In-person in HCI J 4 or on Zoom: https://ethz.zoom.us/j/69001008210

Birth of solids studied by liquid-phase electron microscopy

Rolf Erni (Electron Microscopy Center — EMPA)

There are thermodynamic reasons why solids are solid and why they can be crystalline. How this happens, however, is less evident. Classical nucleation theory extrapolates thermodynamic properties of bulk materials to systems that merely consist of dozens of atoms or even less. We are interested in studying how solids become solid and how they adopt crystallinity. We try to mimic the formation of nanocrystals in small systems that we can observe by electron microscopy and aim at monitoring the atomic mechanisms that lead to the formation of crystalline matter. We study Pt and Au atoms in the vacuum environment of the microscope and activate them by temperature and the electron beam to form clusters. Approaching more realistic systems, we use small nanoreactors made either of nanodroplets of vacuum-compatible ionic liquids or graphene-based liquid cells to study the formation of nanocrystals in liquids. Although the inevitable electron beam might complicate the data interpretation, all our observations reveal that realistic nucleation reactions are more complex than what the classical nucleation theory predicts, and this enhanced complexity is further reflected in the variety of pathways nanocrystals follow in their growth process.

Frustrated frustules: Geometrical frustration in the diatom cell wall

Maria Feofilova (Soft and Living Materials — D-MATL)

Diatoms are single-celled organisms with a cell wall made of silica, called the frustule. Their elaborate patterns have fascinated scientists for years, however little is known about the biological and physical mechanisms involved in their organizations.

In this work, we take a top-down approach and examine the micron- scale organization of diatoms from the Coscinodiscus family. We find two competing tendencies of organization, which appear to be controlled by distinct biological pathways. On one hand, micron-scale pores organize locally on a triangular lattice. On the other, lattice vectors tend to point globally toward a center of symmetry. This com- petition results in a frustrated triangular lattice, populated with geo- metrically necessary defects whose density increases near the center.

In-person in HCI J 4 or on Zoom: https://ethz.zoom.us/j/66595365893

Accelerating the search for functional materials using machine learning methods

Aria Mansouri (Materials Theory — D-MATL)

I will show several examples of the application of machine learning in materials design.
First, I discuss how screening crystal structure databases using machine learning models constructed based on compositional and structural features led to the development of two superhard materials. Further, to screen for compounds beyond the existing databases, we used an ensemble learning method to directly predict the load-dependent Vickers hardness, using the composition as input. Such a composition-based model is useful to rapidly screen through composition spaces to focus the search space; however, neglecting the influence of crystal structure limits its application. One such example is ferroelectricity. Therefore crystal structure prediction is essential to finding new ferroelectric materials.
A ferroelectric candidate possesses a polar crystal structure, is insulating and thermodynamically favorable. To search for candidates that fulfill these requirements, we developed a framework that consists of a series of machine learning models in conjunction with high-throughput DFT calculations and group theoretical analyses capable of predicting the crystal structure of any given composition. Finally, we developed a machine learning model based on distortion modes to predict new meta-stable crystal structures of BiFeO3.

Soft magnetic Fe-based bulk glassy alloys: a comprehensive case study

Mihai Stoica (Metal Physics and Technology — D-MATL)

The Fe-based metallic glasses (MGs) are very promising for applications. Although they are metallic alloys consisting of common chemical elements, the amorphous structure that characterizes the MGs can be obtained only upon quenching the master alloy from the molten state. The resulted samples are usually in form of ribbons, having thicknesses up to few tens of micrometers.

They are soft magnetic materials, showing extremely low coercivity and high permeability, combined with a relatively high saturation magnetization. Therefore, they possess low magnetic losses, making them attractive for electrical transformers, sensors and actuators. Moreover, the soft magnetic properties may be further enhanced upon nanocrystallization. Also, they can be tailored by a proper control of the resulting nanostructure. This particular structure can be achieved only upon annealing the amorphous precursor. A major drawback is that by nanocrystallization the MGs become mechanically brittle and difficult to manipulate. Therefore, a bulk glassy alloy would be more feasible for applications as small parts in e.g. magnetic clutches or actuators.

