Category Materials Colloquium

Lecture Hall HCI J4

Dr. Hyun Suk Wang

Laboratory for Polymeric Materials , ETH

Depolymerization of Commercial Polymethacrylates Triggered by Visible Light

The reversion of vinyl polymers with C–C backbones to their monomers represents an ideal path to alleviate the growing plastic waste stream. However, depolymerizing such stable materials remains a challenge, with state-of-the-art methods relying on “designer” polymers [1-2] that are neither commercially produced nor suitable for real-world applications. Here, we report a main-chain-initiated, visible-light-triggered depolymerization directly applicable to commercial polymers containing undisclosed impurities (e.g. comonomers, additives, dyes).[3] By in-situ generation of chlorine radicals directly from the solvent, near-quantitative (>98%) depolymerization of polymethacrylates could be achieved regardless of their synthetic route (e.g. radical or ionic polymerization), end-group, and molecular weight (up to 1.6 million Da). The possibility to perform multi-gram scale depolymerizations and confer temporal control renders this methodology a versatile and general route to recycling.

1. Wang, H.S.; Truong, N.P.; Pei, Z.; Coote, M.L.; Anastasaki, A.; Reversing RAFT Polymerization: Near-Quantitative Monomer Generation Via a Catalyst-Free Depolymerization Approach. J. Am. Chem. Soc. 2022, 144, 4678–4684
2. Wang, H.S.; Parkatzidis, K.; Junkers, T.; Truong, N.P.; Anastasaki, A.; Controlled radical depolymerization: Structural differentiation and molecular weight control. Chem 2024, 10, 388-401
3. Wang, H.S.; Agrachev, M.; Kim, H.; Truong, N.P.; Choi, T.-L.; Jeschke, G.; Anastasaki, A.; Visible-Light-Triggered Depolymerization of Commercial Polymethacrylates. Science 2025, Accepted

Dr. Tenorio Tunas Maria

Laboratory for Magnetism and Interface Physics, ETH

1D and 2D graphene nanoarchitectures with atomic precision: from on-surface synthesis to quantum electronic properties

Bottom-up synthesis has proven to be a highly efficient method to build graphene nanoarchitectures with atomic precision, giving rise to tunable quantum electronic properties. The most illustrative example is the vast array of graphene nanoribbons (GNRs) that have been synthesised almost à la carte. However, achieving greater structural complexity and scaling up to two dimensions remains a significant challenge.

The on-surface synthesis of 2D nanoporous graphene (NPG) [1] marked a breakthrough, demonstrating the feasibility of growing a graphene-based monolayer nanostructure with semiconducting and anisotropic electronic properties, while exhibiting long-range order and uniformly distributed array of atomically precise nanopores.

Here, we present novel synthetic approaches to control the pore shape, size [2], and the dopant distribution [3] of NPG with atomic precision, while preserving the long-range order. This will cover from assessing the on-surface synthesis of doped GNR to 2D NPGs and porous GNRs. Regarding dopant content [3, 4], we demonstrate the stability of heteroatom-containing pores and the ultimate fabrication of a lateral superlattice of heterojunctions, where the junction is reduced to the carbon-carbon bond distance. This novel material exhibits in-gap tunnelling states and interface quantum dipoles, which further refine the heterojunction’s characteristics. For pore size and shape tunability [2, 5], we show that different phenylene-type bridge configurations can lead to different electronic lateral coupling of the NPG components, enabling or suppressing the quantum transport.

Bond-resolved Scanning Tunneling Microscope (STM) has been the tool used to inspect the nanopore characteristics (Figure 1) by functionalising the tip with a single CO molecule. Scanning Tunneling Spectroscopy (STS) has provided key insights into the quantum electronic properties emerging from these novel 2D nanoporous graphene structures, which have been simultaneously corroborated by DFT ab-initio calculations. Our results reflect that the rational design of both specific molecular precursors and reaction pathways can lead to the formation of long-range and multifunctional NPGs.

