Solar power could get boost from new light absorption design

ScienceDaily (Nov. 2, 2011) — Solar power may be on the rise, but solar cells are only as efficient as the amount of sunlight they collect. Under the direction of a new McCormick professor, researchers have developed a new material that absorbs a wide range of wavelengths and could lead to more efficient and less expensive solar technology.

A paper describing the findings, "Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers," was published November 1 in the journal Nature Communications.

"The solar spectrum is not like a laser -- it's very broadband, starting with UV and going up to near-infrared," said Koray Aydin, assistant professor of electrical engineering and computer science and the paper's lead author. "To capture this light most efficiently, a solar cell needs to have a broadband response. This design allows us to achieve that."

The researchers used two unconventional materials -- metal and silicon oxide -- to create thin but complex, trapezoid-shaped metal gratings on the nanoscale that can trap a wider range of visible light. The use of these materials is unusual because on their own, they do not absorb light; however, they worked together on the nanoscale to achieve very high absorption rates, Aydin said.

The uniquely shaped grating captured a wide range of wavelengths due to the local optical resonances, causing light to spend more time inside the material until it gets absorbed. This composite metamaterial was also able to collect light from many different angles -- a useful quality when dealing with sunlight, which hits solar cells at different angles as sun moves from east to west throughout the day.

This research is not directly applicable to solar cell technology because metal and silicon oxide cannot convert light to electricity; in fact, the photons are converted to heat and might allow novel ways to control the heat flow at the nanoscale. However, the innovative trapezoid shape could be replicated in semiconducting materials that could be used in solar cells, Aydin said.

If applied to semiconducting materials, the technology could lead to thinner, lower-cost, and more efficient solar cells, he said.

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The above story is reprinted from materials provided by Northwestern University. The original article was written by Sarah Ostman.

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Journal Reference:

Koray Aydin, Vivian E. Ferry, Ryan M. Briggs, Harry A. Atwater. Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers. Nature Communications, 2011; 2: 517 DOI: 10.1038/ncomms1528

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Asteroid lutetia: Primitive body from solar system's planet-forming period

ScienceDaily (Oct. 28, 2011) — The European Space Agency's Rosetta spacecraft has revealed asteroid Lutetia to be a primitive body, left over as the planets were forming in our Solar System. Results from Rosetta's fleeting flyby also suggest that this mini-world tried to grow a metal heart.

Rosetta flew past Lutetia on 10 July 2010 at a speed of 54 000 km/hr and a closest distance of 3170 km. At the time, the 130 km-long asteroid was the largest encountered by a spacecraft. Since then, scientists have been analysing the data taken during the brief encounter.

All previous flybys went past objects, which were fragments of once-larger bodies. However, during the encounter, scientists speculated that Lutetia might be an older, primitive 'mini-world'.

Now they are much more certain. Images from the OSIRIS camera reveal that parts of Lutetia's surface are around 3.6 billion years old. Other parts are young by astronomical standards, at 50-80 million years old.

Astronomers estimate the age of airless planets, moons, and asteroids by counting craters. Each bowl-shaped depression on the surface is made by an impact. The older the surface, the more impacts it will have accumulated. Some parts of Lutetia are heavily cratered, implying that it is very old.

On the other hand, the youngest areas of Lutetia are landslides, probably triggered by the vibrations from particularly jarring nearby impacts.

Debris resulting from these many impacts now lies across the surface as a 1 km-thick layer of pulverised rock.

There are also boulders strewn across the surface: some are 300-400 m across, or about half the size of Ayers Rock, in Australia.

Some impacts must have been so large that they broke off whole chunks of Lutetia, gradually sculpting it into the battered wreck we see today.

"We don't think Lutetia was born looking like this," says Holger Sierks, Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany. "It was probably round when it formed."

Rosetta's VIRTIS spectrometer found that Lutetia's composition is remarkably uniform across all the observed regions.

"It is striking that an object of this size can bear scars of events so different in age across its surface while not showing any sign of surface compositional variation," says Fabrizio Capaccioni, INAF, Rome, Italy.

This is just the start of the mystery.

Rosetta also let scientists investigate beneath the asteroid's surface. It appears that Lutetia tried to grow an iron core like a bona-fide planet when it formed.

During the encounter, Lutetia's weak gravity tugged on Rosetta. The slight change in Rosetta's path was reflected in radio signals received back at Earth, indicating a mass of 1.7 million billion tonnes.

This was a surprise.

"The mass was lower than expected. Ground-based observations had suggested much higher values," says Martin Pätzold, Universität zu Köln, Germany, leader of the radio science team.

Nevertheless, when combined with its volume, Lutetia still turns out to have one of the highest densities of any known asteroid: 3400 kg per cubic metre. The density implies that Lutetia contains significant quantities of iron, but not necessarily in a fully formed core.

To form an iron core, Lutetia would have had to melt as a result of heat released by radioactive isotopes in its rocks. The dense iron would then sink to the centre and the rocky material would float to the top.

However, VIRTIS indicates that Lutetia's surface composition remains entirely primordial, displaying none of the rocky material expected to form during such a molten phase.

The only explanation appears to be that Lutetia was subjected to some internal heating early in its history but did not melt completely and so did not end up with a well-defined iron core.

These results, all gathered during just a short flyby, make Lutetia a unique asteroid and an invaluable postcard from the past, at a time when Earth was forming.

"We picked a most important member of the asteroid belt," said Rita Schulz, ESA's Rosetta Project Scientist.

"All the asteroids encountered so far were different from each other, but Lutetia is the only one in which both primordial and differentiation features have been found.

"These unexpected results clearly show that there is still much more to investigate before we understand the belt fully."

Having now left Lutetia far behind, Rosetta is in hibernation and en route to its 2014 rendezvous with comet Churyumov-Gerasimenko.

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Journal Reference:

H. Sierks, P. Lamy, C. Barbieri, D. Koschny, H. Rickman, R. Rodrigo, M. F. A'Hearn, F. Angrilli, M. A. Barucci, J.- L. Bertaux, I. Bertini, S. Besse, B. Carry, G. Cremonese, V. Da Deppo, B. Davidsson, S. Debei, M. De Cecco, J. De Leon, F. Ferri, S. Fornasier, M. Fulle, S. F. Hviid, R. W. Gaskell, O. Groussin, P. Gutierrez, W. Ip, L. Jorda, M. Kaasalainen, H. U. Keller, J. Knollenberg, R. Kramm, E. Kuhrt, M. Kuppers, L. Lara, M. Lazzarin, C. Leyrat, J. J. L. Moreno, S. Magrin, S. Marchi, F. Marzari, M. Massironi, H. Michalik, R. Moissl, G. Naletto, F. Preusker, L. Sabau, W. Sabolo, F. Scholten, C. Snodgrass, N. Thomas, C. Tubiana, P. Vernazza, J.- B. Vincent, K.- P. Wenzel, T. Andert, M. Patzold, B. P. Weiss. Images of Asteroid 21 Lutetia: A Remnant Planetesimal from the Early Solar System. Science, 2011; 334 (6055): 487 DOI: 10.1126/science.1207325

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Kepler spacecraft discovers new multi-planet solar system

ScienceDaily (Oct. 5, 2011) — A team of researchers led by Bill Cochran of The University of Texas at Austin has used NASA's Kepler spacecraft to discover an unusual multiple-planet system containing a super-Earth and two Neptune-sized planets orbiting in resonance with each other.

They are announcing the find in Nantes, France at a joint meeting of the European Planetary Science Conference and the American Astronomical Society's Division of Planetary Science. The research will be published in a special Kepler issue of The Astrophysical Journal Supplement Series in November.

Cochran's team is announcing three planets orbiting Kepler-18, a star similar to the Sun. Kepler 18 is just 10 percent larger than the Sun and contains 97 percent of the Sun's mass. It may host more planets than the three just announced.

The planets are designated b, c, and d. All three planets orbit much closer to Kepler-18 than Mercury does to the Sun. Orbiting closest to Kepler-18 with a 3.5-day period, planet b weighs in at about 6.9 times the mass of Earth, and twice Earth's size. Planet b is considered a "super-Earth." Planet c has a mass of about 17 Earths, is about 5.5 times Earth's size, and orbits Kepler-18 in 7.6 days. Planet d weighs in at 16 Earths, at 7 times Earth's size, and has a 14.9-day orbit. The masses and sizes of c and d qualify them as low-density 2Neptune-class" planets.

Planet c orbits the star twice for every one orbit d makes. But the times that each of these planets transit the face of Kepler-18 "are not staying exactly on that orbital period," Cochran says. "One is slightly early when the other one is slightly late, [then] both are on time at the same time, and then vice-versa."

