Nanostructured TiO2 is one of the best materials for photocatalysis, thanks to its high surface area and surface reactivity, but its large energy bandgap (3.2 eV) hinders the use of the entire solar spectrum. Here, it is proposed that defect-engineered nanostructured TiO2 photocatalysts are obtained by hydrogenation strategy to extend its light absorption up to the near-infrared region. It is demonstrated that hydrogenated or colored TiO2 hollow spheres (THS) composed of hierarchically assembled nanoparticles result in much broader exploitation of the solar spectrum (up to 1200 nm) and the engineered surface enhances the photogeneration of charges for photocatalytic processes. In turn, when applied for photodegradation of a targeted drug (Ciprofloxacin) this results in 82% degradation after 6 h under simulated sunlight. Valence band analysis by photoelectron spectroscopy revealed the presence of oxygen vacancies, whose surface density increases with the hydrogenation rate. Thus, a tight correlation between degree of hydrogenation and photocatalytic activity is directly established. Further insight comes from electron paramagnetic resonance, which evidences bulk Ti3+ centers only in hydrogenated THS. The results are anticipated to disclose a new path toward highly efficient photocatalytic titania in a series of applications targeting water remediation and solar fuel production.

Interfacing graphene with ultrathin oxide films is a crucial step towards its integration in novel electronic devices. However, obtaining two-dimensional oxide films on top of graphene is a formidable task, as the extremely low surface free energy of the graphitic substrate favors the formation of oxide clusters. Here, we demonstrate that the oxidation of a Cr carbide film intercalated between graphene and a Ni(1 1 1) substrate triggers the de-intercalation of Cr atoms, which form a continuous and atomically flat Cr oxide wetting layer on top of graphene. Microscopic and spectroscopic analyses demonstrate that the process affects marginally the structural integrity and electronic properties of graphene with respect to the pristine graphene/Ni(1 1 1) case. These findings show a new method to obtain high-quality graphene/Cr oxide interfaces. In perspective, these well-defined junctions could be used to finely control the electrical conductivity of graphene through an insulating oxide gate.

Spin-polarized electrons confined in low-dimensional structures are of high interest for spintronics applications. Here, we investigate the electronic structure of an ordered array of Bi monomer and dimer chains on the Ag(110) surface. By means of spin-resolved photoemission spectroscopy, we find Rashba-Bychkov split bands crossing the Fermi level with one-dimensional constant energy contours. These bands are up-spin polarized for positive wave vectors and down-spin polarized for negative wave vectors, at variance with the Rashba-Bychkov model that predicts a pair of states with opposite spin in each half of the surface Brillouin zone. Density functional theory shows that spin-selective hybridization with the Ag bulk bands originates this unconventional spin texture.

BiPd is a noncentrosymmetric superconductor with Dirac-like surface states on both (010) and (01¯0) faces. The Dirac cone on (010) surface is intense and appears at 0.66 eV binding energy. These states have drawn much attention due to contradictory reports on dimensionality and the momentum of these Dirac fermions. We have studied the properties of these Dirac fermions using varied photon energies and different experimental conditions. The behavior of the Dirac cone is found to be two-dimensional. In addition, we found few more surface states appearing at higher binding energies compared to the Dirac cone.

The equilibrium boundary for the layered VTiSesolid solution was determined according to a set of experimental data of synchrotron radiation X-ray photoelectron spectroscopy and electromotive force of the Li|Li|VTiSeelectrochemical cells. This boundary appears to be 20 mol % of V (x = 0.2). The formation of the VSestructural fragments, which are tightly bounded with VTiSelayers, appears at x > 0.2. A combined analysis of the Fermi-surface maps and the atomic force microscopy images reveals that the (001) planes in VSeand VTiSestructural fragments are parallel to each other. At the same time, these fragments demonstrate turbostratic disordering within the basal plane. The formation of such pseudo-homogeneous materials is associated with the contribution of the free surface energy, which is relatively high in the quasi-two-dimensional materials.

Multilayer structures comprising Bi/Ag bilayers display spin-charge conversion, a phenomenon of interest in spintronics whose origin (inverse Rashba-Edelstein vs. inverse spin Hall effects) is still debated. The Bi/Ag interfaces are assumed to be stable in model calculations, but the experimental determination of their structural properties is missing. Here, we explore by photoemission spectroscopy the stability and electronic structure of a Bi/Ag interface with strong Rashba interaction. We saturate the surface of a Ag(111) crystal with the AgBi alloy, which presents surface-embedded Bi atoms and characteristic Rashba-split surface states, and deposit Ag layers on top of it at room temperature. The intensity of the Rashba-split bands weakens sizably after the deposition of few Ag monolayers, although most of the Bi atoms segregate at the surface and retain the original local coordination. These findings are consistent with the surfactant behavior of Bi atoms, which favors a layer-by-layer Ag growth without preserving the long-range order of the AgBi alloy. They also suggest the need of detailed chemical and structural analysis for an accurate description of Bi/Ag bilayers.

