New Technique Could Harvest More of the Sun's Energy

As solar panels become less expensive and capable of generating more power, solar energy is becoming a more commercially viable alternative source of electricity. However, the photovoltaic cells now used to turn sunlight into electricity can only absorb and use a small fraction of that light, and that means a significant amount of solar energy goes untapped.

A new technology created by researchers from Caltech, and described in a paper published online in the October 30 issue of Science Express, represents a first step toward harnessing that lost energy.

Sunlight is composed of many wavelengths of light. In a traditional solar panel, silicon atoms are struck by sunlight and the atoms' outermost electrons absorb energy from some of these wavelengths of sunlight, causing the electrons to get excited. Once the excited electrons absorb enough energy to jump free from the silicon atoms, they can flow independently through the material to produce electricity. This is called the photovoltaic effect—a phenomenon that takes place in a solar panel's photovoltaic cells.

Although silicon-based photovoltaic cells can absorb light wavelengths that fall in the visible spectrum—light that is visible to the human eye—longer wavelengths such as infrared light pass through the silicon. These wavelengths of light pass right through the silicon and never get converted to electricity—and in the case of infrared, they are normally lost as unwanted heat.

"The silicon absorbs only a certain fraction of the spectrum, and it's transparent to the rest. If I put a photovoltaic module on my roof, the silicon absorbs that portion of the spectrum, and some of that light gets converted into power. But the rest of it ends up just heating up my roof," says Harry A. Atwater, the Howard Hughes Professor of Applied Physics and Materials Science; director, Resnick Sustainability Institute, who led the study.

Now, Atwater and his colleagues have found a way to absorb and make use of these infrared waves with a structure composed not of silicon, but entirely of metal.

The new technique they've developed is based on a phenomenon observed in metallic structures known as plasmon resonance. Plasmons are coordinated waves, or ripples, of electrons that exist on the surfaces of metals at the point where the metal meets the air.

While the plasmon resonances of metals are predetermined in nature, Atwater and his colleagues found that those resonances are capable of being tuned to other wavelengths when the metals are made into tiny nanostructures in the lab.

"Normally in a metal like silver or copper or gold, the density of electrons in that metal is fixed; it's just a property of the material," Atwater says. "But in the lab, I can add electrons to the atoms of metal nanostructures and charge them up. And when I do that, the resonance frequency will change."

"We've demonstrated that these resonantly excited metal surfaces can produce a potential"—an effect very similar to rubbing a glass rod with a piece of fur: you deposit electrons on the glass rod. "You charge it up, or build up an electrostatic charge that can be discharged as a mild shock," he says. "So similarly, exciting these metal nanostructures near their resonance charges up those metal structures, producing an electrostatic potential that you can measure."

This electrostatic potential is a first step in the creation of electricity, Atwater says. "If we can develop a way to produce a steady-state current, this could potentially be a power source. He envisions a solar cell using the plasmoelectric effect someday being used in tandem with photovoltaic cells to harness both visible and infrared light for the creation of electricity.

Although such solar cells are still on the horizon, the new technique could even now be incorporated into new types of sensors that detect light based on the electrostatic potential.

"Like all such inventions or discoveries, the path of this technology is unpredictable," Atwater says. "But any time you can demonstrate a new effect to create a sensor for light, that finding has almost always yielded some kind of new product."

This work was published in a paper titled, "Plasmoelectric Potentials in Metal Nanostructures." Other coauthors include first author Matthew T. Sheldon, a former postdoctoral scholar at Caltech; Ana M. Brown, an applied physics graduate student at Caltech; and Jorik van de Groep and Albert Polman from the FOM Institute AMOLF in Amsterdam. The study was funded by the Department of Energy, the Netherlands Organization for Scientific Research, and an NSF Graduate Research Fellowship.

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Photosynthesis: A Planetary Revolution

Watson Lecture Preview

Two and a half billion years ago, single-celled organisms called cyanobacteria harnessed sunlight to split water molecules, producing energy to power their cells and releasing oxygen into an atmosphere that had previously had none. These early environmental engineers are responsible for the life we see around us today, and much more besides.

At 8:00 p.m. on Wednesday, November 19, in Caltech's Beckman Auditorium, Professor of Geobiology Woodward "Woody" Fischer will describe how they transformed the planet. Admission is free.

 

Q: What do you do?