Scientifically speaking, c and d are orbiting in a 2:1 resonance. "It means they're interacting with each other," Cochran explains. "When they are close to each other ... they exchange energy, pull and tug on each other."

Kepler uses the "transit method" to look for planets. It monitors a star's brightness over time, looking for periodic dips that could indicate a planet passing in front of the star. A large part of the Kepler science team's work is proving that potential planets they find aren't something else that mimics the transit signature (such as a perfectly aligned background star, specifically either an eclipsing binary star or a single star orbited by a giant planet).

That follow-up work to Kepler is done by scores of scientists using ground-based telescopes the world over (including several at The University of Texas at Austin's McDonald Observatory) as well as Spitzer Space Telescope.

Kepler-18's planets c and d did astronomers a favour by proving their planet credentials up front via their orbital resonance; they had to be in the same planetary system as each other for the resonance to occur.

Confirming the planetary bona fides of planet b, the super-Earth, was much more complicated, Cochran says. His team used a technique called "validation," instead of verification. They set out to figure out the probability that it could be something other than a planet.

First, they used the Palomar 5-meter (200-inch) Hale Telescope with adaptive optics to take an extremely high-resolution look at the space around Kepler-18. They wanted to see if anything close to the star could be positively identified as a background object that would cause the transit signal they had attributed to a super-Earth.

"We successively went through every possible type of object that could be there," Cochran says. "There are limits on the sort of objects that can be there at different distances from the star." Astronomers know how many of different types of objects (various kinds of stars, background galaxies, and more) are seen on average in the sky. They didn't find anything in the Palomar image.

"There's a small possibility that [planet b] is due to a background object, but we're very confident that it's probably a planet," Cochran says. His team calculated that the likelihood the object is a planet is 700 times more likely than the likelihood that it's a background object.

The process is called "planet validation," rather than the usual "planet verification." Cochran says it's important to understand the difference -- not just for this system, but for future discoveries from Kepler and other missions.

"We're trying to prepare the astronomical community and the public for the concept of validation," he says. "The goal of Kepler is to find an Earth-sized planet in the habitable zone [where life could arise], with a one-year orbit. Proving that such an object really is a planet is very difficult [with current technology]. When we find what looks to be a habitable Earth, we'll have to use a validation process, rather than a confirmation process. We're going to have to make statistical arguments."

Kepler was selected as the tenth NASA Discovery mission. NASA Ames Research Center, Moffett Field, Calif., is the home organization of the science principal investigator, and is responsible for the ground system development, mission operations and science data analysis. Jet Propulsion Laboratory, Pasadena, Calif., managed the Kepler mission development. Ball Aerospace & Technologies Corp. of Boulder, Colo., developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder. The Space Telescope Science Institute in Baltimore archives, hosts, and distributes the Kepler science data. For more information about the Kepler mission, visit: http://www.nasa.gov/kepler.

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NASA's Spitzer detects comet storm in nearby solar system

ScienceDaily (Oct. 19, 2011) — NASA's Spitzer Space Telescope has detected signs of icy bodies raining down in an alien solar system. The downpour resembles our own solar system several billion years ago during a period known as the "Late Heavy Bombardment," which may have brought water and other life-forming ingredients to Earth.

During this epoch, comets and other frosty objects that were flung from the outer solar system pummeled the inner planets. The barrage scarred our moon and produced large amounts of dust.

Now Spitzer has spotted a band of dust around a nearby bright star in the northern sky called Eta Corvi that strongly matches the contents of an obliterated giant comet. This dust is located close enough to Eta Corvi that Earth-like worlds could exist, suggesting a collision took place between a planet and one or more comets. The Eta Corvi system is approximately one billion years old, which researchers think is about the right age for such a hailstorm.

"We believe we have direct evidence for an ongoing Late Heavy Bombardment in the nearby star system Eta Corvi, occurring about the same time as in our solar system," said Carey Lisse, senior research scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and lead author of a paper detailing the findings. The findings will be published in the Astrophysical Journal. Lisse presented the results at the Signposts of Planets meeting at NASA's Goddard Space Flight Center in Greenbelt, Md., on Oct. 19.

Astronomers used Spitzer's infrared detectors to analyze the light coming from the dust around Eta Corvi. Certain chemical fingerprints were observed, including water ice, organics and rock, which indicate a giant comet source.

The light signature emitted by the dust around Eta Corvi also resembles the Almahata Sitta meteorite, which fell to Earth in fragments across Sudan in 2008. The similarities between the meteorite and the object obliterated in Eta Corvi imply a common birthplace in their respective solar systems.

A second, more massive ring of colder dust located at the far edge of the Eta Corvi system seems like the proper environment for a reservoir of cometary bodies. This bright ring, discovered in 2005, looms at about 150 times the distance from Eta Corvi as Earth is from the sun. Our solar system has a similar region, known as the Kuiper Belt, where icy and rocky leftovers from planet formation linger. The new Spitzer data suggest that the Almahata Sitta meteorite may have originated in our own Kuiper Belt.

The Kuiper Belt was home to a vastly greater number of these frozen bodies, collectively dubbed Kuiper Belt objects. About 4 billion years ago, some 600 million years after our solar system formed, scientists think the Kuiper Belt was disturbed by a migration of the gas-giant planets Jupiter and Saturn. This jarring shift in the solar system's gravitational balance scattered the icy bodies in the Kuiper Belt, flinging the vast majority into interstellar space and producing cold dust in the belt. Some Kuiper Belt objects, however, were set on paths that crossed the orbits of the inner planets.

The resulting bombardment of comets lasted until 3.8 billion years ago. After comets impacted the side of the moon that faces Earth, magma seeped out of the lunar crust, eventually cooling into dark "seas," or maria. When viewed against the lighter surrounding areas of the lunar surface, those seas form the distinctive "Man in the Moon" visage. Comets also struck Earth or incinerated in the atmosphere, and are thought to have deposited water and carbon on our planet. This period of impacts might have helped life form by delivering its crucial ingredients.

"We think the Eta Corvi system should be studied in detail to learn more about the rain of impacting comets and other objects that may have started life on our own planet," Lisse said.

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer mission for the agency's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

For more information about Spitzer, visit http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer .

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Kepler spacecraft discovers new multi-planet solar system

ScienceDaily (Oct. 5, 2011) — A team of researchers led by Bill Cochran of The University of Texas at Austin has used NASA's Kepler spacecraft to discover an unusual multiple-planet system containing a super-Earth and two Neptune-sized planets orbiting in resonance with each other.

They are announcing the find in Nantes, France at a joint meeting of the European Planetary Science Conference and the American Astronomical Society's Division of Planetary Science. The research will be published in a special Kepler issue of The Astrophysical Journal Supplement Series in November.

Cochran's team is announcing three planets orbiting Kepler-18, a star similar to the Sun. Kepler 18 is just 10 percent larger than the Sun and contains 97 percent of the Sun's mass. It may host more planets than the three just announced.

The planets are designated b, c, and d. All three planets orbit much closer to Kepler-18 than Mercury does to the Sun. Orbiting closest to Kepler-18 with a 3.5-day period, planet b weighs in at about 6.9 times the mass of Earth, and twice Earth's size. Planet b is considered a "super-Earth." Planet c has a mass of about 17 Earths, is about 5.5 times Earth's size, and orbits Kepler-18 in 7.6 days. Planet d weighs in at 16 Earths, at 7 times Earth's size, and has a 14.9-day orbit. The masses and sizes of c and d qualify them as low-density 2Neptune-class" planets.

Planet c orbits the star twice for every one orbit d makes. But the times that each of these planets transit the face of Kepler-18 "are not staying exactly on that orbital period," Cochran says. "One is slightly early when the other one is slightly late, [then] both are on time at the same time, and then vice-versa."

Scientifically speaking, c and d are orbiting in a 2:1 resonance. "It means they're interacting with each other," Cochran explains. "When they are close to each other ... they exchange energy, pull and tug on each other."

Kepler uses the "transit method" to look for planets. It monitors a star's brightness over time, looking for periodic dips that could indicate a planet passing in front of the star. A large part of the Kepler science team's work is proving that potential planets they find aren't something else that mimics the transit signature (such as a perfectly aligned background star, specifically either an eclipsing binary star or a single star orbited by a giant planet).

That follow-up work to Kepler is done by scores of scientists using ground-based telescopes the world over (including several at The University of Texas at Austin's McDonald Observatory) as well as Spitzer Space Telescope.

Kepler-18's planets c and d did astronomers a favour by proving their planet credentials up front via their orbital resonance; they had to be in the same planetary system as each other for the resonance to occur.

Confirming the planetary bona fides of planet b, the super-Earth, was much more complicated, Cochran says. His team used a technique called "validation," instead of verification. They set out to figure out the probability that it could be something other than a planet.

