MIT Professor Frances M. Ross is pioneering new techniques to study materials growth and how structure relates to performance.

A hundred years ago, “2d” meant a two-penny, or 1-inch, nail. Today, “2D” encompasses a broad range of atomically thin flat materials, many with exotic properties not found in the bulk equivalents of the same materials, with graphene – the atomically thin form of carbon – perhaps the most prominent. While many researchers at MIT and elsewhere are exploring 2D materials and their special properties, Frances M. Ross, the Ellen Swallow Richards Professor in Materials Science and Engineering, is interested in what happens when these 2D materials and ordinary 3D materials come together.

Research Highlight:
Frances M. Ross

“We’re interested in the interface between a 2D material and a 3D material because every 2D material that you want to use in an application, such as an electronic device, still has to talk to the outside world, which is three-dimensional,” Ross says.

“We’re at an interesting time because there are immense developments in instrumentation for electron microscopy, and there is great interest in materials with very precisely controlled structures and properties, and these two things cross in a fascinating way,” says Ross.

“The opportunities are very exciting,” Ross says. “We’re going to be really improving the characterization capabilities here at MIT.” Ross specializes in examining how nanoscale materials grow and react in both gases and liquid media, by recording movies using electron microscopy. Microscopy of reactions in liquids is particularly useful for understanding the mechanisms of electrochemical reactions that govern the performance of catalysts, batteries, fuel cells, and other important technologies. “In the case of liquid phase microscopy, you can also look at corrosion where things dissolve away, while in gases you can look at how individual crystals grow or how materials react with, say, oxygen,” she says.

Ross joined the Department of Materials Science and Engineering (DMSE) faculty last year, moving from the Nanoscale Materials Analysis department at the IBM Thomas J. Watson Research Center. “I learned a tremendous amount from my IBM colleagues and hope to extend our research in material design and growth in new directions,” she says.

Recording movies

During a recent visit to her lab, Ross explained an experimental set up donated to MIT by IBM. An ultra-high vacuum evaporator system arrived first, to be attached later directly onto a specially designed transmission electron microscope. “This gives powerful possibilities,” Ross explains. “We can put a sample in the vacuum, clean it, do all sorts of things to it such as heating and adding other materials, then transfer it under vacuum into the microscope, where we can do more experiments while we record images. So we can, for example, deposit silicon or germanium, or evaporate metals, while the sample is in the microscope and the electron beam is shining through it, and we are recording a movie of the process.”

While waiting this spring for the transmission electron microscope to be set up, members of Ross’s seven-member research group, including Materials Science and Engineering Postdoc Shu Fen Tan and graduate student Kate Reidy, made and studied a variety of self-assembled structures. The evaporator system was housed temporarily on the fifth level prototyping space of MIT.nano while Ross’s lab was being readied in Building 13. “MIT.nano had the resources and space; we were happy to be able to help,” says Anna Osherov, MIT.nano Assistant Director of User Services.

“All of us are interested in this grand challenge of materials science, which is how do you make a material with the properties you want and, in particular, how do you use nanoscale dimensions to tweak the properties, and create new properties, that you can’t get from bulk materials,” Ross says.

Using the ultra-high vacuum system, graduate student Kate Reidy formed structures of gold and niobium on several 2D materials. “Gold loves to grow into little triangles,” Ross notes. “We’ve been talking to people in physics and materials science about which combinations of materials are the most important to them in terms of controlling the structures and the interfaces between the components in order to give some improvement in the properties of the material,” she notes.

Postdoc Shu Fen Tan synthesized nickel-platinum nanoparticles and examined them using another technique, liquid cell electron microscopy. She could arrange for only the nickel to dissolve, leaving behind spiky skeletons of platinum. “Inside the liquid cell, we are able to see this whole process at high spatial and temporal resolutions,” Tan says. She explains that platinum is a noble metal and less reactive than nickel, so under the right conditions the nickel participates in an electrochemical dissolution reaction and the platinum is left behind.

Platinum is a well-known catalyst in organic chemistry and fuel cell materials, Tan notes, but it is also expensive, so finding combinations with less expensive materials such as nickel is desirable.

“This is an example of the range of materials reactions you can image in the electron microscope using the liquid cell technique,” Ross says. “You can grow materials; you can etch them away; you can look at, for example, bubble formation and fluid motion.”

A particularly important application of this technique is to study cycling of battery materials. “Obviously, I can’t put an AA battery in here, but you could set up the important materials inside this very small liquid cell and then you can cycle it back and forth and ask, if I charge and discharge it 10 times, what happens? It does not work just as well as before - how does it fail?” Ross asks. “Some kind of failure analysis and all the intermediate stages of charging and discharging can be observed in the liquid cell.”

“Microscopy experiments where you see every step of a reaction give you a much better chance of understanding what’s going on,” Ross says.

Moiré patterns

Graduate student Reidy is interested in how to control the growth of gold on 2D materials such as graphene, tungsten diselenide and molybdenum disulfide. When she deposited gold on “dirty” graphene, blobs of gold collected around the impurities. But when Reidy grew gold on graphene that had been heated and cleaned of impurities, she found perfect triangles of gold. Depositing gold on both the top and bottom sides of clean graphene, Reidy saw in the microscope features known as moiré patterns, which are caused when the overlapping crystal structures are out of alignment.

The gold triangles may be useful as photonic and plasmonic structures. “We think this could be important for a lot of applications, and it is always interesting for us to see what happens,” Reidy says. She is planning to extend her clean growth method to form 3D metal crystals on stacked 2D materials with various rotation angles and other mixed layer structures. Reidy is interested in the properties of graphene and hexagonal boron nitride (hBN), as well as two materials that are semiconducting in their 2D single layer form, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2). “One aspect that’s very interesting in the 2D materials community is the contacts between 2D materials and 3D metals,” Reidy says. “If they want to make a semiconducting device or a device with graphene, the contact could be ohmic for the graphene case or a Schottky contact for the semiconducting case, and the interface between these materials is really, really important.”

“You can also imagine devices using the graphene just as a spacer layer between two other materials,” Ross adds.

For device makers, Reidy says it is sometimes important to have a 3D material grow with its atomic arrangement aligned perfectly with the atomic arrangement in the 2D layer beneath. This is called epitaxial growth. Describing an image of gold grown together with silver on graphene, Reidy explains, “We found that silver doesn’t grow epitaxially, it doesn’t make those perfect single crystals on graphene that we wanted to make, but by first depositing the gold and then depositing silver around it, we can almost force silver to go into an epitaxial shape because it wants to conform to what its gold neighbors are doing.”

Electron microscope images can also show imperfections in a crystal such as rippling or bending, Reidy notes. “One of the great things about electron microscopy is that it is very sensitive to changes in the arrangement of the atoms,” Ross says. “You could have a perfect crystal and it would all look the same shade of gray, but if you have a local change in the structure, even a subtle change, electron microscopy can pick it up. Even if the change is just within the top few layers of atoms without affecting the rest of the material beneath, the image will show distinctive features that allow us to work out what’s going on.”

Reidy also is exploring the possibilities of combining niobium - a metal that is superconducting at low temperatures - with a 2D topological insulator, bismuth telluride. Topological insulators have fascinating properties whose discovery resulted in the Nobel Prize in Physics in 2016. “If you deposit niobium on top of bismuth telluride, with a very good interface, you can make superconducting junctions. We’ve been looking into niobium deposition, and rather than triangles we see structures that are more dendritic looking,” Reidy says. Dendritic structures look like the frost patterns formed on the inside of windows in winter or the feathery patterns of some ferns. Changing the temperature and other conditions during the deposition of niobium can change the patterns that the material takes.

