Graphene Replaces Traditional Silicon Substrates in Future Devices


Researchers at the Norwegian University of Science and Technology (NTNU) have patented and are commercializing a method by which gallium arsenide (GaAs) nanowires are grown on graphene.

The method, which was described and published in the journal Nano Letters (“Vertically Aligned GaAs Nanowires on Graphite and Few-Layer Graphene: Generic Model and Epitaxial Growth”), employs Molecular Beam Epitaxy (MBE) to grow the GaAs nanowires layer by layer. A video describing the process can be seen below.

 

“We do not see this as a new product,” says Professor Helge Weman, a professor at NTNU’s Department of Electronics and Telecommunications in the press release. “This is a template for a new production method for semiconductor devices. We expect solar cells and light emitting diodes to be first in line when future applications are planned.”

Whether it is a method or a product, Weman and his colleagues have launched a new company called Crayonano that will be commercializing the hybrid material that the researchers developed.

The researchers contend that replacing traditional semiconductor materials as a substrate will reduce material costs. The silicon materials are fairly expensive and are usually over 500µm thick for 100mm wafers. As the video explains, using graphene reduces the substrate thickness to the width of one atom. Obviously reduction in material is really only a side benefit to the use of graphene. The real advantage is that the electrode is transparent and flexible, thus its targeting for solar cells and LEDs.

Interestingly Weman sees his team’s work as a compliment to the work of companies like IBM that have used graphene “to make integrated circuits on 200-mm wafers coated with a continuous layer of the atom-thick material.”

Weman notes: “Companies like IBM and Samsung are driving this development in the search for a replacement for silicon in electronics as well as for new applications, such as flexible touch screens for mobile phones. Well, they need not wait any more. Our invention fits perfectly with the production machinery they already have. We make it easy for them to upgrade consumer electronics to a level where design has no limits.”

As magnanimous as Weman’s invitation sounds, one can’t help but think it comes from concern. The prospect of a five-year-development period before a product gets to market might be somewhat worrying for a group of scientists who just launched a new startup. A nice licensing agreement from one of the big electronics companies must look appealing right about now.

 

Light might prompt graphene devices on demand


MIKE WILLIAMS

 – OCTOBER 10, 2012

Rice University researchers find plasmonics show promise for optically induced electronics

Rice University researchers are doping graphene with light in a way that could lead to the more efficient design and manufacture of electronics, as well as novel security and cryptography devices.

Graphene circuitry

Nanoscale plasmonic antennas called nonamers placed on graphene have the potential to create electronic circuits by hitting them with light at particular frequencies, according to researchers at Rice University. The positively and negatively doped graphene can be prompted to form phantom circuits on demand.

Manufacturers chemically dope silicon to adjust its semiconducting properties. But the breakthrough reported in the American Chemical Society journal ACS Nano details a novel concept: plasmon-induced doping of graphene, the ultrastrong, highly conductive, single-atom-thick form of carbon.

That could facilitate the instant creation of circuitry – optically induced electronics – on graphene patterned with plasmonic antennas that can manipulate light and inject electrons into the material to affect its conductivity.

The research incorporates both theoretical and experimental work to show the potential for making simple, graphene-based diodes and transistors on demand. The work was done by Rice scientists Naomi Halas, Stanley C. Moore Professor in Electrical and Computer Engineering, a professor of biomedical engineering, chemistry, physics and astronomy and director of the Laboratory for Nanophotonics; and Peter Nordlander, professor of physics and astronomy and of electrical and computer engineering; physicist Frank Koppens of the Institute of Photonic Sciences in Barcelona, Spain; lead author Zheyu Fang, a postdoctoral researcher at Rice; and their colleagues.

“One of the major justifications for graphene research has always been about the electronics,” Nordlander said. “People who know silicon understand that electronics are only possible because it can be p- and n-doped (positive and negative), and we’re learning how this can be done on graphene.

“The doping of graphene is a key parameter in the development of graphene electronics,” he said. “You can’t buy graphene-based electronic devices now, but there’s no question that manufacturers are putting a lot of effort into it because of its potential high speed.”

Researchers have investigated many strategies for doping graphene, including attaching organic or metallic molecules to its hexagonal lattice. Making it selectively – and reversibly – amenable to doping would be like having a graphene blackboard upon which circuitry can be written and erased at will, depending on the colors, angles or polarization of the light hitting it.

Nonamers

Nonamers in the drawings at top and in the photos at bottom are arrays of nine gold nanoparticles deposited on graphene and tuned to particular frequencies of light. When illuminated, the plasmonic particles pump electrons into the graphene, according to researchers at Rice University who say the technology may lead to the creation of on-demand circuitry for electronic devices.

The ability to attach plasmonic nanoantennas to graphene affords just such a possibility. Halas and Nordlander have considerable expertise in the manipulation of the quasiparticles known as plasmons, which can be prompted to oscillate on the surface of a metal. In earlier work, they succeeded in depositing plasmonic nanoparticles that act as photodetectors on graphene.

These metal particles don’t so much reflect light as redirect its energy; the plasmons that flow in waves across the surface when excited emit light or can create “hot electrons” at particular, controllable wavelengths. Adjacent plasmonic particles can interact with each other in ways that are also tunable.

