Probing the non-pairwise interactions between CO molecules moving on a Cu(111) surface

The coverage dependent dynamics of CO on a Cu(111) surface are studied on an atomic scale using helium spin-echo spectroscopy. CO molecules occupy top sites preferentially, but also visit intermediate bridge sites in their motion along the reaction coordinate. We observe an increase in hopping rate as the CO coverage grows; however, the motion remains uncorrelated up to at least 0.10 monolayers (ML). From the temperature dependence of the diffusion rate, we find an effective barrier of 98 ± 5 meV for diffusion. Thermal motion is modelled with Langevin molecular dynamics, using a potential energy surface having adsorption sites at top and bridge positions and the experimental data are well represented by an adiabatic barrier for hopping of 123 meV. The sites are not degenerate and the rate changes observed with coverage are modelled successfully by changing the shape of the adiabatic potential energy surface in the region of the transition state without modifying the energy barrier. The results demonstrate that sufficient detail exists in the experimental data to provide information on the principal adsorption sites as well as the energy landscape in the region of the transition state.     

第一性原理计算Ni原子吸附在Al(111)表面原子结构和电子态

利用密度泛函理论和广义梯度近似研究镍吸附在Al(111)表面。在覆盖率为0.25ML下,分析了Ni吸附在Al(111)表面的面心立方洞位、六角密排洞位、顶位和桥位四个高对称位的原子结构和吸附能。比较不同高对称位的吸附能发现,六角密排洞位的吸附能最大,是5.76 eV,是最稳定的吸附位置。详细讨论了两个最低能量结构-三重洞位的电子结构、功函数、表面偶极距和Ni-Al键的特性。在费米能级附近,Ni-3d和Al-3s,3p轨道产生杂化,形成金属间化合键。由于吸附导致双金属体系表面偶极距和功函数的变化。我们发现：Ni原子与Al(111)表面原子间成建主要是共价键,没有表现出明显的静电荷跃迁,相应的产生非常小的表面偶极距。与面心立方洞位相比,六角密排洞位在费米能级附近产生较低的态密度,在键态附近产生较大的杂化。     

Phase diagram of molecular oxygen adsorption on the (111) platinum surface

Molecular oxygen adsorption on the Pt(111) surface is studied based on ab initio computations and thermodynamics. An O2 adsorption phase diagram is determined. There are two possible chemisorbed molecular states: one at a bridge site and another one at an fcc hollow site. While some population in the bridge sites persists at all coverages, the states coexist through the intermediate coverage phases. The relative coverage of the two species on the surface is determined by the competition between the Pt lattice distortion energy (that results from O2 adsorption) and the O2 repulsion energy. Our results give a reasonable explanation for the seemingly contradictory findings in previous experimental and theoretical work.     

Aluminium adsorption on Ir(1 1 1) at a quarter monolayer coverage: A first-principles study

We present an ab initio density-functional study for aluminium adsorption on Ir(1 1 1) at high symmetry sites, namely, the fcc-, hcp-hollow, top and bridge sites. In each case, we calculate the atomic geometry, average binding energy, work function, and surface dipole moment at the coverage of 0.25 monolayer. We find the favourable structure to be Al at threefold hcp-hollow site, with a corresponding binding energy of 4.46 eV. We present and compare the electronic properties of the two lowest energy structures, i.e., at the threefold hollow sites and discuss the nature of the Al-Ir bond and binding site preference. In particular, we observe a large hybridization of Al-3s, 3p and Ir-5d states near Fermi level, forming an inter-metallic bonds. This results in a significant electron transfer from the Al atoms to the Ir(1 1 1) substrate, inducing an outward pointing surface dipole moment and a large decrease in the work function of 1.69 eV for Al in the hcp-hollow site. Compared to the fcc-hollow site, adsorption in the hcp-hollow site results in a lower density-of-states at the Fermi level, as well as a greater hybridization in the bonding states.     

