Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Low energy electron diffraction and auger electron spectroscopy studies of the structure of adsorbed gases on solid surfaces

Low energy electron diffraction (LEED) studies of the structure of adsorbed molecules on crystal surfaces revealed that ordered surface structures predominate under most conditions of the experiments. In the absence of chemical reactions with the substrate, the degree of ordering depends on the heats of adsorption, ΔH_ads, and the activation energies for surface diffusion, ΔE_D1. Since ΔH_ads is usually markedly larger than ΔE_D1, small changes of substrate temperature facilitate ordering without appreciable increase in desorption rates. The surface structures of adsorbed gases that have been reported so far have been tabulated. For molecules whose size is compatible with the interatomic distance of the substrate, rules of ordering can be proposed that permit prediction of the structure of the adsorbed layer that is likely to form. These rules indicate close packing due to attractive interactions in the adsorbed layer, and that the rotational multiplicity of the substrate is likely to be maintained by the adsorbate structure. When molecules whose dimensions are larger than the substrate interatomic distance are adsorbed, the conditions that control ordering are more complex and simple rules may not be readily applicable.The surface structures of adsorbed gases have also been studied on high Miller Index substrate surfaces. These surfaces are characterized by ordered steps separated by terraces of low index surface orientation. Many gases have different ordering characteristics on stepped surfaces than on low index crystal faces due to the stronger substrate-adsorbate interactions in these surfaces. The dissociation of diatomic molecules at steps induces the formation of new types of surface structures (frequently one-dimensional) and the dehydrogenation of hydrocarbons at steps induces the formation of ordered carbonaceous surface structures that would not nucleate on low index substrate planes.So far, mostly work function changes upon adsorption gave indication of the magnitude of charge transfer upon adsorption and on forming of new surface chemical bonds. Most recently, chemical shifts of the Auger transitions of the substrate atoms and of the adsorbed molecules upon chemisorption, have been found to provide additional information on charge redistribution during adsorption.     

First-principles studies of BN sheets with absorbed transition metal single atoms or dimers: stabilities, electronic structures, and magnetic properties

BN sheets with absorbed transition metal (TM) single atoms, including Fe, Co, and Ni, and their dimers have been investigated by using a first-principles method within the generalized gradient approximation. All of the TM atoms studied are found to be chemically adsorbed on BN sheets. Upon adsorption, the binding energies of the Fe and Co single atoms are modest and almost independent of the adsorption sites, indicating the high mobility of the adatoms and isolated particles to be easily formed on the surface. However, Ni atoms are found to bind tightly to BN sheets and may adopt a layer-by-layer growth mode. The Fe, Co, and Ni dimers tend to lie (nearly) perpendicular to the BN plane. Due to the wide band gap of the pure BN sheet, the electronic structures of the BN sheets with TM adatoms are determined primarily by the distribution of TM electronic states around the Fermi level. Very interesting spin gapless semiconductors or half-metals can be obtained in the studied systems. The magnetism of the TM atoms is preserved well on the BN sheet, very close to that of the corresponding free atoms and often weakly dependent on the adsorption sites. The present results indicate that BN sheets with adsorbed TM atoms have potential applications in fields such as spintronics and magnetic data storage due to the special spin-polarized electronic structures and magnetic properties they possess.     

Dynamics of adsorption,exchange reactions,and defect formation at solid surfaces

In real time computer simulations of reactive atoms incident on solid surfaces, several interesting fundamental processes have been observed. These include (1) a cascade of inelastic collisions between incident atom and substrate atoms that eventually results in equilibration and adsorption; (2) for sufficiently reactive atoms, and sufficiently high substrate temperatures, an exchange reaction in which the adsorbed atom replaces a substrate atom; and (3) the formation of defects as a result of adsorption or exchange events.     

T1-relaxation of 129Xe on metal single crystal surfaces-multilayer experiments on iridium and monolayer considerations

The surface of a typical laboratory single crystal has about 10(15) surface atoms or adsorption sites, respectively, and is thus far out of reach for conventional NMR experiments using thermal polarization. It should however be in reach for NMR of adsorbed laser polarized (hyperpolarized) 129Xe, which is produced by spin transfer from optically pumped rubidium. With multilayer experiments of xenon adsorbed on an iridium surface we do not only demonstrate that monolayer sensitivity has been obtained, we also show that such surface experiments can be performed under ultra high vacuum conditions with the crystal being mounted in a typical surface analysis chamber on a manipulator with far-reaching sample heating and cooling abilities. With only four spectra summed up we present an NMR signal from at most 4x10(14) atoms of 129Xe, four layers of naturally abundant xenon, respectively. The fact that no monolayer signal has been measured so far is explained by a fast Korringa relaxation due to the Fermi contact interaction of the 129Xe nuclei with the electrons of the metal substrate. T(1)-relaxation times in the order of several ms have been calculated using all electron density functional theory for several metal substrates.     

