Diffusion en surface de l'argent sur les plans (001), (111), (110) et des surfaces vicinales du cuivre

The diffusion coefficients of silver on singular and vicinal surfaces of copper have been measured using an oxidation method to determine the concentration profiles. The diffusion was carried out under ultra high vacuum at low temperature (250 to 500 °C) with a low concentration of the diffusing species (less than 0.5 monolayers). Comparison of the results with the terrace-ledge-kink model shows that the diffusion occurs along the ledges and also explains the diffusion on the vicinal surfaces. The differences between this model and the diffusion observed on the singular surfaces are explained by the presence of separated ledge loops on these surfaces.     

Theoretical study of the Ni growth on Pt stepped surfaces

We have investigated the growth of Ni on Pt stepped surfaces with (1 1 1) terraces by means of potentials derived from the second moment approximation in a tight-binding model. The activation energies associated to these processes are determined. The Schwoebel barriers of Ni atoms descending steps of Pt stepped surfaces are calculated for different kinds of straight steps (A and B steps) differing by the orientation of the ledge. In addition, we study the diffusion of Ni adatoms at fcc or hcp sites in the presence of small adislands on the terraces, in the vicinity of the A and B steps. We show that a good estimate of the potential wells and diffusion barriers could be given by introducing a lateral effective pair interaction model, the interactions extending up to the next nearest neighbors. Finally, we have carried out Kinetic Monte-Carlo simulations to investigate the Ni wire formation at Pt step edges and the influence of the exchange processes in the alloy formation.     

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.     

High-temperature morphology of stepped gold surfaces

Molecular dynamics simulations with a classical many-body potential are used to study the high-temperature stability of stepped non-melting metal surfaces. We have studied in particular the Au(111) vicinal surfaces in the (M + 1,M− 1,M) family and the Au(100) vicinals in the (M,1,1) family. Some vicinal orientations close to the non-melting Au(111) surface become unstable close to the bulk melting temperature and facet into a mixture of crystalline (111) regions and localized surface-melted regions. On the contrary, we do not find high-temperature faceting for vicinals close to Au(100), also a non-melting surface. These (100) vicinal surfaces gradually disorder with disappearance of individual steps well below the bulk melting temperature. We have also studied the high-temperature stability of ledges formed by pairs of monatomic steps of opposite sign on the Au(111) surface. It is found that these ledges attract each other, so that several of them merge into one larger ledge, whose edge steps then act as a nucleation site for surface melting.     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.     

The instability of vicinal electrode surfaces against step bunching II: Theory

It is shown that vicinal surfaces with a regular step array in contact with an electrolyte are intrinsically unstable against a phase separation into areas of flat terraces and areas of step bunches. The effect is caused by the step dipole moment which lowers the potential of zero charge (pzc) of stepped surfaces. Specific calculations performed for vicinal surfaces of silver are qualitatively in keeping with experimental observations. Step bunching is likewise expected for vicinal surfaces of gold and platinum.     

A LEED study of UO2(∼100) vicinal surfaces

The ordering and faceting properties of UO₂(～100) vicinal surfaces have been studied via LEED and Auger measurements. The measurements have demonstrated a reduced tendency for step ordering on UO₂(～100) vicinal surfaces when compared to step ordering on UO₂(～111) vicinal surfaces. The UO₂(～100) vicinal surfaces were observed to decompose irreversibly into low-index facets, including prominent (100) facets, at temperatures below those needed for creation of lowest index faceting on UO₂(～111) vicinal surfaces. These properties suggest that (100) terraces, in contrast to (111) terraces, act as surface diffusion barriers that limit longrange surface communication while growing at the expense of intermediate faceting stages.     

The study of vicinal GaAs( 100) surfaces by PAX

The adsorption of xenon at low temperatures on both GaAs(100) and vicinal surfaces has been studied using ultraviolet photoemission spectroscopy. The Xe 5p peaks show a characteristic shift to lower binding energy with surface As-depletion. Additional weak emission features seen on the vicinal planes, shifted by around 0.45 eV to higher binding energy, are attributed to Xe adsorbed at the step sites on such surfaces.     

Interplay between atomic and mesoscopic order on gold vicinal surfaces

Self-organization on Au(1,1,1) vicinal surfaces provides a unique opportunity to study the interplay between atomic and mesoscopic order. First, experimental results demonstrate the different interactions between steps and surface reconstruction on Au(1,1,1) vicinal surfaces. Depending on the step atomic structure, lines of discommensurations are found to be either parallel or perpendicular to the step edges. This leads to a complete understanding of the mesoscopic self-organization on theses surfaces, which drastically depends on the step structure. This points out the crucial role played by the edge energy cost which can monitor the faceting periodicity in a wide range of values.     

