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Messages - Rohan Sarker

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76
EEE / Team visualizes complex electronic state
« on: May 09, 2018, 11:22:21 AM »
A material called sodium manganese dioxide has shown promise for use in electrodes in rechargeable batteries. Now a team of researchers has produced the first detailed visualization — down to the level of individual atoms — of exactly how the material behaves during charging and discharging, in the process elucidating an exotic molecular state that may help in understanding superconductivity.

The new findings are reported this week in the journal Nature Materials, in a paper by MIT postdoc Xin Li, professors Young Lee and Gerbrand Ceder, also of MIT, and 12 others.

The phenomenon the team investigated — known as the cooperative Jahn-Teller effect —“is a basic piece of physics that has been well-known historically,” explains Ceder, the R.P. Simmons Professor of Materials Science and Engineering. It describes how the positions of atoms in certain compounds can be slightly distorted, changing the material’s electrical and magnetic properties.

“It is associated with a lot of interesting phenomena,” Ceder says — so a better understanding could be useful both in advancing our knowledge of physics and in potential applications, from improved batteries to new kinds of electronics.

While the Jahn-Teller phenomenon is well-known, Ceder says it’s a bit unusual to see it in battery compounds such as the sodium manganese dioxide now under investigation as a possible lower-cost substitute for the lithium-based electrodes in lithium-ion batteries.

Such rechargeable batteries work when an electrical current pulls ions out of an electrode during charging, then returns them to the electrode as the battery is used. The arrangement of atoms within the material “is very ordered, and normally the ordering is driven by fairly standard physics,” Ceder says. “But in this material, the order is completely driven by the Jahn-Teller effect.”

Understanding how that difference affects charging and discharging could be important in guiding teams around the world who are seeking to improve the performance of such batteries, but it proved a daunting challenge for the MIT team.

The team combined density functional theory with technologies including electron diffraction; synchrotron X-ray diffraction; neutron diffraction; and aberration-corrected atomic-resolution scanning microscopy for direct visualization. Using these methods, the researchers showed that the material produces a “superstructure” governed by the Jahn-Teller effect; at very low temperatures, it produces a kind of “magnetic stripe sandwich,” with alternating stripes of ferrimagnetic and antiferromagnetic atomic chains.

“This is fundamental work,” Li says, to determine “any intrinsic capacity limits to sodium manganese dioxide” — such as how much charge it can hold, or how many times it can go through the charge-discharge cycle without degradation. The ultimate goal is to find out “how [to] make a higher-capacity sodium-ion battery electrode,” Li says.

In addition to possible battery applications, the work led to the finding that sodium manganese dioxide forms bands of magnetic domains at temperatures of 60 kelvins (-352 degrees Fahrenheit) or less. This finding, Li says, may be important to the emerging field of spin electronics, where the spin states of electrons, rather than their electrical charges, carry and store information.

Even before this new research, Li says, batteries made of this sodium-ion composition “showed comparable capacity to the commercial lithium-ion batteries,” which are one of the leading technologies in production today. While no companies are now producing sodium-ion batteries, the technology has great potential: Sodium is more abundant, less expensive, and safer to work with than lithium.

“This is still fairly basic research,” Li says, adding: “Understanding always pushes us forward, especially in this field. You only make progress by understanding these materials better.”

The work also included researchers Dong Su at Brookhaven National Laboratory, Juan-Carlos Idrobo at Oak Ridge National Laboratory, and Jeffrey W. Lynn at the National Institute of Standards and Technology, as well as seven others from MIT. It was partly funded by the Samsung Advanced Institute of Technology and the U.S. Department of Energy.

77
EEE / Making a wire-free future
« on: May 09, 2018, 11:21:42 AM »
More than a century ago, engineer and inventor Nikola Tesla proposed a global system of wireless transmission of electricity — or wireless power. But one key obstacle to realizing this ambitious vision has always been the inefficiency of transferring power over long distances.

Near the end of the last decade, however, a team of MIT researchers led by Professor of Physics Marin Soljacic took definitive steps toward more practical wireless charging. First, in 2007, the team wirelessly lit a 60-watt light bulb from eight feet away using two large copper coils, with similarly tuned resonant frequencies, that transferred energy from one to the other over the magnetic field. Then, in 2010, they shrunk the coils down and significantly increased the efficiency of the system, noting future applications in consumer products.

Now, this “wireless electricity” (or “WiTricity”) technology — licensed through the researchers’ startup, WiTricity Corp. — is coming to mobile devices, electric vehicles, and potentially a host of other applications.

The aim is to forge toward a “wire-free world,” says Soljacic. Primarily, this means consumers need not carry wires and power bricks. But it could also lead to benefits such as smaller batteries and less hardware — which would lower costs for manufacturers and consumers.

“It’s probably a dream of any professor at MIT to help change the world for a better place,” says Soljacic, a WiTricity co-founder who now serves on its board of directors. “We believe wireless charging has a potential to do that.”

He is not alone. Last month, WiTricity signed a licensing agreement with Intel to integrate WiTricity technology into computing devices powered by Intel. Back in December, Toyota licensed WiTricity technology for a future line of electric cars. Several more publicized and unpublicized companies have recently joined in the licensing parade for this technology, including Thoratec for their implantable ventricular assisting devices, and TDK for wireless electric vehicle-charging systems. There’s even talk of a helmet powered wirelessly via backpack, specifically for military applications.

At present, WiTricity technology charges devices at around 6 to 12 inches with roughly 95 percent efficiency — 12 watts for mobile devices and up to 6.6 kilowatts for cars. But, with growing research and development, the company is increasing distance, scale, and efficiency. It’s also developed repeaters: passive devices that extend the distance of the power transfer. These can be developed into a wide variety of shapes and can be embedded in a carpet to “hop” the power across a room.


