2p X-ray Absorption Spectroscopy Research Articles

Disorder and mixed valence properties of Sr2FeMoO6 studied by photoelectron diffraction and x-ray absorption spectroscopy

A single crystal of the double peroviskite Sr2FeMoO6 has been prepared and was studied by using two-dimensional photoelectron diffraction (2DPED) and x-ray absorption spectroscopy (XAS). We found clear evidence for disorder of Fe3+/Mo5+ by 2DPED experiments, supporting a disorder picture for Sr2FeMoO6. On the other hand, Fe 2p XAS spectra suggested the existence of Fe2+ components (i.e. the mixed valence states) at the surface, but it was suppressed in the bulk. It seems that this fact has complicated a controversy so far concerning the reduction of saturation magnetization of this material.

Direct experimental evaluation of ligand-induced backbonding in nickel metallacyclic complexes.

The details of ligand-induced backbonding in nickel diphosphine π complexes are explored using nickel L-edge (3d←2p) X-ray absorption spectroscopy as a means of quantifying the degree of backbonding derived from direct Ni 3d donation into the π ligand. It is observed that backbonding into weakly π acidic ligands such as alkenes and arenes is dominated by contributions from the diphosphine ligand via σ-donation, leading to activated metallacycles with a Ni(0) d10 metal centre. With more strongly π acidic ligands, however, metal contributions to backbonding increase substantially leading to a more electron-deficient metal centre that is best described as having a Ni(i) spectroscopic oxidation state.

XMCD and PES study of a compensated-ferrimagnetic half-metal Mn3Ga

By employing soft X-ray magnetic circular dichroism (XMCD), soft X-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES), we have investigated the electronic structure of the candidate zero-moment half-metallic Mn3Ga. We have studied the ball-milled and annealed Mn3Ga powder samples that exhibit nearly zero magnetization. Mn 2p XAS revealed that Mn ions in Mn3Ga are nearly divalent for both of the Mn ions having the locally octahedral symmetry and those having the locally tetrahedral symmetry. The measured Mn 2p XMCD spectrum of Mn3Ga is very similar to that of ferrimagnetic MnFe2O4 having divalent Mn ions. The sum-rule analysis of the Mn 2p XMCD spectrum shows that both the spin and orbital magnetic moments of Mn ions in Mn3Ga are negligibly small, in agreement with the nearly compensated-ferrimagnetic ground state of Mn3Ga. The valence-band PES spectrum of Mn3Ga agrees well with the calculated density of states, supporting the half-metallic electronic structure of Mn3Ga.

Experimental evidence for mixed-valent Cr ions in half-metallic CrO2: Temperature-dependent XMCD study

By employing soft X-ray magnetic circular dichroism (XMCD) and soft X-ray absorption spectroscopy (XAS), we have investigated the temperature (T)-dependent valence and spin states of Cr ions in half-metallic ferromagnet CrO2 nanorod powder. The lineshape-analysis of both the Cr 2p XAS and XMCD spectra of CrO2 reveals that the Cr ions in bulk CrO2 have both Cr4+t2g2↑ and Cr3+t2g3↑ valence configurations. This work provides the direct experimental evidence for the Cr3+-Cr4+ mixed-valent states in bulk CrO2(Cr3.8+), demonstrating that the half-metallic ferromagnetism in CrO2 originates from the double-exchange interaction. The T-dependent intensity variation of the Cr 2p XMCD spectrum agrees well with the bulk magnetization M(T) and the Curie temperature, indicating that the measured Cr 2p XMCD data represent the intrinsic bulk ferromagnetism in CrO2, and support the above finding of the XMCD lineshape analysis.

