C60 could form a new kind of gel

Enlarge

(PhysOrg.com) -- C₆₀, the spherical carbon molecule also known as a buckminsterfullerene, has intrigued scientists for its unique properties and potential applications in nanotechnology and electronics. Now scientists have found that C₆₀may have another unusual property: it may take the form of a one-component gel under certain conditions. To date, all known gels consist of two components: an evenly distributed substance (a colloid) and a substance that dissolves the colloid (a solvent).

Scientists have previously discovered that C₆₀can take the form of different phases of matter, including solids and liquids. Here, chemists Patrick Royall from the University of Bristol and Stephen Williams from the Australian National University found that C₆₀can theoretically exist in a dense liquid phase containing clusters, which bind together to form a gel structure, specifically a"spinodal"gel. The gel is made entirely of carbon.

In their study, the scientists performedcomputer simulationsshowing that C₆₀can form a gel at moderately high temperatures and very high quench rates. The simulations showed that C₆₀gels form in about 10 nanoseconds and are stable atroom temperaturefor at least 100 nanoseconds, which is the maximum time that the simulations were run. Although the gel showed some coarsening, the scientists predict that it would remain stable for more than 100 nanoseconds. Eventually, however, the gel would separate into a crystal and a gas.

As far as experimentally demonstrating the C₆₀gel, the scientists predict that it will be a challenge, largely due to the extremely high quench rates required, which are not currently experimentally feasible. However, they may investigate creative ways to lower the quench rate, or try to use larger fullerenes such as C₅₄₀, which may also become a carbon gel. In any case, the potential existence of a one-componentgelcould lead to an overall better understanding of the nature of gels.

Source

Introducing youths to big ideas about a nano-sized world through video games

(PhysOrg.com) -- Working on a science fair project with his lab partner Nikki, Harold Biggums finds himself transformed into a tiny superhero and flung into the midst of an alien plot to take over the world— a plot that he and Nikki can foil only by defying gravity, walking on water and charging across electric fields.

This narrative dilemma is the basic storyline forGeckoman!, an online video gamedeveloped by Northeastern University researchers at the Center for High-rate Nanomanufacturing (CHN), which seeks to educate middle-school students about nanoscience and technology.

CHN director Ahmed Busnaina and associate director Jacqueline Isaacs led an interdisciplinary team of educators and game designers to develop the game, which is available in English and Spanish.

“Geckoman! is both engaging and challenging, and along the way, students pick up a lot of nanoscience fundamentals,” said Busnaina, the William Lincoln Smith Professor of Mechanical and Industrial Engineering at Northeastern.

“We had excellent teachers working with us to develop four lesson plans that guide student learning,” said Isaacs, a professor in the Department of Mechanical and Industrial Engineering. “The results of student play tests indicate that students are learning new concepts.”

Game players follow Harold on an adventurous journey, after he has been shrunk to the nanoscale following an explosion in his laboratory. Players must navigate Harold through various levels across three different worlds, while also collecting scattered notebook pages that provide nanoscience tips to help him progress.

The game was created with funding help from the National Science Foundation; and 15 Days LLC, a company founded by Northeastern alumni and faculty, collaborated with CHN faculty on design. Staff members at Boston’s Museum of Science helped match the game content to national and Massachusetts K-12 science standards.

How did the game get its name? Early in the game development process, the team worked on incorporating a key concept in nanoscience— the“van der Waals” adhesion force, which dominates other forces at the nanoscale. In fact, it is this force that enables geckos to run up walls; the pads of their feet have millions of nanoscale extensions. The game developers decided that Harold would have to become Geckoman, enabling him to move with greater ease between all the unusual surfaces he must navigate in addressing the game’s multi-level challenges. 

Source

Researchers develop new technology for cheaper, more efficient solar cells

The sun provides more than enough energy for all our needs, if only we could harness it cheaply and efficiently. Solar energy could provide a clean alternative to fossil fuels, but the high cost of solar cells has been a major barrier to their widespread use.

Stanford researchers have found that adding a single layer oforganic moleculesto a solar cell can increase its efficiency three-fold and could lead to cheaper, more efficient solar panels. Their results were published online inACS Nanoon Feb. 7.

Professor of chemical engineering Stacey Bent first became interested in a new kind ofsolar technologytwo years ago. These solar cells used tiny particles of semiconductors called"quantum dots."Quantum dot solar cells are cheaper to produce than traditional ones, as they can be made using simple chemical reactions. But despite their promise, they lagged well behind existing solar cells in efficiency.

"I wondered if we could use our knowledge of chemistry to improve their efficiency,"
Bent said. If she could do that, the reduced cost of these solar cells could lead to mass adoption of the technology.

Bent will discuss her research on Sunday, Feb. 20, at the annual meeting of the American Association for the Advancement of Science in Washington, D.C.

In principle, quantum dot cells can reach much higher efficiency, Bent said, because of a fundamental limitation of traditional solar cells.

Solar cells work by using energy from the sun to excite electrons. The excited electrons jump from a lower energy level to a higher one, leaving behind a"hole"where the electron used to be. Solar cells use a semiconductor to pull an electron in one direction, and another material to pull the hole in the other direction. This flow of electron and hole in different directions leads to an electric current.

But it takes a certain minimum energy to fully separate the electron and the hole. The amount of energy required is specific to different materials and affects what color, or wavelength, of light the material best absorbs. Silicon is commonly used to make solar cells because the energy required to excite its electrons corresponds closely to the wavelength of visible light.

But solar cells made of a single material have a maximum efficiency of about 31 percent, a limitation of the fixed energy level they can absorb.

