Saturday, December 31, 2011

First electronic optical fibers with hydrogenated amorphous silicon are developed

 A new chemical technique for depositing a non-crystalline form of silicon into the long, ultra-thin pores of optical fibers has been developed by an international team of scientists in the United States and the United Kingdom. The technique, which is the first of its kind to use high-pressure chemistry for making well-developed films and wires of this particular kind of silicon semiconductor, will help scientists to make more-efficient and more-flexible optical fibers.

The findings, by an international team led by John Badding, a professor of chemistry at Penn State University, will be published in a future print edition of the Journal of the American Chemical Society.

Badding explained that hydrogenated amorphous silicon -- a noncrystalline form of silicon -- is ideal for applications such as solar cells. Hydrogenated amorphous silicon also would be useful for the light-guiding cores of optical fibers; however, depositing the silicon compound into an optical fiber -- which is thinner than the width of a human hair -- presents a challenge. "Traditionally, hydrogenated amorphous silicon is created using an expensive laboratory device known as a plasma reactor," Badding explained. "Such a reactor begins with a precursor called silane -- a silicon-hydrogen compound. Our goal was not only to find a simpler way to create hydrogenated amorphous silicon using silane, but also to use it in the development of an optical fiber."

Because traditional, low-pressure chemistry techniques cannot be used for depositing hydrogenated amorphous silicon into a fiber, the team had to find another approach. "While the low-pressure plasma reactor technique works well enough for depositing hydrogenated amorphous silicon onto a surface to make solar cells, it does not allow the silane precursor molecules to be pushed into the long, thin holes in an optical fiber," said Pier J. A. Sazio of the University of Southampton in the United Kingdom and one of the team's leaders. "The trick was to develop a high-pressure technique that could force the molecules of silane all the way down into the fiber and then also convert them to amorphous hydrogenated silicon. The high-pressure chemistry technique is unique in allowing the silane to decompose into the useful hydrogenated form of amorphous silicon, rather than the much less-useful non-hydrogenated form that otherwise would form without a plasma reactor. Using pressure in this way is very practical because the optical fibers are so small."

Optical fibers with a non-crystalline form of silicon have many applications. For example, such fibers could be used in telecommunications devices, or even to change laser light into different infrared wavelengths. Infrared light could be used to improve surgical techniques, military countermeasure devices, or chemical-sensing tools, such as those that detect pollutants or environmental toxins. The team members also hope that their research will be used to improve existing solar-cell technology. "What's most exciting about our research is that, for the first time, optical fibers with hydrogenated amorphous silicon are possible; however, our technique also reduces certain production costs, so there's no reason it could not help in the manufacture of less-expensive solar cells, as well," Badding said.

In addition to Badding and Sazio, other members of the research team include Neil F. Baril, Rongrui He, Todd D. Day, Justin R. Sparks, Banafsheh Keshavarzi, Mahesh Krishna-murthi, Ali Borhan, and Venkatraman Gopalan of Penn State; and Anna C. Peacock and Noel Healy of the University of Southampton in the United Kingdom.

Story Source:

The above story is reprinted from materials provided by Penn State.

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

The onset of electrical resistance

Researchers at the Max-Born-Institute, Berlin, Germany, observed the extremely fast onset of electrical resistance in a semiconductor by following electron motions in real-time.

When you first learned about electric currents, you may have asked how the electrons in a solid material move from the negative to the positive terminal. In principle, they could move ballistically or 'fly' through the solid, without being affected by the atoms or other charges of the material. But this actually never happens under normal conditions because the electrons interact with the vibrating atoms or with impurities. These collisions typically occur within an extremely short time, usually about 100 femtoseconds (10 -13 seconds, or a tenth of a trillionth of a second). So the electron motion along the material, rather than being like running down an empty street, is more like trying to walk through a very dense crowd. Typically, electrons move only with a speed of 1m per hour, they are slower than snails.

