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Researchers have developed an ultrafast light-emitting device that can flip on and off 90 billion times a second and could form the basis of optical computing.

At its most basic level, your smart phone’s battery is powering billions of transistors using electrons to flip on and off billions of times per second. But if microchips could use photons instead of electrons to process and transmit data, computers could operate even faster.

But first engineers must build a light source that can be turned on and off that rapidly. While lasers can fit this requirement, they are too energy-hungry and unwieldy to integrate into computer chips.

Duke University researchers are now one step closer to such a light source. In a new study, a team from the Pratt School of Engineering pushed semiconductor quantum dots to emit light at more than 90 billion gigahertz. This so-called plasmonic device could one day be used in optical computing chips or for optical communication between traditional electronic microchips.

The study was published online on July 27 in Nature Communications.

“This is something that the scientific community has wanted to do for a long time,” said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke. “We can now start to think about making fast-switching devices based on this research, so there’s a lot of excitement about this demonstration.”

The new speed record was set using plasmonics. When a laser shines on the surface of a silver cube just 75 nanometers wide, the free electrons on its surface begin to oscillate together in a wave. These oscillations create their own light, which reacts again with the free electrons. Energy trapped on the surface of the nanocube in this fashion is called a plasmon.

The plasmon creates an intense electromagnetic field between the silver nanocube and a thin sheet of gold placed a mere 20 atoms away. This field interacts with quantum dots — spheres of semiconducting material just six nanometers wide — that are sandwiched in between the nanocube and the gold. The quantum dots, in turn, produce a directional, efficient emission of photons that can be turned on and off at more than 90 gigahertz.

“There is great interest in replacing lasers with LEDs for short-distance optical communication, but these ideas have always been limited by the slow emission rate of fluorescent materials, lack of efficiency and inability to direct the photons,” said Gleb Akselrod, a postdoctoral research in Mikkelsen’s laboratory. “Now we have made an important step towards solving these problems.”

“The eventual goal is to integrate our technology into a device that can be excited either optically or electrically,” said Thang Hoang, also a postdoctoral researcher in Mikkelsen’s laboratory. “That’s something that I think everyone, including funding agencies, is pushing pretty hard for.”

The group is now working to use the plasmonic structure to create a single photon source — a necessity for extremely secure quantum communications — by sandwiching a single quantum dot in the gap between the silver nanocube and gold foil. They are also trying to precisely place and orient the quantum dots to create the fastest fluorescence rates possible.

Aside from its potential technological impacts, the research demonstrates that well-known materials need not be limited by their intrinsic properties.

“By tailoring the environment around a material, like we’ve done here with semiconductors, we can create new designer materials with almost any optical properties we desire,” said Mikkelsen. “And that’s an emerging area that’s fascinating to think about.”

Scientists studying thin layers of phosphorus have found surprising properties that could open the door to ultrathin and ultralight solar cells and LEDs.

The team used sticky tape to create single-atom thick layers, termed phosphorene, in the same simple way as the Nobel-prize winning discovery of graphene.

Unlike graphene, phosphorene is a semiconductor, like silicon, which is the basis of current electronics technology.

“Because phosphorene is so thin and light, it creates possibilities for making lots of interesting devices, such as LEDs or solar cells,” said lead researcher Dr Yuerui (Larry) Lu, from The Australian National University (ANU).

“It shows very promising light emission properties.”

The team created phosphorene by repeatedly using sticky tape to peel thinner and thinner layers of crystals from the black crystalline form of phosphorus.

As well as creating much thinner and lighter semiconductors than silicon, phosphorene has light emission properties that vary widely with the thickness of the layers, which enables much more flexibility for manufacturing.

“This property has never been reported before in any other material,” said Dr Lu, from ANU College of Engineering and Computer Science, whose study is published in the Nature serial journal Light: Science and Applications.

“By changing the number of layers we can tightly control the band gap, which determines the material’s properties, such as the colour of LED it would make.

“You can see quite clearly under the microscope the different colours of the sample, which tells you how many layers are there,” said Dr Lu.

