Dr. Barbastathis' work with 3D Optical Systems Group is focused on fundamental research and application development within optics and nanofabrication. More specifically, digital holography and volume holography for imaging of complex biological and fluidic systems, and subwavelength optical engineering using novel 3D assembly methods, such as nanostructured origami. Nanostructured origami leverages precision patterning by two-dimensional lithography with folding to achieve 3D structures primarily for optics, but also for sensing and energy storage.

Now, new findings by researchers at MIT are a major step toward making graphene with this coveted property. The work could also lead to revisions in some theoretical predictions in graphene physics.

From left: Prof. Ray Ashoori, postdocs Andrea Young and Ben Hunt, graduate student Javier Sanchez-Yamagishi, and Prof. Pablo Jarillo-Herrero. Photo: Jarillo-Herrero and Ashoori groups.

The new technique involves placing a sheet of graphene -- a carbon-based material whose structure is just one atom thick -- on top of hexagonal boron nitride, another one-atom-thick material with similar properties. The resulting material shares graphene's amazing ability to conduct electrons, while adding the band gap necessary to form transistors and other semiconductor devices.

The work is described in a paper in the journal Science co-authored by Pablo Jarillo-Herrero, the Mitsui Career Development Assistant Professor of Physics at MIT, Professor of Physics Ray Ashoori, and 10 others.

"By combining two materials," Jarillo-Herrero says, "we created a hybrid material that has different properties than either of the two."

Graphene is an extremely good conductor of electrons, while boron nitride is a good insulator, blocking the passage of electrons. "We made a high-quality semiconductor by putting them together," Jarillo-Herrero explains. Semiconductors, which can switch between conducting and insulating states, are the basis for all modern electronics.

To make the hybrid material work, the researchers had to align, with near perfection, the atomic lattices of the two materials, which both consist of a series of hexagons. The size of the hexagons (known as the lattice constant) in the two materials is almost the same, but not quite: Those in boron nitride are 1.8 percent larger. So while it is possible to line the hexagons up almost perfectly in one place, over a larger area the pattern goes in and out of register.

At this point, the researchers say they must rely on chance to get the angular alignment for the desired electronic properties in the resulting stack. However, the alignment turns out to be correct about one time out of 15, they say.

"The qualities of the boron nitride bleed over into the graphene," Ashoori says. But what's most "spectacular," he adds, is that the properties of the resulting semiconductor can be "tuned" by just slightly rotating one sheet relative to the other, allowing for a spectrum of materials with varied electronic characteristics.

Others have made graphene into a semiconductor by etching the sheets into narrow ribbons, Ashoori says, but such an approach substantially degrades graphene's electrical properties. By contrast, the new method appears to produce no such degradation.

The band gap created so far in the material is smaller than that needed for practical electronic devices; finding ways of increasing it will require further work, the researchers say.

"If ... a large band gap could be engineered, it could have applications in all of digital electronics," Jarillo-Herrero says. But even at its present level, he adds, this approach could be applied to some optoelectronic applications, such as photodetectors.

The results "surprised us pleasantly," Ashoori says, and will require some explanation by theorists. Because of the difference in lattice constants of the two materials, the researchers had predicted that the hybrid's properties would vary from place to place. Instead, they found a constant, and unexpectedly large, band gap across the whole surface.

In addition, Jarillo-Herrero says, the magnitude of the change in electrical properties produced by putting the two materials together "is much larger than theory predicts."

The MIT team also observed an interesting new physical phenomenon. When exposed to a magnetic field, the material exhibits fractal properties -- known as a Hofstadter butterfly energy spectrum -- that were described decades ago by theorists, but thought impossible in the real world. There is intense research in this area; two other research groups also report on these Hofstadter butterfly effects this week in the journal Nature.

Eva Andrei, a professor of physics at Rutgers University who was not involved in this work, says that until recently, "decades-old theoretical predictions of novel and surprising physical phenomena, expected to occur in 2-D electron systems [such as graphene], have lain dormant." But the MIT team's work clearly demonstrates some of these phenomena, she says.

"Perhaps most significant is their observation of a band gap in zero magnetic field," she says. "The ability to induce a zero-field band gap in graphene may one day allow its use as a switch in transistor applications, providing a viable and inexpensive alternative to silicon electronics."

The research included postdocs Ben Hunt and Andrea Young and graduate student Javier Sanchez-Yamagishi, as well as six other researchers from the University of Arizona, the National Institute for Materials Science in Tsukuba, Japan, and Tohoku University in Japan. The work was funded by the U.S. Department of Energy, the Gordon and Betty Moore Foundation and the National Science Foundation.

--David L. Chandler, MIT News Office; 5/16/2013

"Typically at a university, every faculty member has a private lab," says Dr. Vicky Diadiuk, MTL's Associate Director, Operations. "But MTL is different in that we're a shared facility."

Vicky Diadiuk
Principal Research Engineer
Associate Director, Operations
MIT Microsystems Technology LaboratoriesMost of the students who use the facilities are from MIT's Schools of Engineering and Science, but users have come from 29 departments, labs and centers, says Diadiuk. In addition to providing economy of scale in facilities costs, MTL provides 15 full-time staff members who maintain tools, train students, and generate basic processes. This provides a strong competitive advantage for recruiting faculty, who may not want to spend their first few years setting up a facilities-intensive lab, says Diadiuk.

"Everything is already set up when they come in, including the professional staff, machines, basic processes, and sometimes even the specific recipes they need," she says. "Most faculty would rather spend their time doing research."

Diadiuk should know. She spent many years doing optoelectronic device research at MIT Lincoln Laboratory prior to starting at MTL in 1996. As Associate Director, she oversees MTL's daily operations, reporting to MTL Director and Electrical Engineering professor Vladimir Bulovic.

