Orbital moments of 3d adatoms and Co nanostructures on Cu(0 0 1) surfaces

We use ab initio calculations to investigate spin and orbital moments of 3d transition-metal adatoms and Co nanostructures on Cu(0 0 1) surfaces. For Fe and Co adatoms on Cu(0 0 1) we predict extremely large orbital moments, comparable to the spin moments at these sites. For Mn and Cr adatoms the orbital moments are extremely small and can be neglected in face of their rather large spin moments. Ni adatoms on Cu(0 0 1) were found to be non-magnetic. Our investigations for adsorbed flat clusters of Co on Cu(0 0 1) address the persistence and extent of these large orbital moments in the clusters as a function of their size. We find that, the average orbital moment (M_orb) per Co atom is strongly correlated with the coordination number, decreasing drastically and monotonically as the average number of first Co neighbors around the sites in the cluster (N_Co) is increased.     

Hydrogen adsorption on rhodium: Hydride formed on the surface of thin rhodium films

H₂ interaction with thin Rh films deposited on Pyrex glass under UHV conditions has been studied by simultaneous measurement of work function changes ΔΦ and hydrogen pressure P, at selected constant temperatures: 78 and 298 K. Prior to the adsorption experiments the thin film topography was illustrated using the AFM and STM methods. The influence of hydrogen adsorption on the resistance of thin Rh film was examined in the course of an independent experiment. The number of sites accessible for adsorption on the thin Rh film surface was found determining population of oxygen adatoms within the monolayer at 78 K, when incorporation of these adspecies below the surface is negligible. It was established that at all examined temperatures hydrogen adsorption led to coverage Θ approaching 1 under equilibrium pressure below 10⁻³ Pa, increasing the work function. Under higher H₂ pressure an additional uptake of hydrogen leading to Θ ∼ 1.68 at 298 K, and to Θ ∼ 2 at 78 K is reached. On this surface at low temperatures there exist weakly bound, reversibly adsorbed, positively charged adspecies characteristic for hydrogen adsorption on transition metal hydrides. The change of thin Rh film resistance caused by hydrogen adsorption was not measurable.     

Coverage temperature phase diagram of LEED spot and arced streak intensities for Li on Cu(0 0 1) surface using lattice gas model

It has been established that the arced streaks connecting four spots observed in LEED for a Li system adsorbed on a Cu(0 0 1) surface originate from the Bragg reflection from parallel adatomic lines on a c(2 × 2) lattice site. For example, one streak at about k_y = π/a originates from the parallel atomic lines including two atoms separated at a distance of d_y = 2a, which is the second-neighbor distance in a c(2 × 2) lattice.The c(2 × 2) structure sites consist of two sublattices with y = 2na and y = (2n + 1)a. Here, the difference in the number of adatoms on the two sublattices is the cause of the intensity of the midpoint of the streak, where the differences depend on the coverage of adatoms, Θ.In this study, using a lattice gas model on the substrate lattice with Monte Carlo simulation, we obtain the coverage and temperature dependence of intensities of the spots for the c(2 × 2) structure and the streaks.We found that the intensity of the streaks increase and decrease within the coverage range 0 < Θ < 0.5. That of the spots increases monotonically in this coverage range. These theoretical findings are similar to the experimental results.On the other hand, as temperature is increased, the intensity of the streaks increases and becomes saturated. We found a similar phenomenon using analytical calculation by statistical mechanics. In addition, the intensity of the spots decreased with the second-order transition.     

Effects of evolving surface morphology on yield during focused ion beam milling of carbon

We investigate evolving surface morphology during focused ion beam bombardment of C and determine its effects on sputter yield over a large range of ion dose (10¹⁷-10¹⁹ ions/cm²) and incidence angles (Θ = 0-80°). Carbon bombarded by 20 keV Ga⁺ either retains a smooth sputtered surface or develops one of two rough surface morphologies (sinusoidal ripples or steps/terraces) depending on the angle of ion incidence. For conditions that lead to smooth sputter-eroded surfaces there is no change in yield with ion dose after erosion of the solid commences. However, for all conditions that lead to surface roughening we observe coarsening of morphology with increased ion dose and a concomitant decrease in yield. A decrease in yield occurs as surface ripples increase wavelength and, for large Θ, as step/terrace morphologies evolve. The yield also decreases with dose as rippled surfaces transition to have steps and terraces at Θ = 75°. Similar trends of decreasing yield are found for H₂O-assisted focused ion beam milling. The effects of changing surface morphology on yield are explained by the varying incidence angles exposed to the high-energy beam.     

