Making Colors with Magnets

A new nanomaterial could lead to novel types of displays.
By Kevin Bullis
Rainbow rust: A solution of nanoscopic iron-oxide particles changes color as a magnet gets closer, causing the particles to rearrange. The color changes from red to blue as the magnetic field’s strength increases.
Credit: Yin laboratory, University of California, Riverside

A material developed by researchers at the University of California, Riverside can take on any color of the rainbow, simply by the scientists changing the distance between the material and a magnet. It could be used in sensors or, encapsulated in microcapsules, in rewritable posters or other large color displays.
The researchers made the material using a high-temperature method to synthesize nanoscale, crystalline particles of magnetite, a form of iron oxide. Each particle was made about 10 nanometers in diameter because, as they get much larger than this, magnetite particles become permanent magnets, and therefore would cluster together and fall out of solution. The 10-nanometer particles group together to form uniformly sized spherical clusters, each about 120 nanometers across; in tests, these clusters have stayed suspended in solution for months.
By coating these clusters with an electrically charged surfactant, the researchers cause the clusters to repel each other. When researchers use a magnet to counteract the repellent forces, the clusters rearrange and move closer together, changing the color of the light they reflect. The stronger the magnetic field, the closer the particles, with the color changing from the red end of the spectrum toward the blue, opposite end, as the magnet gets closer to the material. Moving the magnet away allows the electrostatic charge to force the particles apart again, returning the system to its original condition.
"The beauty of this system is that it is so simple," says Orlin Velev, a chemistry and biomolecular-engineering professor at North Carolina State University. "It can be used over large areas because it's very inexpensive and very easy to make." The work is published in the early online edition of the journal Angewandte Chemie.
A number of other researchers have developed color-changing materials, some of which are also controlled with magnetic forces; others use electrical or mechanical forces. The Riverside researchers, led by Yadong Yin, a professor of chemistry, however, are able to pack far more magnetic material per spherical building block that was previously possible. Sanford Asher, a professor of chemistry and materials science at the University of Pittsburgh who has encapsulated magnetite particles in polymer spheres, says that the new approach increases the amount of magnetic material by fivefold.
As a result, the new materials can be tuned to a larger number of colors than previously made materials. Indeed, North Carolina State's Velev, who works on materials that change color in response to electronic signals, says he knows of no other material capable of taking on such a wide range of colors.
The Riverside researchers found that processing the materials at high temperatures ensured that the 10-nanometer particles formed with a crystalline atomic structure. It also caused the particles to group together to form similarly sized clusters. In contrast, more commonly used room-temperature synthesis results in particles that form irregular agglomerations. The uniformity of the clusters and the crystallinity of the particles seem to improve the magnetic response of the materials, Yin says, although he and his colleagues are still looking into the underlying mechanisms involved.
The materials can switch colors at a rate of twice a second, which is still too slow for use in TVs and computer monitors. Yin hopes to increase switching speeds still more by using smaller amounts of material, perhaps in microscopic capsules. Such small amounts will make it easier to present a uniform magnetic field to the entire sample, potentially aiding the rearrangement of the clusters. Also, such microcapsules could be arranged to form pixels in a display, as is done now with E-Ink, a type of electronic paper used in some electronic book readers and a cell phones. (See "A Good Read.")
But even with faster speeds, Yin doesn't expect the materials to replace current computer-monitor technology. Rather, he has his sights set on larger-scale applications that would take advantage of the low cost of the materials. Examples could include posters that can be rewritten but don't have to change as fast as displays of video.
One significant drawback of the current materials is that they would need a constant power supply to preserve the magnetic field and hold the microcapsules at a set color. Yin's next step is to develop a version of the materials that remains stable after their color is changed--that is, until they're switched to a new color. If this is possible, then a poster could be printed with something like the read-write head on a hard drive, Yin says. It would preserve the image until it's rewritten with another pass of the print head, using no power in between.
"At this stage it's fun to play with," Velev says. "Maybe at later stages it could be used for some decorative purpose, such as paint that changes color, or some new types of labels or display boards. Right now it's a beautiful piece of research."