First, they used the Palomar 5-meter (200-inch) Hale Telescope with adaptive optics to take an extremely high-resolution look at the space around Kepler-18. They wanted to see if anything close to the star could be positively identified as a background object that would cause the transit signal they had attributed to a super-Earth.

"We successively went through every possible type of object that could be there," Cochran says. "There are limits on the sort of objects that can be there at different distances from the star." Astronomers know how many of different types of objects (various kinds of stars, background galaxies, and more) are seen on average in the sky. They didn't find anything in the Palomar image.

"There's a small possibility that [planet b] is due to a background object, but we're very confident that it's probably a planet," Cochran says. His team calculated that the likelihood the object is a planet is 700 times more likely than the likelihood that it's a background object.

The process is called "planet validation," rather than the usual "planet verification." Cochran says it's important to understand the difference -- not just for this system, but for future discoveries from Kepler and other missions.

"We're trying to prepare the astronomical community and the public for the concept of validation," he says. "The goal of Kepler is to find an Earth-sized planet in the habitable zone [where life could arise], with a one-year orbit. Proving that such an object really is a planet is very difficult [with current technology]. When we find what looks to be a habitable Earth, we'll have to use a validation process, rather than a confirmation process. We're going to have to make statistical arguments."

Kepler was selected as the tenth NASA Discovery mission. NASA Ames Research Center, Moffett Field, Calif., is the home organization of the science principal investigator, and is responsible for the ground system development, mission operations and science data analysis. Jet Propulsion Laboratory, Pasadena, Calif., managed the Kepler mission development. Ball Aerospace & Technologies Corp. of Boulder, Colo., developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder. The Space Telescope Science Institute in Baltimore archives, hosts, and distributes the Kepler science data. For more information about the Kepler mission, visit: http://www.nasa.gov/kepler.

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Solar changes help create cold northern winters

Fluctuations in ultraviolet light can set up frigid, snowy conditionsWeb edition : Monday, October 10th, 2011

Harsh winters in the United States and northern Europe may partly be the result of changes in ultraviolet radiation coming from the sun.

A new climate simulation study shows how fluctuations in ultraviolet light linked to the sun’s 11-year activity cycle could change winter weather patterns across the Northern Hemisphere. The work appears online October 9 in Nature Geoscience.

“We hope this will open the door to improving ultralong-range predictions,” says co-author Adam Scaife, a climate modeler at the Met Office’s Hadley Centre in Exeter, England.

Scientists have long noted anecdotal links between low solar activity and cold European winters: Part of the Little Ice Age, which gripped the region between about 1550 and 1850, coincided with a record low number of sunspots, which are one measure of solar activity.  But until now, Scaife says, no one had found a physical explanation for how subtle changes in radiation hitting the top of Earth’s atmosphere could translate to changes in weather patterns at the surface.

The answer came from the Solar Radiation and Climate Experiment satellite. From 2004 to 2007, during the low points of the last solar cycle, the satellite measured a surprising drop-off in the amount of ultraviolet radiation coming from the sun, roughly five times greater than previously thought. “I thought, if that’s true, that’s going to do something interesting to the climate system,” Scaife says.

To test what might happen, the scientists put the big ultraviolet decline into the Met Office’s climate model, a massive computer program that can simulate how the ocean and atmosphere respond to such changes. With less ultraviolet radiation, the simulation suggested, parts of the upper atmosphere cooled more than usual and allowed winds to blow more from the east. The anomaly then got bigger and started to burrow down through the atmosphere to altitudes where weather patterns form.  There, the changes affected how storms would normally grow, allowing cold weather to form over northern Europe and the United States.

These changes occurred only in winter, and not during every solar cycle minimum the model analyzed. But over time, the scientists found, more winters saw these cold patterns form during solar minimum than during solar maximum. “It’s changing the odds of what kind of winter you’re going to get by a significant amount,” Scaife says.

At the same time, weather patterns over southern Europe and Canada were milder than normal, essentially canceling out the chill of northern Europe and the United States. The new work thus can’t say much about whether changes in solar radiation affect global temperatures, Scaife says. Other natural factors also affect the severity of winters, including volcanic eruptions and semi-regular weather patterns like El Niño.

Kunihiko Kodera, a sun-climate researcher at Nagoya University in Japan, says the new model seems to capture all the steps in the atmosphere, but small details as to how warm or cold it got over particular landmasses may make it difficult to predict localized weather changes.

If the Met Office computer model can accurately reproduce past forecasts, the researchers hope to start incorporating solar variability into long-term weather predictions. Solar activity is currently increasing toward an expected weak maximum in 2013.

Found in: Earth and Earth Science

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Measuring elusive solar neutrinos flowing through the Earth, physicists learn more about the sun

ScienceDaily (Oct. 11, 2011) — Using one of the most sensitive neutrino detectors on the planet, an international team including physicists Laura Cadonati and Andrea Pocar at the University of Massachusetts Amherst are now measuring the flow of solar neutrinos reaching earth more precisely than ever before. The detector probes matter at the most fundamental level and provides a powerful tool for directly observing the sun's composition.

Pocar, Cadonati and colleagues report in the current issue of Physical Review Letters that the Borexino instrument has now measured with high precision the flux of the beryllium seven (7Be) solar neutrino, abundant, low-energy particles once below the observable threshold. With this advance, they can now precisely study the behavior of solar neutrinos with kinetic energy below 1 megaelectron volt (MeV). Borexino scientists also recently reported the first observation of neutrinos produced in a little-studied solar nuclear process known as proton-electron-proton, or pep, and set of stringent limit on reactions involving carbon, nitrogen and oxygen (the CNO cycle) in the sun.

Cadonati says, "Borexino is the only detector capable of observing the entire spectrum of solar neutrinos at once. Our results, the culmination of 20 years of research, greatly narrow the observation precision. The data confirm the neutrino oscillations, flavor changes and flow predicted by models of the sun and particle physics."

Of particular interest, Pocar and Cadonati note, is the Borexino instrument's ability to more thoroughly test neutrino oscillation parameters, allowing an exploration of their characteristic non-zero mass, which does not fit the Standard Model of particle physics. "Our data can tell us about fundamental micro physics at the particle level," says Cadonati. "Borexino is using neutrinos to explore the interior of the sun, looking for new, exciting clues to the mysteries of the universe we cannot see." Pocar adds, "Our detector provides stringent tests of the three-neutrino oscillations model."

Solar neutrinos are produced in nuclear processes and radioactive decays of several elements during fusion reactions at the sun's core. As many as 65 billion of them stream out of the sun and hit every square centimeter of Earth's surface [or 420 billion every square inch] every second. But because they only interact through the nuclear weak force they pass through matter unaffected, making them very difficult to detect and to distinguish from the trace nuclear decays of ordinary materials. The weak force is one of the four fundamental forces of nature, with gravity, electromagnetism and the strong force. It is responsible for the radioactive decay of unstable subatomic particles, with a short range of influence, about 1 percent of the diameter of a typical atomic nucleus.

The Borexino instrument, housed far beneath Italy's Apennine Mountains, detects neutrinos as they interact with an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 2,000 tons of water. Its great depth and many onion-like protective layers maintain the core as the most radiation-free medium on the planet.

There are three neutrino types, or "flavors": electron, muon and tau. Those produced in the sun are the electron type. As they travel away from their birthplace, they oscillate, or change from one flavor to another. A detector like Borexino can observe all three types in real time and measures each one's energy, but it cannot distinguish between them. It's more sensitive to the electron type so they are more likely to be seen.

The 7Be neutrino flux now being detected by Borexino is predicted by the standard solar model to make up about 10 percent of solar flow, Cadonati says. Earlier instruments in Canada and Japan designed to detect higher-energy neutrinos had already observed evidence of their flavor oscillations, probing 1/10,000 of the solar neutrino flux and their oscillations as they travel through solar matter. However, without data in the low-energy range as scanned by Borexino, physicists were not able to confirm the specific energy-dependent effect of solar neutrino oscillations. Borexino has now filled this gap and for the first time observed evidence of neutrino oscillation in vacuum, as they travel between the sun and Earth.

Pocar says that from the astrophysics angle, Borexino's ability to conduct "precision physics" experiments and collect a large number of observations, with concomitant higher statistical power, is yielding data that show how our sun works. As for the possibility of discovering a new kind of neutrino coming from the sun, which is allowed by some theoretical extensions to the Standard Model of particle physics, he adds, "You always have the hope of seeing surprises, some small deviation from the expectations. And this you can only have if your accuracy and precision are good enough to see very small variations."

In a companion paper, the Borexino team says their 7Be solar neutrino flux measurements show no flow differences between day and night. Some had hypothesized that one might exist because neutrinos pass through Earth's bulk at night. But Pocar says, "The traverse through the Earth seems not to change neutrinos' flavor."

