|Angela Belcher, chair of MIT’s Department of Biological Engineering, and Neel Bardhan, postdoctoral fellow in MIT's Koch Institute, with an oversized model of carbon nanotube probes for detecting tiny ovarian tumors. Image, Koch Institute/MIT.|
An MIT Koch Institute team, led by Angela Belcher, chair of MIT’s Department of Biological Engineering, won the 2020 STAT Madness competition with technology to see tiny ovarian tumors.
Read more on STAT News.
MIT’s vice president for research identifies three areas that show particular promise for climate action.
|While superior battery technology is still in our future, MIT Vice President for Research Maria Zuber is already encouraged by the increased use of renewable energy.|
Climate change is a very personal issue to Maria Zuber, MIT’s vice president for research. A native of eastern Pennsylvania, she watched her grandfathers, both coal miners, battle black-lung disease. “The burning of anthracite coal drove my community and was a central part of my childhood,” says Zuber. “Yet it’s been known since the 1800s that combustion of fossil fuels puts CO2 into the atmosphere, and that the effects can be damaging.”
Today, the catastrophic effects of climate change are showing up even faster than models predicted, she observes. “If you just look at it that way, it’s easy to despair.”
Yet Zuber, also the E. A. Griswold Professor of Geophysics, remains optimistic. “People are looking at those effects based on what we know now, but I think about the actions that will be taken when we have technological breakthroughs and an improved understanding of the climate system,” she explains. “Those breakthroughs will happen — we just don’t know exactly when.”
Zuber identifies three areas of MIT-based research that show particular promise for climate action: battery technology, renewable energy, and fusion.
“Batteries are key to combating climate change because in the deployment of renewable energy, one of the greatest challenges is intermittency,” she explains. “We need to store the power of wind and sun so we can use them when the sun isn’t shining and the wind isn’t blowing. We need better battery capacity, efficiency, and design, and we need batteries that are made out of common-earth materials as opposed to rare ones.”
At the third in a series of climate action-focused symposia sponsored by the Institute this academic year, Yet-Ming Chiang ’80, ScD ’85, the Kyocera Professor of Ceramics in MIT’s Department of Materials Science and Engineering, provided examples of this effort. Chiang is a cofounder of Form Energy, one of the portfolio companies of The Engine, an MIT-based innovation hub for startups focused on technology with potential for changing the world. Chiang described research and development of batteries based on materials such as sulfur and zinc, which are cheaper and more abundant than the lithium commonly used today.
Solar and wind
While superior battery technology is still in our future, Zuber is already encouraged by the increased use of renewable energy. “Solar and wind energy are really penetrating into society, and a lot of new jobs are being created as a result,” she says. “Just renewables won’t solve all the problems, though. We also have to move toward decarbonizing other parts of the energy system where we haven’t made as much progress. But you can really see the tide starting to turn.”
Zuber cites the work of Vladimir Bulović, the Fariborz Maseeh Chair in Emerging Technology and founding faculty director of MIT.nano, who works to create next-generation, lightweight, flexible photovoltaics that could dramatically improve solar energy systems. “We have a lot going on in solar energy,” she says.
On the topic of fusion, Zuber’s enthusiasm is boundless. “Fusion is the process that powers the sun, and we need to bring that process down to Earth,” she explains. “The fuel is hydrogen, a component of water, so it’s practically free and virtually inexhaustible.” The greatest obstacle to working with fusion is designing a device that creates more energy than it uses for power. “This is a really difficult challenge,” she admits. “But fusion could be an important contributor to limiting the change in our climate. It doesn’t put CO2 in the atmosphere, and there’s no radioactive fuel waste involved.”
Zuber highlights the Institute’s collaboration with Commonwealth Fusion Systems (CFS), a startup spun out of MIT’s Plasma Science and Fusion Center. MIT’s role in CFS was conceived by researchers led by center director Dennis Whyte, the Hitachi America Professor of Engineering and cofounder of CFS.
“People never took fusion seriously, but they seem to be taking it seriously now,” says Zuber. “We have considerable investment in CFS. The fact that the private sector is also investing heavily in fusion energy indicates optimism that the technology has matured to the point where it’s a reasonable longer-term investment.”
In addition to research, Zuber described the Institute’s successful collaboration with other organizations and governments. In one example, MIT joined forces with two local organizations, Boston Medical Center and the Post Office Square Redevelopment Corporation, in a 2016 power-purchase agreement that resulted in the construction of a 650-acre, 60-megawatt solar farm on the site of a former tobacco farm in North Carolina. The power generated by the solar farm replaces power previously supplied by a coal-fired plant.
