NEWS FROM THE WORLD OF MATERIALS

Materials in Focus

Photonic edge states allow photons to bypass defects in optical circuits
Joint Quantum Institute (JQI) at the University of Maryland and the National Institute of Standards and Technology (NIST). See also the press release by Chad Boutin of NIST.

Image credit: Joint Quantum Institute. Click image to enlarge.

Image caption: Artist's rendering of the proposed JQI fault-tolerant photon delay device for a future photon-based microchip. The devices ordinarily have a single row of resonators; using multiple rows like this provides alternative pathways for the photons to travel around any physical defects. 

In the quest to make robust optical chips for computers and other electronic devices, defects in the materials have proven to be a major challenge to the efficient transmission of photons. “In nanofabrication, there will always be errors in the system,” says Mohammad Hafezi of the Joint Quantum Institute (JQI) at the University of Maryland and the National Institute of Standards and Technology (NIST). “The question we asked ourselves was, ‘can we make a system that doesn’t care about defects?’”

Hafezi and his colleagues, including Jacob Taylor of JQI and Eugene Demler and Mikhail Lukin from Harvard University, answered the question affirmatively in a recent paper in Nature Physics. The answer came by considering a two dimensional array of coupled resonator optical waveguides (CROWs), which are typically used as optical delay components to slow down the transmission of digital data until it is needed. Instead of the common linear arrangement of resonator rings, they simulated a two-dimensional array of resonators. In a linear arrangement, a single defect might be enough to deflect a photon from its path. Now, simply by changing the architecture of the device and not the material, the researchers provided alternate paths, known as “photonic edge states,” that the photon could use to bypass a defect in the system.

But not just any two dimensional array would work in this case. To be effective, the device architecture must be able to make the photons experience the same two-dimensional physics as electrons experience in two dimensions in a magnetic field. “We simulated the quantum Hall effect (QHE) physics for photons,” Hafezi says. “In this way, the robustness that the electron has in the quantum Hall effect is experienced by photons, eliminating the certain effects of nanofabrication errors.” Optical delay lines were used as a first example of this potential technology; in the future, it is possible that the robust photonic architecture could be used in many photonic device components. On a more fundamental level, Hafezi is particularly interested in obtaining a better understanding of the QHE in electrons by analogy with the behavior of the photons in his simulations. [Nature Physics]

Nano Focus

Millimeter-long GaN nanowires grow horizontally on sapphire substrate
(Weizmann Institute of Science, Israel. See also the press release issued by the Weizmann Institute of Science.)
Image credit: Ernesto Joselevich. Click image to enlarge.

Image caption: Illustration of nanowires growing horizontally along nanogrooves. 

Most nanowires are born standing up, rising vertically from a substrate to reach heights in the range of tens of micrometers; they typically require post-fabrication processing to form aligned arrays of nanowires suitable for use in an electronic or optical device. Attempts to grow nanowires horizontally on a surface have had some success, but the resulting nanowires were still in the micrometer length range, with limited control over their crystallographic orientation. Now, researchers at the Weizmann Institute of Science in Israel, led by Ernesto Joselevich, have reported in Science the development of a process for producing millimeter-long GaN nanowires by guided growth on various crystallographic planes of a sapphire surface. The process allows the researchers to grow “very long and perfectly aligned horizontal nanowires with exquisite control of their crystallographic orientation,” according to Joselevich.

The research team, which included Ph.D. student David Tsivion, postdoctoral fellow Mark Schvartzman, and staff scientists Ronit Popovitz-Biro and Palle von Huth, used chemical vapor deposition of GaN on eight different sapphire planes seeded with Ni catalysts to achieve these results. Analysis of the nanowires produced on these various planes revealed that those formed on surface steps and grooves were better aligned than those formed on a smooth plane. For instance, on a well-cut, smooth sapphire C-plane, nanowires grew in random triangular patterns following six isomorphic directions. By miscutting the same C-plane by 2°, the nanowires grew along only two directions, forming parallel arrays. “We found that when the substrate is cut in a slightly tilted or unstable plane,” Joselevich says, “the surface wrinkled up upon heating, and the tiny steps and grooves that formed on it made the alignment of the nanowires much better than on a smooth surface.” The authors explained this effect in the paper very simply: “graphoepitaxy overrules epitaxy.”

They report that their GaN nanowires have few defects and excellent optical and electronic properties, making them excellent potential candidates for nanoscale high-power circuits, LEDs, lasers, photovoltaic cells, photodetectors, and radio-frequency, photonic and non-linear optical devices. The relative absence of defects is atypical for semiconductors grown on a substrate, because stresses usually develop that produce defects. “We think this is because, unlike a two-dimensional film, which usually gets stressed, a nanowire can relax by shrinking or swelling sidewise, making the system much more tolerant to mismatch than one is used to seeing in continuous two-dimensional films,” Joselevich speculates. “This is a new one-dimensional nanoscale effect, which, together with the effect of graphoepitaxy, somehow changes the paradigm not only in the new field of nanowires, but also in the well-established fields of epitaxy and thin films.” [Science] 

Room-temperature multiferroic materials created at interface
(Helmoltz-Zentrum Berlin (HZB). See also the press release by Eric Verbeten of Helmoltz-Zentrum Berlin.)
Image credit: HZB. Click image to enlarge.

Image caption: HZB staff scientist Florin Radu checks the BaTiO₃ sample alignment in the ALICE diffractometer. 

