Application of factor analysis to XPS valence band of superparamagnetic iron oxide nanoparticles

X-Ray photoelectron spectra of nano-sized superparamagnetic iron oxide nanoparticles were examined with the aim to discriminate the different degree of iron oxidation. Careful analysis of the valence band regions reveals the presence of both Fe₃O₄ and Fe₂O₃. The application of factor analysis enabled us to extract the relative molar concentrations of these oxides in the nanoparticles. This is of particular interest in improving the magnetic properties of iron oxide nanoparticles whose superparamagnetic character can be optimized to obtain better contrast in images from nuclear magnetic resonance. As a result, the factor analysis allows tuning the nanoparticle synthesis conditions in order to obtain the optimal magnetic properties for imaging. Results obtained by the XPS valence band analysis were compared to the transmission electron microscopy, X-ray diffraction and Raman measurements.     

New interpretations of XPS spectra of nickel metal and oxides

A current interpretation of XPS spectra of Ni metal assumes that the main 6 eV satellite is due to a two hole c3d⁹4s² (c is a core hole) final state effect. We report REELS observation in AES at low voltages of losses (plasmons and inter-band transitions) corresponding to the satellite structures in Ni metal 2p spectra. The satellite near 6 eV is attributed to a predominant surface plasmon loss. A current interpretation of Ni 2p spectra of oxides and other compounds is based on charge transfer assignments of the main peak at 854.6 eV and the broad satellite centred at around 861 eV to the cd⁹L and the unscreened cd⁸ final-state configurations, respectively (L is a ligand hole). Multiplet splittings have been shown to be necessary for assignment of Fe 2p and Cr 2p spectral profiles and chemical states. The assignments of Ni 2p states are re-examined with intra-atomic multiplet envelopes applied to Ni(OH)₂, NiOOH and NiO spectra. It is shown that the free ion multiplet envelopes for Ni²⁺ and Ni³⁺ simulate the main line and satellite structures for Ni(OH)₂ and NiOOH. Fitting the NiO Ni 2p spectral profile is not as straightforward as the hydroxide and oxyhydroxide. It may involve contributions from inter-atomic, non-local electronic coupling and screening effects with multiplet structures significantly different from the free ions as found for MnO. A scheme for fitting these spectra using multiplet envelopes is proposed.     

High resolution XPS studies on hexadecafluoro-copper-phthalocyanine deposited onto Si(1 1 1)7×7 surface

High resolution X-ray photoelectron spectroscopy measurements have been performed onto ultrathin films of hexadecafluoro copper phthalocyanine deposited, at room temperature and in ultrahigh vacuum conditions, onto clean Si(1 1 1)7×7 substrate (silicon, Si). The measurements, performed at various film thicknesses, show a strong interaction between the molecule and the Si substrate. All the core level peaks present strong modifications induced by the substrate interaction. In particular the fluorine (F) spectrum clearly presents the effect of the interaction of some F atoms of the molecule with the substrate, which determines the formation of F–Si bonds while the copper spectrum indicates a charge transfer from the Si substrate. The changes observed in the other core level spectra have been attributed to a different charge distribution in the molecule, after the formation of F–Si bonds. We suggest a planar growth of these molecules on the Si substrate starting from the first layer.     

High resolution XPS study of the large-band-gap semiconductor stibnite (Sb2S3): Structural contributions and surface reconstruction