“We have also convened investment firms, fossil fuel companies, climate-scenario producers, environmental advocates, and NGOs, along with academics like ourselves, to explore the role of corporate disclosures with regard to climate change,” she says. “Companies are taking major risks if they don’t consider the financial consequences of global warming.”
Continued engagement with outside organizations and populations is key. “Climate change affects everybody on Earth, and MIT can’t solve a global problem alone,” Zuber points out. “A solution that might work here in Cambridge might not work in India or Africa, so we’ve sought out partners from different areas of the developing world. We need to consider those perspectives in energy solutions.”
Hope for the planet
Much of Zuber’s hope for climate action, she says, comes from MIT students.
“The greatest thing about our students is that they believe they can solve this problem,” she says. “We are not dispirited — we will keep working to find a solution.”
– MIT Resource Development | MIT News Office
April 22, 2020
2019 Summer Scholar Jared Bowden will pursue a PhD at the University of California, Berkeley.
During Summer 2019, Jared Bowden worked on slow release, targeted drug delivery in the lab of MIT Professor Paula T. Hammond, who is head of the department of chemical engineering and the David H. Koch (1962) Professor in Engineering, through the NSF-funded MIT MRL Research Experience for Undergraduates program.
What graduate school program do you intend to pursue?
After graduating from the University of Massachusetts, Amherst, this spring, I will be pursuing my PhD in Chemical and Biochemical Engineering at the University of California, Berkeley, next fall. I am really excited to continue my studies and grateful for all of the people and experiences which have helped me along the way to get to this point.
What about your MIT MRL Summer Scholar experience was most enjoyable?
The MIT MRL Summer Scholar program allowed me to dive more in depth into a research project than I had ever had the ability to do before and I really enjoyed the freedom I had to take on the project as my own and lead its direction. This experience was also instrumental in helping me decide to continue my studies in graduate school.
What are your future plans or ambitions?
I am really interested in continuing to conduct research with applications in improving human and environmental health, whether it be through an academic path beyond graduate school or in industry.
2019 MIT MRL Summer Scholar Leah Borgsmiller chooses Northwestern University for graduate school.
During Summer 2019, Leah Borgsmiller worked on niobium-aluminum thin films for superconducting nanowire single photon detectors in Electrical Engineering and Computer Science Professor Karl K. Berggren’s lab through the NSF-funded MIT MRL Research Experience for Undergraduates program.
What graduate school will you be attending?
I have decided to continue my studies at Northwestern University by starting in their PhD program in Materials Science and Engineering in the fall. I’m not completely sure what I want to do in the future, but I’m confident that I want to pursue a PhD in the area of electronic materials and then build a career in materials research.
What awards have you received?
I received the Northwestern University Outstanding MSE Junior Award in May of 2019. I have also been awarded a National Science Foundation Graduate Research Fellowship to fund my graduate studies.
What about your MIT MRL Summer Scholar experience was most enjoyable?
The most enjoyable part of my summer experience at MIT was being exposed to so many different cutting edge researchers and studies being conducted at MIT. I loved hearing about what my peers were researching during their time at MIT in addition to getting the opportunity to dive deeply into my project.
How did your Summer Scholars experience contribute to getting you where you are today?
The MIT MRL Summer Scholars experience confirmed that I wanted to apply to graduate schools and has given me a new confidence in myself as a researcher, as well as providing me with practical research skills that have helped me in research projects since then. This experience was invaluable in preparing me to start my PhD in the fall.
Device for harnessing terahertz radiation might enable self-powering implants, cellphones, other portable electronics.
|This schematic figure, from the researchers’ paper, shows a green square that represents graphene on top of a square of another material. The red lines represent terahertz waves. The blue triangles represent antenna that surround the square to capture the terahertz waves and focus the waves to the square. Courtesy of the researchers.|
Any device that sends out a Wi-Fi signal also emits terahertz waves — electromagnetic waves with a frequency somewhere between microwaves and infrared light. These high-frequency radiation waves, known as “T-rays,” are also produced by almost anything that registers a temperature, including our own bodies and the inanimate objects around us.
Terahertz waves are pervasive in our daily lives, and if harnessed, their concentrated power could potentially serve as an alternate energy source. Imagine, for instance, a cellphone add-on that passively soaks up ambient T-rays and uses their energy to charge your phone. However, to date, terahertz waves are wasted energy, as there has been no practical way to capture and convert them into any usable form.
Now physicists at MIT have come up with a blueprint for a device they believe would be able to convert ambient terahertz waves into a direct current, a form of electricity that powers many household electronics.