A: I'm a geobiologist of the historical variety. I'm trying to understand both how the earth works, and why it works that way. The whys are hard, because you can't redo this planetary experiment. You have to create clever ways to work backward from what you can observe to answer the question you've posed.

When you boil down the earth's history, there are maybe a half-dozen singularities—fundamental changes in how our planet and the life on it interact. Photosynthetic cyanobacteria reengineered the planet. Photosynthesis led to two more singularities—plants and animals appeared. The remaining singularities are mass extinctions as a result of something happening to the global environment, and photosynthesis likely caused one of those as well. Oxygen can be highly toxic because it's so reactive. It chews up your DNA, and it binds to the metal compounds that cells use to shuttle electrons around. Any microbes that couldn't cope with this new pollutant died off, or were forced to hide in oxygen-depleted environments.

Atmospheric oxygen resulted from a change to a microbe's metabolism that evolved once, at a specific time in the earth's history. We want to know why that happened. What were those bacteria doing beforehand? What forced them to develop this radically new way of making a living?

Bacteria don't leave fossils, per se, but they can leave behind metabolic signatures that sedimentary rocks preserve. They impact the rock's elemental composition, and they alter the ratios between heavier and lighter isotopes of certain elements as well. We can work backward from that information to deduce what the bacteria were doing on the ocean floor and in the seawater above it as those sediments were being laid down.

 

Q: If the earth has had breathable oxygen for billions of years, why should we care where it came from?

A: There are two really good reasons.

One has to do with meeting society's energy demands. There's a tremendous effort at Caltech and elsewhere to develop "solar fuels." Can we do better than green plants? If cyanobacteria did the best they could under tight constraints, maybe not. But if there are a variety of ways to do that chemistry, maybe we can clear the slate and do something entirely different.

The deeper reason is that atmospheric oxygen rewrote life's recipe book. Oxygen-based metabolism provides extra energy that can be invested in cellular specialization. A group of specialized cells can become a tissue, and eventually you have complex creatures with limbs. It's like agriculture—when you start growing crops, you have surplus food. Villages spring up. Craftsmen appear.

It gets to the Big Question—how rare are we? The earth is 4.5 billion years old, and the oldest evidence for life is about 3.5 billion years old. It took another billion years until photosynthesis, and two billion more for animals to develop. Is it possible to evolve advanced creatures under a different set of constraints leading to completely different metabolisms? If we're looking for life on worlds that play by different rules, will we recognize it?

 

Q: How did you get into this line of work?

A: As a small kid, I always loved science. That disappeared somewhere in middle school, so I went to Colorado College in Colorado Springs—a small, liberal-arts school with a really intense curriculum called the block plan. You take one class at a time for a month. You're completely immersed—lecture from nine to twelve, break for lunch, afternoon labs, evening homework. Lather, rinse, repeat. I took a geology class on a whim, because my grandfather had once taught paleontology there. The class vanished into the mountains for a month, and I was hooked.

In graduate school at Harvard, I worked with Andy Knoll, a Precambrian paleontologist who's trying to understand what the world looked like before animals. Andy's primary appointment is actually in the biology department, and I built on my sedimentary-geology background with a lot of biology classes—molecular biology, biochemistry, genomics, comparative biology, evolutionary biology. And then I came here as an Agouron Postdoctoral Scholar in Geobiology in 2007. I was fortunate that they invited me to stay.

 


Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.
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Rock-Dwelling Microbes Remove Methane from Deep Sea

Methane-breathing microbes that inhabit rocky mounds on the seafloor could be preventing large volumes of the potent greenhouse gas from entering the oceans and reaching the atmosphere, according to a new study by Caltech researchers.

The rock-dwelling microbes, which are detailed in the Oct. 14 issue of Nature Communications, represent a previously unrecognized biological sink for methane and as a result could reshape scientists' understanding of where this greenhouse gas is being consumed in subseafloor habitats, says Professor of Geobiology Victoria Orphan, who led the study.

"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," Orphan says.

Orphan's team has been studying methane-breathing marine microorganisms for nearly 20 years. The microbes they focus on survive without oxygen, relying instead on sulfate ions present in seawater for their energy needs. Previous work by Orphan's team helped show that the methane-breathing system is actually made up of two different kinds of microorganisms that work closely with one another. One of the partners, dubbed "ANME" for "ANaerobic MEthanotrophs," belongs to a type of ancient single-celled creatures called the archaea.