Quantum dot solar cells do not share this limitation and can in theory be far more efficient. The energy levels of electrons in quantum dot semiconductors depends on their size– the smaller the quantum dot, the larger the energy needed to excite electrons to the next level.

So quantum dots can be tuned to absorb a certain wavelength of light just by changing their size. And they can be used to build more complex solar cells that have more than one size of quantum dot, allowing them to absorb multiple wavelengths of light.

Because of these advantages, Bent and her students have been investigating ways to improve the efficiency of quantum dot solar cells, along with associate Professor Michael McGehee of the department of Materials Science and Engineering.

The researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a very thin single layer of organic molecules. These molecules were self-assembling, meaning that their interactions caused them to pack together in an ordered way. The quantum dots were present at the interface of this organic layer and the semiconductor. Bent's students tried several different organic molecules in an attempt to learn which ones would most increase the efficiency of the solar cells.

But she found that the exact molecule didn't matter– just having a single organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells."We were surprised, we thought it would be very sensitive to what we put down,"said Bent.

But she said the result made sense in hindsight, and the researchers came up with a new model– it's the length of the molecule, and not its exact nature, that matters. Molecules that are too long don't allow the quantum dots to interact well with the semiconductor.

Bent's theory is that once the sun's energy creates an electron and a hole, the thin organic layer helps keep them apart, preventing them from recombining and being wasted. The group has yet to optimize the solar cells, and they have currently achieved an efficiency of, at most, 0.4 percent. But the group can tune several aspects of the cell, and once they do, the three-fold increase caused by the organic layer would be even more significant.

Bent said the cadmium sulfide quantum dots she is currently using are not ideal for solar cells, and the group will try different materials. She said she would also try other molecules for the organic layer, and could change the design of the solar cell to try to absorb more light and produce more electrical charge. Once Bent has found a way to increase the efficiency of quantum dotsolar cells, she said she hopes their lower cost will lead to wider acceptance of solarenergy.

Source

A nano-Solution to global water problem: Nanomembranes could filter bacteria

(PhysOrg.com) -- New nanomaterials research from the University at Buffalo could lead to new solutions for an age-old public health problem: how to separate bacteria from drinking water.

To the naked eye, bothwater moleculesand germs are invisible -- objects so tiny they are measured by the nanometer, a unit of length about 100,000 times thinner than the width of a human hair.

But at the microscopic level, the two actually differ greatly in size. A single water molecule is less than a nanometer wide, while some of the most diminutive bacteria are a couple hundred.

Working with a special kind of polymer called a block copolymer, a UB research team has synthesized a new kind of nanomembrane containing pores about 55nanometersin diameter -- large enough for water to slip through easily, but too small for bacteria.

The pore size is the largest anyone has achieved to date using block copolymers, which possess special properties that ensure pores will be evenly spaced, said Javid Rzayev, the UB chemist who led the study. The findings were published online on Jan. 31 inNano Lettersand will appear in the journal's print edition later this year, with UB chemistry graduate student Justin Bolton as lead author.

"These materials present new opportunities for use as filtration membranes,"said Rzayev, an assistant professor of chemistry."Commercial membranes have limitations as far as pore density or uniformity of the pore size. The membranes prepared from block copolymers have a very dense distribution of pores, and the pores are uniform."

"There's a lot of research in this area, but what our research team was able to accomplish is to expand the range of available pores to 50 nanometers in diameter, which was previously unattainable by block-copolymer-based methods,"Rzayev continued."Making pores bigger increases the flow of water, which will translate into cost and time savings. At the same time, 50 to 100 nm diameter pores are small enough not to allow any bacteria through. So, that is a sweet spot for this kind of application."

The new nanomembrane owes its special qualities to the polymers that scientists used to create it. Block copolymers are made up of two polymers that repel one another but are"stitched"together at one end to form the single copolymer.

When many block copolymers are mixed together, their mutual repulsion leads them to assemble in a regular, alternating pattern. The result of that process, called self-assembly, is a solid nanomembrane comprising two different kinds of polymers.

To create evenly spacedporesin the material, Rzayev and colleagues simply removed one of the polymers. The pores' relatively large size was due to the unique architecture of the originalblock copolymers, which were made from bottle-brush molecules that resemble round hair brushes, with molecular"bristles"protruding all the way around a molecular backbone.

Source

Researchers discover new way to design metal nanoparticle catalysts

Tiny metal nanoparticles are used as catalysts in many reactions, from refining chemicals to producing polymers and biofuels. How well these nanoparticles perform as catalysts for these reactions depend on which of their crystal faces are exposed.

But previous attempts to design thesenanoparticlesby changing their shape have failed because the structures are unstable and will revert back to their equilibrium shape.

Now, researchers at Northwestern University's Institute for Catalysis in Energy Processing have discovered a new strategy for fabricatingmetal nanoparticlesin catalysts that promises to enhance the selectivity and yield for a wide range of structure-sensitive catalytic reactions. The team, led by Laurence D. Marks, professor of materials science and engineering at the McCormick School of Engineering and Applied Science, discovered that they could design nanoparticles by designing the particle's support structure.

"Instead of trying to engineer the nanoparticles, we've engineered the substrate that the nanoparticle sits on,"Marks said."That changes what faces are exposed."Their results were published in February in the journalNano Letters.

This solution was a bit of a discovery: the team created the nanoparticle samples, discovered that they didn't change their shape (as the laws of thermodynamics caused previously designed nanoparticles to do), then set out figuring how it worked. It turns out that epitaxy— the relationship between the position of the atoms in the nanoparticle and the position of the atoms on the substrate— was more important to design than previously thought.