MTL consists of three cleanroom laboratories, offering different levels of cleanliness and sophistication. The crown jewel is the second-floor lab, which provides "what you would find in a commercial fab, except for the vastly different scale," says Diadiuk. On the fourth floor, the Technology Research Lab offers fewer restrictions and more flexibility. "Some of our innovations in processing and materials start there, and if they're compatible with mainstream fabrication, they can come down to the second floor," says Diadiuk.

The fifth floor lab is "kind of a garage works, where anything goes as long as it's safe," says Diadiuk. "There's no barrier to entry or contamination concerns, so people can play there and develop processes. When they discover that cleanliness matters, which is a more natural way of learning than being told, some of them migrate to the cleaner labs downstairs."

Moving Beyond Silicon
As with the semiconductor industry at large, MTL typically depends on silicon as the substrate material. Yet, the lab is increasingly using light-emitting and -responding materials like III-V compounds, says Diadiuk. Gallium nitride is a "very exciting material right now," says Diadiuk. "The wafers are very expensive, but they can be used on the same equipment we use for fabricating devices on silicon and the other III-V semiconductor substrates like indium phosphide and gallium arsenide."

Because MTL generates a wide variety of devices for experimental purposes, researchers seldom use a full 25-wafer lot, even though the tools can handle them. "We use six-inch diameter or smaller wafers whereas modern industrial tools are set up for eight or 12 inches," says Diadiuk. "For this and other reasons, like the availability of replacement parts, we must constantly renew our toolset."

Last year the lab acquired a "really cool state-of-the-art e-beam writer," says Diadiuk. This direct-write lithography machine does not require a mask, and supports features as small as a few nanometers.

In recent years, MTL has increasingly applied semiconductor fabrication technologies to the manufacture of a variety of tiny devices. "We use the same processes used to make transistors to make things like micro-reactors and DNA chips," says Diadiuk. MTL is also "pushing the envelope with other materials like plastics and polymers."

The range of devices built at MTL is impressive. In the first three months of 2013 alone, MTL announced participation in projects including nanowire-based solar cells, optical phased arrays for medical-imaging, and a super-fast new p-type transistor. MTL also houses four special research centers, focusing on integrated circuits (CICS), medical devices (MEDRC), graphene devices (MIT-CG) and gallium nitride (MIT-GaN), respectively.

One of the most ambitious projects MTL ever undertook was a MEMS micro-engine project led by MIT's Martin Schmidt. "We had about 42 researchers and many more students working on it, and we used essentially every technology we know," recalls Diadiuk. "The project led us to think about designing semiconductor devices in three dimensions. We aligned and bonded up to seven wafers, which was fairly heroic, and fabricated a turbine the size of a shirt button. The experience made us really good at wafer bonding and deep etching."

Safety First
Having a single shared facility for fabrication not only reduces costs and improves recruiting, but it also pays off with greater safety. "We often use dangerous chemicals and gases that can explode and burn and do all sorts of bad things," says Diadiuk. "You really don't want to have 22 different labs, each one with its own cylinder of toxic gases."

Diadiuk is proud of MTL's safety record, which she says has been kept up with the assistance of MIT's Environmental Health and Safety (EH&S) office, as well as MIT Facilities, which maintains the detection systems. Among other precautions, the facility has toxic gas detectors on every floor and near every tool that uses them, supported with redundant systems. Hydrogen is kept in an explosion-proof bunker, and there are numerous hydrogen detectors.

Perhaps the most important safety measure is training. This is a challenge, says Diadiuk, considering that about 500 students per year use the lab. All users must have hands-on experience in microfabrication or take a regular semester-long course, Introduction to Microfabrication (6.152); if curricular credit is not desired, users can instead opt for an intensive one-week lab-only version. Before entering the lab, students, faculty, and other users are trained on lab-specific EH&S requirements. Once inside, they are taught how to use specific machines.

Researchers from other universities are also welcome to apply for lab access, as long as they go through the same training regimen. Most visitors are from Boston-area schools, says Diadiuk, but some have come from other parts of the country and overseas as well.

Open to Industry, from Start-ups to MIG
Industry can gain access to MTL's labs via the Fabrication Facilities Access (FFA) program. Since larger companies tend to have their own facilities, the labs are primarily attractive to start-ups, says Diadiuk. "Companies can send engineers to use our cleanroom as long as their processes don't pose a risk of cross-contamination, and they pass all safety exams. There's very little IP entanglement, which the start-ups really like."

Sometimes larger manufacturers use the facilities for special projects they can't develop with their own pilot lines. Here, MTL's expertise in non-traditional fabrication is particularly attractive. "Making a material change for them is really hard, but for us, it's pretty straightforward," says Diadiuk.

Some companies are members of a sponsor group called the Microsystems Industrial Group (MIG), which advises and helps fund the lab. Sometimes MIG members direct part of their contributions to particular research programs. "It's a highly interactive participation," says Diadiuk.

A number of MIG members donate equipment to help keep the lab up to date. Although it's usually "at least one generation behind," according to Diadiuk, it's still close enough so that "by the time our students graduate they are often fully trained on essentially the same equipment they would use in the workplace."

MIG's financial contributions, meanwhile, help offset general infrastructure costs, which are typically beyond the charter of research grants. Despite this funding, plus several million dollars a year from MIT, MTL continues to seek new ways to stay competitive. In fact, MIT is now in the planning stages to build a brand new MTL facility.

In this and other projects, MIG members provide valuable feedback and direction, and both sides keep each other up to date on the latest developments, says Diadiuk. "Our interactions with industry provide a sanity check," she says. "They bring us back to reality."

