Giant and Tunable Out-of-Plane Spin Polarization of Topological Antimonene.

Topological insulators are bulk insulators with metallic and fully spin-polarized surface states displaying Dirac-like band dispersion. Due to spin-momentum locking, these topological surface states (TSSs) have a predominant in-plane spin polarization in the bulk fundamental gap. Here, we show by spin-resolved photoemission spectroscopy that the TSS of a topological insulator interfaced with an antimonene bilayer exhibits nearly full out-of-plane spin polarization within the substrate gap. We connect this phenomenon to a symmetry-protected band crossing of the spin-polarized surface states. The nearly full out-of-plane spin polarization of the TSS occurs along a continuous path in the energy-momentum space, and the spin polarization within the gap can be reversibly tuned from nearly full out-of-plane to nearly full in-plane by electron doping. These findings pave the way to advanced spintronics applications that exploit the giant out-of-plane spin polarization of TSSs.

Facile Electron Transfer in Atomically Coupled Heterointerface for Accelerated Oxygen Evolution.

An efficient and cost-effective approach for the development of advanced catalysts has been regarded as a sustainable way for green energy utilization. The general guideline to design active and efficient catalysts for oxygen evolution reaction (OER) is to achieve high intrinsic activity and the exposure of more density of the interfacial active sites. The heterointerface is one of the most attractive ways that plays a key role in electrochemical water oxidation. Herein, atomically cluster-based heterointerface catalysts with strong metal support interaction (SMSI) between WMn2 O4 and TiO2 are designed. In this case, the WMn2 O4 nanoflakes are uniformly decorated by TiO2 particles to create electronic effect on WMn2 O4 nanoflakes as confirmed by X-ray absorption near edge fine structure. As a result, the engineered heterointerface requires an OER onset overpotential as low as 200 mV versus reversible hydrogen electrode, which is stable for up to 30 h of test. The outstanding performance and long-term durability are due to SMSI, the exposure of interfacial active sites, and accelerated reaction kinetics. To confirm the synergistic interaction between WMn2 O4 and TiO2 , and the modification of the electronic structure, high-resolution transmission electron microscopy (HR-TEM), X-ray photoemission spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) are used.

Alcohol Vapor Sensor Based on Quasi-2D Nb2O5 Derived from Oxidized Nb2CTz MXenes.

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.

Spin tuning of electron-doped metal-phthalocyanine layers.

The spin state of organic-based magnets at interfaces is to a great extent determined by the organic environment and the nature of the spin-carrying metal center, which is further subject to modifications by the adsorbate-substrate coupling. Direct chemical doping offers an additional route for tailoring the electronic and magnetic characteristics of molecular magnets. Here we present a systematic investigation of the effects of alkali metal doping on the charge state and crystal field of 3d metal ions in Cu, Ni, Fe, and Mn phthalocyanine (Pc) monolayers adsorbed on Ag. Combined X-ray absorption spectroscopy and ligand field multiplet calculations show that Cu(II), Ni(II), and Fe(II) ions reduce to Cu(I), Ni(I), and Fe(I) upon alkali metal adsorption, whereas Mn maintains its formal oxidation state. The strength of the crystal field at the Ni, Fe, and Mn sites is strongly reduced upon doping. The combined effect of these changes is that the magnetic moment of high- and low-spin ions such as Cu and Ni can be entirely turned off or on, respectively, whereas the magnetic configuration of MnPc can be changed from intermediate (3/2) to high (5/2) spin. In the case of FePc a 10-fold increase of the orbital magnetic moment accompanies charge transfer and a transition to a high-spin state.

Doping dependent intrinsic magnetization in silicon in Ni/Si heterostructures.

This investigation delves into the complex interaction at metal-semiconductor interfaces, highlighting the magnetic proximity effect in Ni/Si interfaces through systematic X-ray magnetic circular dichroism (XMCD) studies at Ni and Si edges. We analyzed two Ni/Si heterostructures with differing semiconductor doping, uncovering a magnetic proximity effect manifesting as equilibrium magnetization in the semiconductor substrate induced by the adjacent Ni layer. Our results display distinct magnetization signs corresponding to the doping levels: low-doped samples show parallel alignment to the Ni layer, while high-doped samples align antiparallel, indicating a nuanced interplay of underlying magnetization mechanisms. These findings pinpoint the roles of electron tunneling and exchange splitting modification in the magnetization behavior. The study enriches the understanding of ferromagnetic-semiconductor interface behavior, setting a precedent for the design of advanced spintronic devices that leverage the nuanced magnetic properties of these hybrid systems.

Chemical Interactions at the Interface of Au on Bi2Se3 Topological Insulator.

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 Se0. 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 advancements in the field of devices based on TIs.

Electronic Band Engineering of Two-Dimensional Kagomé Polymers.

