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  • richardmitnick 6:01 pm on November 26, 2014 Permalink | Reply
    Tags: , , , , Plasmonics   

    From Caltech: “New Technique Could Harvest More of the Sun’s Energy” 

    Caltech Logo
    Caltech

    11/26/2014
    Jessica Stoller-Conrad

    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.

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    An ultra-sensitive needle measures the voltage that is generated while the nanospheres are illuminated.
    Credit: AMOLF/Tremani – Figure: Artist impression of the plasmo-electric effect.

    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.

    See the full article here.

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    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”
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  • richardmitnick 8:43 am on November 1, 2014 Permalink | Reply
    Tags: , , Plasmonics   

    From physicsworld.com: “Plasmons convert light into a voltage” 

    physicsworld
    physicsworld.com

    Oct 30, 2014
    Tim Wogan

    A new way of creating a voltage by shining light on a solid has been developed by researchers in the US and Europe. Unlike most photovoltaic devices, the new system does not rely on semiconductors but rather on surface plasmons in tiny metal nanostructures. The team is now working to create new types of devices that convert light into electrical energy.

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    Bright idea: Harry Atwater is working on a plasmonic solar cell

    Surface plasmons are collective excitations of electrons at the surface of a metal that interact very strongly with light. As a result, plasmons are of great technological interest as an interface between photonics and electronics. This interaction is strongest at the plasmon-resonance frequency, which is defined by the size and shape of an object and its charge density. In 2009 Paul Mulvaney and colleagues at the University of Melbourne in Australia applied an electrical potential to gold nanoparticles, and found that they could tune the plasmon-resonance frequency by injecting or removing electrons.

    Sweeping laser

    In the new work, applied physicist Harry Atwater and colleagues at California Institute of Technology, together with researchers in the Netherlands, show that the reverse can also occur: a surface potential can be induced by using light to modify the charge density of a nanoparticle. The team made its plasmonic material by attaching gold nanorods with a plasmon-resonance wavelength of 550 nm to an indium-tin-oxide substrate. Then the researchers fired a tuneable laser at the structure, and swept the laser wavelength from 480 nm to 650 nm. During illumination, the electric potential on the surface of the material was monitored using the conductive tip of an atomic force microscope.

    When the laser was on resonance with the surface plasmon, no voltage was induced. Irradiation either side of the resonant frequency, however, did produce a voltage. When the wavelength was below 550 nm a negative potential was measured on the gold nanorods, while longer-wavelength light created a positive potential. The team found that the magnitude of the potential related to the rate at which the light absorbance changed with respect to the frequency of the light. The largest potential (which was negative) was produced by illumination at 500 nm. Atwater offers a thermodynamic explanation for this observation: “If you shine light on the structure, free-energy minimization will cause the structure to try to adjust its charge density to bring itself into resonance with the exciting light.” The researchers have dubbed this phenomenon the plasmoelectric effect.

    Successful model

    The team then used this model to predict the frequency at which the maximum potentials should be generated in its set-up, and found broad agreement with its experimental results. The researchers also checked that the model could be applied generally, by testing it in a different type of plasmonic material: a thin gold sheet studded with a periodic pattern of 10 μm holes mounted on a glass substrate. This too showed a plasmoelectric effect, with the peak negative and positive potentials as predicted by the model.

    While the devices reported by the team simply produce a potential difference when illuminated, the team is now working on a device that will deliver usable electrical energy and thereby function as a solar cell. Atwater believes that such a device could complement traditional semiconductor photovoltaic cells: “Any given single-material solar cell can only convert power from photons that have energy greater than the band-gap energy,” he says, “[Our device] could potentially be used behind a conventional photovoltaic cell to harvest the infrared part of the spectrum, because I can design a plasmonic structure to have a resonance at pretty much any frequency.”

    Fascinating physics

    Nano-optics specialist Thomas Ebbesen of the University of Strasbourg, says: “I find it to be very impressive work. If something like this could become efficient as an energy conversion process that would of course be technologically important. But independent of that, I find the underlying physics very interesting just from a thermodynamic point of view.”

    Ortwin Hess of Imperial College London is also impressed, and wants to know more: “Thermodynamics seems to be supporting their experimental and simulation work, and I’m really happy about that,” he says. “Nevertheless, from the microscopic perspective, plasmons are made up of electrons, and in the end I would like to see how that works.” The researchers are working on this question, and Atwater says there will be “a forthcoming theory paper in the near future”.

    The research is published in Science.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

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  • richardmitnick 8:02 pm on July 10, 2014 Permalink | Reply
    Tags: , , , Plasmonics   

    From physicsworld.com: “Plasmons excite hot carriers” 

    physicsworld
    physicsworld.com

    Jul 10, 2014
    Belle Dumé

    The first complete theory of how plasmons produce “hot carriers” has been developed by researchers in the US. The new model could help make this process of producing carriers more efficient, which would be good news for enhancing solar-energy conversion in photovoltaic devices, making better photocatalysts and for applications like water splitting to produce hydrogen, to name but a few.

    chart
    Feeling hot hot hot: how hot carriers are distributed

    Plasmons are quantized collective oscillations of conduction electrons on the surface of metallic nanostructures that interact strongly with light. Such enhanced interaction allows them to concentrate light into subwavelength volumes, well below the diffraction limit of light. The phenomenon could be put to good use in a range of technologies, such as light detection and modulation, optical communications, photovoltaics and spectroscopy.