In the last several years we investigated the possibility to create such bulk nanocrystalline alloys, with customizable structures and magnetic properties. Although the magnetic behavior is understood, the mechanism of the nanocrystallization could not have been resolved without employing state-of-the-art investigation methods, as time-resolved X-ray diffraction in transmission configuration using synchrotron radiation, in-situ and ex-situ transmission electron microscopy and atom probe tomography. The current presentation will guide the listener through these studies and will shed light on the nanocrystallization mechanism.

In-person in HCI J 4 or on Zoom: https://ethz.zoom.us/j/68765823059

Sodium ion batteries: Opportunities and challenges

Eldho Edison (Multifunctional Materials — D-MATL)

Among the existing energy storage technologies, lithium-ion batteries (LIBs) have unmatched energy density and versatility. From the time of their first commercialization in 1991, the growth in LIBs has been driven by portable devices. In recent years, however, large-scale electric vehicle and stationary applications have emerged. These large-scale applications have put unprecedented pressure on the LIB value chain, resulting in the need for alternative energy storage chemistries.

The Sodium-ion battery (SIB) chemistry is one of the most promising “beyond-lithium” energy storage technologies. In this talk, the technological evolutions of both LIBs and SIBs, the key differences/similarities between the two battery chemistries and the prospects and challenges for the commercialization of SIBs will be presented. The research progress and the challenges of high-capacity alloying anodes for Sodium-ion batteries will be discussed.

Magnetic vortices: into the third dimension

Sebastian Gliga (Laboratory for Condensed Matter — PSI)

Vortices are familiar phenomena in fluids and gases, apparent for example in tornadoes, hurricanes, and whirlpools. Vortices also exist in ferromagnets, where they are characterized by a circulating in-plane magnetization structure. The resulting pattern leads to a very stable state whose study is motivated by both fundamental and technological interest.
Over the past decades, vortices have been extensively studied in thin-film structures, where the magnetization is accessible with two-dimensional imaging methods. Recently, the development of X-ray based magnetic nanotomography with a spatial resolution of 100 nm has enabled the non-destructive imaging of bulk magnetic structures. We have uncovered three-dimensional structures forming vortex loops corresponding to magnetic vorticity rings that are formally analogous to hydrodynamic vortex rings in fluids. Remarkably, we have also observed structures that have no counterparts in incompressible fluids: stable vortex loops intersected by magnetic singularities.

While vortices have been studied in bulk magnets since at least the 1970s, our results shed new light onto their rich physics and open possibilities for further studies of complex three-dimensional solitons, enabling the development of applications based on three-dimensional magnetic structures.

Zoom: https://ethz.zoom.us/j/69333396880

Tuning polymer dispersity by photoinduced ATRP: monomodal distributions with ppm copper concentration

Richard Whitfield (Polymeric Materials – D-MATL)

Unlike natural biopolymers, such as DNA and proteins, synthetic polymers have a distribution of different molecular weight species. This distribution is measured by a dispersity value and has a significant influence on polymer properties. It is therefore highly beneficial to develop strategies to systematically tune the dispersity, however, current methods are limited to bimodal molecular weight distributions, adulterated polymer chains, or low end‐group fidelity and rely on feeding reagents, flow‐based, or multicomponent systems. To overcome these limitations, we have developed a simple batch system whereby photo-induced atom transfer radical polymerisation is exploited as a convenient and versatile technique to control the dispersity of both homopolymers and block copolymers. By varying the concentration of the copper complex, a wide range of monomodal molecular weight distributions can be obtained. In all cases, high end‐group fidelity was confirmed by MALDI‐ToF‐MS and exemplified by efficient block copolymer formation. Importantly, our approach utilises ppm levels of copper (as low as 4 ppm), can be tolerant to oxygen and exhibits perfect temporal control, representing a major step forward in tuning polymer dispersity for various applications.