Generally adopted strategies for enhancing the photocatalytic activity are aimed at tuning the visible light response, the exposed crystal facets, and the nanocrystal shape. Here, we present a different approach for designing efficient photocatalysts displaying a substrate-specific reactivity upon defect engineering. The platinized, defective anisotropic brookite TiO 2 photocatalysts are tested for alcohol photoreforming showing up to an 11-fold increase in methanol oxidation rate, compared with the pristine one, while presenting much lower ethanol or isopropanol specific oxidation rates. We demonstrate that the substrate-specific alcohol oxidation and hydrogen evolution reactions are tightly related, and when the former is increased, the latter is boosted. The reduced anisotropic brookite shows up to 18-fold higher specific photoactivity with respect to anatase and brookite with isotropic nanocrystals. Advanced in situ characterizations and theoretical investigations reveal that controlled engineering over oxygen vacancies and lattice strain produces large electron polarons hosting the substrate-specific active sites for alcohol photo-oxidation.

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-mu m MXene multilayer for different organic vapors and humidity at 10(1)-10(4) ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2O down to 10 ppm. Moreover, humidity suppresses the response of Mo2CTx to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10(-2)-10(6) Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo2CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo2C surface interaction with organic analytes and H2O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

The question on whether there exists a unique photoelectron reference plane for a stepped solid surface is discussed on the basis of angle-resolved photoelectron spectroscopy data for Ag films grown on Pt(997). Different step morphologies at the surface and interface, revealed by low-energy electron diffraction measurements, result in distinctly different band dispersions of the sp-like quantum well states and of the Shockley surface state. Quantum well standing waves form between the parallel optical surface and interface planes, while the surface state follows the orientation of a local plane tilted with respect to the optical surface. These findings show the connection of the photoelectron reference plane with the local morphology of a solid surface and the spatial extent of the electron wave functions.

Quantum materials having Dirac fermions in conjunction with superconductivity is believed to be the candidate material to realize exotic physics as well as advanced technology. Angle-resolved photoemission spectroscopy (ARPES), a direct probe of the electronic structure, has been extensively used to study these materials. However, experiments often exhibit conflicting results on dimensionality and momentum of the Dirac fermions (e.g., Dirac states in BiPd, a novel noncentrosymmetric superconductor), which is crucial for the determination of the symmetry, time-reversal invariant momenta, and other emerging properties. Employing high-resolution ARPES at varied conditions, we demonstrated a methodology to identify the location of the Dirac node accurately and discover that the deviation from two dimensionality of the Dirac states in BiPd proposed earlier is not a material property. These results helped to reveal the topology of the anisotropy of the Dirac states accurately. We have constructed a model Hamiltonian considering higher-order spin-orbit terms and demonstrate that this model provides an excellent description of the observed anisotropy. Intriguing features of the Dirac states in a noncentrosymmetric superconductor revealed in this study are expected to have significant implications regarding the properties of topological superconductors.

Recent theoretical calculations predict the presence of Dirac nodal lines with ? Berry phase and related topological surface states in elemental alkaline-earth metals. Here we provide experimental and theoretical evidence for the existence of similar nodal lines also in hexagonal close-packed Yb, an element of the lanthanide series, in the limit of zero spin-orbit coupling. These topological properties emerge after taking into account self-energy corrections, which permit one to correctly describe the experimental low-energy electronic structure of Yb. By angle-resolved photoemission spectroscopy we demonstrate the occurrence of topological surface states in Yb which are robust in the presence of the large spin-orbit coupling.

The "two-step" growth technique has been used to grow atomically uniform Ag films on 7 × 7 Si(111) and 8 × 8 ?-Si3N4(0001)/Si(111) surfaces. Angle-resolved photoemission spectroscopy reveals the formation of sp quantum well states in the Ag films with distinct properties in the two cases. It is shown that the valence electrons in silver can be confined in the fundamental gap of a less than 1-nm-thin nitride layer, effectively decoupling the Ag and Si states.

The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS bilayer forms at the boundaries of single-layer single-domain MoS islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems.

We investigate the electronic structure of a noncentrosymmetric superconductor, BiPd using photoemission spectroscopy with multiple photon energies ranging from ultraviolet to hard x-ray. Experimental data exhibit interesting difference in the surface and bulk electronic structures of this system. While the surface Bi core level peaks appear at lower binding energies, the surface valence band features are found at the higher binding energy side of the bulk valence band; valence band is primarily constituted by the Pd 4d states.These changes in the electronic structure cannot be explained by the change in ionicity of the constituent elements via charge transfer. Analysis of the experimental data indicates that the Bi-Pd hybridization physics plays the key role in deriving the anomalous spectral evolution and the electronic properties of this system.

