Lună: noiembrie 2017

Promițătoare nanosenzori pentru submarine, mine și nave spațiale(Promising nanosensors for submarines, mines and spacecraft)

au descoperit un mecanism care permite senzorilor de gaz, bazați pe oxizi de metale nanocristaline, să lucreze la temperatura camerei. Această invenție va spori eficiența monitorizării mediului la centralele nucleare, submarine și nave spațiale.Cercetătorii au descoperit că compozitele bazate pe oxizi de zinc și indiu pot crește semnificativ sensibilitatea senzorului la hidrogen

Posted: Nov 10, 2017

Promising nanosensors for submarines, mines and spacecraft

(Nanowerk News) Researchers from the Physics Department of Moscow State University and their colleagues have discovered a mechanism that allows gas sensors, based on nanocrystalline metal oxides, to work at room temperature. This invention will raise the efficiency of environmental monitoring at nuclear power plants, on submarines and spacecrafts. The discovery was reported in Scientific Reports („Green light activated hydrogen sensing of nanocrystalline composite ZnO-In2O3 films at room temperature”).
Hydrogen Sensor Scheme
This is a hydrogen sensor scheme. (Image: Alexander Ilyin)
Scientists have proposed a new fundamental principle of operation of hydrogen sensors. Unlike most resistive gas detectors, it does not need to be heated and only requires visible light. This discovery will significantly reduce the energy consumption of the sensor and expand its scope.
„Such sensors can be used in explosive environments or be built into mobile devices without constructing additional heat sink systems,” said Alexander Ilyin, a co-author of the study, a PhD student at the Physics Department of Moscow State University.
Researchers have found out that composites based on zinc and indium oxides can significantly increase the sensitivity of the sensor to hydrogen. Physicists have also proposed an explanation for the hypersensitivity of the designed composite. In their opinion, the sensor response mechanism consists in a change in the processes of generation and recombination of nonequilibrium electrons when the composite is interacting with hydrogen. Composites with a certain structure provide a more significant change in these processes.
Samples for the sensor were made from nanocrystalline indium and zinc oxide powders. The structure and particle size were studied by the means of transmission electron microscopy and x-ray diffraction. The electrical and sensor characteristics of the structureswere studied in the new designed setup, in which the required temperature of the composite and the concentration of hydrogen were well controlled.
The obtained results would allow the scientists to develop a new type of resistive hydrogen sensor that works under additional illumination without heating. Such sensors are promising not only for effective monitoring of environmental pollution in industrial plants, but also for constant monitoring of air in closed facilities (submarines, mines, spacecraft) where the slightest change in the chemical composition can lead to human casualties.
Source: Lomonosov Moscow State University

Foarte flexibilă memorie flash organică pentru electronice pliabile și de unică folosință(Highly flexible organic flash memory for foldable and disposable electronics)

Acest studiu ilustrează foarte bine faptul că o memorie flash extrem de flexibilă poate fi făcută pentru a avea un nivel practic de performanță viabil, astfel încât să contribuie la aparatele electronice portabile și la hârtia electronică inteligentă”.