Entropic effects in diffusion-adsorption processes in micropores

We analyze the kinetics of diffusion-adsorption processes of one component systems in micropores. In realistic situations, the pores exhibit irregular forms which give rise to inhomogeneous adsorption with preferred sites for the adsorbing particles. By modeling the tortuosity of the pores by means of entropic barriers, we obtain a kinetic equation for the averaged concentration of particles along the pore and on its surface. The analysis performed yields expressions for the adsorption rate, the effective diffusion coeffcient, the adsorption isotherms and the concentration of the adsorbed particles. It is shown that this last quantity strongly depends on the form of the pore. This feature opens the possibility to design micropores with an optimal adsorption rate at selected sites. Our results show that to consider the geometry of the pore in the reaction-diffusion scheme is crucial to reproduce experimental observations of the concentration of adsorbed particles in micropores.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

Comparison of S, Pt, and Hf adsorption on NiAl(1 1 0)

First-principles periodic slab density-functional theory (DFT) calculations with a plane-wave basis are used to predict the properties of S, Pt, and Hf adsorption on NiAl(1 1 0). Stable adsorption sites are identified, and adsorbate binding energies and structures are predicted. We find that while S adsorbs in a threefold site, the metals prefer to adsorb in the Ni-Ni twofold bridge site. The latter finding is consistent with scanning tunneling microscopy experiments for adsorption of various transition metals on NiAl(1 1 0) by Ho and coworkers. S is predicted to easily diffuse between threefold sites. We find that Pt and Hf both induce significant changes in the local surface structure, changing twofold bridge sites into fourfold coordination sites by drawing next-nearest-neighbor atoms nearly equidistant with the nearest-neighbor atoms. We find Pt favors interaction with Al slightly more than Ni, while Hf shows a particularly strong affinity for Ni compared to Al. We also predict that Hf may diffuse one-dimensionally along Ni rows with a barrier of ≈0.6 eV.     

Computational approach to surface diffusion

A computational model treating atomic and molecular diffusion on planar fcc (111) and (100) surfaces in terms of a sum of pairwise Morse interactions is examined. Atomic adsorbates have an energetic preference for the hollow site with a diffusion barrier determined by the energy difference to the bridge site. This energy barrier varies up to about 30% of the heat of adsorption, depending upon the surface structure and the form of calculation. A donor diatomic molecule behaves essentially atomic-like, but acceptor diatomic molecules behave significantly differently. Under various circumstances the energetically preferred adsorption site for an acceptor molecule is on-top or bridge with nonmonotonic energy pathways between the high-symmetry sites. These results are compared with experiment and analytical model results.     

Interaction of H2O with the RuO2(1 1 0) surface studied by HREELS and TDS

The adsorption of water on a RuO₂(1 1 0) surface was studied by using high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). The first thermal desorption peak observed between 350 and 425 K is attributed to molecular water adsorbed on fivefold coordinated Ru_cus sites. Higher coverages of water give rise to TDS peaks between 190 and 160 K, which we attribute to water in the second layer bound to bridge oxygen, and multilayers, respectively. HREELS shows that H₂O chemisorbs on Ru_cus sites through oxygen inducing a slight red shift of the vibrational frequency of O_bridge atoms. Molecular adsorption is also confirmed by the presence of both the scissor and the libration modes showing the expected isotopic shift for D₂O. The water adsorbed on the Ru_cus sites also forms hydrogen bonds with the bridge oxygen indicated by the broad intensity at the lower frequency side of the O-H stretch mode. HREELS and TDS results suggest that on the perfect RuO₂(1 1 0) surface water dissociation is almost negligible.     

Island formation during Al deposition on 5-fold Al-Cu-Fe quasicrystalline surfaces: Kinetic Monte Carlo simulation of a disordered-bond-network lattice-gas model

Scanning tunneling microscopy experiments have previously revealed the formation of pseudomorphic starfish-shaped islands during the initial stages of Al deposition on 5-fold icosohedral Al-Cu-Fe quasicrystal surfaces. To simulate this process, we first identify appropriate 5-fold surface terminations of Al-Cu-Fe from a model for bulk structure, and construct associated potential energy surfaces for the binding of Al adatoms on these terminations. We then identify a ‘disordered-bond-network’ (DBN) connecting neighboring local adsorption sites for Al on Al-Cu-Fe, and determine site binding energies as well as activation barriers for Al adatom hopping between neighboring sites. Al-Al adsorbate interactions, which stabilize islands, are also prescribed. Then, within the framework of a DBN lattice-gas model, we simulate the deposition and diffusion of Al on Al-Cu-Fe. We explore the competition between starfish and incomplete starfish ensembles of sites (which provide traps in the form of deep potential energy wells for diffusing Al) and isolated trap sites, with regard to the heterogeneous nucleation and aggregation of Al into islands.     

