Category: Electronics

Buckminsterfullerene (C₆₀), aka fullerene and buckyball (credit: St Stev via Foter.com / CC BY-NC-ND)

A Dartmouth College scientist and his collaborators* have created the first high-resolution co-assembly between a protein and buckminsterfullerene (C₆₀), aka fullerene and buckyball (a sphere-like molecule composed of 60 carbon atoms and shaped like a soccer ball).

“This is a proof-of-principle study demonstrating that proteins can be used as effective vehicles for organizing nanomaterials by design,” says senior author Gevorg Grigoryan, an assistant professor of computer science at Dartmouth and senior author of a study discussed in an open-access paper in the journal in Nature Communications.

Proteins organize and orchestrate essentially all molecular processes in our cells. The goal of the new study was to create a new artificial protein that can direct the self-assembly of fullerene into ordered superstructures.

COP, a stable tetramer (a polymer derived from four identical single molecule) in isolation, interacts with C60 (fullerene) molecules via a surface-binding site and further self-assembles into a co-crystalline array called C₆₀Sol–COP (credit: Kook-Han Kim et al./Nature Communications)

Grigoryan and his colleagues show that that their artificial protein organizes a fullerene into a lattice called C₆₀Sol–COP. COP, a protein that is a stable tetramer (a polymer derived from four identical single molecules), interacted with fullerene molecules via a surface-binding site and further self-assembled into an ordered crystalline superstructure. Interestingly, the superstructure exhibits high charge conductance, whereas both the protein-alone crystal and amorphous C₆₀ are electrically insulating.

Grigoryan says that if we learn to do the programmable self-assembly of precisely organized molecular building blocks more generally, it will lead to a range of new materials with properties such as higher strength, lighter weight, and greater chemical reactivity, resulting in a host of applications, from medicine to energy and electronics.

Fullerenes are currently used in nanotechnology because of their high heat resistance and electrical superconductivity (when doped), but the molecule has been difficult to organize in useful ways.

* The study also included researchers from Dartmouth College, Sungkyunkwan University, the New Jersey Institute of Technology, the National Institute of Science Education and Research, the University of California-San Francisco, the University of Pennsylvania, and the Institute for Basic Science.

Abstract of Protein-directed self-assembly of a fullerene crystal

Learning to engineer self-assembly would enable the precise organization of molecules by design to create matter with tailored properties. Here we demonstrate that proteins can direct the self-assembly of buckminsterfullerene (C₆₀) into ordered superstructures. A previously engineered tetrameric helical bundle binds C₆₀ in solution, rendering it water soluble. Two tetramers associate with one C₆₀, promoting further organization revealed in a 1.67-Å crystal structure. Fullerene groups occupy periodic lattice sites, sandwiched between two Tyr residues from adjacent tetramers. Strikingly, the assembly exhibits high charge conductance, whereas both the protein-alone crystal and amorphous C₆₀ are electrically insulating. The affinity of C₆₀ for its crystal-binding site is estimated to be in the nanomolar range, with lattices of known protein crystals geometrically compatible with incorporating the motif. Taken together, these findings suggest a new means of organizing fullerene molecules into a rich variety of lattices to generate new properties by design.

World’s fastest silicon-based flexible transistors, shown here on a plastic substrate (credit: Jung-Hun Seo/UW–Madison)

A team headed by University of Wisconsin—Madison engineers has fabricated a flexible transistor that operates at a record 38 gigahertz, but may be able to operate at 110 gigahertz.

The process could allow manufacturers to easily and cheaply fabricate high-performance transistors with wireless capabilities, using a radical fabrication method based on huge rolls of flexible plastic.

The new transistor can also transmit data or transfer power wirelessly, which could unlock advances in a whole host of applications ranging from wearable electronics to sensors.

Low-cost radical method uses less energy, achieves higher transistor density

The researchers’ nanoscale fabrication method (based on a simple, low-cost process called nanoimprint lithography) replaces conventional lithographic approaches — which use light and chemicals to pattern flexible transistors — overcoming such limitations as light diffraction, imprecision that leads to short circuits of different contacts, and the need to fabricate the circuitry in multiple passes.

The researchers — led by Zhenqiang (Jack) Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering, and research scientist Jung-Hun Seo —  published details of the advance Wednesday April 20 in an open-access paper in the journal Scientific Reports.

