Symposium X: Frontiers of Materials Research

Symposium X Alessandra Lanzara_800x800Alessandra Lanzara, University of California, Berkeley, and Lawrence Berkeley National Laboratory

Engineering Two-Dimensional Heterostructures with a Twist 

Written by Arthur L. Robinson

The past few years have seen exciting new opportunities emerging from simply stacking and/or twisting together atom-thick layers of the same or different materials. The lattice mismatch or rotational misalignment introduced by such stacking gives rise to long-range Moiré patterns that lead to modification of the electronic band structure, which in turn gives rise to the appearance of unexpected properties, such as Mott-like behavior and superconductivity, even in weakly interacting systems such as graphene.

In her Thursday Symposium X presentation, Alessandra Lanzara of the University of California, Berkeley, and the Lawrence Berkeley National Laboratory described recent investigations by her group on twisted and strained graphene and transition metal dichalcogenide (TMD) heterostructures as a function of twisting angle and gating. Using angle-resolved photoemission spectroscopy, the group studied the effect of such misalignments on the electronic structure of these materials, yielding insight on the key parameters that lead to the onset of strong correlation and novel behavior in these materials.

Lanzara opened her talk by introducing the importance of topology as an essential theoretical tool in understanding the properties of materials. Until recently, thinking about transitions in crystalline solids has been based on order parameters related to symmetry breaking and correlations. Topology has now joined these as an organizing principle of matter. In general, topological properties are those that are preserved under continuous deformation. For example, in a topological insulator there is no sharp phase transition, but the insulator property is preserved as the electronic band structure is continuously deformed.

Symposium X Alessandra Lanzara 2_800x800

With the addition of topology, said Lanzara, it is now possible to describe the various states of materials now on a single diagram with a correlation energy on one axis and the spin–orbit coupling on the other. Close to the origin, conventional metals and insulators are well described by band theory. As the correlations increase, Mott insulators come to the fore, whereas topological insulators and semimetals come to the fore when spin–orbit coupling increases. But the future may lie in the panoply of exotic behaviors like Weyl insulators that arise as both correlations and spin–orbit coupling grow.

“What new cooperative phenomena and particles will occur when you bring together correlation, spin orbit coupling, and topology?” Lanzara asked next. Taking a hint from physicist Richard Feynman’s famed questions about two-dimensional pages, the question became “What would the properties of materials be if we could really arrange the atoms the way we want them?” But how would one go about exploring this immense space? One way to arrange atoms is by means of heterostructures consisting of stacks of materials with different properties with relevant aspects being dimensionality, coupling to the lattice, order (spin, charge, orbitals, and Cooper pairs in superconductors), and electrostatic doping. Outcomes of building these structures include emergent phenomena at interfaces, such as ferromagnetism, superconductivity, and metal-to-insulator transitions.

From here Lanzara rapidly reviewed some considerations, such as electron screening, and methods for controlling the electronic structure in the context of searching for new phenomena. In particular, her group found that twisting the layers in the heterostructure provided a new level of band-structure control. In fact, when Lanzara was being introduced as the speaker for this Symposium X, the MC used the term twist-tronics.

After discussing engineering of topology and strong correlation, including local inversion-symmetry breaking in the heterostructure layers that gives rise to spin–orbit coupling, Lanzara turned toward the possibility of an even larger phase space for materials design, moving from periodic crystals with both long- and short-range order, to quasiperiodic crystals with order but are not periodic, to Floquet crystals that are periodic in time, and ending with amorphous materials with no long-range order but perhaps some short-range order. After asking if amorphous systems can be used for materials engineering, she reported some early results on the amorphous topological insulator Bi2Se3. One task was to find a replacement for the momentum quantum numbers (kx, ky, kz) in crystals. The group was thinking of an (average) bulk Hamiltonian as spherically symmetric in k-space, resulting in a wavefunction parameterized by k2 and the angles q and f in a spherical coordinate system.

Lanzara summed up her presentation by declaring that two-dimensional heterostructures constitute an incredible, highly tunable platform for exploring correlation, symmetry breaking, and topology. The electronic structure of two-dimensional van der Waals materials is extremely easy to modify, including effects such as symmetry breaking to induce gap opening and renormalization effects due to screening, and spin–orbit coupling and other many-body interactions. But questions still ripe for investigation include: Can we design new types of many-body topological properties and new particles? And what new phases can result from the interplay between them?

