Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型，用第一性原理计算方法，计算并分析了Pd在CeO₂(111)面上不同覆盖度时的吸附能，价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明：1)在单层吸附时，Pd的最佳吸附位置是O的顶位偏向Ce的桥位；在1/4单层吸附时，Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时，吸附原子Pd之间的作用较强；1/4单 关键词： 三元催化剂 Pd 2')" href="#">CeO₂ 吸附 密度泛函理论     

AES and ELS study of gold deposition on top of hydrogen saturated Si(1 1 1) surfaces

Gold was deposited on top of hydrogen saturated Si(1 1 1) clean surfaces. The electronic nature and atomic intermixing between gold overlayer and silicon substrate were studied by AES and ELS. It is claimed that there is a critical thickness (～ 2 ML), where 1 ML = 7.8 × 10¹⁴ atoms cm^?2 for Si(1 1 1), for gold to induce alloyed metallic overlayer formation on the surface of a specimen due to intermixing reaction independently whether a surface of Si(1 1 1) substrate is saturated with hydrogen atoms or not.     

A leed-aes study of thin Pd films on Si(111) and (100) substrates

A system Pd (deposit)-Si (substrate) has been studied by LEED and AES. Pd₂Si formed on Si(111) became epitaxial after a short time of annealing at a temperature between 300 and 700°C, while the Pd₂Si formed on Si(100) did not, in both cases the surfaces of the Pd₂Si being covered with a very thin Si layer. A sequence of superstructures (3√3 × 3√3), (1 × 1), and (2√3 × 2√3) was observed successively in Pd/Si(111) as the annealing temperature was increased. A (√3 × √3) structure was obtained by sputtering the 3√3 surface slightly. It was found that the √3 structure corresponds to Pd²Si(0001)-(1 × 1) grown epitaxially on Si(111), and that the 3√3 structure comes from the thin Si layer accumulated over the silicide surface, while the 2√3 and 1 structures arise from a submonolayer of Pd adsorbed on Si(111). Superstructures observed on a Pd/Si(100) system are also studied.     

Yttrium silicide films into Si(111) — Fabrication and properties

The formation of a thin layer of hexagonal Y Si_2?x phase on a single-crystal Si(111) substrate by implantation of 195 keV Y ions with a dose of 5×10¹⁶Y ⁺/cm² at room temperature (RT) is investigated. The structural characterization of the as-implanted and annealed samples is performed using Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) pole figure and cross-sectional transmission electron microscopy (XTEM). The results show that the orientation relationship between the Y Si_2?x layer and Si substrate is Y Si_2?x(0 0 0 1)//Si(111) and Y Si_2?x[1 1 -2 0]//Si [110].     

Giant core-exciton effects on Si(111) 7 × 7 surfaces

We present the first direct experimental evidence for a large surface influenced core-exciton effect on silicon. The Si(111) 7 × 7 L_2,3 absorption edge has been studied with core-level electron energy loss (ELS) and X-ray photoemission spectroscopy (XPS). An excitonic shift of ～1–2 eV have been found for transitions from Si(2p) to empty surface states.     

Scanning tunneling microscopy observation of ultrathin epitaxial CoSi<Subscript>2</Subscript>(111) films grown at a high temperature

Scanning tunneling microscopy (STM) is used to study the basic laws of growth of ultrathin epitaxial CoSi₂(111) films with Co coverages up to 4 ML formed upon sequential deposition of Co and Si atoms taken in a stoichiometric ratio onto the Co–Si(111) surface at room temperature and subsequent annealing at 600–700°C. When the coverage of Co atoms is lower than ~2.7 ML, flat CoSi₂ islands up to ~3 nm high with surface structure 2 × 2 or 1 × 1 grow. It is shown that continuous epitaxial CoSi₂ films containing 3–4 triple Si–Co–Si layers grow provided precise control of deposition. CoSi₂ films can contain inclusions of the local regions with (2 × 1)Si reconstruction. At a temperature above 700°C, a multilevel CoSi₂ film with pinholes grows because of vertical growth caused by the difference between the free energies of the CoSi₂(111) and Si(111) surfaces. According to theoretical calculations, structures of A or B type with a coordination number of 8 of Co atoms are most favorable for the CoSi₂(111)2 × 2 interface.     

