Paolo Moras

High-resolution photoemission spectroscopy combined with density functional theory is used to elucidate how chemical bonding governs the electronic structure of the semimetallic topological candidate β-PtBi₂. On the low-temperature-cleaved (111) surface, the Bi4f core level remains at the binding energy of elemental Bi, whereas Pt4f exhibits an anomalous positive shift of approximately 0.5 eV relative to elemental Pt. Reproducing the experimental valence-band spectrum also requires shifting the calculated density of states by about 0.3 eV toward higher binding energy. These observations indicate that the measured binding-energy shifts reflect not only charge redistribution but also inefficient photohole screening associated with covalent-like Pt5d–Bi6p hybridization. Surface-sensitive Bi5d measurements further suggest enhanced surface metallicity without evidence of a chemically distinct surface phase. In contrast, room-temperature cleavage and sputtering/annealing treatments generate additional Bi and Pt core-level components, indicating a non-stoichiometric PtBi₂−x surface phase. The associated core-level shifts and valence-band evolution indicate that Bi vacancies induce Pt5d–Bi6p rehybridization toward a more metallic bonding regime. These findings establish β-PtBi₂ as a model system for understanding how chemical bonding, surface stoichiometry, charge redistribution, and photohole screening collectively determine the photoemission response of semimetallic intermetallic compounds.

Spinodal and binodal decomposition were used here to drive the formation of self-assembled bulk Bi2Se3/CdSe (i.e. topological insulator / trivial semiconductor) heterojunctions. As a first step, we determined the formation conditions of the stable layered compound, whose stoichiometry was quantified to be Bi2.4Cd0.8Se4.4, and we investigated its crystalline, surface chemical and electronic properties. After this, a systematic study allowed us to establish the thermodynamic and kinetic conditions necessary to achieve phase separation of Bi2.4Cd0.8Se4.4 in the spinodal and binodal regimes. In the spinodal bulk heterojunction material the widths of the CdSe and Bi2Se3 lamellar structures were ~25 to 100 nm and ~80 to 130 nm, respectively. In the binodal bulk heterojunction material, we observed drop-like CdSe crystallites ranging in size from ~ 20 nm to 1 m within the Bi2Se3 matrix. Moreover, the junction between Bi2Se3 and CdSe, was found to be atomically abrupt for both spinodal and binodal patterns. This approach offers a facile self-driven one-step alternative procedure to the complex and expensive multi-step artificial growth methods typically used to produce nanostructured all-inorganic bulk heterojunction materials for future applications in the field of solar energy.

Topological semimetals, being characterized by symmetry‐protected band crossings, represent a fascinating class of materials with extraordinary electronic properties. Type‐II Dirac semimetals, featuring highly tilted Dirac cones, offer unique opportunities for both fundamental research and technological advancements. Platinum ditelluride (PtTe ₂ ) has emerged as a promising candidate for a type‐II Dirac semimetal, exhibiting relevant properties for future spintronic and optoelectronic devices. While the existence of type‐II Dirac cones in PtTe ₂ has been confirmed by cryogenic temperature angle‐resolved photoemission spectroscopy (ARPES), practical applications necessitate their stability at ambient conditions. Here, we present a melt growth method for the synthesis of high‐quality PtTe ₂ crystals and we perform ARPES characterization both at cryogenic and at room temperature, providing compelling evidence for the robust nature of its topological electronic structure under realistic operating conditions. This demonstration paves the way for the development of PtTe ₂ ‐based devices leveraging its distinctive topological properties in practical settings.

Understanding charge separation dynamics in organic semiconductor blends is crucial for optimizing the performance of organic photovoltaic solar cells. In this study, the optoelectronic properties and charge separation dynamics of a PCE10:FOIC blend, by combining steady‐state and time‐resolved spectroscopies with high‐level DFT calculations. Femtosecond transient absorption spectroscopy revealed a significant reduction of the exciton‐exciton annihilation recombination rate in the acceptor when incorporated into the blend, compared to its pristine form. This reduction is attributed to a decrease in exciton density within the acceptor, driven by an efficient hole‐separation process that is characterized by following the temporal evolution of the transient signals associated with the excited states of the donor when the acceptor is selectively excited within the blend. The analysis of these dynamics enabled the estimation of the hole separation time constant from the acceptor to the donor, yielding a time constant of (1.3 ± 0.3) ps. Additionally, this study allowed the quantification of exciton diffusion and revealed a charge separation efficiency of ≈60%, providing valuable insights for the design of next‐generation organic photovoltaic materials with enhanced charge separation and improved device efficiency.

Magnetic order engineering in two-dimensional Dirac systems is of great interest for theoretical and technological exploration. Up to now, the experimental advances in this field mostly concerned graphene monolayers. Here, we report a comprehensive study of a monolayer-thick germanene-like sheet in contact with gadolinium atoms. Direct observations supported by first-principles calculations reveal the fingerprints of the Dirac fermions in the electronic structure and noncollinear antiferromagnetism. The hybridization of the germanene layer with Gd atoms leads to a large and tunable gap in the Dirac states that carry a nonzero spin-Berry curvature. We discovered that cesium-induced controlled electron doping can switch the system into a ferromagnetic state and then back to the antiferromagnetism at saturated cesium monolayer limit. We explain these reversible magnetic transitions by the oscillatory behavior of the Ruderman-Kittel-Kasuya-Yosida interaction and suggest that this system could find application in magnetoelectronics and spintronics.

Water splitting is a promising and sustainable technology that can address energy and environmental challenges by producing clean H₂ without emissions of harmful pollutants. However, alkaline water oxidation is the most relevant process at the industrial level, and it faces obstacles due to unfavorable thermodynamics and high overpotential. The search for new environmentally friendly materials with high activity and low cost is a significant challenge. Herein, Mn₂P₄O₁₂ microspheres are synthesized from MnO₂ nanosheets via hydrothermal and chemical vapor deposition processes by regulating phosphorization as a new material in water‐splitting catalysis. The spherical Mn₂P₄O₁₂ microstructures act as pre‐catalysts and undergo surface reconstruction during electrochemical activation, leading to the formation of β‐MnO₂ as the true active phase. Once stabilized, they exhibit outstanding catalytic performance (250 and 510 mV at 10 and 100 mA cm⁻² with Tafel slope as low as 40.80 mV dec⁻¹) and stability for more than 32 h at different potentials. The in situ surface reconstruction highlighted by a multi‐technique analysis ensures the catalyst's stability and results in efficient catalytic active sites for adsorbed oxygen, which enhances overall performance. This study provides insights into cyclotetraphosphate catalysis and offers a pathway for developing efficient and cost‐effective materials for water electrolysis.