Symposium X—Frontiers of Materials Research features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.


Symposium X: Frontiers of Materials Research

IMG_9094_800x800Francesco Stellacci, École Polytechnique Fédérale de Lausanne

Using Materials Science Concepts to Design New Drugs

Written by Don Monroe

Materials scientists have contributed to innovations in delivery of drugs, but only rarely to the drugs themselves. Francesco Stellacci described the efforts of his group to develop drugs against viruses. Viral diseases kill millions of people each year, mostly in the developing world, and new viruses are continually evolving or crossing over from animal reservoirs.

Although the best remedy is vaccines, these are not available for many important viruses. Once a disease is established, Stellacci said, there are only two drugs available: the cocktail against HIV, and drugs for Hepatitis C. These antiviral drugs work by interfering with specific machinery inside the cell that the viruses exploit.

A second approach is directly destroying viruses before infection, but such virucidal agents are toxic. A third strategy is blocking viral attachment to cells with a virustatic compound, such as heparin, that coats the virus. This idea had been explored in an HIV-blocking cream, but in a clinical trial it actually increased the rate of infection by concentrating viruses and releasing them into the bloodstream.

For Stellacci the key thermodynamic problem is that binding is reversible when the concentrations decrease. His inspiration was to build on binding by irreversibly opening up the virus. “We can always take a reversible process and build a cascade of events that make it irreversible,” he said, just as reversible electrochemistry becomes irreversible during corrosion.

The ligands through which viruses attach to cells vary widely, but among viral families there are a few that are highly conserved. These “attachment ligands” point to cell surface components that also rarely mutate, such as sialic acids (bound by influenza and others) and sulfonic-acid–bearing heparan sulfate proteoglycans (bound by dengue, Ebola, herpes simplex, HIV, HPV, and others). Targeting these ligands could allow broad-spectrum antiviral drugs.

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Stellacci’s idea was to introduce multiple targets for attachment-ligand binding (initially sulfonic acid groups), each tethered by a large, flexible, hydrophobic chain to a common platform (initially a gold nanoparticle). The enforced proximity of the hydrophobic chains will create local pressure that could induce the virus to break up.

Experiments confirmed that this structure killed viruses in vitro at nanomolar concentrations, which is sensitive enough to be interesting but could have been achieved with heparin. To demonstrate irreversibility, Stallacci’s team developed an assay by killing 99% of the viruses and then highly diluting the mixture to see if the inhibition persisted. This assay proved that the effect was indeed irreversible for a broad spectrum of viruses.

As expected, the effect became reversible when the researchers replaced the hydrophobic chain with ethylene glycol or replaced the sulfonic acid group with a carboxylic acid group.

The researchers went on to demonstrate effectiveness against HSV-2 in human vaginal tissue ex vivo. They also successfully treated mice with respiratory RSV infection in vivo using an intranasal spray.

Importantly, Stellaci’s team showed that the initial gold-nanoparticle anchor was not necessary, achieving similar effectiveness with a cyclodextrin acting as a rigid backbone for the sulfonated chains. This permits larger scale production, which let the researchers demonstrate that treatment releases the viral DNA payload, confirming the breakup of the viruses.

Although the results so far have been positive and support further experiments, Stellacci cautioned that there is still much work to be done, for example to deliver the necessary local and global concentrations in large animals. But he stressed that even if this particular approach falls short, the work demonstrates the effectiveness of using materials-science principles in drug design. Stellacci also urged researchers not to limit their attention to the problems of rich populations.

Symposium X—Frontiers of Materials Research features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.

 


Symposium X: Frontiers of Materials Research

IMG_8698_800x800Evelyn N. Wang, Massachusetts Institute of Technology

Nanoengineered Materials for Advanced Energy and Water Technologies 

Written by Arthur L. Robinson

Evelyn Wang, head of MIT’s Mechanical Engineering Department, opened her Tuesday Symposium X presentation by stating that the goal for her research programs was to take advantage of nanoengineered materials and their new functionalities to realize exciting, untapped potentials for advancing energy and water technologies. The emphasis of her talk was more on using nanoengineered materials to solve problems than on methods for developing new materials. In particular, she provided examples of how her groups were leveraging nanoscale manipulation capabilities to develop devices for advanced thermal management, solar thermal energy conversion, and water harvesting.

