Chemists Create “Comb” that Detects Terahertz Waves with Extreme Precision

Light can come in many frequencies, only a small fraction of which can be seen by humans. Between the invisible low-frequency radio waves used by cell phones and the high frequencies associated with infrared light lies a fairly wide swath of the electromagnetic spectrum occupied by what are called terahertz, or sometimes submillimeter, waves. Exploitation of these waves could lead to many new applications in fields ranging from medical imaging to astronomy, but terahertz waves have proven tricky to produce and study in the laboratory. Now, Caltech chemists have created a device that generates and detects terahertz waves over a wide spectral range with extreme precision, allowing it to be used as an unparalleled tool for measuring terahertz waves.

The new device is an example of what is known as a frequency comb, which uses ultrafast pulsed lasers, or oscillators, to produce thousands of unique frequencies of radiation distributed evenly across a spectrum like the teeth of a comb. Scientists can then use them like rulers, lining up the teeth like tick marks to very precisely measure light frequencies. The first frequency combs, developed in the 1990s, earned their creators (John Hall of JILA and Theordor Hánsch of the Max Planck Institute of Quantum Optics and Ludwig Maximilians University Munich) the 2005 Nobel Prize in physics. These combs, which originated in the visible part of the spectrum, have revolutionized how scientists measure light, leading, for example, to the development of today's most accurate timekeepers, known as optical atomic clocks.

The team at Caltech combined commercially available lasers and optics with custom-built electronics to extend this technology to the terahertz, creating a terahertz frequency comb with an unprecedented combination of spectral coverage and precision. Its thousands of "teeth" are evenly spaced across the majority of the terahertz region of the spectrum (0.15-2.4 THz), giving scientists a way to simultaneously measure absorption in a sample at all of those frequencies.

The work is described in a paper that appears in the online version of the journal Physical Review Letters and will be published in the April 24 issue. The lead author is graduate student and National Science Foundation fellow Ian Finneran, who works in the lab of Geoffrey A. Blake, professor of cosmochemistry and planetary sciences and professor of chemistry at Caltech.

Blake explains the utility of the new device, contrasting it with a common radio tuner. "With radio waves, most tuners let you zero in on and listen to just one station, or frequency, at a time," he says. "Here, in our terahertz approach, we can separate and process more than 10,000 frequencies all at once. In the near future, we hope to bump that number up to more than 100,000."

That is important because the terahertz region of the spectrum is chock-full of information. Everything in the universe that is warmer than about 10 degrees Kelvin (-263 degrees Celsius) gives off terahertz radiation. Even at these very low temperatures molecules can rotate in space, yielding unique fingerprints in the terahertz. Astronomers using telescopes such as Caltech's Submillimeter Observatory, the Atacama Large Millimeter Array, and the Herschel Space Observatory are searching stellar nurseries and planet-forming disks at terahertz frequencies, looking for such chemical fingerprints to try to determine the kinds of molecules that are present and thus available to planetary systems. But in just a single chunk of the sky, it would not be unusual to find signatures of 25 or more different molecules.

To be able to definitively identify specific molecules within such a tangle of terahertz signals, scientists first need to determine exact measurements of the chemical fingerprints associated with various molecules. This requires a precise source of terahertz waves, in addition to a sensitive detector, and the terahertz frequency comb is ideal for making such measurements in the lab.

"When we look up into space with terahertz light, we basically see this forest of lines related to the tumbling motions of various molecules," says Finneran. "Unraveling and understanding these lines is difficult, as you must trek across that forest one point and one molecule at a time in the lab. It can take weeks, and you would have to use many different instruments. What we've developed, this terahertz comb, is a way to analyze the entire forest all at once."

After the device generates its tens of thousands of evenly spaced frequencies, the waves travel through a sample—in the paper, the researchers provide the example of water vapor. The instrument then measures what light passes through the sample and what gets absorbed by molecules at each tooth along the comb. If a detected tooth gets shorter, the sample absorbed that particular terahertz wave; if it comes through at the baseline height, the sample did not absorb at that frequency.

"Since we know exactly where each of the tick marks on our ruler is to about nine digits, we can use this as a diagnostic tool to get these frequencies really, really precisely," says Finneran. "When you look up in space, you want to make sure that you have such very exact measurements from the lab."

In addition to the astrochemical application of identifying molecules in space, the terahertz comb will also be useful for studying fundamental interactions between molecules. "The terahertz is unique in that it is really the only direct way to look not only at vibrations within individual large molecules that are important to life, but also at vibrations between different molecules that govern the behavior of liquids such as water," says Blake.