Figure 1: Bond-resolved constant height Scanning Tunnelling Microscope image of two covalently coupled GNRs: a nitrogen-doped one (left) and a carbon pristine one (right), showing the formation of nanopores at the junction. (STM conditions: 8 nm X 4 nm, 3 mV)

1. C.Moreno M.Vilas-Varela, B.Kretz, A.Garcia-Lekue, M.V.Costache, M.Paradinas, M. Panighel, G.Ceballos, S.O.Valenzuela, D.Peña, A.Mugarza, Science, 360, 199-203 (2018); C. Moreno, M. Panighel, M. Vilas-Varela, G. Sauthier, M. Tenorio, G. Ceballos, D. Peña, A. Mugarza, Chem. Mater. 31, 331-341 (2019)
2. C. Moreno, X. Diaz de Cerio, M. Vilas-Varela, M. Tenorio, A. Sarasola, M. Brandbyge, D. Peña, A. García-Lekue, A. Mugarza, JACS, 145, 8988-8995 (2023)
3. M. Tenorio, C. Moreno, P. Febrer, J. Castro-Esteban, P. Ordejón, D. Peña, M. Pruneda, A. Mugarza, Advanced Materials, 19 (2022)
4. M. Tenorio, C. Moreno, M. Vilas-Varela, J. Castro-Esteban, P. Febrer, M. Pruneda, D. Peña, A. Mugarza, Small Methods 8 (2023)
5. C. Moreno, X. Diaz de Cerio, M. Tenorio, F. Gao, M. Vilas-Varela, A. Sarasola, D. Peña, A. García-Lekue, A. Mugarza (2024)

The 7th edition of the Materials Colloquium 2024 will take place next Tuesday November 5th, at 4.30 PM in HCI G3.

It is a common belief that damage caused by freezing is due to water’s expansion as it freezes. However, in most soft materials, this expansion actually plays a rather insignificant role. Instead, freezing damage is typically caused by the (far less well-known) process of ‘cryosuction’, where ice crystals grow by sucking nearby water towards them. This suction can lead to huge amounts of ice expansion inside soft, wet materials. We study freezing damage with an experimental system that allows us to directly image individual ice crystals growing inside soft, wet materials. The results reveal a range of insights — for example, we find that the freezing damage is surprisingly similar to the damage that arises during the drying of wet materials. Furthermore, unexpected factors, such as the polycrystallinity of ice, can play an important role in the freezing process.

Marianne Liebi (Laboratory for X-ray Characterization of Materials, EPFL)

Imaging with X-ray scattering contrast for the characterization of hierarchical material structure

Microscopy in scanning mode allows to image different contrasts from the sample, by probing not only absorption and phase contrast of the sample but for example a full X-ray fluorescence spectrum or a 2D scattering pattern. Small- and wide-angle X-ray scattering (SAXS/WAXS) imaging is in particular valuable for heterogeneous samples, in which structural elements in the nano- or Ångström-scale change over macroscopic length scales, thus several millimeters or centimeters. The scattering patterns provide statistical information in each scan point, making this method complementary to high-resolution imaging techniques. The information extracted from each scattering pattern can be used to create images with different contrast, such as density of nano-scale features or orientation of nanostructures. These methods can also be combined with computed tomography to study the inside of three-dimensional samples in tensor or texture tomography. A range of applications will be shown from biological tissues to various soft-matter systems.

In-person in HCI J4:

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Elastic Microphase Separation Produces Bi-continuous Materials

Carla Fernández-Rico (Laboratory for Complex Materials — D-MATL)

Phase separation is a fascinating physical process that is not only responsible for the internal organization of living cells but also a promising tool for structuring soft materials. The central challenge in making meso-structured materials via phase separation is the lack of structural control at length scales beyond the macromolecular scale. In this talk, I will introduce Elastic MicroPhase Separation (EMPS) as a route to create bi-continuous structures in the scale of hundreds of nanometers inside elastomeric networks. I will present the analysis of the microstructure, phase equilibria, and kinetics of the system, and show how it features aspects of both nucleation and growth and spinodal decomposition. Finally, I will demonstrate the potential our approach by toughening polymeric materials, and making bi-continuous structures with controlled structural and anisotropic gradients. 

