March 3, 2011 — Computer scientists at the National Institute of Standards and Technology (NIST) have improved software that can take much of the guesswork out of difficult materials development and environmental simulation problems. 

Figure: Once provided with a microscope’s image of a composite material, OOF software can help analyze the effects of the material’s internal structure on stress. Using OOF, researchers can identify the different substances (blue and gray areas) that make up the material and compute their response to stress or other effects, providing clues about how the overall sample will behave. Credit: Langer, NIST

The software package, OOF (Object-Oriented Finite element analysis) is a specialized tool to help materials designers understand how stress and other factors act on a material with a complex internal structure, as is the case with many alloys and ceramics. As its starting point, OOF uses micrograph images of a material taken by a microscope. At the simplest level, OOF is designed to answer questions like, “I know what this material looks like and what it’s made of, but I wonder what would happen if I pull on it in different ways?” or “I have a picture of this stuff and I know that different parts expand more than others as temperature increases; I wonder where the stresses are greatest?”

OOF has been available in previous versions since 1998. The new version (2.1) adds a number of improvements:

• improves OOF’s ability to envision non-linear behavior, such as large-scale deformation, which plays a significant role in many types of stress response;
• allows users to analyze a material’s performance over time, not just under static conditions as was the case previously;
• templates allow programmers to plug in their own details and formulas describing a particular substance.

Later this year, the team expects to enable users to analyze three-dimensional micrographs of a material, rather than the 2D "slices" that can be analyzed at this point.

OOF is available for free download at http://www.ctcms.nist.gov/oof/oof2/. The package runs on Unix like systems, including Linux, OS X and Linux-like environments within Windows.

The National Institute of Standards and Technology (NIST) is an agency of the U.S. Commerce Department.

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March 3, 2011 – BUSINESS WIRE — Excelitas Technologies, optoelectronics provider, achieved record, world-class performance results for high photon detection efficiency (PDE) and low dark counts in the development and commercialization of its solid state silicon photomultipliers (SiPM) technology. SiPMs are an important element in Excelitas’ low light level detection (L3D) suite of technologies and products targeting the medical and analytical market space.

The results have been published and presented at the "CERN Industry-Academia Matching Event on SiPM and Related Technologies," in Geneva, Switzerland in February. The event brought together 120 experts active in cutting-edge photon detection work from both industry and academia.

In 2009, the company entered into an exclusive agreement with Max Planck Innovation, the technology transfer organization of the Max Planck Society, for the licensing of its ultra-fast, low cross-talk SiPM technology. Silicon photomultipliers offer very high PDE, ultra-short response time, and low power consumption, suiting low-light level applications such as fluorescence and molecular imaging in life sciences, clinical diagnostics, and analytical instrument sectors.

Excelitas Technologies is showcasing its broad array of light emission and low light level detection solutions tailored specifically for the analytical and life sciences markets at Pittcon 2011, Booth 5060. Pittcon 2011 takes place March 13-18 at the Georgia World Congress Center in Atlanta, GA.

Excelitas Technologies targets illumination, detection and other high-performance technology needs of OEM customers. The company was previously the Illumination and Detection Solutions (IDS) business unit of PerkinElmer, and is now owned by Veritas Capital. Learn more at www.excelitas.com

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March 2, 2011 — Researchers at North Carolina State University have developed a cheap and easy method for assembling nanowires, controlling their alignment and density. Dr. Yong Zhu, an assistant professor of mechanical and aerospace engineering at NC State, deposited the nanowires on a stretched rubber substrate, then released the tension on the substrate. When the nanowires settled, they aligned at a right angle to where the tension originated.

The researchers hope the findings will foster additional research into a range of device applications using nanowires, from nanoelectronics to nanosensors, especially on unconventional substrates such as rubber, plastic and paper.

"Alignment is a critical first step for developing devices that use nanowires," says Zhu, who co-authored a paper describing the research. "Hopefully our simple and cost-effective method will facilitate research in this field."

Aligning nanowires is challenging because they are created as a profusion of randomly oriented nanoscale wires between 10 and 100nm in diameter. Before any practical applications can be pursued, the user must assemble the nanowires in an orderly way. Specifically, users need to align the nanowires in a common direction and define their density. Controlling alignment and density is commonly called "assembling" the nanowires.