78
EEE / A world of wireless power
« on: May 09, 2018, 11:20:21 AM »
If you buy a 2016 Toyota Prius, you won’t need to worry about keeping your hybrid car charged — just get the option for wireless power transfer that lets you drive into your garage and have your battery automatically topped up from a pad on the floor.

A year or two from now you’ll also be able to purchase laptops, tablets, mobile phones and other consumer electronic devices that don’t need any wires, because their power needs will be met by wireless transmission.

“Instead of having a different charging cord for every device you own, you can have one location where you put your mobile phone or your laptop, and it will stay charged automatically,” says Morris Kesler, chief technology officer at WiTricity of Watertown, Mass. “There’s no reason that these devices need a cord anymore.”

WiTricity, an MIT spinoff, offers highly resonant wireless power transfer technology that “is applicable in any situation where a device has a cord or a battery that needs to be charged,” Kesler says.

An idea that resonated

In magnetic induction, an alternating magnetic field is generated in a transmitter coil and then converted into electrical current in a receiver coil. Wireless power systems that exploit this technique have been around for decades, with cordless toothbrushes offering one example. But traditional wireless power systems based on magnetic induction come with severe operational limitations, especially in transfer distance and positioning.

In 2006, MIT physics professor Marin Soljačić and his colleagues demonstrated a highly resonant form of magnetic induction that can carry wireless power efficiently over larger distances — the breakthrough being commercialized by WiTricity.

“The use of resonance enables efficient use of energy transfer over greater distances and with greater positional freedom than you get with a traditional inductive system,” says Kesler. “For example, your cordless toothbrush only works when the toothbrush is in the holder. Resonance technology lets you move that receiver farther apart and still transfer energy efficiently, and the orientation of the device is less critical than it is in a traditional system. You also can transfer energy from one source to more than one device, the source and the devices don’t have to be the same size, and you can charge through materials like tables.”

Most importantly, “the technology allows you to charge things without even thinking about it,” he emphasizes. “You put your device on a table or a workspace, and it charges as you go.”

Like other magnetic inductive power transmissions, the WiTricity technology interacts only very weakly with the human body, Kesler adds. From a safety perspective, it satisfies the same regulatory limits as common household electronics and appliances.

As the holder of the foundational patents, WiTricity is helping to drive standardization efforts around wireless power transfer over distance using magnetic resonance, including those for automobiles run by the Society of Automotive Engineers and those for consumer electronics pursued by the Alliance for Wireless Power, whose Rezence™ specification incorporates WiTricity technology.

Powering up under difficult conditions

In addition to offering compelling increases in convenience for cars and consumer electronics, the WiTricity technology will provide dramatic enhancements in applications where power is difficult to deliver.

In one example, WiTricity licensee Thoratec is leveraging the improved wireless power transfer to develop better heart-assist pumps. Today, such pumps are typically powered by implanted wires that exit the body. Wireless power transfer offers the potential to improve quality of life for patients, giving them greater freedom of movement, and removing the wires that are uncomfortable and likely to trigger infections. Medical devices implanted several centimeters below the skin could be charged safely and with high efficiency, Kesler says.

In addition to a host of medical applications, the technology is finding many uses in industrial settings. Wireless power transfer that works over a distance offers important advantages, for instance, in powering equipment that gets wet. “You don’t necessarily want to have a charge port on a device like that,” Kesler points out. “By embedding our technology into that device, you can charge it wirelessly without having to plug it in, which basically offers a safer usage model.”

For example, the remotely-operated undersea vehicles employed in offshore petroleum operations must dock very precisely to connect up for charging. “WiTricity technology would allow you to charge them without requiring that precise positioning and without having any electrical components exposed,” Kesler says.

The company also envisions a host of military applications, ranging from powering remotely operated vehicles to rationalizing the collections of batteries carried by foot combatants.

Readying for fast-growing markets

WiTricity’s publicly announced licensees include Intel and Mediatek for consumer electronics and Delphi, IHI, TDK, and Toyota for automotive applications. The total market for wireless power systems of all kinds will reach $8.5 billion in 2018, driven most strongly by adoption in mobile phones and tablet computers, predicts IHS Technology. In this highly competitive market, numerous companies will offer different technologies and system designs. Many products will work by traditional magnetic induction, but those using magnetic resonance technology will need a WiTricity license, Kesler says.

“The market has started to catch up with the technology now, and we are working on standardized licensing agreements to make it easier for our customers to put it into practice,” he says. The firm develops prototypes and reference designs that help licensees get started on their applications, and offers the WiCAD simulation environment, a design tool that allows companies to create specifications for their designs virtually before building expensive prototypes.

WiTricity also sells demonstration products that allow companies considering the technology to see it in action. “Additionally, at our facility, we can demonstrate the technology in ways that are difficult to explain on a piece of paper,” Kesler says. “Usually when people see the technology they say, ‘Wow, that looks like magic, how do you do that?’”

79
EEE / Squitching behavior
« on: May 09, 2018, 11:19:44 AM »
A longstanding problem in designing nanoscale electromechanical switches is the tendency for metal-to-metal contacts to stick together, locking the switch in an “on” position. MIT electrical engineering graduate student Farnaz Niroui has found a way to exploit that tendency to create electrodes with nanometer-thin separations. By designing a cantilever that can collapse and permanently adhere onto a support structure during the fabrication process, Niroui's process leaves a controllable nanoscale gap between the cantilever and electrodes neighboring the point of adhesion.