With a unique, three-dimensional current-flow pattern, the high-performance transistor consumes less energy and operates more efficiently. And because the researchers’ method enables them to slice much narrower trenches than conventional fabrication processes can, it also could enable semiconductor manufacturers to squeeze an even greater number of transistors onto an electronic device.

Ultimately, says Ma, because the mold can be reused, the method could easily scale for use in a technology called roll-to-roll processing (think of a giant, patterned rolling pin moving across sheets of plastic the size of a tabletop), and that would allow semiconductor manufacturers to repeat their pattern and mass-fabricate many devices on a roll of flexible plastic.

“Nanoimprint lithography addresses future applications for flexible electronics,” says Ma, whose work was supported by the Air Force Office of Scientific Research. “We don’t want to make them the way the semiconductor industry does now. Our step, which is most critical for roll-to-roll printing, is ready.”

The process

1. Using low-temperature processes, the researchers patterned the transistor circuitry using nanoimprint lithography.
2. Using selective doping, the researchers introduced impurities into materials in precise locations to enhance their properties — in this case, electrical conductivity. Currently, the dopant sometimes merges into areas of the material it shouldn’t, causing what is known as the “short channel” effect. The researchers took an unconventional approach: They blanketed their single crystalline silicon with a dopant, rather than selectively doping it.
3. They added a light-sensitive material, or photoresist layer, and used a technique called electron-beam lithography — which uses a focused beam of electrons to create shapes as narrow as 10 nanometers wide — on the photoresist to create a reusable mold of the nanoscale patterns they desired. They applied the mold to an ultrathin, very flexible silicon membrane to create a photoresist pattern.
4. They finished with a dry-etching process — essentially, a nanoscale knife — that cut precise, nanometer-scale trenches in the silicon following the patterns in the mold, and added wide gates, which function as switches, atop the trenches.

Additional authors are at UW–Madison, the University of Michigan, the University of Texas at Arlington, and the University of California, Berkeley.

Abstract of Fast Flexible Transistors with a Nanotrench Structure

The simplification of fabrication processes that can define very fine patterns for large-area flexible radio-frequency (RF) applications is very desirable because it is generally very challenging to realize submicron scale patterns on flexible substrates. Conventional nanoscale patterning methods, such as e-beam lithography, cannot be easily applied to such applications. On the other hand, recent advances in nanoimprinting lithography (NIL) may enable the fabrication of large-area nanoelectronics, especially flexible RF electronics with finely defined patterns, thereby significantly broadening RF applications. Here we report a generic strategy for fabricating high-performance flexible Si nanomembrane (NM)-based RF thin-film transistors (TFTs), capable of over 100 GHz operation in theory, with NIL patterned deep-submicron-scale channel lengths. A unique 3-dimensional etched-trench-channel configuration was used to allow for TFT fabrication compatible with flexible substrates. Optimal device parameters were obtained through device simulation to understand the underlying device physics and to enhance device controllability. Experimentally, a record-breaking 38 GHz maximum oscillation frequency f_max value has been successfully demonstrated from TFTs with a 2 μm gate length built with flexible Si NM on plastic substrates.

Top: Schematic diagram of all-nanowire-based capacitor (similar to a battery), using gold-manganese dioxide conductors and PMMA gel layer. Bottom: photograph of the capacitor containing 750 parallel nanowire loops patterned onto a glass microscope slide. (credit: Mya Le Thai/ACS Energy Lett.)

University of California, Irvine researchers have invented a new nanowire-based battery material that can be recharged hundreds of thousands of times, moving us closer to a battery that would never require replacement.

It could lead to commercial batteries with greatly lengthened lifespans for computers, smartphones, appliances, cars, and spacecraft.

The design is based on nanowires, which are highly conductive and feature a large surface area for the storage and transfer of electrons.

Currently, nanowires are extremely fragile and don’t hold up well to repeated discharging and recharging (cycling). In a typical lithium-ion battery, they expand and grow brittle, which leads to cracking.

UCI researchers have solved this problem by coating a gold nanowire in a manganese dioxide shell and encasing the assembly in an electrolyte made of a Plexiglas-like gel. The liquid electrolyte is replaced with a poly(methyl methacrylate) (PMMA) gel electrolyte. The combination is reliable and resistant to failure.

The study leader, UCI doctoral candidate Mya Le Thai, cycled the testing electrode up to 200,000 times over three months without detecting any loss of capacity or power and without fracturing any nanowires. The findings were published Wednesday Apr. 20 in an open-access paper in the American Chemical Society’s Energy Letters.