Posted: Nov 07, 2017

Highly flexible organic flash memory for foldable and disposable electronics

(Nanowerk News) A KAIST team reported ultra-flexible organic flash memory that is bendable down to a radius of 300 µm. The memory exhibits a significantly-long projected retention rate with a programming voltage on par with the present industrial standards.
A joint research team led by Professor Seunghyup Yoo of the School of Electrical Engineering and Professor Sung Gap Im of the Department of Chemical and Biomolecular Engineering said that their memory technology can be applied to non-conventional substrates, such as plastics and papers, to demonstrate its feasibility over a wide range of applications.
With Dr. Seungwon Lee and Dr. Hanul Moon playing the role of leading authors, the research was published in Nature Communications („Organic flash memory on various flexible substrates for foldable and disposable electronics”).
Foldable flash memory
Foldable flash memory. (Image: KAIST)
Flash memory is a non-volatile, transistor-based data-storage device that has become essential in most electronic systems in daily life. With straightforward operation mechanisms and easy integration into NAND or NOR array architecture, flash memory has been established as the most successful and dominant non-volatile memory technology by far.
Despite promising demonstrations in the early stages of organic electronics, the overall progress in this field has been far slower than that of thin-film transistors (TFTs) or other devices based on flexible materials. It has been challenging, in particular, to develop flash memory that simultaneously exhibits a significant level of flexibility and performance. This is mainly due to the scarcity of flexible dielectric layers, which are responsible for the tunneling and blocking of charges.
The solution processing used for the preparation of most of the polymeric dielectric layers also makes it difficult to use them in flash memory due to the complexity involved in the formation of the bilayer dielectric structure, which is the key to flash memory operations.
The research team tried to overcome these hurdles and realize highly flexible flash memory by employing thin polymeric insulators grown with initiated chemical vapor deposition (iCVD), a vapor-phase growth technique for polymers that was previously shown to be promising for the fabrication of flexible TFTs. It was further shown that these iCVD-based polymeric insulators, when coupled with rational device design and material choice, can make a significant contribution to flash memory as well.
Memory using conventional polymer insulating films has often required a voltage as high as 100 V (volt) in order to attain long memory retention. If the device is made to operate at a low voltage, the short retention period of less than a month was problematic.
Structure of flexible flash memory
Structure of flexible flash memory. (Image: KAIST)
The KAIST team produced flash memory with programming voltages around 10 V and a projected data retention time of over 10 years, while maintaining its memory performance even at a mechanical strain of 2.8%. This is a significant improvement over the existing inorganic insulation layer-based flash memory that allowed only a 1% strain.
The team demonstrated the virtually foldable memory devices by fabricating the proposed flash memory on a 6-micrometer-thick ultrathin plastic film. In addition, it succeeded in producing them on printing paper, opening a way for disposable smart electronic products such as electronic paper and electronic business card.
Professor Yoo said, ” This study well illustrates that even highly flexible flash memory can be made to have a practically viable level of performance, so that it contributes to full-fledged wearable electronic devices and smart electronic paper.”
Source: The Korea Advanced Institute of Science and Technology (KAIST)

Tehnica de transfer produce senzori de gaz de nitrură de galiu care pot fi purtați(Transfer technique produces wearable gallium nitride gas sensors)

O tehnica de transfer bazata pe straturi subtiri de sacrificiu de nitrura de bor ar putea permite senzorilor de gaz nitridiu de inalta performanta sa fie crescuti pe substraturi de safir si apoi transferate pe materiale polimerice sau flexibile de polimer. Tehnica ar putea facilita producerea de dispozitive de detectare mobile, mobile și de unică folosință cu costuri reduse pentru o gamă largă de aplicații de mediu.

