Friday, April 24, 2015
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Memorial Service for Thomas A. Tombrello, Jr., Robert H. Goddard Professor of Physics

Caltech Scientists Develop Cool Process to Make Better Graphene

A new technique invented at Caltech to produce graphene—a material made up of an atom-thick layer of carbon—at room temperature could help pave the way for commercially feasible graphene-based solar cells and light-emitting diodes, large-panel displays, and flexible electronics.

"With this new technique, we can grow large sheets of electronic-grade graphene in much less time and at much lower temperatures," says Caltech staff scientist David Boyd, who developed the method.

Boyd is the first author of a new study, published in the March 18 issue of the journal Nature Communications, detailing the new manufacturing process and the novel properties of the graphene it produces.

Graphene could revolutionize a variety of engineering and scientific fields due to its unique properties, which include a tensile strength 200 times stronger than steel and an electrical mobility that is two to three orders of magnitude better than silicon. The electrical mobility of a material is a measure of how easily electrons can travel across its surface.

However, achieving these properties on an industrially relevant scale has proven to be complicated. Existing techniques require temperatures that are much too hot—1,800 degrees Fahrenheit, or 1,000 degrees Celsius—for incorporating graphene fabrication with current electronic manufacturing. Additionally, high-temperature growth of graphene tends to induce large, uncontrollably distributed strain—deformation—in the material, which severely compromises its intrinsic properties.   

"Previously, people were only able to grow a few square millimeters of high-mobility graphene at a time, and it required very high temperatures, long periods of time, and many steps," says Caltech physics professor Nai-Chang Yeh, the Fletcher Jones Foundation Co-Director of the Kavli Nanoscience Institute and the corresponding author of the new study. "Our new method can consistently produce high-mobility and nearly strain-free graphene in a single step in just a few minutes without high temperature. We have created sample sizes of a few square centimeters, and since we think that our method is scalable, we believe that we can grow sheets that are up to several square inches or larger, paving the way to realistic large-scale applications."

The new manufacturing process might not have been discovered at all if not for a fortunate turn of events. In 2012, Boyd, then working in the lab of the late David Goodwin, at that time a Caltech professor of mechanical engineering and applied physics, was trying to reproduce a graphene-manufacturing process he had read about in a scientific journal. In this process, heated copper is used to catalyze graphene growth. "I was playing around with it on my lunch hour," says Boyd, who now works with Yeh's research group. "But the recipe wasn't working. It seemed like a very simple process. I even had better equipment than what was used in the original experiment, so it should have been easier for me."

During one of his attempts to reproduce the experiment, the phone rang. While Boyd took the call, he unintentionally let a copper foil heat for longer than usual before exposing it to methane vapor, which provides the carbon atoms needed for graphene growth.

When later Boyd examined the copper plate using Raman spectroscopy, a technique used for detecting and identifying graphene, he saw evidence that a graphene layer had indeed formed. "It was an 'A-ha!' moment," Boyd says. "I realized then that the trick to growth is to have a very clean surface, one without the copper oxide."

As Boyd recalls, he then remembered that Robert Millikan, a Nobel Prize–winning physicist and the head of Caltech from 1921 to 1945, also had to contend with removing copper oxide when he performed his famous 1916 experiment to measure Planck's constant, which is important for calculating the amount of energy a single particle of light, or photon, contains. Boyd wondered if he, like Millikan, could devise a method for cleaning his copper while it was under vacuum conditions.



Schematic of the Caltech growth process for graphene.
(Courtesy of Nature Communications)

The solution Boyd hit upon was to use a system first developed in the 1960s to generate a hydrogen plasma—that is, hydrogen gas that has been electrified to separate the electrons from the protons—to remove the copper oxide at much lower temperatures. His initial experiments revealed not only that the technique worked to remove the copper oxide, but that it simultaneously produced graphene as well.

At first, Boyd could not figure out why the technique was so successful. He later discovered that two leaky valves were letting in trace amounts of methane into the experiment chamber. "The valves were letting in just the right amount of methane for graphene to grow," he says.

The ability to produce graphene without the need for active heating not only reduces manufacturing costs, but also results in a better product because fewer defects—introduced as a result of thermal expansion and contraction processes—are generated. This in turn eliminates the need for multiple postproduction steps. "Typically, it takes about ten hours and nine to ten different steps to make a batch of high-mobility graphene using high-temperature growth methods," Yeh says. "Our process involves one step, and it takes five minutes."

Work by Yeh's group and international collaborators later revealed that graphene made using the new technique is of higher quality than graphene made using conventional methods: It is stronger because it contains fewer defects that could weaken its mechanical strength, and it has the highest electrical mobility yet measured for synthetic graphene.