The team is currently testing the nanoparticles in a catalytic reactor, and early results look promising, Marks says. The nanoparticles appear to be stable enough to survive the rigors of long-term use as catalysts.

"It opens the door to designing better catalysts,"Marks said."This method could be used with a variety of different metal nanoparticles. It's a new strategy, and it could have a very big impact."

Source

Metallic molecules to nanotubes: Spread out!

Enlarge

(PhysOrg.com) -- A lab at Rice University has stepped forward with an efficient method to disperse nanotubes in a way that preserves their unique properties -- and adds more.

The new technique allows inorganicmetal complexeswith different functionalities to remain in close contact with single-walled carbon nanotubes while keeping them separated in a solution.

That separation is critical to manufacturers who want to spin fiber from nanotubes, or mix them into composite materials for strength or to take advantage of theirelectrical properties. For starters, the ability to functionalize the nanotubes at the same time may advance imaging sensors, catalysis and solar-activatedhydrogen fuel cells.

Better yet, a batch of nanotubes can apparently stay dispersed in water for weeks on end.

Keeping carbon nanotubes from clumping in aqueous solutions and combining them with molecules that add novel abilities have been flies in the ointment for scientists exploring the use of these highly versatile materials.

They've tried attaching organic molecules to the nanotubes' surfaces to add functionality as well as solubility. But while these techniques can separate nanotubes from one another, they take a toll on the nanotubes' electronic, thermal and mechanical properties.

Angel Marti, a Rice assistant professor of chemistry and bioengineering and a Norman Hackerman-Welch Young Investigator, and his students reported this month in the Royal Society of Chemistry journalChemical Communicationsthat ruthenium polypyridyl complexes are highly effective at dispersing nanotubes in water efficiently and for long periods. Ruthenium is a rare metallic element.

One key is having just the right molecule for the job. Marti and his team created ruthenium complexes by combining the element with ligands, stable molecules that bind tometal ions. The resulting molecular complex is part hydrophobic (the ligands) and part hydrophilic (the ruthenium). The ligands strongly bind to nanotubes while the attached ruthenium molecules interact with water to maintain the tubes in solution and keep them apart from one another.

Another key turned out to be moderation.

Originally, Marti said, he and co-authors Disha Jain and Avishek Saha weren't out to solve a problem that has boggled chemists for decades, but their willingness to"do something crazy"paid off big-time. Jain is a former postdoctoral researcher in Marti's lab, and Saha is a graduate student.

The researchers were eyeing ruthenium complexes as part of a study to track amyloid deposits associated with Alzheimer's disease."We started to wonder what would happen if we modified the metal complex so it could bind to a nanotube,"Marti said."That would provide solubility, individualization, dispersion and functionality."

It did, but not at first."Avishek put this together with purified single-walled carbon nanotubes (created via Rice's HiPco process) and sonicated. Absolutely nothing happened. The nanotubes didn't get into solution -- they just clumped at the bottom.

"That was very weird, but that's how science works -- some things you think are good ideas never work."

Saha removed the liquid and left the clumped nanotubes at the bottom of the centrifuge tube."So I said, 'Well, why don't you do something crazy. Just add water to that, and with the little bit of ruthenium that might remain there, try to do the reaction.' He did that, and the solution turned black."

A low concentration of ruthenium did the trick."We found out that 0.05 percent of the ruthenium complex is the optimum concentration to dissolve nanotubes,"Marti said. Further experimentation showed that simple ruthenium complexes alone did not work. The molecule requires its hydrophobic ligand tail, which seeks to minimize its exposure to water by binding with nanotubes."That's the same thing nanotubes want to do, so it's a favorable relationship,"he said.

Marti also found the nanotubes' natural fluorescence unaffected by the ruthenium complexes."Even though they've been purified, which can introduce defects, they still exhibit very good fluorescence,"he said.

He said that certain ruthenium complexes have the ability to stay in an excited state for a long time -- about 600 nanoseconds, or 100 times longer than normalorganic molecules."It means the probability that it will transfer an electron is high. That's convenient for energy transfer applications, which are important for imaging,"he said.

Thatnanotubesstay suspended for a long time should catch the eye of manufacturers who use them in bulk."They should stay separated for weeks without problems,"Marti said."We have solutions that have been sitting for months without any signs of crashing."

Source

Simpler way of making proteins could lead to new nanomedicine agents

Enlarge

Researchers have developed a simple method of making short protein chains with spiral structures that can also dissolve in water, two desirable traits not often found together. Such structures could have applications as building blocks for self-assembling nanostructures and as agents for drug and gene delivery.

Led by Jianjun Cheng, a professor of materials science and engineering at the University of Illinois, the research team will publish its findings in the Feb. 22 edition of the journalNature Communications.

Materials scientists have been interested in designing largepolymer moleculesthat could be used as building blocks for self-assembling structures. The challenge has been that the molecules generally adopt a globular, spherical shape, limiting their ability to form orderly aggregates. However, polypeptides– chains of amino acids such as proteins– can form helical structures. Short polypeptide chains that adopt a spiral shape act like cylindrical rods.

"If you have two rigid rods, one positive and one negative, right next to each other, they're going to stick to each other. If you have a way to put the charge on the surface then they can pack together in a close, compact way, so they form a three-dimensional structure,"Cheng said.