Additional coauthors on the paper, "Decade-Spanning High-Precision Terahertz Frequency Comb," include current Caltech graduate students Jacob Good, P. Brandon Carroll, and Marco Allodi, as well as recent graduate Daniel Holland (PhD '14). The work was supported by funding from the National Science Foundation.

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Inhibitor for Abnormal Protein Points the Way to More Selective Cancer Drugs

Nowhere is the adage "form follows function" more true than in the folded chain of amino acids that makes up a single protein macromolecule. But proteins are very sensitive to errors in their genetic blueprints. One single-letter DNA "misspelling" (called a point mutation) can alter a protein's structure or electric charge distribution enough to render it ineffective or even deleterious.

Unfortunately, cells containing abnormal proteins generally coexist alongside those containing the normal (or "wild") type, and telling them apart requires a high degree of molecular specificity. This is a particular concern in the case of cancer-causing proteins.

"With present technologies, developing a drug that will target only the mutant version of a protein is difficult," notes Blake Farrow, a graduate student in materials science at Caltech and a Howard Hughes Medical Institute Fellow. "Most anticancer agents indiscriminately attack both mutant and healthy proteins and tissues."

Farrow is part of a Caltech-led team that recently created a new type of highly selective molecule that can actively distinguish a mutated protein from the wild type by binding only the mutated protein.

The work was described in a paper that appeared in Nature Chemistry on April 13.

The project was begun by Kaycie Deyle (PhD '14), now a postdoctoral fellow at École Polytechnique Fédérale de Lausanne, and utilized a number of novel technologies, including click chemistry and protein-catalyzed capture (PCC). Click chemistry is a technique for rapidly and reliably constructing molecular assemblies from modular components. PCC screening, which was developed by the Caltech researchers, uses click chemistry to reveal which of several candidate molecules will bind (and "click") most strongly to a given region of a specific protein.

As a test case, the researchers investigated a point mutation called E17K, which occurs in a specific region of Akt1, a protein hundreds of amino acids long. Akt1 plays a key role in cell growth and proliferation, and its E17K mutation is closely linked to an increase in the development of tumors and the survival of cancer cells.

Using a synthesized fragment of the cancer-causing form of Akt1, the researchers replaced an amino acid close to the E17K mutation with a structure that could act as a "click handle." The goal was to create a short molecule that could wedge itself into the folds of the protein, binding to the E17K mutation at one end and "clicking" to the handle at the other.

Candidate molecules were constructed by splicing amino acids together into short chains five amino acids long, which is just enough to reach from the click handle to the mutation site. With almost two dozen amino acids to choose from for each of the five slots, the researchers were faced with over a million possible configurations.

A multistep PCC screening process narrowed this large number of candidates down to one combination that bound to the mutant version of the protein 10 times more strongly than to the wild type. The code letters representing the five amino acids making up the molecule gave it its informal name: "yleaf."

Next, a second PCC screening process was used to find amino acid chains that could be used to extend the yleaf molecule, giving it the ability to grip multiple naturally occurring features of the Akt1 molecule, rather than requiring a click handle to be artificially inserted. Testing of this wider-wingspan yleaf showed that not only did it bind almost exclusively to its intended target (and nowhere else, including the unmutated form of Akt1), but also that in doing so it inhibited the protein's activity and hence could be expected to impair its ability to support tumor growth. In fact, the extended yleaf molecule inhibited the mutated protein a thousand times better than it did the wild-type form.

James Heath, the Elizabeth W. Gilloon Professor and Professor of Chemistry at Caltech and corresponding author of the paper, says this selective inhibitor strategy "is certainly a very important first step" toward new cancer drug modalities. With additional design considerations to facilitate passage through the cell membrane, compounds of this sort could become the basis of new drugs for targeting and inhibiting abnormal protein molecules in living cells, he says.

In addition to Farrow, Deyle, and Heath, other authors on the Nature Chemistry paper, "A protein-targeting strategy used to develop a selective inhibitor of the E17K point mutation in the PH domain of Akt1," are Michelle Wong, Aiko Umeda, Arundhati Nag, and Samir Das from Caltech; Ying Qiao Hee and Jeremy Work, who contributed to the work at Caltech as a Summer Undergraduate Research Fellow and an Amgen Scholar, respectively; Bert Lai from Indi Molecular; and Steven W. Millward from the University of Texas MD Anderson Cancer Center. The work was supported by the Institute for Collaborative Biotechnologies, the National Cancer Institute, the Defense Advanced Research Projects Agency and the Jean Perkins Foundation. Research was performed in collaboration with Indi Molecular (founded by Heath), which seeks to commercialize PCC technology.