Posted: Nov 09, 2017

Transfer technique produces wearable gallium nitride gas sensors

(Nanowerk News) A transfer technique based on thin sacrificial layers of boron nitride could allow high-performance gallium nitride gas sensors to be grown on sapphire substrates and then transferred to metallic or flexible polymer support materials. The technique could facilitate the production of low-cost wearable, mobile and disposable sensing devices for a wide range of environmental applications.
Transferring the gallium nitride sensors to metallic foils and flexible polymers doubles their sensitivity to nitrogen dioxide gas, and boosts response time by a factor of six. The simple production steps, based on metal organic vapor phase epitaxy (MOVPE), could also lower the cost of producing the sensors and other optoelectronic devices.
Sensors produced with the new process can detect ammonia at parts-per-billion levels and differentiate between nitrogen-containing gases. The gas sensor fabrication technique was reported November 9 in the journal Scientific Reports („Gas sensors boosted by two-dimensional h-BN enabled transfer on thin substrate foils: towards wearable and portable applications”).
Image shows wafer-scale processed AlGaN/GaN sensors being tested
Image shows wafer-scale processed AlGaN/GaN sensors being tested. (Image: Georgia Tech Lorraine)
„Mechanically, we just peel the devices off the substrate, like peeling the layers of an onion,” explained Abdallah Ougazzaden, director of Georgia Tech Lorraine in Metz, France and a professor in Georgia Tech’s School of Electrical and Computer Engineering (ECE). „We can put the layer on another support that could be flexible, metallic or plastic. This technique really opens up a lot of opportunity for new functionality, new devices – and commercializing them.”
The researchers begin the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1,300 degrees Celsius. The boron nitride surface coating is only a few nanometers thick, and produces crystalline structures that have strong planar surface connections, but weak vertical connections.
Aluminum gallium nitride (AlGaN/GaN) devices are then grown atop the monolayers at a temperature of about 1,100 degrees Celsius, also using an MOVPE process. Because of the boron nitride crystalline properties, the devices are attached to the substrate only by weak Van der Waals forces, which can be overcome mechanically. The devices can be transferred to other substrates without inducing cracks or other defects. The sapphire wafers can be reused for additional device growth.
„This approach for engineering GaN-based sensors is a key step in the pathway towards economically viable, flexible sensors with improved performances that could be integrated into wearable applications,” the authors wrote in their paper.
So far, the researchers have transferred the sensors to copper foil, aluminum foil and polymeric materials. In operation, the devices can differentiate between nitrogen oxide, nitrogen dioxide, and ammonia. Because the devices are approximately 100 by 100 microns, sensors for multiple gases can be produced on a single integrated device.
„Not only can we differentiate between these gases, but because the sensor is very small, we can detect them all at the same time with an array of sensors,” said Ougazzaden, who expects that the devices could be modified to also detect ozone, carbon dioxide and other gases.
The gallium nitride sensors could have a wide range of applications from industry to vehicle engines – and for wearable sensing devices. The devices are attractive because of their advantageous materials properties, which include high thermal and chemical stability.
„The devices are small and flexible, which will allow us to put them onto many different types of support,” said Ougazzaden, who also directs the International Joint Research Lab at Georgia Tech CNRS.
To assess the effects of transferring the devices to a different substrate, the researchers measured device performance on the original sapphire wafer and compared that to performance on the new metallic and polymer substrates. They were surprised to see a doubling of the sensor sensitivity and a six-fold increase in response time, changes beyond what could be expected by a simple thermal change in the devices.
„Not only can we have flexibility in the substrate, but we can also improve the performance of the devices just by moving them to a different support with appropriate properties,” he said. „Properties of the substrate alone makes the different in the performance.”
In future work, the researchers hope to boost the quality of the devices and demonstrate other sensing applications. „One of the challenges ahead is to improve the quality of the materials so we can extend this to other applications that are very sensitive to the substrates, such as high-performance electronics.”
The Georgia Tech researchers have previously used a similar technique to produce light-emitting diodes and ultraviolet detectors that were transferred to different substrates, and they believe the process could also be used to produce high-power electronics. For those applications, transferring the devices from sapphire to substrates with better thermal conductivity could provide a significant advantage in device operation.
Ougazzaden and his research team have been working on boron-based semiconductors since 2005. Their work has attracted visits from several industrial companies interested in exploring the technology, he said.
„I am very excited and lucky to work on such hot topic and top-notch technology at GT-Lorraine,” said Taha Ayari, a Ph.D. student in the Georgia Tech School of ECE and the paper’s first author.
Source: Georgia Institute of Technology

Tehnica de transfer produce senzori de gaz de nitrură de galiu care pot fi purtați(Transfer technique produces wearable gallium nitride gas sensors)

Senzorii produse prin noul proces pot detecta amoniacul la nivelele pe miliarde de hectare și pot diferenția între gazele care conțin azot.Transferul senzorilor de nitrură de galiu la foliile metalice și polimerii flexibili își dublează sensibilitatea la gazul de dioxid de azot și crește timpul de răspuns cu un factor de șase. Etapele simple de producție, bazate pe epitaxia cu fază metalică cu vapori organici (MOVPE), ar putea, de asemenea, să reducă costul producerii senzorilor și a altor dispozitive optoelectronice.

