Ethers on Si(001): A Prime Example for the Common Ground between Surface Science and Molecular Organic Chemistry

By using computational chemistry it has been shown that the adsorption of ether molecules on Si(001) under ultrahigh vacuum conditions can be understood with classical concepts of organic chemistry. Detailed analysis of the two‐step reaction mechanism—1) formation of a dative bond between the ether oxygen atom and a Lewis acidic surface atom and 2) nucleophilic attack of a nearby Lewis basic surface atom—shows that it mirrors acid‐catalyzed ether cleavage in solution. The O−Si dative bond is the strongest of its kind, and the reactivity in step 2 defies the Bell–Evans–Polanyi principle. Electron rearrangement during C−O bond cleavage has been visualized with a newly developed method for analyzing bonding, which shows that the mechanism of nucleophilic substitutions on semiconductor surfaces is identical to molecular S_N2 reactions. Our findings illustrate how surface science and molecular chemistry can mutually benefit from each other and unexpected insight can be gained.     

Dissociation of N2 on a Si(111)-7x7 Surface at Room Temperature

We demonstrate that the strong N₂ bond can be efficiently dissociated at low pressure and ambient temperature on a Si(111)-7x7 surface. The reaction was experimentally investigated by scanning tunnelling microscopy and X-ray photoemission spectroscopy. Experimental and density functional theory results suggest that relatively low thermal energy collision of N₂ with the surface can facilitate electron transfer from the Si(111)-7x7 surface to the π*-antibonding orbitals of N₂ that significantly weaken the N₂ bond. This activated N₂ triple bond dissociation on the surface leads to the formation of a Si₃N interface.     

金属-有机配合物分子在Au(111)表面的吸附行为

用STM对含氧桥的金属-有机配合物[Cu₂(μ-O)(dptap)4(NO₃)₂]分子在Au(111)表面的吸附行为进行了研究. STM结果表明, 该分子同时存在非解离吸附和解离吸附, 大部分分子在Au(111)面形成有规则的排列, 少量分子发生解离吸附, 并形成(√3×√3)R30°Cu原子吸附结构. 探讨了两种吸附现象共存的起因.     

Mo(001)表面的CaO(001)薄膜模型催化体系的STM研究

氧化物单晶化薄膜的制备与表征是研究氧化物表面性质的重要方法,也是模型催化研究的前沿领域。本文主要综述了Fritz-Haber研究所的Hajo Freund小组在过去几年间围绕着以Mo(001)为衬底制备的CaO(001)薄膜模型催化体系而进行的表面结构和化学性质的系列研究。其中既包含了氧化物薄膜研究的共同特点,如界面效应、膜厚效应等,也包含有CaO/Mo体系独特的性质,如Mo的自发掺杂对表面性质的调控作用。在该系列研究中低温扫描隧道显微镜(LT-STM)技术的应用贯穿了方方面面,从原子结构表征到电子性质研究,从杂质缺陷的鉴别到表面物种荷电性质的分析等。STM所获得的微观信息直接从原子分子水平揭示了调控薄膜表面性质的各种控因。特别的,在理论计算的辅助下,不断深化认识氧化物掺杂调控的原理和机制,为设计新型催化剂提供重要思路。     

Surface Chemistry of tert‐Butylphosphine (TBP) on Si(001) in the Nucleation Phase of Thin‐Film Growth

We combine density functional theory calculations and scanning tunneling microscopy investigations to identify the relevant chemical species and reactions in the nucleation phase of chemical vapor deposition. tert‐Butylphosphine (TBP) was deposited on a silicon substrate under conditions typical for surface functionalization and growth of semiconductor materials. On the activated hydrogen‐covered surface H/Si(001) it forms a strong covalent P?Si bond without loss of the tert‐butyl group. Calculations show that site preference for multiple adsorption of TBP is influenced by steric repulsion of the adsorbate's bulky substituent. STM imaging furthermore revealed an anisotropic distribution of TBP with a preference for adsorption perpendicular to the surface dimer rows. The adsorption patterns found can be understood by a mechanism invoking stabilization of surface hydrogen vacancies through electron donation by an adsorbate. The now improved understanding of nucleation in thin‐film growth may help to optimize molecular precursors and experimental conditions and will ultimately lead to higher quality materials.     

