The Sn/Si(111)-( 3 x 3)R30 degrees surface, a 2D Mott insulator, has long been predicted and then found experimentally to metallize and even turn superconducting upon boron doping. To clarify the structural, spectroscopic, and theoretical details of that evolution, here we present angle resolved photoemission spectra data supplementing morphology and scanning tunneling measurements. These combined experimental results are compared with predictions from a variety of electronic structure approaches, mostly density functional DFT+U, but not neglecting Mott-Hubbard models, both ordered and disordered. These theoretical pictures address different spectroscopic aspects, including the 2D Fermi surface, the Hubbard bands, etc. While no single picture accounts for all observations at once, the emergent hypothesis compatible with all data is that metallization arises from sub-subsurface boron doping, alternatively to the main standard subsurface boron geometry that would leave the surface insulating. These results advance the indispensable frame for the further understanding of this fascinating system.

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.

Antimonene, the two-dimensional phase of antimony, appears in two distinct allotropes when epitaxially grown on Bi2Se3: the puckered asymmetric washboard (α) and buckled honeycomb (β) bilayer structures. As-deposited antimony films exhibit varying proportions of single α and β structures. We identify the conditions necessary for ordered, pure-phase growth of single to triple β-antimonene bilayers. Additionally, we determine their electronic structure, work function, and characteristic core-level binding energies, offering an explanation for the relatively large chemical shifts observed among the different phases. This study not only establishes a protocol for achieving a single β phase of antimonene but also provides key signatures for distinguishing between the different allotropes using standard spectroscopic and microscopic techniques.

An in-depth understanding of the electronic structure of 2H-MoTe2 at the atomic layer limit is a crucial step towards its exploitation in nanoscale devices. Here, we show that millimeter-sized monolayer (ML) MoTe2 samples, as well as smaller sized bilayer (BL) samples, can be obtained using the mechanical exfoliation technique. The electronic structure of these materials is investigated by angle-resolved photoemission spectroscopy (ARPES) for the first time and by density functional theory (DFT) calculations. The comparison between experiments and theory allows us to describe ML MoTe2 as a semiconductor with a direct gap at the K point. This scenario is reinforced by the experimental observation of the conduction band minimum at K in Rb-doped ML MoTe2, resulting in a gap of at least 0.924 eV. In the BL MoTe2 system, the maxima of the bands at Γ and K show very similar energies, thus leaving the door open to a direct gap scenario, in analogy to WSe2. The monotonic increase in the separation between spin-split bands at K while moving from ML to BL and bulk-like MoTe2 is attributed to interlayer coupling. Our findings can be considered as a reference to understand quantum anomalous and fractional quantum anomalous Hall effects recently discovered in ML and BL MoTe2 based moiré heterostructures.

The studies of electronic effects in reduced dimensionality have become a frontier in nanoscience due to exotic and highly tunable character of quantum phenomena. Recently, a new class of 2D ultrathin Ln$X_2$ metalloxenes composed of a triangular lattice of lanthanide ions (Ln) coupled with 2D-Xenes of silicene or germanene ($X_2$) was introduced and studied with a particular focus on magnetic and transport properties. However, the electronic properties of metalloxenes and their effective functionalization remain mainly unexplored. Here, using a number of experimental and theoretical techniques, we trace the evolution of electronic properties and magnetic ground state of metalloxenes triggered by external perturbations. We demonstrate that the band structure of Ln$X_2$ films can be uniquely modified by controlling the Xenes stacking, thickness, varying the rare-earth and host elements, and applying an external electric field. Our findings suggest new pathways to manipulate the electronic properties of 2D rare-earth magnets that can be adjusted for spintronics applications.

This study explores the intricate chemical processes at the interface between the topological insulator Bi2Se3 and deposited Au. The study mainly focused on room-temperature interactions that can cause the aging of, e.g., gold contacts on electronic devices based on the topological insulators (TIs) or spintronic devices. Our investigation uncovers a complex mechanism involving redox reactions, diffusion, and structural changes akin to the vapor-liquid-solid process. We observe the precipitation of metallic bismuth on the top of the Au layer and also a similar process, albeit at a slower rate, involving Se-0. The resulting non-stoichiometry in the interfacial layers is compensated with the formation of an intermetallic compound low on Bi. As the temperature increases, Se diffusion intensifies, now leading to a selenium deficiency at the interfacial region and subsequent restructuring of the interface. These findings provide valuable insights crucial for optimizing material design and device performance, thereby guiding future research endeavors and technological