Conventional X-ray photoelectron spectroscopy (XPS) and synchrotron radiation XPS (SRXPS) were used to probe the chemical state properties of stibnite (Sb₂S₃), a large-band-gap semiconductor of complex structure. The conventional spectra were obtained with a Kratos Axis Ultra XPS with magnetic confinement charge neutralization, which is very effective in minimizing both uniform charging and differential charging on this large-band-gap semiconductor. The narrow linewidths (much narrower than previously obtained) for single doublet fits (e.g. Sb 4d_5/2 of 0.57 eV and S 2p_3/2 of 0.63 eV) enabled the observation of a small peak on the low binding energy side of the Sb 3d and Sb 4d lines. With the aid of the very surface-sensitive Sb 4d SRXPS spectra, these low energy peaks are assigned to small Sb metal clusters at the surface after cleavage; the signal for these clusters increases with X-ray dose on the sample.A detailed analysis of the Sb 4d and S 2p linewidths concludes that the Sb 4d_5/2 linewidth is larger than expected based on the inherent linewidth of the instrument and the Sb 4d lifetime width, and on comparison with the As 3d linewidth (0.52 eV) for the analogous As₂S₃. Also, the S 2p_3/2 linewidth is substantially broader than the Sb 4d_5/2 linewidth. These larger than expected linewidths are due to two structurally distinct Sb atoms and three structurally distinct S atoms in the Sb₂S₃ crystal structure. Accordingly, the Sb 4d and S 2p spectra have been fitted to two and three doublets respectively, and the linewidth for all peaks is 0.53 eV. Using recent molecular orbital calculations, the doublets have been assigned to the different structural Sb and S sites.     

The XPS valence band spectra of NbC

The x-ray photoemission valence band spectra of NbC have been measured and are compared with the x-ray emission spectra and with the results from band structure calculations. This comparison leads to a large enhancement of theC2 _p photoabsorption cross section (at=1,487 eV) compared to the value calculated for the free atom. The effect of the nonmetal vacancy in the valence band can be described very well with vacancy cluster calculations.     

基于XPS与XAS的稀磁半导体GaMnN电子结构研究

采用基于同步辐射技术的X射线光电子能谱(XPS)与X射线吸收谱(XAS)测试由金属有机化学气相沉积(MOCVD)技术制备的不同Mn掺杂浓度的稀磁半导体GaMnN薄膜的电子结构,探究Mn掺杂浓度对磁性原子Mn周围的局域环境和电子态等方面的影响,并阐述材料铁磁性变化的机理. XPS和XAS图谱分析表明：Mn²⁺和Mn³⁺共存于薄膜样品内,样品D中Mn²⁺占比高达70%-80%,N空位随Mn掺杂浓度增加而增多且N空位能够使空穴浓度降低,导致Mn 3d和N 2p轨道间的相互交换作用减小,从而减弱体系铁磁性.此外,Mn不同的掺杂浓度会影响GaMnN薄膜p-d耦合杂化能力的强弱,当掺Mn 1.8%时具有较强的p-d耦合杂化能力.     

Comparison of the background corrected valence band XPS spectra of Fe and Co aluminides and silicides with their electronic structures

The background corrected valence band XPS spectra and the electronic structures of FeAl, FeSi, CoAl and CoSi were studied. Clean surfaces of the polycrystalline samples were obtained by in situ fracturing of the samples in an XPS spectrometer. The energy loss parts of the Fe 2p, Co 2p and valence band spectra were removed by the deconvolution method using Al 2s or Si 2s spectra as response functions. CoAl exhibited a satellite peak in the Co 2p region, but the other compounds had no clear satellite peaks in the Co 2p and Fe 2p regions. The experimentally background corrected valence band spectra were compared with the calculated spectra using the first-principle band calculation. There were large discrepancies between the spectra above the binding energy of 5 eV. These indicated that the experimental spectra could not be explained by the electronic structures of the ground states alone.     