Formation of strontium atomic chains on the singular and stepped Si(111) surfaces

The surface structures 3 × 2, 5 × 2, 7 × 2, and 9 × 2 formed on the Si(111) plane during adsorption of submonolayer strontium have been investigated using scanning tunneling microscopy. The experimental data obtained are in agreement with the models of surface reconstructions proposed earlier for the structures induced by the adsorption of divalent metal atoms. An important constituent of these structures is provided by strontium atomic chains in the 〈1$ \bar 1 $ \bar 1 0〉 direction. Three types of domains of surface structures are formed on the Si(111) plane, which correspond to the rotational symmetry C ₃ of the surface under consideration. The reduction of the symmetry of the substrate to C ₁ with the use of the stepped Si(7 7 10) surface makes it possible to form strontium atomic chains, which are oriented with respect to the substrate in a similar manner.     

A theoretical investigation of the binding of TiCln to MgCl2

The structure of the (0 0 0 1) surface of the -MgCl₂ crystal has been investigated using DFT-GGA periodic calculations. The calculated surface relaxation is in agreement with LEED measurements. Motivated for the use of MgCl₂ as support for the Ziegler–Natta reaction, we have studied the adsorption of the catalyst (titanium chlorides as monomers or dimers) on the (1 0 0) and (1 1 0) MgCl₂ surfaces. The structures of adsorbed species are close to those previously found on cluster models: bridging chlorine atoms connect the Ti to the Mg atoms and the systems remain in high spin states. The (0 0 0 1) surface is the most stable face of the -MgCl₂ crystal; however it is Cl-terminated and henceforth poorly reactive; it had been suggested to deposit metallic Mg in order to improve its reactivity. Our modelling explains the failure of this tentative; the interaction between the deposited metal and the surface is repulsive and uncharged Mg atom does not bind.     

NiTi(110)表面氧原子吸附的第一性原理研究

为了研究给定的NiTi的表面氧化过程,在保持体系中Ni和Ti原子总数相等的条件下,构建了一系列Ti原子在表面反位的c(2×2)-NiTi(110)缺陷体系,并利用第一性原理计算研究了氧原子在各种NiTi(110)反位缺陷体系的吸附行为以及表面形成能.计算结果表明:吸附氧原子的稳定性与表面Ti原子的富集程度有很大的关联性,体系表面Ti原子富集程度越高,氧原子吸附的稳定性越高;当覆盖度较高时,由于氧原子的吸附,可使Ni和Ti原子在表面出现反位.在富氧条件(μ_o≥-9.35 eV)下,氧原子在表面第1层中的全部Ni原子与第3层全部Ti换位的反位缺陷体系上的吸附最稳定,此时随着氧原子的吸附,表面上的Ti原子升高,导致向上膨胀生长形成二氧化钛层,且在其下方形成富Ni层,由此可合理地解释实验上发现NiTi合金氧化形成二氧化钛层的可能原因.     

On interfacial free energy versus interfacial stress of fluids adsorbed on chemically patterned surfaces: lessons from the special case of striped substrates

The statistical mechanics of fluids adsorbed on chemically patterned surfaces is far from straightforward when attempting to develop virial theorems for the interfacial free energy. This is because in general one would have to devise new procedures to distinguish surface tension from surface stress, even for wall-fluid models where the substrate atoms are replaced by an effective external field. However, the distinction may be made explicit for the special case of striped walls. This paper makes use of striped wall-fluid models to discuss the significance of fluid interfacial stress to patterned inhomogeneous fluids. In particular, it considers the adsorption of hemicylindrical drops on an array of high energy stripes. For the planar stripe geometry it is also possible to make effective use of the pressure tensor formalism. Beyond the special case of striped patterns it is not possible to use standard procedures based on virial theorems to directly evaluate interfacial free energy. The lessons learnt from these exercises apply especially to computer simulation studies of patterned inhomogeneous fluids.     