--Eric Brown, MIT Industrial Liaison Program, 5/16/2013

One solution to that problem is the 15-year-old idea of "cognitive radio," in which wireless devices would scan their environments for vacant frequencies and use these for transmissions. Different proposals for cognitive radio place different emphases on hardware and software, but the chief component of many hardware approaches is a bank of filters that can isolate any frequency in a wide band.

Researchers at MIT's Microsystems Technology Laboratory (MTL) have developed a new method for manufacturing such filters that should improve their performance while enabling 14 times as many of them to be crammed on a single chip. That's a vital consideration in handheld devices where space is tight. But just as important, the new method uses techniques already common in the production of signal-processing chips, so it should be easy for manufacturers to adopt.

Laura Popa (left) and Dana Weinstein test their experimental chip using a cryogenic vacuum radio-frequency probe station. PHOTO: M. Scott Brauer

There are two main approaches to hardware-based radio-signal filtration: one is to perform the filtration electronically; the other is to convert the radio signal to an acoustic signal -- a physical vibration -- and then convert it back to an electrical signal. In work to be presented in June at the International Conference on Solid-State Sensors, Actuators and Microsystems, Dana Weinstein, the Steve and Renee Finn Career Development Assistant Professor of Electrical Engineering and Computer Science, and Laura Popa, a graduate student in physics, adopted the second approach.

Resonant ideas

Both types of filtration use devices called resonators, and acoustic resonators have a couple of clear advantages over electronic ones. One is that their filtration is more precise.

"If I pluck a guitar string -- that's the easiest resonator to think of -- it's going to resonate at some frequency, and it's going to die down due to losses," Weinstein explains. "That loss is related to, basically, energy leaked away from that resonance mode into all other frequencies. Less loss means better frequency selectivity, and mechanical acoustic resonators have less loss than electrical resonators."

Acoustic resonators' other advantage is that, in principle, they can be packed more densely than electrical-filtration circuits. "Acoustic wavelengths are much smaller than electromagnetic wavelengths," Weinstein says. "So for a given frequency, my mechanical resonator is going to be much smaller."

But in practice, the number of acoustic resonators in a filtration bank has been limited. The heart of any device that converts electrical signals to mechanical vibrations, or vice versa, is a capacitor, which can be thought of as two parallel metal plates separated by a small distance.

"The capacitors change the impedance" -- a measure of the ease with which a wave propagates -- "that the antenna sees, so you may have unwanted reflections back into the antenna," Weinstein says. "Each capacitor from each filter is going to affect the antenna, and that's no good. It means I can only have so many filters, and therefore so many frequencies that I can separate my signal into."

Another problem with acoustic resonators is that turning them on or off -- a necessary step in the isolation of a particular transmission frequency -- requires giving each resonator its own electrical switch. Traditionally, an incoming radio-frequency signal has had to pass through that switch before reaching the resonator, suffering some loss of quality in the process.

Switching channels

Weinstein and Popa solve both these problems at a stroke. Moreover, they do it by adapting a technology already common in wireless devices: a gallium nitride transistor.

Almost all commercial transistors use semiconductors: materials, like gallium nitride, that can be switched between a conductive and a nonconductive state by the application of a voltage. In Weinstein and Popa's new resonator, the lower "plate" of the capacitor is in fact a gallium nitride channel in its conductive state.

Switching that channel to its nonconductive state is like removing the lower plate of the capacitor, which drastically reduces the capacitors' effect on the quality of the radio signal. In experiments, the MTL researchers found that their resonators had only one-fourteenth the "capacitive load" of conventional resonators. "The radio can now afford to have 14 times as many filters attached to the antenna," Weinstein says, "so we can span more frequencies."

Switching the channel to its nonconductive state also turns the resonator off, so the researchers' new design requires no additional switch in the path of the incoming signal, improving signal quality.

Finally, the new resonator uses only materials already found in the gallium arsenide transistors common in wireless devices, so mass-producing it should require no major modifications of existing manufacturing processes.

Commercial adoption of cognitive radio has been slow for a number of reasons. "Part of it is being able to get the frequency-agile components and do it in a cost-effective manner," says Thomas Kazior, a principal engineering fellow at Raytheon. "Plus the size constraint: Filters tend to be big to begin with, and banks of tunable filters just make things even bigger."

The MTL researchers' work could help with both problems, Kazior says. "We're talking about making filters that are directly integrated onto, say, a receiver chip, because the little resonator devices are literally the size of a transistor," he says. "These are all on a tiny scale."

"They can help with the cost problem because these resonator-type structures almost come for free," Kazior adds. "Building them is part of the semiconductor fabrication process, using pretty much the existing fabrication steps that you're using to build the transistor and the rest of the circuits. You just may need to add one, or two at the most, additional steps -- out of 100 or more steps."

--Larry Hardesty, MIT News Office; 5/15/2013

Games! Barbecue! Tim the Beaver! Cotton candy!
Fun for MTL students, faculty, staff, and affiliates alike!

The Grossman Group (also known as G²E) focuses on the application and development of cutting-edge simulation tools and experimental techniques to understand, predict, and design novel materials with applications in energy conversion, energy storage, thermal transport, surface phenomena, and synthesis. They aim to understand the key optical, electronic and mechanical behaviors of energy conversion in order to design new materials with greater efficiencies and lower costs.

Now in its fourth year, the Early Career Awards support the development of individual research programs by outstanding scientists who are in the early stages of their careers, and stimulates research careers in the disciplines supported by the DOE's Office of Science. Across the Office of Basic Energy Sciences divisions, 61 awards were made from about 770 proposals that went out for peer review.
Learn more about the awards: http://science.energy.gov/early-career/

Tisdale, the Charles and Hilda Roddey Career Development Assistant Professor of Chemical Engineering, will use his award to support work over five years to develop a novel ultrafast microscopy technique for visualizing electronic processes at interfaces in next-generation solar cells.

