Oxygen adsorption on clean Mo (100) surfaces

Oxygen adsorption on clean Mo (100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2 × 2) phase growing simultaneously with a (6 × 2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3×1) structure together with a p (1×1) structure, probably due to an oxide phase. Even in the low temperature range (370–500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4×4) structure which is followed by a (√5 × √5)?R 26° 33 structure. The same occurs when the inital c (2 × 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2×2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and Mo.     

Preparation and chemisorptive properties of the clean normal and reconstructed surfaces of Ir(100) — role of multiplets

The preparation and properties of the clean normal (1 × 1)Ir(100) and the reconstructed (1 × 5)Ir(100) metal surfaces are reported. Photoemission studies of the chemisorptive bonding of carbon monoxide, of hydrogen and of oxygen on the Ir(100) surface are discussed. The possible occurrence of multiplets in the photoemission spctra for chemisorbed oxygen on cesium; Ni(111) and Ir(100) is proposed and evidence for their importance is presented.     

V(001)表面上(4×1)-O,(2×2)-S两超结构的角分辨光电子谱

用角分辨紫外光电子谱详细研究了V(001)表面上,S和O偏析引起的(4×1)-O,(2×2)-S两超结构,确定了S,O各自引起的吸附态的峰位和对称性;得到了O占据V(001)表面上空位的实验证据。所得实验结果与理论计算相等。 关键词：     

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.     

Anomalous oxidation properties of the ZnSe (100) surface

We have observed a unique, pressure-dependent adsorption isotherm of oxygen on the ZnSe (100) surface, which consists of an unmeasurable uptake followed by an irreversible, step-like uptake for pressures exceeding a critical value of ～ 0.08 torr at room temperature. A partial depletion of Se accompanies this adsorption process. For incomplete oxidation, oxygen induced (2 × 1) and (3 × 1) surface reconstructions may be generated, the first such structures to be observed for semiconductors. The electron-energy-loss spectra for these surfaces and for the clean ZnSe (100)c(2 × 2) surface are presented. The clean surface exhibits a dangling-bond-derived empty surface state ～ 1 eV above the conductor band edge, and filled surface states near 3.2, 6.5, and 15 eV below the valence band edge.     

Ge(100) surfaces prepared in vapor phase epitaxy process ambient

Ge(100) substrates essential for subsequent III–V integration were studied in the hydrogen ambient of a metalorganic vapor phase epitaxy reactor. We confirmed thermal oxide and carbon removal by X‐ray photoelectron spectroscopy, characterized the (2 × 1)/(1 × 2) surface reconstruction by low energy electron diffraction, and employed reflection anisotropy spectroscopy for optical in situ analysis. Our Ge(100) spectra de‐ viate from reference data of clean surfaces prepared in ultra‐high vacuum, most probably due to the presence of hydrogen bonds. The observation was correlated with Fourier‐transform infrared spectroscopy showing coupled H–Ge–Ge–H stretch modes associated with a monohydride termination of the Ge(100) surface. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

LEED investigation of germanium surfaces cleaned by sublimation of sulphide films; structural transitions on clean Ge(110) surface

Sublimation of deposited germanium sulphide films at the temperatures as low as 350°C results in the appearance of LEED patterns of clean surfaces of germanium. In the interface between Ge(111) or Ge(110) and germanium sulphide, ordered structures are observed, namely Ge(111)?(2 × 1)S and Ge(110)?(10 × 5)S. The conclusion about the structure of the Ge(100) germanium sulphide interface cannot be made unambiguously. The structures of clean Ge(110) surfaces are described. The annealing of clean surfaces of Ge(110) at different temperatures leads to the formation of one of two possible surface structures. After annealing at temperatures below 380°C and above 430°C the Ge(110)?c(8 × 10) clean superstructure is observed. After annealing at temperatures from 380 up to 430°C the surface (110) is rearranged in vicinal planes of the (17 15 1) type with the (2 × 1) superstructure. These structures undergo reversible transitions from one to another at temperatures of about 380 and 430°C.     