Source: http://www.technologyreview.com

Storing Light

A new optical device could make high-speed computing and communications possible.
By Kevin Bullis

A microscopic device for storing light developed by researchers at Cornell University could help free up bottlenecks in optical communications and computing. This could potentially improve computer and communications speeds by an order of magnitude.
The new device relies on an optically controlled "gate" that can be opened and closed to trap and release light. Temporarily storing light pulses could make it possible to control the order in which bits of information are sent, as well as the timing, both of which are essential for routing communications via fiber optics. Today, such routing is done, for the most part, electronically, a slow and inefficient process that requires converting light pulses into electrons and back again. In computers, optical memory could also make possible optical communication between devices on computer chips.
Switching to optical routing has been a challenge because pulses of light, unlike electrons, are difficult to control. One way to slow down the pulses and control their movement would be to temporarily confine them to a small continuous loop. (See "Tiny Device Stores Light.") But the problem with this approach is getting the light in and out of such a trap, since any entry point will also serve as an exit that would allow light to escape. What's needed is a way to close the entryway once the light has entered, and to do so very quickly--in less time than it takes for the light to circle around the loop and escape. Later, when the light pulse is needed, the entryway could be opened again.
The Cornell researchers, led by Michal Lipson, a professor of electrical and computer engineering at the university, use a very fast, 1.5-picosecond pulse of light to open and close the entryway. The Cornell device includes two parallel silicon tracks, each 560 nanometers wide. Between these two tracks, and nearly touching them, are two silicon rings spaced a fraction of the width of a hair apart. To trap the light in these rings, the researchers turned to some of their earlier work, in which they found that the rings can be tuned to detour different colors by shining a brief pulse of light on them.
Light of a certain color passes along the silicon track, takes a detour through one of the rings, and then rejoins the silicon track and continues on its way. However, if the rings are retuned to the same frequency the moment after a light pulse enters a ring, the light pulse will circulate between the rings in a continuous loop rather than rejoin the silicon track and escape. Tuning the rings to different frequencies again, such as by shining another pulse on one of the rings, allows the light to escape this circuit and continue on to its destination.
Work remains to be done before such a device will function in a commercial system. So far, the rings only capture part of a pulse of light. As a result, any information encoded in the shape of the overall pulse is lost. This can be solved by compressing the pulse and using a cascade of rings, says Mehmet Yanik, a professor of electrical engineering and computer science at MIT.
The other issue is that the length of time a light pulse can be stored is relatively short, Lipson says. If the light stays in the ring for too long, it will be too weak to use. Lipson says it might be possible to make up for light losses by amplifying the light signal after it leaves the rings to restore any lost power.
Other schemes for storing light have been demonstrated in the past, but these were impractical, requiring carefully controlled conditions, for example, or a large, complicated system. The new approach is an important step forward because it makes it possible to store light in ambient conditions and in a very small device, says Marin Soljacic, a professor of physics at MIT. Once you've done that, he says, "then it becomes interesting to industry."

Nano Memory

A nanowire device 100 times as dense as today's memory chips.
By Kevin Bullis

Two layers of 400 nanowires (blue and gray areas) encode data on molecules where they cross. Red lines are electrodes.
Credit: Jonathan E. Green and Habib Ahmad

Researchers at Caltech and the University of California, Los Angeles, have reached a new milestone in the effort to use individual molecules to store data, an approach that could dramatically shrink electronic circuitry. One hundred times as dense as today's memory chips, the Caltech device is the largest-ever array of memory bits made of molecular switches, with 160,000 bits in all. In the device, information is stored in molecules called rotaxanes, each of which has two components. One is barbell shaped; the other is a ring of atoms that moves between two stations on the bar when a voltage is applied. Two perpendicular layers of 400 nanowires deliver the voltage, reading or writing information. It's a big step forward from earlier prototype arrays of just a few thousand bits. "We thought that if we weren't able to make something at this scale, people would say that this is just an academic exercise," says James Heath, professor of chemistry at Caltech and one of the project's researchers. He cautions, however, that "there are problems still. We're not talking about technology that you would expect to come out tomorrow."

Nanowire LEDs

Infrared light-emitting nanowires could lead to optical communications on microchips.

By Kevin Bullis

Microscopic LED: A thin indium-nitride nanowire spans two electrodes. When a current is applied, it emits infrared light.
Credit: IBM Research

Researchers at IBM Research in Yorktown Heights, NY, have demonstrated a new way to convert electricity into light in nanowire-based light-emitting devices (LEDs). The nanowire LEDs could eventually be used for telecommunications and for faster communications between devices on microchips. The approach could also pave the way for a new type of bright, efficient display.

