采用(meta)-GGA泛函对Ag(111)、Rh(111)和Ir(111)上乙烯吸附的理论研究

本文针对在多种催化反应的重要中间体乙烯,使用(meta)-GGA等级的包含PBE,BEEF-vdW,SCAN以及SCAN+rVV10在内的多种交换关联泛函,系统研究了在过渡金属表面(Ag,Rh和Ir)上乙烯吸附势能面对泛函的依赖关系. 研究发现,对于乙烯在贵金属Ag(111)上的吸附,除了PBE外,BEEF-vdW,SCAN以及SCAN+rVV10均能预测出物理吸附态的存在. 对于乙烯在Rh(111)面的吸附,SCAN和SCAN+rVV10预测在化学吸附位之前存在有物理吸附前驱态,而基于PBE和BEEF-vdW的势能面并没有发现前驱态的存在. 而对于乙烯在Ir(111)上的吸附,BEEF-vdW也能微弱地预测出化学吸附前驱态的存在. 研究结果表明,无论在哪一种金属表面上,四种泛函中SCAN+rVV10给出的吸附能最强,其次是SCAN,最后是PBE或者BEEF-vdW.     

Work functions at facet edges

We perform ab initio pseudopotential calculations for metal crystals with finite facets of different crystallographic orientation to investigate the work function profile near crystal edges. We examine local edge effects, and address the problem of the coexistence of different face-dependent local work functions at crystal edges. By modeling the electronic dipoles at the metal surface, we show how nonvanishing surface charges spontaneously appear on metals with inequivalent facets. Our studies of Al crystal nanowires with (100) and (111) facets are extended to derive the dependence of the work function on the crystal morphology in the macroscopic limit.     

Contrast inversion of the h-BN nanomesh investigated by nc-AFM and Kelvin probe force microscopy

Single sheets of hexagonal boron nitride (h-BN) on transition metals provide a model system for layered insulating materials as well as a functional substrate for molecules and metal clusters. The progress in the understanding of h-BN layers on transition metals was mainly driven by scanning tunnelling microscopy (STM) and photoelectron spectroscopy (PES) measurements within the last decade, while direct measurements of mechanical and electrical properties are still rare. Our investigations of the two-dimensional (2D) h-BN nanomesh on a Rh(111) substrate by high-resolution noncontact atomic force microscopy (nc-AFM) reveal a complex surface structure including a frequently observed contrast inversion. Detailed 2D force spectroscopy measurements are revealing towards a mechanical elastic deformation of the h-BN monolayer caused by the tip-sample interaction. Furthermore, Kelvin probe force microscopy (KPFM) and spectroscopy measurements show local work function variations of the nanomesh, proving the results obtained by PES but additionally providing detailed local information.     

Elastic-plastic deformation and fracture of shock-compressed single-crystal and polycrystalline copper near melting

The Hugoniot elastic limit, the yield strength, and the spall strength of polycrystalline M1 copper and single-crystal (110) and (111) copper are determined during shock compression up to 8 GPa in the temperature range 20–1080°C from an analysis of the free-surface velocity profiles recorded with VISAR laser velocimeter. The measurements show that all copper samples exhibit strong athermal hardening (increase in the Hugoniot elastic limit) near the melting temperature. Copper single crystals have a very low elastic limit in the temperature range up to 600°C, this limit increases sharply as the temperature increases to 1000°C, and it depends on the crystallographic orientation of a single crystal. The temperature dependence of the spall strength has a threshold character for all copper samples. Copper single crystals demonstrate higher resistance to spall fracture; however, near the melting temperature, the difference between the spall strengths of the copper single crystals and M1 copper becomes insignificant, 50% of the initial level.     

Nonuniform scaling applied to surface energies of transition metals

We construct a generalized gradient approximation of the exchange-correlation energy that satisfies the nonuniform scaling in one dimension and is accurate in the whole quasi-two-dimensional (Q2D) regime. Using spatial and energetic analyses of metal (111) surfaces, we show that the Q2D behavior is important at the surface of most transition metals, and that the here proposed Q2D-generalized gradient approximation functional predicts for these metals accurate surface energies as well as bulk properties.     