--MIT News Office; 5/8/2013

Now, researchers at MIT have shown that there is a way to blow past that limit as easily as today's jet fighters zoom through the sound barrier -- which was also once seen as an ultimate limit.

Their work appears this week in a report in the journal Science, co-authored by graduate students including Daniel Congreve, Nicholas Thompson, Eric Hontz and Shane Yost, alumna Jiye Lee '12, and professors Marc Baldo and Troy Van Voorhis.

The principle behind the barrier-busting technique has been known theoretically since the 1960s, says Baldo, a professor of electrical engineering at MIT. But it was a somewhat obscure idea that nobody had succeeded in putting into practice. The MIT team was able, for the first time, to perform a successful "proof of principle" of the idea, which is known as singlet exciton fission. (An exciton is the excited state of a molecule after absorbing energy from a photon.)

In a standard photovoltaic (PV) cell, each photon knocks loose exactly one electron inside the PV material. That loose electron then can be harnessed through wires to provide an electrical current.

But in the new technique, each photon can instead knock two electrons loose. This makes the process much more efficient: In a standard cell, any excess energy carried by a photon is wasted as heat, whereas in the new system the extra energy goes into producing two electrons instead of one.

While others have previously "split" a photon's energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth's surface. The new work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels.

This was accomplished using an organic compound called pentacene in an organic solar cell. While that material's ability to produce two excitons from one photon had been known, nobody had previously been able to incorporate it within a PV device that generated more than one electron per photon.

"Our whole project was directed at showing that this splitting process was effective," says Baldo, who is also the director of the Center for Excitonics, sponsored by the U.S. Department of Energy. "We showed that we could get through that barrier."

The theoretical basis for this work was laid long ago, says Congreve, but nobody had been able to realize it in a real, functioning system. "In this system," he says, "everyone knew you could, they were just waiting for someone to do it."

"This is the landmark event we had all been waiting to see," adds Richard Friend, the Cavendish Professor of Physics at the University of Cambridge, who was not involved in this research. "This is really great research."

Since this was just a first proof of principle, the team has not yet optimized the energy-conversion efficiency of the system, which remains less than 2 percent. But ratcheting up that efficiency through further optimization should be a straightforward process, the researchers say. "There appears to be no fundamental barrier," Thompson says.

While today's commercial solar panels typically have an efficiency of at most 25 percent, a silicon solar cell harnessing singlet fission should make it feasible to achieve efficiency of more than 30 percent, Baldo says -- a huge leap in a field typically marked by slow, incremental progress. In solar cell research, he notes, people are striving "for an increase of a tenth of a percent."

Solar panel efficiencies can also be improved by stacking different solar cells together, but combining solar cells is expensive with conventional solar-cell materials. The new technology instead promises to work as an inexpensive coating on solar cells.

The work made use of a known material, but the team is now exploring new materials that might perform the same trick even better. "The field is working on materials that were chanced upon," Baldo says -- but now that the principles are better understood, researchers can begin exploring possible alternatives in a more systematic way.

Christopher Bardeen, a professor of chemistry at the University of California at Riverside who was not involved in this research, calls this work "very important" and says the process used by the MIT team "represents a first step towards incorporating an exotic photophysical process (fission) into a real device. This achievement will help convince workers in the field that this process has real potential for boosting organic solar cell efficiencies by 25 percent or more."

The research was performed in the Center for Excitonics and supported by the U.S. Department of Energy. MIT has filed for a provisional patent on the technology.

--David L. Chandler, MIT News Office; 4/18/2013

This Scanning Electron Microscope image shows an array of nanowires. Photo: Kristian Molhave/Opensource Handbook of Nanoscience and Nanotechnology

The initial discovery of carbon nanotubes -- tiny tubes of pure carbon, essentially sheets of graphene rolled up unto a cylinder -- is generally credited to a paper published in 1991 by the Japanese physicist Sumio Ijima (although some forms of carbon nanotubes had been observed earlier). Almost immediately, there was an explosion of interest in this exotic form of a commonplace material. Nanowires -- solid crystalline fibers, rather than hollow tubes -- gained similar prominence a few years later.

Due to their extreme slenderness, both nanotubes and nanowires are essentially one-dimensional. "They are quasi-one-dimensional materials," says MIT associate professor of materials science and engineering Silvija Gradečak: "Two of their dimensions are on the nanometer scale." This one-dimensionality confers distinctive electrical and optical properties.

For one thing, it means that the electrons and photons within these nanowires experience "quantum confinement effects," Gradečak says. And yet, unlike other materials that produce such quantum effects, such as quantum dots, nanowires' length makes it possible for them to connect with other macroscopic devices and the outside world.

The structure of a nanowire is so simple that there's no room for defects, and electrons pass through unimpeded, Gradečak explains. This sidesteps a major problem with typical crystalline semiconductors, such as those made from a wafer of silicon: There are always defects in those structures, and those defects interfere with the passage of electrons.

Made of a variety of materials, nanowires can be "grown" on many different substrates through a vapor deposition process. Tiny beads of molten gold or other metals are deposited on a surface; the nanowire material, in vapor, is then absorbed by the molten gold, ultimately growing from the bottom of that bead as a skinny column of the material. By selecting the size of the metal bead, it is possible to precisely control the size of the resulting nanowire.

In addition, materials that don't ordinarily mix easily can be grown together in nanowire form. For example, layers of silicon and germanium, two widely used semiconductors, "are very difficult to grow together in thin films," Gradečak says. "But in nanowires, they can be grown without any problems." Moreover, the equipment needed for this kind of vapor deposition is widely used in the semiconductor industry, and can easily be adapted for the production of nanowires.