In the future, the researchers hope to identify the origin of every neutrino type coming from the sun, particularly to assess the relative levels of carbon, nitrogen and oxygen there, to deepen understanding of how the sun evolved and how its workings are related to that of larger stars.

Borexino is an international collaboration funded by the U.S. National Science Foundation, the Italian National Institute for Nuclear Physics (INFN) which manages the Gran Sasso labs and similar organizations in Germany, Russia and Poland.

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G. Bellini, J. Benziger, D. Bick, S. Bonetti, G. Bonfini, M. Buizza Avanzini, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, P. Cavalcante, A. Chavarria, D. D’Angelo, S. Davini, A. Derbin, A. Etenko, K. Fomenko, D. Franco, C. Galbiati, S. Gazzana, C. Ghiano, M. Giammarchi, M. Goeger-Neff, A. Goretti, L. Grandi, E. Guardincerri, S. Hardy, Aldo Ianni, Andrea Ianni, V. Kobychev, D. Korablev, G. Korga, Y. Koshio, D. Kryn, M. Laubenstein, T. Lewke, E. Litvinovich, B. Loer, F. Lombardi, P. Lombardi, L. Ludhova, I. Machulin, S. Manecki, W. Maneschg, G. Manuzio, Q. Meindl, E. Meroni, L. Miramonti, M. Misiaszek, D. Montanari, P. Mosteiro, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, M. Pallavicini, L. Papp, C. Peña-Garay, L. Perasso, S. Perasso, A. Pocar, R. Raghavan, G. Ranucci, A. Razeto, A. Re, A. Romani, A. Sabelnikov, R. Saldanha, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, R. Tartaglia, G. Testera, D. Vignaud, R. Vogelaar, F. von Feilitzsch, J. Winter, M. Wojcik, A. Wright, M. Wurm, J. Xu, O. Zaimidoroga, S. Zavatarelli, G. Zuzel. Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino. Physical Review Letters, 2011; 107 (14) DOI: 10.1103/PhysRevLett.107.141302

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Measuring elusive solar neutrinos flowing through the Earth, physicists learn more about the sun

ScienceDaily (Oct. 11, 2011) — Using one of the most sensitive neutrino detectors on the planet, an international team including physicists Laura Cadonati and Andrea Pocar at the University of Massachusetts Amherst are now measuring the flow of solar neutrinos reaching earth more precisely than ever before. The detector probes matter at the most fundamental level and provides a powerful tool for directly observing the sun's composition.

Pocar, Cadonati and colleagues report in the current issue of Physical Review Letters that the Borexino instrument has now measured with high precision the flux of the beryllium seven (7Be) solar neutrino, abundant, low-energy particles once below the observable threshold. With this advance, they can now precisely study the behavior of solar neutrinos with kinetic energy below 1 megaelectron volt (MeV). Borexino scientists also recently reported the first observation of neutrinos produced in a little-studied solar nuclear process known as proton-electron-proton, or pep, and set of stringent limit on reactions involving carbon, nitrogen and oxygen (the CNO cycle) in the sun.

Cadonati says, "Borexino is the only detector capable of observing the entire spectrum of solar neutrinos at once. Our results, the culmination of 20 years of research, greatly narrow the observation precision. The data confirm the neutrino oscillations, flavor changes and flow predicted by models of the sun and particle physics."

Of particular interest, Pocar and Cadonati note, is the Borexino instrument's ability to more thoroughly test neutrino oscillation parameters, allowing an exploration of their characteristic non-zero mass, which does not fit the Standard Model of particle physics. "Our data can tell us about fundamental micro physics at the particle level," says Cadonati. "Borexino is using neutrinos to explore the interior of the sun, looking for new, exciting clues to the mysteries of the universe we cannot see." Pocar adds, "Our detector provides stringent tests of the three-neutrino oscillations model."

Solar neutrinos are produced in nuclear processes and radioactive decays of several elements during fusion reactions at the sun's core. As many as 65 billion of them stream out of the sun and hit every square centimeter of Earth's surface [or 420 billion every square inch] every second. But because they only interact through the nuclear weak force they pass through matter unaffected, making them very difficult to detect and to distinguish from the trace nuclear decays of ordinary materials. The weak force is one of the four fundamental forces of nature, with gravity, electromagnetism and the strong force. It is responsible for the radioactive decay of unstable subatomic particles, with a short range of influence, about 1 percent of the diameter of a typical atomic nucleus.

The Borexino instrument, housed far beneath Italy's Apennine Mountains, detects neutrinos as they interact with an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 2,000 tons of water. Its great depth and many onion-like protective layers maintain the core as the most radiation-free medium on the planet.

There are three neutrino types, or "flavors": electron, muon and tau. Those produced in the sun are the electron type. As they travel away from their birthplace, they oscillate, or change from one flavor to another. A detector like Borexino can observe all three types in real time and measures each one's energy, but it cannot distinguish between them. It's more sensitive to the electron type so they are more likely to be seen.

The 7Be neutrino flux now being detected by Borexino is predicted by the standard solar model to make up about 10 percent of solar flow, Cadonati says. Earlier instruments in Canada and Japan designed to detect higher-energy neutrinos had already observed evidence of their flavor oscillations, probing 1/10,000 of the solar neutrino flux and their oscillations as they travel through solar matter. However, without data in the low-energy range as scanned by Borexino, physicists were not able to confirm the specific energy-dependent effect of solar neutrino oscillations. Borexino has now filled this gap and for the first time observed evidence of neutrino oscillation in vacuum, as they travel between the sun and Earth.

Pocar says that from the astrophysics angle, Borexino's ability to conduct "precision physics" experiments and collect a large number of observations, with concomitant higher statistical power, is yielding data that show how our sun works. As for the possibility of discovering a new kind of neutrino coming from the sun, which is allowed by some theoretical extensions to the Standard Model of particle physics, he adds, "You always have the hope of seeing surprises, some small deviation from the expectations. And this you can only have if your accuracy and precision are good enough to see very small variations."

In a companion paper, the Borexino team says their 7Be solar neutrino flux measurements show no flow differences between day and night. Some had hypothesized that one might exist because neutrinos pass through Earth's bulk at night. But Pocar says, "The traverse through the Earth seems not to change neutrinos' flavor."

In the future, the researchers hope to identify the origin of every neutrino type coming from the sun, particularly to assess the relative levels of carbon, nitrogen and oxygen there, to deepen understanding of how the sun evolved and how its workings are related to that of larger stars.

Borexino is an international collaboration funded by the U.S. National Science Foundation, the Italian National Institute for Nuclear Physics (INFN) which manages the Gran Sasso labs and similar organizations in Germany, Russia and Poland.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Massachusetts at Amherst.

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G. Bellini, J. Benziger, D. Bick, S. Bonetti, G. Bonfini, M. Buizza Avanzini, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, P. Cavalcante, A. Chavarria, D. D’Angelo, S. Davini, A. Derbin, A. Etenko, K. Fomenko, D. Franco, C. Galbiati, S. Gazzana, C. Ghiano, M. Giammarchi, M. Goeger-Neff, A. Goretti, L. Grandi, E. Guardincerri, S. Hardy, Aldo Ianni, Andrea Ianni, V. Kobychev, D. Korablev, G. Korga, Y. Koshio, D. Kryn, M. Laubenstein, T. Lewke, E. Litvinovich, B. Loer, F. Lombardi, P. Lombardi, L. Ludhova, I. Machulin, S. Manecki, W. Maneschg, G. Manuzio, Q. Meindl, E. Meroni, L. Miramonti, M. Misiaszek, D. Montanari, P. Mosteiro, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, M. Pallavicini, L. Papp, C. Peña-Garay, L. Perasso, S. Perasso, A. Pocar, R. Raghavan, G. Ranucci, A. Razeto, A. Re, A. Romani, A. Sabelnikov, R. Saldanha, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, R. Tartaglia, G. Testera, D. Vignaud, R. Vogelaar, F. von Feilitzsch, J. Winter, M. Wojcik, A. Wright, M. Wurm, J. Xu, O. Zaimidoroga, S. Zavatarelli, G. Zuzel. Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino. Physical Review Letters, 2011; 107 (14) DOI: 10.1103/PhysRevLett.107.141302

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New Saudi Arabias of solar energy: Himalaya Mountains, Andes, Antarctica

ScienceDaily (Oct. 13, 2011) — Mention prime geography for generation of solar energy, and people tend to think of hot deserts. But a new study concludes that some of the world's coldest landscapes -- including the Himalaya Mountains, the Andes, and even Antarctica -- could become Saudi Arabias of solar. The research appears in the ACS journal Environmental Science & Technology.