“Mya was playing around, and she coated this whole thing with a very thin gel layer and started to cycle it,” said senior author Reginald Penner, chair of UCI’s chemistry department. “She discovered that just by using this gel, she could cycle it hundreds of thousands of times without losing any capacity. That was crazy, because these things typically die in dramatic fashion after 5,000 or 6,000 or 7,000 cycles at most.”

The researchers think the gel plasticizes the metal oxide in the battery and gives it flexibility, preventing cracking.

“The coated electrode holds its shape much better, making it a more reliable option,” Thai said. “This research proves that a nanowire-based battery electrode can have a long lifetime and that we can make these kinds of batteries a reality.”

Abstract of 100k Cycles and Beyond: Extraordinary Cycle Stability for MnO₂ Nanowires Imparted by a Gel Electrolyte

We demonstrate reversible cycle stability for up to 200 000 cycles with 94–96% average Coulombic efficiency for symmetrical δ-MnO₂ nanowire capacitors operating across a 1.2 V voltage window in a poly(methyl methacrylate) (PMMA) gel electrolyte. The nanowires investigated here have a Au@δ-MnO₂ core@shell architecture in which a central gold nanowire current collector is surrounded by an electrodeposited layer of δ-MnO₂ that has a thickness of between 143 and 300 nm. Identical capacitors operating in the absence of PMMA (propylene carbonate (PC), 1.0 M LiClO₄) show dramatically reduced cycle stabilities ranging from 2000 to 8000 cycles. In the liquid PC electrolyte, the δ-MnO₂ shell fractures, delaminates, and separates from the gold nanowire current collector. These deleterious processes are not observed in the PMMA electrolyte.

Researchers at The Ohio State University are developing embroidered antennas and circuits with 0.1 mm precision — the perfect size to integrate electronic components such as communication devices, sensors, and computer memory devices into clothing.  (credit: Photo by Jo McCulty, courtesy of The Ohio State University)

Ohio State University researchers have taken a key step in the design of  “functional textiles” — clothes that gather, store, or transmit digital information. They’ve developed a breakthrough method of weaving electronic components into fabric with 0.1mm precision — small enough to integrate components such as sensors and computer memory devices into clothing.

Imagine shirts that act as antennas for your smart phone or tablet, workout clothes that monitor your fitness level, sports equipment that monitors athletes’ performance, a bandage that tells your doctor how well the tissue beneath is healing, or a flexible fabric cap that senses or stimulates activity in the brain (eliminating restrictive tethered external wiring on the patient’s body).

“A revolution is happening in the textile industry,” said John Volakis, director of the ElectroScience Laboratory and the Roy & Lois Chope Chair Professor of Electrical Engineering at Ohio State. “We believe that functional textiles are an enabling technology for communications and sensing — and one day, even medical applications like imaging and health monitoring.”

Recently, he and research scientist Asimina Kiourti refined their patented fabrication method to create prototype wearables at a fraction of the cost and in half the time compared to two years ago. They published the new results in the journal IEEE Antennas and Wireless Propagation Letters.

Cell-phone antennas

Asimina Kiourti, a research scientist at The Ohio State University, demonstrates the embroidery technique she and John Volakis invented for integrating electronics into clothing (photo credit: Jo McCulty/The Ohio State University)

In Volakis’ lab, the functional textiles, also called “e-textiles,” are created in part on a typical tabletop sewing machine. Like other modern sewing machines, it embroiders thread into fabric automatically based on a pattern loaded from a computer file. The researchers substitute the thread with fine silver metal wires that, once embroidered, feel the same as traditional thread to the touch.

“We started with a technology that is very well known — machine embroidery — and we asked: how can we functionalize embroidered shapes? How do we make them transmit signals at useful frequencies, like for cell phones or health sensors?” Volakis said. “Now, for the first time, we’ve achieved the accuracy of printed metal circuit boards, so our new goal is to take advantage of the precision to incorporate receivers and other electronic components.”

Broadband antenna constructed from multiple embroidered geometric shapes, each resonating at a different frequency (photo credit: Jo McCulty/The Ohio State University)

The shape of the embroidery determines the frequency of operation of the antenna or circuit, explained Kiourti. The shape of one broadband antenna, for instance, consists of more than half a dozen interlocking geometric shapes, each a little bigger than a fingernail, that form an intricate circle a few inches across. Each piece of the circle transmits energy at a different frequency, so that they cover a broad spectrum of energies when working together — achieving a “broadband” capability of the antenna for cell phone and Internet access.

