Valence band of molybdenum by photoelectron spectroscopy

Experimental photoelectron spectra of a clean polycrystalline Mo surface excited by monochromatized Al K _α X-rays are presented. The spectra are compared with valence bands obtained by UPS and by band structure calculations within the 5 eV region below the Mo Fermi level. All results mentioned above display peaks at 0.3, 1.7, 2.8 and 4 eV belowE _F. The energy distribution of the valence band does not vary with photon energy and electron emission angle for the four different polycrystalline Mo surfaces compared. It is concluded that the four peaks representing the Mo valence band are predominantly of bulk origin.     

Photoemission from α and β phases of the GaAs(0 0 1)-c(4 × 4) surface

We prepared α- and β surface phases of GaAs(0 0 1)-c(4 × 4) reconstruction by molecular beam epitaxy (MBE) using As₄ and As₂ molecular beams, respectively, and examined them by angle-resolved ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation as an excitation source. The UPS valence band spectra and the XPS 3d core level data show pronounced differences corresponding to the surface composition and the atomic structure of the two phases, as proposed in the literature. In UPS, the β phase is characterized by an intensive surface state 0.5 eV below the top of the valence band at low photon energy, while an analogous peak in the α phase spectra is missing. The surface state is interpreted in terms of dangling bonds on As dimers. The As3d and Ga3d core level photoelectron lines exhibit phase-specific shapes as well as differences in the number, position and intensity of their deconvoluted components. The location of various atoms in the surface and subsurface layers is discussed; our photoemission results support models of the β phase and the α phase with As-As dimers and Ga-As heterodimers, respectively.     

X-ray diffraction and photoelectron spectroscopy studies of MoO2 as catalyst for the partial oxidation of isooctane

X-ray diffraction (XRD), X-ray photoemission (XPS) as well as ultraviolet photoemission (UPS) spectroscopy experiments on MoO₂ powders were carried out to examine the bulk, the core level energies, and the electronic structure of MoO₂ samples that were employed as catalysts for the partial oxidation of isooctane. Five fresh 0.5-g MoO₂ samples were exposed for 0, 0.5, 9, 20, and 43 h to identical reforming environments and their spent samples were analyzed using the techniques mentioned above. Our results indicate the rapid appearance of an intermediate Mo phase with a binding energy of 228.5 eV and whose concentration progressively increases with time. The oxidation state for this new phase was graphically estimated to approximately +2.6 and assigned to the compound Mo₂O₃, which forms on the catalyst surface as a result of its exposure to the reforming environment. The electronic structure probed by UPS reveals two bands, one at 1.62 eV and another at 0.55 eV below the Fermi level, that decrease with the increasing time on stream. These results correlate very well with the drop in the catalytic performance of MoO₂ for the partial oxidation of isooctane and with the decline in the concentration of dioxide (Mo⁴⁺) detected not only on the catalyst surface, but also in the bulk structure, as confirmed by our XRD analysis.     

O2的化学吸附对2H—MoS2(0001)清洁表面和离子溅射表面电子结构的影响

利用低能N⁺(0.5keV)离子轻微轰击2H-MoS₂(0001)清洁表面,从UPS(HeⅠ,HeⅡ)得到d电子峰向E_F移动,价带顶出现明显的“肩膀”或带尾,它随轰击时间的增加而增强,同时使d_(z²)带变宽。UPS的结果表明,这种表面在室温下有明显的O₂吸附活性,O₂吸附后这个肩膀明显下降。结合XPS,AES和LEED的研究,我们认为这个“肩膀”态与次表面原子层的Mo原子的d电子的暴露和最外表面原子层s原子空位缺陷的产生有关。这些新的表面电子态与加氢脱硫(HDS)催化活性中心有密切的关系。 关键词：     

Electronic structure of PdO studied by photoemission (XPS,UPS)

In this paper we present the results of photoemission studies (XPS and UPS) performed on a polycrystalline surface of PdO. The electron density of states (EDOS) deduced both from XPS and UPS (He_I and He_II) are very similar. The valence band of PdO, which differs significantly from the Pd one, can be built up by four structures located at 0.5 eV, 2.2eV, 4.5 eV and 6.5 eV below E_F. The various electronic contributions (p or d) in the band are considered and, in order to explain our spectra, we discuss several hypothesis taking into account the possible existence of satellite lines or crystal field effects. Our XPS and UPS spectra show that the energy bands of PdO are narrow (～ 2–3 eV), moreover the energy shift of the core levels (|ΔE^F_B| = 2 eV) is important : these results suggest that the correlations between the d electrons may be important in PdO.     