While nanowires' and nanotubes' diameters are negligible, their length can extend for hundreds of micrometers, even reaching lengths visible to the unaided eye. No other known material can produce such extreme length-to-diameter ratios: millions of times longer than they are wide.

Because of this, the wires have an extremely high ratio of surface area to volume. That makes them very good as detectors, because all that surface area can be treated to bind with specific chemical or biological molecules. The electrical signal generated by that binding can then easily be transmitted along the wire.

Similarly, nanowires' shape can be used to produce narrow-beam lasers or light-emitting diodes (LEDs), Gradečak says. These tiny light sources might someday find applications within photonic chips, for example -- chips in which information is carried by light, instead of the electric charges that relay information in today's electronics.

Compared to solid nanowires, nanotubes have a more complex structure: essentially one-atom-thick sheets of pure carbon, with the atoms arranged in a pattern that resembles chicken wire. They behave in many ways as one-dimensional materials, but are actually hollow tubes, like a long, nanometer-scale drinking straw.

The properties of carbon nanotubes can vary greatly depending on how they are rolled up, a property called chirality. (It's similar to the difference between forming a paper tube by rolling a sheet of paper lengthwise versus on the diagonal: The different alignments of fibers in the paper produce different strength in the resulting tubes.) In the case of carbon nanotubes, chirality can determine whether the tubes behave as metals or as semiconductors.

But unlike the precise manufacturing control that is possible with nanowires, so far methods for making nanotubes produce a random mix of types, which must be sorted to make use of one particular kind. Besides single-walled nanotubes, they also exist in double-walled and multi-walled forms.

In addition to their useful electronic and optical properties, carbon nanotubes are exceptionally strong, and are used as reinforcing fibers in advanced composite materials. "In any application where one-dimensionality is important, both carbon nanotubes and nanowires would provide benefits," Gradečak says.

--David L. Chandler, MIT News Office; 4/11/2013

The pilot recipients are Regina Barzilay, Skolkovo Foundation Associate Professor of Computer Science and Engineering; Duane Boning, Skolkovo Foundation Professor of Electrical Engineering and Computer Science; David Gamarnik, Skolkovo Foundation Professor of Operations Research; Fiona Murray, Skolkovo Foundation Associate Professor of Entrepreneurship; Bruce Tidor, Skolkovo Foundation Professor of Computer Science and Biological Engineering; and Forest White, Skolkovo Foundation Associate Professor of Biological Engineering.

Faculty were informed of the appointments by MIT Vice President Claude Canizares; terms began on Jan. 1 and continue through 2013.

As faculty lead for the MIT Skoltech Initiative, Boning hopes that the honor draws attention to other opportunities for MIT faculty and researchers to work with Skoltech. "I am grateful for the support and excited about what it represents for the MIT community. The collaboration with Russia is creating new ways to experiment, from support for collaborative research and course development, to sabbatical and leave opportunities in Moscow," he says.

Tidor, who advises on faculty search, adds, "The mission to create a new innovation-based university in Moscow is compelling. Working cross-culturally with colleagues, developing a global recruiting strategy, and trying to build some of the best features of MIT in a different place is exciting, challenging, and rewarding. In working to create an effective environment for faculty recruitment, our search committees have become a mechanism for bridging departments and exchanging ideas within MIT and for building global and local networks."

Working with Skoltech faculty and leadership, the six recipients have helped launch joint research, innovation and educational programs, MISTI-Russia activities, and other mechanisms for collaboration.

--MIT Skoltech Initiative; 4/2/2013

--MIT News Office, 3/28/2013

Scanning Electron Microscope images show an array of zinc-oxide nanowires (top) and a cross-section of a photovoltaic cell made from the nano wires, interspersed with quantum dots made of lead sulfide (dark areas). A layer of gold at the top (light band) and a layer of indium-tin-oxide at the bottom (lighter area) form the two electrodes of the solar cell.
Images courtesy of Jean, et al/Advanced Materials

Photovoltaics (PVs) based on tiny colloidal quantum dots have several potential advantages over other approaches to making solar cells: They can be manufactured in a room-temperature process, saving energy and avoiding complications associated with high-temperature processing of silicon and other PV materials. They can be made from abundant, inexpensive materials that do not require extensive purification, as silicon does. And they can be applied to a variety of inexpensive and even flexible substrate materials, such as lightweight plastics.

But there's a tradeoff in designing such devices, because of two contradictory needs for an effective PV: A solar cell's absorbing layer needs to be thin to allow charges to pass readily from the sites where solar energy is absorbed to the wires that carry current away -- but it also needs to be thick enough to absorb light efficiently. Improved performance in one of these areas tends to worsen the other, says Joel Jean, a doctoral student in MIT's Department of Electrical Engineering and Computer Science (EECS).

"You want a thick film to absorb the light, and you want it thin to get the charges out," he says. "So there's a huge discrepancy."

That's where the addition of zinc oxide nanowires can play a useful role, says Jean, who is the lead author of a paper to be published in the journal Advanced Materials. The paper is co-authored by chemistry professor Moungi Bawendi, materials science and engineering professor Silvija Gradečak, EECS professor Vladimir Bulović, and three other graduate students and a postdoc.

These nanowires are conductive enough to extract charges easily, but long enough to provide the depth needed for light absorption, Jean says. Using a bottom-up growth process to grow these nanowires and infiltrating them with lead-sulfide quantum dots produces a 50 percent boost in the current generated by the solar cell, and a 35 percent increase in overall efficiency, Jean says. The process produces a vertical array of these nanowires, which are transparent to visible light, interspersed with quantum dots.

"If you shine light along the length of the nanowires, you get the advantage of depth," he says. But also, "you decouple light absorption and charge carrier extraction, since the electrons can hop sideways onto a nearby nanowire and be collected."

