Category: Nanotech/Materials Science

Researchers have developed a new method for doping (integrating elements to change a semiconductor’s properties) single crystals of diamond with boron at relatively low temperatures, without degradation.

Diamonds have properties that could make them ideal semiconductors for power electronics. They can handle high voltages and power, and electrical currents also flow through diamonds quickly, meaning the material would make for energy-efficient devices. And they are thermally conductive, which means diamond-based devices would dissipate heat quickly and easily (no need for bulky, expensive cooling methods). However. diamond’s rigid crystalline structure makes doping difficult.*

Doping a diamond with boron

Zhengqiang (Jack) Ma, a University of Wisconsin-Madison electrical and computer engineering professor, and his colleagues describe a solution in the Journal of Applied Physics, from AIP Publishing.

They discovered that if you bond a single-crystal diamond with a piece of silicon doped with boron, and heat it to 800 degrees Celsius (low compared to conventional techniques), the boron atoms will migrate from the silicon to the diamond. It turns out that the boron-doped silicon has defects such as vacancies, where an atom is missing in the lattice structure. Carbon atoms from the diamond will fill those vacancies, leaving empty spots for boron atoms.

This technique also allows for selective doping, which means more control when making devices. You can choose where to dope a single-crystal diamond simply by bonding the silicon to that spot.

The new method currently only works for P-type doping, where the semiconductor is doped with an element that provides positive charge carriers (in this case, the absence of electrons, called holes). The researchers are already working on a simple device using P-type single-crystal diamond semiconductors.

But to make electronic devices like transistors, you need N-type doping, which gives the semiconductor negative charge carriers (electrons). And other barriers remain: diamond is expensive and single crystals are very small.

Still, Ma says, achieving P-type doping is an important step, and might inspire others to find solutions for the remaining challenges. Eventually, he said, single-crystal diamond could be useful everywhere — perfect, for instance, for controlling power in the electrical grid.

* Currently, you can dope diamond by coating the crystal with boron and heating it to 1450 degrees Celsius. But it’s difficult to remove the boron coating at the end. This method only works on diamonds consisting of multiple crystals stuck together. Because such polydiamonds have irregularities between the crystals, single crystals would be superior semiconductors. You can dope single crystals by injecting boron atoms while growing the crystals artificially. The problem is the process requires powerful microwaves that can degrade the quality of the crystal.

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Abstract of Thermal diffusion boron doping of single-crystal natural diamond

With the best overall electronic and thermal properties, single crystal diamond (SCD) is the extreme wide bandgap material that is expected to revolutionize power electronics and radio-frequency electronics in the future. However, turning SCD into useful semiconductors requires overcoming doping challenges, as conventional substitutional doping techniques, such as thermal diffusion and ion implantation, are not easily applicable to SCD. Here we report a simple and easily accessible doping strategy demonstrating that electrically activated, substitutional doping in SCD without inducing graphitization transition or lattice damage can be readily realized with thermal diffusion at relatively low temperatures by using heavily dopedSi nanomembranes as a unique dopant carrying medium. Atomistic simulations elucidate a vacancyexchange boron doping mechanism that occurs at the bonded interface between Si and diamond. We further demonstrate selectively doped high voltage diodes and half-wave rectifier circuits using such dopedSCD. Our new doping strategy has established a reachable path toward using SCDs for future high voltage power conversion systems and for other novel diamond based electronic devices. The novel dopingmechanism may find its critical use in other wide bandgap semiconductors.

A new electronic material created by an international team headed by Penn State scientists can heal all its functions automatically, even after breaking multiple times. The new material could improve the durability of wearable electronics.

Electronic materials have been a major stumbling block for the advance of flexible electronics because existing materials do not function well after breaking and healing.

“Wearable and bendable electronics are subject to mechanical deformation over time, which could destroy or break them,” said Qing Wang, professor of materials science and engineering, Penn State. “We wanted to find an electronic material that would repair itself to restore all of its functionality, and do so after multiple breaks.”

In the past, researchers have been able to create self-healable materials (such as these, covered on KurzweilAI) that can restore a single function after breaking. But restoring a suite of functions is critical for creating effective wearable electronics. For example, if a dielectric material retains its electrical resistivity after self-healing but not its thermal conductivity, that could put electronics at risk of overheating.

The material that Wang and his team created restores all properties needed for use as a dielectric in wearable electronics — mechanical strength, breakdown strength to protect against surges, electrical resistivity, thermal conductivity, and dielectric (insulating) properties. They published their findings online in Advanced Functional Materials.

“Most research into self-healable electronic materials has focused on electrical conductivity but dielectrics have been overlooked,” said Wang. “We need conducting elements in circuits but we also need insulation and protection for microelectronics.” Most self-healable materials are also soft or “gum-like,” said Wang, but the material he and his colleagues created is very tough in comparison.

His team added boron nitride nanosheets to a base material of plastic polymer. The material is able to self-heal because boron nitride nanosheets connect to one another with hydrogen bonding groups functionalized onto their surface. When two pieces are placed in close proximity, the electrostatic attraction naturally occurring between both bonding elements draws them close together. When the hydrogen bond is restored, the two pieces are “healed.”

Depending on the percentage of boron nitride nanosheets added to the polymer, this self-healing may require additional heat or pressure, but some forms of the new material can self-heal at room temperature when placed next to each other.

Unlike other healable materials that use hydrogen bonds, boron nitride nanosheets are impermeable to moisture. This means that devices using this dielectric material can operate effectively within high humidity contexts such as in a shower or at a beach.

“This is the first time that a self-healable material has been created that can restore multiple properties over multiple breaks, and we see this being useful across many applications,” said Wang.

Harbin Institute of Technology researches also collaborated on this research, which was supported by the China Scholarship Council.

Penn State Research Communications | Flexible Insulator

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Abstract of Self-Healable Polymer Nanocomposites Capable of Simultaneously Recovering Multiple Functionalities

The continuous evolution toward electronics with high power densities and integrated circuits with smaller feature sizes and faster speeds places high demands on a set of material properties, namely, the electrical, thermal, and mechanical properties of polymer dielectrics. Herein, a supramolecular approach is described to self-healable polymer nanocomposites that are mechanically robust and capable of restoring simultaneously structural, electrical, dielectric, and thermal transport properties after multiple fractures. With the incorporation of surface-functionalized boron nitride nanosheets, the polymer nanocomposites exhibit many desirable features as dielectric materials such as higher breakdown strength, larger electrical resistivity, improved thermal conductivity, greater mechanical strength, and much stabilized dielectric properties when compared to the pristine polymer. It is found that the recovery condition has remained the same during sequential cycles of cutting and healing, therefore suggesting no aging of the polymer nanocomposites with mechanical breakdown. Moreover, moisture has a minimal effect on the healing and dielectric properties of the polymer nanocomposites, which is in stark contrast to what is typically observed in the hydrogen-bonded supramolecular structures.

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.

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.

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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.

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.

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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.

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.”

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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.

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

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.”

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.

 

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.

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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.