ELECTRONIC STRUCTURE OF BISMUTH MOLYBDENUM OXIDE SINGLE CRYSTAL Bi0.19MoO3

Single crystal Bi_0.19MoO₃ has been grown by fused salt electrolytic technique. X-ray powder diffraction shows that the unit cell parameters are: a=1.9985nm, b=0.4085nm and c=1.4437nm. The temperature dependence of resistivity demonstrates a semiconductor characteristic. X-ray photoemission spectroscopy studies provide that the valence band of Bi_0.19MoO₃ are made up of oxygen p_π and the π^*, π and σ bonding bands formed by orbital combination. The shoulder at 0.4 eV near the top of valence band may be formed from the non-bonding d_xy orbitals of some Mo atoms. The O1s core-electron spectrum reveals the presence of two inequivalent bonds of oxygen ions in Bi_0.19MoO₃. Bi4f core-level spectrum shows two bonding characters of Bi atoms in bismuth molybdenum oxide single crystals. Mo3d core-level spectrum could be decomposed into two kinds of valence states of molybdenum(Mo⁺⁵ and Mo⁺⁶).     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

Interaction of oxygen with a Mo(111) surface

Oxygen adsorption on a Mo(111) surface is investigated at low pressures (10^?7 to 10^?5 Pa) and room temperature by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS). In agreement with previous studies it is established that the surface is not reconstructed during adsorption and the oxygen forms no ordered structures. On the basis of kinetic and spectroscopy data, the formation of two adsorption states on the surface within 1 monolayer is established. The valence band of a clean surface is studied in detail. An attempt is made to ascribe the peaks obtained to definite d states. The interaction between O₂ and Mo(111) is discussed in terms of the results obtained and a comparison with the O₂/W(111) system is made.     

The interaction of oxygen with gadolinium: UPS and XPS studies

The interaction of oxygen with evaporated Gd films at 300 K has been studied for the first time using AlK α XPS and Hel and Hell UPS. The characteristic changes in the Gd(6s5d) and O(2p) valence bands, Gd(4f), Gd(5p) and Gd(4d) core levels, and O(2s) and O(1s) core levels were studied. Evidence is presented for the initial formation of an intermediate oxidation state at low exposure (characterized by a new Gd valence band with a maximum in the DOS at ～ 2.5 ev below E_F and an ～ 0.6 eV shift in Gd(4f)) prior to formation of Gd₂O₃ where the Gd(6s5d) valence band disappears completely, as expected for Gd³⁺. In the higher exposure range there is little further increase in the oxide thickness, which is estimated as $\text{? 20}\text{A}\text{?}$, but there is a slow replacement of O by OH, as characterized by a second O(1s) feature at 532.3 eV and OH 1π and 3σ orbitals in UPS at ～ 6.7 and 11.5 eV. The interpretations are supported by parallel studies on bulk Gd₂O₃ and by Ar⁺ sputtering experiments. Comparisons are made to other rare earth oxidation studies.     

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

Electronic structure of β-RbNd(MoO4)2 by XPS and XES

β-RbNd(MoO₄)₂ microplates have been prepared by the multistage solid state synthesis method. The phase composition and micromorphology of the final product have been evaluated by XRD and SEM methods. The electronic structure of β-RbNd(MoO₄)₂ molybdate has been studied employing the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). For the molybdate, the XPS core-level and valence-band spectra, as well as XES bands representing energy distribution of the Mo 4d- and O 2p-like states, have been measured. It has been established that the O 2p-like states contribute mainly to the upper portion of the valence band with also significant contributions throughout the whole valence-band region. The Mo 4d-like states contribute mainly to a lower valence band portion.     