Origin of the enhanced exchange bias in polycrystalline-BiFeO3/Co bilayers by X-ray absorption Spectroscopy

Polycrystalline-BiFeO3(BFO)/Co bilayers were grown by pulsed laser deposition (PLD) and magnetron sputtering, with fast laser annealing under magnetic field. The enhanced exchange bias (EB) had been found in the BFO/Co bilayers (Appl. Surf. Sci. 367 (2016) 418). In order to reveal the origin of the enhanced EB in the samples, X-ray absorption Spectroscopy (XAS) of Fe 2p, Co 2p and O 1s were performed. The Co 2p XAS indicated the increase of Co oxidation state and the Fe 2p XAS of sample A and B under laser annealing processes showed that crystal field splitting energy decreased and led to the weakening of spin-orbit coupling with the increasing of the laser fluence. It was considered that the appearance of the oxidation state of Co and Fe2+ ions and the existence of the unidirectional anisotropy due to the laser fluence was responsible for the results and also for the enhanced EB.

Resonant photoemission and X-ray absorption spectroscopies of lithiated magnetite thin film

Resonant photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) were used to investigate the effect of lithiation on the electronic structure of Fe3O4 thin film relevant to the operation mechanism of nanoionic devices to enable magnetic property tuning. Comparison of the Fe 2p XAS spectrum for lithiated Fe3O4 (Li–Fe3O4) with that for pristine Fe3O4 clearly demonstrated that the number of Fe2+ ions at octahedral B sites is increased by lithiation. The valence band RPES spectra of Li–Fe3O4 further showed that lithiation increases the density of states near the Fermi level originating Fe2+ ions at octahedral B sites. These findings agree well with the observed decrease in the saturation magnetization in the magnetization–magnetic field (M–H) loop of Li–Fe3O4 thin film, indicating that minority spins (down spins) increase (i.e., total spins decrease) due to lithiation. The variation in the number of Fe2+ ions at B sites is suggested to be an underlying operating mechanism of a nanoionics-based magnetic property tuning device.

반쪽 금속 호이슬러 화합물 Mn3Ga의 연 X선 방사광 분광 연구

이 연구에서는 방사광을 이용한 연 X-선 흡수 분광법(soft X-ray absorption spectroscopy: XAS)과 광전자 분광법(photoemission spectroscopy: PES) 을 이용하여 반쪽금속 반강자성체 후보 물질인 <TEX>$Mn_3Ga$</TEX> 호이슬러 화합물의 전자구조를 연구하였다. 이 연구에 사용된 시료는 full-Heusler <TEX>$Mn_3Ga$</TEX>로 ball milling 후 열처리하지 않은 시료와 ball milling 후 <TEX>$400^{\circ}C$</TEX>에서 열처리한 두 시료를 사용하였다. XAS 분석에 의하면 <TEX>$Mn_3Ga$</TEX>에서 Mn 이온들의 원자가는 <TEX>$Mn^{2+}$</TEX> 상태임을 알 수 있었으며, 국소적으로 팔면체 대칭성을 가진 Mn 이온들과 사면체 대칭성을 가진 Mn 이온들이 섞여 있음을 알 수 있었다. 그리고 <TEX>$Mn_3Ga$</TEX>의 가전자띠 PES 스펙트럼은 전자구조 계산에 의한 반쪽 금속성 상태밀도와 대체로 유사함을 발견하였다. By employing photoemission spectroscopy (PES) and soft X-ray absorption spectroscopy (XAS), the electronic structure of the candidate half-metallic antiferromagnet of <TEX>$Mn_3Ga$</TEX> Heusler compound has been investigated. We have studied two ball-milled <TEX>$Mn_3Ga$</TEX> powder samples, one after annealing and the other without annealing, respectively. Based on the Mn 2p XAS study, we have found that Mn ions are nearly divalent in <TEX>$Mn_3Ga$</TEX> and that the Mn ions having the locally octahedral symmetry and those having the locally tetrahedral symmetry are both present in <TEX>$Mn_3Ga$</TEX>. We have found relatively good agreement between the measured valence-band PES spectrum of <TEX>$Mn_3Ga$</TEX> and the calculated density of states, which is in agreement with the half-metallic electronic structure of <TEX>$Mn_3Ga$</TEX>.

