Effects of atomic-scale roughness on adhesion between diamond surfaces

CNST Project Leader Rachel Cannara and collaborators from the United States Naval Academy (USNA) and the University of Pennsylvania have shown that atomic-scale surface roughness has a strong influence on adhesion for diamond, amorphous carbon, and model diamond nanocomposites.

Usingatomic force microscope(AFM) measurements performed at the University of Wisconsin-Madison, molecular dynamics (MD) simulations, and ab initiodensity functional theory(DFT), they investigated the adhesive physics and mechanics of nanoscale interfaces between diamond surfaces.

For atomically smooth surfaces, the greater density of atoms in the (111) plane would be expected to lead to a higher electrostaticdipole momentper unit area and a higher work of adhesion than the (001) orientation. However, the AFM measurements, supported by detailed simulations of model diamond nanocomposites, challenge this assumption in a way that can only be explained by variations in atomic-level surface roughness, which for single crystals can arise from orientation-dependent growth mechanisms.

Unlike previous ab initio studies that compared surface energies for diamond (111)(1×1)-H surfaces and unreconstructed diamond (001)(1×1)-H surfaces, the MD simulations performed at USNA simulate the (2×1)-reconstructed C(001)-H surface. The simulations predict that the C(001)(2x1)-H surface is energetically favorable to the unreconstructed surface. Corroborating these simulations, high-precision AFM lateral force images of the (001) surface revealed (2×1) dimer-row domains.

In addition to using the appropriate (001) surface structure, the MD simulations account for long-range van der Waals interactions, as well as surface energies, when calculating the work of adhesion for each interface. Moreover, the ab initio DFT calculations reveal the presence of bond dipoles on single-crystal diamond surfaces.

Using AFM, the contact mechanics of the interface was extracted from the load-dependence of the contact area during sliding friction experiments. Works ofadhesionwere then calculated from the appropriate contact mechanics model and from pull-off forces measured during the sliding experiments and quasistatic force-displacement measurements. These results have broad implications for the design of MEMS/NEMS devices that incorporate diamond or diamond-like materials.

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Self-assembling structures open door to new class of materials

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Researchers at the University of Illinois and Northwestern University have demonstrated bio-inspired structures that self-assemble from simple building blocks: spheres.

The helical"supermolecules"are made of tiny colloid balls instead of atoms or molecules. Similar methods could be used to makenew materialswith the functionality of complex colloidal molecules. The team will publish its findings in the Jan. 14 issue of the journalScience.

"We can now make a whole new class ofsmart materials, which opens the door to new functionality that we couldn't imagine before,"said Steve Granick, Founder Professor of Engineering at the University of Illinois and a professor of materials science and engineering, chemistry, and physics.

Granick's team developed tiny latex spheres, dubbed"Janus spheres,"which attract each other in water on one side, but repel each other on the other side. The dual nature is what gives the spheres their ability to form unusual structures, in a similar way to atoms and molecules.

In pure water, the particles disperse completely because their charged sides repel one another. However, when salt is added to the solution, the salt ions soften therepulsionso the spheres can approach sufficiently closely for their hydrophobic ends to attract. The attraction between those ends draws the spheres together into clusters.

At low salt concentrations, small clusters of only a few particles form. At higher levels, larger clusters form, eventually self-assembling into chains with an intricate helical structure.

"Just like atoms growing into molecules, these particles can grow into supracolloids,"Granick said."Such pathways would be very conventional if we were talking about atoms and molecules reacting with each other chemically, but people haven't realized that particles can behave in this way also."

The team designed spheres with just the right amount of attraction between their hydrophobic halves so that they would stick to one another but still be dynamic enough to allow for motion, rearrangement, and cluster growth.

"The amount of stickiness really does matter a lot. You can end up with something that's disordered, just small clusters, or if the spheres are too sticky, you end up with a globular mess instead of these beautiful structures,"said graduate student Jonathan Whitmer, a co-author of the paper.

One of the advantages of the team's supermolecules is that they are large enough to observe in real time using a microscope. The researchers were able to watch the Janus spheres come together and the clusters grow– whether one sphere at a time or by merging with other small clusters– and rearrange into different structural configurations the team calls isomers.

