Order-disorder transition on Si(001): c(4 × 2) to (2 × 1)

A phase transition between c(4 × 2) and (2 × 1) structures on the clean Si(001) surface has been observed at about 200 K by low energy electron diffraction. From the temperature dependence of the width and intensity of the diffraction spots this transition is concluded to be a second order order-disorder transition. The transition proceeds by a single stage, which is in strong contrast with the case of Ge(001) for which a two-stage transition has been reported.     

NH3-induced c(4 × 2) to (2 × 2) transition on the Ge(001) surface by a domino-like dimer flip

Under the conditions of thermodynamic adsorption-desorption equilibrium, the first strongly bound molecular adsorption state of ammonia on Ge(001) saturates at one molecule per Ge reconstruction dimer (1/2 ML). High-resolution electron diffraction studies show that this adsorption is accompanied by a structural transition from c(4 × 2) on the clean surface to a (2 × 2) structure which is already completed for a coverage of about 0.04 ML, far below saturation. We propose a model implying the formation of NH₃ islands locally covered with 1/2 ML and a (2 × 2) periodicity caused by a flip of the dimer tilt direction of every second dimer Beyond the edge of these islands, the dimer flip continues domino-like along the dimer rows over the clean parts of the surface. Elongated (2 × 2) domains about 280–330Å long and 30–65Å wide are formed, depending on coverage.     

Surface phonon dispersion of Cu(100)c(2×2)N

The Cu(100) surface covered with atomic nitrogen has been studied with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and inelastic electron scattering (EELS). Atomic nitrogen, formed by thermal dissociation of NH₃ adsorbed at 100 K, forms a c(2×2) overlayer on the Cu(100) surface. The dispersion of adsorbate and substrate associated modes of the Cu(100) surface covered with a c(2×2) nitrogen overlayer has been measured along the two dimensional Brillouin zone in the - by inelastic electron scattering. The experimental data are compared to a lattice dynamical slab calculation. The Rayleigh-mode (S₄-phonon) is only slightly changed by the N-overlayer. An optimum fit for the perpendicular and parallel nitrogen modes (=320 cm^–1 and =740 cm^–1 at =0.1) is obtained when the nitrogen atom is placed 0.6 Å above the first copper layer.     

Carbon monoxide chemisorption on Cu(100)-c(2 × 2)Pd

Chemisorption of carbon monoxide on Cu(100)-c(2 × 2)Pd was investigated using ultraviolet photoelectron spectroscopy. Adsorption was found to occur molecularly with a saturation coverage at 100 K which is approximately the same as that on Cu(100). Desorption spectra indicate that the CO-surface bond strength for the majority of the molecules is intermediate to that for the pure metals. The 4σ satellite, characteristic of photoemission from CO on Cu, was not observed.     

Cu(100)表面c(2×2)-N原子结构与吸附行为研究

用密度泛函理论的总能计算研究了金属铜(100)面的表面原子结构以及氮原子的c(2×2)吸附状态.研究结果表明：在Cu(100) c(2×2)-N表面系统中，氮原子处于四度配位的空洞(FFH)位置，距离最表面铜原子层的垂直距离为0.20?，最短的Cu—N键长度为1.83?.结构优化的计算否定了被吸附物导致的表面再构模型，即c(2×2)元胞的两个铜原子在垂直于表面方向发生相对位移，一个铜原子运动到氮原子之上的模型.该吸附表面的功函数约为4.65eV, 氮原子的平均吸附能为4.92 eV(以孤立氮原子为能量参考点).计算结果还说明，Cu—N杂化形成的表面局域态的位置在费米面以下约1.0 eV附近出现，氮原子和第一层以及第二层铜原子均有不同程度的杂化作用.该结果为最近有关该表面的STM图像的争论提供了判据性的第一性原理计算结果. 关键词： Cu(100) c(2×2)-N 表面吸附态 密度泛函总能计算     

Dispersion of the oxygen-induced bands on Cu (110) - an angle-resolved ups study of the system p(2×1)O/Cu(110)

The two-dimensional electronic energy bands induced by adsorption of atomic oxygen on Cu(110) are measured with angle-resolved photoelectron spectroscopy (ARUPS). Three oxygen-derived bonding bands and two Cu - derived antibonding bands are found below the Fermi energy. The dispersions can be described by assuming nearest-neighbour σ and π CuO bonds along the [001] direction with oxygen in a long bridge site.     

Spin-resolved photoemission form ferromagnetic Ni(110)c(2 × 2)-S: Theory and comparison with experiment

Changes in spin-resolved photoemission spectra, which had been observed experimentally upon chemisorption of a c(2 × 2)-S layer on ferromagnetic Ni(110), are investigated theoretically by means of dynamical one-step-model photoemission calculations. Two large peaks produced about 1 and 2 eV below the clean Ni peak can, with the aid of the bulk band structure along ΓX and along the “surface-Umklapp” direction LW, be explained as due to Umklapp coupling in the initial state and in the final state, respectively.     

The growth of Ag films on a TiO2(110)-(1×1) surface

We report here the growth of Ag film and its thermal stability on the TiO₂(1 1 0)-(1×1) surface using combination techniques of low-energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At a surface temperature as low as 125 K, a 2D growth of Ag films seems to occur for submonolayer coverages up to ∼0.8 ML. Annealing of low temperature grown Ag films to 500 K for coverage of 1–2.4 ML would result in the formation of metastable Ag layers with rest of Ag forming 3D needle-like islands on top of this Ag film.     

