Formation of subsurface carbon induced by oxygen adsorption on carbon-covered W(001)

The adsorption of oxygen on the carbon-covered W(001) surface was studied by AES and low energy ion scattering. At low carbon and oxygen coverages, both species can be accommodated on the surface. At higher coverages, the oxygen displaces the carbon into the near-surface (selvedge) region. When oxygen is adsorbed on the W(001)-p(5 × 1)C surface (formed by exposure to more than 50 L of ethylene at 1500 K), carbon is displaced in a nearly one for one manner. Annealing the oxygen-covered p(5 × 1)C surface to 950 K removes up to 0.5 ML oxygen as CO. Interestingly, the surface carbon coverage is unchanged by CO desorption at 950 K and subsurface carbon is partially replenished by diffusion from the bulk. Oxygen adsorption in excess of 0.5 ML suppresses carbon segregation from the bulk at 950 K. The additional oxygen does not desorb until 1300 K. Surface carbon is restored by annealing to 1500 K.. The degree and rate of carbon segregation depend on the initial ethylene exposure even though the resulting W(001)-p(5 × 1)C surfaces are identical according to AES, LEED, and ion scattering.     

Spin-polarized ion scattering spectroscopy of CCl4 adsorption on Fe(0 0 1) surfaces

The interaction of CCl₄ molecules with Fe(0 0 1) surfaces was investigated by spin-polarized ion scattering spectroscopy (SP-ISS). It was observed that CCl₄ molecules adsorb dissociatively on the surface at ambient temperature (290 K), and consequently, iron and chlorine were major surface constituents. It was found that the chlorine adatoms are located atop of iron atoms of the second surface layer (hollow sites of the surface). It is indicated that the spin state of iron atoms at the surface is not affected by exposure to a CCl₄ atmosphere, while almost no spin is induced in the chlorine adatoms. Similar behavior is observed in the spin states of iron and chlorine on an oxygen preadsorbed-Fe(0 0 1) surface. The difference in the spin states of iron and chlorine clarifies the local property of the incidence ion neutralization and element selectivity of SP-ISS in this CCl₄/Fe system.     

Behavior of zirconium in the ZrOW(100) system at high temperature, studied by ISS, AES and work-function measurements

The behavior of zirconium atoms at the W(100) surface associated with oxygen adsorption at different sample temperatures has been studied by Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and the relative change of the work function (Δф) measured by the onset of the secondary electron energy distribution. The results have revealed: (i) adsorption of zirconium onto the W(100) surface followed by the elevation of the sample temperature up to 1710 K in an oxygen partial pressure of 2.7 × 10⁻⁴ induces complete diffusion of zirconium atoms into the W(100) substrate; (ii) further exposure of oxygen induces co-existence of oxygen and tungsten on the surface at 1710 K, resulting in a work function of 4.37 eV; (iii) keeping the sample temperature at 1710 K, simple evacuation of the system has resulted in surface segregation of zirconium atoms to the surface to form a zirconium atomic layer on the top-most surface, reducing the work function to 2.7 eV. The results have revealed that this specific behavior of zirconium atoms at high temperature assures, with very good reproducibility, the highly stable performance and long service life of Zr---O/W(100)-emitters in practical use, even in a low vacuum of 10⁻⁶ Pa.     

Investigation of oxygen adsorption on Cu(110) by low-energy ion bombardment

The interaction of molecular oxygen with a Cu(110) surface is investigated by means of low energy ion scattering (LEIS) and secondary ion emission. The position of chemisorbed oxygen relative to the matrix atoms of the Cu(110) surface could be determined using a shadow cone model, from measurements of Ne⁺ ions scattered by adsorbed oxygen atoms. The adsorbed oxygen atoms are situated 0.6 ± 0.1 Å below the midpoint between two adjacent atoms in a 〈100〉 surface row. The results of the measurements of the ion impact desorption of adsorbed oxygen suggest a dominating contribution of sputtering processes. Ion focussing effects also contributes to the oxygen desorption. The ion induced and the spontaneous oxygen adsorption processes are studied using different experimental methods. Sticking probability values obtained during ion bombardment show a strong increase due to the ion bombardment.     

