Resonant Auger spectromicroscopy in ultrathin Fe films on W(110)

L3M 2,3 M 2,3 Auger transition is measured near the L3 resonance of ferromagnetic Fe films on W(110). The kinetic energies of the Auger peaks display the typical Raman behaviour for photon energies well below the absorption threshold, where the Auger energy follows the changes in the photon energy. Classical Auger behaviour with constant kinetic energy sets in at about 1.5 eV below the L3 resonance independently from the number of Fe layers down to the monolayer thickness. Strong x-ray circular magnetic dichroism is observed at the L3 edge in the entire L3M 2,3 M 2,3 Auger spectrum. Different Auger features originating from the final state with two 3p core holes show slight variations in the dichroic signal, which is attributed to the exchange interaction between the core holes and the valence band. Finally, XMCD-PEEM magnetic domain imaging using Auger electrons is demonstrated with a high level of contrast and lateral resolution approaching that of imaging with secondary photoelectrons

Synchrotron radiation photoemission spectroscopy of the oxygen modified CrCl3 surface

We investigate the experimentally challenging CrCl3 surface by photon energy dependent photoemission (PE). The core and valence electrons after cleavage of a single crystal, either in a ultra-high vacuum (UHV) or in air, are studied by keeping the samples at 150 degrees C, aiming at confirming the atomic composition with respect to the expected bulk atomic structure. A common spectroscopic denominator revealed by data is the presence of a stable, but only partially ordered Cl-O-Cr surface. The electronic core levels (Cl 2p, Cr 2p and 3p), the latter ones of cumbersome component determination, allowed us to quantify the electron charge transfer to the Cr atom as a net result of this modification and the increased exchange interaction between metal and ligand atoms. In particular, the analysis of multiplet components by the CMT4XPS code evidenced the charge transfer to be favored, and similarly the reduced crystal field due to the established polarization field. Though it is often claimed that a significant amount of Cl and Cr atomic vacancies has to be included, such a possibility can be excluded on the basis of the sign and the importance of the shift in the binding energy of core level electrons. The present methodological approach can be of great impact to quantify the structure of ordered sub-oxide phases occurring in mono or bi-layer Cr trihalides

Kink far below the Fermi level reveals new electron-magnon scattering channel in Fe

Many properties of real materials can be modeled using ab initio methods within a single-particle picture. However, for an accurate theoretical treatment of excited states, it is necessary to describe electron-electron correlations including interactions with bosons: phonons, plasmons, or magnons. In this work, by comparing spin- and momentum-resolved photoemission spectroscopy measurements to many-body calculations carried out with a newly developed first-principles method, we show that a kink in the electronic band dispersion of a ferromagnetic material can occur at much deeper binding energies than expected (E_b=1.5 eV). We demonstrate that the observed spectral signature reflects the formation of a many-body state that includes a photohole bound to a coherent superposition of renormalized spin-flip excitations. The existence of such a many-body state sheds new light on the physics of the electron-magnon interaction which is essential in fields such as spintronics and Fe-based superconductivity.Comment: 6 pages, 2 figure

MoS2 Nanosheets Uniformly Anchored on NiMoO4 Nanorods, a Highly Active Hierarchical Nanostructure Catalyst for Oxygen Evolution Reaction and Pseudo-Capacitors

Hierarchical nanostructures have attracted considerable research attention due to their applications in the catalysis field. Herein, we design a versatile hierarchical nanostructure composed of NiMoO4 nanorods surrounded by active MoS2 nanosheets on an interconnected nickel foam substrate. The as-prepared nanostructure exhibits excellent oxygen evolution reaction per-formance, producing a current density of 10 mA cm−2 at an overpotential of 90 mV, in comparison with 220 mV necessary to reach a similar current den-sity for NiMoO4. This behavior originates from the structural/morphological properties of the MoS2 nanosheets, which present numerous surface-active sites and allow good contact with the electrolyte. Besides, the structures can effectively store charges, due to their unique branched network providing accessible active surface area, which facilitates intermediates adsorptions. Particularly, NiMoO4/MoS2 shows a charge capacity of 358 mAhg−1 at a current of 0.5 A g−1 (230 mAhg−1 for NiMoO4), thus suggesting promising applications for charge-storing devices

Semiconductor Halogenation in Molecular Highly-Oriented Layered p–n (n–p) Junctions

Organic p–n junctions attract widespread interest in the field of molecular electronics because of their unique optoelectronic singularities. Importantly, the molecular donor/acceptor character is strongly correlated to the degree of substitution, e.g., the introduction of electron-withdrawing groups. Herein, by gradually increasing the degree of peripheral fluorination on planar, D4h−symmetric iron(II) phthalocyanato (FePc) complexes, the energy level alignment and molecular order is defined in a metal-supported bilayered Pc-based junction using photoemission orbital tomography. This non-destructive method selectively allows identifying molecular levels of the hetero-architectures. It demonstrates that, while the symmetric fluorination of FePc does not disrupt the long-range order and degree of metal-to-molecule charge transfer in the first molecular layer, it strongly impacts the energy alignment in both the interface and topmost layer in the bilayered structures. The p–n junction formed in the bilayer of perhydrogenated FePc and perfluorinated FeF16Pc may serve as an ideal model for understanding the basic charge-transport phenomena at the metal-supported organic–organic interfaces, with possible application in photovoltaic devices

Reversible redox reactions in metal-supported porphyrin: The role of spin and oxidation state

