Übersicht vergangener Kolloquien

Complex Photonic Systems (COPS), MESA+ Institute, University of Twente, Enschede 

The study of the propagation of light through complex composite materials – e.g., paint, foam, bio­logical tissue – is a topic that has become a field of its own. This may seem surprising: if a material is so opaque that it scrambles images, how can one see through? If a laser beam is scram­bled, how does optical interference survive? The answer is in essence that interferences survive even millions of scattering events, observable as speckle or enhanced back scattering. Know-how of light scattering serves to address challenges in high-tech industry – from lighting, CMOS metrology, to atmospheric sensing – for sustainable technology. 

Three complementary tools are crucial to control light interference in complex materials: nanostructure, shape (freeform), wavefront shaping. (1) Much progress is made to realize nanostructures: periodic, cav­ity superlattices, or chiral. The chal­lenge to get calibrated densities is met by using in situ X-ray imaging. Since X-ray methods are non-destructive, devices are available for further study or integration [Sch2024]. (2) External sample shape, long neglected, is remarkably crucial, as known in industry, and now also in wavefront shaping [Rat2023a]. (3) Wavefronts shaped with SLMs offer many control parameters to meet a desi­red goal like a highly optimized focus (Fig. 1). This is used to send light deep into a forbidden gap [Upp2021] or do secure optical communication. A new topic is mutual scattering where extinction (shadow) is control­led to make objects more transparent or darker, or sense particle displacement in opaque materials. 

Finally, even the transport of light intensity without interference – at the basis of most scattering optics – still holds puzzles, notably when well-known models like diffusion break down [Akd2024].

[Akd2024] O. Akdemir, M. D. Truong, A. Rates, A. Lagendijk & WLV, Phys. Rev. A 110 (2024) 033520

[Rat2023a] A. Rates, A. Lagendijk, A. J. L. Adam, W. L. Ijzerman & WLV, Opt. Express 31 (2023) 43351

[Sch2024] A. S. Schulz, M. Kozoň, G. J. Vancso, J. Huskens & WLV, J. Phys. Chem. C 128 (2024) 9142

[Upp2021] R. Uppu, M. Adhikary, C.A.M. Harteveld & WLV, Phys. Rev. Lett. 126 (2021) 177402

INM – Leibniz Institute for New Materials, 66123 Saarbrücken, Germany

High-resolution force microscopy reveals the mechanisms of adhesion and friction at the molecular scale. We will discuss recent results for two systems which are currently in the focus of materials physics, namely and hydrogels and 2D material. The mechanical properties of soft matter depend on the collective interactions of polymers in random networks. The randomness of the networks is reflected in ensemble modelling of their properties, often in the form of scaling laws. However, important functions such as the interaction of biomaterials with cells or the lubrication of contact lenses depend on the interaction at the molecular level. It is therefore important to study soft matter mechanics also at the single molecule level. We will report one study on the mechanics of single cross-links which mediate cell attachment at a hydrogel surface [1,2]. A second study establishes single-polymer friction force microscopy for a dsDNA in dynamic interaction with a nanoporous membrane [3].

The fascinating properties of 2D materials originate from strong bonds and coupling in the plane of atoms, and weak coupling normal to the atomic planes. This anisotropy leads to spectacular lubrication by graphene. However, when the pressure of the AFM exceeds 10 GPa, we observe a sudden increase in friction which can be attributed to the intermittent formation of covalent bonds across the atomic plane by a pressure-induced sp2-to-sp3 transition – the tip locally induces reactive diamond [4].    

[1]    A. Çolak et al., The mechanics of single cross-links which mediate cell attachment at a hydrogel surface, Nanoscale 11, 11596 (2019).

[2]    Bin Li et al., Molecular stiffness cues of an interpenetrating network hydrogel for cell adhesion, Materials Today Bio 15, 100323 (2022).

[3]    K. Schellnhuber et al., Single-Polymer Friction Force Microscopy of dsDNA Interacting with a Nanoporous Membrane, Langmuir 40, 968 (2024).

[4]    B. Szczefanowicz et al., Formation of intermittent covalent bonds at high contact pressure limits superlow friction on epitaxial graphene, Physical Review Research 5, L012049 (2023).

Max Planck Institute for Sustainable Materials, Dusseldorf 40237, Germany

Research Center Future Energy Materials and Systems, Ruhr University Bochum, Bochum 44801, Germany

Faculty of Physics and Astronomy, Ruhr University Bochum, Bochum 44801, Germany

Observing the behavior of materials under realistic conditions is quintessential to improve their properties and understand their intrinsic nature. Transmission electron microscopy (TEM) offers pathways to study even complex materials down to the atomic level. In combination with in situ holder platforms, it is possible to explore material evolution at variable temperatures, exposed to strain, electric or magnetic fields and in gas or liquid environments. Combining such in situ experiments with spectroscopic and scanning diffraction techniques facilitates to gain quantitative insights into the transformation of their microstructure.