Posted: Nov 09, 2017

Transfer technique produces wearable gallium nitride gas sensors

(Nanowerk News) A transfer technique based on thin sacrificial layers of boron nitride could allow high-performance gallium nitride gas sensors to be grown on sapphire substrates and then transferred to metallic or flexible polymer support materials. The technique could facilitate the production of low-cost wearable, mobile and disposable sensing devices for a wide range of environmental applications.
Transferring the gallium nitride sensors to metallic foils and flexible polymers doubles their sensitivity to nitrogen dioxide gas, and boosts response time by a factor of six. The simple production steps, based on metal organic vapor phase epitaxy (MOVPE), could also lower the cost of producing the sensors and other optoelectronic devices.
Sensors produced with the new process can detect ammonia at parts-per-billion levels and differentiate between nitrogen-containing gases. The gas sensor fabrication technique was reported November 9 in the journal Scientific Reports („Gas sensors boosted by two-dimensional h-BN enabled transfer on thin substrate foils: towards wearable and portable applications”).
Image shows wafer-scale processed AlGaN/GaN sensors being tested
Image shows wafer-scale processed AlGaN/GaN sensors being tested. (Image: Georgia Tech Lorraine)
„Mechanically, we just peel the devices off the substrate, like peeling the layers of an onion,” explained Abdallah Ougazzaden, director of Georgia Tech Lorraine in Metz, France and a professor in Georgia Tech’s School of Electrical and Computer Engineering (ECE). „We can put the layer on another support that could be flexible, metallic or plastic. This technique really opens up a lot of opportunity for new functionality, new devices – and commercializing them.”
The researchers begin the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1,300 degrees Celsius. The boron nitride surface coating is only a few nanometers thick, and produces crystalline structures that have strong planar surface connections, but weak vertical connections.
Aluminum gallium nitride (AlGaN/GaN) devices are then grown atop the monolayers at a temperature of about 1,100 degrees Celsius, also using an MOVPE process. Because of the boron nitride crystalline properties, the devices are attached to the substrate only by weak Van der Waals forces, which can be overcome mechanically. The devices can be transferred to other substrates without inducing cracks or other defects. The sapphire wafers can be reused for additional device growth.
„This approach for engineering GaN-based sensors is a key step in the pathway towards economically viable, flexible sensors with improved performances that could be integrated into wearable applications,” the authors wrote in their paper.
So far, the researchers have transferred the sensors to copper foil, aluminum foil and polymeric materials. In operation, the devices can differentiate between nitrogen oxide, nitrogen dioxide, and ammonia. Because the devices are approximately 100 by 100 microns, sensors for multiple gases can be produced on a single integrated device.
„Not only can we differentiate between these gases, but because the sensor is very small, we can detect them all at the same time with an array of sensors,” said Ougazzaden, who expects that the devices could be modified to also detect ozone, carbon dioxide and other gases.
The gallium nitride sensors could have a wide range of applications from industry to vehicle engines – and for wearable sensing devices. The devices are attractive because of their advantageous materials properties, which include high thermal and chemical stability.
„The devices are small and flexible, which will allow us to put them onto many different types of support,” said Ougazzaden, who also directs the International Joint Research Lab at Georgia Tech CNRS.
To assess the effects of transferring the devices to a different substrate, the researchers measured device performance on the original sapphire wafer and compared that to performance on the new metallic and polymer substrates. They were surprised to see a doubling of the sensor sensitivity and a six-fold increase in response time, changes beyond what could be expected by a simple thermal change in the devices.
„Not only can we have flexibility in the substrate, but we can also improve the performance of the devices just by moving them to a different support with appropriate properties,” he said. „Properties of the substrate alone makes the different in the performance.”
In future work, the researchers hope to boost the quality of the devices and demonstrate other sensing applications. „One of the challenges ahead is to improve the quality of the materials so we can extend this to other applications that are very sensitive to the substrates, such as high-performance electronics.”
The Georgia Tech researchers have previously used a similar technique to produce light-emitting diodes and ultraviolet detectors that were transferred to different substrates, and they believe the process could also be used to produce high-power electronics. For those applications, transferring the devices from sapphire to substrates with better thermal conductivity could provide a significant advantage in device operation.
Ougazzaden and his research team have been working on boron-based semiconductors since 2005. Their work has attracted visits from several industrial companies interested in exploring the technology, he said.
„I am very excited and lucky to work on such hot topic and top-notch technology at GT-Lorraine,” said Taha Ayari, a Ph.D. student in the Georgia Tech School of ECE and the paper’s first author.
Source: Georgia Institute of Technology

Cercetătorii dezvoltă dispozitive fotonice flexibile și extensibile(Researchers develop flexible, stretchable photonic devices)

