If you’re not on the edge, you’re taking up too much space…
Written by Omar Zahr
As a serial materials science entrepreneur, Dan Button had a lot to share with iMatSci’s audience of innovators and entrepreneurs at the MRS Fall Meeting this year. He has raised over $100M in financing for four materials science startups, and has overseen two of the companies make their successful exits by being bought by, or merging with, another company. Calling on his experience at Fortune 500 companies Dupont and Corning, as well as his time commercializing five university inventions (now working on his fifth in three-dimensional computer vision), Button emphasized the need for a fledgling startup to be on the cutting-edge of technology if it ever hopes to compete against established industry leaders. Yet this is simply a starting point. A central challenge is financing. Investors often balk at the prospect of investing in a materials science-based startup, as they perceive a risk of low capital efficiency and a protracted time-to-market.
Button highlighted five practical strategies to address those fears: Focusing on a single market, application and product at entry; Dropping-In to existing industry ecosystems wherever possible; Designing-In the solution across the value chain; Practicing production and application of your product to learn, improve and validate; and Diversifying one’s options, finances, and capabilities.
Ultimately, the essence of Button’s message is to do what it takes to get to the market as quickly as possible, with as little equity capital as possible.
Juchen Guo, University of California-Riverside
Reversible Electrochemical Aluminum Intercalation in Metal Sulfides
Written by Armin VahidMohammadi
Aluminum-ion batteries are considered one of the potential candidates to substitute lithium-ion batteries due to their high volumetric capacity, abundant amount of aluminum in earth, and also higher safety compared to flammable lithium-based batteries. Juchen Guo from the University of California-Riverside in the last session of this symposium (Friday), went over their interesting research on utilizing metal sulfides as potential cathodes for aluminum-ion batteries. Chevrel phase (Mo6S8) was the cathode material that was used in this research for reversible aluminum intercalation using aluminum metal as the anode. An ionic liquid was used to enable the intercalation of aluminum inside the system, and different ratios of salt to the ionic liquid were investigated to find the optimum electrolyte composition and exact mechanism of charge storage inside the system. The talk summarizes different capacitances of Chevrel phase material as well as other metal sulfides that were tested in Guo’s group for aluminum batteries and discusses the intercalation sites for the ions species inside the structure of the cathodes. The future direction for these types of cathode materials, such as making their particle size smaller to overcome the diffusion limitations, were discussed. To conclude and by considering the different talks given in this year’s MRS Fall Meeting, we can say that the future of energy storage devices lies in different battery systems beyond lithium-ion batteries due to the many advantages they can provide but there is still a long way left to make them actually market-ready.
Multiferroics—Past, Present and Future
Written by Aditi Risbud
At Thursday’s lunchtime Symposium X talk, Nicola Spaldin conveyed her enthusiasm for the field of multiferroics—materials that combine two or more of the primary ferroic order parameters—ferromagnets, ferroelectrics, and ferroelastics—simultaneously in the same phase. Piezoelectrics are an example of multiferroic materials.
Spaldin noted the scarcity of materials exhibiting both ferromagnetic and ferroelectric behavior. Coexistence of these two properties could result in single-component inductors or capacitors, or integrated dielectric/storage devices.
Taking the idea one step further, magnetoelectric coupling would lead to electrical field-based control over magnetism—the “holy grail” of the field from a technological point of view. Spaldin cited “frightening” projections of annual information technology consumption in the decades ahead suggesting IT will consume 50% of the world’s energy by 2030 if scientists do not come up with entirely new device paradigms.
“if we want to keep information technology consumption to what is it today, around ten percent of world energy consumption, not even Moore’s Law scaling will suffice—we have to come up with new ideas for devices based on new materials,” noted Spaldin.
Although the term “multiferroics” did not appear in the scientific literature until a 2000 publication by Spaldin, Russian scientists Lev Landau and Evgeny Lifshitz had introduced the concept of magnetoelectrics in the 1960s. The search for magnetic ferroelectric materials took much longer, because the chemistry that promotes one functionality—filled or empty d-orbitals—often prohibits the other.
The multiferroic researcher community has come up with several approaches to address this contradiction. For example, two cations—one magnetic, one ferroelectric—can be used in a transition metal oxide to combine the two properties in one material. (This is “the least clever approach, and I can say that because it’s the way I worked on it,” Spaldin said.) Transition metal oxides are ideal because they are stable and have strong correlations between electrons.
