Huang Z.,National University of Singapore |
Han K.,National University of Singapore |
Zeng S.,National University of Singapore |
Motapothula M.,National University of Singapore |
And 13 more authors.
Nano Letters | Year: 2016
Since the discovery of two-dimensional electron gas (2DEG) at the oxide interface of LaAlO3/SrTiO3 (LAO/STO), improving carrier mobility has become an important issue for device applications. In this paper, by using an alternate polar perovskite insulator (La0.3Sr0.7) (Al0.65Ta0.35)O3 (LSAT) for reducing lattice mismatch from 3.0% to 1.0%, the low-temperature carrier mobility has been increased 30 fold to 35 000 cm2 V-1 s-1. Moreover, two critical thicknesses for the LSAT/STO (001) interface are found, one at 5 unit cells for appearance of the 2DEG and the other at 12 unit cells for a peak in the carrier mobility. By contrast, the conducting (110) and (111) LSAT/STO interfaces only show a single critical thickness of 8 unit cells. This can be explained in terms of polar fluctuation arising from LSAT chemical composition. In addition to lattice mismatch and crystal symmetry at the interface, polar fluctuation arising from composition has been identified as an important variable to be tailored at the oxide interfaces to optimize the 2DEG transport. © 2016 American Chemical Society.
Balke N.,Center for Nanophase Materials science |
Balke N.,Institute for Functional Imaging of Materials |
Maksymovych P.,Center for Nanophase Materials science |
Maksymovych P.,Institute for Functional Imaging of Materials |
And 11 more authors.
ACS Nano | Year: 2015
Ferroelectricity in functional materials remains one of the most fascinating areas of modern science in the past several decades. In the last several years, the rapid development of piezoresponse force microscopy (PFM) and spectroscopy revealed the presence of electromechanical hysteresis loops and bias-induced remnant polar states in a broad variety of materials including many inorganic oxides, polymers, and biosystems. In many cases, this behavior was interpreted as the ample evidence for ferroelectric nature of the system. Here, we systematically analyze PFM responses on ferroelectric and nonferroelectric materials and demonstrate that mechanisms unrelated to ferroelectricity can induce ferroelectric-like characteristics through charge injection and electrostatic forces on the tip. We will focus on similarities and differences in various PFM measurement characteristics to provide an experimental guideline to differentiate between ferroelectric material properties and charge injection. In the end, we apply the developed measurement protocols to an unknown ferroelectric material. © 2015 American Chemical Society.
The ORNL study published in the journal Small demonstrates how scanning transmission electron microscopes, normally used as imaging tools, are also capable of precision sculpting of nanometer-sized 3-D features in complex oxide materials. By offering single atomic plane precision, the technique could find uses in fabricating structures for functional nanoscale devices such as microchips. The structures grow epitaxially, or in perfect crystalline alignment, which ensures that the same electrical and mechanical properties extend throughout the whole material. "We can make smaller things with more precise shapes," said ORNL's Albina Borisevich, who led the study. "The process is also epitaxial, which gives us much more pronounced control over properties than we could accomplish with other approaches." ORNL scientists happened upon the method as they were imaging an imperfectly prepared strontium titanate thin film. The sample, consisting of a crystalline substrate covered by an amorphous layer of the same material, transformed as the electron beam passed through it. A team from ORNL's Institute for Functional Imaging of Materials, which unites scientists from different disciplines, worked together to understand and exploit the discovery. "When we exposed the amorphous layer to an electron beam, we seemed to nudge it toward adopting its preferred crystalline state," Borisevich said. "It does that exactly where the electron beam is." The use of a scanning transmission electron microscope, which passes an electron beam through a bulk material, sets the approach apart from lithography techniques that only pattern or manipulate a material's surface. "We're using fine control of the beam to build something inside the solid itself," said ORNL's Stephen Jesse. "We're making transformations that are buried deep within the structure. It would be like tunneling inside a mountain to build a house." The technique offers a shortcut to researchers interested in studying how materials' characteristics change with thickness. Instead of imaging multiple samples of varying widths, scientists could use the microscopy method to add layers to the sample