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A liquid Eu precursor, bis(normal-propyl-tetramethylcyclopentadienyl)europium has been synthesized. This precursor exists as a liquid at temperatures higher than 49 °C, has a moderately high vapor pressure, contains no oxygen in its molecular structure, and can be distilled to high purity. These properties make it ideal for doping using a chemical vapor or atomic layer deposition method, and provide a degree of control previously unavailable. As a precursor the Eu exists in the divalent valance state, however, once doped into GaN by organometallic vapor phase epitaxy, the room-temperature photoluminescence of the Eu-doped GaN exhibited the typical red emission due to the intra-4f shell transition of trivalent Eu. After variation of the growth temperature, it was found that divalent Eu could be stabilized in the GaN matrix. By tuning the Fermi level through donor doping, the ratio of Eu2+ to Eu3+ could be controlled. The change in valence state of the Eu ions was confirmed using X-ray absorption near-edge structure.

The iron (Fe) electrodeposition-electrochemical dissolution has been employed on nitrogen-doped carbon material (P-PI) prepared via multi-step pyrolysis of a polyimide precursor to achieve the introduction of Fe species, and its influence on the oxygen reduction reaction (ORR) is investigated by cyclic and rotating ring-disk electrode voltammetry in 0.5 M H2SO4. After the electrochemical treatment, the overpotential and H2O2 production percentage of ORR on the P-PI are decreased and the number of electrons transferred is increased in the meanwhile. In combination with the results of X-ray absorption fine structure spectra, the presence of Fe-N-x sites (Fe ions coordinated by nitrogen) is believed to be responsible for the improved ORR performance. Further kinetic analysis indicates that a two-electron reduction of O-2 is predominant on the untreated P-PI with coexistence of a direct four-electron transformation of O-2 to H2O, while the introduction of Fe species leads to a larger increase in the rate constant for the four-electron reduction than that for the two-electron process, being in good agreement with the view that Fe-N-x sites are active for four-electron ORR.

We have succeeded in situ Eu doping into Al x Ga 1 − x N grown by organometallic vapor phase epitaxy and investigated the effects of the growth pressure and Al composition on the photoluminescence (PL) properties of Eu 3+ ions. The intensity of red emission due to the 5 D 0 – 7 F 2 transition of Eu 3+ ions decreased with increasing Al content in the case of growth at atmospheric pressure. The effect of the reactor pressure on the Eu concentration and Eu 3+ PL properties showed that low-pressure growth is effective in increasing the Eu doping concentration and luminescence efficiency. The PL intensity increased with the Al composition and reached a maximum intensity at approximately x = 0.24. At Al compositions of higher than 0.24, X-ray absorption near-edge structure analysis revealed that the concentration of Eu 2+ ions increased with increasing Al composition, leading to a reduction in the concentration of optically active Eu 3+ ions.

The electronic structure of the residual metal atoms in Fe phthalocyanine (FePc)-based carbon catalysts, prepared by pyrolyzing a mixture of FePc and phenolic resin polymer at 800 °C, before and after acid washing have been investigated using X-ray absorption fine structure (XAFS) spectroscopy to clarify the role of Fe in the oxygen reduction reaction (ORR) activity. The Fe K X-ray emission intensity suggests that the acid washing process reduces 36% of the total amount of residual Fe in the FePc-based catalysts. The decomposition analyses for the XAFS spectra reveal that the composition ratio of each Fe component is unaltered by the acid washing, indicating that the residual Fe components were removed by the acid washing irrespective of their chemical states. Because the oxygen reduction potential was approximately unchanged by the acid washing, the residual Fe itself does not seem to contribute directly to the ORR activity of the samples. The residual Fe is composed mainly of metallic Fe components (Fe metal and iron carbide Fe3C), which can act as a catalyst to accelerate the growth of the sp2 carbon network during pyrolysis. The results imply that light elements such as C and N are components of the ORR active sites in the FePc-based carbon catalysts pyrolyzed at high temperatures where the metal-N4 structures in the macrocycles are mostly decomposed.

