Initial oxidation of Mn surface studied by ultraviolet photoelectron spectroscopy and metastable induced electron spectroscopy

Metastable induced electron spectroscopy (MIES) was combined to ultraviolet photoelectron spectroscopy (UPS) to study the initial steps of manganese oxidation. Oxygen exposure directly led to the formation of MnO with no intermediate states. The MnO feature saturation observed by MIES and UPS techniques showed noticeable differences and proved the formation of several oxide layers. The oxidation kinetics was studied by measuring MnO features by UPS, which depend on the surface coverage by oxygen. We observe a decrease of oxygen adsorption probability with oxygen exposure. Oxidation proceeds by oxygen dissolution into the first layers to form a three-dimension MnO. This hypothesis was confirmed by our work function measurements.     

Surface analysis of LiF films by electron stimulated desorption and x-ray photoelectron spectroscopy

Electron stimulated desorption and X-ray photoelectron spectroscopy studies of evaporated LiF films show that the films are Li-rich in the surface region (Li/F atomic ratio ∼ 3:1) with a contaminant overlayer which includes hydrocarbons and water. Substantial amounts of hydrogen are also directly bonded to LiF surface. Exposure of the films to RF-activated Ar plasma results in reduction of the Li/F stoichiometry by a factor of ∼ 2 and the removal of carbon from the surface. H₂O and possibly other oxygen-containing species remain.     

Surface valence changes in rare earths monitored by giant-resonance electron energy losses

Electron energy loss spectroscopy of 4d giant resonances in the rare earths is shown to be a sensitive monitor of valence changes in surface reactions with O₂ and S. Yb is found to be divalent on exposure to S but forms a trivalent oxide, while Gd is consistently trivalent. Eu forms a monosulfide, and an oxide of possibly mixed divalent and trivalent stoichiometry, while for Sm mixture of divalent and trivalent spectral features is consistently observed.     

Chemical analysis of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy

We studied the chemical state of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy (XPS), which were fabricated using dual-target magnetron sputtering by controlling the power applied to both metal and chalcogenide targets. On the basis of the valence band spectra obtained, it was suggested that semiconducting SmS has the final state corresponding to the Sm²⁺(4f⁶) configuration below the Fermi level, and metallic SmS has mainly the Sm³⁺(4f⁵) final state and a virtual band state in the Sm 5d band, contributing to the delocalization of 4f electrons and the emergence of metallic conductivity (4f⁶d⁰-4f⁵d¹). Thus, the spectra of our fabricated SmS thin films correspond to the band structure obtained from the dielectric property. This is the first work performed on the intrinsically prepared metallic SmS while the former works done for the sample transformed from semiconductor to metal phase by hard polishing.     

Bandwidth of free electron metals as measured by photoelectron spectroscopy

Recent accurate measurements of the bandwidth of nearly free electron metals Na, Mg and Al by photoelectron spectroscopy show reductions from calculated bandwidth. It is suggested that at least part of this discrepancy can be accounted for by taking the Mahan-Nozières-DeDominicis effect into account.     

Influence of an electrostatic potential at the metal/electrolyte interface on the electron binding energy of adsorbates as probed by X-ray photoelectron spectroscopy

We present a study of the influence of an electrostatic potential across the Au(1 1 1)/4,4′-dithiodipyridine (PySSPy)-SAM/0.01 M NaOH interface on the electron binding energies of the N 1s and S 2p orbitals of the adsorbate, as probed by X-ray photoelectron spectroscopy (XPS). The observed systematic binding energy shift for N(1s) with electrode potential of −0.9 eV/V has been assigned to a so-called electrochemical shift, i.e., an apparent shift of the binding energy due to the potential drop in the electric double layer. No systematical shift was observed for the S(2p) core level of the thiol with electrode potential. The results are contrasted by measurements in 0.1 M H₂SO₄, where no electrochemical shift was found for the protonated nitrogen.     

Elucidation of surface structure and bonding by photoelectron spectroscopy?

Information obtained on the adsorption of small and medium-sized molecules (CO, O₂, CO₂, NO, C₂H₄, C₆H₆) at three metal surfaces (Mo, W, Ni) by X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) is discussed in an attempt to establish what has been achieved, and what might be achieved in the elucidation of adsorption bonding at metal surfaces. Some of the individual results have been published by the present author with co-workers, and by other authors, but some results represent new work. Emphasis is placed on the detection of different states of adsorption, and then on interpretations of structure and bonding. The latter is divided into two areas, empirical interpretation by analogy between different adsorption systems, and in more absolute terms by consideration of the differences in electronic energy levels between the absorbate molecule in its gaseous and adsorbed state as well as differences between the metal levels in the clean and surface bonded state. Note is made of the problems of suitable reference levels and the phenomena of relaxation energies. Mo and W are taken as representative examples of adsorption confined usually to the monolayer regime; Ni is taken as an example of the situation where further reaction may occur. Most of the work described refers to polycrystalline films since little work has yet been done by XPS on adsorption at single crystal surfaces. A short general review of the quantitative aspects of XPS and UPS for analysis is given.     