NO adsorption and diffusion on unreconstructed Pt{1 0 0} surface. A density functional theory investigation

Ab initio density functional theory was used to investigate the adsorption and diffusion of a single NO molecule on the unreconstructed Pt{1 0 0}-(1 × 1) surface. To our knowledge this is the first theoretical study of the NO diffusion activation energy on the Pt{1 0 0} surface. The most stable adsorption position for NO corresponds to the bridge site with the axis of the molecule perpendicular to the surface. The bond of the NO molecule to the surface is through the N-atom. We found that there is a low adsorption energy when the NO molecule is bonded through the O-atom and the axis is perpendicular to the surface, for the three high symmetry sites investigated. NO diffusion between bridge-hollow sites, bridge-atop sites, and hollow-atop sites was also investigated. The barrier for NO diffusion is 0.41 eV, which corresponds to the energy difference between the bridge and hollow sites. This value is around 15% of the highest adsorption energy found on this surface. NO stretch frequencies are also calculated for the three high symmetry sites investigated.     

Elementary processes at semiconductor/electrolyte interfaces: perspectives and limits of electron spectroscopy

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.     

Chemisorption of isocyanate (NCO) on the Pd(1 0 0) surface at different coverages

The chemisorption of isocyanate (NCO) species on Pd(1 0 0) was studied within the density functional formalism (DFT) using a periodic slab model. The NCO was adsorbed on top, bridge and hollow sites of the metal surface at different coverages. At low coverages, the adsorption energies are in the range of ?2.5/?3.0 eV, indicating an important adsorbate–substrate interaction for the three sites studied. The lateral repulsive interaction between neighboring NCO species is almost negligible or weak at lower and intermediate coverages, and very strong at complete monolayer. At low coverages, both bridge and hollow sites are energetically preferred; yet the bridge site becomes the only favoured site at intermediate and complete coverages. Work function and dipole moment calculations can be interpreted by an important charge transfer from the metal surface to NCO. Interestingly, while on hollow site the charge taken by NCO is essentially the same over all the range of coverage, an increasing depolarization is observed on bridge and top sites as the coverage increases. Symmetric and asymmetric NCO stretching modes were also calculated and compared with recent infrared spectroscopy measurements.     

Rate processes on a square lattice with top and bridge sites for adsorption

Thermal desorption and surface diffusion on a square lattice are analyzed in the framework of the lattice-gas model taking into account top and bridge sites for adsorption (with the difference in the partition functions for frustrated translations on these sites) and the nearest-neighbour and next-nearest-neighbour adsorbate-adsorbate interactions. For physically reasonable sets of parameters, the calculated thermal desorption spectra are rather insensitive with respect to the relative population of top and bridge sites. The same conclusion is applicable to the coverage dependence of the chemical diffusion coefficient. General results and discussion are accompanied by simulation of CO adsorption on Ni(001).     

Sulfur adsorption on Fe(1 1 0): a DFT study

The adsorption of atomic S on the Fe(1 1 0) surface is examined using density functional theory (DFT). Three different adsorption sites are considered, including the atop, hollow and bridge sites and the S is adsorbed at a quarter monolayer coverage in a p(2 × 2) arrangement. The hollow site is found to be the most stable, followed by the bridge and atop sites. At all three sites, S adsorption results in relatively minor surface reconstruction, with the most significant being that for the hollow site, with lateral displacements of 0.09 Å. Comparisons between S-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the S. At the hollow site, the presence of S causes an increase in the surface Fe d-orbital density of states between 4 and 5 eV. However, S adsorption has no significant effect on the structure and magnetic properties of the lower substrate layers.     

Hopping motion of chlorine atoms on Si(1 0 0)-(2 × 1) surfaces induced by carrier injection from scanning tunneling microscope tips

Injection of tunneling electrons and holes from the probe tips of a scanning tunneling microscope was found to enhance the hopping motion of Cl atoms between neighboring dangling-bond sites of Si dimers on Si(1 0 0)-(2 × 1) surfaces, featured by the rate of hopping linearly dependent on the injection current. The hopping rate formed peaks at sample biases of V_S∼+1.25 and −0.85 V, which agree with the peaks in the local density of states spectrum measured by scanning tunneling spectroscopy. The Cl hopping was enhanced at Cl-adsorbed sites even remote from the injection point. The Cl hopping by hole injection was more efficiently enhanced by sweeping the tip along the Si dimer row than by tip-sweeping along the perpendicular direction. Such anisotropy, on the other hand, was insignificant in the electron injection case. All of these findings can be interpreted by the model that the holes injected primarily into a surface band originated from the dangling bonds of Si dimers propagate quite anisotropically along the surface, and become localized at Cl sites somehow to destabilize the Si-Cl bonds causing hopping of the Cl atoms. The electrons injected into a bulk band propagate in an isotropic manner and then get resonantly trapped at Si-Cl antibonding orbitals, resulting in bond destabilization and hopping of the Cl atoms.     

