Novel Semiconductor Structure Bends Light 'Wrong' Way -- Exciting Application Potential

Source:

http://www.sciencedaily.com/releases/2007/10/071014163634.htm

Science Daily — A Princeton-led research team has created an easy-to-produce material from the stuff of computer chips that has the rare ability to bend light in the opposite direction from all naturally occurring materials. This startling property may contribute to significant advances in many areas, including high-speed communications, medical diagnostics and detection of terrorist threats.

The new substance is in a relatively new class of materials called "metamaterials," which are made out of traditional substances, such as metals or semiconductors, arranged in very small alternating patterns that modify their collective properties. This approach enables metamaterials to manipulate light in ways that cannot be accomplished by normal materials.
Previous metamaterials were two-dimensional arrangements of metals, which limited their usefulness. The Princeton invention is the first three-dimensional metamaterial constructed entirely from semiconductors, the principal ingredient of microchips and optoelectronics.
"To be useful in a variety of devices, metamaterials need to be three-dimensional," said Princeton electrical engineering professor Claire Gmachl, one of the researchers on the study. "Furthermore, this is made from semiconductors, which are extremely functional materials. These are the things from which true applications are made."
The research team, led by Princeton engineering graduate student Anthony Hoffman, will publish its findings online Oct. 14 in the journal Nature Materials. Other Princeton researchers on the team include graduate students Leonid Alekseyev, Scott Howard and Kale Franz; former Council of Science and Technology fellow Dan Wasserman, now at the University of Massachusetts-Lowell; and former electrical engineering professor Evgenii Narimanov, now at Purdue University. The team also includes collaborators from Oregon State University and telecommunications firm Alcatel-Lucent.
Light waves and other forms of electromagnetic radiation bend whenever they pass from one medium to another. This phenomenon, called refraction, is readily observable when a straw placed into a glass of water appears to be bent or broken. Lenses in reading glasses or a camera work because of refraction.
All materials have an index of refraction, which measures the degree and direction that light is bent as it passes through them. While materials found in nature have positive refractive indices, the material recently invented by Princeton researchers has a negative index of refraction.
In the case of the straw in a glass, normal water would make the underwater portion of the straw appear to bend toward the surface. If water were able to refract light negatively, as the newly invented semiconductor does, the segment of straw under the water would appear as if it were bending away from the surface
Far more than a neat optical illusion, negative refraction holds promise for the development of superior lenses. The positive refractive indices of normal materials necessitate the use of curved lenses, which inherently distort some of the light that passes through them, in telescopes and microscopes. Flat lenses made from materials that exhibit negative refraction could compensate for this aberration and enable far more powerful microscopes that can "see" things as small as molecules of DNA.
In addition, the Princeton metamaterial is capable of negative refraction of light in the mid-infrared region, which is used in a wide range of sensing and communications applications. Its unique composition results in less lost light than previous metamaterials, which were made of extremely small arrangements of metal wires and rings. The semiconductors that constitute the new material are grown from crystals using common manufacturing techniques, making it less complex, more reliable and easier to produce.
"Currently, the typical infrared lens is a massive object -- the setups are bulky," Hoffman said. "This new material may enable more compact mid-infrared optics because we now have a new material with an entirely new set of optical parameters in our toolkit."
The research is part of a multi-institutional research center called Mid-Infrared Technologies for Health and the Environment (MIRTHE). Researchers at MIRTHE are developing compact sensors that detect trace amounts of gases in the atmosphere and human breath. These could one day be used in devices that monitor air quality and enhance homeland security, as well as in non-invasive and on-the-spot medical tests for diabetes and lung disease.
The research relies on a new type of laser that emits mid-infrared light. Gmachl, who directs the MIRTHE project, said the new material could be used to make the lasers better and smaller.
Next, the team plans to incorporate the new metamaterial into lasers. Additionally, the researchers will continue to modify the material in attempts to make features ever smaller in an effort to expand the range of light wavelengths they are able to manipulate.
The work was supported by the MIRTHE center and the Princeton Center for Complex Materials (PCCM), both sponsored by the National Science Foundation.
Note: This story has been adapted from material provided by Princeton University, Engineering School.

