Simon Wins International Mathematics Prize

Barry Simon, the IBM Professor of Mathematics and Theoretical Physics at Caltech, has been awarded the International János Bolyai Prize of Mathematics for 2015 by the Hungarian Academy of Sciences. The prize is given every five years and honors internationally outstanding works in mathematics. As the award was discontinued for almost a century following World War I, Simon, whose work focuses on mathematical physics, is its sixth recipient.

In particular, Simon is being recognized for his book titled Orthogonal Polynomials on the Unit Circle, in which he connects two important fields of mathematics: the theory of orthogonal polynomials and operator theory. Orthogonal polynomials are important in solving, expanding, and interpreting solutions to many kinds of differential equations. Operator theory has fundamental applications in the study of solutions to the Schrödinger equation, which is crucial to an understanding of quantum mechanics. Simon's connection between the fields has led to diverse applications, from probability theory to theoretical physics.

Simon first arrived at Caltech as a Sherman Fairchild Distinguished Visiting Scholar in 1980, joining the faculty permanently in 1981. He is a fellow of the American Academy of Arts and Sciences. He also received the Poincaré Prize in 2012, named for mathematician Henri Poincaré. Poincaré was, incidentally, the first recipient of the Bolyai Prize in 1905.

Hungarian Academy of Sciences President László Lovász will award Simon with the prize at a public session of the academy's Section of Mathematics conference in the second half of 2015.

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JPL News: NuSTAR Captures Possible Beacons from Dead Stars

Peering into the heart of the Milky Way galaxy, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) has spotted a mysterious glow of high-energy X-rays that, according to scientists, could be the "howls" of dead stars as they feed on stellar companions.

"We may be witnessing the beacons of a hitherto hidden population of pulsars in the galactic center," said co-author Fiona Harrison of Caltech, principal investigator of NuSTAR. "This would mean there is something special about the environment in the very center of our galaxy."

Read the full story from JPL News

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Sean Carroll Awarded Guggenheim Fellowship

Sean Carroll, a research professor of physics, has been named a 2015 Fellow of the John Simon Guggenheim Memorial Foundation. Established in 1925, the Guggenheim Fellowship Program awards mid-career fellowships for those who have "demonstrated exceptional capacity for productive scholarship or exceptional creative ability in the arts." This year, the Guggenheim Foundation awarded 173 fellowships, two of which went to physicists.

Carroll came to Caltech in 2006. His research interests are broadly spread across theoretical physics, ranging from cosmology and general relativity to quantum mechanics and particle physics. His proposal to the Guggenheim Foundation, titled "Emergent Structures and the Laws of Physics," focuses on the concept of emergence: how the deepest levels of reality—quantum mechanics, field theory, and space-time—are connected to higher and more complex phenomena, like statistical mechanics and organized structures.

"Since the very notion of complexity does not have a universally-agreed-upon definition, any progress we can make in understanding its basic features is potentially very important," Carroll says in his Guggenheim application.

Carroll has also done research into the relationship between philosophy and physics, particularly within the developing field of philosophy of cosmology. Studies in the field take philosophical approaches to traditional physics problems, such as the arrow of time—the idea that there is a distinction between past and future throughout the observable universe, although the laws of physics would be the same if the direction of time were reversed.

"While science was my first love and remains my primary passion, the philosophical desire to dig deep and ask fundamental questions continues to resonate strongly with me," Carroll says in the "Career Narrative" portion of his application. "I'm convinced that familiarity with modern philosophy of science can be invaluable to physicists trying to tackle questions at the foundations of the discipline."

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Understanding the Earth at Caltech

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Credit: Courtesy J. Andrade/Caltech

The ground beneath our feet may seem unexceptional, but it has a profound impact on the mechanics of landslides, earthquakes, and even Mars rovers. That is why civil and mechanical engineer Jose Andrade studies soils as well as other granular materials. Andrade creates computational models that capture the behavior of these materials—simulating a landslide or the interaction of a rover wheel and Martian soil, for instance. Though modeling a few grains of sand may be simple, predicting their action as a bulk material is very complex. "This dichotomy…leads to some really cool work," says Andrade. "The challenge is to capture the essence of the physics without the complexity of applying it to each grain in order to devise models that work at the landslide level."

