. Has this been considered by the Texas team?

2.  University Communications said on Nov. 21, 2013

Response by Prof. Rodney S. Ruoff, co-corresponding author of the November 8 Science publication:

This is an excellent question, thank you.

(Let me first offer a few background comments, realizing that Tom Billings might not need them, but that others might appreciate them.). Strength is a kinetic parameter, not a thermodynamic parameter. At times the strength of a material is confused with the stiffness of the same material. (The Wikipedia entries “Strength of Materials” and “Stiffness” make clear the difference.) There are a variety of factors that influence, thus, the ‘strength’ of a material. For how long does it bear the load applied before breaking? What is the rate of loading (the ‘strain rate’), is there cyclical loading, and so on? On small length scales, there can be samples that are essentially free of any defects (because they are small in size) and thus measured strength values can approach the ideal ‘high end’ limit. While this is important and interesting, what is technologically important for macroscale versus microscale-nanoscale applications, is the presence or absence of defects, and the types of defects present, over the relevant length scale. And for ‘macroscale’ this means about centimeters or more in size.

With this background, I suggest that if researchers, hopefully in the near future, report very high strengths (such as above 10 Gpa or so, see below) from graphene-based samples that are of the sort of ‘standard’ lengths used for testing macroscale samples (such as ‘dog bone’ samples prepared for tensile testing that are about 10 centimeters long), and including under a variety of testing conditions such as in the presence of different chemical environments, at different and technologically relevant temperatures, at different rates of loading, and eventually also with cyclic loading and also loaded over long times, then there is reason for tremendous interest and excitement. The current highest strength macroscale materials that are in commercial use are the highly graphitized carbon fibers and also fibers comprised of rigid rod polymers, with ultimate strength values around 5.6 GPa (GPa is the abbreviation for Gigapascals; for comparison a high strength steel might have a strength of about 2 GPa). Such fibers are used, e.g., in the composites in the “Boeing Dreamliner”.

There is another macroscale material (‘glass fiber’) that can have much higher strengths than 5.6 GPa, but these higher values occur only under unique loading conditions. Remarkably, glass (glass fibers that have been carefully polished to remove surface flaws) can exhibit a strength of about 20 GPa, but only when loaded at, say, liquid helium (about 4 Kelvin) or liquid nitrogen (about 77 Kelvin) temperatures to scrupulously remove any water. The presence of even tiny amounts of water molecules can lead to failure at much lower values of stress—way below 5.6 GPa. The ideal strength of graphene should be about 130 GPa or so. (It has been said at times, including in the mass media, that ideal graphene would be the strongest material that could ever exist. However, it is likely that diamond, if defect free, would have a strength when loaded along the (100) direction of roughly around 225 GPa, per first principles calculations; see, e.g., Telling, R. H.; Pickard, C. J.; Payne, M. C. and Field, J. E. (2000). "Theoretical Strength and Cleavage of Diamond". Physical Review Letters 84 (22): 5160–5163.

I close my remarks by noting that, yes, I have been thinking about the strength of graphene and its possibilities, since about 1992 because I foresaw then that it might eventually be made in very large scale. (Thank you for asking. This article may be of further interest: Ruoff, Rodney S. Personal perspectives on graphene: New graphene-related materials on the horizon. MRS Bulletin (December 2012 special issue: Graphene: Fundamentals and Functionalities), 37, 1314-1318). My team published several papers in 1999 oriented towards this question of the ultimate strength of graphene. And with the large area growth of graphene by CVD on copper foils that we and others have contributed to, if such samples have relatively complete bonding at the grain boundaries (no “holes” in the material!) and large crystal size to minimize the number of grain boundaries, then graphene should find use in many structural applications, with aerospace immediately coming to mind, but in many others as well (of course based on availability and cost).

Indeed, at this juncture of graphene R&D, I suggest that it is now VERY timely for creative people to be thinking about “What kind of applications benefit from a strength that may be in the range of, say, 10 to 100 GPa, and keeping in mind that at a stress of 100 GPa the material is elongated about 10% and where the material is of low density (graphene: about 2g/cc)”.

3.  Johnny Giddings said on Nov. 22, 2013

My name is Johnny Giddings, I am a 13 year old homeschool student who is involved in 2 STEM competitions. Both competitions encourage the use of futuristic materials. I have strongly suggested the use of graphene. I found lots of data on the internet, but am unable to find answers to these questions: What is pure graphene's weight per cubic inch, and what is graphene's electrical conductivity. In my research I found that you're one of the leading scientists studying graphene and am hoping you can help me out with this information. Thank you for you time.