Spaldin went on to discuss promising systems with the potential for multiferroic properties, including BiFeO3, which her group has extensively studied with her long-time experimental collaborator Ramamoorthy Ramesh at the University of California, Berkeley. Another potential route is to make a composite multiferroic, made of alternating layers of ferroelectric and ferromagnetic materials, where the coupling is mediated by strain. Darrell Schlom’s group at Cornell University achieved this concept with “designer multiferroic composites” in the lutetium iron oxide system.
Lastly, Spaldin discussed her intriguing work in using multiferroics to understand cosmology. By studying the ferroelectric domain structure of yttrium manganese oxide, she determined the meeting points of these domains in the oxide are actually one-dimensional strings. What’s more, the structural phase transition in multiferroic YMnO3 provides an exact mathematical analog to the formation of cosmic strings in the early universe.
Spaldin now collaborates with cosmologists—previously “thwarted by their inability to replay the Big Bang,” joked Spaldin—to study the early universe. By cooling YMnO3 at different rates through the structural phase transition and counting how many domain intersections form, the team can simulate expansion at different rates across the “Grand Unification Transition.”
For the students and early-career researchers in the audience, Spaldin discussed how she became interested in multiferroic materials, when she and a fellow postdoc at Yale University pondered the idea of a material being both ferroelectric and ferromagnetic. A weekend’s worth of library research unveiled no such material.
The quest to find multiferroics became Spaldin’s “obsession,” and she made it an integral part of her research when she became an assistant professor at UC Santa Barbara. The moral of the story? “Don’t neglect drinking coffee with your colleagues—it’s very important!”
“It’s exciting to be involved in a field from the very beginning, watching it evolve, sometimes being able to nurture the field in the direction you’d like it to go, and seeing it blossom into directions I hadn’t envisaged,” she said.
Designer Matter—Fascinating Interactions of Light and Sound with Metamaterials
Written by Don Monroe
The past 15 years have seen revolutionary new ideas about the interactions of waves with matter. Assembling metamaterials that have emergent properties very different from their constituents promises negative–refractive-index lenses, invisibility cloaks, and other surprises. But practical applications that show these effects over large scales have been slow in coming, said Andrea Alù of the University of Texas at Austin, the recipient of the Kavli Foundation Early Career Lectureship in Materials Science, in his talk on Thursday.
These exotic structures tend to have high losses and limited bandwidth, lack reconfigurability, be nearly linear, and be limited by symmetry constraints, Alù said, but his group has been varying additional parameters to extend the possibilities. Their extensive results include theory and simulation as well as acoustic, radio-frequency (RF), and optical experiments.
In one example, Alù described patterning tiny structures on multi–quantum-well structures to enhance and control their nonlinear properties. The results showed efficient wavelength conversion for a mirror that was one-twentieth of the wavelength, avoiding the usual challenges of phase matching for exploiting nonlinear processes.
A large part of Alù’s presentation addressed reciprocity, which means that the strength of wave propagation is equally strong when source and detector are swapped. A well-known theorem concludes that this always occurs if the medium is symmetric, time-invariant, and linear. Current nonreciprocal devices, such as isolators that protect lasers from back-reflection, exploit magnetic materials that break time-reversal symmetry, but because magneto-optic effects are weak, these devices tend to be large.
A magnetic field breaks the degeneracy between clockwise- and counterclockwise-traveling modes in a circular cavity. Alù’s team produced the same effect in an acoustic cavity by introducing fans to spin the air in a circular acoustic waveguide. They achieved 40 dB isolation with an air speed that was only a tiny fraction of the speed of sound. A similar effect is expected for light in ring waveguides by introducing a modulation that moves very rapidly around the ring to replicate the effect of a moving medium.
In addition to allowing compact isolators and circulators, the breaking of time-reversal symmetry allows an analog of the topological insulators that have generated tremendous excitement in condensed-matter physics. Specifically, a lattice of symmetry-broken rings will support a mode that propagate in only one direction around the edge of the lattice. Such topological modes are insensitive to the details of how the lattice is terminated, because there are no corresponding modes for scattering into the bulk or in the reverse direction.
