Noua analiză explică rolul defectelor în oxizii metalici(New analysis explains role of defects in metal oxides)

Înțelegerea acestor efecte este importantă și pentru alte aplicații, cum ar fi împărțirea moleculelor de apă pentru a produce hidrogen la interfețele solid-lichide, dispozitive electronice care se bazează pe interfațe de oxid de oxid sau alte procedee electrochimice care utilizează aceste materiale ca catalizatori, permiteți interacțiunile.
Materialele pe care echipa le-a studiat fac parte dintr-o clasa cunoscuta sub denumirea de oxizi binari alcalino-pamant, ale caror componente sunt „printre cele mai abundente clase de materiale de pe Pamant”, spune Yildiz. „[Această clasă este] ieftină, abundentă și are proprietăți reglabile”, făcându-l promițătoare pentru multe aplicații. Dar ea adaugă că abordarea teoretică pe care au luat-o vor fi aplicate în prezent mult mai larg, la multe alte tipuri de materiale oxidice și la alte tipuri de defecte în interiorul lor, pe lângă vacanțele neutre pentru oxigen.

Posted: Sep 22, 2017

New analysis explains role of defects in metal oxides

(Nanowerk News) Sometimes things that are technically defects, such as imperfections in a material’s crystal lattice, can actually produce changes in properties that open up new kinds of useful applications. New research from a team at MIT shows that such imperfections in a family of materials known as insulating metal oxides may be key to their performance for a variety of high-tech applications, such as nonvolatile memory chips and energy conversion technologies.
The findings are reported this week in the journal Physical Review Letters („Polarizing Oxygen Vacancies in Insulating Metal Oxides under a High Electric Field”), in a paper by MIT Associate Professor Bilge Yildiz, Professor and Associate Provost Krystyn Van Vliet, and former postdoc Mostafa Youssef.
the atomic lattice of a crystal of barium oxide is depicted, with atoms of oxygen and barium represented by red and gray spheres
In this diagram, the atomic lattice of a crystal of barium oxide is depicted, with atoms of oxygen and barium represented by red and gray spheres. A neutral oxygen vacancy, a place where an oxygen atom should appear in the lattice but is instead replaced by two electrons, is represented by the yellow shape, which depicts the charge density of those electrons. At left, the crystal is seen with no electric field applied, and at right, with an applied field of 21.8 megavolts per centimeter. The distortions of the lattice reveal the effects of that applied electric field. (Image: Felice Frankel)
These metal oxide materials have been investigated by many researchers, Yildiz says, and „their properties are highly governed by the number and the kind of defects that are present.” When subjected to strong driving forces, such as strong electric fields, „the behavior of such defects had not been well-understood,” she says.
Researchers do have a well-established theoretical understanding of how perfectly structured versions of these insulating metal oxides function under a variety of conditions, such as in strong electric fields, but there was no such theory to describe the materials when they contain common types of defects, according to Yildiz. Understanding these effects quantitatively is important in order to develop this promising family of materials for potential applications including new types of low-energy computer memory and processing devices, electrically based refrigeration, and electro-catalytic energy-conversion devices such as fuel cells.
The team demonstrated a theoretical framework and showed how the stability and structure of a point defect is altered under strong electric fields. They took a common defect called a neutral oxygen vacancy – a place where an oxygen atom should appear in the lattice but instead two electrons are trapped. Their results have quantified the polarization behavior of the material with this defect, in an electric field.
„The oxygen vacancies in particular are very important in electronic and electrochemical applications,” says Yildiz, who holds joint appointments in the departments of Nuclear Science and Engineering and Materials Science and Engineering.
In many of these applications, she says, there can be an internal voltage gradient created within the thin-film material, and this „electric potential” gradient causes strong electric fields. Understanding the effects of those fields is essential for the design of certain new devices.
„Most of the work in this area is experimental,” Yildiz says. „You take a thin film, you put it in an electric field, and you do measurements.” But in such experiments, the effects of the local electric potential and the electric field are convoluted, making it very hard to understand the results. „It’s impossible to resolve them from each other, so you need to have a theory” to account for the effects, she adds.
The researchers have now devised a new theoretical framework that allows them to isolate the electric field effect from the electric potential effect, and quantify both independently. This allowed them to make very specific predictions that are different from those produced by classical theory and should make it possible to validate the new model experimentally within a year, Yildiz says.
The findings should help enable the development of some important potential applications, she says. One is in a new type of computer memory device known as resistive switching memory, which provides fast switching speeds using very little energy. These memory devices rely on the presence of defects.
„The way they switch their resistance state [to record data] depends on the defect type, content, and distribution,” she says. „In order to model the device behavior, you should be able to model how the applied strong electric fields alter the defect structure, concentration, and distribution.” That’s what this new work enables: „If you know quantitatively the effects of both the potential and the field, then you can design your operating conditions to benefit from these effects.”
Understanding these effects is also important for other applications such as splitting water molecules to produce hydrogen at solid-liquid interfaces, electronic devices that rely on oxide-oxide interfaces, or other electrochemical processes using these materials as catalysts, where defects serve as the sites that enable the interactions.
The materials the team studied belong to a class known as alkaline-earth-metal binary oxides, whose constituents are „among the most abundant class of materials on Earth,” Yildiz says. „[This class is] cheap, abundant, and has tunable properties,” making it promising for many applications. But she adds that the theoretical approach they took will now be applied much more broadly, to many other kinds of oxide materials and to other kinds of defects within them besides the neutral oxygen vacancies.
Source: By David L. Chandler, MIT