That effect can easily be seen in graphs of the material’s Fano resonance, where the plasmonic antennas called nonamers, each a little more than 300 nanometers across, clearly scatter light from a laser source except at the specific wavelength to which the antennas are tuned. For the Rice experiment, those nonamers – eight nanoscale gold discs arrayed around one larger disc – were deposited onto a sheet of graphene through electron-beam lithography. The nonamers were tuned to scatter light between 500 and 1,250 nanometers, but with destructive interference at about 825 nanometers.

At the point of destructive interference, most of the incident light energy is converted into hot electrons that transfer directly to the graphene sheet and change portions of the sheet from a conductor to an n-doped semiconductor.

Arrays of antennas can be affected in various ways and allow phantom circuits to materialize under the influence of light. “Quantum dot and plasmonic nanoparticle antennas can be tuned to respond to pretty much any color in the visible spectrum,” Nordlander said. “We can even tune them to different polarization states, or the shape of a wavefront.

“That’s the magic of plasmonics,” he said. “We can tune the plasmon resonance any way we want. In this case, we decided to do it at 825 nanometers because that is in the middle of the spectral range of our available light sources. We wanted to know that we could send light at different colors and see no effect, and at that particular color see a big effect.”

Nordlander said he foresees a day when, instead of using a key, people might wave a flashlight in a particular pattern to open a door by inducing the circuitry of a lock on demand. “Opening a lock becomes a direct event because we are sending the right lights toward the substrate and creating the integrated circuits. It will only answer to my call,” he said.

Rice co-authors of the paper are graduate students Yumin Wang and Andrea Schlather, research scientist Zheng Liu, and Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry.

The research was supported by the Robert A. Welch Foundation, the Office of Naval Research, the Department of Defense National Security Science and Engineering Faculty Fellows program and Fundacio Cellex Barcelona.

Topological Superconductors: NIST and the U of Maryland


Seeking a robust home for qubits | October 8th, 2012

If quantum computers are ever going to perform all those expected feats of code-breaking and number crunching, then their component qubits—tiny ephemeral quantum cells held in a superposition of internal states—will have to be protected from intervention by the outside world. In other words, decoherence, the loss of the qubits’ quantum integrity, has to be postponed. Now theoretical physicists at the Joint Quantum Institute (JQI) and the University of Maryland have done an important step forward to understand qubits in a real-world setup. In a new study they show, for the first time, that qubits can successfully exist in a so called topological superconductor material even in the presence of impurities in the material and strong interactions among participating electrons.

To see how qubits can enter into their special coherence-protection program, courtesy of “Majorana particles,” an exotic form of excitation, some groundwork has to be laid.

Quantum Materials

topsup-01Figure 1
Credit: Emily Edwards
Click for Hi-Res

Most designs for qubits involve materials where quantum effects are important. In one such material, superconductors (SC), electrons pair up and can then enter into a large ensemble, a supercurrent, which flows through the material without suffering energy loss. Another material is a sandwich of semiconductors which support the quantum Hall effect (QHE). Here, very low temperatures and a powerful external magnetic field force electrons in a thin boundary layer to execute tiny cyclone motions (not exactly, but ok—also isn’t a cyclone a storm?). At the edge of these layers, the electrons, unable to trace out a complete circular path, will creep along the edge, where they constitute a net electrical current.

 

One of the most interesting and useful facts about these electrons at the edge is that they move in one direction. They cannot scatter backwards no matter how many impurities (which in ordinary conductors can lead to energy dissipation) may be in the material. If, furthermore, the electrons can be oriented according to their spin—their intrinsic angular momentum—then we get what is called the quantum spin Hall effect (QSH). In this case all electrons with spin up will circulate around the material (at the edge) in one direction, while electrons with spin down will circulate around in the opposite direction.

The QHE state is depicted in figure 1.

Topological Materials

 

topsup-02Figure 2
Credit: Emily Edwards
Click for Hi-Res

In some materials the underlying magnetism of the nuclei in the atoms making of the material is so strong than no external magnet is needed to create the Hall effects. Mercury-cadmium-telluride compounds are examples of materials called topological insulators. Insulators (not sure how this sentence was supposed to start, but grammatically is currently confusing) because even as electrons move around the edge of the material with very little loss of energy, the interior of these 3-dimensional structures is an insulator; no current flows. The “topological” is a bit harder to explain. Partly the flow of current on the outside bespeaks of geometry: the electrons flow only at the edge and are unable (owing to quantum interactions) from scattering backwards if they meet an impediment.

 

But topology in this case has more to do with the way in which the motion of the electrons in these materials are described in terms of “dispersion relations.” Just as waves of white light will be dispersed into a spectrum of colors when the waves strike the oblique side of a prism, so electron waves (electrons considered as quantum waves) will be “dispersed,” in the sense that electrons with the same energy might have different momenta, depending on how the electrons move through the material in question.

The idea of electron dispersal is often depicted in the form of an energy-level diagram. In insulators (the left panel of Figure 2) electrons remain in a valence band; they don’t have enough energy to visit the conduction band of energies; hence the electrons do not move; the material is an insulator against electricity. In a conductor (middle part) the conduction and valence bands overlap. In the QHE (right panel) electrons in the interior of the material also do not move along; the bulk of the material is an insulator. But for electrons at the edge there is a chance for movement into the conduction band.