Bridge-bonded CO in Ni(110)-p2mg(2× 1)-CO: a SEXAFS study

Surface-extended X-ray-absorption fine-structure measurements have been performed to determine the adsorption site of CO in the densely-packed system Ni(110)-p2mg(2 × 1)-CO. The analysis shows that CO adsorbs in a tilted configuration on short-bridge sites yielding a nearest-neighbour O-Ni distance of 2.84 ± 0.05 Å. Tilted CO molecules in on-top positions can be definitely ruled out.     

The electronic properties of Co nanowires on Cu(110)-p(2 × 3)N

We present the first measurements of the differential conductance of Co wires grown on top of Cu(110)-p(2 × 3)N (Cu₃N). We apply scanning tunneling spectroscopy (STS) in constant height and constant current mode to access the electronic density of states of the sample over a wide energy range. All measurements have been performed at 7 K. Our study reveals that the differential conductance of the Co wires is very similar to that of Cu₃N. Spectra of the differential conductance measured on the Co wires and on Cu₃N reveal that both systems exhibit the same characteristic features near + 1.8V and + 3.5 V.     

Structural,electronic and magnetic properties of the Mn-Ni(110) c(2×2) surface alloy

<正>Using first-principles total energy method,we study the structural,the electronic and the magnetic properties of the MnNi(110) c(2×2) surface alloy.Paramagnetic,ferromagnetic,and antiferromagnetic surfaces in the top layer and the second layer are considered.It turns out that the substitutional alloy in the outermost layer with ferromagnetic surface is the most stable in all cases.The buckling of the Mn-Ni(110) c(2×2) surface alloy in the top layer is as large as 0.26 A(1 A=0.1 nm) and the weak rippling is 0.038 A in the third layer,in excellent agreement with experimental results.It is proved that the magnetism of Mn can stabilize this surface alloy.Electronic structures show a large magnetic splitting for the Mn atom,which is slightly higher than that of Mn-Ni(100) c(2×2) surface alloy(3.41 eV) due to the higher magnetic moment.A large magnetic moment for the Mn atom is predicted to be 3.81μB.We suggest the ferromagnetic order of the Mn moments and the ferromagnetic coupling to the Ni substrate,which confirms the experimental results.The magnetism of Mn is identified as the driving force of the large buckling and the work-function change.The comparison with the other magnetic surface alloys is also presented and some trends are predicted.     

Influence of the scattering potential model on low energy electron diffraction from Cu(001)−c(2 × 2)-Pb

A dynamical LEED intensity analysis is reported for Cu(001)−c(2 × 2)-Pb. The adsorbate layer distance from the substrate is determined as 2.29 Å, and the topmost interlayer spacing for the substrate is the same as in bulk Cu, in contrast to a contraction for clean Cu(001). This structural result is, within the accuracy reached, insensitive to changes in the assumed scattering potential models. The r-factors suggest a weak preference for an energy-dependent exchange correlation and a moderate one for adding a localized adsorption part inside the muffin-tin spheres. The sensitivity of spectra and r-factors to changes in the assumed isotropic Debye temperature for Pb suggests that vibrational anisotropy should be taken into account in order to improve the accuracy of the analysis. Calculated spin polarization spectra are very sensitive to the exchange approximation, the localized absorption and the Debye temperature. Together with experimental data, they should be useful in particular for determining the vibrational anisotropy.     

Structure determination of Ni(110)-(2 × 3)-N by use of SEXAFS measurements: on-surface and sub-surface sites on a pseudo-(100) reconstructed surface

Surface-extended X-ray-absorption fine-structure measurements have been performed on the Ni(110)-(2 × 3)-N system. The data are consistent with a model in which nitrogen chemisorbs on a pseudo-(100) reconstructed and largely corrugated surface with a nearest-neighbour N-Ni bond length of 1.86 ± 0.03 Å. The corrugation results in two raised and two lowered [11&#x0304;0] Ni rows which are not uniformly distributed in the [001] direction. Nitrogen chemisorbs in four-fold hollow sites slightly (0.19Å) above the lowered Ni rows and in pseudo-four-fold hollow sites slightly (0.38 Å) below the plane defined by a raised and a lowered Ni row. In both sites N is equidistantly bonded to Ni atoms in the second layer. Structural models with long-bridge adsorption sites can be safely excluded.     

Correlated development of a (2 × 2) reconstruction and a charge accumulation layer on the InAs(111)–Bi surface

We have studied the formation of a Bi-induced (2 × 2) reconstruction on the InAs(111)B surface. In connection to the development of the (2 × 2) reconstruction, a two dimensional charge accumulation layer located at the bottom of the InAs conduction band appears as seen through a photoemission structure at the Fermi level. Not well ordered Bi layers do not induce a charge accumulation. The Bi-induced reconstruction reduces the polarization of the pristine surface and changes the initial charge distribution. InAsBi alloying occurs below the surface where Bi acts as charge donor leading to the charge accumulation layer.     

Monitoring surface reactions with scanning tunneling microscopy: CO oxidation on p(2 × 1)-O pre-covered Cu(110) at 400 K

Scanning tunneling microscopy (STM) is used to study the oxidation of CO on O pre-covered Cu(110) in the steady state at 400 K at the atomic level. There is a strong preference for CO to react with oxygen along the p(2 × 1) oxygen rows. The reaction appears to occur initially at the outer edge of an oxygen island, creating defects in the overlayer structure. Once created, oxygen overlayer defects are more reactive than non-defect sites and play a dominant role in sustaining the reaction. Copper atoms that make-up the p(2 × 1) oxygen overlayer structure add to terrace edges after oxygen supply is exhausted and do not form small Cu islands that could eventually lead to surface roughening.