Materials to Do Anything Under the Sun
By Syl Kacapyr
Peek inside any materials laboratory at Cornell Engineering and you’ll see how researchers are breaking the very rules of nature to create new materials that would not exist otherwise. These materials include metals, semiconductors, ceramics, polymers and nanomaterials, and are designed to give manufacturers, scientists and medical professionals new tools and new possibilities.
Cornell has long been a pioneer within materials science and engineering, and has done so by taking an unconventional approach to the field.
“We got into the business of creating centers just as the notion of the center was born,” said Lance Collins, the Joseph Silbert Dean of Engineering, referring to the Cornell Center for Materials Research (CCMR) established in 1960.
At that time, one of the most important revolutions in materials history was taking the electronics industry by storm. Scientists and engineers had discovered that ‘doping’ the cheap and abundant element silicon, that is, mixing it with atoms from other elements such as arsenic or gallium, altered its electrical properties so that it could be used to engineer more reliable transistors. Scientists had also perfected methods of purifying silicon by removing trace amounts of oxygen from its raw oxide form. By 1960, the first integrated circuit had been invented, giving way to processors and, eventually, the modern computer.
Companies and universities quickly jumped on the opportunity to have a hand in the growing field, but Cornell challenged the conventional thought at the time, which was to focus exclusively on silicon.
“If it were just the electrical engineers, it would look like the other materials operations. We were saying ‘no,’” said Collins. “We had the applied physicists, we had the chemists, and they didn’t want just a silicon-oriented shop, which is what all our competitors did. They wanted a broad-based set of tools to work with any materials to do anything under the sun.”
Cornell Engineering’s interdisciplinary vision garnered international exposure within the scientific community. The college became the first institution to grow a single layer of graphene and determine its structure, the first to achieve laboratory pressures exceeding those at the center of the Earth, and its research into polymer-clay nanocomposites laid the foundation for an entirely new industry.
Cornell’s Department of Materials Science and Engineering is now over a half-century-old, and by collaborating with other departments, colleges and institutions around the world, its latest achievements include materials that work inside the body to detect cancer, capture carbon from the atmosphere to battle climate change, and shrink electronics to once-unimaginable scales.
And with a recent focus by the U.S. government to accelerate the field, Cornell is taking the lead by operating a new platform where the next generation of advanced materials will be developed “to do anything under the sun.”
New Materials, New Paradigm
Most engineers will concede that silicon-based circuits, as they exist today, are reaching their limitations. With billions of transistors—each nearing atomic dimensions—crammed onto computer chips the size of a fingernail, improving the functionality and performance of today’s circuits depends partly on new materials and the researchers who are discovering them.
Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry in the Department of Materials Science and Engineering, is leading one such effort through the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM). The $25 million initiative, funded this year by the National Science Foundation, invites materials researchers from around the nation to take advantage of the world-class expertise and facilities at Cornell and its partner institutions to create entirely new materials, from the bottom up.
The particular focus of PARADIM is the interface between oxide or chalcogenide-based, two-dimensional materials and the active substrates they’re deposited upon. It’s there, according to Schlom, that previously unrealized properties can exist. He analogizes the interface to the thin layer of water that exists between an ice-skating blade and the ice. “The same sort of excitement happens at our interfaces. We put material A on top of material B and for the right combination, a wacky property emerges at the interface between these materials that neither material had by itself,” said Schlom. Sometimes those “wacky” properties can turn out to be quite beneficial for electronics or other applications.
Schlom has dedicated much of his career to creating new materials and exploring, or characterizing, their properties. He and a team of researchers published a study in 2013 that demonstrated how a new material—strontium titanium oxide—could be used to engineer better tunable capacitors to improve cellphone reception and open up higher frequency channels for wireless service providers.
The new material was first theorized with the help of Craig Fennie, associate professor of applied and engineering physics, who used computational methods to design the strontium titanium oxide using first-principles calculations. Rather than create a material and then see what its properties are, Fennie begins with a set of desired properties and then works backwards to theorize a material that could possess those properties.
Computational theory is a relatively new tool in materials science and engineering, but it’s already changing the way new materials are sought. Fennie’s work has had such an impact that he was prestigiously bestowed the so-called “genius award” from the MacArthur Foundation. The foundation cited his research that correctly predicted ferroelectric and ferromagnetic characteristics of several previously unexplored metal oxides, including that of europium titanate—a ferroelectric-ferromagnetic material synthesized by Schlom to display electrical properties 1,000 times better than the previously best-known material of its kind.
Coincidentally, it was the same year Fennie was given the MacArthur Award that he theorized the strontium titanium oxide for Schlom. It was at that point Schlom used a method known as molecular-beam epitaxy to stack atomic layers of the material according to Fennie’s blueprint. The new material was synthesized with layers of strontium titanate separated by a mono-layer of strontium oxide. The interface of these layers is what gives the material its ability to record high frequencies, enabling cellphones to have greater bandwidth, less interference and fewer dropped calls.
