Nature Materials: Smallest lattice structure worldwide

The smallest lattice in the world is visible under the microscope only. Struts and braces are 0.2 µm in diameter. Total size of the lattice is about 10 µm. Photo: J. Bauer / KIT

KIT scientists now present the smallest lattice structure made by man in the Nature Materials journal. Its struts and braces are made of glassy carbon and are less than 1 µm long and 200 nm in diameter. They are smaller than comparable metamaterials by a factor of 5. The small dimension results in so far unreached ratios of strength to density. Applications as electrodes, filters or optical components might be possible. (DOI: 10.1038/nmat4561)

"Lightweight construction materials, such as bones and wood, are found everywhere in nature," Dr.-Ing. Jens Bauer of Karlsruhe Institute of Technology (KIT), the first author of the study, explains. "They have a high load-bearing capacity and small weight and, hence, serve as models for mechanical metamaterials for technical applications."

Metamaterials are materials, whose structures of some micrometers (millionths of a meter) in dimension are planned and manufactured specifically for them to possess mechanical or optical properties that cannot be reached by unstructured solids. Examples are invisibility cloaks that guide light, sound or heat around objects, materials that counterintuitively react to pressure and shear (auxetic materials) or lightweight nanomaterials of high specific stability (force per unit area and density).

The smallest stable lattice structure worldwide presented now was produced by the established 3D laser lithography process at first. The desired structure of micrometer size is hardened in a photoresist by laser beams in a computer-controlled manner. However, resolution of this process is limited, such that struts of about 5 - 10 µm length and 1 µm in diameter can be produced only. In a subsequent step, the structure was therefore shrunk and vitrified by pyrolysis. For the first time, pyrolysis was used for manufacturing microstructured lattices. The object is exposed to temperatures of around 900°C in a vacuum furnace. As a result, chemical bonds reorient themselves. Except for carbon, all elements escape from the resist. The unordered carbon remains in the shrunk lattice structure in the form of glassy carbon. The resulting structures were tested for stability under pressure by the researchers.

"According to the results, load-bearing capacity of the lattice is very close to the theoretical limit and far above that of unstructured glassy carbon," Prof. Oliver Kraft, co-author of the study, reports. Until the end of last year, Kraft headed the Institute for Applied Materials of KIT. This year, he took over office as KIT Vice President for Research. "Diamond is the only solid having a higher specific stability."

Microstructured materials are often used for insulation or shock absorption. Open-pored materials may be used as filters in chemical industry. Metamaterials also have extraordinary optical properties that are applied in telecommunications. Glassy carbon is a high-technology material made of pure carbon. It combines glassy, ceramic properties with graphite properties and is of interest for use in electrodes of batteries or electrolysis systems.

KIT

SLAC's ultrafast 'electron camera' visualizes ripples in 2-D material

Researchers have used SLAC’s experiment for ultrafast electron diffraction (UED), one of the world’s fastest “electron cameras,” to take snapshots of a three-atom-thick layer of a promising material as it wrinkles in response to a laser pulse. Understanding these dynamic ripples could provide crucial clues for the development of next-generation solar cells, electronics and catalysts. (SLAC National Accelerator Laboratory)

Understanding Motions of Thin Layers May Help Design Solar Cells, Electronics and Catalysts of the Future

New research led by scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University shows how individual atoms move in trillionths of a second to form wrinkles on a three-atom-thick material. Revealed by a brand new “electron camera,” one of the world’s speediest, this unprecedented level of detail could guide researchers in the development of efficient solar cells, fast and flexible electronics and high-performance chemical catalysts.

The breakthrough, accepted for publication Aug. 31 in Nano Letters, could take materials science to a whole new level. It was made possible with SLAC’s instrument for ultrafast electron diffraction (UED), which uses energetic electrons to take snapshots of atoms and molecules on timescales as fast as 100 quadrillionths of a second.

“This is the first published scientific result with our new instrument,” said scientist Xijie Wang, SLAC’s UED team lead. “It showcases the method’s outstanding combination of atomic resolution, speed and sensitivity.”