Alù showed that introducing a moving modulation in a planar surface avoids reciprocity for free-space modes as well, enabling absorbers such as photovoltaics that do not emit or antennas that transmit without receiving.
Another approach for breaking reciprocity exploits nonlinearity. Among other results, Alù and his co-workers constructed an array that acts as a topological insulator for high intensities of RF.
Using a single nonlinear resonance to create asymmetric propagation imposes a fundamental penalty with insertion loss, Alù and his colleagues have shown. However, by using a pair of resonances they designed an isolator that is fully transmissive in one direction and fully reflective in the other, which they verified in an RF structure.
Alù also described a strategy for cloaking that uses an active medium to cancel scattering and to fill in the shadow of an object. This technique avoids the tradeoff that limits the useful bandwidth of a passive cloak of a particular size. Applied to two planar surfaces, one with loss and one with gain, this method provides a way to achieve negative refractive index.
With these and other diverse examples, Alù showed that there are still many opportunities for novel manipulations of wave propagation.
The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.
A Materials Perspective on Topological Insulators and Related Electronic Materials
Written by Arthur L. Robinson
For his MRS Medal Award presentation, Robert J. Cava covered three topics: geometrically frustrated magnets, topological insulators, and magnetoresistive materials that collectively show the kinds of challenges faced by crystal growers trying to synthesize novel materials with specific properties, often at the request of their colleagues. Cava illustrated the challenges by retracing the routes that resulted in success, that is, quality crystals with the properties sought.
To illustrate geometrical frustration, Cava first considered the problem of placing magnetic moments that want the nearest neighbors to align antiferromagnetically on a two-dimensional triangular lattice. There is no solution to the problem, and the magnetic moments have to compromise with some ferromagnetic alignment, a situation called geometric frustration. A three-dimensional lattice of tetrahedrons has the same issue. The most complicated lattice based on triangles is called a kagome lattice, and the three-dimensional analog is the pyrochlore lattice, which can be visualized as kagome planes formed by tetrahedron bases separated by planes comprising tetrahedron vertices.
Rare-earth kagome compounds are well-known frustrated magnets, but no rare-earth kagome compounds had been found until recently. The trick was to create a pyrochlore with alternately magnetic and nonmagnetic kagome planes. A comparison of the magnetic properties of a Nd2(ScNb)O7 pyrochlore and the Nd3Sb2Mg2O14 kagome compound showed no gross difference in behavior. However the magnetic ordering temperature of around 0.4 K for both materials was too low for materials studies, suggesting the use of divalent 3d transition elements rather than the f-electron rare earths. This strategy also called for using fluorine to help with the charge balance. After a brave student found a way to make HF gas (a neurotoxin) for the synthesis process, several crystals were made based on cobalt and nickel (e.g., NaCaCo2F7, NaSrCo2F7, NaCaNi2F7), all with magnetic interactions larger than 100 K.
Cava then moved to topological insulators. Normal insulators have roughly parabolic (E ~ k2) conduction and valence bands near the Fermi level. In two- and three-dimensional topological insulators, there are additional metallic edge and surface states with linear dispersion (E ~ k) within the bulk bandgap that arise when the bulk bands would overlap in energy but are separated by strong spin-orbit coupling. Where the surface-state bands cross is called the Dirac point. Symmetry protects these states because their wave vector and spin are coupled, so that backscattering cannot occur without forbidden spin flipping and is thus prevented.
The first three-dimensional topological insulator was Bi0.9Sb0.1. However, the material was difficult for materials physicists to study, owing to a complicated surface-state Fermi surface. Looking for materials with a heavy metal, small bandgap, and the same two-dimensional symmetry as bismuth led to Bi2Se3, which was a topological insulator, but the bulk was too conductive to allow detailed study of the surface states. Cava then began a seven-year search for the perfect bulk-crystal topological insulator based on four primary materials requirements: Very high bulk resistivity so that the transport of charge is dominated by surface states, the surface Dirac point energy is isolated from the bulk energies so that there is no interference from bulk electrons, the surface states “show up” in transport measurements, and the crystals are controllable and reproducible to grow. These requirements led to the Bi2Te3‑xSex series of which Bi2Te2Se turned out to be a very good topological insulator, but still the Dirac point energy was in the valence band. Additional substitutions solved the problem with BiSbTe2S, which Cava said has met all the requirements to be the ideal bulk-crystal topological insulator.