In the search for multiferroic materials—those in which ferroelectric polarization and magnetic order exist simultaneously—researchers have traditionally pursued two paths: the study of intrinsically multiferroic materials, and the fabrication of artificial multiferroics by mixing materials having magnetic and ferroelectric properties into a single structure. While these methods have shown some success, the multiferroic properties have been observed only at very low temperatures (-270 °C), making them impractical for use in electronic devices. Now researchers in Germany, France, and the United Kingdom have produced room-temperature multiferroics by following a third path. “Our method profits from interface effects in thin films,” says Sergio Valencia of Helmoltz-Zentrum Berlin, leader of one of the groups participating in this research. “We show that, as theoretically predicted, electronic effects occurring at the interface of a ferromagnet with a ferroelectric can lead to multiferroicity in the latter.”

As reported in Nature Materials, the ferroelectric they chose was a thin film of BaTiO₃. By depositing a thin layer of ferromagnetic materials such as Fe or Co on BaTiO₃, the researchers were able to induce a remanent magnetic moment along with ferroelectricity spontaneously in the BaTiO₃ film at room temperature. Soft x-ray resonant magnetic scattering and piezoresponse force microscopy revealed remanent magnetization and hysteretic properties.  “Most known multiferroic materials have virtually zero remanent magnetization (being antiferromagnets or weak ferromagnets) at room temperature,” Valencia says.

This new material offers two possibilities for practical applications in magnetic data storage devices. First, if the magnetic and ferroelectric materials are not coupled, their states could vary independently, producing a four-state memory bit in place of the two-state one now available. Alternatively, if the magnetoelectric coupling is strong, then the magnetic state of a memory bit could be changed by controlling electric fields, which consumes much less power than the current practice of altering magnetic fields.  Valencia says that the next step in this investigation is to determine the strength of the electromagnetic coupling.

This research was a joint project of the Helmholtz-Zentrum-Berlin für Materialen und Energie, Berlin, Germany; the Unité Mixte de Physique CNRS/Thales, Palaiseau, France; the Université Paris-Sud, Orsay, France; the University of Cambridge, United Kingdom; the Université d’Evry-Val d’Essonne, Evry cedex, France; and the Ruhr-Universität Bochum, Bochum, Germany. [Nature Materials] 

Bio Focus

Resin-based coatings have high modulus and glass transition temperature
(North Dakota State University, Fargo, ND)
Image credit: Dean Webster, North Dakota State University. Click image to enlarge.

Image caption: Molecular structure of ESEFA resin-based coating.

As part of the ongoing effort to find bio-based replacements for traditionally petrochemical-based products, researchers in the Department of Coatings and Polymeric Materials at North Dakota State University, Fargo, recently announced in Biomacromolecules (an ACS publication) the fabrication of high modulus coatings from epoxidized sucrose esters of fatty acids (ESEFA). Dean C. Webster and his colleagues Xiao Pan and Partha Sengupta crosslinked various vegetable oils through their epoxy groups to form hard, thermosetting resins that could be used as coating materials. According to Webster, earlier attempts to crosslink vegetable oils usually resulted in coatings with low moduli and glass transition temperatures (T_g). “In our paper, we have a few materials with moduli over one GPa, which I think is pretty incredible,” Webster says. “We can get fairly high glass transition temperatures out of these materials as well.” The maximum T_g reported was 103.7 °C.

Webster attributes these properties to the large number of fatty acid (CH₃-xCH₂-COOH) groups—as many as eight—that they attached to sucrose, yielding a high number of epoxy groups. “When we crosslink through those epoxy groups we can get a much higher crosslink density,” he says, “and thus a higher modulus.” The resins are also compact so the viscosity is in the moderate range, making possible a sprayable coating with very little solvent for industrial purposes. While much more testing and characterization must be done to determine viable applications and limitations of these resins before commercialization, Webster foresees possibilities for coatings that are applied and baked on metal surfaces in factory settings. In addition, he and a colleague were recently awarded an NSF grant to look at these resins as matrices for bio-based composites.[Biomacromolecules]

Energy Focus

Pipeline Alternatives to Reduce Carbon Emissions during the Operations of Liquid and Gas Fuels Transmission and Distribution in Mexico

by Lorenzo Martinez-Gomez
www.corrosionyproteccion.com

Last November at the 2010 International Materials Research Congress in Cancun, many countries confirmed strong environmental commitments and established long-range initiatives to reduce global CO₂ emissions into the atmosphere. The Cancun meeting triggered many initiatives in Mexico after the government increased the market value of Mexican carbon bonds. While Mexican carbon bonds are still priced lower than ones traded in Europe, the Mexican valuation is now priced significantly higher than U.S. carbon offsets.

The petroleum industry is a major contributor to the greenhouse gas (GHG) emissions of Mexico. Currently, production practices in the region involve large quantities of gas being burned or released to the atmosphere. Refineries and petrochemical plants are also major sources of GHGs. Transmission and distribution of liquid and gas fuels by trucking are still common practices in Mexico. Cancun and the Riviera Maya together consume over 7 million liters per day of jet fuel, diesel, and gasoline. These fuels have traditionally been transported over land from cities on the Gulf of Mexico such as Merida, Coatzacoalcos, or even Salina Cruz on the Pacific coast, averaging over 400 to 500 km in trucking transport distances per month. In central Mexico, the highly populated and industrialized valley of Cuernavaca, Cuautla, and large parts of Guerrero also rely on fuels transported by truck from Mexico City, Tlaxcala, Puebla, or Toluca.