SLAC Director Chi-Chang Kao said, “Together with complementary data from SLAC’s X-ray laser Linac Coherent Light Source, UED creates unprecedented opportunities for ultrafast science in a broad range of disciplines, from materials science to chemistry to the biosciences.” LCLS is a DOE Office of Science User Facility.

This animation explains how researchers use high-energy electrons at SLAC to study faster-than-ever motions of atoms and molecules relevant to important materials properties and chemical processes.

Extraordinary Material Properties in Two Dimensions

Monolayers, or 2-D materials, contain just a single layer of molecules. In this form they can take on new and exciting properties such as superior mechanical strength and an extraordinary ability to conduct electricity and heat. But how do these monolayers acquire their unique characteristics? Until now, researchers only had a limited view of the underlying mechanisms.

“The functionality of 2-D materials critically depends on how their atoms move,” said SLAC and Stanford researcher Aaron Lindenberg, who led the research team. “However, no one has ever been able to study these motions on the atomic level and in real time before. Our results are an important step toward engineering next-generation devices from single-layer materials.” The research team looked at molybdenum disulfide, or MoS₂, which is widely used as a lubricant but takes on a number of interesting behaviors when in single-layer form – more than 150,000 times thinner than a human hair.

For example, the monolayer form is normally an insulator, but when stretched, it can become electrically conductive. This switching behavior could be used in thin, flexible electronics and to encode information in data storage devices. Thin films of MoS₂ are also under study as possible catalysts that facilitate chemical reactions. In addition, they capture light very efficiently and could be used in future solar cells.

Because of this strong interaction with light, researchers also think they may be able to manipulate the material’s properties with light pulses.

“To engineer future devices, control them with light and create new properties through systematic modifications, we first need to understand the structural transformations of monolayers on the atomic level,” said Stanford researcher Ehren Mannebach, the study’s lead author.Visualization of laser-induced motions of atoms (black and yellow spheres) in a molybdenum disulfide monolayer: The laser pulse creates wrinkles with large amplitudes – more than 15 percent of the layer’s thickness – that develop in a trillionth of a second. (K.-A. Duerloo/Stanford)

Electron Camera Reveals Ultrafast Motions

Previous analyses showed that single layers of molybdenum disulfide have a wrinkled surface. However, these studies only provided a static picture. The new study reveals for the first time how surface ripples form and evolve in response to laser light.

Researchers at SLAC placed their monolayer samples, which were prepared by Linyou Cao’s group at North Carolina State University, into a beam of very energetic electrons. The electrons, which come bundled in ultrashort pulses, scatter off the sample’s atoms and produce a signal on a detector that scientists use to determine where atoms are located in the monolayer. This technique is called ultrafast electron diffraction.

The team then used ultrashort laser pulses to excite motions in the material, which cause the scattering pattern to change over time.

“Combined with theoretical calculations, these data show how the light pulses generate wrinkles that have large amplitudes – more than 15 percent of the layer’s thickness – and develop extremely quickly, in about a trillionth of a second. This is the first time someone has visualized these ultrafast atomic motions,” Lindenberg said.

Once scientists better understand monolayers of different materials, they could begin putting them together and engineer mixed materials with completely new optical, mechanical, electronic and chemical properties.

The research was supported by DOE’s Office of Science, the SLAC UED/UEM program development fund, the German National Academy of Sciences, and the U.S. National Science Foundation.

To study ultrafast atomic motions in a single layer of molybdenum disulfide, researchers followed a pump-probe approach: They excited motions with a laser pulse (pump pulse, red) and probed the laser-induced structural changes with a subsequent electron pulse (probe pulse, blue). The electrons of the probe pulse scatter off the monolayer’s atoms (blue and yellow spheres) and form a scattering pattern on the detector – a signal the team used to determine the monolayer structure. By recording patterns at different time delays between the pump and probe pulses, the scientists were able to determine how the atomic structure of the molybdenum disulfide film changed over time. (SLAC National Accelerator Laboratory)

SLAC

Will 2-D Tin be the Next Super Material?

Stanene Lattice

Adding fluorine atoms (yellow) to a single layer of tin atoms (grey) 
should allow a predicted new material, stanene, to conduct 
electricity perfectly along its edges (blue and red arrows) at  
temperatures up to 100 degrees Celsius (212 Fahrenheit). 

