3d Electronic States Research Articles

Influence of film thickness and air exposure on the transport gap of manganese phthalocyanine

The interface formation between manganese phthalocyanine (MnPc) and cobalt was investigated combining ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The transport band gap of the MnPc increases with the film thickness up to a value of (1.2 ± 0.3) eV while the optical band gap as determined from spectroscopic ellipsometry amounts to 0.5 eV. The gap values are smaller compared to other phthalocyanines due to metallic Mn 3d states close to the Fermi level. The transport band gap was found to open upon air exposure as a result of the disappearance of the occupied 3d electronic states.

Crystal growth and electronic properties of a 3D Rashba material, BiTeI, with adjusted carrier concentrations

3D Rashba materials can be a leading player in spin-related novel phenomena, ranging from the metallic extreme (unconventional superconductivity) to the transport intermediate (spin Hall effects) to the novel insulating variant (3D topological insulating states). As the essential backbone for both fundamental and applied research of such a 3D Rashba material, this study established the growth of sizeable single crystals of a candidate compound BiTeI with adjusted carrier concentrations. Three techniques (standard vertical Bridgman, modified horizontal Bridgman, and vapour transport) were employed, and BiTeI crystals (>1 × 1 × 0.2 mm3) with fundamentally different electronic states from metallic to insulating were successfully grown by the chosen technique. The 3D Rashba electronic states, including the Fermi surface topology, for the corresponding carrier concentrations of the obtained BiTeI crystals were revealed by relativistic first-principles calculations.

Identical-Location Transmission Electron Microscopy Study of Pt/C and Pt–Co/C Nanostructured Electrocatalyst Aging: Effects of Morphological and Compositional Changes on the Oxygen Reduction Reaction Activity

In this study, the activity for the oxygen reduction reaction (ORR) and the structural, morphological, and compositional changes of Pt, Pt3Co, and PtCo nanoparticles deposited on high surface area carbon (Vulcan XC72) are investigated after they were submitted to accelerated electrochemical aging tests. These tests consisted of stepping the potential in 1 min successively between potentials of 0.9 and 0.1 V vs. reversible hydrogen electrode (RHE) or between 0.9 and 0.6 V vs. RHE for 15 h in 1.0 M H2SO4 at 60 °C. Transmission electron microscopy, identical-location transmission electron microscopy, X-ray energy-dispersive spectroscopy analyses, and in situ X-ray absorption spectroscopy are used to characterize the changes in the morphological, compositional, and in the Pt 5d electronic states before and after the electrochemical aging tests. The experimental results show that the Pt/C and Pt–Co/C electrocatalysts are modified upon aging, following changes in particle size, geometry, and composition. The 0.9- to 0.6-V protocol is more aggressive in reducing the ORR activity, and this seems to be strongly related to the carbon corrosion and not to changes in the metallic particles. After the 0.9- to 0.1-V aging procedure, the ORR activity of the Pt/C particles is improved, while that of Pt3Co/C particles only slightly changes. In the case of Pt/C, these effects are related to the increase of the particle sizes, surface reconstruction, and, therefore, smaller oxide formation, which potentially induces an increase of the ORR activity. On the contrary, on Pt3Co/C, these positive effects are counterbalanced by a detrimental (and large) effect of Co dissolution.

Detection of Fe 3d electronic states in the valence band and magnetic properties of Fe-doped ZnO film

Fe-doped ZnO film has been grown by laser molecular beam epitaxy (L-MBE) and structurally characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), all of which reveal the high quality of the film. No secondary phase was detected. Resonant photoemission spectroscopy (RPES) with photon energies around the Fe 2p–3d absorption edge is performed to detect the electronic structure in the valence band. A strong resonant effect at a photon energy of 710 eV is observed. Fe3+ is the only valence state of Fe ions in the film and the Fe 3d electronic states are concentrated at binding energies of about 3.8 eV and 7 eV∼8 eV. There are no electronic states related to Fe near the Fermi level. Magnetic measurements reveal a typical superparamagnetic property at room temperature. The absence of electronic states related to Fe near the Fermi level and the high quality of the film, with few defects, provide little support to ferromagnetism.

Non-collinear ferrimagnetic phases in the Fe2−xMnxAs system

The stabilization mechanisms of non-collinear magnetically ordered phases observed in the Fe2−xMnxAs system with 1.19 ≤ x ≤ 1.365 are considered within the framework of a model approach that uses the number of d-electrons and the form of density of their electron states, obtained from ab initio calculations. It is found that the energy stability of non-collinear structures and the kind of order-order phase transitions are determined by electron filling of the d-band and by the form of density of d-electron states, which depend on the manganese content. It is demonstrated that the pressure features of spontaneous and magnetic field-induced order-order transitions are related to a character of renormalization of the electronic structure under pressure.