Dr Lu’s team found the optical gap for monolayer phosphorene was 1.75 electron volts, corresponding to red light of a wavelength of 700 nanometers. As more layers were added, the optical gap decreased. For instance, for five layers, the optical gap value was 0.8 electron volts, a infrared wavelength of 1550 nanometres. For very thick layers, the value was around 0.3 electron volts, a mid-infrared wavelength of around 3.5 microns.

The behaviour of phosphorene in thin layers is superior to silicon, said Dr Lu.

“Phosphorene’s surface states are minimised, unlike silicon, whose surface states are serious and prevent it being used in such a thin state.”

Researchers at Chalmers University of Technology have developed a method for efficiently cooling electronics using graphene-based film. The film has a thermal conductivity capacity that is four times that of copper. Moreover, the graphene film is attachable to electronic components made of silicon, which favors the film’s performance compared to typical graphene characteristics shown in previous, similar experiments.

Electronic systems available today accumulate a great deal of heat, mostly due to the ever-increasing demand on functionality. Getting rid of excess heat in efficient ways is imperative to prolonging electronic lifespan, and would also lead to a considerable reduction in energy usage. According to an American study, approximately half the energy required to run computer servers, is used for cooling purposes alone.

A couple of years ago, a research team led by Johan Liu, professor at Chalmers University of Technology, were the first to show that graphene can have a cooling effect on silicon-based electronics. That was the starting point for researchers conducting research on the cooling of silicon-based electronics using graphene.

“But the methods that have been in place so far have presented the researchers with problems,” Johan Liu said. “It has become evident that those methods cannot be used to rid electronic devices off great amounts of heat, because they have consisted only of a few layers of thermal conductive atoms. When you try to add more layers of graphene, another problem arises, a problem with adhesiveness. After having increased the amount of layers, the graphene no longer will adhere to the surface, since the adhesion is held together only by weak van der Waals bonds.”

“We have now solved this problem by managing to create strong covalent bonds between the graphene film and the surface, which is an electronic component made of silicon,” he continues.

The stronger bonds result from so-called functionalization of the graphene, i.e. the addition of a property-altering molecule. Having tested several different additives, the Chalmers researchers concluded that an addition of (3-Aminopropyl) triethoxysilane (APTES) molecules has the most desired effect. When heated and put through hydrolysis, it creates so-called silane bonds between the graphene and the electronic component.

Moreover, functionalization using silane coupling doubles the thermal conductivity of the graphene. The researchers have shown that the in-plane thermal conductivity of the graphene-based film, with 20 micrometer thickness, can reach a thermal conductivity value of 1600 W/mK, which is four times that of copper.

“Increased thermal capacity could lead to several new applications for graphene,” says Johan Liu. “One example is the integration of graphene-based film into microelectronic devices and systems, such as highly efficient Light Emitting Diodes (LEDs), lasers and radio frequency components for cooling purposes. Graphene-based film could also pave the way for faster, smaller, more energy efficient, sustainable high power electronics.”

The latest manufacturing, materials and production developments in semiconductor and related technologies will be featured at SEMICON West 2015 on July 14-16 at Moscone Center in San Francisco, Calif.  Semiconductor processing is at a crossroads and is changing how companies operate to be competitive. Learning about breakthrough technology and networking is essential to remain ahead of the curve.  

More than 25,000 professionals are expected, and over 600 companies will exhibit the latest in semiconductor manufacturing.  Major semiconductor manufacturers, foundry, fabless companies, equipment and materials suppliers — plus leading companies in MEMS, displays, printed/flexible electronics, PV, and other emerging technologies — attend SEMICON West.