Credit: Kelly Lance ©2013 MBARI

Geobiologist Victoria Orphan looks deep into the ocean to learn how microbes influence carbon, nitrogen, and sulfur cycling. For more than 20 years, her lab has been studying methane-breathing marine microorganisms that inhabit rocky mounds on the ocean floor. "Methane is a much more powerful greenhouse gas than carbon dioxide, so tracing its flow through the environment is really a priority for climate models and for understanding the carbon cycle," says Orphan. Her team recently discovered a significantly wider habitat for these microbes than was previously known. The microbes, she thinks, could be preventing large volumes of the potent greenhouse gas from entering the oceans and reaching the atmosphere.

Credit: NASA/JPL-Caltech

Researchers know that aerosols—tiny particles in the atmosphere—scatter and absorb incoming sunlight, affecting the formation and properties of clouds. But it is not well understood how these effects might influence climate change. Enter chemical engineer John Seinfeld. His team conducted a global survey of the impact of changing aerosol levels on low-level marine clouds—clouds with the largest impact on the amount of incoming sunlight Earth reflects back into space—and found that varying aerosol levels altered both the quantity of atmospheric clouds and the clouds' internal properties. These results offer climatologists "unique guidance on how warm cloud processes should be incorporated in climate models with changing aerosol levels," Seinfeld says.

Credit: Yan Hu/Aroian Lab/UC San Diego

Tiny parasitic worms infect nearly half a billion people worldwide, causing gastrointestinal issues, cognitive impairment, and other health problems. Biologist Paul Sternberg is on the case. His lab recently analyzed the entire 313-million-nucleotide genome of the hookworm Ancylostoma ceylanicum to determine which genes turn on when the worm infects its host. A new family of proteins unique to parasitic worms and related to the early infection process was identified; the discovery could lead to new treatments targeting those genes. "A parasitic infection is a balance between the parasites trying to suppress the immune system and the host trying to attack the parasite," Sternberg observes, "and by analyzing the genome, we can uncover clues that might help us alter that balance in favor of the host."

Credit: K.Batygin/Caltech

Earth is special, not least because our solar system has a unique (as far as we know) orbital architecture: its rocky planets have relatively low masses compared to those around other sun-like stars. Planetary scientist Konstantin Batygin has an explanation. Using computer simulations to describe the solar system's early evolution, he and his colleagues showed that Jupiter's primordial wandering initiated a collisional cascade that ultimately destroyed the first generation population of more massive planets once residing in Earth's current orbital neighborhood. This process wiped the inner solar system's slate clean and set the stage for the formation of the planets that exist today. "Ultimately, what this means," says Batygin, "is that planets truly like Earth are intrinsically not very common."

Credit: Nicolás Wey-Gόmez/Caltech

Human understanding of the world has evolved over centuries, anchored to scientific and technological advancements and our ability to map uncharted territories. Historian Nicolás Wey-Gόmez traces this evolution and how the age of discovery helped shape culture and politics in the modern era. Using primary sources such as letters and diaries, he examines the assumptions behind Europe's encounter with the Americas, focusing on early portrayals of native peoples by Europeans. "The science and technology that early modern Europeans recovered from antiquity by way of the Arab world enabled them to imagine lands far beyond their own," says Wey-Gómez. "This knowledge provided them with an essential framework to begin to comprehend the peoples they encountered around the globe."

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At Caltech, researchers study the Earth from many angles—from investigating its origins and evolution to exploring its geology and inner workings to examining its biological systems. Taken together, their findings enable a more nuanced understanding of our planet in all its complexity, helping to ensure that it—and we—endure. This slideshow highlights just a few of the Earth-centered projects happening right now at Caltech.

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Two Caltech Seniors Win Hertz Fellowships

Adam Jermyn and Charles Tschirhart join the 51st class of Hertz fellows

Caltech seniors Adam Jermyn and Charles Tschirhart have been named 2015 Hertz Fellowship winners. Selected from a pool of approximately 800 applicants, the awardees will receive up to five years of support for their graduate studies. According to the Hertz Foundation, fellows are chosen for their intellect, their ingenuity, and their potential to bring meaningful improvement to society. Jermyn and Tschirhart bring the number of Caltech undergraduate Hertz fellows to 60.