In another design, tests showed that an embroidered spiral antenna (top photo) measuring approximately six inches across transmitted signals at frequencies of 1 to 5 GHz with near-perfect efficiency,  well-suited to broadband Internet and cellular communication.

Proposed embroidery process to achieve 0.1mm geometrical precision (credit: Asimina Kiourti et al./IEEE Antennas Wireless Propag. Lett.)

On problem they had was that fine wires couldn’t provide as much surface conductivity as thick wires. So they had to find a way to work the fine thread into embroidery densities and shapes that would boost the surface conductivity and, thus, the antenna/sensor performance.

The new threads have a 0.1-mm diameter, made with only seven filaments. Each filament is copper at the center, enameled with pure silver. They purchase the wire by the spool at a cost of 3 cents per foot; Kiourti estimated that embroidering a single broadband antenna like the one mentioned above consumes about 10 feet of thread, for a material cost of around 30 cents per antenna. That’s 24 times less expensive than when Volakis and Kiourti created similar antennas in 2014.

In part, the cost savings comes from using less thread per embroidery. The researchers previously had to stack the thicker thread in two layers, one on top of the other, to make the antenna carry a strong enough electrical signal. But by refining the technique that she and Volakis developed, Kiourti was able to create the new, high-precision antennas in only one embroidered layer of the finer thread. So now the process takes half the time: only about 15 minutes for the broadband antenna mentioned above.

She’s also incorporated some techniques common to microelectronics manufacturing to add parts to embroidered antennas and circuits. One prototype antenna looks like a spiral and can be embroidered into clothing to improve cell phone signal reception. Another prototype, a stretchable antenna with an integrated RFID (radio-frequency identification) chip embedded in rubber, takes the applications for the technology beyond clothing — for tires, in this case.

The work fits well with Ohio State’s role as a founding partner of the Advanced Functional Fabrics of America Institute, a national manufacturing resource center for industry and government. The new institute, which joins some 50 universities and industrial partners, was announced earlier this month by U.S. Secretary of Defense Ashton Carter.

Syscom Advanced Materials in Columbus provided the threads used in Volakis and Kiourti’s initial work. The finer threads used in this study were purchased from Swiss manufacturer Elektrisola. The research is funded by the National Science Foundation, and Ohio State will license the technology for further development.

Abstract of Fabrication of Textile Antennas and Circuits With 0.1 mm Precision

We present a new selection of E-fibers (also referred to as E-threads) and associated embroidery process. The new E-threads and process achieve a geometrical precision down to 0.1 mm. Thus, for the first time, accuracy of typical printed circuit board (PCB) prototypes can be achieved directly on textiles. Compared to our latest embroidery approach, the proposed process achieves: 1) 3 × higher geometrical precision; 2) 24 × lower fabrication cost; 3) 50% less fabrication time; and 4) equally good RF performance. This improvement was achieved by employing a new class of very thin, 7-filament, Elektrisola E-threads ( diameter ≈ 0.12 mm, almost 2 × thinner than before). To validate our approach, we “printed” and tested a textile spiral antenna operating between 1-5 GHz. Measurement results were in good agreement with simulations. We envision this textile spiral to be integrated within a cap and unobtrusively acquire neuropotentials from wireless fully-passive brain implants. Overall, the proposed embroidery approach brings forward new possibilities for a wide range of applications.

Because this process works at relatively low temperatures, many transistors can be made on a flexible backing at once. (credit: University of Pennsylvania)

University of Pennsylvania engineers have developed a simplified new approach for making transistors by sequentially depositing their components in the form of liquid nanocrystal “inks.” The new process open the door for transistors and other electronic components to be built into flexible or wearable applications. It also avoids the highly complex current process for creating transistors, which requires high-temperature, high-vacuum equipment. Also, the new lower-temperature process is compatible with a wide array of materials and can be applied to larger areas.

Transistors patterned on plastic backing

The researchers’ nanocrystal-based field effect transistors were patterned onto flexible plastic backings using spin coating, but could eventually be constructed by additive manufacturing systems, like 3D printers.

Published in the journal Science,  the study was lead by Cherie Kagan, the Stephen J. Angello Professor in the School of Engineering and Applied Science, and Ji-Hyuk Choi, then a member of her lab, now a senior researcher at the Korea Institute of Geoscience and Mineral Resources. Researchers at Korea University Korea’s Yonsei University were also involved.