Can 2-D Materials Save Moore’s Law?

Mathieu Luisier (Integrated Systems Laboratory – D-ITET)

Since the first experimental demonstration of a monolayer MoS 2 transistor in 2011, transition metal dichalcogenides (TMDs) have received a wide attention from the scientific community as potential replacement for Silicon FinFETs at the end of the semiconductor roadmap. As graphene, TMDs exhibit excellent electrostatic properties due to their 2-D nature, but contrary to it, they are characterized by large band gaps, while keeping decent mobilities. However, so far, no transistor based on a TMD channel could outperform the Si technology. While this limitation can be partly attributed to technical issues, the TMD bandstructure also explains this behavior: electrons/holes are not fast enough to allow for large ON-state currents. Through density functional theory (DFT), the existence of more than 1,800 2-D materials was recently predicted. Among them there might be components with better transport properties than TMDs. We therefore selected 100 monolayers out of this database, combined DFT and quantum transport to simulate their “current vs. voltage” characteristics, and identified 13 candidates with both n- and p-type ON-state currents larger than what Si FinFETs are expected to deliver in the future. In this talk, I will present the results of this study.

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Zoom: https://ethz.zoom.us/j/63151988666

From binary lipid-water phase diagrams to lipid nanoparticle-based mRNA
COVID-19 vaccines

Peter Walde (Laboratory for Multifunctional Materials – D-MATL)

The aim of the talk is to emphasize that basic research on the aggregation behavior of amphiphilic lipids in aqueous solution and on the controlled formation of lipid vesicles (liposomes) for drug delivery applications was essential for the successful development of lipid nanoparticle-based mRNA COVID-19 vaccines.

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Zoom: https://ethz.zoom.us/j/64504425191

Fuel cells, batteries, electrolyzers, etc.: some insights from a materials science point of view

Lorenz Gubler (Electrochemistry Laboratory – PSI)

Electrochemical storage & conversion technologies are expected to play a pivotal role in the energy transition and defossilization of our economy. In addition to batteries used for grid-scale energy storage and electromobility, electrochemical conversion devices using or producing hydrogen, i.e. fuel cells and electrolyzers, can contribute to reducing the carbon footprint of the transport sector and chemical industry. In this seminar, we will be looking at the state-of-the-art of these devices, and highlight selected challenges regarding the choice of cell materials and components. Examples from the research on these topics will be shown to illustrate current limitations of the technology and future prospects.

Do soft solids have strain-dependent surface tension?

Nicolas Bain (Soft and Living Materials – D-MATL)

Despite its importance in any adhesion and wetting phenomena, there is a fundamental property that is not yet understood in soft solids: surface elasticity. Also called the Shuttleworth effect, surface elasticity can be boiled down to one question. Does stretching the surface of a soft solid change its surface tension? In 2017, Xu et. al designed an experiment in which the opening angle of a wetting ridge was a proxy to evidence a dramatic increase of surface tension with stretch. In 2019, however, Masurel et al. claimed that the coupling between nonlinear mechanics and the singular nature of the wetting ridge suffice to explain the behavior of the opening angle observed by Xu et al, without invoking the Shuttleworth effect. The question, therefore, remains open. This presentation will focus on an experimental setup with no geometric singularity, that leaves no doubt on the existence or absence of surface elasticity in soft solids, hopefully closing this long-lasting controversy.

Q. Xu, K. E. Jensen, R. Boltyanskiy, R. Sarfati, R. W. Style, and E. R. Dufresne, Nature communications 8, 1 (2017).
R. Masurel, M. Roché, L. Limat, I. Ionescu, and J. Dervaux, Physical review letters 122, 248004 (2019).

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