Their design takes advantage of the quantum mechanical, or atomic behavior of the carbon material graphene. They found that by combining graphene with another material, in this case, boron nitride, the electrons in graphene should skew their motion toward a common direction. Any incoming terahertz waves should “shuttle” graphene’s electrons, like so many tiny air traffic controllers, to flow through the material in a single direction, as a direct current.
The researchers have published their results in the journal Science Advances, and are working with experimentalists to turn their design into a physical device.
“We are surrounded by electromagnetic waves in the terahertz range,” says lead author Hiroki Isobe, a postdoc in MIT’s Materials Research Laboratory. “If we can convert that energy into an energy source we can use for daily life, that would help to address the energy challenges we are facing right now.”
Isobe’s co-authors are Liang Fu, the Lawrence C. and Sarah W. Biedenharn Career Development Associate Professor of Physics at MIT; and Su-yang Xu, a former MIT postdoc who is now an assistant professor of chemistry at Harvard University.
Breaking graphene’s symmetry
Over the last decade, scientists have looked for ways to harvest and convert ambient energy into usable electrical energy. They have done so mainly through rectifiers, devices that are designed to convert electromagnetic waves from their oscillating (alternating) current to direct current.
Most rectifiers are designed to convert low-frequency waves such as radio waves, using an electrical circuit with diodes to generate an electric field that can steer radio waves through the device as a DC current. These rectifiers only work up to a certain frequency, and have not been able to accommodate the terahertz range.
A few experimental technologies that have been able to convert terahertz waves into DC current do so only at ultracold temperatures — setups that would be difficult to implement in practical applications.
Instead of turning electromagnetic waves into a DC current by applying an external electric field in a device, Isobe wondered whether, at a quantum mechanical level, a material’s own electrons could be induced to flow in one direction, in order to steer incoming terahertz waves into a DC current.
Such a material would have to be very clean, or free of impurities, in order for the electrons in the material to flow through without scattering off irregularities in the material. Graphene, he found, was the ideal starting material.
To direct graphene’s electrons to flow in one direction, he would have to break the material’s inherent symmetry, or what physicists call “inversion.” Normally, graphene’s electrons feel an equal force between them, meaning that any incoming energy would scatter the electrons in all directions, symmetrically. Isobe looked for ways to break graphene’s inversion and induce an asymmetric flow of electrons in response to incoming energy.
Looking through the literature, he found that others had experimented with graphene by placing it atop a layer of boron nitride, a similar honeycomb lattice made of two types of atoms — boron and nitrogen. They found that in this arrangement, the forces between graphene’s electrons were knocked out of balance: Electrons closer to boron felt a certain force while electrons closer to nitrogen experienced a different pull. The overall effect was what physicists call “skew scattering,” in which clouds of electrons skew their motion in one direction.
Isobe developed a systematic theoretical study of all the ways electrons in graphene might scatter in combination with an underlying substrate such as boron nitride, and how this electron scattering would affect any incoming electromagnetic waves, particularly in the terahertz frequency range.
He found that electrons were driven by incoming terahertz waves to skew in one direction, and this skew motion generates a DC current, if graphene were relatively pure. If too many impurities did exist in graphene, they would act as obstacles in the path of electron clouds, causing these clouds to scatter in all directions, rather than moving as one.
“With many impurities, this skewed motion just ends up oscillating, and any incoming terahertz energy is lost through this oscillation,” Isobe explains. “So we want a clean sample to effectively get a skewed motion.”
|Terahertz waves are pervasive in our daily lives, and if harnessed, their concentrated power could potentially serve as an alternate energy source. Imagine, for instance, a cellphone add-on that passively soaks up ambient T-rays and uses their energy to charge your phone. Illustration, José-Luis Olivares, MIT.|
They also found that the stronger the incoming terahertz energy, the more of that energy a device can convert to DC current. This means that any device that converts T-rays should also include a way to concentrate those waves before they enter the device.
With all this in mind, the researchers drew up a blueprint for a terahertz rectifier that consists of a small square of graphene that sits atop a layer of boron nitride and is sandwiched within an antenna that would collect and concentrate ambient terahertz radiation, boosting its signal enough to convert it into a DC current.
“This would work very much like a solar cell, except for a different frequency range, to passively collect and convert ambient energy,” Fu says.
The team has filed a patent for the new “high-frequency rectification” design, and the researchers are working with experimental physicists at MIT to develop a physical device based on their design, which should be able to work at room temperature, versus the ultracold temperatures required for previous terahertz rectifiers and detectors.
“If a device works at room temperature, we can use it for many portable applications,” Isobe says.
He envisions that, in the near future, terahertz rectifiers may be used, for instance, to wirelessly power implants in a patient’s body, without requiring surgery to change an implant’s batteries. Such devices could also convert ambient Wi-Fi signals to charge up personal electronics such as laptops and cellphones.