All the researchers are eager for new electron microscopes to arrive at MIT.nano to give further insights into the behavior of these materials. “Many things will happen within the next year, things are ramping up already, and I have great people to work with. One new microscope is being installed now in MIT.nano and another will arrive next year. The whole community will see the benefits of improved microscopy characterization capabilities here,” Ross says.

MIT.nano’s Osherov notes that two cryogenic transmission electron microscopes (cryo-TEM) are installed and running. “Our goal is to establish a unique microscopy-centered community. We encourage and hope to facilitate a cross-pollination between the cryo-EM researchers, primarily focused on biological applications, and ‘soft’ material as well as other research communities across campus,” she says. The latest addition of a scanning transmission electron microscope with enhanced analytical capabilities (ultrahigh energy resolution monochromator, 4D STEM detector, Super-X EDS detector, tomography, and several in situ holders) brought in by John Chipman Associate Professor of Materials Science and Engineering James M. LeBeau, once installed, will substantially enhance the microscopy capabilities of the MIT campus. “We consider Professor Ross to be an immense resource for advising us in how to shape the in situ approach to measurements using the advanced instrumentation that will be shared and available to all the researchers within the MIT community and beyond,” Osherov says.

Little drinking straws

“Sometimes you know more or less what you are going to see during a growth experiment, but very often there’s something that you don’t expect,” Ross says. She shows an example of zinc oxide nanowires that were grown using a germanium catalyst. Some of the long crystals have a hole through their centers, creating structures which are like little drinking straws, circular outside but with a hexagonally shaped interior. “This is a single crystal of zinc oxide, and the interesting question for us is why do the experimental conditions create these facets inside, while the outside is smooth?” Ross asks. “Metal oxide nanostructures have so many different applications, and each new structure can show different properties. In particular, by going to the nanoscale you get access to a diverse set of properties.”

“Ultimately, we’d like to develop techniques for growing well-defined structures out of metal oxides, especially if we can control the composition at each location on the structure,” Ross says. A key to this approach is self-assembly, where the material builds itself into the structure you want without having to individually tweak each component. “Self-assembly works very well for certain materials but the problem is that there’s always some uncertainty, some randomness or fluctuations. There’s poor control over the exact structures that you get. So the idea is to try to understand self-assembly well enough to be able to control it and get the properties that you want,” Ross says.

“We have to understand how the atoms end up where they are, then use that self-assembly ability of atoms to make a structure we want. The way to understand how things self-assemble is to watch them do it and that requires movies with high spatial resolution and good time resolution,” Ross explains. Electron microscopy can be used to acquire structural and compositional information and can even measure strain fields or electric and magnetic fields. “Imagine recording all of these things, but in a movie where you are also controlling how materials grow within the microscope. Once you have made a movie of something happening, you analyze all the steps of the growth process and use that to understand which physical principles were the key ones that determined how the structure nucleated and evolved and ended up the way it does.”

Future directions

Ross hopes to bring in a unique high-resolution, high vacuum TEM with capabilities to image materials growth and other dynamic processes. She intends to develop new capabilities for both water-based and gas-based environments. This custom microscope is still in the planning stages but will be situated in one of the rooms in the Imaging Suite in MIT.nano.

“Professor Ross is a pioneer in this field,” Osherov says. “The majority of TEM studies to-date have been static rather than dynamic. With static measurements you are observing a sample at one particular snapshot in time, so you don’t gain any information about how it was formed. Using dynamic measurements, you can look at the atoms hopping from state to state until they find the final position. The ability to observe self-assembling processes and growth in real-time provides valuable mechanistic insights. We’re looking forward to bringing these advanced capabilities to MIT.nano.” she says.

“Once a certain technique is disseminated to the public, it brings attention,” Osherov says. “When results are published, researchers expand their vision of experimental design based on available state-of-the-art capabilities, leading to many new experiments that will be focused on dynamic applications.”

Rooms in MIT.nano feature the quietest space on the MIT campus, designed to reduce vibrations and electromagnetic interference to as low a level as possible. “There is space available for Professor Ross to continue her research and to develop it further,” Osherov says. “The ability of in situ monitoring the formation of matter and interfaces will find applications in multiple fields across campus, and lead to a further push of the conventional electron microscopy limits.”

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Denis Paiste, Materials Research Laboratory
August 26, 2019

 

 

MIT researchers have developed a new technique to reveal the uncharted dynamics of electrons in materials.
Time resolved XUV ARPES
Time-resolved XUV ARPES setup developed by researchers in MIT Professor Nuh Gedik’s research group at MIT. The researchers use infrared (IR) light pulses to “pump” the electrons to the excited state and XUV (extreme ultraviolet) light pulses to “probe” the photoemitted electrons after a time delay. Their new technique enables full access to the electronic band structure of all materials with unprecedentedly narrow energy resolution on femtosecond timescales. Image, Edbert Jarvis Sie/Nature Communications.

A new technique developed by a team at MIT can map the complete electronic band structure of materials at high resolution. This capability is usually exclusive to large synchrotron facilities but now it is available as a tabletop laser-based setup at MIT. This technique, which uses extreme ultraviolet (XUV) laser pulses to measure the dynamics of electrons via angle-resolved photoemission spectroscopy (ARPES), is called time-resolved XUV ARPES.

Unlike the synchrotron-based setup, this laser-based setup further provides a time-resolved feature to watch the electrons inside a material on a very fast, femtosecond (quadrillionth of a second) timescale. Comparing this fast technique on a time and distance scale, while light can travel in roughly one second from the Moon to the Earth, in one femtosecond, light can only travel as far as the thickness of a single sheet of regular copy paper.

The MIT team evaluated their instrument resolution using four exemplary materials representing a wide spectrum of quantum materials: topological Weyl semimetal (WTe2), high critical temperature superconductor (Bi2Sr2CaCu2O8+δ), layered semiconductor (WSe2), and charge density wave system (TiSe2).

The technique is described in a paper appearing in the journal Nature Communications, authored by MIT physicists Edbert Jarvis Sie, PhD ’17, former postdoc Timm Rohwer, Changmin Lee, PhD ’18, and MIT Physics Professor Nuh Gedik.

A central goal of modern condensed matter physics is to discover novel phases of matter and exert control over their intrinsic quantum properties. Such behaviors are rooted in the way the energy of the electrons change as a function of their momentum inside different materials. This relationship is known as the electronic band structure of materials and can be measured using photoemission spectroscopy. This technique uses light with high photon energy to knock the electrons away from the material surface – a process formerly known as the photoelectric effect for which Albert Einstein received the Nobel Prize in Physics in 1921. Nowadays, the speed and direction of the outgoing electrons can be measured in an angle-resolved manner to determine the energy and momentum relationship inside the material.

The collective interaction between electrons in these materials often goes beyond textbook predictions. One method to study such non-conventional interactions is by promoting the electrons to higher energy levels and watching how they relax back to the ground state. This is called a pump-and-probe method, which basically is the same method people use in their everyday lives to perceive new objects around them. For example, anyone can drop a pebble on the surface of water and watch how the ripples decay to observe the surface tension and acoustics of water. The difference in the MIT setup is that the researchers use infrared (IR) light pulses to “pump” the electrons to the excited state and the XUV light pulses to “probe” the photoemitted electrons after a time delay.

Time- and angle-resolved photoemission spectroscopy (trARPES) captures movies of the electronic band structure of the solid with femtosecond time resolution. This technique provides invaluable insights into the electron dynamics which is crucial to understand the properties of the materials. However, it has been difficult to access high momenta electrons with narrow energy resolution via laser-based ARPES, severely constraining the type of phenomena that can be studied with this technique.

