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Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe2. We present a many-body approximation for the Green’s function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.

A team of researchers around scientists at the Physical Chemistry Department of the Fritz Haber Institute has found a new method of observing the electron-hole-pairs, called excitons, that are key for the function of photovoltaic cells. They managed to film them using the free-electron laser FLASH at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg.

In the world of sustainable energy, solar cells have gained a lot of importance in the last few years. In 2019, 9% of Germany’s net electricity supply came from solar energy. That may seem relatively low, but it’s actually huge compared to ten years ago. Solar energy is one of the fastest growing energy sectors.

One of the reasons for this success story is the large amount of research on the inner workings of photovoltaic cells. Many of the insights in this field initially come from fundamental science as is undertaken at the Physical Chemistry Department of the Fritz Haber Institute. There, a team around Ralph Ernstorfer has just found a way of imaging a key element of all optoelectronic applications: the formation and movement of excitons.

“An exciton describes an electron and the hole it leaves behind when it moves from its original place,” explains Ernstorfer. Excitons are important in the process of producing electricity in solar panels. At the core of a photovoltaic cell, or what is more commonly called a solar panel, is a semiconductor (silicon for example) that on one side has many loose electrons and on the other more atoms with holes. Electricity is generated when a current runs between them, i.e. when the electrons move to fill the holes. But that process would be over in a few seconds without a new supply of electrons and holes. “The sunlight that shines on the photovoltaic cell prompts the creation of new excitons by lifting electrons to a new energy level,” says Ernstorfer, “where they leave a vacancy in their original states. That guarantees a constant supply of electrons and holes, thus enabling an electric current.”

While this principle has been known for decades, physics research has been a little slow in measuring how exactly this happens. An international collaboration of researchers around a team from the Fritz Haber Institute in Berlin now conceived a new approach for observing the generation of excitons in real time. “We wanted to know how excitons either broke up or how they formed from independent particles”, says Maciej Dendzik, lead author of the study. Using the free-electron laser FLASH at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, they emitted light on to a semiconductor (tungsten diselenide – WSe2) and took billions of snapshots of the electrons’ arrangement in the material. “We essentially made a movie of the semiconductor’s electronic structure during and after light absorption,” adds Dendzik excitedly.

Distinguishing of excitons from quasi-free particles is not an easy task. The researchers took an unconventional approach to this problem by monitoring not only the excited states population, but also strongly bound electrons from the atomic core at the same time. The latter turned out to be essential for seeing the exciton breaking dynamics due to subtle changes in interaction strength as this process unfolds. This experiment resulted in development of a novel model describing how core electrons feel the presence of excitons, which paves the way for exciting new research in the field of semiconductor physics and, ultimately, the design of more efficient solar cells.

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Electrochemistry is the chemistry of electrons and charge transfer. For a molecular level understanding one would like to probe the dynamics of electron transfer at the electrochemical interface on all relevant time scale.  While conventional electrochemical characterization methods such as impedance spectroscopy are very useful in sampling “slow” dynamics, they are not fast enough for processes such as interfacial electron transfer or solvent reorganization. In this study a novel optoelectronic technique has been developed combining femtosecond lasers and conventional electrochemical electronics to characterize the birth and ultrafast structural evolution of solvated electrons at the gold/water interface. Transient spectra with a time resolution of 50 fs reveal novel aspects of the properties of solvated electrons at the interface, like trapping in a “hot state” and its subsequent evolution. The technique will enable a better understanding of hot electron-driven reactions at electrochemical interfaces.

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Tip-enhanced Raman scattering (TERS) combined with highly stable scanning tunneling microscopy is a powerful tool to obtain local chemical information at surfaces with exceptional high spatial resolution. Here, we investigate the evolution of TERS at Angstrom-scale distances between tip and surface up to the point where tip apex touches the surface. A stable atomic point contact is reversibly formed in the junction between our plasmonic Ag tip and ultrathin ZnO films on a Ag(111) surface. A dramatic enhancement and abrupt increase of the TERS intensity occurs upon contact formation for ZnO, which attributed to strong hybridization between the Ag tip and ZnO enhancing the Raman polarizability. This is further corroborated by the appearance of a new vibrational mode arising from the atomic contact. In contrast, related control experiments on chemically inert NaCl films show no such enhancement and the absence of the point contact mode.

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The Award Committee of the Japan Society of Vacuum and Surface Science (JVSS) has selected Dr. Takashi Kumagai (Fritz Haber Institute of the Max Planck Society, Germany) as the Rising Medalist of The Heinrich Rohrer Medal for the his achievements to combine plasmonic near-field physics with low-temperature scanning tunneling microscopy (STM). The prize will be awarded in a ceremony at the International Symposium on Surface Science(ISSSS-9) in November 2020, Takamatsu, Japan.
The work by Takashi Kumagai combines highly original research on naoscale surface science with the physics of locally enhanced electromagnetic fields in atonically controlled STM junctions. A key development technical development are nano-fabricated plasmonics tips allowing to localize of light down to the atomistic (i.e. Angstrom) length scale. This enabled highly reliable and reproducible experiments and the discovery of a novel processes like plasmon-assisted resonance electron transfer, near-field induced chemical reactions, and the demonstration of tip-enhanced resonance Raman spectroscopy in quantum point contacts.