Patru elemente fac platforma optică 2-D(Four elements make 2-D optical platform)

au descoperit un aliaj bidimensional cu o bandgap optica care poate fi reglat de temperatura folosita pentru ao creste.A dezvoltat aliajul de patru componente din metale tranziționale, molibden și tungsten, cu sulfuri de calciu și seleniu într-un cuptor de depunere chimică de vapori. Ei au descoperit că schimbările de temperatură au făcut modificări subtile în modul în care atomii au asamblat și, de asemenea, au modificat proprietățile care determină modul în care absorb și emit lumină.

Posted: Sep 21, 2017

Four elements make 2-D optical platform

(Nanowerk News) Rice University scientists have discovered a two-dimensional alloy with an optical bandgap that can be tuned by the temperature used to grow it.
The Rice lab of materials scientist Pulickel Ajayan grew the four-component alloy of transition metals molybdenum and tungsten with chalcogens sulfur and selenium in a chemical vapor deposition furnace. They found changes in temperature made subtle changes in the way atoms assembled and also altered the properties that determine how they absorb and emit light.
Subtle changes in growth temperature alters the form of a four-component alloy
Subtle changes in growth temperature alters the form of a four-component alloy created at Rice University. The alloy can be tuned to alter its optical bandgap, which may be of use in the design of solar cells and light-emitting diodes. (Illustration by Alex Kutana)
Their experiments were built upon work by the lab of Rice theoretical physicist Boris Yakobson, which created scores of models to predict how various combinations of the four elements should work.
The process should be of interest to engineers looking to make smaller, more-efficient devices. Because the bandgap falls in the optical range of the electromagnetic spectrum, the researchers said solar cells and light-emitting diodes might be the first beneficiaries.
The paper appears as a cover story in the current issue of Advanced Materials („Quaternary 2D Transition Metal Dichalcogenides (TMDs) with Tunable Bandgap”).
The team led by co-lead author and Rice research scientist Alex Kutana generated 152 random models of the material that showed the bandgap could be tuned from 1.62 to 1.84 electron volts by varying the growth temperature from 650 and 800 degrees Celsius (1,202 to 1,472 degrees Fahrenheit).
The experimental team led by Sandhya Susarla then made and tested the thermodynamically stable materials in a furnace at 50-degree increments. Scientists at Oak Ridge National Laboratory led by postdoctoral researcher Jordan Hachtel produced microscope images that identified and detailed the position of each atom in the materials.
“Labs have made 2-D materials with two or three components, but we don’t believe anyone has tried four,” said co-author and Rice postdoctoral researcher Chandra Sekhar Tiwary. “Having four components gives us an additional degree of freedom. With fewer materials, every adjustment you make to change the bandgap turns it into a different material. That’s not the case here.”
“What we’ve made should be very useful,” added Susarla, a Rice graduate student. “For applications like solar cells and LEDs, you need a material that has a broad bandgap.”
Tiwary said the material can be tuned to cover the entire spectrum of visible light, from 400- to 700-nanometer wavelengths. “That’s a huge range we can cover by just changing this composition,” he said. “If we choose the composition correctly, we can hit the correct bandgap or correct emission point.”
“These materials are arguably the most important 2-D semiconductors because of their excellent optoelectronic properties and low cost,” Kutana said. “Our high-throughput calculations permitted us to avoid prior assumptions about how the alloy bandgap behaved. The surprising outcome was how regular the bandgap changes were, resulting in optical properties that are both useful and predictable.”
Source: By Mike Williams, Rice University