Adsorption of iso-butane on ZnO(0 0 0 1)-Zn

Presented are thermal desorption spectroscopy (TDS) and adsorption probability measurements of iso-butane on the Zn-terminated surface of ZnO. The initial adsorption probability, S₀, decreases linearly from 0.57 to 0.22 (±0.02) with impact energy, E_i = 0.74-1.92 eV, and is independent of adsorption temperature, T_s = 91-114 K (±5 K), indicating non-activated molecular adsorption. The coverage, Θ, dependent adsorption probabilities, S(Θ), show a cross-over from adsorbate-assisted adsorption (S increases with Θ) to Kisliuk-like dynamics at about the desorption temperature of iso-butane bi-layers (∼110 K). Thus, the adsorption dynamics are precursor-mediated. The enhanced (gas-surface) mass-match, caused by forming a second layer of the alkane, leads to adsorbate-assisted adsorption. A direct fitting procedure of the TDS data yields a pre-exponential factor of 2.5 × 10¹³/s and a coverage dependent heat of adsorption of E_d(Θ) = 39 − 6 ∗ Θ + 2.5 ∗ exp(−Θ/0.07) kJ/mol.     

Angular distribution of desorbing/permeating deuterium from modified Pd(1 1 1) surfaces

The angular distribution of desorbing deuterium molecules was investigated for the clean Pd(1 1 1) surface and for modified Pd(1 1 1) surfaces, either pre-covered with 0.2 ML potassium or with an ultra-thin V₂O₃ surface oxide. The palladium sample was part of a permeation source and the angular distribution was measured by lateral displacement of the sample in front of a differentially pumped flux detector. For the clean surface at 523 K, the angular distribution is close to a cosine distribution, but changes to a cos^1.9Θ distribution at 700 K. Potassium on the surface alters the angular distribution to a cos³Θ function at 523 K. The ultra-thin vanadium oxide layer on the Pd(1 1 1) surface has no significant influence on the angular distribution of deuterium desorption. The experimental results were compared with existing data of the energy dependent sticking coefficient and the energy distribution of the desorption flux as measured by time-of-flight spectroscopy. This made it possible to get information on the applicability of detailed balance and normal energy scaling.     

Initial stages of oxidation of Cu(1 1 1)

Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O₂ at low pressure (<5 × 10⁻⁶ mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu-Cu distance (∼0.3 nm), consistent with local Cu₂O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu₂O(1 1 1)-like structure.     

Atom geometry of nanostructured Fe films grown on c(2 × 2)-N/Cu(1 0 0) surface: An investigation by X-ray absorption spectroscopy with multishell analysis

We investigated the growth of Fe nanostructured films on c(2 × 2)-N/Cu(1 0 0) surface with Fe K-edge X-ray absorption fine structure (XAFS) in the near edge and in the extended energy region. The high photon flux of the incident X-rays allowed us to perform multishell analysis of the XAFS oscillations for Fe coverage Θ_Fe < 1 ML. This data analysis yields a detailed investigation of the atom geometry and some insights in the film morphology. At Θ_N < 0.5 ML (N saturation coverage) there is absence of contribution to XAFS from N atoms. First shell analysis of linearly polarized XAFS gives Fe-Fe (or Fe-Cu) bond length values varying between R₁ = 2.526 ± 0.006 Å at the highest Fe coverage (3 ML ) and R₁ = 2.58 ± 0.01 Å at Θ_Fe = 0.5 ML, Θ_N = 0.3 ML, with incidence angle Θ = 35°. These values are different from the case of bcc Fe (R = 2.48 Å), and compatible with fcc Fe (R₁ = 2.52 Å) and fcc Cu (R₁ = 2.55 Å). At the Fe lowest coverage (Θ_Fe = 0.5 ML) the dependence of R₁ on the incidence angle indicates expansion of the outmost layer. Near edge spectra and multishell analysis can be well reproduced by fcc geometry with high degree of static disorder. At N saturation pre-coverage (Θ_N = 0.5 ML) the XAFS analysis has to keep into account the Fe-N bonding. The results suggest two different adsorption sites: one with Fe in a fcc hollow site, surrounded by other metal atoms as nearest neighbours, and one resulting from an exchange with a Cu atom underneath the N layer.     

