Local composition fluctuations in random alloys become crucial when one or more dimensions are reduced to the nanoscale. Using extended Hückel theory, we study the semiconductor random alloy SiGe sandwiched between Si due to its relevance for transistor devices. We evaluate the effects of the alloy composition, layer thickness, and local fluctuations of the Ge concentration on the band alignment and the band gap. The results are compared with the finite quantum well model. That model captures the essential physics and can act as a computationally faster alternative.

Epitaxial bismuthene on SiC(0001) hosts symmetry-protected metallic edge states within a large bulk band gap, establishing it as a promising two-dimensional topological insulator for hightemperature quantum spin Hall (QSH) transport. Here we realize bismuthene islands by intercalating Bi beneath zero-layer graphene on SiC(0001) followed by hydrogen treatment, yielding well-defined edges with controlled terminations. Spectroscopic measurements demonstrate that the edge states reside inside the bulk band gap and remain charge neutral. The graphene overlayer interacts only weakly with the bismuthene, preserving its topological character while providing environmental protection. Notably, the one-dimensional edge channels exhibit signatures of enhanced electronic correlations relative to freestanding bismuthene, suggesting proximity-induced modification of the QSH edge physics. These results establish graphene-capped bismuthene as a robust and tunable platform for correlated quantum spin Hall states.

Intercalating a Pb monolayer between graphene and SiC(0001) creates a densely packed metallic layer in close proximity to graphene. Using low-temperature four-point-probe scanning tunneling microscopy and density functional theory, we correlate the local conductivity of this two-dimensional heterostructure with spatially resolved spectroscopy. By varying the tunneling gap, we distinguish the density-of-states contributions of the decoupled graphene sheet and the buried Pb interface layer. At large tip-sample separations, the spectra resemble those of charge-neutral, quasi-freestanding graphene with a small contribution of the metallic Pb layer beneath. This separation confirms the presence of a 5 meV energy gap in graphene, primarily arising from symmetry breaking induced by the epitaxial Pb layer. A proximity-induced intrinsic spin-orbit coupling appears negligible or is compensated by Rashba-type interactions.

Recent studies have revealed that a confined bismuth layer at the graphene/SiC interface can be reversibly switched between a topologically trivial precursor phase and the quantum spin Hall insulator bismuthene. Here, we present a detailed spin- and angle-resolved photoelectron spectroscopy study of both structures, resolving the spin texture of their low-energy electronic states. Owing to the strong intrinsic spin-orbit coupling of bismuth and the asymmetric confinement potential at the interface, the valence bands of both structures are Rashba-split. We demonstrate the expected spin-momentum locking for both phases and Kramers' doublets through the investigation of the valence bands' spin polarization at multiple positions in reciprocal space.

The adhesion of joining engineering plastics to the surface of aluminum components depends on the characteristics of the interface. Using first-principles density functional theory (DFT), the adsorption, surface reconstruction, and local bond stiffness of six polymer-constituent adatoms (H, C, N, O, Si, and S) are investigated at the top, bridge, hcp, and fcc sites of pristine and vacancy-defective Al(111) surfaces. All adatoms favor the threefold hollow sites, and the average adatom-aluminum bond length changes with the atomic radius, while C, N, and Si lead to the strongest first-layer reconstruction. The electronegative atoms N and O bind most strongly (about-7.9 eV), and a single surface vacancy effectively traps the adatoms C, N, and O with small covalent radii below the surface plane, marking the incipient stages of oxide, nitride, and carbide formation. Furthermore, the vertical bond stiffness is found to be inversely correlated with the binding strength. The top site with low coordination yields the stiffest but weakest bond and the highest vibrational frequency (up to 1679 cm-1 for H), while the stable hollow sites present softer bonds. Electron localization function and crystal orbital Hamilton population analyses corelate this inverse relationship to bond directionality which governs the bond stiffness, and to the total covalent overlap which governs the binding strength.