In the present talk, I will show examples of in situ experiments probing nanostructured materials at high temperature, under varying magnetic field and in the liquid. In the first example, aberration-corrected TEM is used to study the transformation dynamics of a complex alloy microstructure at temperatures as high as 900°C. Atomic resolution observations provide insights into how nano-carbides are forming and dissolving and it is possible to monitor the role of interfacial dynamics in the dissolution process.

The second example addresses the interaction of magnetic domain walls in a dual-phase ferromagnetic alloy. In situ Lorentz TEM experiments are coupled to differential phase contrast scanning TEM (STEM) imaging to uncover the magnetic domain evolution and how magnetic domain walls in the different phases interact dynamically when varying the magnetic field.

In the last example, I will highlight recent experiments performed in probing the evolution of nanostructured copper under electrochemical potential by in situ liquid cell microscopy. I will demonstrate how copper nanoparticles are forming in the liquid solution under negative potential and show how 4D-STEM can be used to observe the nanocrystalline structure, as well as phase formation under open circuit potential even in a ∼500 nm liquid layer.

Institute of Physics, University of Rostock

The discovery of graphene almost two decades ago has opened an exciting field of research: single atomic layers of various bulk crystals can easily be prepared, manipulated, and stacked using remarkably simple techniques. This allows us to build novel kinds of heterostructures, one atomic layer at a time. Even though we are dealing with atomically thin objects, their interaction with light is, in many cases, surprisingly strong: for example, the absorption provided by a single atomic layer of graphene is easily visible to the naked eye.  

Many two-dimensional crystals can also emit light in the visible and infrared spectrum, making them potentially suitable for novel optoelectronic devices. Among these, transition metal dichalcogenides (TMDCs) have garnered a lot of attention. Their monolayers host optically bright, tightly bound electron-hole pairs (so-called excitons) that dominate their optical properties even at room temperature.

Heterobilayers of these materials may host interlayer excitons, in which electrons and holes are spatially separated in the constituent monolayers. The unique degrees of freedom we have in assembling these heterobilayers allow us to tailor the properties of interlayer excitons.

Stacking TMDC monolayers with parallel orientation leads to ferroelectric fields at the interfaces of adjacent layers, whose orientation can be switched by in-plane sliding, potentially giving rise to a novel type of nonvolatile data storage. Remarkably, the ferroelectric order can be mapped using optical spectroscopy.

In this talk, I will introduce the field of light-matter interaction with these novel materials and discuss recent advances.

Fraunhofer-Institute for Mechanics of Materials IWM, MicroTribology Center μTC, Freiburg  &  Karlsruhe Institute of Technology (KIT), IAM - Institute for Applied Materials, Karlsruhe

Graphite is a well-known solid lubricant that has been studied for decades. At low loads, graphite’s lubricity depends on humidity. Classical models like e.g. the adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies [1]. To explore the humidity dependence and validate the adsorption model for high loads (loads around 1 GPa as typically found in rolling bearing elements), a commercial graphite solid lubricant was studied using microtribometry [2]. Even at very high contact pressures, a high and low friction regime is observed – depending on humidity.

Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) reveal transformation of the polycrystalline graphite lubricant into turbostratic carbon after high but also after low load (50 MPa) sliding. Quantum molecular dynamics simulations relate high friction and wear to cold welding and shear-induced formation of turbostratic carbon, while low friction originates in molecular water films on surfaces. The combined experiments and simulations lead to a novel, generalized adsorption model including turbostratic carbon formation.

[1] R. H. Savage, J. Appl. Phys. 19, 1-10 (1948).

[2] C. Morstein, A. Klemenz, M. Dienwiebel, M. Moseler, Nature Comm. 13, 5958 (2022).

2nd Physics Institute, University of Stuttgart Max Planck Institute for Solid State Research, Stuttgart

A fundamental design rule that nature has developed for biological machines is the intimate correlation between motion and function. One class of biological machines is molecular motors in living cells, which directly convert chemical energy into mechanical work. They coexist in every eukaryotic cell, but differ in their types of motion, the filaments they bind to, the cargos they carry, as well as the work they perform. Such natural structures offer inspiration and blueprints for constructing DNA-assembled artificial systems, which mimic their functionality.

In this talk, I will discuss DNA nanostructures with distinct motion and functions that interrogate synthetic cells. The interplay between the dynamic behavior of DNA nanostructures and synthetic cells gives rise to peculiar phenomena, which may rejuvenate the field of synthetic biology and greatly enhance the technological value of DNA nanotechnology.