Valence band and core level X-ray photoelectron spectroscopy of lead sulfide nanoparticle-polymer composites

Valence band and core level X-ray photoelectron spectroscopy (XPS) were used to probe lead sulfide (PbS) nanoparticle-polymer nanocomposites. Composite materials were prepared by trapping commercially available monodisperse 3 and 10 nm PbS nanoparticles in two polymers, the non-conducting polymer, polystyrene, and the conjugated polymer, poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene (referred to below as MEH-PPV). The nanocomposites prepared from commercial nanoparticles underwent oxidation, mainly to form lead sulfate. However, the narrow size distributions of the commercial nanoparticles allowed observation of distinct changes in the valence band from the 3 to 10 nm nanoparticles. Nanocomposites of 2-5 and 4-7 nm PbS nanoparticles were synthesized by growing the particles in poly(vinyl alcohol) (referred to below as PVA) and MEH-PPV, respectively. These composites both indicated the formation of lead sulfide nanoparticles. Furthermore, the XP spectra for the PVA/PbS composite displayed bonding between the PbS nanoparticles and the polymer while MEH-PPV showed no PbS-polymer bonding. The nanoparticles synthesized in MEH-PPV did not undergo oxidation. The particle size distributions of the synthesized nanoparticles were too broad to display size-dependent changes in the valence band.     

Electronic structure of non-centrosymmetric AgCd2GaS4 and AgCd2GaSe4 single crystals

X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion irradiated (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals grown, respectively, by the Bridgman method and the method of direct crystallization have been measured in the present work. The X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals. Electronic structure of AgCd₂GaS₄ has been calculated employing the full potential linearized augmented plane wave method. For the AgCd₂GaS₄ compound, the X-ray emission bands representing the energy distribution of the valence Ag d-, Cd d-, Ga p- and S p-like states were recorded and compared on a common energy scale with the XPS valence-band spectrum. The theoretical and experimental data regarding the occupation of the valence band of AgCd₂GaS₄ were found to be in excellent agreement to each other. Second harmonic generation (SHG) efficiency of AgCd₂GaS₄ by using the 320 ns CO laser at 5.5 μm has been recorded within the temperature range 80–300 K. Substantial increase of the photoinduced SHG which in turn is substantially dependent on the temperature has been detected for the AgCd₂GaS₄ compound.     

High resolution study of near-infrared emission spectra of NdO

High resolution near-infrared emission spectra of ¹⁴²NdO have been recorded using a Fourier transform spectrometer. Molecules were produced by heating ¹⁴²Nd₂O₃ to about 2100 K in a special furnace in presence of argon gas. Some 44 bands appear in the region between 7500 and 12 600 cm⁻¹, 19 of which classified into 11 systems involving 11 Ω states have been analyzed in rotation. Three of these systems were known from previous studies at moderate resolution which settled the symmetry Ω=4 of the ground state [L.A. Kaledin, E.A. Shenyavskaya, I. Kovacs, Acta Phys. Hung. 54 (1983) 189]. From the present analysis, information is obtained for the ground state in the levels v=0,1,2, and for the low-lying (1)5, (1)3, and (2)4 states in the levels v=0,v=0,1, and v=0, respectively; the (1)5 and (1)3 states show measurable Λ-doubling. The seven upper states either are perturbed or present anomalous Λ-doubling in the levels investigated; for three of them the perturbations could be reduced using a simple 2 × 2 interaction model. Their Ω symmetry could be ascribed (two states with Ω=3, four states with Ω=4, and one state with Ω=5).     

XPS analyses of lanthanides phosphates

XPS measurements were performed on lanthanide orthophosphates LnPO₄ (Ln: La, Ce, Nd, Gd), and correlated with XRD and some EDS analyses. Single lanthanide phosphates LnPO₄ and mixed lanthanide phosphates LnxLn^′1−xPO₄, all crystallized in a monoclinic structure similar to the monazite mineral. Results were examined qualitatively and quantitatively, by considering the Ln 3d lines, P 2p line and O 1s line.Ionic sputtering does not induce significant broadening nor an additional shift of the lanthanide peaks and does not have an effect on their chemical environments, except in the case of cerium. However, sputtering seems to significantly reduce the intensity of the O 1s and P 2p peaks, while Auger peaks of carbon and oxygen atoms interfere, respectively, with the 3d line of gadolinium and neodymium. Those phenomena are all the more important when the Nd and Gd content is weak and must be taken into account for a quantitative analysis of the spectra.The quantitative analyses reveal the expected results for single monazites. On mixed phosphates, the measured phosphorous and oxygen amounts are generally more valid for non-sputtered sample surfaces than for sputtered surfaces, while the calculated relative amount of lanthanides [Ln]/[Ln′] are correct in all the cases.One of the important goals of this work was to have a satisfactory value for the atomic ratio La/Gd, and particularly in the case of (La,Ce)PO₄ and (La,Gd)PO₄, to obtain good agreement between the calculated lanthanides ratio Nd/Gd and ratios estimated from XRD and EDS measurements.     