On-surface molecular functionalization paved the way for the stabilization of chelated ions in different oxidation and spin states, allowing for the fine control of catalytic and magnetic properties of metalorganic networks. Considering two model systems, a reduced Co(i) and an open-shell Co(ii) metal-supported 2D molecular array, we investigate the interplay between the low valence oxidation and unpaired spin state in the molecular reactivity. We show that the redox reaction taking place at the cobalt tetraphenylporphyrin/Cu(100) interface, stabilizing the low-spin Co(i) state with no unpaired electrons in its valence shell, plays a pivotal role in changing the reactivity. This goes beyond the sole presence of unpaired electrons in the valence state of the Co(ii) metal-organic species, often designated as being responsible for the reactivity towards small molecules like NO and NO2. The reversible Co-NO2 interaction, established with the Co(i) leads to the stabilization of the Co(iii) oxidation state

Coupling Borophene to Graphene in Air-Stable Heterostructures

Artificial 2D van der Waals heterostructures with controllable vertical stacking and rotational orientation exhibit multifaceted electronic properties that are appealing for applications in fields ranging from optoelectronics to energy storage. Along with transition metal dichalcogenides and graphene, borophene has recently emerged as a promising building block for 2D devices due to its conductive nature as well as its exceptional mechanical and electronic properties. Here, it is demonstrated that the combination of the dissolution-segregation process and chemical vapor deposition allows for the synthesis of graphene/borophene heterostructures of the highest crystalline and chemical quality, in which graphene sits on top of the borophene layer with metallic character. The formation of laterally distinct micron-sized areas allows a comparative study of borophene, graphene, and the graphene–borophene heterostack in terms of their electronic properties and stability in a reactive environment. Whereas pristine borophene is particularly prone to oxidation, the graphene–borophene heterostack is chemically inert and enables the conservation of borophene's character even after exposure to air. This study opens up new perspectives for the scalable synthesis of graphene–borophene heterostacks with enhanced ability to preserve the metallic character and electronic properties of borophene

Extended π-conjugation: a key to magnetic anisotropy preservation in highly reactive porphyrins

In this study, the magnetic anisotropy of metal complexes is explored for its crucial role in the development of molecular materials for cutting-edge applications in spintronics, memory storage, and quantum computing. The challenge of achieving maximum magnetic anisotropy for paramagnetic single nickel ion sites is addressed and realized through an on-surface thermally induced planarization reaction in tetraphenylporphyrin, which maintains the nickel species in a square planar coordination environment. At the same time, the effective ligand field reduction due to the increased π-conjugation results in a lower reactivity of the molecular species. The results herein reported showcase the synergy between magnetic anisotropy and chemical robustness in single-site magnetic materials, thus opening exciting prospects for the development of stable uniaxial anisotropy in these materials. Such a finding represents a relevant advance in the field and validates a protocol for exploring magnetic anisotropy in metal complexes

Enhancing Electron Correlation at a 3d Ferromagnetic Surface

Spin-resolved momentum microscopy and theoretical calculations are combined beyond the one-electron approximation to unveil the spin-dependent electronic structure of the interface formed between iron (Fe) and an ordered oxygen (O) atomic layer, and an adsorbate-induced enhancement of electronic correlations is found. It is demonstrated that this enhancement is responsible for a drastic narrowing of the Fe d-bands close to the Fermi energy (EF) and a reduction of the exchange splitting, which is not accounted for in the Stoner picture of ferromagnetism. In addition, correlation leads to a significant spin-dependent broadening of the electronic bands at higher binding energies and their merging with satellite features, which are manifestations of a pure many-electron behavior. Overall, adatom adsorption can be used to vary the material parameters of transition metal surfaces to access different intermediate electronic correlated regimes, which will otherwise not be accessible. The results show that the concepts developed to understand the physics and chemistry of adsorbate–metal interfaces, relevant for a variety of research areas, from spintronics to catalysis, need to be reconsidered with many-particle effects being of utmost importance. These may affect chemisorption energy, spin transport, magnetic order, and even play a key role in the emergence of ferromagnetism at interfaces between non-magnetic systems

NiMoO4@Co3O4 Core–Shell Nanorods: In Situ Catalyst Reconstruction toward High Efficiency Oxygen Evolution Reaction

The sluggish kinetics of the oxygen evolution reaction (OER) is the bottleneck for the practical exploitation of water splitting. Here, the potential of a core–shell structure of hydrous NiMoO4 microrods conformally covered by Co3O4 nanoparticles via atomic layer depositions is demonstrated. In situ Raman and synchrotron-based photoemission spectroscopy analysis confirms the leaching out of Mo facilitates the catalyst reconstruction, and it is one of the centers of active sites responsible for higher catalytic activity. Post OER characterization indicates that the leaching of Mo from the crystal structure, induces the surface of the catalyst to become porous and rougher, hence facilitating the penetration of the electrolyte. The presence of Co3O4 improves the onset potential of the hydrated catalyst due to its higher conductivity, confirmed by the shift in the Fermi level of the heterostructure. In particular NiMoO4@Co3O4 shows a record low overpotential of 120 mV at a current density of 10 mA cm−2, sustaining a remarkable performance operating at a constant current density of 10, 50, and 100 mA cm−2 with negligible decay. Presented outcomes can significantly contribute to the practical use of the water-splitting process, by offering a clear and in-depth understanding of the preparation of a robust and efficient catalyst for water-splitting