"We design these smart materials to fall into useful shapes that nature wouldn't choose,"Granick said.

Surprisingly, theoretical calculations and computer simulations by Erik Luijten, Northwestern University professor of materials science and engineering and of engineering sciences and applied mathematics, and Whitmer, a student in his group, showed that the most common helical structures are not the most energetically favorable. Rather, the spheres come together in a way that is the most kinetically favorable– that is, the first good fit that they encounter.

Next, the researchers hope to continue to explore the colloid properties with a view toward engineering more unnatural structures. Janus particles of differing sizes or shapes could open the door to building other supermolecules and to greater control over their formation.

"These particular particles have preferred structures, but now that we realize the general mechanism, we can apply it to other systems– smallerparticles, different interactions– and try to engineer clusters that switch in shape,"Granick said.

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Cracking a tooth: 3-D map of atoms sheds light on nanoscale interfaces in teeth, may aid materials design

Teeth and bone are important and complex structures in humans and other animals, but little is actually known about their chemical structure at the atomic scale. What exactly gives them their renowned toughness, hardness and strength? How do organisms control the synthesis of these advanced functional composites?

Now, using a highly sophisticated atomic-scale imaging tool on a sea creature's tooth, two Northwestern University researchers have peeled away some of the mystery of organic/inorganic interfaces that are at the heart of tooth andbone structure. They are the first to produce a three-dimensional map of the location and identity of millions of individual atoms in the complex hybrid material that allows the animal to literally chew rock.

Demonstrating that atom-probe tomography (APT) can be used to interrogate such materials opens up the possibility of tracking fluoride in teeth and cancer and osteoporosis drugs in bone (at previously inaccessible length scales). The detailed knowledge of organic/inorganic interfaces also will help scientists rationally design usefulnew materials-- flexible electronics, polymers and nanocomposite materials, such as organicphotovoltaics-- that combine the best properties of organic andinorganic materials.

The results will be published Jan. 13 by the journalNature.

"The interface between the organic and inorganic materials plays a large role in controlling properties and structure,"said Derk Joester, senior author of the paper."How do organisms make and control these materials? We need to understand this architecture on the nanoscale level to design new materials intelligently. Otherwise we really have no idea what is going on."

Joester is the Morris E. Fine Junior Professor in Materials and Manufacturing at the McCormick School of Engineering and Applied Science. Lyle Gordon, a doctoral student in Joester's lab, is the other author of the paper.

The two set out to find the organic fibers they knew to be an important part of the tooth's structure, buried in the tough outer layer of the tooth, made of magnetite. Their quantitative mapping of the tooth shows that the carbon-based fibers, each 5 to 10 nanometers in diameter, also contained either sodium or magnesium ions. Joester and Gordon are the first to have direct proof of the location, dimension and chemical composition of organic fibers inside the mineral.

They were surprised by the chemical heterogeneity of the fibers, which hints at how organisms modulate chemistry at the nanoscale. Joester and Gordon are anxious to learn more about how the organic fibers interface with the inorganic minerals, which is key to understanding hybrid materials.

"The tooth's toughness comes from this mix of organic and inorganic materials and the interfaces between them,"Joester said."While this is in principle well known, it is intriguing to think we may have overlooked how subtle changes in the chemical makeup of nanoscale interfaces may play a role in, for instance, bone formation or the diffusion of fluoride into tooth enamel. In this regard, atom-probe tomography has the potential to revolutionize our understanding."

Atom-probe tomography (APT) produces an atom-by-atom, 3-D reconstruction of a sample with sub-nanometer resolution. But many in the field didn't think APT would work to analyze a material made up of organic and inorganic parts.

Fortunately for Joester and Gordon, Northwestern has both David Seidman, a leader in the field who uses APT to study metals, and two of the few APT instruments in the country. (There are less than a dozen.) Seidman, Walter P. Murphy Professor of Materials Science and Engineering, encouraged Joester to take the risk and use APT to study biological architectures. The scientists also were able to exchange ideas with the engineers developing 3-D atom-probe instruments at CAMECA, a scientific instrumentation company in nearby Madison, Wis.