Now for the topology: just as a coffee cup is equivalent to a donut topologically—either can be transformed into the other by stretching but not by any tearing—so here the valence band can be transformed into a conduction band (at least for edge states) no matter what impurities might be present in the underlying material. In other words, the “topological” nature of the material offers some protection for the flow of electrons against the otherwise-dissipating effects of impurities.

The marvelous properties of superconductors and topological materials can be combined. If a one-dimensional topological specimen—a nanowire made from indium and arsenic—is draped across a superconductor (niobium, say) then the superconductivity can extend into the wire (proximity effect). And in this conjunction of materials, still another hotly-pursued effect can come into play.

Majorana Particles

 

topsup-03Figure 3
The orange balls (the letter gamma) mark the two places, at either end of the nanowire, where the Majorana excitations appear. The wire sits atop the superconductor. (Credit: Alejandro Lobos)
Click for Hi-Res

One last concept is needed here—Majorana particles—named for the Italian physicist Ettore Majorana, who predicted in 1937 the existence of a class of particle that would serve as its own antiparticle. Probably this object would not exist usefully in the form of a single real particle but would, rather, appear in a material as a quasiparticle, an ensemble excitation of many electrons.

 

Some scientists believe that qubits made from Majorana pulses excited in topological materials (and benefitting from the same sort of topological protection that benefits, say, electrons in QHE materials) would be much more immune from decoherence than other qubits based on conventional particles.

Specifically Sankar Das Sarma and his colleagues at the University of Maryland (JQI and the Condensed Matter Theory Center) predicted that Majorana particles would appear in topological quantum nanowires. In fact part of the Majorana excitation would appear at both ends of the wire. These predictions were borne out. It is precisely the separation of these two parts (each of which constitutes a sort of “half electron”) that confers some of the anticipated coherence-protection: a qubit made of that Majorana excitation would not be disrupted by merely a local irregularity in the wire.

A recent experiment in Holland provides preliminary evidence for exactly this occurrence (***).

Robust Qubits Amid Disorder

 

topsup-04Figure 4
Here is a picture of the prospective experimental setup in which Majorana particles could be made in a hybrid superconductor-nanowire material. In (a) the topological semiconductor bridges a gap between two parts of a superconductor. The letter phi represents an external magnetic field which can tailor conditions in the semiconductor. In (b) the immediate nanowire (gray rod) environment is shown. L is the size of the gap between the superconductor halves while L1 is the distance over which the underlying superconductivity will persist within the overlying semiconductor. (c) shows how the superconductor (SC)-semiconductor (SM) sandwich can be further tuned by a nearby electrical circuit. (Credit: Lutchyn, Sau and Das Sarma)
Click for Hi-Res

One of the authors of the new study, Alejandro Lobos, said that the earlier Maryland prediction, useful as it was, was still somewhat idealistic in that it didn’t fully grapple with the presence of impurities, a fact of life which all engineers of actual computers must confront. This is what the new paper, which appears in the journal Physical Review Letters, addresses.

 

The problem of impurities or defects (which flowing electrons encounter as a form of disorder) is especially important for components which are two or even one dimensional in nature. The same is true for the repulsive force among electrons. “In 3-dimensional materials,” said Lobos, “electrons (and their screening clouds of surrounding holes) can avoid each other thanks to the availability of space. They can just go around each other. In 1-D materials, this is not possible, since electrons cannot pass each other. In 1D, if one electron wants to move, it has to move all the other electrons! This ensures that excitations in a 1D metal are necessarily collective, as opposed to the single-particle excitations existing in a 3D metal.

So, in summary, the new Maryland work shows that disorder and electron interactions, two things that normally work to disrupt superconductivity, can be overcome with careful engineering of the material. “A number of important theoretical studies before ours have focused on the destabilizing effects of either disorder or interaction on topological superconductors,” said Lobos. “These studies showed the extent to which a topological superconductor could survive under these effects separately. But to make contact with real materials, disorder and interactions have to be considered on equal footing and simultaneously, a particular requirement imposed by the one-dimensional geometry of the system. It was then an important question to determine if it was possible to stabilize a topological superconductor under their simultaneous presence. The good news is that the answer is yes: despite their detrimental effect, there is still a sizable range of parameters where topological superconductors hosting Majorana excitations can exist. That’s the main result of our study, which will be useful to understand and characterize topological superconductors in more realistic situations.”

 

(*) The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.

(**) “Interplay of disorder and interaction in Majorana quantum wires,” Alejandro M. Lobos, Roman M. Lutchyn, and S. Das Sarma, Physical Review Letters, 5 October 2012

(***) See earlier Majorana JQI press release and several pertinent research papers

Alejandro M. Lobos, (301)405-0603, alobos@umd.edu

Press contact: Phillip F. Schewe, pschewe@umd.edu, 301-405-0989. http://jqi.umd.edu/

MIT team builds most complex synthetic biology circuit yet


New sensor can detect four different molecules, could be used to program cells to precisely monitor their environments.