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Unravelling Plasmonic Photoelectrochemistry at Macro- and Micro-scales

Giulia Tagliabue (Laboratory of Nanoscience for Energy Technologies, EPFL)

In the last decade, plasmonic and dielectric nanoantennas have revolutionized light manipulation and control at the nanoscale. Interestingly, hot carriers and photoluminescence in metals have opened new pathways for controlling photo(electro)chemical processes and monitoring temperatures. Yet, fundamental questions remain about the microscopic details of these complex light-matter interactions. We have recently developed a well-controlled experimental platform based on ultrathin monocrystalline gold flakes and scanning electrochemical microscopy that, in combination with theory, enables deeper insight into light absorption and emission processes as well as photochemical transformations. In particular, I will present micro-scale photoelectrochemical measurements clarifying the interplay of hot carrier generation/transport and quantifying the injection probability of high-energy d-band holes at the metal-electrolyte interface, connecting it to the ultrafast dynamics of these carriers in monocrystalline metals. Overall, this microscopic insight is critical to advance the design of plasmonic-based energy devices. I will also show complementary results unraveling the origin of luminescence in gold and how crystallinity improves hot carrier dynamics and transport.

In-person in HCI G4:

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Machine Learning for the Inverse Design of (Meta-)Materials

Dennis M. Kochmann (Mechanics and Materials Lab – D-MAVT)

Multiscale material modeling has two primary goals: (i) understanding and predicant a material’s properties based on its small-scale architecture, and (ii) identifying those small-scale structural features that enable us to control and optimize a material’s properties and performance. Owing to the rise of additive manufacturing, metamaterials (or architected materials) have emerged as a special class of artificial materials with interesting or tunable properties, and as a new playground for computational modeling. While the forward problem (i) has produced many successful modeling techniques across scales, the inverse problem (ii) has remained a challenge: how do we design (meta-)materials with target properties? At the core of this challenge are the abundant design and property spaces, as well as the fact that the map from structure to properties cannot be inverted. Here, we will discuss how in recent years machine learning has offered new opportunities for the inverse design of (meta-)materials through generative models that predict metamaterial architectures with extreme, peculiar, or as-designed mechanical properties

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Harnessing Size Effects: Where Nanostructures Meet Additive Manufacturing

Rebecca Gallivan (Laboratory for Nanometallurgy – D-MATL)

Nanostructured materials promise to unlock new functionality that can address modern challenges in electronics, health, and infrastructure. However, to fully hardness the power of size-based effects, we need to develop new methods for designing microstructurally complex and heterogeneous nanostructures. Furthermore, deep fundamental understanding of designed nanoscale materials is critical for bringing these tools to the materials engineers of tomorrow. Using novel nanoscale additive manufacturing and nanomechanical techniques, we will explore new pathways for nanoscale materials design and investigate the emergingmaterials behaviors that arise at the intersection of geometric and microstructural size-effects. Insights will focus on beginning to untangle the intricate roles of processing and microstructure in advancing nanoscale materials performance.

In-person in HCI G3:

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Soft magnetic carpets for transport of solids, liquids and droplets

Ahmet F. Demirörs (Complex Materials — D-MATL)

One of the major interests of modern robotics is micromanipulation by active and adaptive materials. An example to such micromanipulation is observed in respiratory system, where pathogen clearance actuation enabled by means of motile cilia. While various types of artificial cilia have been engineered recently, they often involve complex manufacturing protocols and focus on transporting liquids only. Here, we create soft magnetic carpets via an easy self-assembly route based on the Rosensweig instability. These carpets can transport liquids but also solid objects that are larger and heavier than the artificial cilia, using a crowd-surfing effect. This amphibious transportation is locally and reconfigurably tuneable by simple micromagnets or advanced programmable magnetic fields with a high degree of spatial resolution.

While our active carpets are generally applicable to integrated control systems for transport, mixing, and sorting, these effects could also be exploited for microfluidic viscosimetry and elastometry. Finally, I also show briefly how we extended this system to a droplet manipulation platform by simply infusing the soft carpets with a liquid repellent oil.

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How electrons get heavy and die

Hans Kroha (University of Bonn)

Usually we think of a metal as a liquid of electrons, particles familiar as individual entities from elementary particle physics. However, in certain materials, magnetic interactions can lead to an increase of the effective mass of the charge carriers by factors of 100s or 1000. Even more striking, near a magnetic phase transition in such, so-called heavy-fermion compounds, the very concept of a particle can break down and give way to an exotic quantum fluid without quantized charge carriers with corresponding, unusual properties. In this talk we give a brief review of this physics and the resulting properties. We then show how this breakdown dynamics is monitored and analyzed by time-resolved terahertz (THz) spectroscopy developed and performed at the Materials Department of the ETH.

 

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

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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.

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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.

 

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

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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.

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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.

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