Fe Adatoms Research Articles

Molecular dynamics and kinetic Monte Carlo simulations of Fe island growth on Cu(111)

We have performed extensive molecular dynamics (MD) simulations of a 0.07 ML coverage of Fe adatoms on Cu(111) at 300 K using a many-body interaction potential from the molecular dynamics/Monte-Carlo corrected effective medium theory. The Fe atoms were deposited randomly and then followed for two hundred picoseconds (ps) while annealing at a temperature of 300 K. The results of these MD simulations were analyzed to understand the general features of island growth. One finding was that islands as large as four atoms are only one of magnitude less mobile than monomers. Thus, the mobility of small islands played a significant role in the growth of large islands. In addition, the presence of next-nearest neighbor attractions was found to increase the speed of this growth. Another finding was that the Fe islands obtained their equilibrium structures only very slowly compared to the times of aggregation or coalescence. The MD results were used to construct and test several kinetic Monte Carlo models that incorporated the newly identified processes. Considering these MC models, we discuss some details of island structure, stability and aggregation.

Surface adatom-adatom coordination and orientation determined by low energy Auger electron and photoelectron diffraction due to adatom emission

The Auger electron and photoelectron diffraction patterns due to surface adatom emission at low kinetic energies may be used to distinguish isolated adatoms, neighboring surface site occupation, the formation of adatom chains or clusters and their orientation relative to the substrate. The evolution of the diffraction pattern with neighboring site occupancy is discussed for Cu and Fe adatoms on Cu(001) within the context of the single scattering cluster approximation. The ability to detect short-range adatom coordination in the absence of long range order has direct applications for studies of surface diffusion, surface ordering and thin film growth.

Thermal stability and intermixing of ultrathin Fe films on a Au(001) surface

Thermal stabilities and atomic intermixing of ultrathin ferromagnetic Fe films grown on a Au(001) surface have been studied using high-resolution low-energy electron diffraction (HRLEED). Both the average terrace size and the average inhomogeneity length can be quantitatively determined as a function of temperature by properly decomposing the energy dependent angular profiles of the (00) beam measured from Fe/Au(001) films at various temperatures. Atomic place exchange between Au and Fe is processed in Fe films up to ~ 2.5 ML thick at room temperature. For higher coverage Fe films (3.5–5.0 ML), the Fe adatoms bury the intermixed interface (⩽ 2.5 ML thick) formed at room temperature. For all the films studied (1.5–5.0 ML), the occurrence of intermixing — i.e., intermixed Fe and Au patches due to interdiffusion of Fe adatoms into Au bulk and exchanges of Au atoms to the surface — is observed clearly in different elevated temperature ranges. This conclusion is supported by the results that dominant surface steps for the thicker Fe films change from Fe/Fe steps to Au/Fe steps, and eventually, to Au/Au steps as the annealing temperature is increased. The average inhomogeneity length as a result of intermixing is extracted and is in the order of 20 Å over a wide range of temperatures.

Spectroscopy of a single adsorbed atom.

We have used a low-temperature ultrahigh-vacuum scanning tunneling microscope to perform atomically localized spectroscopic measurements on single Fe atoms adsorbed onto the Pt(111) surface. Using a simple tunneling model, we are able to quantitatively deconvolute the tip and adatom local densities of states (LDOS) from the dI/dV spectra. We find that a resonance occurs in the adatom LDOS that is centered 0.5 eV above the Fermi energy. This feature has a width of approximately 0.6 eV, and occurs only when the tip is within angstroms (laterally) of the center of an Fe adatom

Structural and magnetic properties of Fe/Ni(111).

Structural, electronic, and magnetic properties of Fe/Ni(111) were studied using the total-energy local-spin-density full-potential linearized-augmented-plane-wave method. The Fe adatoms are found to be located on the hcp sites on the Ni(111) surface with a 3.8% downward relaxation relative to the average Fe and Ni distances in bulk. The spin magnetic moments of the Ni(111) surface atoms are enhanced to 0.63${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ from the bulk value of 0.58${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. This moment is further increased to 0.67${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ after the Fe adsorption because of the enhanced exchange interaction. By contrast, the magnetic moment of the Fe overlayer decreases to 2.33${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ from the value of 2.49${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for the isolated Fe monolayer. Layer-by-layer contact hyperfine-field values are presented that show the differing influence of the valence-electron contributions in affecting the proportionality of the hyperfine field and the magnetic moment.

Oxygen withdrawal, copper valency, and interface reaction for Fe/La1.85Sr0.15CuO4.

High-resolution x-ray photoemission spectroscopy was used to study reactive interface formation for Fe adatoms deposited onto the high-temperature superconductor ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_{4}$. Fe deposition reduces the emission within \ensuremath{\approx}3 eV of the Fermi level, indicating disruption of the hybrid $\mathrm{Cu} 3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{-}\mathrm{O} 2{p}_{x,y}$ states responsible for superconductivity. Strong reaction of Fe adatoms with substrate oxygen is observed for coverages ranging from 0.25 to 8 \AA{}, and oxygen is withdrawn to form Fe-O. The chemical states of Cu are selectively changed with a quenching of the ${\mathrm{Cu}}^{2+}$ configuration concurrent with the persistence of the ${\mathrm{Cu}}^{1+}$ configuration.