A multi-scale Monte Carlo study of oxide structures on the Cu(1 0 0) surface

We present a multi-scale Monte Carlo study of the oxidation of the Cu(1 0 0) surface based on the Bortz-Kalos-Lebowitz model with the equilibrium energetics obtained from ab initio calculations. The radial and island size distribution functions are examined and Cu-O structures are analyzed at different temperatures and coverages. We concentrate on the coverages of 0.3 monolayer O or less, with variable sub-monolayer coverages of Cu. The results show that even though the ab initio calculations yield a higher barrier for O than for Cu adatom diffusion on Cu(1 0 0), the stability of Cu structures causes the O adatoms to be more mobile on the Cu(1 0 0) surface than the Cu adatoms. We are able to reproduce the c(2 × 2)-O domains seen in the experiments. However, we give an alternative explanation based on the repulsive interactions of O that, on one hand, cause the local ordering and, on the other hand, prohibits large well-ordered domains. We also give interpretation on the formation of the R45°-O reconstruction of Cu(1 0 0) above the O coverages of 0.3 monolayer based on the ab initio energetics.     

Growth of Cr on Ir(1 1 1) studied by scanning tunneling microscopy

We have studied the room-temperature growth of Cr on Ir(1 1 1) by scanning tunneling microscopy. Even in the low-coverage regime, up to a total coverage of 2 monolayers (ML), Cr does not grow in the layer-by-layer mode. Instead, we observe islands with local coverages Θ between 1 ML and 5 ML. While the 1st layer growth is pseudomorphic, sporadic defect lines are observed in the 2nd layer. For Θ ? 3 ML periodic one-dimensional dislocation lines appear indicating the onset of strain relief. Scanning tunneling spectroscopy reveals that islands with Θ = 1 ML exist in two modifications. Though their tunneling spectra are qualitatively rather similar, direct comparison shows that the main peak is shifted by about 15 mV, resulting in peak positions of −0.255 V and −0.270 V. We interpret these two modifications as regular fcc Cr and Cr which exhibits a faulted hcp stacking on Ir(1 1 1), respectively. The assignment of fcc to areas directly attached to substrate steps together with the evolution of the ratio of the different ML-areas with coverage leads to the conclusion that hcp is the more favorable stacking.     

Initial stages of hydroxide formation and its reduction on Co(0 0 0 1) studied by in situ STM and XPS in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the initial stages of hydroxide formation and its reduction on Co(0 0 0 1) in 0.1 M NaOH. XPS investigations give chemical information about the adlayer composition after oxidation and at the different reduction stages. In the subpotential range of oxidation at E<−0.55 V (SHE) the formation of a Co(OH)₂(0 0 0 1) superstructure is observed. It shows a hexagonal symmetry with an average periodicity of P=1.25±0.20 nm. The coincidence cell of the observed structure consists of 16 unit cells Co(OH)₂(0 0 0 1) showing an average lattice parameter of a=0.33±0.05 nm and thus the Co(OH)₂ monolayer forms a 5 × 5 superstructure with respect to the underlying metallic Co(0 0 0 1) substrate. XPS results clearly prove the presence of hydroxide and exclude the formation of CoO in the subpotential range.At the very beginning of the reduction process small two-dimensional metal clusters and islands can be observed. It is assumed that they are crystallization nuclei for metal formation. They enlarge and grow together with other islands or larger terraces. During this reduction process two-dimensional adatomic arrays consisting of OH⁻-Co²⁺- OH⁻ trimers appear on the surface. Some of these trimers accumulate at step edges, and finally decorate them. This decoration builds up an energy barrier for further metal incorporation and prevents further growth of the terraces with remaining metal clusters on their surfaces. The reduction of the Co(OH)₂ layer is found to be not completed which is confirmed by XPS results.     