Kotaro Kawajiri and colleagues explain that the potential for generating electricity with renewable solar energy depends heavily on geographic location. Arid and semi-arid areas with plenty of sunshine long have been recognized as good solar sites. However, the scientists point out that, as a result of the limited data available for critical weather-related conditions on a global scale, gaps still exist in knowledge about the best geographical locations for producing solar energy. To expand that knowledge, they used one established technique to estimate global solar energy potential using the data that are available. The technique takes into account the effects of temperature on the output of solar cells. Future work will consider other variables, such as transmission losses and snow fall.

As expected, they found that many hot regions such as the U.S. desert southwest are ideal locations for solar arrays. However, they also found that many cold regions at high elevations receive a lot of sunlight -- so much so that their potential for producing power from the sun is even higher than in some desert areas. Kawajiri and colleagues found, for instance, that the Himalayas, which include Mt. Everest, could be an ideal locale for solar fields that generate electricity for the fast-expanding economy of the People's Republic of China.

The authors acknowledge funding from the National Institute of Advanced Industrial Science and Technology and the Japan Society for the Promotion of Science.

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Kotaro Kawajiri, Takashi Oozeki, Yutaka Genchi. Effect of Temperature on PV Potential in the World. Environmental Science & Technology, 2011; 110818104437050 DOI: 10.1021/es200635x

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Nature offers key lessons on harvesting solar power, say chemists

ScienceDaily (Sep. 24, 2011) — Clean solutions to human energy demands are essential to our future. While sunlight is the most abundant source of energy at our disposal, we have yet to learn how to capture, transfer and store solar energy efficiently. According to University of Toronto chemistry professor Greg Scholes, the answers can be found in the complex systems at work in nature.

"Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules," said Scholes, the D.J. LeRoy Distinguished Professor at U of T. "The energy is stored fleetingly as vibrating electrons and then transferred to a suitable reactor. It is the same in biological systems. In photosynthesis, for example, antenna complexes composed of chlorophyll capture sunlight and direct the energy to special proteins called reaction centres that help make oxygen and sugars. It is like plugging those proteins into a solar power socket."

In an article in Nature Chemistry to be published Sept. 23, Scholes and colleagues from several other universities examine the latest research in various natural antenna complexes. Using lessons learned from these natural phenomena, they provide a framework for how to design light harvesting systems that will route the flow of energy in sophisticated ways and over long distances, providing a microscopic "energy grid" to regulate solar energy conversion.

A key challenge is that the energy from sunlight is captured by coloured molecules called dyes or pigments, but is stored for only a billionth of a second. This leaves little time to route the energy from pigments to molecular machinery that produces fuel or electricity. How can we harvest sunlight and utilize its energy before it is lost?

"This is why natural photosynthesis is so inspiring," said Scholes. "More than 10 million billion photons of light strike a leaf each second. Of these, almost every red-coloured photon is captured by chlorophyll pigments which feed plant growth." Learning the workings of these natural light-harvesting systems fostered a vision, proposed by Scholes and his co-authors, to design and demonstrate molecular "circuitry" that is 10 times smaller than the thinnest electrical wire in computer processors. These energy circuits could control, regulate, direct and amplify raw solar energy which has been captured by human-made pigments, thus preventing the loss of precious energy before it is utilized.

Last year, Scholes led a team that showed that marine algae, a normally functioning biological system, uses quantum mechanics in order to optimize photosynthesis, a process essential to its survival. These and other insights from the natural world promise to revolutionize our ability to harness the power of the sun.

"Lessons from nature about solar light harvesting" was written by Scholes, Graham Fleming of the University of California, Berkeley, Alexandra Olaya-Castro of University College, London UK and Rienk van Grondelle of VU University in Amsterdam, The Netherlands.

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Gregory D. Scholes, Graham R. Fleming, Alexandra Olaya-Castro, Rienk van Grondelle. Lessons from nature about solar light harvesting. Nature Chemistry, 2011; 3 (10): 763 DOI: 10.1038/nchem.1145

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Cheap and efficient solar cell made possible by linked nanoparticles

ScienceDaily (Sep. 26, 2011) — Researchers of the Chemical Engineering department and the Kavli institute of the TU DElft have demonstrated that electrons can move freely in layers of linked semiconductor nanoparticles under the influence of light. This new knowledge will be very useful for the development of cheap and efficient quantum dot solar cells.

The researchers published their findings on September 25 on the website of the scientific journal Nature Nanotechnology.

Cheap and efficient

The current crystalline silicon solar panels are expensive to produce. Cheaper solar cells are available, but these are inefficient. For example, an organic solar cell has a maximum efficiency of 8%. One way of increasing the efficiency of cheap solar cells is the use of semiconductor nanoparticles, quantum dots. In theory, the efficiency of these cells can be increased to 44%. This is in part due to the avalanche effect, demonstrated by researchers from TU Delft and the FOM Foundation in 2008. In the current solar cells, an absorbed light particle can only excite one electron (creating an electron-hole pair), while in a quantum dot solar cell a light particle can excite several electrons. The more electrons that are excited, the greater the efficiency of the solar cell.

Linked nanoparticles

Up to now, the creation of electron-hole pairs under the influence of light was only demonstrated within the limits of a quantum dot. To be usable in solar cells, it is essential that electrons and holes are able to move. This is what creates an electrical current that can be collected at an electrode. Researchers from the same research group have now demonstrated that the electron-hole pairs can also move as free charges between the nanoparticles. To this end they linked nanoparticles together, using very small molecules, so that they were very densely clustered while still remaining separate from each other. The nanoparticles are so close together that every single light particle that is absorbed by the solar cell actually causes electrons to move.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Delft University of Technology.

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Elise Talgorn, Yunan Gao, Michiel Aerts, Lucas T. Kunneman, Juleon M. Schins, T. J. Savenije, Marijn A. van Huis, Herre S. J. van der Zant, Arjan J. Houtepen, Laurens D. A. Siebbeles. Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.159

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View the original article here

Cheap and efficient solar cell made possible by linked nanoparticles

ScienceDaily (Sep. 26, 2011) — Researchers of the Chemical Engineering department and the Kavli institute of the TU DElft have demonstrated that electrons can move freely in layers of linked semiconductor nanoparticles under the influence of light. This new knowledge will be very useful for the development of cheap and efficient quantum dot solar cells.

The researchers published their findings on September 25 on the website of the scientific journal Nature Nanotechnology.

Cheap and efficient

The current crystalline silicon solar panels are expensive to produce. Cheaper solar cells are available, but these are inefficient. For example, an organic solar cell has a maximum efficiency of 8%. One way of increasing the efficiency of cheap solar cells is the use of semiconductor nanoparticles, quantum dots. In theory, the efficiency of these cells can be increased to 44%. This is in part due to the avalanche effect, demonstrated by researchers from TU Delft and the FOM Foundation in 2008. In the current solar cells, an absorbed light particle can only excite one electron (creating an electron-hole pair), while in a quantum dot solar cell a light particle can excite several electrons. The more electrons that are excited, the greater the efficiency of the solar cell.

Linked nanoparticles

Up to now, the creation of electron-hole pairs under the influence of light was only demonstrated within the limits of a quantum dot. To be usable in solar cells, it is essential that electrons and holes are able to move. This is what creates an electrical current that can be collected at an electrode. Researchers from the same research group have now demonstrated that the electron-hole pairs can also move as free charges between the nanoparticles. To this end they linked nanoparticles together, using very small molecules, so that they were very densely clustered while still remaining separate from each other. The nanoparticles are so close together that every single light particle that is absorbed by the solar cell actually causes electrons to move.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Delft University of Technology.

Journal Reference:

Elise Talgorn, Yunan Gao, Michiel Aerts, Lucas T. Kunneman, Juleon M. Schins, T. J. Savenije, Marijn A. van Huis, Herre S. J. van der Zant, Arjan J. Houtepen, Laurens D. A. Siebbeles. Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.159

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Copper film could lower touch screen, LED and solar cell costs

ScienceDaily (Sep. 28, 2011) — Copper nanowires may be coming to a little screen near you. These new nanostructures have the potential to drive down the costs of displaying information on cell phones, e-readers and iPads, and they could also help engineers build foldable electronics and improved solar cells, according to new research.

Duke chemist Ben Wiley and his graduate student have developed a technique to organize copper atoms in water to form long, thin, non-clumped nanowires. The nanowires are then transformed into transparent, conductive films and coated onto glass or plastic.

The new research shows that the copper nanowire films have the same properties as those currently used in electronic devices and solar cells, but are less expensive to manufacture. The results were published online Sept. 23 in Advanced Materials.

