ORNL recipe for oxide interface perfection opens path to novel materials

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Media Contact: Ron Walli Communications and Media Relations 865.576.0226

ORNL recipe for oxide interface perfection opens path to novel materials

OAK RIDGE, Tenn., Nov. 16, 2012 — By tweaking the formula for growing oxide thin films, researchers at the Department of Energy's Oak Ridge National Laboratory achieved virtual perfection at the interface of two insulator materials.

This finding, published in the journal Advanced Materials, could have significant ramifications for creation of novel materials with applications in energy and information technologies, leading to more efficient solar cells, batteries, solid oxide fuel cells, faster transistors and more powerful capacitors.

The research team, led by ORNL's Ho Nyung Lee, demonstrated that a single unit cell layer of lanthanum aluminate grown on a strontium titanate substrate is sufficient to stabilize a chemically and atomically sharp interface. A unit cell is the smallest group of atoms that possess the properties of a crystalline material.

"This means that we can now create new properties by precisely conditioning the boundary in the process of stacking different oxides on top of each other," said Lee, a member of the Materials Science and Technology Division.

What's especially noteworthy is that a layer even one unit cell thick could serve as a buffer and dramatically improve the interface quality.

For this research, Lee and colleagues used pulsed laser deposition to deposit lanthanum aluminate thin films on strontium titanate substrates. They were able to demonstrate that a mundane variable such as the oxygen pressure during deposition of lanthanum aluminate is the key factor for achieving atomically sharp interfaces and changing the interface properties on a single unit cell level. Importantly, this finding is not limited to fine-tuning this particular interface, but also applies to a broad range of oxide heterostructures in a class of minerals known as perovskites.

The discovery of electrical properties in oxides - ordinarily insulators - has generated excitement and potentially creates the possibility that oxide electronics could become an alternative to the current semiconductor technology based on silicon.

Making this finding possible was Argonne National Laboratory's Advanced Photon Source and the extreme brightness of synchrotrons that allowed scientists to study the structure and composition at the interface.

"The sophisticated surface X-ray diffraction methods available at the Advanced Photon Source were key to zeroing in on the origin of the interface behavior," said co-author and colleague Gyula Eres.

While previous research with lanthanum aluminate thin film growth used low oxygen pressures, Lee and colleagues systematically explored the effects of oxygen pressure in a wide range. They determined that a shielding layer of lanthanum aluminate grown at high oxygen pressure followed by continued growth at a lower pressure resulted in a highly ordered atomically and chemically sharp - essentially defect-free -- interface.

Other ORNL authors of the paper, titled "Atomic Layer Engineering of Perovskite Oxides for Chemically Sharp Heterointerfaces," are Woo Seok Choi, the first author, Christopher Rouleau and Sung Seok Seo. Other institutions contributing to the paper are the University of Kentucky, Argonne National Laboratory and the University of Science and Technology of China.

Funding for this research was provided by the DOE Office of Science, which also supports the Advanced Photon Source. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov/. UT-Battelle manages ORNL for DOE's Office of Science.

Primary Standards Laboratory: Sandia’s the word for precision measurements, calibrations

Sandia Labs News Releases

November 15, 2012

Primary Standards Laboratory: Sandia’s the word for precision measurements, calibrations

ALBUQUERQUE, N.M. — You probably never gave roundness a thought.

 

But when it’s crucial that something be really round, federal labs and agencies can turn to the Department of Energy’s Primary Standards Laboratory (PSL), operated by Sandia. The PSL is often the last word on measurements, particularly in the world of nuclear weapons.

The image of project lead Hy Tran is reflected in a polished quartz ball that is the standard for roundness. The Primary Standards Laboratory at Sandia uses a specialized instrument to measure roundness deviation against the roundness standard, which has been certified to national standards to be round within about 20 nanometers. Click on the thumbnail for a high-resolution image. (Photo by Randy Montoya)

The PSL develops and maintains precision measurement standards, does measurement assurance training and consulting, provides calibrations and technical support and performs technical surveys and measurement audits. Work there ranges from electrical, physical, dimensional and thermodynamic calibrations for Sandia organizations to reference standards for nuclear weapons plants at Pantex, Y-12 and other nuclear security sites inside and outside DOE, to projects for such agencies as NASA.

Measurement and calibration are critical because they affect the quality of published scientific and technical data, conclusions drawn from that data and certification of product-to-performance requirements. Sandia’s calibrations largely trace to reference standards from the National Institute of Standards and Technology (NIST) for just about anything that can be measured.

“It doesn’t matter what the discipline is, whether it’s voltage or mass or pressure or temperature, you have to quantify it and it has to meet some type of standard” of accuracy, said PSL distinguished technologist Jim Novak.

Take, for example, the standard for roundness, or deviation from a circle. The PSL uses a specialized instrument to measure roundness deviation against a roundness standard, a polished quartz ball nestled in a padded box. That ball, certified to national standards, is round within about 20 nanometers.

“You can’t measure the actual diameter of a circle in here, but we can measure how far off that circle is from being a perfect circle,” said project lead Hy Tran. “We are capable of resolving a tenth of a nanometer from this piece of equipment. … The resolution is exquisite” — to the point that if the quartz ball were Earth size, the instrument could detect hills and valleys about 1¼ inch high.

Most measurements and calibrations are based on comparison.

“You’re always comparing with a standard,” said the PSL’s Bud Burns. “You have a standard, you know what that uncertainty is and you compare an unknown with that standard.”

