Category Archives: electronics

Light-based computation made better with silver

It’s pretty amazing to imagine a future where computers run on light but according to a May 16, 2017 news item on ScienceDaily the idea is not beyond the realms of possibility,

Tomorrow’s computers will run on light, and gold nanoparticle chains show much promise as light conductors. Now Ludwig-Maximilians-Universitaet (LMU) in Munich scientists have demonstrated how tiny spots of silver could markedly reduce energy consumption in light-based computation.

Today’s computers are faster and smaller than ever before. The latest generation of transistors will have structural features with dimensions of only 10 nanometers. If computers are to become even faster and at the same time more energy efficient at these minuscule scales, they will probably need to process information using light particles instead of electrons. This is referred to as “optical computing.”

The silver serves as a kind of intermediary between the gold particles while not dissipating energy. Capture: Liedl/Hohmann (NIM)

A March 15, 2017 LMU press release (also one EurekAlert), which originated the news item, describes a current use of light in telecommunications technology and this latest research breakthrough (the discrepancy in dates is likely due to when the paper was made available online versus in print),

Fiber-optic networks already use light to transport data over long distances at high speed and with minimum loss. The diameters of the thinnest cables, however, are in the micrometer range, as the light waves — with a wavelength of around one micrometer — must be able to oscillate unhindered. In order to process data on a micro- or even nanochip, an entirely new system is therefore required.

One possibility would be to conduct light signals via so-called plasmon oscillations. This involves a light particle (photon) exciting the electron cloud of a gold nanoparticle so that it starts oscillating. These waves then travel along a chain of nanoparticles at approximately 10% of the speed of light. This approach achieves two goals: nanometer-scale dimensions and enormous speed. What remains, however, is the energy consumption. In a chain composed purely of gold, this would be almost as high as in conventional transistors, due to the considerable heat development in the gold particles.

A tiny spot of silver

Tim Liedl, Professor of Physics at LMU and PI at the cluster of excellence Nanosystems Initiative Munich (NIM), together with colleagues from Ohio University, has now published an article in the journal Nature Physics, which describes how silver nanoparticles can significantly reduce the energy consumption. The physicists built a sort of miniature test track with a length of around 100 nanometers, composed of three nanoparticles: one gold nanoparticle at each end, with a silver nanoparticle right in the middle.

The silver serves as a kind of intermediary between the gold particles while not dissipating energy. To make the silver particle’s plasmon oscillate, more excitation energy is required than for gold. Therefore, the energy just flows “around” the silver particle. “Transport is mediated via the coupling of the electromagnetic fields around the so-called hot spots which are created between each of the two gold particles and the silver particle,” explains Tim Liedl. “This allows the energy to be transported with almost no loss, and on a femtosecond time scale.”

Textbook quantum model

The decisive precondition for the experiments was the fact that Tim Liedl and his colleagues are experts in the exquisitely exact placement of nanostructures. This is done by the DNA origami method, which allows different crystalline nanoparticles to be placed at precisely defined nanodistances from each other. Similar experiments had previously been conducted using conventional lithography techniques. However, these do not provide the required spatial precision, in particular where different types of metals are involved.

In parallel, the physicists simulated the experimental set-up on the computer – and had their results confirmed. In addition to classical electrodynamic simulations, Alexander Govorov, Professor of Physics at Ohio University, Athens, USA, was able to establish a simple quantum-mechanical model: “In this model, the classical and the quantum-mechanical pictures match very well, which makes it a potential example for the textbooks.”

Here’s a link to and c citation for the paper,

Hotspot-mediated non-dissipative and ultrafast plasmon passage by Eva-Maria Roller, Lucas V. Besteiro, Claudia Pupp, Larousse Khosravi Khorashad, Alexander O. Govorov, & Tim Liedl. Nature Physics (2017) doi:10.1038/nphys4120 Published online 15 May 2017

This paper is behind a paywall.

Gently measuring electrical signals in small animals with nano-SPEARs

This work comes from Rice University (Texas, US) according to an April 17, 2017 news item on Nanowerk,

Microscopic probes developed at Rice University have simplified the process of measuring electrical activity in individual cells of small living animals. The technique allows a single animal like a worm to be tested again and again and could revolutionize data-gathering for disease characterization and drug interactions.

The Rice lab of electrical and computer engineer Jacob Robinson has invented “nanoscale suspended electrode arrays” — aka nano-SPEARs — to give researchers access to electrophysiological signals from the cells of small animals without injuring them. Nano-SPEARs replace glass pipette electrodes that must be aligned by hand each time they are used.”