The newly developed XUV trARPES setup at MIT, which is about 9 to 10 feet long, can generate a femtosecond extreme ultraviolet light source at high energy resolution. “XUV will be quickly absorbed by air, so we house the optics in vacuum,” Sie says, “Every component from the light source to the sample chamber is projected on the computer drawing on a millimeter precision.” This technique enables full access to the electronic band structure of all materials with unprecedentedly narrow energy resolution on femtosecond timescales. “To demonstrate the resolution of our setup, it is not sufficient to measure the resolution of the light source alone,” Sie says, “We must verify the true resolutions from real photoemission measurements using a wide range of materials – the results are very satisfying!”

The final assembly of the MIT setup comprises several emerging instruments that are being developed concurrently in industry: femtosecond XUV light source (XUUS) from KMLabs, XUV monochromator (OP-XCT) from McPherson, and angle-resolved time-of-flight (ARToF) electron analyzer from Scienta Omicron. “We believe that this technique has the potential to push the boundary of condensed matter physics,” Gedik says, “So we worked with relevant companies to achieve this spearheading capability.” 

Edbert Jarvis Sie (right) and Timm Rohwer (left) in the Gedik research group with the diagnostic chamber from their time-resolved XUV ARPES setup.
Edbert Jarvis Sie (right) and Timm Rohwer (left) of the Gedik research group with the diagnostic chamber from their time-resolved XUV ARPES setup. The MIT researchers worked with state-of-the-art scientific equipment makers to complete their setup and push the boundary of condensed matter physics. Photo, Ilkem Ozge Ozel.

The MIT setup can accurately measure the energy of electrons with high momenta. “The combination of time-of-flight electron analyzer and XUV femtosecond light source gives us the ability to measure the complete band structure of almost all materials,” Rohwer says, “Unlike some other setups, we don’t have to repeatedly tilt the sample to map the band structure – and this saves us a lot of time!”

Another significant advance is the ability to change the photon energy. “Photoemission intensity often varies significantly with the photon energy used in the experiment. This is because the photoemission cross section depends on the orbital character of the elements forming the solid,” Lee says. “The photon energy tunability provided by our setup is extremely useful in enhancing the photoemission counts of particular bands that we are interested in.”

Stanford Institute for Materials and Energy Science Staff Scientist Dr. Patrick S. Kirchmann, who is an expert in ARPES techniques, says, “as a practitioner I believe that trARPES is profoundly useful. Any quantum material, topological insulator, or superconductivity question profits from understanding the band structure in non-equilibrium.”

“The basic idea of trARPES is simple: by detecting the emission angle and energy of photoemitted electrons we can record the electronic band structure. Done after exciting the sample with light we can record changes of the band structure that provide us with ‘electron movies,’ which are filmed at frame rates of their natural femtosecond time scale,” Kirchmann explains.

Commenting on the Gedik research group’s new findings at MIT, Kirchmann says, “The work of Sie and Gedik sets a new standard by achieving 30 meV (milli electron volt) bandwidth while maintaining 200 femtosecond time resolution. By incorporating exchangeable gratings in their setup, it will also be possible to change that partitioning of the time-bandwidth product. These achievements will enable long-needed high-definition studies of quantum materials with high enough energy resolution to provide profound insights."

The work was supported by the U.S. Department of Energy, Army Research Office, and the Gordon and Betty Moore Foundation.

back to newsletter– Materials Research Laboratory | MIT News
August 19, 2019

 

Noninvasive device could benefit patients with kidney disease, congestive heart failure, or dehydration.
MIT researchers have developed a noninvasive hydration sensor that is based on the same technology as MRI, but, unlike MRI scanners, it can fit in a doctor’s office.  Lina Colucci, Andrew Hall, image.
MIT researchers have developed a noninvasive hydration sensor that is based on the same technology as MRI, but, unlike MRI scanners, it can fit in a doctor’s office. Image: Lina Colucci, Andrew Hall.

For patients with kidney failure who need dialysis, removing fluid at the correct rate and stopping at the right time is critical. This typically requires guessing how much water to remove and carefully monitoring the patient for sudden drops in blood pressure.

Currently there is no reliable, easy way to measure hydration levels in these patients, who number around half a million in the United States. However, researchers from MIT and Massachusetts General Hospital have now developed a portable sensor that can accurately measure patients’ hydration levels using a technique known as nuclear magnetic resonance (NMR) relaxometry.

Such a device could be useful for not only dialysis patients but also people with congestive heart failure, as well as athletes and elderly people who may be in danger of becoming dehydrated, says Michael Cima, the David H. Koch Professor of Engineering in MIT’s Department of Materials Science and Engineering.

“There’s a tremendous need across many different patient populations to know whether they have too much water or too little water,” says Cima, who is the senior author of the study and a member of MIT’s Koch Institute for Integrative Cancer Research. “This is a way we could measure directly, in every patient, how close they are to a normal hydration state.”

The portable device is based on the same technology as magnetic resonance imaging (MRI) scanners but can obtain measurements at a fraction of the cost of MRI, and in much less time, because there is no imaging involved.

Lina Colucci, a former graduate student in health sciences and technology, is the lead author of the paper, which appears in the July 24 issue of Science Translational Medicine. Other authors of the paper include MIT graduate student Matthew Li; MGH nephrologists Kristin Corapi, Andrew Allegretti, and Herbert Lin; MGH research fellow Xavier Vela Parada; MGH Chief of Medicine Dennis Ausiello; and Harvard Medical School assistant professor in radiology Matthew Rosen.

Hydration status

Cima began working on this project about 10 years ago, after realizing that there was a critical need for an accurate, noninvasive way to measure hydration.

Currently, the available methods are either invasive, subjective, or unreliable. Doctors most frequently assess overload (hypervolemia) by a few physical signs such as examining the size of the jugular vein, pressing on the skin, or examining the ankles where water might pool.

The MIT team decided to try a different approach, based on NMR. Cima had previously launched a company called T2 Biosystems that uses small NMR devices to diagnose bacterial infections by analyzing patient blood samples. One day, he had the idea to use the devices to try to measure water content in tissue, and a few years ago, the researchers got a grant from the MIT-MGH Strategic Partnership to do a small clinical trial for monitoring hydration. They studied both healthy controls and patients with end-stage renal disease who regularly underwent dialysis.

One of the main goals of dialysis is to remove fluid in order bring patients to their “dry weight,” which is the weight at which their fluid levels are optimized. Determining a patient’s dry weight is extremely challenging, however. Doctors currently estimate dry weight based on physical signs as well as through trial-and-error over multiple dialysis sessions.

The MIT/MGH team showed that quantitative NMR, which works by measuring a property of hydrogen atoms called T2 relaxation time, can provide much more accurate measurements. The T2 signal measures both the environment and quantity of hydrogen atoms (or water molecules) present.

“The beauty of magnetic resonance compared to other modalities for assessing hydration is that the magnetic resonance signal comes exclusively from hydrogen atoms. And most of the hydrogen atoms in the human body are found in water molecules,” Colucci says.

The researchers used their device to measure fluid volume in patients before and after they underwent dialysis. The results showed that this technique could distinguish healthy patients from those needing dialysis with just the first measurement. In addition, the measurement correctly showed dialysis patients moving closer to a normal hydration state over the course of their treatment.

Furthermore, the NMR measurements were able to detect the presence of excess fluid in the body before traditional clinical signs — such as visible fluid accumulation below the skin — were present. The sensor could be used by physicians to determine when a patient has reached their true dry weight, and this determination could be personalized at each dialysis treatment.

Better monitoring

The researchers are now planning additional clinical trials with dialysis patients. They expect that dialysis, which currently costs the United States more than $40 billion per year, would be one of the biggest applications for this technology. This kind of monitoring could also be useful for patients with congestive heart failure, which affects about 5 million people in the United States.