The Peculiarities of Americium Electronic Structure and Magnetic Susceptibility

By implementing a self-consistent procedure that combines the calculations of electronic spectrum within LDA+U+SO approximation with a generalized spin-fluctuation s(p)df-model, we have calculated the densities of states and temperature dependence of magnetic susceptibility of Americium. It was revealed that in the vicinity of the Fermi energy the densities of states of Am d-electrons become comparable to its f-electron densities of states that leads to that both bands make comparable contributions to the spin magnetic susceptibility. It is shown that the unusual transition from Curie-like to Pauli-like magnetic susceptibility, which is observed in Am, is caused by coexistence in Am of electronic states of two types—the localized, responsible for the formation of Curie-like susceptibility and the itinerant, which lead to temperature-independent magnetic susceptibility.

X-ray absorption and resonant photoemission studies of Mn doped SrTiO3 epitaxial films

Synchrotron radiation was used to measure X-ray absorption and resonant photoemission of epitaxial Mn doped SrTiO3 film. The spectra together with the modeling revealed that Mn valence state depends on the ions localization in the film and on the annealing temperature. The XAS spectra can be fitted by the composition of Mn3+ and Mn2+. Surface region was richer in trivalent Mn, while deeper layers and the film annealed in UHV at 630°C exhibited mostly divalent Mn ions. Partial density of Mn 3d electronic states derived from the resonant photoemission showed a significant contribution to the in-gap states.

In situ filling of metallic single‐walled carbon nanotubes with ferrocene molecules

AbstractWe present a stepwise in situ filling of metallic single‐walled carbon nanotubes (SWCNTs) with a mean diameter of 1.4 nm with metallocenes and a concomitant characterisation of their electronic properties using photoemission spectroscopy (PES) and X‐ray absorption spectroscopy (XAS) without breaking the ultra‐high vacuum environment. The changes in the SWCNTs' electronic structure upon ferrocene (FeCp2) filling are suggested by a broadening of the C 1s spectral line. Weak n‐type charge transfer from the ferrocene to the SWCNT is evidenced by a reduction of the intensity of the first van Hove singularity (vHs) in the fine structure of the unoccupied density of states as well as an increase in the valency of Fe in the FeCp2 molecule. Additionally, we evidence the presence of clean ferrocene with a resonance photoemission study showing the appearance of electronic states of localised Fe 3d electrons. This gives further insight into the electrical properties of functionalised carbon nanotubes fabricated in a clean environment avoiding inclusion of impurities.

ChemInform Abstract: Elementary Reactions of N Atoms with Hydrocarbons: First Steps Towards the Formation of Prebiotic N‐Containing Molecules in Planetary Atmospheres

AbstractReview: 85 refs.

Phase Transformation and Lithiation Effect on Electronic Structure of LixFePO4: An In-Depth Study by Soft X-ray and Simulations

Through soft X-ray absorption spectroscopy, hard X-ray Raman scattering, and theoretical simulations, we provide the most in-depth and systematic study of the phase transformation and (de)lithiation effect on electronic structure in Li(x)FePO(4) nanoparticles and single crystals. Soft X-ray reveals directly the valence states of Fe 3d electrons in the vicinity of Fermi level, which is sensitive to the local lattice distortion, but more importantly offers detailed information on the evolution of electronic states at different electrochemical stages. The soft X-ray spectra of Li(x)FePO(4) nanoparticles evolve vividly with the (de)lithiation level. The spectra fingerprint the (de)lithiation process with rich information on Li distribution, valency, spin states, and crystal field. The high-resolution spectra reveal a subtle but critical deviation from two-phase transformation in our electrochemically prepared samples. In addition, we performed both first-principles calculations and multiplet simulations of the spectra and quantitatively determined the 3d valence states that are completely redistributed through (de)lithiation. This electronic reconfiguration was further verified by the polarization-dependent spectra collected on LiFePO(4) single crystals, especially along the lithium diffusion direction. The evolution of the 3d states is overall consistent with the local lattice distortion and provides a fundamental picture of the (de)lithiation effects on electronic structure in the Li(x)FePO(4) system.

Synthesis and characterization of structural and luminescence properties of blue — green BaAlxOy:Eu2+ phosphor by solution — combustion method

Abstract Europium-doped barium aluminate (BaAlxOy:Eu2+) phosphors were obtained at low temperatures (500°C) using the solution — combustion of corresponding metal nitrate-urea solution mixtures. The particle size and morphology and the structural and luminescent properties of the synthesized phosphors were examined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), Electron diffraction spectroscopy (EDS) and photoluminescence (PL). It was found that the change in Ba: Al molar ratios showed greatly influence not only on the particle size and morphology, but also on their PL spectra and crystalline structure. The structure of BaAlxOy nanophosphors changes from a hexagonal Ba2Al10O17 phase for samples with 6:100 molar ratios to a hexagonal BaAl2O4 one with an increase in Ba content. The peak of the emission band occurs at a longer wavelength (around 615 nm) with a decrease in Ba concentration but displays a broad blue-green emission band composed from two emissions with the maximum at 495 and 530nm coming from Eu2+ in two sites for increasing Ba content. The blue-green emission is probably due to the influence of 5d electron states of Eu2+ in the crystal field because of atomic size variation causing crystal defects while the red emission is due to f - f transitions. These findings clearly demonstrate the possibility of fine tuning the colour emission.