There’s a gray area with measurements, and instruments are calibrated to keep those uncertainties within acceptable tolerances, Novak said. Over the decades, new instruments and techniques have shrunk the uncertainties.

There are basically two types of measurement standards: those based on an artifact and those that are intrinsic. Measurements often are based on an artifact — something physical such as that polished quartz ball that could vary in the tiniest way from another object based on it. An intrinsic standard, on the other hand, means a measurement can be reproduced anywhere on the planet with the same result because it relies on inherent and reproducible properties of a phenomenon or substance.

The 45,000-square-foot PSL building at Sandia includes 30,000 square feet of specially designed lab space for measurements and calibrations representing more than 100 metrology disciplines — physical and mechanical quantities such as gas flow, acceleration or vacuum standards; radiation, including alpha radiation, laser pulse energy, neutron pulse and solar power; electrical quantities such as DC and AC voltage and current; and microwave electrical quantities. To ensure accurate calibrations, temperature and humidity are rigidly controlled in each PSL lab and the building is shielded from radio frequency waves and electromagnetic radiation and isolated from vibration. Even gravity has been calculated at specific locations within some individual labs because of its importance to precise calibrations.

The PSL has unique capabilities to support the nuclear weapons complex, including pulsed neutrons for neutron generators, microwave devices for radar systems and gas leak measurements for components that must retain seal integrity at different temperatures and pressures, such as from sea level to space.

It also tests how proficiently other DOE laboratories perform their own measurements, based on standards provided by Sandia. If a particular laboratory’s core capability is measuring DC voltage, the PSL sends it a voltage source. The PSL knows what the voltage is, and it’s up to the other lab to measure it. Then the PSL checks to see that the results are what they should be, Novak said.

Sandia has performed calibrations since the 1950s. In 1968, the Atomic Energy Commission, a DOE precursor, designated Sandia to maintain the Primary Standards Laboratory for the weapons complex. That makes the PSL the technical arm of the National Nuclear Security Administration for measurements, Novak said.

For more information on the PSL, visit http://www.sandia.gov/psl/.

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Sandia National Laboratories is a multiprogram laboratory operated and managed by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.

ORNL pushes the boundaries of electron microscopy to unlock the potential of graphene

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Media Contact: Jennifer Brouner Communications and Media Relations 865-241-9515

ORNL pushes the boundaries of electron microscopy to unlock the potential of graphene

	The atomic resolution Z-contrast images show individual silicon atoms bonded differently in graphene. (hi-res image)

OAK RIDGE, Tenn., Nov. 15, 2012 — Electron microscopy at the Department of Energy's Oak Ridge National Laboratory is providing unprecedented views of the individual atoms in graphene, offering scientists a chance to unlock the material's full potential for uses from engine combustion to consumer electronics.

Graphene crystals were first isolated in 2004. They are two-dimensional (one-atom in thickness), harder than diamonds and far stronger than steel, providing unprecedented stiffness, electrical and thermal properties. By viewing the atomic and bonding configurations of individual graphene atoms, scientists are able to suggest ways to optimize materials so they are better suited for specific applications.

In a paper published in Physical Review Letters, a team of researchers from Oak Ridge National Laboratory and Vanderbilt University used aberration-corrected scanning transmission electron microscopy to study the atomic and electronic structure of silicon impurities in graphene.

"We have used new experimental and computational tools to reveal the bonding characteristics of individual impurities in graphene. For instance, we can now differentiate between a non-carbon atom that is two-dimensionally or three-dimensionally bonded in graphene. In fact, we were finally able to directly visualize a bonding configuration that was predicted in the 1930s but has never been observed experimentally," said ORNL researcher Juan-Carlos Idrobo. Electrons in orbit around an atom fall into four broad categories - s, p, d and f - based on factors including symmetry and energy levels.

"We observed that silicon d-states participate in the bonding only when the silicon is two-dimensionally coordinated," Idrobo said. "There are many elements such as chromium, iron, and copper where the d-states or d-electrons play a dominant role in determining how the element bonds in a material."

By studying the atomic and electronic structure of graphene and identifying any impurities, researchers can better predict which elemental additions will improve the material's performance.

Slightly altering the chemical makeup of graphene could customize the material, making it more suitable for a variety of applications. For example, one elemental addition may make the material a better replacement for the platinum catalytic converters in cars, while another may allow it to function better in electronic devices or as a membrane.

Graphene has the potential to replace the inner workings of electronic gadgets people use every day because of its ability to conduct heat and electricity and its optical transparency. It offers a cheaper and more abundant alternative to indium, a limited resource that is widely used in the transparent conducting coating present in almost all electronic display devices such as digital displays in cars, TVs, laptops and handheld gadgets like cell phones, tablets and music players.

Researchers expect the imaging techniques demonstrated at ORNL to be used to understand the atomic structures and bonding characteristics of atoms in other two-dimensional materials, too.

The authors of the paper are Wu Zhou, Myron Kapetanakis, Micah Prange, Sokrates Pantelides, Stephen Pennycook and Idrobo.

This research was supported by National Science Foundation and the DOE Office of Science. Researchers also made use of Oak Ridge National Laboratory's Shared Research Equipment User Facility along with Lawrence Berkeley National Laboratory's National Energy Research Scientific Computing Center, both of which are also supported by DOE's Office of Science.

ORNL is managed by UT-Battelle for the Department of Energy's Office of Science. DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visithttp://science.energy.gov.