Through a mechanism that is still unclear, ANME work closely with bacteria to consume methane using sulfate from seawater. "Without this biological process, much of that methane would enter the water column, and the escape rates into the atmosphere would probably be quite a bit higher," says study first author Jeffrey Marlow, a geobiology graduate student in Orphan's lab.

Until now, however, the activity of ANME and their bacterial partners had been primarily studied in sediments located in cold seeps, areas on the ocean bottom where methane is escaping from subseafloor sources into the water above. The new study marks the first time they have been observed to oxidize methane inside carbonate mounds, huge rocky outcroppings of calcium carbonate that can rise hundreds of feet above the seafloor.

If the microbes are living inside the mounds themselves, then the distribution of methane consumption is significantly different from what was previously thought. "Methane-derived carbonates represent a large volume within many seep systems, and finding active methane-consuming archaea and bacteria in the interior of these carbonate rocks extends the known habitat for methane-consuming microorganisms beyond the relatively thin layer of sediment that may overlay a carbonate mound," Marlow says.

Orphan and her team detected evidence of methane-breathing microbes in carbonate rocks collected from three cold seeps around the world: one at a tectonic plate boundary near Costa Rica; another in the Eel River basin off the coast of northwestern California; and at Hydrate Ridge, off the Oregon coast. The team used manned and robotic submersibles to collect the rock samples from depths ranging from 2,000 feet to nearly half a mile below the surface.

Marlow has vivid memories of being a passenger in the submersible Alvin during one of those rock-retrieval missions. "As you sink down, the water outside your window goes from bright blue surface water to darker turquoise and navy blue and all these shades of blue that you didn't know existed until it gets completely dark," Marlow recalls. "And then you start seeing flashes of light because the vehicle is perturbing the water column and exciting florescent organisms. When you finally get to the seafloor, Alvin's exterior lights turn on, and this crazy alien world is illuminated in front of you."

The carbonate mounds that the subs visited often serve as foundations for coral and sponges, and are home to rockfishes, clams, crabs, and other aquatic life. For their study, the team members gathered rock samples not only from carbonate mounds located within active cold seeps, where methane could be seen escaping from the seafloor into the water, but also from mounds that appeared to be dormant.

Once the carbonate rocks were collected, they were transported back to the surface and rushed into a cold room aboard a research ship. In the cold room, which was maintained at the temperature of the deep sea, the team cracked open the carbonates in order to gather material from their interiors. "We wanted to make sure we weren't just sampling material from the surface of the rocks," Marlow says.

Using a microscope, the team confirmed that ANME and sulfate-reducing bacterial cells were indeed present inside the carbonate rocks, and genetic analysis of their DNA showed that they were related to methanotrophs that had previously been characterized in seafloor sediment. The scientists also used a technique that involved radiolabeled 14C-methane tracer gas to quantify the rates of methane consumption in the carbonate rocks and sediments from both the actively seeping sites and the areas appearing to be inactive. They found that the rock-dwelling methanotrophs consumed methane at a slower rate than their sediment-dwelling cousins.

"The carbonate-based microbes breathed methane at roughly one-third the rate of those gathered from sediments near active seep sites," Marlow says. "However, because there are likely many more microbes living in carbonate mounds than in sediments, their contributions to methane removal from the environment may be more significant."

The rock samples that were harvested near supposedly dormant cold seeps also harbored microbial communities capable of consuming methane. "We were surprised to find that these marine microorganisms are still viable and, if exposed to methane, can continue to oxidize this greenhouse gas long after surface expressions of seepage have vanished." Orphan says.

Along with Orphan and Marlow, additional coauthors on the paper, "Carbonate-hosted methanotrophy represents an unrecognized methane sink in the deep sea," include former Caltech associate research scientist Joshua Steele, now at the Southern California Coastal Water Research Project; Wiebke Ziebis, an associate professor at the University of Southern California; Andrew Thurber, an assistant professor at Oregon State University; and Lisa Levin, a professor at the Scripps Institution of Oceanography. Funding for the study was provided by the National Science Foundation; NASA's Astrobiology Institute; the Gordon and Betty Moore Foundation Marine Microbiology Initiative grant; and the National Research Council of the National Academies. 

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Swimming Sea-Monkeys Reveal How Zooplankton May Help Drive Ocean Circulation

Brine shrimp, which are sold as pets known as Sea-Monkeys, are tiny—only about half an inch long each. With about 10 small leaf-like fins that flap about, they look as if they could hardly make waves.