Though the electrons collide with something very frequently in the material, these collisions do take a finite time to occur. Just like if you are walking through a crowd, sometimes there are small empty spaces where you can walk a little faster for a short distance. If it were possible to follow the electrons on an extremely fast (femtosecond) time scale, then you would expect to see that when the battery is first turned on, for a very short time, the electrons really do fly unperturbed through the material before they bump into anything. This is exactly what scientists at the Max-Born-Institute in Berlin recently did in a semiconductor material and report in the current issue of the journal Physical Review Letters [volume 107, 256602 (2011)]. Extremely short bursts of terahertz light (1 terahertz = 10 12 Hz, 1 trillion oscillations per second) were used instead of the battery (light has an electric field, just like a battery) to accelerate optically generated free electrons in a piece of gallium arsenide. The accelerated electrons generate another electric field, which, if measured with femtosecond time resolution, indicates exactly what they are doing. The researchers saw that the electrons travelled unperturbed in the direction of the electric field when the battery was first turned on. About 300 femtoseconds later, their velocity slowed down due to collisions.

In the attached movie, we show a cartoon of what is happening in the gallium arsenide crystal. Electrons (blue balls) and holes (red balls) show random thermal motion before the terahertz pulse hits the sample. The electric field (green arrow) accelerates electrons and holes in opposite directions. After onset of scattering this motion is slowed down and results in a heated electron-hole gas, i.e., in faster thermal motion.

The present experiments allowed the researchers to determine which type of collision is mainly responsible for the velocity loss. Interestingly, they found that the main collision partners were not atomic vibrations but positively charged particles called holes. A hole is just a missing electron in the valence band of the semiconductor, which can itself be viewed as a positively charged particle with a mass 6 times higher than the electron. Optical excitation of the semiconductor generates both free electrons and holes which the terahertz bursts, our battery, move in opposite directions. Because the holes have such a large mass, they do not move very fast, but they do get in the way of the electrons, making them slower.

Such a direct understanding of electric friction will be useful in the future for designing more efficient and faster electronics, and perhaps for finding new tricks to reduce electrical resistance.

Story Source:

The above story is reprinted from materials provided by Forschungsverbund Berlin e.V. (FVB).

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

Journal Reference:

P. Bowlan, W. Kuehn, K. Reimann, M. Woerner, T. Elsaesser, R. Hey, C. Flytzanis. High-Field Transport in an Electron-Hole Plasma: Transition from Ballistic to Drift Motion. Physical Review Letters, 2011; 107 (25) DOI: 10.1103/PhysRevLett.107.256602

Inspired by insect cuticle, scientists develop material that's tough and strong

 Researchers at the Wyss Institute for Biologically Inspired Engineering at Harvard University have developed a new material that replicates the exceptional strength, toughness, and versatility of one of nature's more extraordinary substances -- insect cuticle. Also low-cost, biodegradable, and biocompatible, the new material, called "Shrilk," could one day replace plastics in consumer products and be used safely in a variety of medical applications.

The research findings appear December 13 in the online issue of Advanced Materials. The work was conducted by Wyss Institute postdoctoral fellow, Javier G. Fernandez, Ph.D., with Wyss Institute Founding Director Donald Ingber, M.D., Ph.D. Ingber is the Judah Folkman Professor of Vascular Biology at Harvard Medical School and Children's Hospital Boston and is a Professor of Bioengineering at the Harvard School of Engineering and Applied Sciences.

Natural insect cuticle, such as that found in the rigid exoskeleton of a housefly or grasshopper, is uniquely suited to the challenge of providing protection without adding weight or bulk. As such, it can deflect external chemical and physical strains without damaging the insect's internal components, while providing structure for the insect's muscles and wings. It is so light that it doesn't inhibit flight and so thin that it allows flexibility. Also remarkable is its ability to vary its properties, from rigid along the insect's body segments and wings to elastic along its limb joints.

Insect cuticle is a composite material consisting of layers of chitin, a polysaccharide polymer, and protein organized in a laminar, plywood-like structure. Mechanical and chemical interactions between these materials provide the cuticle with its unique mechanical and chemical properties. By studying these complex interactions and recreating this unique chemistry and laminar design in the lab, Fernandez and Ingber were able to engineer a thin, clear film that has the same composition and structure as insect cuticle. The material is called Shrilk because it is composed of fibroin protein from silk and from chitin, which is commonly extracted from discarded shrimp shells.