The more the rubber substrate is stretched, the more aligned the nanowires will be, and the greater the nanowire density will be. "Our method is cost-effective," says Feng Xu, a Ph.D. student working on this project. "It can also be used for nanowires synthesized by different methods or processed in different conditions, for instance, silver nanowires synthesized in solution and silicon nanowires synthesized by the vapor-liquid-solid method, as demonstrated in our work."

Previous research has presented a number of other methods for assembling nanowires. The new method can be used in conjunction with previous methods to achieve even better nanowire assembly.

The use of a rubber substrate in this method facilitates broad research and manufacturing sectors. For example, a key element of research into stretchable nanoelectronics involves aligning nanowires on a stretchable rubber substrate. Similarly, rubber is also the material used as stamps in transfer printing, a fabrication method used in manufacturing nanodevices on diverse substrates ranging from silicon to glass to plastic.

Zhu notes that the initial step of the method, when the nanowires are first deposited on stretched rubber, sometimes yields an inconsistent degree of nanowire alignment. The team is currently working to understand the fundamental interface mechanics, including adhesion and static friction, between nanowires and rubber substrates, which is expected to lead to a better control of the assembly process and hence a higher yield of the nanowire assembly.

The paper, "Strain-Release Assembly of Nanowires on Stretchable Substrates," was published Feb. 22 in ACS Nano. The paper was co-authored by Zhu, Xu, NC State Ph.D. student John Durham, and Dr. Benjamin Wiley, an assistant professor at Duke University. Access it online here: http://pubs.acs.org/doi/abs/10.1021/nn103183d
Abstract: A simple yet effective method for assembly of highly aligned nanowires (NWs) on stretchable substrates is reported. In this method, NWs were first transferred to a strained stretchable substrate. After the strain was released, the NWs aligned in the transverse direction and the area coverage of the NWs on the substrate increased. This method can be applied to any NWs deposited on a stretchable film and can be repeated multiple times to increase the alignment and density of the NWs. For silver (Ag) and silicon (Si) NWs on poly(dimethylsiloxane) (PDMS) substrates, the probability of NW alignment increased from 29% to 90% for Ag NWs, and from 25% to 88% for Si NWs after two assembly steps; the density increased by 60% and 75% for the Ag and Si NWs, respectively. The large-strain elasticity of the substrate and the static friction between the NWs and the substrate play key roles in this assembly method. We find that a model that takes into account the volume incompressibility of PDMS reliably predicts the degree of NW alignment and NW density. The utility of this assembly method was demonstrated by fabricating a strain sensor array composed of aligned Si NWs on a PDMS substrate, with a device yield of 95%.

The research was funded by the National Science Foundation.

NC State’s Department of Mechanical and Aerospace Engineering is part of the university’s College of Engineering.

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March 1, 2011 — Electrical energy can be generated from a temperature difference in a circuit with suitable materials. In simulations, ETH Zurich scientists show which materials are most likely to succeed in a thermoelectric process.

 
 
The computer-simulated thermoelectric material is an atomically thin germanium (Ge) coating on a thin silicon nanowire, which dramatically reduces the wire’s thermal conductivity. Only the red spots in the cross-section indicate high heat flux areas. The nanowires look like long french fries: elongated rectangular cuboids more than 300 silicon unit cells long and with a cross-section 9 unit cells wide and 9 high. The wires are 160nm long and about 5nm in equivalent diameter. The nanowires are coated with an atomically-thin layer of germanium, the thickness of the layer being only one to two unit cells of the semiconductor material.

A SiGe nanowire with this construction would be a worthy candidate for use in thermoelectricity. The only problem is that, up to now, the tiny semiconductor wire in this form exists only in the computer of Ming Hu, a post-doctoral scientist in the group of Dimos Poulikakos, Professor of Thermodynamics at the Institute of Energy Technology. Professor K. Giapis of the California Institute of Technology, USA, who spent his Sabbatical leave with the Poulikakos group at ETH Zurich, also collaborated in performing the research that led to the development of this wire.

Thermoelectricity exploits the fact that temperature and electricity can be under certain conditions inter-convertible. Due to the Seebeck effect, a small electrical voltage occurs in a circuit when a temperature difference is present between the contact points of two different kinds of electrical conductors in the circuit. However, not all conducting or semiconducting materials are suitable for thermoelectricity generation. To exhibit high conversion coefficients rendering a material viable to realistic applications the material thermal conductivity must be as small as possible, whereas its electrical conductivity must be large. Dimos Poulikakos says, "Such materials are practically non-existent in nature."