“The water retention issues of congestive heart failure patients are very significant,” Cima says. “Our sensor may offer the possibility of a direct measure of how close they are to a normal fluid state. This is important because identifying fluid accumulation early has been shown to reduce hospitalization, but right now there are no ways to quantify low-level fluid accumulation in the body. Our technology could potentially be used at home as a way for the care team to get that early warning.”
Sahir Kalim, a nephrologist and assistant professor of medicine at Massachusetts General Hospital, described the MIT approach as “highly novel.”

“The development of a bedside device that can accurately inform providers about how much fluid a patient should ideally have removed during their dialysis treatment would likely be one of the most significant developments in dialysis care in many years,” says Kalim, who was not involved in the study. “Colucci and colleagues have made a promising innovation that may one day yield this impact.”

In their study of the healthy control subjects, the researchers also incidentally discovered that they could detect dehydration. This could make the device useful for monitoring elderly people, who often become dehydrated because their sense of thirst lessens with age, or athletes taking part in marathons or other endurance events. The researchers are planning future clinical trials to test the potential of their technology to detect dehydration.

The research was funded by the MGH-MIT Strategic Partnership Grand Challenge, the Air Force Medical Services/Institute of Soldier Nanotechnologies, the National Science Foundation Graduate Research Fellowships Program, the National Institute of Biomedical Imaging and Bioengineering, the Koch Institute Support (core) Grant from the National Cancer Institute, and Harvard University.

back to newsletterAnne Trafton | MIT News Office 
July 24, 2019

Tuesday, 23 July 2019 15:15

New insights into bismuth’s character

Theorists at MIT and collaborators uncover hidden topological insulator states in bismuth crystals.
A crystal of bismuth has a staircase-like appearance because of the repeating honeycomb-like structure of its atoms. Researchers at MIT and colleagues have conducted a theoretical analysis to reveal several previously unidentified topological properties of bismuth.
A crystal of bismuth has a staircase-like appearance because of the repeating honeycomb-like structure of its atoms. Researchers at MIT along with colleagues in Boston, Singapore and Taiwan have conducted a theoretical analysis to reveal several previously unidentified topological properties of bismuth. One of these topological properties makes bismuth a robust electronic conductor along its edges where horizontal and vertical faces meet, which physicists call a hinge state. Image, Denis Paiste, Materials Research Laboratory.

The search for better materials for computers and other electronic devices has focused on a group of materials known as “topological insulators” that have a special property of conducting electricity on the edge of their surfaces like traffic lanes on a highway. This can increase energy efficiency and reduce heat output.

The first experimentally demonstrated topological insulator in 2009 was bismuth-antimony, but only recently did researchers identify pure bismuth as a new type of topological insulator. A group of researchers in Europe and the U.S. provided both experimental evidence and theoretical analysis in a 2018 Nature Physics report.

Now, researchers at MIT along with colleagues in Boston, Singapore and Taiwan have conducted a theoretical analysis to reveal several more previously unidentified topological properties of bismuth. The team was led by senior authors MIT Associate Professor of Physics Liang Fu, MIT Professor of Physics Nuh Gedik, Northeastern University Distinguished Professor Arun Bansil (Physics) and Research Fellow Hsin Lin at Academica Sinica (Taiwan).

“It’s kind of a hidden topology where people did not know that it can be that way,” says MIT Postdoc Su-Yang Xu, a coauthor of the paper published recently in PNAS.

Topology is a mathematical tool that physicists use to study electronic properties by analyzing electrons’ quantum wave functions. The “topological” properties give rise to a high degree of stability in the material and make its electronic structure very robust against minor imperfections in the crystal such as impurities or minor distortions of its shape such as stretching or squeezing.

“Let’s say I have a crystal that has imperfections. Those imperfections, as long as they are not so dramatic, then my electrical property will not change,” Xu explains. “If there is such topology and if the electronic properties are uniquely tied to the topology rather than the shape, then it will be very robust.”

“In this particular compound, unless you somehow apply pressure or something to distort the crystal structure, otherwise this conduction will always be protected,” Xu says.

Since the electrons carrying a certain spin can only move in one direction in these topological materials, they cannot bounce backwards or scatter, which is the behavior that makes silicon and copper based electronic devices heat up.

While materials scientists seek to identify materials with fast electrical conduction and low heat output for advanced computers, physicists want to classify the types of topological and other properties that underlie these better performing materials.

In the new paper, “Topology on a new facet of bismuth,” the authors calculated that bismuth should show a state known as a “Dirac surface state,” which is considered a hallmark of these topological insulators. They found that the crystal is unchanged by a half circle rotation (180 degrees). This is called a twofold rotational symmetry. Such a twofold rotational symmetry protects the Dirac surface states. If this twofold rotation symmetry of the crystal is disrupted, these surface states lose their topological protection.

Bismuth also features a topological state along certain edges of the crystal where two vertical and horizontal faces meet called a “hinge” state. To fully realize the desired topological effects in this material, the hinge state and other surface states must be coupled to another electronic phenomenon known as “band inversion” that the theorists’ calculations show also is present in bismuth. They predict that these topological surface states could be confirmed by using an experimental technique known as photoemission spectroscopy.

If electrons flowing through copper are like a school of fish swimming through a lake in summer, electrons flowing across a topological surface are more like ice skaters crossing the lake’s frozen surface in winter. For bismuth, however, in the hinge state, their motion would be more akin to skating on the corner edge of an ice cube.

The researchers also found that in the hinge state, as the electrons’ move forward, their momentum and another property called spin, which defines a clockwise or counterclockwise rotation of the electrons, is “locked.” “Their direction of spinning is locked with respect to their direction of motion,” Xu explains.

These additional topological states might help explain why bismuth lets electrons travel through it in much farther than most other materials and why it conducts electricity efficiently with many fewer electrons than materials such as copper.

“If we really want to make these things useful and significantly improve the performance of our transistors, we need to find good topological materials, good in terms of they are easy to make, they are not toxic, and also they are relatively abundant on earth,” Xu suggests. Bismuth, which is an element that is safe for human consumption in the form of remedies to treat heartburn, for example, meets all these requirements.

“This work is a culmination of a decade and a half’s worth of advancement in our understanding of symmetry protected topological materials,” says David Hsieh, Professor of Physics at Caltech, who was not involved in this research.

“I think that these theoretical results are robust and it is simply a matter of experimentally imaging them using techniques like angle-resolved photoemission spectroscopy, which Prof. Gedik is an expert in,” Hsieh adds.

Northeastern University Professor of Physics Gregory Fiete notes that “Bismuth-based compounds have long played a starring role in topological materials, though bismuth itself was originally believed to be topologically trivial.”

“Now, this team has discovered that pure bismuth is multiply topological with a pair of surface Dirac cones untethered to any particular momentum value,” says Fiete, who also was not involved in this research. “The possibility to move the Dirac cones through external parameter control may open the way to applications that exploit this feature."

Caltech Prof. Hsieh notes that the new findings add to the numbers of ways that topologically protected metallic states can be stabilized in materials. “If bismuth can be turned from semimetal into insulator, then isolation of these surface states in electrical transport can be realized, which may be useful for low power electronics applications,” Hsieh explains.

Also contributing to the bismuth topology paper were MIT Postdoc Qiong Ma; Tay-Rong Chang of the Department of Physics, National Cheng Kung University, Taiwan, and the Center for Quantum Frontiers of Research & Technology, Taiwan; Xiaoting Zhou, Department of Physics, National Cheng Kung University, Taiwan; and Chuang-Han Hsu, Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore.

This work was partly supported by the Center for Integrated Quantum Materials and the U.S. Department of Energy, Materials Sciences and Engineering division.

back to newsletterDenis Paiste, Materials Research Laboratory
July 29, 2019

The incubator’s winding journey to success helped its startup community grow closer while addressing environmental challenges.