An April 17, 2017 Rice University news release (also on EurekAltert), which originated the news item, details the work,

“One of the experimental bottlenecks in studying synaptic behavior and degenerative diseases that affect the synapse is performing electrical measurements at those synapses,” Robinson said. “We set out to study large groups of animals under lots of different conditions to screen drugs or test different genetic factors that relate to errors in signaling at those synapses.”

Robinson’s early work at Rice focused on high-quality, high-throughput electrical characterization of individual cells. The new platform adapts the concept to probe the surface cells of nematodes, worms that make up 80 percent of all animals on Earth.

Most of what is known about muscle activity and synaptic transmission in the worms comes from the few studies that successfully used manually aligned glass pipettes to measure electrical activity from individual cells, Robinson said. However, this patch clamp technique requires time-consuming and invasive surgery that could negatively affect the data that is gathered from small research animals.

The platform developed by Robinson’s team works something like a toll booth for traveling worms. As each animal passes through a narrow channel, it is temporarily immobilized and pressed against one or several nano-SPEARS that penetrate its body-wall muscle and record electrical activity from nearby cells. That animal is then released, the next is captured and measured, and so on. Robinson said the device proved much faster to use than traditional electrophysiological cell measurement techniques.

The nano-SPEARs are created using standard thin-film deposition procedures and electron-beam or photolithography and can be made from less than 200 nanometers to more than 5 microns thick, depending on the size of animal to be tested. Because the nano-SPEARs can be fabricated on either silicon or glass, the technique easily combines with fluorescence microscopy, Robinson said.

The animals suitable for probing with a nano-SPEAR can be as large as several millimeters, like hydra, cousins of the jellyfish and the subject of an upcoming study. But nematodes known as Caenorhabditis elegans were practical for several reasons: First, Robinson said, they’re small enough to be compatible with microfluidic devices and nanowire electrodes. Second, there were a lot of them down the hall at the lab of Rice colleague Weiwei Zhong, who studies nematodes as transparent, easily manipulated models for signaling pathways that are common to all animals.

“I used to shy away from measuring electrophysiology because the conventional method of patch clamping is so technically challenging,” said Zhong, an assistant professor of biochemistry and cell biology and co-author of the paper. “Only a few graduate students or postdocs can do it. With Jacob’s device, even an undergraduate student can measure electrophysiology.”

“This meshes nicely with the high-throughput phenotyping she does,” Robinson said. “She can now correlate locomotive phenotypes with activity at the muscle cells. We believe that will be useful to study degenerative diseases centered around neuromuscular junctions.”

In fact, the labs have begun doing so. “We are now using this setup to profile worms with neurodegenerative disease models such as Parkinson’s and screen for drugs that reduce the symptoms,” Zhong said. “This would not be possible using the conventional method.”

Initial tests on C. elegans models for amyotrophic lateral sclerosis and Parkinson’s disease revealed for the first time clear differences in electrophysiological responses between the two, the researchers reported.

Testing the efficacy of drugs will be helped by the new ability to study small animals for long periods. “What we can do, for the first time, is look at electrical activity over a long period of time and discover interesting patterns of behavior,” Robinson said.

Some worms were studied for up to an hour, and others were tested on multiple days, said lead author Daniel Gonzales, a Rice graduate student in Robinson’s lab who took charge of herding nematodes through the microfluidic devices.

“It was in some way easier than working with isolated cells because the worms are larger and fairly sturdy,” Gonzales said. “With cells, if there’s too much pressure, they die. If they hit a wall, they die. But worms are really sturdy, so it was just a matter of getting them up against the electrodes and keeping them there.”

The team constructed microfluidic arrays with multiple channels that allowed testing of many nematodes at once. In comparison with patch-clamping techniques that limit labs to studying about one animal per hour, Robinson said his team measured as many as 16 nematodes per hour.

“Because this is a silicon-based technology, making arrays and producing recording chambers in high numbers becomes a real possibility,” he said.

A scanning electron micrograph shows a nano-SPEAR suspended midway between layers of silicon (grey) and photoresist material (pink) that form a recording chamber for immobilized nematodes. The high-throughput technology developed at Rice University can be adapted for other small animals and could enhance data-gathering for disease characterization and drug interactions. Courtesy of the Robinson Lab

Here’s a link to and a citation for the paper,

Scalable electrophysiology in intact small animals with nanoscale suspended electrode arrays by Daniel L. Gonzales, Krishna N. Badhiwala, Daniel G. Vercosa, Benjamin W. Avants, Zheng Liu, Weiwei Zhong, & Jacob T. Robinson. Nature Nanotechnology (2017) doi:10.1038/nnano.2017.55 Published online 17 April 2017

This paper is behind a paywall.