But get billions of similarly tiny organisms together and they can move oceans.

It turns out that the collective swimming motion of Sea-Monkeys and other zooplankton—swimming plankton—can generate enough swirling flow to potentially influence the circulation of water in oceans, according to a new study by Caltech researchers.

The effect could be as strong as those due to the wind and tides, the main factors that are known to drive the up-and-down mixing of oceans, says John Dabiri, professor of aeronautics and bioengineering at Caltech. According to the new analysis by Dabiri and mechanical engineering graduate student Monica Wilhelmus, organisms like brine shrimp, despite their diminutive size, may play a significant role in stirring up nutrients, heat, and salt in the sea—major components of the ocean system.

In 2009, Dabiri's research team studied jellyfish to show that small animals can generate flow in the surrounding water. "Now," Dabiri says, "these new lab experiments show that similar effects can occur in organisms that are much smaller but also more numerous—and therefore potentially more impactful in regions of the ocean important for climate."

The researchers describe their findings in the journal Physics of Fluids.

Brine shrimp (specifically Artemia salina) can be found in toy stores, as part of kits that allow you to raise a colony at home. But in nature, they live in bodies of salty water, such as the Great Salt Lake in Utah. Their behavior is cued by light: at night, they swim toward the surface to munch on photosynthesizing algae while avoiding predators. During the day, they sink back into the dark depths of the water.


A. salina (a species of brine shrimp, commonly known as Sea-Monkeys) begin a vertical migration, stimulated by a vertical blue laser light.

To study this behavior in the laboratory, Dabiri and Wilhelmus use a combination of blue and green lasers to induce the shrimp to migrate upward inside a big tank of water. The green laser at the top of the tank provides a bright target for the shrimp to swim toward while a blue laser rising along the side of the tank lights up a path to guide them upward.

The tank water is filled with tiny, silver-coated hollow glass spheres 13 microns wide (about one-half of one-thousandth of an inch). By tracking the motion of those spheres with a high-speed camera and a red laser that is invisible to the organisms, the researchers can measure how the shrimp's swimming causes the surrounding water to swirl.

Although researchers had proposed the idea that swimming zooplankton can influence ocean circulation, the effect had never been directly observed, Dabiri says. Past studies could only analyze how individual organisms disturb the water surrounding them.

But thanks to this new laser-guided setup, Dabiri and Wilhelmus have been able to determine that the collective motion of the shrimp creates powerful swirls—stronger than would be produced by simply adding up the effects produced by individual organisms.

Adding up the effect of all of the zooplankton in the ocean—assuming they have a similar influence—could inject as much as a trillion watts of power into the oceans to drive global circulation, Dabiri says. In comparison, the winds and tides contribute a combined two trillion watts.

Using this new experimental setup will enable future studies to better untangle the complex relationships between swimming organisms and ocean currents, Dabiri says. "Coaxing Sea-Monkeys to swim when and where you want them to is even more difficult than it sounds," he adds. "But Monica was undeterred over the course of this project and found a creative solution to a very challenging problem."

The title of the Physics of Fluids paper is "Observations of large-scale fluid transport by laser-guided plankton aggregations." The research was supported by the U.S.-Israel Binational Science Foundation, the Office of Naval Research, and the National Science Foundation.

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Caltech's Green Revolving Fund: Financing Energy Efficiency

For the last six years, Caltech Facilities has managed a green revolving fund called the Caltech Energy Conservation Investment Program (CECIP) to finance energy efficiency projects around campus. To date the program has invested $18 million in such projects that can pay back their costs in less than six years. These investments have received $3 million in rebates and have returned more than $4.5 million to the loan fund in avoided utility costs. The program has been recognized with a number of awards, including, most recently, an Innovation Award from the National Association of College and University Business Officers (NACUBO).

"This award recognizes CECIP's unique combination of innovation in both facilities and finance," says Dean Currie, vice president for business and finance at Caltech. "CECIP is intensely rigorous in its measurement of actual building performance, both pre- and post-investment, and in its recalculation of savings based on actual energy prices, not just those that prevailed when the project was approved. The Caltech board was so impressed with the CECIP concept, that it authorized the continued investment of millions of dollars in the program right through the 2008-2009 financial crisis."