Greentown Labs is the largest clean technology incubator in North America, a fact that’s easy to accept when you walk inside. The massive, open entrance of Greentown’s Somerville, Massachusetts, headquarters gives visitors the impression they’ve entered the office of one of Greater Boston’s most successful tech companies.

North America's largest clean technology incubator

Beyond the modern entryway are smaller working spaces — some cluttered with startup prototypes, others lined with orderly lab equipment — to enable foundational, company-building experiments.

In addition to the space and equipment, Greentown offers startups equity-free legal, information technology, marketing, and sales support, and a coveted network of corporations and industry investors.

But what many entrepreneurs say they like most about Greentown is the people.

“Greentown offers a lot of different things, but first and foremost among them is a community of entrepreneurs who are striving to solve big challenges in climate, energy, and the environment,” says Greentown Labs CEO Emily Reichert MBA ’12.

Greentown is full of stories of peers bumping into each other in the kitchen only to find they’re struggling with similar problems or, even better, that one of them already grappled with the problem and found a solution.

MIT has played a pivotal role in Greentown’s success since its inception. Reichert estimates about 60 percent of Greentown’s more than 90 current startups were founded by MIT alumni.

The current version of Greentown looks like the result of some well-funded, grand vision set forth long ago. But Greentown’s rise was every bit as spontaneous — and tenuous — as the early days of any startup.

A space for building

In 2010, Sorin Grama SM ’07 and Sam White were looking for office space to work on a new chiller design for their startup, Promethean Power Systems, which still develops off-grid refrigeration systems in India. They needed a place to build the big, leaky refrigeration prototypes they’d thought up. It also needed to be close to MIT, where the company founders connected with advisors and interns.

Eventually, White found “a dilapidated warehouse” on Charles Street in Cambridge for the right price. What the space lacked in beauty it made up for in size, so the founders decided to use an MIT email list to see if other founders would like to join them. Some founders building an app were first to respond. Their first reaction was to ask White and Grama to clean up a bit, and they were politely shown the door.

Without exactly intending to, Grama and White had made their warehouse a builder space. Over the next week, a few more founders came in, including Jason Hanna, the co-founder of building efficiency company Embue; Jeremy Pitts SM ’10, MBA ’10, who was creating more efficient compressor systems for the oil and gas industry as the founder of Oscomp Systems; and Adam Rein MBA ’10 and Ben Glass ’07 SM ’10, whose company Altaeros was building airborne wind turbines. The warehouse looked perfect to them.

“What we all had in common was we just needed a space to prototype and build stuff, where we could spill stuff, make noise, and share tools,” Grama says. “Pretty quickly it became a nice band of startups that appreciated the same thing.”

The winter of 2010-2011 was a freezing one in the warehouse, made worse by icy cement floors, but the founders couldn’t help but notice the benefits of working together. Any time an intern or investor came to see one company, they were introduced to the others. Founders with expertise in areas like grant writing or funding rounds would give lunchtime presentations to help the others.

Rein remembers thinking he was in the perfect environment to succeed despite the sometimes comical dysfunction of the space. One day an official with the United States Agency for International Development (USAID) stopped by to evaluate one of the startups for a grant. The visit went well enough — until she got locked in the bathroom. The founders eventually got her out, but they didn’t think the incident boded for their chances of getting that grant.

When the landlord kicked them out of Charles Street, they found a similar space in South Boston, recruiting friends and employees to help strip wires, scrape walls, and paint over the course of a week. Rein recalls his regular duties included ordering toilet paper for the building.

The space was also twice as large as the one in Cambridge, so as Greentown’s reputation spread throughout 2011, five startups became 15, then 20.

“It really took on a life of its own,” Grama says.

Among the curious MIT students who journeyed to Greentown that year was Reichert. Having worked as a chemist for 10 years in spotless, safety-certified labs before coming to MIT, she was shocked to see the condition of Greentown.
“The first time I walked in I had two gut reactions,” Reichert says. “The first was I felt this amazing energy and passion, and kind of a buzzing. If you walk into Greentown today you still feel those things. The second was, ‘Oh my god, this place is a death trap.’”

After earning her MBA, Reichert initially helped out as a consultant at Greentown. By February of 2013, she joined Greentown to run it full time. It was a critical time for the growing co-op: White and Grama were getting ready to move to India to work on Promethean, and Hanna, who had primarily led Greentown to that point, was expecting the birth of his first child.

At the same time, real estate prices in South Boston were skyrocketing, and Greentown was again being forced to move.

Reichert, who worked as CEO without a salary for more than a year, remembers those first six months on the job as the most stressful of her life. With no money to put toward a new space, she was able to partner with the City of Somerville to secure some funding and find a new location. Reichert signed a construction contract to renovate the Somerville space before she knew where the money would come from, and began lobbying state and corporate officials for sponsorships.

She still remembers the day Greentown was to be evicted from South Boston, with everyone scrambling to clean out the cluttered warehouse and a few determined founders running one last experiment until 7 p.m. before throwing the last of the equipment in a U-Haul truck and beginning the next phase of Greentown’s journey.

Growing up

Within 15 months of the move to Somerville, Greentown’s 40,000 square feet were completely filled and Reichert began the process of expanding the headquarters. Today, Greentown’s three buildings make up more than 100,000 square feet of prototyping, office, and event space and feature a wet lab, electronics lab, and machine shop.

Since its inception, Greentown has supported more than 200 startups that have created around 2,800 jobs, many in the Boston area. The original founders still serve on Greentown’s board of directors, ensuring every dollar Greentown makes goes toward supporting startups.

Of the founding companies, only Promethean and Altaeros are still housed in Greentown, although they’re all still operating in some form.

“We probably should’ve moved out, but it’s important to work in a place you really enjoy,” Rein says of Altaeros.

Grama, meanwhile, has come full circle. After ceding the reigns of Promethean and returning from India, last year he started another company, Transaera, that’s developing efficient, environmentally friendly cooling systems based on research from MIT.

This time, it took him a lot less time to find office space.

back to newsletter– Zach Winn | MIT News Office
June 25, 2019

MIT’s Senthil Todadri and Xiao-Gang Wen will study highly entangled quantum matter in a collaboration supported by the Simons Foundation.
An artistic impression depicts ultra-quantum matter: from the cold topological matter (blue) to hot, strongly correlated metal (red). Image, Harald Ritsch/University of Innsbruck
An artistic impression depicts ultra-quantum matter: from the cold topological matter (blue) to hot, strongly correlated metal (red). Image, Harald Ritsch/University of Innsbruck.

MIT professors Senthil Todadri and Xiao-Gang Wen are members of the newly established Simons Collaboration on Ultra-Quantum Matter. The effort, funded by the Simons Foundation, is an $8 million four-year award, renewable for three additional years, and will support theoretical physics research across 12 institutions, including MIT.

The science of the collaboration is based on a series of recent developments in theoretical physics, revealing that even large macroscopic systems that consist of many atoms or electrons — matter — can behave in an essentially quantum way. Such ultra-quantum matter (UQM) allows for quantum phenomena beyond what can be realized by individual atoms or electrons, including distributed storage of quantum information, fractional quantum numbers, and perfect conducting boundary.

While some examples of UQM have been experimentally established, many more have been theoretically proposed, ranging from highly entangled topological states to unconventional metals that behave like a complex soup. The Simons Collaboration on Ultra-Quantum Matter will classify possible forms of UQM, understand their physical properties, and provide the key ideas to enable new realizations of UQM in the lab.

Ultra dream team

In particular, the collaboration will draw upon lessons from recently discovered connections between topological states of matter and unconventional metals, and seeks to develop a new theoretical framework for those phases of ultra-quantum matter. Achieving these goals requires ideas and tools from multiple areas of theoretical physics, and accordingly the collaboration brings together experts in condensed matter physics, quantum field theory, quantum information, and atomic physics to forge a new interdisciplinary approach.