The researchers built an LED resembling a transistor that consists of an indium-nitride nanowire stretched between two electrodes on top of a silicon substrate. The nanowire is about 100 nanometers wide and spans a distance of less than 10 micrometers. When the researchers apply a current to the nanowire, it emits light. While nanowires that emit light have been made before, the new devices rely on different physical mechanisms that are simpler; as a result, the nanowire LED could be more efficient and have improved performance. What's more, the device succeeds in emitting infrared light, which has been particularly difficult for nanowires to do, says Phaedon Avouris, one of the IBM researchers.

Typically, light in LEDs is produced by injecting both electrons and their positive counterparts, holes, into an active material, where they combine and emit light. With the new devices, the researchers only have to inject electrons; these cause electrons and holes to form locally, inside the nanowires. The mechanism could be more efficient because a single electron can be used to generate more than one electron-hole pair. What's more, the researchers have demonstrated that the nanowires can produce more intense light emission than other LEDs.

The nanowires' small size and compatibility with silicon make them attractive for integration on chips, says Eugene Fitzgerald, a professor of materials science and engineering at MIT. The nanowires also emit infrared light, which makes them ideal for fiber-optic telecommunications and for optical communications between devices on microchips that could help dramatically speed up computers.

The nanowire LEDs extend the range of colors that can be emitted from nitride-based materials, Fitzgerald says. Nitride materials are the basis of the blue lasers in high-definition DVD players, he says, and they have also been useful for emitting green light. If the nanowires can be tuned to emit red light, as seems likely, then red, green, and blue LEDs could all be created with variations of the same material, making it practical to manufacture them all on the same substrate. Eventually, it may be possible to arrange such LEDs into the pixels of full-color displays that are brighter, more efficient, and better looking than today's flat-panel LCD displays, Fitzgerald says.

Not only did the wires emit infrared light, but they also showed a peculiar ability to emit more intense light as temperatures rose; ordinarily, at high temperatures light emission dims or stops. This could lead to LEDs that can withstand high temperatures, a property that could be useful for certain military applications, Avouris says.

The novel physical mechanisms underlying the indium-nitride nanowires' ability to emit light might have wider implications for nanowire research. If the mechanism used here works in other materials, it could expand the number of materials that might be used to create LEDs, Fitzgerald says. That could make LEDs cheaper and give researchers far greater versatility in creating devices with improved performance.

Ultrastrong Paper from Graphene

A new paperlike material could lead to novel types of light and flexible materials.

By Prachi Patel-Predd

The right stuff: Researchers at Northwestern University have reassembled one-atom-thick graphene sheets that make up soft and flaky graphite crystals in order to create a tough, flexible, paperlike material.
Credit: Dmitriy Dikin

Using graphite--the black flaky stuff employed in pencils--researchers at Northwestern University have created a strong, flexible, and lightweight paperlike material. It could be used as electrolytes or hydrogen storage materials in fuel cells, electrodes in supercapacitors and batteries, and super-thin chemical filters. It could also be mixed with polymers or metals to make materials for use in aircraft fuselages, cars, and buildings.

The new material is made of overlapping layers of graphene, one-atom-thick sheets of carbon atoms arranged in honeycomb-like hexagons. In contrast, graphite, which becomes powdery under pressure, is made of graphene sheets stacked one on top of the other.

Rodney Ruoff, a Northwestern nanoengineering professor who led the work, published in Nature this week, says that the methods behind making the novel graphene paper could lead to even stronger versions. Right now, water molecules hold together the individual 10-nanometer-thick graphene flakes to create the micrometers-thick graphene paper. By using other chemicals as glues, the researchers could make ultrastrong paperlike materials with various properties. "The future is particularly bright because the system is very flexible ... The chemistry is almost infinite," Ruoff says.

Individual sheets of graphene were not known to exist until three years ago, when Andre Geim, a professor of physics at the University of Manchester, in the UK, used adhesive tape to get a few flakes of graphene from a graphite crystal. Researchers still don't understand all of graphene's properties, but they know that it can conduct electrons extremely well and is known to be exceptionally strong. "Graphene is the toughest material in the world--tougher than diamond," Geim says. But in graphite, the graphene sheets are assembled in such a way that they do not bind strongly to each other. So they simply flake off under friction, creating a pencil's black marks.