Launched in 2008, CECIP began as a concept for funding energy efficiency work on campus in a way that would not affect the operating budget. As a green revolving fund, the idea was to borrow an initial allocation from Caltech's endowment in order to finance projects that produce a return on investment of 15 percent or more. "The investments made in energy efficiency reduce campus-wide utility costs," says Matt Berbée, director of maintenance management and energy services at Caltech. "These utility reductions go back into the fund and can then be used to finance additional projects."

CECIP plays a key role in Caltech's Greenhouse Gas Mitigation Strategy. To date, Caltech has reduced direct emissions by more than 20 percent since 2008, putting the Institute well on its way to achieving its 2020 emissions reduction target.

The first CECIP-funded project was an LED lighting retrofit in the parking structures on Wilson Avenue. The project was completed in 2009 and has returned more in avoided utility costs than it cost to implement. Since that initial pilot, CECIP has funded dozens of projects across campus including full building automation controls and mechanical system upgrades in Broad Center, Moore Lab, and Beckman Institute.

Since CECIP's inception, Caltech's energy density—the Btu used per square foot of space—has dropped by about 10 percent. Another way to look at the impact of the CECIP projects is that, without them, the Institute would consume about 18 more gigawatt-hours of electricity every year (enough to power more than 1,600 homes).

"As energy efficiency projects are completed, the amount of energy supplied by the grid decreases, and this improves Caltech's carbon footprint," says Berbée. "This is not being green just to be green. These investments are about verified financial performance, reducing environmental impact, and keeping the focus on the main thing: efficiently operated and maintained space for research and education."

Currently, more than 30 CECIP projects are paying back into the fund, producing about half a million dollars every quarter in avoided utility costs.

Berbée says documenting and verifying those savings is necessary to CECIP's success. "We have the numbers to prove that this is working."

And those performance numbers have made CECIP a model for other organizations and institutions looking for ways to finance energy efficiency projects. On October 30, Caltech will host its fourth energy efficiency forum, where managers from other higher-education campuses, private research centers, real estate firms, and companies with campus-like facilities will come to learn about Caltech's efforts and to see CECIP projects firsthand.

"The energy efficiency forum is one of the ways we demonstrate our commitment to continuous improvement," says Berbée. "It allows for the exchange of best practices among industry leaders and offers us the ability to highlight the value of integrating energy management throughout all aspects of a building's life cycle."

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Checking the First Data from OCO-2

On July 2, NASA successfully launched its first satellite dedicated to measuring carbon dioxide in Earth's atmosphere. The Orbiting Carbon Observatory-2 (OCO-2) mission—operated by NASA's Jet Propulsion Laboratory—will soon provide atmospheric carbon dioxide measurements from thousands of points all over the planet. Last week, the satellite reached its proper orbit—meaning that it is now beginning to return its first data to Earth.

Data from the satellite will be used to help researchers understand the anthropogenic and natural sources of CO2, and how changing levels of the greenhouse gas may affect Earth's climate. But before OCO-2 provides scientists with such a global picture of the carbon cycle—where carbon is being produced and absorbed on Earth—researchers have to convert raw satellite data into a CO2 reading and then, just as importantly, make sure that the reading is accurate. A team of Caltech researchers is playing an instrumental role in this effort.

As it orbits, OCO-2 provides data about levels of atmospheric CO2 by measuring the sunlight that reflects off Earth, below. "OCO-2 measures something that is related to the CO2 measurement we want but it's not directly what we want. So from the reflected light, we have to extract the information about CO2," says Yuk Yung, the Smits Family Professor of Planetary Science.

The process begins with the satellite's instrument, a set of high-resolution spectrometers that measure the intensity of sunlight at different wavelengths, or colors, after it has passed twice through the atmosphere—once from the sun to the surface, and then back from the surface to space. As the satellite orbits, systematically slicing over sections of Earth's atmosphere, it will collect millions of these measurements.

"OCO-2 will provide the measurements of this light at different wavelengths in millions of what we call spectra, but spectra aren't what we really want—what we really want is to know how much carbon dioxide is in the atmosphere," Yung says. "But to get the CO2 information from the spectra, we have to do what's called data retrieval—and that's one of my jobs."

The data retrieval method that Yung and his colleagues designed for OCO-2 compares the light spectra collected by the satellite to a model of how light spectra would look—based on the laws of physics and knowledge of how efficiently CO2 absorbs sunlight. This knowledge, in turn, is derived from laboratory measurements made by Caltech professor of chemical physics Mitchio Okumura and his colleagues at JPL and the National Institute of Standards and Technology.