Images of early-stage growth of graphene on copper. The lines of hexagons are graphene nuclei, with increasing magnification from left to right, where the scale bars from left to right correspond to 10 μm, 1 μm, and 200 nm, respectively. The hexagons grow together into a seamless sheet of graphene. (Courtesy of Nature Communications)

The team thinks one reason their technique is so efficient is that a chemical reaction between the hydrogen plasma and air molecules in the chamber's atmosphere generates cyano radicals—carbon-nitrogen molecules that have been stripped of their electrons. Like tiny superscrubbers, these charged molecules effectively scour the copper of surface imperfections providing a pristine surface on which to grow graphene.

The scientists also discovered that their graphene grows in a special way. Graphene produced using conventional thermal processes grows from a random patchwork of depositions. But graphene growth with the plasma technique is more orderly. The graphene deposits form lines that then grow into a seamless sheet, which contributes to its mechanical and electrical integrity.

A scaled-up version of their plasma technique could open the door for new kinds of electronics manufacturing, Yeh says. For example, graphene sheets with low concentrations of defects could be used to protect materials against degradation from exposure to the environment. Another possibility would be to grow large sheets of graphene that can be used as a transparent conducting electrode for solar cells and display panels. "In the future, you could have graphene-based cell-phone displays that generate their own power," Yeh says.



Atomically resolved scanning tunneling microscopic images of graphene grown on a copper (111) single crystal, with increasing magnification from left to right. (Courtesy of Nature Communications)

Another possibility, she says, is to introduce intentional imperfections into graphene's lattice structure to create specific mechanical and electronic attributes. "If you can strain graphene by design at the nanoscale, you can artificially engineer its properties. But for this to work, you need to start with a perfectly smooth, strain-free sheet of graphene," Yeh says. "You can't do this if you have a sheet of graphene that has uncontrollable defects in different places."

Along with Yeh and Boyd, additional authors on the paper, "Single-Step Deposition of High-Mobility Graphene at Reduced Temperatures," include Caltech graduate students Wei Hsiang Lin, Chen Chih Hsu and Chien-Chang Chen; Caltech staff scientist Marcus Teague; Yuan-Yen Lo, Tsung-Chih Cheng, and Chih-I Wu of National Taiwan University; and Wen-Yuan Chan, Wei-Bing Su, and Chia-Seng Chang of the Institute of Physics, Academia Sinica. Funding support for the study at Caltech was provided by the National Science Foundation, under the Institute of Quantum Information and Matter, and by the Gordon and Betty Moore Foundation and the Kavli Foundation through the Kavli Nanoscience Institute. The work in Taiwan was supported by the Taiwanese National Science Council.

Images reprinted from Nature Communications, "Single-Step Deposition of High-Mobility Graphene at Reduced Temperatures," March 18, 2015, with permission from Nature Communications.

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A Cool Process to Make Better Graphene
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Kent and Joyce Kresa Endow Leadership Chair for Caltech’s Division of Physics, Mathematics and Astronomy

Gift will transform back-of-the-napkin ideas into groundbreaking science

The Kent and Joyce Kresa Leadership Chair for Caltech's Division of Physics, Mathematics and Astronomy (PMA) will generate funds to give the division chair agility to respond immediately when singular opportunities arise. The unrestricted $10 million endowment for PMA, one of Caltech's six academic divisions, is named for senior trustee and board chair emeritus Kent Kresa and his late wife, Joyce, who established the fund.

"Kent and Joyce Kresa have given to Caltech as institutional leaders and now as major benefactors," says President Thomas F. Rosenbaum, holder of Caltech's Sonja and William Davidow Presidential Chair and professor of physics. "Deeply understanding the special nature of Caltech, their gift will empower generations of faculty and students to push the boundaries of discovery and change the world."

According to Kresa, the gift's potential impact hinges on a quality that he and his wife found to be unique to Caltech: the fact that such a small place with such a small faculty and student body can do such big science—and make such a big difference in the world.

"Caltech is a nurturer of good ideas and new things," Kresa says. "Of anywhere in the world, the ideas per student and ideas per faculty member are probably the highest at Caltech.

"We wanted to give a gift that would allow the faculty to decide where they should spend the money, and have that available in perpetuity."

B. Thomas Soifer, the inaugural holder of the Kent and Joyce Kresa Leadership Chair, says that this kind of support will have a dramatic impact. "For years, I've kept a wish list—literally a black notebook on my desk—where I capture the most exciting ideas I hear from faculty and students. This gift will transform notes like those into tangible realities by providing unrestricted funds to support groundbreaking projects. The division's greatest contributions to science all started as ventures that Caltech had the foresight to encourage, and the Kresas' gift helps ensure that this will always be true."