Influence of a single adatom on sputtering at grazing incidence - A molecular-dynamics case study of 5 keV Ar impact on Pt (1 1 1)

Grazing incidence ion impact on a flat terrace lets the projectile reflect specularly off the surface, leading to little or no damage production or sputtering. The presence of isolated surface defects may change this behaviour drastically. We investigate this phenomenon for the specific case of 5 keV Ar ions impinging at 83° towards the surface normal onto the Pt (1 1 1) surface. Molecular-dynamics simulations allow to study the influence of isolated adatoms in detail. The scattering of the projectile from the adatom can redirect the projectile, or let the adatom recoil, such that either of them deposits considerable energy in the target surface, leading to abundant damage production and sputtering. Two distinct collision zones are identified: (i) When the projectile hits the surface in front of the adatom, it may collide with the adatom indirectly (after being specularly reflected off the surface); (ii) alternatively, it may hit the adatom directly. We quantify our results by measuring the zone of influence (≅13 Å²) around the adatom, into which the projectile must hit in order to collide with the adatom, and by the sputter cross section of roughly . The data compare well with previous simulation results of sputtering from an atomically rough surface.     

Diffusion of rhodium layers on the stepped surface of a tungsten micro-crystal

We have investigated the spreading of rhodium at coverages of 0.25, 0.5, 1, 2 and 3 ML over the curved surface of a field emitter tip using field electron microscopy. We have found that the activation energy of spreading as well as the prefactor for the diffusivity depend strongly on the thickness of the layer diffusing, due to a change in interactions in the adsorbate-substrate system. The derived average activation energy for spreading first decreases from E_dif = 1.32 eV/atom at Θ = 0.25 ML to E_dif = 0.71 eV at Θ ≈ 2 ML and than rises again to E_dif = 1.20 eV at Θ ≈ 3 ML. The prefactor for the diffusivity D₀ also decreases with increasing coverage from 0.5 to 1 ML, and stays almost constant for multilayer diffusion in a range of few orders of magnitude lower than for single atom diffusion. We register typical spreading behavior with a sharp moving boundary in the (0 1 1)-(0 0 1) zone of the tip and an unusual picture of diffusion in the (0 1 1)-(1 1 2) region of the tip. In the second region diffusion proceeds without a sharp boundary, independent of the thickness of the moving layer. We think that such an unusual picture can be caused by the change in composition of the second and next layers of adsorbing material due to the early stage of faceting observed in this region of the tip at higher temperature. The results are compared with data for diffusion of individual Rh atoms and small clusters; to understand the observed diffusion we propose taking account of the atomic surface structure of the substrate, modified by strong interactions of the Rh adsorbate with the W micro-crystal surface.     

Effect of surface steps on sputtering and surface defect formation: molecular-dynamics study of 5 keV Xe bombardment of Pt(1 1 1) at glancing incidence angles

Using molecular-dynamics simulation, we study sputtering and defect formation induced by 5 keV Xe⁺ ion impact on a Pt(1 1 1) surface at oblique and glancing incidence angles. Impact on a terrace produces yield maxima at ?=60-65° incidence angle towards the surface normal. Beyond 75-80°, no damage is produced due to projectile ion reflection. Impact on a dense-packed step, however, produces defects in sizeable numbers up to glancing incidence, ?=85°. The dependence of the yields on the incidence angle and distance of the impact point of the projectile to the step are discussed.     