One advantage of quantum dot-based PVs is that they can be tuned to absorb light over a much wider range of wavelengths than conventional devices, Jean says. This is an early demonstration of a principle that, through further optimization and improved physical understanding, might lead to practical, inexpensive new kinds of photovoltaic devices, he says.

Mark Thompson, a professor of chemistry at the University of Southern California who was not involved in this research, says, "The MIT team has made a real advance." While other groups have tried a similar approach, he says, the MIT team was able to achieve much better control of nanowire dimensions and density. "This required a careful and deliberate study, and the results speak for themselves. I suspect that this is only the beginning, and as they continue to improve their process, we will see even higher efficiencies."

Already, the test devices have produced efficiencies of almost 5 percent, among the highest ever reported for a quantum-dot PV based on zinc oxide, he says. With further development, Jean says, it may be possible to improve the devices' overall efficiency beyond 10 percent, which is widely accepted as the minimum efficiency for a commercially viable solar cell. Further research will, among other things, explore using longer nanowires to make thicker films, and also work on better controlling the spacing of the nanowires to improve the infiltration of quantum dots between them.

The team, which also included postdoc Sehoon Chang and graduate students Patrick Brown, Jayce Cheng and Paul Rekemeyer, was supported by the National Science Foundation; the MIT Center for Materials Science and Engineering; the Samsung Group; the MIT/Masdar Institute Cooperative Program; the MIT Energy Initiative; the Hertz Foundation; and the Agency for Science, Technology and Research of Singapore.

--David Chandler, MIT News Office; 3/25/2013

Jongyoon Han; Associate Professor, Electrical Engineering and Computer Science/Biological Engineering

Those working in selective membranes have known about the ion concentration polarization phenomenon for some time. By definition membranes pass one type of material or molecule while blocking another. A unique type of membrane, known as ion selective membranes, repels or blocks the passage of positive ions, for example. These membranes have found use in engineering fuel cells.

When an electric field passes across an ion selective membrane, ions are driven in both directions. The membrane in the middle blocks one type of ion while passing the other in the process creating a charge imbalance. Explains Han, "If you create a group of positive ions on one side and negative ions on the other side of the membrane there is a strong electrostatic interaction between them creating the condition of ion concentration polarization." But when ion concentration changes so does the entire electric field distribution which can induce strong fluid motions around it.

With a background in physics, Han joined the MIT faculty in 2002 in the Department of Electrical Engineering and Computer Science. He later joined the newly created Department of Biological Engineering where his work on ion concentration polarization has progressed rapidly, and today he is also Head of the Micro/Nanofluidic BioMEMS Group at the MIT Research Lab of Electronics. Since 2005, Han has homed in on the science of ion concentration polarization to find practical solutions to two of today's scientific challenges.

Water Desalinization and Purification

Han's first attempt at engineering ion concentration polarization led to creating a biosensing device that concentrated biomolecules in one location. Instead of having a low concentration biomolecule sample, this device concentrated these biomolecules in a small region making it easier to detect them.

Later, Han realized that is was not just moving the biomolecules but the ions itself, or the salt contained in the system. From there sprang the idea of moving the salt in the water to actually desalinate it. The team successfully produced a technology and published their first desalinization paper in 2010.

"It is the same principle but using it for different industry," Han says. "Because you can move the ions away or draw them in, you can use the phenomena to push the ions around and desalinate the water, even from the sea water to the fresh water level."

One of the benefits of ion concentration polarization technology is that not only are salts removed from the water but it also clears larger charged colloid particles like cells, bacteria, and proteins. "It is one of the very clear benefits of this technology," he explains. "In a single same step you can actually remove all the salt and bacteria from the cells together."

To commercialize the technology, Han and colleagues are in the process of launching a small start-up company for small-scale household or personal use water purification. The team is aiming to produce the clarified water with "good power efficiency" that may include solar panel technology instead of using larger amounts of electricity. The other necessary metric is to build a portable technology that can be used anywhere, including in disaster-stricken regions. "You can bring the device to the problem and apply it without the need for electricity or infrastructure support."

Han realizes that the technology has potential to be competitive for large-scale water desalinization and purification production but it has yet to be demonstrated on that scale. To do so, he is seeking to raise government or industry support to fund the necessary engineering work to bring this new desalinization idea to the larger markets.

Neuroprosthetics

In 2010, Han thought of applying ion concentration polarization science in the context of nerve cells. The nerve is a circuit that conducts electrical impulses based on ion current in and out of the membrane. The nerve cell naturally maintains a certain ion concentration imbalance between the inside and outside of the cell. When a signal is received, the nerve opens up the ion channel protein allowing ions to flow into the nerve cell.

"We understood that this process is critically dependent on the concentration of different ions around the nerve cell," Han explains. For more than 100 years scientists have known that decreasing the calcium concentration around the nerve causes it to become hypersensitive. The nerve can then be excited and triggered with lower amounts of current.

"That is where we got the idea that if we deplete the calcium concentration around the nerve and make it more hypersensitive then we could make better prosthetics," Han recalls. The goal of neuroprosthetics is to artificially excite often-damaged nerves in the area to allow muscle activation. Today, that is achieved by positioning electrical current into the nerve.

Working with clinical collaborators from Beth Israel Hospital, Han's team published their first paper related to neuroengineered prosthetics in 2011. The paper focused on the use of a new calcium-selective ion membrane. The team designed a metal electrode coated with this membrane material. "It depletes the calcium ions around the nerve so that when we excite the nerve we clearly see the decrease of the threshold current," he explains.

"If we can control the ion concentration of that location, you can make the nerve locally hypersensitive so you don't need to apply higher amount of current." The benefits could make the use of prosthetics potentially more comfortable. Since the entire area conducts electricity, applying higher levels may trigger another nerve, including a sensory nerve that will register pain or heat. "I think we can make a clear impact on those parasitic activation issues because we can potentially lower the amount of current significantly," Han concludes.