Antiferromagnetic Ground State of a C60‐Covered Si(001) Surface

Modeling magnetism: The antiferromagnetic ground state of the C₆₀/Si(001)‐c(4×4) surface is predicted by means of density functional theory calculations. Two adjacent dangling bonds (DBs) generated by the adsorption of C₆₀ are antiferromagnetically coupled with each other. This study demonstrates that magnetic Si surfaces can be prepared by engineering single Si DBs with unpaired electrons.

     

Size Effects for the Adsorption of Alkali Metal Atoms on the Si(001) Surface

The adsorptions of a series of alkali metal(AM) atoms, Li, Na, K, Rb and Cs, on a Si(001)-2×2 surface at 0.25 monolayer coverage have been investigated systematically by means of density functional theory calculations. The effects of the size of AM atoms on the Si(001) surface are focused in the present work by examining the most stable adsorption site, diffusion path, band structure, charge transfer, and the change of work function for different adsorbates. Our results suggest that, when the interactions among AM atoms are neglectable, these AM atoms can be divided into three classes. For Li and Na atoms, they show unique site preferences, and correspond to the strongest and weakest AM–Si interactions, respectively. In particular, the band structure calculation indicates that the nature of Li–Si interaction differs significantly from others. For the adsorptions of other AM atoms with larger size(namely, K, Rb and Cs), the similarities in the atomic and electronic structures are observed, implying that the atom size has little influence on the adsorption behavior for these large AM atoms on the Si(001) surface.     

Au(111)面甲烷硫醇吸附的密度泛函理论研究

采用基于密度泛函理论的第一性原理方法和平板模型研究了CH₃SH分子在Au(111)表面的吸附构型和电子结构. 系统地计算了S原子在不同位置以不同方式吸附的系列构型, 计算结果表明, CH₃SH分子倾向于吸附在top位上, S-C键相对于Au表面法线的夹角为62°～78°|而S-H键断裂后CH₃S_H则倾向于吸附在bri-fcc位上, S-C键相对于Au(111)表面法线的夹角为49°～57°. 比较分析CH₃SH分子和CH₃S_H的吸附, 发现CH₃SH分子倾向于不解离吸附, 表面温度的提升和缺陷的出现可能促使S-H键的断裂. 通过比较S原子在独立的CH₃SH分子和吸附状态下的局域态密度, 发现S-H键断裂后S原子和表面的键合强于S-H键未断裂时S原子和表面的键合. 扫描隧道显微镜(STM)图像模拟显示了CH₃SH和CH₃S_H在Au(111)表面吸附的3个典型的STM图像.     

Iron adsorption on SiO2/Si(111)

The adsorption of ferric and ferrous iron onto the native oxide of the SiO₂/Si(111) surface has been evaluated using X‐ray photoelectron spectroscopy (XPS). Through a series of immersion experiments, performed at room temperature and pH 1, it has been shown that the ferric species is strongly adsorbed onto the hydrophilic surface, while ferrous iron remains in solution. Dehydroxylation of the silica surface by etching with hydrofluoric acid reduces the concentration of receptive Si‐OH groups, thereby limiting iron adsorption. The experiments were reproduced in a combined ultrahigh vacuum‐electrochemical system (UHV‐EC), which allowed a carbon‐free surface to be prepared before contacting the iron solutions, and confirmed the strong affinity of ferric iron towards the SiO₂/Si(111) surface. Copyright © 2007 John Wiley & Sons, Ltd.     