“It is clear that we have discovered a killer material,” Schlom told the Cornell Chronicle after the study was first published, “but it is likely that even better tunable dielectrics can be found using our approach.”
Not every university can take that approach, however, because not every university has access to molecular-beam epitaxy or facilities for characterization. PARADIM’s aim is to change that, empowering engineers with a platform to theorize, synthesis and characterize a material. That theory-to-characterization cycle is what Schlom and colleagues have been doing with great success, and now they’re changing the paradigm for others.
While some Cornell engineers are focused on new materials, others are thinking about established materials in new ways.
Gallium nitride—a semiconductor with a wide energy bandgap—has been a recent focus of the electronics community, and for a good reason. When considering options for a material that could improve electronics beyond what silicon can offer, gallium nitride fits the bill. Its properties allow it to operate quickly within a circuit, accommodating relatively large amounts of electricity and operating at high temperatures.
“The challenge with silicon-based electronics is that if you want to support a reasonably large voltage, the entire circuit has to be slow, which means you have to use large, passive capacitors and inductors,” said Huili Grace Xing, the Richard Lunquist Sesquicentennial Professor of Electrical and Computer Engineering, and of materials science and engineering.
And it’s not just consumer electronics such as cellphones and computers that could benefit from gallium nitride’s unique properties. Dwindling fossil fuels along with climate change have intensified efforts to improve alternative energy devices such as electric motors, smart grids and solar cells—all devices where the inefficiency of silicon has left room for improvement.
But gallium nitride has a tendency for defects that, for some applications, make it unreliable. Xing has been working to improve the semiconductor’s properties, and in 2015, she and Debdeep Jena, also a professor within the same two departments, published a study demonstrating a gallium nitride power diode—a circuit component used to regulate the flow of electricity.
The semiconductor community quickly took notice of Xing and Jena’s diode because it could support over 1,400 volts of electricity despite only being about one-tenth the width of a human hair. It was the first time anybody had demonstrated a diode at such a ‘figure of merit,’—a scale used to measure the quality of a material. “We demonstrated a figure of merit that had never even been predicted, let alone electrically demonstrated. It surpassed expectations of semiconductors,” said Xing, who added that every step of the research, from the material’s atomic arrangement of layers to fabrication testing, contributed to the successful end result.
Such materials are important for green technologies that depend on efficiency. Electric cars are currently limited by their weight as well as the amount of power they can handle. Xing says wide bandgap semiconductors like gallium nitride could also be applied toward smart grids that more efficiently manage electricity. “We can think about a power distribution station for an entire city that can be potentially smaller. Instead of building size, now maybe it can be room size,” she said.
Solar Cells and Synchrontron Source
The space industry now favors gallium alloys over silicon for use in the solar cells that power satellites because a smaller, lighter system can save money when launched into space. However, gallium nitride hasn’t been one of those preferred alloys. Materials engineers at Cornell have instead investigated other ways to make solar cells more efficient, using help from one of Cornell’s most powerful research tools: the Cornell High Energy Synchrotron Source, also known as CHESS.
In 2008, a group of researchers engineered a solar cell as a nano-manufactured polymer film just 400 nanometers thick. The rate at which it could convert sunlight to electricity was low compared to silicon-based photocells, but it demonstrated a novel method for developing low-cost, thin solar cells.
Leading that research group was Uli Wiesner, the Spencer T. Olin Professor of Materials Science and Engineering, who published a related study in 2015 demonstrating how to optimize the fabrication conditions of a thin-film solar cell.
Wiesner focused on using metal halide perovskites, which have recently garnered attention from the materials community for their unique crystal structure, giving them properties prime for solar applications. But in order for them to be as defect-free as possible, Wiesner used the Cornell synchrotron to characterize how different lead-salt solutions helped optimize the perovskite films.
Buried 40 feet below Cornell’s surface, the high-energy x-ray facility spans a half-mile circumference that loops under the south campus athletics fields. And while CHESS is one of many facilities used by Cornell’s materials scientists and engineers, it’s the only one that can send electrons traveling at 99.9999995 percent the speed of light in order to emit powerful x-rays that can be directed toward materials.
Teams of researchers use the synchrotron facility around the clock to characterize new materials, and like Wiesner, they’re trying to get a closer look at crystal structures in order to better understand how they relate to the material properties they hope to optimize.
“We have researchers studying materials and systems across any spectrum you choose to define—from soft matter, like protein molecules floating in solution, all the way to super-hard, super-strong materials used to build airplane engines,” said Ernie Fontes, associate director of CHESS.