Niroui, who works in Professor Vladimir Bulović’s Organic and Nanostructured Electronics Laboratory (ONE Lab), presented her most recent findings Jan. 20 at the IEEE Micro Electro Mechanical Systems (MEMS) Conference in Portugal. MIT collaborators include professors Jeffrey Lang in electrical engineering and Timothy M. Swager in chemistry. Their paper is titled, “Controlled Fabrication of Nanoscale Gaps Using Stiction.”

Stiction, as permanent adhesion is called, is a very important challenge in electromechanical systems and often results in device failure. Niroui turned stiction to her advantage by using a support structure to make nanoscale gaps. "Initially the cantilever is fabricated with a relatively larger gap which is easier to fabricate, but then we modulate the surface adhesion forces to be able to cause a collapse between the cantilever and the support. As the cantilever collapses, this gap reduces to width much smaller than patterned," she explains.

"We can get sub-10-nanometer gaps," she says. "It's controllable because by choosing the design of the cantilever, controlling its mechanical properties and the placement of the other electrodes, we can get gaps that are different in size. This is useful not only for our application, which is in tunneling electromechanical switches, but as well for molecular electronics and contact-based electromechanical switches. It’s a general approach to develop nanoscale gaps.”

Niroui's latest work builds on her earlier work showing a design for a squeezable switch — or "squitch" — which fills the narrow gap between contacts with an organic molecular layer that can be compressed tightly enough to allow current to tunnel, or flow, from one electrode to another without direct contact — the "on" position — but that will spring back to open a gap wide enough that current cannot flow between electrodes — the "off" position. The softer the filler material is, the less voltage is needed to compress it. The goal is a low-power switch with repeatable abrupt switching behavior that can complement or replace conventional transistors.

Niroui designed, fabricated, tested, and characterized the cantilevered switch in which one electrode is fixed and the other moveable with the switching gap filled with a molecular layer. She presented her initial findings at the IEEE MEMS Conference in San Francisco last year in a paper titled, "Nanoelectromechanical Tunneling Switches Based on Self-Assembled Molecular Layers." "We're working right now on alternative designs to achieve an optimized switching performance," Niroui says.

"For me, one of the interesting aspects of the project is the fact that devices are designed in very small dimensions," Niroui adds, noting that the tunneling gap between the electrodes is only a few nanometers. She uses scanning electron microscopy at the MIT Center for Materials Science and Engineering to image the gold-coated electrode structures and the nanogaps, while using electrical measurements to verify the effect of the presence of the molecules in the switching gap.

Building her switch on a silicon/silcon-oxide base, Niroui added a top layer of PMMA, a polymer that is sensitive to electron beams. She then used electron beam lithography to pattern the device structure and wash away the excess PMMA. She used a thermal evaporator to coat the switch structure with gold. Gold was the material of choice because it enables the thiolated molecules to self-assemble in the gap, the final assembly step.

For the initial tunneling current demonstration, Niroui used an off-the-shelf molecule in the gap between electrodes. Work is continuing with collaborators in Swager's chemistry lab to synthesize new molecules with optimal mechanical properties to optimize the switching performance.

"Our project uses this design to have two metal electrodes with a single layer of molecules in the middle," Niroui explains. "We use self-assembly of molecules that allows the gap to be fabricated very small. By choosing the molecule and its properties such as the molecular length, we can control the gap thickness very precisely in the few-nanometer regime. The reason we want the gap small is that it allows us to reduce the switching voltage. The smaller the gap, the smaller the switching voltage and the less energy you are going to consume to switch on and off your device, which is very desirable."

The molecules filling the gap act as tiny springs. When an electrostatic force is applied, the electrodes compress the filler, squishing all the molecules. "These molecules are going to prevent the two metals coming into contact. At the same time the compressed layer is going to provide a restoring force, so it's going to avoid the typical sticking problem, permanent adhesion between the two electrodes, that is otherwise very common in electromechanical systems," she says.

Tunneling electromechanical switches work by controlling the gap between two metal electrodes that never come into direct contact. "You always will have a gap between the two electrodes. Because of the gap, the current that you modulate is the tunneling current," Niroui says.

Niroui tested a version of her original device without a molecular gap filler and the two electrodes immediately stuck together. By filling the gap, current-voltage tests showed characteristics that were reproducible and repeatable, so the devices didn't short. "By comparing to theoretical models, we observe that we get some compression of the molecules, and we extract mechanical properties of molecules that match what is reported experimentally in the literature," she says. While the device established proof of concept, improvements are needed in the filler material for practical use.

Niroui, 26, is from Toronto, Canada, and received her bachelor's in nanotechnology engineering at the University of Waterloo. She received a master's in electrical engineering at MIT in 2013. She hopes to complete her doctoral work in 2016.

80
Weng Hong Teh really chipped in during his Leaders for Global Operations (LGO) internship project at SanDisk Corporation — his work on silicon wafers earned him and his advisors a special award and is already being used in production of SanDisk memory products.

Teh will graduate soon from the MIT LGO program, in which students earn an MBA from the Sloan School of Management plus an MS in one of seven engineering disciplines in two years. The program includes a six-month on-site internship at an LGO partner company where students work on a real-world operations or manufacturing problem. The research and solutions stemming from the internship lead to the dual-degree master's thesis.

During his internship project at one of partner company SanDisk’s test-and-assembly factories in Shanghai, Teh developed a better method for cutting up large round silicon wafers into square microprocessor chips. The chips are as thin as 25 microns, which is one-third to one-fourth the diameter of an average human hair. When the wafers are cut with a saw or even a laser, some of the resulting microprocessors must be discarded because of defects cased by the cutting process — for example, when the edges are not precisely smooth.

