Magnetic Interactions of Vanadyl Phthalocyanine with Ferromagnetic Iron, Cobalt, and Nickel Surfaces

We have investigated the molecular orientation and magnetic properties of vanadyl phthalocyanine (VOPc) deposited on ferromagnetic Fe, Co, and Ni films, which were epitaxially grown on Cu(001) using X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The results reveal that the VOPc framework lies flat on the surfaces with oxygen-up configurations. The spin of the central V atoms in VOPc is antiferromagnetically coupled with those of Fe or Co atoms in the ferromagnetic surface. In contrast, the magnetic coupling between V and Ni is not effective, exhibiting simple paramagnetism of VOPc

Direct Synthesis of Vanadium Phthalocyanine and Its Electronic and Magnetic States in Monolayers and Multilayers on Ag(111)

Vanadium phthalocyanine (VPc) monolayers and multilayers were synthesized on Ag(111), and the electronic and magnetic states of an unachieved VPc with a divalent state of V were investigated. The VPc monolayer was fabricated by directly depositing the V atoms on a metal-free phthalocyanine (H₂Pc) monolayer under ultra-high-vacuum conditions. The VPc multilayer was synthesized by repeated VPc monolayer deposition and subsequent sample annealing at approximately 450 K. The N 1s X-ray photoelectron spectra (XPS) of these samples showed a remarkable reduction in the peak assigned to H-bonded N atoms, concomitant with the appearance of a new peak attributed to V-bonded N atoms close to the peak of iminic N. Additionally, the oxidation state of V estimated from the V 2p XPS peak position corresponded to 1.6 and 2.4 in the monolayer and multilayer samples, respectively. These results clearly imply that VPc monolayers and multilayers were successfully obtained. The main ground-state electronic configuration of the V center was found to be ²E_g by angle-dependent V L-edge X-ray absorption spectroscopy. Furthermore, X-ray magnetic circular dichroism (XMCD) measurements suggest that this ²E_g state was mixed with the ²A_1g state by spin–orbit coupling in the ground state. Data revealed that VPc shows a paramagnetic state on the Ag surface and in an H₂Pc film but an antiferromagnetic state in the multilayer. Partial electron charge transfer was also observed from the Ag surface to the V center at the VPc/Ag(111) interface, leading to a significant decrease in XMCD signals in the monolayer

In Operando X-ray Absorption Fine Structure Studies of Polyoxometalate Molecular Cluster Batteries: Polyoxometalates as Electron Sponges

We carried out in operando Mo K-edge X-ray absorption fine structure measurements on the rechargeable molecular cluster batteries (MCBs) of polyoxometalates (POMs), in which a Keggin-type POM, [PMo₁₂O₄₀]^3–, is utilized as a cathode active material with a lithium metal anode. The POM-MCBs exhibit a large capacity of ca. 270 (A h)/kg in a voltage range between <i>V</i> = 4.0 V and <i>V</i> = 1.5 V. X-ray absorption near-edge structure analyses demonstrate that all 12 Mo⁶⁺ ions in [PMo₁₂O₄₀]^3– are reduced to Mo⁴⁺ in the discharging process. This means the formation of a super-reduced state of the POM, namely, [PMo₁₂O₄₀]^27–, which stores 24 electrons, and this electron number can explain the large capacity of the POM-MCBs. Furthermore, extended X-ray absorption fine structure analyses reveal the molecular structure of [PMo₁₂O₄₀]^27–, which is slightly reduced in size compared to the original [PMo₁₂O₄₀]^3– and involves Mo⁴⁺ metal–metal-bonded triangles. Density functional theory calculations suggest that these triangles are formed because of the large number of additional electrons in the super-reduced state

Rate Enhancements in Structural Transformations of Pt–Co and Pt–Ni Bimetallic Cathode Catalysts in Polymer Electrolyte Fuel Cells Studied by in Situ Time-Resolved X‑ray Absorption Fine Structure