Effects of metal-support interactions on the electronic structures of metal atoms adsorbed on the perfect and defective MgO(1 0 0) surfaces

The electronic structures of Ni, Pd, Pt, Cu, and Zn atoms adsorbed on the perfect MgO(1 0 0) surface and on a surface oxygen vacancy have been studied at the DFT/B3LYP level of theory using both the bare cluster and embedded cluster models. Ni, Pd, Pt, and Cu atoms can form stable adsorption complexes on the regular O site of the perfect MgO(1 0 0) surface with the binding energies of 19.0, 25.2, 46.7, and 17.3 kcal/mol, respectively, despite very little electron transfer between the surface and the metal atoms. On the other hand, adsorptions of Ni, Pd, Pt, and Cu atoms show strong interaction with an oxygen vacancy on the MgO(1 0 0) surface by transferring a significant number of electron charges from the vacancy to the adsorbed metal atoms and thus forming ionic bonds with the vacancy site. These interactions on the vacancy site for Ni, Pd, Pt, and Cu atoms increase the binding energies by 25.8, 59.7, 85.2, and 19.1 kcal/mol, respectively, compared to those on the perfect surface. Zn atom interacts very weakly with the perfect surface as well as the surface oxygen vacancy. We observed that the interaction increases from Ni to Pt in the same group and decreases from Ni to Zn in the same transition metal period in both perfect and vacancy systems. These relationships correlate well with the degrees of electron transfer from the surface to the adsorbed metal atom. The changes in the ionization potentials of the surface also correlate with the adsorption energies or degrees of electron transfers. Madelung potential is found to have significant effects on the electronic properties of metal atom adsorptions on the MgO(1 0 0) surface as well as on an oxygen vacancy, though it is more so for the latter. Furthermore, the Madelung potential facilitates electron transfer from the surface to the adsorbed metal atoms but not in the other direction.     

α-P/GaAs(100)界面的光电子能谱研究

本文采用X射线光电子能谱、紫外光电子能谱和低能电子衍射对室温下P在GaAs(100)表面上的生长进行了研究。结果表明,在生长初期P是成团吸附的,随着淀积量的增加而生长成α-P薄膜,该薄膜的价带结构与等离子体淀积的α-P:H薄膜的价带结构相似。在界面处有约一单层的P与衬底表面的Ga成键。α-P覆盖层使GaAs表面势垒下降约0.2eV。 关键词：     

FT-IR、XPS和DFT研究乙酰氧肟酸钠在针铁矿或赤铁矿表面的吸附机理

采用FT-IR、X射线光电子能谱以及基于周期平面波的DFT方法分别研究了乙酰氧肟酸钠(aHA)在针铁矿或赤铁矿表面上的吸附结构,并将计算得到的光电子能谱移动和电荷转移实验得到的XPS结果进行对比.红外结果显示aHA可能以五元环螯合物形式吸附于针铁矿或赤铁矿表面,DFT计算结果表明aHA中氧肟酸基团的两个氧原子与针铁矿(101)或(100)表面上的一个铁原子形成五元环结构,同时氧肟酸基团中的两个氧还相应的与针铁矿表面邻近两个铁原子成键,但氧肟酸基团中的两个氧原子却只与赤铁矿(001)表面上的一个铁原子形成五元环结构.对于针铁矿(101)、(100)和赤铁矿(001)表面上吸附位点对应的铁原子,计算得到的光电子能谱移动与实验得到的光电子能谱移动基本相等,说明了计算结果的可靠性和所得吸附结构的合理性.     

硫在Fe(100)面吸附的第一性原理研究

基于密度泛函理论的第一性原理方法,在广义梯度近似下,计算了硫原子在Fe(100)面吸附的结构和电子性质,并计算了其分子轨道和吸附能.同时讨论了相关吸附性质与硫原子表面覆盖度(0.25-1.0ML)的关系.结果表明：硫原子吸附在H位最稳定,吸附能均随浓度的增加而单调增加;B位吸附的硫原子与Fe(100)表面的距离随浓度非单调变化,在0.5ML时达到最大,是由较高的局域电子云重叠产生的排斥作用所导致的;对比分析吸附前后硫和Fe的s及p电子的态密度,显示了硫化亚铁的生成.     

