Tuesday, January 3, 2012

Discovery of a 'dark state' could mean a brighter future for solar energy

The efficiency of conventional solar cells could be significantly increased, according to new research on the mechanisms of solar energy conversion led by chemist Xiaoyang Zhu at The University of Texas at Austin.

Zhu and his team have discovered that it's possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material.

"Plastic semiconductor solar cell production has great advantages, one of which is low cost," said Zhu, a professor of chemistry. "Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies."

Zhu and his team published their groundbreaking discovery Dec. 16 in Science.

The maximum theoretical efficiency of the silicon solar cell in use today is approximately 31 percent, because much of the sun's energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of "hot electrons," is instead lost as heat. Capturing hot electrons could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent.

Zhu and his team previously demonstrated that those hot electrons could be captured using semiconductor nanocrystals. They published that research in Science in 2010, but Zhu says the actual implementation of a viable technology based on that research is very challenging.

"For one thing," said Zhu, "that 66 percent efficiency can only be achieved when highly focused sunlight is used, not just the raw sunlight that typically hits a solar panel. This creates problems when considering engineering a new material or device."

To circumvent that problem, Zhu and his team have found an alternative. They discovered that a photon produces a dark quantum "shadow state" from which two electrons can then be efficiently captured to generate more energy in the semiconductor pentacene.

Zhu said that exploiting that mechanism could increase solar cell efficiency to 44 percent without the need for focusing a solar beam, which would encourage more widespread use of solar technology.

The research team was spearheaded by Wai-lun Chan, a postdoctoral fellow in Zhu's group, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov, Loren Kaake and Luis Miaja-Avila. The research was supported by the National Science Foundation and the Department of Energy.

Science Behind the Discovery:

Absorption of a photon in a pentacene semiconductor creates an excited electron-hole pair called an exciton.The exciton is coupled quantum mechanically to a dark "shadow state" called a multiexciton.This dark shadow state can be the most efficient source of two electrons via transfer to an electron acceptor material, such as fullerene, which was used in the study.Exploiting the dark shadow state to produce double the electrons could increase solar cell efficiency to 44 percent.

Story Source:

The above story is reprinted from materials provided by University of Texas at Austin.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

W.-L. Chan, M. Ligges, A. Jailaubekov, L. Kaake, L. Miaja-Avila, X.- Y. Zhu. Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer. Science, 2011; 334 (6062): 1541 DOI: 10.1126/science.1213986

Glow of recognition: New detectors could provide easy visual identification of toxins or pathogens

 Researchers at MIT have developed a new way of revealing the presence of specific chemicals -- whether toxins, disease markers, pathogens or explosives. The system visually signals the presence of a target chemical by emitting a fluorescent glow.

The approach combines fluorescent molecules with an open scaffolding called a metal-organic framework (MOF). This structure provides lots of open space for target molecules to occupy, bringing them into close proximity with fluorescent molecules that react to their presence.

The findings were reported in the Journal of the American Chemical Society in a paper by assistant professor of chemistry Mircea Dincă, with postdoc Natalia Shustova and undergraduate student Brian McCarthy, published online in November and to appear in a forthcoming print issue.

The work could have significant applications in sensors attuned to specific compounds whose detection could be read at a glance simply by watching for the material to glow. "A lot of known sensors work in reverse," Dincă says, meaning they "turn off" in the presence of the target compound. "Turn-on sensors are better," he says, because "they're easier to detect, the contrast is better."

Mark Allendorf, a research scientist at Sandia National Laboratory, who was not involved in this work, agrees. "Present materials generally function via luminescence quenching," and thus "suffer from reduced detection sensitivity and selectivity," he says. "Turn-on detection would address these limitations and be a considerable advance."

For example, if the material is tuned to detect carbon dioxide, "the more gas you have, the more intensity in the response," making the device's readout more obvious. And it's not just the presence or absence of a specific type of molecule: The system can also respond to changes in the viscosity of a fluid, such as blood, which can be an important indicator in diseases such as diabetes. In such applications, the material could provide two different indications at once -- for example, changing in color depending on the presence of a specific compound, such as glucose in the blood, while changing in intensity depending on the viscosity.