Evidence for long-range surface state mediated interaction between sodium atoms on copper (0 0 1)

Inelastic helium atom scattering from sodium atoms on the Cu(0 0 1) surface at 50 K reveals a remarkable 15% increase in the frequency of the frustrated translational vibrations (T-mode) from ?ω=5.56 to 6.34 meV with increasing coverage from Θ_Na=0.008 to 0.125. The coverage dependence and the negligible dispersion of the frequency cannot be explained by the direct dipole-dipole coupling but are well-understood in terms of the Lau-Kohn effective long-range interaction via intrinsic surfaces states.     

The processes of ordering and formation of two-dimensional glasses at metal surfaces

Using the experimental results obtained for the Dy-Mo(1 1 2) system, we discuss the possibilities and mechanisms of formation of two-dimensional (2D) glasses on metal surfaces. It has been found that in the coverage range 0.07 < θ < 0.58, ordered Dy superstructures formed and observed at T < 400 K are irreversibly destroyed by annealing to higher temperatures and turn into an amorphous (glass) structure on cooling. It is supposed that this conversion is caused by the formation, at T > 400 K, of a Dy-Mo surface alloy in which the rate of Dy surface diffusion is strongly reduced in comparison with its value in the absence of alloying. As a result, the mobility of Dy adatoms becomes too low at the temperatures corresponding to the ordered equilibrium state of the surface, and this state cannot be achieved in reasonable relaxation time. This interpretation is corroborated by the experimental data on substantial suppression of surface diffusion in some coadsorbed layers. Since surface glasses contact with ordered (crystalline) substrates, their structure may have a peculiar character different from that of “conventional” metal glasses. Surface glasses can find a number of applications as rather stable systems that combine low dimensionality, specific electronic structure of their constituents and extremely high density of defects.     

Calculation of the surface energy of fcc-metals with the empirical electron surface model

The empirical electron surface model (EESM) based on the empirical electron theory and the dangling bond analysis method has been used to establish a database of surface energy for low-index surfaces of fcc-metals such as Al, Mn, Co, Ni, Cu, Pd, Ag, Pt, Au, and Pb. A brief introduction of EESM will be presented in this paper. The calculated results are in agreement with experimental and other theoretical values. Comparison of the experimental results and calculation values shows that the average relative error is less than 10% and these values show a strong anisotropy. As we predicted, the surface energy of the close-packed plane (1 1 1) is the lowest one of all index surfaces. For low-index planes, the order of the surface energies is γ_(1 1 1) < γ_(1 0 0) < γ_(1 1 0) < γ_(2 1 0). It is also found that the dangling bond electron density and the spatial distribution of covalent bonds have a great influence on surface energy of various index surfaces.     

Hyperthermal scattering of Cs from Pt(1 1 1): the effect of image charge on the ion-surface energy transfer

We have studied the energy exchange between hyperthermal (5-100 eV) Cs⁺ projectiles and a Pt(1 1 1) surface by measuring the kinetic energy of the scattered ions. The scattering geometry was chosen to be in-plane with specular scattering angles, and the energy of the scattered ions was analyzed as functions of incidence energy and angle. For low incidence energy (<40 eV), the energy transfer to the Pt surface is substantially enhanced due to the attractive image charge force between Cs⁺ and the surface. The image charge effects are highlighted by the different energy transfer on Pt(1 1 1) and Si(1 1 1) surfaces. Analysis of the experimental results using two- and three-dimensional theoretical models revealed a well depth of 1 eV for the image charge potential. Hyperthermal Cs⁺ ions scatter from Pt(1 1 1) predominantly via double collisions with Pt atoms, though the scattering phenomena are insensitive to the impact site at the surface.     

