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.
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.
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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]
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.
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.
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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]
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.
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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
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.
(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.
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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 (Tg). “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 Tg 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]
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.
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.)
(Click image to enlarge.)
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