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Abedi Receives Fellowship for New Americans

Mohamad Abedi, a PhD candidate in bioengineering, has received a Paul & Daisy Soros Fellowship for New Americans. Thirty fellows were selected from nearly 1,200 applicants "for their potential to make significant contributions to US society, culture, or their academic field," according to the fellowship program description. Each Soros Fellow will receive up to $90,000 to help cover two years of tuition, and other educational and living expenses, while studying any subject at any university in the United States. The fellowship was established to assist young new Americans—permanent residents, naturalized citizens, or children of naturalized citizen parents—at critical points in their educations.

"I'm honored and excited to receive this fellowship. Coming to the United States provided me with a plethora of opportunities and support that allowed me to pursue my dream and be here at Caltech today," Abedi says.

Abedi was born to Palestinian refugees in the United Arab Emirates. As a child he frequently visited family in the Beddawi refugee camp in Lebanon. The lack of adequate health care resources he saw there motivated him to pursue a degree in bioengineering.

"As a bioengineer, I hope to develop low-cost medical technologies that could provide people with health care regardless of their geographical location and financial capabilities," Abedi says.

After moving with his family to California during his final year of high school, Abedi began a degree in biomedical engineering at UC Irvine, where he worked on building affordable diagnostic devices that could run on air instead of electricity. At UC Irvine, he also ventured into synthetic biology to study bacterial genetic circuits—interacting genes and proteins that enable cells to sense and communicate with one another.

Now a first-year graduate student in the lab of Mikhail Shapiro, assistant professor of chemical engineering, Abedi aims to develop tools for the noninvasive modulation of brain circuitry. These would eventually allow scientists to understand and treat neurological and psychiatric diseases involving the dysfunction of local neural circuits, such as depression or obsessive-compulsive disorder.

"Understanding the human brain, where trillions of cells work in harmony to form a magnificent structure, is arguably the most ambitious target of scientific inquiry," Abedi says. "I am interested in utilizing tools from cellular and molecular engineering to study the brain, with the overarching goal of improving human health and welfare worldwide."

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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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A Spotlight on Inventing

On Thursday, March 19, Caltech is hosting the fourth annual conference of the National Academy of Inventors. The NAI is a nonprofit organization that was founded in 2010 by the U.S. Patent and Trademark Office to encourage inventors and also enhance the visibility and understanding of the value of academic technology and innovation.

Its annual conference will bring hundreds of NAI members—inventors, researchers, scientists, engineers, and scholars from more than 200 institutions around the country—to Caltech. While here, they will share big ideas, discuss opportunities for future innovations, and also celebrate the newest class of NAI fellows. According to the NAI, election to fellow status is a "high professional distinction accorded to academic inventors who have demonstrated a highly prolific spirit of innovation in creating or facilitating outstanding inventions that have made a tangible impact on quality of life, economic development, and the welfare of society." The newest class of 170 fellows includes four Caltech professors.

The conference is "a very exciting opportunity for Caltech," says Caltech vice provost Morteza Gharib (PhD '83), the Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering, and an NAI charter fellow. "Having an organization that brings some of our greatest minds together to look at the problems we are facing and support them in finding solutions is a noble cause, and we at Caltech are proud to be supporters of that."

Advancing innovation and the transfer of new technologies and ideas to society and industry is both a personal and professional passion for Gharib. The holder of nearly 100 patents, he leads a research group at Caltech that studies examples from the natural world—fins, wings, blood vessels, embryonic structures, and entire organisms—to gain inspiration for inventions that have practical uses in power generation, drug delivery, dentistry, and more. As vice provost, he also oversees Caltech's Office of Technology Transfer and Corporate Partnerships. OTT plays an instrumental role in helping Caltech's researchers commercially realize their ideas, making sure that their work is protected, patented, and licensed along the way. As of the close of fiscal year 2014, Caltech managed more than 1,700 active U.S. patents. Since the office was established in 1995, its staff has helped launch more than 150 start-up companies.

To learn more about what it means to be an inventor, we recently chatted with Gharib and two of Caltech's newest NAI fellows—Frances Arnold and Carver Mead (BS '56, MS '57, PhD '60).

Arnold, the Dick and Barbara Dickinson Professor of Chemical Engineering, Bioengineering and Biochemistry, and director of the Donna and Benjamin M. Rosen Bioengineering Center, pioneered methods of "directed evolution" to engineer new proteins in the lab. The method is now widely used to create catalysts for industrial processes, including the production of fuels and chemicals from renewable resources.