However, it is difficult to make helical polypeptides that are water-soluble so they can be used in solution. Polypeptides gain their solubility from side chains– molecular structures that stem from each amino acid link in the polypeptide chain. Amino acids with positive or negative charges in their side chains are needed to make a polypeptide disperse in water.

The problem arises when chains with charged side chains form helical structures. The charges cause a strong repulsion between the side chains, which destabilizes the helical conformation. This causes water-soluble polypeptides to form random coil structures instead of the desired helices.

In exploring solutions to the riddle of helical, water-soluble polypeptides, researchers have tried several complicated methods. For example, scientists have attempted grafting highly water-soluble chemicals to the side chains to increase the polypeptides' overall solubility, or creating helices with charges only on one side.

"You can achieve the helical structure and the solubility but you have to design the helical structure in a very special way. The peptide design needs a very specific sequence. Then you're very limited in the type of polypeptide you can build, and it's not easy to design or handle these polypeptides,"Cheng said.

In contrast, Cheng's group developed a very straightforward solution. Since the close proximity of the charges causes the repulsion that disrupts the helix, the researchers simply elongated the side chains, moving the charges farther from the backbone and giving them more freedom to keep their distance from one another.

The researchers observed that as they increased the length of the side chains with charges on the end, the polypeptides' propensity for forming helices also increased.

"It's such a simple idea– move the charge away from the backbone,"Cheng said."It's not difficult at all to make the longer side chains, and it has amazing properties for winding up helical structures simply by pushing the distance between the charge and the backbone."

The group found that not only do polypeptides with long side chains form helices, they display remarkable stability even when compared to non-charged helices. The helices seem immune to temperature, pH, and other denaturing agents that would unwind most polypeptides.

This may explain why amino acids with large hydrophobic side chains are not found in nature. Such immutability would preclude dynamic winding and unwinding of protein structures, which is essential to many biological processes. However, rigid stability is a desirable trait for the types of applications Cheng's group explores: nanostructures for drug andgene delivery, particularly targeting cancerous tumors and stem cells.

"We want to test the correlation of the lengths of the helices and the circulation in the body to see what's the impact of the shape and the charge and the side chains for clearance in the body,"Cheng said."Recent studies show that the aspect ratio of the nanostructures– spherical structures versus tubes– has a huge impact on their penetration of tumor tissues and circulation half-lives in the body."

Cheng plans to create a library of short helical polypeptides of varying backbone lengths, side chain lengths and types of charge. He hopes to simplify the chemistry even further and make the materials widely accessible. His lab already has demonstrated that helical structures can be effective gene delivery and membrane transduction agents, and building the library of soluble helical molecules will allow further investigation of tailoring suchnanostructuresfor specific biomedical applications.

Source

More News

Find more news articles via<a href="http://www.physorg.com/sort/date/all/">sort by date</a>page

Source

More news stories

[Annonce]

An inexpensive and rapid testing method can effectively identify a sub-group of never-smoking lung cancer patients whose tumors express a molecule associated with increased risk of disease progression or recurrence, US researchers ...

Medicine& Health/Cancer

12 minutes ago |not rated yet |0

Source

Taming carbon nanotubes

Carbon nanotubes have many attractive properties, and their structure and areas of application can be compared with those of graphene, the material for whose discovery the most recent Nobel Prize was awarded. In order to be able to exploit these properties, however, it is necessary to have full control of the manufacturing process. Scientists at the University of Gothenburg (Sweden) are closing in on the answer.

"Our results show that the metal particles that form the basis of the manufacture of carbon nanotubes must have a certain minimum size, in order for growth to start and to continue. It is also probable that the particles are in liquid form at a manufacturing temperature of around 800 C, even though the metals used may have much higher melting points", says Anders Börjesson from the Department of Physics at the University of Gothenburg.

The scientists have used various computer models to study in detail properties that are difficult or impossible to examine in experimental conditions. Only when we fully understand the manufacturing process will we be able to exploit this material fully.

The diameter of the nanotubes is of the order of one billionth of a metre, and they can be as thin as a single carbon layer. The length of the tubes, in contrast, can extend from the nanometre scale up to several decimetres. Carbon nanotubes can be regarded, quite simply, as thin threads of pure carbon, whose length can be a billion times greater than their thickness.

Interest for nanotubes is based on their outstanding properties: they are among the strongest materials known and have extremely high conductivity for both electric current and heat.

The strength can be used to reinforce other materials, just as the strength of glass and carbon fibres is used in plastics, and steel reinforcement is used in concrete. Carbon nanotubes, however, would enable plastics to be manufactured that are ten times stronger than the strongest materials available today. Suchmaterialscould be used not only in exclusive sports equipment but also in the construction of buildings that appear to come from science fiction: a lift between the Earth and space could be anchored using a material based on nanotubes.

The carbon nanotubes may also replace other material when it comes to conducting very high electrical currents, since they do not become hot, nor do they catch fire. Certain nanotubes have semiconducting properties and could be used to build nanoelectronic circuits, giving much smaller and faster processors to be used in computers.

One way of combining the strength and electrical properties of the carbon nanotubes would be to mix them with polymer material, and by weaving threads that also contain electronic circuits. It would be possible, for example, to weave instruments for monitoring heart function directly into clothes.

Source

Long and narrow, free of defects, and soluble: graphene nanoribbons by bottom-up synthesis

Enlarge

(PhysOrg.com) -- Electronic components based on graphene could render our current silicon-based electronics obsolete. Graphene, a more recently discovered form of carbon, consists of two-dimensional sheets of aromatic six-membered carbon rings in a honeycomb arrangement. In contrast to extended graphene layers, narrow graphene nanoribbons have semiconducting properties and could thus be candidates for electronic applications.