In situ NMR study of hydrogenation/dehydrogenation of ZrCr2 and physisorbed hydrogen

In situ proton nuclear magnetic resonance (NMR) was employed to study the hydrogenation and dehydrogenation kinetics of ZrCr2. The powder samples prepared by arc melting followed by homogenizing heat treatment and grinding were put inside a boron nitride (BN) crucible, which was placed inside a quartz tube attached to a high pressure assembly specially designed for in situ NMR measurements. The samples were activated at 573K under vacuum inside the high temperature probe and then exposed to the required hydrogen gas pressure at room temperature for hydrogenation. Gradual hydrogenation of the samples was recorded through in situ NMR. Before activation of the sample, the exposure to high pressure showed two peaks with NMR shifts of 8.5 and −3.3ppm, which were assigned to hydrogen gas and physisorbed hydrogen, respectively. After activation, the sample showed absorption manifested through the appearance of a broad peak with an NMR shift of −141ppm. The NMR shift of absorption peak increased to higher magnetic fields as the hydrogen absorption proceeded. It was also accompanied by a change in NMR shift of physisorbed hydrogen peak in the same direction. Both the shifts, i.e., of absorbed hydrogen as well as of physisorbed hydrogen were linearly related with the degree of hydrogenation. This change in NMR shift of physisorbed hydrogen was explained on the basis of increase in density of d-electronic states at Fermi level with an increase in the absorbed hydrogen.

Correlation between electronic structure and magnetic properties of Fe-doped ZnO films

Fe-doped ZnO films with different Fe concentrations that display ferromagnetism at room temperature have been prepared by plasma assisted molecular beam epitaxy (p-MBE) techniques. Synchrotron-based measurements of photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), resonant photoemission spectroscopy (RPES), and superconducting quantum interference device (SQUID) were performed to investigate the electronic structure and magnetic properties of the films. It was found by Fe 2p PES and XAS that the dominant valence state of Fe ions is Fe3+ and that the configuration of Fe ions varies from tetrahedral sites to octahedral sites as the Fe concentration increases. Results of RPES indicate that the electronic states related to Fe2+ also exist near the Fermi level and that the distribution of Fe 3d electronic states in the valence band varies with different Fe concentrations. Correlations of the magnetic properties with the electronic structure of Fe-ZnO films have established that the electronic states related to Fe2+ and localized defects like Zn vacancies play an important role for ferromagnetism of Fe-ZnO films, while Fe3+ ions at octahedral sites destabilize the ferromagnetic interactions.

New Magnetoresistance Results in GdB<sub>6</sub>

Detailed study of transverse magnetoresistance (MR) and magnetic susceptibility have been done on the high quality single crystals of antiferromagnet GdB6 (TN15.5K) in the wide range of temperatures 2-40K in magnetic fields up to 8T. The data obtained allow to establish the low temperature antiferromagnetic (AFMII) phase below T*~4.7K with the complex behavior of MR including the field hysteresis of the magnetoresistance. It was shown that MR behavior depends on the cooling-warming prehistory in AFM state of GdB6. The analysis of experimental data allowed us to deduce three contributions to MR in AFM(I), (T*) and paramagnetic (PM) phases of GdB6. In addition to the main negative component Δρ˿ρ~ H2 interpreted in terms of Yosida model both the linear and nonlinear magnetic contributions were also established. The anomalies of MR found in AFM(I) state seem to be associated with the local spin polarization of 5d-electron states of Gd3+ ion.

Electronic-structure origin of the anisotropic thermopower of nanolaminated Ti3SiC2determined by polarized x-ray spectroscopy and Seebeck measurements