MOF materials were first produced about 15 years ago, but their amazing porosity has made them a very active area of research. Although they simply look like little rocks, the sponge-like structures have so much internal surface area that one gram of the material, if unfolded, would cover a football field, Dincă says.

The material's inner pores are about one nanometer (one billionth of a meter) across, making them "about the size of a small molecule" and well suited as molecular detectors, he says.

The new material is based on the MIT team's discovery of a way to bind a certain type of fluorescent molecules, also known as chromophores, onto the MOF's metal atoms. While these particular chromophores cannot emit light by themselves, they become fluorescent when bunched together. When in bunches or clumps, however, target molecules cannot reach them and therefore cannot be detected. Attaching the chromophores to nodes of the MOF's open framework keeps them from clumping, while also keeping them close to the empty pores so they can easily respond to the arrival of a target molecule.

Ben Zhong Tang, a professor of chemistry at the Hong Kong University of Science and Technology, who was not involved in this work, says the MIT researchers have taken "an elegant approach" to producing functional MOFs, and "have already demonstrated the utility of their MOFs for detection and differentiation of normally difficult-to-distinguish" molecules called volatile organic compounds.

Tang says the new system still needs further refinement to improve the efficiency of production, which he says should be easily accomplished. Once that is achieved, he says, it could find many uses. "Many more applications may be envisioned: For example, the MOFs may serve as smart vehicles and monitors for controlled drug deliveries," with the additional benefit that "the fluorescence should be gradually weakened in intensity along with progressive release of the drugs, thus enabling in situ real-time monitoring of the drug release profiles." But for now, he says, "the work is excellent in terms of proof of concept."

The work was supported by MIT's Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, and by the National Science Foundation.

Story Source:

The above story is reprinted from materials provided by Massachusetts Institute of Technology. The original article was written by David L. Chandler, MIT News Office.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Natalia B. Shustova, Brian D. McCarthy, Mircea Dincă. Turn-On Fluorescence in Tetraphenylethylene-Based Metal–Organic Frameworks: An Alternative to Aggregation-Induced Emission. Journal of the American Chemical Society, 2011; 133 (50): 20126 DOI: 10.1021/ja209327q

First certified reference material for single-wall carbon nanotubes

The National Institute of Standards and Technology (NIST) has issued the world's first reference material for single-wall carbon nanotube soot. Distantly related to the soot in your fireplace or in a candle flame, nanotube-laden soot is the primary industrial source of single-wall carbon nanotubes, perhaps the archetype of all nanoscale materials. The new NIST material offers companies and researchers a badly needed source of uniform and well-characterized carbon nanotube soot for material comparisons, as well as chemical and toxicity analysis.

With walls of carbon only one atom thick and looking like a sheet of chicken wire curled into a cylinder, single-wall carbon nanotubes are one of several families of pure carbon materials that, because of their nanoscale size, have special properties. "Single-wall carbon nanotubes," says NIST chemical engineer Jeffery Fagan, "have exquisite optical, mechanical, thermal and electronic properties, and because of their small width but long lengths -- think of something like a long piece of hair but 10,000 times thinner -- full development of these materials should enable lighter, stronger materials, as well as improve many technologies from sensors to electronics and batteries."

Unfortunately, nanotubes are difficult to produce without significant impurities or in large quantities. Single-wall nanotubes, in particular, have been notorious for their relatively low quality and batch-to-batch variability. They typically are produced in complex processes using small particles of metal catalysts that promote the growth of the nanotubes. The resulting material -- often a powder not unlike the soot you would find in your fireplace -- has frequently contained large amounts of impurities, such as other forms of carbon, and sometimes significant levels of catalysts.

"One of the issues that this reference material addresses is that there's no homogeneous lot that people can buy to do comparative measurements," says Fagan. "Even batch-to-batch, raw carbon nanotube powder samples have varied so much that there is no interlaboratory consistency. And that's particularly a problem for comparisons such as toxicity measurements. If you bought carbon nanotubes, you were pretty much guaranteed that your sample could be so different from anyone else's samples that either your measurements could be specific to some flaw of your material, or that others might not be able to reproduce what you were doing."