Ruoff's idea was to "disassemble graphite into individual layers and reassemble them in a different way than they are in graphite." The goal was to find a way to glue the graphene platelets together while reassembling them, which would create a tough and flexible material.

Since it's hard to separate the graphene sheets in graphite, the researchers first used an acid to oxidize graphite and make graphite oxide. Then they put the graphite oxide in water. Individual graphene-oxide sheets easily separated in water.

When the researchers filtered the suspension, the graphene-oxide flakes settled down on the filter, randomly overlapping with each other. Water glued the flakes together; its hydrogen atoms bonded with the carbon atoms in adjacent flakes. The result was a dark-brown, thin, flexible graphene-oxide paper. By adjusting the concentration of graphite oxide in the water, the researchers changed the thickness of the paper, ranging from 1 to 100 micrometers.

In an effort to develop superstrong lightweight materials, others have used carbon nanotubes. And the new graphene-oxide paper is not as strong as carbon-nanotube films, Geim says. "The advantage of materials made from carbon nanotubes is they're much tougher, because they entangle like spaghetti," he says. "When you're dealing with flat sheets, they entangle very little and are breakable."

But the graphene-oxide paper has other key advantages. Graphite is a cheap raw material, and the filtration method is simple and leads to lots of graphene. Most important, the Northwestern researchers' work opens up a way to manipulate graphene sheets and make paperlike materials with different properties.

When Ruoff and his colleagues oxidize graphene into graphene oxide, for instance, the carbon-based material goes from being an electrical conductor to being an insulator. Ruoff says that he can alter graphene's chemistry in other ways to change its electrical properties and make it an insulator, a conductor, or even a semiconductor.

That electrical versatility combines with an ultrastrong material has some observers excited. "They haven't used any tough glue between the [graphene platelets]," Geim says. "I expect very, very tough materials if a proper glue between graphene is used."

Self-Assembling Nanostructures

Researchers find an easy route to complex nanomaterials.

By Kevin Bullis

No assembly required: Nanorods of cadmium sulfide with silver-sulfide quantum dots (dark spots) form automatically when researchers mix together the right starting chemicals.
Credit: Paul Alivisatos/University of California, Berkeley

Researchers at the University of California, Berkeley, have found an easy way to make a complex nanostructure that consists of tiny rods studded with nanocrystals. The new self-assembly synthesis method could lead to intricate nanomaterials for more-efficient solar cells and less expensive devices for directly converting heat into electricity.

In the structures, the quantum dots are all about the same size and are spaced evenly along the rods--a feat that in the past required special conditions such as a vacuum, with researchers carefully controlling the size and spacing of different materials, says Paul Alivisatos, the professor of chemistry and materials science at Berkeley who led the work. In contrast, Alivisatos simply mixes together the appropriate starting materials in a solution; these materials then arrange themselves into the orderly structure.

Such solution-processing techniques can lead to manufacturing methods in which materials, such as those used in solar cells, are printed on continuous sheets, driving down costs compared with other methods. "Anytime you make something in solution, rather than in a vacuum, it becomes a lot easier and cheaper," says Moungi Bawendi, a chemistry professor at MIT who was not involved in this work.

To make the rods, Alivisatos mixes a combination of methanol and a silver salt into a solution that already contains cadmium-sulfide nanorods. Cadmium ions have a strong affinity for methanol. As a result, when the materials are mixed, the methanol draws cadmium out of the nanorods. Silver ions then fill in the vacant spots left by the cadmium, forming areas of silver sulfide within the rod. At the same time, differences in the crystalline structures of the cadmiun-sulfide rods and the silver-sulfide quantum dots regulate the dots' size and spacing. This is the first time such differences have been used to control the self-assembly of materials in solution.

The nanocrystal-studded rods could prove useful for solar cells and thermoelectric devices that convert heat directly into electricity. For example, in conventional solar cells, each photon only generates a single electron. But certain kinds of quantum dots convert single photons into multiple electrons, which could more than double the efficiency of solar cells. (See "Silicon and Sun.") The problem has been capturing those electrons to create an electrical current. Embedding quantum dots inside rods of another material could help with this problem, says Alivisatos. The quantum dots would absorb the light, while the other material would capture the electrons that the dots generate.

