Dirk Hauschild

The electronic gap of metal–organic frameworks (MOFs), i.e., the energy difference between the highest occupied and lowest unoccupied electronic states, is crucial in many MOF applications. The magnitude and the orbital nature of the electronic gap determine the fundamental chemistry of the MOFs. Hence, local, atomic-level information about the electronic states is critical for MOFs with tailored properties or targeted functionalities; however, such information is not accessible through standard ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy and is usually derived from quantum chemical calculations. Here, we introduce resonant inelastic (soft) X-ray scattering (RIXS) as a direct experimental tool to locally probe electronic gaps in MOFs. Using NH2-MIL-101(Cr) as a test case, we compare our RIXS results with published UV-Vis-NIR spectra. The transitions observed in the UV-Vis-NIR data are also present in our RIXS data. By locally probing the chromium L2,3 and carbon, nitrogen, and oxygen K edges, we can identify specific local sites for each transition.

Using nonclassical divalent lanthanide precursors as multielectron reducing agents, complexes [K(18-crown-6)(Cp″2LnTe3)] with Ln = La, Ce, and Nd; Cp″ = 1,3-bis(trimethylsilyl)cyclopentadienyl were synthesized and investigated to elucidate the mechanism of lanthanide-tellurium bonding and the role of 4f-element electron density in stabilizing small chalcogenide chains. Density functional theory (DFT) reveals that the frontier molecular orbitals of these complexes are predominantly localized on the [Te3]2– fragment, while the trivalent lanthanide ions stabilize the tellurium chain through weak but measurable metal–ligand interactions. To experimentally resolve these interactions, we focus on complementary ligand- and metal-centered X-ray spectroscopic approaches. Ln L3-edge high-resolution XANES (HR-XANES) and valence-band resonant inelastic X-ray scattering (VB-RIXS) demonstrate that the Ln-Te/C interaction has substantial Ln 5d orbital contributions, particularly for the Ln-Te bond, and remain largely constant across the three complexes. DFT-based bond analysis provides a mechanistic interpretation of these trends. The Ln–C interaction exhibits increasing electron density at the bond critical point and a higher delocalization index (QTAIM analysis) from the lighter to the heavier lanthanides, reflecting enhanced Ln 4f participation within an energy-driven covalency regime. The Ln–Te interaction is predominantly electrostatic, with meaningful orbital contributions arising mainly from Ln 5d participation within an orbital-overlap-driven covalency regime. These results demonstrate that the Ln–C and Ln–Te bonding in the present complexes follow fundamentally distinct covalency mechanisms, which together enable the stabilization and isolation of the small [Te3]2– fragment chain.

Defect states are crucial in many electronic devices. Oxygen vacancies, for example, are common in transparent conductive oxides and can both be beneficial (e.g., as a dopant to enhance conductivity) as well as detrimental (e.g., leading to reduced device performance by recombination at interfaces). Using soft and hard X-ray photoelectron spectroscopy (PES and HAXPES) with excitation photon energies ranging from 0.1 to 6.3 keV, we study the electronic surface structure of differently processed Ga₂O₃ samples in a depth-resolved fashion. Specifically, we investigate a Ga₂O₃ thin film, as used in Cu-(In,Ga)-Se₂-based thin-film solar cells, as well as a β-Ga₂O₃ single crystal before and after a defect-inducing Ar⁺-ion treatment. Spectra calculations based on density functional theory (DFT) are used to explain the PES and HAXPES valence band signatures as a function of the excitation photon energy. The β-Ga₂O₃ single crystal spectra can be well described by the DFT calculations. In contrast, the Ga₂O₃ thin film and Ar⁺-ion treated β-Ga₂O₃ show significant spectral broadening of the valence band features and additional spectral intensity close to the valence band maximum, which we assign to defect states. This additional intensity varies as a function of probing depth, suggesting that these defects are mostly localized at the surface. We also discuss the importance of DFT-based spectra calculations to verify the proper determination of valence band maxima using a linear extrapolation.

S L2,3 X-ray emission spectroscopy (XES) and spectra calculations based on density functional theory are used to study the electronic valence structure of Mn, Fe, Co, and Cu transition metal sulfides, without (MnS, FeS, CoS, and Cu2S) and with (MnS2, FeS2, CoS2, and CuS) sulfur dimers in the crystal structure. We find the upper valence band region in the experimental spectra to be very sensitive to the transition metal, while a splitting in the transitions from the S 3s derived bands indicates the presence of sulfur dimers. This makes S L2,3 XES particularly well suited to the chemical speciation of these compounds. The spectral changes are reproduced by our spectra calculations, which allows for a detailed understanding of the electronic structure based on the band structure and the projected density of states. The splitting of the S 3s derived bands in the dimer compounds is found to increase with decreasing sulfur–sulfur distance, indicating an increase of covalency. The splitting in our experimental XES spectra is found to be smaller than in photoemission data and our spectra calculations, which we attribute to nuclear dynamics on the time scale of the X-ray emission process. In particular, the high sensitivity of S L2,3 XES to the presence of sulfur dimers, together with its suitability for operando studies, makes it a key technique for future studies on a wide range of relevant material systems.