SEMICON West will feature valuable on-exhibition floor technical sessions and programs that are included in the  $100 registration “expo pass” (registration fee increases on July 11).  Keynote events include: 

·         “Scaling the Walls of Sub-14nm Manufacturing” with panelists from Qualcomm, Stanford University, ASE and IBM, moderated by imec’s Jo de Boeck, senior VP of Corporate Technology (July 14, 9:00-10:00am)

·         “The Internet of Things and the Next Fifty Years of Moore’s Law“ by Intel’s Doug Davis, senior VP and GM of loT (July 15, 9:00am-9:45am)

TechXPOTs will provide updates in areas including test, advanced materials and processes, advanced packaging, productivity and emerging markets and technologies. TechXPOTs include:

·      What’s Next for MEMS? With speakers from ASE, CEA-Leti, EV Group, MEMS Industry Group, Silicon Valley Band of Angels, Teledyne DALSA, and Yole Developpement (July 14, 10:30am-12:30pm)

·      Automating Semiconductor Test Productivity with speakers from ASE, Optimal+, Texas Instruments, and Xcerra (July 14, 10:30am-12:30pm)

·      Materials Session: Contamination Control in the Sub-20nm Era with speakers from Entegris, Intel, JSR Micro, Matheson, and Nanometrics; moderated by Mike Corbett, Linx (July 14, 1:30pm-3:30pm)

·      Emerging Generation Memory Technology: Update on 3DNAND, MRAM, and RRAM (July 14, 1:30pm-3:40pm).

·      The Evolution of the New 200mm Fab for the Internet of Everything with speakers from Entrepix, Genmark Automation, Lam Research, Qorvo, and Surplus Global (July 15, 2:00pm-4:00pm)

·      Monetizing the IoT: Opportunities and Challenges for the Semiconductor Sector with Amkor, Cadence Design Systems, Ernst & Young, Freescale Semiconductor, and Gartner; moderated by Edward Sperling, Semiconductor Engineering (July 16, 10:30am-12:30pm)

·      The Factory of the (Near) Future: Using Industrial IoT and 3D Printing  with speakers from AirLiquide, Applied Materials, Lawrence Livermore National Laboratory, Oak Ridge National Laboratory, and Proto Cafe (July 16, 1:00pm-3:00pm) 

The Silicon Innovation Forum will be held on July 14-15.  A special exposition segment, this area will include exhibits and two days of presentations.  The first day will be a forum where start-up companies seeking investment capital will present to a panel of investors.  Open to all attendees, this session will feature exciting new technologies.  The second day will be a forum on new research. Attendees can hear presentations on advanced research from SLAC National Accelerator Laboratory, International Consortium for Advanced Manufacturing Research, SUNY Network of Excellence – Materials & Advanced Manufacturing, Novati Technologies, MIST Center, Micro/Nano Electronics Metrology at NIST, Texas State University and Georgia Tech Heat Lab. 

On July 16, University Day welcomes students and faculty to learn about the microelectronics industry, connect with industry representatives, and explore career opportunities. University Day is on the Keynote Stage (North Hall E). The agenda includes career networking, exploration forum, expo and SEMICON West tours.

For the eighth year, SEMICON West will be co-located with Intersolar North America, the leading solar technology conference and exhibition in the U.S.  Premier sponsors of SEMICON West 2015 include Applied Materials, KLA-Tencor, and Lam Research.  Register now at www.semiconwest.org.

Dow Corning reported today that the Korean Intellectual Property Office (KIPO) granted a patent protecting the company’s high refractive index (RI) phenyl-based optical silicone encapsulant technology, which targets advanced LED lighting applications. Specifically, the patent protects the composition of curable organopolysiloxane chemistry used to formulate Dow Corning Optical Encapsulant products, which offer numerous high-value benefits to LED devices. These benefits include improved light output, excellent mechanical protection of LED components and enduring gas barrier properties for enhanced reliability.  The patent granted by the KIPO ensures only Dow Corning products are authorized to contain the patented technology.

“The KIPO’s decision is only the latest milestone in Dow Corning’s ongoing efforts to rigorously protect its diverse and multi-layered intellectual property family of advanced optical materials,” said Kaz Maruyama, global marketing director, Lighting Solutions, Dow Corning. “We applaud the KIPO’s action, which helps to validate prior decisions from patent offices in the European Union, the United States, Taiwan and Malaysia, as well as Japan, where we began developing this advanced technology more than a decade ago.”