STM and LEED observations of a c(2 × 2) Ge overlayer on Pt(1 0 0)

We have investigated surface structures formed by deposition of 0.2 and 0.5-ML Ge on Pt(1 0 0) by using scanning tunneling microscopy (STM) and low electron energy diffraction (LEED). In addition, their temperature dependence and reactivity to CO have been studied. We observed the formation of disordered domains for Ge adatom coverages below 0.25-ML and complete c(2 × 2) structures at 0.25 to 0.5-ML Ge after annealing at 600-1200 K. Deposition of 0.2-ML Ge on a clean, hexagonally reconstructed (5 × 20)-Pt(1 0 0) substrate at 400 K lifts the reconstruction and ejects excess Pt atoms from the first layer into the adlayer. After annealing this surface to 600 K, the deposited Ge formed Ge adatoms on flat terraces and on round Pt adislands with incomplete c(2 × 2) structures, in addition to the presence of clean (1 × 1)-Pt(1 0 0) domains that were several nanometers across. Some domains of the unreconstructed (5 × 20)-Pt(1 0 0) surface still remained. After the deposition of 0.5-ML Ge and annealing at 600 K, disordered Ge domains disappeared and a c(2 × 2) Ge overlayer was produced all over the surface. Square terraces with square domains of the clean (1 × 1)-Pt(1 0 0) surface extended for nanometers. Annealing this surface to 900 K produced disordered Ge domains, and this was associated with an increase in Ge vacancies. When surfaces with 0.2-ML Ge were heated to 900 or 1200 K, or when a surface with 0.5-ML Ge was heated to 1200 K, larger domains of (5 × 20)-Pt(1 0 0) were formed with the agglomeration of disordered Ge adatoms. Pt clusters were observed in the Ge domains, and we consider these to be composed of those excess Pt atoms formed by lifting the reconstruction of the (5 × 20)-Pt(1 0 0) surface upon Ge agglomeration during cooling. A paper published elsewhere [T. Matsumoto, C. Ho, M. Batzill, B.E. Koel, Physical Review B, submitted for publication.] describes Na⁺-ion scattering spectroscopy (Na⁺-ISS) and X-ray photoelectron diffraction (XPD) experiments that distinguish between Ge present in an overlayer from incorporation into the top Pt layer to form a surface alloy for the surface structures reported here. Furthermore, these investigations revealed that disordered Ge adatoms observed herein might be associated with incomplete c(2 × 2) structures. Therefore, our observations of the formation of complete and incomplete domains of c(2 × 2) Ge adatoms indicate that interactions between Ge adatoms are repulsive at nearest neighbor distances and attractive at second-nearest neighbor distances. Regarding the reactivity of these surfaces, CO does not chemisorb on a Pt(1 0 0) surface with a c(2 × 2)-Ge overlayer and no measurable CO uptake was observed under UHV conditions at 220 K.     

Diffusion and desorption of submonolayer platinum deposited on the multi-faceted surface of a tungsten micro-crystal

Diffusion and desorption of platinum on the tungsten micro-crystal in the form of the W(1 1 1) oriented emitter tip has been studied using the field electron microscopy (FEM) technique. Diffusion of small dose of platinum (average thickness about 0.18 geometrical ML after spreading) on the thermally clean W emitter tip was studied at temperatures 648-742 K. Average activation energy for diffusion E_diff was found to lie between 1.16 ± 0.08 eVand 1.30 ± 0.16 eV. During annealing at the diffusion temperatures Pt-induced faceting of the emitter surface was visible in the neighbourhood of the {1 1 1} pole. The layer equilibrated in the diffusion process was stable at temperatures up to 1100 K where reduction of the high voltage at a fixed emission current, characteristic of alloying of Pt with W, was detected. Submonolayer of platinum (Θ_Pt = 0.18 ML) started to desorb at tip temperature ≥1780 K. The measurements of average activation energy for desorption of ‘zero coverage’ Pt (0.03 ML ≤ Θ_Pt ≤ 0.06 ML) from the entire W emitter surface were carried out at temperatures 1990-2170 K and yield the value of E_des = 5.19 ± 0.22 eV to 5.33 ± 0.19 eV. The results are compared with data for diffusion of individual Pt atoms and small clusters and with data for adsorption of Pt atoms on a planar W(1 1 0) surface. In discussion the atomic surface structure of the substrate, modified by the strong interaction of Pt with the W micro-crystal, is also taken into account.