Surface reconstruction of Cu (111) upon oxygen adsorption

Low-energy He⁺ ion scattering (IS) in combination with AES, LEED and work function measurements has been applied for the determination of surface reconstruction of Cu(111) upon oxygen adsorption. IS data clearly indicate that oxygen is not significant incorporated into the bulk at room temperature adsorption, however the surface shows reconstruction by displacement of Cu atoms by 0.3 Å. The disappearance of structure of both Cu_IS and O_IS in the “?_in pattern” demonstrate the development of a disordered layer of reconstruction centres. At saturation coverage, a rough and dis-ordered oxygen-copper surface layer is present.     

Unusual spin transitions

New heterospin complexes of Cu(hfac)₂ (hfac, hexafluoroacetylacetonate) with pyrazole-substituted nitronyl nitroxides have been found that in the solid state exhibit thermally induced spin transitions analogous to spin crossover. For the first complex, [Cu(hfac)₂L^i-Pr], at room temperature, the Cu—O_L distances, where O_L is the oxygen atom of the nitroxyl group, are very short (2.143 Å). This leads to a strong antiferromagnetic exchange (～-120cm^?1) in the > N—^·O—Cu²⁺—O^·—N < exchange clusters. The CuO₆ coordination units formed by four O atoms of the two hfac anions and by the nitroxyl O atoms of the two bridging nitroxides have a rare form of flattened octahedra, transformed at low temperatures into elongated octahedra with shorter Cu—O_L distances (2.143 Å→2.002Å) and two longer Cu—O_hfac distances (2.130 Å→2.293 Å). For the second complex, [Cu(hfac)₂L^Bu·0.5C₆H₁₄], unusual low temperature structural dynamics of heterospin systems have been found. It is characterized by the formation of two types of CuO₆ unit. The axial Cu—O_L distances are lengthened in one unit (2.250 Å→2.347 Å) and shortened in the other (2.250 Å → 2.006 Å). This leads to a sophisticated μ_eff(T) dependence with μ_eff drastically decreased at 163 K as a result of full coupling of two spins in half of all >N—^·O—Cu²⁺—O^·—N < exchange clusters and to a shift from antiferromagnetic to ferromagnetic exchange in the other half.     

Anomalies in the magnetic properties of copper impurity clusters in barium fluoride crystals

Copper impurity trimers with antiferromagnetic coupling between Cu²⁺ ions are synthesized in BaF₂ crystals. In some of the BaF₂: Cu crystals thus prepared, the concentration of copper stable trimers turns out to be higher than that of copper dimers and copper single centers. The BaF₂: Cu crystals are studied using electron paramagnetic resonance (EPR) spectroscopy at frequencies of 9.3, 23.0, and 37.0 GHz in the temperature range 4.2–77 K. It is found that the components of the effective tensor g for copper trimers depend strongly on the external magnetic field and temperature. A static model of the molecular structure of the copper trimer is proposed on the basis of the experimental data obtained.     

Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar⁺ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.     

The electronic structure of CuCl: An “ab initio” Hartree-Fock SCF-MO study of the CuCl43− cluster

Results from restricted Hartree-Fock SCF-MO calculations on the CuCl₄^3? cluster occurring in solid CuCl are reported. The chemical bonding is discussed on the basis of a population analysis of the ground state orbitals. Calculations on the first ionized states show the Cu 3d electrons to be less tightly bound than the Cl 3p electrons in contrast to Koopmans' theorem predictions and in agreement with experiment. The high degree of localization of the 3d hole orbitals obtained is in conflict with earlier estimates of d-bond covalency. It is suggested that the “Ham effect” rather than covalency is responsible for the reduced spin orbit splitting that can be deduced from the observed exciton absorption. The potential surface for a Cu⁺ ion moving in the field of neighbouring Cl^? ions, has been determined from calculations on the CuCl₄^3? cluster in which the metal ion has been displaced towards a face, an edge or a vertex of the surrounding tetrahedron. The vibration frequency and the activation energy for the diffusion of Cu⁺ ions obtained are in fair agreement with the experimental data. These results support the viewpoint that the anomalous change in scattering intensity with temperature observed in X-ray and neutron diffraction studies is due to the presence of a substantial fraction of the Cu⁺ ions on interstitial lattice sites.     