(Yong Xu/Tsinghua University; Greg Stewart/SLAC)

Theorists Predict New Single-Layer Material Could Go Beyond Graphene, Conducting Electricity with 100 Percent Efficiency at Room Temperature

A single layer of tin atoms could be the world’s first material to conduct electricity with 100 percent efficiency at the temperatures that computer chips operate, according to a team of theoretical physicists led by researchers from the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory and Stanford University.

Researchers call the new material "stanene," combining the Latin name for tin (stannum) with the suffix used in graphene, another single-layer material whose novel electrical properties hold promise for a wide range of applications.

"Stanene could increase the speed and lower the power needs of future generations of computer chips, if our prediction is confirmed by experiments that are underway in several laboratories around the world," said the team leader, Shoucheng Zhang, a physics professor at Stanford and the Stanford Institute for Materials and Energy Sciences (SIMES), a joint institute with SLAC. The team’s work was published recently in Physical Review Letters.

The Path to Stanene

For the past decade, Zhang and colleagues have been calculating and predicting the electronic properties of a special class of materials known as topological insulators, which conduct electricity only on their outside edges or surfaces and not through their interiors. When topological insulators are just one atom thick, their edges conduct electricity with 100 percent efficiency. These unusual properties result from complex interactions between the electrons and nuclei of heavy atoms in the materials.

“The magic of topological insulators is that by their very nature, they force electrons to move in defined lanes without any speed limit, like the German autobahn,” Zhang said. “As long as they’re on the freeway – the edges or surfaces – the electrons will travel without resistance.”

In 2006 and 2009, Zhang’s group predicted that mercury telluride and several combinations of bismuth, antimony, selenium and tellurium should be topological insulators, and they were soon proven right in experiments performed by others. But none of those materials is a perfect conductor of electricity at room temperature, limiting their potential for commercial applications.

Earlier this year, visiting scientist Yong Xu, who is now at Tsinghua University in Beijing, collaborated with Zhang’s group to consider the properties of a single layer of pure tin.

“We knew we should be looking at elements in the lower-right portion of the periodic table,” Xu said. “All previous topological insulators have involved the heavy and electron-rich elements located there.”

Their calculations indicated that a single layer of tin would be a topological insulator at and above room temperature, and that adding fluorine atoms to the tin would extend its operating range to at least 100 degrees Celsius (212 degrees Fahrenheit).

Ultimately a Substitute for Silicon?

Zhang said the first application for this stanene-fluorine combination could be in wiring that connects the many sections of a microprocessor, allowing electrons to flow as freely as cars on a highway. Traffic congestion would still occur at on- and off-ramps made of conventional conductors, he said. But stanene wiring should significantly reduce the power consumption and heat production of microprocessors.

Manufacturing challenges include ensuring that only a single layer of tin is deposited and keeping that single layer intact during high-temperature chip-making processes.

“Eventually, we can imagine stanene being used for many more circuit structures, including replacing silicon in the hearts of transistors,” Zhang said. “Someday we might even call this area Tin Valley rather than Silicon Valley.”

Additional contributors included researchers from Tsinghua University in Beijing and the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany. The research was supported by the Mesodynamic Architectures program of the Defense Advanced Research Projects Agency.

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science. To learn more, please visitwww.slac.stanford.edu.

The Stanford Institute for Materials and Energy Sciences (SIMES) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. SIMES studies the nature, properties and synthesis of complex and novel materials in the effort to create clean, renewable energy technologies. For more information, please visit simes.slac.stanford.edu.

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 visit science.energy.gov.

Source: https://www6.slac.stanford.edu/news/2013-11-21-tin-super-material-stanene.aspx

Clay key to high-temperature supercapacitors

Rice University lab creates energy storage that may find use in oil discovery, space, military applications 

Clay, an abundant and cheap natural material, is a key ingredient in a supercapacitor that can operate at very high temperatures, according to Rice University researchers who have developed such a device.