Joester and Gordon imaged teeth of the chiton, a tiny marine mollusk, because much is known about the biomineralization process. The chiton lives in the sea and feeds on algae found on rocks. It continually makes new rows of teeth -- one a day -- to replace mature but worn teeth; in conveyor-belt fashion, the older teeth move down the creature's tongue-like radula toward the mouth where it feeds.

Chiton teeth resemble human teeth in that they have a hard and tough outer layer -- equivalent to our enamel -- and a softer core. Instead of enamel, the rock-chewing chitons use magnetite, a very hard iron oxide, which gives their teeth a black luster.

The researchers extracted micron-sized samples from the leading edge of the tooth. Using a focused ion beam tool at the Northwestern University Atomic and Nanoscale Characterization Experimental Center core facility, these samples were fashioned into very sharp tips (less than 20 nanometers across). The process is reminiscent of sharpening a pencil, albeit with a supercharged stream of gallium ions.

The APT technique applies an extremely high electric field to the sample; atoms on the surface ionize, fly off and hit an imaging detector (similar to those found in night-vision equipment). The atoms are stripped off atom-by-atom and layer-by-layer, like peeling an onion. Computer methods then are used to calculate the original location of the atoms, producing a 3-D map or tomogram of millions of atoms within the sample.

Joester and Gordon now are studying the tooth enamel of a vertebrate and plan to apply APT to bone, which is also made of organic and inorganic parts, to learn more about its nanoscale structure.

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Electron gas on insulator's surface opens way to multifunctional transistors

French researchers have succeeded in creating a conductive layer on the surface of strontium titanate (SrTiO₃), a transparent insulating material considered to be very promising for the development of future microelectronics applications. Two nanometers thick, this conductive layer is a two-dimensional metallic electron gas (2DEG) that is part of the insulating material. Easy to produce, it opens new possibilities for electronics based on transition metal oxides (the SrTiO₃family), taking advantage of these materials' vast range of physical properties (superconductivity, magnetism, thermoelectricity, etc.) to integrate a number of different functions in a single microelectronic device. A paper explaining this unexpected discovery, arising from research at the SOLEIL synchrotron, was published in the January 13, 2011 issue of<i>Nature</i>magazine.

Today's microelectronic components consist of layers of semiconductors on asilicon substrate. In order to sustain the pace of periodic upgrades in the performance of microelectronic devices beyond 2020, alternative technological solutions are being investigated. Researchers are increasingly turning their attention to transition metal oxides , which offer promising physical properties such as superconductivity, magnetoresistance, thermoelectricity, multiferroicity and photocatalytic capacity.

Within this family of materials, strontium titanate (SrTiO₃) has been the subject of extensive research. Thisinsulating materialbecomes a good conductor when it is doped, for example by creating a few surface oxygen vacancies. The interfaces between SrTiO3 and other oxides (LaTiO3 or LaAlO3) are conductive, even though the two materials are insulators. Moreover, they offer properties like superconductivity,magnetoresistanceand thermoelectricity, with very good performances at room temperature. The problem, however, is that interfaces between oxides are very difficult to produce.

Now an unexpected discovery has burst through this technological barrier. An international team led by researchers at CNRS and Université Paris-Sud 11 has produced a two-dimensional metallicelectron gas(2DEG) on the surface of SrTiO₃. This conductive layer, approximately two nanometers thick, was obtained by vacuum-cleaving a piece of strontium titanate, a very simple and economical process. The constituent elements of SrTiO₃are natural resources available in large quantities, and the compound is non-toxic, unlike the materials most widely used in microelectronics today (bismuth tellurides). In addition, 2DEGs could probably be created on the surface of other transition metal oxides using a similar technique.

The discovery of a conductive layer of this type (not requiring the addition of a layer of another material) is a significant step forward for oxide-based microelectronics. It could make it possible to combine the intrinsic multifunctional properties of transition metal oxides with those of the two-dimensional metal on their surface. Possible developments could include the coupling of a ferroelectric oxide with the electron gas on its surface to produce non-volatile memories, or the inclusion of transparent circuits on the surface of solar cells or touch screens.