For context, Wang reviewed the current breakdown of US energy consumption (97.4 quadrillion BTU) by production technology, showing that renewable-energy technologies accounted for only 10% of the total. Energy for residential and commercial buildings, transportation, and industry is dominated by fossil fuels (petroleum, natural gas, and coal), with the consequence that the resulting greenhouse-gas emissions are driving the dire climate-warming future that we will inherit if nothing changes. In working to mitigate climate change, Wang’s goal is to do so while retaining our current, comfortable daily lives as much as possible.

Of the various renewable-energy technologies, solar energy remains underutilized at about 6% of the overall 10% renewable share. Wang’s group aims to develop more efficient and lower-cost solutions to make solar energy utilization more viable. They chose to focus on solar thermal rather than photovoltaic power in part because solar thermal is currently more amenable to low-cost energy storage and thus make solar thermal a reliable, around-the-clock technology. The idea is to harness solar energy in the form of heat from a field of solar collectors to generate electricity via a steam cycle. At present it is still a low-efficiency (overall solar-to-electricity conversion efficiency of 10-15%, and high-cost technology. Wang’s group looked at two ways to boost the efficiency.

First, they identified a possibility to increase the efficiency of the electricity-generation side by decreasing the steam-condenser temperature and turbine backpressure. It turns out that condensation heat transfer is hampered by a liquid film, whereas droplet-covered surfaces are more efficient, but droplets tend to wet the surface, resulting in a spread-out film in the end. Wang discussed her group’s recent work that takes advantage of novel surface designs to control and manipulate wettability and liquid-vapor phase-change processes. They demonstrated high-flux evaporation from ultrathin nanoporous membranes and nanostructured “non-wicking” surfaces that can repel liquids even during condensation, preventing wetting of the surface. The pores are the key, tending to support the droplets rather than letting them spread out.

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Second, the group looked at how nanoengineered materials could be used to increase the efficiency on the solar-collector side. Specifically, they worked on optically transparent, thermally insulating silica aerogel solar receivers to enhance the solar-collection efficiency over that of the traditional trough-shaped mirror arrays. The basic high-efficiency, air-stable device comprises a sandwich of protective glass covered with the aerogel on top of which a painted black absorbed layer interfaces with an array of fluid-filled pipes covered by black absorber layers. Solar radiation incident on an array of linear Fresnel reflectors reflects the light to the aerogel device, where it heats the fluid in the pipes. The keys are the high transparency and the low thermal conductivity of the group’s specially developed aerogel, which allows the sunlight to pass through the aerogel and heat the fluid in the pipes without heat losses through the aerogel.

Related work with aerogels included double-pane windows with aerogel between the panes. Here the key was to find a solution to the haze that can degrade the view through the pane. With the MIT aerogel, insulation performance nearly equaled that of triple-pane windows at a cost closer to that of conventional double-pane windows. A second application was the use of polyethylene aerogels in radiative cooling devices. Here, the key is an overlap between the blackbody emission spectrum and an infrared atmospheric window to allow radiative cooling on earth. Daytime cooling to below the ambient temperature requires optically selective surfaces or geometries to achieve high solar reflectivity and high infrared emissivity. High solar absorption and poor thermal insulation are challenges to cooling. A porous, low-density aerogel made of polyethylene was a good fit, demonstrated by successful tests at a site in the arid Atacama Desert region of Chile.

Speaking of arid climates, Wang finished up with a description of water harvesting powered by natural sunlight. In the most arid regions of the earth, water scarcity is so extreme that it is present all year with no relief. Overall water scarcity affects 20% of the global population. Water in the air is a resource equivalent to 14% of freshwater in the lakes of the earth (13,000 trillion liters). To address water-scarcity challenges in arid climates, Wang’s approach to the problem was to look at a water-harvesting device that leverages the unique properties of metal–organic frameworks (MOFs). Dewing is one existing water-harvesting technology, but it is only suited for high relative humidity conditions and thus not appropriate for arid environments. Working with researchers at the University of California-Berkeley, the group devised a water-harvesting device based on porous MOF in a reflective enclosure that adsorbs water molecules at night and releases it as the sun heats it during the day. The water vapor then condenses and is collected. Depending on the MOF, the device is effective at relative humidities below 20% in tests in Tempe, Arizona.