Mead, the Gordon and Betty Moore Professor of Engineering and Applied Science, Emeritus, has significantly advanced the technology of integrated circuits by developing a method called very-large-scale integration (VSLI) that allows engineers to combine thousands of transistors onto a single microchip, thus exponentially expanding computer processing power.

 

What does it mean to be an inventor?

Arnold: It means I get to play—with ideas—and create new things that solve problems.

Gharib: An inventor is someone who has the ability to summarize what had not been before into something that has a new form and is novel.

Inventing is not a sudden process either; you don't just come up with an invention. It comes from where you have been, all the influences you have received from your education, your community, and the environment that you are in—from whether you have been challenged or excited by problems.

Mead: I have never thought of myself as an inventor! I always thought of myself as a guy who figures things out and then it just turned out that every once in a while something that I "figured out" would be important. Some of those things turned into inventions. I am just a creative person. I'm someone who likes to solve problems.

 

How does the invention process relate to the scientific process?

Arnold: Many scientists pursue the answer to a question: "How does this work?" Inventors often pursue an answer to a problem: "How can I get this to work?"

Gharib: It's not the same path. Remember that engineers basically invented locomotives, and it wasn't until half a century later that we actually understood the laws of thermodynamics and why this works.

The scientific process is systematic. It relies on certain logical steps that you take—from defining the problem, testing what works, eliminating problems—and that pushes you to be able to be in a position to discover. But in inventing, you see the solutions without knowing or needing to understand why and pursue that.

Mead: For me, it's all the same. I have the same approach for all of my work—it's all just about figuring things out.

 

How has Caltech supported you as an inventor?

Arnold: Caltech has provided me with great students and with the financial support to pursue new ideas, and then not placed the traditional academic constraints on what we can pursue.

Gharib: Our inventors see an environment that is conducive to inventing. Caltech supports them to get their idea translated from the lab into something that is useful, something that is protected, and something that will have a societal impact.

The best example I have of that is that I didn't own a single patent before I came to Caltech—even though I was in academia for 10 years before coming here. It wasn't that I didn't have ideas, it was just that I didn't have a motivation for pursuing those ideas. When I came here, I saw that you can really take your ideas and make them into something for industry, for society, for faculty, and so on, to benefit from.

Mead: I think the best thing Caltech did for me is leave me alone, because I could pursue the things that I felt strongly about. It's always taken a very long time for me to move after having the first inkling of some direction or idea that I am drawn to. I might work for five years on something before I can fully explain to people why I am working on it.

I think Caltech is very special in that way; it doesn't interfere with the creative process.

 

What invention of yours are you most proud of? Why? 

Arnold: I am lucky to have been the first to show how evolution can be used to construct a whole slew of new and useful catalysts. This is a fundamental process that can be used to solve so many important problems. It has been picked up and used by hundreds of academic and industrial labs, all over the world, for everything from making better laundry detergents to producing fuels from renewable resources.

Gharib: Seventy-thousand people have a shunt in their eyes that I developed, and that is helping them avoid having to deal with issues of glaucoma. In addition to that, a 3-D imaging device that I originally developed for naval underwater surveillance is now being used for making dental crowns.

It is a good feeling when you see one of your patents have societal impact.

Mead: My work in the area of very-large-scale integration—figuring out how transistors could scale up and how you could build them better—has affected the world in a profound way, and I am pleased to be a part of that. You can think of it as inventing a method, a way through, but not as a tangible invention of the usual sort.

In regards to more traditional "inventions," there are a couple of other things that have gone out into the world and made a difference. The first was the Schottky-gate field-effect transistor, which I created over Thanksgiving in 1965. It is in the transmitters of all cell phones. The second was an advancement that a graduate student of mine led. He came up with a way of making semiconductor charge-coupled devices (CCDs) work more efficiently, which enabled them to be used in the imaging world. CCDs are still the imaging sensor that is used in astronomical instruments.

Of course, you never know when you are doing something whether it will really be accepted and if people will move on it. So when it happens—when people take something that you do seriously—it's kind of surprising.

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Friday, April 10, 2015
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One Step Closer to Artificial Photosynthesis and "Solar Fuels"

Caltech scientists, inspired by a chemical process found in leaves, have developed an electrically conductive film that could help pave the way for devices capable of harnessing sunlight to split water into hydrogen fuel.

When applied to semiconducting materials such as silicon, the nickel oxide film prevents rust buildup and facilitates an important chemical process in the solar-driven production of fuels such as methane or hydrogen.