The oxygen partial pressure ( P (O 2 )) dependence of in situ X-ray absorption spectroscopy (XAS) at the Co and Fe K edges was measured simultaneously and continuously at 900 and 1000 K. These experiments, which were performed during reduction, changing P (O 2 ) from 1 to 10 −4 atm, were used to investigate each valence related to Co and Fe in (La 0.6 Sr 0.4 )(Co 0.2 Fe 0.8 )O 3−δ (LSCF). The absorption edge shift of the Co K edge was more than twice that of the Fe K edge at 1000 K during reduction. For quantitative analysis, X-ray absorption near-edge structure spectroscopy was carried out at the Co and Fe K edges; the results indicated that the Co valence decreased more easily than the Fe valence; that is, the oxygen preferentially left from the oxygen sites around Co.

We studied the relationships between the temperature dependence of the charge density, the atomic isotropic displacement parameter ( U iso ) of oxygen, and conductivity in (La 0.75 Sr 0.25 )MnO 3.00 (LSM) and (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 2.33− δ (BSCF) using the maximum-entropy method (MEM) and Rietveld refinements of in situ synchrotron X-ray diffraction (SR-XRD) measurements taken at 500–900 K. MEM revealed that the Mn–O plane in LSM has a strong isotropic covalent bond that enables high electron conductivity. The charge density at the O sites in LSM decreased slightly with increasing temperature. The charge density at the O2(8 d ) site in BSCF was independent of temperature. In contrast, the charge density at the O1(4 c ) site in BSCF decreased significantly with increasing temperature. We suggest that the O1(4 c ) site and not the O2(8 d ) site has decreasing occupancy. We also estimated U iso for the oxygen sites with and without taking the occupancy factor into consideration. We presumed that the temperature dependence of U iso for the O1(4 c ) site was strongly related to the site occupancy. There was no temperature dependence for U iso of the O1(4 c ) site when taking the decreasing site occupancy into account; however, the U iso value of the O1(4 c ) site was three times that of the O2(8 d ) site. On the basis of the analysis results for the temperature dependences of the charge density, site occupancy, and U iso of each oxygen site in BSCF, we propose that the O1(4 c ) site is responsible for oxygen ion conduction.

The crystal structure and exchange bias effect in Fe films deposited on Si(100) substrates have been investigated. X-ray diffraction and fluorescence-extended X-ray absorption fine structure measurements reveal that the as-deoposited Fe films are polycrystalline with a preferred (110) texture. In addition, we observe a shift in the magnetic hysteresis loop of Fe films deposited on Si (100) where there is existance of a thin oxidized layer. By comparison, for Fe film deposited on Si(100) lacking the oxidized layer, it does not exhibit any features of shift in the magnetic hysteresis loop. We postulate the effect results from a coupling between the ferromagnetic Fe film and the antiferromagnetic Fe oxide that forms spontaneously in the interface. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have fabricated MnAs ferromagnetic nanoscaled dots on sulfur-passivated semiconductor substrates such as GaAs(0 0 1), GaAs(1 1 1)B, and InP(0 0 1) by low temperature molecular beam epitaxy (MBE). Atomic force microscopy (AFM) results reveal that the shape of MnAs dots depends on the surface energy of sulfur-passivated substrates. Extended X-ray absorption fine structure (EXAFS) and superconducting quantum interference device (SQUID) results suggest that MnAs dots on sulfur-passivated GaAs(0 0 1) are zb-type, while MnAs dots on sulfur-passivated GaAs(1 1 1)B and InP(0 0 1) are NiAs-type. We have found that the balance between substrate surface energy and lattice mismatch is essential to successfully fabricate zb-MnAs.

Thin films of α-Mn were epitaxially grown on the 4H-SiC (0 0 0 1) homoepitaxial substrates by using the molecular beam epitaxy. In situ reflection high-energy electron diffraction (RHEED), ex situ X-ray diffraction (XRD), atomic force microscopy (AFM) and the fluorescence extended X-ray absorption fine structure (EXAFS) are used to characterize their structural properties. We found that the α-Mn films with mirror-like appearance keep epitaxial growth on the 4H-SiC substrates despite of their very dissimilar nature of the crystal structure, bonding and chemical properties. The epitaxial relationship between body-centered-cubic (bcc) Mn and 4H-SiC is Mn (1 1 0)//4H-SiC(0 0 0 1) with Mn[0 0 1]//4H-SiC [ 1 1 ¯ 0 0 ] .

