X-ray Photoemission Spectroscopy Research Articles

Nanoscale Networks of Metal Oxides by Polymer Imprinting and Templating for Future Adaptable Electronics

While network formation is prevalent in nature, networks are generally not expected in inorganic structures. Exceptions are those cases in which surface states become important, such as nanoparticles. However, even in these cases, the morphology of these networks is difficult to control and they show a large degree of disorder. In this work, we show that highly ordered and interconnected nanoscale networks of functional metal oxides can be fabricated by a combination of polymer imprinting and polymer templating through solution processable methods. We report the fabrication of a number of functional oxide networks (i.e., BiFeO3, SrTiO3, La0.7Ca0.3MnO3, and HfO2) from solution, showing that all the oxide materials tried so far are able to follow the self-assembled network morphology dictated by the polymer structure. These networks were characterized for the overall structure by scanning electron microscopy and atomic force microscopy (AFM). Grazing incidence small angle X-ray scattering showed a good imprint quality on the mm2 scale for the combined networks, which is challenging given that multiple processing steps were involved during the fabrication. The material stoichiometries were investigated by X-ray photoemission spectroscopy and the crystal phases by grazing incidence wide angle X-ray scattering. When electronic functionality is anticipated, the networks behave as expected: conducting AFM on the La0.7Ca0.3MnO3 networks confirmed the conductive character of this composition; and piezoresponse force microscopy of the BiFeO3 network is consistent with the presence of ferroelectric behavior. These nanoscale networks show promise for future applications in adaptable electronics, such as neuromorphic computing or brain-inspired information processing.

An S-scheme CdS/K2Ta2O6 heterojunction photocatalyst for production of H2O2 from water and air

The hydrogen peroxide (H2O2) production using solar energy is of great significance in chemical industry and environmental remediation. However, this technique requires that photocatalysts have efficient charge separation and migration efficiency. Herein, we designed a flower-like CdS/K2Ta2O6 (CdS/KTO) S-scheme heterojunction with promoted charge separation and migration by a facile two-step hydrothermal strategy. The CdS/KTO photocatalyst exhibited an outstanding performance with a H2O2 production rate of 160.89 μmol g−1 h−1 without using any sacrificial agents and adding additional O2. The combination of the in situ irradiated X-ray photoemission spectroscopy (XPS), electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations revealed that the formation of an S-scheme heterojunction between CdS and KTO could greatly promote the separation of photogenerated electron-hole pairs toward efficient H2O2 production. This work provides insights for the charge transfer mechanism of S-scheme heterojunction and an innovative stratagem for green, energy-saving and sustainable H2O2 production.

Rocksalt CeO epitaxial thin film as a heavy-fermion system transiting from p -type metal to partially compensated n -type metal by 4f delocalization

Rocksalt CeO (001) epitaxial thin films were synthesized and their electronic properties were investigated. A simple $4{f}^{1}{5d}^{1}$ electronic configuration with $4f--5d$ hybridization in CeO was confirmed by x-ray photoemission and absorption spectroscopy. While $5d$ conduction holes governed the metallic conduction at high temperatures, a partially compensated $n$-type conduction appeared below \ensuremath{\sim}10 K with a rapid decrease in resistivity corresponding to a Fermi liquid state. The hole mobility was as high as $646\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{--1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{--1}$ at 2 K in contrast to the two decades lower electron mobility, reflecting the large and small dispersion of $5d$ and $4f$ bands, respectively. At 0.8 K, a resistivity minimum was observed as a manifestation of the Kondo effect, indicating partial $4f$ localization. These results represented that the significant $4f--5d$ hybridization induced a Kondo coherent state in CeO with a short Ce-Ce interionic distance unlike the other antiferromagnetic Ce chalcogenides.

Dehydroxylation of Kaolinite Tunes Metal Oxide–Nanoclay Interactions for Enhancing Antibacterial Activity

Engineered nanoparticle–support interaction is an effective strategy for tuning the structures and performance of engineered nanoparticles. Here, we show that tuning the dehydroxylation of kaolinite nanoclay as the support could induce zinc oxide–kaolinite interactions. We used free energy theory, electron microscopy, and X-ray photoemission spectroscopy to identify interaction strengths between metal oxides and the underlying nanoclay induced by dehydroxylation. Desirable exposure of nanoparticle sites and the geometrical and crystal structure were obtained by tuning the interface interactions between ZnO nanoparticles and nanoclay. The surface free energy of zinc oxide–nanoclay results in different interfacial interactions, and the properties of the surface free energy electron-donating (γ−) and electron-accepting (γ+) parameters have significant effects on the electron acceptor. This could, in turn, promote stronger interactions between zinc oxide and the kaolinite surface, which produce more active (0001) Zn-polar surfaces with promoting zinc oxide nanoparticles growing along the <0001> direction. Reactive oxygen species, leached zinc ions, and electron transfer can modulate the antibacterial activities of the samples as a function of interface free energy. This further demonstrates the interfacial interactions induced by dehydroxylation. This work has new application potential in biomedicine and materials science.