Dispozitivele ar putea găsi utilizări în cabluri pentru a conecta dispozitive de calcul sau în sisteme de diagnosticare și monitorizare care ar putea fi atașate pe piele sau implantate în corp, flexându-se ușor cu țesutul natural.Astfel de circuite fotonice flexibile și elastice ar putea fi, de asemenea, utile pentru aplicațiile în care dispozitivele trebuie să se conformeze suprafețelor inegale ale altui material, cum ar fi în manometrele de tensiune

Posted: Nov 08, 2017

Researchers develop flexible, stretchable photonic devices

(Nanowerk News) Researchers at MIT and several other institutions have developed a method for making photonic devices — similar to electronic devices but based on light rather than electricity — that can bend and stretch without damage. The devices could find uses in cables to connect computing devices, or in diagnostic and monitoring systems that could be attached to the skin or implanted in the body, flexing easily with the natural tissue.
The findings, which involve the use of a specialized kind of glass called chalcogenide, are described in two papers by MIT Associate Professor Juejun Hu and more than a dozen others at MIT, the University of Central Florida, and universities in China and France. The paper is slated for publication soon in Light: Science and Applications („Monolithically integrated stretchable photonics”).
flexible, stretchable photonic device material
A new material produced by Juejun Hu and his team can be repeatedly stretched without losing its optical properties. (Image: MIT)
Hu, who is the Merton C. Flemings Associate Professor of Materials Science and Engineering, says that many people are interested in the possibility of optical technologies that can stretch and bend, especially for applications such as skin-mounted monitoring devices that could directly sense optical signals. Such devices might, for example, simultaneously detect heart rate, blood oxygen levels, and even blood pressure.
Photonics devices process light beams directly, using systems of LEDs, lenses, and mirrors fabricated with the same kinds of processes used to manufacture electronic microchips. Using light beams rather than a flow of electrons can have advantages for many applications; if the original data is light-based, for example, optical processing avoids the need for a conversion process.
But most current photonics devices are fabricated from rigid materials on rigid substrates, Hu says, and thus have an “inherent mismatch” for applications that “should be soft like human skin.” But most soft materials, including most polymers, have a low refractive index, which leads to a poor ability to confine a light beam.
Instead of using such flexible materials, Hu and his team took a novel approach: They formed the stiff material — in this case a thin layer of a type of glass called chalcogenide — into a spring-like coil. Just as steel can be made to stretch and bend when formed into a spring, the architecture of this glass coil allows it to stretch and bend freely while maintaining its desirable optical properties.
“You end up with something as flexible as rubber, that can bend and stretch, and still has a high refractive index and is very transparent,” Hu says. Tests have shown that such spring-like configurations, made directly on a polymer substrate, can undergo thousands of stretching cycles with no detectable degradation in their optical performance. The team produced a variety of photonic components, interconnected by the flexible, spring-like waveguides, all in an epoxy resin matrix, which was made stiffer near the optical components and more flexible around the waveguides.
Other kinds of stretchable photonics have been made by embedding nanorods of a stiffer material in a polymer base, but those require extra manufacturing steps and are not compatible with existing photonic systems, Hu says.
Such flexible, stretchable photonic circuits could also be useful for applications where the devices need to conform to the uneven surfaces of some other material, such as in strain gauges. Optics technology is very sensitive to strain, according to Hu, and could detect deformations of less than one-hundredth of 1 percent.
This research is still in early stages; Hu’s team has demonstrated only single devices at a time thus far. “For it to be useful, we have to demonstrate all the components integrated on a single device,” he says. Work is ongoing to develop the technology to that point so that it could be commercially applied, which Hu says could take another two to three years.
In another paper published last week in Nature Photonics („Chalcogenide glass-on-graphene photonics”), Hu and his collaborators have also developed a new way of integrating layers of photonics, made of chalcogenide glass and two-dimensional materials such as graphene, with conventional semiconductor photonic circuitry. Existing methods for integrating such materials require them to be made on one surface and then peeled off and transferred to the semiconductor wafer, which adds significant complexity to the process. Instead, the new process allows the layers to be fabricated directly on the semiconductor surface, at room temperature, allowing for simplified fabrication and more precise alignment.
The process can also make use of the chalcogenide material as a “passivation layer,” to protect 2-D materials from degradation caused by ambient moisture, and as a way to control the optoelectronic characteristics of 2-D materials. The method is generic and could be extended to other emerging 2-D materials besides graphene, to expand and expedite their integration with photonic circuitry, Hu says.
Source: By David L. Chandler, MIT