The discovery of two-dimensional materials has revolutionized condensed matter physics, with transition metal dichalcogenides (TMDCs) offering tunable electronic and optical properties. Monolayer tungsten diselenide WSe $_2$ , a direct bandgap semiconductor with strong excitonic effects, is especially promising for optoelectronics. When two WSe $_2$ monolayers are stacked with controlled twisting angles, the resulting twisted bilayer (tB) material forms a moiré superlattice that significantly modifies its electronic structure and optical response through interlayer coupling and band reconstruction. In a systematic study employing photoluminescence (PL) and differential micro-reflectance contrast ( $\mu$ RC) spectroscopy and supported by first-principles calculations, we investigate the optical properties of tB WSe $_2$ for twisting angles $ 0^\circ \lt \theta \lt 60^\circ $ . Excitonic peaks (A, B, C, D) exhibit angle-dependent energy shifts. Notably, A and C excitons show characteristic energy splittings that reflect twisting-angle-modulated interlayer hybridization and spin–orbit coupling effects. The A exciton shows local minima at $0^\circ$ and $60^\circ$ , and a maximum near $30^\circ$ . This pattern reflects variations in interlayer hybridization—stronger coupling at $0^\circ$ and $ 60^\circ$ , weaker at intermediate angles—consistent with moiré-induced modifications. Our work reveals the periodic modulation of exciton energies in WSe $_2$ homo-bilayers across a wide range of twisting angles, directly linking these variations to interlayer coupling strength and spin–orbit splitting. Our findings provide clear experimental–theoretical consistency, identifying the twisting angle as an effective tuning knob for excitonic transitions and interlayer interactions in TMDC bilayers. The work contributes to the understanding of the structure–property relationships in twisted TMDC materials, and the results may lead to new design principles for next-generation, moiré-engineered optoelectronic and quantum devices.

The nucleation of coherent L12 precipitates in aluminum alloys is investigated by first-principles supercell modeling, with density functional theory (DFT) calculations in the generalized gradient approximation, employing a plane wave basis set and pseudopotentials. Distance-resolved solute-solute interaction maps were obtained for Zr in Al, revealing a strong energetic preference for the 2nd coordination shell, consistent with the atomic arrangement in the metastable L12-Al3Zr phase. Stepwise cluster assembly shows a linear stabilization energy gain, while configurations corresponding to the equilibrium D023 structure are unfavorable at early nucleation stages. Similar 2nd-shell attraction was confirmed for Sc, Ti, and Hf, in contrast to V, Cr, and Cu, which lack strong ordering tendencies. These results provide atomistic insight into precipitate formation pathways and establish predictive criteria for identifying alloying elements with a high propensity to form thermally stable, coherent L12 phases in aluminum alloys.

Self-assembled monolayers of polyalanine α-helices exhibit distinct structural phases with implications for chiral-induced spin selectivity. We combine scanning tunneling microscopy and theoretical modeling to reveal how chiral composition governs supramolecular organization. Enantiopure systems form hexagonal lattices, while racemic mixtures organize into rectangular phases with stripe-like features. Our interaction potentials derived from density-functional based tight binding calculations show that opposite-handed helix pairs exhibit stronger binding and closer packing, explaining the denser racemic structures. Crucially, we demonstrate that the observed STM contrast arises from anti-parallel alignment of opposite-handed helices rather than physical height variations. These findings establish fundamental structure-property relationships for designing peptide-based spintronic materials.

A new parameterization for Si and Ge in the framework of the extended Hückel theory (EHT) is presented and its application in calculating the band structure of strained and unstrained SiGe alloys with varying Ge content is shown. The variation of the bandgap and electron/hole effective masses caused by the random distribution of Si and Ge atoms in the alloy is investigated. The parameter set is further verified by combining it with an existing parameterization for C, highlighting the key advantage of EHT over standard tight binding methods: the atom‐centered EHT approach allows an easy combination of parameter sets. By unfolding the alloy supercell band structure back into the primitive cell Brillouin zone, the effect of alloy composition on the electronic structure is visualized and the effect of neighboring C atoms on the bandgap is investigated.