Influence of one CO molecule on structural and electronic properties of monatomic Cu chain

By using first-principles calculations, we have systematically investigated the structural and electronic properties of an infinite linear monatomic Cu chain with an adsorbed CO molecule. We find that the bridge geometry is energeticabsally favored not only when the Cu–Cu bond below the molecule is unstretched, but also for a wide range of d_Cu–Cu up to about 4.20 Å, while the substitutional geometry is favored only in the hyperstretched situation d_Cu–Cu>4.80 Å. Charge density differences point out the electron transfer is from the Cu atoms to the adsorbed CO molecule. The binding mechanism of CO to Cu chain can be described by the Blyholder’s model, in terms of σ-donation of electron density from the nonbonding CO-5σ orbital into empty metal orbitals and π-backdonation from the occupied metal d orbitals to empty CO-2π^⁎ orbital. The donation/backdonation process leads to the formation of bonding/antibonding pairs, 5σ_b/5σ_a and 2π_b^⁎/2π_a^⁎, with the 5σ_a lying above E_f and the 2π_b^⁎ below E_f.     

Measurements of the electric field gradient at cadmium in the high Tc superconductor YBa2Cu3O6+x

The electric field gradient (EFG) at¹¹¹Cd probe atoms in YBa₂Cu₃O_6+x has been studied by the perturbed angular correlation (PAC) spectroscopy. After annealing at temperatures below 1000 K the probe atoms are located at the Cu(1) site; above 1100 K they preferentially occupy the Y site, whereas in the intermediate temperature range they also reside on the Ba site. A reduction of the oxygen content from x=1.0 to 0.2 resulted in a decrease of the asymmetry parameter measured at the Ba site, but in an increase of it at the Y site, indicating there a local distortion of the YBa₂Cu₃O_6+x lattice by the Cd impurity. Two different surroundings for the Cd atoms at the Cu(1) site were observed.     

Effect of helium/argon gas ratio in a He-Ar-Cu<Superscript>+</Superscript> IR hollow-cathode discharge laser: modeling study and comparison with experiments

The He-Ar-Cu⁺ IR laser operates in a hollow-cathode discharge, typically in a mixture of helium with a few-% Ar. The population inversion of the Cu⁺ ion levels, responsible for laser action, is attributed to asymmetric charge transfer between He⁺ ions and sputtered Cu atoms. The Ar gas is added to promote sputtering of the Cu cathode. In this paper, a hybrid modeling network consisting of several different models for the various plasma species present in a He-Ar-Cu hollow-cathode discharge is applied to investigate the effect of Ar concentration in the gas mixture on the discharge behavior, and to find the optimum He/Ar gas ratio for laser operation. It is found that the densities of electrons, Ar⁺ ions, Ar_m ^* metastable atoms, sputtered Cu atoms and Cu⁺ ions increase upon the addition of more Ar gas, whereas the densities of He⁺ ions, He₂ ⁺ ions and He_m ^* metastable atoms drop considerably. The product of the calculated Cu atom and He⁺ ion densities, which determines the production rate of the upper laser levels, and hence probably also the laser output power, is found to reach a maximum around 1–5 % Ar addition. This calculation result is compared to experimental measurements, and reasonable agreement has been reached. Received: 14 October 2002 / Revised version: 28 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +32-3/820-23-76, E-mail: annemie.bogaerts@ua.ac.be     

钴原子及其团簇在Rh(111)和Pd(111)表面的扫描隧道显微学研究

利用扫描隧道显微镜和扫描隧道谱(STM/STS)及单原子操纵,系统研究了单个钴原子(Co) 及其团簇在Rh (111)和Pd (111)两种表面的吸附和自旋电子输运性质. 发现单个Co原子在Rh (111)上有两种不同的稳定吸附位,分别对应于hcp和fcc空位, 他们的高度明显不同,在针尖的操纵下单个Co原子可以在两种吸附位之间相互转化. 在这两种吸附位的单个Co原子的STS谱的费米面附近都存在很显著的峰形结构, 经分析认为Rh (111)表面单个Co原子处于混价区,因此这一峰结构是d轨道共振 和近藤共振共同作用的结果.对于Rh (111)表面上的Co原子二聚体和三聚体, 其费米面附近没有观测到显著的峰,这可能是由于原子间磁交换相互作用 和原子间轨道杂化引起的体系态密度改变所共同导致.与Rh (111)表面不同, 在Pd (111)表面吸附的单个Co原子则表现出均一的高度.并且对于Pd (111)表面所有 单个Co原子及其二聚体和三聚体,在其STS谱的费米面附近均未探测到显著的电子结构, 表明Co原子吸附于Pd (111)表面具有与Rh (111)表面上不同的原子-衬底相互作用与自旋电子输运性质.     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.     

Mössbauer study of the HgBa2Ca n−1CunO2n+2 ceramics

The parameters of the electric-field gradient tensor at the copper, barium, and mercury sites in the HgBa₂Ca _n?1Cu_nO_2n+2 lattices (n=1,2,3) have been determined by Mössbauer emission spectroscopy on the ⁶⁷Cu(⁶⁷Zn), ¹³³Ba(¹³³Cs), and ¹⁹⁷Hg(¹⁹⁷Au) isotopes, and calculated in the point-charge approximation. An analysis of these results combined with available ⁶³Cu NMR data showed that the experimental and theoretical data can be reconciled by assuming that the holes originating from defects in the material are localized primarily on the sublattice of the oxygen lying in the copper plane [for HgBa₂Ca₂Cu₃O₈, in the plane of the Cu(2) atoms].     