We have grown ErP on Ga 0.52 In 0.48 P (001) lattice-matched to GaAs and on InP (001) by low-pressure organometallic vapor phase epitaxy and investigated the surface morphology of ErP layers. Lattice-mismatch in ErP/Ga 0.52 In 0.48 P/GaAs heterostructures (Δ a / a = − 0.8%) is much less than that of ErP/InP heterostructures (− 4.5%). Extended X-ray absorption fine structure measurement revealed that Er exists in the form of ErP rock-salt structure in both samples. The estimated growth rate of ErP on Ga 0.52 In 0.48 P is 1.8 ML/h. Our results demonstrate that ErP on Ga 0.52 In 0.48 P has smaller surface roughness than ErP on InP. Moreover, an ErP layer exists underneath the surface about 2 ML and the surface roughness does not depend on the ErP thickness in the range of our experiments (2.2–13.7 ML).

We report on the growth and characterization of Mn-doped cubic-GaN films. The n-type carriers, which are resulting from the formation of nitrogen vacancies, were intentionally utilized. For a 3% Mn sample with a high carrier density (∼1×1020 cm−3), we found substitutional Mn atoms on both the Ga-site and N-site, i.e., (Ga,Mn)N and Ga(N,Mn) exist together. A ferromagnetic behavior was observed in this sample at low temperature (∼5 K), although a 3% Mn semi-insulating sample in which Mn atoms substitute only the Ga-sites in the cubic-GaN lattice showed paramagnetism even at low temperature. The structural and magnetic properties of the relatively high Mn composition (⩾5%) samples were found to be governed by precipitate clusters of antiferromagnetic GaMn3N and ferromagnetic Mn4N.

The lowest limit for detection (LLD) of the impurity concentration doped in semiconductors in the case of fluorescence XAFS measurements has been investigated as a function of the matrix of the impurity and the geometry of the measurement. When the impurity concentration is very low and other background noise is well suppressed, X-ray resonant Raman scattering by the constituent atoms of the matrix remains as a major background for the fluorescence-detected XAFS measurement. For example, in the fluorescence-detected XAFS measurement for Er-doped semiconductors at the Er L(III)-edge, the LLD of the Er concentration was about 5 x 10(14) to 1 x 10(15) cm(-2) for GaAs and GaP, and lower than 1 x 10(14) cm(-2) for InP. The resonant Raman scattering of Ga atoms in the host semiconductor determines the LLD.

We have investigated local structures of ErP grown by organometallic vapor phase epitaxy Er source: tris(ethylcyclopentadienyl)erbium (Er(EtCp)3 by extended X-ray absorption fine structure (EXAFS) measurement. The EXAFS analysis revealed that NaCl-type ErP and Er–O(–C) compounds coexisted in the case of ErP growth by using Er(EtCp)3. The NaCl-type ErP was preferentially formed on InP(1 1 1)A compared with InP(0 0 1) and InP(1 1 1)B. It is considered that formation of unexpected Er–O(–C) compounds is due to low but significant concentration of residual O and/or C in Er(EtCp)3.

In this work, geometric structures for a half-metallic ferromagnet “zinc-blende CrAs”, which showed ferromagnetic behavior beyond room temperature, were investigated using fluorescence extended X-ray absorption fine structure (EXAFS) measurement. The EXAFS measurements revealed that As atoms around Cr atoms in the 2 nm CrAs film grown on a GaAs(0 0 1) substrate were coordinated tetrahedrally, indicating formation of zinc-blende CrAs. The Cr–As bond length in the zinc-blende CrAs is 2.49 A. This value is close to that which was estimated from the lattice constant (5.82 A) of ferromagnetic zinc-blende CrAs calculated by full-potential linearized augmented-plane wave method. The EXAFS analysis show that the theoretically predicted zinc-blende CrAs can be fabricated on GaAs(0 0 1) substrate by low-temperature molecular-beam epitaxy.