CHESS is one of only five facilities in the country that conducts synchrotron x-ray research, so securing time inside remains coveted. Fontes says the facility’s database has nearly 500 active projects, and out of the more than 1,000 researchers that visit CHESS annually, ten percent come from other countries. “That shows the need for capabilities that users can’t find elsewhere,” said Fontes. “In addition, we’re pleased that almost half that number are post-doctoral associates or graduate students, which says that there’s a healthy generation of young scientists who are learning the trade and value of x-ray measurements. The need for high-energy, high-powered x-ray light sources is growing daily.”
Wiesner says the Cornell facility has been essential for breakthroughs in a number of his research areas, including the analysis of his thin films, which were as small as mere hundredths of a nanometer. “The value of close proximity to a synchrotron with exquisite and highly dedicated staff can not be overstated,” he said. “Our work on the perovskites is a particularly important example where this had tremendous impact. Work at CHESS allowed us to get unprecedented insights into the structural evolution of perovskite thin films that otherwise would have been impossible to obtain.”
Nanomaterials to Diagnose Cancer
Wiesner is somewhat of a jack of all trades when it comes to materials. Aside from his work on thin-film perovskites, he has developed a new class of hybrid materials described as “flexible ceramics,” created a polymer mold that can produce perfect silicon nanostructures, and his research group produced the first-ever self-assembled superconductor structure. Wiesner even engineered an iridescent material film that was used exclusively for a sculpture by renowned Korean artist Kimsooja.
But Wiesner’s most advanced research project isn’t a material meant to be placed inside an electronic device, solar cell or artistic sculpture. It’s a silica-organic hybrid nanoparticle meant to be placed inside the human body.
The nanoparticles—called Cornell dots, or C dots for short—are smaller than 10 nanometers in size and consist of a core containing several dye molecules, which are surrounded by a protective silicon dioxide shell. Once inside the body, the C dots attach to cancer cells and fluorescently highlight them using the dye.
It’s a complex science that Wiesner and colleagues have continually improved over the last decade. “This has lead to a number of breakthroughs in the area of cancer nano-medicine, including the first-ever human clinical trials with such hybrid optical silica particles,” said Wiesner.
Those human trials were considered to be a great success and in 2015, Wiesner announced with the Memorial Sloan Kettering Cancer Center (MSKCC) the MSKCC-Cornell Center for Translation of Cancer Nanomedicines—a $10 million pre-clinical research center with the goal of bringing together scientists, engineers, biologists and physicians to develop and translate new cancer care applications based on the C dot nanotechnology. This includes altering the nanoparticles to deliver treatment to melanoma and brain cancers, potentially through the use of radioactive isotopes.
“What excites me most is the fact that our work on silica nanoparticles is able to bridge the gap between fundamental science and applications in cancer nano-medicine,” said Wiesner. “There are not many cases in the career of a university professor where the work has the potential to directly touch the quality of life of a lot of people. This clearly is one of those cases.”
Sorbent Sponges for Carbon Capture
Another materials project with the potential to change the quality of life for a lot of people is one that also has the potential to reduce greenhouse gases in the atmosphere.
Carbon capture is a technique that can be used to collect the harmful greenhouse gas carbon dioxide either at its source of production, such as an industrial power plant, or directly from the atmosphere.
Emmanuel Giannelis has developed a strategy for capturing carbon dioxide by using a chemically-engineered sorbent—a material substance that can absorb the gas like a sponge. Giannelis is the associate dean for research and graduate studies, but still finds time to conduct research in his materials science and engineering laboratory.
He and Fernando Escobedo, professor of chemical and biomolecular engineering, are investigating new classes of solid sorbents. They start with a silica scaffold and then fill its pores with a liquid amine—an ammonia compound that replaces one or more hydrogen atoms with a substitute.
The finished material is a dry, white powder that can absorb carbon dioxide and, according to Giannelis, can be recycled for repeated use. He and Escobedo are also exploring ways to convert the carbon dioxide waste into useful byproducts, such as biodegradable plastics or solid carbonates that could be used as substitutes for cement.
Mollusks and Microscopy
While Cornell engineers often create materials not found in the natural world, it is sometimes nature that provides inspiration for a new or improved material.
Bio-inspired materials are synthesized to mimic properties or functions found in plants and animals, which have adapted to their respective environments partly through the evolution of the materials from which they’re made. Leaves are optimized for harvesting light for photosynthesis, providing inspiration for solar cells. Mechanical engineers can learn from bone, which provides structure while remaining light enough to enable mobility.
Lara Estroff, associate professor of materials science and engineering, leads a research group that focuses on bio-inspired materials synthesis, particularly bio-mineralization—how biological organisms control crystal growth in natural objects like bones, teeth and shells.