Effect of PbPc on electron structure and carrier dynamics of black phosphorus

Using lead phthalocyanine (PbPc) as surface doping material on black phosphorous (BP) we observe enhanced photo-excited carriers in the PbPc/BP heterostructure. The interfacial energy level alignment is investigated with ultra violet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS). The heterojunction is type I with gap of BP nested in that of PbPc, facilitating confinement of electrons and holes in BP. Ultrafast time-resolved two-photon photoemission (TR-2PPE) spectroscopy is used to study the influence of PbPc on the photo excited unoccupied electronic states and the dynamics of the relaxation processes. Monolayer PbPc can greatly increase the pump excited hot electrons and the 2 photon emission of BP. The enhanced population in the intermediate states is attributed to the straddling of the band alignment which benefits the photo excited electrons in PbPc transferring to BP. Density functional theory calculations supported the interface dipole and charge redistribution. Our results provide a fundamental understanding of the excellent opto-electrical response of PbPc/BP interface of promising application in the high efficient photo detectors.

Atomic Layer Deposited SiOX-Based Resistive Switching Memory for Multi-Level Cell Storage

Herein, stable resistive switching characteristics are demonstrated in an atomic-layer-deposited SiOX-based resistive memory device. The thickness and chemical properties of the Pt/SiOX/TaN stack are verified by transmission electron microscopy (TEM) and X-ray photoemission spectroscopy (XPS). It is demonstrated that much better resistive switching is obtained using a negative set and positive reset compared to the opposite polarity. In addition, multi-level switching is demonstrated by changing the compliance current (CC) and the reset stop voltage, and potentiation and depression are emulated by applying pulses to achieve a synaptic device. Finally, a pulse endurance of 10,000 cycles and a retention time of 5000 s are confirmed by modulating the pulse input and reading voltage, respectively.

Scavenger-Supported Photocatalytic Evidence of an Extended Type I Electronic Structure of the TiO2@Fe2O3 Interface.

Heterostructures of TiO2@Fe2O3 with a specific electronic structure and morphology enable us to control the interfacial charge transport necessary for their efficient photocatalytic performance. In spite of the extensive research, there still remains a profound ambiguity as far as the band alignment at the interface of TiO2@Fe2O3 is concerned. In this work, the extended type I heterojunction between anatase TiO2 nanocrystals and α-Fe2O3 hematite nanograins is proposed. Experimental evidence supporting this conclusion is based on direct measurements such as optical spectroscopy, X-ray photoemission spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), and the results of indirect studies of photocatalytic decomposition of rhodamine B (RhB) with selected scavengers of various active species of OH•, h•, e–, and •O2–. The presence of small 6–8 nm Fe2O3 crystallites at the surface of TiO2 has been confirmed in HRTEM images. Irregular 15–50 nm needle-like hematite grains could be observed in scanning electron micrographs. Substitutional incorporation of Fe3+ ions into the TiO2 crystal lattice is predicted by a 0.16% decrease in lattice parameter a and a 0.08% change of c, as well as by a shift of the Raman Eg(1) peak from 143 cm–1 in pure TiO2 to 149 cm–1 in Fe2O3-modified TiO2. Analysis of O 1s XPS spectra corroborates this conclusion, indicating the formation of oxygen vacancies at the surface of titanium(IV) oxide. The presence of the Fe3+ impurity level in the forbidden band gap of TiO2 is revealed by the 2.80 eV optical transition. The size effect is responsible for the absorption feature appearing at 2.48 eV. Increased photocatalytic activity within the visible range suggests that the electron transfer involves high energy levels of Fe2O3. Well-programed experiments with scavengers allow us to eliminate the less probable mechanisms of RhB photodecomposition and propose a band diagram of the TiO2@Fe2O3 heterojunction.