Putting "Crazy Ideas" to Work

Regardless of what he studies, Han's research modus operandi is to begin with some mundane, obscure physics phenomenon. "You try to understand the physics behind it and then you realize that there are some things you can do with this phenomenon. Maybe you can make a new device or a different system to capitalize on this unique phenomenon," he explains. "For example, when I see a neuroprosthetic I don't see the prosthetic. I think about what happens between the electrode and the cell."

Han partly credits the MIT culture with focusing his "crazy ideas" into research projects. "MIT places so much emphasis on the practical use of science, engineering, and technologies," he says. "Whenever I see other people present their work, they all talk about what it might be used for. That is the culture of this place and the practically-minded scientist."

--Alice McCarthy, ILP Institute Insider, 3/14/2013

Assistant Professor Sangbae Kim works on the 70-pound 'cheetah' robot designed at MIT.
Photo: M. Scott Brauer

The motors can be programmed to quickly adjust the robot's leg stiffness and damping ratio -- or cushioning -- in response to outside forces such as a push, or a change in terrain. The researchers will present the efficiency results and design principles for their electric motor at the International Conference on Robotics and Automation in May.

Sangbae Kim, the Esther and Harold E. Edgerton Assistant Professor in MIT's Department of Mechanical Engineering, says achieving energy-efficiency in legged robots has proven extremely difficult. Robots such as Boston Dynamic's "Big Dog" carry heavy gasoline engines and hydraulic transmissions, while other electrically powered robots require large battery packs, gears, force sensors and springs to coordinate the joints in a robot's leg. All this weighty machinery can add up to significant wasted energy, particularly when a robot's legs need to make frequent contact with the ground in order to trot or gallop.

"In order to send a robot to find people or perform emergency tasks, like in the Fukushima disaster, you want it to be autonomous," Kim says. "If it could run for more than two hours and search a large field, that would be useful. But one of the reasons why people think it's impossible to make an electric robot that does this is because efficiencies have been pretty bad."

Kim adds that part of the challenge in powering running machines with electric motors is that such robots require a flexible response upon impact, and high power, torque and efficiency -- characteristics that have historically been difficult to achieve with electric motors.

To understand how an electrically powered system might waste little energy while running, the researchers first looked at general sources of energy loss in running robots. They found that most wasted energy comes from three sources: heat given off by a motor; energy dissipated through mechanical transmission, such as losses to friction through multiple gear trains; and inefficient control, such as energy lost through a heavy-footed step, as opposed to a smoother and more gentle gait.

The group then came up with design principles to minimize such energy waste. To combat heat loss from motors, the group proposed a high-torque-density motor -- a motor that produces a significant amount of torque at a given weight and heat production. The team analyzed the relationship between motor size and torque, and designed custom motors that exceed the torque performance of commercially available electric motors.

The team found that such high-torque motors require fewer gears -- a characteristic that would improve efficiency even more, as there would be less machinery through which energy could dissipate. Many researchers have used springs and dampers in series with motors to protect the robot from forceful impacts during locomotion, but it's difficult to control a spring's stiffness and damping ratio -- which can be a problem if a robot has to traverse disparate surfaces, such as asphalt and sand.

"With our system, we can make our robotic leg behave like a spring or damper without having physical springs, dampers or force sensors," Kim says.

Kim is the Esther and Harold E. Edgerton Assistant Professor in MIT's Department of Mechanical Engineering. Photo: M. Scott Brauer

In addition to heat given off by a motor, the group found that another major source of energy loss comes from the force of impact as a robot's leg hits the ground. Such forces can be strong enough to shake a machine and potentially cause damage. Engineers need to use dampers, or shock absorbers, to minimize shaking and stabilize such systems. But Kim says such dampers act to dissipate energy each time a leg meets the ground.

In contrast, the cheetah-bot's electric motors capture this energy, feeding it back to the system to further power the robot.

"The majority of impact energy goes back to the battery because the damping is created by custom-designed electric control of the motor," Kim says. "[The motor] regenerates energy that would have been lost."

Kim adds that mounting motors and gears at the hip joint would also reduce energy loss by minimizing leg inertia: Some legged robots are designed with motors and gearboxes at each joint along a leg, which can be cumbersome and can lose more energy at every impact. With Kim's design, 85 percent of the weight of the leg is concentrated at the hip joint, keeping the rest of the leg relatively lightweight.

The researchers also attached strips of Kevlar to connect sections of the robot's legs, simulating the structure of tendons along a bone. The Kevlar strengthens the leg with little additional weight, and further reduces the leg's inertia. The group also constructed a flexible spine out of rings of polyurethane rubber, sandwiched between vertebra-like segments. Kim hypothesizes that the spine moves along with the rear legs, and can store elastic energy while galloping.

To test the efficiency of the robot, the researchers ran it on a treadmill at a steady 5-mph clip. They measured the voltage and current of the battery, as well as that from each motor. They calculated the robot's efficiency of locomotion -- also known as cost of transport -- and found that it was more efficient than robotic competitors such as Big Dog and Honda's two-legged robot, ASIMO.

After digging through the literature on animal locomotion, the researchers plotted the cost of transport of various running, flying and swimming animals. They found that, not surprisingly, fliers were more efficient than runners, although swimmers were the most efficient movers. The cheetah robot, according to Kim's calculations, falls around the efficiency range of humans, cheetahs and hunting dogs.

Currently the team is assembling a set of new motors, designed by Jeffrey Lang, a professor of electrical engineering at MIT. Kim expects that once the group outfits the robot with improved motors, the cheetah robot will be able to gallop at speeds of up to 35 mph, with an efficiency that rivals even fliers. The researchers are convinced that this approach can exceed biological muscle in many aspects, including power, torque and responsiveness.