"We have developed a new type of protective coating that enables a key process in the solar-driven production of fuels to be performed with record efficiency, stability, and effectiveness, and in a system that is intrinsically safe and does not produce explosive mixtures of hydrogen and oxygen," says Nate Lewis, the George L. Argyros Professor and professor of chemistry at Caltech and a coauthor of a new study, published the week of March 9 in the online issue of the journal the Proceedings of the National Academy of Sciences, that describes the film.

The development could help lead to safe, efficient artificial photosynthetic systems—also called solar-fuel generators or "artificial leaves"—that replicate the natural process of photosynthesis that plants use to convert sunlight, water, and carbon dioxide into oxygen and fuel in the form of carbohydrates, or sugars.

The artificial leaf that Lewis' team is developing in part at Caltech's Joint Center for Artificial Photosynthesis (JCAP) consists of three main components: two electrodes—a photoanode and a photocathode—and a membrane. The photoanode uses sunlight to oxidize water molecules to generate oxygen gas, protons, and electrons, while the photocathode recombines the protons and electrons to form hydrogen gas. The membrane, which is typically made of plastic, keeps the two gases separate in order to eliminate any possibility of an explosion, and lets the gas be collected under pressure to safely push it into a pipeline.

Scientists have tried building the electrodes out of common semiconductors such as silicon or gallium arsenide—which absorb light and are also used in solar panels—but a major problem is that these materials develop an oxide layer (that is, rust) when exposed to water.

Lewis and other scientists have experimented with creating protective coatings for the electrodes, but all previous attempts have failed for various reasons. "You want the coating to be many things: chemically compatible with the semiconductor it's trying to protect, impermeable to water, electrically conductive, highly transparent to incoming light, and highly catalytic for the reaction to make oxygen and fuels," says Lewis, who is also JCAP's scientific director. "Creating a protective layer that displayed any one of these attributes would be a significant leap forward, but what we've now discovered is a material that can do all of these things at once."

The team has shown that its nickel oxide film is compatible with many different kinds of semiconductor materials, including silicon, indium phosphide, and cadmium telluride. When applied to photoanodes, the nickel oxide film far exceeded the performance of other similar films—including one that Lewis's group created just last year. That film was more complicated—it consisted of two layers versus one and used as its main ingredient titanium dioxide (TiO2, also known as titania), a naturally occurring compound that is also used to make sunscreens, toothpastes, and white paint.

"After watching the photoanodes run at record performance without any noticeable degradation for 24 hours, and then 100 hours, and then 500 hours, I knew we had done what scientists had failed to do before," says Ke Sun, a postdoc in Lewis's lab and the first author of the new study.

Lewis's team developed a technique for creating the nickel oxide film that involves smashing atoms of argon into a pellet of nickel atoms at high speeds, in an oxygen-rich environment. "The nickel fragments that sputter off of the pellet react with the oxygen atoms to produce an oxidized form of nickel that gets deposited onto the semiconductor," Lewis says.

Crucially, the team's nickel oxide film works well in conjunction with the membrane that separates the photoanode from the photocathode and staggers the production of hydrogen and oxygen gases.

"Without a membrane, the photoanode and photocathode are close enough to each other to conduct electricity, and if you also have bubbles of highly reactive hydrogen and oxygen gases being produced in the same place at the same time, that is a recipe for disaster," Lewis says. "With our film, you can build a safe device that will not explode, and that lasts and is efficient, all at once."

Lewis cautions that scientists are still a long way off from developing a commercial product that can convert sunlight into fuel. Other components of the system, such as the photocathode, will also need to be perfected.

"Our team is also working on a photocathode," Lewis says. "What we have to do is combine both of these elements together and show that the entire system works. That will not be easy, but we now have one of the missing key pieces that has eluded the field for the past half-century."

Along with Lewis and Sun, additional authors on the paper, "Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films," include Caltech graduate students Fadl Saadi, Michael Lichterman, Xinghao Zhou, Noah Plymale, and Stefan Omelchenko; William Hale, from the University of Southampton; Hsin-Ping Wang and Jr-Hau He, from King Abdullah University in Saudi Arabia; Kimberly Papadantonakis, a scientific research manager at Caltech; and Bruce Brunschwig, the director of the Molecular Materials Research Center at Caltech. Funding was provided by the Office of Science at the U.S. Department of Energy, the National Science Foundation, the Beckman Institute, and the Gordon and Betty Moore Foundation.

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Tuesday, April 7, 2015
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Tuesday, March 31, 2015 to Thursday, April 16, 2015
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Monday, March 31, 2014 to Wednesday, April 16, 2014
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