Screening effects in the Ti 2p core level spectra of Ti-based ternary nitrides

The electronic structure of Ti-based ternary nitrides, Ti-X-N, with X = Si, Al or Cr, has been investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Striking screening effects associated with the decrease of the occupied states at the Fermi level have been observed in the Ti 2p core level spectra of the Ti-Si-N and Ti-Al-N systems with decreasing the Ti content, due to the reduction of the screening ability of conduction electrons. As a consequence of this effect, a strong increase of the ratio between the satellite or “poorly-screened” peak and the main line or “well-screened” peak in the Ti 2p spectra is observed. This behaviour strongly supports the explanation proposed by Porte et al. for the origin of the Ti 2p satellite structure in nearly stoichiometric TiN_x films. Likewise, shifts of the Fermi edge have been observed in the Ti-Si-N and Ti-Al-N systems with decreasing the Ti content. These shifts could be associated with the formation of a nanocomposite structure in which metallic clusters are embedded in an insulator matrix, or just to reflect a metal-to-insulator transition in those ternary systems. Although, the core level and valence band spectra do not allow to univocally decide on this point. The comparative study of the evolution of Ti 2p spectra and of the occupied states at the Fermi level in Ti-Si-N and Ti-Al-N systems suggests that the disappearance of the well-screened peak upon decreasing the Ti content is related not only with the decrease of the occupied states at the Fermi level, but with the formation of isolated metallic clusters embedded in an insulating matrix. The formation of small isolated TiN clusters could change the degree of localization of screening orbitals, therefore promoting the satellite at the expense of the main line in the Ti 2p core level spectra.     

XPS characterization of naturally aged wood

Wood deterioration over time (by a simultaneously biological, chemical or physical attack) is an inevitable continuous process in the environment. This process destroys all heritage resulting in a loss of valuable old wooden structures and their properties. What type of deterioration occurs and how these processes impact the wood are important questions that need consideration if old wooden structures are to be studied and properly preserved.X-ray photoelectron spectroscopy (XPS) was employed to analyze the undegraded (sound wood of ∼6 years) and degraded lime wood (∼150 years, ∼180 years, ∼250 years) from painting supports, differing in terms of the provenance, conservation status and environmental conditions of storage. Elaborated XPS analysis (comparison of C and O individual spectra, decomposition for each atomic component, calculation of O/C ratio) provided a view of the composition of the sample surfaces analyzed. On the basis of these results, it was confirmed that significant changes occurred in the first period of ageing, the ∼150 years lime wood sample having the highest percent of the carbon atoms and the lowest percentage of oxygen atoms and, respectively O/C ratio. According to our previous studies (X-ray diffraction, FTIR spectroscopy, analytical pyrolysis combined with gas chromatography/mass spectrometry and ESR-spectroscopy results), these features could be attributed to the fact that hemicelluloses and amorphous cellulose are degraded in time, whereas the crystalline fraction of cellulose decreases more slowly than the amorphous one. Consequently, the observation may be made that lignin is not so easily degraded under the environmental conditions where paintings are frequently exposed.     