The role of the equatorial ligands and their influence on the electronic structures and bonding properties of uranyl and other actinyls are not well understood and are thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F^- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl^- and uranyl-Br^-. The good energy match between uranyl(VI) and F^- likely enhances the stability of the uranyl-F^- bond, contributing to the higher U-F^- affinity in aqueous solution compared to uranyl-Cl^-/Br^-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides: NaRb₈(UO₂)₅F₁₉·2H₂O, Rb₂UO₂Cl₄·2H₂O, and Rb₂UO₂Br₄·2H₂O. We describe covalent uranium-halide interactions, following the trend Br^- ≈ Cl^- > F^-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M₄ edge HR-XANES and RIXS exhibit unexpected σ* peak trends not directly correlated with U═O bond lengths but well explained by LFDFT RIXS calculations.

The role of the equatorial ligands and their influence on the electronic structure and bonding properties of uranyl and other actinyls are not well understood and thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl- and uranyl-Br-. The good energy match between uranyl(VI) and F- likely enhances the stability of the uranyl-F- bond, contributing to the higher U-F- affinity in aqueous solution compared to uranyl-Cl-/Br-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides—NaRb8(UO2)5F19·2H2O, Rb2UO2Cl4·2H2O, and Rb2UO2Br4·2H2O. We describe covalent uranium-halide interactions, following the trend Br- ≈ Cl- > F-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M4 edge HR-XANES and RIXS exhibit unexpected σ* peak trends, not directly correlated with U=O bond lengths but well explained by LFDFT RIXS calculations.

The bulk electronic structure of β-Ga2O3 single crystals was investigated using oxygen K-edge x-ray emission and absorption spectroscopy as well as resonant inelastic soft x-ray scattering. Spectra were obtained for different orientations of the crystal planes with respect to the polarization vector of the incident x-ray beam. The spectra are analyzed with calculations based on density functional theory and using the Bethe–Salpeter equation in the ocean code to take the core–hole interaction into account. These calculations correctly capture all the main features in the experimental spectra, demonstrating the potential of the approach to predict the electronic properties of similar compounds. We find a pronounced anisotropy as a function of the excitation polarization vector and significant differences in the spectral contributions from the inequivalent oxygen atoms of β-Ga2O3.

The chemical and electronic structure of the CdS/(Ag,Cu)(In,Ga)Se₂ (CdS/ACIGSe) interface for thin‐film solar cells, involving an absorber with a bulk [Ag]/([Ag]+[Cu]) (AAC) ratio of 0.06, a state‐of‐the‐art RbF post‐deposition treatment (PDT), and a chemical‐bath deposited CdS buffer layer, is studied. To gain a detailed and depth‐resolved picture of the CdS/ACIGSe interface, synchrotron‐ and laboratory‐based hard X‐ray, soft X‐ray, and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy are combined. Compared to the bulk of the absorber, a Cu‐ and Ga‐poor ACIGSe surface is found, with a slightly increased AAC ratio. Strong evidence of a Rb–In–Se species (possibly with some Ag) at the absorber surface is compiled, with a corresponding band gap of 2.79 ± 0.12 eV. This finding is in clear contrast to comparable Ag‐free Cu(In,Ga)Se₂ absorbers with RbF‐PDT. The Rb–In–Se surface species is not removed by the (wet‐chemical) CdS deposition process, while some Se diffuses into the CdS layer and segregates at its surface. The CdS buffer layer shows a band gap of 2.48 ± 0.12 eV, and a cliff (≈ −0.4 eV) is determined in the conduction band alignment at the interface between the Rb–In–Se species and the CdS buffer.

A detailed characterization of the impact of a RbF post-deposition treatment (RbF-PDT) on the chemical structure of a wide-gap Cu(In, Ga)Se2 thin-film solar cell absorber surface with a high Ga/(Ga + In) (GGI) ratio of 0.9 is presented. Using synchrotron- and lab-based x-ray photoelectron spectroscopy, as well as x-ray-excited Auger electron spectroscopy, we observe distinct differences to RbF-PDT on absorber surfaces with the common GGI of ∼0.3. In particular, RbF-PDT reduces sodium and oxide content at the surface, while the copper concentration at the surface is not affected. We find no spectral evidence for the formation of a distinct Rb–In–Se surface layer. In addition, we observe that the GGI ratio at the surface is slightly decreased due to a reduction of the Ga and an increase in the In concentration, which may explain the observed improvement in the power conversion efficiency after the PDT (from 6.8% to 7.3%).