"To make scientifically meaningful measurements, OCO-2 has to detect CO2 with better than 0.3 percent precision, and that has meant going back to the lab and measuring the spectral properties with extraordinarily high precision," Okumura says. From this retrieval, the researchers determine the amount of CO2 in the atmosphere above each of OCO-2's sampling points.

However, when OCO-2 sends its first CO2 measurements back to Earth for analysis, they'll still have to go through one more check, says Paul Wennberg, the R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering.

"Although the OCO-2 retrieval will calculate the amount of carbon dioxide above the point where the spectrometers pointed, we know that these initial numbers will be wrong until the data are calibrated," Wennberg says. Wennberg and his team provide this calibration with their Total Carbon Column Observing Network (TCCON), a ground-based network of instruments that measure atmospheric CO2 from approximately 20 locations around the world.

TCCON and OCO-2 provide the same type of CO2 measurement—what is called a column average of CO2. This measurement provides the average abundance of CO2 in a column from the ground all the way up through Earth's atmosphere.

About once per day, the OCO-2 instrument will be commanded to point at one of TCCON's stations continuously as it passes overhead. By comparing the Earth-based and space-based measurements, researchers will evaluate the data that they receive from the satellite and improve the retrieval method.

The complete, high-quality information OCO-2 provides about global CO2 levels will be important for researchers and policymakers to determine how human activity influences the carbon cycle—and how these activities contribute to our changing planet.

"A lot of the very first satellites were developed to study astronomy and planets far away. But there has been a shift. Our changing climate means that we now have a big need to study Earth," and the information OCO-2 provides about our atmosphere will be an important part of filling that need, says Yung.

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Corals Provide Clues for Climate Change Research

Just as growth rings can offer insight into climate changes occurring during the lifespan of a tree, corals have much to tell about changes in the ocean. At Caltech, climate scientists Jess F. Adkins and Nivedita Thiagarajan use manned submersibles, like Alvin operated by the Woods Hole Oceanographic Institution, to dive thousands of meters below the surface to collect these specimens—and to shed new light on the connection between variance in carbon dioxide (CO2) levels in the deep ocean and historical glacial cycles.

A paper describing the research appears in the July 3 issue of Nature.

It has long been known that ice sheets wax and wane as the concentration of CO2 decreases and increases in the atmosphere. Adkins and his team believe that the deep ocean—which stores 60 times more inorganic sources of carbon than is found in the atmosphere—must play a vital role in this variance.

To investigate this, the researchers analyzed the calcium carbonate skeletons of corals collected from deep in the North Atlantic Ocean. The corals were built up from 11,000–18,000 years ago out of CO2 dissolved in the ocean.

"We used a new technique that has been developed at Caltech, called clumped isotope thermometry, to determine what the temperature of the ocean was in the location where the coral grew," says Thiagarajan, the Dreyfus Postdoctoral Scholar in Geochemistry at Caltech and lead author of the paper. "We also used radiocarbon dating and uranium-series dating to estimate the deep-ocean ventilation rate during this time period." 

The researchers found that the deep ocean started warming before the start of a rapid climate change event about 14,600 years ago in which the last glacial period—or most recent time period when ice sheets covered a large portion of Earth—was in the final stages of transitioning to the current interglacial period.

"We found that a warm-water-under-cold-water scenario developed around 800 years before the largest signal of warming in the Greenland ice cores, called the 'Bølling–Allerød,'" explains Adkins. "CO2 had already been rising in the atmosphere by this time, but we see the deep-ocean reorganization brought on by the potential energy release to be the pivot point for the system to switch from a glacial state, where the deep ocean can hold onto CO2, and an interglacial state, where it lets out CO2."  

"Studying Earth's climate in the past helps us understand how different parts of the climate system interact with each other," says Thiagarajan. "Figuring out these underlying mechanisms will help us predict how climate will change in the future." 

Additional authors on the Nature paper, "Abrupt pre-Bølling–Allerød warming and circulation changes in the deep ocean," are geochemist John M. Eiler and graduate student Adam V. Subhas from Caltech, and John R. Southon from UC Irvine. 