Solid solution hardening and softening effects in aluminum alloys by substitutional foreign atoms are studied using quantum-mechanical first-principles modeling of periodically repeated supercells, with full density functional theory (DFT) calculations in the generalized gradient approximation, employing a plane wave basis set and pseudopotentials. In supercell models of the Shockley partial dislocation, the electronic and steric interactions of the alloying atoms are investigated within the dislocation core and the surrounding Cottrell atmosphere. The DFT-derived energy barrier toward dislocation motion is most strongly modified for substitutional sites located close to the dislocation core. The influence of the different substituents increases with the propensity to form binary aluminum compounds, but with an element-specific variation: Si lowers the barrier, indicating a potential solid solution softening effect, whereas Zr increases the barrier, which correlates with an impeded dislocation motion. On the quantum scale, Mg is of little influence with a slight tendency toward a barrier increase, although its classical alloying parameters are identical to those of Zr. Thus, calculating from first principles the additional electronic contributions to the still commonly applied classical size-stress theory is indispensable for alloying near dislocation cores. This indicates that electronic interactions need to be included for refining the classical model.

Intercalation in epigraphene systems is an established technique widely used to modify the electronic structure of graphene and to synthesize otherwise unstable two-dimensional layers with exotic electronic properties. However, capturing the full symmetry of the heterostructure requires a large number of atoms, rendering traditional electronic-structure approaches computationally demanding. Density-functional-based tight-binding (DFTB) offers an efficient alternative, balancing accuracy and reduced computational cost. For heavy elements such as Pb, however, proper parameters for these types of calculations are not available in the open literature. In this work, we developed a Slater-Koster parameter set for the elements Si, C, and Pb, enabling the investigation of the electronic structure of various elemental and binary solid-state structures, such as graphene, plumbene, and silicon carbide. Our results obtained with DFTB for bulk Pb and Pb/SiC structures show good qualitative agreement with pure DFT calculations. Furthermore, the inclusion of spin-orbit coupling (SOC) significantly modifies their electronic properties, aligning with DFT findings. These results underscore the capability of the optimized parameters to accurately model complex systems, allowing investigations of larger systems closer to experimental observations.

SiGeC, a random alloy, is studied by means of density functional theory for different Si:Ge ratios across the whole composition range and small C concentrations of 1.6 and 3.2% in Si-rich alloys. The random occupation of lattice sites in SiGeC results in statistical variations of material properties, which we investigate using approximate to 20,000 random configurations. This is an alternative approach to methods such as the virtual crystal approximation or the special quasi-random structure method because the presented approach allows access to statistical fluctuations. These fluctuations can be seen in the formation energy of the SiGe alloy, and the fluctuations increase with increasing C content. In addition, we can associate those fluctuations to structural features. Stability analysis based on the Gibbs energy shows that an elevated temperature is required to stabilize the random alloy via entropic contributions. The solubility of C is rather small (<1% at room temperature, up to 1.5% at 500 K), depends on the Ge content, and increases with rising temperature. We also study the band gap considering methodological as well as structural implications: Their quantitative values are strongly affected by finite-size effects due to periodic boundary conditions in the simulation, leading to larger band gap variations than potentially observable. Despite this issue, we can conclude qualitative trends such as rising band gap variations with increasing C content.

