Nano research fit for a king: Scientists test strength of composite bonds one nanotube at a time

(PhysOrg.com) -- Arthur pulled a sword from a stone, proving to a kingdom that right beats might. Researchers at Rice University are making the same point in the nanoscale realm.

In this case, the sword is a multiwalledcarbon nanotubeand the stone is a bead of epoxy.

Knowing precisely how much strength is needed to pull the nanotube from the bead is essential to materials scientists’ advancing the art of making stronger, lighter composites for everything from sporting goods to spacecraft.

A team led by Jun Lou, an assistant professor of mechanical engineering and materials science at Rice, and first author Yogeeswaran Ganesan, who recently earned his doctorate in Lou's lab, has published a paper in the American Chemical Society journal Applied Materials and Interfaces describing its work to measure the interface toughness of carbon nanotube-reinforced epoxy composites.

Lou, Ganesan and their colleagues have a second new paper inACS Nanoon using the same technique to measure the effect of nitrogen doping on the mechanical properties of carbon nanotubes.

Nanotubes are finding their way into products as manufacturers bank on their reputation for strength and lightness. One can buy baseball bats, tennis rackets and high-priced bicycles reinforced with nanotubes.

"Carbon nanotubes are so small (a strand of hair is 50,000 times wider) that in order to use them on the human scale, you have to do something to make them bigger,"Lou said.

One such way is to mix them into composites, an imperfect science that involves much trial and error since the possible strength of the interface between every type of nanotube and every type of base material is not well understood. Lou and his team intend to eliminate the guesswork with a way to measure important properties of a composite before the first batch is mixed.

"You don't want to spend a lot of time and money on a fancy chemical treatment without knowing what's happening at the critical interface,"Lou said.

Single-fiber pullout tests have been used since the early days of composite manufacturing to measure not only the strength of a bond but when, why and how it will break. That's hard on the nanoscale. Others have used atomic force microscopes as part of the pulling mechanism, but the method has its limitations, Lou said.

The Rice team has built a better device: a spring-loaded, push-pull micromechanical assembly on a silicon chip that allows researchers to string a multiwalled nanotube to a blanket of epoxy on one side while the other is held firmly in place with a platinum anchor. Pressing down on the spring applies equal force to both sides, allowing researchers to see just how much is needed to pull the tube from the epoxy.

The team reported in the first paper that forces binding multiwalled nanotubes to a general-purpose epoxy called Epon 828 were actually weaker than they expected."We have started to understand that adding nanotubes to bulk material doesn't always give you better properties,"Lou said."You have to be very careful about how you add them in and what kind interface they form."

Because batches of nanotubes tend to stick together, some manufacturers functionalize their surfaces to disperse them before mixing into a material."But that can disrupt the outer wall, and that's a bad thing,"Lou said."If you do something to make nanotubes easily dispersible but decrease their intrinsic strength, you're shooting yourself in the foot."

On the other hand, he said,"If manufacturers need a tough material that absorbs energy without breaking, a weaker interface may not be a bad thing. During this pullout process, there's a lot of friction at the interface of the nanotube and the matrix, and friction is effectively a way to dissipate energy."

Sometimes the end product is better if the nanotube stretches before it breaks. In theACS Nanopaper, the team compared the tensile strength of pristine versus nitrogen-doped multiwalled carbon nanotubes. They found the pristine tubes tend to snap in a brittle fashion, while nitrogen-doped tubes exhibit signs of plasticity --"necking"before they break.

That may be desirable for certain materials, Lou said."You don't build a bridge out of ceramic. You build it out of steel because of its plasticity.

"If we can develop a nanotube composite with room-temperature plasticity, it's going to be fantastic,"he said."It will find many, many uses."

Lou said Rice's versatile technique for carrying out nanomechanical experiments is poised to find many long-sought answers."Developing an ability to engineering nanocomposites with mechanical properties tailored for specific applications is the proverbial holy grail of all structural nanocomposite research,"Ganesan said."The technique essentially takes us one step closer to achieving this goal."

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Stanford scientists see the solar future, and it's all about 'nanodomes' and 'plasmonics'

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(PhysOrg.com) -- Stanford engineers have succeeded in harnessing plasmonics -- an emerging branch of science and technology -- to more effectively trap light within thin solar cells. As a result, we're one step closer to thin, inexpensive solar cells.

Researchers in solar energy speak of a day when millions of otherwise fallow square meters of sun-drenched roofs, windows, deserts and even clothing will be integrated with inexpensive solar cells that are many times thinner and lighter than the bulky rooftop panels familiar today.

So, when your iPod is on the nod, you might plug it into your shirt to recharge. Lost in the Serengeti with a sapped cell phone? No problem; rolled in your backpack is a lightweight solar pad. Sailing the seven seas and your GPS needs some juice? Hoist a solar sail and be one with the gods of geosynchronous orbit.

It is not hard to envision a time when such technologies will be ubiquitous in our increasingly energy-hungry lives. That day may come a bit sooner thanks to a multidisciplinary team of Stanford engineers led by Mike McGehee, Yi Cui and Mark Brongersma, and joined by Michael Graetzel at theÉcole Polytechnique Fédérale de Lausanne (EPFL).

