MRS Medal Award

MedalRobert J. Cava, Princeton University

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.”


ES3: Perovskite Solar Cell Research from Material Properties to Photovoltaic Function

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.


NM4: Nanomaterials-Based Solar Energy Conversion

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.


EC2: Facilitating Charge Transport in Electrochemical Energy Storage Materials

Ilona Acznik, Institute of Non-Ferrous Metals, Poland

Improvement of Power-Energy Characteristic of Li-Ion Capacitor by Structure Modification of the Graphite Anode

Written by Armin VahidMohammadi

Lithium capacitors can be considered as a promising energy storage system to substitute lithium batteries. Ilona Acznik from the Institute of Non-Ferrous Metals in Poland summarized their findings on lithium-ion capacitors that utilize graphite as a negative electrode and activated carbon as positive electrode. The activated carbon is playing the high surface area material and the graphite is the kind of battery material to create the asymmetric capacitor which can present both high power density of supercapacitors and high energy density of batteries. Results of different types of graphite were presented and it was discussed how structural modification can affect the electrochemical behavior in such a system. Acznik also discussed chemically reduced graphite oxide as a promising material in lithium-ion capacitors. She showed that through modification of graphite and by producing reduced graphite oxide her research group was able to maintain the performance even at high rates.


EC2: Facilitating Charge Transport in Electrochemical Energy Storage Materials

Terri Lin, University of California, Los Angeles

MoS2 Nanocrystals as Fast Charging Pseudocapacitors for Li and Na Battery

Written by Armin VahidMohammadi

For the past several years, nanomaterials have been heavily investigated for energy storage devices. By now, it is proven that going to nanosized particles can significantly change the behavior and properties of materials compared to their bulk form. In her talk, Terri Lin from UCLA discussed how to change the electrochemical charge storage behavior of molybdenum disulfide (MoS2) by changing its size. Supercapacitors and batteries share many similar things but it is well known that battery materials cannot be the proper choice for supercapacitors. Based on charge storage mechanism of materials, they can be divided into electrochemical double layer capacitors or pseudocapacitors, where the pseudocapacitors themselves can be considered either as redox or the intercalating type. She explains the properties of the MoS2 and brings up its promising properties for Na and Li batteries and capacitors. She emphasized an important behavior change in the nanosized MoS2 compared to its bulk. The bulk MoS2 is considered as a good battery electrode, but when its size is reduced to nano-range, it shows pseudocapacitive behavior that makes it a good electrode and material for capacitors. She further mentioned that through electrochemical analysis, the researchers have been able to confirm capacitive behavior of the nanosized MoS2. The results also show that the linear change in the lattice spacing in nano-MoS2 represents a solid solution behavior of this material and introduces the intercalation pseudocapacitive behavior for it.


EC2: Facilitating Charge Transport in Electrochemical Energy Storage Materials

Liqiang Mai, Wuhan University of Technology, China

One-Dimensional Nanomaterials for Energy Storage

Written by Armin VahidMohammadi

In recent years, there has been great interest in novel nanomaterials like one- and two-dimensional materials that can exhibit a variety of interesting properties for energy storage applications. Liqiang Mai from Wuhan University of Technology in China explained the problem with current devices utilizing some type of nanomaterial as electrode. Mai’s group has used several strategies like intercalation with chemical species to improve the electrochemical performance of the devices. The concept of nanowire-based batteries (batteries that utilize different types of nanowires as electrode, i.e. vanadium oxide nanowire/lithiated carbon batteries) was explained and some challenges were pointed out. One way to come up with solutions for current challenges is understanding the problem itself through characterizing the materials that are used inside the battery systems. Mai summarized different materials characterization techniques that have been used to evaluate the materials structural changes during cycling which is mostly believed to be the main reason for capacity fade during the lifespan of the battery. The transport properties of sodium and lithium ions in nanowire-based batteries were also discussed to shed light on possible future research directions in the field of one-dimensional electrode materials for lithium and sodium batteries.