过渡族金属掺杂Al(111)表面对氢分子催化分解的影响

为了探求过渡金属催化剂对催化合成储氢材料NaAlH₄效果的影响, 本文采用第一性原理方法研究了多种金属原子取代Al (111)表面铝原子形成的合金表面对氢的催化分解的影响. 计算结果表明, Sc, V, Fe, Ti原子掺杂的表面对氢分子分解具有催化作用. H₂在对应的掺杂表面催化分解所需要的活化能分别为0.54 eV, 0.29 eV, 0.51 eV, 0.12 eV. H原子在Sc, V, Ti掺杂表面扩散需要的活化能分别为0.51 eV, 0.66 eV, 0.57 eV. 同时, 过渡金属掺杂在Al表面时倾向于分散分布, 增加掺杂表面的掺杂原子个数, 掺杂表面的催化效果体现为单个掺杂过渡金属原子的催化效果. 本研究将为金属掺杂Al (111)表面催化加氢合成NaAlH₄提供理论参考.     

To the theory of I–V characteristics of the superlattices

The influence of inelastic scattering on the I–V characteristics of “dirty” and “pure” superlattices (SL) in the quasielastic limit is studied. It is shown that the form of the I–V characteristics of “dirty” SL is determined solely by the frequency of elastic scattering in a wide range of parameters and that the electron-phonon interaction in “pure” SL leads to the formation of two regions with negative differential conductivity (NDC) on the I–V characteristics.     

Theoretical studies of ultrathin film-induced faceting on W(111) surfaces

We have used local volume (or EAM) potentials to study the pyramidal faceting (or reconstruction) of a W(111) surface induced by face center cubic (fcc) metals Pd, Pt, Au, and a body center cubic (bcc) metal Mo. We found that the surface-energy differences of (211) and (111) surfaces of bcc W increases as one or few monolayers of Pd, Pt, Au, and Mo films are deposited. We found that the lateral relaxation which is allowed on the (211) surface further increases the surface energy anisotropy as the thickness of the fcc metal film increases. Our calculated results are consistent with the argument that the surface energy anisotropy is the driving force for the faceting, but do not rule out three-dimensional (3D) island growth as another possible mechanism for the (211) faceting. We also found that there is a possible bilayer growth mode in W(211) surfaces with Pt and Pd films.     

Electric field gradient in transitional dilute alloys

The electric field gradient (EFG) and the asymmetry parameter (η) due to transition metal impurities in the host metals Al, Cu and V are investigated. The valence EFG is formulated using dielectric screening theory in conjunction with pseudopotential theory. The size EFG is evaluated in the elastic continuum limit for a screened point charge model of the host metal. The detailed calculations are carried out forAl (Sc, Fe, Cr),Cu (Ni, Pd, Pt) andV (Ti, Cr, Fe, Nb, Ta, W) alloys.     

The bonding of diethyl ether, ethanol and their fluorinated analogs to zirconium oxide thin films

We report a thermal desorption study of the interaction of water, diethyl ether, ethanol, perfluorodiethyl ether, and 2,2,2-trifluoroethanol with zirconium oxide thin films epitaxially grown on Pt(111) single-crystal surfaces, and with the Pt(111) metal surface for comparison. We find that water and the hydrogenated ether interact chemically with the oxide surface through the O lone pair, whereas the fluorinated ether has only a van der Waals interaction. Ethanol and 2,2,2-trifluoroethanol are decomposed to some extent on zirconium oxide upon heating. Fluorination decreases the bond strength between the molecules and the oxide surface. A similar trend is observed on the Pt(111) surface with regard to relative adsorptivity and reactivity of the adsorbates examined. Although the Pt(111) surface provides weaker interaction with the molecules that interact via O lone pair orbitals (water, ether and alcohol) than the oxide surface, it decomposes diethyl ether as well as the alcohols.     