Cava finished with a discussion of magnetoresistance, the phenomenon in which an applied magnetic field changes a material’s resistivity at a fixed temperature. There are several ways in which this can happen, depending on the materials. Giant magnetoresistance (GMR) in thin-film structures composed of alternating ferromagnetic and nonmagnetic conductive layers, for example, is used in read sensor heads for magnetic storage devices. While looking at WTe2 during topical insulator studies, a member of Cava’s laboratory found that this material exhibited a very large magnetoresistance. In high fields up to 60 Tesla, the magnetoresistance did not saturate, reaching a value of 13 million percent, but the mechanism remains a mystery. With improved purity, nearly defect-free crystals exhibited improved magnetoresistance (1.8 million percent at 2 K and 9 Tesla).
The MRS Medal, endowed by Toh-Ming Lu and Gwo-Ching Wang, is awarded for a specific outstanding recent discovery or advancement that has a major impact on the progress of a materials-related field. Robert J. Cava is being honored “for pioneering contributions in the discovery of new classes of 3D Topological Insulators.”
Li Na Quan, University of Toronto
Reduced Dimensionality Perovskite for Photovoltaics and Light-Emitting Diodes
Written by Xiwen Gong
As an emerging class of semiconducting materials, organic-inorganic hybrid perovskites have so-far led to great advances in the performance of solution-processed optoelectronic devices. However, many as-formed lead halide perovskite thin films lack chemical and structural stability, leading to rapid degradation in the presence of moisture. Therefore, for perovskite materials to make an impact in light emission field, it is necessary to overcome their free-carrier nature of electrons and holes in perovskites at room temperature.
Li Na Quan from the University of Toronto investigated a novel material platform of mixed organic, dimensionally-tunable quasi-two-dimensional (2D) perovskite thin films that can be used to bridge the gap between 2D and 3D materials. Quasi-2D perovskite films exhibit improved stability due to the increased formation energy, while retaining the high performance characteristics of MAPbI3 perovskites. As a result, they achieved an improved stability perovskite solar cell power conversion efficiency around 18%.
As follow-up work with quai-2D perovskites, they studied a mixed materials perovskites that was composed of different quantum-sized, tuned grains. This resulted in an enhanced photoluminescence quantum yield and high performance of light-emitting diode under the 750 nm near-infrared operation conditions. They have explained the charge carrier dynamics of the materials from the ultra-fast transient absorption study.
In this way, they obtained a view of the charge transfer. The results indicate that the multiphase perovskite materials channel energy across an inhomogeneous energy landscape and thus concentrate carriers on smaller bandgap emitters.
Alexander Uhl, University of Washington
Solution-Processed Chalcopyrite-Perovskite Tandem Solar Cells in Bandgap-Matched Two-Terminal Architectures
Written by Xiwen Gong
Tandem solar cells exhibit great promise in improving solar power conversion efficiency by making better use of the solar spectrum. In stark contrast with single junction solar cells, which can only absorb photons above the bandgap, tandem cells combine high bandgap material with a low bandgap one, extending their absorption range further toward the short or long wavelength region.
Cu(In,Ga)(S,Se)2 (CIGS) and perovskite solar cells both show the highest efficiency of 22% in a single junction configuration. If we combine these two cells together in a tandem cell, the highest theoretical power conversion efficiency (PCE) can be raised to 45%, with perovskite (1.6 eV) as top cell and CIGS (1.0 eV) as bottom cell.
Alexander Uhl from the University of Washington introduced their work in chalcopyrite perovskite tandem solar cells in the morning session on Thursday. The researchers started from making high efficiency single junction cells made from solution-processed CIGS or CIS (CuIn(S,Se)2). The PCE of 14.7% and 13.0% were achieved from CIGS and CIS, respectively. However, when illuminated with near-infrared light, the fill factor (FF) of the CIS cells can drop dramatically by almost 50%. The researchers solved this FF issue by increasing the concentration of sulfur: the FF kept constant when illuminated in the near-infrared wavelength region when sulfur ratio was increased from 1% to 6%. By delicate design of the tandem cell structure (MAPbI3 on top of CIS), Uhl reported the record high efficiency of 18.5% among solution-processed two-terminal chalcogenide-perovskite tandem solar cells.