MIT team builds most complex synthetic biology circuit yetUsing genes as interchangeable parts, synthetic biologists design cellular circuits that can perform new functions, such as sensing environmental conditions. However, the complexity that can be achieved in such circuits has been limited by a critical bottleneck: the difficulty in assembling genetic components that don’t interfere with each other. 

 

Unlike electronic circuits on a silicon chip, biological circuits inside a cell cannot be physically isolated from one another. “The cell is sort of a burrito. It has everything mixed together,” saysChristopher Voigt, an associate professor of biological engineering at MIT.

Because all the cellular machinery for reading genes and synthesizing proteins is jumbled together, researchers have to be careful that proteins that control one part of their synthetic circuit don’t hinder other parts of the circuit.

Voigt and his students have now developed circuit components that don’t interfere with one another, allowing them to produce the most complex synthetic circuit ever built. The circuit, described in the Oct. 7 issue of Nature, integrates four sensors for different molecules. Such circuits could be used in cells to precisely monitor their environments and respond appropriately.

“It’s incredibly complex, stitching together all these pieces,” says Voigt, who is co-director of the Synthetic Biology Center at MIT. Larger circuits would require computer programs that Voigt and his students are now developing, which should allow them to combine hundreds of circuits in new and useful ways.

Lead author of the paper is MIT postdoc Tae Seok Moon. Other authors are MIT postdoc Chunbo Lou and Alvin Tamsir, a graduate student at the University of California at San Francisco.

Expanding the possibilities

Previously, Voigt has designed bacteria that can respond to light and capture photographic images, and others that can detect low oxygen levels and high cell density — both conditions often found in tumors. However, no matter the end result, most of his projects, and those of other synthetic biologists, use a small handful of known genetic parts. “We were just repackaging the same circuits over and over again,” Voigt says.

To expand the number of possible circuits, the researchers needed components that would not interfere with each other. They started out by studying the bacterium that causes salmonella, which has a cellular pathway that controls the injection of proteins into human cells. “It’s a very tightly regulated circuit, which is what makes it a good synthetic circuit,” Voigt says.

The pathway consists of three components: an activator, a promoter and a chaperone. A promoter is a region of DNA where proteins bind to initiate transcription of a gene. An activator is one such protein. Some activators also require a chaperone protein before they can bind to DNA to initiate transcription.

The researchers found 60 different versions of this pathway in other species of bacteria, and found that most of the proteins involved in each were different enough that they did not interfere with one another. However, there was a small amount of crosstalk between a few of the circuit components, so the researchers used an approach called directed evolution to reduce it. Directed evolution is a trial-and-error process that involves mutating a gene to create thousands of similar variants, then testing them for the desired trait. The best candidates are mutated and screened again, until the optimal gene is created.

Aindrila Mukhopadhyay, a staff scientist at Lawrence Berkeley National Laboratory, says the amount of troubleshooting the researchers did to create each functional module is impressive. “A lot of people are charmed by the idea of creating complex genetic circuits. This study provides valuable examples of the types of optimizations that they may have to do in order to accomplish such goals,” says Mukhopadhyay, who was not part of the research team.

Layered circuits

To design synthetic circuits so they can be layered together, their inputs and outputs must mesh. With an electrical circuit, the inputs and outputs are always electricity. With these biological circuits, the inputs and outputs are proteins that control the next circuit (either activators or chaperones).

These components could be useful for creating circuits that can sense a variety of environmental conditions. “If a cell needs to find the right microenvironment — glucose, pH, temperature and osmolarity [solute concentration] — individually they’re not very specific, but getting all four of those things really narrows it down,” Voigt says.

The researchers are now applying this work to create a sensor that will allow yeast in an industrial fermenter to monitor their own environment and adjust their output accordingly.

The research was funded by the U.S. Office of Naval Research and the National Institutes of Health.

 

Solar Cell Consisting of a Single Molecule: Individual Protein Complex Generates Electric Current


ScienceDaily (Oct. 2, 2012) — An team of scientists, led by Joachim Reichert, Johannes Barth, and Alexander Holleitner (Technische Universitaet Muenchen, Clusters of Excellence MAP and NIM), and Itai Carmeli (Tel Aviv University) developed a method to measure photocurrents of a single functionalized photosynthetic protein system. The scientists could demonstrate that such a system can be integrated and selectively addressed in artificial photovoltaic device architectures while retaining their biomolecular functional properties.


The proteins represent light-driven, highly efficient single-molecule electron pumps that can act as current generators in nanoscale electric circuits.
 Photosystem-I (green) is optically excited by an electrode (on top). An electron then is transferred step by step in only 16 nanoseconds. (Credit: Christoph Hohmann (NIM))

The interdisciplinary team publishes the results in Nature Nanotechnologythis week.

The scientist investigated the photosystem-I reaction center which is a chlorophyll protein complex located in membranes of chloroplasts from cyanobacteria. Plants, algae and bacteria use photosynthesis to convert solar energy into chemical energy. The initial stages of this process — where light is absorbed and energy and electrons are transferred — are mediated by photosynthetic proteins composed of chlorophyll and carotenoid complexes. Until now, none of the available methods were sensitive enough to measure photocurrents generated by a single protein. Photosystem-I exhibits outstanding optoelectronic properties found only in photosynthetic systems. The nanoscale dimension further makes the photosystem-I a promising unit for applications in molecular optoelectronics.