Fausto Intilla

www.oloscience.com

New Path For Designing Novel Nanomaterials Discovered

Source:

http://www.sciencedaily.com/releases/2007/10/071012095246.htm

Science Daily — A University of Arkansas researcher and his colleagues have found a novel way to “look” at atomic orbitals, and have directly shown for the first time that they change substantially when interacting at the interface of a ferromagnet and a high-temperature superconductor.
This finding opens up a new way of designing nanoscale superconducting materials and fundamentally changes scientific convention, which suggests that only electron spin and atomic charge – not atomic orbitals – influence the properties of superconducting nanostructures. It also has implications for interfaces between other complex oxide materials.
Jacques Chakhalian, assistant professor of physics in the J. William Fulbright College of Arts and Sciences, and his colleagues will publish their findings online at the Science Express Web site, published by the journal Science, Oct. 11.
Until now, materials science researchers believed that an electron’s charge and spin influenced the characteristics of conventional bulk materials. Atomic orbitals, which consist of the patterns of electron density that may be formed in an atom, were previously thought to be inactive.
“In conventional materials like copper or silicon, you could account for everything you could see through charge and spin,” Chakhalian said. Further, orbitals have proved difficult to “see” through physical experimentation, so it wasn’t possible to examine any changes in orbital symmetry that might be taking place at the interface.
Chakhalian’s work has focused on what happens at the interface between two different materials – for instance, superconductors and ferromagnets, two materials with properties that were thought to be incompatible with each other in bulk. In 2006, he and his colleagues created the first high-quality material to have both superconducting and ferromagnetic properties, and they used that material in this experiment.
Chakhalian and his colleagues worked with synchrotron radiation at the Advanced Photon Source, Argonne National Laboratory in Argonne, Ill., to examine the interface between a high-temperature superconducting material containing copper oxide and a ferromagnetic material containing manganese oxide. The synchrotron light is electromagnetic radiation of varying wavelengths that can be tuned to a specific wavelength and polarization for a particular experiment. Unlike conventional X-rays, which diffuse through space, the synchrotron light beams are sharply focused, like a laser beam with extreme brilliance.
The researchers forced the two materials into unusual quantum states. Using a technique called resonant X-ray absorption, they were able to “look” at the atomic orbitals at the interface and determine their symmetry in a non-destructive way.
They found that the atomic orbitals changed the nature of their symmetry at the interface and created a covalent bond between the copper and manganese atoms. This bonding does not exist in the bulk of the individual materials
“When you merge these two materials, the atomic orbitals at the interface become important. They start contributing to the electronic properties of the material,” Chakhalian said. “This opens a new way of designing materials. We can design quantum materials with engineered physical properties.”
The discovery may allow researchers to manipulate nanoscale superconductivity at the interface – opening the possibility of creating room-temperature semiconductors.
Generators that use superconducting materials generate electricity extremely efficiently, at half the size of conventional generators. General Electric estimates the potential market for superconducting generators to be between $20 billion and $30 billion over the next decade.
Chakhalian’s colleagues include J.W. Freeland and M. van Veenendaal of the Advanced Photon Source, Argonne National Laboratory, Argonne, Ill.; and H.-U. Habermeier, G. Cristiani, G. Khaliullin and B. Keimer of the Max Planck Institute for Solid State Research in Stuttgart, Germany.
Note: This story has been adapted from material provided by University of Arkansas, Fayetteville.

Fausto Intilla
www.oloscience.com

New Plastic Is Strong As Steel, Transparent

Source:

http://www.sciencedaily.com/releases/2007/10/071004143114.htm

Science Daily — By mimicking a brick-and-mortar molecular structure found in seashells, University of Michigan researchers created a composite plastic that's as strong as steel but lighter and transparent.