Klaus Müllen and a team from the Max Planck Institute for Polymer Research in Mainz have now introduced a new method for the synthesis of long, narrow graphene ribbons with defined dimensions in the journalAngewandte Chemie.

Previously, graphene ribbons were mainly cut out of larger graphene sheets or were obtained by slitting open carbon nanotubes lengthwise. However, with these methods it is impossible to produce ribbons that have a precisely established ratio of width to length as well as defined edges. These details are important because they determine the electronic properties of the ribbons. The search has thus been on for a method that allows controlled production of very narrow graphene ribbons—an extremely difficult challenge. The German researchers working with Müllen are now well on the way to overcome it. They are not starting with large structures to cut up (top-down); instead they are building their ribbons from smaller components (bottom-up).

The building blocks selected by Müllen and his team are long chains of aromatic six-membered carbon rings called polyphenlyenes. In contrast to previous approaches, they did not produce straight chains; instead they made them with a flexible, zigzagging, bent backbone. Furthermore, they attached hydrocarbon side-chains to the backbone to increase the solubility in organic solvents, which allows the compounds to be synthesized and processed in solution.

The next step is a reaction that splits off hydrogen (dehydrogenation). This causes a ring-closing reaction in each pointy tip of the zigzag, forming a new aromatic six-memberedcarbonring that shares a side with three neighboring rings—the chain transforms in to a narrow ribbon.

In this way, the team was able to produce a series of different nanoribbons with lengths reaching over 40 nm. The width of the ribbon was defined by the size of the“points” of the polyphenylene precursor. The resulting ribbons are free of defects and soluble in common organic solvents.

“We have been the first to demonstrate that structural perfection can be achieved by the classical bottom-up synthesis of definedgraphenenanoribbons,” says Müllen.“The solubility of theribbonsis an important requirement for the large-scale production ofelectronic components.”

Source

Nanonets give rust a boost as agent in water splitting's hydrogen harvest

Enlarge

Coating a lattice of tiny wires called Nanonets with iron oxide– known more commonly as rust– creates an economical and efficient platform for the process of water splitting, an emerging clean fuel science that harvests hydrogen from water, Boston College researchers report in the online edition of the<i>Journal of the American Chemical Society</i>.

Assistant Professor of Chemistry Dunwei Wang and his clean energy lab pioneered the development of Nanonets in 2008 and have since shown them to be a viable new platform for a number of energy applications by virtue of the increased surface area and improved conductivity of the nano-scale netting made from titanium disilicide, a readily available semiconductor.

Wang and his team report that coating the Nanonets with hematite, the plentiful mineral form ofiron oxide, showed the mineral could absorb light efficiently and without the added expense of enhancing the material with an oxygen evolving catalyst.

The results flow directly from the introduction of the Nanonet platform, Wang said. While constructed of wires 1/400th the size of a human hair, Nanonets are highly conductive and offer significant surface area. They serve dual roles as a structural support and an efficient charge collector, allowing for maximum photon-to-charge conversion, Wang said.

"Recent research has shown that the use of a catalyst can boost the performance of hematite,"said Wang."What we have shown is the potential performance of hematite at its fundamental level, without a catalyst. By using this unique Nanonet structure, we have shed new light on the fundamental performance capabilities of hematite in water splitting."

On its own, hematite faces natural limits in its ability to transport a charge. A photon can be absorbed, but has no place to go. By giving it structure and added conductivity, the charge transport abilities of hematite increase, said Wang. Water splitting, a chemical reaction that separates water into oxygen andhydrogengas, can be initiated by passing an electric current through water. But that process is expensive, so gains in efficiency and conductivity are required to make large-scale water splitting an economically viable source for clean energy, Wang said.

"The result highlights the importance of charge transport in semiconductor-based
watersplitting, particularly for materials whose performance is limited by poor charge diffusion,"the researchers report in the journal."Our design introduces material components to provide a dedicated charge transport pathway, alleviates the reliance on the materials' intrinsic properties, and therefore has the potential to greatly broaden where and how various existing materials can be used in energy-related applications."

Source

Simpler fabrication of nanogaps

Enlarge

Plasmons, which are density waves of electrons, are of great interest to pure and applied scientists because of their novel properties, and because of their application to sensing and photonic technologies. These applications are possible because plasmons are sensitive to surface properties, and allow for the concentration of electric fields into small volumes. Fabricating the intricate nanostructures necessary to support plasmons, however, has proved a challenge. Now a straightforward fabrication technique, capable of generating plasmon-supporting nanogap structures over large areas, has been demonstrated by Wakana Kubo and Shigenori Fujikawa from the RIKEN Innovation Center, Wako, and the Japan Science and Technology Agency.

The researchers fabricated many copies of a structure consisting of two nested vertical gold cylinders, with the cylinders spaced apart by tens of nanometers. This structure, called a‘double nanopillar’, was designed to support a highly concentrated electric field in the gap between the cylinders, in response to illumination with light. When the gap was filled with a liquid or gas, the optical properties of the double nanopillar changed, making it a useful sensor.

Typically, closely gapped structures such as the double nanopillar are fabricated individually by carving a polymer resist with an electron beam, but this process is slow and can pattern only small areas. Fujikawa and colleagues used a template-based coating process instead. They etched a silicon wafer to make a mold of periodically spaced holes, and applied the mold to a soft polymer film, resulting in an array of polymer pillars. They then coated these pillars with a gold layer, followed by a spacer, and a second gold layer. Finally, they removed the polymer film and spacer layers, leaving a double nanopillar array (Fig. 1). Using this process, the researchers could make a patterned area as large as the original template, and adapt it to include different spacer materials with finely controlled thicknesses.