Room temperature giant magneto-caloric effect in Ni45Mn44Sn11-XInX (X = 1, 3) disordered Heusler alloy: The role of martensite transition

Here, we report the first order martensite to austenite transition characterized through magnetic and calorimetric studies in (off-stoichiometric) shape memory Heusler alloy Ni45Mn44Sn11-XInX (X = 1; 3) in the proximity of room temperature. The structural properties of the alloys were checked with x-ray diffraction (XRD), x-ray photoemission spectroscopy (XPS) and energy dispersive x-ray analysis (EDXA). The dominance of a particular phase (martensite or austenite) at room temperature can be predicted from the structural study by XRD as two different phases (L21 and 10 M) were found at room temperature with different proportion for the two alloys. Martensite transition was observed near the room temperature for both samples which was monitored with differential calorimetry and magnetization study. A large magneto-caloric effect was found in the system near the room temperature observed from the magnetic measurements. The calculated entropy change (3.24 JKg−1K−1 and 11.2 JKg−1K−1 for X = 1 and 3 respectively) and the refrigerant capacity (27 JKg−1 and 40 JKg−1 for X = 1 and 3 respectively) for 3 T magnetic field were found to be significant for practical applications near room temperature. A small amount of In substitutions in Sn sites have influenced the martensite transition as a significant increase in the martensite transition temperature has been observed. The magnetocaloric effect in these materials has been understood in the realm of sharp magnetization change arising in the vicinity of metamagnetic transition from the martensite phase (weakly magnetic) to the austenite phase (ferromagnetic). The In substitution is thought to influence the hybridization between Ni-Mn bonds which in turn influence the martensite transition and thereby enhancing the magnetocaloric property of the materials.

High temperature crystallographic and low temperature magnetic phase transitions in Fe doped Sr2RuMnO6

Fe doped Sr2RuMnO6 (SRMO) double perovskites (Sr2RuMn1-xFexO6, x = 0, 0.1, 0.2 and 0.3) were prepared by solid-state route. Both x-ray diffraction and Raman spectroscopy were performed to investigate the crystal structure of the synthesized double perovskites. Rietveld refinement of the x-ray diffraction patterns confirmed a phase transition from tetragonal to cubic space group as a function of doping concentration of iron. Raman spectroscopy at room temperature and group theory analysis revealed the phonon modes associated with the space group of the samples. The temperature dependent Raman spectroscopy showed an anharmonic behaviour of the phonon modes of the Fe doped SRMO samples. The temperature evolution of the phononic modes in the range of 300 K–620 K is predominantly influenced by the lattice degrees of freedom. The presence of several oxidation states Mn (2+, 3+ and 4+) and Fe (3+ and 4+) was confirmed by an X-ray photoemission spectroscopy analysis of the highest doped sample (x = 0.3). The magnetic properties measurements showed that the samples were completely paramagnetic at room temperature. The samples exhibit antiferromagnetism at very low temperatures and we conclude that they exhibit ferrimagnetic ground state in the mid temperature region.

Piezoelectric nanogenerator based on lead-free BiFeO3:Sr perovskite

The current work reports the synthesis of Bi1−xSrxFeO3 (SBFO; x = 0.0, 0.15, 0.25, 0.35) nanoparticles by sol-gel method and study of its structural and piezoelectric properties. X-ray diffraction identifies the phase transformation from rhombohedral to cubic of BiFeO3 upon doping of Sr. X-ray photoemission spectroscopy revealed that Sr doped BiFeO3 nanoparticles are rich with oxygen vacancies compared to the bare BiFeO3 nanoparticles. The highest maximal polarization value of 6.74 µC cm−2 and remnant polarization 4.20 µC cm−2 are observed for BiFeO3 with 25% of Sr. The SBFO-PENG with 25% of Sr content device provides output voltage and current of 6.35 V and 0.64 μA under pushing tester with 1 Hz frequency, which is higher to that for a bare BiFeO3 PENG. It also exhibits high mechanical robustness, and much stable and durable output even after 1500 s. A maximum power density of ~ 0.31 µWcm−2 was achieve at a resistive load of 108 Ω. The SBFO-PENG device's feasibility is tested by trigging commercial electronics such as charging capacitors and lighting the LED, and it can drive a 0.47 μF capacitor to store the energy of 7.29 μJ within 500 s under pushing tester.