Adam Jermyn, a physics major from Longmeadow, Massachusetts, works with so-called "emergent phenomena," which "is a broad term referring to situations where we know all of the laws on a fundamental level but where there are so many pieces working together that the consequences aren't known," he says. For example, the basic laws governing fluid mechanics are simple equations that relate such easily measured quantities as density, velocity, and temperature to one another, but simulating the behavior of two gases as they mix in a turbulent flow can tax the capacity of a supercomputer.

Jermyn's senior thesis models how a pulsar—a type of celestial radio source that flashes as fast as a thousand times per second—disrupts the atmosphere of a companion star. Pulsars are neutron stars—supernova cinders that pack the mass of a couple of suns into a sphere roughly the size of Manhattan. The spin imparted by the supernova's explosion and equally violent collapse creates a beam of tightly focused radio waves. If a neutron star were "aimed" at Earth, the beam's fleeting illumination would register as a flash in our radio telescopes every time it swept across us. Meanwhile, the pulsar's intense gravity distorts the companion star, creating a bulge on its surface. Like Earth's moon, the star's rotation is tidally locked, always presenting the same side to its dominant neighbor. The companion star's atmosphere gets siphoned away, layer by layer, forming a turbulent tendril of gas that winds in an ever-tightening spiral around the pulsar as the stolen material accretes onto its surface.

Charles Tschirhart of Naperville, Illinois, is a double major in applied physics and chemistry. His interests lie at the opposite end of the scale—in the world of nanotechnology, where lengths are measured in nanometers, or billionths of a meter. In the summer of 2012, he was part of a team that built nanoelectrodes—tiny silicon needles that penetrate a cell wall without damaging the cell to monitor the electrical activity within.

Tschirhart and Jermyn share an interest in fluid mechanics. "I think the biggest difference between what Adam and I do is that he is a theorist, and I am an experimentalist," Tschirhart says. "Physicists pretend that a fluid is a continuum of infinitely divisible matter and thus doesn't have any 'graininess' to it." But because atoms and molecules do have finite sizes, "once you get down to small enough scales," he says, "even water becomes 'grainy.'" The fluid becomes more viscous, as it takes effort to force the grains past one another. For his senior thesis, Tschirhart determined the nanoviscosity of silicone oil by measuring the thickness of a thin film of oil, smearing it even thinner with a stream of air and measuring its thickness again. The thickness should decrease in a linear manner, but this doesn't happen when the layer gets thin enough. "These films aren't much thicker than the size of a molecule," he says. "This is where noncontinuum effects show up." These effects could affect how engineers approach tasks as diverse as lubricating hard drives and extracting crude oil from porous rocks.

Both students took Physics 11, a course taught by the late Professor Thomas Tombrello. Tombrello launched this class in 1989 to teach encourage freshman to think creatively, and taught it annually until his death in September 2014. This year, Jermyn and Tschirhart are helping teach it. "Physics 11 really shaped the way I ask questions, and I have Tom Tombrello to thank for that," says Jermyn. "He pushed us to think about things obliquely," Tschirhart concurs. "After I got over my initial nerves, I found myself enjoying [the two rounds of Hertz interviews], which made it much easier to answer the questions creatively."

Both plan to defer their Hertz doctoral fellowships while they take advanced degrees in England. Tschirhart will be attending the University of Nottingham as a Fulbright Scholar for one year, where he plans to develop new applications for atomic force microscopy, a powerful technique for "photographing" nanoscale objects. Jermyn will be at the University of Cambridge for two years as a Marshall Scholar investigating the processes by which planets form around binary star systems.

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Caltech Space Challenge: Mission to an Asteroid in Lunar Orbit

For one week at the end of March, 32 students from 20 universities and 14 countries came to Caltech for an intensive training experience in space mission design: the Caltech Space Challenge. Organizers hand-selected the undergraduate and graduate students from a pool of 220 applicants and created two "dream teams" of engineers, scientists, and designers to face off in a competition to see who could design the best mission.

This year, the teams—Team Explorer and Team Voyager—were tasked with designing a manned mission to an asteroid placed in orbit around the moon. Aside from determining details such as the best type of vehicle to use, the optimal launch date, and how to keep the astronauts safe, each team was asked to explain how its mission would explore and make use of the asteroid to enable future missions to more distant locales, such as Mars.