CO₂ emissions associated with liquid or gas hydrocarbon transmission and distribution are remarkably different when the transportation method is considered. Whereas trucking is heavy in fuel consumption, pipeline delivery is by far the most reliable, cost-effective, and environmentally friendly means of fluid transportation.

Engineering projects have calculated the potential tonnage of CO₂ emissions to be saved by constructing pipeline networks to feed hydrocarbons to both the Cancun – Riviera Maya region and Cuernavaca valley, with many projects benefitting from funding based on carbon dioxide bonuses. Accurate calculations of this sort may sustain the costs of important pipeline projects based on the long-term value of the savings of carbon dioxide emissions.

Software has been developed to combine and analyze all research results involved in the calculation of the carbon signatures of pipeline and truck transportation of liquid and gas hydrocarbons. Geographical information systems were employed to perform calculations for alternative potential right-of-way trajectories, as well as the fuel consumption associated with the current trucking routes. Other considerations are related to the carbon signature of trucking as a whole, including the excess of human resources, the differential needs of metering, tanking, and logistics, and the overall critical storage facility involved.

Considering European and Mexican carbon dioxide bond values, the resulting financing opportunities for pipeline projects are significant. For the case of supplying hydrocarbons by pipeline to Cancun and the Rivera Maya, the projected carbon dioxide emission savings over 30 years could finance an important segment of the pipeline construction project. Pipeline construction to supply hydrocarbons to the Cuernavaca – Cuautla Valleys would also result in saving millions of tons of CO₂ emissions. 

Image in Focus

Peppermint Towers

SEM image of VLS-grown Si microwires coated with droplets of wax (which have been false-colored using Adobe Photoshop).

Credit: Emily Warren, California Institute of Technology 
(Click image to enlarge.)

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NEWS FROM THE WORLD OF MATERIALS

Materials in Focus

Hydrogen pressure may control graphene growth
Oak Ridge National Laboratory (ORNL). See also the press release by Ron Walli of ORNL.)
Image credit: ORNL. Click image to enlarge.

Image caption: Graphene grains come in several different shapes. Hydrogen gas controls the grains' appearance. 

In one currently popular way of making graphene, methane (the carbon source) is mixed with molecular hydrogen, and the two gases flow over a copper substrate at a temperature of approximately 1000 °C. The role of hydrogen in this process has been difficult to assess because its concentration has varied from essentially zero to several thousand times the amount of methane in different experiments. Now researchers from Oak Ridge National Laboratory (ORNL) and New Mexico State University, by carefully controlling the hydrogen/methane ratio, have determined that hydrogen performs two roles in the graphene formation process: (1) it acts as a co-catalyst with the copper, and (2) it acts as an etchant to control graphene grain size and shape. The results were reported recently in ACS Nano. 

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NEWS FROM THE WORLD OF MATERIALS

Materials in Focus

High strength gold bridge, one atom long
(State University of New York Buffalo News Center. See also the press release by Charlotte Hsu.)
Image courtesy of University at Buffalo. Click image to enlarge.

Image caption: A bridge made of a single atom of gold has twice the strength of bulk gold, according to new UB research.

Smaller is not necessarily weaker, Harsh Deep Chopra and his colleagues at the State University of New York at Buffalo have discovered. In fact, a bridge made of just a single gold atom between a gold substrate and a gold coated cantilever tip has at least twice the modulus of the bulk, they report in a recent paper in Physical Review B. “You would think that if you make these constrictions so tiny they would become ever more fragile,” Chopra says. “Actually, they become even harder to elastically deform.” They note that when the atoms do not have the full coordination of bulk gold, the gold-gold bonds contract and strengthen, which leads to modulus enhancement. This discovery could prove to be important as nano-devices are made smaller and smaller with each generation.

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NEWS FROM THE WORLD OF MATERIALS

Materials in Focus

Ductile or brittle at the flip of a switch
(Technical University of Hamburg and Shenyang National Laboratory for Materials Science, China)
Photo credit: Technical University of Hamburg. Click image to enlarge.

Generally, a material's basic properties are determined by its composition and microstructure during the manufacturing process. Changes in these properties may occur during extended use, but this is generally a slow process governed by creep, fatigue, or some other factor. Now, however, researchers Jörg Weissmüller at the Technical University of Hamburg and Hai-Jun Jin at the Shenyang National Laboratory for Materials Science in China have developed a hybrid nanostructure material that can change properties at the flip of a switch. As reported recently in Science, they have developed a material consisting of a nanoporous gold backbone filled with a liquid electrolyte that is capable of fast, reversible tuning of its yield strength, flow stress, and ductility through the application of an electric field. "The concept allows the user to select, for instance," the authors wrote, "a soft and ductile state for processing and a high-strength state for service as a structural material." 

Starting with a gold/silver alloy, they removed the silver by corrosion, leaving a monolithic skeleton characterized by contiguous gold ligaments and an equally contiguous pore structure. The pores were then filled with a 1M HClO₄ electrolyte. Compression of this hybrid nanostructure material under different electric potential conditions revealed the change in properties. Using a sample with gold ligaments having a diameter of 20 nm, compression under a constant applied voltage of 1.03 V showed ductility up to high strain conditions. The same process performed with an applied voltage of 1.48 V showed a 36% increase in yield strength and a loss of ductility. Also, the flow stress doubled when switching from 1.03 V to 1.48 V. These changes were completely reversible when the applied voltages were reversed. 