A similar configuration is promising for thermoelectrics, devices that directly convert heat into electricity. The alternating crystal structures in the nanorods could block the transfer of heat while allowing electrons to pass--two key features of such devices.

Having demonstrated the new method for making the structures, Alivisatos and his colleagues are beginning to study the potential photoelectric and thermoelectric properties of the materials. They will likely need to turn to different compounds, such as copper sulfide and cadmium sulfide--a combination that has been used for solar cells in the past, Alivisatos says. There's no guarantee, however, that these materials will form the same orderly structures, or indeed that the structures will perform as the researchers hope they will.

Even if these particular structures do not prove to be the key to low-cost, high-efficiency solar cells, the new self-assembly method for making nanostructures could inspire new materials that are. And Bawendi highlights the need to continue basic research like this to solve today's energy problems. "We don't know what the solution is going to be," he says. But if we create high-quality, carefully described materials as Alivisatos has done, "some of them may be the answer," Bawendi says.

Self-Assembling Nanostructures

Researchers find an easy route to complex nanomaterials.

By Kevin Bullis

No assembly required: Nanorods of cadmium sulfide with silver-sulfide quantum dots (dark spots) form automatically when researchers mix together the right starting chemicals.
Credit: Paul Alivisatos/University of California, Berkeley

Researchers at the University of California, Berkeley, have found an easy way to make a complex nanostructure that consists of tiny rods studded with nanocrystals. The new self-assembly synthesis method could lead to intricate nanomaterials for more-efficient solar cells and less expensive devices for directly converting heat into electricity.

In the structures, the quantum dots are all about the same size and are spaced evenly along the rods--a feat that in the past required special conditions such as a vacuum, with researchers carefully controlling the size and spacing of different materials, says Paul Alivisatos, the professor of chemistry and materials science at Berkeley who led the work. In contrast, Alivisatos simply mixes together the appropriate starting materials in a solution; these materials then arrange themselves into the orderly structure.

Such solution-processing techniques can lead to manufacturing methods in which materials, such as those used in solar cells, are printed on continuous sheets, driving down costs compared with other methods. "Anytime you make something in solution, rather than in a vacuum, it becomes a lot easier and cheaper," says Moungi Bawendi, a chemistry professor at MIT who was not involved in this work.

To make the rods, Alivisatos mixes a combination of methanol and a silver salt into a solution that already contains cadmium-sulfide nanorods. Cadmium ions have a strong affinity for methanol. As a result, when the materials are mixed, the methanol draws cadmium out of the nanorods. Silver ions then fill in the vacant spots left by the cadmium, forming areas of silver sulfide within the rod. At the same time, differences in the crystalline structures of the cadmiun-sulfide rods and the silver-sulfide quantum dots regulate the dots' size and spacing. This is the first time such differences have been used to control the self-assembly of materials in solution.

The nanocrystal-studded rods could prove useful for solar cells and thermoelectric devices that convert heat directly into electricity. For example, in conventional solar cells, each photon only generates a single electron. But certain kinds of quantum dots convert single photons into multiple electrons, which could more than double the efficiency of solar cells. (See "Silicon and Sun.") The problem has been capturing those electrons to create an electrical current. Embedding quantum dots inside rods of another material could help with this problem, says Alivisatos. The quantum dots would absorb the light, while the other material would capture the electrons that the dots generate.

A similar configuration is promising for thermoelectrics, devices that directly convert heat into electricity. The alternating crystal structures in the nanorods could block the transfer of heat while allowing electrons to pass--two key features of such devices.

Having demonstrated the new method for making the structures, Alivisatos and his colleagues are beginning to study the potential photoelectric and thermoelectric properties of the materials. They will likely need to turn to different compounds, such as copper sulfide and cadmium sulfide--a combination that has been used for solar cells in the past, Alivisatos says. There's no guarantee, however, that these materials will form the same orderly structures, or indeed that the structures will perform as the researchers hope they will.

Even if these particular structures do not prove to be the key to low-cost, high-efficiency solar cells, the new self-assembly method for making nanostructures could inspire new materials that are. And Bawendi highlights the need to continue basic research like this to solve today's energy problems. "We don't know what the solution is going to be," he says. But if we create high-quality, carefully described materials as Alivisatos has done, "some of them may be the answer," Bawendi says.