Two-dimensional conjugated polymers (2DCPs) are an emerging class of materials that exhibit properties similar to graphene yet do not have the limitation of zero bandgap. On-surface synthesis provides exceptional control on the polymerization reaction, allowing tailoring properties by choosing suitable monomers. Heteroatom-substituted triangulene 2DCPs constitute a playing ground for such a design and are predicted to exhibit graphene-like band structures with high charge mobility and characteristic Dirac cones in conduction or valence states. However, measuring these properties experimentally is challenging and requires long-range-ordered polymers, preferably with an epitaxial relationship with the substrate. Here, we investigate the electronic properties of a mesoscale-ordered carbonyl-bridged triphenylamine 2DCP (P2TANGO) and demonstrate the presence of a Dirac cone by combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations. Moreover, we measure the absolute energy position of the Dirac cone with respect to the vacuum level. We show that the bridging functionality of the triangulene (ether vs carbonyl) does not significantly perturb the band structure but strongly affects the positioning of the bands with respect to the Au(111) states and allows control of the ionization energy of the polymer. Our results provide proof of the controllable electronic properties of 2DCPs and bring us closer to their use in practical applications.

Giant Rashba-splitting of one-dimensional metallic states in Bi dimer lines on InAs(100).

Bismuth produces different types of ordered superstructures on the InAs(100) surface, depending on the growth procedure and coverage. The (2 × 1) phase forms at completion of one 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)-(2 × 1) phase an intriguing system, where novel transport regimes can be studied.

Interplay between magnetic order and electronic band structure in ultrathin GdGe2 metalloxene films.

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.

Degradation and Self-Healing of FAPbBr3 Perovskite under Soft-X-Ray Irradiation.

The extensive use of perovskites as light absorbers calls for a deeper understanding of the interaction of these materials with light. Here, the evolution of the chemical and optoelectronic properties of formamidinium lead tri-bromide (FAPbBr3 ) films is tracked under the soft X-ray beam of a high-brilliance synchrotron source by photoemission spectroscopy and micro-photoluminescence. Two contrasting processes are at play during the irradiation. The degradation of the material manifests with the formation of Pb0 metallic clusters, loss of gaseous Br2 , decrease and shift of the photoluminescence emission. The recovery of the photoluminescence signal for prolonged beam exposure times is ascribed to self-healing of FAPbBr3 , thanks to the re-oxidation of Pb0 and migration of FA+ and Br- ions. This scenario is validated on FAPbBr3 films treated by Ar+ ion sputtering. The degradation/self-healing effect, which is previously reported for irradiation up to the ultraviolet regime, has the potential of extending the lifetime of X-ray detectors based on perovskites.

Physical Delithiation of Epitaxial LiCoO2 Battery Cathodes as a Platform for Surface Electronic Structure Investigation.

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 < x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtain.

Unidirectional Nano-modulated Binding and Electron Scattering in Epitaxial Borophene.

A complex interplay between the crystal structure and the electron behavior within borophene renders this material an intriguing 2D system, with many of its electronic properties still undiscovered. Experimental insight into those properties is additionally hampered by the limited capabilities of the established synthesis methods, which, in turn, inhibits the realization of potential borophene applications. In this multimethod study, photoemission spectroscopies and scanning probe techniques complemented by theoretical calculations have been used to investigate the electronic characteristics of a high-coverage, single-layer borophene on the Ir(111) substrate. Our results show that the binding of borophene to Ir(111) exhibits pronounced one-dimensional modulation and transforms borophene into a nanograting. The scattering of photoelectrons from this structural grating gives rise to the replication of the electronic bands. In addition, the binding modulation is reflected in the chemical reactivity of borophene and gives rise to its inhomogeneous aging effect. Such aging is easily reset by dissolving boron atoms in iridium at high temperature, followed by their reassembly into a fresh atomically thin borophene mesh. Besides proving electron-grating capabilities of the boron monolayer, our data provide comprehensive insight into the electronic properties of epitaxial borophene which is vital for further examination of other boron systems of reduced dimensionality.

Nearly-freestanding supramolecular assembly with tunable structural properties.

The synthesis and design of two-dimensional supramolecular assemblies with specific functionalities is one of the principal goals of the emerging field of molecule-based electronics, which is relevant for many technological applications. Although a large number of molecular assemblies have been already investigated, engineering uniform and highly ordered two-dimensional molecular assemblies is still a challenge. Here we report on a novel approach to prepare wide highly crystalline molecular assemblies with tunable structural properties. We make use of the high-reactivity of the carboxylic acid functional moiety and of the predictable structural features of non-polar alkane chains to synthesize 2D supramolecular assemblies of 4-(decyloxy)benzoic acid (4DBA;C[Formula: see text]H[Formula: see text]O[Formula: see text]) on a Au(111) surface. By means of scanning tunneling microscopy, density functional theory calculations and photoemission spectroscopy, we demonstrate that these molecules form a self-limited highly ordered and defect-free two-dimensional single-layer film of micrometer-size, which exhibits a nearly-freestanding character. We prove that by changing the length of the alkoxy chain it is possible to modify in a controlled way the molecular density of the "floating" overlayer without affecting the molecular assembly. This system is especially suitable for engineering molecular assemblies because it represents one of the few 2D molecular arrays with specific functionality where the structural properties can be tuned in a controlled way, while preserving the molecular pattern.

