Solar-Powered Battery Woven Into Fabric Overcomes Hurdle for 'Wearable Electronics'

Taek-Soo Kim, Jung-Yong Lee, Jang Wook Choi and colleagues explain that electronic textiles have the potential to integrate smartphone functions into clothes, eyeglasses, watches and materials worn on the skin. Possibilities range from the practical -- for example, allowing athletes to monitor vital signs -- to the aesthetic, such as lighting up patterns on clothing. The bottleneck slowing progress toward development of a wider range of flexible e-fabrics and materials is the battery technology required to power them. Current wearable electronics, such as smartwatches and Google Glass, still require a charger with a cord, and already-developed textile batteries are costly and impractical. To unlink smart technology from the wall socket, the team had to rethink what materials are best suited for use in a flexible, rechargeable battery that's also inexpensive.

They tested unconventional materials and found that they could coat polyester yarn with nickel and then carbon, and use polyurethane as a binder and separator to produce a flexible battery that kept working, even after being folded and unfolded many times. They also integrated lightweight solar cells to recharge the battery without disassembling it from clothing or requiring the wearer to plug in.

The authors acknowledge funding from the National Research Foundation of Korea.

Scientists build world’s smallest FM radio transmitter

US researchers have used graphene to create the world’s smallest FM radio transmitter — a nano-mechanical system that can create FM signals.

A team of researchers from Columbia University, led by mechanical engineering Professor James Hone and electrical engineering Professor Kenneth Shepard, took advantage of graphene’s special properties — its mechanical strength and electrical conduction — and developed a nano-mechanical system that can create FM signals.

“This work is significant in that it demonstrates an application of graphene that cannot be achieved using conventional materials,” Prof. Hone said.

“And it’s an important first step in advancing wireless signal processing and designing ultrathin, efficient cell phones. Our devices are much smaller than any other sources of radio signals, and can be put on the same chip that’s used for data processing,” Prof. Hone added.

In the new study, the team took advantage of graphene’s mechanical ‘stretchability’ to tune the output frequency of their custom oscillator, creating a nano-mechanical version of an electronic component known as a voltage controlled oscillator (VCO).

With a VCO, explained Prof. Hone, it is easy to generate FM signal, exactly what is used for FM radio broadcasting.

The team built a graphene NEMS whose frequency was about 100 megahertz, which lies right in the middle of the FM radio band (87.7 to 108 MHz).

They used low-frequency musical signals (both pure tones and songs from a smartphone) to modulate the 100 MHz carrier signal from the graphene, and then retrieved the musical signals again using an ordinary FM radio receiver.

“This device is by far the smallest system that can create such FM signals,” said Prof. Hone.

The study is published in the journal Nature Nanotechnology.

Scientists Invent Self-Healing Battery Electrode

They reported the advance in the Nov. 19 issue of Nature Chemistry.

"Self-healing is very important for the survival and long lifetimes of animals and plants," said Chao Wang, a postdoctoral researcher at Stanford and one of two principal authors of the paper. "We want to incorporate this feature into lithium ion batteries so they will have a long lifetime as well."

Chao developed the self-healing polymer in the lab of Stanford Professor Zhenan Bao, whose group has been working on flexible electronic skin for use in robots, sensors, prosthetic limbs and other applications. For the battery project he added tiny nanoparticles of carbon to the polymer so it would conduct electricity.

"We found that silicon electrodes lasted 10 times longer when coated with the self-healing polymer, which repaired any cracks within just a few hours," Bao said.

"Their capacity for storing energy is in the practical range now, but we would certainly like to push that," said Yi Cui, an associate professor at SLAC and Stanford who led the research with Bao. The electrodes worked for about 100 charge-discharge cycles without significantly losing their energy storage capacity. "That's still quite a way from the goal of about 500 cycles for cell phones and 3,000 cycles for an electric vehicle," Cui said, "but the promise is there, and from all our data it looks like it's working."

Researchers worldwide are racing to find ways to store more energy in the negative electrodes of lithium ion batteries to achieve higher performance while reducing weight. One of the most promising electrode materials is silicon; it has a high capacity for soaking up lithium ions from the battery fluid during charging and then releasing them when the battery is put to work.