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DOE Awards $15 Million to Caltech's Solar Energy Research

The United States Department of Energy (DOE) announced on Wednesday that it will be awarding $15.2 million to Caltech's Light-Material Interactions in Energy Conversion (LMI) program, one of 32 Energy Frontier Research Centers (EFRCs) nationwide that will receive a combined $100 million over the next four years to pursue innovative energy research.

The LMI-EFRC is directed by Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science, and is a collaborative partnership of researchers in photonics (the generation, manipulation, and detection of light) at Caltech, Lawrence Berkeley National Laboratory, the University of Illinois at Urbana-Champaign, Stanford University, and Harvard University.

The DOE received more than 200 proposals for EFRCs. Caltech is among 22 centers whose initial funding, granted in 2009, is being extended for another four years. During its first funding period, among other accomplishments, LMI-EFRC fabricated complex three-dimensional photonic nanostructure and light absorbers; created a solar cell with world-record-breaking efficiency; and developed the printing-based mechanical assembly of microscale solar cells.

"In recent years the solar energy landscape has been fundamentally altered with the recent growth of a large worldwide photovoltaics industry," says Atwater. "The most important area for basic research advances is now in enumerating the scientific principles and methods for achieving the highest conversion efficiencies. There is a new era emerging in which the science of nanoscale light management plays a critical role in enabling energy conversion to surpass traditional limits. This is where the Light-Material Interactions EFRC has focused its effort and is making advances."

LMI-EFRC will be using its new DOE award to address opportunities for high-efficiency solar energy conversion, with a goal of making scientific discoveries that will enable utilization of the entire visible and infrared solar resource.

"We are proud of the accomplishments of Professor Atwater, his colleagues, postdocs, and students in the Light-Material Interactions effort and are gratified that this effort will go beyond its great accomplishments to date through the renewed funding from the DOE," says Peter Schröder, deputy chair of the Division of Engineering and Applied Science and the Shaler Arthur Hanisch Professor of Computer Science and Applied and Computational Mathematics. "Harry exemplifies the best tradition of engineering at Caltech, creating the interface between fundamental science advances and their realization through engineering for the benefit of society at large."

According to the United States Department of Energy, "transforming the way we generate, supply, transmit, store, and use energy will be one of the defining challenges for America and the globe in the 21st century. At its heart, the challenge is a scientific one. Important as they are, incremental advances in current energy technologies will not be sufficient. History has demonstrated that radically new technologies arise from disruptive advances at the science frontiers. The Energy Frontier Research Centers program aims to accelerate such transformative discovery." 

Energy Secretary Ernest Moniz, in announcing the awards, said, "Today, we are mobilizing some of our most talented scientists to join forces and pursue the discoveries and breakthroughs that will lay the foundation for our nation's energy future."

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Commencement Speaker Daniel Yergin Visits Caltech

This year's commencement speaker, Pulitzer Prize-winning energy expert Daniel Yergin, came to campus a day early to get better acquainted with Caltech's faculty and students, and in particular to learn more about Caltech's energy initiatives.

Yergin spent the morning touring JPL. "I was especially excited to see the Curiosity Rover," says Yergin. "Also, it was remarkable to actually be in the space flight operations center, having seen it in the news so often before."

In the afternoon at Caltech, Yergin specifically asked to spend most of his time meeting with people to learn more about their work. "Caltech is such a preeminent institution," says Yergin, "and I thought that coming for commencement provided a wonderful opportunity to get a sense of the culture here. Caltech has such a large impact for such a small place, and I wanted to experience more of it."

Of course, high on Yergin's agenda was the opportunity to meet with those engaged in research on sustainable energy. A mid-afternoon meeting at the Resnick Sustainability Institute yielded a lively exchange between Yergin, graduate students, and postdoctoral fellows about the future of energy research and the geopolitics of energy policy. Asked what he had learned from writing The Prize and The Quest, his two bestselling volumes on the history of oil, gas, and energy policy, Yergin replied that the main takeaway was that throughout the history of the industry, "everybody will feel they know where everything related to energy is going to go, and then in four or five years, something comes along to completely change it."

Stressing the increasingly important role of innovation in energy technology and geopolitics, especially as global markets continue to grow, Yergin says, "The challenge is that energy is a long-range investment and a long-range research project. It's very hard to say what is actually going to make a difference, but the cumulative impact of what you and people like you are doing is absolutely necessary. It's these kind of centers that will map the future of energy policy. The innovation is not going to come from big companies; it's going to come from universities and research institutes.  So keep doing what you're doing."