Kubo and Fujikawa tested the double nanopillars as sensors of refractive index, which showed sensitivities that were greater than sensors that had equivalent metal surface areas, but which did not have a nanoscale gap. This comparison demonstrated that the electric field in the double nanopillars was indeed highly concentrated. The new fabrication process marks just the beginning of an extended research program, says Fujikawa.“We do not fully understand the optical behavior of thesenanostructures,” he explains.“We will seek out collaborations with other researchers to investigate them further, and will try including magnetic, electric and organic materials into our process.”

Source

Nanosilver: A new name -- well-known effects

Nanosilver is not a new discovery by nanotechnologists -- it has been used in various products for over a hundred years, as is shown by a new Empa study. The antimicrobial effects of minute silver particles, which were then known as"colloidal silver,"were known from the earliest days of its use.

Numerousnanomaterialsare currently at the focus of public attention. In particularsilver nanoparticlesare being investigated in detail, both by scientists as well as by the regulatory authorities. The assumption behind this interest is that they are dealing with a completely new substance. However, Empa researchers Bernd Nowack and Harald Krug, together with Murray Heights of the company HeiQ have shown in a paper recently published in the journalEnvironmental Science&Technologythat nanosilver is by no means the discovery of the 21st century. Silver particles with diameters of seven to nine nm were mentioned as early as 1889. They were used in medications or as biocides to prevent the growth of bacteria on surfaces, for example in antibacterial water filters or in algaecides for swimming pools.

The nanoparticles were known as"colloidal silver"in those days, but what was meant was the same then as now– extremely small particles of silver. The only new aspect is the use today of the prefix"nano"."However,"according to Bernd Nowack,"nano does not mean something new, and nor does it mean something that is harmful."When"colloidal silver"became available on the market in large quantities in the 1920s it was the topic of numerous studies and subject to appropriate regulation by the authorities. Even in those days the significance of the discovery of nanoparticles and how they worked was realized."That is not to say that the possible side-effects of nanoparticles on humans and the environment should be played down or ignored,"adds Nowack. It is important to characterize in exact detail the material properties of nanosilver and not just to believe unquestioningly the doubts and reservations surrounding the product.

The term nanoparticle is understood to refer to particles whose dimensions are less than 100 nm. Because of their minute size nanoparticles have different properties than those of larger particles of the same material. For example, for a given volume nanoparticles have a much greater surface area, so they are frequently much more reactive than the bulk material. In addition, even in small quantities nanosilver produces more silver ions than solid silver. These silver ions are toxic to bacteria. Whether or not nanosilver represents a risk to humans and the environment is currently the subject of a great deal of investigation.

Currently there are hundreds of products in circulation which contain silver nanoparticles. Examples include cosmetics, food packaging materials, disinfectants, cleaning agents and– not least– antibacterial socks and underwear. Every year some 320 tonnes of nanosilver are used worldwide, some of which is released into wastewater, thus finding its way into natural water recirculation systems. What effects solar particles have on rivers, soil and the organisms that live in them has not yet been clarified in detail.

A commentary by Bernd Nowack in the scientific journalSciencediscusses the implications of the newest studies on nanosilver in sewage treatment plants. More than 90% remains bound in the sewage sludge in the form of silver sulfide, a substance which is extremely insoluble and orders of magnitude less poisonous than free silver ions. It apparently does not matter what the original form of the silver in the wastewater was, whether as metallic nanoparticles, assilver ionsin solution or as precipitated insoluble silver salts.

"As far as the environmental effects are concerned, it seems that nanosilver in consumer goods is no different than other forms of silver and represents only a minor problem for eco-systems,"says Nowack. What is still to be clarified, however, is in what form the unbound silver is present in the treated water released from sewage works, and what happens to the silver sulfide in natural waters. Is this stable and unreactive or is it transformed into other forms of silver?

Source

Solar cells can be made thinner and lighter with the help of aluminum particles

Enlarge

Solar cells are a key technology in the drive toward cleaner energy production. Unfortunately, solar technology is not yet economically competitive and the cost of solar cells needs to be brought down. One way to overcome this problem is to reduce the amount of expensive semiconductor material used, but thin-film solar cells tend to have lower performance compared with conventional solar cells.

Yuriy Akimov and Wee Shing Koh at the A*STAR Institute of High Performance Computing (Singapore) have now improved the lightconversion efficiencyof thin-film solar cells by depositingaluminumparticles on the cell surface.

Metallicnanoparticlescan direct light better into the solar cell and prevent light from escaping. In conventional‘thick-film’ solar cells, the nanoparticles would have little effect because all the light is absorbed by the film due to its thickness. For thin films, however, the nanoparticles can make a big difference. Their scattering increases the duration the light stays in the film, bringing the total absorption of light up to a level comparable with that for conventional solar cells.”The strategy allows us to reduce the production costs of solar cells by several times and makes photovoltaics more competitive with respect to other forms of power generation,” says Akimov.

The researchers modeled the light absorption efficiency of solar cells for various nanoparticle materials and sizes. In particular, they compared the properties of silver versus aluminum nanoparticles. In most studies on the subject, silver particles have been preferred. These have optical resonances in the visible part of the spectrum that are even better at focusing the light into the solar cell. Unfortunately, there is a tradeoff: the optical resonances also cause the absorption of light by the nanoparticles, which means the solar cell is less efficient.