Pd층의 두께 변화에 따른 [Co/Pd] 다층박막의 연엑스선 방사광 분광 연구

We have investigated the electronic structures of intermetallic multilayer (ML) films of [Co(2A)/Pd(xA)] (x: the thickness of the Pd sublayer; x = 1 A, 3 A, 5 A, 7 A, 9 A) by employing soft X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD). Both Co 2p XAS and XMCD spectra are found to be similar to one another, as well as to those of Co metal, providing evidence for the metallic bonding of Co ions in [Co/Pd] ML films. By analyzing the measured Co 2p XMCD spectra, we have determined the orbital magnetic moments and the spin magnetic moments of Co ions in [Co(2A)/Pd(xA)] ML films. Based on this analysis, we have found that the orbital magnetic moments are enhanced greatly when x increases from 1 A to 3 A, and then do not change much for x ≥ 3 A. This finding suggests that the interface spin-orbit coupling plays an important role in determining the perpendicular magnetic anisotropy in [Co/Pd] ML films.

Soft X-ray absorption spectroscopy study of multiferroic Bi-substituted Ba1−x Bi x Ti0.9Fe0.1O3

The electronic structures of multiferroic oxides of Ba(1-x)Bi(x)Ti(0.9)Fe(0.1)O(3) (0 < x < 0.12) have been investigated by employing photoemission spectroscopy and soft x-ray absorption spectroscopy (XAS). The measured Fe and Ti 2p XAS spectra show that Ti ions are in the Ti4+ states for all x and that Fe ions are Fe2+-Fe3+ mixed-valent for x > 0. The valence states of Fe ions are found to be nearly trivalent for x=0, and decreases with increasing x from being nearly trivalent (v(Fe)~ 3) for x=0 to v(Fe)~ 2.6 for x=0.12. The valence states of both Ti and Ba ions do not change for all x < 0.12. Based on the obtained valence states of Fe ions, the electronic and magnetic properties of Ba(1-x)Bi(x)Ti(0.9)Fe(0.1)O(3) are explored.

An in situ resonant photoemission and x-ray absorption study of the BiFeO3 thin film

Multiferroic bismuth ferrite (BiFeO3) thin films were prepared using the pulsed laser deposition (PLD) technique. The electronic structure of the film was studied in situ using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Both the Fe 2p PES and XAS spectra show that the Fe ions were initially in a+3 valence state. The Fe 2p and O K edge XAS spectra indicate that the oxygen octahedral crystal ligand field divides the unoccupied Fe 3d state into t2g↓and eg↓states. Valence band Fe 2p–3d resonant photoemission results indicate that hybridization between Fe 3d and O 2p plays an important role in the multiferroic properties of BiFeO3 thin films.

Direct observation of high-spin states in manganese dimer and trimer cations by x-ray magnetic circular dichroism spectroscopy in an ion trap.

The electronic structure and magnetic moments of free Mn2 (+) and Mn3 (+) are characterized by 2p x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap that is coupled to a synchrotron radiation beamline. Our results directly show that localized magnetic moments of 5 μB are created by 3d(5)((6)S) states at each ionic core, which are coupled ferromagnetically to form molecular high-spin states via indirect exchange that is mediated in both cases by a delocalized valence electron in a singly occupied 4s derived antibonding molecular orbital with an unpaired spin. This leads to total magnetic moments of 11 μB for Mn2 (+) and 16 μB for Mn3 (+), with no contribution of orbital angular momentum.