The Rice group of materials scientist Pulickel Ajayan reported in Nature’s online journal, Scientific Reports, that the supercapacitor is reliable at temperatures of up to 200 degrees Celsius (392 degrees Fahrenheit) and possibly beyond. It could be useful for powering devices for use in extreme environments, such as oil drilling, the military and space.

A composite of clay and an electrolyte allowed Rice University researchers to make sheets of material that can serve as both electrolyte and a separator in a new kind of high-temperature supercapacitor. Images courtesy of the Ajayan Group

“Our intention is to completely move away from conventional liquid or gel-type electrolytes, which have been limited to low-temperature operation of electrochemical devices,” said Arava Leela Mohana Reddy, lead author and a former research scientist at Rice.

“We found that a clay-based membrane electrolyte is a game-changing breakthrough that overcomes one of the key limitations of high-temperature operation of electrochemical energy devices,” Reddy said. “By allowing safe operation over a wide range of temperatures without compromising on high energy, power and cycle life, we believe we can dramatically enhance or even eliminate the need for expensive thermal management systems.”

A supercapacitor combines the best qualities of capacitors that charge in seconds and discharge energy in a burst and rechargeable batteries that charge slowly but release energy on demand over time. The ideal supercapacitor would charge quickly, store energy and release it as needed.

“Researchers have been trying for years to make energy storage devices like batteries and supercapacitors that work reliably in high-temperature environments, but this has been challenging, given the traditional materials used to build these devices,” Ajayan said.

In particular, researchers have struggled to find an electrolyte, which conducts ions between a battery’s electrodes, that won’t break down when the heat is on. Another issue has been finding a separator that won’t shrink at high temperatures and lead to short circuits. (The separator keeps the electrolyte on the anode and cathode sides of a traditional battery apart while allowing ions to pass through).

“Our innovation has been to identify an unconventional electrolyte/separator system that remains stable at high temperatures,” Ajayan said.

The Rice researchers led by Reddy and Rachel Borges solved both problems at once. First, they investigated using room-temperature ionic liquids (RTILs) developed in 2009 by European and Australian researchers. RTILs show low conductivity at room temperature but become less viscous and more conductive when heated.

Clay has high thermal stability, high sorption capacity, a large active surface area and high permeability, Reddy said, and is commonly used in muds for oil drilling, in modern construction, in medical applications and as a binder by iron and steel foundries.

After combining equal amounts of RTIL and naturally occurring Bentonite clay into a composite paste, the researchers sandwiched it between layers of reduced graphene oxide and two current collectors to form a supercapacitor. Tests and subsequent electron microscope images of the device showed no change in the materials after heating it to 200 degrees Celsius. In fact, Reddy said, there was very little change in the material up to 300 degrees Celsius.

“The ionic conductivity increases almost linearly until the material reaches 180 degrees, and then saturates at 200,” he said.

Despite a slight drop in capacity observed in the initial charge/discharge cycles, the supercapacitors were stable through 10,000 test cycles. Both energy and power density improved by two orders of magnitude as the operating temperature increased from room temperature to 200 degrees Celsius, the researchers found.

The team took its discovery a step further and combined the RTIL/clay with a small amount of thermoplastic polyurethane to form a membrane sheet that can be cut into various shapes and sizes, which allows design flexibility for devices.

Co-authors of the paper are graduate students Marco-Tulio Rodrigues and Hemtej Gullapalli and former postdoctoral researcher Kaushik Balakrishnan, all of Rice; and Glaura Silva, an associate professor at the Federal University of Minas Gerais, Belo Horizonte, Brazil. Ajayan is the Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry at Rice. Borges is a visiting student from the Federal University of Minas Gerais. Reddy is now an assistant professor at Wayne State University in Detroit.

The Advanced Energy Consortium supported the research.

http://news.rice.edu/2013/09/03/clay-key-to-high-temperature-supercapacitors-2/

New Evidence to Aid Search for Charge 'Stripes' in Superconductors

Findings identify signature that will help scientists investigate and understand materials that carry current with no resistance

Scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have identified a series of clues that particular arrangements of electrical charges known as "stripes" may play a role in superconductivity—the ability of some materials to carry electric current with no energy loss. But uncovering the detailed relationship between these stripe patterns and the appearance or disappearance of superconductivity is extremely difficult, particularly because the stripes that may accompany superconductivity are very likely moving, or fluctuating.