Circuite integrate imprimate direct pe tesatura (Fully integrated circuits printed directly onto fabric)

Cercetatorii au integrat cu succes circuite electronice lavabile, stretchable si respirabil in tesatura, deschizand noi posibilitati pentru textile inteligente si electronice portabile. Circuitele au fost realizate cu cerneluri ieftine, sigure și ecologice și tipărite folosind tehnici convenționale de tipărire cu jet de cerneală.o forma bidimensionala a carbonului – poate fi imprimat direct pe tesatura pentru a produce circuite electronice integrate, care sunt confortabile si pot supravietui la 20 de cicluri într-o mașină de spălat tipică.cernelurile materiale bidimensionale, variind de la tehnologia de sănătate personală și de bunăstare, până la recoltarea și depozitarea energiei purtătoare, îmbrăcămintea militară, computerele portabile și moda.

Posted: Nov 08, 2017

Fully integrated circuits printed directly onto fabric

(Nanowerk News) Researchers have successfully incorporated washable, stretchable and breathable electronic circuits into fabric, opening up new possibilities for smart textiles and wearable electronics. The circuits were made with cheap, safe and environmentally friendly inks, and printed using conventional inkjet printing techniques.
The researchers, from the University of Cambridge, working with colleagues in Italy and China, have demonstrated how graphene – a two-dimensional form of carbon – can be directly printed onto fabric to produce integrated electronic circuits which are comfortable to wear and can survive up to 20 cycles in a typical washing machine.
The new textile electronic devices are based on low-cost, sustainable and scalable inkjet printing of inks based on graphene and other two-dimensional materials, and are produced by standard processing techniques. The results are published in the journal Nature Communications („Fully inkjet-printed two-dimensional material field-effect heterojunctions for wearable and textile electronics”).
Electronic Circuit Printed on Fabric
This is a sample circuit printed on fabric. (Image: Felice Torrisi)
Based on earlier work on the formulation of graphene inks for printed electronics, the team designed low-boiling point inks, which were directly printed onto polyester fabric. Additionally, they found that modifying the roughness of the fabric improved the performance of the printed devices. The versatility of this process allowed the researchers to design not only single transistors but all-printed integrated electronic circuits combining active and passive components.
Most wearable electronic devices that are currently available rely on rigid electronic components mounted on plastic, rubber or textiles. These offer limited compatibility with the skin in many circumstances, are damaged when washed and are uncomfortable to wear because they are not breathable.
„Other inks for printed electronics normally require toxic solvents and are not suitable to be worn, whereas our inks are both cheap, safe and environmentally-friendly, and can be combined to create electronic circuits by simply printing different two-dimensional materials on the fabric,” said Dr Felice Torrisi of the Cambridge Graphene Centre, the paper’s senior author.
„Digital textile printing has been around for decades to print simple colorants on textiles, but our result demonstrates for the first time that such technology can also be used to print the entire electronic integrated circuits on textiles,” said co-author Professor Roman Sordan of Politecnico di Milano. „Although we demonstrated very simple integrated circuits, our process is scalable and there are no fundamental obstacles to the technological development of wearable electronic devices both in terms of their complexity and performance.”
„The printed components are flexible, washable and require low power, essential requirements for applications in wearable electronics,” said PhD student Tian Carey, the paper’s first author.
The work opens up a number of commercial opportunities for two-dimensional material inks, ranging from personal health and well-being technology, to wearable energy harvesting and storage, military garments, wearable computing and fashion.
„Turning textile fibres into functional electronic components can open to an entirely new set of applications from healthcare and wellbeing to the Internet of Things,” said Torrisi. „Thanks to nanotechnology, in the future our clothes could incorporate these textile-based electronics, such as displays or sensors and become interactive.”
The use of graphene and other related 2D material (GRM) inks to create electronic components and devices integrated into fabrics and innovative textiles is at the centre of new technical advances in the smart textiles industry. The teams at the Cambridge Graphene Centre and Politecnico di Milano are also involved in the Graphene Flagship, an EC-funded, pan-European project dedicated to bringing graphene and GRM technologies to commercial applications.
Source: University of Cambridge