Ag在CeO2-x(111)薄膜上的生长与电子结构

利用XPS和RPES技术研究了CeO_2-x(111)薄膜表面上的氧空位对Ag纳米颗粒的生长和电子结构的影响. XPS结果表明,室温下,Ag纳米颗粒在部分还原的CeO_2-x(111)薄膜上呈三维岛状生长, 并且岛密度比完全氧化的CeO₂(111)薄膜表面上的大, 说明氧空位可以作为Ag纳米粒子生长的中心. Ag3d_5/2芯能级的结合能随着Ag颗粒尺寸的减小而增大, 主要来源于终态效应的贡献. Ag和CeO_2-x     

XPS研究Nd表面氧化物的生长过程

用X射线光电子能谱(XPS)研究新鲜Nd表面在不同氧气和空气进气量的情况下,表面氧化物的生长过程.发现氧化层由氧化物、氢氧化物及表面化学吸附水三种成分,这三种成分的增加与进气量都存在正比对数关系,氧化物和吸附水在一定进气量后趋于饱和,而氢氧化物持续不断增加,最终氢氧化物占据主要地位. 关键词： X射线光电子能谱(XPS) Nd 氧化     

Core level and valence band studies of layered Ti3SiC2 by high resolution photoelectron spectroscopy

The ternary compund Ti₃SiC₂ is a prominent representative of a new class of layered ceramics whose extraordinary physical properties has attracted much attention in recent years. Ti₃SiC₂ is electrically and thermally highly conductive, elastically rigid, lightweight, and maintains its strength to high temperatures. It is furthermore damage tolerant and oxidation resistant. We have studied fractured surfaces of coarse-grained Ti₃SiC₂ by means of photoelectron spectroscopy at the MAX-lab synchrotron radiation facility in Lund, Sweden. High-resolution C 1s, Si 2p, Ti 2p, Ti 3s and Ti 3p core-level spectra are reported and interpreted in terms of crystallographic and electronic structure. Valence band spectra confirm the validity of recent band calculations.     

Molecular orientation of thiol-derivatized tetraphenylporphyrin on gold studied by XPS and NEXAFS

Tetraphenylporphyrins bearing four linkers consisting of thioacetyl-functionalized carbon chains were immobilized on a gold surface via thiolate-gold bonds using two different preparation routes. The structure of these molecular layers was characterized in detail with synchrotron radiation based core-level spectroscopy, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results show that the geometry of the molecular layers and the number of linkers that bind to the gold surface depends on the preparation schemes. The deprotection of the linkers through removal of the terminal acetyl group before the molecular adsorption (deprotected systems) resulted in porphyrins bound to the gold surface with on the average three linkers, their normal axis being tilted ∼38° with respect to the surface normal. On the other hand, porphyrin layers prepared directly with the acetyl group still in place on the linkers (protected systems) are made of molecules bound to the gold surface via two linkers on the average. The resulting orientation is more upright, with the normal axis of the porphyrin plane tilted ∼50° with respect to the surface normal. Moreover, NEXAFS measurements revealed that the acetyl deprotected porphyrin layers have a higher degree of ordering than the protected systems.     

The adsorption of oxygen and water on Ca and CaO films studied with MIES, UPS and XPS

Metastable induced electron spectroscopy (MIES), Ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS) are employed to study the adsorption of water on Ca and CaO films as well as the adsorption of oxygen on Ca films. Ca films are prepared by evaporation of Ca onto clean Si(1 0 0) substrates. CaO films are produced by Ca evaporation in an oxygen atmosphere at a substrate temperature of 400 °C. Gas adsorption on the Ca films at room temperature, both for oxygen and water, is initiated by complete dissociation of the impinging molecules leading to the formation of Ca–O bonds. Exposure to water furthermore leads to the formation of hydroxyl groups via hydrogen abstraction from water forming a complete surface layer. Hydroxyl groups are also formed upon exposure of CaO films to water, but to a significantly smaller amount compared to Ca films exposed to water.     

XPS valence band spectra of Nb3Sn,Nb and Sn

XPS valence band spectra of Nb₃Sn, Nb and Sn have been obtained under high resolution. The 4d band structure in the Nb₃Sn spectrum is similar to that seen in Nb metal.