Unraveling the optoelectronic properties of CoSbx intrinsic selective solar absorber towards high-temperature surfaces.

The combination of the ability to absorb most of the solar radiation and simultaneously suppress infrared re-radiation allows selective solar absorbers (SSAs) to maximize solar energy to heat conversion, which is critical to several advanced applications. The intrinsic spectral selective materials are rare in nature and only a few demonstrated complete solar absorption. Typically, intrinsic materials exhibit high performances when integrated into complex multilayered solar absorber systems due to their limited spectral selectivity and solar absorption. In this study, we propose CoSbx (2 < x < 3) as a new exceptionally efficient SSA. Here we demonstrate that the low bandgap nature of CoSbx endows broadband solar absorption (0.96) over the solar spectral range and simultaneous low emissivity (0.18) in the mid-infrared region, resulting in a remarkable intrinsic spectral solar selectivity of 5.3. Under 1 sun illumination, the heat concentrates on the surface of the CoSbx thin film, and an impressive temperature of 101.7 °C is reached, demonstrating the highest value among reported intrinsic SSAs. Furthermore, the CoSbx was tested for solar water evaporation achieving an evaporation rate of 1.4 kg m-2 h-1. This study could expand the use of narrow bandgap semiconductors as efficient intrinsic SSAs with high surface temperatures in solar applications.

A novel free-electron laser single-pulse Wollaston polarimeter for magneto-dynamical studies.

Here, we report on the conceptual design, the hardware realization, and the first experimental results of a novel and compact x-ray polarimeter capable of a single-pulse linear polarization angle detection in the extreme ultraviolet photon energy range. The polarimeter is tested by performing time resolved pump-probe experiments on a Ni80Fe20 Permalloy film at the M2,3 Ni edge at an externally seeded free-electron laser source. Comparison with similar experiments reported in the literature shows the advantages of our approach also in view of future experiments.

2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air.

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2 CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 101 -104  ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2 O down to 10 ppm. Moreover, humidity suppresses the response of Mo2 CTx 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 -106  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 Mo2 CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo2 C surface interaction with organic analytes and H2 O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

Indirect chiral magnetic exchange through Dzyaloshinskii-Moriya-enhanced RKKY interactions in manganese oxide chains on Ir(100).

Localized electron spins can couple magnetically via the Ruderman-Kittel-Kasuya-Yosida interaction even if their wave functions lack direct overlap. Theory predicts that spin-orbit scattering leads to a Dzyaloshinskii-Moriya type enhancement of this indirect exchange interaction, giving rise to chiral exchange terms. Here we present a combined spin-polarized scanning tunneling microscopy, angle-resolved photoemission, and density functional theory study of MnO2 chains on Ir(100). Whereas we find antiferromagnetic Mn-Mn coupling along the chain, the inter-chain coupling across the non-magnetic Ir substrate turns out to be chiral with a 120° rotation between adjacent MnO2 chains. Calculations reveal that the Dzyaloshinskii-Moriya interaction results in spin spirals with a periodicity in agreement with experiment. Our findings confirm the existence of indirect chiral magnetic exchange, potentially giving rise to exotic phenomena, such as chiral spin-liquid states in spin ice systems or the emergence of new quasiparticles.

Temperature Driven Phase Transition at the Antimonene/Bi2Se3 van der Waals Heterostructure.

We report the discovery of a temperature-induced phase transition between the α and ß structures of antimonene. When antimony is deposited at room temperature on bismuth selenide, it forms domains of α-antimonene having different orientations with respect to the substrate. During a mild annealing, the ß phase grows and prevails over the α phase, eventually forming a single domain that perfectly matches the surface lattice structure of bismuth selenide. First-principles thermodynamics calculations of this van der Waals heterostructure explain the different temperature-dependent stability of the two phases and reveal a minimum energy transition path. Although the formation energies of freestanding α- and ß-antimonene only slightly differ, the ß phase is ultimately favored in the annealed heterostructure due to an increased interaction with the substrate mediated by the perfect lattice match.

Electronic States of Silicene Allotropes on Ag(111).

Silicene, a honeycomb lattice of silicon, presents a particular case of allotropism on Ag(111). Silicene forms multiple structures with alike in-plane geometry but different out-of-plane atomic buckling and registry to the substrate. Angle-resolved photoemission and first-principles calculations show that these silicene structures, with (4×4), (√13×√13)R13.9°, and (2√3×2√3)R30° lattice periodicity, display similar electronic bands despite the structural differences. In all cases the interaction with the substrate modifies the electronic states, which significantly differ from those of free-standing silicene. Complex photoemission patterns arise from surface umklapp processes, varying according to the periodicity of the silicene allotropes.