Un nou concept pentru un ghid de undă unidirecțional(A new concept for a unidirectional waveguide)

un nou tip de material a atras atracția de creștere: așa-numitul izolator topologic. Această clasă de materiale prezintă o proprietate foarte ciudată: ele se comportă ca izolații în interior, dar conțin stări de conducere la limitele lor. Deoarece aceste stări sunt protejate „din punct de vedere topologic”, statele sunt foarte robuste împotriva imperfecțiunilor, iar curenții electrici pot circula aproape fără disipare. Acest lucru face ca aceste materiale să fie extrem de interesante pentru sarcina de comunicare cuantică și de calcul cuantic, de exemplu.

Posted: Nov 07, 2017

A new concept for a unidirectional waveguide

(Nanowerk News) In the past decade, a new type of material has attracted raising attraction: the so-called topological insulator. This class of materials exhibits a very peculiar property: they behave like insulators in the interior, but contain conducting states at their boundaries. As these states are “topologically” protected, the states are very robust against imperfections, and electric currents can flow almost without any dissipation. This makes these materials extremely interesting for the task of quantum communication and quantum computing, for example.
Now Dr. Tao Shi (at present Chinese Academy of Sciences, Beijing) and Prof. Ignacio Cirac from the Max Planck Institute of Quantum Optics in Garching, together with Prof. Jeff Kimble from the California Institute of Technology (Pasadena, USA), have developed a detailed scheme for an experimental setup to realize a 2-dimensional topological insulator with classical optical networks (PNAS, „Topological phenomena in classical optical networks”).
concept for a unidirectional waveguide
Scheme of the optical nework. (© PNAS)
“In this network, photon modes play the role of the electronic states in a solid state layer,” explains Dr. Tao Shi. “By preparing chiral photon modes at the boundary, we may be able to build a one-way electromagnetic waveguide, in which light can only propagate in one direction, while the opposite direction is prohibited.“
Solid state crystals are characterized by their band structure. In case of an insulator, the so-called valence band, in which all electronic states are occupied, is separated from the conduction band by a large forbidden zone. This, however, only holds for infinite samples.
In case of a confined crystal or layer, the electronic states at the surface or edge, respectively, differ from those in the interior, and sometimes they emerge in the middle of the forbidden zone. As the shape of the band structure is mathematically described by a certain topological number, these systems are called in short “topological insulators”.
The chirality of the edge states is locked to the spin of the electron, and is consequently protected by time-reversal symmetry: a reverse in direction would imply a flip of the spin direction. For a certain class of materials with a “nontrivial” topological number this is not allowed. Therefore, the states are protected and robust against imperfections or deformations, as long as the perturbations are small.
In a certain class of electronic 2-dimensional topological insulators also the so-called quantum spin Hall (QSH) effect can be observed. Intuitively, this effect describes the phenomenon that electrons with different spins are subjected to oppositely directed magnetic fields.
Different from previous schemes, the scientists propose a setup made of optical passive elements such as fibres, beam splitters, and wave plates, whereby system losses are reduced to a large extent. By constructing the nodes of the network with a “bad” cavity, i.e. a cavity with high damping, they are able to dramatically enhance the topological bandgap to the scale of the free spectral range. As a consequence, the edge modes survive in the larger frequency domain with much longer lifetime. Furthermore, the interplay between the topology and the Kerr non-linearity induces the generation of squeezed edge modes.
“The optical analogue of a topological insulator paves the way towards building the one-way wave guide,” Dr. Shi points out. “Apart from that – our ultimate goal is to realize the fractional quantum Hall effect (FQHE) in this photonic system. To this end, we have to induce strong photon-photon interactions by means of atoms. We would also like to see some exotic topological phases in the photonic system, which can be very different from those observed in conventional condensed matter systems.“
Source: By Olivia Meyer-Streng, Max Planck Institute of Quantum Optics

Materialele compozite pe bază de grafen deblochează calea către tranzistoare cu putere redusă(Graphene-based composite materials unlock the path to ultra-low-power transistors)

a descoperit un nou traseu pentru tranzistoarele cu putere redusă, folosind un material compozit pe bază de grafen. cred că soluția constă în materiale compozite realizate din monostraturi de grafen și dicalcogenide de metale tranziționale (TMDC). Au descoperit că aceste materiale ar putea fi folosite pentru a obține un control electric fin asupra procesului de rotație a electronilor, acul mic al busolei.