Au-Induced Faceting of Si(5 5 12)

The growth of Au on the stable, high-index Si(5512) surface has been studied using scanning tunneling microscopy (STM). At very low coverages and moderate annealing temperatures (0.1ML, 400–500°C), Au appears to decorate the underlying Si rows and form an array of rows that maintains the underlying (5512) periodicity of 5.4nm. For higher annealing temperatures and coverages, however, Au causes faceting to a number of nearby planes. The two primary facets formed at lower (0.15ML) and higher (0.5–2ML) coverages are the (337) and (225) planes, which are tilted 0.7° down [towards (111)] and 1.1° up from (5512), respectively. Both orientations are in fact subunits of the (5512) unit cell, so their presence is not surprising. In addition to these facets, two types of sawtooth morphologies composed of planes oriented further from (5512) are found at very high annealing temperatures (800–900°C). These include (113)+(7715) planes at very low coverage (0.05ml) and (113)+(5511) planes at higher coverage (1ML), where (113) is tilted up by 5.3° and (7715) and (5511) are tilted down by 2.9° and 2.2°, respectively. Au adsorption on Si(5512) therefore results in the formation of five possible facet planes: (113), (225), (337), (5511), and (7715).     

Combined XPS and DFT investigation of the adsorption modes of methyl enol ether functionalized cyclooctyne on Si(001)

The reaction of methyl enol ether functionalized cyclooctyne on the silicon (001) surface was investigated by means of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Three different groups of final states were identified; all of them bind on Si(001) via the strained triple bond of cyclooctyne but they differ in the configuration of the methyl enol ether group. The majority of molecules adsorbs without additional reaction of the enol ether group; the relative contribution of this configuration to the total coverage depends on substrate temperature and coverage. Further configurations include enol ether groups which reacted on the silicon surface either via ether cleavage or enol ether groups which transformed on the surface into a carbonyl group.     

不同覆盖度下Li原子在Si(001)表面上的吸附构型和电子结构

采用基于赝势平面波基组的密度泛函理论, 对不同Li原子覆盖度下Li/Si(001)体系的吸附构型、电子结构以及吸附Li原子对表面性质的影响进行了系统研究. 计算结果表明, 在所考察的覆盖度范围内, Li原子倾向于吸附在相邻两个Si-Si二聚体之间各种对称性较高的空穴位, 其中覆盖度为0.75 ML(monolayer)时具有最小的平均吸附能. 由能带结构分析结果可知, 随着覆盖度的增大, Si(001)表面存在由半导体→导体→半导体的变化过程. 在覆盖度为1.00 ML时, 由于表层二聚体均受到显著破坏, 使得体系带隙明显增大. 吸附后, 有较多电子从Li原子转移到底物, 导致Si(001)表面功函显著下降, 并随着覆盖度的增加表面功函呈现振荡变化. 此外, 从热力学稳定性角度上看, 覆盖度为0.75 ML的Li/Si(001)表面较难形成.     

ANISOTROPIC STRAIN EFFECT ON MORPHOLOGY AND ATOMIC STRUCTURE OF VICINAL Si(001) SURFACE

On vicinal Si(001) surfaces, dependence of growth morphology on the applied strain direction and formation of vacancy lines from Ag-induced missing dimer vacancies are studied. Both phenomena are intimately related to the anisotropic nature of the strain field which originates from the surface dimerization. Strain relief mechanism, reflecting on the surface morphology, is shown to be different in two orthogonal directions. Normal to the steps, step-pair bunching and waving lead to formation of hillocks and pits. Along the step direction, bending of step pairs forms a cusp which later develops into a deep groove. Toward the atomic scale, the formation of the vacancy lines is driven by the short-range attractive interaction between the vacancies in adjacent dimer rows and the long-range repulsive interaction between them in the same dimer row. A full form and magnitudes of the interactions are derived from the thermally-excited wandering of the vacancy lines formed by a nominal amount of Ag depositing onto the surface.     