Aplicarea semiconductorilor sensibili la aer în nanoelectronică(Application of air-sensitive semiconductors in nanoelectronics)

Una dintre domeniile promițătoare ale științei materialelor moderne este studiul materialelor bidimensionale (2D), adică filme subțiri constând din unul sau mai multe straturi atomice. Materialele 2D datorate supraconductivității și rezistenței lor electrice ar putea constitui o bază pentru nanoelectronica modernă. Aplicațiile optice în nanoelectronică necesită materiale avansate capabile să „genereze” mari fluxuri de electroni prin iradiere ușoară. Gallium selenidă (GaSe) este unul dintre semiconductorii 2D care poate face față acestei probleme cel mai eficient.De exemplu, poate fi aplicat în dispozitive încapsulate care sunt fabricate în vid și apoi acoperite cu un strat de protecție care elimină penetrarea aerului.

Posted: Sep 21, 2017

Application of air-sensitive semiconductors in nanoelectronics

(Nanowerk News) A research group consisting of scientists from Tomsk Polytechnic University, Germany and Venezuela proved vulnerability of a two-dimensional semiconductor gallium selenide in air. This discovery will allow manufacturing superconducting nanoelectronics based on gallium selenide, which has never been previously achieved by any research team in the world.
The study was published in Semiconductor Science and Technology („GaSe oxidation in air: from bulk to monolayers”).
Oxidation of Gallium Selenide
Raman spectrum of 8 L GaSe taken at two different times during time evolution map. (a) Red dashed line is drawn as the guide for the eye. The constant intensity ratio peaks indicates that oxidation stops approximately after 16 500 s. Thickness dependent Raman spectrum of GaSe for oxidation investigation. (b) Each spectrum was taken with 700 s × 3 of accumulation time. (Image: Tomsk Polytechnic University)
One of the promising areas of modern materials science is the study of two-dimensional (2D) materials, i.e. thin films consisting of one or several atomic layers. 2D materials due to their electrical superconductivity and strength could be a basis for modern nanoelectronics. Optic applications in nanoelectronics require advanced materials capable of ‘generating’ great electron fluxes upon light irradiation. Gallium selenide (GaSe) is one of the 2D semiconductors that can cope with this problem most efficiently.
‘Some research teams abroad tried to create electronic devices based on GaSe. However, despite extensive theoretical studies of this material, which were published in major scientific journals, the stability of the material in real devices remained unclear,’ says Prof. Raul Rodriguez, the Department of Lasers and Lighting Engineering.
The research team revealed the reasons behind this. They studied GaSe by means of Raman spectroscopy and x-ray photoelectron spectroscopy that allowed proving the existence of chemical bonds between gallium and oxygen. Photoluminescence in oxidized substance is absent that also proves the formation of oxides. It means that the scientists revealed that GaSe oxidizes quickly in air and loses its electrical conductivity necessary for creating nanoeletronic devices.
‘GaSe monolayers become oxidized almost immediately after being exposed to air. Further research of GASe stability in air will allow making proposals how to protect it and maintain its optoelectronic properties,’ emphasize the authors.
According to Prof. Rodriguez, for GaSe not to lose its unique properties it should be placed in a vacuum or inert environment. For example, it can be applied in encapsulated devices that are vacuum-manufactured and then covered with a protective layer eliminating air penetration.
This method can be used to produce next generation optoelectronics, detectors, light sources and solar batteries. Such devices of ultra-small sizes will have very high quantum efficiency, i.e. they will be able to generate large electron fluxes under small external exposure.
Source: Tomsk University