Kent Kresa, who is an alumnus of MIT, believes that both universities make an enormous difference in the world. Initially attracted to Caltech by the proximity of a great research institution to his West Coast home, he soon came to appreciate both the distinctive mental stimulation afforded by Caltech's highly collaborative academic environment and his friendships with faculty and staff. During his 21 years as a trustee, he has involved himself deeply, chairing the board from 2005 to 2012 and cofounding the Institute's new Space Innovation Council.

"Over the years, Kent and Joyce have supported Caltech in many diverse and important ways," says Caltech Provost Edward M. Stolper, holder of the Carl and Shirley Larson Provostial Chair and William E. Leonhard Professor of Geology. "In addition to contributing his leadership, expertise, guidance, and wisdom, Kent is one of the Institute's most thoughtful and enthusiastic supporters. It is no surprise to all of us who have known them that Kent and Joyce once again stepped forward to support one of Caltech's most important priorities."

Although the Kresas' most recent gift focuses on PMA, their philanthropy spans divisions at Caltech. In 2009, the couple endowed the Joyce and Kent Kresa Professorship in Engineering and Applied Science. Kent Kresa is intrigued by the work of the current chairholder, Sergio Pellegrino, who is developing small satellites and devices that organize themselves in space.

"There is a lot still to do in aeronautics and astronautics," Kresa says. "I'm thrilled that there are people pushing the envelope."

A member of the National Academy of Engineering and past president of the American Institute of Aeronautics and Astronautics, Kresa worked for Northrop Grumman Corporation for 28 years, ultimately serving as its president, chairman, and CEO. Kresa has served on the boards of many organizations, including the TCW Group, the W. M. Keck Foundation, General Motors—where he was interim chairman—and Avery Dennison—where he was chairman. He also spent years with the MIT Lincoln Laboratory and the Defense Advanced Research Projects Agency.

Joyce Kresa, trained as a vocalist, became a well-known champion of the arts, education, medicine, and science. She served on the board of the Los Angeles Philharmonic and was president of the Blue Ribbon, a support organization for the Music Center.

After decades of success in aerospace, Kent Kresa sees his latest gift to Caltech as a way to express his thankfulness for a rewarding career in engineering and science and to provide similar opportunities for future generations.

"I can't think of a better place to give back to than a place that will create new things," he says. "For those who have the ability to be philanthropic during their lives or to leave something after their lives are finished, there couldn't be a nicer thing to do than to honor Caltech with some of those resources."

The Kresas join a growing list of donors who have given generously to create leadership chairs that provide funding for the president, provost, and division chairs to use at their discretion. Caltech's other leadership chairs are the Sonja and William Davidow Presidential Chair; the Carl and Shirley Larson Provostial Chair; the Otis Booth Leadership Chair in the Division of Engineering and Applied Science; and the William K. Bowes Jr. Leadership Chair in the Division of Biology and Biological Engineering.

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Kresa Leadership Chair Opens Door for Opportunity
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Friday, April 10, 2015
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Transforming Chemistry Education

Tuesday, April 7, 2015
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Tuesday, March 31, 2015 to Thursday, April 16, 2015
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Caltech Professors Named Fellows of the American Physical Society

John Dabiri and Maria Spiropulu have been named fellows of the American Physical Society (APS) for their exceptional contributions to physics.

The APS Division of Fluid Dynamics nominated Dabiri, professor of aeronautics and bioengineering, for his contributions to "vortex dynamics and biological propulsion, and for pioneering new concepts in wind energy."

Dabiri, the director of the Center for Bioinspired Engineering, studies the mechanics and dynamics of biological propulsion—particularly using jellyfish as a model. His group aims to discover biologically inspired design principles that can be applied in engineering systems.

In addition, Dabiri oversees the Caltech Field Laboratory for Optimized Wind Energy (FLOWE), an experimental wind farm for testing the energy-generating efficiency of various configurations of vertical-axis wind turbines. By optimizing the placement of the wind turbines based on observations of schools of fish, Dabiri and his group demonstrated that power output can be increased tenfold.

Professor of Physics Maria Spiropulu is an experimental particle physicist. She has worked with particle accelerators and detectors for the past 22 years and has pioneered new methods of data analysis in order to learn about the physics of the universe at both astrophysical and atomic scales. She was nominated by the APS Division of Particles and Fields for her work searching for evidence of supersymmetry (a theory that says that every fundamental particle has a supersymmetric partner) and extra dimensions at the Tevatron, a proton-antiproton collider at Fermilab in Illinois. Spiropulu was also noted for her work on the characterization of the Higgs boson—a long-sought fundamental particle thought to give other particles their mass—at the Large Hadron Collider (LHC) in Geneva, Switzerland.

In addition to Dabiri and Spiropulu, 39 other Caltech faculty and researchers have been elected as fellows of the APS since the program began in 1980.

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Monday, April 20, 2015
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Tuesday, April 14, 2015
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