基于蒙特卡罗方法的4H-SiC(0001)面聚并台阶形貌演化机理

针对SiC外延生长中微观原子动力学过程,建立了一个三维蒙特卡罗模型来研究偏向■或■方向4H-SiC(0001)邻晶面上台阶形貌演化过程,并且利用Burton-Cabera-Frank理论分析了其形成机理.在蒙特卡罗模型中,首先建立了一个计算4H-SiC晶体生长过程的晶格网格,用来确定Si原子和C原子晶格坐标以及联系它们之间的化学键;其次,考虑了原子在台阶面上的吸附、扩散,原子在台阶边上的附着、分离以及传输等过程;最后,为了更加详细地捕捉微观原子在晶体表面的动力学过程信息,该模型把Si原子和C原子分别对待,同时还考虑了能量势垒对吸附原子影响.模拟结果表明:在偏向■方向的4H-SiC(0001)邻晶面,有一个晶胞高度的聚并台阶形貌形成,而对于偏向■方向的邻晶面,出现了半个晶胞高度的聚并台阶形貌,该模拟结果与实验中观察到的结果相符合.最后,利用Burton-Cabera-Frank理论对聚并台阶形貌演化机理进行了讨论.     

Identification of step atoms by high resolution core level spectroscopy

Vicinal Rh(111) surfaces are studied with high resolution core level photoemission. We demonstrate the possibility to distinguish between the different kinds of surface atoms on these surfaces by virtue of their 3d core level binding energies. In particular, the low coordinated step atoms are found to exhibit a clear fingerprint in Rh 3d spectra. We demonstrate how this may be used to show that initial oxygen adsorption occurs on the steps and not on the terraces of the vicinal surfaces.     

合成氨钌催化剂的晶体和形貌敏感性

哈伯-博施法合成氨反应是高温高压的耗能过程,因此降低该过程的能量消耗及开发温和条件下合成氨反应催化剂具有重要意义. 金属钌是合成氨反应中最有前途的催化剂之一,一直备受广泛关注. 确定金属钌催化剂的结构敏感性并提高其比质量活性是多相催化中亟待解决的重要问题. 氮气(N₂)活化是合成氨反应中的关键步骤. 本文通过第一性原理理论计算和微观动力学模拟方法系统研究了具有六方密排和面心立方晶体结构的钌催化剂上N₂活化过程和N₂解离反应速率. 理论计算研究表明,在六方密排Ru形貌中,{2130}晶面具有最高的N₂解离活性,其次是{0001}台阶面,它们比六方密排Ru其他表面上N₂解离反应速率高3个数量级以上;在面心立方Ru形貌中,{211}和{311}表面上N₂解离活性最高. 这些结果都表明台阶面/台阶位对氮气活化至关重要. 虽然六方密排Ru {2130}晶面具有最低的N₂解离能垒,然而由于面心立方Ru上可以暴露更高密度的活性位点,使得面心立方Ru比六方密排Ru具有更高的N₂转化速率. 本研究深入理解了N₂解离过程中,金属Ru 催化剂形貌和晶相结构敏感性,这为设计和优化高活性的合成氨Ru催化剂提供了理论基础.     

Growth of copper phthalocyanine on hydrogen passivated vicinal silicon(1 1 1) surfaces

Using ultra-high vacuum scanning tunneling microscopy (UHV-STM), we show that copper-phthalocyanine (CuPc) grows in a well ordered manner on hydrogen passivated vicinal silicon surfaces. CuPc grows one-dimensionally parallel to the monatomic steps on the vicinal silicon surface. Surprisingly, elongated clusters of the CuPc parallel to the step directions are formed even on the middle of the terraces well away from the step edges. The one-dimensional growth mode continues even after the full monolayer coverage on the substrate which results in strongly oriented growth mode of a thin film of CuPc on the vicinal silicon surfaces.     

Rule for structures of open metal surfaces

We present a clear and simple rule for determining the relaxation sequences on open (stepped, vicinal, or high-Miller-index) metal surfaces. At the bulk-truncated configuration of a surface, a surface slab is defined where the coordination of atoms is reduced from the bulk. The rule predicts that the interlayer spacings within this slab contract, while the interlayer spacing between this slab and the substrate expands. By first-principles calculations, we show that this rule is obeyed on all open Cu surfaces with interlayer spacings down to about 0.5 A. We also illustrate a direct relation of the relaxation sequences to the charge redistribution on these surfaces, which is demonstrated to be driving the multilayer relaxations. The applicability of the rule can be extended to other fcc and bcc metals, including unreconstructed and missing-row surfaces.     