Strângerea memoriei: Materialele inovatoare de tip prototip sunt viitorul stocării datelor(Straining the memory: Prototype strain engineered materials are the future of data storage)

Când atomii de la interfață sunt dezordonați, materialul are o rezistență electrică ridicată, în timp ce interfața comandată are o rezistență electrică scăzută. Deoarece numai interfața comută, un subset de straturi din material poate rămâne neschimbat și cristalin. Aceasta înseamnă că interfața poate fi proiectată de straturile fără comutare – întreaga structură nu trebuie să treacă într-o stare dezordonată. Acest lucru face superlattica foarte diferită de aliajele de memorie cu schimbări de fază nestructurate, cum ar fi aliajul Ge 2 Sb 2 Te 5 .

Posted: Sep 20, 2017

Straining the memory: Prototype strain engineered materials are the future of data storage

(Nanowerk News) Researchers from the Singapore University of Technology and Design (SUTD) and Shanghai Institute of Microsystems and Information Technology have nano-engineered a superlattice data storage material. Data is recorded at the interfaces of the superlattice layers.
When the atoms at the interface are disordered, the material has a high electrical resistance while the ordered interface has a low electrical resistance. Since only the interface switches, a subset of layers within the material can remain unchanged and crystalline. This means that the interface can be engineered by the non-switching layers – the entire structure need not switch into a disordered state. This makes the superlattice very different to unstructured phase change memory alloys, such as the Ge2Sb2Te5 alloy.
comparison of the electrical switching current and switching times for the strained superlattices interfacial phase change memory with other state-of-the-art phase change memory materials
A comparison of the electrical switching current and switching times for the strained superlattices interfacial phase change memory with other state-of-the-art phase change memory materials. (Image: Zhou et al.) (click on image to enlarge)
The authors showed in a paper published in Nano Futures („Avalanche atomic switching in strain engineered Sb2Te3–GeTe interfacial phase-change memory cells”) that fast switching in these nanostructured materials is due to avalanche atomic switching at the interface.
The first atom that switches requires a large amount of energy, but subsequent atoms require less energy. As more atoms switch, the energy required for subsequent atoms to switch is lowered. This leads to an exponential increase in the switching probability with the number of atoms switching.
Zhou et al showed that the energy for the first atom to switch can be engineered by straining the layer interfaces. The research team created prototype memory devices, which exploit this effect that outperformed state-of-the-art phase change memory devices. The switching voltage, current, and switching time are substantially reduced while the electrical resistance changes by a factor of 500. Thus these prototype devices are faster and more efficient than current competing technologies.
One of the members of the research team, Assistant Professor Robert Simpson, said: „The superlattices devices are remarkably energy efficient. We foresee this technology impacting new 3D memory architectures, such as Intel’s 3D x-point. We are now building on the success of these data storage materials by optimising similar phase change materials for switchable nano photonics applications.”
Source: Singapore University of Technology and Design

Grafenul și alte nanomateriale de carbon pot înlocui metalele rare(Graphene and other carbon nanomaterials can replace scarce metals)

Nanomaterialele de carbon constau exclusiv sau în principal din carbon și sunt materiale puternice cu o bună conductivitate. Mai multe metale rare au proprietăți similare. Metalele se găsesc, de exemplu, în cabluri, ecrane subțiri, ignifuge, protecție la coroziune și condensatoare.
Rickard Arvidsson și Björn Sandén de la Universitatea de Tehnologie din Chalmers au investigat dacă nanoparticulele de carbon, grafena, fullerenele și nanotuburile de carbon au potențialul de a înlocui 14 metale rare în principalele domenii de aplicare (vezi tabelul din imaginea atașată). Ei au găsit soluții potențiale bazate pe tehnologie pentru a înlocui metalele cu nanomateriale de carbon pentru toate aplicațiile, cu excepția aurului din bijuterii. Metalele pe care le avem cel mai aproape de a putea înlocui sunt indiu, galiu, beriliu și argint.