Effects of oxygen concentration and irradiation defects on the oxidation corrosion of body-centered-cubic iron surfaces: A first-principles study

Oxidation corrosion of steels usually occurs in contact with the oxygen-contained environment, which is accelerated by high oxygen concentration and irradiation. The oxidation mechanism of steels is investigated by the adsorption/solution of oxygen atoms on/under body-centered-cubic (bcc) iron surfaces, and diffusion of oxygen atoms on the surface and in the near-surface region. Energetic results indicate that oxygen atoms prefer to adsorb at hollow and long-bridge positions on the Fe(100) and (110) surfaces, respectively. As the coverage of oxygen atoms increases, oxygen atoms would repel each other and gradually dissolve in the near-surface and bulk region. As vacancies exist, oxygen atoms are attracted by vacancies, especially in the near-surface and bulk region. Dynamic results indicate that the diffusion of O atoms on surfaces is easier than that into near-surface, which is affected by oxygen coverage and vacancies. Moreover, the effects of oxygen concentration and irradiation on oxygen density in the near-surface and bulk region are estimated by the McLean's model with a simple hypothesis.     

Ultra thin Al film on the W(110) surface

We have investigated the surface structure of the Al/W(110) surface using low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (ISS). We observe a p(2 × 1) double domain LEED image for the 0.5 ML Al/W(110) surface at annealing temperature 850 °C. We found that 0.5 ML Al atoms cover on the W(110) surface uniformly but do not form 3 or 2-dimensional islands. We also measured the Al adsorption site at the Al/W(110)-p(2 × 1) surface using ISS. We found that Al atoms adsorbed at the center of the bridge site. The height of the adsorbed Al atoms is determined to be 2.18 ± 0.15 Å above the W surface layer.     

Cl原子在γ-TiAl(111)表面吸附的第一性原理研究

基于密度泛函理论,在广义梯度近似下研究了Cl在γ-TiAl(111)表面的吸附.计算结果表明:γ-TiAl(111)表面的面心立方位置(fcc)和六角密排位置(hcp)为Cl吸附的稳定位置,当覆盖度Θ小于一个单层(ML)时,Cl原子倾向于吸附在γ-TiAl(111)表面近邻为多Ti的位置.电子结构分析发现,Cl原子同表面金属原子形成较强的离子键,并且成键具有一定的方向性.当Cl原子和O原子共同在γ-TiAl(111)表面吸附时,二者都趋     

Coverage-dependent molecular tilt of carbon monoxide chemisorbed on Pt{110}: A combined LEED and DFT structural analysis

The adsorption of carbon monoxide on the Pt{110} surface at coverages of 0.5 ML and 1.0 ML was investigated using quantitative low-energy electron diffraction (LEED IV) and density-functional theory (DFT). At 0.5 ML CO lifts the reconstruction of the clean surface but does not form an ordered overlayer. At the saturation coverage, 1.0 ML, a well-ordered p(2 × 1) superstructure with glide line symmetry is formed. It was confirmed that the CO molecules adsorb on top of the Pt atoms in the top-most substrate layer with the molecular axes tilted by ± 22° with respect to the surface normal in alternating directions away from the close packed rows of Pt atoms. This is accompanied by significant lateral shifts of 0.55 Å away from the atop sites in the same direction as the tilt. The top-most substrate layer relaxes inwards by ? 4% with respect to the bulk-terminated atom positions, while the consecutive layers only show minor relaxations. Despite the lack of long-range order in the 0.5 ML CO layer it was possible to determine key structural parameters by LEED IV using only the intensities of the integer-order spots. At this coverage CO also adsorbs on atop sites with the molecular axis closer to the surface normal (< 10°). The average substrate relaxations in each layer are similar for both coverages and consistent with DFT calculations performed for a variety of ordered structures with coverages of 1.0 ML and 0.5 ML.     