In summing up, Wang pointed to scale-up of materials to larger quantities, cost reduction, robustness over a long life, and improved performance as significant challenges to transferring the technology to the commercial world.

Symposium X—Frontiers of Materials Research features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.


Symposium X: Frontiers of Materials Research

IMG_8471_800x800Brian Litt, University of Pennsylvania

Ghost in the Machine—Translating New Technologies into Next-Generation Neurodevices

Written by Don Monroe

As a medical doctor as well as a professor of neurology, neurosurgery, and bioengineering, Brian Litt was well positioned in his Symposium X talk to share the phenomenal impact that neural interfaces are having on patients’ lives. These devices owe much of their success to the brain’s ability to adapt to their still-primitive signals, he stressed. “It’s because of neuroplasticity patients that can actually use this stuff.”

Litt illustrated the disappointingly low resolution available from retinal implants, and the distorted sounds available from a few frequency bands from a cochlear implant. He also briefly described advances in speech synthesis and motor control through human-machine interfaces. In addition, he listed a host of diseases that might be treated through manipulation of peripheral nerves. “While these are amazing accomplishments,” he said, “there’s a lot more to be done.”

Litt encouraged materials scientists to contribute their expertise to these highly interdisciplinary problems: “We need both incremental and disruptive innovation.” He noted the importance an institutional framework that includes animal models and an infrastructure for ultimately translating innovations from the bench to the clinic, as well as funding for commercialization. “These don’t all have to be at your institution,” he stressed, although he noted that “at Penn, we have most of this together.”

Technical challenges include smaller sensors with higher resolution and exchange of high-bandwidth signals between embedded devices and the outside world. The devices themselves can create tissue injury and degrade in the biological environment. In particular, biocompatibility requires mechanically similar materials to reduce damage during inevitable tissue movements.

All devices that monitor or stimulate neurons face many common issues, but there are additional challenges for implants in the brain, such as those Litt and his collaborators have explored for epilepsy. Litt chronicled one patient’s often unsatisfying history, which included previous crude procedures and failures to identify the precise brain region that initiated seizures. Litt and his colleagues eventually located and laser-ablated the correct region and eliminated daytime seizures, but limitations of imaging, tissue damage, and poor understanding of the disease neurophysiology remain frustrating.

For analysis, Litt envisions combining onboard processing with transmission of salient data to the cloud for more intensive processing. There is also room to improve the algorithms, as illustrated by a recent USD$25K competition to detect or predict seizures from neural data, which significantly improved on previous methodologies.

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Litt listed hopes for future brain technology, including “native delivery” that avoids invasive surgical procedures. Flexible targeting of denser electrodes, or even moving electrodes, could help zoom in on areas of interest. He also mentioned several promising ideas, many discussed at previous MRS meetings. These include electrodes that are flexible or can be injected into the brain, although he stressed the importance of probing the deep folds of the brain and not just its surface. Another proposal would exploit the blood vessels to deliver injected devices that can later be mapped.

There are many other potential approaches, such as “living electrodes,” magnetically activated liposomes, and microfluidically controlled electrodes, magnetothermal delivery, and ultrasonically activated “neural dust.” Optogenetic manipulation of brain circuitry is “absolutely possible,” Litt said, although the path to human use remains undefined. Ultrasound can also directly perform neural modulation or can be used to direct light for optogenetics.

“There’s tremendous opportunity,” Litt said. “What we’re doing is impressive, but still crude. Even this is life-changing.”

Symposium X—Frontiers of Materials Research features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.