“We are taking a quantum material with some asymmetry at the atomic scale, that can now be utilized, which opens up a lot of possibilities,” Fu says.
This research was funded in part by the U.S. Army Research Laboratory and the U.S. Army Research Oﬃce through the Institute for Soldier Nanotechnologies (ISN).
– Jennifer Chu | MIT News Office
March 27, 2020
An MIT team has devised a lithium metal anode that could improve the longevity and energy density of future batteries.
|New research by engineers at MIT and elsewhere could lead to batteries that can pack more power per pound and last longer. Illustration, MIT News.|
New research by engineers at MIT and elsewhere could lead to batteries that can pack more power per pound and last longer, based on the long-sought goal of using pure lithium metal as one of the battery’s two electrodes, the anode.
The new electrode concept comes from the laboratory of Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and professor of materials science and engineering. It is described Feb. 3, 2020, in the journal Nature, in a paper co-authored by Yuming Chen and Ziqiang Wang at MIT, along with 11 others at MIT and in Hong Kong, Florida, and Texas.
The design is part of a concept for developing safe all-solid-state batteries, dispensing with the liquid or polymer gel usually used as the electrolyte material between the battery’s two electrodes. An electrolyte allows lithium ions to travel back and forth during the charging and discharging cycles of the battery, and an all-solid version could be safer than liquid electrolytes, which have high volatilility and have been the source of explosions in lithium batteries.
“There has been a lot of work on solid-state batteries, with lithium metal electrodes and solid electrolytes,” Li says, but these efforts have faced a number of issues.
One of the biggest problems is that when the battery is charged up, atoms accumulate inside the lithium metal, causing it to expand. The metal then shrinks again during discharge, as the battery is used. These repeated changes in the metal’s dimensions, somewhat like the process of inhaling and exhaling, make it difficult for the solids to maintain constant contact, and tend to cause the solid electrolyte to fracture or detach.
Another problem is that none of the proposed solid electrolytes are truly chemically stable while in contact with the highly reactive lithium metal, and they tend to degrade over time.
Most attempts to overcome these problems have focused on designing solid electrolyte materials that are absolutely stable against lithium metal, which turns out to be difficult. Instead, Li and his team adopted an unusual design that utilizes two additional classes of solids, “mixed ionic-electronic conductors” (MIEC) and “electron and Li-ion insulators” (ELI), which are absolutely chemically stable in contact with lithium metal.
The researchers developed a three-dimensional nanoarchitecture in the form of a honeycomb-like array of hexagonal MIEC tubes, partially infused with the solid lithium metal to form one electrode of the battery, but with extra space left inside each tube. When the lithium expands in the charging process, it flows into the empty space in the interior of the tubes, moving like a liquid even though it retains its solid crystalline structure. This flow, entirely confined inside the honeycomb structure, relieves the pressure from the expansion caused by charging, but without changing the electrode’s outer dimensions or the boundary between the electrode and electrolyte. The other material, the ELI, serves as a crucial mechanical binder between the MIEC walls and the solid electrolyte layer.
“We designed this structure that gives us three-dimensional electrodes, like a honeycomb,” Li says. The void spaces in each tube of the structure allow the lithium to “creep backward” into the tubes, “and that way, it doesn’t build up stress to crack the solid electrolyte.” The expanding and contracting lithium inside these tubes moves in and out, sort of like a car engine’s pistons inside their cylinders. Because these structures are built at nanoscale dimensions (the tubes are about 100 to 300 nanometers in diameter, and tens of microns in height), the result is like “an engine with 10 billion pistons, with lithium metal as the working fluid,” Li says.
Because the walls of these honeycomb-like structures are made of chemically stable MIEC, the lithium never loses electrical contact with the material, Li says. Thus, the whole solid battery can remain mechanically and chemically stable as it goes through its cycles of use. The team has proved the concept experimentally, putting a test device through 100 cycles of charging and discharging without producing any fracturing of the solids.
|Reversible lithium (Li) metal plating and stripping in a carbon tubule with an inner diameter of 100 nanometers. Courtesy of the researchers.|
Li says that though many other groups are working on what they call solid batteries, most of those systems actually work better with some liquid electrolyte mixed with the solid electrolyte material. “But in our case,” he says, “it’s truly all solid. There is no liquid or gel in it of any kind.”
The new system could lead to safe anodes that weigh only a quarter as much as their conventional counterparts in lithium-ion batteries, for the same amount of storage capacity. If combined with new concepts for lightweight versions of the other electrode, the cathode, this work could lead to substantial reductions in the overall weight of lithium-ion batteries. For example, the team hopes it could lead to cellphones that could be charged just once every three days, without making the phones any heavier or bulkier.