MXenes are two-dimensional (2D) materials with a great potential for sensor applications due to their high aspect ratio and fully functionalized surface that can be tuned for specific gas adsorption. Here, we demonstrate that the Nb2CTz-based sensor exhibits high performance towards alcohol vapors at temperatures up to 300–350 °C, with the best sensitivity towards ethanol. We attribute the observed remarkable chemiresistive effect of this material to the formation of quasi-2D Nb2O5 sheets as the result of the oxidation of Nb-based MXenes. These findings are supported by synchrotron X-ray photoelectron spectroscopy studies together with X-ray diffraction and electron microscopy observations. For analyte selectivity, we employ a multisensor approach where the gas recognition is achieved by linear discriminant analysis of the vector response of the on-chip sensor array. The reported protocol demonstrates that MXene layers are efficient precursors for the derivation of 2D oxide architectures, which are suitable for developing gas sensors and sensor arrays.

Studies of persistent organic radical films on conductive metal surfaces can pave the way for diverse applications such as improved spin probes and labels, data control and storage, spintronics, and quantum computing. We grew monolayer films of three nitroxyl radicals (NRs), viz. TEMPO and two carbamoyl-proxyl radicals (nit8 and nit9) under ultra-high vacuum conditions on Au(111) and Cu(111) surfaces. The electronic properties of the films and NR adsorption mechanisms were analyzed by means of X-ray photoelectron (XPS) and absorption (NEXAFS) spectroscopies, with the aid of density functional theory (DFT) and time-dependent DFT computations performed on large unit cells (rev-PBE) and clusters (CAM-B3LYP). We found that all three NRs physisorb weakly on Au. In the case of nit8 and nit9, H-bonded monolayers are formed that recline parallel to the Au surface. Stronger interactions with Cu resulted in chemisorption and robust films, with nit8 and nit9 exhibiting upright orientation due to the amide group acting as an efficient binding anchor. Conversely, TEMPO binds to Cu necessarily via NO which is observed to lead to the destruction of the spin-carrying NO functionality. Computational evidence highlighted the decisive role of Cu surface defects in the partial fragmentation of the CONH2 anchor upon chemisorption of nit8 and nit9. Nitroxide radicals' adsorption mechanisms and film properties tunable by appropriately selecting the substrate.

Wide bandgap perovskites have recently gained attention owing to their physical properties, versatility, and potential in various optoelectronic devices including LEDs, detectors, and building-integrated photovoltaics (BIPV). However, BIPV materials must meet conflicting requirements, necessitating high performance, high transparency in the visible spectrum and color neutrality. This study investigates the controlled addition of chlorine in FAPb(Br1-xClx)3 perovskites to achieve band gaps exceeding 2.4 eV. Increasing chlorine content from xCl = 0.00 to xCl = 0.25 widens the band gap from 2.37 to 2.52 eV, effectively improving the visible light transparency. Advanced characterization techniques including X-ray diffraction, synchrotron radiation photoelectron spectroscopy, photoluminescence imaging, and fast transient absorption spectroscopy, complemented by density functional theory, reveal insights into absorption properties, electronic structure, and ultrafast recombination dynamics as a function of the thin film chemical composition. Furthermore, this study evaluates energetic disorder, carrier recombination rates, and non-radiative losses for different compositions by extracting quantitative parameters such as Urbach energy and quasi-Fermi level splitting, offering novel insights and guidelines for the design and optimization of emerging photovoltaic (PV) materials. Optimal PV performance metrics are achieved with a wide bandgap bromine perovskite containing 14% chlorine, striking a balance between morphology, transparency, and voltage losses.

Surface defects engineered nano-Cu/TiO2 photocatalysts are synthesized through an easy and cost-effective microwave-assisted hydrothermal synthesis, mixing commercial P25 titania (TiO2) and oxalic acid (Ox), followed by 2.0 wt% Cu co-catalyst (labeled as Cu-2.0) loading through in situ photodeposition during reaction. The hydrothermal treatment does not affect the catalyst crystalline structure, morphology, nor the surface area. However, depending on the Ox/TiO2 molar ratio used an influence on the optical properties and on the reactivity of the system is detected. The presence of surface defects leads to intraband states formation between valence band and conduction band of bare titania, inducing an important enhancement in the photoactivity. Thus, Cu-2.0/gOx/P25 200 (where g is the weight of Ox and 200 the temperature in Celsius degrees used during the synthesis) have been successfully tested as efficient photocatalysts for hydrogen production through methanol (MeOH) reforming under UV light in a MeOH/ H2O solution (10% v/v) by fluxing the system with N-2, showing an increased reactivity compared to the bare Cu-2.0/P25 system.