Hydrogen adsorption,absorption and diffusion on and in transition metal surfaces: A DFT study

Periodic, self-consistent DFT-GGA(PW91) calculations are used to study the interaction of hydrogen with different facets of seventeen transition metals—the (100) and (111) facets of face-centered cubic (fcc) metals, the (0001) facet of hexagonal-close packed (hcp) metals, and the (100) and (110) facets of body-centered cubic (bcc) metals. Calculated geometries and binding energies for surface and subsurface hydrogen are reported and are, in general, in good agreement with both previous modeling studies and experimental data. There are significant differences between the binding on the close-packed and more open (100) facets of the same metal. Geometries of subsurface hydrogen on different facets of the same metal are generally similar; however, binding energies of hydrogen in the subsurface of the different facets studied showed significant variation. Formation of surface hydrogen is exothermic with respect to gas-phase H₂ on all metals studied with the exception of Ag and Au. For each metal studied, hydrogen in its preferred subsurface state is always less stable than its preferred surface state. The magnitude of the activation energy for hydrogen diffusion from the surface layer into the first subsurface layer is dominated by the difference in the thermodynamic stability of these two states. Diffusion from the first subsurface layer to one layer further into the bulk does not generally have a large thermodynamic barrier but still has a moderate kinetic barrier. Despite the proximity to the metal surface, the activation energy for hydrogen diffusion from the first to the second subsurface layer is generally similar to experimentally-determined activation energies for bulk diffusion found in the literature. There are also some significant differences in the activation energy for hydrogen diffusion into the bulk through different facets of the same metal.     

Indium square root 7 x square root 3 on Si(111): a nearly free electron metal in two dimensions

We present measurements of the Fermi surface and underlying band structure of a single layer of indium on Si(111) with square root 7 x square root 3 periodicity. Electrons from both indium valence electrons and silicon dangling bonds contribute to a nearly free, two-dimensional metal on a pseudo-4-fold lattice, which is almost completely decoupled at the Fermi level from the underlying hexagonal silicon lattice. The mean free path inferred from our data is quite long, suggesting the system might be a suitable model for studying the ground state of two-dimensional metals.     

Biomass-derived oxygenate reforming on Pt(111): A demonstration of surface science using d-glucose and its model surrogate glycolaldehyde

Molecules derived from cellulosic biomass, such as glucose, represent an important renewable feedstock for the production of hydrogen and hydrocarbon-based fuels and chemicals. Development of efficient catalysts for their reformation into useful products is needed; however, this requires a detailed understanding of their adsorption and reaction on catalytically active transition metal surfaces. In this paper we demonstrate that the standard surface science techniques routinely used to characterize the reaction of small molecules on metals are also amenable for use in studying the adsorption and reaction of complex biomass-derivatives on single crystal metal surfaces. In particular, Temperature Programmed Desorption (TPD) and High Resolution Electron Energy Loss Spectroscopy (HREELS) combined with Density Functional Theory (DFT) calculations were used to elucidate the adsorption configuration of d-glucose and glycolaldehye on Pt(111). Both molecules were found to adsorb in an η₁ aldehyde configuration partially validating the use of simple, functionally-equivalent model compounds for surface studies of cellulosic oxygenates.     

Density functional theory study of the water adsorption at Bi(111) electrode surface

The adsorption of a water molecule on a basal Bi(111) electrode surface, crystallising in the rhombohedral system, has been studied in the framework of cluster model. The quantum chemical calculations were performed at the Density Functional Theory (DFT) level and the electrical double layer effects were analysed by using an external electric field. In contrast to computational predictions reported previously for other metal surfaces, crystallising in the face-centred cubic or hexagonal close-packed systems, a hollow site for Bi(111) was found to be energetically the most preferable; the water adsorption energy amounts to ? 28 kJ mol^? 1. In a wide range of surface charge densities the water molecule is bound preferentially through the O atom in orientation perpendicular to the surface plane. The Bi(111) hydrophilic properties are compared with those for other metals. Some adsorption characteristics of a hydrogen atom and a hydroxyl group at Bi(111) are reported as well, which give evidence in favour of the non-dissociative adsorption of water molecules.     