Application of synchrotron radiation to investigation of the mechanism of increase in the yield of alkali metal ions in electron-stimulated desorption

Core-level photoelectron spectroscopy with synchrotron radiation (hv = 140 eV) has been applied to study the variation in the Si⁺ charge state in silicon films deposited on the W(100) surface after thermal annealing of the substrate. The purpose of this study is to check the mechanism responsible for the sharp increase in the yield of Na⁺ ions in electron-stimulated desorption from a sodium layer adsorbed on the Si/W(100) surface after high-temperature annealing. The evolution of the W 4f _7/2 and Si 2p photoelectron spectra and the valence band photoemission spectra is investigated for two silicon coverages (1 and 3 ML) on the W(100) surface in the temperature range 300<T<2200 K. It is shown that annealing of 1 ML Si on the W(100) surface results in the formation of a W-Si covalent bond, which can weaken the Si-Na bond and lead to an increase in the equilibrium distance X ₀ and, hence, to an increase in the yield of Na⁺ ions in electron-stimulated desorption. The variation in the photoelectron spectra under annealing of 3 ML Si differs from that observed after annealing of 1 ML Si in the direction of charge transfer, thus correlating with the opposite effect of annealing of 3 ML Si/W on the Na⁺ yield in electron-stimulated desorption.     

Comparative AES and RHEED study of the formation of Pd silicide on clean and oxide covered Si(100) and (111) surfaces

The formation and epitaxial orientation of Pd silicide on clean and native oxide covered Si(100) and (111) surfaces was studied by Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Pd was vapor deposited in UHV on to the substrates up to thicknesses of about 6 nm. On clean Si substrates, ultra-thin Pd deposits reacted to form Pd₂Si already at room temperature, as detected by a characteristic splitting of the Si LVV Auger peak. However, a polycrystalline structure with very small crystallite sizes was indicated by diffuse ring patterns in RHEED. When the initial thickness of the Pd deposit exceeded about 3 nm, the diffraction ring pattern of unreacted metal developed. During annealing of room temperature deposits of Pd, the (100) and (111) substrates behaved differently. Larger crystallites formed on Si(100), but the films remained polycrystalline, though textured. On Si(111), virtually perfect epitaxial re-orientation of the silicide was found. When the substrates were initially covered with native oxide of about 2 nm thickness, silicide formation started at about 200°C, resulting in polycrystalline, but strongly textured Pd₂Si. Upon further annealing at temperatures up to 600°C, an additional phase of epitaxially oriented Pd₂Si developed on Si(111), similar to that on clean Si(100). In all experiments, extended annealing at temperatures above 250°C caused segregation of Si to the surface. This was accompanied by the development of an additional peak in the Auger electron spectra at about 313 eV, which we assign to a plasmon loss of δE = 17 eV in the Si overlayer, being excited by Pd Auger electrons of energy 330 eV.     

A molecular-beam investigation of the reaction H2 + 12O2 → H2O on Pd(111)

Using molecular-beam relaxation techniques and isotopic exchange experiments, the water-formation reaction on Pd(111) has been shown to proceed via a Langmuir-Hinshelwood mechanism. The reaction product H₂O is emitted from the surface with a cosine distribution. The rate-determining step is the formation of OH_ad in the reaction O_ad + H_ad → OH_ad. The activation energy for this step is 7 kcal/mole with a pre-exponential factor, v, of 4 × 10^?8 cm² atom^?1 sec^?1. This value for v lies well below that observed for simple second-order desorption of dissociatively adsorbed diatomic gases, but is roughly of the order of that obtained for the oxidation of CO on Pd(111). The formation of H₂O proceeds differently under conditions of excess O₂ or H₂. In an excess of H₂, the kinetics is dominated by the transport of atomic hydrogen between the bulk and the surface as was found for the H?D exchange reaction on Pd(111). In an excess of O₂, diffusion of hydrogen into the bulk is blocked by adsorbed oxygen and the hydrogen reservoir available for reaction at the surface is decreased by several orders of magnitude. This results in a drastic reduction of the reaction rate which can be reversed by increasing the partial pressure of H₂.     

Electron energy loss spectroscopy of sodium covered Ge(111) surfaces

The ELS spectra of sodium covered Ge(111) surfaces are studied. It was found that sodium coverage upto 1 ML leads to a extinction of the ELS features of the clean Ge surface. With increasing sodium coverage a metal induced peak at 3.4 eV and a peak at 31.6 eV due to a sodium 2p-core level transitions appear. ELS spectra of a heat-treated surface covered with sodium imply a formation of a surface Na-Ge “compound” as a result of Na-Ge interdiffusion.     