Teh's laser technology actually causes controlled cracking from the inside rather than a complete cut from the outside. He calls his method "stealth dicing" because it employs a laser that cuts the wafer from the inside out, rather than straight across in one direction and then again after turning 90 degrees like a waffle. The laser operates at a wavelength that's transparent to the shiny surface of the silicon bulk material, so the laser beam doesn't get absorbed and cause ablation damage to the wafer.

Although the technique had been used previously for other niche industries, Teh adapted it and combined two tandem processes to achieve a novel integration flow he called "p-SDBG" (partial stealth dicing before grinding) that can circumvent the energy-reflection issues that are usually associated with silicon memory wafers. Testing revealed that stealth dicing resulted in an average of 3.5 percent higher yield of defect-free memory chips, potentially saving the company an estimated $12 million a year.

In a blog entry he wrote whole working on his internship in Sept. 2014, Teh noted that he was initially skeptical of how much impact he could make in a relatively short time, but he found he had considerable freedom to define his project due in part to his prior semiconductor experience at Intel.

"I am an individual contributor, group leader, integrator, supplier manager and everything in between. Sweet!" he wrote. "I dare say that this internship period during MIT LGO likely enabled the most life-changing experience I have had for a while."

Teh, who is earning his MS in electrical enginering and computer science, also has a PhD in physics from Cambridge University and a master's degree in advanced materials from the National University of Singapore.

In appreciation for his efforts, SanDisk gave glass statuettes and memory disk replicas (including one imprinted with Teh's smiling face) to him and his MIT faculty advisors, professor of electrical engineering and computer science Duane S. Boning and professor of statistics and engineering systems Roy Welsch. The trio has submitted papers based on his internship to Applied Physics Letters and two other high-impact factor engineering journals, and Teh presented them at an international SanDisk conference in Nov. 2014.

As if all that weren't enough, he also filed for a patent that grew out of a side project during his internship and did some management education at SanDisk. At the request of K.L. Bock, general manager and vice president for SanDisk Shanghai, he developed and delivered an MBA Leadership Series in which he discussed selected business case studies with the directors of the SanDisk Shanghai site, learning from the best of lessons from the West and the East. His final delivery was based on a case he co-developed with his LGO classmate, Adam Traina, based on Traina's boat-racing experience.

"That was fun," said Teh, who will join SanDisk in Milpitas, California, as director of advanced manufacturing and engineering after he graduates in June.

Teh had high praise for his MIT education ("an MIT strength is problem-solving from first principles," he said) and especially the LGO curriculum. "The leadership literature taught, shared, and discussed is pretty unique to LGO. It facilitates a deliberate experimentation" with different leadership styles and techniques, he said, noting that his classes exposed him for the first time to the thinking of leaders including Martin Luther King Jr. and Burma's Aung San Suu Kyi.

81
EEE / Toward tiny, solar-powered sensors
« on: May 09, 2018, 11:18:07 AM »
The latest buzz in the information technology industry regards “the Internet of things” — the idea that vehicles, appliances, civil-engineering structures, manufacturing equipment, and even livestock would have their own embedded sensors that report information directly to networked servers, aiding with maintenance and the coordination of tasks.

Realizing that vision, however, will require extremely low-power sensors that can run for months without battery changes — or, even better, that can extract energy from the environment to recharge.

Last week, at the Symposia on VLSI Technology and Circuits, MIT researchers presented a new power converter chip that can harvest more than 80 percent of the energy trickling into it, even at the extremely low power levels characteristic of tiny solar cells. Previous ultralow-power converters that used the same approach had efficiencies of only 40 or 50 percent.

Moreover, the researchers’ chip achieves those efficiency improvements while assuming additional responsibilities. Where most of its ultralow-power predecessors could use a solar cell to either charge a battery or directly power a device, this new chip can do both, and it can power the device directly from the battery.

All of those operations also share a single inductor — the chip’s main electrical component — which saves on circuit board space but increases the circuit complexity even further. Nonetheless, the chip’s power consumption remains low.

“We still want to have battery-charging capability, and we still want to provide a regulated output voltage,” says Dina Reda El-Damak, an MIT graduate student in electrical engineering and computer science and first author on the new paper. “We need to regulate the input to extract the maximum power, and we really want to do all these tasks with inductor sharing and see which operational mode is the best. And we want to do it without compromising the performance, at very limited input power levels — 10 nanowatts to 1 microwatt — for the Internet of things.”

The prototype chip was manufactured through the Taiwan Semiconductor Manufacturing Company's University Shuttle Program.

Ups and downs

The circuit’s chief function is to regulate the voltages between the solar cell, the battery, and the device the cell is powering. If the battery operates for too long at a voltage that’s either too high or too low, for instance, its chemical reactants break down, and it loses the ability to hold a charge.

To control the current flow across their chip, El-Damak and her advisor, Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering, use an inductor, which is a wire wound into a coil. When a current passes through an inductor, it generates a magnetic field, which in turn resists any change in the current.

Throwing switches in the inductor’s path causes it to alternately charge and discharge, so that the current flowing through it continuously ramps up and then drops back down to zero. Keeping a lid on the current improves the circuit’s efficiency, since the rate at which it dissipates energy as heat is proportional to the square of the current.

Once the current drops to zero, however, the switches in the inductor’s path need to be thrown immediately; otherwise, current could begin to flow through the circuit in the wrong direction, which would drastically diminish its efficiency. The complication is that the rate at which the current rises and falls depends on the voltage generated by the solar cell, which is highly variable. So the timing of the switch throws has to vary, too.