助催化剂Co与单晶MoS2边缘面区域及离子溅射解理面相互作用

为了阐明Co原子在钼硫化态催化剂表面上的助催化作用,本文利用UPS,XPS和LEED,研究了Co-Mo催化剂活性相的层状硫化物半导体MoS₂单晶边缘面区域的成分较高的表面以及离子溅射解理面上过渡金属Co的亚原子单层淀积过程。在覆盖度为某个亚原子单层时,表面上存在的与淀积上去的Co有关的界面态,改变了E_F能级的钉扎位置(提高了0.30—0.35eV),使表面势垒下降,表面功函数减小。在E_F附近电子结构的明显变化和LEED研究的结果表明,在MoS₂表面的无序缺陷位置上,可能形成了有利于催化过程的Co-Mo-S类合金键型的活性相。 关键词：     

Reaction of molecular hydrogen with the 100 face of MoO3: II. Kinetics initiated by atomic hydrogen and characterization of the surface electronic state

In order to provide a physical background to the model proposed in Part I, the kinetics of the reaction between the 100 face of a MoO₃ single crystal and, first a mixture of 4% atomic hydrogen in H₂, and later molecular hydrogen only, has been studied. The rate processes of the activation step and of the stationary step are entirely comparable with those observed for one single crystal surface loaded with platinum particles. Thus atomic hydrogen in the gas phase as well as atomic hydrogen produced by the dissociation of molecular hydrogen on a Pt particle may prepare the favourable surface state able to dissociatively chemisorb molecular hydrogen, ruling out — once again — the classical model of the hydrogen spillover process. This “favourable” state has a Fermi level which is 0.25 eV lower than that of initial MoO₃, as shown by measuring the work function with a Kelvin probe. This lowering is in good agreement with the variation of the free energy between MoO₃ and H_1.6MoO₃, measured electrochemically by others. This suggests that the protons inserted into the surface layers transform the initial MoO₃ layers into layers with composition H_1.6MoO₃. The starting material is thus transformed into a biphasic system, the diffusion of the reaction boundary between the two types of layers being the overall rate limiting process. The Fermi energy of H_1.6MoO₃ being known, it is possible to show that in transformed surface layers the conduction and the valence bands overlap, in agreement with the approximate profile of this band observed by XPS for a reacted single crystal surface. The d character of this band would explain why molecular hydrogen can be dissociatively chemisorbed when this favourable surface state is obtained. The fast electron delocalization within the Mo-O-Mo bonds yields fast oscillations in the oxidation states of the molybdenum atom in the surface layer, accounting for the presence of “oxidized” and “reduced” sites. The formal equation observed in Part I for the rate of stationary step is therefore explained, the impinging H₂ molecules reducing temporarily the oxidized fraction of the surface.     

A study of surface reproducibility and impurity segregation using ion neutralization and photoemission spectroscopies

We have studied the reproducibility of electron spectra obtained from a series of Si(111) surfaces on lightly doped crystals that were subjected to a common sputtering procedure but different subsequent heat treatments. Each of the surfaces displayed a sharp 7 × 7 LEED pattern and showed no impurities above the minimum detection limit of our Auger electron spectrograph. Ion neutralization spectroscopy (INS) and photoemission (UPS) at ?ω = 16.8 and 21.2 eV were used to obtain the electron spectra. From the observed differences in the electron spectra, the known characteristics of these spectroscopies, and a comparison with theory we conclude that these surfaces had small and differing amounts of impurity located principally in the selvedge or near-surface bulk rather than directly on or in the surface monolayer. The surface was cleaner than the near surface bulk. Longer heating of one sample to higher temperatures brought to the surface detectable amounts of Mo impurity that had diffused into the crystal from the Mo mounting clamps, changing the LEED pattern to √3 × √3(R 30°) and producing large modifications of both the INS and UPS spectra.     

Reductive-annealing-induced changes in Mo valence states on the surfaces of MoO3 single crystals and their high temperature transport

Reduction of MoO₃ in extreme reducing condition is a way to achieve Mo metal. However, effect of less extreme reductive-annealing, where it allows to keep the crystal structure, on physical and chemical properties of MoO₃ has not been well-studied. In this work, we studied the evolution of Mo valence state during reductive annealing and its effect on high temperature transport. We found the formation of oxygen vacancies on surface of MoO₃ single crystals at the low temperature, which is evidenced by increase of Mo⁵⁺ and color change. In addition, formation of Mo⁴⁺ was at the elevated temperature. For understanding the relation between bulk conductivity and Mo valence state, real-time impedance spectroscopy is employed. Use of two different gases makes it possible to distinguish impedance responses of MoO₃ from those of reduced MoO_3-x. Also, from time-dependent impedance measurements, we observed the evolution of transport behaviour by evolution of Mo valence state.     