Kagan’s group developed four nanocrystal inks that comprise the transistor, then deposited them on a flexible backing. (credit: University of Pennsylvania)

The researchers began by dispersing a specific type of nanocrystals in a liquid, creating nanocrystal inks. They developed a library of four of these inks: a conductor (silver), an insulator (aluminum oxide), a semiconductor (cadmium selenide), and a conductor combined with a dopant (a mixture of silver and indium). (“Doping” the semiconductor layer of a transistor with impurities controls whether the device creates a positive or negative charge.)

“These materials are colloids just like the ink in your inkjet printer,” Kagan said, “but you can get all the characteristics that you want and expect from the analogous bulk materials, such as whether they’re conductors, semiconductors or insulators.” Although the electrical properties of several of these nanocrystal inks had been independently verified, they had never been combined into full devices. “Our question was whether you could lay them down on a surface in such a way that they work together to form functional transistors.”

Laying down patterns in layers

Such a process entails layering or mixing them in precise patterns.

First, the conductive silver nanocrystal ink was deposited from liquid on a flexible plastic surface that was treated with a photolithographic mask, then rapidly spun to draw it out in an even layer. The mask was then removed to leave the silver ink in the shape of the transistor’s gate electrode.

The researchers followed that layer by spin-coating a layer of the aluminum oxide nanocrystal-based insulator, then a layer of the cadmium selenide nanocrystal-based semiconductor and finally another masked layer for the indium/silver mixture, which forms the transistor’s source and drain electrodes. Upon heating at relatively low temperatures, the indium dopant diffused from those electrodes into the semiconductor component.

“The trick with working with solution-based materials is making sure that, when you add the second layer, it doesn’t wash off the first, and so on,” Kagan said. “We had to treat the surfaces of the nanocrystals, both when they’re first in solution and after they’re deposited, to make sure they have the right electrical properties and that they stick together in the configuration we want.”

Because this entirely ink-based fabrication process works at lower temperatures than existing vacuum-based methods, the researchers were able to make several transistors on the same flexible plastic backing at the same time.

The inks’ specialized surface chemistry allowed them to stay in configuration without losing their electrical properties. (credit: University of Pennsylvania)

“Making transistors over larger areas and at lower temperatures have been goals for an emerging class of technologies, when people think of the Internet of things, large area flexible electronics and wearable devices,” Kagan said. “We haven’t developed all of the necessary aspects so they could be printed yet, but because these materials are all solution-based, it demonstrates the promise of this materials class and sets the stage for additive manufacturing.”

Because this entirely ink-based fabrication process works at lower temperatures than existing vacuum-based methods, the researchers were able to make several transistors on the same flexible plastic backing at the same time.

3D-printing transistors for wearables

“This is the first work,” Choi said, “showing that all the components, the metallic, insulating, and semiconducting layers of the transistors, and even the doping of the semiconductor, could be made from nanocrystals.”

“Making transistors over larger areas and at lower temperatures have been goals for an emerging class of technologies, when people think of the Internet of things, large area flexible electronics and wearable devices,” Kagan said. “We haven’t developed all of the necessary aspects so they could be printed yet, but because these materials are all solution-based, it demonstrates the promise of this materials class and sets the stage for additive manufacturing.”

The research was supported by the National Science Foundation, the U.S. Department of Energy, the Office of Naval Research, and the Korea Institute of Geoscience and Mineral Resources funded by the Ministry of Science, ICT, and Future Planning of Korea.

Abstract of Exploiting the colloidal nanocrystal library to construct electronic devices

Synthetic methods produce libraries of colloidal nanocrystals with tunable physical properties by tailoring the nanocrystal size, shape, and composition. Here, we exploit colloidal nanocrystal diversity and design the materials, interfaces, and processes to construct all-nanocrystal electronic devices using solution-based processes. Metallic silver and semiconducting cadmium selenide nanocrystals are deposited to form high-conductivity and high-mobility thin-film electrodes and channel layers of field-effect transistors. Insulating aluminum oxide nanocrystals are assembled layer by layer with polyelectrolytes to form high–dielectric constant gate insulator layers for low-voltage device operation. Metallic indium nanocrystals are codispersed with silver nanocrystals to integrate an indium supply in the deposited electrodes that serves to passivate and dope the cadmium selenide nanocrystal channel layer. We fabricate all-nanocrystal field-effect transistors on flexible plastics with electron mobilities of 21.7 square centimeters per volt-second.