Granted in early March, Patent 101499709 covers the composition of industry leading products such as Dow Corning OE-6630, OE-7620 and OE-7651N Encapsulants. All deliver high RIs in the range of 1.53 to 1.55, compared to the lower RI of 1.41 that is typical of methyl-based silicone chemistries. While seemingly small, that difference can translate into about 7 percent more light output. Achieving a comparable improvement from an LED chip would require significant investment.

In addition to higher RI, Dow Corning’s portfolio of phenyl silicone packaging materials delivers photothermal stability suitable for many middle- and high-power general lighting applications. Compared to methyl-based technology, phenyl-based silicone encapsulants generally offer a stronger gas barrier, which helps protect key LED components such as silver electrodes and phosphor against moisture deterioration and sulfur corrosion. LED electrodes double as reflective elements, and phosphor is a key element of light conversion. As a result, enhanced gas barrier protection helps maintain both light output performance and reliability of LED packages.

“Patenting these high RI phenyl-based optical silicone encapsulants in Korea is an important step for Dow Corning and for its customers, who depend on the consistent high-quality and reliable high-performance that our LED encapsulants provide,” Maruyama said. “Supply chain integrity and consistent material quality will be critical competitive benefits as LED lighting aims to offer a credible, cost-effective alternative to conventional light sources.”

Led by Young Duck Kim, a postdoctoral research scientist in James Hone’s group at Columbia Engineering, a team of scientists from Columbia, Seoul National University (SNU), and Korea Research Institute of Standards and Science (KRISS) reported today that they have demonstrated — for the first time — an on-chip visible light source using graphene, an atomically thin and perfectly crystalline form of carbon, as a filament. They attached small strips of graphene to metal electrodes, suspended the strips above the substrate, and passed a current through the filaments to cause them to heat up. The study, “Bright visible light emission from graphene,” is published in the Advance Online Publication (AOP) on Nature Nanotechnology‘s website on June 15.

“We’ve created what is essentially the world’s thinnest light bulb,” says Hone, Wang Fon-Jen Professor of Mechanical Engineering at Columbia Engineering and co-author of the study. “This new type of ‘broadband’ light emitter can be integrated into chips and will pave the way towards the realization of atomically thin, flexible, and transparent displays, and graphene-based on-chip optical communications.”

Creating light in small structures on the surface of a chip is crucial for developing fully integrated “photonic” circuits that do with light what is now done with electric currents in semiconductor integrated circuits. Researchers have developed many approaches to do this, but have not yet been able to put the oldest and simplest artificial light source — the incandescent light bulb — onto a chip. This is primarily because light bulb filaments must be extremely hot — thousands of degrees Celsius — in order to glow in the visible range and micro-scale metal wires cannot withstand such temperatures. In addition, heat transfer from the hot filament to its surroundings is extremely efficient at the microscale, making such structures impractical and leading to damage of the surrounding chip.

By measuring the spectrum of the light emitted from the graphene, the team was able to show that the graphene was reaching temperatures of above 2500 degrees Celsius, hot enough to glow brightly.

“The visible light from atomically thin graphene is so intense that it is visible even to the naked eye, without any additional magnification,” explains Young Duck Kim, first and co-lead author on the paper and postdoctoral research scientist who works in Hone’s group at Columbia Engineering.

Interestingly, the spectrum of the emitted light showed peaks at specific wavelengths, which the team discovered was due to interference between the light emitted directly from the graphene and light reflecting off the silicon substrate and passing back through the graphene. Kim notes, “This is only possible because graphene is transparent, unlike any conventional filament, and allows us to tune the emission spectrum by changing the distance to the substrate.”

The ability of graphene to achieve such high temperatures without melting the substrate or the metal electrodes is due to another interesting property: as it heats up, graphene becomes a much poorer conductor of heat. This means that the high temperatures stay confined to a small ‘hot spot’ in the center.

“At the highest temperatures, the electron temperature is much higher than that of acoustic vibrational modes of the graphene lattice, so that less energy is needed to attain temperatures needed for visible light emission,” Myung-Ho Bae, a senior researcher at KRISS and co-lead author, observes. “These unique thermal properties allow us to heat the suspended graphene up to half of temperature of the sun, and improve efficiency 1000 times, as compared to graphene on a solid substrate.”