Topological insulators containing single atoms of magnetic metals are useful for studying the effects of magnetism on the topological properties of matter. Controlling the distribution of magnetic atoms remains a major issue during the synthesis of these materials, as the formation of clusters and/or unwanted precipitates competes energetically with the formation of dilute phases. Here, we report the synthesis of large Bi2Se3 single crystals with rhombohedral structure and extremely low concentration (0.15 at%) of substitutional Cr ions using the Bridgman method. After exfoliation, the crystals exhibit a topological surface state with an energy-momentum dispersion different from that of pristine Bi2Se3, due to the presence of an apparent energy gap at the Dirac point. The properties of this state are discussed in comparison with previous spectroscopic measurements of Cr-doped Bi2Se3 films.

In this study, the electronic band structure and the dynamics of the excited carriers in the formamidinium lead bromide (FAPbBr3) perovskite are investigated by combining the information obtained from steady-state absorption, photoluminescence, femtosecond transient absorption spectroscopy, photoelectron spectroscopy, and density functional theory calculations. A detailed description of the electronic transitions in the UV-vis energy range has been provided, giving an estimation of the exciton binding energy (40 +/- 5 meV), the transition energy from the first valence band (VB1) to the conduction band (CB1) (electronic bandgap at 2.37 +/- 0.01 eV) and assigning the broad peak at approximate to 3.4 eV to the transition from the second innermost valence band (VB2) to CB1. The temporal dynamics of the excited carriers involved in these transitions are investigated using different excitation energies and carrier densities. Different trends are observed in the dynamics of the transient signals associated with the VB1 -> CB1 (PB1) and VB2 -> CB1 (PB2) transitions. As the carrier density increased, PB1 exhibited a slowing down of its rise time, while PB2 showed an acceleration attributed to the thermalization of the excited holes in VB2. These valuable findings have the potential to unlock new strategies aimed at maximizing the efficiencies and performance of perovskite-based solar cell devices. With the use of a synergic and multiscale approach that brings together experiments and theory, a detailed insight into the electronic structure of FAPbBr3 along with the description of the ultrafast dynamics of photoexcited carriers is provided. The multiscale methodology allows the exploration of novel strategies for the design of perovskite-based solar cells.image

We present a detailed analysis of the electronic properties of graphene/Eu/Ni(111). By using angle- and spin-resolved photoemission spectroscopy and ab initio calculations, we show that the intercalation of Eu in the graphene/Ni(111) interface gives rise to a gapped freestanding dispersion of the ππ^{*} Dirac cones at the K[over ¯] point with an additional lifting of the spin degeneracy due to the mixing of graphene and Eu states. The interaction with the magnetic substrate results in a large spin-dependent gap in the Dirac cones with a topological nature characterized by a large Berry curvature and a spin-polarized Van Hove singularity, whose closeness to the Fermi level gives rise to a polaronic band.

The Zhang-Rice (ZR) state is a strongly hybridized bound state formed by transition -metal and oxygen atoms. The spin fluctuations within the ZR state are known to play an important role in high- T c superconductivity in cuprates. Here, we employ a combination of angle -resolved photoemission spectroscopy (ARPES), x-ray photoemission spectroscopy (XPS), and ab initio embedded dynamical mean -field theory (eDMFT) to investigate the influence of magnetic ordering on the spectral characteristics of the valence band and Mn 2 p core -level in MnO (001) ultrathin films. Our results demonstrate that a complex spin -selective evolution of Mn 3 d - O 2 p hybridization develops due to the long-range antiferromagnetic (AFM) ordering. This hybridization significantly alters the spectral shape and weight of the ZR state. Specifically, in the AFM phase, we observed the sharpening of the ZR state and band folding with the periodicity of the AFM unit cell of MnO(001). We also demonstrated a strong connection between the spectral evolution of the ZR state and the non -local screening channels of the photoexcited core holes. Further, our detailed temperature -dependent study reveals the presence of short-range antiferromagnetic correlations that exist at much higher temperatures than Neel temperature ( T N ) and shows the evolution of the ZR state across the magnetic transitions and its implication to the core -hole screening in 3 d binary transition metal oxides.