Directed by Professor Ashvin Vishwanath at Harvard University, the collaboration comprises researchers at MIT, Harvard, Caltech, the Institute for Advanced Study, Stanford University, University of California at Santa Barbara, University of California at San Diego, University of Chicago, University of Colorado at Boulder, University of Innsbruck, University of Maryland, and University of Washington.

“I am looking forward to scientific interactions with MIT theorists Senthil and Wen, who are key members of our Simons collaboration on Ultra-Quantum Matter, and hope this will further strengthen collaborations within the Cambridge area and beyond. Their research on highly entangled quantum materials is of fundamental significance, and may provide new directions for device applications, quantum computing, and high-temperature superconductors,” says collaboration director Ashvin Vishwanath of Harvard University.

“They have also been mentors for several collaboration members,” says Vishwanath, who worked with Senthil as a Pappalardo Fellow in physics from 2001 to 2004.

Senthil has played a leading role in the field of non-Fermi liquids, in the classification of strongly interacting topological insulators and related topological phases, and in the development of field theory dualities with diverse applications in condensed matter physics.

Wen is one of the founders of the field of topological phases of matter, introducing the concept of topological order in 1989 and opening up a new research direction in condensed matter physics. Wen’s research has often exposed mathematical structures that have not appeared before in condensed matter physics problems.

MIT-grown

Of the 17 faculty members who are participating in the collaboration, more than half, including Senthil, Wen, and Vishwanath, have MIT affiliations.

Michael Hermele, the collaboration’s deputy director and an associate professor at the University of Colorado at Boulder, was a postdoc in the MIT Condensed Matter Theory group.

Associate professors Xie Chen PhD ’12 and Michael Levin PhD ’06, at Caltech and the University of Chicago, respectively, earned their doctorates at MIT under Wen.

Other principal investigators include alumni Subir Sachdev ’82, now chair of the Department of Physics at Harvard, and Leon Balents ’89, a physics professor at UC Santa Barbara's Kavli Institute for Theoretical Physics. John McGreevy, a string theorist who conducted research in the Center for Theoretical Physics (CTP), is now a professor of physics at UC San Diego. Dam Thanh Son and Andreas Karch, former CTP postdocs, are now with the University of Chicago and the University of Washington, respectively.

The collaboration is part of the Simons Collaborations in Mathematics and Physical Sciences program, which aims to “stimulate progress on fundamental scientific questions of major importance in mathematics, theoretical physics and theoretical computer science.” The Simons Collaboration on Ultra-Quantum Matter is one of 12 such collaborative grants ranging across these fields.

The first meeting of the newly established collaboration will take place Sept. 12-13 in Cambridge, Massachusetts.

back to newsletterJulia C. Keller | School of Science 
May 29, 2019 | MIT News Office

RELATED: 

Faculty highlight: Senthil Todadri

 

Tuesday, 25 June 2019 11:38

A very good ring resonator

MIT, SUTD researchers show high-quality photonic device based on amorphous silicon carbide
ACS Photonics Cover 05152019 Web
Researchers at MIT and Singapore University of Technology (SUTD) have demonstrated a micro ring resonator made of amorphous silicon carbide with the highest quality factor to date. Illustration, Dawn Tan, SUTD. Reproduced with permission of ACS Photonics.

Researchers at MIT and Singapore University of Technology (SUTD) have demonstrated a micro ring resonator made of amorphous silicon carbide with the highest quality factor to date. The resonator shows promise to be used as an on-chip photonic light source at the infrared telecom wavelength of 1,550 nanometers.

Ordinary daylight will pass through a window unaltered, which is called linear transmission, but the same light passing through a prism will split into a rainbow of colors. Similarly in photonic devices, infrared light from a laser can pass through in linear fashion without changing its “color,” but at high intensity, the light can exhibit nonlinear behavior, generating additional colors, or wavelengths. For example, a single yellow laser coupled to a photonic device can generate blue, green, yellow or orange colors.

Researchers led by MIT Materials Research Laboratory Research Scientist Anuradha M. Agarwal fabricated the amorphous silicon carbide ring resonators and researchers at SUTD led by Associate Professor Dawn T.H. Tan analyzed the device’s linear and nonlinear properties.

“We are able to show one order of magnitude higher nonlinear effect than measured before in any of the silicon carbide substrates,” Agarwal says.

Their findings are described in a paper, featured on the cover of ACS Photonics, by Agarwal, Tan, MIT materials science and engineering graduate student Danhao Ma, and three others in Singapore and Malaysia.

Quality factor is a measure of how strongly the resonator produces nonlinear effects. “The larger the quality factor, the better the nonlinear effect,” says Tan, who leads the Photonics Devices and Systems Group at SUTD. “So in this case, the quality factor was pretty good. It was actually much better than we expected.”

Resonator advantage

High intensity of light is needed to trigger nonlinear properties for photonic devices, which can be achieved either by ramping up the power of the laser or using a device such as a ring resonator. “A ring allows for that high intensity because it traps the photons for a long time,” Agarwal explains. “More and more photons build up to like a crescendo and that allows for the evaluation of nonlinear optical properties.”

Like a fiber optic cable, which transmits light by wrapping one material that carries the light inside a different material that won’t allow the light to escape through it, the amorphous silicon carbide ring resonator and straight waveguide for carrying the infrared light are surrounded by a layer of silicon oxide that minimizes the amount of light that can escape. The refractive indices of different materials determine how well they work together as the carrier layer and protective outer layer.

“We are trying to create this kind of a fiber optic waveguide on chip,” Agarwal explains. “So it’s like a fiber, but on a chip, and therefore what we need is a core with a high (refractive) index and a cladding with a low index.” Silicon carbide and silicon oxide have a large enough difference in their refractive indices that they work together well as the core and cladding for a waveguide.

The researchers achieved the record quality factor in this study using the plasma enhanced chemical (PECVD) process to deposit the silicon carbide, at a temperature that is compatible with CMOS silicon chip processing, and developing a method to pattern and etch the silicon carbide ring resonator, which is coupled to a straight waveguide.

Overcoming challenges

MIT graduate student Ma overcame several processing challenges to make the high-quality resonator. When Ma began working on silicon carbide materials for this study about three years ago, there was no existing recipe for how to etch a pattern into the amorphous silicon carbide material when it is deposited on a silicon dioxide substrate. “Silicon carbide is a very rigid and physically and chemically hard material, so in other words, it’s very difficult for it to be removed or etched,” Ma says.

To deposit and etch the silicon carbide waveguide on silicon oxide, Ma first used electron beam lithography to pattern the waveguides and reactive ion dry etching to remove excess silicon carbide. But his first attempts using a typical polymer-based mask didn’t work because the process removed more of the mask than it removed of the silicon carbide. Ma then tried a metal mask, but grain boundaries from the mask carried over to the silicon carbide leaving behind rough sidewalls in the waveguides. Roughness is undesirable because it increases photon scattering and light loss. To resolve the issue, Ma developed a technique using a silicon dioxide-based mask for the reactive ion etching. During the process development, Ma worked closely with his colleague Postdoctoral Associate Qingyang Du and Mark K. Mondol, assistant director, NanoStructures Laboratory, Research Laboratory of Electronics.

“We came up with the right type of chemistries in this reaction and controlled the gas flows and the plasma, or in other words, the details of the processing recipe,” Ma says. “This recipe is really selective to etch silicon carbide compared to silicon dioxide which made it possible for us to shape the silicon carbide photonic devices and have a smooth waveguide sidewall,” Ma says. The smooth sidewall is critical for maintaining the optical signals in the photonic device, he notes.

MRL Anu Agarwal 0523 DP Web
MIT Materials Research Laboratory Research Scientist Anuradha M. Agarwal partnered with Singapore University of Technology and Design Assistant Professor Dawn T.H. Tan to demonstrate a high-quality amorphous silicon carbide ring resonator. Photo, Denis Paiste, Materials Research Laboratory.