Electric hourglass

To control the switches’ timing, El-Damak and Chandrakasan use an electrical component called a capacitor, which can store electrical charge. The higher the current, the more rapidly the capacitor fills. When it’s full, the circuit stops charging the inductor.

The rate at which the current drops off, however, depends on the output voltage, whose regulation is the very purpose of the chip. Since that voltage is fixed, the variation in timing has to come from variation in capacitance. El-Damak and Chandrakasan thus equip their chip with a bank of capacitors of different sizes. As the current drops, it charges a subset of those capacitors, whose selection is determined by the solar cell’s voltage. Once again, when the capacitor fills, the switches in the inductor’s path are flipped.

“In this technology space, there’s usually a trend to lower efficiency as the power gets lower, because there’s a fixed amount of energy that’s consumed by doing the work,” says Brett Miwa, who leads a power conversion development project as a fellow at the chip manufacturer Maxim Integrated. “If you’re only coming in with a small amount, it’s hard to get most of it out, because you lose more as a percentage. [El-Damak’s] design is unusually efficient for how low a power level she’s at.”

“One of the things that’s most notable about it is that it’s really a fairly complete system,” he adds. “It’s really kind of a full system-on-a chip for power management. And that makes it a little more complicated, a little bit larger, and a little bit more comprehensive than some of the other designs that might be reported in the literature. So for her to still achieve these high-performance specs in a much more sophisticated system is also noteworthy.”

82
EEE / SMART electronics research
« on: May 09, 2018, 11:16:38 AM »
The Singapore-MIT Alliance for Research and Technology (SMART) has entered its eighth year of investigating new technologies that could serve Singapore and other future-looking cities. At the same time, this collaboration between MIT and the National Research Foundation of Singapore (NRF) is exploring new concepts in the process of research and development itself. SMART is not only better funded than most R&D projects; it’s also distinct in its interdisciplinary and iterative approach to innovation.

MIT may push interdisciplinary collaboration more than most universities, but SMART takes the concept to a whole new level. “SMART uses a different model than anything in the U.S.,” says Eugene A. Fitzgerald, the Merton C. Flemings-SMA Professor of Materials Science and Engineering at MIT, who heads up the SMART Low Energy Electronic Systems (LEES) group. “We bring together faculty with expertise in different areas to collaborate closely from start to finish. In larger, multi-investigator programs, we typically simulate that sort of interdisciplinary approach, but we end up dividing the programs into different disciplines.”

The SMART LEES research program is marked by an iterative process of discovery in which researchers periodically reconsider the potential applications of an emerging technology. This involves evaluating how the latest changes to an emerging design, as well as new technological, economic, and social trends, might affect its purpose or its manufacturing and distribution.

The iterative process dovetails nicely with SMART’s interdisciplinary approach. As a former SMA Fellow in the Singapore-MIT Alliance (SMA) from 1999-2009, Fitzgerald has seen this in action. “A circuit designer might imagine a particular kind of circuit, which then motivates materials and device people to build prototypes, which then induce new ideas in the designer,” he says. “The idea is to create a new white space of innovation that industry would not converge upon naturally.”

SMART will soon begin evaluating what new and existing programs will be funded in the third five-year term starting in 2017. SMART programs are chosen through a competitive process at MIT, with further review of finalists by Singapore’s NRF.

Some programs may take only five or 10 years while others could extend to a 15-year period. “True fundamental innovation takes 10 to 15 years to reach market, so giving these projects enough time is important,” says Fitzgerald. “In the U.S., programs like this tend to last only three years, at which point valuable innovations can be terminated or interuppted.”

SMART’S labs at NRF’s Campus for Research Excellence and Technological Enterprise (CREATE) represent the first major MIT facility outside of Massachusetts. Approximately 50 MIT faculty members are involved in SMART, more or less equally split between LEES and four other programs: BioSystems and Micromechanics (BioSyM); the Center for Environmental Sensing and Modeling (CENSAM); Future Urban Mobility (FM); and Infectious Diseases (ID). There is always at least one resident MIT professor per program working at the SMART labs at CREATE, and all 50 SMART faculty members spend one month a year in Singapore.

The interchange benefits MIT as much as Singapore, says Fitzgerald. “SMART offers MIT faculty an opportunity to work with considerable resources and an interesting new research model.”

The current projects follow Singapore’s goal of creating a “smart nation” with a knowledge-based economy that enables a higher GDP per person, says Fitzgerald. While projects are developed with Singapore’s future in mind, they all have broader applicability. Projects tackle global challenges such as the need to more effectively communicate knowledge, improve efficiency, and increase mobility in the city of the future, he adds.

SMART has already developed several interesting new technologies, sometimes in collaboration with other institutes, such as the National University of Singapore (NUS), Nanyang Technological University (NTU), and various programs at MIT. The BioSyM group recently developed a technique for detecting malaria within minutes; the ID group recently joined with a multinational research team including Duke University, NUS, NTU, and MIT to announce the development of a viable dengue therapeutic for all dengue serotypes; CENSAM built a multifunction “LEDIF” sensor device for analyzing water quality, which is being deployed on a NUS autonomous underwater vehicle; the FM group worked with NUS to launch the first driverless cars authorized for use on Singapore’s streets; and LEES is combining new integrated circuit technology with existing fabrication methods.

SMART LEES: Optical chips from III-V and silicon

At the four-year old SMART LEES program, Fitzgerald is leading a team working on incorporating new materials into semiconductor systems. An early focus is on lattice-mismatched materials, the process of integrating layers in electronic materials and devices with different lattice parameters. Holding true to SMART’s interdisciplinary charter, LEES draws on faculty and students from material sciences, devices, processing, and circuit design.