Bismuth produces different types of ordered superstructures on the InAs(100) surface, depending on the growth procedure and coverage. The (2x1) phase forms at completion of a Bi monolayer and consists of a uniformly oriented array of parallel lines of Bi dimers. Scanning tunneling and core level spectroscopies demonstrate its metallic character, in contrast with the semiconducting properties expected on the basis of the electron counting principle. The weak electronic coupling among neighboring lines gives rise to quasi one-dimensional Bi-derived bands with open contours at the Fermi level. Spin- and angle-resolved photoelectron spectroscopy reveals a giant Rashba splitting of these bands, in good agreement with ab-initio electronic structure calculations. The very high density of the dimer lines, the metallic and quasi one-dimensional band dispersion and the Rashba-like spin texture make the Bi/InAs(100)-(2x1) phase an intriguing system, where novel transport regimes can be studied.

Topological insulators in which the Fermi level is in the bulk gap and intersects only a topological surface state (the Dirac cone) are of special interest in the current research. In the last decades, a fine-tuning of the chemical composition of topological insulators has been carefully explored in order to control the Fermi level position with respect to the Dirac surface state. Taking the SnBi2Te4 crystal as a case study, we provide a characterization of its electronic structure by means of angle-resolved photoemission spectroscopy and first-principles calculations. We show that, going away from the Brillouin zone center, bulk band states energetically overlap with the Dirac cone at the Fermi level, thus providing an unwanted as well as hidden contribution to the transport properties of the material. In addition, the comparison between experimental results of the band structure with state-of-the-art simulations, implemented taking into account the number of defects, leads to useful insights on the existing limits in the description of this material.

Dimensionality can strongly influence the magnetic structure of solid systems. Here, we predict theoretically and confirm experimentally that the antiferromagnetic (AFM) ground state of bulk gadolinium germanide metalloxene, which has a quasi-layered defective GdGe2 structure, is preserved in the ultrathin film limit. Ab initio calculations demonstrate that ultrathin GdGe2 films present in-plane intra-layer ferromagnetic coupling and AFM inter-layer coupling in the ground state. Angle-resolved photoemission spectroscopy finds the AFM-induced band splitting expected for the 2 and 3 GdGe2 trilayer (TL) films, which disappear above the Néel temperature. The comparative analysis of isostructural ultrathin DyGe2 and GdSi2 films confirms the magnetic origin of the observed band splitting. These findings are in contrast with the recent report of ferromagnetism in ultrathin metalloxene films, which we ascribe to the presence of uncompensated magnetic moments.

The sluggish kinetics associated with the oxygen evolution reaction (OER) limits the sustainability of fuel production and chemical synthesis. Developing catalysts based on Earth abundant elements with a reasonable strategy could solve the challenge. Here, we present a heterostructure built from CrOx and CuS whose interface gives rise to the advent of new functionalities in catalytic activity. Using X-ray photoelectron and absorption spectroscopies, we identified the multiple oxidation states and low coordination number of Cr metal in CrOx-CuS heterostructure. Benefitting from these features, CrOx-CuS generates oxygen gas through water splitting with a low over potential of 190 mV vs RHE at a current density of 10 mA cm-2. The catalyst shows no evident deactivation after a 36-hours operation in alkaline medium. The high catalytic activity, inspired by first principles calculations, and long-time durability make it one of the most effective OER electrocatalysts.

Silicene, the silicon equivalent of graphene, is attracting increasing scientific and technological attention in view of the exploitation of its exotic electronic properties. This novel material has been theoretically predicted to exist as a free-standing layer in a low-buckled, stable form, and can be synthesized by the deposition of Si on appropriate crystalline substrates. By employing low-energy electron diffraction and microscopy, we have studied the growth of Si on Ag(111) and observed a rich variety of rotationally non-equivalent silicene structures. Our results highlight a very complex formation diagram, reflecting the coexistence of different and nearly degenerate silicene phases, whose relative abundance can be controlled by varying the Si coverage and growth temperature. At variance with other studies, we find that the formation of single-phase silicene monolayers cannot be achieved on Ag(111).