In situ time-resolved X-ray absorption fine structure spectra of Pt/C, Pt₃Co/C, and Pt₃Ni/C cathode electrocatalysts in membrane electrode assemblies (catalyst loading: 0.5 mg_metal cm^–2) were successfully measured every 100 ms for a voltage cycling process between 0.4 and 1.0 V. Systematic analysis of in situ time-resolved X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra in the molecular scale revealed the structural kinetics of the Pt and Pt₃M (M = Co, Ni) bimetallic cathode catalysts under polymer electrolyte fuel cell operating conditions, and the rate constants of Pt charging, Pt–O bond formation/breaking, and Pt–Pt bond breaking/re-formation relevant to the fuel cell performances were successfully determined. The addition of the 3d transition metals to Pt reduced the Pt oxidation state and significantly enhanced the reaction rates of Pt discharging, Pt–O bond breaking, and Pt–Pt bond re-forming in the reductive process from 1.0 to 0.4 V

Kinetics and Mechanism of Redox Processes of Pt/C and Pt₃Co/C Cathode Electrocatalysts in a Polymer Electrolyte Fuel Cell during an Accelerated Durability Test

The degradation of Pt electrocatalysts in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells under working conditions is a serious problem for their practical use. Here we report the kinetics and mechanism of redox reactions at the surfaces of Pt/C and Pt₃Co/C cathode electrocatalysts during catalyst degradation processes by an accelerated durability test (ADT) studied by operando time-resolved X-ray absorption fine structure (XAFS) spectroscopy. Systematic analysis of a series of Pt L_III-edge time-resolved XAFS spectra measured every 100 ms at different degradation stages revealed changes in the kinetics of Pt redox reactions on Pt/C and Pt₃Co/C cathode electrocatalysts. In the case of Pt/C, as the number of ADT cycles increased, structural changes for Pt redox reactions (charging, surface, and subsurface oxidation) became less sensitive because of the agglomeration of catalyst particles. It was found that their rate constants were almost constant independent of the agglomeration of the Pt electrocatalyst. On the other hand, in the case of Pt₃Co/C, the rate constants of the redox reactions of the cathode electrocatalyst gradually reduced as the number of ADT cycles increased. The differences in the kinetics for the redox processes would be differences in the degradation mechanism of these cathode electrocatalysts

Controlled Modification of Superconductivity in Epitaxial Atomic Layer–Organic Molecule Heterostructures

Self-assembled organic molecules can potentially be an excellent source of charge and spin for two-dimensional (2D) atomic-layer superconductors. Here we investigate 2D heterostructures based on In atomic layers epitaxially grown on Si and highly ordered metal-phthalocyanine (MPc, M = Mn, Cu) through a variety of techniques: scanning tunneling microscopy, electron transport measurements, angle-resolved photoemission spectroscopy, X-ray magnetic circular dichroism, and ab initio calculations. We demonstrate that the superconducting transition temperature (<i>T</i>_c) of the heterostructures can be modified in a controllable manner. Particularly, the substitution of the coordinated metal atoms from Mn to Cu is found to reverse the <i>T</i>_c shift from negative to positive directions. This distinctive behavior is attributed to a competition of charge and spin effects, the latter of which is governed by the directionality of the relevant d-orbitals. The present study shows the effectiveness of molecule-induced surface doping and the significance of microscopic understanding of the molecular states in these 2D heterostructures

Operando Time-Resolved X-ray Absorption Fine Structure Study for Surface Events on a Pt₃Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes

The structural kinetics of surface events on a Pt₃Co/C cathode catalyst in a polymer electrolyte fuel cell (PEFC) was investigated by operando time-resolved X-ray absorption fine structure (XAFS) with a time resolution of 500 ms. The rate constants of electrochemical reactions, the changes in charge density on Pt, and the changes in the local coordination structures of the Pt₃Co alloy catalyst in the PEFC were successfully evaluated during fuel-cell voltage-operating processes. Significant time lags were observed between the electrochemical reactions and the structural changes in the Pt₃Co alloy catalyst. The rate constants of all the surface events on the Pt₃Co/C catalyst were significantly higher than those on the Pt/C catalyst, suggesting the advantageous behaviors (cell performance and catalyst durability) on the Pt₃Co alloy cathode catalyst

Large-Gap Magnetic Topological Heterostructure Formed by Subsurface Incorporation of a Ferromagnetic Layer

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi₂Se₄/Bi₂Se₃ heterostructure, which was fabricated by self-assembling a MnBi₂Se₄ layer on top of the Bi₂Se₃ surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic <i>ab initio</i> calculations, demonstrate that the fabricated MnBi₂Se₄/Bi₂Se₃ heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number <i>C</i> = −1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future “topotronics” devices