To address these issues, a multidisciplinary research team at NIST has worked to develop the metrology necessary for quantitative single-wall carbon nanotube measurements through a three-prong approach: basic measurement and separation science, documentary protocols and standards through international standards organizations, and now certified reference materials.

The new NIST product, Standard Reference Material (SRM) 2483, "Single-Wall Carbon Nanotubes (Raw Soot)," will directly address the issue of comparability. It is possibly the world's single largest supply of homogeneous, chemically analyzed, carbon nanotube soot where the uniformity of the samples from unit to unit is assured. Each unit of SRM 2483, a glass vial containing 250 milligrams of soot, is certified by NIST for the mass fraction values of several common contaminants: barium, cerium, chlorine, cobalt, dysprosium, europium, gadolinium, lanthanum, molybdenum and samarium. Reference values (values believed to be accurate, but not rising to the level of confidence that NIST certifies) are provided for an additional seven elements.

NIST also provides additional reference data useful for nanotube analysis, including thermal gravimetric and Raman data, as well as informational values for ultraviolet-visible-near-infrared absorbance spectra, near-infrared fluorescence spectra, Raman scattering spectra and scanning electron microscopy images. With these sets of information, purchasers of the material should be able to compare their results against the NIST values and against those from suppliers or after processing, ensuring a consistent point of comparison.

Single units of SRM 2483, "Single-Wall Carbon Nanotubes (Raw Soot)," are available from the NIST Standard Reference Materials Program at www.nist.gov/srm/.

Standard Reference Materials are among the most widely distributed and used products from NIST. The agency prepares, analyzes and distributes more than a thousand different materials that are used throughout the world to check the accuracy of instruments and test procedures used in manufacturing, clinical chemistry, environmental monitoring, electronics, criminal forensics and dozens of other fields.

Story Source:

The above story is reprinted from materials provided by National Institute of Standards and Technology (NIST).

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Chemists solve an 84-year-old theory on how molecules move energy after light absorption

The same principle that causes figure skaters to spin faster as they draw their arms into their bodies has now been used by Michigan State University researchers to understand how molecules move energy around following the absorption of light.

Conservation of angular momentum is a fundamental property of nature, one that astronomers use to detect the presence of satellites circling distant planets. In 1927, it was proposed that this principle should apply to chemical reactions, but a clear demonstration has never been achieved.

In the current issue of Science, MSU chemist Jim McCusker demonstrates for the first time the effect is real and also suggests how scientists could use it to control and predict chemical reaction pathways in general.

"The idea has floated around for decades and has been implicitly invoked in a variety of contexts, but no one had ever come up with a chemical system that could demonstrate whether or not the underlying concept was valid," McCusker said. "Our result not only validates the idea, but it really allows us to start thinking about chemical reactions from an entirely different perspective."

The experiment involved the preparation of two closely related molecules that were specifically designed to undergo a chemical reaction known as fluorescence resonance energy transfer, or FRET. Upon absorption of light, the system is predisposed to transfer that energy from one part of the molecule to another.

McCusker's team changed the identity of one of the atoms in the molecule from chromium to cobalt. This altered the molecule's properties and shut down the reaction. The absence of any detectable energy transfer in the cobalt-containing compound confirmed the hypothesis.

"What we have successfully conducted is a proof-of-principle experiment," McCusker said. "One can easily imagine employing these ideas to other chemical processes, and we're actually exploring some of these avenues in my group right now."

The researchers believe their results could impact a variety of fields including molecular electronics, biology and energy science through the development of new types of chemical reactions.

Dong Guo, a postdoctoral researcher, and Troy Knight, former graduate student and now research scientist at Dow Chemical, were part of McCusker's team. Funding was provided by the National Science Foundation.

Story Source:

The above story is reprinted from materials provided by Michigan State University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Sharpening the lines: Advance could lead to smaller features in the quest for more compact, faster microchips

 The microchip revolution has seen a steady shrinking of features on silicon chips, packing in more transistors and wires to boost chips' speed and data capacity. But in recent years, the technologies behind these chips have begun to bump up against fundamental limits, such as the wavelengths of light used for critical steps in chip manufacturing.