Local structures around Tb ions implanted in SiO 2 on Si, which exhibited strong green luminescence (∼540 nm) at room temperature, were investigated by using fluorescence X-ray absorption fines structure (XAFS) analysis at Tb L III -edge. The X-ray absorption near-edge structure spectra showed that Tb ions implanted in SiO 2 were incorporated as Tb 3+ . The extended X-ray absorption fines structure analysis revealed that two types of local structures coexisted, in which two and six oxygen atoms were coordinated around Tb ions, respectively. Amount of Tb–6O structure in the annealed sample increased compared with that in the as-implanted one. Photoluminescence measurement showed enhancement of luminescence intensity by annealing. Therefore, it is considered that formation of Tb–6O structure is related to the enhancement of luminescence intensity.

We have investigated geometric structure of Eu-doped GaN grown by OMVPE at different growth temperature by using fluorescence XAFS measurement in order to elucidate the relationship between the local structure around Eu atoms and the PL properties. It is found that majority of Eu atoms doped in GaN are fundamentally substituted on Ga-site in GaN lattice. Moreover, it is revealed that degree of disorder in the layer grown at 900°C is larger than that in the layers grown at 1000 and 1050°C. These results indicate that the local structure around Eu atoms is closely related to the luminescence properties of the Eu-doped GaN layers. [DOI: 10.1380/ejssnt.2011.51]

Electronic structures of Pt–Co cathode and Pt–Ru anode catalysts in membrane electrolyte assemblies (MEAs) for polymer electrolyte fuel cell have been investigated using X-ray absorption near edge structure (XANES) spectroscopy, and the changes of electronic structures accompanied with degradation have been observed by comparison between spectra obtained by fluorescence-yield (FY) and conversion-electron-yield (CEY) methods, probing depths of which are several hundreds μm and ∼100 nm, respectively. The Co K XANES spectra of the as-fabricated MEA show that the Co atoms in the cathode are metallic and oxidized Co ions exist at the interface between the cathode and electrolyte. The spectra of the long-time operated MEA suggest that the oxidation of Co makes progress with degradation of the cathode catalysts. In contrast to the Co K XANES spectra, the line shape of the Ru K XANES spectra is unchanged even after the long-time operation.

Novel short-period MnF2–CaF2 superlattices (SLs) on Si(111) substrates have been grown by molecular beam epitaxy. The thickness of a MnF2 layer was 1–3 molecular layers. Reflection high-energy electron diffraction studies indicated the fluorite type of crystal structure of these layers. Fluorescent extended X-ray absorption fine structure measurements supported this observation. Atomic force microscopy measurements showed a flat surface morphology of the SLs. X-ray diffraction measurements revealed well-pronounced superstructural reflections. The width of their ω-curve did not exceed 2.5 arc min, which indicated a good crystal quality of the SLs.

Local structures around Mn atoms in MnF2–CaF2 superlattices and double heterostructures (DHSs) grown by molecular beam epitaxy (MBE) were investigated by fluorescence extended X-ray absorption fine structure (EXAFS) measurement. The EXAFS analysis revealed that in thin MnF2 layers (≤3 ML) fluorite structure that does not exist at normal conditions prevailed over the rutile structure, and the content ratio of the fluorite MnF2 strongly depended on the thickness of the MnF2 layer. Therefore, the thin MnF2 layers grew in fluorite structure on CaF2 and changed to the rutile structure with the increase of the MnF2 layer thickness.

We have investigated Er- and O-doped GaAs samples by fluorescence extended X-ray absorption fine structure (EXAFS) measurements in order to clarify the local structures around the Er atoms. The EXAFS analysis revealed that in the range of Er concentration from 2×10 18 cm −3 to 3×10 20 cm −3 the majority of the Er atoms substituted Ga sublattice with two adjacent oxygen atoms (Er–2O center). On the contrary, in the same range of the Er concentration the PL intensity decreased rapidly with the increase of the Er concentration. The EXAFS analysis also indicated that the averaged arrangements of second or third neighbor atoms are different in each sample. Therefore, the Er–2O centers are considered to be further modified by the local structures of those second and even third nearest neighbors, which in turn change their optical characteristics.