One-atom-thick rare-earth/noble metal (RE-NM) compounds are attractive materials to investigate two-dimensional magnetism, since they are easy to synthesize into a common RE-NM2 structure with high crystal perfection. Here we perform a comparative study of the GdAu2, HoAu2, and YbAu2 monolayer compounds grown on Au(111). We find the same atomic lattice quality and moiré superlattice periodicity in the three cases, but different electronic properties and magnetism. The YbAu2 monolayer reveals the characteristic electronic signatures of a mixed-valence configuration in the Yb atom. In contrast, GdAu2 and HoAu2 show the trivalent character of the rare-earth and ferromagnetic transitions below 22 K. Yet, the GdAu2 monolayer has an in-plane magnetic easy-axis, versus the out-of-plane one in HoAu2. The electronic bands of the two trivalent compounds are very similar, while the divalent YbAu2 monolayer exhibits different band features. In the latter, a strong 4f-5d hybridization is manifested in neatly resolved avoided crossings near the Fermi level. First principles theory points to a residual presence of empty 4f states, explaining the fluctuating valence of Yb in the YbAu2 monolayer.

Topological surface states usually emerge at the boundary between a topological and a conventional insulator. Their precise physical character and spatial localization depend on the complex interplay between the chemical, structural and electronic properties of the two insulators in contact. Using a lattice-matched heterointerface of single and double bilayers of ?-antimonene and bismuth selenide, we perform a comprehensive experimental and theoretical study of the chiral surface states by means of microscopy and spectroscopic measurements complemented by first-principles calculations. We demonstrate that, although ?-antimonene is a trivial insulator in its free-standing form, it inherits the unique symmetry-protected spin texture from the substrate via a proximity effect that induces outward migration of the topological state. This "topologization" of ?-antimonene is found to be driven by the hybridization of the bands from either side of the interface.

Metal monochalcogenides (MX) have recently been rediscovered as two-dimensional materials with electronic properties highly dependent on the number of layers. Although some intriguing properties appear in the few-layer regime, the carrier mobility of MX compounds increases with the number of layers, motivating the interest in multilayered heterostructures or bulk materials. By means of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory calculations, we compare the electronic band structure of bulk ?-GaSe and ?-InSe semiconductors. We focus our attention on the top valence band of the two compounds along main symmetry directions, discussing the effect of spin-orbit coupling and contributions from post-transition-metal (Ga or In) and Se atoms. Our results show that the top valence band at ? point is dominated by Se pz states, while the main effect of Ga or In appears more deeply in binding energy, at the Brillouin zone corners, and in the conduction band. These findings explain also the experimental observation of a hole effective mass rather insensitive to the post-transition metal. Finally, by means of spin-resolved ARPES and surface band structure calculations we describe Rashba-Bychkov spin splitting of surface states in ?-InSe.

We report on a comparative theoretical and experimental investigation of the electronic band structure of a family of three-dimensional topological insulators, AIVBi4Te7-xSex(AIV= Sn, Pb; x = 0, 1). We prove by means of density functional theory calculations and angle-resolved photoemission spectroscopy measurements that partial or total substitution of heavy atoms by lighter isoelectronic ones affects the electronic properties of topological insulators. In particular, we show that the modification of the Dirac cone position relative to the Fermi level and the bulk band gap size can be controlled by varying the stoichiometry of the compound. We also demonstrate that the investigated systems are inert to oxygen exposure.

We investigated the electronic structure of the Si ( 111 ) - 7 × 7 surface below 20 K by scanning tunneling and photoemission spectroscopies and by density functional theory calculations. Previous experimental studies have questioned the ground state of this surface, which is expected to be metallic in a band picture because of the odd number of electrons per unit cell. Our differential conductance spectra instead show the opening of an energy gap at the Fermi level and a significant temperature dependence of the electronic properties, especially for the adatoms at the center of the unfaulted half of the unit cell. Complementary photoemission spectra with improved correction of the surface photovoltage shift corroborate the differential conductance data and demonstrate the absence of surface bands crossing the Fermi level at 17 K. These consistent experimental observations point to an insulating ground state and contradict the prediction of a metallic surface obtained by density functional theory in the generalized gradient approximation. The calculations indicate that this surface has or is near a magnetic instability, but remains metallic in the magnetic phases even including correlation effects at mean-field level. We discuss possible origins of the observed discrepancies between experiments and calculations.

Two-dimensional materials with high charge carrier mobility and tunable band gaps have attracted intense research effort for their potential use in nanoelectronics. Two-dimensional ?-conjugated polymers constitute a promising subclass because the band structure can be manipulated by varying the molecular building blocks while preserving key features such as Dirac cones and high charge mobility. The major barriers to the application of two-dimensional ?-conjugated polymers have been the small domain size and high defect density attained in the syntheses explored so far. Here, we demonstrate the fabrication of mesoscale ordered two-dimensional ?-conjugated polymer kagome lattices with semiconducting properties, Dirac cone structures and flat bands on Au(111). This material has been obtained by combining a rigid azatriangulene precursor and a hot dosing approach, which favours molecular diffusion and eliminates voids in the network. These results open opportunities for the synthesis of two-dimensional ?-conjugated polymer Dirac cone materials and their integration into devices.