Thermal evolution of the band edges of 6H-SiC: X-ray methods compared to the optical band gap

The band gap of semiconductors like silicon and silicon carbide (SiC) is the key for their device properties. In this research, the band gap of 6H-SiC and its temperature dependence were analyzed with silicon 2p X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) allowing for a separate analysis of the conduction-band minimum (CBM) and valence-band maximum (VBM) components of the band gap. The temperature-dependent asymmetric band gap shrinking of 6H-SiC was determined with a valence-band slope of +2.45×10−4eV/K and a conduction-band slope of −1.334×10−4eV/K. The apparent asymmetry, e.g., that two thirds of the band-gap shrinking with increasing temperature is due to the VBM evolution in 6H-SiC, is similar to the asymmetry obtained for pure silicon before. The overall band gap temperature-dependence determined with XAS and non-resonant XES is compared to temperature-dependent optical studies. The core-excitonic binding energy appearing in the Si 2p XAS is extracted as the main difference. In addition, the energy loss of the onset of the first band in RIXS yields to values similar to the optical band gap over the tested temperature range.

Electronic structure of V2O3 thin film prepared by RF magnetron sputtering using oxygen radical and V-metal

We have prepared V2O3 thin films on Al2O3 (0001) substrate by RF magnetron sputtering using oxygen radical and V-metal target, and studied their electronic structures by X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The as-deposited V2O3 thin films deposited at 600 °C exhibits high crystallization by oxygen radical irradiation during deposition. The electrical resistivity decreases with increasing deposition temperature, and this result is due to decreasing electron scattering. The crystal field splitting (10Dq) of the V2O3 thin film estimated from the fitting of the V 2p XAS spectrum is larger than that of the bulk crystal. In the XPS spectrum of the valence band region, the density of state (DOS) of the orbital in the V 3d state increases at around the Fermi level. These results contribute to the lattice distortion between the thin film and the substrate.

Back Cover: A Multispectroscopic Study of 3 d Orbitals in Cobalt Carboxylates: The High Sensitivity of 2p3d Resonant X-ray Emission Spectroscopy to the Ligand Field (Angew. Chem. Int. Ed. 4/2013)

3d transition-metal-ion coordination is crucial in many chemical processes and in the understanding of their reaction mechanisms. In their Communication on page 1170 ff. M. M. van Schooneveld, F. M. F. de Groot et al. show for cobalt(II) carboxylates that synchrotron-based 2p3d resonant X-ray emission spectroscopy (RXES) allows for a far more judicious determination of the metal-ion coordination environment than benchmark ultraviolet/visible (UV/Vis) and 2p X-ray absorption (XAS) spectroscopy do.

A multispectroscopic study of 3d orbitals in cobalt carboxylates: the high sensitivity of 2p3d resonant X-ray emission spectroscopy to the ligand field.

Determination of the ligand coordination number and symmetry of a transition metal ion is important to understand reaction mechanisms in inorganic chemistry. The problem can be addressed through diffraction techniques or by spectroscopy. Here we limit ourselves to the latter and note that, historically, the problem has been studied with UV/Vis, or optical absorption or electronic spectroscopy.1 Recently however the field of resonant X-ray emission spectroscopy (RXES) developed at a high pace.2 Here we show that metal 2p3d RXES is highly sensitive to the metal ion ligand field. We present a comparison of UV/Vis, 2p X-ray absorption spectroscopy (XAS), and 2p3d RXES on a series of cobalt(II) carboxylates. The X-ray data were acquired at the state-of-the-art ADRESS beamline.3 We show that 2p XAS and UV/Vis have a limited discriminative power compared to 2p3d RXES. Through ligand field multiplet (LFM) calculations we show that 2p3d RXES allows the most judicious analysis of the ligand field. While previous 2p3d RXES studies on metal oxides revealed its d–d sensitivity,4 this is the first such observation on inorganic complexes. More importantly, the notion that 2p3d RXES measures element-selective, more as well as more intense d–d excitations than UV/Vis, and that this allows a more reliable determination of the ligand field, is novel. 2p3d RXES will allow unraveling reaction mechanisms of important 3d-metal-mediated chemical processes.