Although the thermal conductivity of bulk silicon is high, this thermal conductivity deteriorates as soon as the semiconductor is converted into a wire-like nanostructure. "Even pure silicon nanowires are not good enough for efficient energy conversion," says Ming Hu.

Through computer simulations, Hu Ming has showed that silicon nanowires conduct heat even more poorly if they are coated with an atomically thin layer of germanium, another semiconductor. The thermal conductivity decreases by 75% compared to pure silicon nanowires, and does so at room temperature. When Hu added more layers of germanium in his model, the thermal conductivity increased again.

The researchers showed that the reason for the dramatic reduction in the thermal conductivity of germanium-coated silicon nanowires lies in the altered vibration modes of the phonons that transport heat through the crystal lattice. The wavelengths of the particles were shortened and compressed at the interfacial layer between the silicon and the germanium, which blocked the heat transport to a very large degree.

Therefore, the researchers conclude that thin silicon nanowires should be coated with one or two layers of germanium to enable a significant step toward achieving viable thermoelectric processes.

The SiGe nanowires still exist only in Ming Hu’s computer. However, the plan is to manufacture them soon in Poulikakos’ laboratory for real experiments. Thermoelectric methods could make an important contribution to alternative energy production in the future. For example, the ETH Zurich professor can envisage that, with the aid of suitable installations, they could be used to exploit the waste heat from machines or buildings to generate electricity, which can be stored or fed into the grid. Based on the present state of knowledge, one could imagine devices that supply electricity to individual houses or portable equipment. Thermoelectric modules, e.g. as big as a kitchen table, could also act as solar panels to generate electrical energy from solar energy. Poulikakos warns that "such practical applications are still a rather long way into the future."

Reference

Hu M, Giapis KP, Goicochea JV, Zhang X and Poulikakos D., Significant Reduction of Thermal Conductivity in Si/Ge Core-Shell Nanowires., Nano Letters, 2011, 11, 618–623. doi: 10.1021/nl103718a

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March 1, 2011 – Business Wire — CVD Equipment Corporation (NASDAQ: CVV) and Graphene Laboratories Inc. now offer commercial CVDGraphene research starter materials and customization services for transparent and conductive CVD-grown graphene films, transferred onto substrates such as insulating silicon dioxide, plastic, and glass.

These materials and services target accelerated development of novel graphene-enabled products that are compatible with standard semiconductor processing. CVD Equipment Corporation manufactures CVD-grown graphene materials in its Application Laboratory. The transfer of Graphene onto a variety of standard and customer-supplied substrates is performed jointly with Graphene Laboratories Inc., which directly markets the products and services worldwide using CVD Equipment Corporation’s CVDGraphene trademark.

Since our first commercial offering of 2D CVDGraphene materials in Q2 2010, our CVDGraphene material has been supplied to several hundred researchers worldwide. Interest in graphene materials was further enhanced by the awarding of the Nobel Prize in Physics in October 2010 for the discovery and isolation of graphene. By offering a Graphene transfer service onto customer-specified materials, we will fulfill strong demand by academic and industrial researchers, said the companies’ representatives. CVD-grown graphene yields high-quality large-scale graphene coatings, stated Dr. Elena Polyakova, president and CEO of Graphene Laboratories, Inc. "In addition to offering off-the-shelf products, we also offer contract process development and customized Graphene products," Polyakova added.

This transfer service will enable better customer support for graphene incorporation into electronic, solar, bio and medical fields. Single-layer graphene films on an insulator like silicon dioxide are a starting material for developing graphene-based transistors, NEMS and MEMS, and sensors. Transparent graphene films on glass and plastics are of interest to developers of touch-screen displays, flexible electronics, and solar cells, says Karlheinz Strobl, VP of business development for CVD Equipment Corporation.

Graphene Laboratories Inc. brings functional graphene materials and devices to market. For further information about Graphene Laboratories Inc., visit www.graphenelab.com

CVD Equipment Corporation (NASDAQ: CVV) is a designer and manufacturer of standard and custom state-of-the-art equipment used in the development, design and manufacture of advanced electronic components, materials and coatings for research and industrial applications. Learn more at www.CVDequipment.com

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