ES1: Materials Science and Chemistry for Grid-Scale Energy Storage

Panpan Dong, Washington State University

Selenium Sulfide Composites as Promising Electrode Materials for Rechargeable Batteries with Enhanced Electrochemical Performance

Written by Armin VahidMohammadi

Sulfur has always been of high interest for the lithium battery community. Panpan Dong from Washington State University talked about their research on selenium disulfide (SeS2) for Li batteries. Selenium has a better cycle-ability and electrochemical life-span compared to sulfur; however, sulfur has a higher capacity. Therefore, researchers have been trying to incorporate the advantages of both of these and find the proper selenium-sulfur composite electrodes. In their research, SeS2 composite was prepared through a two-step heat treatment. The effect of different electrolytes with different solvents, salts, and additives were investigated on the performance and life cycle of the fabricated lithium batteries. Furthermore, after 50 cycles, they had tried to analyze the possible reactions happening in the cell during the several cycles. They prepared the selenium sulfide copolymers as well. It was mentioned that the color of different compositions of their copolymer electrode is different due to different degrees of polymerization. After materials synthesis through several materials characterization techniques such as x-ray diffraction (XRD) and scanning electron microscopy (SEM), structure of the produced electrodes was investigated to compare the differences with electrodes with no polymer. Dong showed that the cycling performance of the cells with copolymer electrodes was greatly improved.


NM4: Nanomaterials-Based Solar Energy Conversion

Jeremy Munday, University of Maryland

Tunable Photonic Elements for Solar Energy

Written by Xiwen Gong

Around 40% of total energy consumption comes from residential and commercial buildings for the purpose of lighting and heating, for example. Smart windows, which can switch between transparent to opaque states, can reduce energy consumption inside buildings by utilizing external lighting and heating sources from the sun. However, smart windows usually consume external energy sources and cannot fully make use of the solar energy during the off (opaque) state.

Jeremy Munday from the University of Maryland designed a device to get around these problems. The researchers chose a polymer dispersed liquid crystal (PDLC) as the active material for smart windows: the windows turn transparent when there is a voltage applied, and stay opaque to protect privacy at the intrinsic state. The PDLC devices were then stacked on top of polycrystalline silicon solar cells. In this way, smart windows function in a self-supporting manner by letting the light through when it is on the on (transparent) state, which can be absorbed by the solar cell and then generate electricity to power the windows in turn.

Solar energy cannot only help power buildings, but also space crafts: a photon can generate a force when it gets reflected or absorbed by an object, then impart its momentum onto the object to provide propulsion. Munday outlined his recent work on the precise measurement of radiation pressure in ambient conditions. This technique provides an effective method to study optical forces of solar sails.


TC4: Advances in Spatial, Energy and Time Resolution in Electron Microscopy

Peter Crozier, Arizona State University

Probing Vibrational and Electronic States with Monochromated Electron Energy-Loss Spectroscopy

Written by Yuanyuan Zhu

Vibrational spectroscopies conventionally carried out using sources such as infrared or neutrons are powerful techniques that can probe many important properties of materials, including bonding arrangements and chemical identity. However, the spatial resolution of these spectroscopies is typically limited and they average over the materials examined. Recent breakthroughs in improving the energy resolution of electron energy-loss spectroscopy in transmission electron microscopes using monochromators has open up the study of vibrational modes in nanostructures. In this talk, microscopist Peter Crozier from Arizona State University presented their application of the state-of-the-art vibrational electron energy-loss spectroscopy in exploring one of the most important challenges in physical and life sciences involving the role of water, hydrate and hydroxyl species on nanoparticle surfaces and interfaces.

Crozier showed that the hydrogen-oxygen fingerprint can be correlated with highly localized structural and morphological information obtained from electron imaging. The use of aloof beam mode for spectral acquisition avoids direct electron irradiation of the sample thus greatly reducing beam damage to the OH bond. These findings open the door for using electron microscopy to probe local hydroxyl and hydrate species on nanoscale organic and inorganic structures.