The team also demonstrated the scalability of their technique by realizing large-scale of arrays of chemical-vapor-deposited (CVD) graphene light emitters.

Yun Daniel Park, professor in the department of physics and astronomy at Seoul National University and co-lead author, notes that they are working with the same material that Thomas Edison used when he invented the incandescent light bulb: “Edison originally used carbon as a filament for his light bulb and here we are going back to the same element, but using it in its pure form — graphene — and at its ultimate size limit — one atom thick.”

The group is currently working to further characterize the performance of these devices — for example, how fast they can be turned on and off to create “bits” for optical communications — and to develop techniques for integrating them into flexible substrates.

Hone adds, “We are just starting to dream about other uses for these structures — for example, as micro-hotplates that can be heated to thousands of degrees in a fraction of a second to study high-temperature chemical reactions or catalysis.”

A Si quantum dot (QD)-based hybrid inorganic/organic light-emitting diode (LED) that exhibits white-blue electroluminescence has been fabricated by Professor Ken-ichi SAITOW (Natural Science Center for Basic Research and Development, Hiroshima University), Graduate student Yunzi XIN (Graduate School of Science, Hiroshima University), and their collaborators.

Professor Ken-ichi Saitow, Natural Science Center for Basic Research and Development, Hiroshima University and Graduate student Yunzi Xin, Graduate School of Science, Hiroshima University, have fabricated an Si QD hybrid LED. CREDIT: Natural Science Center for Basic Research and Development, Hiroshima University

Professor Ken-ichi Saitow, Natural Science Center for Basic Research and Development, Hiroshima University and Graduate student Yunzi Xin, Graduate School of Science, Hiroshima University, have fabricated an Si QD hybrid LED.
CREDIT: Natural Science Center for Basic Research and Development, Hiroshima University

 

A hybrid LED is expected to be a next-generation illumination device for producing flexible lighting and display, and this is achieved for the Si QD-based white-blue LED. For details, refer to “White-blue electroluminescence from a Si quantum dot hybrid light-emitting diode,” in Applied Physics Letters; DOI: 10.1063/1.4921415.

The Si QD hybrid LED was developed using a simple method; almost all processes were solution-based and conducted at ambient temperature and pressure. Conductive polymer solutions and a colloidal Si QD solution were deposited on the glass substrate. The current and optical power densities of the LED are, respectively, 280 and 350 times greater than those reported previously for such a device at the same voltage (6 V). In addition, the active area of the LED is 4 mm2, which is 40 times larger than that of a typical commercial LED; the thickness of the LED is 0.5 mm.

“QD LED has attracted significant attention as a next-generation LED,” Professor Saitow said. “Although several breakthroughs will be required for achieving implementation, a QD-based hybrid LED allows us to give so fruitful feature that we cannot imagine.”

BY DR. RANDHIR THAKUR, Executive Vice President, General Manager, Silicon Systems Group, APPLIED MATERIALS, INC

For 50 years, Moore’s Law has served as a guide for technologists everywhere in the world, setting the pace for the semiconductor industry’s innovation cycle. Moore’s Law has made a tremendous impact not only on the electronics industry, but on our world and our everyday life. It led us from the infancy of the PC era, through the formative years of the internet, to the adolescence of smartphones. Now, with the rise of the Internet of Things, market researchers forecast that in the next 5 years, the number of connected devices per person will more than double, so even after 50 years we don’t see Moore’s Law slowing down.

As chipmakers work tirelessly to continue device scaling, they are encountering daunting technical and economic hurdles. Increasing complexity is driving the need for new materials and new device architectures. Enabling these innovations and the node-over-node success of Moore’s Law requires advance- ments in precision materials engineering, including precision films, materials removal, materials modification and interface engineering, supported by metrology and inspection.