and simultaneously observe what happens. "The whole premise of nanoscience is that sometimes when you shrink a material it exhibits properties that are very different than the bulk material," Borisevich said. "Here we can control that. If we know there is a certain dependence on size, we can determine exactly where we want to be on that curve and go there." Theoretical calculations on ORNL's Titan supercomputer helped the researchers understand the process's underlying mechanisms. The simulations showed that the observed behavior, known as a knock-on process, is consistent with the electron beam transferring energy to individual atoms in the material rather than heating an area of the material. "With the electron beam, we are injecting energy into the system and nudging where it would otherwise go by itself, given enough time," Borisevich said. "Thermodynamically it wants to be crystalline, but this process takes a long time at room temperature." The study is published as "Atomic-level sculpting of crystalline oxides: towards bulk nanofabrication with single atomic plane precision." Explore further: Electron microscopy inspires flexoelectric theory behind 'material on the brink'
Multi-institutional partnerships that accelerate the pathway to innovation are key to solving the world’s energy crisis. In a Keynote speech at the R&D 100 Awards and Technology Conference in late November, Thom Mason, director of Oak Ridge National Laboratory (ORNL) said energy “is the most important challenge of our time.” And he expects scientific innovation and technology to solve it. Energy boasts an extra helping of importance given how central it is to other compelling challenges of this advanced 21st century, including—environmental impacts of energy production, distribution and use; national and global security implications of energy scarcity; economic consequences of energy prices; and energy access in developing nations. Specifically, Mason pointed to the lack of energy in developing nations as one of the world’s biggest hurdles. “Our concerns about CO emissions mean we can’t simply take the way we generate, distribute and use energy today to the developing world, and mount that on to a population of 9 to 10 billion people,” he said. “You will only exacerbate the potential for conflict. We need a transformation of the way energy is used in the world in order for us to evolve to a point where a much larger fraction of the world can enjoy the standard of living we enjoy.” According to Mason, the transformation can only come about through advances in the following areas: • Electrification of transportation • Renewable energy: solar, wind, geothermal • Biofuels and bioproducts • Advanced liquid fuels from fossil resources • Carbon management • Next-generation nuclear power: fission and fusion So, how do we enable advances in these areas? The short answer is innovative materials development. But it’s not as simple as that. Materials solution All of the aspects of materials development are intertwined with other scientific concepts. To improve existing energy technologies, research requires robust and reliable materials. To understand complex materials and systems, research demands increasingly sophisticated tools. To deliver novel capabilities, research requires a detailed understanding of materials structure and dynamics at the molecular level. Luckily, in the past few years, there has been tremendous leaps made to understand, measure, characterize and modify advanced materials—in part due to new resources like neutron sources, light sources, atomic resolution microscopy, nanoscale science and high-performance computing. Therefore, while materials development may be key to the energy crisis, it’s major advances in basic R&D and supporting technologies that are key to materials development. You can’t have one without the other. “You can’t put all your eggs in one basket,” Mason said. “You can’t get there with just one piece of the solution.” For ORNL, the overall solution is a better pathway to innovation. The head of the government laboratory said he feels a close coupling of both basic and applied R&D can accelerate innovation, but it requires multiple parties. Academia should be involved for their emphasis on early discovery (basic science), and industry should be involved given their emphasis on near-term solutions (serial production). National laboratories funded by the government should be involved to bridge the gap between academia and industry, providing multi-disciplinary, long-term solutions that span fundamental to applied R&D. “It’s important to establish an environment that supports this,” Mason said. “The ability for all these different sectors to interact and exchange people back and forth [is important] so they can be the carriers of these currents along the innovation chain.” Additive Manufacturing