The researchers noted that at 1.48 V the gold ligaments are covered with adsorbed oxygen, while at 1.03 V they are "clean." This led them to investigate the role of surface stress and surface tension on these property variations; they concluded that neither surface stress nor surface tension was responsible. The most likely explanation according to the researchers is that the adsorbed oxygen exerts a drag on dislocations that intersect the surface, resulting in "adsorption locking," which increases the yield strength and the flow stress at the higher voltage. "For the first time we have succeeded in in producing a material which, while in service, can switch back and forth between a state of strong and brittle behavior and one of soft and malleable," Weismuller said in a press release issued by the Technical University of Hamburg. "We are still at the fundamental research stage but our discovery may bring significant progress in the development of so-called smart materials." [Science]

Nano Focus

Si nanowire-based non-volatile memory devices reduce power consumption
(NIST and George Mason University)
Photo credit: Bonevich/NIST. Click image to enlarge.

Using small, 20-nm diameter Si nanowires wrapped in HfO₂ and Al₂O₃, researchers Curt Richter at NIST and Qiliang Li at George Mason University may have found a path to creating low-power, fast-writing, non-volatile memories that could eventually replace DRAM and SRAM. The DRAM devices require frequent refreshing to retain stored data, which consumes a large part of the power. The SRAM devices used for cache memory in computers' central processing units (CPUs) are volatile and need to be powered to retain data. The standby power for data remanence is a significant part of the total power dissipation. The lower power consumption of non-volatile memory could mean longer intervals between recharging batteries in computers and other electronic devices. This is very attractive for portable and stand-alone electronics. 

As reported recently in Nanotechnology, Richter, Li, and their colleagues took advantage of electrical properties of the materials and the geometry of a small diameter nanowire to improve the electrostatics of gate control. The dielectric properties of HfO₂ make it a good charge-trapping layer, and an Al₂O₃ layer acts as a blocking oxide. Richter says they can tune the stack through band engineering to produce the best possible charge trapping dielectric stacks. Then they take advantage of the small diameter of the Si nanowire to achieve 3D electrostatics, which Richter says, gives them better control than traditional 2D planar devices. "Better electrostatic control means faster, more effective turning on and off," Richter says. "And we've also tuned the gate stack so that most of the electric field is dropped just over the tunnel barrier so we have better control. That means we can hopefully operate at lower voltages and reduce power compared to more traditional dielectric stacks in planar structures." 

Their goal is to achieve faster write/erase speeds for non-volatile memory with reduced power consumption. "Our plan is two-fold," Li says. "One is to reduce the channel length so we can achieve higher memory density. The second is to do more engineering on the dielectric stack so that we can get the non-volatile memory programming speed to below 1 ns, similar to SRAMs."[Nanotechnology]

Bio Focus

Attaching proteins to electrodes in ambient conditions
(University of Pennsylvania)
Image credit: Bonnell/University of Pennsylvania. Click image to enlarge.

Most research involving the attachment of proteins to electrodes to measure their electrical properties has been done in liquid solutions to understand the biological principles of the operation of proteins inside cells. But for other potential applications, such as energy harvesting or toxic chemical sensing, the protein/electrode device must function in ambient, open air conditions. Now researchers at the University of Pennsylvania led by Dawn A. Bonnell have demonstrated successful operation of a single molecular layer of artificial proteins attached to electrodes as optoelectronic devices in an ambient environment. What's more, they've developed a new AFM-based technique to quantitatively measure the resistance, capacitance, and dielectric constant of such devices, as reported in ACS Nano. 

Researcher Bodhana Discher fabricated the artificial proteins used in these experiments. Device manufacture involved the self-assembly of amphiphilic protein helices in groups of four on a highly oriented pyrolitic graphite surface using microcontact printing. A single molecular layer of these helices measured 6.6 ± 0.5 nm—the height of a protein helix standing vertically on the graphite surface. The optically active molecule zinc (II) protroporphyrin (ZnPP) was inserted into the interior of the scaffold formed by these four helices; later measurements showed that approximately five ZnPP molecules occupied a single scaffold. 

Using their new AFM-based technique called torsional resonance nanoimpedance microscopy, the researchers oscillated a metal AFM tip sideways rather than up and down, so as not to damage the delicate protein structures. A blue LED with a wavelength of 425 nm emitted light near the sample-tip junction to excite the ZnPP molecules. "We use a technique we call 'force stabilization' to get very near the surface," Bonnell says," without disrupting or damaging it. We call it 'soft contact.'" When combined with special circuitry that maximized the signal-to-noise ratio at a higher frequency, they were able to measure the dielectric constant quantitatively by "measuring the polarization volume change between the ground state when there is no light on the ZnPP and the excited state when the light is on it and it is absorbing photons," Bonnell says. 

"You'll see lots of characterization papers on lots of different properties in these systems," Bonnell concludes, "but what was different here and I think is going to be generalized in a broader context is that we developed a technique that can measure the dielectric constant of a single-molecule-thick layer." [ACS Nano]

To hear Dawn Bonnell explain her views of the possible applications of this research (mp3, 59 sec.), click the soundwave icon:

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Energy Focus

Dark plasmons trap more light
(Northwestern University)
Photo credit: Northwwestern University. Click image to enlarge.