Posted: Sep 19, 2017

Graphene and other carbon nanomaterials can replace scarce metals

(Nanowerk News) Scarce metals are found in a wide range of everyday objects around us. They are complicated to extract, difficult to recycle and so rare that several of them have become „conflict minerals” which can promote conflicts and oppression. A survey at Chalmers University of Technology now shows that there are potential technology-based solutions that can replace many of the metals with carbon nanomaterials, such as graphene (Journal of Cleaner Production, „Carbon nanomaterials as potential substitutes for scarce metals”).
They can be found in your computer, in your mobile phone, in almost all other electronic equipment and in many of the plastics around you. Society is highly dependent on scarce metals, and this dependence has many disadvantages.
Scarce metals such as tin, silver, tungsten and indium are both rare and difficult to extract since the workable concentrations are very small. This ensures the metals are highly sought after – and their extraction is a breeding ground for conflicts, such as in the Democratic Republic of the Congo where they fund armed conflicts.
In addition, they are difficult to recycle profitably since they are often present in small quantities in various components such as electronics.
Graphene and Other Carbon Nanomaterials Can Replace Scarce Metals
Scarce metals are found in a wide range of everyday objects around us. A survey at Chalmers University of Technology now shows that there are potential technology-based solutions that can replace many of the metals with carbon nanomaterials, such as graphene. (Image: Jonas Sandwall) (click on image to enlarge)
Rickard Arvidsson and Björn Sandén, researchers in environmental systems analysis at Chalmers University of Technology, have now examined an alternative solution: substituting carbon nanomaterials for the scarce metals. These substances – the best known of which is graphene – are strong materials with good conductivity, like scarce metals.
„Now technology development has allowed us to make greater use of the common element carbon,” says Sandén. „Today there are many new carbon nanomaterials with similar properties to metals. It’s a welcome new track, and it’s important to invest in both the recycling and substitution of scarce metals from now on.”
The Chalmers researchers have studied the main applications of 14 different metals, and by reviewing patents and scientific literature have investigated the potential for replacing them by carbon nanomaterials. The results provide a unique overview of research and technology development in the field.
According to Arvidsson and Sandén the summary shows that a shift away from the use of scarce metals to carbon nanomaterials is already taking place.
„There are potential technology-based solutions for replacing 13 out of the 14 metals by carbon nanomaterials in their most common applications. The technology development is at different stages for different metals and applications, but in some cases such as indium and gallium, the results are very promising,” Arvidsson says.
„This offers hope,” says Sandén. „In the debate on resource constraints, circular economy and society’s handling of materials, the focus has long been on recycling and reuse. Substitution is a potential alternative that has not been explored to the same extent and as the resource issues become more pressing, we now have more tools to work with.”
The research findings were recently published in the Journal of Cleaner Production. Arvidsson and Sandén stress that there are significant potential benefits from reducing the use of scarce metals, and they hope to be able to strengthen the case for more research and development in the field.
„Imagine being able to replace scarce metals with carbon,” Sandén says. „Extracting the carbon from biomass would create a natural cycle.”
„Since carbon is such a common and readily available material, it would also be possible to reduce the conflicts and geopolitical problems associated with these metals,” Arvidsson says.
At the same time they point out that more research is needed in the field in order to deal with any new problems that may arise if the scarce metals are replaced.
„Carbon nanomaterials are only a relatively recent discovery, and so far knowledge is limited about their environmental impact from a life-cycle perspective. But generally there seems to be a potential for a low environmental impact,” Arvidsson says.