One new concept for a lighter cathode was described by another team led by Li, in a paper that appeared last month in the journal Nature Energy, co-authored by MIT postdoc Zhi Zhu and graduate student Daiwei Yu. The material would reduce the use of nickel and cobalt, which are expensive and toxic and used in present-day cathodes. The new cathode does not rely only on the capacity contribution from these transition-metals in battery cycling. Instead, it would rely more on the redox capacity of oxygen, which is much lighter and more abundant. But in this process the oxygen ions become more mobile, which can cause them to escape from the cathode particles. The researchers used a high-temperature surface treatment with molten salt to produce a protective surface layer on particles of manganese- and lithium-rich metal-oxide, so the amount of oxygen loss is drastically reduced.
Even though the surface layer is very thin, just 5 to 20 nanometers thick on a 400 nanometer-wide particle, it provides good protection for the underlying material. “It’s almost like immunization,” Li says, against the destructive effects of oxygen loss in batteries used at room temperature. The present versions provide at least a 50 percent improvement in the amount of energy that can be stored for a given weight, with much better cycling stability.
The team has only built small lab-scale devices so far, but “I expect this can be scaled up very quickly,” Li says. The materials needed, mostly manganese, are significantly cheaper than the nickel or cobalt used by other systems, so these cathodes could cost as little as a fifth as much as the conventional versions.
The research teams included researchers from MIT, Hong Kong Polytechnic University, the University of Central Florida, the University of Texas at Austin, and Brookhaven National Laboratories in Upton, New York. The work was supported by the National Science Foundation.
– David L. Chandler | MIT News Office
February 3, 2020
The new method could impact devices used in imaging, machine learning, and more.
|This 8-inch wafer contains phase-change pixels that can be controlled to modulate light. Researchers are studying the properties and behaviors of the pixels to inform the creation of future devices that use phase-change materials. Image, Nicole Fandel|
In the 1950s, the field of electronics began to change when the transistor replaced vacuum tubes in computers. The change, which entailed replacing large and slow components with small and fast ones, was a catalyst for the enduring trend of miniaturization in computer design. No such revolution has yet hit the field of infrared optics, which remains reliant on bulky moving parts that preclude building small systems.
However, a team of researchers at MIT Lincoln Laboratory, together with Professor Juejun Hu and graduate students from MIT's Department of Materials Science and Engineering, is devising a way to control infrared light by using phase-change materials instead of moving parts. These materials have the ability to change their optical properties when energy is added to them.
“There are multiple possible ways where this material can enable new photonic devices that impact people’s lives,” says Hu. “For example, it can be useful for energy-efficient optical switches, which can improve network speed and reduce power consumption of internet data centers. It can enable reconfigurable meta-optical devices, such as compact, flat infrared zoom lenses without mechanical moving parts. It can also lead to new computing systems, which can make machine learning faster and more power-efficient compared to current solutions.”
A fundamental property of phase-change materials is that they can change how fast light travels through them (the refractive index). “There are already ways to modulate light using a refractive index change, but phase-change materials can change almost 1,000 times better,” says Jeffrey Chou, a team member formerly in the laboratory's Advanced Materials and Microsystems Group.
The team successfully controlled infrared light in multiple systems by using a new class of phase-change material containing the elements germanium, antimony, selenium, and tellurium, collectively known as GSST. This work is discussed in a paper published in Nature Communications.
A phase-change material's magic occurs in the chemical bonds that tie its atoms together. In one phase state, the material is crystalline, with its atoms arranged in an organized pattern. This state can be changed by applying a short, high-temperature spike of thermal energy to the material, causing the bonds in the crystal to break down and then reform in a more random, or amorphous, pattern. To change the material back to the crystalline state, a long- and medium-temperature pulse of thermal energy is applied.
“This changing of the chemical bonds allows for different optical properties to emerge, similar to the differences between coal (amorphous) and diamond (crystalline),” says Christopher Roberts, another Lincoln Laboratory member of the research team. “While both materials are mostly carbon, they have vastly different optical properties.”
Currently, phase-change materials are used for industry applications, such as Blu-ray technology and rewritable DVDs, because their properties are useful for storing and erasing a large amount of information. But so far, no one has used them in infrared optics because they tend to be transparent in one state and opaque in the other. (Think of the diamond, which light can pass through, and coal, which light cannot penetrate.) If light cannot pass through one of the states, then that light cannot be adequately controlled for a range of uses; instead, a system would only be able to work like an on/off switch, allowing light to either pass through the material or not pass through at all.
