Klaus Ellmer

Excitonics is an emerging field focused on exploiting and manipulating excitons generated through light-matter interactions. Advancing the field into X-ray excitonics requires precise energy and time control over core-exciton resonances. Such control enables the use of core excitons to enhance non-linear optical effects, including X-ray second harmonic generation, and has broad applications in material characterization and quantum information processing. To achieve these objectives, it is essential to comprehend the role of many-body effects governing core-exciton dynamics. In this work, we address this challenge by combining experiments with a novel \emph{ab initio} approach specifically developed to interpret pump-probe excitations. Applied to the prototypical wide-bandgap semiconductor ZnO, first-principles calculations reproduce experimental results and unveil how the density and distribution of photoexcited carriers can tune the dynamical Coulomb screening, thereby controlling core-exciton binding energies, while Pauli blocking remains negligible. These insights lead us to propose a method for dynamically controlling core-exciton resonances at absorption edges, achieving either a uniform spectral blue shift, driven by thermalized carrier distributions on picosecond timescales, or individual spectral blue shifts, driven by time-dependent carrier distributions on femtosecond timescales.

Excitonics is an emerging field focused on exploiting and manipulating excitons generated through light-matter interactions. Advancing the field into X-ray excitonics requires precise energy and time control of core-exciton resonances, enabling non-linear X-ray phenomena such as element-specific X-ray transient gratings, and advancing material characterization. To achieve these objectives, it is essential to comprehend the role of many-body effects governing core-exciton dynamics. In this work, we address this challenge by combining experiments with an ab initio approach specifically developed to interpret pump-probe excitations. Applied to the prototypical wide-bandgap semiconductor ZnO, first-principles calculations reproduce experimental results and unveil how the density and distribution of photoexcited carriers dynamically tune Coulomb screening, thereby controlling core-exciton binding energies, while Pauli blocking remains negligible. These insights inform a method for dynamically controlling core-exciton resonances at absorption edges, achieving either a uniform spectral blue shift caused by thermalized carrier distributions on picosecond timescales, or distinct blue shifts for individual resonances, driven by time-dependent carrier distributions on femtosecond timescales.

The efficiencies of transition metal dichalcogenide thin films and single crystals (MoS ₂ , WS ₂ , MoSe ₂ , WSe ₂ ) are evaluated for photoelectrochemical hydrogen evolution.

By reactive magnetron sputtering from a ceramic SnO₂:Ta target onto unheated substrates, X-ray amorphous SnO:Ta films were prepared in gas mixtures of Ar/O₂(N₂O, H₂O). The process windows, where the films exhibit the lowest resistivity values, were investigated as a function of the partial pressure of the reactive gases O₂, N₂O and H₂O. We found that all three gases lead to the same minimum resistivity, while the width of the process window is broadest for the reactive gas H₂O. While the amorphous films were remarkably conductive (ρ ≈ 5 × 10⁻³ Ωcm), the films crystallized by annealing at 500 °C exhibit higher resistivities due to grain boundary limited conduction. For larger film thicknesses (d ≳ 150 nm), crystallization occurs already during the deposition, caused by the substrate temperature increase due to the energy influx from the condensing film species and from the plasma (ions, electrons), leading to higher resistivities of these films. The best amorphous SnO₂:Ta films had a resistivity of lower than 4 × 10⁻³ Ωcm, with a carrier concentration of 1.1 × 10²⁰ cm⁻³, and a Hall mobility of 16 cm²/Vs. The sheet resistance was about 400 Ω/□ for 100 nm films and 80 Ω/□ for 500 nm thick films. The average optical transmittance from 500 to 1000 nm is greater than 76% for 100 nm films, where the films, deposited with H₂O as reactive gas, exhibit even a slightly higher transmittance of 80%. These X-ray amorpous SnO₂:Ta films can be used as low-temperature prepared transparent and conductive protection layers, for instance, to protect semiconducting photoelectrodes for water splitting, and also, where appropriate, in combination with more conductive TCO films (ITO or ZnO).

Al-doped ZnO and Zn_1 − xMg_xO films, deposited by magnetron sputtering at substrate temperatures from about 25 °C to 500 °C and exhibiting a strong variation of the carrier concentration, were investigated by X-ray absorption spectroscopy (XAS), analyzing the X-ray absorption near-edge structures. The near-order structure, derived from the X-ray absorption spectra, shows that the dopant Al was built-in onto Zn lattice sites, independently of the deposition temperature up to 450 °C. FEFF calculations confirm the lattice location of the dopant on zinc lattice sites. The formation of secondary (spinel) phases occurs only at deposition temperatures above 450 °C, more pronounced for the Zn_1 − xMg_xO:Al films. In contrast to the XAS measurements, the electrical parameters of the films—carrier concentration and Hall mobility—exhibit a strong dependence on the substrate temperature, with a variation of the electron concentration by a factor of 4.5 from 25 °C to about 300 °C. This shows that the XAS method is significantly less sensitive compared to electrical measurements for the characterization of electrically active dopant elements. This fact is tentatively explained by the formation of oxygen interstitial defects O_i during deposition, annealing out at low temperatures, and by the formation of defect complexes of dopant atoms with intrinsic lattice defects, like Al-O_i or Al-Mg-O_i, which induce the observed electrical effects but are not detectable by XAS methods.

The transparent conducting oxides ZnO and ZnO:Al are interesting materials for a wide range of applications. Several of these applications need a large area, single crystalline, and specially doped thin layers. A common technique for the fabrication of those layers is RF (radio frequency) -magnetron sputtering. The investigation of the crystal quality of such layers requires methods of analysis that are destruction free and that are able to obtain information about the concentration and type of defects versus depth. One such option is the Rutherford backscattering spectroscopy (RBS) in channelling mode. In this work, we exploit the channelling effect and its energy dependence, which are sensitive to the type of defects. By using appropriate software and measuring RBS channelling spectra with different beam energies, we were able to determine the depth distribution of point defects and dislocation loops. The presence of dislocation loops was proven using other previously applied analysis methods. The main advantage of RBS in channelling mode is the quantification of point defects, which can be important for defining the electrical and optical properties of such layers. The technique demonstrated is applicable to other defective crystals or thin crystalline layers.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Highly (001)-textured, photoactive WSe₂ thin films have been prepared by an amorphous solid-liquid-crystalline solid process promoted by palladium. By increasing the thickness of the Pd promoter film (≥10 nm) the structure and texture of the WSe₂ films can be improved significantly. However, these as-crystallized WSe₂ films are only weakly photoactive in a 0.5 М H₂SO₄ electrolyte under AM 1.5 solar irradiation which we attribute to an inefficient photogenerated charge transfer across the WSe₂/electrolyte interface via the prevailing van der Waals planes of the WSe₂ crystallites. In this work photochemically deposited platinum on the p-type WSe₂ photocathode is used for an efficient electron transfer thus inducing the hydrogen evolution reaction. Upon illuminating the WSe₂ photocathodes in a Pt-ion containing electrolyte, the photogenerated electrons reduce Pt⁺ to Pt leading to the precipitation of Pt islands, preferentially at edge steps of the WSe₂, i.e. at the grain boundaries of the WSe₂ crystallites. The increasing amount of Pt islands at the grain boundaries linearly enhances the photocurrent density up to 2.5 mA cm⁻² at 0 V_RHE in sulfuric acid, the highest reported value up to now for WSe₂ thin films.