Skyrmions and ultra-thin multilayer film

The National University of Singapore (NUS) and skyrmions are featured in an April 10, 2017 news item on phys.org,

A team of scientists led by Associate Professor Yang Hyunsoo from the Department of Electrical and Computer Engineering at the National University of Singapore’s (NUS) Faculty of Engineering has invented a novel ultra-thin multilayer film which could harness the properties of tiny magnetic whirls, known as skyrmions, as information carriers for storing and processing data on magnetic media.

The nano-sized thin film, which was developed in collaboration with researchers from Brookhaven National Laboratory, Stony Brook University, and Louisiana State University, is a critical step towards the design of data storage devices that use less power and work faster than existing memory technologies. The invention was reported in prestigious scientific journal Nature Communications on 10 March 2017.

An April 10, 2017 NUS press release on EurekAlert, which originated the news item, describes the work in more detail,

Tiny magnetic whirls with huge potential as information carriers

The digital transformation has resulted in ever-increasing demands for better processing and storing of large amounts of data, as well as improvements in hard drive technology. Since their discovery in magnetic materials in 2009, skyrmions, which are tiny swirling magnetic textures only a few nanometres in size, have been extensively studied as possible information carriers in next-generation data storage and logic devices.

Skyrmions have been shown to exist in layered systems, with a heavy metal placed beneath a ferromagnetic material. Due to the interaction between the different materials, an interfacial symmetry breaking interaction, known as the Dzyaloshinskii-Moriya interaction (DMI), is formed, and this helps to stabilise a skyrmion. However, without an out-of-plane magnetic field present, the stability of the skyrmion is compromised. In addition, due to its tiny size, it is difficult to image the nano-sized materials.

To address these limitations, the researchers worked towards creating stable magnetic skyrmions at room temperature without the need for a biasing magnetic field.

Unique material for data storage

The NUS team, which also comprises Dr Shawn Pollard and Ms Yu Jiawei from the NUS Department of Electrical and Computer Engineering, found that a large DMI could be maintained in multilayer films composed of cobalt and palladium, and this is large enough to stabilise skyrmion spin textures.

In order to image the magnetic structure of these films, the NUS researchers, in collaboration with Brookhaven National Laboratory in the United States, employed Lorentz transmission electron microscopy (L-TEM). L-TEM has the ability to image magnetic structures below 10 nanometres, but it has not been used to observe skyrmions in multilayer geometries previously as it was predicted to exhibit zero signal. However, when conducting the experiments, the researchers found that by tilting the films with respect to the electron beam, they found that they could obtain clear contrast consistent with that expected for skyrmions, with sizes below 100 nanometres.

Dr Pollard explained, “It has long been assumed that there is no DMI in a symmetric structure like the one present in our work, hence, there will be no skyrmion. It is really unexpected for us to find both large DMI and skyrmions in the multilayer film we engineered. What’s more, these nanoscale skyrmions persisted even after the removal of an external biasing magnetic field, which are the first of their kind.”

Assoc Prof Yang added, “This experiment not only demonstrates the usefulness of L-TEM in studying these systems, but also opens up a completely new material in which skyrmions can be created. Without the need for a biasing field, the design and implementation of skyrmion based devices are significantly simplified. The small size of the skyrmions, combined with the incredible stability generated here, could be potentially useful for the design of next-generation spintronic devices that are energy efficient and can outperform current memory technologies.”

Next step

Assoc Prof Yang and his team are currently looking at how nanoscale skyrmions interact with each other and with electrical currents, to further the development of skyrmion based electronics.

Here’s a link to and a citation for the paper,

Observation of stable Néel skyrmions in cobalt/palladium multilayers with Lorentz transmission electron microscopy by Shawn D. Pollard, Joseph A. Garlow, Jiawei Yu, Zhen Wang, Yimei Zhu & Hyunsoo Yang. Nature Communications 8, Article number: 14761 (2017) doi:10.1038/ncomms14761 Published online: 10 March 2017

This is an open access paper.

2D printed transistors in Ireland

2D transistors seem to be a hot area for research these days. In Ireland, the AMBER Centre has announced a transistor consisting entirely of 2D nanomaterials in an April 6, 2017 news item on Nanowerk,

Researchers in AMBER, the Science Foundation Ireland-funded materials science research centre hosted in Trinity College Dublin, have fabricated printed transistors consisting entirely of 2-dimensional nanomaterials for the first time. These 2D materials combine exciting electronic properties with the potential for low-cost production.

