Magnetism of Mg atomic chains on the NaCl(100) surface

Based on the first-principles calculations within the density-functional theory, we have investigated the magnetism of 1D Mg nanowires, both unsupported and supported by the NaCl(100) surface. It is shown that Mg can exhibit magnetism in both the geometry of linear and zigzag chains. The freestanding Mg linear chain shows magnetization when the bonds are compressed. For an Mg zigzag chain, however, ferromagnetism is predicted at the equilibrium bond length. It is found that the magnetism of Mg chains can be maintained when deposited on the NaCl(100) surface. We have also analyzed the results by using the Stoner theory and the calculated electronic band structures.     

FeS2(100)表面原子几何与电子结构的理论研究

采用密度泛函理论研究了FeS2(100)表面原子几何与电子结构.理论计算结果表明:FeS2(100)表面无弛豫、无重构,是体相原子几何的自然终止.与体相电子结构相比,FeS2(100)表面电子特性明显不同,禁带中央产生新的表面态,且表面态局域性强,主要由Fe原子的3d分波贡献.配位场理论定性分析表明:FeS2(100)完整晶面表面态产生机制是Fe原子的配位数减少、局部对称性下降所致 关键词： 密度泛函理论 表面电子结构 FeS2     

Local probing of adsorbate electronic structure using soft X-ray emission; atomic nitrogen on Ni(100) and Cu(100)

The N 2p partial density of states of p4g-N/Ni(100) and c(2 × 2)-N/Cu(100) overlayers have been studied by soft X-ray emission spectroscopy (SXES) and comparisons with corresponding UV photoelectron spectra are made. Broad states (∼10 eV) are observed due to the hybridisation between the N 2p and the substrate 3d and 4sp bands. For both N/Cu and N/Ni the intensity stretches out to the Fermi level, where a prominent peak for N/Cu is observed. These states close to the Fermi level are interpreted as antibonding 2p-3d hybridised states. The higher occupancy of these states in N/Cu is expected due to the higher binding energy of the 3d band in Cu than in Ni. For a complete interpretation of the SXE spectra, especially for N/Cu where many features are observed, further theoretical studies are required.     

Enhanced surface atomic step motion observed in real time after nanoindentation of NaCl(100)

The nanometer-scale indentation of a crystalline surface produces nanostructures that evolve on a timescale that is inaccessible to existing imaging methods for the vast majority of surfaces. We have been able to observe the dynamic evolution of the freshly cleaved surface of a NaCl(100) crystal after indentation with an atomic force microscope (AFM) in air. Here we present sequential AFM images featuring vertical atomic resolution which show that atomic terrace motion is greatly enhanced by the AFM indentation. Moreover, some of the nanometric features generated by the indentation become reassimilated into the crystalline surface structure of the surroundings of the indentation over a period of time of the order of several minutes.     

Some studies of lead and iron adsorption on the W(100) surface by field emission microscopy

The behaviour of lead and iron adsorbed on the W(100) surface has been studied by probe hole field emission microscopy, field desorption, and by measurement of the total energy distribution (TED) of field-emitted electrons. Lead adsorbed at 300 K which reduces the work function of W(100) can be completely removed at 78 K by field desorption below 3.2 V Å^?1 and the resulting surface has both the work function and TED, which are characteristic of the clean plane. Condensation at 800 K followed by field desorption, results in a plane surface of work function 4.17 eV and an altered TED. This effect is attributed to the microfacetting, which is observed by LEED. The Swanson peak in the W(100) TED which is removed by submonolayer amounts of lead re-emerges at monolayer coverage when lead adopts the (1 × 1) structure. Such behaviour is consistent with the model proposed by Kar and Soven. A spectral peak observed when lead is adsorbed on the reconstructed W(100) surface is thought to derive from the atomic ¹D state. Adsorption of iron on a W(100) surface reduces φ considerably due to dipole formation and efficiently quenches the Swanson peak. Higher coverages introduce other peaks in the TED enhancement curve, and by adopting an energy scale based on the work of Hagstrum, an attempt is made to interpret the observed peaks in terms of the known energy structure of the free iron atom. One of the three spectral peaks is assigned to the 4s² ground state of the iron atom, and the remaining two peaks are tentatively attributed to atomic p-states. It is concluded that while the excited state structure of the iron atom is too complex to permit complete interpretation of the spectra, this approach offers the hope that, for simpler atoms, such features may be interpreted in this way.     

Nucleation of Pd dimers at defect sites of the MgO(100) surface

Point defects on oxide surfaces are presumed to be preferential nucleation sites for supported metal clusters. Under typical growth conditions, dimers constitute the first step in island nucleation. First-principles calculations on the formation of Pd dimers on regular and defect sites of the MgO(100) surface show that nucleation occurs with large dimer binding energies at divacancies and charged oxygen vacancies (F+ centers), while it is less favorable on steps and neutral F centers. The extensive database of defect trapping/attachment properties gives a firm basis to rationalize recent atomic-force microscopy findings and provides guidelines valid, in general, for ionic substrates.     

Neutral particle emission from zinc oxide surface induced by high-density electronic excitation

Emission of neutral zinc and oxygen atoms and oxygen molecules emitted from zinc oxide surface by laser irradiation have been measured. It is found that the emission intensity starts to increase when the laser intensity exceeds a certain threshold value, indicating that the particle emission is a high density electronic excitation effect. The energy spectrum of emitted particles is found not to follow the Maxwellian distribution.     