The first challenge the physicists had to master was the development of a method to electrically contact single molecules in strong optical fields. The central element of the realized nanodevice are photosynthetic proteins self-assembled and covalently bound to a gold electrode via cysteine mutation groups. The photocurrent was measured by means of a gold-covered glass tip employed in a scanning near-field optical microscopy set-up. The photosynthetic proteins are optically excited by a photon flux guided through the tetrahedral tip that at the same time provides the electrical contact. With this technique, the physicists were able to monitor the photocurrent generated in single protein units.

The research was supported by the German Research Foundation (DFG) via the SPP 1243 (grants HO 3324/2 and RE 2592/2), the Clusters of Excellence Munich-Centre for Advanced Photonics and Nanosystems Initiative Munich, as well as ERC Advanced Grant MolArt (no. 47299).

Why quantum dots can join every aspect of everyday life


 

Sheet of semiconductor crystals

Tiny bits of semiconductor crystals – so-called quantum dots – have such remarkable properties that scientists think they will soon be used in everything from light bulbs to the design of ultra-efficient solar cells. Photograph: Science Photo Library

The properties of a material were once thought to be defined only by its chemical composition. But size matters too, especially for semiconductors. Make crystals of silicon small enough – less than 10 nanometres – and their tiny dimensions can start to dictate how the atoms behave and react in the presence of other things.

These tiny bits of semiconductor crystals – so-called quantum dots – have such remarkable, novel properties that scientists think they will soon be used in everything from light bulbs to imaging of cancer cells or in the design of ultra-efficient solar cells.

Semiconductors such as silicon or indium arsenide are chosen to build electronic circuits because of the discrete energy levels at which they can give off electrons or photons. This makes them useful in building switches, transistors and other devices. It was once thought these energy levels – known as band gaps – were fixed. But shrinking the physical size of the semiconductor material to quantum-dot level seems able to change the band gaps, altering the wavelengths of light the material can emit or changing the energy it takes to change a material from an insulator to a conductor.

Instead of looking for brand new materials to build different devices, then, quantum dots make it possible to use a single type of semiconductor to produce a range of different characteristics. Researchers could tune dots made from silicon to emit a range of different colours in different situations, for example, instead of having to use a range of materials with different chemical
compositions.

“The main application for quantum dots at the moment is biological tagging of cells,” says Paul O’Brien, a professor of inorganic materials at the University of Manchester and co-founder of Nanoco Technologies a quantum dot manufacturer also based in Manchester. They are used in the same way as fluorescent dyes, to label agents, he says, but with the advantage that a single laser source can be used to illuminate many different tags each with a specific wavelength.

By attaching different types of quantum dots to proteins that target and attach to specific cell types in the body, these bits of semiconductor can be used by doctors to monitor different kinds of cells. When a laser is then directed on to tagged cells, doctors can see what colour they glow.

The ability to shine also makes quantum dots well suited to produce white light. Existing white bulbs based on low energy light emitting diode (LED) technology tend to produce a garish and bluish form of light that notoriously feels cold, says O’Brien. This is because these LEDs use a phosphor that produces an artificial white light that contains less red wavelengths than natural white light. By embedding quantum dots into a film that is placed over a bulb containing blue LEDs, it is possible to get a much warmer colour of white light. The blue light from the LED stimulates the quantum dots which, in turn, emit light in a range of colours. Provided you have chosen your dots carefully, these will combine to form white light.

The first of these quantum dot lights hit the market in 2010, a partnership between QD Vision, an MIT spinout in Lexington, Massachusetts, and Nexxus Lighting of Charlotte, North Carolina.

Backlights for laptops, tablets and mobile devices are next in line, and they should appear in products before the end of 2012 says VJ Sahi, head of corporate development at materials design company Nanosys of Palo Alto, California. Besides the colour advantages, quantum-dot-based backlights can be three times more efficient than traditional backlights.

Eventually, says Sahi, quantum dots will do more than just light up displays. The long-term aim is use them to create each red, green and blue sub-pixel that makes up a coloured display. This should produce much brighter colours and consume less power than LCD or even the latest state-of-the-art organic LED (OLED) displays. They should also have no problems with viewing angles, he adds.

The interesting properties of quantum dots come from the fact that they behave like tuning forks for photons, a result of a phenomenon called confinement. At less than 10 nanometres in size – about 50 atoms – they fall within the dimensions of a critical quantum characteristic of the material known as the exciton Bohr radius. The energy levels of electrons within the material’s atoms are constrained and, when a photon or electron hits an atom and excites it, the atom re-emits the energy as a photon of a very specific energy level.

Quantum dots also have another trick up their sleeve. Besides converting photons of one energy into photons of another, they can also be used to release electrons and create electrical currents: in other words they can be used to make solar cells. Arthur Nozik at the National Renewable Energy Laboratory in Boulder, Colorado, says that quantum-dot solar cells would be much more efficient at converting the energy from photons and therefore boost the amount of power they can produce.