But this high capacity comes at a price: Silicon electrodes swell to three times normal size and shrink back down again each time the battery charges and discharges, and the brittle material soon cracks and falls apart, degrading battery performance. This is a problem for all electrodes in high-capacity batteries, said Hui Wu, a former Stanford postdoc who is now a faculty member at Tsinghua University in Beijing, the other principal author of the paper.

To make the self-healing coating, scientists deliberately weakened some of the chemical bonds within polymers -- long, chain-like molecules with many identical units. The resulting material breaks easily, but the broken ends are chemically drawn to each other and quickly link up again, mimicking the process that allows biological molecules such as DNA to assemble, rearrange and break down.

Researchers in Cui's lab and elsewhere have tested a number of ways to keep silicon electrodes intact and improve their performance. Some are being explored for commercial uses, but many involve exotic materials and fabrication techniques that are challenging to scale up for production.

The self-healing electrode, which is made from silicon microparticles that are widely used in the semiconductor and solar cell industry, is the first solution that seems to offer a practical road forward, Cui said. The researchers said they think this approach could work for other electrode materials as well, and they will continue to refine the technique to improve the silicon electrode's performance and longevity.

Large Graphene Crystals With Exceptional Electrical Properties Created

The team used surface oxygen to grow centimeter-size single graphene crystals on copper. The crystals were about 10,000 times as large as the largest crystals from only four years ago. Very large single crystals have exceptional electrical properties.

"The game we play is that we want nucleation (the growth of tiny 'crystal seeds') to occur, but we also want to harness and control how many of these tiny nuclei there are, and which will grow larger," said Rodney S. Ruoff, professor in the Cockrell School of Engineering. "Oxygen at the right surface concentration means only a few nuclei grow, and winners can grow into very large crystals."

The team -- led by postdoctoral fellow Yufeng Hao and Ruoff of the Department of Mechanical Engineering and the Materials Science and Engineering Program, along with Luigi Colombo, a material scientist with Texas Instruments -- worked for three years on the graphene growth method. The team's paper, "The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper," is featured on the cover of the Nov. 8, 2013, issue of Science.

One of the world's strongest materials, graphene is flexible and has high electrical and thermal conductivity that makes it a promising material for flexible electronics, solar cells, batteries and high-speed transistors. The team's understanding of how graphene growth is influenced by differing amounts of surface oxygen is a major step toward improved high-quality graphene films at industrial scale.

The team's method "is a fundamental breakthrough, which will lead to growth of high-quality and large area graphene film," said Sanjay Banerjee, who heads the Cockrell School's South West Academy of Nanoelectronics (SWAN). "By increasing the single-crystal domain sizes, the electronic transport properties will be dramatically improved and lead to new applications in flexible electronics."

Graphene has always been grown in a polycrystalline form, that is, it is composed of many crystals that are joined together with irregular chemical bonding at the boundaries between crystals ("grain boundaries"), something like a patch-work quilt. Large single-crystal graphene is of great interest because the grain boundaries in polycrystalline material have defects, and eliminating such defects makes for a better material.

By controlling the concentration of surface oxygen, the researchers could increase the crystal size from a millimeter to a centimeter. Rather than hexagon-shaped and smaller crystals, the addition of the right amount of surface oxygen produced much larger single crystals with multibranched edges, similar to a snowflake.

"In the long run it might be possible to achieve meter-length single crystals," Ruoff said. "This has been possible with other materials, such as silicon and quartz. Even a centimeter crystal size -- if the grain boundaries are not too defective -- is extremely significant."

"We can start to think of this material's potential use in airplanes and in other structural applications -- if it proves to be exceptionally strong at length scales like parts of an airplane wing, and so on," he said.

Another major finding by the team was that the "carrier mobility" of electrons (how fast the electrons move) in graphene films grown in the presence of surface oxygen is exceptionally high. This is important because the speed at which the charge carriers move is important for many electronic devices -- the higher the speed, the faster the device can perform.

Yufeng Hao says he thinks the knowledge gained in this study could prove useful to industry.

"The high quality of the graphene grown by our method will likely be developed further by industry, and that will eventually allow devices to be faster and more efficient," Hao said.

Single-crystal films can also be used for the evaluation and development of new types of devices that call for a larger scale than could be achieved before, added Colombo.

"At this time, there are no other reported techniques that can provide high quality transferrable films," Colombo said. "The material we were able to grow will be much more uniform in its properties than a polycrystalline film."