Waves of energy

In an article published inAdvance Energy Materials, the Stanford/EPFL team announced a new type of thin solar cell that could offer a new direction for the field. They succeeded in harnessing plasmonics– an emerging branch of science and technology– to more effectively trap light within thin solar cells to improve performance and push them one step closer to daily reality.

"Plasmonics makes it much easier to improve the efficiency of solar cells,"said McGehee, an associate professor of materials science and engineering at Stanford.

McGehee is the director of CAMP– the Center for Advanced Molecular Photovoltaics– a multidisciplinary, multi-university team tackling the challenges of thin-film solar cells.

"Using plasmonics we can absorb the light in thinner films than ever before,"McGehee said."The thinner the film, the closer the charged particles are to the electrodes. In essence, more electrons can make it to the electrode to become electricity."

Plasmonics is the study of the interaction of light and metal. Under precise circumstances, these interactions create a flow of high-frequency, dense electrical waves rather than electron particles. The electronic pulse travels in extremely fast waves of greater and lesser density, like sound through the air.

A perfect solar waffle

The lightbulb moment for the team came when they imprinted a honeycomb pattern of nanoscale dimples into a layer of metal within the solar cell. Think of it as a nanoscale waffle, only the bumps on the waffle iron are domes rather than cubes– nanodomes to be exact, each only a few billionths of a meter across.

To fashion their waffle, McGehee and team members spread a thin layer of batter on a transparent, electrically conductive base. This batter is mostly titania, a semi-porous metal that is also transparent to light. Next, they use their nano waffle iron to imprint the dimples into the batter. Next, they layer on some butter– a light-sensitive dye– which oozes into the dimples and pores of the waffle. Lastly, the engineers add some syrup– a layer of silver, which hardens almost immediately.

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Titania within the solar cell is imprinted by the silicon nanodomes like a waffle imprinted by the iron.

When all those nanodimples fill up, the result is a pattern of nanodomes on the light-ward side of the silver.

This bumpy layer of silver has two primary benefits. First, it acts as a mirror, scattering unabsorbed light back into the dye for another shot at collection. Second, the light interacts with the silver nanodomes to produce plasmonic effects. Those domes of silver are crucial. Reflectors without them will not produce the desired effect. And any old nanodomes won't do either; they must be just the right diameter and height, and spaced just so, to fully optimize the plasmonics.

If you imagine your nanoself observing one of these solar cells in slow motion, you would see photons enter and pass through the transparent base and the titania (the waffle), at which point some photons would be absorbed by the light-sensitive dye (the butter), creating an electric current. Most of the remaining photons would hit the silver back reflector (the hardened syrup) and bounce back into the solar cell. A certain portion of the photons that reach the silver, however, will strike the nanodomes and cause plasmonic waves to course outward. And there you have it– the first-ever plasmonic dye-sensitized solar cell.

Trapping the light fantastic

It is easy to see why researchers are focused on thin-film solar technology. In recent years, much hope has been directed toward these lightweight, flexible cells that use photosensitive dyes to generate electricity. These cells have many advantages: They are less energy intensive and less costly to produce, flowing like newsprint off huge roll presses. They are thinner even than other"thin"solar cells. They are also printable on flexible bases that can be rolled up and taken virtually anywhere. Many use non-toxic, abundantly available materials, as well– a huge plus in the push for sustainability.

Dye-sensitized solar cells are not without challenges, however. First off, the very best convert only a small percentage of light into electricity– about 8 percent. The bulkier commercial technologies available today have reached 25 percent efficiency, and certain advanced applications have topped 40 percent. And then there is durability. The latest thin solar cell will last about seven years under continuous exposure to the elements. Not bad until you consider that 20 to 30 years is the commercial standard.

Both efficiency and reliability will have to improve. Nonetheless, engineers like McGehee believe that if they can convert just 15 percent of the light into electricity– a figure that is not out of reach– and tease the lifespan to a decade, we might soon find ourselves in the age of personal solar cells. An advance likeplasmonicsjust might provide the spark necessary to take the field down a new and exciting path.

A matter of economics

Cheaper and cleaner will be the keys. Coal-based power is plentiful and cheap, but also comes at a steep environmental cost in gouged landscapes and polluted skies. At today's commercial rates, however, even the best solar alternatives cost five times more per kilowatt-hour than coal. It is clear that economics, and not technology, is what stands between us and our solar future.

But McGehee and others are confident they can make thinsolar cellsmore attractive.

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A nanoscale rope, and another step toward complex nanomaterials that assemble themselves

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(PhysOrg.com) -- Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have coaxed polymers to braid themselves into wispy nanoscale ropes that approach the structural complexity of biological materials.

Their work is the latest development in the push to develop self-assembling nanoscale materials that mimic the intricacy and functionality of nature’s handiwork, but which are rugged enough to withstand harsh conditions such as heat and dryness.