Fitzgerald was one of the key developers of silicon-germanium lattice mismatch strain engineering at Bell Labs in the early '90s, and he continued his research into the new millennium at MIT, where he helped spin off AmberWave to commercialize strained silicon. Now, he is leading SMART LEES in experimenting with new monolithic, lattice-mismatched integrated circuits using III-V (3-5) semiconductor materials combined with silicon complimentary metal oxide semiconductor (CMOS) technology.

“We’re looking at III-V materials that have great optical properties and are better at power management and communication than silicon,” Fitzgerald says.

Until recently, the integration of III-V semiconductors with silicon has been stymied by the fact that the sizes of the lattice constant are fundamentally different, thereby producing a high level of defects. In recent years, Fitzgerald has modified the process he used to control defect densities in strained silicon for III-V semiconductors. Each principal investigator in the SMART LEES program can contribute his or her expertise to the goal of creating new circuits incorporating new materials, resulting in new functionality in electronic circuits.

Today’s processor cores communicate using silicon circuits, which consume a great deal of power. “A III-V device with optimized lattice matching and a very small footprint could span the tiny currents inside the core all the way to higher on-chip voltages needed for communication,” Fitzgerald says. The technology could reduce cellphone power consumption by offloading communication and sync, he adds.

“With integrating III-V and other materials with silicon technology, we are designing circuits that nobody could conceive of, such as building lighting systems on a chip,” Fitzgerald explains. “We’re making the silicon fab the entire supply chain for the lighting system, so you get a complete programmable lighting surface, including driver circuitry and intelligence in a very small footprint. Ideally, you could drop a III-V devices into circuits whenever you want to communicate with the outside world, by wireless or optical communication.”

The monolithic integration of LEDs with silicon electronics could greatly reduce the cost of smart LED lighting, and potentially lead to thin displays with their own optical inter-device communications via light pulses. The result might be a more secure alternative to WiFi, he adds.

“Once the display and computer merge into a single device, you can define new capabilities,” says Fitzgerald. “For example, a very thin display with multiple functionality in a single device is possible. This opens up possibilities such as a smartwatch display that is also a solar cell that charges the device. You could even reprogram the same display to act as a scanner.”

Initially, the technology will be used for a variety of task-specific devices with similar hardware. “But once volume ramps up you could have common platforms where all these capabilities reside,”  Fitzgerald says. “It would be like a tricorder where you can program it to do a lot of different things.”

SMART LEES is also experimenting with a business model that makes it easier for novel integrated circuits to reach market. “Today we have huge fabs that ship huge amounts of wafers,” Fitzgerald says. “The problem is that whenever someone comes up with a new device or material, it’s difficult to insert it into the manufacturing infrastructure. They’re making sugar cookies and you’re telling them you want chocolate chip cookies — and they don’t want chocolate chips anywhere near their facilities.”

The SMART LEES solution is to use a standard design kit to build upon a standard wafer made in a real fab, but then stop the manufacturing process in the middle. “At that point, we bring the wafer back to R&D to add new materials or devices in a way that can be compatible with the fab,” Fitzgerald explains. “The device can then be finished using the standard processes. This way, you can produce new ICs for smaller markets without having to build a new process or a $5 billion plant for every new product.”

The combination of the integrating new materials and devices with silicon, combined with new approaches to fabrication, could lead to the same sort of long-lasting benefits as Moore’s Law, says Fitzgerald. He notes, however, that “the next paradigm usually comes from where you least expect it.”

83
EEE / Charging up random access memory
« on: May 09, 2018, 11:15:47 AM »
Just as magnetic materials have opposing North and South poles, ferroelectric materials have opposing positive charges and negative charges that exhibit measurable differences in electric potential. Researchers at MIT and colleagues in China recently demonstrated this ferroelectric behavior along the edges of atomically thin tin-tellurium film at room temperature.

Measurements showed the energy gap, or bandgap, of this ultra-thin (2-D) film to be about eight times higher than the bandgap in bulk (3-D) tin-tellurium, with an on/off ratio as high as 3,000, they report July 15 in the journal Science. Their findings hold promise for making random access memory (RAM) devices from this special semiconductor material, which is known as a topological crystalline insulator.

“This discovery is very exciting because usually when you decrease the thickness from the 3-D to 2-D, the phase transition temperature always decreases and therefore could destroy the ferroelectricity. But in this case, the [ferroelectric] phase transition temperature increased. It’s quite unusual,” explains MIT postdoc Junwei Liu, a first author of the paper. “As far as we know, this might be the first time to observe this very unusual property.” MIT assistant professor of physics Liang Fu is one of the paper’s senior authors.

Three years’ work

These results follow three years of work based on a prediction by Fu, former student Timothy Hsieh PhD ’15, postdoc Liu (who was then a graduate student at Tsinghua University), and collaborators, that ferroelectric structural distortion in tin-tellurium and similar topological crystalline insulators, would open a tunable bandgap on the surface. Hsieh is now a postdoc at University of California at Santa Barbara. Researchers in the U.S. and Europe, including Vidya Madhavan and Ilija Zeljkovic at Boston College, confirmed this prediction experimentally in bulk materials. “The importance of ferroelectricity in topological crystalline insulators led us to study thin films of tin telluride,” Fu says. 

Tin-tellurium, which is also known as tin telluride, is classified as a IV-VI semiconductor, because tin (Sn) is from Group IV on the periodic table and tellurium (Te) from Group VI-A. At extremely cold temperatures, below about -283 degrees Fahrenheit, bulk tin-tellurium is a ferroelectric material, which means it becomes polarized with positive and negative electric charges splitting into opposing alignments, but it is not practical for room temperature applications.