Facts
Carbon nanomaterials consist solely or mainly of carbon, and are strong materials with good conductivity. Several scarce metals have similar properties. The metals are found, for example, in cables, thin screens, flame-retardants, corrosion protection and capacitors.
Rickard Arvidsson and Björn Sandén at Chalmers University of Technology have investigated whether the carbon nanomaterials graphene, fullerenes and carbon nanotubes have the potential to replace 14 scarce metals in their main areas of application (see table in attached image). They found potential technology-based solutions to replace the metals with carbon nanomaterials for all applications except for gold in jewellery. The metals which we are closest to being able to substitute are indium, gallium, beryllium and silver.
Source: Chalmers University of Technology

 

O celulă solară pe care o poți pune în spălare(A solar cell you can put in the wash)

au dezvoltat un nou tip de dispozitiv fotovoltaic ultra-subțire, acoperit pe ambele părți cu folii stretchable și impermeabile, care pot continua să furnizeze energie electrică din lumina soarelui chiar și după ce sunt înmuiate în apă sau întinse și comprimate. Ar putea deschide calea către celulele solare care să poată fi alimentate de dispozitive precum monitoarele de sănătate încorporate în îmbrăcăminte.sperăm că aceste fotovoltaice organice lavabile, ușor și elastice va deschide o nouă cale pentru utilizarea ca sistem de sursă de alimentare pe termen lung pentru senzori care pot fi purtați și alte dispozitive

Posted: Sep 18, 2017

A solar cell you can put in the wash

(Nanowerk News) Scientists from RIKEN and the University of Tokyo have developed a new type of ultra-thin photovoltaic device, coated on both sides with stretchable and waterproof films, which can continue to provide electricity from sunlight even after being soaked in water or being stretched and compressed.
The work, published in Nature Energy („Stretchable and waterproof elastomer-coated organic photovoltaics for washable electronic textile applications”), could open the way to wearable solar cells, which will provide power to devices such as health monitors incorporated into clothing. One of the requirements of the Internet of Things–referring to a world where devices of all sorts are connected to the Internet–is the development of power sources for a host of devices, including devices that can be worn on the body.
According to Takao Someya, the leader of the research group, these could include sensors that record heartbeats and body temperature, for example, providing early warning of medical problems. In the past, attempts have been made to create photovoltaics that could be incorporated into textiles, but typically they lacked at least one of the important properties–long-term stability in both air and water, energy efficiency, and robustness including resistance to deformation–that are key to successful devices.
For the present work, the members of the research group developed extremely thin and flexible organic photovoltaic cells, based on a material called PNTz4T, which they had developed in earlier work. They deposited the device in an inverse architecture, which they had previously developed, onto a 1-um-thick parylene film. The ultra-thin device was then placed onto acrylic-based elastomer and the top side of the device was coated with an identical elastomer, giving it a coating on both sides to prevent water infiltration.. The elastomer, while allowing light to enter, prevented water and air from leaking into the cells, making them more long-lasting than previous experiments.
The researchers then subjected the device to a variety of tests, finding first that it had a strong energy efficiency of 7.9 percent, producing a current of 7.86 milliwatts per square centimeter, as the current density was 13.8 milliamperes per square centimeter at 0.57 volts, based on a simulated sunlight of 100 milliwatts per square centimeter. To test its resistance to water, they soaked it in water for two hours, and found that the efficiency decreased by just 5.4 percent. And to test the durability, they subjected it to compression, and found that after compressing by nearly half for twenty cycles while placing drops of water on it, it still had 80 percent of the original efficiency.
According to Kenjiro Fukuda of the RIKEN Center for Emergent Matter Science, „We were very gratified to find that our device has great environmental stability while simultaneously having a good efficiency and mechanical robustness. We very much hope that these washable, lightweight and stretchable organic photovoltaics will open a new avenue for use as a long-term power source system for wearable sensors and other devices.”
Source: RIKEN

O nouă abordare a impulsurilor ultrascurte de lumină(A new approach to ultrafast light pulses)