However, the research team found that that by adding the element selenium to the original material (called GST), the material's absorption of infrared light in the crystalline phase decreased dramatically — in essence, changing it from an opaque coal-like material to a more transparent diamond-like one. What's more, the large difference in the refractive index of the two states affects the propagation of light through them.
“This change in refractive index, without introducing optical loss, allows for the design of devices that control infrared light without the need for mechanical parts,” Roberts says.
As an example, imagine a laser beam that is pointing in one direction and needs to be changed to another. In current systems, a large mechanical gimbal would physically move a lens to steer the beam to another position. A thin-film lens made of GSST would be able change positions by electrically reprogramming the phase-change materials, enabling beam steering with no moving parts.
The team has already tested the material successfully in a moving lens. They have also demonstrated its use in infrared hyperspectral imaging, which is used to analyze images for hidden objects or information, and in a fast optical shutter that was able to close in nanoseconds.
The potential uses for GSST are vast, and an ultimate goal for the team is to design reconfigurable optical chips, lenses, and filters, which currently must be rebuilt from scratch each time a change is required. Once the team is ready to move the material beyond the research phase, it should be fairly easy to transition it into the commercial space. Because it's already compatible with standard microelectronic fabrication processes, GSST components could be made at a low cost and in large numbers.
Recently, the laboratory obtained a combinatorial sputtering chamber — a state-of-the-art machine that allows researchers to create custom materials out of individual elements. The team will use this chamber to further optimize the materials for improved reliability and switching speeds, as well as for low-power applications. They also plan to experiment with other materials that may prove useful in controlling visible light.
The next steps for the team are to look closely into real-world applications of GSST and understand what those systems need in terms of power, size, switching speed, and optical contrast.
“The impact [of this research] is twofold,” Hu says. "Phase-change materials offer a dramatically enhanced refractive index change compared to other physical effects — induced by electric field or temperature change, for instance — thereby enabling extremely compact reprogrammable optical devices and circuits. Our demonstration of bistate optical transparency in these materials is also significant in that we can now create high-performance infrared components with minimal optical loss.” The new material, Hu continues, is expected to open up an entirely new design space in the field of infrared optics.
– Anne McGovern | Lincoln Laboratory
MIT News Office, January 30, 2020
Physics experiment with ultrafast laser pulses produces a previously unseen phase of matter.
|An artist's impression of a light-induced charge density wave (CDW). The wavy mesh represents distortions of the material’s lattice structure caused by the formation of CDWs. Glowing spheres represent photons. In the center, the original CDW is suppressed by a brief pulse of laser light, while a new CDW appears at right angles to the first. Image, Alfred Zong.|
Adding energy to any material, such as by heating it, almost always makes its structure less orderly. Ice, for example, with its crystalline structure, melts to become liquid water, with no order at all.
But in new experiments by physicists at MIT and elsewhere, the opposite happens: When a pattern called a charge density wave in a certain material is hit with a fast laser pulse, a whole new charge density wave is created — a highly ordered state, instead of the expected disorder. The surprising finding could help to reveal unseen properties in materials of all kinds.
The discovery is being reported today in the journal Nature Physics, in a paper by MIT professors Nuh Gedik and Pablo Jarillo-Herrero, postdoc Anshul Kogar, graduate student Alfred Zong, and 17 others at MIT, Harvard University, SLAC National Accelerator Laboratory, Stanford University, and Argonne National Laboratory.
The experiments made use of a material called lanthanum tritelluride, which naturally forms itself into a layered structure. In this material, a wavelike pattern of electrons in high- and low-density regions forms spontaneously but is confined to a single direction within the material. But when hit with an ultrafast burst of laser light — less than a picosecond long, or under one trillionth of a second — that pattern, called a charge density wave or CDW, is obliterated, and a new CDW, at right angles to the original, pops into existence.
This new, perpendicular CDW is something that has never been observed before in this material. It exists for only a flash, disappearing within a few more picoseconds. As it disappears, the original one comes back into view, suggesting that its presence had been somehow suppressed by the new one.
Gedik explains that in ordinary materials, the density of electrons within the material is constant throughout their volume, but in certain materials, when they are cooled below some specific temperature, the electrons organize themselves into a CDW with alternating regions of high and low electron density. In lanthanum tritelluride, or LaTe3, the CDW is along one fixed direction within the material. In the other two dimensions, the electron density remains constant, as in ordinary materials.
The perpendicular version of the CDW that appears after the burst of laser light has never before been observed in this material, Gedik says. It “just briefly flashes, and then it’s gone,” Kogar says, to be replaced by the original CDW pattern which immediately pops back into view.