Researchers Teri Odom and Wei Zhou of Northwestern University recently reported in Nature Nanotechnology a new type of subradiant (dark) plasmon that is easily tunable by modification of the height of gold nanoparticles arranged in a large-scale, two-dimensional array. Previous attempts to make dark plasmons have involved structuring single nanoparticles or nanoparticle arrays in complex ways, in an attempt to take advantage of broken symmetries in the structure. "In our case we just change the height of the nanoparticles," Odom says. "That's easier than trying to manipulate sub-wavelength features in individual particles." 

Abandoning the traditional electron beam lithographic methods, which limit the height of nanoparticles that can be made, the researchers used a template-stripping nanofabrication technique to obtain two-dimensional arrays of gold particles with heights ranging from 65 to 175 nm on transparent substrates. Experimenting with an array of 100-nm high, 160-nm diameter gold particles spaced at 400-nm intervals and covering a total area greater than 18 cm², Odom and Zhou found an out-of-plane (E0z) electric component of tranverse-magnetic polarized light that excited out-of-plane plasmon modes. These plasmon modes are narrow (FWHM~5 nm) at resonance, and strong coupling between their dipolar moments suppresses the radiative decay of the radiant (bright) plasmons, trapping light in the x-y plane of the nanoparticle array. "We're finally accessing the third dimension," Odom says. "Because we could make the gold nanoparticles so tall, we were able to discover this out-of-plane lattice mode which happens to have this dark plasmon character. We're uncovering some of the unique outcomes of being able to manipulate structure in the z-dimension." 

Odom thinks these arrays, with their concentrated, in-plane local energy fields, might be valuable platforms on which to study the mechanisms of chemical reactions. Also, the scalability of the fabrication technique could lead to a coupling of plasmonic and photovoltaic applications. "Because these arrays can trap the light in a much more efficient way and because we can scale them," Odom says, speculating about the distant future, "they could provide a practical first step for plasmonics-based photovoltaics." [Nature Nanotechnology]

Graphene oxide "glue" makes stacking tandem solar cells easier
(Northwestern University)
Photo credit: Huang/Northwestern University. Click image to enlarge.

Mixing graphene oxide (GO) and the common polymer PEDOT:PSS in water produces a sticky thin film upon casting that may make it simpler to fabricate tandem solar cells, according to research published recently in the Journal of the American Chemical Society. Jiaxing Huang and his colleagues at Northwestern University describe a proof-of-concept using direct adhesive lamination of the layers of tandem devices with GO/PEDOT gel as the glue, a process which they say is much easier than creating tandem architectures via solution processes, as is now commonly done. 

Tandem solar cells are multijunction devices in which two sub-cells are stacked for increased solar energy absorption. This stacking requires that the "glue" interlayers be orthogonally processable, which is not easy to achieve in solution with organic solar cells. Also, careful choice of solvents is needed at each step to avoid damaging components in other layers. No such problems arise when aqueous solutions of GO (0.1 -2 wt%) and PEDOT:PSS (1.3-1.7wt.%) are mixed to form a viscous gel that can be easily applied to many substrates. Heat treatment at 60°C turns the gel into a sticky adhesive to bond stacks together. Furthermore, despite the electrically insulating nature of GO, the conductivity of PEDOT:PSS films increases by an order of magnitude when GO is added. The authors suggest that this may be due to a conformational change in PEDOT upon contact with GO. More generally, the GO:PEDOT gel could serve as a non-metallic solder for electrical and mechanical connections in any organic electronic device. [Journal of the American Chemical Society]

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NEWS FROM THE WORLD OF MATERIALS

Materials in Focus

Thermoelectric properties of half-Heusler alloys enhanced
(Physics World)

To be of practical use, a thermoelectric material must be good at conducting electricity but poor at conducting heat. "Half-Heusler" alloys have promising thermoelectric properties but they suffer from having relatively high thermal conductivities. One way of reducing their conductivity is to squish together a fine powder of the material to form a nanocomposite containing many small grains. Heat has a hard time travelling across grain boundaries, thereby reducing the overall thermal conduction of the nanocomposite. Researchers have now used this technique on an extremely fine powder of a half-Heusler alloy, producing a nanocomposite with the best ZT (thermoelectric figure of merit) yet for a half-Heusler. [Nano Letters]

Embedded microvoids make LEDs more efficient
(North Carolina State University)
LED lighting relies on GaN thin films to create the diode structure that produces light. A new technique now reduces the number of defects in GaN films by two to three orders of magnitude by embedding microvoids. This improves the quality of the material that emits light, and for a given input of electrical power, the output of light can be increased by a factor of two – which is very big. This is particularly true for low electrical power input and for LEDs emitting in the ultraviolet range. The researchers started with a GaN film that was two microns thick and embedded half of that thickness with large voids – empty spaces that were one to two microns long and 0.25 microns in diameter. The researchers found that defects in the film were drawn to the voids and became trapped – leaving the portions of the film above the voids with far fewer defects. [Applied Physics Letters]