This breakthrough could unlock the potential for applications such as food packaging that displays a digital countdown to warn you of spoiling, wine labels that alert you when your white wine is at its optimum temperature, or even a window pane that shows the day’s forecast. …

An April 7, 2017 AMBER Centre press release (also on EurekAlert), which originated the news item, expands on the theme,

Prof Jonathan Coleman, who is an investigator in AMBER and Trinity’s School of Physics, said, “In the future, printed devices will be incorporated into even the most mundane objects such as labels, posters and packaging.

Printed electronic circuitry (constructed from the devices we have created) will allow consumer products to gather, process, display and transmit information: for example, milk cartons could send messages to your phone warning that the milk is about to go out-of-date.

We believe that 2D nanomaterials can compete with the materials currently used for printed electronics. Compared to other materials employed in this field, our 2D nanomaterials have the capability to yield more cost effective and higher performance printed devices. However, while the last decade has underlined the potential of 2D materials for a range of electronic applications, only the first steps have been taken to demonstrate their worth in printed electronics. This publication is important because it shows that conducting, semiconducting and insulating 2D nanomaterials can be combined together in complex devices. We felt that it was critically important to focus on printing transistors as they are the electric switches at the heart of modern computing. We believe this work opens the way to print a whole host of devices solely from 2D nanosheets.”

Led by Prof Coleman, in collaboration with the groups of Prof Georg Duesberg (AMBER) and Prof. Laurens Siebbeles (TU Delft,Netherlands), the team used standard printing techniques to combine graphene nanosheets as the electrodes with two other nanomaterials, tungsten diselenide and boron nitride as the channel and separator (two important parts of a transistor) to form an all-printed, all-nanosheet, working transistor.

Printable electronics have developed over the last thirty years based mainly on printable carbon-based molecules. While these molecules can easily be turned into printable inks, such materials are somewhat unstable and have well-known performance limitations. There have been many attempts to surpass these obstacles using alternative materials, such as carbon nanotubes or inorganic nanoparticles, but these materials have also shown limitations in either performance or in manufacturability. While the performance of printed 2D devices cannot yet compare with advanced transistors, the team believe there is a wide scope to improve performance beyond the current state-of-the-art for printed transistors.

The ability to print 2D nanomaterials is based on Prof. Coleman’s scalable method of producing 2D nanomaterials, including graphene, boron nitride, and tungsten diselenide nanosheets, in liquids, a method he has licensed to Samsung and Thomas Swan. These nanosheets are flat nanoparticles that are a few nanometres thick but hundreds of nanometres wide. Critically, nanosheets made from different materials have electronic properties that can be conducting, insulating or semiconducting and so include all the building blocks of electronics. Liquid processing is especially advantageous in that it yields large quantities of high quality 2D materials in a form that is easy to process into inks. Prof. Coleman’s publication provides the potential to print circuitry at extremely low cost which will facilitate a range of applications from animated posters to smart labels.

Prof Coleman is a partner in Graphene flagship, a €1 billion EU initiative to boost new technologies and innovation during the next 10 years.

Here’s a link to and a citation for the paper,

All-printed thin-film transistors from networks of liquid-exfoliated nanosheets by Adam G. Kelly, Toby Hallam, Claudia Backes, Andrew Harvey, Amir Sajad Esmaeily, Ian Godwin, João Coelho, Valeria Nicolosi, Jannika Lauth, Aditya Kulkarni, Sachin Kinge, Laurens D. A. Siebbeles, Georg S. Duesberg, Jonathan N. Coleman. Science  07 Apr 2017: Vol. 356, Issue 6333, pp. 69-73 DOI: 10.1126/science.aal4062

This paper is behind a paywall.

Ultra-thin superconducting film for outer space

Truth in a press release? But first, there’s this April 6, 2017 news item on Nanowerk announcing research that may have applications in aerospace and other sectors,

Experimental physicists in the research group led by Professor Uwe Hartmann at Saarland University have developed a thin nanomaterial with superconducting properties. Below about -200 °C these materials conduct electricity without loss, levitate magnets and can screen magnetic fields.

The particularly interesting aspect of this work is that the research team has succeeded in creating superconducting nanowires that can be woven into an ultra-thin film that is as flexible as cling film. As a result, novel coatings for applications ranging from aerospace to medical technology are becoming possible.