Wireless Device Converts 'Lost' Energy Into Electric Power: Metamaterial Cells Provide Electric Power as Efficiently as Solar Panels

The device wirelessly converts the microwave signal to direct current voltage capable of recharging a cell phone battery or other small electronic device, according to a report appearing in the journal Applied Physics Letters in December 2013.

It operates on a similar principle to solar panels, which convert light energy into electrical current. But this versatile energy harvester could be tuned to harvest the signal from other energy sources, including satellite signals, sound signals or Wi-Fi signals, the researchers say.

The key to the power harvester lies in its application of metamaterials, engineered structures that can capture various forms of wave energy and tune them for useful applications.

Undergraduate engineering student Allen Hawkes, working with graduate student Alexander Katko and lead investigator Steven Cummer, professor of electrical and computer engineering, designed an electrical circuit capable of harvesting microwaves.

They used a series of five fiberglass and copper energy conductors wired together on a circuit board to convert microwaves into 7.3V of electrical energy. By comparison, Universal Serial Bus (USB) chargers for small electronic devices provide about 5V of power.

"We were aiming for the highest energy efficiency we could achieve," said Hawkes. "We had been getting energy efficiency around 6 to 10 percent, but with this design we were able to dramatically improve energy conversion to 37 percent, which is comparable to what is achieved in solar cells."

"It's possible to use this design for a lot of different frequencies and types of energy, including vibration and sound energy harvesting," Katko said. "Until now, a lot of work with metamaterials has been theoretical. We are showing that with a little work, these materials can be useful for consumer applications."

For instance, a metamaterial coating could be applied to the ceiling of a room to redirect and recover a Wi-Fi signal that would otherwise be lost, Katko said. Another application could be to improve the energy efficiency of appliances by wirelessly recovering power that is now lost during use.

"The properties of metamaterials allow for design flexibility not possible with ordinary devices like antennas," said Katko. "When traditional antennas are close to each other in space they talk to each other and interfere with each other's operation. The design process used to create our metamaterial array takes these effects into account, allowing the cells to work together."

With additional modifications, the researchers said the power-harvesting metamaterial could potentially be built into a cell phone, allowing the phone to recharge wirelessly while not in use. This feature could, in principle, allow people living in locations without ready access to a conventional power outlet to harvest energy from a nearby cell phone tower instead.

"Our work demonstrates a simple and inexpensive approach to electromagnetic power harvesting," said Cummer. "The beauty of the design is that the basic building blocks are self-contained and additive. One can simply assemble more blocks to increase the scavenged power."

For example, a series of power-harvesting blocks could be assembled to capture the signal from a known set of satellites passing overhead, the researchers explained. The small amount of energy generated from these signals might power a sensor network in a remote location such as a mountaintop or desert, allowing data collection for a long-term study that takes infrequent measurements.

Diamond Imperfections Pave the Way to Technology Gold

Using two-dimensional electronic spectroscopy on pico- and femto-second time-scales, a research team led by Graham Fleming, Vice Chancellor for Research at UC Berkeley and faculty scientist with Berkeley Lab's Physical Biosciences Division, has recorded unprecedented observations of energy moving through the atom-sized diamond impurities known as nitrogen-vacancy (NV) centers. An NV center is created when two adjacent carbon atoms in a diamond crystal are replaced by a nitrogen atom and an empty gap.

"Our use of 2D electronic spectroscopy allowed us to essentially map the flow of energy through the NV center in real time and observe critical quantum mechanical effects," Fleming says. "The results hold broad implications for magnetometry, quantum information, nanophotonics, sensing and ultrafast spectroscopy."

Fleming is the corresponding author of a paper in Nature Physics that describes this research entitled "Vibrational and electronic dynamics of nitrogen-vacancy centres in diamond revealed by two-dimensional ultrafast spectroscopy." The lead author is Vanessa Huxter, former member of Fleming's research group and now a professor at the University of Arizona. Other co-authors are Thomas Oliver and Dmitry Budker, both of whom holds joint appointments with Berkeley Lab and UC Berkeley.

These 2D electronic spectroscopy measurements have provided us with the first window into the ultrafast dynamics of NV centers in diamond," says Huxter. "We were able to observe previously hidden vibrational and electronic properties of the NV center system, including the discovery of vibrational coherences lasting about two picoseconds, which on a quantum mechanical scale is a surprisingly long time."