It's made of layers of clay nanosheets and a water-soluble polymer that shares chemistry with white glue.
Engineering professor Nicholas Kotov almost dubbed it "plastic steel," but the new material isn't quite stretchy enough to earn that name. Nevertheless, he says its further development could lead to lighter, stronger armor for soldiers or police and their vehicles. It could also be used in microelectromechanical devices, microfluidics, biomedical sensors and valves and unmanned aircraft.
Kotov and other U-M faculty members are authors of a paper on this composite material, "Ultrastrong and Stiff Layered Polymer Nanocomposites," published in the Oct. 5 edition of Science.
The scientists solved a problem that has confounded engineers and scientists for decades: Individual nano-size building blocks such as nanotubes, nanosheets and nanorods are ultrastrong. But larger materials made out of bonded nano-size building blocks were comparatively weak. Until now.
"When you tried to build something you can hold in your arms, scientists had difficulties transferring the strength of individual nanosheets or nanotubes to the entire material," Kotov said. "We've demonstrated that one can achieve almost ideal transfer of stress between nanosheets and a polymer matrix."
The researchers created this new composite plastic with a machine they developed that builds materials one nanoscale layer after another.
The robotic machine consists of an arm that hovers over a wheel of vials of different liquids. In this case, the arm held a piece of glass about the size of a stick of gum on which it built the new material.
The arm dipped the glass into the glue-like polymer solution and then into a liquid that was a dispersion of clay nanosheets. After those layers dried, the process repeated. It took 300 layers of each the glue-like polymer and the clay nanosheets to create a piece of this material as thick as a piece of plastic wrap.
Mother of pearl, the iridescent lining of mussel and oyster shells, is built layer-by-layer like this. It's one of the toughest natural mineral-based materials.
The glue-like polymer used in this experiment, which is polyvinyl alcohol, was as important as the layer-by-layer assembly process. The structure of the "nanoglue" and the clay nanosheets allowed the layers to form cooperative hydrogen bonds, which gives rise to what Kotov called "the Velcro effect." Such bonds, if broken, can reform easily in a new place.
The Velcro effect is one reason the material is so strong. Another is the arrangement of the nanosheets. They're stacked like bricks, in an alternating pattern.
"When you have a brick-and-mortar structure, any cracks are blunted by each interface," Kotov explained. "It's hard to replicate with nanoscale building blocks on a large scale, but that's what we've achieved."
Collaborators include: mechanical engineering professor Ellen Arruda; aerospace engineering professor Anthony Waas; chemical, materials science and biomedical engineering professor Joerg Lahann; and chemistry professor Ayyalusamy Ramamoorthy. Kotov is a professor of chemical engineering, materials science and engineering, and biomedical engineering.
The nanomechanical behavior of these materials is being modeled by professor Arruda's group; Waas and his group are working on applications in aviation.
Note: This story has been adapted from material provided by University of Michigan.

Fausto Intilla

www.oloscience.com

Two Nanostructures Are Better Than One

Source:

http://www.sciencedaily.com/releases/2007/09/070904114617.htm

Science Daily — Imagine using minuscule structures the size of molecules to harvest sunlight and convert it into electricity. Or employing the same structures to store hydrogen fuel so that it fits into a car's gas tank. Or replacing today's semiconductors with these structures, ushering in the next generation of small, powerful electronics.