The chemisorption of N2 on the (110) surface of iridium

The molecular chemisorption of N₂ on the reconstructed Ir(110)-(1 × 2) surface has been studied with thermal desorption mass spectrometry, XPS, UPS, AES, LEED and the co-adsorption of N₂ with hydrogen. Photoelectron spectroscopy shows molecular levels of N₂ at 8.0 (5σ + 1π) and 11.8 (4σ) eV in the valence band and at 399.2 eV with a satellite at 404.2 eV in the N(1s) region, where the binding energies are referenced to the Ir Fermi level. The kinetics of adsorption and desorption show that both precursor kinetics and interadsorbate interactions are important for this chemisorption system. Adsorption occurs with a constant probability of adsorption of unity up to saturation coverage (4.8 × 10¹⁴ cm^?2), and the thermal desorption spectra give rise to two peaks. The activation energy for desorption varies between 8.5 and 6.0 kcal mole^?1 at low and high coverages, respectively. Results of the co-adsorption of N₂ and hydrogen indicate that adsorbed N₂ resides in the missing-row troughs on the reconstructed surface. Nitrogen is displaced by hydrogen, and the most tightly bound state of hydrogen blocks virtually all N₂ adsorption. A p1g1(2 × 2) LEED pattern is associated with a saturated overlayer of adsorbed N₂ on Ir(110)-(1 × 2).     

Spectroscopic analysis of CIGS2/CdS thin film solar cell heterojunctions on stainless steel foil

This paper presents a spectroscopic analysis of the interface between a CuIn_1−xGa_xS₂ (CIGS2) absorber and a CdS buffer layer on stainless steel foil by Auger electron spectroscopy (AES), inverse photoemission spectroscopy (IPES) and photoelectron spectroscopy (PES) such as X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). By combining these spectroscopic techniques, detailed information about the electronic and chemical properties of the CIGS2 surface and the CdS/CIGS2 interface can be obtained. The gallium concentration in CIGS2 films was found to increase continuously towards the Mo back contact. XPS analysis showed the presence of KCO₃ on the surface of CdS, deposited on etched and un-oxidized samples indicating diffusion of potassium. No potassium was observed on oxidized as well as samples having thicker CdS (50 nm) indicating the effectiveness of oxidation and chemical bath deposition (CBD) process in cleaning the sample surface effectively. In addition, investigation of the electronic level alignment at the interface has been carried out by combining PES and IPES. Conduction band offset of −0.45 (±0.15) eV and a valence band offset of −1.06 (±0.15) eV were measured. These unfavorable conditions limit efficiency of CIGS2 thin film solar cells.     

Fundamental processes of aluminium corrosion studied under ultra high vacuum conditions

Surface sensitive electron spectroscopy was applied to study the fundamental processes of aluminium corrosion. We used metastable induced electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) for the investigation of the densities of states of surface and bulk, respectively. Furthermore we applied X-ray photoelectron spectroscopy (XPS) to investigate the chemical composition of the top surface layers. All measurements were performed under ultra high vacuum conditions.Al films with thicknesses of 7 nm were investigated. Both the interaction of oxygen and water with these films leads to the formation of an aluminium-oxygen layer, which is partly composed of stoichiometric Al₂O₃. Weak heat treatment at 770 K transforms the surface layer into Al₂O₃ with a thickness of about 2 nm. Further gas offer does not lead to an increase of this thickness, neither for oxygen nor for water. Additional to the oxygen offer, water exposure leads to the formation of OH species in the top aluminium-oxygen layer to a small amount. Weak heat treatment to 770 K removes this species completely. Water exposure leads to a much faster oxide formation than oxygen exposure. We try to give a model for the fundamental corrosion processes on a molecular scale.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.     

Alkali metal intercalation into SnS2

The intercalation of sodium and potassium into the layered semiconductor SnS₂ has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). After deposition of the alkali metals onto (0001) cleavage planes of SnS₂ in ultra high vacuum (UHV), semiconducting intercalation phases were formed. They seem to be homogeneous and disordered under the given experimental conditions. The valence electrons of the alkali metals are transferred into electronic states of the host lattice, whose valence band density of states changes significantly during intercalation. The underlying changes of the binding properties of the host lattice are discussed. The course of intercalation can be separated into three phases. During an induction period the concentration of the alkali metal on the surface remains very small, the electronic states of the substrate are shifted by band bending. During an intercalation period the topotactic reaction proceeds. After reaching saturation compositions of the intercalation phase at the surface, the alkali metal diffuses into the bulk. Crystal or surface defects seem to have a significant influence on the kinetics of intercalation and on the stoichiometry of the intercalation compounds.