The research team will be exhibiting their superconducting film at Hannover Messe from April 24th to April 28th [2017] (Hall 2, Stand B46) and are looking for commercial and industrial partners with whom they can develop their system for practical applications.

An April 6, 2017 University of Saarland press release (also on EurekAlert), which originated the news item, provides more details along with a line that rings with the truth,

A team of experimental physicists at Saarland University have developed something that – it has to be said – seems pretty unremarkable at first sight. [emphasis mine] It looks like nothing more than a charred black piece of paper. But appearances can be deceiving. This unassuming object is a superconductor. The term ‘superconductor’ is given to a material that (usually at a very low temperatures) has zero electrical resistance and can therefore conduct an electric current without loss. Put simply, the electrons in the material can flow unrestricted through the cold immobilized atomic lattice. In the absence of electrical resistance, if a magnet is brought up close to a cold superconductor, the magnet effectively ‘sees’ a mirror image of itself in the superconducting material. So if a superconductor and a magnet are placed in close proximity to one another and cooled with liquid nitrogen they will repel each another and the magnet levitates above the superconductor. The term ‘levitation’ comes from the Latin word levitas meaning lightness. It’s a bit like a low-temperature version of the hoverboard from the ‘Back to the Future’ films. If the temperature is too high, however, frictionless sliding is just not going to happen.
Many of the common superconducting materials available today are rigid, brittle and dense, which makes them heavy. The Saarbrücken physicists have now succeeded in packing superconducting properties into a thin flexible film. The material is a essentially a woven fabric of plastic fibres and high-temperature superconducting nanowires. ‘That makes the material very pliable and adaptable – like cling film (or ‘plastic wrap’ as it’s also known). Theoretically, the material can be made to any size. And we need fewer resources than are typically required to make superconducting ceramics, so our superconducting mesh is also cheaper to fabricate,’ explains Uwe Hartmann, Professor of Nanostructure Research and Nanotechnology at Saarland University.

The low weight of the film is particularly advantageous. ‘With a density of only 0.05 grams per cubic centimetre, the material is very light, weighing about a hundred times less than a conventional superconductor. This makes the material very promising for all those applications where weight is an issue, such as in space technology. There are also potential applications in medical technology,’ explains Hartmann. The material could be used as a novel coating to provide low-temperature screening from electromagnetic fields, or it could be used in flexible cables or to facilitate friction-free motion.

In order to be able to weave this new material, the experimental physicists made use of a technique known as electrospinning, which is usually used in the manufacture of polymeric fibres. ‘We force a liquid material through a very fine nozzle known as a spinneret to which a high electrical voltage has been applied. This produces nanowire filaments that are a thousand times thinner than the diameter of a human hair, typically about 300 nanometres or less. We then heat the mesh of fibres so that superconductors of the right composition are created. The superconducting material itself is typically an yttrium-barium-copper-oxide or similar compound,’ explains Dr. Michael Koblischka, one of the research scientists in Hartmann‘s group.

The research project received €100,000 in funding from the Volkswagen Foundation as part of its ‘Experiment!’ initiative. The initiative aims to encourage curiosity-driven, blue-skies research. The positive results from the Saarbrücken research team demonstrate the value of this type of funding. Since September 2016, the project has been supported by the German Research Foundation (DFG). Total funds of around €425,000 will be provided over a three-year period during which the research team will be carrying out more detailed investigations into the properties of the nanowires.

I’d say the “unremarkable but appearances can be deceiving” comments are true more often than not. I think that’s one of the hard things about science. Big advances can look nondescript.

What looks like a pretty unremarkable piece of burnt paper is in fact an ultrathin superconductor that has been developed by the team lead by Uwe Hartmann (r.) shown here with doctoral student XianLin Zeng. Courtesy: Saarland University

In any event, here’s a link to and a citation for the paper,

Preparation of granular Bi-2212 nanowires by electrospinning by Xian Lin Zeng, Michael R Koblischka, Thomas Karwoth, Thomas Hauet, and Uwe Hartmann. Superconductor Science and Technology, Volume 30, Number 3 Published 1 February 2017

© 2017 IOP Publishing Ltd

This paper is behind a paywall.