    Surface plasmons only live for a short while, after which they either decay radiatively by emitting a photon or non-radiatively by generating electron–hole (charge carrier) pairs, explains team leader Peter Nordlander from Rice University. In the non-radiative case, hot charge carriers are produced. These carriers are electrons and holes that have been excited by photons with high energies.
    Capturing hot-carrier energy

    In bulk materials, hot carriers quickly cool in a matter of picoseconds, releasing phonons (vibrations of the crystal lattice, or heat). Indeed, such wasted heat can account for up to 50% of the energy losses in present-day solar cells. If the energy of hot carriers could be captured before it converts into wasted heat, solar-to-electric power-conversion efficiencies might be greatly increased.

    Hot carriers can also induce chemical reactions – that would otherwise be too energetically demanding – in molecules near the surface of plasmonic nanostructures. Such reactions might help in water splitting, for example. Here, water is separated into oxygen and hydrogen using sunlight, which is a clean and renewable way to produce energy. They might also be used to transfer electrons into molecules or structures nearby – and so act as dopants.


    Simple model

    To fully exploit these carriers for such applications, researchers need to understand the physical processes behind plasmon-induced hot-carrier generation. Nordlander’s team has now developed a simple model that describes how plasmons produce hot carriers in spherical silver nanoparticles and nanoshells. The model describes the conduction electrons in the metal as free particles and then analyses how plasmons excite hot carriers using Fermi’s golden rule – a way to calculate how a quantum system transitions from one state into another following a perturbation.

    The model allows the researchers to calculate how many hot carriers are produced as a function of the light frequency used to excite the metal, as well as the rate at which they are produced. The spectral profile obtained is, to all intents and purposes, the “plasmonic spectrum” of the material.


    Particle size and hot-carrier lifetimes

    “Our analyses reveal that particle size and hot-carrier lifetimes are central for determining both the production rate and the energies of the hot carriers,” says Nordlander. “Larger particles and shorter lifetimes produce more carriers with lower energies and smaller particles produce fewer carriers, but with higher energies.”

    The team says that it has also succeeded in characterizing how efficient the hot-carrier generation process is, thanks to a figure of merit that measures how many high-energy carriers are produced per plasmon.

    “Our results could help provide strategies for making the hot-carrier generation process more efficient,” says team member Alejandro Manjavacas. “Indeed, we are now busy developing another theory for how hot carriers are produced in transition-metal particles and a third one that describes how the hot carriers evolve over time.” Identifying the timescales involved in carrier decay will be another essential element for optimizing the carrier-generation process, he adds.

    The results are published in ACS Nano.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics


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  • richardmitnick 3:12 pm on November 23, 2011 Permalink | Reply
    Tags: , , , , , Plasmonics   

    From Berkeley Lab: “On the Road to Plasmonics With Silver Polyhedral Nanocrystals” 


    Berkeley Lab

    Berkeley Lab Researchers Find Simpler Approach to Making Plasmonic Materials

    Lynn Yarris
    November 22, 2011

    “The question of how many polyhedral nanocrystals of silver can be packed into millimeter-sized supercrystals may not be burning on many lips but the answer holds importance for one of today’s hottest new high-tech fields – plasmonics! Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) may have opened the door to a simpler approach for the fabrication of plasmonic materials by inducing polyhedral-shaped silver nanocrystals to self-assemble into three-dimensional supercrystals of the highest possible density.”

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    On the left are micrographs of supercrystals of silver polyderal nanocrystals and on the right the corresponding diagrams of their densest known packings for (from top-down) cubes, truncated cubes and cuboctahedra. (Image courtesy of Berkeley Lab)

    Plasmonics is the phenomenon by which a beam of light is confined in ultra-cramped spaces allowing it to be manipulated into doing things a beam of light in open space cannot. This phenomenon holds great promise for superfast computers, microscopes that can see nanoscale objects with visible light, and even the creation of invisibility carpets. A major challenge for developing plasmonic technology, however, is the difficulty of fabricating metamaterials with nano-sized interfaces between noble metals and dielectrics.

    Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division, led a study in which silver nanocrystals of a variety of polyhedral shapes self-assembled into exotic millimeter-sized superstructures through a simple sedimentation technique based on gravity.

    ‘We have shown through experiment and computer simulation that a range of highly uniform, nanoscale silver polyhedral crystals can self-assemble into structures that have been calculated to be the densest packings of these shapes,’ Yang says.”

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    Peidong Yang, a chemist who holds joint appointments with Berkeley Lab and UC Berkeley, is a recognized nanoscience authority. (Photo by Roy Kaltschmidt, Berkely Lab Public Affairs)

    See the full post here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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