철원소를 함유한 분자기반 생체물질 나노입자들의 연 x선 방사광 분광 연구

이 연구에서는 헬리코박터 파이로리 페리틴, 헴, <TEX>$NaM[Fe(CN)_6]{\cdot}H_2O$</TEX> 프러시안 블루(M=Co, Ni) 등의 분자기반 생체물질 나노입자들의 전자 구조를 연구하기 위하여 방사광을 광원으로 사용한 연 x선 광흡수 분광(soft x-ray absorption spectroscopy: XAS)과 연 x선 자기 원편광 이색성(soft x-ray magnetic circular dichroism: XMCD) 분광법 실험을 수행하였다. 이 연구로부터 페리틴 나노 입자들을 구성하고 있는 Fe 이온들은 모두 거의 3가 (<TEX>$Fe^{3+}$</TEX>)의 원자가 상태에 있으며, 프러시안 블루 나노 입자들 내의 Fe 이온들은 <TEX>$Fe^{2+}-Fe^{3+}$</TEX>의 혼합원자가 상태에 있다는 것을 알 수 있었다. 또한 페리틴 내의 <TEX>$Fe^{3+}$</TEX> 이온들은 한 종류의 대칭성을 가진 국소 구조를 가지며, Fe 이온들의 자기모멘트는 모두 한 방향으로 정렬되어 있다는 사실을 발견하였다. 그리고 프러시안 블루 나노 입자들의 Fe 이온의 국소적 결합은 주로 <TEX>$(CN)^-$</TEX> 리건드와의 결합에 의하여 결정된다는 것을 분광학적으로 알 수 있었다. By employing soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD), the electronic structures of molecule-based nano bioparticles, such as Helicobacter pylori ferritin (H. pylori ferritin), Heme, <TEX>$NaM[Fe(CN)_6]{\cdot}H_2O$</TEX>-type Prussian Blue (M=Co, Ni) analogue, have been investigated. The measured Fe 2p XAS spectra reveal that Fe ions are trivalent (<TEX>$Fe^{3+}$</TEX>) in H. pylori ferritins, while they are in the <TEX>$Fe^{2+}-Fe^{3+}$</TEX> mixed-valent states in <TEX>$NaM[Fe(CN)_6]{\cdot}H_2O$</TEX> Prussian Blue analogues (M=Co, Ni). According to the Fe 2p XMCD spectrum of high-state H. pylori ferritin, all the <TEX>$Fe^{3+}$</TEX> ions have the same local symmetry and their magnetic moments are aligned in the same direction. It is also found that the Fe 3d orbitals in <TEX>$NaM[Fe(CN)_6]{\cdot}H_2O$</TEX> have a strong covalent bonding to <TEX>$(CN)^-$</TEX> ligands, but with a very weak bonding to the 2p orbitals of O ligands.

Copper and xanthate adsorption onto pyrite surfaces: Implications for mineral separation through flotation

The findings of synchrotron based spectromicroscopic investigations of Cu and xanthate adsorption onto pyrite surfaces are reported. Synchrotron based photoemission electron microscopy measurements have revealed that Cu adsorbs only onto specific surface sites on pyrite. The Cu 2p X-ray absorption near edge structure and position (2p3/2 peak at 934.3eV) confirms the presence of Cu+ with transitions from 2p to 4s states being apparent. Moreover, the peak position is typical of Cu+ bonded to S, Cu(I)S. This is confirmed by S 2p X-ray photoelectron spectroscopy with oxidised S2− species (S−), most likely resulting from electrochemical interaction of solution Cu2+ and surface S2− species, being observed. Fe 2p X-ray absorption spectroscopy shows that surface Fe does not take part in the Cu adsorption or subsequent restructuring processes.Synchrotron FTIR microscopy analysis of xanthate (potassium ethyl xanthate, KEX) adsorbed onto Cu activated surfaces also shows a heterogeneous distribution on pyrite. The use of a dilute (10−5M) KEX concentration shows Cu(I)–xanthate as the only surface xanthate containing species. This species may have formed through a chemical process involving previously adsorbed Cu+ and the xanthate from solution. When a greater (10−3M) concentration of KEX is used significant concentrations of Cu(I)–xanthate precipitate from solution onto the pyrite surfaces and cannot be easily removed. Under this condition, the predominant surface species remains Cu(I)–xanthate with smaller concentrations of diethyl dixanthogen also being present. The latter species may form in solution although some previous studies also suggest direct surface formation.