Though scaling is getting harder, I am confident Moore’s Law will continue because equipment suppliers and chipmakers never cease to innovate. As we face the increasing challenges of new technology inflections, earlier engagement in the development cycle between equipment suppliers and chipmakers is required to uncover new solutions. Such early and deep collaboration is critical to delivering complex precision materials engineering solutions on time. In fact, in the mobility era, earlier and deeper collaboration across the entire value chain is essential (applications, system/hardware, fabless, foundry/IDM, equipment supplier, chemical supplier, component supplier, etc.) to accelerate time to market and extend Moore’s Law.

Today, new 3D architectures, FinFET and 3D NAND, are enabling the extension of Moore’s Law. Dense 3D structures with high aspect ratios create fundamental challenges in device manufacturing. Further, the industry has shifted much of its historical reliance from litho-enabled scaling to materials-enabled scaling, requiring thinner precision films with atomic-scale accuracy. The emphasis on thin conformal films, which can be 2000 times smaller than a human hair, makes it increasingly critical to engineer film properties and manage film interactions between adjacent film surfaces. Selective processing is also a growing requirement, particularly for the deposition and removal of films. We expect more selective applications beyond Epitaxy and Cobalt liner deposition. There will also be a major expansion of new materials in addition to the key inflection of high-k metal gate that helped to reduce power leakage issues associated with scaling.

Gordon Moore’s prediction that ignited an industry will continue to influence our way of life through a combination of architecture and material changes. New process designs and new ways to atomically deposit materials are needed. More processes will be integrated on the same platform without vacuum breaks to create pristine interfaces. As an equipment supplier, we have to manage longer R&D cycles to support the industry’s roadmap, and plan for faster ramp and yield curves. Of utmost importance is staying close to our customers to ensure we deliver solutions with the desired economic and technical benefits.

Looking at the electronics industry from where it is today out to 2020, many more devices will be in use, the world will be more connected and, particularly in emerging markets, there will be greater consumer appetite for more products with advanced features. Given these transformations and demand, I think the growth and excitement in our industry will continue for many more years, thanks to Moore’s Law.

UPDATE:15 December 2015: Minor changes made to reflect correct ARM product nomenclature.

By Jeff Dorsch, Contributing Editor

Those 16-nanometer chips with FinFETs? Yesterday’s news. Taiwan Semiconductor Manufacturing wants you to know that they’re ready, willing, and able to help you design chips with 10-nanometer features.

The foundry presented Monday morning with its long-time partners, ARM Holdings and Synopsys, on its preparations for the 10nm process node.

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“The N10 design ecosystem is ready for customer design starts,” said Willy Chen, TSMC’s deputy director of Design & Technology Platform. He noted that TSMC has been collaborating with Synopsys for 15 years, while ARM and TSMC together offer “the most advanced ARM processor cores in the most advanced TSMC technology.”

Rob Aitken of ARM added, “10-nanometer enablement needs an ecosystem,” which the three companies are prepared to provide. He said ARM has “some cool things under development to make chip design faster,” without elaborating.

Haroon Gahur, principal design engineer at ARM, began the program by describing attributes of the ARM Cortex-A72 processor design, which he said consumes 75% less energy than previous ARM cores.

Joe Walston of Synopsys said ARM used the DC Graphical, IC Compiler I, and IC Compiler II tools from Synopsys in developing Cortex-A72, with signoff performed by PrimeTime SI. ARM’s Gahur noted that IC Compiler II provided a significant runtime advantage over its predecessor, IC Compiler I, by completing its run in five hours, compared with about 24 hours for IC Compiler I.

The program also featured Denny Liu, deputy general manager of Design Technology at MediaTek, who spoke of his company’s involvement with Synopsys and TSMC. He detailed MediaTek’s Helio X20, introduced last month, which is a tri-cluster mobile processor with 10 cores. MediaTek also employed IC Compiler II in designing the chip.

For all the 10nm talk, TSMC is hitting its stride with the N16FF+ process. Synopsys and TSMC announced Monday that the IC Compiler II place-and-route tool is certified for the foundry’s 16nm FinFET Plus process.

“The 16FF+ design flow is here,” TSMC’s Chen said.