Reactions of formaldehyde and methanol on clean,carburized and oxidized Mo(100) surfaces

The reactions of formaldehyde and methanol have been studied on clean, carburized, and oxidized Mo(100) surfaces using temperature programmed reaction spectroscopy (TPRS). The thermal cracking of ethylene at 550 K and the adsorption of molecular oxygen at 1050 K were used to carburize and oxidize, respectively, the clean surface to saturation. Both the carbide and oxide surfaces showed (1×1) LEED features. Methanol decomposed to give hydrogen atoms and methoxy intermediates upon adsorption on the clean Mo(100) surface at 200 K. The methoxy intermediate was stable up to 340 K. Adsorbed carbon and oxygen suppressed the dissociation of the hydroxyl hydrogen from the alcohol and yielded a significantly different activity and selectivity compared to the very reactive clean surface. The binding energies for both formaldehyde and methanol on the three surfaces were similar, demonstrating the weak sensitivity of donor-acceptor bonds to surface modifiers. The results in this study were very similar to those previously observed for W(100) though different adlayer structures were present. This similarity suggested that the modification in surface reactivity was primarily a compositional effect.     

Interaction of O2 with Pt(100): I. Equilibrium measurements

Two newly discovered phases on the Pt(100) surface produced by the adsorption of oxygen have been investigated using Rutherford baekscattering (RBS), nuclear microanalysis (NMA), work function changes (Δφ) and LEED. One phase is associated with the oxygensaturated surface (0.63 ± 0.03 monolayers0.81 × 10¹⁵ O atoms cm^?2), where a very complex LEED pattern is observed; the other is observed at an average coverage of 0.44 ± 0.05 monolayers and gives rise to a (3 × 1) LEED pattern (when observed at room temperature). For both surfaces, RBS measurements indicate large (? 0.025 nm) Pt atom displacements. Also discussed is a new method for preparing the “clean” (1 × 1)-Pt(100) surface without the need for NO adsorption/decomposition.     

The superstructures of the clean Pt(100) and Ir(100) surfaces

The clean and reconstructed surfaces of Pt(100) and Ir(100) were investigated by low energy electron diffraction (LEED). It is shown that two superstructures can be observed in the case of platinum. The structure Pt(100)-hex, which is commonly called Pt(100)-(5 × 20), transforms to Pt(100)-hex-R0.7° above 1100 K. It is shown that this stable phase differs from the first one by a slight rotation of the hexagonal surface layer by 0.7°. For Ir(100) only the well known (1 × 5) superstructure is observed without any rotation of the outer layer. The rotation angle of 0.7° for platinum and the stability of the unrelated structure for iridium can be interpreted by simple calculations of the coordination of surface atoms with those of the second layer. The method assumes that the surface layer is of ideal hexagonal structure in the case of platinum and nearly hexagonal in the case of iridium. The results are in good agreement with the experiment.     

Initial growth of Cu films on oxygen covered W(110)

The growth of Cu films on the p(2 × 1)O-W(110) and (1 × 1)O-W(110) surfaces has been monitored using specular He scattering. Deposition at temperatures below 200 K leads to rough films on both surfaces. Deposition at T > 300 K leads to phase separation of Cu and O on the p(2 × 1)O-W(110) surface. Deposition at 300 K on the higher oxygen coverage surface leads to films which are nearly as smooth as these deposited on clean W(110). Deposition at 800 K leads to needle-like crystallite formation without an initial continuous film.     

Density Functional Calculation of the 0.5ML-Terminated Allyl Mercaptan/Si（100）-（2 × 1） Surface

The structural and electronic properties of the 0.5 ML-terminated allyl mercaptan （ALM）/Si（IO0）-（2 x 1） surface are studied using the density functional method. The calculated absorption energy of the ALM molecule on the 0.5 ML-terminated ALM/Si（IO0）-（2 x 1） surface is 3.36eV, implying that adsorption is strongly favorable. The electronic structure calculations show that the ALM/Si（IO0）-（2 x 1）, the clean Si（100）-（2 x 1）, and the fully-terminated H/Si（IO0）-（2 ~ 1） surfaces have the nature of an indirect band gap semiconductor. The highest occupied molecular orbital is dominated by the ALM, confirming the mechanism proposed by Hossain for its chain reaction.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

RHEED studies of Si(100) surface structures induced by Ga evaporation

Si(100) surface structures induced by Ga molecular beam deposition in an ultra-high vacuum have been investigated using a reflection high-energy electron diffraction system (RHEED). It has been found that the Ga evaporation of submonolayer thickness on a clean Si(100) 2 × 1 surface produces surface structures of Si(100) 3 ×2, 5 × 2, 2 ×2 and 8 × 1 sequentially in the temperature range from 350 to 680° C. The RHEED patterns and a two-dimensional phase diagram including five superstructures are presented.     