Such applications are many years from becoming commercial reality. But they serve to demonstrate that no material technology stands still; sometimes all you have to do is cut it down to size.

Flexible lithium-ion battery technology is on the march


By 

August 10, 2012

A newly developed bendable thin-film lithium-ion battery could help bridge the gap to high...

A newly developed bendable thin-film lithium-ion battery could help bridge the gap to high-performance bendable electronics (Photo: Keon Jae Lee/KAIST)

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Researchers at the Korea Advanced Institute of Science and Technology (KAIST) have developed a promising solid state, thin-film lithium-ion battery that claims the highest energy density ever achieved for a flexible battery. The new design, which showed for the first time that high-performance thin films can be used for flexible batteries, may be commercialized as early as next year.

Lithium-ion batteries are a strong candidate for powering the flexible electronics of the future. A high-performance lithium-ion flexible batterywould be a giant step toward fully-fledged flexible electronics systems and would open the door to flexible e-paper, wearable devices, and better piezoelectric systems that harvest energy from mechanical forces.

Research is progressing, but seems to have hit an invisible – though very real – performance wall. This is because most designs employ either low-performance flexible organic materials, or polymer binders that take up too much space and decrease the battery’s power density. In addition, the cathodes have to be treated at high temperatures to improve performance, but this can’t be done effectively on substrates made of flexible polymers.

The new approach developed at KAIST uses high energy density inorganic thin films that can be treated at high temperatures, resulting in the highest-performance flexible lithium-ion batteries yet. “There is no performance difference in energy density, capacity, and cycle life between our flexible battery and bulk batteries,” Prof. Keon Jae Lee, who led the research efforts, told Gizmag. “On the contrary, performance is improved by about 10 percent because of the stress release effect.”

The batteries are built by sequentially depositing several layers – a current collector, a cathode, an electrolyte, an anode, and a protective layer – on a brittle substrate made of mica. Then, the mica is manually delaminated using adhesive tape, and the battery is enclosed between two polymer sheets to improve mechanical resistance.

Bending the battery affects performance, but not to disastrous levels. With the battery constantly bent at a radius of sixteen millimeters (about the same curvature of a fifty-cent coin) the discharge capacity drops by about seven percent after 100 charge-discharge cycles, compared to a three percent drop when the battery is not bent. Voltage was shown to remain almost constant, dropping by a very modest 0.02 V after the battery was bent and released 20,000 times.

“The technology for commercializing this battery could come in a relatively short time, about a year,” says Prof. Lee. But first, the researchers need to find a better, automated way to delaminate the mica substrate – the manual method, involving adhesive tape, is very unpractical and can take up to ten minutes per battery.

“We are investigating a laser lift-off [delamination] process to facilitate mass production of large area flexible lithium-ion batteriesr” says Lee. “Its feasibility is already proven and will be reported in a later paper.”

The team is also interested in stacking the structures on top of each other to improve charge density.

A paper describing the battery was recently published on the journal Nano Letters. The video below illustrates the voltage performance of the batteries under mechanical stress.

Source: KAIST

 

Viewpoint: Quantum Dots Tuned for Entanglement


 

Published October 1, 2012  |  Physics 5, 109 (2012)  |  DOI: 10.1103/Physics.5.109

Researchers have applied a combination of an electric field and mechanical strain to a system of quantum dots in order to correct for asymmetries that usually prevent these semiconductor nanostructures from emitting entangled photons.

 

Universal Recovery of the Energy-Level Degeneracy of Bright Excitons in InGaAs Quantum Dots without a Structure Symmetry

R. Trotta, E. Zallo, C. Ortix, P. Atkinson, J. D. Plumhof, J. van den Brink, A. Rastelli, and O. G. Schmidt

Published October 1, 2012 | PDF (free)
+Enlarge imageFigure 1

Figure 1 A quantum dot (red) can emit two entangled photons (yellow) when its internal energy states are tuned by a mechanical strain and an electric field.

Entanglement distinguishes quantum mechanics from classical physics and as such is the core resource in most applications of quantum information science. In previous demonstrations, entangled photons have allowed fundamental tests of quantum mechanics, provided secure communication protocols, enabled computations using algorithms no classical computer could perform efficiently, and provided improvements in optical sensing and imaging [1]. Moving forward, scientists will need reliable, mass-producible sources of entangled photons. Quantum dots are a leading candidate for this role, but these “artificial atoms” suffer from structural irregularities that spoil entanglement by causing a mismatch in the energies of emitted photon pairs. New research in Physical Review Letters[2] overcomes this challenge by using two control knobs—an applied electric field and mechanical strain—that together reshape the electronic structure of the dots so that the possibility for entanglement is recovered.

A pair of entangled photons can be physically separated, but their quantum identities remain locked together. Any measurement of the polarization of one photon, for example, will instantaneously determine the measurement of the corresponding polarization of the other. Today, most experiments employing entangled photons generate them using a weak process of spontaneous frequency conversion in a nonlinear crystal, whereby one parent photon at a given wavelength is converted into two longer-wavelength daughter photons that are entangled. This requires a strong pump-laser intensity and heavy spectral and spatial filtering at the output. If one pumps this process too weakly, then on most attempts no entangled photons result. If one pumps the process too strongly, then too many photon pairs emerge. The statistics of this process provide the most fundamental difficulty in expanding existing photon-entanglement-based quantum information technologies, and these technologies must be scaled up to compete with corresponding classical technologies.