Carbon nanotubes self-assembling into transistors on a gold substrate

I’m not sure this work is ready for commercialization (I think not) but it’s certainly intriguing. From an April 5, 2017 news item on ScienceDaily,

Carbon nanotubes can be used to make very small electronic devices, but they are difficult to handle. University of Groningen scientists, together with colleagues from the University of Wuppertal and IBM Zurich, have developed a method to select semiconducting nanotubes from a solution and make them self-assemble on a circuit of gold electrodes. …

An April 5, 2017 University of Groningen (Netherlands) press release on EurekAlert, which originated the news item, explains the work in more detail,

The results look deceptively simple: a self-assembled transistor with nearly 100 percent purity and very high electron mobility. But it took ten years to get there. University of Groningen Professor of Photophysics and Optoelectronics Maria Antonietta Loi designed polymers which wrap themselves around specific carbon nanotubes in a solution of mixed tubes. Thiol side chains on the polymer bind the tubes to the gold electrodes, creating the resultant transistor.

Patent

‘In our previous work, we learned a lot about how polymers attach to specific carbon nanotubes’, Loi explains. These nanotubes can be depicted as a rolled sheet of graphene, the two-dimensional form of carbon. ‘Depending on the way the sheets are rolled up, they have properties ranging from semiconductor to semi-metallic to metallic.’ Only the semiconductor tubes can be used to fabricate transistors, but the production process always results in a mixture.

‘We had the idea of using polymers with thiol side chains some time ago’, says Loi. The idea was that as sulphur binds to metals, it will direct polymer-wrapped nanotubes towards gold electrodes. While Loi was working on the problem, IBM even patented the concept. ‘But there was a big problem in the IBM work: the polymers with thiols also attached to metallic nanotubes and included them in the transistors, which ruined them.’

Solution

Loi’s solution was to reduce the thiol content of the polymers, with the assistance of polymer chemists from the University of Wuppertal. ‘What we have now shown is that this concept of bottom-up assembly works: by using polymers with a low concentration of thiols, we can selectively bring semiconducting nanotubes from a solution onto a circuit.’ The sulphur-gold bond is strong, so the nanotubes are firmly fixed: enough even to stay there after sonication of the transistor in organic solvents.

The production process is simple: metallic patterns are deposited on a carrier , which is then dipped into a solution of carbon nanotubes. The electrodes are spaced to achieve proper alignment: ‘The tubes are some 500 nanometres long, and we placed the electrodes for the transistors at intervals of 300 nanometres. The next transistor is over 500 nanometres away.’ The spacing limits the density of the transistors, but Loi is confident that this could be increased with clever engineering.

‘Over the last years, we have created a library of polymers that select semiconducting nanotubes and developed a better understanding of how the structure and composition of the polymers influences which carbon nanotubes they select’, says Loi. The result is a cheap and scalable production method for nanotube electronics. So what is the future for this technology? Loi: ‘It is difficult to predict whether the industry will develop this idea, but we are working on improvements, and this will eventually bring the idea closer to the market.’

Here’s a link to and a citation for the paper,

On-Chip Chemical Self-Assembly of Semiconducting Single-Walled Carbon Nanotubes (SWNTs): Toward Robust and Scale Invariant SWNTs Transistors by Vladimir Derenskyi, Widianta Gomulya, Wytse Talsma, Jorge Mario Salazar-Rios, Martin Fritsch, Peter Nirmalraj, Heike Riel, Sybille Allard, Ullrich Scherf, and Maria A. Loi. Advanced Materials DOI: 10.1002/adma.201606757 Version of Record online: 5 APR 2017

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

Improving silver nanowires for flexible transparent conducting electrodes (FTCEs)

This is a very pretty image from the Korea Advanced Institute of Science and Technology (KAIST),

Picture 1: Artistic Rendtition of Light Interaction with Nanomaterials (This image shows flash-induced plasmonic interactions with nanowires to improve silver nanowires (Ag NWs).) Courtesy: KAIST

An April 4, 2017 news item on plys.org announces the research,

Flexible transparent conducting electrodes (FTCEs) are an essential element of flexible optoelectronics for next-generation wearable displays, augmented reality (AR), and the Internet of Things (IoTs). Silver nanowires (Ag NWs) have received a great deal of attention as future FTCEs due to their great flexibility, material stability, and large-scale productivity. Despite these advantages, Ag NWs have drawbacks such as high wire-to-wire contact resistance and poor adhesion to substrates, resulting in severe power consumption and the delamination of FTCEs.

A Korean research team led by Professor Keon Jae Lee of the Materials Science and Engineering Department at KAIST and Dr. Hong-Jin Park from BSP Inc., has developed high-performance Ag NWs (sheet resistance ~ 5 Ω /sq, transmittance 90 % at λ = 550 nm) with strong adhesion on plastic (interfacial energy of 30.7 J m-2) using flash light-material interactions.