Application of low energy ion blocking for adsorption site determination of Na Atoms on a Cu(111) surface

Ion blocking in the low keV energy range is demonstrated to be a sensitive method for probing surface adsorption sites by means of the technique of time-of-flight scattering and recoiling spectroscopy (TOF-SARS). Adsorbed atoms can block the nearly isotropic backscattering of primary ions from surface atoms in the outmost layers of a crystal. The relative adsorption site position can be derived unambiguously by simple geometrical constructs between the adsorbed atom site and the surface atom sites. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) and molecular dynamics (MD) simulations provide the detailed ion trajectories. Herein we present a quantitative analysis of the blocking effects produced by sub-monolayer Na adsorbed on a Cu(111) surface at room temperature. The results show that the Na adsorption site preferences are different at different Na coverages. At a coverage θ = 0.25 monolayer, Na atoms preferentially populate the fcc threefold surface sites with a height of 2.7 ± 0.1 Å above the 1st layer Cu atoms. At a lower coverage of θ = 0.10 monolayer, there is no adsorption site preference for the Na atoms on the Cu(111) surface.     

Adsorption states and Ga depletion during oxygen adsorption on clean polar GaAs surfaces

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS) and Auger (AES) measurements were used to study oxygen adsorption on sputtered an annealed GaAs(111)Ga, (1&#x0304;1&#x0304;1&#x0304;)As, and (100) surfaces. Two forms of adsorbed oxygen are seen in UPS. One of them is associatively bound and desorbs at 400–550 K mainly as molecular O₂. It is most probably bound to surface As atoms as indicated by the small amounts of AsO which desorb simultaneously. The second form is atomic oxygen bound in an oxidic environment. It desorbs at 720–850 K in the form of Ga₂O. Electron irradiation of the associatively bound oxygen transforms it into the oxidic form. This explains the mechanism of the known stimulating effect of low energy electrons on the oxidation of these surfaces. During oxygen exposure a Ga depletion occurs at the surface which indicates that oxygen adsorption is a more complex phenomenon then is usually assumed. The following model for oxygen adsorption is proposed: oxygen impinges on the surface, removes Ga atoms and thus creates sites which are capable of adsorbing molecular oxygen on As atoms of the second layer and are surrounded by Ga atoms of the first layer. This molecular oxygen is stable and simultaneously forms the precursor state for the dissociation to the oxidic form.     

氧化铝与Al（100）基体间界面原子结构研究

本文介绍了用M e v 离子散射和沟道效应研究单晶铝表面无定型氧化层与基体之间界面原子结构的方法, 报道了AI_2_O_3_/ Al ( 1 0 0) 界面原子结构的实验结果.实验表明, 在纯氧气氛围中400 ℃ 下生成的氧化铝膜, 铝和氧原子浓度比例严格为2 与3 之比;AI_2_O_3_ 膜和Al (100)基体之间的界面极其陡峭, 氧化铝膜下Al (1 0 0 ) 基体表面的再构层不大于一个原子层. 由实验测量与用Monte corlo 方法计算结果比较, 得到再构层原子离开原来晶格位置距离为0.18埃. 室温下,Al ( 1 0 0 ) 氧吸附层的元素成份在氧原子覆盖度较低时主要为AlO. 关键词：     

The adsorption of NO on an oxygen pre-covered Pt(1 1 1) surface: in situ high-resolution XPS combined with molecular beam studies

Adsorption of NO on a Pt(1 1 1) surface pre-covered with a p(2 × 2) atomic oxygen layer has been studied in situ by high-resolution X-ray photoelectron spectroscopy and temperature-programmed XPS using third-generation synchrotron radiation at BESSY II, Berlin, combined with molecular beam techniques and ex situ by low energy electron diffraction and temperature-programmed desorption. O 1s XP spectra reveal that an ordered p(2 × 2)-O layer dramatically changes the adsorption behavior of NO as compared to the clean surface. The atomic oxygen occupies fcc hollow sites, and therefore blocks NO adsorption on these sites, which are energetically preferred on clean Pt(1 1 1). As a consequence, NO populates on-top sites at low coverage. At 110 K for higher coverages, NO can additionally adsorb on hcp hollow sites, thereby inducing a shift of the O 1s binding energy of atomic oxygen towards lower energies by about 0.25 eV. The bond strength of the hcp hollow NO species to the substrate is weakened by the presence of atomic oxygen. A sharp p(2 × 2) LEED pattern is observed for NO adsorption on the oxygen pre-covered surface, up to saturation coverage. The total saturation coverage of NO on Pt(1 1 1) pre-covered with varying amounts of oxygen (below 0.25 ML) decreases linearly with the coverage of oxygen. The initial sticking coefficient of NO is reduced from 0.96 on clean Pt(1 1 1) to 0.88 on a p(2 × 2) oxygen pre-covered surface.     

Adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface

The adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface has been studied with scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). At 0.1 ML Cs coverage the whole surface exhibits a mixture of (1 × 2) and (1 × 3) reconstructed structures, indicating that Cs atoms exert a cooperative effect on the surface structures. Real-time STM observation shows that silver atoms on the Cs-covered surface are highly mobile on the nanometer scale at 300 K. The Cs-reconstructed Ag(1 1 0) surface alters the structure formed by dissociative adsorption of oxygen from p(2 × 1) or c(6 × 2) to a p(3 × 5) structure which incorporates 1/3 ML Ag atoms, resulting in the formation of nanometer-sized (10–20 nm) islands. The Cs-induced reconstruction facilitates the adsorption of CO₂, which does not adsorb on unreconstructed, clean Ag(1 1 0). CO₂ adsorption leads to the formation of locally ordered (2 × 1) structures and linear (2 × 2) structures distributed inhomogeneously on the surface. Adsorbed CO₂ desorbs from the Cs-covered surface without accompanied O₂ desorption, ruling out carbonate as an intermediate. As a possible alternative, an oxalate-type surface complex [OOC–COO] is suggested, supported by the occurrence of extensive isotope exchange between oxygen atoms among CO₂(a). Direct interaction between CO₂ and Cs may become significant at higher Cs coverage (>0.3 ML).     

Surface reconstruction of Cu (111) upon oxygen adsorption

Low-energy He⁺ ion scattering (IS) in combination with AES, LEED and work function measurements has been applied for the determination of surface reconstruction of Cu(111) upon oxygen adsorption. IS data clearly indicate that oxygen is not significant incorporated into the bulk at room temperature adsorption, however the surface shows reconstruction by displacement of Cu atoms by 0.3 Å. The disappearance of structure of both Cu_IS and O_IS in the “?_in pattern” demonstrate the development of a disordered layer of reconstruction centres. At saturation coverage, a rough and dis-ordered oxygen-copper surface layer is present.     

Theoretical studies of fast He atom scattering from a W{100} surface

Theoretical calculations of the scattering of fast neutral He atoms from W{100} are presented which are directly compared to the results from recent experiments of Nielsen and Delchar. The experiments which were performed for He atom energies between 150 and 1000 eV, and for incident polar angles between 0° and 65° as measured from the surface normal, displayed peaks in the polar angle distributions at 72°, 56° and ±10°. The results from classical dynamics calculations are employed here to explain the scattering phenomena that give rise to these peaks in the polar distributions. The calculations indicate that the peak at 72° is primarily due to scattering from the first layer W atoms. The peaks at ± 10° and 56° are unusual in that there are a multitude of different collision paths that result in the He atom being scattered into the same final angle. The peaks at ± 10° result from He atoms scattering mainly from the second, third and fourth layers of W atoms. The He atoms are focused on the outward path into the near normal direction by two first and two second layer W atoms. Subsurface scattering is also responsible for the peak at 56°. In this case the channel of first and second layer W atoms that focuses the outgoing He atoms is oriented at 54.7° with respect to the surface normal. It is proposed that slight variations of the experimental data from the calculated values are due to surface reconstruction of W{100} and that a more thorough analysis could reveal the microscopic nature of this structure.     

Ferromagnetism in defective yttria-stabilized zirconia

Significant research attention has been devoted to identifying and synthesizing new magnetic materials via doping of non-magnetic materials. The material defects offer an approach to stabilize ferromagnetism in non-magnetic materials such as oxygen-deficient HfO₂ and oxygen-deficient ZrO₂. In this study, we demonstrated room-temperature ferromagnetism via nitrogen ion implantation on yttria-stabilized zirconia (YSZ) single crystals. The results of structural and chemical analyses indicate the formation of a distinct surface layer through the implantation of nitrogen ions and potential oxygen vacancies. The lattice constant in this surface layer increased by 0.6% compared to the bulk value. Nitrogen ions were observed in this region, and their concentration was estimated to be 2.32 atoms per unit cell. In contrast to the lack of magnetic hysteresis in a YSZ single crystal, ferromagnetic hysteresis was observed in the ion-implanted YSZ crystals, owing to defects—nitrogen ions and oxygen vacancies in the surface layer.     