Nanolaminated materials exhibit characteristic magnetic, mechanical, and thermoelectric properties, with large contemporary scientific and technological interest. Here, we report on the anisotropic Seebeck coefficient in nanolaminated Ti3SiC2 single-crystal thin films and trace the origin to anisotropies in element-specific electronic states. In bulk polycrystalline form, Ti3SiC2 has a virtually zero Seebeck coefficient over a wide temperature range. In contrast, we find that the in-plane (basal ab) Seebeck coefficient of Ti3SiC2, measured on single-crystal films has a substantial and positive value of 4-6 muV/K. Employing a combination of polarized angle-dependent x-ray spectroscopy and density functional theory we directly show electronic structure anisotropy in inherently nanolaminated Ti3SiC2 single-crystal thin films as a model system. The density of Ti 3d and C 2p states at the Fermi level in the basal ab-plane is about 40 % higher than along the c-axis. The Seebeck coefficient is related to electron and hole-like bands close to the Fermi level but in contrast to ground state density functional theory modeling, the electronic structure is also influenced by phonons that need to be taken into account. Positive contribution to the Seebeck coefficient of the element-specific electronic occupations in the basal plane is compensated by 73 % enhanced Si 3d electronic states across the laminate plane that give rise to a negative Seebeck coefficient in that direction. Strong phonon vibration modes with three to four times higher frequency along the c-axis than along the basal ab-plane also influence the electronic population and the measured spectra by the asymmetric average displacements of the Si atoms. These results constitute experimental evidence explaining why the average Seebeck coefficient of Ti3SiC2 in polycrystals is negligible over a wide temperature range.

Influence of Mn concentration on the electronic and magnetic properties of Mn doped [formula omitted]: A first-principles study

The influence of Mn concentration on the electronic and magnetic properties of Mn doped β-Ge3N4 was investigated by using first-principles calculations based on density functional theory. Our results show that Mn atoms prefer occupying Ge sites and have a tendency to cluster. The electronic and magnetic properties of the system with Mn doping are closely related to Mn concentration. As Mn concentration increases from 5.56% to 11.11%, the conductivity of the system transforms from semiconductivity to half-metallicity, and the spin states of Mn 3d electrons also transit from low-spin to high-spin states. Our detailed analyses of electronic structure reveal that the ferromagnetic coupling between the Mn atoms induces the high spin-states of Mn 3d electrons and the half-metallicity of the system.

Direct Observation of Interband Spin-Orbit Coupling in a Two-Dimensional Electron System

We report the direct observation of interband spin-orbit (SO) coupling in a two-dimensional (2D) surface electron system, in addition to the anticipated Rashba spin splitting. Using angle-resolved photoemission experiments and first-principles calculations on Bi-Ag-Au heterostructures, we show that the effect strongly modifies the dispersion as well as the orbital and spin character of the 2D electronic states, thus giving rise to considerable deviations from the Rashba model. The strength of the interband SO coupling is tuned by the thickness of the thin film structures.

Fabrication of $\hbox{Fe}_{16}\hbox{N}_{2}$ Films by Sputtering Process and Experimental Investigation of Origin of Giant Saturation Magnetization in $\hbox{Fe}_{16}\hbox{N}_{2}$

We present a systematic study to address a longstanding mystery in magnetic materials and magnetism, whether there is giant saturation magnetization in Fe16N2 and why. Experimental results based on sputtered thin film samples are presented. The magnetism of Fe16N2 is discussed systematically from the aspects of material processing, magnetic characterization and theoretical investigation. It is observed that thin films with Fe16N2+Fe8N mixture phases and high degree of N ordering, exhibit a saturation magnetization up to 2.68T at room temperature, which substantially exceeds the ferromagnetism limit based on the traditional band magnetism understanding. From X-ray magnetic circular Dichorism (XMCD) experiment, transport measurement and first-principle calculation based on LDA+U method, it is both experimentally and theoretically justified that the origin of giant saturation magnetization is correlated with the formation of highly localized 3d electron states in this Fe-N system. A large magnetocrystalline anisotropy for such a material is also discussed. Our proposed “cluster+atom” theory provides promising directions on designing novel magnetic materials with unique performances.

Hall effect study in antiferromagnets RB6 (R-Pr, Nd)

We report the results of precision measurements of Hall effect carried out on high quality single crystals of PrB6 and NdB6 at temperatures 2–300 K in magnetic fields up to 1 T. The Hall coefficient RH demonstrates weak temperature dependence in paramagnetic (PM) phase of both compounds. When temperature decreases below TN the second harmonic contribution of Hall resistivity was detected. The results of the work are discussed in terms of effect of spin polarization of 5d-electron states in PrB6 and NdB6.

A comparative study of electronic structure and magnetic properties of SrCrO3 and SrMoO3

A comparative study of electronic structure and magnetic properties of SrCrO3 and SrMoO3 has been carried out using FPLAPW method with density-functional theory. The calculated results suggest that both compounds are nonmagnetic (NM) metal in cubic structures at room temperature, and they exhibit very similar band structure and electronic properties except more extend Mo 4d orbitals than Cr 3d electronic states. However, the electronic structure and magnetic properties exhibit remarkable differences between them in the low temperature phases. SrCrO3 is with a C-AFM ground state with magnetic moment of 1.18μB/Cr in the tetragonal structure, while SrMoO3 is with a NM ground state in the orthorhombic structure. It is assumed that the extend 4d orbitals may be the reason which results in NM solution at low temperature phase of SrMoO3.