Posted: Nov 07, 2017

Graphene-based composite materials unlock the path to ultra-low-power transistors

(Nanowerk News) An international team of scientists has discovered a new route to ultra-low-power transistors using a graphene-based composite material.
As transistors are squeezed into ever smaller areas within computer chips, the semiconductor industry struggles to contain overheating in devices.
Now researchers from the University of York and Roma Tre University believe the solution lies in composite materials built from monolayers of graphene and the transition metal dichalcogenide (TMDC).They discovered these materials could be used to achieve a fine electrical control over the electron’s spin – its tiny compass needle.
The new research, published today in the journal Physical Review Letters („Optimal Charge-to-Spin Conversion in Graphene on Transition-Metal Dichalcogenides”), could lead the way to much needed low-energy consumption electronics.
Lead researcher Dr Aires Ferreira, of the University of York’s Department of Physics, said: „For many years, we have been searching for good conductors allowing efficient electrical control over the electron’s spin.
„We found this can be achieved with little effort when two-dimensional graphene is paired with certain semiconducting layered materials. Our calculations show that the application of small voltages across the graphene layer induces a net polarization of conduction spins.
„We believe that our predictions will attract substantial interest from the spintronics community. The flexible, atomically thin nature of the graphene-based structure is a major advantage for applications. Also, the presence of a semiconducting component opens up the possibility for integration with optical communication networks.”
The electron’s spin is like a tiny, point-like magnet which can point only in two directions, up or down. In materials where a major fraction of electrons’ spins is aligned, a magnetic response is produced, which can be used to encode information.
‘Spin currents’ – built from ‘up’ and ‘down’ spins flowing in opposite directions – carry no net charge, and therefore in theory, produce no heating. The control of spin information would therefore open the path towards ultra-energy-efficient computer chips. The team of researchers showed that when a small current is passed through the graphene layer, the electrons’ spin polarize in plane due to ‘spin-orbital’ forces brought about by the proximity to the TMDC base. They also showed that the efficiency of charge-to-spin conversion can be quite high even at room temperature.
Manuel Offidani, a PhD student with York’s Department of Physics, carried out most of the complex calculations in this study. He said: „The current-induced polarization of the electron’s spin is an elegant relativistic phenomenon that arises at the interface between different materials.
„We chose graphene mainly because of its superb structural and electronic properties. In order to enhance the relativistic effects experienced by charge carriers in graphene, we investigated the possibility of matching it with recently discovered layered semiconductors.”
Professor Roberto Raimondi, who leads the spintronics group at Roma Tre University, said: „The possibility of orienting the electron spin with electrical currents is attracting a lot of attention in the spintronics community and arises generally as a consequence of specific symmetry conditions.
„As such this phenomenon represents a perfect example where fundamental and applied research go happily together. In this respect, our calculations demonstrate that graphene combined with the transition metal dichalcogenides is an ideal platform where abstract theoretical principles may find immediate application in showing the way to experimental and technological development.”
Current-induced spin polarization in non-magnetic media was first demonstrated in 2001 in semiconductors and, more recently, in metallic hetero-interfaces. Now the researchers predict that a similar effect occurs in graphene on TMDC monolayer.
Surprisingly they found that the unique character of electronic states in graphene enable charge-to-spin conversion efficiency of up to 94 per cent. This opens up the possibility of a graphene-based composite material becoming the basis for ultra-compact and greener spin-logic devices.
Dr Mirco Milletarì, a former member of the spintronics group at Roma Tre University, said: „This work follows insights gained from understanding fundamental laws that enabled us to envisage systems where the efficiency of charge-to-spin conversion can be optimal for technological applications. In particular, the much needed low-energy consumption electronics that will improve durability and performances of future devices.”
Source: University of York