单原子Pt吸附于不同原子暴露终端BiOBr{001}面的第一性原理研究

基于密度泛函理论(density functional theory, DFT)的第一性原理方法研究了暴露不同原子终端的BiOBr{001}表面以及单原子Pt吸附于BiOBr{001}-BiO不同位置的几何构型、电子结构、光学性质和电荷转移.计算结果表明:BiOBr{001}面BiO终端暴露可诱导产生表面态且价带和导带能级向低能方向移动,光氧化性增强,尤其导带下方出现的表面态能级有助于光生电子-空穴对的分离和迁移,光吸收显著增强,且BiOBr{001}面BiO终端的功函数远低于贵金属Pt,有利于电荷定向转移.其次,单原子Pt吸附于BiOBr{001}-BiO为基底的表面,在禁带中间诱导产生杂质能级, Pt吸附于穴位时吸附能最小,光响应能力最好且电荷转移量最大,吸附于顶位和桥位时,形成开放性的贫电子区域,因此可预测穴位为Pt原子的吸附位点,预示其良好的降解有机污染物效果, Pt吸附于BiOBr{001}-BiO的顶位和桥位,具有潜在的CO_2还原或固氮等领域应用.     

Site-specific core level spectroscopy of CO and NO adsorption on Pt(110)(1×2) and (1×1) surfaces

The adsorption of CO and NO on the (1×2) and (1×1) modifications of the Pt(110) surface was studied by x-ray photoemission spectroscopy, LEED and work-function change measurements. The O(1s) binding energy of adsorbed CO is site-specific and differentiates between on-top and bridge adsorbed species. CO adsorption on Pt(110)(1×2) at 120 K occurred sequentially into on-top and bridge sites yielding an orderedc(8×4) layer at the maximum coverage. At 300 K only on-top bonded CO was present after CO adsorption on the (1×2) surface. CO adsorption on the (1×1) surface at 120 K showed a transient bridge adsorbed CO and on-top CO at saturation, with an ordered (2×1)p1g1 LEED pattern. Heating the (2×1)p1g1 CO layer to 400 K also showed this transient bridge CO species. Work function changes generally correlated with the appearance of different CO species but were complex in detail. The findings for CO adsorption are consistent with the missing row model of the (1×2) surface.Parallel data for NO adsorption on (1×2) and (1×1) surfaces at 120 K were less informative than those for CO because O(1s) spectra showed single broad peaks. Peak contributions due to bridge and on-top bonded NO could be estimated.     

Special facets of diffusion and energy transfer in macromolecular systems

We consider energy transfer in polymers in dilute solutions and model it as a purely incoherent, hopping exciton motion which ends at permanent traps, whose positions along the polymer chain are fixed. The transport involves not only steps along the chain but also steps between sites which are far away along the chain's backbone but near to each other in Euclidean space. This possibility leads to very peculiar features of the excitation’s diffusion along the chain, due to strong correlations between subsequent steps. The process depends strongly on whether the chain is frozen or moves. We show that conformational changes influence trapping strongly, even if they are relatively slow on the time scale of the hopping motion. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997.     

含锰钢表面氢原子/氢分子吸附行为的第一性原理研究

采用基于密度泛函理论的第一性原理方法研究了氢原子和氢分子在纯铁表面和锰原子掺杂表面的吸附与解离行为.研究结果表明,氢原子可在纯铁(001)表面稳定吸附,吸附能按照顶位,桥位和心位依次增强;而溶质原子锰降低了氢原子距离表面的位置并强化了氢原子的吸附行为.氢分子在纯铁表面的吸附解离行为取决于氢分子距离模型表面的初始距离和初始空间构型.氢分子平行于纯铁(001)表面时,距离心位1.2?发生解离,而桥位、顶位均不会发生解离;氢分子垂直放置时,距离桥位0.6?、顶位1.0?发生解离,心位不会发生解离.氢分子平行于锰掺杂纯铁(001)表面时,距离桥位0.6?、顶位0.7?、心位1.2?发生解离;氢分子垂直放置时,距离桥位、心位0.8?发生解离,而顶位放置氢分子不发生解离.归纳可知,锰溶质原子掺杂会增加铁基体表面氢原子和氢分子的吸附作用并促进氢分子发生分解.