Formation of surface adspecies at furan and tetrahydrofuran adsorption on the Pt surface studied by work function changes and Auger electron spectroscopy

The adsorption of tetrahydrofuran (THF) and furan (F) on polycrystalline Pt-foil has been studied by work function change (Δφ) measurements and Auger electron spectroscopy (AES). The results were compared with those obtained earlier for the adsorption of cyclic hydrocarbons. The general character of the adsorption process (i.e. the dependence of ∣Δφ∣ and C/Pt ratio on temperature and exposure, and the ∣Δφ∣ vs. C/Pt correlation) is similar to the one observed during adsorption of cyclic hydrocarbons. THF produces σ-bonded adspecies in a similar manner to cyclopentane. F forms first a π-bonded surface complex, as do benzene and toluene. The π-adsorbed species are then transformed into σ-bonded ones under exposure to excess F. The numeric values characterizing the adsorption of the two heterocycles (mainly the C/Pt values) differ substantially from those measured for the adsorption of cyclic hydrocarbons.     

Near surface composition of some alloys by X-ray photoelectron spectroscopy

Chemical compositions of the alloys of CuNi (Cu_0.10Ni_0.90, Cu_0.30Ni_0.70, Cu_0.70Ni_0.30) and BiSb (Bi_0.80Sb_0.20, Bi_0.64Sb_0.34, Bi_0.55Sb_0.45) are determined by X-ray photoelectron spectroscopy. The stoichiometries are determined and are compared with the bulk compositions. Possible sources of systematic errors contributing to the results are discussed. Errors arising out of preferential etching in these alloys have been investigated. It has been inferred from such studies that the preferential etching does not enrich the surface composition with a particular component for the two systems reported here. Quantitative results of CuNi system indicate that the surface regions of the Cu_0.70Ni_0.30 alloy is Cu-rich, although no such evidence is observed in case of BiSb system.     

Characterization of thin films on the nanometer scale by Auger electron spectroscopy and X-ray photoelectron spectroscopy

We describe two NIST databases that can be used to characterize thin films from Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) measurements. First, the NIST Electron Effective-Attenuation-Length Database provides values of effective attenuation lengths (EALs) for user-specified materials and measurement conditions. The EALs differ from the corresponding inelastic mean free paths on account of elastic-scattering of the signal electrons. The database supplies “practical” EALs that can be used to determine overlayer-film thicknesses. Practical EALs are plotted as a function of film thickness, and an average value is shown for a user-selected thickness. The average practical EAL can be utilized as the “lambda parameter” to obtain film thicknesses from simple equations in which the effects of elastic-scattering are neglected. A single average practical EAL can generally be employed for a useful range of film thicknesses and for electron emission angles of up to about 60°. For larger emission angles, the practical EAL should be found for the particular conditions. Second, we describe a new NIST database for the Simulation of Electron Spectra for Surface Analysis (SESSA) to be released in 2004. This database provides data for many parameters needed in quantitative AES and XPS (e.g., excitation cross-sections, electron-scattering cross-sections, lineshapes, fluorescence yields, and backscattering factors). Relevant data for a user-specified experiment are automatically retrieved by a small expert system. In addition, Auger electron and photoelectron spectra can be simulated for layered samples. The simulated spectra, for layer compositions and thicknesses specified by the user, can be compared with measured spectra. The layer compositions and thicknesses can then be adjusted to find maximum consistency between simulated and measured spectra, and thus, provide more detailed characterizations of multilayer thin-film materials. SESSA can also provide practical EALs, and we compare values provided by the NIST EAL database and SESSA for hafnium dioxide. Differences of up to 10% were found for film thicknesses less than 20 Å due to the use of different physical models in each database.     

Surface analysis of Ni/Al alloys by X-ray photoelectron spectroscopy

The surface composition of various pretreated NiAl alloys was investigated by the XPS method. Surface enrichment of Al is observed in freshly polished samples. Storage in air (or H₂O), especially at elevated temperatures, greatly increases the thickness of the aluminium oxide (or aluminium hydroxide) layer and reduces the nickel intensities correspondingly. Treatment with NaOH dissolves aluminium out of the surface, which is thus left with a high nickel content. The changes in this surface caused by exposure in air and H₂O are observed.     

沉积态铀薄膜表面氧化的X射线光电子能谱

含铀(U)薄膜在激光惯性约束聚变的实验研究中有重要的用途.研究其在不同气氛下的氧化性能可以为微靶制备、储存及物理实验提供关键的实验数据.通过超高真空磁控溅射技术制备了纯U薄膜及金-铀(Au-U)复合平面膜,将其在大气、高纯氩(Ar)气及超高真空度环境中暴露一段时间后,利用X射线光电子能谱仪结合Ar~+束深度剖析技术考察U层中氧(O)元素分布及价态,分析氧化产物及机理.结果显示,初始状态的U薄膜中未检测到O的存在.Au-U复合薄膜中的微观缺陷减弱了Au防护层的屏蔽效果,使其在3周左右时间内严重氧化,产物为U表面致密的氧化膜及缺陷周围的点状腐蚀物,主要成分均为二氧化铀(UO_2).在高纯Ar气中纯U薄膜仅暴露6 h后表面即被严重氧化,生成厚度不均匀的UO_2.在超高真空度环境下保存12 h后,纯U薄膜表面也发生明显氧化,生成厚度不足1 nm的UO_2.Ar~+束对铀氧化物的刻蚀会因择优溅射效应而使UO_2被还原成非化学计量的UO_(2-x),但这种效应受O含量的影响.     