The films that currently connect pixels in electronic screens are made of indium tin oxide, or ITO. It is highly transparent, which transmits the information well. But the ITO film must be deposited from a vapor in a process that is a thousand times slower than newspaper printing, and, once the ITO is in the device, it cracks easily. Indium is also an expensive rare earth element, costing as much as $800 per kilogram.

These problems have driven worldwide efforts to find less expensive materials that can be coated or printed like ink at much faster speeds to make low-cost, transparent conducting films, Wiley said.

One alternative to an ITO film is to use inks containing silver nanowires. The first cell phone with a screen made from silver nanowires will be on the market this year. But silver, like indium, is still relatively expensive at $1400 per kilogram.

Copper, on the other hand, is a thousand times more abundant than indium or silver, and about 100 times less expensive, costing only $9 per kilogram.

In 2010, Wiley and his graduate student Aaron Rathmell showed that it was possible to form a layer of copper nanowires on glass to make a transparent conducting film.

But at that time, the performance of the film was not good enough for practical applications because the wires clumped together. The new way of growing the copper nanowires and coating them on glass surfaces eliminates the clumping problem, Wiley said.

He and Rathmell also created the new copper nanowires to maintain their conductivity and form when bent back and forth 1,000 times. In contrast, ITO films' conduction and structure break after a few bends.

Wiley said the low-cost, high-performance, and flexibility of copper nanowires make them a natural choice for use in the next generation of displays and solar cells. He co-founded a company called NanoForge Corp in 2010 to manufacture copper nanowires for commercial applications.

In early 2011, NanoForge received a $45,000 North Carolina IDEA grant for refinement and scale-up of the manufacturing process of copper nanowires, and it is now filling orders.

With continuing development, copper nanowires could be in screens and solar cells in the next few years, which could lead to lighter and more reliable displays and also to making solar energy more competitive with fossil fuels, Wiley said.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Duke University.

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Rathmell, A. R. and Wiley, B. J. he Synthesis and Coating of Long, Thin Copper Nanowires to Make Flexible, Transparent Conducting Films on Plastic Substrates. Advanced Materials, 2011 DOI: 10.1002/adma.201102284

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Researchers use carbon nanotubes to make solar cells affordable, flexible

ScienceDaily (Sep. 27, 2011) — Researchers from Northwestern University have developed a carbon-based material that could revolutionize the way solar power is harvested. The new solar cell material -- a transparent conductor made of carbon nanotubes -- provides an alternative to current technology, which is mechanically brittle and reliant on a relatively rare mineral.

Due to Earth's abundance of carbon, carbon nanotubes have the potential to boost the long-term viability of solar power by providing a cost-efficient option as demand for the technology increases. In addition, the material's mechanical flexibility could allow solar cells to be integrated into fabrics and clothing, enabling portable energy supplies that could impact everything from personal electronics to military operations.

The research, headed by Mark C. Hersam, professor of materials science and engineering and professor of chemistry, and Tobin J. Marks, Vladimir N. Ipatieff Professor of Catalytic Chemistry and professor of materials science and engineering, is featured on the cover of the October 2011 issue of Advanced Energy Materials, a new journal that specializes in science about materials used in energy applications.

Solar cells are composed of several layers, including a transparent conductor layer that allows light to pass into the cell and electricity to pass out; for both these actions to occur, the conductor must be both electrically conductive and also optically transparent. Few materials concurrently possess both of these properties.

Currently, indium tin oxide is the dominant material used in transparent conductor applications, but the material has two potential limitations. Indium tin oxide is mechanically brittle, which precludes its use in applications that require mechanical flexibility. In addition, Indium tin oxide relies on the relatively rare element indium, so the projected increased demand for solar cells could push the price of indium to problematically high levels.

"If solar technology really becomes widespread, as everyone hopes it will, we will likely have a crisis in the supply of indium," Hersam said. "There's a great desire to identify materials -- especially earth-abundant elements like carbon -- that can take indium's place in solar technology."

Hersam and Marks' team has created an alternative to indium tin oxide using single-walled carbon nanotubes, tiny, hollow cylinders of carbon just one nanometer in diameter.

The researchers have gone further to determine the type of nanotube that is most effective in transparent conductors. Nanotubes' properties vary depending on their diameter and their chiral angle, the angle that describes the arrangement of carbon atoms along the length of the nanotube. These properties determine two types of nanotubes: metallic and semiconducting.

Metallic nanotubes, the researchers found, are 50 times more effective than semiconducting ones when used as transparent conductors in organic solar cells.

"We have now identified precisely the type of carbon nanotube that should be used in this application," Hersam said.

Because carbon nanotubes are flexible, as opposed to the brittle indium tin oxide, the researchers' findings could pave the way for many new applications in solar cells. For example, the military could incorporate the flexible solar cells into tent material to provide solar power directly to soldiers in the field, or the cells could be integrated into clothing, backpacks, or purses for wearable electronics.

"With this mechanically flexible technology, it's much easier to imagine integrating solar technology into everyday life, rather than carrying around a large, inflexible solar cell," Hersam said.

Researchers are now examining other layers of the solar cell to explore also replacing these with carbon-based nanomaterials.

Besides Hersam and Marks, other authors include Timothy P. Tyler, Ryan E. Brock, and Hunter J. Karmel. This work was supported by the Argonne Northwestern-Northwestern Solar Energy Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

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Timothy P. Tyler, Ryan E. Brock, Hunter J. Karmel, Tobin J. Marks, Mark C. Hersam. Organic Solar Cell Characterization: Electronically Monodisperse Single-Walled Carbon Nanotube Thin Films as Transparent Conducting Anodes in Organic Photovoltaic Devices (Adv. Energy Mater. 5/2011). Advanced Energy Materials, 2011; 1 (5): 701 DOI: 10.1002/aenm.201190021

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Copper film could lower touch screen, LED and solar cell costs

ScienceDaily (Sep. 28, 2011) — Copper nanowires may be coming to a little screen near you. These new nanostructures have the potential to drive down the costs of displaying information on cell phones, e-readers and iPads, and they could also help engineers build foldable electronics and improved solar cells, according to new research.

Duke chemist Ben Wiley and his graduate student have developed a technique to organize copper atoms in water to form long, thin, non-clumped nanowires. The nanowires are then transformed into transparent, conductive films and coated onto glass or plastic.

The new research shows that the copper nanowire films have the same properties as those currently used in electronic devices and solar cells, but are less expensive to manufacture. The results were published online Sept. 23 in Advanced Materials.

The films that currently connect pixels in electronic screens are made of indium tin oxide, or ITO. It is highly transparent, which transmits the information well. But the ITO film must be deposited from a vapor in a process that is a thousand times slower than newspaper printing, and, once the ITO is in the device, it cracks easily. Indium is also an expensive rare earth element, costing as much as $800 per kilogram.

These problems have driven worldwide efforts to find less expensive materials that can be coated or printed like ink at much faster speeds to make low-cost, transparent conducting films, Wiley said.

One alternative to an ITO film is to use inks containing silver nanowires. The first cell phone with a screen made from silver nanowires will be on the market this year. But silver, like indium, is still relatively expensive at $1400 per kilogram.

Copper, on the other hand, is a thousand times more abundant than indium or silver, and about 100 times less expensive, costing only $9 per kilogram.

In 2010, Wiley and his graduate student Aaron Rathmell showed that it was possible to form a layer of copper nanowires on glass to make a transparent conducting film.

But at that time, the performance of the film was not good enough for practical applications because the wires clumped together. The new way of growing the copper nanowires and coating them on glass surfaces eliminates the clumping problem, Wiley said.

He and Rathmell also created the new copper nanowires to maintain their conductivity and form when bent back and forth 1,000 times. In contrast, ITO films' conduction and structure break after a few bends.

Wiley said the low-cost, high-performance, and flexibility of copper nanowires make them a natural choice for use in the next generation of displays and solar cells. He co-founded a company called NanoForge Corp in 2010 to manufacture copper nanowires for commercial applications.

In early 2011, NanoForge received a $45,000 North Carolina IDEA grant for refinement and scale-up of the manufacturing process of copper nanowires, and it is now filling orders.

With continuing development, copper nanowires could be in screens and solar cells in the next few years, which could lead to lighter and more reliable displays and also to making solar energy more competitive with fossil fuels, Wiley said.

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Journal Reference:

Rathmell, A. R. and Wiley, B. J. he Synthesis and Coating of Long, Thin Copper Nanowires to Make Flexible, Transparent Conducting Films on Plastic Substrates. Advanced Materials, 2011 DOI: 10.1002/adma.201102284

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Nature offers key lessons on harvesting solar power, say chemists

ScienceDaily (Sep. 24, 2011) — Clean solutions to human energy demands are essential to our future. While sunlight is the most abundant source of energy at our disposal, we have yet to learn how to capture, transfer and store solar energy efficiently. According to University of Toronto chemistry professor Greg Scholes, the answers can be found in the complex systems at work in nature.

"Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules," said Scholes, the D.J. LeRoy Distinguished Professor at U of T. "The energy is stored fleetingly as vibrating electrons and then transferred to a suitable reactor. It is the same in biological systems. In photosynthesis, for example, antenna complexes composed of chlorophyll capture sunlight and direct the energy to special proteins called reaction centres that help make oxygen and sugars. It is like plugging those proteins into a solar power socket."

In an article in Nature Chemistry to be published Sept. 23, Scholes and colleagues from several other universities examine the latest research in various natural antenna complexes. Using lessons learned from these natural phenomena, they provide a framework for how to design light harvesting systems that will route the flow of energy in sophisticated ways and over long distances, providing a microscopic "energy grid" to regulate solar energy conversion.

A key challenge is that the energy from sunlight is captured by coloured molecules called dyes or pigments, but is stored for only a billionth of a second. This leaves little time to route the energy from pigments to molecular machinery that produces fuel or electricity. How can we harvest sunlight and utilize its energy before it is lost?

"This is why natural photosynthesis is so inspiring," said Scholes. "More than 10 million billion photons of light strike a leaf each second. Of these, almost every red-coloured photon is captured by chlorophyll pigments which feed plant growth." Learning the workings of these natural light-harvesting systems fostered a vision, proposed by Scholes and his co-authors, to design and demonstrate molecular "circuitry" that is 10 times smaller than the thinnest electrical wire in computer processors. These energy circuits could control, regulate, direct and amplify raw solar energy which has been captured by human-made pigments, thus preventing the loss of precious energy before it is utilized.

Last year, Scholes led a team that showed that marine algae, a normally functioning biological system, uses quantum mechanics in order to optimize photosynthesis, a process essential to its survival. These and other insights from the natural world promise to revolutionize our ability to harness the power of the sun.

"Lessons from nature about solar light harvesting" was written by Scholes, Graham Fleming of the University of California, Berkeley, Alexandra Olaya-Castro of University College, London UK and Rienk van Grondelle of VU University in Amsterdam, The Netherlands.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Toronto.

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Gregory D. Scholes, Graham R. Fleming, Alexandra Olaya-Castro, Rienk van Grondelle. Lessons from nature about solar light harvesting. Nature Chemistry, 2011; 3 (10): 763 DOI: 10.1038/nchem.1145

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Researchers use carbon nanotubes to make solar cells affordable, flexible

ScienceDaily (Sep. 27, 2011) — Researchers from Northwestern University have developed a carbon-based material that could revolutionize the way solar power is harvested. The new solar cell material -- a transparent conductor made of carbon nanotubes -- provides an alternative to current technology, which is mechanically brittle and reliant on a relatively rare mineral.

Due to Earth's abundance of carbon, carbon nanotubes have the potential to boost the long-term viability of solar power by providing a cost-efficient option as demand for the technology increases. In addition, the material's mechanical flexibility could allow solar cells to be integrated into fabrics and clothing, enabling portable energy supplies that could impact everything from personal electronics to military operations.

The research, headed by Mark C. Hersam, professor of materials science and engineering and professor of chemistry, and Tobin J. Marks, Vladimir N. Ipatieff Professor of Catalytic Chemistry and professor of materials science and engineering, is featured on the cover of the October 2011 issue of Advanced Energy Materials, a new journal that specializes in science about materials used in energy applications.

Solar cells are composed of several layers, including a transparent conductor layer that allows light to pass into the cell and electricity to pass out; for both these actions to occur, the conductor must be both electrically conductive and also optically transparent. Few materials concurrently possess both of these properties.

Currently, indium tin oxide is the dominant material used in transparent conductor applications, but the material has two potential limitations. Indium tin oxide is mechanically brittle, which precludes its use in applications that require mechanical flexibility. In addition, Indium tin oxide relies on the relatively rare element indium, so the projected increased demand for solar cells could push the price of indium to problematically high levels.

"If solar technology really becomes widespread, as everyone hopes it will, we will likely have a crisis in the supply of indium," Hersam said. "There's a great desire to identify materials -- especially earth-abundant elements like carbon -- that can take indium's place in solar technology."

Hersam and Marks' team has created an alternative to indium tin oxide using single-walled carbon nanotubes, tiny, hollow cylinders of carbon just one nanometer in diameter.

The researchers have gone further to determine the type of nanotube that is most effective in transparent conductors. Nanotubes' properties vary depending on their diameter and their chiral angle, the angle that describes the arrangement of carbon atoms along the length of the nanotube. These properties determine two types of nanotubes: metallic and semiconducting.

Metallic nanotubes, the researchers found, are 50 times more effective than semiconducting ones when used as transparent conductors in organic solar cells.

"We have now identified precisely the type of carbon nanotube that should be used in this application," Hersam said.

Because carbon nanotubes are flexible, as opposed to the brittle indium tin oxide, the researchers' findings could pave the way for many new applications in solar cells. For example, the military could incorporate the flexible solar cells into tent material to provide solar power directly to soldiers in the field, or the cells could be integrated into clothing, backpacks, or purses for wearable electronics.

"With this mechanically flexible technology, it's much easier to imagine integrating solar technology into everyday life, rather than carrying around a large, inflexible solar cell," Hersam said.

Researchers are now examining other layers of the solar cell to explore also replacing these with carbon-based nanomaterials.

Besides Hersam and Marks, other authors include Timothy P. Tyler, Ryan E. Brock, and Hunter J. Karmel. This work was supported by the Argonne Northwestern-Northwestern Solar Energy Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Northwestern University, via EurekAlert!, a service of AAAS.

Journal Reference:

Timothy P. Tyler, Ryan E. Brock, Hunter J. Karmel, Tobin J. Marks, Mark C. Hersam. Organic Solar Cell Characterization: Electronically Monodisperse Single-Walled Carbon Nanotube Thin Films as Transparent Conducting Anodes in Organic Photovoltaic Devices (Adv. Energy Mater. 5/2011). Advanced Energy Materials, 2011; 1 (5): 701 DOI: 10.1002/aenm.201190021

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New solar cell: Engineers crack full-spectrum solar challenge

ScienceDaily (June 27, 2011) — In a paper published in Nature Photonics, U of T Engineering researchers report a new solar cell that may pave the way to inexpensive coatings that efficiently convert the sun's rays to electricity.

The U of T researchers, led by Professor Ted Sargent, report the first efficient tandem solar cell based on colloidal quantum dots (CQD). "The U of T device is a stack of two light-absorbing layers -- one tuned to capture the sun's visible rays, the other engineered to harvest the half of the sun's power that lies in the infrared," said lead author Dr. Xihua Wang.

"We needed a breakthrough in architecting the interface between the visible and infrared junction," said Sargent, a Professor of Electrical and Computer Engineering at the University of Toronto, who is also the Canada Research Chair in Nanotechnology. "The team engineered a cascade -- really a waterfall -- of nanometers-thick materials to shuttle electrons between the visible and infrared layers."

According to doctoral student Ghada Koleilat, "We needed a new strategy -- which we call the Graded Recombination Layer -- so that our visible and infrared light-harvesters could be linked together efficiently, without any compromise to either layer."

The team pioneered solar cells made using CQD, nanoscale materials that can readily be tuned to respond to specific wavelengths of the visible and invisible spectrum. By capturing such a broad range of light waves -- wider than normal solar cells -- tandem CQD solar cells can in principle reach up to 42 per cent efficiencies. The best single-junction solar cells are constrained to a maximum of 31 per cent efficiency. In reality, solar cells that are on the roofs of houses and in consumer products have 14 to 18 per cent efficiency. The work expands the Toronto team's world-leading 5.6 per cent efficient colloidal quantum dot solar cells.

"Building efficient, cost-effective solar cells is a grand global challenge. The University of Toronto is extremely proud of its world-class leadership in the field," said Professor Farid Najm, Chair of The Edward S. Rogers Sr. Department of Electrical & Computer Engineering.

Sargent is hopeful that in five years solar cells using the graded recombination layer published in the Nature Photonics paper will be integrated into building materials, mobile devices, and automobile parts.

"The solar community -- and the world -- needs a solar cell that is over 10% efficient, and that dramatically improves on today's photovoltaic module price points," said Sargent. "This advance lights up a practical path to engineering high-efficiency solar cells that make the best use of the diverse photons making up the sun's broad palette."

The publication was based in part on work supported by an award made by the King Abdullah University of Science and Technology (KAUST), by the Ontario Research Fund Research Excellence Program, and by the Natural Sciences and Engineering Research Council (NSERC) of Canada. Equipment from Angstrom Engineering and Innovative Technology enabled the research.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Toronto Faculty of Applied Science & Engineering, via EurekAlert!, a service of AAAS.