Electrodeposited copper nanocubes on multi-layer graphene: A novel nanozyme for ultrasensitive dopamine detection from biological samples

In this paper, copper nanocubes (CuNCs) were grown on top of few-layer and multi-layer graphene (FLG and MLG) nanomaterial by electrodeposition, to develop a sensitive and selective nanozyme based electrochemical sensor for the determination of dopamine from plasma samples. The copper electrodeposition process was intensively studied and optimized to obtain cubic shape CuNPs on top of graphene that increased the active area of the sensor. The surface of the nanomaterial was investigated with several microphysical characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray (EDX), and Fourier-transform infrared (FTIR), Raman, and X-ray photoemission spectroscopies (XPS). Further, the nanozyme sensor based on CuNCs-Gr/SPCE was electrochemically characterized using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), and the response towards the oxidation of DA was investigated using the differential pulse voltammetry (DPV) technique. The signal recorded with CuNCs-Gr/SPCE was linear in a wide range (0.001 – 100 µM), with a low limit of detection of 0.33 nM and a very high sensitivity of 196 mA/moL-1. The developed sensor showed high selectivity towards DA in presence of other neurotransmitters. The detection of DA in human plasma samples was obtained with 95.8–100.2 % recovery percentage, proving the reliability of the method.

Bi0.5Pb0.5FeO3 with Unusual Pb Charge Disproportionation: Indication of a Systematic Charge Distribution Change in Bi0.5Pb0.5MO3 (M: 3d Transition Metal).

Bi0.5Pb0.5FeO3 with 1:1 mixture of Bi and Pb having charge degrees of freedom at the A-site of perovskite oxide ABO3 is obtained for the first time by high-pressure synthesis. Comprehensive synchrotron X-ray powder diffraction, optical second harmonic generation, Mössbauer spectroscopy, and hard X-ray photoemission spectroscopy measurements revealed that Bi0.5Pb0.5FeO3 is a canted antiferromagnetic insulator crystalizing in a nonpolar tetragonal I4/mcm structure with √2a × √2a × 2a unit cell and has unusually Pb charge disproportionated Bi3+0.5Pb2+0.25Pb4+0.25Fe3+O3 charge distribution. The valence of transition metal M in Bi0.5Pb0.5MO3 changes from 3.5+ to 3+ and finally to 2+ from Mn to Fe and to Ni, from left to right in the periodic table as the 3d-level becomes deeper. The valences of Bi and Pb increase to compensate for the decrease in the M's valence, and Pb changes from 6s2 (2+) to 6s0 (4+) before Bi changes.

Flower-like ZnO Nanostructures Local Surface Morphology and Chemistry

This work presents the results of comparative studies using complementary methods, such as scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS), and thermal desorption spectroscopy (TDS) to investigate the local surface morphology and chemistry of flower-like ZnO nanostructures synthesized by the thermal oxidation technique on native Si/SiO2 substrates. SEM studies showed that our flower-like ZnO nanostructures contained mostly isolated and irregular morphological low-dimensional forms, seen as rolled-up floss flowers, together with local, elongated, complex stalks similar to Liatris flowers, which contained joined short flosses in the form of nanodendrites. Beyond this, XPS studies showed that these nanostructures exhibited a slight surface nonstoichiometry, mostly related to the existence of oxygen-deficient regions, combined with strong undesired C surface contamination. In addition, the TDS studies showed that these undesired surface contaminations (including mainly C species and hydroxyl groups) are only slightly removed from the surface of our flower-like ZnO nanostructures, causing an expected modification of their nonstoichiometry. All of these effects are of great importance when using our flower-like ZnO nanostructures in gas sensor devices for detecting oxidizing gases because surface contamination leads to an undesired barrier for toxic gas adsorption, and it can additionally be responsible for the uncontrolled sensor aging effect.

Theoretical and experimental approaches for the determination of functional properties of MgSnN2 thin films

MgSnN2 thin films were deposited by reactive magnetron co-sputtering on fused silica and silicon substrates. The properties of the films were determined as a function of the deposition temperature. Although the films exhibited a strong preferred orientation along the [002] direction, cumulative X-ray diffractograms at various χ angles evidenced that the deposited films crystallized in a wurtzite-like structure. Regardless of the substrate temperature during deposition, MgSnN2 films had a columnar microstructure. Mössbauer spectrometry and X-ray photoemission spectroscopy were used to extract the chemical environment of magnesium and tin atoms and to identify their oxidation states. Electrical properties were deduced from Hall effect measurements. An increase of the electron mobility was noticed at high deposition temperature. The optical band gap of MgSnN2 films increased from approx. 2.28 to 2.43 eV in the 300–500 °C range. The experimental optical properties of MgSnN2 films were compared to those obtained by ab initio calculations.