Transparent SiC:Mn films were synthesized on 4H-SiC wafers and their magnetic and structural properties at the interface were investigated. It was found that a clear magnetic hysteresis loop can still be obtained in samples for which the top unintentionally synthesized second phases have been removed by etching. The temperature dependence of magnetization indicates that the etched sample has a Curie temperature (Tc) of 300 K. Further results from the extended X-ray absorption fine structure (EXAFS) data analysis suggest that the Mn incorporates solely on the interstitial site in the 4H-SiC lattice. Being optically transparent with the above properties, Mn-doped silicon carbide (SiC) emerges as a viable candidate for the development of spin electronics.

We have investigated OMVPE-grown InP doped with Er by fluorescence EXAFS (extended X-ray absorption fine structure) in order to study the local structures around Er atoms, so as to understand the luminescence property of Er atoms in semiconductors. The EXAFS analysis revealed that the Er atoms substituted into the In-site of InP, in the samples grown below 550°C, which exhibited high efficiency of luminescence, while the Er atoms constructed the NaCl-structure ErP, in the samples grown above 580°C, which exhibited low efficiency of luminescence.

[fcc-Fe/Cu] multilayers on Cu(001) were investigated by fluorescence EXAFS (extended X-ray absorption fine structure) in order to study relationship between local structures around the Fe atoms and their magnetic properties. The EXAFS analysis revealed that the Fe–Fe bond length of fcc-Fe becomes longer with the increase of the surface roughness of the Cu substrate, thus resulting in increase of the magnetic moment per Fe atom in fcc-Fe. The magnetic moments per Fe atom which were estimated from the volume ratio obtained by EXAFS results agreed with those measured by VSM.

We have investigated OMVPE-grown InP δ-doped with Er by EXAFS. The EXAFS measurement revealed that Er atoms which were δ-doped in InP by exposing to Er source for 15 min at 530°C (15 min Er-exposed sample) formed NaCl-structure ErP whose ErP bond length was 2.80 ± 0.03 A. The 10 min Er-exposed sample at 580°C showed a spectrum similar to cubic bixbyite structure Er 2 O 3 which was not observed in uniformly Er-doped InP samples. For the 40 min Er-exposed sample at 530°C, Er atoms formed both ErP and Er 2 O 3 .

Er-doped GaAs samples were grown by molecular beam epitaxy at 400°C and 590°C. The samples were investigated by photoluminescence (PL) and fluorescence extended X-ray absorption fine structure (EXAFS) measurements in order to find the relationship between the local structures around the Er atoms and the PL properties. Er-related PL peaks were observed only for the samples grown at a low temperature (400°C). The fluorescence EXAFS measurements showed the Er atoms were located on tetrahedral interstitial site when the growth temperature was low. On the other hand, when the growth temperature was high (590°C), the Er atoms substituted the Ga site in GaAs lattice.

(La0.6Sr0.4)(Co0.2Fe0.8)O3–δ (LSCF) has been promised as a cathode material of solid oxide fuel cells at intermediate temperatures. Despite the many previous studies of LSCF that have been reported, the role of Co and Fe atoms in the oxygen ion conduction is still unclear. In this work, we aimed at presenting each valence, oxygen chemical diffusion coefficient (Dchem) and activation energy (Ea) related to Co and Fe in LSCF by in situ X-ray absorption spectroscopy (XAS) at high temperatures and during reduction. For quantitative analysis of X-ray absorption near edge structure (XANES) spectroscopy, these results indicated that the Co valence decreased more easily than the Fe valence. On the other hand, from relaxation plots of the Co and Fe valence during reduction, the values of Dchem and Ea related to Co and Fe were nearly equal. Considering equations showing the oxygen ion conductivity, these results would indicate that oxygen ion conductivity was contributed by Co with more oxygen vacancies rather than Fe. According to these results, a structural model with and without oxygen vacancies and the oxygen ion conduction mechanism of LSCF was speculated, that is, we found that oxygen ion conductivity was more closely related to Co than Fe in LSCF by direct observations of in situ XAS.

A novel ferromagnetic II‐VI diluted magnetic semiconductor, (Cd,Cr)Te, has been combined with a two‐dimensional electron system. The anomalous Hall effect accompanied by the hysteresis loop was successfully observed in the Hall measurement. This fact shows the ferromagnetic exchange interaction between carrier and magnetic moment was realized in the present system.