The characterisation of model Cu/Ru(001) bimetallic catalysts by static SIMS with XPS and TPD

The deposition of Cu onto Ru(001) at 540 and 1080 K has been studied by SSIMS supplemented by XPS and TPD. The SSIMS mass spectrum of the Cu/Ru surfaces showed the presence of Cu⁺ and Ru⁺ ions as well as the cluster ions RuCu⁺, Ru⁺₂, Cu⁺₂ and lower intensities of Cu⁺₃, RuCu⁺₂ and Ru₂Cu⁺. Copper coverage was independently monitored by XPS and TPD. It was shown that Cu coverage in the sub-monolayer region was directly proportional to $\text{RuCu}{}^{\mathrm{+}}\text{Ru}{}^{\mathrm{+}}$. Secondary ion emission was strongly influenced by work function changes consequent on Cu adsorption.Studies of the variation of the elemental and cluster ion intensities as a function of Cu coverage, at the two temperatures, has provided detailed information on the dispersion of Cu in the sub-monolayer region, and on the physical form of the ad-layer above a monolayer coverage.     

机械合金化Fe100-xCux体系的X射线吸收精细结构研究

利用X射线吸收精细结构(XAFS)方法研究机械合金化制备的Fe_100-xCu_x(x=0,10,20,40,60,70,80,100,x为原子百分比)合金中Fe和Cu原子的局域环境结构.对于Fe_100-xCu_x(x≥40)二元混合物,球磨160h后,Fe原子的近邻配位结构从bcc转变为fcc,但Cu原子的近邻结构保持其fcc不变.与之相反,在Fe₈₀Cu₂₀和Fe₉₀Cu₁₀(x≤20)合金中,Fe原子的近邻配位保持其bcc结构而Cu原子的近邻配位结构从fcc转变为bcc结构.XAFS结果还表明,fcc结构的Fe_100-xCu_x样品中Fe的无序因子σ(0.0099nm)比bcc结构的Fe_100-xCu_x中的σ(0.0081nm)大得多;并且在机械合金化Fe_100-xCu_x(x≥40)样品中Fe原子的σ(0.0099nm)比其Cu原子的σ(0.0089nm)大.这表明机械合金化Fe_100-xCu_x样品中Fe和Cu原子可以有相同的局域结构环境但不是均匀的过饱和固溶体,而是由Fe富集区和Cu富集区组成的合金.我们提出互扩散和诱导相变机理来解释在球磨过程中Fe_100-xCu_x合金产生从bcc到fcc和从fcc到bcc变化的结构相变 关键词： XAFS 100-xCu_x合金')" href="#">Fe_100-xCu_x合金 机械合金化     

Magnetism of Fe clusters embedded in Cu,Ag, and Au fcc matrices: density functional calculations

We present extensive first principles density functional theory (DFT) calculations dedicated to analyze the magnetic properties of small Fe _n clusters (n = 2,3) embedded in Cu fcc, Ag fcc and Au fcc matrices. We consider several dimers and trimers having different interatomic distances. In all cases the Fe atoms are embedded as substitutional impurities in the metallic network. For the case of the Fe dimers we have considered two magnetic configurations: ferromagnetic (antiferromagnetic), when the atomic magnetic moment of the Fe atoms are parallel (antiparallel) each other. For the case of dimers immersed in Cu and Ag matrices, the ground state corresponds to the ferromagnetic Fe dimer whose interatomic distance is a/√2. For Fe dimer immersed in the Au matrix the ground state corresponds to a ferromagnetic coupling when the interatomic distance is a√(3/2). In the case of the Fe trimers we have considered three or four magnetic configurations, depending on the Fe cluster geometry. For the case of Fe trimer immersed in Cu and Ag matrices we have found that the ground state corresponds to the ferromagnetic trimer forming an equilateral triangle with an interatomic distance equal to a/√2. The ground state for the Fe trimer immersed in the Au matrix corresponds to the ferromagnetic Fe trimer forming a right angle triangle.     