Given the ubiquitous presence of weak magnetic fields, a sufficiently sensitive detector could be used in a wide range of applications including medical diagnostic and treatment procedures, chemical analyses, energy exploration and homeland security (to detect explosives). Diamond NV centers are held to be one of the finest magnetic sensors possible on the nanoscale. Diamond NV centers are also highly promising candidates for the creation of qubits -- data encoded through quantum-spin rather than electrical charge that will be the heart and soul of quantum computing. Qubits can store exponentially more data and process it billions of times faster than classical computer bits. However, for these rich promises to be fully met, a much better fundamental understanding is needed of the electronic-state dynamics when an NV center is energized.

Says co-author Budker, a UC Berkeley physics professor with Berkeley Lab's Nuclear Sciences Division and leading authority on NV center physics, "NV centers in diamond are already becoming a workhorse in magnetometry and other sensor fields, but they remain somewhat of a black box in that we still don't know understand some important features of their energy levels and dynamics. Our findings in this study provide a starting point for new insights into such critical electronic-state phenomena as dephasing, spin addressing and relaxation."

This study was made possible by the unique 2D electronic spectroscopy technique, which was first developed by Fleming and his research group to study the quantum mechanical underpinnings of photosynthesis. This ultrafast technique enables researchers to track the transfer of energy between atoms or molecules that are coupled (connected) through their electronic and vibrational states. Tracking is done through both time and space. It is accomplished by sequentially flashing light from three laser beams on a sample while a fourth beam serves as a local oscillator to amplify and phase-match the resulting spectroscopic signals.

"By providing femtosecond temporal resolution and nanometer spatial resolution, 2D electronic spectroscopy allows us to simultaneously follow the dynamics of multiple electronic states," says Fleming, who has compared this technology to the early super-heterodyne radios.

In this new study, the use of 2D electronic spectroscopy revealed that the vibrational modes of NV centers in diamond -- a subject of keen scientific interest because these modes directly affect optical and material properties -- are strongly coupled to the defect.

"We were able to identify a number of individual vibrational modes and found that these modes were almost all local to the defect centers and that they were coherent -- quantum mechanically coupled -- for about two picoseconds," says Huxter.

"Through a combination of theory and observation, researchers had suspected that NV center vibrational modes were primarily local to the defect, but our direct observation of the vibrations and their coupling to the excitation states confirms this idea."

In addition, the researchers also were able to measure non-radiative relaxation in the excited state, a property that must be understood and exploited for the creation of qubits.

"We found that the non-radiative relaxation timescale for NV centers in diamond was around four picoseconds, which was slower than we had expected given the number of vibrational states," Huxter says.

The information acquired from this study should make it possible to tune the properties of NV centers in diamonds and open up new avenues for research.

"For example, by optically pumping the NV centers we could specifically excite phonon modes based on their coupling factors," Fleming says. "This would allow the development of diamonds with NV centers that can be used for quantum storage and information processing based on both phonons and spin."

This research was supported primarily by a grant from the National Science Foundation.

Synaptic Transistor Learns While It Computes

Our brains have upwards of 86 billion neurons, connected by synapses that not only complete myriad logic circuits; they continuously adapt to stimuli, strengthening some connections while weakening others. We call that process learning, and it enables the kind of rapid, highly efficient computational processes that put Siri and Blue Gene to shame.

Materials scientists at the Harvard School of Engineering and Applied Sciences (SEAS) have now created a new type of transistor that mimics the behavior of a synapse. The novel device simultaneously modulates the flow of information in a circuit and physically adapts to changing signals.

Exploiting unusual properties in modern materials, the synaptic transistor could mark the beginning of a new kind of artificial intelligence: one embedded not in smart algorithms but in the very architecture of a computer. The findings appear in Nature Communications.

"There's extraordinary interest in building energy-efficient electronics these days," says principal investigator Shriram Ramanathan, associate professor of materials science at Harvard SEAS. "Historically, people have been focused on speed, but with speed comes the penalty of power dissipation. With electronics becoming more and more powerful and ubiquitous, you could have a huge impact by cutting down the amount of energy they consume."

The human mind, for all its phenomenal computing power, runs on roughly 20 Watts of energy (less than a household light bulb), so it offers a natural model for engineers.