The 2DEG on the surface of strontium titanate was identified and studied in experiments using angle-resolved photoemission spectroscopy (ARPES) at the SOLEIL synchrotron in Saint-Aubin, France, and the Synchrotron Radiation Center at the University of Wisconsin, USA.

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Researchers settle argument over mobility of flexible filaments (w/ Video)

(PhysOrg.com) -- Theo Odijk, you win. The professor of biotechnology at Delft University of Technology in the Netherlands has a new best friend in Rice University's Matteo Pasquali.

Together with collaborators at the French National Center for Scientific Research (CNRS), the University of Bordeaux, France, and Vrije University, Amsterdam, the Rice professor and his team have settled a long-standing controversy in the field of polymer dynamics: The researchers proved once and for all that Odijk was correct in proclaiming that a little flexibility goes a long way for stifffilamentsin a solution.

The study in the current issue of the journalScienceshows that even a small ability to bend givesnanotubesand other tiny, stiff filaments the means to navigate through crowded environments, or even such fixed networks as cell matrices.

The work by Pasquali, a professor in chemical and biomolecular engineering and in chemistry, may bring about new ways to influence the motion of tiny filaments by tailoring their stiffness for a given environment.

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A video of a reptating nanotube

Nanotubes are being studied for potential use in all kinds of sensing, even in the seemingly disparate fields of biological applications and oil exploration. In both, the ability of nanotubes and other fine, filamentous particles to move through their environments is critical, Pasquali said.

Understanding the motion of a single,flexible polymerchain in a network has been key to scientific advances by Odijk and others on, for example, the behavior of DNA. The Rice researchers expect their revelation to have no less impact.

Pasquali and lead author Nikta Fakhri, a former graduate student at Rice now doing postdoctoral research at the University of Gottingen, Germany, set out to break the deadlocked theories by Odijk and two other scientists who disagreed on the Brownian motion of stiff filaments in a crowded environment, and whether stiffness itself played any part.

"There's a long-standing, fundamental question: How does this threadlike object move when it gets crowded? It could be crowded because it's in a gel, or because there are a lot of threadlike objects with it -- which to that one object looks like a gel,"he said.

Crowding constrains the ability of a filament to travel. Think of trying to get from the back to the front of a crowded bus; it takes a certain amount of agility to weave your way through the packed bodies."It turns out that with a little flexibility, a filament can explore the space around it much more effectively,"Pasquali said.

That becomes important when the goal is to get filaments to find and enter a cellular pore to deliver a dose of medication or to act as a fluorescent sensor.

"If you look at the human body, they say we're made of 60 percent water, but we don't slosh around,"Pasquali explained."That's because the water is trapped in pores. Almost all the water in our body is in gel-like structures: inside our cells, which are laden with filamentous networks, or in the interstitial fluid surrounding these cells. We are a big, squishy, porous medium. We need to understand how the nanoparticles move in this medium."

Pasquali and Fakhri mimicked biological networks by using varying concentrations of agarose gel, a porous material often used as a filter in biochemistry and molecular biology for DNA and proteins. The gel forms a matrix of controllable size through which molecules can move.

Nanotubes served as a stand-in for any type of filament, albeit one whose stiffness can be controlled. Like a PVC pipe in the macro world, nanotubes get stiffer as they get thicker; but even the stiffest tubes can flex a bit with length, and these tubes were thousands of times longer than they were wide.

The study started somewhat serendipitously when co-author Laurent Cognet, a researcher at CNRS and the University of Bordeaux, tried to immobilize nanotubes in agarose gels. He noticed in a failed experiment that the nanotubes moved in a"funny way"and discussed it with Pasquali.

Pasquali asked whether the nanotubes were reptating -- scientist lingo for a snakelike motion -- and Cognet said yes. Fakhri, who was studying the dynamics of nanotubes, traveled to the Bordeaux laboratory of Cognet and co-author Brahim Lounis to capture images of the nanotubes in motion.