An April 5, 2017 KAIST press release (also on EurekAlert), which originated the news item, explains more about the research,

The broad ultraviolet (UV) spectrum of a flash light enables the localized heating at the junctions of nanowires (NWs), which results in the fast and complete welding of Ag NWs. Consequently, the Ag NWs demonstrate six times higher conductivity than that of the pristine NWs. In addition, the near-infrared (NIR) of the flash lamp melted the interface between the Ag NWs and a polyethylene terephthalate (PET) substrate, dramatically enhancing the adhesion force of the Ag NWs to the PET by 310 %.

Professor Lee said, “Light interaction with nanomaterials is an important field for future flexible electronics since it can overcome thermal limit of plastics, and we are currently expanding our research into light-inorganic interactions.”

Meanwhile, BSP Inc., a laser manufacturing company and a collaborator of this work, has launched new flash lamp equipment for flexible applications based on the Prof. Lee’s research.

The results of this work entitled “Flash-Induced Self-Limited Plasmonic Welding of Ag NW Network for Transparent Flexible Energy Harvester (DOI: 10.1002/adma.201603473)”(http://onlinelibrary.wiley.com/doi/10.1002/adma.201603473/pdf) were published in the February 2, 2017 issue of Advanced Materials as the cover article.

Professor Lee also contributed an invited review in the same journal of the April 3 2017 online issue, “Laser-Material Interaction for Flexible Applications,” overviewing the recent advances in light interactions with flexible nanomaterials.

Lately, It seems I’ve stumbled across quite a few stories about wearable technologies and research to improve them.

Recycle electronic waste by crushing it into nanodust

Given the issues with e-waste this work seems quite exciting. From a March 21, 2017 Rice University news release (also on EurekAlert), Note: Links have been removed,

Researchers at Rice University and the Indian Institute of Science have an idea to simplify electronic waste recycling: Crush it into nanodust.

Specifically, they want to make the particles so small that separating different components is relatively simple compared with processes used to recycle electronic junk now.

Chandra Sekhar Tiwary, a postdoctoral researcher at Rice and a researcher at the Indian Institute of Science in Bangalore, uses a low-temperature cryo-mill to pulverize electronic waste – primarily the chips, other electronic components and polymers that make up printed circuit boards (PCBs) — into particles so small that they do not contaminate each other.

Then they can be sorted and reused, he said.

Circuit boards from electronics, like computer mice, can be crushed into nanodust by a cryo-mill, according to researchers at Rice and the Indian Institute of Science. The dust can then be easily separated into its component elements for recycling.

Circuit boards from electronics, like computer mice, can be crushed into nanodust by a cryo-mill, according to researchers at Rice and the Indian Institute of Science. The dust can then be easily separated into its component elements for recycling. Courtesy of the Ajayan Research Group

The process is the subject of a Materials Today paper by Tiwary, Rice materials scientist Pulickel Ajayan and Indian Institute professors Kamanio Chattopadhyay and D.P. Mahapatra. 

The researchers intend it to replace current processes that involve dumping outdated electronics into landfills, or burning or treating them with chemicals to recover valuable metals and alloys. None are particularly friendly to the environment, Tiwary said.

“In every case, the cycle is one way, and burning or using chemicals takes a lot of energy while still leaving waste,” he said. “We propose a system that breaks all of the components – metals, oxides and polymers – into homogenous powders and makes them easy to reuse.”

The researchers estimate that so-called e-waste will grow by 33 percent over the next four years, and by 2030 will weigh more than a billion tons. Nearly 80 to 85 percent of often-toxic e-waste ends up in an incinerator or a landfill, Tiwary said, and is the fastest-growing waste stream in the United States, according to the Environmental Protection Agency.

The answer may be scaled-up versions of a cryo-mill designed by the Indian team that, rather than heating them, keeps materials at ultra-low temperatures during crushing.

Cold materials are more brittle and easier to pulverize, Tiwary said. “We take advantage of the physics. When you heat things, they are more likely to combine: You can put metals into polymer, oxides into polymers. That’s what high-temperature processing is for, and it makes mixing really easy.

A transparent piece of epoxy, left, compared to epoxy with e-waste reinforcement at right. A cryo-milling process developed at Rice University and the Indian Institute of Science simplifies the process of separating and recycling electronic waste.