Deoarece reacția materialului este atât de puternic influențată de apropierea exactă a substratului metalic din apropiere, astfel de sisteme ar putea fi de asemenea folosite pentru instrumente de măsurare foarte precise. „Interacțiunea este redusă ca o funcție a mărimii decalajului, astfel încât aceasta ar putea fi acum utilizată dacă vrem să măsuram apropierea unei suprafețe”, spune Fang.
Pe măsură ce echipa își continuă studiile cu privire la aceste materiale, următorul pas este de a studia efectele pe care ar putea avea modelul suprafeței metalice, deoarece testele au utilizat până acum numai suprafețe plane.

Posted: Sep 18, 2017

A new approach to ultrafast light pulses

(Nanowerk News) Two-dimensional materials called molecular aggregates are very effective light emitters that work on a different principle than typical organic light-emitting diodes (OLEDs) or quantum dots. But their potential as components for new kinds of optoelectronic devices has been limited by their relatively slow response time. Now, researchers at MIT, the University of California at Berkeley, and Northeastern University have found a way to overcome that limitation, potentially opening up a variety of applications for these materials.
The findings are described in the journal Proceedings of the National Academy of Sciences(„Ultrafast fluorescent decay induced by metal-mediated dipole–dipole interaction in two-dimensional molecular aggregates”), in a paper by MIT associate professor of mechanical engineering Nicholas X. Fang, postdocs Qing Hu and Dafei Jin, and five others.
light strikes a molecular lattice deposited on a metal substrate
In this image, light strikes a molecular lattice deposited on a metal substrate. The molecules can quickly exchange energy with the metal below, a mechanism that leads to a much faster response time for the emission of fluorescent light from the lattice. (Image: MIT)
The key to enhancing the response time of these 2-D molecular aggregates (2DMA), Fang and his team found, is to couple that material with a thin layer of a metal such as silver. The interaction between the 2DMA and the metal that is just a few nanometers away boosts the speed of the material’s light pulses more than tenfold.
These 2DMA materials exhibit a number of unusual properties and have been used to create exotic forms of matter, known as Bose-Einstein condensates, at room temperature, while other approaches required extreme cooling. They have also been applied in technologies such as solar cells and light-harvesting organic antennas. But the new work for the first time identifies the strong influence that a very close sheet of metal can have on the way these materials emit light.
In order for these materials to be useful in devices such as photonic chips – which are like semiconductor chips but carry out their operations using light instead of electrons – „the challenge is to be able to switch them on and off quickly,” which had not been possible before, Fang says.
With the metal substrate nearby, the response time for the light emission dropped from 60 picoseconds (trillionths of a second) to just 2 picoseconds, Fang says: „This is pretty exciting, because we observed this effect even when the material is 5 to 10 nanometers away from the surface,” with a spacing layer of polymer in between. That’s enough of a separation that fabricating such paired materials in quantity should not be an overly demanding process. „This is something we think could be adapted to roll-to-roll printing,” he says.
If used for signal processing, such as sending data by light rather than radio waves, Fang says, this advance could lead to a data transmission rate of about 40 gigahertz, which is eight times faster than such devices can currently deliver. This is „a very promising step, but it’s still very early” as far as translating that into practical, manufacturable devices, he cautions.
The team studied only one of the many kinds of molecular aggregates that have been developed, so there may still be opportunities to find even better variations. „This is actually a very rich family of luminous materials,” Fang says.
Because the responsiveness of the material is so strongly influenced by the exact proximity of the nearby metal substrate, such systems could also be used for very precise measuring tools. „The interaction is reduced as a function of the gap size, so it could now be used if we want to measure the proximity of a surface,” Fang says.
As the team continues its studies of these materials, one next step is to study the effects that patterning of the metal surface might have, since the tests so far only used flat surfaces. Other questions to be addressed include determining the useful lifetimes of these materials and how they might be extended.
Fang says a first prototype of a device using this system might be produced „within a year or so.”
Source: By David L. Chandler, Massachusetts Institute of Technology