Symposium X: Frontiers of Materials Research

SymposiumX2_800x533Bart Biebuyck, The Fuel Cells and Hydrogen Joint Undertaking

Development of Fuel Cells and Hydrogen Technologies in Europe Toward Commercialization from 2020 Onward

Written by Prachi Patel

Bart Biebuyck gave an excellent overview of the progress on fuel cell and hydrogen technologies in Europe. The Joint Undertaking is a public-private partnership of the European Commission, and the industry and research arms of the Hydrogen Europe group. With 1.7 billion Euros in funding, the partnership’s mission is to accelerate research and development (R&D) and bring these technologies to market readiness by 2020.

The goals of the Undertaking are to produce hydrogen in a green way by using less critical raw materials; low-cost fuel cells for transportation, heat, and electricity; and, the key driver for Europe, hydrogen storage for integrating renewables on the grid.

Several ongoing projects on electrolysis to produce hydrogen have already generated materials breakthroughs and slashed the cost of electrolysers since 2011, he said. This has boosted capacity. Megawatt scale electrolysers that produce green hydrogen fuel are now operating at various industrial plants around Europe. A large bakery in Völs, Austria, for instance, uses the hydrogen from its 3.4 MW hydro-electricity-powered electrolyser to heat bread ovens. This offsets the carbon emissions from baking bread, each gram of which produces a gram of carbon dioxide, Biebuyck said.

Research is also underway on using solar power to split water, but improvements in efficiency are needed, he said. The EU industry has launched an initiative to have a 40 GW electrolyser by 2040.

Next, he addressed progress in the area of fuel cells for transport. The focus of materials research here is to find catalysts that use little or, ideally, no platinum. Today’s platinum-based catalysts make up a third of fuel cell cost. Another avenue to reduce cost is to eliminate rare-earth materials found in some components.

The Joint Undertaking also supports fuel cell vehicles and infrastructure. Asian car manufacturers dominate the market today, but some European auto companies plan to have hydrogen car prototypes by 2025. As for refueling stations, there are 120 now in Europe, but 50 member states have committed to building more, reaching a target of about 850 by 2025. “We are also focusing on fuel cell buses to clean up cities,” Biebuyck said. Fuel-cell buses are expected to reach cost-parity with diesel and battery buses in the next 2–3 years. Meanwhile, the first fuel-cell garbage trucks are starting to appear on the market. 

Biebuyck went on to talk about the potential of fuel cells in railway transport, and promising demonstrations in hydrogen-powered aircraft and ships. For ships, there needs to be regulations, and there is a need for research on liquid-hydrogen storage and megawatt-scale fuel cells for ships.

Finally, in the area of heating and cooling, there is a need for research advances in solid-oxide fuel cells. Breakthrough concepts like 3D-printing are being funded by the Joint Undertaking. And installations of “washing machine-sized” micro combined heat and power systems are going up steadily in Europe.

Biebuyck ended by giving a glimpse into the future. Last year, 28 European countries signed an agreement to work on hydrogen research. In the 100 billion Euro Horizon Europe research program “you will find hydrogen and fuel cells many times in the text, even more than batteries,” he said.

But for solid progress to be made in this area, international cooperation is going to be critical, he stressed. And there is a dearth of talented materials scientists and engineers in the area. “We really need you,” he said to the audience, “because in hydrogen and fuel cells, materials research is very important. Look at hydrogen fuel cells, because I guarantee you it will be a successful future.”


Symposium X: Frontiers of Materials Research

Thursday_Symposium X_800x533Jonathan Arenberg, Northrop Grumman Aerospace Systems

NASA’s James Webb Space Telescope

Written by Prachi Patel

Many scientists and space enthusiasts eagerly await the launch of the James Webb Space Telescope, which NASA has now set to March 2021. In his talk, Jon Arenberg described the mission, design challenges, and development of “NASA’s next great leap in space science.”

He described the four fundamental science objectives of this powerful astronomical observatory. The JWST aims to detect light from the first glowing objects in the universe, the earliest stars and galaxies, to “see the beginning of time,” he said. It will also help scientists understand the assembly of galaxies; the birth of stars and planets; and investigate planetary systems to understand the origins of life.

Meeting these lofty goals poses many design challenges. Detecting very dim objects equates to being able to detect one or two photons. This requires large mirrors, an infrared telescope since the earliest stars and galaxies are red-shifted, and to keep equipment cold and stable. As an example of the strict materials property and behavior prediction needs for the telescope, Arenberg showed how, in 2006, Northrop Grumman demonstrated that the bonded composite backplane had a predictable distortion of just a few nm/K.