In the case of silver, this resonance is right in the key part of the solar spectrum, so that light absorption is considerable. But not so for aluminum nanoparticles, where these resonances are outside the important part of the solar spectrum. Furthermore, the aluminum particles handle oxidation well and their properties change little with variations in shape and size. And more importantly, their scattering properties are robust in comparison with silver nanoparticles.“We found that nanoparticles made of aluminum perform better than those made of other metals in enhancing light trapping in thin-film solar cells,” says Akimov.“We believe aluminum particles can help make thin-filmsolar cellscommercially viable.”

Source

Complexity in core-shell nanomagnets

Enlarge

The magnetic exchange bias coupling between core and shell depends critically on the"frozen spins"that reside at the interface between the two different magnetic nanomaterials, according to users from Purdue University working with the Electronic&Magnetic Materials&Devices Group.

The relative population of such frozen spins can be modulated by external physical parameters, such as the strength of the applied cooling field and the cycling history of magnetic field sweeps (training effect).

A more complex change occurs when core-shell nanoparticles are aged under ambient conditions. Along with structural evolution from well-defined core-shell nanostructures to nanoparticles containing multiple voids at the interface, there is a significant increase in the population of frozen spins, both of which affect the magnetic properties.

Core-shell Fe@Fe₃O₄nanoparticles exhibit substantial exchange bias at low temperatures, mediated by unidirectionally aligned moments at the core-shell interface. These spins are frozen into magnetic alignment with field cooling and are depinned in a temperature-dependent manner.

The population of such frozen spins has a direct impact on both coercivity (H_C) and the exchange-bias field (H_E), which are modulated by external physical parameters, such as the strength of the applied cooling field and the cycling history of magnetic field sweeps (training effect).

Aging of the core-shell nanoparticles under ambient conditions results in a gradual decrease in magnetization but overall retention of H_Cand H_E, as well as a large increase in the population of frozen spins.

These changes are accompanied by a structural evolution from well-defined core-shell structures to particles containing multiple voids, attributable to the Kirkendall effect. Energy-filtered and high-resolution transmission electron microscopy both indicate further oxidation of the shell layer, but the iron core is remarkably well preserved.

The increase in frozen spin population with age is responsible for the overall retention of exchange bias, despite void formation and other oxidation-dependent changes. The exchange-bias field becomes negligible upon deliberate oxidation of Fe@Fe₃O₄nanoparticles into yolk-shell particles, with a nearly complete physical separation of core and shell.

Source

Versatility of a new material makes for more efficient solar cells

Enlarge

(PhysOrg.com) -- A Colorado State University professor has successfully turned a mineral compound into a material that can pass current through a nanoparticle network– an important discovery into more efficient, inexpensive solar cell materials.

Amy Prieto, professor of chemistry and founder of Prieto Battery, discovered that dramatic reactions occur with copper selenide at the nanoscale, according to the cover story in the Feb. 9 issue of theJournal of the American Chemical Society. Reactions with air allow Prieto and her students to manipulate or“tune” the properties of the device– such as a solar cell– containing the copper compound.

That’s an important discovery for looking further into earth-abundant, non-toxic materials that could help makesolar cellsinexpensive and absorb sunlight more efficiently than silicon, Prieto said.

“Nanoparticlesare so small, therefore most of the surface reactions that you would never notice in bulk materials are pretty dramatic in a nanoparticle,” Prieto said.“There is getting to be a lot of interest in making devices like solar cells from nanoparticles.

"There is still much to be understood about how the material we're now using works– how it absorbs photons and converts them to current, that then has to traverse a tortuous path through the nanoparticle network” she said.

Prieto and her team tested the copper nanomaterial by attaching electrodes to thin films of copper selenide nanoparticles and watching how the thin films pass electric charges. They found that, with prolonged air exposure, the current changed dramatically.

Prieto’s research focuses on creating new inorganic materials– to replace silicon or cadmium telluride, for example– that could be incorporated into solar panels to produce electrical current.

She joined Colorado State University in 2005 as an assistant professor. She is part of the university’s Clean Energy Supercluster commercialization arm, Cenergy. In 2009, Prieto co-founded Cenergy’s first startup company, Prieto Battery, a company expected to produce batteries theoretically up to 1,000 times more powerful and 10 times longer lasting and cheaper than traditional batteries. The development of this technology could revolutionize the military, automobile and healthcare industries.

Source

Tuning graphene film so it sheds water

Windshields that shed water so effectively that they don't need wipers. Ship hulls so slippery that they glide through the water more efficiently than ordinary hulls.

These are some of the potential applications for graphene, one of the hottestnew materialsin the field ofnanotechnology, raised by the research of James Dickerson, assistant professor of physics at Vanderbilt.

Dickerson and his colleagues have figured out how to create a freestanding film of graphene oxide and alter itssurface roughnessso that it either causeswaterto bead up and run off or causes it to spread out in a thin layer.

"Graphene films are transparent and, because they are made of carbon, they are very inexpensive to make,"Dickerson said."The technique that we use can be rapidly scaled up to produce it in commercial quantities."

His approach is documented in an article published online by the journalACS Nanoon Nov. 26.

Graphene is made up of sheets ofcarbon atomsarranged in rings– something like molecular chicken wire. Not only is this one of the thinnest materials possible, but it is 10 times stronger than steel and conducts electricity better at room temperature than any other known material. Graphene's exotic properties have attracted widespread scientific interest, but Dickerson is one of the first to investigate how it interacts with water.