"The transistor we've demonstrated is really an analog to the synapse in our brains," says co-lead author Jian Shi, a postdoctoral fellow at SEAS. "Each time a neuron initiates an action and another neuron reacts, the synapse between them increases the strength of its connection. And the faster the neurons spike each time, the stronger the synaptic connection. Essentially, it memorizes the action between the neurons."

In principle, a system integrating millions of tiny synaptic transistors and neuron terminals could take parallel computing into a new era of ultra-efficient high performance.

While calcium ions and receptors effect a change in a biological synapse, the artificial version achieves the same plasticity with oxygen ions. When a voltage is applied, these ions slip in and out of the crystal lattice of a very thin (80-nanometer) film of samarium nickelate, which acts as the synapse channel between two platinum "axon" and "dendrite" terminals. The varying concentration of ions in the nickelate raises or lowers its conductance -- that is, its ability to carry information on an electrical current -- and, just as in a natural synapse, the strength of the connection depends on the time delay in the electrical signal.

Structurally, the device consists of the nickelate semiconductor sandwiched between two platinum electrodes and adjacent to a small pocket of ionic liquid. An external circuit multiplexer converts the time delay into a magnitude of voltage which it applies to the ionic liquid, creating an electric field that either drives ions into the nickelate or removes them. The entire device, just a few hundred microns long, is embedded in a silicon chip.

The synaptic transistor offers several immediate advantages over traditional silicon transistors. For a start, it is not restricted to the binary system of ones and zeros.

"This system changes its conductance in an analog way, continuously, as the composition of the material changes," explains Shi. "It would be rather challenging to use CMOS, the traditional circuit technology, to imitate a synapse, because real biological synapses have a practically unlimited number of possible states -- not just 'on' or 'off.'"

The synaptic transistor offers another advantage: non-volatile memory, which means even when power is interrupted, the device remembers its state.

Additionally, the new transistor is inherently energy efficient. The nickelate belongs to an unusual class of materials, called correlated electron systems, that can undergo an insulator-metal transition. At a certain temperature -- or, in this case, when exposed to an external field -- the conductance of the material suddenly changes.

"We exploit the extreme sensitivity of this material," says Ramanathan. "A very small excitation allows you to get a large signal, so the input energy required to drive this switching is potentially very small. That could translate into a large boost for energy efficiency."

The nickelate system is also well positioned for seamless integration into existing silicon-based systems.

"In this paper, we demonstrate high-temperature operation, but the beauty of this type of a device is that the 'learning' behavior is more or less temperature insensitive, and that's a big advantage," says Ramanathan. "We can operate this anywhere from about room temperature up to at least 160 degrees Celsius."

For now, the limitations relate to the challenges of synthesizing a relatively unexplored material system, and to the size of the device, which affects its speed.

"In our proof-of-concept device, the time constant is really set by our experimental geometry," says Ramanathan. "In other words, to really make a super-fast device, all you'd have to do is confine the liquid and position the gate electrode closer to it."

In fact, Ramanathan and his research team are already planning, with microfluidics experts at SEAS, to investigate the possibilities and limits for this "ultimate fluidic transistor."

He also has a seed grant from the National Academy of Sciences to explore the integration of synaptic transistors into bioinspired circuits, with L. Mahadevan, Lola England de Valpine Professor of Applied Mathematics, professor of organismic and evolutionary biology, and professor of physics.

"In the SEAS setting it's very exciting; we're able to collaborate easily with people from very diverse interests," Ramanathan says.

For the materials scientist, as much curiosity derives from exploring the capabilities of correlated oxides (like the nickelate used in this study) as from the possible applications.

"You have to build new instrumentation to be able to synthesize these new materials, but once you're able to do that, you really have a completely new material system whose properties are virtually unexplored," Ramanathan says. "It's very exciting to have such materials to work with, where very little is known about them and you have an opportunity to build knowledge from scratch."

"This kind of proof-of-concept demonstration carries that work into the 'applied' world," he adds, "where you can really translate these exotic electronic properties into compelling, state-of-the-art devices."

This research was supported by the National Science Foundation (NSF), the Army Research Office's Multidisciplinary University Research Initiative, and the Air Force Office of Scientific Research. The team also benefited from the facilities at the Harvard Center for Nanoscale Systems, a member of the NSF-supported National Nanotechnology Infrastructure Network. Sieu D. Ha, a postdoctoral fellow at SEAS, was the co-lead author; additional coauthors included graduate student You Zhou and Frank Schoofs, a former postdoctoral fellow.