Photoelectron spectroscopy and diffraction of surface nanoscale NbO/Nb(110) structures

X-ray photoelectron spectroscopy and diffraction (XPS and XPD) are applied to analyze oxygen-induced surface structures on the Nb(110) face formed due to oxygen segregation from the crystal bulk on thermal annealing to 2000 K in vacuum and/or oxygen adsorption in situ at temperatures above 1100 K. The Nb3d, O1s electronic states and valence band spectra of the NbO _x /Nb(110) surface are studied by XPS, and the results are compared with data for NbO, NbO₂, and Nb₂O₅ oxides. It is shown that niobium atoms entering the composition of surface oxide structures on Nb(110), from the standpoint of the nearest environment and chemical bond, are similar to metal states in NbO. The NbO _x layer thickness is estimated to be 0.5 nm. Two chemically inequivalent oxygen states are distinguished on Nb(110), which are, presumably, atomic chemisorbed oxygen on the parts of the clean surface of the Nb monolayer with hexagonal packing and oxygen in the composition of NbO _x -like linear clusters on Nb(110). A model of the NbO _x /Nb(110) surface takes into account a distortion of the structure of NbO _x clusters: a periodic vertical shift of metal atoms in Nb-chains and changes in Nb-O bond angles. Original Russian Text Copyright ? 2009 by M. V. Kuznetsov, A. S. Razinkin, and E. V. Shalaeva __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 50, No. 3, pp. 536–543, May–June, 2009.     

Theoretical Investigation on the Adsorption of Ag^＋ and Hydrated Ag^＋ Cations on Clean Si（111） Surface

In this paper, the adsorption of Ag^＋ and hydrated Ag^＋ cations on clean Si（111） surface were investigated by using cluster （Gaussian 03） and periodic （DMol^3） ab initio calculations. Si（111） surface was described with cluster models （Si14H17 and Si22H21） and a four-silicon layer slab with periodic boundary conditions. The effect of basis set superposition error （BSSE） was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Ag^＋ cations and clean Si（111） surface are large, suggesting a strong interaction between hydrated Ag^＋ cations and the semiconductor surface. With the increase of number, water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Ag^＋ cation. The Ag^＋ cation in aqueous solution will safely attach to the clean Si（111） surface.     

CO分子在TiC(001)表面上的吸附构型与电子结构

采用第一性原理方法和平板模型对CO分子在TiC(001)表面的吸附构型和电子结构进行了详细研究. 结果表明, CO分子倾向于采用C端吸附在表层Ti原子上方. 对于该吸附方式, 计算得到的吸附能、CO各电子态所处能级位置以及C—O键伸缩振动频率的红移值均与实验观测结果相吻合. 由能带结构和Mvlliken布居分析结果可知, 当采用C端吸附时, CO的5σ和2π鄢态受到底物影响最为显著, 尤其是C端的桥位吸附方式. 此外, 还进一步对底物表面态在CO吸附过程中的作用进行了探讨.     

Chemistry of O‐ and H‐Containing Species on the (001) Surface of Anatase TiO2: A DFT Study

The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO₂ (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H₂O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO₂ surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O₂ adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O₂. In the adsorbed OOH, the O? O bond is considerably lengthened and thus weakened.     

NiO(001)面吸附NO电子结构的理论研究

用嵌入点电荷的簇为模型,采用B3LYP方法研究了NO在NiO(001)面上不同吸附位置和不同吸附方式的电子结构,并计算了振动频率。结果表明在完整表面上最稳定的吸附方式是N端歪斜吸附在Ni位,这与实验结果相符;而在缺陷表面上,NO以直线方式吸附在角位上最为稳定。振动频率计算表明,NO吸附后NO振动频率红移约50cm-1。我们根据价轨道电子密度图解释了NO吸附后振动频率红移的原因,这个解释完全不同于传统认为的3d对2π的反馈键理论。