The resulting spectroscopic and direct still and video images of 35 fluorescent single-walled nanotubes showed them snaking through the gel, probing pores and paths. The nanotubes, like all filaments, obeyed the rules of thermal-induced Brownian motion; they were pushed and pulled by the ever-changing states of the molecules around them.

The research established that flexibility significantly enhances the nanotubes' ability to navigate around obstacles and speeds up their exploration.

Pasquali said Fakhri doggedly pursued her analysis of the nanotubes' motion through computerized image recognition and motion tracking, as well as old-fashioned pencil-and-paper dynamical analysis. He said his longtime collaborator, co-author Frederick MacKintosh, a theoretical physicist at Vrije University, was a tremendous help. MacKintosh has been studying the dynamics of biological networks for nearly two decades.

Pasquali intends to replace the gel with real rocks to see how nanotubes, which can be used as oil-detecting sensors, move in a more structured environment."Rocks can be a little more complicated,"he said."The question here is, what can nanotubes do better than nanoparticles? The answer may be that slender nanotubes may interact with electromagnetic fields more strongly than other nanoparticles of the same volume."

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T-Shirt replaces battery: Fiber-based electrochemical micro-supercapacitor

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Will we soon be plugging our mobile phone into our t-shirt instead of putting in a battery? This vision is not totally out of reach: the first steps in this direction have already been taken.

Now a team led by Zhong Lin Wang at the Georgia Institute of Technology (Atlanta, USA) and Jong Min Kim of Samsung Electronics in South Korea is introducing a prototype for a flexibleenergy storage devicethat can be worked into textiles. As the scientists report in the journalAngewandte Chemie, thissupercapacitoris made of a very special arrangement of zinc oxide nanowires grown on conventional fibers.

Although smaller, lighter components are constantly being developed, most devices forenergy generationand storage are much too bulky and heavy for increasingly miniaturized electronic devices of the future. Supercapacitors are an interesting alternative to batteries andrechargeable batteriesfor energy storage. They can be recharged almost endlessly and extremely fast; however, previous examples have not been flexible or light enough.

The research team has now developed a prototype for a high-efficiency fiber-based electrochemical micro-supercapacitor that uses zinc oxide nanowires as electrodes. The substrate for one of the electrode is a flexible, fine plastic wire; for the other electrode it is a fiber made of Kevlar. Kevlar is the material used to make bulletproof vests. The researchers were able to grow zinc oxide nanowires on each of these substrates. Additional coatings with materials like gold andmanganese oxidecould further improve the charge capacitance. Using tweezers, the researchers then wrapped each of the plastic wires with a Kevlar fiber. This assembly was then embedded in a solid gel electrolyte that separates the two electrodes and allows for the necessary charge transport. A bundle of these fibers could be processed to form a thread.

Zinc oxide has special advantages over conventional supercapacitor materials,: it can be grown on any desired substrate in any form at low temperature (below 100°C) and it is both biocompatible and environmentally friendly.

A particularly intriguing application would be the use of these new charge-storage media in combination with flexible fiber nanogenerators, which Wang and his team have previously developed. The wearer’s heartbeat and steps, or even a light wind, would be enough to move the piezoelectriczinc oxidenanowiresin the fibers, generating electrical current.

In the form of a"power shirt"such a system could deliver enough current for small electronic devices, such as mobile phones or small sensors like those used to warn firemen of toxins.

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'Nanoscoops' could spark new generation of electric automobile batteries

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An entirely new type of nanomaterial developed at Rensselaer Polytechnic Institute could enable the next generation of high-power rechargeable lithium (Li)-ion batteries for electric automobiles, as well as batteries for laptop computers, mobile phones, and other portable devices.

The new material, dubbed a"nanoscoop"because its shape resembles a cone with a scoop of ice cream on top, can withstand extremely high rates of charge and discharge that would cause conventional electrodes used in today's Li-ion batteries to rapidly deteriorate and fail. The nanoscoop's success lies in its unique material composition, structure, and size.

