Edvin Lundgren

Surface oxides on close-packed surfaces of late transition metals

In recent years, the formation of thin, well-ordered but complex surface oxides on late transition metals has been discovered. The driving force for this line of research has been the strong incentive to increase the partial pressure of oxygen from ultra-high vacuum to conditions more relevant for heterogeneous catalysis. Here we review the present status of the research field. Compared to oxygen adatom superstructures, the structure of the surface oxides has proven to be extremely complex, and the investigations have therefore relied on a combination of several experimental and theoretical techniques. The approach to solving the structures formed on close-packed surfaces of Pd and Rh is presented in some detail. Focusing on the structures found, we show that the surface oxides share some general properties with the corresponding bulk oxides. Nevertheless, of all surface oxide structures known today, only the two-dimensional surface oxides on Pd(100) and Pt(110) have the same lattice as the bulk oxides (PdO and PtO, respectively). In addition to two-dimensional oxides, including the O-Rh-O trilayers found on Rh, one-dimensional oxides were observed at ridges or steps of open surfaces such as (110) or vicinal surfaces. Finally, we briefly report on a few studies of the reactivity of surface oxides with well-known structure.

Oxygen-induced surface phase transformation of Pd(111): sticking, adsorption and desorption kinetics

Adsorption and desorption of oxygen on Pd(111) were studied by high-flux molecular beam adsorption, LEED, TDS and scanning tunnelling microscopy (STM) between 300 and 623 K sample temperature for oxygen coverages Θ O up to 1 ML. While adsorption below Θ O = 0.25 is precursor mediated and proceeds without changes of the Pd substrate, it is activated for Θ O > 0.25 and induces a massive change of the surface structure. STM reveals the formation of a new surface phase which consists of islands with a local oxygen coverage of 1 ML but less Pd atoms than the bulk-terminated (111) layer. Its formation and decay require activated mass transport of Pd and O atoms over mesoscopic distances. Due to island growth of this phase the oxygen sticking decreases linearly between Θ O = 0.25 and 1 ML. For Θ O > 0.25 ML the TPD rate maxima are shifted towards higher temperature with increasing initial coverage, indicating autocatalytic desorption kinetics. Desorption occurs preferentially from a dilute chemisorbed phase on Pd(111) terraces, with the islands of the high oxygen-density phase acting as a reservoir for O. The experimental TPD data can be well described by a simple mathematical model considering phase equilibrium during desorption.

Surface and subsurface alloy formation of vanadium on Pd(111)

Abstract We have studied the submonolayer growth of vanadium on the Pd(111) surface at different substrate temperatures. By means of low-energy ion spectroscopy (LEIS), Auger electron spectroscopy (AES) scanning tunnelling microscopy (STM), X-ray photoelectron diffraction (XPD) and ab initio local density functional calculation we find that, at room temperature, deposited vanadium atoms substitute surface palladium atoms. In addition, islands are formed on the surface that consist mostly of the substituted palladium atoms. At higher temperatures vanadium diffuses into subsurface layers and, at a temperature of 300°C, only a small amount of vanadium is observed in the topmost layer. STM revealed the formation of a ( 3 × 3 ) R 30° superstructure and XPD measurements demonstrated that this structure is due to vanadium atoms incorporated in the second layer. This finding is confirmed by ab initio calculations. A model for the ( 3 × 3 ) R 30° structure based on the experiments and the ab initio calculations is given.

Interaction of H2, CO and O2 with a vanadium (111) surface

Abstract Adsorption of hydrogen and deuterium on V(111) is governed by dynamical steering at low molecular energies whereas at high beam energies direct adsorption is observed. Strong rotational effects and clear isotope effects in the adsorption dynamics of H 2 and D 2 can be seen. At elevated surface temperatures hydrogen dissolves in the bulk; the absorption coefficient is strongly dependent on surface temperature. For CO a large fraction dissociates upon adsorption. At the clean surface an intrinsic precursor facilitates adsorption and at finite coverages an extrinsic precursor leads to a sticking coefficient independent of coverage. Oxygen can adsorb up to a saturation coverage of 3.8 monolayers followed by surface oxidation. For all three gases calibration procedures for surface coverages are presented as well as quantitative values for the sticking coefficients.