Then he detailed the telescope’s design: a 6.5 m telescope, a mirror that is more than 7 times Hubble’s area and weighs about half. The cooling requirement is especially tricky, with a sun-facing side that is 340–370 K, and a cold dark side that need cryogenic temperatures of 25–90 K. This temperature difference occurs over a 4.5-foot separation.

Materials science helps meet these challenges, Arenberg said. The five-layer sunshield is silicon-clad plastic, while the cold-facing side is vapor-deposited aluminum. These materials plus the four V shapes formed by the sunshield allow the drastic 300-K-drop in temperature. The large mirror is made of beryllium, which he said they chose because it is lightweight and has very low thermal expansion over the 45 K range of operational temperatures.

Arenberg then described other challenges that JWST engineers have addressed, such as the factors for determining the JWST’s circumsolar orbit, and the reliability needs to have such a large telescope that can be folded and unfolded.

Through a series of splendid photos, he illustrated the development and testing of the JWST’s parts of tests in NASA and Northrop Grumman’s immense test facilities. The entire telescope has also been tested end-to-end at NASA’s Johnson Space Center. Once all the tests are done, the telescope will be shipped to South America for launch.

Arenberg ended by looking into the future. “Science is very keen on answering questions such as how did the universe start, how does it work, and are we alone?” He said that answering such fundamental questions requires making nano- and pico-meter level measurements and predictions, which will need systems far beyond the level of Webb. “Uncertainty is expensive,” he said. “Uncertainty can be a mission-killer.” Removing this uncertainty will come down to clever system design and precisely understanding the fundamental properties and behavior of materials.


Symposium X: Frontiers of Materials Research

SymposiumX3_800x533Sunita Satyapal, US Department of Energy

Hydrogen and Fuel-Cell Technology Perspectives

Written by Prachi Patel

The energy landscape is changing in the United States. The country uses almost 100 Quadrillion BTUs of energy, an increasingly larger share of which is coming from natural gas and oil. Renewables form about 11% of the mix. And hydrogen fuel is one small part of that portfolio, Sunita Satyapal told her audience during Symposium X.

As a brief introduction to the first element, Satyapal outlined hydrogen’s good and bad. The gas has one of the highest energy content by weight of all conventional fuels. But tables turn when it comes to volumetric capacity, with hydrogen faring four times worse in energy content when compared to gasoline.

Satyapal focused on four main messages: the progress in and current status of hydrogen and fuel cell technologies; the US Department of Energy’s (DOE) H2@Scale initiative that aims to enable innovations that could make hydrogen a cost-competitive fuel; pressing research and development (R&D) needs and challenges; and how materials science and research collaborations could help address those.

Satyapal first highlighted the tremendous progress and commercial success in the area of fuel cells. Statistics include 12,000 fuel cell cars, more than a quarter million residential fuel cells in Japan, and more than 25,000 fuel-cell forklifts. She mentioned that steady market growth in the area is mostly in the transportation sector.

After showing the audience a sample of a fuel-cell membrane electrode assembly, Satyapal presented her main takeaway: that hydrogen fuel cells bring immense carbon emission benefits. In terms of life-cycle emissions, today’s gasoline vehicles give 450 grams of carbon dioxide equivalent emissions per mile. By comparison, due to the 60% efficiency of fuel cells, even if hydrogen is produced from natural gas, that carbon burden goes down to 252 grams of CO2e per mile.

Next Satyapal introduced DOE’s H2@Scale program, which aims to provide reliable and affordable hydrogen to all sectors. DOE has assessed hydrogen demand and supply across the U.S. and found that most regions have enough resources, she said.

Moving on to R&D needs, she said DOE’s main goal is to reduce cost. The costs of fuel cells, hydrogen fuel production and delivery infrastructure, and onboard storage systems are all still too high. The H2@Scale program plans to focus on R&D to cut costs in all three areas with an increased budget for DOE’s Fuel Cell Technologies Program in 2019.