The geometric structures for the Mn‐Si films synthesized on a 4H‐SiC homoepitaxial wafers have been investigated by fluorescence EXAFS measurements. The EXAFS analysis revealed that the synthesized Mn‐Si layer formed Mn5Si2 and the Mn atoms in the interlayer between the Mn‐Si and the SiC layer were incorporated on the interstitial site in the 4H‐SiC lattice.

The Restriction of the use of certain Hazardous Substances (RoHS) directive will take effect on July 1 of this year. From that date, the use of chromate conversion coatings containing hexavalent chromium will not be permitted. By comparing the concentration of Cr6+ determined by the diphenyl carbazide color test and by fluorescence XANES (X‐Ray Absorption Near Edge Structure) measurement, we can correct for the Cr6+ content of the color test. This will enable the use of the diphenyl carbazide color test to check product shipments in compliance with the RoHS directive.

For understanding the luminescence of Er atoms in III–V semiconductors, OMVPE-grown InP doped with Er has been investigated by fluorescence EXAFS (extended X-ray absorption fine structure) in order to study the local structure around Er atoms. The local structures around the Er atoms doped in InP, with doping as dilute as 3 × 1012 Er atoms in a 1.5 mm × 1.0 mm spot, were successfully measured by fluorescence EXAFS. The EXAFS analysis revealed that the Er atoms doped in InP above 853 K (which showed low luminescence) formed the rock-salt-structure ErP, while the Er atoms doped in InP below 823 K (which showed high luminescence) substituted on the In site of InP. The dependence of the local structure on growth temperature was observed for the samples doped with 3 × 1012 atoms and 1.2 × 1013 atoms of Er.

A MnSiC compound with a Curie temperature above room temperature is synthesized using the carbonization technique. EDX and EXAFS measurements indicate that the host manganese silicide is Mn/sub 4/Si/sub 7/. Carbonization is found to play an important role in the observed ferromagnetism and anomalous Hall effect although no effect of carbon on the structure is detected.

We have successfully observed radiant Er-related low-temperature photoluminescence (PL) dominated by numerous extremely sharp emission lines due to the intra-4f shell transitions of Er/sup 3+/ ions in Er-doped GaP (GaP:Er) grown by OMVPE with TBP. The intensity of the emission lines depended strongly on the growth temperature, the Er concentration and the reactor pressure, indicating coexistence of various Er-related luminescence centers in the samples. The fluorescence-detected EXAFS analysis on the samples revealed clearly that the majority of Er atoms doped are substitutionally incorporated into Ga sites in the GaP lattice. Effects of In-addition to GaP:Er have also been investigated.

Geometric structures for II-VI diluted magnetic semiconductor iodine-doped Zn1-xCrxTe films (x = 0.05) grown by molecular-beam epitaxy with high-temperature ferromagnetism were investigated by using fluorescence x-ray absorption fine structure (XAFS) measurement in order to elucidate the relationship between the geometric structure and the magnetic properties. The XAFS analysis has revealed that the local structures around Cr atoms are dependent on the growth temperature. For the samples grown at 360 and 390 °C, the majority of Cr atoms are tetrahedrally coordinated to Te atoms, indicating the formation of substitutional Cr on Zn-site in ZnTe lattice and/or zinc-blende (ZB) CrTe. On the contrary, for the samples grown at 300 and 330 °C the additional formation of secondary phases such as Cr-Te compounds was suggested. Therefore, it is deduced that the formation of secondary phase such as Cr-Te compounds are related to the increase of Curie temperature. The XAFS analysis has also revealed that the local structures do not depend on the I concentration. For the samples with different I concentrations, the Cr atoms form substitutional Cr on Zn-site in ZnTe lattice and/or ZB CrTe, corresponding to the result of TEM measurement.

The temperature dependence of the exchange bias has been investigated in Fe films deposited on the Si substrates with the native oxidize layer. A crossover from negative to positive exchange bias has been observed with increasing temperature at 30 K, accompanied with the maximum of the coercivity. This effect could be understood as a result of the exchange coupling between the ferromagnetic Fe film and the spin-glasslike Fe oxides particles formed spontaneously at the interface, where the direction of their end spins controls the sign of exchange bias.