Growth, characterization of LiMnAs: A useful pyramid scheme
(Physics)
All electronics technologies have, at their heart, critical materials that make their function possible. These can be "old" materials such as silicon, whose major materials development was achieved by previous generations, or "new" materials such as gallium-nitride, which has been developed by our contemporaries.If the discovery and development of new materials comes to a stop, then the introduction and growth of new technologies will almost certainly come to a halt as well. Spintronics is an example of such a critical current technology, driving the creation of increased density, faster electronic memories through the electronic manipulation of magnetic moments. Researchers now report the successful growth and characterization of LiMnAs, a new candidate material for spintronic applications. They show convincing evidence of epitaxy and good film quality, and show that LiMnAs is a semiconductor, by performing optical spectroscopy. They also show that it is antiferromagnetic in thin film form by measuring its temperature-dependent magnetization. [Physical Review B]

Electrical phenomena in silicon oxide in electronics explored
(Eurekalert/ACS)
Researchers have found that silicon dioxide in computer chips, long regarded as an electrical insulator, can actually be made to act like a switch and take part in electronic processes. They have documented various electrical phenomena such as resistive switching and related nonlinear conduction, current hysteresis, and negative differential resistance, that are intrinsic to a thin layer of SiOx. This is more crucial in the area of nanoelectronics, wherein researchers thought that switching observed was due to the nano-additive but it turns out that the source of the switching might be from the underlying silicon oxide itself. The work clarifies the possible nature behind switching events in molecular and nano-scale systems investigated so far, that were not well understood. [J. American Chemical Society]

Nano Focus

Silver nanoparticles-coated paper for food packaging
(American Chemical Society)
It is known that silver nanoparticles show excellent microbicidal properties, much better than those of larger particles. Researchers have now demonstrated an effective, long-lasting method for depositing silver nanoparticles on the surface of paper that involves ultrasound waves. The coated paper showed potent antibacterial activity against E. coli and S. aureus, two causes of bacterial food poisoning, killing all of the bacteria in just three hours. This suggests its potential application as a food packaging material for promoting longer shelf life. [Langmuir]

Bio Focus

Nanoparticle divides to penetrate into tumors
(Chemistry World)

Researchers have created a nanoparticle that breaks up into smaller units once it reaches its target, allowing it to penetrate deeper into tumor tissue and deliver treatment more effectively. The new nanoparticles are 100 nm balls of gelatin that contain small particles that are only 10 nm in diameter. The gelatin nanoparticles get to the tumors, and then tumor enzymes digest the gelatin and release the smaller constituents, that then move through the tumor. In vitro studies showed that the particles penetrated tumor tissue much better traditional larger nanoparticles that remain one size. [Proceedings of the National Academy of Sciences]

New method for tethering and stretching DNA
(Nanotechweb.org)
Researchers have developed a reproducible surface chemistry technique for tethering DNA molecules onto surfaces and a new way to stretch the molecules to various lengths. DNA can be used as a molecular scaffold to make metal contacts to organic semiconductors. A key step in this process involves being able to tether the DNA to various surfaces and stretch the molecule to varying lengths. The new strategy involves synthesizing hybrid DNA-organic molecule-DNA (DOD) structures, then stretching and tethering the DOD assemblies between two microscopic metal electrodes. The researchers then make metal electrode-organic molecule-metal electrode (MOM) structures by further metallizing the DNA segments within the DOD structures. The team then exploited so-called biotin-Streptavidin linkage chemistry to tether the DNA assemblies to device surfaces. The method could eventually be used to make large-scale nanoelectronic devices based on single organic molecules. [ACS Nano]

Nanoscale transistors used to study single-molecule interactions
(Columbia University/Eurekalert)
Researchers have figured out a way to study single-molecule interactions on very short time scales using nanoscale transistors. They show how, for the first time, transistors can be used to detect the binding of the two halves of the DNA double helix with the DNA tethered to the transistor sensor. The transistors directly detect and amplify the charge of these single biomolecules. Previously, scientists have used fluorescence techniques to look at interactions at the level of single molecules. But these techniques require that the target molecules being studied be labeled with fluorescent reporter molecules, and the bandwidths for detection are limited by the time required to collect the very small number of photons emitted by these reporters. The transistors employed in this study were fashioned from carbon nanotubes which are exquisitely sensitive because the biomolecule can be directly tethered to the carbon nanotube wall creating enough sensitivity to detect a single DNA molecule. [Nature Nanotechnology]

Energy Focus 

Packings of carbon nanotubes for hydrogen storage
(Chemistry World)

Researchers have designed a 3D carbon nanotube matrix that can store and release hydrogen extremely efficiently. They used a computer-based approach to design a 3D carbon nanotube structure that can store more hydrogen at room temperature than any other carbon-based material. This is a top down approach from advanced mathematics, to geometry, to computer modeling, to chemical properties. The US Department of Energy's target for hydrogen storage materials by 2015 is 6wt% while the new nanotube material has a total hydrogen uptake of 5.5wt% at room temperature. Inspired by natural sponges, the team designed a computer model that placed carbon nanotubes in the hole positions of a theoretical sponge network. [Advanced Materials]

Relativity powers lead-acid battery
(Physical Review Focus)
The lead-acid battery that starts most car engines gets about 80 percent of its voltage from relativity, according to theoretical work using computer simulations. The relativistic effect comes from fast-moving electrons in the lead atom. The computer simulations also explain why tin-acid batteries do not work, despite apparent similarities between tin and lead. The researchers are the first to derive theoretical models of the lead-acid battery from fundamental physics principles. By switching relativistic parts of their models "on" and "off", the team found that relativity accounts for 1.7 volts of a single cell, which means that about 10 of the 12 volts in a car battery come from relativistic effects. [Applied Physics Letters]