As a step toward solving this problem, the Brookhaven team used an indirect method to detect fluctuating stripes of charge density in a material closely related to a superconductor. The research, described in a paper published online in Physical Review Letters August 30, 2013, identifies a key signature to look for in superconductors as scientists seek ways to better understand and engineer these materials for future energy-saving applications.

"In previous experiments, we've seen evidence of fluctuating "magnetic spin" stripes—patterns of how adjacent atoms' spin directions are arranged—that are compatible with superconductivity," said Brookhaven physicist John Tranquada, a senior collaborator on the research team. "Now we're trying to look at the arrangements of charge density, to see if there are alternating stripes of densely and more loosely packed charges. The charge part is harder to see."

To get an idea of the difficulty of tracking moving stripes, think of the cars in a supermarket parking lot. The lines delineating the parking spots are like the positions of atoms making up a crystal, and the cars are like the electrons. If there's a pattern to the arrangement—say alternating colors of cars in adjacent spots—it would be easy to spot in a single snapshot. But if you took a single photo (with a very long exposure) over the course of a busy shopping day as cars moved into and out of spots, all you'd see is a blur. You wouldn't be able to tell if they continued to park in alternating order, if the details of the parking pattern were changing, or even whether there was a pattern at all. 

A series of individual snapshots might make the details more discernable. But in the case of analyzing materials science samples, the "snapshots" are often made with very intense x-rays or neutron beams.  And access to beam time at these imaging facilities is limited, and expensive. "You can't throw enough 'light' on the problem to see it," Tranquada said.

Instead, the scientists tried a completely different approach. Rather than looking directly at the stripes they looked for a telltale signal that indicates the presence of the stripes by association, but in a different measurement that can be done quickly and with much less precious beam time. They started these experiments on a material they knew had a static striped pattern below a certain temperature to make sure that the signal was evident in this new measurement. They then studied what happened as the temperature rose to see whether the stripes would either disappear or persist but start to move.

The scientists ground up crystals of the test material into a fine powder and placed samples of it in line with a beam of neutrons at the Los Alamos Neutron Scattering Center at Los Alamos National Laboratory. Similar to the way light reflecting off an object enters your eyes to create an image, the neutron beams diffracted by the crystals' atoms yield information about the positions of the atoms. The scientists used that information to infer the material's electronic structure, and repeated the experiment at gradually warmer temperatures.

"We're looking at the average crystal structure, the height-to-width aspect ratio of that structure, and how different the positions of the atoms are from that average," said Milinda Abeykoon, lead author on the paper. 

In the static striped arrangement, the atoms are displaced from the average in a regular way—like parking spots that are alternatingly wider or narrower than average. Such atomic displacements force the electrons to follow a stripe-ordered arrangement—the way smaller cars would fill the narrow parking spots alternating with wider SUVs.

With increasing temperature, the scientists found that while the aspect ratio of the crystal structure changed, the displacements from average structure persisted, leading them to conclude by inference that the striped pattern of charge density also remained, even though it was no longer static.

"This is the first powder diffraction scattering evidence for fluctuating charge stripes above the temperature where we see static order," said co-author Simon Billinge, referring to the new measurement. Billinge, who holds a joint appointment with Brookhaven Lab and Columbia University's School of Engineering and Applied Science, leads the collaboration that performed this study.

"One of the most critical aspects of this experiment is that we had lots of different data points, lots of temperatures—so you can catch small deviations," said co-author Emil Bozin of Brookhaven. He also noted how improvements in detector technology made it possible to collect a lot of data within a fixed amount of time. "Ten years ago we would have needed a couple of weeks of beam time to do this experiment; we collected all our data in just a few days."

The next step: Return to searching for stripes in superconductors. "This model system teaches us what diffraction-scattering signature to look for in copper-based superconductors to see if these fluctuations exist," Bozin said.

That search should lead to better understanding of the role of stripes in superconductivity, and possibly to new approaches to engineer advanced superconductors for energy applications.

http://www.bnl.gov/newsroom/news.php?a=11570