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Cynthia Eller
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JCAP Stabilizes Common Semiconductors For Solar Fuels Generation

Caltech researchers devise a method to protect the materials in a solar-fuel generator

Researchers around the world are trying to develop solar-driven generators that can split water, yielding hydrogen gas that could be used as clean fuel. Such a device requires efficient light-absorbing materials that attract and hold sunlight to drive the chemical reactions involved in water splitting. Semiconductors like silicon and gallium arsenide are excellent light absorbers—as is clear from their widespread use in solar panels. However, these materials rust when submerged in the type of water solutions found in such systems.

Now Caltech researchers at the Joint Center for Artificial Photosynthesis (JCAP) have devised a method for protecting these common semiconductors from corrosion even as the materials continue to absorb light efficiently. The finding paves the way for the use of these materials in solar-fuel generators.

"For the better part of a half century, these materials have been considered off the table for this kind of use," says Nate Lewis, the George L. Argyros Professor and professor of chemistry at Caltech, and the principal investigator on the paper. "But we didn't give up on developing schemes by which we could protect them, and now these technologically important semiconductors are back on the table."

The research, led by Shu Hu, a postdoctoral scholar in chemistry at Caltech, appears in the May 30 issue of the journal Science.

In the type of integrated solar-fuel generator that JCAP is striving to produce, two half-reactions must take place—one involving the oxidation of water to produce oxygen gas; the other involving the reduction of water, yielding hydrogen gas. Each half-reaction requires both a light-absorbing material to serve as the photoelectrode and a catalyst to drive the chemistry. In addition, the two reactions must be physically separated by a barrier to avoid producing an explosive mixture of their products.

Historically, it has been particularly difficult to come up with a light-absorbing material that will robustly carry out the oxidation half-reaction. Researchers have tried, without much success, a variety of materials and numerous techniques for coating the common light-absorbing semiconductors. The problem has been that if the protective layer is too thin, the aqueous solution penetrates through and corrodes the semiconductor. If, on the other hand, the layer is too thick, it prevents corrosion but also blocks the semiconductor from absorbing light and keeps electrons from passing through to reach the catalyst that drives the reaction.

At Caltech, the researchers used a process called atomic layer deposition to form a layer of titanium dioxide (TiO2)—a material found in white paint and many toothpastes and sunscreens—on single crystals of silicon, gallium arsenide, or gallium phosphide. The key was that they used a form of TiO2 known as "leaky TiO2"—because it leaks electricity. First made in the 1990s as a material that might be useful for building computer chips, leaky oxides were rejected as undesirable because of their charge-leaking behavior. However, leaky TiO2 seems to be just what was needed for this solar-fuel generator application. Deposited as a film, ranging in thickness between 4 and 143 nanometers, the TiO2 remained optically transparent on the semiconductor crystals—allowing them to absorb light—and protected them from corrosion but allowed electrons to pass through with minimal resistance.

On top of the TiO2, the researchers deposited 100-nanometer-thick "islands" of an abundant, inexpensive nickel oxide material that successfully catalyzed the oxidation of water to form molecular oxygen.

The work appears to now make a slew of choices available as possible light-absorbing materials for the oxidation side of the water-splitting equation. However, the researchers emphasize, it is not yet known whether the protective coating would work as well if applied using an inexpensive, less-controlled application technique, such as painting or spraying the TiO2 onto a semiconductor. Also, thus far, the Caltech team has only tested the coated semiconductors for a few hundred hours of continuous illumination.

"This is already a record in terms of both efficiency and stability for this field, but we don't yet know whether the system fails over the long term and are trying to ensure that we make something that will last for years over large areas, as opposed to weeks," says Lewis. "That's the next step."

The work, titled "Amorphous TiO2 Coatings Stabilize Si, GaAs, and GaP Photoanodes for Efficient Water Oxidation," was supported by the Office of Science of the U.S. Department of Energy through an award to JCAP, a DOE Energy Innovation Hub. Some of the work was also supported by the Resnick Sustainability Institute and the Beckman Institute at Caltech. Additional coauthors on the paper are graduate students Matthew Shaner, Joseph Beardslee, and Michael Lichterman, as well as Bruce S. Brunschwig, director of the Molecular Materials Resource Center at Caltech.

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Kimm Fesenmaier
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Stabilizing Semiconductors for Solar Fuels Generation
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