Here we present new findings of a comprehensive study of the fundamental physicochemical properties for GeS and GeSe in bulk form. UV and X-ray photoelectron spectroscopies (UPS/XPS) were employed for the experiments, which were carried out on in situ cleaned (100) surfaces free from contamination. This allowed to obtain reliable results, also unchanged by effects related to charging of the samples. The work functions, electron affinities and ionization energies as well as core level lines were found. The band gaps of the investigated materials were determined by photoreflectance and optical absorption methods. As a result, band energy diagrams relative to the vacuum level for GeS and GeSe were constructed. The diagrams provide information about the valence and conduction band offsets, crucial for the design of various electronic devices and semiconducting heterostructures.

The Fermi level position at the interface of a heterostructure is a critical factor for device functionality, strongly influenced by surface-related phenomena. In this study, contactless electroreflectance (CER) was utilized for the first time to investigate the built-in electric field in MXene/GaN structures with the goal of understanding the carrier transfer across the MXene/GaN interface. Five MXenes with high work functions were examined: Cr₂C, Mo₂C, V₂C, V₄C₃, and Ti₃C₂. The physicochemical properties of the MXene/GaN structures were analyzed by using X-ray and UV photoelectron spectroscopies. It was shown that upon the coverage of the GaN surface by all investigated MXenes, a shift in the position of the surface Fermi level occurs, consequently raising the interface barrier. Additionally, the physicochemical stability of MXenes on the GaN surface was studied after annealing the structures at 750 °C. Our findings indicate that the annealing process increases the barrier height and the ionization energies of all studied structures. Furthermore, it has been shown that removing excess MXene material from the surface did not significantly impact the built-in electric field, emphasizing the robust physicochemical stability of the MXenes on the GaN surface. To validate the potential of engineering of MXene/GaN interface barrier, Schottky diodes with MXenes exhibiting the highest barrier height (Mo₂C and V₂C) were demonstrated.

Van der Waals crystals offer opportunities to engineer material properties by stacking diverse 2D layers into heterostructures. Charge redistribution at these interfaces, governed by band alignment and Fermi levels, enables control over optical, electronic, and magnetic behavior. In this study, we investigated the band alignment and excitation spectra of exfoliated MPS₃ (M = Mn, Fe, Co, Ni) using X-ray and UV photoelectron spectroscopy, optical absorption, and DFT + U calculations. Ionization potentials from 5.4 eV (FePS₃) to 6.2 eV (NiPS₃) were determined. The resulting band diagrams differentiate localized d states from hybridized p-d states, offering insights for designing functional heterostructures. Selective occupation of unoccupied 3d states provides a pathway to tune magnetic order. Theoretical results show that MnPS₃ is well-aligned for both hydrogen and oxygen evolution reactions (HER and OER), while other MPS₃ are promising for OER. MnPS₃/NiPS₃ heterostructure exhibits optimal band alignment for efficient water splitting across a broad pH range.

GaNAs layers with a low As concentration (As ≤ 0.6%) have been grown by molecular beam epitaxy and studied by structural and optical methods. It has been observed that the incorporation of a small amount of As atoms into the GaN host leads to a significant reduction of the bandgap due to the formation of an As-related band above the valence band of the GaN host. The position of this band does not change with temperature, and therefore, a reduced temperature dependence of the bandgap is observed for As-diluted GaN compared to the pure GaN host, which is ∼40 meV vs ∼70 meV in the 10–295 K temperature range. The observed effect is explained within the band anticrossing model. It is expected that the reduced temperature dependence of the bandgap in As-diluted GaN can be utilized in lasers with improved thermal stability.