Simpler fabrication of nanogaps

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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.”

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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?

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

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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.”

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Complexity in core-shell nanomagnets

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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.

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Versatility of a new material makes for more efficient solar cells

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(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.

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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.

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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.

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