Although still early in the development stage, their research could lead to new applications that combine the best of both worlds. Perhaps they’ll be used as scaffolds to guide the construction of nanoscale wires and other structures. Or perhaps they’ll be used to develop drug-delivery vehicles that target disease at the molecular scale, or to develop molecular sensors and sieve-like devices that separate molecules from one another.

Specifically, the scientists created the conditions for synthetic polymers called polypeptoids to assemble themselves into ever more complicated structures: first into sheets, then into stacks of sheets, which in turn roll up into double helices that resemble a rope measuring only 600 nanometers in diameter (a nanometer is a billionth of a meter).

“This hierarchichal self assembly is the hallmark of biological materials such as collagen, but designing synthetic structures that do this has been a major challenge,” says Ron Zuckermann, who is the Facility Director of the Biological Nanostructures Facility in Berkeley Lab’s Molecular Foundry.

In addition, unlike normal polymers, the scientists can control the atom-by-atom makeup of the ropy structures. They can also engineer helices of specific lengths and sequences. This“tunability” opens the door for the development of synthetic structures that mimicbiological materials’ ability to carry out incredible feats of precision, such as homing in on specific molecules.

“Nature uses exact length and sequence to develop highly functional structures. An antibody can recognize one form of a protein over another, and we’re trying to mimic this,” adds Zuckermann.

Zuckermann and colleagues conducted the research at The Molecular Foundry, which is one of the five DOE Nanoscale Science Research Centers premier national user facilities for interdisciplinary research at the nanoscale. Joining him were fellow Berkeley Lab scientists Hannah Murnen, Adrianne Rosales, Jonathan Jaworski, and Rachel Segalman. Their research was published in a recent issue of theJournal of the American Chemical Society.

The scientists worked with chains of bioinspired polymers called a peptoids. Peptoids are structures that mimic peptides, which nature uses to form proteins, the workhorses of biology. Instead of using peptides to build proteins, however, the scientists are striving to use peptoids to build synthetic structures that behave like proteins.

The team started with a block copolymer, which is a polymer composed of two or more different monomers.

“Simple block copolymers self assemble intonanoscalestructures, but we wanted to see how the detailed sequence and functionality of bioinspired units could be used to make more complicated structures,” says Rachel Segalman, a faculty scientist at Berkeley Lab and professor of Chemical and Biomolecular Engineering at University of California, Berkeley.

With this in mind, the peptoid pieces were robotically synthesized, processed, and then added to a solution that fosters self assembly.

The result was a variety of self-made shapes and structures, with the braided helices being the most intriguing. The hierarchical structure of the helix, and its ability to be manipulated atom-by-atom, means that it could be used as a template for mineralizing complex structures on a nanometer scale.

“The idea is to assemble structurally complex structures at the nanometer scale with minimal input,” says Hannah Murnen. She adds that the scientists next hope is to capitalize on the fact that they have minute control over the structure’s sequence, and explore how very small chemical changes alter the helical structure.

Says Zuckermann,“These braided helices are one of the first forays into making atomically defined block copolymers. The idea is to take something we normally think of as plastic, and enable it to adopt structures that are more complex and capable of higher function, such as molecular recognition, which is what proteins do really well.”

X-ray diffraction experiments used to characterize the structures were conducted at beamlines 8.3.1 and 7.3.3 of Berkeley Lab’s Advanced Light Source, a national user facility that generates intense x-rays to probe the fundamental properties of substances. This work was supported in part by the Office of Naval Research.

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No longer just a spectator, silicon oxide gets into the electronics action on computer chips

In the materials science equivalent of a football fan jumping onto the field and scoring a touchdown, scientists are documenting that one fundamental component of computer chips, long regarded as a passive bystander, can actually be made to act like a switch. That potentially allows it to take part in the electronic processes that power cell phones, iPads, computers, and thousands of other products.

In a report in theJournal of the American Chemical Society, the scientists document the multiple ways in which silicon dioxide, long regarded simply as an electric insulator, gets involved in the action. This behavior had formerly confused scientists working in the area of nanoelectronics— they thought that the switching was due to the nano-additive but it turns out that the source of the switching might be from the underlying silicon oxide itself.

Jun Yao, Douglas Natelson, Lin Zhong, and James Tour explain that manufacturers have long used silicon oxide, normally a very poor conductor of electricity, as both a supportive and insulating material in electronics. Silicon, a primary component of beach sand, is the semiconductor material at the heart of modern electronics. When bound to oxygen, the resulting silicon oxide is generally one of the highest quality electronic insulating materials.

The scientists recently showed, however, that the oxide material can be converted to a switchable conductor by an electrical process. This phenomenon may hold the key to developing a new generation of smaller, more powerfulcomputer chips, but the mechanism behind this switching was unclear, until now. It also clarifies the possible nature behind the switching events in former molecular and nano-scale systems.

The scientists sandwiched a nano-sized layer of silicon oxide, thousands of times smaller than the width of a human hair, between two electrodes and exposed the device to increasing amounts of electrical current. They demonstrated that electricity can cause thesilicon oxideto breakdown into smaller components, nano-sized crystals of silicon, in a way that boosts its electrical conductivity and makes it a player in the working processes of computer chips.

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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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