The Rensselaer research team, led by Professor Nikhil Koratkar, demonstrated how a nanoscoop electrode could be charged and discharged at a rate 40 to 60 times faster than conventional battery anodes, while maintaining a comparable energy density. This stellar performance, which was achieved over 100 continuous charge/discharge cycles, has the team confident that their new technology holds significant potential for the design and realization of high-power, high-capacity Li-ion rechargeable batteries.

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Researchers at Rensselaer Polytechnic Institute developed an entirely new type of nanomaterial that could enable the next generation of high-power rechargeable lithium (Li)-ion batteries for electric automobiles, laptop computers, mobile phones and other devices. The material, called a"nanoscoop"because it resembles a cone with a scoop of ice cream on top, is shown in the above scanning electron microscope image. Nanoscoops can withstand extremely high rates of charge and discharge that would cause today's Li-ion batteries to rapidly deteriorate and fail. Credit: Rensselaer/Koratkar

"Charging my laptop or cell phone in a few minutes, rather than an hour, sounds pretty good to me,"said Koratkar, a professor in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer."By using our nanoscoops as the anode architecture for Li-ionrechargeable batteries, this is a very real prospect. Moreover, this technology could potentially be ramped up to suit the demanding needs of batteries for electric automobiles."

Batteries for all-electric vehicles must deliver high power densities in addition to high energy densities, Koatkar said. These vehicles today use supercapacitors to perform power-intensive functions, such as starting the vehicle and rapid acceleration, in conjunction with conventional batteries that deliver high energy density for normal cruise driving and other operations. Koratkar said the invention of nanoscoops may enable these two separate systems to be combined into a single, more efficient battery unit.

Results of the study were detailed in the paper"Functionally Strain-Graded Nanoscoops for High Power Li-Ion Battery Anodes,"published Thursday by the journalNano Letters.

The anode structure of a Li-ion battery physically grows and shrinks as the battery charges or discharges. When charging, the addition of Li ions increases the volume of the anode, while discharging has the opposite effect. These volume changes result in a buildup of stress in theanode. Too great a stress that builds up too quickly, as in the case of a battery charging or discharging at high speeds, can cause the battery to fail prematurely. This is why most batteries in today's portable electronic devices like cell phones and laptops charge very slowly– the slow charge rate is intentional and designed to protect the battery from stress-induced damage.

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Researchers at Rensselaer Polytechnic Institute developed an entirely new type of nanomaterial that could enable the next generation of high-power rechargeable lithium (Li)-ion batteries for electric automobiles, laptop computers, mobile phones and other devices. The material, called a"nanoscoop"because it resembles a cone with a scoop of ice cream on top, is shown in the above scanning electron microscope image. Nanoscoops can withstand extremely high rates of charge and discharge that would cause today's Li-ion batteries to rapidly deteriorate and fail. Credit: Rensselaer/Koratkar

The Rensselaer team's nanoscoop, however, was engineered to withstand this buildup of stress. Made from a carbon (C) nanorod base topped with a thin layer of nanoscale aluminum (Al) and a"scoop"of nanoscale silicon (Si), the structures are flexible and able to quickly accept and discharge Li ions at extremely fast rates without sustaining significant damage. The segmented structure of the nanoscoop allows the strain to be gradually transferred from the C base to the Al layer, and finally to the Si scoop. This natural strain gradation provides for a less abrupt transition in stress across the material interfaces, leading to improved structural integrity of the electrode.

The nanoscale size of the scoop is also vital since nanostructures are less prone to cracking than bulk materials, according to Koratkar.

"Due to their nanoscale size, our nanoscoops can soak and release Li at high rates far more effectively than the macroscale anodes used in today's Li-ion batteries,"he said."This means our nanoscoop may be the solution to a critical problem facing auto companies and other battery manufacturers– how can you increase the power density of a battery while still keeping theenergy densityhigh?"

A limitation of the nanoscoop architecture is the relatively low total mass of the electrode, Koratkar said. To solve this, the team's next steps are to try growing longer scoops with greater mass, or develop a method for stacking layers of nanoscoops on top of each other. Another possibility the team is exploring includes growing the nanoscoops on large flexible substrates that can be rolled or shaped to fit along the contours or chassis of the automobile.

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