To this end, DOE has created and is funding several different consortia. Satyapal talked about the promising work that has emerged from some of the materials-focused research consortia. The ElectroCat consortium, for example, aims to eliminate expensive platinum catalysts from fuel cells. In a recent article in the journal Science, members of the consortium reported an Iron-N complex that shows promise. “This is really exciting news,” she said. Another bit of exciting research comes from the HydroGEN consortium, with researchers at the University of Colorado Boulder showing successful use of machine learning to screen stable perovskites that can split water. The Hydrogen Materials­–Advanced Research consortium (HyMARC), meanwhile, is accelerating the development of breakthrough hydrogen storage materials.

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Satyapal then addressed an area of research where the MRS audience might be able to help: hydrogen embrittlement, the phenomenon in which metals take up hydrogen, lose their ductility, and then crack. This is where DOE’s latest consortium, H-MAT, could help. Researchers in this consortium are trying to understand degradation mechanisms in storage materials, which include metal and polymers. Understanding hydrogen embrittlement could allow the innovation of better materials for hydrogen storage and delivery.

Finally, Satyapal highlighted the importance of collaborations. Over 20 countries are now members of an organization called the International Partnership for Hydrogen and Fuel Cells in the Economy (IPHE). She focused on Japan and Korea as countries with promising progress in the area. Japan has a goal to steeply increase the use of hydrogen in transportation, heating, and industrial processes by 2030 (the torch at the 2020 Olympics in Japan will be fueled by hydrogen), while Korea has announced USD $1.8 billion for hydrogen research.

To end, she pulled up a slide with a simple “Δt” on it, representing the concept of time. “The past always seems closer than the future,” she said. The last 20 years have gone by fast. And so will the next 20, even though 2040 looks far away right now. “We really need to accelerate progress.”


Symposium X Speaker and National Academy of Engineering Member Molly Stevens

Molly M. Stevens, Imperial College London, discusses her Symposium X talk, "Designing Bio-Responsive Hybrid Materials."

 

MRS TV presents new broadcasts each day during the Meeting.  View it on monitors throughout the convention center, online at mrs.org/spring2019, and in the following hotels:

  • Sheraton Phoenix – channel 89
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  • Renaissance Phoenix – TV in lobby

Symposium X: Frontiers of Materials Research

Symposium-X_Tuesday_800x533Molly M. Stevens, Imperial College London

Designing Bio-Responsive Hybrid Materials

Written by Don Monroe

In her Symposium X presentation, Molly M. Stevens described a wide range of examples from her group employing materials in biomedical applications, as well as applying advanced materials characterization techniques to understand and improve the materials. She is also helping direct a project to bring advanced diagnostic capabilities enabled by cell phones to people in Africa.

Developing materials for regenerative medicine requires a balance between the complexity of the materials and the need to translate them into clinical use, Stevens stressed. A simple system that exploits the body’s natural regeneration can be effective, as illustrated by a calcium-contained gel that creates an in vivo bio-reactor to grow bone. “Sometimes you can get away with very simple materials systems,” she said, “but there are many other applications where that’s not going to work.”

Stem cells, for example, vary their differentiation in response to numerous mechanical and chemical cues, and different shapes of adhesion for stem cells were known to direct them toward bone-like or fat-like differentiation. Stevens and her research team used high-resolution microscopy, focused-ion-beam, and other tools to reveal how the adhesion geometry affects the cell-substrate interface, as well as the relevant developmental pathways.

They also studied the hierarchical organization of bone, and the relationship between its organic and mineral components, to inform scaffold design. In addition to providing a porous environment matched to the cells, the chemical influences can be profound, as Stevens showed with recent successful bone scaffolds incorporating strontium. She is also collaborating with chemists to create a versatile toolbox for functionalizing polymers in various ways, for example so they interact directly with cells in the body, she said, “because a lot of these conventional polymer systems are not necessarily inclined to do that.”

Stevens also collaborated with researchers from the Howard Hughes Medical Institute to explore arrays of very small needles made of porous silicon. The nanoscale porosity of the needles enables delivery of molecules, because the needles induce formation of vesicles that are taken into cells. Incorporation of a vascular growth factor in this way induced growth of new blood vessels.