Gedik points out that “this is quite unusual. In most cases, when you add energy to a material, you reduce order.”
“It’s as if these two [kinds of CDW] are competing — when one shows up, the other goes away,” Kogar says. “I think the really important concept here is phase competition.”
The idea that two possible states of matter might be in competition and that the dominant mode is suppressing one or more alternative modes is fairly common in quantum materials, the researchers say. This suggests that there may be latent states lurking unseen in many kinds of matter that could be unveiled if a way can be found to suppress the dominant state. That is what seems to be happening in the case of these competing CDW states, which are considered to be analogous to crystal structures because of the predictable, orderly patterns of their subatomic constituents.
Normally, all stable materials are found in their minimum energy states — that is, of all possible configurations of their atoms and molecules, the material settles into the state that requires the least energy to maintain itself. But for a given chemical structure, there may be other possible configurations the material could potentially have, except that they are suppressed by the dominant, lowest-energy state.
“By knocking out that dominant state with light, maybe those other states can be realized,” Gedik says. And because the new states appear and disappear so quickly, “you can turn them on and off,” which may prove useful for some information processing applications.
The possibility that suppressing other phases might reveal entirely new material properties opens up many new areas of research, Kogar says. “The goal is to find phases of material that can only exist out of equilibrium,” he says — in other words, states that would never be attainable without a method, such as this system of fast laser pulses, for suppressing the dominant phase.
Gedik adds that “normally, to change the phase of a material you try chemical changes, or pressure, or magnetic fields. In this work, we are using light to make these changes.”
The new findings may help to better understand the role of phase competition in other systems. This in turn can help to answer questions like why superconductivity occurs in some materials at relatively high temperatures, and may help in the quest to discover even higher-temperature superconductors. Gedik says, “What if all you need to do is shine light on a material, and this new state comes into being?”
The work was supported by the U.S. Department of Energy, SLAC National Accelerator Laboratory, the Skoltech-MIT NGP Program, the Center for Excitonics, and the Gordon and Betty Moore Foundation.
– David L. Chandler | MIT News Office
November 11, 2019
Study of minerals widely used in industrial processes could lead to discovery of new materials for catalysis and filtering.
Zeolites are a class of natural or manufactured minerals with a sponge-like structure, riddled with tiny pores that make them useful as catalysts or ultrafine filters. But of the millions of zeolite compositions that are theoretically possible, so far only about 248 have ever been discovered or made. Now, research from MIT helps explain why only this small subset has been found, and could help scientists find or produce more zeolites with desired properties.
The new findings are reported in the journal Nature Materials, in a paper by MIT graduate students Daniel Schwalbe-Koda and Zach Jensen, and professors Elsa Olivetti and Rafael Gomez-Bombarelli.
|Traditional structure-based representations of the many forms of zeolites, some of which are illustrated here, provide little guidance as to how they can convert to other forms, but a new graph-based system does a much better job. Illustration courtesy of the researchers.|
Previous attempts to figure out why only this small group of possible zeolite compositions has been identified, and to explain why certain types of zeolites can be transformed into specific other types, have failed to come up with a theory that matches the observed data. Now, the MIT team has developed a mathematical approach to describing the different molecular structures. The approach is based on graph theory, which can predict which pairs of zeolite types can be transformed from one to the other.
This could be an important step toward finding ways of making zeolites tailored for specific purposes. It could also lead to new pathways for production, since it predicts certain transformations that have not been previously observed. And, it suggests the possibility of producing zeolites that have never been seen before, since some of the predicted pairings would lead to transformations into new types of zeolite structures.
Zeolites are widely used today in applications as varied as catalyzing the “cracking” of petroleum in refineries and absorbing odors as components in cat litterbox filler. Even more applications may become possible if researchers can create new types of zeolites, for example with pore sizes suited to specific types of filtration.
All kinds of zeolites are silicate minerals, similar in chemical composition to quartz. In fact, over geological timescales, they will all eventually turn into quartz — a much denser form of the mineral — explains Gomez-Bombarelli, who is the Toyota Assistant Professor in Materials Processing. But in the meantime, they are in a “metastable” form, which can sometimes be transformed into a different metastable form by applying heat or pressure or both. Some of these transformations are well-known and already used to produce desired zeolite varieties from more readily available natural forms.
Currently, many zeolites are produced by using chemical compounds known as OSDAs (organic structure-directing agents), which provide a kind of template for their crystallization. But Gomez-Bombarelli says that if instead they can be produced through the transformation of another, readily available form of zeolite, “that’s really exciting. If we don’t need to use OSDAs, then it’s much cheaper [to produce the material]. The organic material is pricey. Anything we can make to avoid the organics gets us closer to industrial-scale production.