The Space Challenge takes place at Caltech every two years. For the inaugural challenge in 2011, participants designed a manned mission to a near-Earth asteroid. Two years later, the challenge involved planning a mission to one of Mars's moons.

This year, organizers based the challenge on NASA's Asteroid Redirect Mission (ARM), proposed for launch in 2020. The concept of that mission is to send a robotic spacecraft to a near-Earth asteroid, have it remove a large boulder from the asteroid's surface, and then move it into a lunar orbit. A version of a mission originally considered by the Keck Institute for Space Studies (KISS) at Caltech, NASA's ARM is part of a larger strategy to use asteroids as a stepping-stone to manned missions to Mars and beyond.

"KISS came up with this idea to redirect an asteroid and bring it here as a way to fulfill President Obama's vision of people going to an asteroid by 2025," explains Hayden Burgoyne, a graduate student in space engineering at Caltech and one of two student lead organizers for this year's challenge. "Basically, they said, 'It's hard to send people to an asteroid; it's easier to bring an asteroid to us.' But people are looking toward the end goal of Mars, and they want to know how the Asteroid Redirect Mission will help us get there. So we framed this challenge as a resource utilization challenge to show how this resource that they bring back—this asteroid—can be used to benefit future human exploration."

Throughout the week, the students attended lectures delivered by scientists and engineers from JPL and the aerospace industry on topics related to the challenge, such as mission formulation, human space exploration, asteroid mining, and chemical propulsion. They were also able to consult with mentors working in related fields who were available to help the teams troubleshoot.

"Basically, we brought together the best of the best," says Niccolo Cymbalist, a graduate student in aeronautics at Caltech and the event's other student lead organizer. "But one of the neat things is that the students had the opportunity to interact with sort of their future selves. The speakers and mentors who came in from JPL and from industry are also at the top of their fields, and many participants from previous years have gone on to work in space-related fields."

This year, the teams also had the opportunity to complete a half-day formalized study with a group in the Innovation Foundry at JPL, known as the A-Team. These JPL scientists and engineers help explore, develop, and evaluate early mission concepts and were able to advise the students on science, implementation, and programmatic elements of their respective missions.

At the end of the week, both teams turned in written reports and presented their mission concepts to an audience that included jurors from Caltech, JPL, the Planetary Society, Lockheed Martin, Northrop Grumman, SpaceX, and Millennium Space Systems.

In their mission plans, both groups opted to use two rockets—one to launch scientific cargo and another at a later date to deliver the crew. They also both decided that three astronauts would be optimal for this mission.

Beyond those similarities, though, the two teams had quite different approaches to the challenge. Team Explorer had the idea to use an autonomous swarm of robots to characterize the topology of the asteroid and to collect samples both at the surface and at depth, using a specially designed chamber to extract volatiles. They planned to purify water found on the asteroid, demonstrating that it could be used in a variety of ways, including to water a lettuce garden—something that might capture the attention of the general public. The mission would also determine whether the asteroid could be used as a resource depot for other missions, or as part of the Deep Space Network to help facilitate communication between Earth and operating spacecraft.

In contrast, Team Voyager planned to join their mission's cargo and crew vehicles with an inflatable habitat brought along as cargo once their astronauts reached the asteroid. The astronauts would then spend five days using a robotic arm to drill and to conduct seismic surveys as they determined whether it was safe to explore the asteroid further. They also would bring a suite of scientific instruments with them, including a device to extract oxygen, hydrogen, and methanol from the asteroid, and they would collect and return samples to Earth from the asteroid's subsurface core. Team Voyager's plan for engaging the public included social media and a live feed from a 3-D HD 360-degree camera mounted on an astronaut's helmet.

The organizers say both teams presented outstanding missions. "I was blown away by the quality of the work that the students produced," says Burgoyne.

The final results were presented at a closing reception and banquet at the Athenaeum on March 27. In the end, Team Voyager came out slightly ahead of Team Explorer. According to the jury, the deciding factor was Team Voyager's presentation and success in turning their technically detailed report into a compelling story for the audience.