First-principles study of carbon atoms adsorbed on MgO(100) related to graphene growth

We have performed density functional theory calculations to understand the initial growth of graphene by studying the adsorption of carbon atoms on the oxide substrates such as magnesium oxide. For adsorption behaviors of carbon atoms on the MgO(100) surface, their adsorption geometries and binding energies are calculated. The binding of a carbon atom is the most stable at the on-top oxygen site on MgO(100). Such strong C–O binding is analyzed by examining the projected density of states. Then, we also increase the number of carbon atoms on MgO(100) to investigate their adsorption behaviors. Due to strong binding between carbon atoms, adsorbed carbon atoms form chain-like or graphene-like structures on the surface. Combined with relatively strong C–O binding, this result may explain the graphene growth on MgO(100) observed in available experiments.     

H2S在Fe(100)面吸附的第一性原理研究

基于密度泛函理论第一性原理, 在广义梯度近似下, 研究了表面覆盖度为0.25 ML (monolayer)时硫化氢分子在Fe(100)面吸附的结构和电子性质, 并与单个硫原子吸附结果进行了对比. 结果表明: 硫化氢分子吸附在B2位吸附能最小为-1.23 eV, 最稳定, B1位吸附能最大为-0.01 eV, 最不稳定; 并对硫化氢分子在B1位和B2位吸附后的电子态密度进行了分析, 也表明了吸附在B2位稳定, 且吸附在B2位后硫化氢分子几何结构变化不大; 将硫化氢中硫原子吸附与单个硫原子吸附的电子性质进行了比较, 发现前者吸附作用非常微弱; 同时对吸附后的Fe(100)面进行了对比, 单个硫原子吸附的Fe(100)面电子态密度出现了一系列峰值且离散分布, 生成了硫化亚铁, 表明在硫化氢环境下, 主要是硫化氢析出的硫原子发生了吸附. 关键词： 第一性原理 Fe(100)表面 吸附能 硫化氢     

Xe and Kr adsorption on the Si(111) 7 × 7 surface

A study of adsorption of Xe and Kr on the Si(111) 7 × 7 surface is presented. Low energy electron diffraction indicates the substrate structure is not appreciably modified by the adsorption. The data consist primarily of isobars, i.e. the amount adsorbed as a function of temperature at fixed pressure. The stepwise adsorption of each of the first several adatoms per surface unit mesh is resolved. This indicates a sequence of distinguishable adsorption sites in the mesh each of which can be occupied by a single atom. There are appreciable differences in the measured binding energies between the various sites. Pairwise sums of 6–12 potentials have been used to estimate binding energies expected for sites presented by proposed structural models of the surface. The observed sequence of sites seems to exclude buckled models and those models which involve only arrays of identical entities. No presently published model is consistent with these data in detail. The triangular island models with some modifications might present sufficiently large binding energies and rich sequences of sites. Whatever the actual structure, it must be more complex than those which have usually been considered.     

CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal-CO binding energy and the C-O stretching frequency change as function of the substrate site where the metal atom is bound.     

吡嗪分子在硅(100)面吸附构型的XPS和NEXAFS谱理论研究

本文利用X射线谱研究了吡嗪(C₄H₄N₂)分子共价吸附于硅(100)面的几种吸附构型的几何结构和电子结构. 利用密度泛函理论结合团簇模型,对预测的吸附结构的碳K壳层(1s)X射线光电子能谱(XPS)和近边X射线吸收精细结构(NEXAFS)谱进行了模拟. 计算结果阐明了XPS和NEXAFS谱与不同吸附构型的对应关系. 与XPS谱相比,NEXAFS谱对所研究的吡嗪/硅(100)体系的结构有明显的依赖性,可以很好地用于结构鉴定. 根据碳原子的分类,研究了在NEXAFS光谱中不同化学环境下碳原子的光谱成分.     

Structural Features of Boron-Doped Si（113） Surfaces Simulated by ab initio Calculations

Based on ab initio calculations, boron-doped Si（113） surfaces have been simulated and atomic structures of the surfaces have been proposed. It has been determined that surface features of empty and filled states that are separately localized at pentamers and adatoms indicates a low surface density of B atoms, while it is attributed to heavy doping of B atoms at the second layer that pentamers and adatoms are both present in an image of scanning tunnelling microscopy. B doping at the second layer should be balanced by adsorbed B or Si atoms beside the adatoms and inserted B interstitials below the adatoms.