Density functional study of oxygen adsorption on the Mg3Nd (0 0 1) surface

The adsorption of oxygen atoms on Mg₃Nd (0 0 1) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the generalized gradient approximation (GGA) in the Perdew and Wang (PW91). The most preferred adsorption position was at the top-hollow site. Upon the optimization on top-hollow site with different coverage, it was found that the adsorption energy decreased with oxygen coverage. The density of states analysis showed that obvious charge transfer took place between O atom and the nearest Nd atom and chemical bond formed between O atom and the nearest Nd atom after O adsorption. The result of surface energy as a function of chemical potential change of oxygen indicated the clean Mg₃Nd (0 0 1) surface was easy to adsorb oxygen and form 1.00 ML surface.     

氯原子在Cu(111)表面的吸附结构和电子态

密度泛函理论(DFT)总能计算研究了不同覆盖度下氯原子在Cu(111)表面的吸附结构和表面电子态。计算结果表明,清洁Cu(111)表面自由能 为15.72 ,表面功函数φ为4.753eV。在1/4ML和1/3ML覆盖度下,每个氯原子在Cu(111)表面fcc谷位的吸附能分别等于3.278eV/atom和3.284eV/atom。在1/2ML覆盖度下,两个紧邻氯原子分别吸附于fcc和hcp谷位,氯原子的平均吸附能为2.631eV/atom。在1/3ML覆盖度下,fcc和hcp两个位置每个氯原子吸附能的差值约为2meV/atom,与正入射X光驻波实验结合蒙特卡罗方法得到结果(<10meV/atom)基本一致。在1/4ML、1/3ML和1/2ML覆盖度下,吸附后Cu(111)表面的功函数依次为5.263eV、5.275eV和5.851eV。吸附原子和衬底价轨道杂化形成的局域表面电子态位于费米能级以下约1.2eV、3.6eV和4.5eV等处。吸附能和电子结构的计算结果表明,氯原子间的直接作用和表面铜原子紧邻氯原子数目是决定表面结构的两个重要因素。     

Oxygen adsorption on (110) silver

The adsorption of oxygen on a (110)Ag surface is investigated by means of Auger electron spectroscopy, LEED and low energy helium ion scattering (IS). With LEED two ordered structures, i.e. (3×1) and (2×1) were observed at oxygen exposures of 1700 L and 7000 L respectively. The oxygen signal observed by AES and IS increases monotonically with oxygen exposure. The signals can be related to absolute coverage by comparison with Δφ measurements and by the use of the LEED data. With this calibration and with theoretical scattering cross-sections the IS measurements allow the position of the adsorbed oxygen to be estimated. The observation of a strong azimuthal anisotropy of the IS signal, e.g. a large oxygen signal if the plane of scattering is parallel to the [110] direction and a relatively small oxygen signal in the [100] direction, leads to the conclusion that the oxygen is adsorbed in a bridge position between two Ag atoms of the [110] surface channels, its centre being slightly below the centres of the Ag atoms.     

The interaction of carbon with Si(1 1 1):As and Si(1 1 1):H surfaces, a theoretical study

Density functional theory calculations have been performed to determine the adsorption site of carbon at the Si(1 1 1):As and Si(1 1 1):H surfaces at different coverages. The As- and H-passivated surfaces were simulated by replacing the topmost Si layer by As or by saturating the Si dangling bonds with hydrogen atoms, respectively. Different high symmetry sites were considered. Carbon was placed successively in the fourfold (T₄) or threefold coordinated (H₃), the ontop (T₁) sites or substituted for a Si atom in the S₅ position located underneath the Si adatom in the T₄ site. We found that the preferred carbon adsorption site depends on the coverage of the passivated surfaces. At low coverages i.e. at 1/16 ML and 1/3 ML, it prefers a distorted T₄ position whereas at 1 ML, it occupies an H₃ site. This contrasts with the clean surface where the most energetically favored site is the S₅ at all coverages. Carbon adsorption induces a significant change in the structural geometry of the surface atoms, leading to a charge re-arrangement in the surface layers.