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.

Selective Response of Mesoporous Silicon to Adsorbants with Nitro Groups

We demonstrate that the electronic structure of mesoporous silicon is affected by adsorption of nitro-based explosive molecules in a compound-selective manner. This selective response is demonstrated by probing the adsorption of two nitro-based molecular explosives (trinitrotoluene and cyclotrimethylenetrinitramine) and a nonexplosive nitro-based aromatic molecule (nitrotoluene) on mesoporous silicon using soft X-ray spectroscopy. The Si atoms strongly interact with adsorbed molecules to form Si-O and Si-N bonds, as evident from the large shifts in emission energy present in the Si L(2,3) X-ray emission spectroscopy (XES) measurements. Furthermore, we find that the energy gap (band gap) of mesoporous silicon changes depending on the adsorbant, as estimated from the Si L(2,3) XES and 2p X-ray absorption spectroscopy (XAS) measurements. Our ab initio molecular dynamics calculations of model compounds suggest that these changes are due to spontaneous breaking of the nitro groups upon contacting surface Si atoms. This compound-selective change in electronic structure may provide a powerful tool for the detection and identification of trace quantities of airborne explosive molecules.

방사광을 이용한 FeV2O4스피넬 산화물의 덩치상태와 분말상태의 전자구조 차이 연구

연 x선 광흡수 분광법(soft x-ray absorption spectroscopy: XAS)과 연 x선 자기 원편광 이색성(soft x-ray magnetic circular dichroism: XMCD)을 이용하여 스피넬 준강자성 산화물인 <TEX>$FeV_2O_4$</TEX>의 전자 구조를 연구하였다. Fe 2p XAS와 V 2p XAS 측정으로 부터 <TEX>$FeV_2O_4$</TEX>에서 Fe 이온과 V이온의 고유한 원자가는 각각 약 <TEX>$Fe^{2.3+}$</TEX>의 혼합원자가 상태와 약 <TEX>$V^{3+}$</TEX>의 상태임을 알 수 있었다. 실험적으로 측정된 Fe 2p XMCD의 신호는 거의 <TEX>$Fe^{2+}$</TEX> 상태에서 기인하였으며, <TEX>$Fe^{3+}$</TEX> 상태는 Fe 2p XMCD의 신호에 거의 기여하지 않는다는 사실이 발견되었다. 그러므로 <TEX>$FeV_2O_4$</TEX>의 자성 특성을 결정함에 있어서 자기적으로 정렬된 <TEX>$Fe^{2+}$</TEX> 상태 이온들이 중요한 역할을 한다고 생각된다. The electronic structure of ferrimagnetic spinel oxide of <TEX>$FeV_2O_4$</TEX> has been investigated by employing soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD). The Fe 2p and V 2p XAS spectra show that the valence states of Fe and V ions are <TEX>${\sim}Fe^{2.3+}$</TEX> mixed-valent states and <TEX>${\sim}V^{3+}$</TEX> states, respectively. In Fe 2p XMCD spectra, finite XMCD signals are observed for divalent <TEX>$Fe^{2+}$</TEX> states only, but not for <TEX>$Fe^{3+}$</TEX> states. This finding indicates that the magnetic moments of <TEX>$Fe^{2+}$</TEX> ions are ordered ferromagnetically but that those of <TEX>$Fe^{3+}$</TEX> ions are cancelled, implying that <TEX>$Fe^{2+}$</TEX> ions play an important role in determining magnetic properties of <TEX>$FeV_2O_4$</TEX>.