Image in Focus 

  

ZnO Nanoflowers
Stem of nanoflowers made by coloring and combining different SEM images of a variety of ZnO nanostructures grown by thermal Chemical Vapor Deposition.
Credit: Abhishek Prasad, Michigan Technological University
(One of three Science as Art competition first place winners at the 2010 MRS Fall Meeting)

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Materials - New Trends

Materials in Focus

Artificial skin made of semiconductor nanowires
(University of California, Berkeley)
Researchers have developed a pressure-sensitive electronic material from semiconductor nanowires. The artificial skin, dubbed "e-skin", is the first such material to be made out of inorganic single crystalline semiconductors. They utilized an innovative fabrication technique that works somewhat like a lint roller in reverse. Instead of picking up fibers, nanowire "hairs" are deposited. The researchers started by growing germanium/silicon nanowires on a cylindrical drum, which was then rolled onto a sticky substrate. As the drum rolled, the nanowires were deposited, or "printed," onto the substrate in an orderly fashion, forming the basis from which thin, flexible sheets of electronic materials could be built. The researchers demonstrated the ability of the e-skin to detect pressure from 0 to 15 kilopascals, a range comparable to the force used for such daily activities as typing on a keyboard or holding an object. [Nature Materials]

Ultrasensitive, highly flexible electronic skin developed
(Stanford University)
By sandwiching a precisely molded, highly elastic rubber layer between two parallel electrodes, researchers were able to create an electronic sensor that can detect the slightest touch. It was able to detect pressures well below the pressure exerted by a 20 milligram bluebottle fly carcass that they experimented with, and with unprecedented speed. The key innovation in the new sensor is the use of a thin film of rubber molded into a grid of tiny pyramids. The thin rubber film between the two electrodes stores electrical charges, much like a battery. When pressure is exerted on the sensor, the rubber film compresses, which changes the amount of electrical charges the film can store. That change is detected by the electrodes and is what enables the sensor to transmit what it is "feeling." [Nature Materials]

Cracking the case on fracture
(Physics)
Many material engineering studies are carried out within a model of continuum plasticity, yet such models often lack sufficient microscopic detail to account for crack propagation and fracture resistance. A study now reports computer simulations showing more clearly what processes influence fracture in plastic deformation, and on what length scales. The authors modeled plastic deformation as the movement of discrete dislocations along slip planes. Specifically, a set of "obstacles" arrayed with some selected spacing restricts the movement of dislocations and modifies the plasticity. They then examined fracture by including an initial crack in the material and observing it propagate as a function of material cohesive strength, fracture energy, and obstacle spacing. They found that it is the obstacle spacing length scale that most strongly affects fracture toughness. Moreover, they propose that their model could serve as a more general simulation environment for fracture studies in various materials. [Physical Review Letters]

Energy Focus

Self-repairing photovoltaics rival conventional solar cells
(NanotechWeb)
During photosynthesis, plants harness solar radiation and convert it into energy. However, the Sun's rays damage and gradually destroy solar-cell components over time. Naturally occurring plants have developed a highly elaborate self-repair mechanism to overcome this problem that involves constantly breaking down and reassembling photodamaged light-harvesting proteins. Researchers have now succeeded in mimicking this process for the first time by creating novel self-assembling complexes that convert light into electricity. The complexes can be repeatedly broken down and reassembled by simply adding a surfactant (a solution of soap molecules). The researchers found that they can indefinitely cycle between assembled and disassembled states by adding and removing the surfactant, but the complexes are only photoactive in the assembled state. [Nature Chemistry]

Laser welding boosts efficiency of TiO₂ solar cells
(NanotechWeb)
Dye-sensitized solar cells (DSSCs) have excellent charge collection capabilities, high open-circuit voltages and good fill-factors. However, they do not completely absorb all of the photons from visible and near-infrared ranges and consequently have lower short-circuit photocurrent densities than inorganic photovoltaics. Increasing the short-circuit current density of DSSCs is a key factor in improving the efficiency of these devices. Researchers have recently demonstrated that the inter-electrode contact resistance arising from poor interfacial adhesion is responsible for a considerable portion of the total resistance in the DSSC. The group has shown that the current flow can be greatly improved by welding the interface with a laser. TiO₂ films formed on transparent conducting oxide (TCO)-coated glass substrates were irradiated with a pulsed UV laser beam at 355 nm, which transmits through TCO and glass, but is strongly absorbed by TiO₂. It was found that a thin continuous TiO₂ layer is formed at the interface as a result of the local melting of TiO₂ nanoparticles. This layer completely bridges the gap between the two electrodes and improves current flow by reducing the contact resistance. [Nanotechnology]

Carbon dioxide-free production of iron
(Highlights in Chemical Technology)
Iron metal has been conventionally produced by melting iron ore at temperatures over 2000°C in a blast furnace. This however produces large amounts of CO₂, which is released into the atmosphere and contributes to climate change. A research team has demonstrated that iron ores (Fe₂O₃ and Fe₃O₄) can be dissolved in molten lithium carbonate at temperatures of around 800°C - a process that was previously thought impossible. Adding an electrical current to the molten mix separates the iron ore into its component parts, iron and oxygen, which can be collected by two electrodes in the solution. Less energy is required to generate the lower temperatures and power the electrolysis, but the researchers also demonstrate that these can be achieved using renewable energy. The team employed their recently developed solar technique, called solar thermal electrochemical photo (STEP) - which uses the Sun's thermal energy to melt the lithium carbonate solution while the visible light energy powers the electrolysis. Using the STEP process no CO₂ is produced. [Chemical Communications]