Ultrathin films of Co on Pt(111) : An STM view

The growth, structure and morphology of ultrathin Co layers with a thickness up to 15 layers deposited at room temperature on Pt(111) have been studied by using scanning tunnelling microscopy (STM) with atomic resolution and chemical discrimination between Co and Pt. This chemical contrast has been confirmed by simulations with an FLAPW (Full Potential Linearized Augmented Plane Waves) ab-initio computer code based on density functional theory. By the help of this contrast between Pt and Co atoms in STM constant current images it is shown that in the early stages of submonolayer growth Co is incorporated into the Pt surface, thereby forming dislocation lines. We were also able to demonstrate that Co atoms descend from the upper terrace to the lower one by an exchange diffusion process with the Pt atoms at the step edges. It is shown that this interlayer diffusion does not take place at straight steps, but rather at corners or kinks. The first completed Co monolayer (ML) is almost pseudomorphic (Co in the Pt fcc lattice sites) with a high density of defects due to the lattice mismatch. The second layer exhibits a moire structure, with the Co in-plane lattice distance close to that of bulk Co. The step edges which are very rough at a coverage of two monolayers become smoother with increasing Co deposition. The growth mode is two-dimensional (layer-by-layer) around two to three monolayers and changes afterward into three-dimensional growth (island growth). We observe that the change of the step edge morphology is also correlated to this change from 2D to 3D growth mode. The reason for the 2D growth at the beginning is attributed to the strained interface between the Co overlayer and the Pt(111) surface which hinders the formation of straight steps. Therefore, many kinks and corners are formed, increasing the probability for interlayer diffusion by the above mentioned exchange process. With increasing number of layers the strain decreases, steps become smoother, interlayer diffusion decreases and therefore island growth develops. Up to the highest coverage (15 ML) studied the growth is characterised by a mainly twinned fcc-like stacking. Only a small amount of hcp stacking has been observed. Further experiments showed that preadsorption of carbon monoxide acts as a surfactant which extends the layer-by-layer growth up to higher Co coverages.

Segregation and ordering at Fe1−xAlx(100) surfaces – a model case for binary alloys

Abstract The geometrical structure, chemical order and composition of (1xa00xa00) oriented surfaces of the binary alloy system Fe1−xAlx were investigated in the Fe-rich regime (x=0.03, 0.15, and 0.30) using quantitative low-energy electron diffraction. Low-energy He+ ion scattering and scanning tunneling microscopy were additionally employed to characterize the x=0.15 sample. The equilibrium structures developing with increasing bulk Al content can be consistently explained by the interplay between Al surface segregation and ordering processes which are controlled by atomic interactions similar to those in the bulk. These interactions divide the process of Al segregation to the very surface into two steps whereby Al atoms occupy sites of two different sublattices of c(2×2) periodicity with different probability. Whilst one sublattice is already completely filled at low bulk Al concentration, the other sublattice fills only gradually with increasing bulk Al content. The local order in deeper layers is consistent with the bulk phase diagram.

An atomic-scale study of the Co induced dendrite formation on Pt(111)

Abstract We have studied the initial growth of Co on the Pt(111) surface in a temperature range of 300–400xa0K with scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. In agreement with previous work by Grutter and Durig, when depositing 0.1xa0ML Co on Pt(111) at 400xa0K, the formation of large dendrites is observed. These dendrites are formed in conjunction with a Co induced local reconstruction of the Pt(111) surface, resulting in dislocations. The dendrites, however, show no evidence for any dislocations below their surface, the local reconstruction is observed to be lifted by the formation of the dendrites. LEIS data suggest that the dendrites consist mainly of Pt, implying, together with the STM data, that Co is incorporated underneath the Pt. A model for this process is proposed.

Adatom interlayer diffusion on Pt(111): an embedded atom method study

Abstract We use embedded atom method potentials to calculate the Schwoebel barriers for a large number of hopping and exchange processes of Pt and Ni adatoms descending steps of the Pt(1xa01xa01) surface. The barriers we find for hopping processes are too high to play any role in homo- and heteroepitaxy, but we find very low and even negative Schwoebel barriers for exchange processes at concave corners and kinks. On straight steps we find the process taking place on B-steps rather than on A-steps, with very similar Schwoebel barriers for Ni and Pt adatoms. We also find a strong dependence of the Schwoebel barrier on the lateral relaxation of step edges as caused by surface stress. For vicinal surfaces with high step density this effect causes an increase of the Schwoebel barrier if the width of the (1xa01xa01)-terraces is reduced. Due to the same effect, the barriers should be different for the small cell sizes of todays ab initio calculations as compared to larger terraces.