Journal Reference:

Xihua Wang, Ghada I. Koleilat, Jiang Tang, Huan Liu, Illan J. Kramer, Ratan Debnath, Lukasz Brzozowski, D. Aaron R. Barkhouse, Larissa Levina, Sjoerd Hoogland, Edward H. Sargent. Tandem colloidal quantum dot solar cells employing a graded recombination layer. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.123

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‘Cling-film’ solar cells could lead to advance in renewable energy

ScienceDaily (July 4, 2011) — A scientific advance in renewable energy which promises a revolution in the ease and cost of using solar cells, has been announced on July 4, 2011. A new study shows that even when using very simple and inexpensive manufacturing methods -- where flexible layers of material are deposited over large areas like cling-film -- efficient solar cell structures can be made.

The study, published in the journal Advanced Energy Materials, paves the way for new solar cell manufacturing techniques and the promise of developments in renewable solar energy. Scientists from the Universities of Sheffield and Cambridge used the ISIS Neutron Source and Diamond Light Source at STFC Rutherford Appleton Laboratory in Oxfordshire to carry out the research.

Plastic (polymer) solar cells are much cheaper to produce than conventional silicon solar cells and have the potential to be produced in large quantities. The study showed that when complex mixtures of molecules in solution are spread onto a surface, like varnishing a table-top, the different molecules separate to the top and bottom of the layer in a way that maximises the efficiency of the resulting solar cell.

Dr Andrew Parnell of the University of Sheffield said, "Our results give important insights into how ultra-cheap solar energy panels for domestic and industrial use can be manufactured on a large scale. Rather than using complex and expensive fabrication methods to create a specific semiconductor nanostructure, high volume printing could be used to produce nano-scale (60 nano-meters) films of solar cells that are over a thousand times thinner than the width of a human hair. These films could then be used to make cost-effective, light and easily transportable plastic solar cell devices such as solar panels."

Dr. Robert Dalgliesh, one of the ISIS scientists involved in the work, said, "This work clearly illustrates the importance of the combined use of neutron and X-ray scattering sources such as ISIS and Diamond in solving modern challenges for society. Using neutron beams at ISIS and Diamond's bright X-rays, we were able to probe the internal structure and properties of the solar cell materials non-destructively. By studying the layers in the materials which convert sunlight into electricity, we are learning how different processing steps change the overall efficiency and affect the overall polymer solar cell performance. "

"Over the next fifty years society is going to need to supply the growing energy demands of the world's population without using fossil fuels, and the only renewable energy source that can do this is the Sun," said Professor Richard Jones of the University of Sheffield. " In a couple of hours enough energy from sunlight falls on the Earth to satisfy the energy needs of the Earth for a whole year, but we need to be able to harness this on a much bigger scale than we can do now. Cheap and efficient polymer solar cells that can cover huge areas could help move us into a new age of renewable energy."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Science and Technology Facilities Council (STFC).

Journal Reference:

Paul A. Staniec, Andrew J. Parnell, Alan D. F. Dunbar, Hunan Yi, Andrew J. Pearson, Tao Wang, Paul E. Hopkinson, Christy Kinane, Robert M. Dalgliesh, Athene M. Donald, Anthony J. Ryan, Ahmed Iraqi, Richard A. L. Jones, David G. Lidzey. The Nanoscale Morphology of a PCDTBT:PCBM Photovoltaic Blend. Advanced Energy Materials, 2011; DOI: 10.1002/aenm.201100144

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New insights on how solar minimums affect Earth

ScienceDaily (June 14, 2011) — Since 1611, humans have recorded the comings and goings of black spots on the sun. The number of these sunspots waxes and wanes over approximately an 11-year cycle -- more sunspots generally mean more activity and eruptions on the sun and vice versa. The number of sunspots can change from cycle to cycle, and 2008 saw the longest and weakest solar minimum since scientists have been monitoring the sun with space-based instruments.

Observations have shown, however, that magnetic effects on Earth due to the sun, effects that cause the aurora to appear, did not go down in synch with the cycle of low magnetism on the sun. Now, a paper in Annales Geophysicae that appeared on May 16, 2011 reports that these effects on Earth did in fact reach a minimum -- indeed they attained their lowest levels of the century -- but some eight months later. The scientists believe that factors in the speed of the solar wind, and the strength and direction of the magnetic fields embedded within it, helped produce this anomalous low.

"Historically, the solar minimum is defined by sunspot number," says space weather scientist Bruce Tsurutani at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who is first author on the paper. "Based on that, 2008 was identified as the period of solar minimum. But the geomagnetic effects on Earth reached their minimum quite some time later, in 2009. So we decided to look at what caused the geomagnetic minimum."

Geomagnetic effects basically amount to any magnetic changes on Earth due to the sun, and they're measured by magnetometer readings on the surface of Earth. Such effects are usually harmless, with the only obvious sign of their presence being the appearance of auroras near the poles. However, in extreme cases, they can cause power grid failures on Earth or induce dangerous currents in long pipelines, so it is valuable to know how the geomagnetic effects vary with the sun.

Three things help determine how much energy from the sun is transferred to Earth's magnetosphere from the solar wind: the speed of the solar wind, the strength of the magnetic field outside Earth's bounds (known as the interplanetary magnetic field) and which direction it is pointing, since a large southward component is necessary to connect successfully to Earth's magnetosphere and transfer energy. The team -- which also included Walter Gonzalez and Ezequiel Echer of the Brazilian National Institute for Space Research in São José dos Campos, Brazil -- examined each component in turn.

First, the researchers noted that in 2008 and 2009, the interplanetary magnetic field was the lowest it had been in the history of the space age. This was an obvious contribution to the geomagnetic minimum. But since the geomagnetic effects didn't drop in 2008, it could not be the only factor.

To examine the speed of the solar wind, they turned to NASA's Advanced Composition Explorer (ACE), which is in interplanetary space outside Earth's magnetosphere, approximately 1 million miles toward the sun. The ACE data showed that the speed of the solar wind stayed high during the sunspot minimum. Only later did it begin a steady decline, correlating to the timing of the decline in geomagnetic effects.

The next step was to understand what caused this decrease. The team found a culprit in something called coronal holes. Coronal holes are darker, colder areas within the sun's outer atmosphere. Fast solar wind shoots out the center of coronal holes at speeds up to 500 miles per second, but wind flowing out of the sides slows down as it expands into space.

"Usually, at solar minimum, the coronal holes are at the sun's poles," says Giuliana de Toma, a solar scientist at the National Center for Atmospheric Research whose research on this topic helped provide insight for this paper. "Therefore, Earth receives wind from only the edges of these holes, and it's not very fast. But in 2007 and 2008, the coronal holes were not confined to the poles as normal."

Those coronal holes lingered at low latitudes to the end of 2008. Consequently, the center of the holes stayed firmly pointed towards Earth, sending fast solar wind in Earth's direction. Only as they finally appeared closer to the poles in 2009 did the speed of the solar wind at Earth begin to slow down. And, of course, the geomagnetic effects and sightings of the aurora along with it.

Coronal holes seem to be responsible for minimizing the southward direction of the interplanetary magnetic field as well. The solar wind's magnetic fields oscillate on the journey from the sun to Earth. These fluctuations are known as Alfvén waves. The wind coming out of the centers of the coronal holes has large fluctuations, meaning that the southward magnetic component -- like that in all the directions -- is fairly large. The wind that comes from the edges, however, has smaller fluctuations, and comparably smaller southward components. So, once again, coronal holes at lower latitudes would have a better chance of connecting with Earth's magnetosphere and causing geomagnetic effects, while mid-latitude holes would be less effective.

Working together, these three factors -- low interplanetary magnetic field strength, combined with slower solar wind speed and smaller magnetic fluctuations due to coronal hole placement -- create the perfect environment for a geomagnetic minimum.

Knowing what situations cause and suppress intense geomagnetic activity on Earth is a step toward better predicting when such events might happen. To do so well, Tsurutani points out, requires focusing on the tight connection between such effects and the complex physics of the sun. "It's important to understand all of these features better," he says. "To understand what causes low interplanetary magnetic fields and what causes coronal holes in general. This is all part of the solar cycle. And all part of what causes effects on Earth."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by NASA/Jet Propulsion Laboratory. The original article was written by Karen C. Fox, NASA's Goddard Space Flight Center.

Journal Reference:

B. T. Tsurutani, E. Echer, W. D. Gonzalez. The solar and interplanetary causes of the recent minimum in geomagnetic activity (MGA23): a combination of midlatitude small coronal holes, low IMF BZ variances, low solar wind speeds and low solar magnetic fields. Annales Geophysicae, 2011; 29 (5): 839 DOI: 10.5194/angeo-29-839-2011

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