A static SIMS/TPD study of the kinetics of methoxy formation and decomposition on O/Pt(111)

The kinetics and mechanism of the decomposition of methanol (CH₃OD) on oxygen-covered Pt(111) were studied using static secondary ion mass spectrometry (SIMS) and temperature programmed desorption (TPD). The initial sticking coefficient and the saturation first layer coverage of CH₃OD are unity and 0.36 ML, respectively. The maximum amounts decomposed in TPD on O/Pt(111) and clean Pt(111) are 0.19 and 0.047 ML, respectively. At low methanol coverages (< 0.05 ML) on O/Pt(111) the only reaction products were CO₂, H₂O and D₂O, whereas at saturation CO, H₂O, D₂O and H₂ were observed. The decomposed amount did not saturate at or before the onset of molecular methanol desorption, but increeased monotonically until saturation of the first layer. No oxygen exchange was observed between CH₃OD and preadsorbed ¹⁸O. A decomposition mechanism involving methoxy and hydroxyl type species is proposed. Methanol coverages as low as 0.002 ML could be detected with SIMS. The CH₃⁺ ion was the most intense ion in the positive SIMS spectrum of both methanol and methoxy. Larger ion clusters such as (CH₃OD)_n⁺ (n = 2, 3) developed successively at specific multilayer coverages. At low coverages on O/Pt(111), methoxy formation occurs above 125 K and its decomposition becomes detectable above 150 K during temperature programming. In the isothermal mode, the SIMS CH₃⁺ ion was used to follow the kinetics. Over the temperature range 120–145 K, the second order Arrhenius rate parameters for methoxy formation are E = 5.5±0.7 kcal/mol and A = 1.5×10^−7±0.6 cm²/s·molecule for an initial methanol coverage of 0.05 ML. Methoxy decomposition was studied in the temperature range 150–165 K and at an initial coverage of 0.015 ML. The first order kinetic parameters, E = 11.4±0.5 kcal/mol and A = 5.3×10^13±1 s⁻¹ were derived. Advantages and limitations of using SIMS as a tool for kinetic studies are discussed.     

New evaluation of fullerene molecules on Si(111)-(7 × 7) reconstructed structure using non-contact scanning non-linear dielectric microscopy

Fullerene (C₆₀) molecules on an Si(111)-(7 × 7) surface have been investigated using non-contact scanning non-linear dielectric microscopy (NC-SNDM) under an ultra-high vacuum. The topography, the interface between the C₆₀ molecule and Si adatoms, and the internal structure of the C₆₀ molecules were successfully investigated. For ～ 0 ML and ～ 0.4 ML coverage, both phase reversal sites and sites without phase reversal could be observed in the first order phase (θ1) image. On the other hand, for 1 ML coverage, phase reversal could not be identified. These results indicate that charge transfer only occurred from Si adatoms to C₆₀ molecules at three-fold symmetric sites on the Si(111)-(7 × 7) surface, and the electric dipole moment is reflected in the electronic state of the C₆₀ molecules. The internal structure of C₆₀ molecules was clearly observed in topography by the second order amplitude (A₂) feedback signal for 1 ML coverage, reflecting the LDOS originating from the t_1u orbital.     

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √p_H2, indicating atomic adsorption. Initial heats of H₂ adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases E_ad being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D₂ is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.     

Atomic-scale X-ray structural analysis of self-assembled monolayers on Silicon

Two related self-assembled monolayers (SAMs), 4-bromostyrene (BrSty) and 4-bromophenylacetylene (BPA), are photochemically grown from solution on to the monohydride-terminated Si(111) surface. The atomic-scale structures of the resulting SAMs are examined by X-ray standing waves (XSW), X-ray reflectivity (XRR), X-ray fluorescence, atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). The coverage is 0.5 ML. The results show that in each case the molecule covalently bonds to a single Si T₁ site and stands up-right with a slight molecular tilt of 17^○ that leaves the Br terminal end over a neighboring T₄ site. The Br height is 8.5?Å (BrSty) and 8.6?Å (BPA) above the top surface Si atom. The combined XSW and XRR results rule-out two alternative bonding models predicted by DFT that have the root of the molecule bonded to two neighboring top Si surface atoms. Based on the XSW 111 and 333 coherent fractions, the BPA/Si(111) has a reduced vertical Br distribution width in comparison to BrSty. This greater rigidity in the molecular structure is correlated to a C=C bond at the root.     