Alicia Lanz, a member of Team Voyager and a graduate student in physics at Caltech, says the best part of the experience was meeting and working with people from various parts of the world and with different scientific training. "It was so interesting to learn from people with different backgrounds and to see everyone work together to create a viable mission that was greater than anything a single individual could have contributed," she says. "The Caltech Space Challenge was an amazing opportunity."

The student technical lead for this year's Space Challenge was Jay Qi, a graduate student in mechanical engineering at Caltech. The faculty advisor was Beverley McKeon, professor of aeronautics at Caltech and associate director of the Graduate Aerospace Laboratories of the California Institute of Technology (GALCIT). Leon Alkalai of JPL was the program mentor. The Space Challenge is organized by GALCIT and supported by Caltech and its Division of Engineering and Applied Science, JPL, KISS, and corporate sponsors including Northrop Grumman, Lockheed Martin, SpaceX, Millennium Space Systems, and AGI.

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New NSF-Funded Physics Frontiers Center Expands Hunt for Gravitational Waves

The search for gravitational waves—elusive ripples in the fabric of space-time predicted to arise from extremely energetic and large-scale cosmic events such as the collisions of neutron stars and black holes—has expanded, thanks to a $14.5-million, five-year award from the National Science Foundation for the creation and operation of a multi-institution Physics Frontiers Center (PFC) called the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).

The NANOGrav PFC will be directed by Xavier Siemens, a physicist at the University of Wisconsin–Milwaukee and the principal investigator for the project, and will fund the NANOGrav research activities of 55 scientists and students distributed across the 15-institution collaboration, including the work of four Caltech/JPL scientists—Senior Faculty Associate Curt Cutler; Visiting Associates Joseph Lazio and Michele Vallisneri; and Walid Majid, a visiting associate at Caltech and a JPL research scientist—as well as two new postdoctoral fellows at Caltech to be supported by the PFC funds. JPL is managed by Caltech for NASA.

"Caltech has a long tradition of leadership in both the theoretical prediction of sources of gravitational waves and experimental searches for them," says Sterl Phinney, professor of theoretical astrophysics and executive officer for astronomy in the Division of Physics, Mathematics and Astronomy. "This ranges from waves created during the inflation of the early universe, which have periods of billions of years; to waves from supermassive black hole binaries in the nuclei of galaxies, with periods of years; to a multitude of sources with periods of minutes to hours; to the final inspiraling of neutron stars and stellar mass black holes, which create gravitational waves with periods less than a tenth of a second."

The detection of the high-frequency gravitational waves created in this last set of events is a central goal of Advanced LIGO (the next-generation Laser Interferometry Gravitational-Wave Observatory), scheduled to begin operation later in 2015. LIGO and Advanced LIGO, funded by NSF, are comanaged by Caltech and MIT.

"This new Physics Frontier Center is a significant boost to what has long been the dark horse in the exploration of the spectrum of gravitational waves: low-frequency gravitational waves," Phinney says. These gravitational waves are predicted to have such a long wavelength—significantly larger than our solar system—that we cannot build a detector large enough to observe them. Fortunately, the universe itself has created its own detection tool, millisecond pulsars—the rapidly spinning, superdense remains of massive stars that have exploded as supernovas. These ultrastable stars appear to "tick" every time their beamed emissions sweep past Earth like a lighthouse beacon. Gravitational waves may be detected in the small but perceptible fluctuations—a few tens of nanoseconds over five or more years—they cause in the measured arrival times at Earth of radio pulses from these millisecond pulsars.

NANOGrav makes use of the Arecibo Observatory in Puerto Rico and the National Radio Astronomy Observatory's Green Bank Telescope (GBT), and will obtain other data from telescopes in Europe, Australia, and Canada. The team of researchers at Caltech will lead NANOGrav's efforts to develop the approaches and algorithms for extracting the weak gravitational-wave signals from the minute changes in the arrival times of pulses from radio pulsars that are observed regularly by these instruments.

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Friday, May 22, 2015
Dabney Hall, Garden of the Associates – The Garden of the Associates

Memorial Service for Gerry Neugebauer, Robert Andrews Millikan Professor of Physics, Emeritus

Friday, April 24, 2015
Athenaeum – The Athenaeum

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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