Shrilk is similar in strength and toughness to an aluminum alloy, but it is only half the weight. It is biodegradable and can be produced at a very lost cost, since chitin is readily available as a shrimp waste product. It is also easily molded into complex shapes, such as tubes. By controlling the water content in the fabrication process, the researchers were even able to reproduce the wide variations in stiffness, from elasticity to rigidity.

These attributes could have multiple applications. As a cheap, environmentally safe alternative to plastic, Shrilk could be used to make trash bags, packaging, and diapers that degrade quickly. As an exceptionally strong, biocompatible material, it could be used to suture wounds that bear high loads, such as in hernia repair, or as a scaffold for tissue regeneration.

"When we talk about the Wyss Institute's mission to create bioinspired materials and products, Shrilk is an example of what we have in mind," said Ingber. "It has the potential to be both a solution to some of today's most critical environmental problems and a stepping stone toward significant medical advances."

Story Source:

The above story is reprinted from materials provided by Wyss Institute for Biologically Inspired Engineering at Harvard.

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

Journal Reference:

Javier G. Fernandez, Donald E. Ingber. Unexpected Strength and Toughness in Chitosan-Fibroin Laminates Inspired by Insect Cuticle. Advanced Materials, 2011; DOI: 10.1002/adma.201104051

New method significantly reduces production costs of fuel cells

 Researchers at Aalto University in Finland have developed a new and significantly cheaper method of manufacturing fuel cells. A noble metal nanoparticle catalyst for fuel cells is prepared using atomic layer deposition (ALD).

This ALD method for manufacturing fuel cells requires 60 per cent less of the costly catalyst than current methods.

"This is a significant discovery, because researchers have not been able to achieve savings of this magnitude before with materials that are commercially available," says Docent Tanja Kallio of Aalto University.

Fuel cells could replace polluting combustion engines that are presently in use. However, in a fuel cell, chemical processes must be sped up by using a catalyst. The high price of catalysts is one of the biggest hurdles to the wide adoption of fuel cells at the moment.

The most commonly used fuel cells cover anode with expensive noble metal powder which reacts well with the fuel. By using the Aalto University researchers' ALD method, this cover can be much thinner and more even than before which lowers costs and increases quality.

With this study, researchers are developing better alcohol fuel cells using methanol or ethanol as their fuel. It is easier to handle and store alcohols than commonly used hydrogen. In alcohol fuel cells, it is also possible to use palladium as a catalyst.

The most common catalyst for hydrogen fuel cells is platinum, which is twice as expensive as palladium. This means that alcohol fuel cells and palladium will bring a more economical product to the market.

Fuel cells can create electricity that produces very little or even no pollution. They are highly efficient, making more energy and requiring less fuel than other devices of equal size. They are also quiet and require low maintenance, because there are no moving parts.

In the future, when production costs can be lowered, fuel cells are expected to power electric vehicles and replace batteries, among other things. Despite their high price, fuel cells have already been used for a long time to produce energy in isolated environments, such as space crafts. These results are based on preliminary testing with fuel cell anodes using a palladium catalyst. Commercial production could start in 5-10 years.

This study was published in the Journal of Physical Chemistry C. The research has been funded by Aalto University's MIDE research program and the Academy of Finland.

Story Source:

The above story is reprinted from materials provided by Aalto University.

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

Journal Reference:

Emma Rikkinen, Annukka Santasalo-Aarnio, Sanna Airaksinen, Maryam Borghei, Ville Viitanen, Jani Sainio, Esko I. Kauppinen, Tanja Kallio, A. Outi I. Krause. Atomic Layer Deposition Preparation of Pd Nanoparticles on a Porous Carbon Support for Alcohol Oxidation. The Journal of Physical Chemistry C, 2011; : 111103103634002 DOI: 10.1021/jp2083659

Twisting molecules by brute force: A top-down approach

Molecules that are twisted are ubiquitous in nature, and have important consequences in biology, chemistry, physics and medicine. Some molecules have unique and technologically useful optical properties; the medicinal properties of drugs depend on the direction of the twist; and within us -- think of the double helix -- twisted DNA can interact with different proteins.