RAIRS studies of CO adsorption on Pd/CeO2−x(1 1 1)/Pt(1 1 1)

Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on CeO_2−x-supported Pd nanoparticles at room temperature. The results show that when CeO_2−x is initially grown on Pt(1 1 1), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto CeO_2−x/Pt(1 1 1), most of the Pd grows directly on top of the CeO_2−x(1 1 1). RAIR spectra of CO adsorption on 1 ML Pd/CeO_2−x/Pt(1 1 1) show a broad CO-Pd band, which is inconsistent with a single crystal Pd surface. However, the 5 ML and 10 ML Pd/CeO_2−x/Pt(1 1 1) spectra show vibrational bands consistent with the presence of Pd(1 1 1) and (1 0 0) faces, suggesting the growth of Pd nanostructures with well defined facets.     

Magnetic hyperfine fields on 111Cd in the Pd1+xMnSb heusler alloys

The time-differential perturbed angular correlation technique has been used to measure the magnetic hyperfine fields (mhf) acting on ¹¹¹Cd impurity on the series of alloys Pd_1+xMnSb with 0 ? x ? 1. We observe three diffrent mhf's which for small values of x are of the order of +250, ?250 and ?150 kOe, decreasing slowly for larger values of x. These three fields are associated with ¹¹¹Cd occupying the sites of Mn, Sb and Pd, respectively. In addition, magnetization measurements on three of the allows (x = 0, 0.4 and 0.8) gave magnetic moments on the Mn atoms of 3.88 ± 0.08, 4.18 ± 0.08, (4.14 ± 0.08)_μB and Curie temperatures of 470 ± 2,422 ± 5 and (288 ± 2) K, respectively. Our results together with the existing data for mhf's on various 5sp elements in the Pd₂MnSb Heusler alloy are compared with the Blandin-Campbell model and show an agreement which is only qualitative.     

Pd-Si薄膜固相反应的透射电子显微镜研究

本工作利用透射电子显微术研究了Pd-Si薄膜固相反应的初始生成相及生成相Pd₂Si与(111)取向Si衬底的取向关系随Pd膜厚度、退火温度等因素的变化规律。实验结果表明:在衬底保持室温的条件下,Pd沉积到Si(111)上时也能够生成一层外延的Pd₂Si,其厚度足以在常规的选区电子衍射中产生明显的信号。在170℃退火时,Pd-Si反应即可持续到生成200nm厚的外延的Pd₂Si。在Pd膜厚度为400nm的条件下,Pd₂Si与Si(111)衬底的取向关系为[0001]_(Pd2Si)轴织构。 关键词：     

Absolute coverages for the various surface phases of CO and O adsorbed on Pd(110)

The absolute surface coverages of CO and O on Pd(110) have been measured by nuclear reaction analysis (NRA) using the ¹²C(d, p)¹³C and ¹⁶O(d, p₁)¹⁷O¹ reactions. The CO coverages of the (2 × 1) and (4 × 2) phases of CO on Pd(110) are 1.00 ±0.05 and 0.73 ±0.05 ML (1 ML = 1 monolayer = 9.4 × 10¹⁴ CO molecules cm⁻²) respectively. The oxygen coverage in the c(4 × 2) phase of O on Pd(110) is 0.50 ±0.05 ML.     

Pd adsorption on Si(1 1 3) surface: STM and XPS study

Pd-induced surface structures on Si(1 1 3) have been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In the initial process of the Pd adsorption below 0.10 ML, Pd silicide (Pd₂Si) clusters are observed to form randomly on the surface. By increasing the Pd coverage to 0.10 ML, the clusters cover the entire surface, and an amorphous layer is formed. After annealing the Si(1 1 3)-Pd surface at 600 °C, various types of islands and chain protrusions appears. The agglomeration, coalescence and crystallization of these islands are observed by using high temperature (HT-) STM. It is also found by XPS that the islands correspond to Pd₂Si structure. On the basis of these results, evolution of Pd-induced structures at high temperatures is in detail discussed.