A transparent piece of epoxy, left, compared to epoxy with e-waste reinforcement at right. A cryo-milling process developed at Rice University and the Indian Institute of Science simplifies the process of separating and recycling electronic waste. Courtesy of the Ajayan Research Group

“But in low temperatures, they don’t like to mix. The materials’ basic properties – their elastic modulus, thermal conductivity and coefficient of thermal expansion – all change. They allow everything to separate really well,” he said.

The test subjects in this case were computer mice – or at least their PCB innards. The cryo-mill contained argon gas and a single tool-grade steel ball. A steady stream of liquid nitrogen kept the container at 154 kelvins (minus 182 degrees Fahrenheit).

When shaken, the ball smashes the polymer first, then the metals and then the oxides just long enough to separate the materials into a powder, with particles between 20 and 100 nanometers wide. That can take up to three hours, after which the particles are bathed in water to separate them.

“Then they can be reused,” he said. “Nothing is wasted.”

Here’s a link to and a citation for the paper,

Electronic waste recycling via cryo-milling and nanoparticle beneficiation by C.S. Tiwary, S. Kishore, R. Vasireddi, D.R. Mahapatra, P.M. Ajayan, K. Chattopadhyay. Materials Today         http://dx.doi.org/10.1016/j.mattod.2017.01.015 Available online 20 March 2017

This paper is behind a paywall.

Transparent silver

This March 21, 2017 news item on Nanowerk is the first I’ve heard of transparent silver; it’s usually transparent aluminum (Note: A link has been removed),

The thinnest, smoothest layer of silver that can survive air exposure has been laid down at the University of Michigan, and it could change the way touchscreens and flat or flexible displays are made (Advanced Materials, “High-performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications”).

It could also help improve computing power, affecting both the transfer of information within a silicon chip and the patterning of the chip itself through metamaterial superlenses.

A March 21, 2017 University of Michigan  news release, which originated the news item, provides details about the research and features a mention about aluminum,

By combining the silver with a little bit of aluminum, the U-M researchers found that it was possible to produce exceptionally thin, smooth layers of silver that are resistant to tarnishing. They applied an anti-reflective coating to make one thin metal layer up to 92.4 percent transparent.

The team showed that the silver coating could guide light about 10 times as far as other metal waveguides—a property that could make it useful for faster computing. And they layered the silver films into a metamaterial hyperlens that could be used to create dense patterns with feature sizes a fraction of what is possible with ordinary ultraviolet methods, on silicon chips, for instance.

Screens of all stripes need transparent electrodes to control which pixels are lit up, but touchscreens are particularly dependent on them. A modern touch screen is made of a transparent conductive layer covered with a nonconductive layer. It senses electrical changes where a conductive object—such as a finger—is pressed against the screen.

“The transparent conductor market has been dominated to this day by one single material,” said L. Jay Guo, professor of electrical engineering and computer science.

This material, indium tin oxide, is projected to become expensive as demand for touch screens continues to grow; there are relatively few known sources of indium, Guo said.

“Before, it was very cheap. Now, the price is rising sharply,” he said.

The ultrathin film could make silver a worthy successor.

Usually, it’s impossible to make a continuous layer of silver less than 15 nanometers thick, or roughly 100 silver atoms. Silver has a tendency to cluster together in small islands rather than extend into an even coating, Guo said.

By adding about 6 percent aluminum, the researchers coaxed the metal into a film of less than half that thickness—seven nanometers. What’s more, when they exposed it to air, it didn’t immediately tarnish as pure silver films do. After several months, the film maintained its conductive properties and transparency. And it was firmly stuck on, whereas pure silver comes off glass with Scotch tape.

In addition to their potential to serve as transparent conductors for touch screens, the thin silver films offer two more tricks, both having to do with silver’s unparalleled ability to transport visible and infrared light waves along its surface. The light waves shrink and travel as so-called surface plasmon polaritons, showing up as oscillations in the concentration of electrons on the silver’s surface.

Those oscillations encode the frequency of the light, preserving it so that it can emerge on the other side. While optical fibers can’t scale down to the size of copper wires on today’s computer chips, plasmonic waveguides could allow information to travel in optical rather than electronic form for faster data transfer. As a waveguide, the smooth silver film could transport the surface plasmons over a centimeter—enough to get by inside a computer chip.

Here’s a link to and a citation for the paper,

High-Performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications by Cheng Zhang, Nathaniel Kinsey, Long Chen, Chengang Ji, Mingjie Xu, Marcello Ferrera, Xiaoqing Pan, Vladimir M. Shalaev, Alexandra Boltasseva, and Jay Guo. Advanced Materials DOI: 10.1002/adma.201605177 Version of Record online: 20 MAR 2017

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.