A. Biedermann

Oxygen adsorption on Al(111): low transient mobility

Abstract Adsorption of oxygen on Al(111) is studied by scanning tunneling microscopy at 80 and 300 K. After adsorption at 130–195 K, STM images taken at 80 K show pairs of oxygen adatoms with interatomic distances mainly between one and three Al interatomic spacings. This clearly shows that dissociation of the oxygen molecules results in a rather low transient mobility of the two oxygen atoms, a fact which is in contrast to previous work [Phys. Rev. Lett. 68 (1992) 624]. We also find evidence for oxygen atoms in a second metastable adsorption site at these temperatures. At room temperature, we find groups of two or more oxygen atoms in adjacent fcc hollow sites, but no single oxygen atoms. We therefore explain the room-temperature results by part of the oxygen pairs remaining or becoming nearest neighbors, whereas others separate by diffusion and their oxygen atoms attach to other pairs or groups, forming the larger groups found. The pairs and larger groups are stable due to an attractive interaction of oxygen atoms in adjacent fcc hollow sites.

Segregation and reconstructions of PtxNi1 − x(100)

Abstract It is known that on (100) surfaces of PtxNi1 − x single crystals Pt segregates. With increasing Pt concentration in the surface the transition from unreconstructed Ni(100) to the pseudo hexagonal Pt(100) reconstruction occurs via a shifted row reconstruction and several pseudo hexagonal (n × 1) superstructures (n = 7, 12 and 19) consisting of similar (7 × 1) and (5 × 1) subcells. This was revealed by atomically resolved scanning tunnelling microscopy (STM). From low energy ion scattering measurements it becomes clear that the formation of the pseudo hexagonal structure leads to strong amplification of Pt segregation. Chemically resolved STM on the atomic scale shows that Pt prefers the highly coordinated four-fold hollow sites in the pseudo hexagonal structures and Ni is pushed into nearly on-top or bridge sites. Therefore the strong tendency of Pt to increase its coordination is proposed as the driving force of the reconstructions. Corrugations and chemical ordering measured by STM within the pseudo hexagonal reconstructions are confirmed by simulations based on embedded atom method potentials.

The shifted-row reconstruction of PtxNi1−x(100)

Scanning tunneling microscopy on Pt10Ni90(100) and Pt25Ni75(100) single crystals reveals close-packed rows of atoms, which are shifted by 14 〈110〉 along the direction of the rows into a bridge position and slightly outward of the surface. Maximum entropy deconvolution of atomically resolved STM data shows that all atoms between the shifted rows are close to the unreconstructed positions. The density of the shifted rows increases with increasing Pt surface concentration up to a maximum value of each 5th row shifted. The reconstruction shows little dependence on the carbon contamination of the surface, but it is lifted by a full c(2 × 2) coverage of carbon monoxide, which can be imaged simultaneously with the substrate, indicating an on-top position of CO. The driving force of the shifted-row reconstruction is discussed.

Preferential sputtering of Pt-Ni alloy single crystals studied by scanning tunneling microscopy

Abstract Due to its composition, the altered layer of preferentially sputtered alloys has a lattice constant different from that of the bulk. This lattice mismatch can lead to the formation of dislocations or reconstructions, which have been studied on different crystallographic faces. While a subsurface dislocation network exists on the (111) plane, parallel dislocations are found below the (110) surface. The (100) surface exhibits a shifted-row reconstruction, which is tentatively attributed to the stress induced by lattice mismatch between the bulk and the Pt-enriched surface. The annealing process of the sputtered Pt25Ni75(111) surface is studied in detail by evaluation of the mismatch dislocations and low energy ion scattering data.

Limits of perturbation theory: first principles simulations of scanning tunneling microscopy scans on Fe(100)

Scanning tunneling microscopy (STM) scans on Fe(100) are compared with first principles calculations of the tunneling current based on the transfer Hamiltonian method. Experimentally, we find a reversal of corrugation for separations between sample and tip below 400 pm. In the simulations we can reproduce these topographies only in a distance range above 400 pm. The approach therefore fails to describe the observed corrugation reversal. We suggest that this failure is due to the quenching of surface states by the approaching STM tip.

Segregated carbon on Pt10Ni90(100) studied by scanning tunneling microscopy

Abstract Scanning tunneling microscopy is used to study the arrangement of segregated carbon atoms with atomic resolution. Individual carbon atoms are visible only under special tip conditions, while they normally do not directly appear on STM topographs. Under all tip conditions, carbon atoms affect the corrugation of their metal neighbours, reducing the apparent height by 20 to 40 pm in the p(2 × 2) and 40 to 70 pm in the c(2 × 2) superstructure. Therefore the existence, structure and amount of carbon can be also derived from images without directly visible carbon atoms. A substrate lattice distortion in regions of the carbon c(2 × 2) superstructure was observed, exhibiting areas of the p4g structure known from earlier LEED studies of Ni(100).

An STM study of the step structure of Pb(110) and Pb(111)

The (110) and (111) surfaces of lead are investigated by scanning tunneling microscopy. They are atomically resolved with corrugations of 0.1A and 0.6A respectively. The STM images allow conclusions about the motion of the atoms at step edges and step step interaction. At room temperature the steps on both surfaces are below the roughening transition. The influence of impurities and tip surface interaction on the step fluctuation is discussed.

Investigation of the segregation on a Fe-3.5at% Si bicrystal with AES and SAM

A fundamental investigation of transport phenomena involved in the surface segregation in polycrystalline systems has been performed, studying a Fe-3.5at% Si bicrystal with one (001) and one (011) oriented surface, separated by an asymmetric grain boundary. The segregation kinetics of Si, P and S was investigated by means of AES (Auger electron spectroscopy). A combined mechanism of bulk diffusion and diffusion across the grain boundary was found to be responsible for the complex segregation behaviour.

Chemical analysis of PtxNi1−x alloy single crystal surfaces by scanning tunnelling microscopy

Two STM investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the STM. They allow a study of surface ordering of a Pt25Ni75(111) crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a Pt10Ni90(100) surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images.

Domain wall structures in an ordered SiFe(110) surface alloy

During the annealing process of an Fe 96.5 Si 3.5 (100)/(110) bicrystal, silicon and impurity carbon segregate to the surface. The structures formed by the segregands on the (110) surface have been studied by STM (geometry) and AES (chemical information). Silicon substitutes iron surface atoms and forms a two-dimensional alloy, whereas carbon occupies hollow sites in the first monolayer, leading to a distortion of the substrate lattice. The structures are based on a c(1 × 3)Si θ = 1/3 ordered surface alloy. Additional silicon as well as the co-segregating impurity carbon are inserted into this structure by formation of domain walls. If the density of these nearly straight and parallel domain walls becomes high enough, commensurate domain wall structures with c(1 × n) supercells can be observed