The main sources of light loss in these resonators are absorption of photons in the ring material and/or scattering of photons caused by edge roughness of the ring device. “Danhao’s processing yielded smooth sidewalls, which enabled low loss and a high Q (Quality) factor resonator,” Agarwal explains.

“The beauty of this silicon carbide material and the technique that we used here in the paper is that the PECVD process of silicon carbide is an inexpensive process, standard in the silicon microelectronics industry,” says Ma, whose research concentration is materials design and engineering for integrated photonics. “Use of the existing microelectronics processes will make the adoption of silicon carbide into the integrated photonic and integrated electronic platforms easier.” The PECVD and reactive dry ion etching processes he used don’t require the lattice matching and other critical demands of epitaxial growth on silicon, and is substrate-agnostic, Ma says.

Better performance

Tan has studied silicon nitride materials and other complementary metal-oxide semiconductor (CMOS) materials for their nonlinearity for several years. “For (amorphous) silicon carbide, you would have a better enhancement when cast as a resonator compared to ultra-silicon-rich nitride (USRN), and it also has a higher nonlinear refractive index than stoichiometric silicon nitride which is prolific in nonlinear optics,” Tan says.

Several kinds of photon absorption known as two-photon and three-photon absorption are typically present in these devices. In this study, Tan says, loss was dominated by three-photon absorption, which is a relatively weak nonlinear loss mechanism, while two-photon absorption, which can be a problem in many crystalline silicon and amorphous silicon materials, was suppressed.

Agarwal and Tan began collaborating while Tan was a visiting scholar at MIT from August 2013 through August 2014. “We were very fortunate to be paired with Prof. Tan’s team, and we benefited a great deal from this collaboration, and we continue to collaborate,” Agarwal says. Agarwal’s team previously worked on using silicon carbide in a sensor for harsh environments.

For the current work, the Singapore team measured the additional wavelengths of light generated in the ring resonator – a phenomenon called spectral broadening which is quantified by a term called Kerr nonlinearity. The researchers found the Kerr nonlinearity of their silicon carbide film to be almost 10 times that of previously reported crystalline and amorphous silicon carbide.

“With this you see a spectral broadening effect, which we can leverage to our advantage because now instead of having just one frequency, we are generating several other frequencies which can provide a super continuum light source,” Agarwal says.

Exciting development

Professor David J. Moss, director of the Centre for Micro-Photonics at Swinburne University of Technology in Australia, who studies photonic materials, says, “This paper presents new results for amorphous silicon carbide as a promising CMOS compatible platform for nonlinear optics, particularly focused on the important telecommunications window.”

“The achievement of a high Kerr nonlinearity – comparable to crystalline silicon – along with negligible two-photon absorption, together with record high (for silicon carbide) Q factor ring resonators, is an exciting development in the continuing quest for ever more efficient platforms for nonlinear optics at 1,550 nanometers,” adds Moss, who was not involved in this research.

Associate Professor Andrea Melloni, who heads the Photonics Devices Group at Politecnico di Milano in Italy, says, “Amorphous SiC (silicon carbide) deposited with PECVD is of great interest. The refractive index is extremely appealing (2.45 is not a common value) because it is high enough to allow large-scale integration, but not as high as silicon, thus minimizing problems associated with the very high index contrast of SOI (silicon-on-insulator) structures.” Melloni, who also did not participate in this research, published a paper last year on Silicon Oxycarbide Photonic Waveguides.

Looking ahead, Ma hopes to make a thicker silicon carbide waveguide for a broader set of applications, for example, creating more wavelengths (multiplexing) within the single waveguide.

“As a first demonstration of what we’ve done together, I think it’s a very promising platform where if we can continue refining the platform and device design, I think we probably would be able to demonstrate very good resonator enhancement because we have demonstrated very good quality factors,” Tan says. “If we wanted to do something like a frequency comb or an optical parametric oscillator, the threshold power becomes a lot smaller if the quality factor is large.”

“If this work can be jointly funded then we can think about making an integrated light source, sensor and detector, so there are a lot of exciting next steps in this,” Agarwal says.

This work was supported by SUTD–MIT International Design Center, the Singapore National Research Foundation and the Singapore Ministry of Education.

back to newsletterDenis Paiste, Materials Research Laboratory
June 27, 2019

 

Monday, 24 June 2019 16:02

Enriching solid-state batteries

MIT researchers show method to make a lithium-rich ceramic electrolyte that is smaller, safer and faster.
MIT Associate Professor Jennifer Rupp stands in front of a pulsed laser deposition chamber. Photo, Denis Paiste, MIT MRL.
MIT Associate Professor Jennifer Rupp stands in front of a pulsed laser deposition chamber in which her team developed a new lithium garnet electrolyte material with the fastest reported ionic conductivity of its type. The new technique pioneered by Rupp and her colleagues produces a thin film that is about 330 nanometers thick. “Having the lithium electrolyte as a solid-state very fast conductor allows you to dream out loud of anything else you can do with fast lithium motion,” Rupp says. Photo, Denis Paiste, Materials Research Laboratory.

Researchers at MIT have come up with a new pulsed laser deposition technique to make thinner lithium electrolytes using less heat, that promises faster charging and potentially higher voltage solid-state lithium ion batteries.

Key to the new technique for processing the solid-state battery electrolyte is alternating layers of the active electrolyte lithium garnet component (chemical formula, Li6.25Al0.25La3Zr2O12, or LLZO), with layers of lithium nitride (chemical formula, Li3N). First, these layers are built up like a wafer cookie using a pulsed laser deposition process at about 300 degrees Celsius (572 degrees Fahrenheit). Then they are heated to 660 C and slowly cooled, a process known as annealing.

During the annealing process, nearly all of the nitrogen atoms burn off into the atmosphere and the lithium atoms from the original nitride layers fuse into the lithium garnet forming a single lithium-rich, ceramic thin film. The extra lithium content in the garnet film allows the material to retain the cubic structure needed for positively charged lithium ions (cations) to move quickly through the electrolyte. The findings were reported in a Nature Energy paper published online May 20, 2019, by MIT Associate Professor Jennifer L. M. Rupp and her students Reto Pfenninger, Michal M. Struzik, Inigo Garbayo and collaborator Evelyn Stilp.

“The really cool new thing is that we found a way to bring the lithium into the film at deposition by using lithium nitride as an internal lithiation source,” senior author Rupp says. Rupp holds joint appointments at MIT in the Materials Science and Engineering department and the Electrical Engineering and Computer Science department.

“The second trick to the story is that we use lithium nitride, which is close in bandgap to the laser that we use in the deposition, whereby we have a very fast transfer of the material, which is another key factor to not lose lithium to evaporation during a pulsed laser deposition,” Rupp explains.

Safer technology

Lithium batteries with commonly used electrolytes made by combining a liquid and a polymer can pose a fire risk when the liquid is exposed to air. Solid-state batteries are desirable because they replace the commonly used liquid polymer electrolytes in consumer lithium batteries with a solid material that is safer. “So we can kick that out, bring something safer in the battery, and decrease the electrolyte component in size by a factor of 100 by going from the polymer to the ceramic system,” Rupp explains.

Although other methods to produce lithium-rich ceramic materials on larger pellets or tapes, which are heated using a process called sintering, can yield a dense microstructure that retains a high lithium concentration, they require higher heat and result in bulkier material. The new technique pioneered by Rupp and her students produces a thin film that is about 330 nanometers thick (less than 1.5 hundred-thousandths of an inch). “Having a thin film structure instead of a thick ceramic is attractive for battery electrolyte in general because it allows you to have more volume in the electrodes where you want to have the active storage capacity. So the holy grail is be thin and be fast,” she says.