The needles can further be used for monitoring tissue. Stevens showed on example where a needle-laden patch loaded with a biomarker identifies the margins of a tumor to assist surgeons. The needles were also effective for introducing quantum-dot biosensors the group had previously developed for lab drug screening, with the potential for real-time, in vivo, intracellular monitoring.

In her final topic, Stevens described the use of nanoparticle-based diagnostics in developing countries. The widespread availability of cell phones and their high-quality cameras, she said, creates an “opportunity to help democratize access to health care.” She is deputy director of a center that aims to implement these tools for early diagnosis.

For example, the center developed a tool based on nanoparticle-based “nanozymes” that are more temperature-stable than traditional enzymes and can directly detect one of the coat proteins of HIV at the point of care. “We were able to develop the most sensitive point-of-care test for HIV that had been developed to date.” She and her colleagues are working to distribute the test in South Africa, where workers have already performed 40,000 tests.


Symposium X—Frontiers of Materials Research

SymposiumX_800WidthErik Bakkers, Technische Universiteit Eindhoven

Bottom-Up Grown Nanowire Quantum Devices

Written by Don Monroe

Despite years of effort, scientists have yet to agree on a physical system for realizing the tremendous potential of quantum computing. Erik Bakkers described a relatively new strategy based on networks of epitaxial III-V nanowires that could exploit exotic collective electronic excitations to enable these applications.

In principle, quantum computing provides exponential increases in computational power from a relatively small number of quantum bits, or qubits. Such qubits can simultaneously represent two possible quantum states, but only until interaction with the environment destroys their coherent relationship. “Decoherence is the big problem of a quantum computer,” Bakkers said. “This is really the fundamental bottleneck.”

Bakkers and his collaborators have been exploring a strategy for overcoming this challenge using “Majorana fermions,” which are their own antiparticle and should be highly resistant to decoherence because they have “no charge, no spin, and no energy.” These entities were proposed decades ago as a model for neutrinos, but have recently been suggested to occur as quasiparticles in condensed-matter systems, in particular in one-dimensional superconductors. “If we can find this particle and control the quantum state, we could have very long decoherence time,” Bakkers said.

His group has been looking for this elusive particle in the proximity-induced superconductive state of InSb nanowires, whose electrons have very low effective mass, strong coupling to magnetic fields, and high spin-orbit coupling. Tunneling spectroscopy revealed the expected conductance peak in the center of the superconducting energy gap. “The data is all consistent with having Majoranas,” Bakkers concluded, although the first experiments were limited by a high density of states in the gap.

To make the wires, Bakkers’ team used an established method in which a ball of metal forms a eutectic with a semiconductor, and subsequent layer-by-layer growth on (111) facets produces long, highly uniform single-crystal nanowires. They developed ways to reduce As and P impurities, grow wires up to 60 µm long, and improve the interfaces, achieving low-temperature mobilities as high as 60,000 cm2/V·s. These nanowires produced extremely clean-induced superconducting gaps, and a magnetic field produces a clear mid-gap state with the predicted quantum conductance of 2e2/h. “We believe this is a very strong signature of having these Majorana states,” Bakkers said.

Exploiting these states for quantum computing will require fashioning these high-quality wires into loops and other circuits. Bakkers showed how to create such structures within the evacuated growth chamber. The gold balls that seed the nanowire growth were defined lithographically on (111) facets of a V-groove etched into a (100) substrate. Choosing the size and location of these seeds let the team create long wires that cross, merge, or shadow each other during later deposition.

In particular, Bakkers described “hashtag” structures (#) with pairs of InSb wires from opposing faces that merge to make “very high-quality wire-wire junctions,” although there can be twin boundaries at the interfaces. Painstakingly dislodging these structures in the electron microscope with a micromanipulator and transferring them to a separate substrate for electrical contacting allows their electrical characterization. The junctions showed the quantized conductance characteristic of ballistic transport, and the oscillating conductance as a function of magnetic field through the loop of the hashtag indicated a coherence length as large as 60 µm. Majorana-based qubits will require somewhat more complex structures, but these appear to be feasible.

Still, Bakkers noted that these techniques are “nice for academic studies, but not really scalable.” He and his collaborators are therefore exploring an alternative future path based on in-plane selective-area growth to create arbitrarily complex circuits in vacuum.

Symposium X—Frontiers of Materials Research features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.