This twisting is called chirality and researchers at Case Western Reserve University have found they can use a macroscopic blunt force to impose and induce a twist in an otherwise non-chiral molecule.

Their new "top-down" approach is described in the Dec. 2 issue of Physical Review Letters.

"The key is that we used a macroscopic force to create chirality down to the molecular level," said Charles Rosenblatt, professor of physics at Case Western Reserve and the senior author on the paper. Rosenblatt started the research with no application in mind. He simply wanted to see if it could be done -- essentially scientific acrobatics.

But, he points out, since antiquity chirality has played a role in health, energy, technology and more -- but until now, chirality always has been a bottom-up phenomenon. This new top-down approach, if it can be scaled up, could lead to custom designed chirality -- and therefore desired properties -- in all kinds of things.

Rosenblatt worked with post-doctoral researcher Rajratan Basu, graduate student Joel S. Pendery, and professor Rolfe G. Petschek, of the physics department at Case Western Reserve, and Chemistry Professor Robert P. Lemieux of Queen's University, Kingston, Ontario.

Chirality isn't as simple as a twist in a material. More precisely, a chiral object can't be superimposed on its mirror image. In a "thought experiment," if one's hand can pass through a mirror (like Alice Through the Looking Glass), the hand cannot be rotated so that it matches its mirror image. Therefore one's hand is chiral.

Depending on the twist, scientists define chiral objects as left-handed and right-handed. Objects that can superimpose themselves on their mirror image, such as a wine goblet, are not chiral.

In optics, chiral molecules rotate the polarization of light -- the direction depends on whether the molecules are left-handed or right-handed. Liquid crystal computer and television screen manufacturers take advantage of this property to enable you to clearly see images from an angle.

In the drug industry, chirality is crucial. Two drugs with the identical chemical formula have different uses. Dextromethorphan, which is right-handed, is a cough syrup and levomethorphan, which is lefthanded, is a narcotic painkiller.

The reason for the different effects? The drugs interact differently with biomolecules inside us, depending on the biomolecules' chirality.

After meeting with Lemieux at a conference, the researchers invented a method to create chirality in a liquid crystal at the molecular level.

They treated two glass slides so that cigar-shaped liquid crystal molecules would align along a particular direction. They then created a thin cell with the slides, but rotated the two alignment directions by approximately a 20 degree angle.

The 20-degree difference caused the molecules' orientation to undergo a right-handed helical rotation, like a standard screw, from one side to the other. This is the imposed chiral twist.

The twist, however, is like a tightened spring and costs energy to maintain. To reduce this cost, some of the naturally left-handed molecules in the crystal became right-handed. That's because, inherently, right-handed molecules give rise to a macroscopic right-handed twist, Rosenblatt explained. This shift of molecules from left-handed to right-handed is the induced chirality.

Although the law of entropy suggests there would be nearly identical numbers of left-handed and right-handed molecules, in order to keep total energy cost at a minimum, the right-handed molecules outnumbered the left, he said.

To test for chirality, the researchers applied an electrical field perpendicular to the molecules. If there were no chirality, there would be nothing to see. If there were chirality, the helical twist would rotate in proportion to the amount of right-handed excess.

They observed a modest rotation, which became larger when they increased the twist.

"The effect was occurring everywhere in the cell, but was strongest at the surface," Rosenblatt said.

Scientists have built chirality into optical materials, electrooptic devices, and more by starting at the molecular level. But the researchers are not aware of other techniques that use a macroscopic force to bring chiralty down to molecules.

The researchers are continuing to investigate ways this can be done.

Story Source:

The above story is reprinted from materials provided by Case Western Reserve University.

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

Journal Reference:

Rajratan Basu, Joel Pendery, Rolfe Petschek, Robert Lemieux, Charles Rosenblatt. Macroscopic Torsional Strain and Induced Molecular Conformational Deracemization. Physical Review Letters, 2011; 107 (23) DOI: 10.1103/PhysRevLett.107.237804