The Damage Analysis for Irradiation Tolerant Spin-Driven Thermoelectric Device Based on Single-Crystalline Y₃Fe₅O₁₂/Pt Heterostructures

Spin-driven thermoelectric (STE) generation based on the combination of the spin Seebeck effect (SSE) and inverse spin Hall effect is an alternative to conventional semiconductor-based thermocouples as it is large scale, low cost, and environment-friendly. The STE device is thought to be radiation hard, making it attractive for space and nuclear technology applications. By using magnetometry, transmission electron microscopy, and the hard X-ray photoemission spectroscopy (HAXPES) measurements, we show that an STE device made of single-crystalline Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (YIG)/Pt heterostructures has tolerance to irradiation of high-energy heavy ion beams. We used 200 MeV gold ion beams modeling cumulative damages due to fission products emitted from the surface of spent nuclear fuels. By varying the dose level, we confirmed that the thermoelectric and magnetic properties of the single-crystalline YIG/Pt STE device are finite at the ion-irradiation dose up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> fluence. In addition, the HAXPES measurements were performed to understand the effects at the interface of YIG/Pt. The HAXPES data suggest that the chemical reaction regarding Fe and O that diminishes the SSE signals is promoted with the increase of the irradiation dose. The understandings of the damage analysis in YIG/Pt are beneficial for developing better STE devices applicable to harsh environmental usages.

Intense and stable room-temperature photoluminescence from nanoporous vanadium oxide formed by in-ambient degradation of VI3 crystals

Vanadium oxides have attracted research interest because their optoelectronic properties make them optically active with room-temperature photoluminescence (PL) emission, which, however, is not sufficiently intense for real applications. For this reason, many nanostructured vanadium oxides are currently fabricated through several precursors and different treatments to improve the PL efficiency and enhance the PL intensity. Herein, we propose an alternative and facile route to the fabrication of nanoporous vanadium oxide flakes through the spontaneous in-ambient degradation of layered van der Waals VI3 crystal, which is composed of a mixture of V2O5 and V3O7 phases. The as-grown VI3 crystals and the formed nanostructured vanadium oxide have been thoroughly studied using X-ray diffraction (XRD) and Raman spectroscopy to access the structural properties, X-ray photoemission spectroscopy (XPS) and synchrotron-based XPS to analyze the electronic core levels and valence bands, scanning electron microscopy (SEM) to access the morphology, and PL spectroscopy to grasp the optoelectronic properties. The nanoporous vanadium oxide system reveals an intense room-temperature PL emission in the red light visible range between 1.7 and 2.0 eV (620 and 730 nm), which is consistent with the V2O5 PL response. Remarkably, the PL emission reaches high intensity compared with those of different V2O5 nanostructures and is stable for months without intensity quenching and energy shifting. This discovery easies the integration of nanostructured vanadium oxides in optoelectronic nanodevices. Besides, the facile methodology proposed here promises to be applied to realize other nanostructured transition metal oxides.

Observation of structural change-driven Griffiths to non-Griffiths-like phase transformation in Pr2-xSrxCoFeO6 (x = 0 to 1)

The study of crystal structure, electronic structure, transport, and magnetic properties of heterovalent Sr2+ doped Pr2-xSrxCoFeO6 (x = 0.0 to 1.0) system have been done. Crystal structure study reveals an occurrence of structural change from orthorhombic (Pnma) to tetragonal (I4/m) phase above x = 0.6. A sudden transformation of the Griffiths-like to non-Griffiths-like magnetic phase is observed as the system changes its crystal structure from Pnma to I4/m. The X-ray photoemission spectroscopy (XPS) study suggests for the existence of mixed oxidation states of the B-site ions viz., Co3+/Co4+ and Fe3+/Fe4+, and it also indicates an increase in the mean oxidation states owing to the hole substitution (Sr2+). The temperature variation of the electrical resistivity of the studied systems follows two different transport mechanisms, such as the variable range hopping (VRH) (in the lower temperature region) and small polaron hoping (SPH) (in the higher temperature region) models. Dc magnetization study shows that a local competing ferromagnetic (FM) exchange interaction increases with Sr doping. Finally, the ac susceptibility study reveals breaking of the long-range-ordering in the system x = 1.0, which appears to be related to the structural change and enhanced spin frustration due to increased competing local FM exchange interactions. In addition, electronic density of states (DOS) calculations of PrSrCoFeO6 (i.e. x = 1.0) using the density functional theory (DFT) have been performed for various Co/Fe atomic distributions. For most of the Co/Fe atomic distributions studied, the calculations show that the total energy of the system with FM coupling among spins has slightly lower energy than that for antiferromagnetic (AFM) coupling.