Confocal Raman imaging and atomic force microscopy of the surface reaction of NO2 and NaCl(100) under humidity

Polarized confocal Raman imaging combined with non‐contact atomic force microscopy (AFM) was used to study the three‐dimensional evolution of the NaCl(100) surface during its reaction with NO₂ at low pressure as a function of relative humidity (RH) from 0% to nearly 80%. Sea salt particles containing NaCl as the main constituent are believed to be the major source of reactive tropospheric chlorine and nitrate fallouts. At an RH of 0%, the reaction of dry NO₂ generates surface conversion to NaNO₃ monolayer capping the NaCl(100) surface and releases NOCl. The subsequent exposure of this NaNO₃ layer to RH below ∼45% induces the formation of rare NaNO₃ tetrahedral crystals less than 0.5 µm in size. The crystallization occurs through two‐dimensional NO₃⁻ migration under the H₂O monolayer regime. After another subsequent exposure to RH above 45% and below 75%, supermicrometric NaNO₃ rhombohedral plates were obtained under the H₂O multilayer regime. On the other hand, the simultaneous exposure of NaCl(100) to NO₂ and H₂O below ∼45% RH rapidly generates numerous submicrometric NaNO₃ tetrahedra on the NaCl(100) surface. The dramatic increase of NaNO₃ production in the presence of water vapour is explained by the formation of HNO₃ and its easy reaction with the NaCl(100) surface. For RH above 45% and below 75%, the tetrahedra evolve to rhombohedral plates of supermicrometric size. The exposure of NaCl(100) to NO₂/H₂O mixtures under RH above 75% induces the coexistence of both solid‐state NaNO₃ and dissolved NO₃⁻ in droplets. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical studies of the bonding of oxygen to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for an O atom bonded to Ni clusters (consisting of one to five atoms) designed to model bonding to the (100) surface of Ni. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. For the (100) surface, we find that the charge distribution for the full O overlay er is consistent with taking a positively charged cluster. The four surface atoms in the surface unit cell and the atom beneath the surface are important in determining the geometry, leading to a Ni⁺₅O cluster as the model for the (100) surface. The optimum oxygen position with this model is 0.96 Å above the surface (four-fold coordinate site) in good agreement with the value (0.90 ± 0.10 Å) from dynamic LEED intensity analysis. The atom beneath the surface allows important polarization effects for the positively charged cluster. The bonding to the surface involves bridging two diagonal surface Ni atoms. There is an O(2pπ) pair which overlaps the other diagonal pair of Ni atoms leading to nonbonded repulsions which increase the distance above the surface. There are two equivalent such structures, the resonance leading to a c(2 × 2) structure for the O overlayer.     

Theoretical studies of the bonding of sulfur to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for a S atom bonded to Ni clusters consisting of 1 to 4 atoms designed to model bonding to the Ni(100) surface. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. Modeling the (100) surface with four Ni atoms, we find the optimum S position to be 1.33 Å above the surface, in good agreement with the value (1.30 ± 0.10 Å) from dynamic LEED intensity calculations. The bonding is qualitatively like that in H₂S with two covalent bonds to one diagonal pair of Ni atoms. There is a S pπ pair overlapping the other diagonal pair of Ni atoms. [Deleting this pair the S moves in to a position 1.04 Å from the surface.] There are two equivalent such structures, the resonance leading to equivalent S atoms and a c(2 × 2) structure for the S overlayer. The Ni in the layer beneath the surface seems to have little effect (~0.03 Å) on the calculated geometry. Bonding the S directly above a single Ni atom leads to a much weaker bond (D_e = 3.32 eV) than does bonding in a bridge position (D_e = 5.37 eV).     

Theoretical studies of the geometries of O and S overlayers on the (100) surface of nickel

Geometries for O and S overlayers on the (100) surface of Ni have been calculated using $\text{a b initio}$ wavefunctions for O and S bonded to small clusters of Ni atoms (1 to 5 Ni atoms). The calculated distance of the adatom from the surface is 0.96 Å and 1.33 Å for O and S, respectively, in excellent agreement with the results of dynamic LEED intensity calculations, 0.9 ± 0.1 Å and 1.3 ± 0.1 Å, respectively. This indicates that accurate geometries of chemisorbed atoms may be obtained from calculations using clusters.     

Dynamics of exciton formation at the Si(100) c(4 x 2) surface

Carrier recombination at the Si(100) c(4 x 2) surface and the underlying surface electronic structure is unraveled by a combination of two-photon photoemission and many-body perturbation theory: An electron excited to the silicon conduction band by a femtosecond infrared laser pulse scatters within 220 ps to the unoccupied surface band, needs 1.5 ps to jump to the band bottom via emission of optical phonons, and finally relaxes within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds.     

AES and LEED studies of xenon adsorption on (100)NaCl

The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N₂ and O₂ in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N₂ production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.     

Theoretical analysis of ARUPS of a Si(100) surface at room temperature

ARUPS spectra of a Si(100) surface with disorder perpendicular to the dimer row are studied by the tight-binding method for the ensemble of surface buckled dimers. We assume order along the row. Results of ARUPS experiment on the surface at room temperature agree qualitatively with calculated results for disordered structure in which the distance to the next buckled dimer along the row and the height of the buckled dimer change alternately along the row. Peaks of two branches observed in a recent experiment at room temperature appear distinctly in calculated results as long as the short range order among the rows is not weak. The intensity of the two branches is very small compared to the other for conventional structure of the surface.