Martin Knupfer

MoOCl 2 is a metallic, layered van der Waals material with a strong in-plane structural anisotropy. The reflectivity of a MoOCl 2 single crystal has been determined in a wide spectral range from the infrared to the visible regime. This reflectivity is extremely anisotropic, reflecting the anisotropic crystal structure and electronic properties of MoOCl 2 . Using a Drude–Lorentz fit, we extracted the dielectric function quantitatively for the two in-plane directions of MoOCl 2 , and we determined the anisotropic optical effective mass of the conduction electrons. Our data additionally demonstrate the presence of interband excitations in the measured spectral range.

Functionalization of 2D materials and of transition metal dichalcogenides (TMDs) in particular is central for next generation devices and other applications. Here we study the effect of calcium dosing on the surfaces of clean WSe 2 and WS 2 in ultrahigh vacuum by x-ray photoelectron spectroscopy, low energy electron diffraction, and scanning electron microscopy. We find that Ca reacts with the chalcogens to CaSe x or CaS x and reduces the remaining tungsten. Ca forms an overlayer on top and causes an outdiffusion of the chalcogen. The overlayer oxidizes easily. We obtained detailed understanding of the interactions of archetypal TMD with an abundant alkali-earth element, which expands our knowledge of metal-surface interactions and of surface manipulation of 2D materials.

Bound electron–hole pairs called excitons determine the photophysical behavior of semiconducting materials. In this study, we present a combination of electron energy-loss spectroscopy in transmission and optical absorption spectroscopy for the investigation of excitons in bulk 2H-MoTe ₂ . The exciton dispersion E ( q ) was measured for momentum transfers oriented along the ΓK and ΓM directions at 20 K. From the extracted dispersion relations, the in-plane exciton effective mass was determined. Our results reveal that for 2H-MoTe ₂ the exciton effective mass is anisotropic. In consideration of the complementary optical results the exciton reduced mass, binding energy and radius have been determined.

A combination of optical absorption and electron energy-loss spectroscopy has been used to investigate the exciton in highly ordered diindenoperylene films. The exciton dispersion is shown to be negative, and the data allow the determination of the total inter-molecular exciton coupling. This coupling is H-aggregate-like, which explains the variation of the spectral profile when going from diindenoperylene (DIP) molecules in solution to the crystal. Momentum dependent spectral weight changes indicate a sizable contribution from charge-transfer exciton coupling, which reduces the total coupling to some extent. We derived a consistent set of model parameters for the description of the exciton behavior in DIP.

Electron energy-loss spectroscopy in transmission is applied to investigate the dependence of the exciton energies on the momentum, q , in the bulk of 2 H − WSe 2 . The exciton dispersion E ( q ) for momentum vectors parallel to the Γ K direction and the Γ M direction at room temperature as well as at 20 K close to the K point is investigated. In addition, the exciton spectra at room temperature are measured for larger momenta parallel to the Γ K direction approaching the indirect gap excitation. The exciton effective mass can be determined from the measured dispersion curves as a function of temperature and in-plane direction. Our data reveal that the exciton effective mass for the excitation close to the K point is virtually isotropic and temperature independent. The indirect band-gap exciton is characterized by a significantly larger effective mass compared to the direct exciton close to the K point. Published by the American Physical Society 2025

Heteroanionic materials show potential as 2D semiconductors due to their tunable band gaps, making them excellent candidates for photocatalytic water splitting applications.

Pentacene is one of the most investigated organic semiconductors. It is well known that the motion of excitons in pentacene and other organic semiconductors is determined by inter-molecular exciton coupling based on charge-transfer processes. In the present study, we demonstrate the impact of the admixture of tetracene, which has a larger band gap and interrupts the pentacene–pentacene interaction, on the exciton behavior in pentacene. Using a combination of optical absorption and electron energy-loss spectroscopy, we show that both the Davydov splitting and the exciton band width in pentacene strongly decrease with increasing tetracene concentration, while the decrease of the exciton band width is substantially larger.

Covalent bonding between transition metal atoms is a common phenomenon in honeycomb lattices of layered materials, which strongly affects their electronic and magnetic properties. This work presents a detailed spectroscopic study of α-MoCl₃, 2D van der Waals material with covalently bonded Mo₂ dimers, with a particular focus on the Mo–Mo bonding. Raman spectra of α-MoCl₃ were studied with multiple excitation laser lines chosen in different parts of the absorption spectrum, while polarization measurements aided in the symmetry assignment of the observed modes. Furthermore, far-IR measurements and (Density Functional Theory) DFT phonon computations were performed to complete vibrational assignment. Polarized absorption, PL, and photoelectron spectroscopy supported by DFT calculations were employed to understand the consequences of the Mo–Mo bonding for the electronic structure and the localization/delocalization balance in d³–d³ interactions. A coupling of dimerization-related structural and electronic properties was revealed in the strong resonance Raman enhancement of the Mo–Mo stretching mode at 153 cm⁻¹ when the excitation laser matched the electronic transition between σ-bonding and antibonding orbitals of the Mo₂ dimer (σ → σ*). The deeper understanding of the metal–metal bonding and identification of the vibrational and electronic spectroscopic signatures of the dimerization will be of great use for the studies of electron delocalization in magnetic van der Waals materials.