These technologies don't exist yet, but the work of a researcher at the University of Wisconsin-Milwaukee (UWM) is bringing them closer to reality with hybrid materials made with carbon nanotubes (CNTs).
Junhong Chen, assistant professor of mechanical engineering, is pioneering better methods of making CNTs more predictable.
CNTs are invisibly thin sheets of graphite that are rolled into a cylindrical shape. Chen's laboratory focuses on new uses for CNTs combined with nanoparticles, bits of matter that are nanoscale in all three dimensions. (A few nanometers are roughly 50,000 times smaller than the width of a human hair.)
"These interesting multi-component structures will open up new opportunities in several interdisciplinary fields," Chen says, "including medical diagnostics, green energy technology, and sensors for everything from food flavor to invisible toxic gas."
Big and small worlds collide
CNTs are the potential superstar structures of molecular engineering because of their remarkable electronic and mechanical properties. Already they are used in making flat panel display screens and sensing devices that can detect substances in very low concentrations.
They conduct electricity like either copper or silicon, are stronger than steel, pliable like polymers (kinds of plastics) and can be made from a range of raw materials.
The challenge is to coax them to behave in predictable ways.
With the help of graduate student Ganhua Lu, Chen has devised a method for creating hybrid structures by coating CNTs with aerosol nanoparticles. His lab also has produced a low-cost way to make "custom" nanoparticles that gives them full control over the structure's final properties.
Manipulating CNTs and nanoparticles is tricky business because so many conditions affect their behavior. Just below nanoscale, at the level of individual atoms, matter acts quite differently than it does lumped together in bulk.
Already they have devised a gas sensor using only nanoparticles of tin oxide.
Their process for producing hybrid structures is far superior to the method currently available, and their work advances understanding of how materials in the quantum world interact with those in the "seen" world.
Surface science cluster at UWM
"My goal is to make something real, that people can see and use and that has tangible results," says Chen, who came to UWM in 2003 after a year as a post-doctoral scholar at the California Institute of Technology.
With two patents pending and funding from sources such as the National Science Foundation and the Xerox Corporation, he is well on his way.
Chen is one of a cluster of UWM scientists -- in engineering, chemistry and physics -- who conduct research into nano- and surface science. In fact, UWM's Laboratory for Surface Studies, a University of Wisconsin System Center of Excellence, brings together the work of 13 faculty who explore the structure and properties of solid surfaces and the interaction of surfaces with atoms and molecules.
The lab's research encompasses topics such as thin films and laminates, spintronics, molecular wires, optical fiber sensing, and properties such as catalysis, corrosion and friction.
New materials, cool features
When it comes to size, there's tiny and then there's tiny all over. Nanostructures can be at nanoscale in either one, two or three dimensions, or any combination. The structure's properties are determined by the number of dimensions at nanoscale, its shape, and the material it's made of.
CNTs, for example, can behave like an electrical conductor or a semiconductor, depending on the diameter and the twist of the tube. Nanoparticles have their own unique characteristics. Once attached to CNTs, they can transfer their abilities to the tube.
Nanoparticles could potentially give an insulator like silicon the ability to conduct electricity.
A happy union of the two structures offers a chance to brainstorm new applications, says Chen. For example, the hybrid can be altered to absorb and emit various wavelengths of light, giving it optical properties.
"You get an opportunity to make a material that could potentially display not only the properties of the CNT and the nanoparticles, but also some additional properties because of the interaction between the two.
"So one plus one may be greater than two," he says. "That's the whole idea."
Molecular building with control
Before Chen's technique for fusing the two nanostructures, the process took hours.
CNTs are produced in a gas phase from carbon precursors, but nanoparticles are made in a solution, he says. "You had to combine the dry with the wet before you can make something out of it," he says. "It's not very compatible."
Also the surface of each structure had to be modified, and the chemistry would be different for each new material involved.
"What you would get at the end could be very different from what you were trying to get," he says.
So Chen and his lab developed a way to make nanoparticles in the gas (dry) phase. Then they applied an electrostatic force to attract any kind of nanoparticle to the CNT. The one-step process takes only minutes.
"It works like an indoor air cleaner, using the electric field to capture dust particles," he says.
Another advantage of the process is Chen can control the size of the nanoparticle that is fastened to the tube, and that determines the final properties of the hybrid structure. The higher the electric field, the larger the particles it will attract.
Chen believes the research has huge potential.
"Many of our projects lie at the intersection of fundamental science and industrial applications with ample opportunities for new discoveries."
Note: This story has been adapted from a news release issued by University of Wisconsin, Milwaukee.

Fausto Intilla

www.oloscience.com