Classical trajectory calculations of the energy distribution of ejected atoms from ion bombarded single crystals

The energy distribution of particles ejected from single crystal surfaces has been calculated using classical dynamics. The model utilizes a microcrystallite of 4 layers with ~60 atoms/layer which is bombarded by 600 eV Ar⁺ at normal incidence. Calculations have been performed for the clean (100) face of copper as well as for copper with oxygen placed in various coverages and site geometries. The energy distributions for Cu, O, Cu₂, CuO and Cu₃ are reported for this system. The distribution for clean Cu exhibits structure which is shown to arise from preferred ejection mechanisms in the crystal. For oxygen adsorbates, the effect of the oxygen binding energy on the peak in the energy distribution of the ejected oxygen atoms is examined by arbitrarily varying the well-depth of the Cu-O pair potential. In general, higher values of the binding energy produce a maximum in the curve at higher energies and also produce a broader energy distribution. The O₂ and Cu₂ dimer distributions are found to peak at approximately the same energy as the O and Cu curves when compared on a kinetic energy/particle basis, although their widths are considerably smaller. Finally, we predict that the CuO energy distribution should be wider than either the Cu₂ or O₂ distributions since it results from the convolution of the Cu and O distributions which are quite different.     

Formation of copper/gold solid solutions by ion beam mixing

The formation of copper/gold solid solutions due to ion beam mixing was studied by Rutherford backscattering, high-voltage electron microscopy and transmission high-energy electron diffraction. Irradiation of multilayered Cu/Au thin films were performed with Xe⁺ ions or Ar⁺ ions at room temperature to doses ranging from 5×10¹⁵ to 2.5×10¹⁶ ions/cm² and energies from 100 to 300 keV. The ion beam mixing leads to uniformly mixed metal layers. The grain size of mixed layers is pronounced increase. It was found that Cu/Au solid solutions are formed with different composition in dependence on itinial composition and implantation dose. Cu-rich and Au-rich solid solutions are induced by ion beam mixing at an initial composition Cu _x Au_100–x withx70. In addition to these solid solutions, a solid solution of middle composition Cu₆₀...₄₀Au₄₀...₆₀ is formed for an initial composition withx<70. The kinetics of formation of solid solution is discussed as a function of the initial composition and implantation dose. Post-annealing experiments of mixed Cu₅₀Au₅₀ multilayers lead to lattice transformations and provide a superlattice structure CuAuI of the L1₀-type. With this process of ordering is associated the formation of dislocation loops.     

Theoretical investigations of the EPR parameters and local structure for Cu2+ in BeO

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors, the hyperfine structure parameters and the quadrupole coupling constant Q) and local structure for Cu²⁺ in BeO are theoretically investigated from the perturbation formulas of these parameters for a 3d⁹ ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are included in the basis of the cluster approach, in view of the strong covalency of the [CuO₄]^6? cluster. From the calculations, the impurity Cu²⁺ is suggested not to occupy exactly the ideal Be²⁺ site but to suffer a slight inward displacement (≈0.024 Å) toward the ligand triangle along the C₃ axis. The theoretical EPR parameters show good agreement with the experimental data.     

Molecular dynamics study of sputtering of Cu (111) under Ar ion bombardment

Abstract

We have used the molecular dynamics (MD) technique using many-body interaction potentials to analyse in detail the processes leading to sputter emission, in order to gain a microscopic understanding of low energy bombardment phenomena. Calculations were performed for a Cu (111) single crystal surface bombarded with Ar atoms in the energy range from 10–1000 eV. The results presented for low bombarding energies are mainly concerned with the near sputtering threshold behaviour, yields and depth of origin of sputtered atoms. Furthermore, it is found, that in addition to sputtered atoms, a large number of ad-atoms at the surface are generated during the evolution of the collision cascade. At higher energies the question of cluster emission and especially their energy distribution and angular distribution are addressed. It was found that the energy distributions for the dimers and monomer atoms exhibit a similar dependence on emission energy as has been observed recently also experimentally. For atoms good agreement with the theoretical Sigmund-Thompson energy distribution was observed. However, for dimers we found that the energy distributions exhibit an asymptotic behaviour at high energies with E^?3 rather than with E^?5, as predicted in previous modelling of cluster emission. Concerning the angular distributions six emission spots, three strong ones in the <110> and three weak ones in the <100> direction were found for atoms, but for dimers only emission spots in the <110> direction were observed, in agreement with experimental results.     

Ion-beam induced mixing of Cu/Au bilayer thin film (kinematics and formation of solid solutions)

Ion-beam induced atomic mixing of Cu/Au bilayer thin film is studied using combined electrical resistivity measurements and Rutherford Backscattering Spectrometry (RBS). 400 keV Kr⁺ ion irradiation with fluences ranging from 3.3×10¹⁵ to 7.6×10¹⁶ ions/cm² at room temperature have been used. Ion beam mixing lead to a uniformly mixed metal alloy. The formation of Cu/Au solid solutions depends on the initial composition and on the fluence of irradiating ions. For an initial composition of Cu₄₂Au₅₈, a Cu-rich solid solution of composition Cu₇₂Au₂₈ is formed after irradiation with 7.6×10¹⁶ ions/cm². The kinematics of the intermixing process is also studied by in situ electrical resistivity measurements which confirmed the formation of the Cu/Au solid solutions.