“We think of crystals as being brittle and fragile and not necessarily what you would want your bones and teeth to be made out of,” said Estroff. “And yet in biology, these crystals are combined with organic materials to make them good structural materials.”
One such example is the unique crystal growth within mollusks. Estroff studied the marine organism hoping to uncover the mystery of how it can produce nacre—the material more commonly known as “mother of pearl” that is found on its inner shell.
While both nacre and mollusk shells had been previously characterized, Estroff used electron microscopy—a technique that uses accelerated electrons to produce high magnification—to examine the interface where the two materials meet. It was there she and a group of researchers discovered that mollusks produce nacre one layer at a time, depositing calcium-carbonate nanoparticles that fuse into crystals between layers of organic material.
“That was pretty exciting because a lot of people had suggested that this might be a route to form nacre, but by combining sample preparation with the amazing electron microscope facility, we were able to actually see how these initial layers were formed,” said Estroff.
That “amazing” facility is the Cornell Center for Materials Research (CCMR), and the microscope was operated by a student working with David Muller, professor of applied and engineering physics. The facility contains some of the most powerful electron microscopes in the world, and by using them, Muller helps scientists and engineers understand the behaviors of materials at the atomic scale by giving them a view of the unpredictable microcosm that exists at the interface of two materials.
Such was the case when examining the mollusk shells. Muller says he and Estroff had originally intended to use electron microscopy to create the first 3-D imaging of how the shells might grow. “We never actually got around to doing this because in preparing the screening samples because we saw something much more interesting elsewhere in the sample,” explained Muller. “We had essentially, and unexpectedly, caught the transition zone where shell builds itself up from the inside-out, and it was showing fibrous structures decorated by tiny nanocrystals that no one had ever seen or described before. The first images were so incredibly striking and different, it was a drop-everything-else-and-focus-on-this moment.”
Estroff says having access to such facilities is why she first decided to come to Ithaca. “I came to Cornell for the collaborations and the facilities, and both of those have paid off in spades,” she said.
A 2016 study by Estroff followed up on her characterization of nacre when her collaborators at the University of Leeds in England were able to synthesize the crystal structure mollusks use in their calcite shells. What they found was that the synthetic shells they had created possessed the same mechanical strength as the biological shells they had hoped to mimic.
“Now that we understand the mechanism by which calcite is strengthened, can we apply it to more technologically relevant materials,” asked Estroff, who added that the research can be used as a stepping stone for future projects.
Help Discover New Materials
In many ways, Cornell University is a playground for materials researchers. The Cornell NanoScale Facility, Center for Materials Research, High Energy Synchrotron Source and various other facilities are filled with chemists, physicists, computer scientists and other engineers all exploring new ideas together, theorizing materials and arranging atoms with absolute precision.
And for the first time in Cornell’s prestigious materials history, a very unique group of researchers has joined the ranks of those making discoveries: the general public. Citizen science is a growing concept in which people with little to no scientific experience contribute to research by providing crowd-sourced information under the guidance of a professional scientist.
Hundreds of citizen scientists without any formal materials engineering background are advancing the field through Materials Discovery—an initiative led by Carla Gomes, professor of computer and information science, and Bruce van Dover, chair of materials science and engineering.
Searching through a seemingly infinite amount of x-ray diffraction data from the synchrotron and other sources involves tremendous amounts of computational power and human insight. But by combining computational techniques with a citizen science community that can identify simple patterns within images, Gomes’ lab can analyze over one-million different combinations of materials in a single day. Using the UDiscoverIt platform on their home computers, citizen scientists can volunteer—and sometimes earn cash—by combing through images, searching for patterns representing crystal structures that are optimized for sustainable technologies like hydrogen fuel cells and solar cells.
“They don’t have to know anything about materials. Even a kid can identify the little patterns,” said Gomes, who added that a 14-hour computational data-analysis project can turn into a five-minute task thanks to visual insights provided by the public.
“Even the quality of the solutions tends to be better because sometimes humans can identify solutions that a machine gets confused,” said Gomes. The problem is that even the best computer algorithms will have trouble searching for crystal patterns because of the “noise” created by x-ray diffraction. It’s like a television signal that doesn’t come in perfectly clear. The noise sometimes produces additional data points that don’t actually belong to the materials being analyzed, creating muddy results.
Humans can be better than computers at ignoring the noise and deciphering what is a crystal pattern versus what isn’t. And although citizen scientists are only contributing to one stage of Gomes’ material analysis project, it’s an important one that proves the future of materials science and engineering could rely, in part, on the keen eye of the general public.
It’s also another example of how Cornell Engineering is taking a unique scientific approach, breaking the rules to advance engineering science and creating “materials to do anything under the sun.”