Multiplet splitting patterns exhibited by the first row transition metal oxides in X-ray photoelectron spectroscopy

High resolution photoelectron spectra from transition metal ions in TiO₂, V₂O₅, VO₂, V₂O₃, MnO, Mn₂O₃, MnO₂, Cr₂O₃, FeO, CoO, NiO, CuO, Cu₂O, FeSrO₃, and Cu doped CaTiO₃ were re-examined using a constrained curve fitting approach. Effective fits of the multiplet splitting present could be attained for the oxides with unfilled 3d bands if multiple final states were assumed. The type of transitions implied, also suggest the classification of these oxides during core level photoelectron emission as either; Mott–Hubbard (V₂O₅, VO₂, V₂O₃, Cr₂O₃, and FeO), intermediate (MnO, Mn₂O₃, and MnO₂) or charge transfer type compounds (FeSrO₃, CoO, NiO, CuO and Cu doped CaTiO₃). These transitions along with relationships defining the splitting energy with respect to the total spin and binding energy are discussed.     

Surface characterisation of carbon monoxide hydrogenation catalysts under reducing conditions,using X-ray photoelectron spectroscopy and auger electron spectroscopy

Promoted, fused iron catalysts, active for CO hydrogenation have been monitored by electron spectroscopy during reduction in 1 bar flowing hydrogen, using an integrated reaction cell/surface analysis system. XPS chemical shift data clearly reveal the extent of reduction and show that it is virtually complete after 6 h reaction at the highest temperature employed (673 K). Apart from reduction, hydrogen also causes segregation of many promoters to the surface with the following order of mobilities: K>Ca≅Si>Mg≅A1. Due to this segregation, surface oxide remains the dominant species, even after reduction, with a comparatively low level of metallic iron. The presence of bulk carbon in the catalyst was also indicated.     

Studies on the ZrO/W(100) surface by means of low-energy electron diffraction and X-ray photoelectron spectroscopy

The low work-function ZrO/W(100) surface was examined with the aim of understanding the reducing mechanism of the work function. Low-energy electron diffraction (LEED) was employed to analyze the surface atomic arrangement, and X-ray photoelectron spectroscopy (XPS) was used to identify the surface chemical condition. The ZrO/W(100) surface was made as follows: (i) around three monolayers of Zr were deposited on a clean W(100) surface, (ii) the sample was heat treated in an oxygen ambience of 1.3x10⁻⁵ Pa for several tens of minutes at 1500 K, and (iii) the sample was flash heated at 2000 K in ultrahigh vacuum (UHV). During heat treatment in O₂, the deposited Zr was oxidized to ZrO₂, and the LEED pattern formed was p(2×1). The work function increased to 5.3 eV. Subsequent flash heating in UHV changed the p(2×1) LEED pattern into a c(4×2) pattern, and transformed ZrO₂ into the so-called Zr–O complex, the oxidized level of which is between ZrO₂ and metallic Zr. A drastic decrease in the work function to 2.7 eV ensued. The angular dependence of XPS showed that the Zr–O complex segregated within a few monolayers at the surface.     

Alignment of the photoelectron spectroscopy beamline at NSRL

Tile photoelectron spectroscopy beamline at National Synchrotron Radiation Laboratory （NSRL） is equipped with a spherical grating monochromator with the included angle of 174. Three gratings with line density of 200, 700 and 1200 lines/mm are used to cover the energy region from 60 eV to 1000 eV. After several years＇ operation, the spectral resolution and flux throughput were deteriorated, and realignment was necessary to improve the performance. First, the wavelength scanning mechanism, the optical components position and the exit slit guide direction are aligned according to tile design value. Second, the gratings are checked by Atomic Force Microscopy （AFM） and then the gas absorption spectrum is measured to optimize the focusing condition of the monoehromator. The spectral resolving power E/AE is recovered to the designed value of 1000@244 eV. The flux at the end station for the 200 lines/ram grating is about 1010 photons/sec/200 mA, which is in accordance with the design. The photon flux for the 700 lines/mm grating is about 5 108 photons/sec/200mA, which is lower than expected. This poor flux throughput may be caused by carbon contamination on the optical components. The 1200 lines/ram grating has roughness much higher than expected so the diffraction efficiency is too low to detect any signal. A new grating would be ordered. After the alignment, the beamline has significant performance improvements in both the resolving power aim the flux throughput for 200 and 700 lines/ram gratings and is provided to users.