The development of efficient solar cells is limited by the inability of the materials to absorb light from the entire solar spectrum. InGaN solar cells have become promising, due to the broad bandgap coverage of the solar spectrum from 0.7 eV to 3.42 eV. The performance of InGaN devices is simulated using SCAPS-1D software. The impacts of layer thickness and defect density on the performance of p-n and p-p-n junction InGaN solar cells were investigated, including holistic optimization of power conversion efficiency and quantum efficiency. A notable observation was the improvement in conversion efficiency with rising indium content, peaking at 23.8 % for In0.6Ga0.4N. For p-p-n junction cells, a thicker p-layer plus an additional thin top p-layer with a larger bandgap proved advantageous. The n-layer defect density in p-n junction cells showed minimal effects on open-circuit voltage and fill factor but reduced short-circuit current and efficiency as it increased. Conversely, the p-layer defect density influenced performance only at high densities beyond 1016 cm−3, while for p-p-n junctions, the top p-layer's defect density had minimal impact. The optimized designs for both p-n and p-p-n junction cells, incorporating graded bandgaps, achieved optimal conversion efficiencies of 33.89 % and 34.07 %, respectively. The p-p-n design showed an enlarged high-efficiency area for suitable indium concentrations, offering broader indium concentration tuning possibilities and better lattice constant tuning. Quantum efficiency evaluations show the differences of defect densities and thicknesses across specific wavelength intervals, reaffirming the potential for strategic cell design choices.

The stability of aluminum alloy surfaces has a critical influence on their ability to join to another material. This study employs first-principles density functional theory calculations to investigate the surface stability, configuration, and diffusion tendencies of alloying elements Mg, Si, Zr, and Sc on the crystal planes (001), (110), and (111) of an aluminum alloy. Utilizing asymmetric slab models, we introduce and apply an energy difference concept (AE2D) to assess the stability of surface alloys relative to pure element covered surfaces. The findings reveal that surface stability is strongly dependent on both composition and atomic arrangement. Generally, mixed surface compositions, particularly a 50% coverage Al2M2, exhibit enhanced stability. And the next nearest neighbor is favored over the nearest-neighbor position of alloying atoms which are precursors to nearest precipitates. An analysis of the diffusion paths indicates that Mg and Si preferentially segregate to the outermost surface layer with Mg more strongly, whereas Zr and Sc exhibit a lower driving force to diffuse towards the outermost aluminum surface.

Quantum spin Hall insulators have been extensively studied both theoretically and experimentally because they exhibit robust helical edge states driven by spin-orbit coupling and offer the potential for applications in spintronics through dissipationless spin transport. Here we show that a single layer of elemental Bi, formed by intercalation of an epitaxial graphene buffer layer on SiC(0001), is a promising candidate for a quantum spin Hall insulator. This layer can be reversibly switched between an electronically inactive precursor state and a bismuthene state, the latter exhibiting the predicted band structure of a true two-dimensional bismuthene layer. Switching is accomplished by hydrogenation (dehydrogenation) of the sample. A partial passivation (activation) of Si dangling bonds causes a lateral shift of Bi atoms involving a change of the adsorption site. In the bismuthene state, the Bi honeycomb layer is a prospective quantum spin Hall insulator, inherently protected by the graphene sheet above and the H-passivated substrate below.

In the present study, we investigate the influence of boron (B) and phosphorus (P) (p- and n-type, respectively) doping on the electronic properties of ultra-thin silicon nanowires (SiNWs) by gradient-corrected density functional calculations with the Perdew–Burke–Ernzerhof (PBE) approximation. In the limit of very small diameters (5–8 Å), both pristine and highly active unsaturated SiNWs with orientations along the [001] and [111] directions exhibit electronic states around the Fermi level, indicative of conductive properties. Conduction is further enhanced by the introduction of doping atoms, as demonstrated by the relative change in the band structures of SiNWs with and without B and P doping. This investigation provides an important insight into the electronic states of SiNWs, which are candidates for future electronics or sensing applications.