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The “electron dynamiχ” group leader from the Department of Physical Chemistry was appointed professor by the Humboldt-Universität zu Berlin on April 1st, 2020.

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The Gaede Prize 2020 is awarded to Dr. Takashi Kumagai for his outstanding work on near-field physics and chemistry in plasmonic nano-junctions. The Gaede award recognizes excellent work in the area of vacuum science and is awarded by the German Physical Society (DPG) in cooperation with the Gaede foundation to scientists in an early career stage.
Takashi Kumagai studied chemistry at the University of Kyoto and received his PhD degree in 2011 on visualization of hydrogen bond dynamics at surfaces. He subsequently joined our department as a postdoc with Leonhard Grill and is heading the “Nanoscale Surface Chemistry” group since 2013. He performed fundamental studies on tautomerization reactions on single molecules and developed new instrumentation in in scanning tunneling microscopy (STM) to address fundamental questions in nanoscale light–matter interactions using highly controlled plasmonic STM junctions.

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Phase-Change materials such as the Ge3Sb2Te6 compound (GST) can be reversibly switched between the amorphous and crystalline crystal phases by ultrashort laser pulses. At infrared wavelengths, the two phases of GST exhibit a large permittivity contrast, which can be used to control the dielectric background sensed by guided optical modes that propagate in close proximity.

In their work, the authors demonstrate experimentally and theoretically that a thin GST film on a polaritonic substrate (SiC) serves as an actively tunable material system for the infrared spectral range. The system not only supports index-shifted p-polarized surface phonon polaritons (SPhPs), but additionally also s-polarized waveguide modes. Intriguingly, the waveguide modes replicate the properties of the SPhP that define its suitability for nanophotonic applications, such as the enhancement of local electric fields and subwavelength confinement.

The authors show experimentally that both modes can be actively tuned by switching the GST phase, with an exceptionally high tuning figure of merit of up to 7.7. With this unique tunability of its omnipolarized guided modes, the material system provides a promising platform for nanophotonic applications such as in-plane metasurfaces or polariton lenses.

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Atomic scale heterostructures offer unique possibilities for new material properties that emerge from the chemical bonding at the material interfaces rather than from the bulk. Often this idea is brought forward using two-dimensional van-der-Waals bonded crystals. In contrast here, researches show that the same concept can also be applied to traditional semiconductors with atomic-level control over the individual few-monolayer-thick sheets using epitaxial methods. The enhanced interlayer bonding strength leads to tremendous modifications of the material properties. The heterostructure behaves as a new crystal hybrid material dominated by the hetero-interfaces, with properties which cannot be derived from the individual constituents.
Using the Nitride semiconductors GaN and AlN, the authors demonstrate this concept experimentally for the first time, focusing on the optical phonons. In the crystal hybrid, new optical phonon modes emerge and others get modified significantly. These phonon modes, in turn, determine the infrared dielectric function and thereby the characteristics of phonon polaritons supported by the new material. Apart from shifted and newly emerging surface polariton bands, the strong anisotropy of the layered structure additionally introduces extreme birefringence, with several pronounced hyperbolic bands emerging. Thus, the new material provides rich physics which is absent its constituents, and which can be controlled via the layer thicknesses. This first demonstration of the crystal hybrid concept paves the way for a new era of material design for infrared-to-terahertz photonics.

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Nanotechnologies utilize physical and chemical properties which arise by shrinking a material in size to the nanometer length scale. The structural stability of nanoscale building blocks is a central aspect in nanoscience as it governs the long-term stability of a device. Function, however, is only achieved once a device is energized, which is why nanotechnologies require materials which are structurally stable under operation conditions.

On the atomic level, the structure is prone to ultrafast motions that range from atomic vibrations and diffusion to translations and rotations of entire nanostructures. These motions, for instance, governs the sintering of nanoparticles to larger complexes which occurs at elevated temperatures. To study the motion of entire nanoparticles, we establish femtosecond electron diffraction as goniometer of ultrafast nanocrystal rotations. We have investigated nanoclusters containing 923 gold atoms, which have been deposited on a graphene sheet, with ultrashort pulses of electrons. We observe motions of the atoms in the cluster as well as constrained rotations, so-called librations, of the entire nanoclusters. This motion is induced by suddenly energizing the material, which is realized by illumination with an ultrashort laser pulse.

These experiments are compared to molecular dynamics and electron diffraction simulations. Through the combination of the different approaches, the mechanism for the conversion of laser energy to rotational motions could be revealed. This study adds a new aspect to the understanding of nanoscopic heterostructures in out-of-equilibrium conditions and to the mechanisms of heat- and mass-transport in nanoscale materials.

This research results from an international collaboration between the research group Structural & Electronic Surface Dynamics at the Fritz Haber Institute in Berlin headed by Ralph Ernstorfer, the group of Richard Palmer at Swansea University in the UK, and the group of Vlasios Mavrantzas located at the University of Patras (Greece) and the ETH Zurich. This project was supported from the European Union through the ERC grant FLATLAND.