Liu, whose work involves theoretical calculations, says in 2013 MIT researchers partnered with experimentalists at Tsinghua University in China to explore ferroelectricity in thin films of the tin-tellurium material. Kai Chang of Tsinghua University in China conducted experiments and is a first author of the Science paper. The paper’s co-authors also include a dozen colleagues at Tsinghua and Renmin Universities and other research facilities in China, and RIKEN Center for Emergent Matter Science in Japan.

A unit cell is the smallest repeating pattern of atoms in the tin-tellurium molecular structure. Built on a base of graphene and silicon carbide, the tin-tellurium layers in the experiments ranged in size from 1 to 8 unit cells. In an actual device, the tin-tellurium would be capped with insulating material, as seen in the illustration in the slideshow above. The new study shows this ferroelectric state persists up to only about 26 F in the single-unit cell thin film tin-tellurium material, but in 2-, 3- and 4-unit cells, the ferroelectric state was robust to 300 kelvins, or 80 F, the highest temperature the experimental apparatus could measure.

“Room-temperature devices could have a very large commercial application. That’s why we are very excited about this work; it’s really robust even in room temperature,” Liu says.

Tests showed that memory could be written through a top gate voltage, which “flips” the in-plane polarization of the ferroelectric film, and read by a voltage tunneling through an edge without erasing the memory state. Evidence of this behavior in a sample that was a mere 16-nanometers in width means that tin-tellurium memory cells could be densely packed. Nanosensors and electronics are also possible.

Zeros and ones

Advantages of ferroelectric memory include lower-power consumption, fast write operations, and durable storage, Liu says. The MIT-Tsinghua results show the separation of positive and negative charges, or polarization, in their sample was in-plane, or parallel, with the atomically flat sample, creating a potential change on the edges of square-shaped islands of the material. Since this potential difference along edges is measurably different, one with large tunneling current, the other small, it can realize two different states that represent either a zero or a one, and these states can be detected simply by measuring the current.

“Based on this property, we proposed a new kind of random access memory. We call it ferroelectric tunneling random access memory,” says Liu, who proposed the initial architecture for this kind of memory, along with Fu and three coauthors at Tsinghua University: Kai Chang, Xi Chen, and Shuai-Hua Ji. MIT has filed for provisional patent protection and is in the process of filing a utility covering the findings regarding in-plane polarization and tunneling current. “It’s very simple, and it’s really practical, and I think it could be realized in the near future,” Liu says.

Previous conventional capacitive ferroelectric random access memory technologies had to destroy a state to read it, Liu says, which meant an extra step of rewriting the information stored in memory after reading it. “In our case, we read the signal without destroying it,” Liu says. “This is the intrinsic advantage of our approach. … Therefore it can have much higher read operation performance.”

“In our experiments, we found that ferroelectricity persists for the very small islands, as small as 25 nanometers by 25 nanometers by 0.5 nanometers; even in these very small islands, the ferroelectricity persists. We could achieve much higher storage density because it is really small,” he explains.

“The authors and their collaborators use a state-of-the-art combination of molecular beam epitaxy and scanning tunneling microscopy to demonstrate a completely unexpected enhancement of ferroelectricity in ultrathin films of [tin-tellurium],” comments Ilija Zeljkovic, an assistant professor of physics at Boston College, who was not involved in this research. “This discovery can potentially be employed in nanodevices, such as the ferroelectric RAM nanodevice the authors describe [in Figure 4 of the Science report].”

Although the 3-D bulk form of tin-tellurium has been studied for decades, the new results in ultrathin 2-D film of the same material exhibit this surprising new phenomenon, Zeljkovic notes. “The study itself is extremely thorough, and the data presented is of the highest quality in spite of the high difficulty of the experiment performed. The study also highlights the recent effort in the condensed matter physics community to search for novel interface phenomena in ultrathin films of existing materials, for example, graphite versus graphene.”

Fewer tin vacancies

Mathematical calculations known as density functional theory matched the experimental findings that there are 1/20 to 1/30 as many tin vacancies in the atomically thin tin-tellurium film than in the bulk form of the material. This lack of defects is believed to contribute to formation of the ferroelectric state.

The next step will be to show these results in actual devices. Future challenges include how to easily and inexpensively produce high quality tin-tellurium thin films and how to precisely control the polarization direction.

The research was supported by the U.S. Department of Energy, the STC Center for Integrated Quantum Materials, the National Natural& Science Foundation, and Ministry of Science and Technology of China.

84
It was the eclipse felt ‘round the world. The August 21, 2017, total solar eclipse that crossed the United States launched a wave in the upper atmosphere that was detected nearly an hour later from Brazil (SN Online: 8/11/17).

“The eclipse itself is a local phenomenon, but our study shows that it had effects around the world,” says space scientist Brian Harding of the University of Illinois at Urbana-Champaign.

Harding watched the eclipse from St. Louis. But he and his colleagues activated a probe near São João do Cariri, Brazil, to observe uncharged particles 250 kilometers high in a part of the atmosphere called the thermosphere.

The probe recorded a fast-moving wave in the thermosphere go by half an hour after sunset in São João do Cariri and 55 minutes after the end of the total eclipse, the team reported April 24 in Geophysical Research Letters. The wave is produced by the motion of the moon’s shadow, which cooled the atmosphere below it. That cold spot then acted like a sink, sucking in the warmer air ahead of it and causing a ripple in the atmosphere as the cold spot moved across the globe.

Previous eclipses also have launched waves at similar altitudes in the ionosphere, the charged plasma of the atmosphere, which overlaps with the electrically neutral thermosphere (SN Online: 8/13/17). This is the first time that scientists have observed a wave in the uncharged part of the atmosphere. Neutral particles are 100 to 1,000 times denser than plasma in the atmosphere, and it’s important to know how they behave too, Harding says.

