SFB 1073 Atomic scale control of energy conversion
Highlight publications of the 2nd funding period
The publications presented here prominently represent the overarching goal of the CRC 1073 which is to understand and control the elementary steps of energy conversion in materials with tunable excitations and interactions:
Understanding microscopic processes in materials and devices that can be switched by light requires experimental access to dynamics on nanometer length and femtosecond time scales. Here, we introduce ultrafast dark-field electron microscopy to map the order parameter across a structural phase transition. We use ultrashort laser pulses to locally excite a 1T-TaS2 (1T-polytype of tantalum disulfide) thin film and image the transient state of the specimen by ultrashort electron pulses. A tailored dark-field aperture array allows us to track the evolution of charge-density wave domains in the material with simultaneous femtosecond temporal and 5-nanometer spatial resolution, elucidating relaxation pathways and domain wall dynamics. The distinctive benefits of selective contrast enhancement will inspire future beam-shaping technology in ultrafast transmission electron microscopy.
Despite the huge importance of friction in regulating movement in all natural and technological processes, the mechanisms underlying dissipation at a sliding contact are still a matter of debate. Attempts to explain the dependence of measured frictional losses at nanoscale contacts on the electronic degrees of freedom of the surrounding materials have so far been controversial. Here, it is proposed that friction can be explained by considering the damping of stick‐slip pulses in a sliding contact. Based on friction force microscopy studies of La(1−x)SrxMnO3 films at the ferromagnetic‐metallic to a paramagnetic‐polaronic conductor phase transition, it is confirmed that the sliding contact generates thermally‐activated slip pulses in the nanoscale contact, and argued that these are damped by direct coupling into the phonon bath. Electron‐phonon coupling leads to the formation of Jahn–Teller polarons and to a clear increase in friction in the high‐temperature phase. There is neither evidence for direct electronic drag on the atomic force microscope tip nor any indication of contributions from electrostatic forces. This intuitive scenario, that friction is governed by the damping of surface vibrational excitations, provides a basis for reconciling controversies in literature studies as well as suggesting possible tactics for controlling friction.
We report active control of the friction force at the contact between a nanoscale asperity and a La0.55Ca0.45MnO3 (LCMO) thin film using electric fields. We use friction force microscopy under ultrahigh vacuum conditions to measure the friction force as we change the film resistive state by electric-field-induced resistive switching. Friction forces are high in the insulating state and clearly change to lower values when the probed local region is switched to the conducting state. Upon switching back to an insulating state, the friction forces increase again. Thus we demonstrate active control of friction without having to change the contact temperature or pressure. By comparing with measurements of friction at the metal-to-insulator transition and with the effect of applied voltage on adhesion, we rule out electronic excitations, electrostatic forces, and changes in contact area as the reasons for the effect of resistive switching on friction. Instead, we argue that friction is limited by phonon relaxation times, which are strongly coupled to the electronic degrees of freedom through distortions of the MnO6 octahedra. The concept of controlling friction forces by electric fields should be applicable to any materials where the field produces strong changes in phonon lifetimes.
Fast Mn adatom hopping at the interface of a La1-xSrxMnO3 electrode to H2O is a consequence of partial solvation of surface Mn in liquid H2O. The adatom hopping is revealed by in situ environmental transmission electron microscopy (ETEM) and quantified by image simulation. Such a dynamic interface has a big impact on understanding highly efficient pathways for the oxygen evolution reaction.
Adsorption of CO to Au(111) proceeds through a metastable chemisorption state. Conventional wisdom holds that molecules are bound more strongly by chemisorption than by physisorption. In this system, the chemisorption binding well (82 meV) is shallower than the physisorption well (120 meV). However, when gas-phase CO collides with Au(111), it must pass through the chemisorption well before reaching equilibrium in the physisorption well. This can be understood by application of the principle of detailed balance; the low entropy chemisorption-state exhibits a rate of thermal desorption that is much higher than that from the high entropy physisorption state. Therefore, by detailed balance, the thermal adsorption is also more efficient to the chemisorption well than to the physisorption state. Once formed, the chemisorption state rapidly converts to the physisorption state.
Here, using ultrafast low-energy electron diffraction (ULEED) in combination with sequential optical excitation, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional surface system, namely atomic indium wires on the (111) surface of silicon. To govern the transition, we harness vibrational coherence in amplitude modes connecting the insulating (8×2) hexagon and the metallic (4×1) zigzag phase. We identify possible control mechanisms and propose a two-dimensional potential energy model for the transition. Our work highlights strong conceptual links between molecular chemistry and solid-state physics, demonstrating control over the conversion of light into chemical energy during the making and breaking of bonds in solids.
Organic photovoltaic devices operate by absorbing light and generating current. These two processes are governed by the optical and transport properties of the organic semiconductor. Despite their common microscopic origin—the electronic structure—disclosing their dynamical interplay is far from trivial. In this publication, we address this issue by time-resolved photoemission to directly investigate the correlation between the optical and transport response in organic materials. We reveal that optical generation of non-interacting excitons in a fullerene film results in a substantial redistribution of all transport levels (within 0.4 eV) of the non-excited molecules. As all observed dynamics evolve on identical timescales, we conclude that optical and transport properties are completely interlinked. This finding paves the way for developing novel concepts for transport level engineering on ultrafast time scales that could lead to novel functional optoelectronic devices.
Viewing the atomic-scale motion and energy dissipation pathways involved in forming a covalent bond is a longstanding challenge for chemistry. We performed scattering experiments of H atoms from graphene and observed a bimodal translational energy loss distribution. Using accurate first-principles dynamics simulations, we show that the quasi-elastic channel involves scattering through the physisorption well where collision sites are near the centers of the six-membered C-rings. The second channel results from transient C-H bond formation, where H atoms lose 1 to 2 electron volts of energy within a 10-femtosecond interaction time. This remarkably rapid form of intramolecular vibrational relaxation results from the C atom's rehybridization during bond formation and is responsible for an unexpectedly high sticking probability of H on graphene.
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