Integrated Energy Most people rely on two energy sources as we go about our day—one as we drive our car to and fro, and the second as we conduct daily life in our house. Typically, these two energy streams are independent of one another—until now. Teaming with 20 industry partners in unprecedented fashion, ORNL turned normality on its head when it debuted its Additive Manufacturing Integrated Energy (AMIE) demonstration project two months ago. The research team manufactured and connected a natural gas-powered hybrid electric vehicle with a solar-powered building to create an integrated energy system. Power can flow in either direction between the vehicle and building through a laboratory-developed wireless technology. Essentially, on overcast or rainy days, your car can power your house; on sunny days, your house can power your car. The demonstration also showcased additive manufacturing’s rapid prototyping potential in architecture and vehicle design, as the car and house were both built using large-scale 3-D printers. ORNL researchers said their integrated energy approach was developed to inspire potential improvements to the modern electric grid. The technology would be a vital step toward renewable energy (away from fossil fuels) in the transportation sector, which is one of the critical industries Mason mentioned at the beginning of his Keynote. Additionally, and also in line with Mason’s message, the integrated technology could bring clean, reliable and stable energy production to nations that are trying to move toward the standard of living. Theoretically, it could be the bridge from developing to developed nation. Perhaps as impressive as AMIE is the timeline of the project. It was just a dream in August 2014, an idea in September 2015 and a full-blown, completed demo project by September 2015. Institute for Functional Imaging of Materials In addition to working with industry partners, the Dept. of Energy has created Bioenergy Research Centers and Energy Innovation Hubs combining resources from all sectors, and ORNL itself has established institutes dedicated to societal challenges. The Institute for Functional Imaging of Materials is one of five institutes created by ORNL to exploit its capabilities and expertise in the ultimate goal of accelerating the discovery, design and deployment of new materials. The Institute is built upon the principle of connecting physical imaging with theory via big data and data analytics to develop a deep knowledge base for materials discovery. From data analytics to data modeling, the Institute boasts many microscopy and imaging techniques across various disciplines, from chemistry and environmental to biomedical, security and biology. The national laboratory in Tennessee is the perfect place to house the Institute as it is home to several major DOE user facilities, including America’s fastest supercomputer, the world’s most intense pulsed neutron beam and high-performance facilities for electron/atom probe and scanning probe microscopy, mass spectrometry and optical, x-ray and chemical imaging. The integration is helpful for researchers when, for example, they use multiple imaging tools to explore a material, only to be confronted by an immense amount of resulting data. Now, capturing and analyzing those data streams can be done in real-time with high-performance computing capabilities and data experts right next door.
Yang B.,Oak Ridge National Laboratory |
Keum J.,Oak Ridge National Laboratory |
Ovchinnikova O.S.,Oak Ridge National Laboratory |
Ovchinnikova O.S.,Institute for Functional Imaging of Materials |
And 8 more authors.
Journal of the American Chemical Society | Year: 2016
Organometallic halide perovskites (OHPs) hold great promise for next-generation, low-cost optoelectronic devices. During the chemical synthesis and crystallization of OHP thin films, a major unresolved question is the competition between multiple halide species (e.g., I-, Cl-, Br-) in the formation of the mixed-halide perovskite crystals. Whether Cl- ions are successfully incorporated into the perovskite crystal structure or, alternatively, where they are located is not yet fully understood. Here, in situ X-ray diffraction measurements of crystallization dynamics are combined with ex situ TOF-SIMS chemical analysis to reveal that Br- or Cl- ions can promote crystal growth, yet reactive I- ions prevent them from incorporating into the lattice of the final perovskite crystal structure. The Cl- ions are located in the grain boundaries of the perovskite films. These findings significantly advance our understanding of the role of halogens during synthesis of hybrid perovskites and provide an insightful guidance to the engineering of high-quality perovskite films, essential for exploring superior-performing and cost-effective optoelectronic devices. © 2016 American Chemical Society.