Surface mass transport of Ag and In on vicinal Si(111)

Mass transport of Ag and In on vicinal Si(111) has been investigated by scanning Auger microscopy (SAM). Highly anisotropic surface diffusion and surface electromigration due to direct current were observed for Ag and In adatoms on 0°−, 0.5°−, 3°− and 6°−off vicinal Si(111) surfaces. The diffusion on the intermediate layer is strongly enhanced in the direction parallel to the step edge for Ag adatoms, while it is remarkably suppressed in the direction perpendicular to the step edge for In adatoms. The activation energy of the diffusion for the Ag adatoms ranged between 0.81 and 1.3 eV, while that for In adatoms increased from 0.31 to 0.66 eV with increasing the vicinal angle. The anisotropic diffusion transport is explained in terms of the step structure and the difference in the binding energy at the step site and the terrace site.     

Instability of vicinal crystal surfaces with transparent steps: Transient kinetics and non-local electromigration

The steps at the crystal surfaces could be partially transparent in the sense that the migrating adatoms can cross the steps without visiting a kink position. In the case of significant transparency the velocity of a given step in a given moment is affected by atomic processes which took place at rather distant terraces in rather earlier moments. The reason is that an adatom needs time to cross many terraces before attaching to a kink position. That is why the theory of crystallization with non-local kinetics should account for the time dependence of the adatom concentrations on the terraces. Such a transient growth model (going beyond the quasi-static approximation of the BCF theory of crystal growth) is developed here for the limit of slow kinetics at the steps and fast surface diffusion. This model predicts instability of the vicinal surface (in agreement with the earlier studies) but rather different expression for the wavelength of the most unstable mode, which depends only on the ratio between the step transparency and the step kinetic coefficients.     

The three-dimensional equilibrium crystal shape of Pb: Recent results of theory and experiment

The three-dimensional equilibrium crystal shape (ECS) is constructed from a set of 35 orientation-dependent surface energies of fcc Pb which are calculated by density functional theory in the local-density approximation and distributed over the [110] and [001] zones of the stereographic triangle. Surface relaxation has a pronounced influence on the equilibrium shape. The (111), (100), (110), (211), (221), (411), (665), (15,1,1), (410) and (320) facets are present after relaxation of all considered surfaces, while only the low-index facets (111), (100) and (110) exist for the unrelaxed ECS. The result for the relaxed Pb crystal state is in support of the experimental ECS of Pb at 320–350 K. On the other hand, approximating the surface energies of vicinal surfaces by assuming a linear relationship between the Pb(111) first-principles surface energy and the number of broken bonds of surface atoms leads to a trivial ECS that shows only (111) and (100) facets, with a sixfold symmetric (111) facet instead of the correct threefold symmetry. It is concluded that the broken bond rule in this simple linear form is not a suitable approximation for obtaining the proper three-dimensional ECS and correct step formation energies. PACS 05.70.Np; 61.50.Jr; 68.35.Md; 71.15.Mb     

LEED studies of adsorption on vicinal copper surfaces

As a means for studying the role of atomic steps in adsorption phenomena LEED has been used to investigate the properties of vicinal copper surfaces. Single crystalline surfaces were cut at angles up to 20° from the (100) pole along [001] and [011̄] zones. The diffraction patterns obtained for the clean surfaces and after adsorption of oxygen, nitrogen ions, carbon and sulphur are described. The emphasis of the paper is on the method of interpretation of the geometry of the patterns, which may be done by straightforward kinematic analyses. In the case of nitrogen it is found that if the steps are widely separated the structure of the layer adsorbed on the terrace is the same as that on the low index surface. When the step spacing is small, and comparable with the crystalline parameter of the adsorbed layer, modifications occur which give rise to different superlattices which extend over several terraces. Adsorption of sulphur on 〈11〉 steps can produce a change in the periodicity of the adsorbed layer parallel to the step direction. The study of diffraction patterns for vicinal surfaces with different step spacings may provide an interesting technique for verifying the interpretation of patterns for low index surfaces.     

LEED studies of UO2(∼111) vicinal surfaces

The ordering/disordering of terraces, ledges and kinks on three UO₂(~ 111) vicinal surfaces were examined by LEED. Optical transforms were used as an indication of the types of diffraction displays expected. On surfaces with high kink and ledge densities and unheated above 600 °C, the ledges on the average are equally spaced and parallel, but the kink-site positions are random. At higher temperatures of 700 < T < 900 °C, the ledges rotate and decompose irreversibly into {553} and {311} microfacets. By transform simulations it was inferred that ledges on the UO₂{553} surfaces are highly ordered and contain less than 7% kink-sites.