85
EEE / How a backyard pendulum saw sliced into a Bronze Age mystery
« on: May 09, 2018, 11:13:12 AM »
Nicholas Blackwell and his father went to a hardware store about three years ago seeking parts for a mystery device from the past. They carefully selected wood and other materials to assemble a stonecutting pendulum that, if Blackwell is right, resembles contraptions once used to build majestic Bronze Age palaces.

With no ancient drawings or blueprints of the tool for guidance, the two men relied on their combined knowledge of archaeology and construction.

Blackwell, an archaeologist at Indiana University Bloomington, had the necessary Bronze Age background. His father, George, brought construction cred to the project. Blackwell grew up watching George, a plumber who owned his own business, fix and build stuff around the house. By high school, the younger Blackwell worked summers helping his dad install heating systems and plumbing at construction sites. The menial tasks Nicholas took on, such as measuring and cutting pipes, were not his idea of fun.

But that earlier work paid off as the two put together their version of a Bronze Age pendulum saw — a stonecutting tool from around 3,300 years ago that has long intrigued researchers. Power drills, ratchets and other tools that George regularly used around the house made the project, built in George’s Virginia backyard, possible.

“My father enjoyed working on the pendulum saw, although he and my mother were a bit concerned about what the neighbors would think when they saw this big wooden thing in their backyard,” Blackwell says. Anyone walking by the fenceless yard had a prime view of a 2.5-meter-tall, blade-swinging apparatus reminiscent of Edgar Allan Poe’s literary torture device.

86
As opioid-related deaths rise in the United States, so has the role of synthetic opioids — primarily illicit fentanyl, mixed into heroin or made into counterfeit pills (SN Online: 3/29/18). In 2016, synthetics surged past prescription opioids and were involved in 19,413 deaths, compared with 17,087 deaths involving prescription opioids, researchers report May 1 in JAMA. The study is based on data from the National Vital Statistic System’s record of all U.S. deaths.

“Synthetic opioids are much deadlier than prescription opioids,” says emergency physician Leana Wen, Health Commissioner of Baltimore, who was not involved in the study. Fentanyl, for example, is about 50 to 100 times more potent than morphine. The illicit origins of many synthetic opioids make the public health response more difficult, she says. “We can track prescriptions; it’s much harder to track illegally trafficked drugs.”

87
EEE / Neutron stars shed neutrinos to cool down quickly
« on: May 09, 2018, 11:11:54 AM »
For some neutron stars, the quickest way to cool off isn’t with a frosty beverage, but with lightweight, subatomic particles called neutrinos.

Scientists have spotted the first solid evidence that some neutron stars, the collapsed remnants of exploded stars, can rapidly cool their cores by emitting neutrinos. The result adds to evidence that scientists are gathering to understand the ultradense matter that is squished deep within a neutron star’s center.

The new evidence comes from a neutron star that repeatedly gobbled material from a neighboring star. The neutron star rapidly cooled off after its meals, scientists determined. X-rays emitted by the neutron star showed that the fast cooldown rate was consistent with a theorized effect called the direct Urca process, in which neutrinos quickly ferry energy away from a collapsed star, astrophysicist Edward Brown and colleagues report in the May 4 Physical Review Letters.

Neutron stars are known to emit neutrinos by a similar process that cools the star slowly. But previously, there wasn’t clear evidence for faster cooling. The team analyzed observations of the neutron star, located about 35,000 light-years from Earth, as it cooled during a 15-year interlude between feeding sessions. Neutrinos carried away energy about 10 times faster than the rate energy is radiated by the sun’s light — or about 100 million times quicker than the slow process, says Brown, of Michigan State University in East Lansing.

Although some other neutron stars have shown hints of such a quick chill, “this is basically the first object for which we can see the star actively cooling before our eyes,” says astrophysicist James Lattimer of Stony Brook University in New York, who was not involved with the research.

The direct Urca process, named by physicists George Gamow and Mário Schenberg in the 1940s, took its moniker from the now-defunct Urca casino in Rio de Janeiro. “The joke being that this process removes heat from the star the way the casino removes money from tourists’ pockets,” Brown says.

In the process, neutrons in the star’s core convert into protons and emit electrons and antineutrinos (the antimatter partners of neutrinos). Likewise, protons convert into neutrons and emit antielectrons and neutrinos. Because neutrinos and antineutrinos interact very rarely with matter, they can escape the core, taking energy with them. “The neutrino is a thief; it robs energy from the star,” says physicist Madappa Prakash of Ohio University in Athens, who was not involved with the research.

The observation may help scientists understand what goes on deep within neutron stars, the cores of which are squeezed to densities far beyond those achievable in laboratories. Although the simplest theory holds that the cores are crammed with neutrons and a smaller number of protons and electrons, scientists have also proposed that the collapsed stars may consist of weird states of matter, containing rare particles called hyperons or a sea of free-floating quarks, the particles that make up protons and neutrons (SN: 12/23/17, p. 7).

The direct Urca process can happen only if the fraction of protons in the center of the neutron star is larger than about 10 percent. So if the process happens, “that already tells us a lot,” says astrophysicist Wynn Ho of Haverford College in Pennsylvania, who was not involved in the research. Such observations could eliminate theories that would predict lower numbers of protons.

However, the scientists weren’t able to determine the mass of the neutron star, limiting the conclusions that can be drawn. But, says Prakash, if the mass of such a quickly cooling neutron star is measured, the neutron star’s interior makeup could be nailed down.

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