Many scientists studying graphene make it using a dry method, called"mechanical cleavage,"that involves rubbing or scraping graphite against a hard surface. The technique produces sheets that are both extremely thin and extremely fragile. Dickerson's method can produce sheets equally as thin but considerable stronger than those made by other techniques. It is already used commercially to produce a variety of different coatings and ceramics. Known as electrophoretic deposition, this"wet"technique combines an electric field within a liquid medium to create nanoparticle films that can be transferred to another surface.

Dickerson and his colleagues found that they could change the manner in which the graphene oxide particles assemble into a film by varying the pH of the liquid medium and the electric voltage used in the process. One pair of settings lay down the particles in a"rug"arrangement that creates a nearly atomically smooth surface. A different pair of settings causes the particles to clump into tiny"bricks"forming a bumpy and uneven surface. The researchers determined that the rug surface causes water to spread out in athin layer, while the brick surface causes water to bead up and run off.

Dickerson is pursuing an approach that could create film that enhances these water-associated properties, making them even more effective at either spreading out water or causing it to bead up and run off. There is considerable academic and commercial interest in the development of coatings with these enhanced properties, called super-hydrophobic and super-hydrophilic. Potential applications range from self-cleaning glasses and clothes to antifogging surfaces to corrosion protection and snow-load protection on buildings. However, effective, low-cost and durable coatings have yet to make it out of the laboratory.

Dickerson's idea is to apply his basic procedure to"fluorographene"– a fluorinated version of graphene that is a two-dimensional version of Teflon– recently produced by Kostya S. Novoselov and Andre K. Geim at the University of Manchester, who received the 2010 Nobel Prize for the discovery of graphene. Normal fluorographene under tension should be considerably more effective in repelling water thangrapheneoxide. So there is a good chance a"brick"version and a"rug"version would have extreme water-associated effects, Dickerson figures.

Source

Researcher investigates new material grown from sugar

(PhysOrg.com) -- Ordinary table sugar could be a key ingredient to developing much lighter, faster, cheaper, denser and more robust computer electronics for use on U.S. military aircraft.

Though admittedly far in the future, recent results from a program led by chemist and Rice University professor, Dr. James Tour demonstrate another example of the cutting-edge basic research.

Tour and his colleagues at Rice have developed a relatively easy and controllable method for making pristine sheets ofgraphene--- the one-atom-thick form of carbon --- from regular tablesugarand other solid carbon sources.

"Dr. Tour is exploring a chemical approach to producing high quality carbon based nanostructures such as nanotubes and graphenes with well defined properties,"said AFOSR program manager, Dr. Charles Lee.

In their method, a small amount of sugar is placed on a tiny sheet of copper foil. The sugar is then subjected to flowing hydrogen and argon gas under heat and low pressure. After 10 minutes, the sugar is reduced to a pure carbon film, or a single layer of graphene. Adjusting thegas flowallowed the researchers to control the thickness of the film.

The use of solid carbon sources like sugar has allowed Tour to stay away from the more cumbersomechemical vapor depositionmethod and the high temperatures associated with it. His one-step, low-temperature process makes graphene considerably easier to manufacture.

"In a traditional CVD point of view, it was straightforward to optimize the pristine graphene's quality through adjusting the growth conditions and themetal catalystswith continuous gas sources (CH₄or C₂H₂),"explained Tour."With this technique using different kinds of solidcarbonsources, more benefits such as graphene doping and thickness control could be realized."

According to Tour, doped graphene opens more possibilities for both Air Force and commercial electronics applications. Pristine graphene has no bandgap, but doped graphene allows for manipulation of electronic and optical properties, important factors for making switching and logic devices.

"These materials can be used in advanced electronics, photonics as well as structural applications for the Air Force,"explained Lee.

While the Air Force is focusing primarily on potential electronics applications, many other commercial and medical uses could be possible, including transparent touch screen devices, special biocompatible films for surgery of traumatic brain injuries, faster transistors in personal computers or thin materials for solar energy harvesting.

Source

Chemists turn gold to purple -- on purpose

Enlarge

Professor Richard Watt and his chemistry students suspected that a common protein could potentially react with sunlight and harvest its energy– similar to what chlorophyll does during photosynthesis.

The story of how they proved it sounds as colorful as the legend of the leprechaun who hid his pot ofgoldat the end of the rainbow.

They started with citric acid from oranges and mixed it with the protein. Next they dissolved gold powder into the solution. Then they put vials of the yellow-colored mixture in direct sunlight and crossed their fingers in the hope that it would turn purple.

Here's the reason why: If it turned purple, that would signal that the gold atoms had received electrons and used the donated energy to bunch together as small, purple-colorednanoparticles. And that would mean that the protein used the sunlight to excite the citric acid and trigger a transfer of energy.

While direct sunlight did the trick in about 20 minutes, a high-powered tungsten mercury lamp worked much faster.

"We set the system up, turned on the light, and the solution turned purple,"Watt said."We knew that we'd proved the concept."

The beauty of this experiment lies not in its colors– unless, of course, you're thinking of it as a potential"green"energy source that keeps the environment clean.

The BYU researchers published their experiments in theJournal of Nanoparticle Research. The final step of this project will involve connecting theproteinto an electrode to channel the energy into a battery or fuel cell. The BYU chemists will partner with Jae-Woo Kim of the National Institute of Aerospace for this next stage of the work.

Professor Watt's pedigree includes a post-doc at Princeton, a father who developed a fuel cell that runs on sugar and weed-killer and a more distant ancestor credited with inventing the first practical steam engine. That ancestor is also the Scottish engineer for whom the unit of power"watt"is named.

Source