This work presents a combined optical and magneto-optical spectroscopic study of thulium iron garnet (Tm3Fe5O12, TmIG) films on substituted gadolinium gallium garnet (Gd2.6Ca0.4Ga4.1Mg0.25Zr0.65O12, sGGG) substrates. Spectroscopic ellipsometry, transmission spectroscopy, magneto-optical Kerr effect spectroscopy and Raman spectroscopy results are presented for TmIG films with a thickness in the range from 20 to 300 nm grown on sGGG by pulsed laser deposition. The complex dielectric functions of TmIG and sGGG are determined and compared with previously published results for bulk yttrium iron garnet and GGG, respectively. The magneto-optical spectroscopy corroborated with Raman spectroscopy sheds light on strain-induced changes as a function of TmIG film thickness.

Intercalation is a promising approach for tailoring the electronic structure of epitaxial graphene on SiC. It enables the formation of otherwise unstable 2D phases of elements and allows the investigation of the interplay between the 2D materials and the substrate. Detailed studies have been conducted on the Pb intercalation process, as well as the structure and electronic properties of the 2D Pb layer using low‐energy electron microscopy and photoelectron spectroscopy. The low‐energy bands of Pb show good agreement with density‐functional theory calculations. A uniform Pb intercalation layer with (1 × 1) periodicity with respect to the SiC substrate is found. The quasifreestanding graphene is effectively screened from the doping influence of the substrate, leading to charge neutrality. Instead, the 2D Pb layer compensates for the spontaneous polarization of the substrate, allowing for the doping of a metal layer under cover. A phase transformation from the (1 × 1) intercalation phase into a bubble phase with quasi‐tenfold periodicity with respect to graphene occurs if the system is provided with sufficient energy. These results experimentally quantify the interaction between the 2D Pb layer, the substrate, and the graphene layer, demonstrating a first step toward controlling the diversity of 2D Pb phases.

The current-voltage characteristics of a single-molecule junction are determined by the electronic coupling Γ between the electronic states of the electrodes and the dominant transport channel(s) of the molecule. Γ is profoundly affected by the choice of the anchoring groups and their binding positions on the tip facets and the tip-tip separation. In this work, mechanically controllable break junction experiments on the N,N'-bis(5-ethynylbenzenethiol-salicylidene)ethylenediamine are presented, in particular, the stretch evolution of Γ with increasing tip-tip separation. The stretch evolution of Γ is characterized by recurring local maxima and can be related to the deformation of the molecule and sliding of the anchoring groups above the tip facets and along the tip edges. A dynamic simulation approach is implemented to model the stretch evolution of Γ, which captures the experimentally observed features remarkably well and establishes a link to the microscopic structure of the single-molecule junction.

Acceptor copolymers with low lowest unoccupied molecular orbital (LUMO) energy levels are key materials for organic electronics. In the present work, quaternization of pyridine -flanked diketopyrrolopyrrole (PyDPPPy) is used to lower the LUMO energy level of the resulting monomer (MePyDPPPy) by as much as 0.7 eV. The drastically changed electronic properties of MePyDPPPy hinder a second methylation step even in an excess of trimethyloxonium tetrafluoroborate and thereby give access to the asymmetric functionalization of N-heterocycle -flanked DPP building blocks. The corresponding n-type polymeric ionene PMePyDPPPyT2 with bithiophene as comonomer forms thixotropic organogels with the p-type polythiophene P(g(4)2T-TT), indicative of specific cross-interactions between this couple of copolymers. Gelation of polymer blend solutions, which is absent for other couples of p-type/ n-type polymers, is of general interest for (co)processing and orientation of different electronic polymers simultaneously into films or filaments. Detailed optical and electronic characterization reveals that films processed from organogels exhibit ground-state electron transfer (GSET) enabled by suitably positioned highest occupied molecular orbital (HOMO) and LUMO energy levels of P(g(4)2T-TT) (-4.07 eV) and PMePyDPPPyT2 (-4.20 eV), respectively. Furthermore, molecular interactions related to gelation and GSET do not appear to significantly influence the morphology of the polymer blend films.