Transparent conductive SnO2 thin films via resonant Ta doping

Transparent conductive oxide (TCO) thin films are highly desired as electrodes for modern flat-panel displays and solar cells. Alternative indium-free TCO materials are highly needed, because of the scarcity and the high price of indium. Based on the mechanism of resonant doping, Ta has been identified as an effective dopant for SnO2 to achieve highly conductive and transparent TCO. In this work, we fabricated a series of Ta-doped SnO2 thin films (Sn1−xTaxO2, x = 0.001, 0.01, 0.02, 0.03) with high conductivity and high optical transparency via a low-cost sol-gel spin coating method. The Sn0.98Ta0.02O2 film achieves the highest electrical conductivity of 855 S cm−1 with a carrier concentration of 2.3 × 1020 cm−3 and high mobility of 23 cm2 V−1 s−1. The films exhibit a very high optical transparency of 89.5% in the visible light region. High-resolution X-ray photoemission spectroscopy and optical spectroscopy were combined to gain insights into the electronic structure of the Sn1−xTaxO2 films. The optical bandgaps of the films are increased from 3.96 eV for the undoped SnO2 to 4.24 eV for the Sn0.98Ta0.02O2 film due to the occupation of the bottom of conduction band by free electrons, i.e., the Burstein-Moss effect. Interestingly, a bandgap shrinkage is also directly observed due to the bandgap renormalization arising from many-body interactions. The double guarantee of transparency and conductivity in Sn1−xTaxO2 films and the low-cost growth method provide a new platform for optoelectronic and solar cell applications.

Hard x-ray photoemission spectroscopy of the ferrimagnetic series Gd6(Mn1−xFex)23

We study the evolution of the electronic structure of the intermetallic series ${\mathrm{Gd}}_{6}{({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})}_{23}, x=0.0--0.75$, which shows nonmonotonic ferrimagnetic ordering temperatures ${T}_{C}$ but with a systematic reduction of the total bulk magnetization upon increasing Fe content, $x$. We have carried out hard x-ray photoemission spectroscopy to elucidate the relation between electronic structure and properties of the series. The Gd $3d$ and Gd $4d$ core-level spectra indicate trivalent ${\mathrm{Gd}}^{3+}$ multiplets in the intermediate-coupling scheme with features due to $L\text{\ensuremath{-}}S$ and $j\text{\ensuremath{-}}J$ coupling. The Fe $2p$ core levels show asymmetric single peak metal-like spectra, while the Mn $2p$ core levels show asymmetric doublet peaks. The relative intensities of the Mn $2p$ doublets as a function of $x$ indicate occupancy changes of distinct crystallographic sites associated with Mn up-spin and down-spin states. The valence band spectra identify the Gd $4f$ states at high binding energies ($\ensuremath{\sim}7.4$ eV). The Mn $3d$ states occur at the Fermi level and as a broad feature between 2 and 5 eV binding energy in ${\mathrm{Gd}}_{6}{\mathrm{Mn}}_{23}$. Upon substitution, the Fe $3d$ states show up as small shifts to higher binding energies compared to Mn $3d$ states. The Fe $3s$ and Mn $3s$ spectra show exchange split peaks, allowing an estimate of the Mn and Fe magnetic moments using a Van Vleck analysis, which also provides a quantification of occupancy changes with $x$. The overall results are consistent with the bulk net magnetization, indicating that Mn up-spin sites become Fe down-spin sites on substitution, while the nonmonotonic ${T}_{C}$ originates in a change from Mn sublattice to Fe sublattice derived ordering.

A Quintuple-Layered Binary Chalcogenide Sb2Te3 Single Crystal and Its Transport Properties for Thermoelectric Applications.

The binary chalcogenide material Sb2Te3 was synthesized via the melting technique. The synthesized materials were converted to a single crystal through the Bridgman–Stockbarger technique. The phase purity and structural properties of the grown crystal were analyzed using powder X-ray diffraction and single-crystal X-ray diffraction measurements. X-ray photoemission spectroscopy reveals the local intermolecular bonding, changes in the stoichiometry, and the oxidation states of the elements present in the crystal. Transport properties like electrical resistivity, Seebeck coefficient, and thermal conductivity were measured. The power factor and figure of merit (ZT) of the grown crystal were calculated.