"There are so many ways to design, and each legged robot has a different system," Kim says. "If you design the motor properly, it's more powerful, simpler robotics."

Ron Fearing, a professor of electrical engineering and computer science at the University of California at Berkeley, says that simple springs can work well in small robots running on smooth terrain. But for rougher, more unpredictable terrain, he says the energy-recovery system of the MIT cheetah has big advantages.

"The cheetah robot has really pushed the technology in efficient motor design, low-loss transmissions, and low-inertia legs," says Fearing, who did not contribute to the research. "By combining these with the regenerative motor drive system, so that mechanical energy from the leg can recharge the battery, that in my opinion has made a huge difference in efficiency, [and] an important step forward in making efficient, electrically driven running robots."

In addition to Kim and Lang, the paper's co-authors include Sangok Seok, Albert Wang, Meng Yee Chuah and David Otten, all of MIT.

This research was funded by the Defense Advanced Research Projects Agency's Maximum Mobility and Manipulation (M3) program.

--Jennifer Chu, MIT News Office; 3/8/2013

Such a system would use sunlight to produce a storable fuel, such as hydrogen, instead of electricity for immediate use. This fuel could then be used on demand to generate electricity through a fuel cell or other device. This process would liberate solar energy for use when the sun isn't shining, and open up a host of potential new applications.

The new work is described in a paper this week in the Proceedings of the National Academy of Sciences by associate professor of mechanical engineering Tonio Buonassisi, former MIT professor Daniel Nocera (now at Harvard University), MIT postdoc Mark Winkler (now at IBM) and former MIT graduate student Casandra Cox (now at Harvard). It follows up on 2011 research that produced a "proof of concept" of an artificial leaf -- a small device that, when placed in a container of water and exposed to sunlight, would produce bubbles of hydrogen and oxygen.

The device combines two technologies: a standard silicon solar cell, which converts sunlight into electricity, and chemical catalysts applied to each side of the cell. Together, these would create an electrochemical device that uses an electric current to split atoms of hydrogen and oxygen from the water molecules surrounding them.

The goal is to produce an inexpensive, self-contained system that could be built from abundant materials. Nocera has long advocated such devices as a means of bringing electricity to billions of people, mostly in the developing world, who now have little or no access to it.

"What's significant is that this paper really describes all this technology that is known, and what to expect if we put it all together," Cox says. "It points out all the challenges, and then you can experimentally address each challenge separately."

Winkler adds that this is a "pretty robust analysis that looked at what's the best you could do with market-ready technology."

The original demonstration leaf, in 2011, had low efficiencies, converting less than 4.7 percent of sunlight into fuel, Buonassisi says. But the team's new analysis shows that efficiencies of 16 percent or more should now be possible using single-bandgap semiconductors, such as crystalline silicon.

"We were surprised, actually," Winkler says: Conventional wisdom held that the characteristics of silicon solar cells would severely limit their effectiveness in splitting water, but that turned out not to be the case. "You've just got to question the conventional wisdom sometimes," he says.

The key to obtaining high solar-to-fuel efficiencies is to combine the right solar cells and catalyst -- a matchmaking activity best guided by a roadmap. The approach presented by the team allows for each component of the artificial leaf to be tested individually, then combined.

The voltage produced by a standard silicon solar cell, about 0.7 volts, is insufficient to power the water-splitting reaction, which needs more than 1.2 volts. One solution is to pair multiple solar cells in series. While this leads to some losses at the interface between the cells, it is a promising direction for the research, Buonassisi says.

An additional source of inefficiency is the water itself -- the pathway that the electrons must traverse to complete the electrical circuit -- which has resistance to the electrons, Buonassisi says. So another way to improve efficiency would be to lower that resistance, perhaps by reducing the distance that ions must travel through the liquid.

"The solution resistance is challenging," Cox says. But, she adds, there are "some tricks" that might help to reduce that resistance, such as reducing the distance between the two sides of the reaction by using interleaved plates.

"In our simulations, we have a framework to determine the limits of efficiency" that are possible with such a system, Buonassisi says. For a system based on conventional silicon solar cells, he says, that limit is about 16 percent; for gallium arsenide cells, a widely touted alternative, the limit rises to 18 percent.

Models to determine the theoretical limits of a given system often lead researchers to pursue the development of new systems that approach those limits, Buonassisi says. "It's usually from these kinds of models that someone gets the courage to go ahead and make the improvements," he says.

"Some of the most impactful papers are ones that identify a performance limit," Buonassisi says. But, he adds, there's a "dose of humility" in looking back at some earlier projections for the limits of solar-cell efficiency: Some of those predicted "limits" have already been exceeded, he says.

"We don't always get it right," Buonassisi says, but such an analysis "lays a roadmap for development and identifies a few 'levers' that can be worked on."

James Barber, the Ernst Chain Professor of Biochemistry at Imperial College London, who was not connected with this work, says, "It is generally agreed that for an effective technology to emerge, the efficiency of the device must be 10 percent or more." The MIT team's work suggests such devices "can provide efficiencies as high as 15 percent. This level of energy conversion is considered very good and practical."

Barber adds that a next step, demonstrating these improvements in a functioning device, is crucial: "It is very important to construct a working system which has a large surface area and operates with solar energy under open field conditions for a long period of time, as is done with the testing of solar cells." If this can be achieved, he says, "the construction of robust and efficient solar-driven modules which produce hydrogen from water on a large industrial scale would have considerable impact on human society."

The work was supported by the National Science Foundation, the Air Force Office of Scientific Research, the Singapore National Research Foundation through the Singapore-MIT Alliance for Research and Technology, and the Chesonis Family Foundation.

--David L. Chandler, MIT News Office; 3/4/2013