不同密度氢吸附金刚石(100)表面的微观结构

利用基于广义梯度近似的密度泛函理论,计算了金刚石(100)表面不同氢吸附密度的平衡态几何结构和态密度.结果表明对于2×1构型,在平行和垂直表面两个方向上发生弛豫,而1×1构型仅在垂直表面方向上发生弛豫.另外,清洁2×1,2×1 ∶0.5H和1×1 ∶1.5H表面,带隙中存在空表面态;而对于1×1 ∶2H和2×1 ∶H两种表面结构,空表面态上移进入导带,带隙中不存在表面态.结合电荷密度分布,探讨了金刚石(100)不同构型和氢吸附密度表面的表面态诱发机理. 关键词： 氢吸附 金刚石 弛豫 表面态     

Phase transitions on clean Si(110) surfaces

The properties of the structure of clean Si(110) surfaces have been investigated by LEED. The phase transitions between surface structures Si(110)?(4 × 5), Si(110)?(2 × 1) and Si(110)(5 × 1) take place at about 600 and 750°C. The time of reconstruction from the high temperature phase to the low temperature phase may exceed the time of the sample cooling. That explains why the Si(110)?(2 × 1) and the Si(110)?(5 × 1) superstructures may be seen at room temperature. Surface defects favour the retaining of high temperature phases on the surface at room temperature. The transition from the Si(110)?(5 × 1) structure to the Si(110)?(2× 1) structure and conversely in the temperature range of 720–750°C apparently occurs through formation of the intermediate structures Si(110)?(7 × 1) and Si(110)?(9 × 1). The models are given of superstructures observed by LEED.     

Relationship between vibrational states of O-Ni systems and their superficial structure on (100) face of nickel single crystal

High resolution energy loss spectra of 4 eV electrons reflected in the specular direction from Ni(100) surface clean or covered by the ordered structures obtained in the different stages of the metal oxidation, are analysed with reference to LEED patterns. At room temperature, the successive p(2 × 2) and c(2 × 2) structures associated with the chemisorption of oxygen have been observed without modification of the energy loss spectra, in respect of the clean nickel surface. Surface phonons are known to occur in the case of the c(2 × 2)S ordered layer and their absence in the case of Ni-O corresponding system is discussed. After short exposures to oxygen between 200 to 500° C, the surface exhibits a so called “intermediate oxide”. It is identified by its hexagonal unit mesh (～5 Å) with two equivalent orientations along the [100] and [110] directions of the substrate and its vibrational spectra characterized by a loss peak at ? 112.5 meV (± 2.5 meV). Subsequent exposures to oxygen lead to the formation of the (100) face of NiO (in epitaxy on the Ni(100) face) accurately identified by its LEED pattern. The obtained typical multiple loss spectra with spacing 67.5 meV (± 15 meV) reveal a scattering of low energy electrons by long wavelength optical phonons associated to the oxide. The characteristic energy loss (67.5 meV) is in relative good agreement with the energy of the Fuchs-Kliewer surface phonon calculated from the optical constants of the nickel oxide.     

Initial stages of oxidation of binary alloys: The case of the stepped Pt3Ti(510) single crystal surface

The oxidation of the (510) oriented surface of the ordered fcc Pt₃Ti alloy was studied by low energy electron diffraction and other surface sensitive techniques. The clean Pt₃Ti(510) surface showed an ordered array of surface steps. Exposing the surface to oxygen at pressures in the range from 1×10⁻⁷ to 1×10⁻⁵ Torr at temperatures of 800 K or higher caused the formation of titanium oxide islands on the surface. This process caused the progressive disappearance of the ordered array of steps, which was replaced by large facets oriented along the (100) and (210) planes.     

RHEED study of In-induced superstructures on Ge(111) surfaces

When submonolayer and monolayer amounts of indium were deposited onto clean Ge(111) surfaces at room temperature and then heated, (13 × 2√3), (12 × 2√3), (11 × 2√3), (10 × 2√3), (4√3 × 4√3) R30°-related, (√31 × √31) R(±9°), (√61 × √61) R(30 ± 4°) and (4.3 × 4.3) structures appeared on the surfaces at fixed In coverages and at fixed surface temperatures. General intensity features of superlattice reflections are derived from intensity estimations by eye of superlattice spots in their RHEED patterns, and some structural characteristics of the superstructures are clarified from the analysis of the general intensity features. The former four superstructures are long-period (2 × 2)-related antiphase structures whose period changes, depending on the coverage. The wavevector characterizing the (13 × 2√3) structure, which appears at the smallest coverage, almost coincides with those of structural fluctuation emerging at the clean Ge(111) (1 × 1) surface around 350°C. The coincidence suggests that the longperiod (2 × 2)-related antiphase structures have a close relationship to the structural fluctuation and, besides, to the (2 × 8) structure in their origin.