Secondary ion emission processes of sputtered alkali ions from alkali/Si(100) and Si(111)

The secondary alkali ion yield vs. the work function change (Δφ) of Na, K and Cs/Si(100) and Si(111) was measured to discuss the details of secondary ion emission processes. In the case of alkali/metal systems, the secondary ion emission is explained by the electron tunneling model. In this model, the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal, and the secondary ion yield depends on exponentially the work function change of metal surface. For alkali/Si(100) systems, the secondary ion emission processes are explained in terms of the electron tunneling model since the secondary alkali ion yield vs. the work function change (Δφ) follows the exponential manner. However, it is not easy to apply the simple electron tunneling model to our experimental results for alkali/Si(111) systems. There is the essential difference in surface structures between Si(100) and Si(111). Therefore, it is suggested that the local electronic environment around the adsorbates might be taken into consideration for alkali/Si(111) systems.     

Interface effect between blue phosphorus and metals

The contacted properties of metal substrates with single layer (monolayer) blue phosphorus are calculated by first principles. We analyze the charge transfer, atomic orbital overlap, electronic properties and potential barrier at the interface of metal contacted blue phosphorene (BuleP) to understand how to effectively inject electrons from the metal into the contacted blue phosphorus. We inquire into interfacial effect of blue phosphorene directly in contact with five representative metallic substrates – Au (111), Ag(111), Al(111), Co(111) and Sc(0001), which are having minimal lattice mismatch with the BlueP. We find that the contact properties of these five metals are ohmic contact and schottky contact. Of the five different contact metals, Co-BlueP heterojunction has the best electrical conductivity. The lower SBH in the Al contact can also lead to a good substrate for a Schottky contact for the heterojunction. These results can provide guidance for the future design of BlueP-based electronic devices and for the exploration of new low-dimensional semiconductor transport processes.     

Vibrational dynamics and surface structure of Bi(111) from helium atom scattering measurements

The Bi(111) surface was studied by elastic scattering of helium atoms at temperatures between 118 and 423 K. The observed diffraction patterns with clear peaks up to third order were used to model the surface corrugation using the eikonal approximation as well as the GR method. Best fit results were obtained with a rather large corrugation height compared to other surfaces with metallic character. The corrugation shows a slight enhancement of the surface electron density in between the positions of the surface atoms. The vibrational dynamics of Bi(111) were investigated by measurements of the Debye-Waller attenuation of the elastic diffraction peaks and a surface Debye temperature of (84 ± 8) K was determined. A decrease of the surface Debye temperature at higher temperatures that was recently observed on Bi nanofilms could not be confirmed in the case of our single-crystal measurements.     

Spontaneous assembly of perfectly ordered identical-size nanocluster arrays

A method, by which periodic two-dimensional arrays of identical metal clusters of nanometer size and spacing could be spontaneously obtained by taking advantage of surface mediated clustering, is reported. The versatility of the method is demonstrated for a broad range of metals on Si(111)-(7 x 7) substrates. In situ scanning tunneling microscopy analysis of In clusters, combined with first-principles total energy calculations, unveils unique initial-stage atomic structures of the surface-supported clusters and the vital steps that lead to the success of this method. A strong interaction between the clusters and the surface holds the key to the observed cluster sizes.     

Theoretical strength and cleavage of diamond

The theoretical strength of diamond has been calculated for the <100>, <110>, and <111> directions using a first principles approach and is found to be strongly dependent on crystallographic direction. This elastic anisotropy, found at large strains, and particularly the pronounced minimum in cohesion in the <111> direction, is believed to be the reason for the remarkable dominance of the 111 cleavage plane when diamond is fractured. The extra energy required to cleave a crystal on planes other than 111 is discussed with reference to simple surface energy calculations and also the introduction of bond-bending terms.     

Thermal motion and energetics of self-assembled domain structures: Pb on Cu(111)

Low energy electron microscope measurements of the thermal motion of 50-200 nm diameter Pb islands on Cu(111) are used to establish the nature and determine the strength of interactions that give rise to self-assembly in this two-dimensional, two-phase system. The results show that self-assembled patterns arise from a temperature-independent surface stress difference of approximately 1.2 N/m between the two phases. With increasing Pb coverage, the domain patterns evolve in a manner consistent with models based on dipolar repulsions caused by elastic interactions due to a surface stress difference.