Now, a new technique developed by researchers at MIT and the University of Utah offers a way to break through one of these limits, possibly enabling further leaps in the computational power packed into a tiny sliver of silicon. A paper describing the process was published in the journal Physical Review Letters in November.

Postdoc Trisha Andrew PhD '10 of MIT's Research Laboratory of Electronics, a co-author of this paper as well as a 2009 paper that described a way of creating finer lines on chips, says this work builds on that earlier method. But unlike the earlier technique, called absorbance modulation, this one allows the production of complex shapes rather than just lines, and can be carried out using less expensive light sources and conventional chip-manufacturing equipment. "The whole optical setup is on a par with what's out there" in chip-making plants, she says. "We've demonstrated a way to make everything cheaper."

As in the earlier work, this new system relies on a combination of approaches: namely, interference patterns between two light sources and a photochromic material that changes color when illuminated by a beam of light. But, Andrew says, a new step is the addition of a material called a photoresist, used to produce a pattern on a chip via a chemical change following exposure to light. The pattern transferred to the chip can then be etched away with a chemical called a developer, leaving a mask that can in turn control where light passes through that layer.

While traditional photolithography is limited to producing chip features larger than the wavelength of the light used, the method devised by Andrew and her colleagues has now been shown to produce features one-eighth that size. Others have achieved similar sizes before, Andrew says, but only with equipment whose complexity is incompatible with quick, inexpensive manufacturing processes.

The new system uses "a materials approach, combined with sophisticated optics, to get large-scale patterning," she says. And the technique should make it possible to reduce the size of the lines even further, she says.

The key to beating the limits usually imposed by the wavelength of light and the size of the optical system is an effect called stimulated emission depletion imaging, or STED, which uses fluorescent materials that emit light when illuminated by a laser beam. If the power of the laser falls below a certain level, the fluorescence stops, leaving a dark patch. It turns out that by carefully controlling the laser's power, it's possible to leave a dark patch much smaller than the wavelength of the laser light itself. By using the dark areas as a mask, and sweeping the beam across the chip surface to create a pattern, these smaller sizes can be "locked in" to the surface.

That process has previously been used to improve the resolution of optical microscopes, but researchers had thought it inapplicable to photolithographic chip making. The innovation by this MIT and Utah team was to combine STED with the earlier absorbance-modulation technique, replacing the fluorescent materials with a special polymer whose molecules change shape in response to specific wavelengths of light.

In addition to enabling the manufacture of chips with finer features, the technique could also be used in other advanced technologies, such as the production of photonic devices, which use patterns to control the flow of light rather than the flow of electricity. "It can be used for any process that uses optical lithography," Andrew says.

Professor Stefan Hell, head of the Department of NanoBiophotonics at the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany, calls this work "strikingly simple and elegant" and "a most impressive demonstration of the idea of using photochromic molecules to create features that are both finer and closer together than half the wavelength of the light."

"The work shows a concrete pathway to creating tiny and dense features at the nanoscale." he adds. "Because of its future potential it needs to be actively pursued. ... These methods have the potential of shifting the paradigm of what we think that focused light can do for making nanosized features and hence mastering the nanoworld."

In addition to Andrew, the paper's authors include Rajesh Menon, formerly a research engineer at MIT and now an assistant professor of electrical engineering and computer science at Utah, and Utah postdoc Nicole Brimhall and graduate student Rajakumar Varma Manthena. The work was supported in part by grants from the U.S. Defense Advanced Research Projects Agency and the National Science Foundation.

Story Source:

The above story is reprinted from materials provided by Massachusetts Institute of Technology. The original article was written by David L. Chandler, MIT News Office.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Nicole Brimhall, Trisha Andrew, Rajakumar Manthena, Rajesh Menon. Breaking the Far-Field Diffraction Limit in Optical Nanopatterning via Repeated Photochemical and Electrochemical Transitions in Photochromic Molecules. Physical Review Letters, 2011; 107 (20) DOI: 10.1103/PhysRevLett.107.205501