The program finished with a presentation by Henry Sheng, group director of research and development at Synopsys, who noted that 90 percent of FinFET tapeouts are done with Synopsys place-and-route tools. Touting his company’s “healthy working relationship with TSMC,” Sheng said that emerging process nodes present a number of challenges, specifically new yield and manufacturing rules, process scaling, and new FinFET devices. Of FinFETs, he said, “These things are electrically different.”

Separately, Synopsys announced Sunday that it has agreed to acquire Atrenta, without disclosing financial terms. The transaction is expected to close this summer.

Researchers at Lehigh University have identified for the first time that a performance gain in the electrical conductivity of random metal nanowire networks can be achieved by slightly restricting nanowire orientation. The most surprising result of the study is that heavily ordered configurations do not outperform configurations with some degree of randomness; randomness in the case of metal nanowire orientations acts to increase conductivity.

The study, Conductivity of Nanowire Arrays under Random and Ordered Orientation Configurations, is published in the current issue of Nature‘s journal Scientific Reports. The research was carried out by Nelson Tansu, Daniel E. ’39 and Patricia M. Smith Endowed Chair Professor in Lehigh’s Center for Photonics and Nanoelectronics and Department of Electrical and Computer Engineering, and lead author Milind Jagota, a Bethlehem-area high school student.

Transparent conductors are needed widely for flat screen displays, touch screens, solar cells, and light-emitting diodes, among many other technologies. Currently, Indium Tin Oxide (ITO) is the most widely used material for transparent conductors due to its high conductivity and high transparency. However, ITO-based technology has several issues. The material is scarce, expensive to manufacture and brittle, a particularly undesirable characteristic for anything being used in this modern age of flexible electronics.

Researchers searching for a replacement for ITO are increasingly employing random networks of metal nanowires to match ITO in both transparency and conductivity. Metal nanowire-based technologies display better flexibility and are more compatible with manufacturing processes than ITO films. The technology, however, is still in an early phase of development and performance must be improved. Current research is focused on the effect of rod orientation on conductivity of networks to improve performance.

In this work, Lehigh researchers developed a computational model for simulation of metal nanowire networks, which should speed the process towards idealizing the configuration of nanowires. The model predicts existing experimental results and previously published computational results.

The researchers then used this model to extract results for the first time on how conductivity of random metal nanowire networks is affected by different orientation restrictions of varying randomness. Two different orientation configurations are reported.

In the first, a uniform distribution of orientations over the range (?θ, θ) with respect to a horizontal line is used. In the second, a distribution of orientations over the range [?θ] _ [θ] is used, also with respect to a horizontal line. In each case θ is gradually decreased from 90° to 0°. Conductivity is measured both in directions parallel and perpendicular to alignment.

Researchers found that a significant improvement in conductivity parallel to direction of alignment can be obtained by slightly restricting orientation of the uniform distribution. This improvement, however, comes at the expense of a larger drop in perpendicular conductivity. The general form of these results matches that demonstrated by researchers experimenting with carbon nanotube films. Surprisingly, it was found that the highly ordered second case is unable to outperform isotropic networks for any value of θ; thus demonstrating that continuous orientation configurations with some degree of randomness are preferable to highly ordered configurations.

Prior research in this field has studied the effects of orientation on conductivity of 3D carbon nanotube composites, finding that a slight degree of alignment improves conductivity. Computational models have been used to study how percolation probability of 2D random rod dispersions is affected by rod orientation. Others have developed a more sophisticated computational model capable of calculating conductivity of 3D rod dispersions, again finding that a slight degree of axial alignment improves conductivity.

“Metal nanowire networks show great potential for application in various forms of technology,” said Jagota. “This computational model, which has proven itself accurate through its good fit with previously published data, has demonstrated quantitatively how different orientation configurations can impact conductivity of metal nanowire networks.”

“Restriction of orientation can improve conductivity in a single direction by significant amounts, which can be relevant in a variety of technologies where current flow is only required in one direction,” said Tansu. “Surprisingly, heavily controlled orientation configurations do not exhibit superior conductivity; some degree of randomness in orientation in fact acts to improve conductivity of the networks. This approach may have tremendous impacts on improving current spreading in optoelectronics devices, specifically on deep ultraviolet emitter with poor p-type contact layer.”