Not all sources of entanglement suffer from these same statistics, however. An atomic cascade process in a single atom, in which two photons result from a single decay, can produce entangled photons with an almost certain guarantee against multiple photon pairs. These processes rely critically on atomic symmetries that leave the emitted photons indistinguishable in all but one degree of freedom, usually polarization. In 2000, Oliver Benson and co-workers [3] pointed out that similar processes could occur in a semiconductor quantum dot. There are many reasons to prefer a semiconductor source of on-demand single pairs of entangled photons, rather than one based on atoms in a trap. Semiconductor fabrication techniques add the convenience of electrical pumping and the scalability of chip-scale integrated optics. Moreover, a single cold microchip could contain large arrays of sources of one-pair-at-a-time entangled photons and could thus replace all the assorted bulky hardware that might otherwise be used in trapped-atom techniques. Unfortunately, the dream of semiconductor-based entangled sources quickly faced a problem: the first attempts at this technology were unable to generate entanglement in photon pairs, due to the lack of atomlike symmetry in real quantum dots [4].

The importance of symmetry stems from the way in which the photons are produced. The process begins by the creation of excitons, which are short-lived bound states between a conduction electron and a valence hole. These states may be optically or electrically pumped, and a state in which a pair of excitons occupies the dot may be selected spectrally. Since these two excitons occupy the same highly confined dot, their lowest energy state has the same spatial wave function and, by the Pauli exclusion principle, opposite spin. When the electrons and holes of the two excitons recombine, the polarization of the photons that are emitted is directly correlated to that spin, resulting in two photons of opposite polarization. If those photons cannot be distinguished by their wavelength, they will be polarization-entangled. However, asymmetries in the quantum dot can change the energy of each single-exciton spin state, resulting in an entanglement-spoiling correlation between each photon’s polarization and its emission wavelength. The energy landscape of an asymmetric dot may also be complicated by couplings between the excitons.

To recover entanglement, one must match the energy of both exciton spin states. Unfortunately, the asymmetries that prevent spin degeneracy in self-assembled InGaAs quantum dots are hard to avoid due to the random, strain-induced process by which they are grown. Researchers have managed in the past to engineer spin degeneracy and subsequent entanglement through a variety of post-growth tuning methods, including adding strain [5], dc magnetic fields [6], dc electric fields [7], and optical fields [8]. Despite these fixes, the dream of large arrays of engineered quantum-dot entanglement sources has so far remained doubtful, in large part because these demonstrated tuning methods only work on a few “hero” dots, namely those whose asymmetry happens to be “just right” for the tuning knob employed.

The advance reported by Rinaldo Trotta of the Leibniz Institute for Solid State and Materials Research, Germany, and his colleagues consists of the application of not just one tuning knob but two [2]. This combination provides sufficiently universal tuning to engineer exciton spin degeneracy in almost any quantum dot. The first knob in this new design is a dc electric field oriented in the dot’s growth direction (illustrated by the battery and metal plates in Fig. 1). The research team achieves this field by growing the dot in a diodelike structure. The second knob is a mechanical strain applied in a direction orthogonal to the dc field (illustrated by the clamp in Fig. 1). This strain is delivered by piezoelectric actuators that are mechanically bonded to the device. The authors describe the basic principle of the device as one knob setting the direction of the entanglement-spoiling perturbation, and the other tuning away the perturbation’s amplitude, so that the two together can eliminate it entirely, regardless of the dot’s detailed structure. To show this, the team closely examined the energies and polarizations of photons emitted from a few different quantum dots as the strain and electric field were varied. They then compared these observations to a simple but adequate theoretical model. Although the experiments did not demonstrate actual entanglement, the results indicated that sufficient degeneracy for creation of entangled photon pairs is attainable. And previous work [5678] leaves little doubt that such a dual-knob device could produce entanglement.

However, realizing large arrays of on-demand entangled pairs from semiconductor quantum dots still requires more effort. To make efficient sources, the photon emission needs to be limited to a single desired direction, a problem usually addressed by adding a microcavity structure of higher quality than used here by Trotta et al. Many other engineering issues remain to be solved as well, such as controlling quantum dot placement with high yield [9], engineering schemes for electrical pumping [7], and compensating for the highly inhomogeneous character of photons from different devices [10]. Overcoming these hurdles will require continued research by many groups from around the world. But this pursuit of highly efficient and reliable entangled photon sources is worth the effort, since the realization of large entanglement-based technologies for quantum information and measurement applications could be revolutionary.

Acknowledgment

The author would like to thank Charles Santori for valuable discussions.

Nanotechnology device aims to prevent malaria deaths through rapid diagnosis


26 September 2012

A pioneering mobile device using cutting-edge nanotechnology to rapidly detect malaria infection and drug resistance could revolutionise how the disease is diagnosed and treated.

Around 800,000 people die from malaria each year after being bitten by mosquitoes infected with malaria parasites. Signs that the parasite is developing resistance to the most powerful anti-malarial drugs in south-east Asia and sub-Saharan Africa mean scientists are working to prevent the drugs becoming ineffective.