An xps study of the interaction of SO 2 with CaO(100): Effect of temperature and metal adsorbates (Fe, Na)

Using X-ray photoelectron spectroscopy (XPS), the chemisorption of SO 2 on CaO(100) has been studied in the temperature range from 25 to 400° C. The chemical state of sulfur has been identified as that of sulfate. The kinetics of SO 2 chemisorption on CaO are discussed. The binding states of Fe and Na on CaO were determined to be Fe 2+ and Na + respectively. The effect of the metal adsorbates (Fe, Na) on the chemisorption of SO 2 was also studied. In the presence of Fe (coverage ⩾ 0.5 monolayer), both sulfate and sulfide were observed; in the presence of Na, however, only sulfate was observed. The saturated amount of sulfate formed depends on the Fe coverage, but it is independent of the Na coverage. At low Fe coverages (< 0.5 monolayer), there is a large increase in the rate of sulfate formation at low SO 2 exposures (< 3 L). This increase is explained by the “activation” of SO 2 chemisorption sites by Fe adatoms. At increased Fe coverage, the rate of sulfate formation decreases due to the blocking effect of the Fe islands formed on the surface. Similarly, the effect of Na adatoms on SO 2 uptake can also be explained by the same mechanism. However, the inability of the Na islands to block the SO 2 chemisorptive sites and the lack of chemisorptive bond formation between SO 2 and Na are explained by the diffusion of SO 2 through the Na adlayer.

Catalytic active sites on sputtered metal interfaces

By the use of Hartree-Fock-Slater (HFS) cluster calculations, the symmetry of adatom induced electronic states and their possible role in catalytic processes have been investigated. For iron adsorbed on a close packed iron single crystal surface we find an increased density of occupied “π” as well as “σ” states at the Fermi level. Simple group theoretical arguments give that the “π” states, which represent the highest occupied molecular orbitals (HOMO) will mix with the lowest unoccupied molecular orbital (LUMO) of CO, 2π ∗ , assuming CO bound normal to the surface in a terminal position with respect to the adatom. Such a system with an adsorbed atom on a flat surface is a model for a sputtered surface and these “π” states may thus explain the experimentally observed high rate of dissociation for CO on sputtered iron surfaces. A comparative study shows that no such increased density of states (DOS) is found when the Fe adatom is replaced by potassium. Finally for Cu adsorbed on Cu(111) only 4s derived “σ” states are introduced close to the Fermi level.

Catalytic active sites on sputtered metal interfaces

By the use of Hartree-Fock-Slater (HFS) cluster calculations, the symmetry of adatom induced electronic states and their possible role in catalytic processes have been investigated. For iron adsorbed on a close packed iron single crystal surface we find an increased density of occupied “π” as well as “σ” states at the Fermi level. Simple group theoretical arguments give that the “π” states, which represent the highest occupied molecular orbitais (HOMO) will mix with the lowest unoccupied molecular orbital (LUMO) of CO, 2π ∗, assuming CO bound normal to the surface in a terminal position with respect to the adatom. Such a system with an adsorbed atom on a flat surface is a model for a sputtered surface and these “π” states may thus explain the experimentally observed high rate of dissociation for CO on sputtered iron surfaces. A comparative study shows that no such increased density of states (DOS) is found when the Fe adatom is replaced by potassium. Finally for Cu adsorbed on Cu(111) only 4s derived “σ” states are introduced close to the Fermi level.

The effect of pH on the electrochemicalbehaviour of iron in hydrochloric acid

The anodic and cathodic behaviour of pure Fe electrodes has been investigated in solutionsof constant ionic strength (CHl + NaCl) at pH values from 0 to 3 by means of the potentiokinetic polarization method (3V/h). The following parameters have been obtained: ∂ε corr/∂ pH = −51 mV; ∂ log i corr/∂pH = − 0·06; b − =120 mV ∂ log i −/∂ log a H − ) ε = 0·5; (∂ε/∂pH) i − = − 59 mV; (∂ε/∂ pH) i + = − 58 mV. As the ph value increases, ∂ log i +/∂ log a 0H −) ε changes from 0·83 to 1·32 and b + changes from 70 to 44 mV. The results, which confirm the importance of the surface treatment, are discussed and compared with those obtained in sulphuric solutions. The competitive adsoprtion of Cl − and OH − ions is evident and in solutions at pH > 2 the effect of the latter ion prevails. On the contrary, in solutions of pH < 2, a strong interaction between the Cl − ions and the Fe adatoms must be assumed. Nevertheless, it is not necessary to assume that these ions directly participate in the dissolution via a desorption process. The dissolution reaction may take place in the following way: Fe + OH −FeOH ads − FeOH ads − → FeOH+ + 2 e