Traditional chemical modeling of the structure of different zeolite compounds, researchers have found, provides no real clue to finding the pairs of zeolites that can readily transform from one to the other.
Compounds that appear structurally similar sometimes are not subject to such transformations, and other pairs that are quite dissimilar turn out to easily interchange. To guide their research, the team used an artificial intelligence system previously developed by the Olivetti group to “read” more than 70,000 research papers on zeolites and select those that specifically identify interzeolite transformations. They then studied those pairs in detail to try to identify common characteristics.
What they found was that a topological description based on graph theory, rather than traditional structural modeling, clearly identified the relevant pairings. These graph-based descriptions, based on the number and locations of chemical bonds in the solids rather than their actual physical arrangement, showed that all the known pairings had nearly identical graphs. No such identical graphs were found among pairs that were not subject to transformation.
The finding revealed a few previously unknown pairings, some of which turned out to match with preliminary laboratory observations that had not previously been identified as such, thus helping to validate the new model. The system also was successful at predicting which forms of zeolites can intergrow — forming combinations of two types that are interleaved like the fingers on two clasped hands. Such combinations are also commercially useful, for example for sequential catalysis steps using different zeolite materials.
Ripe for further research
The new findings might also help explain why many of the theoretically possible zeolite formations don’t seem to actually exist. Since some forms readily transform into others, it may be that some of them transform so quickly that they are never observed on their own. Screening using the graph-based approach may reveal some of these unknown pairings and show why those short-lived forms are not seen.
Some zeolites, according to the graph model, “have no hypothetical partners with the same graph, so it doesn’t make sense to try to transform them, but some have thousands of partners” and thus are ripe for further research, Gomez-Bombarelli says.
In principle, the new findings could lead to the development of a variety of new catalysts, tuned to the exact chemical reactions they are intended to promote. Gomez-Bombarelli says that almost any desired reaction could hypothetically find an appropriate zeolite material to promote it.
“Experimentalists are very excited to find a language to describe their transformations that is predictive,” he says.
This work is “a major advancement in the understanding of interzeolite transformations, which has become an increasingly important topic owing to the potential for using these processes to improve the efficiency and economics of commercial zeolite production,” says Jeffrey Rimer, an associate professor of chemical and biomolecular engineering at the University of Houston, who was not involved in this research.
Manuel Moliner, a tenured scientist at the Technical University of Valencia, in Spain, who also was not connected to this research, says: “Understanding the pairs involved in particular interzeolite transformations, considering not only known zeolites but also hundreds of hypothetical zeolites that have not ever been synthesized, opens extraordinary practical opportunities to rationalize and direct the synthesis of target zeolites with potential interest as industrial catalysts.”
This research was supported, in part, by the National Science Foundation and the Office of Naval Research.
– David L. Chandler | MIT News Office
October 7, 2019
MIT Materials Research Laboratory 2019 Poster Session winners are Mechanical Engineering graduate student Erin Looney, Media Arts and Sciences graduate student Bianca Datta, and Materials Science and Engineering Postdoctoral Associate Michael Chon.
The Poster Session was held immediately after the Materials Day Symposium on Oct. 9, 2019. Winners, who were selected by non-MIT affiliated attendees, each receive a $500 award.
Media Arts and Sciences graduate student
POSTER: “Simulation-based optimization towards fabrication of bio-inspired nanostructures exhibiting structural coloration.”
Datta is using simulation techniques and rapid prototyping to design surfaces that display color like butterfly wings.
Advisor: Christine Ortiz, Morris Cohen Professor of Materials Science and Engineering
POSTER: “High capacity CMOS-compatible thin film batteries on flexible substrates”
Chon is developing all solid-state flexible microbatteries that combine a germanium anode, a ruthenium dioxide cathode and lithium phosphorous oxynitride (LiPON) solid electrolyte. The thin film batteries can be stacked and folded or incorporated directly into integrated circuits.
Advisor: Carl V. Thompson, Stavros Salapatas Professor of Materials Science and Engineering, and Director, Materials Research Laboratory
Erin E. Looney
Mechanical Engineering graduate student
POSTER: “Machine learning-based classification of environmental conditions for PV module testing and design”
Looney simulates solar cell material operation under real world conditions by combining temperature, solar spectra and humidity data to estimate performance with 95 percent accuracy. She showed that a statistical method called a k-means algorithm can produce these results with 1,000 times fewer data inputs.
Advisor: Tonio Buonassisi, Professor of Mechanical Engineering
– Materials Research Laboratory
Updated October 28, 2019