The €5.2million (£4million) Nanomal project – launched today – is planning to provide an affordable hand-held diagnostic device to swiftly detect malaria infection and parasites’ drug resistance. It will allow healthcare workers in remote rural areas to deliver effective drug treatments to counter resistance more quickly, potentially saving lives.

The device – the size and shape of a mobile phone – will use a range of latest proven nanotechnologies to rapidly analyse the parasite DNA from a blood sample. It will then provide a malaria diagnosis and comprehensive screening for drug susceptibility in less than 20 minutes, while the patient waits. With immediately available information about the species of parasite and its potential for drug resistance, a course of treatment personally tailored to counter resistance can be given.

Currently for malaria diagnosis, blood samples are sent to a central referral laboratory for drug resistance analysis, requiring time as well as specialised and expensive tests by skilled scientists. Additionally, confirmation of malaria is often not available where patients present with fever. Very often, drug treatments are prescribed before the diagnosis and drug resistance are confirmed, and may not be effective. Being able to treat effectively and immediately will prevent severe illness and save lives.

The Nanomal consortium is being led by St George’s, University of London, which is working with UK handheld diagnostics and DNA sequencing specialist QuantuMDx Group and teams at the University of Tuebingen in Germany and the Karolinska Institute in Sweden. It was set up in response to increasing signs that the malaria parasite is mutating to resist the most powerful class of anti-malaria drugs, artemisinins. The European Commission has awarded €4million (£3.1million) to the project.

Nanomal lead Professor Sanjeev Krishna, from St George’s, said: “Recent research suggests there’s a real danger that artemisinins could eventually become obsolete, in the same way as other anti-malarials. New drug treatments take many years to develop, so the quickest and cheapest alternative is to optimise the use of current drugs. The huge advances in technology are now giving us a tremendous opportunity to do that and to avoid people falling seriously ill or dying unnecessarily.”

QuantuMDx’s CEO Elaine Warburton said: “Placing a full malaria screen with drug resistance status in the palm of a health professional’s hand will allow instant prescribing of the most effective anti-malaria medication for that patient. Nanomal’s rapid, low-cost test will further support the global health challenge to eradicate malaria.”

The handheld device will take a finger prick of blood, extract the malarial DNA and then detect and sequence the specific mutations linked to drug resistance, using a nanowire biosensor. The chip electrically detects the DNA sequences and converts them directly into binary code, the universal language of computers. The binary code can then be readily analysed and even shared, via wireless or mobile networks, with scientists for real-time monitoring of disease patterns.

The device should provide the same quality of result as a referral laboratory, at a fraction of the time and cost. Each device could cost about the price of a smart phone initially, but may be issued for free in developing countries. A single-test cartridge will be around €13 (£10) initially, but the aim is to reduce this cost to ensure affordability in resource-limited settings.

In addition to improving immediate patient outcomes, the project will allow the researchers to build a better picture of levels of drug resistance in stricken areas. It will also give them information on population impacts of anti-malarial interventions.

Clinical trials of the device are expected to begin within three years, after which it will be brought to market. The technology could be adapted afterwards for use with other infectious diseases.

The Nanomal website can be found at www.nanomal.org.

Nanoparticles Enable Drug Activation by NIR Light


SINGAPORE, Sept. 28, 2012 — Nanoparticles that convert near-infrared to ultraviolet or visible light could help overcome the skin penetration limits of conventional photodynamic therapy.

Photodynamic therapy (PDT) uses visible light — usually in the red wavelength — to activate a light-sensitive drug but is limited to a penetration depth of about 1 cm. Because of this, photodynamic therapy (PDT) is currently used to treat certain types of cancer just under the skin, such as in the esophagus. It generally can’t be used to treat cancers that have spread or large tumors.

The new technique, developed at the National University of Singapore, manipulates gene expression by using nanoparticles to convert near-infrared (NIR) to UV or visible light could offer a safe, noninvasive therapy for deeper cancers.


A team at the National University of Singapore uses nanoparticles to convert near-infrared light to the visible or ultraviolet wavelengths to allow cancer treatments such as photodynamic therapy (PDT) to penetrate deeper into tissue. Courtesy of Muthu Kumara Gnananasammandhan.


“NIR, besides being nontoxic, is also able to penetrate deeper into our tissues,” said associate professor and team leader Zhang Yong. “When NIR reaches the desired places in the body of the patient, the nanoparticles which we have invented are able to convert the NIR back to UV light (up-conversion) to effectively activate the genes in the way desired — by controlling the amount of proteins expressed each time, when this should take place, as well as how long it should take place.”

The researchers successfully controlled gene expression and hindered tumor growth in mice and were also able to target the nanoparticles to specific locations. The method can be customized for a wide range of applications, such as PDT or bioimaging, they said.

The team now is working with the National Cancer Center Singapore to evaluate the technique’s efficacy and safety for clinical trials. It is also developing point-of-care diagnostics for detecting biomarkers and bacteria.

The study appeared online Sept. 17 in Nature MedicineDOI: 10.1038/nm.2933

For more information, visit: www.nus.edu.sg