Compared to the classic ceramic coffee mug, which under high magnification shows metal oxide particles with a grain size of 10s to 100s of microns, the lithium (garnet) oxide thin films processed using Rupp’s methods show nanometer scale grain structures that are 1,000 to 10,000 times smaller. That means Rupp can engineer thinner electrolytes for batteries. “There is no need in a solid-state battery to have a large electrolyte,” she says.

Faster ionic conduction

Instead what is needed is an electrolyte with faster conductivity. The unit of measurement for lithium ion conductivity is expressed in Siemens. The new multilayer deposition technique produces a lithium garnet (LLZO) material that shows the fastest ionic conductivity yet for a lithium-based electrolyte compound, about 2.9 x 10-5 Siemens (0.000029 Siemens) per centimeter. This ionic conductivity is competitive with solid-state lithium battery thin film electrolytes based on LIPON (lithium phosphorus oxynitride electrolytes) and adds a new film electrolyte material to the landscape.

“Having the lithium electrolyte as a solid-state very fast conductor allows you to dream out loud of anything else you can do with fast lithium motion,” Rupp says.

A battery’s negatively charged electrode stores power. The work points the way toward higher voltage batteries based on lithium garnet electrolytes both because its lower processing temperature opens the door to using materials for higher voltage cathodes that would be unstable at higher processing temperatures, and its smaller electrolyte size allows physically larger cathode volume in the same battery size.

Co-authors Michal Struzik and Reto Pfenninger carried out processing and Raman spectroscopy measurements on the lithium garnet material. These measurements were key to showing the material’s fast conduction at room temperature as well as understanding the evolution of its different structural phases.

“One of the main challenges in understanding the development of the crystal structure in LLZO was to develop appropriate methodology. We have proposed a series of experiments to observe development of the crystal structure in the (LLZO) thin film from disordered or 'amorphous' phase to fully crystalline, highly conductive phase utilizing Raman Spectroscopy upon thermal annealing under controlled atmospheric conditions,” says co-author Struzik, who was a postdoc working at ETH and MIT with Rupp’s group, and is now a professor at Warsaw University of Technology in Poland. “That allowed us to observe and understand, how the crystal phases are developed and, as a consequence, the ionic conductivity improved,” he explains.

Their work shows that during the annealing process, lithium garnet evolves from the amorphous phase in the initial multilayer processed at 300 C through progressively higher temperatures to a low conducting tetragonal phase in a temperature range from about 585 C to 630 C and to the desired highly conducting cubic phase after annealing at 660 C. Notably, this temperature of 660 C to achieve the highly conducting phase in the multilayer approach is nearly 400 C lower than the 1,050 C needed to achieve it with prior sintering methods using pellets or tapes.

“One of the greatest challenges facing the realization of solid-state batteries lies in the ability to fabricate such devices. It is tough to bring the manufacturing costs down to meet commercial targets that are competitive with today's liquid electrolyte based lithium-ion batteries, and one of the main reasons is the need to use high temperatures to process the ceramic solid electrolytes,” says Professor Peter Bruce, Wolfson Chair of the Department of Materials at the University of Oxford, who was not involved in this research.

Optical microscopy image of a solid lithium garnet (LLZO) thin film electrolyte deposited on a magnesium oxide. Reprinted with permission of Nature Energy.
Optical microscopy image of a solid lithium garnet (LLZO) thin film electrolyte deposited on a magnesium oxide (MgO) substrate. Reproduced with permission of Nature Energy.

“This important paper reports a novel and imaginative approach to addressing this problem by reducing the processing temperature of garnet-based solid-state batteries by more than half, that is, by hundreds of degrees,” Bruce adds. “Normally high temperatures are required to achieve sufficient solid-state diffusion to intermix the constituent atoms of ceramic electrolyte. By interleaving lithium layers in an elegant nanostructure the authors have overcome this barrier.”

After demonstrating the novel processing and high conductivity of the lithium garnet electrode, the next step will be to test the material in an actual battery to explore how the material reacts with a battery cathode and how stable it is. “There is still a lot to come,” Rupp predicts.

Understanding aluminum dopant sites

A small fraction of aluminum is added to the lithium garnet formulation because aluminum is known to stabilize the highly conductive cubic phase in this high-temperature ceramic. The researchers complemented their Raman spectroscopy analysis with another technique, known as negative-ion time-of-flight secondary ion mass spectrometry (TOF-SIMS), which shows that the aluminum retains its position at what were originally the interfaces between the lithium nitride and lithium garnet layers before the heating step expelled the nitrogen and fused the material.

“When you look at large scale processing of pellets by sintering, then everywhere where you have a grain boundary, you will find close to it a higher concentration of aluminum. So we see a replica of that in our new processing but on a smaller scale at the original interfaces,” Rupp says. “These little things are what adds up also not only to my excitement in engineering but my excitement as a scientist to understand phase formations, where that goes and what that does,” Rupp says.

“Negative TOF-SIMS was indeed challenging to measure since it is more common in the field to perform this experiment with focus on positively charged ions,” explains Pfenninger, who worked at ETH and MIT with Rupp’s group. “However, for the case of the negatively charged nitrogen atoms we could only track it in this peculiar setup. The phase transformations in thin films of LLZO have so far not been investigated in temperature-dependent Raman spectroscopy – another insight towards the understanding thereof.”

The paper’s other authors are Inigo Garbayo, who is now at CIC EnergiGUNE in Minano, Spain, and Evelyn Stilp, who was then with Empa, Swiss Federal Laboratories for Materials Science and Technology, in Dubendorf, Switzerland.

Rupp began this research while serving as a Professor of Electrochemical Materials at ETH Zurich (the Swiss Federal Institute of Technology) before she joined the MIT faculty in February 2017. MIT and ETH have jointly filed for two patents on the multi-layer lithium garnet/lithium nitride processing. This new processing method, which allows precise control of lithium concentration in the material, can also be applied to other lithium oxide films such as lithium titanate and lithium cobaltate that are used in battery electrodes. “That is something we invented. That’s new in ceramic processing,” Rupp says.

“It is a smart idea to use Li3N as a lithium source during preparation of the garnet layers, as lithium loss is a critical issue during thin film preparation otherwise,” comments University Professor Jürgen Janek at Justus Liebig University Giessen in Germany. Janek, who was not involved in this research, adds that “the quality of the data and the analysis is convincing.”

“This work is an exciting first step in preparing one of the best oxide-based solid electrolytes in an intermediate temperature range,” Janek says. “It will be interesting to see, whether the intermediate temperature of about 600 °C is sufficient to avoid side reactions with the electrode materials.”

Oxford Prof. Bruce notes the novelty of the approach, adding “I'm not aware of similar nanostructured approaches to reduce diffusion lengths in solid-state synthesis.”

“Although the paper describes specific application of the approach to the formation of lithium rich and therefore highly conducting garnet solid electrolytes, the methodology has more general applicability and therefore significant potential beyond the specific examples provided in the paper,” Bruce says. Commercialization may be needed to be demonstrate this approach at larger scale, he suggests.

While the immediate impact of this work is likely to be on batteries, Rupp predicts another decade of exciting advances based on applications of her processing techniques to devices for neuromorphic computing, artificial intelligence and fast gas sensors. “The moment the lithium is in a small solid-state film, you can use the fast motion to trigger other electrochemistry,” she says.

Several companies have already expressed interest in using the new electrolyte approach. “It’s good for me to work with strong players in the field so they can push out the technology faster than anything I can do,” Rupp says.

This work was funded by the MIT Lincoln Laboratory, the Thomas Lord Foundation, Competence Center Energy and Mobility, and Swiss Electrics.

back to newsletter Denis Paiste, Materials Research Laboratory
June 27, 2019

 

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