Nano Focus

High-strength Al-alloy includes core/double-shell nanoparticles
(Northwestern University/Small)

Researchers have created a new high-strength aluminum alloy by engineering it at the nano level to give it high-strength and corrosion resistance to high temperatures. They combined aluminum with lithium, scandium, and ytterbium and they were able to create nano-particles with a core surrounded by two shells. The core is ytterbium-rich, while the first shell is rich in scandium and the second shell contains mostly lithium. This core/shell-shell structure has been achieved previously in liquid solutions but this is the first time it has been achieved by processing solely in the solid-state. They also found that some nano-particles had an unexpected structure — a single particle with two cores and two outer shells, like a double-yolked egg. This novel structure consists of two Yb-rich Al3(Li,Yb,Sc) cores with 4--5 nm diameter, two Sc-rich Al3(Li,Sc,Yb) inner shells surrounding their respective cores and one Li-rich Al3Li outer shell enfolding the previous regions and contained within an Al matrix. This is the first time this type of structure has been observed. [Small]

Nanoscale ion diffusion behavior in Li-ion battery revealed
(Oak Ridge National Laboratory)
A research team has developed the new electrochemical strain microscopy (ESM) to examine the movement of lithium ions through a battery's cathode material. The method can provide a detailed picture of ionic motion in nanometer volumes, which exceeds state-of-the-art electrochemical techniques by six to seven orders of magnitude. They achieved the results by applying voltage with an ESM probe to the surface of the battery's layered cathode. By measuring the corresponding electrochemical strain, or volume change, the team was able to visualize how lithium ions flowed through the material. Conventional electrochemical techniques, which analyze electric current instead of strain, do not work on a nanoscale level because the electrochemical currents are too small to measure. These are the first measurements of lithium ion flow at this spatial resolution, according to the authors. [Nature Nanotechnology]

Electric shock resets nanotube sensor
(Chemistry World)

Single-walled carbon nanotube (SWNTs) can be used in very small, highly sensitive chemical sensors for a variety of gases and other chemicals. The SWNTs, attached to a silicon substrate, absorb chemicals onto their surface, however many chemicals are irreversibly absorbed resulting in lengthy processes before the sensor can be reused. A study now shows that the SWNTs could be 'reset' at the simple flick of a switch. The team found that organic molecules bound to the nanotube surface are shaken off when an electric current is passed through the material, resetting the sensor ready for further use. Their technique - current-stimulated desorption (CSD) - passes a strong electric current through the SWNTs. As electrons jump across defects built into the nanotubes, they collide with molecules on the surface. When they hit an absorbed molecule, they transfer excess energy to it, and it flies off the surface. [Science]

High-speed filter uses electrified nanostructures to purify water
(Stanford University)
By dipping plain cotton cloth in a broth full of silver nanowires and carbon nanotubes, researchers have developed a new high-speed, low-cost filter that could easily be implemented to purify water. Instead of physically trapping bacteria as most existing filters do, the new filter lets them flow on through with the water. By the time the pathogens have passed through, the device kills them with an electrical field that runs through the highly conductive "nano-coated" cotton. In lab tests, over 98 percent of Escherichia coli bacteria that were exposed to 20 volts of electricity in the filter for several seconds were killed. Multiple layers of fabric were used to make the filter 2.5 inches thick. [Nano Letters]

Image in Focus

  
Dark Night in Desert
Colorized SEM image (5,000x) of a nano-skyline of cactus in a seemingly extraterrestrial landscape formed by Si pillars created by deep reactive ion etching decorated with Ge nanowires by vapor-liquid-solid growth.
Credit: Lucia Romano, University of Catania, Italy

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Industry Focus

LED technology used in illumination system for fluorescence microscopy
Fluorescence microscopy requires an intense light source at the specific wavelength that will excite fluorescent dyes and proteins. The traditional method employs a white light, typically from a Mercury or Xenon arc lamp. Although such broad spectrum lamps can generate ample light at desired wavelengths, only a small percentage of the projected light is useful in any particular application. The other wavelengths need to be suppressed to avoid background noise that reduces image contrast and obscures the fluorescent light emissions. This process of suppressing extraneous light is complex, expensive and only partially effective: even after decades of refinements, the best filters are not 100% percent successful at blocking the bleed through of non-specific photons. Some mitigation techniques end up not only suppressing peripheral light, but also significantly diminishing the intensity of the desired wavelengths. A radically different approach is now coming to light. Recent advances in high performance Light Emitting Diode (LED) technology have enabled the practical implementation of this theoretical model. High-intensity monochromatic LEDs are now available in a variety of colors that match the excitation bandwidth of many commonly-used fluorescent dyes and proteins.

Carl Zeiss MicroImaging has incorporated this new LED technology in their Colibri illumination system, a light source system for widefield fluorescence microscopy that uses specific wavelength windows with a considerably decreased need to suppress unwanted peripheral wavelengths from a white light arc lamp. The modular Colibri system employs up to four LEDs, without any of the mechanical switching devices like filterwheels or shutters required by traditional illumination systems. With LED technology, users can now take advantage of an excellent alternative for live cell imaging, high-speed or multi-channel fluorescence microscopy, and many other applications.

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