Using scanning tunneling microscopy and low-energy electron diffraction it is demonstrated that the deposition of nickel on the hydrogen-stabilized (5x1)-H phase of Ir(100) leads to the formation of nickel nanowires of tunable widths and of lengths up to the order of 0.1 mu m. The tuning parameter is the number n of full monolayers deposited on the substrate: with that layer completed the next layer starts to grow by forming nanowires of n-atomic width whereby n is in the range 2 < n < 5. The formation of the wires is interpreted as decoration or induction of stair rod dislocations which-as we reported earlier-develop to release the 9.0% pseudomorphic tensile strain of the full nickel layers. They lead to long sunk-in wedges at the surface whose width grows linearly with the number of layers deposited and so, by their decoration, nanowires of corresponding width develop. The wedges, and so the Ni wires, appear above single-atomic and macroscopically long Ir wires of the Ir(100)-(5x1)-H substrate which pin the dislocations. Above n=6, however, the films become rough and develop a squarelike patterning. It is speculated that the deposition of other metals with the same order of pseudomorphic strain should develop similar nanowires.

The nanostructured phase $\text{Ir}(100)\text{\ensuremath{-}}(5\ifmmode\times\else\texttimes\fi{}1)\text{-H}$ is used as a template for the self-organized formation of nanometer-scaled lateral superlattices of the transition metals (TMs) Fe, Co, and Ni. Initially, Fe atoms decorate the monatomic Ir wires which reside in fivefold periodicity on the template. This is in contrast to Co and Ni which form islands within the adsorption stripes between and across the Ir wires. In no case there is intermixing with substrate atoms. At 0.8 monolayer coverage all stripe sites are filled so that lateral ${{\text{TM}}_{4}\text{Ir}}$ superlattices are formed. With further deposition a second TM layer grows, whereby the Ir wires at the interface remain immobile. Due to the different radii of atoms within the interface layer a substantial buckling is imprinted in the growing film as well as in the supporting substrate. The related morphology and crystallographic structure of the films are determined by atomically resolved scanning tunneling microscopy and quantitative low-energy electron diffraction.

Epitaxial Ni films deposited on Ir(100) were investigated by tunneling microscopy (STM), quantitative low-energy electron diffraction (LEED), and density-functional theory (DFT). For film thicknesses beyond 3 monolayers the large tensile strain (approximate to 9%) is relieved by the formation of stair-rod-like dislocations. Their favorable energetics is revealed by DFT calculations which also determine the defects' structural parameters. On the unstructured Ir(100)-(1x1) surface they develop in an irregular way, i.e., without long-range order. In STM they are visible as shallow depressions or by decoration with further adsorbed adatoms. In contrast to this case of missing long-range order, the dislocations are ordered in films on the Ir(100)-(5x1)-H surface, whereby (5x1)-periodic Ir wires at the interface act as pinning centers. So, their detailed atomic structure is accessible experimentally by quantitative LEED with crystallographic precision. Features similar to Ni are also observed for Co films.

A structural study by quantitative low-energy electron diffraction (LEED) and density functional theory (DFT) has been performed on the Ir(100)-(5x1)-hex surface with 0.6 ML of hydrogen adsorbed at low temperature (< 180 K). The theory-experiment fit of LEED intensities based on calculations applying the two mirror symmetry planes of the uncovered surface is not satisfactory. Instead, the real structure has a unit cell with only one mirror plane yielding an excellent R factor (R-P=0.16). The DFT investigation of the energetics also confirms this spontaneous symmetry breaking, yields quantitatively the same structure for the substrate as the LEED analysis and, moreover, retrieves the hydrogen positions. It was found that adsorption of hydrogen leads to an asymmetric increase of the buckling of the top hexagonal layer, with one of the previously protruding surface Ir atoms extruded even more by about 0.1 A. This may be regarded as a precursor of the temperature-activated phase transition above 180 K to a structure in which the most protruding atom of the (5x1) surface unit cell is ejected from the quasihexagonal top layer. These ejected atoms form single-atom-wide iridium wires on the remaining atoms, which rearrange to form a bulklike fcc(100) layer, leading to a complete lifting of the quasihexagonal reconstruction.

The adsorption of hydrogen on the metastable, unreconstructed Ir(100)-(1x1) surface is investigated by density functional theory (DFT), quantitative low-energy diffraction (LEED), and thermal desorption spectrometry (TDS) complemented by scanning tunneling microscopy (STM). The bridge site is unequivocally identified as the adsorption site, rather unusual for metallic fcc(100) surfaces. There is excellent quantitative agreement between calculated and experimentally determined structural parameters both for the clean surface and the adsorbate covered surface. Given the uncertainty of DFT to reproduce absolute energies there is also good agreement with the measured adsorption energy (460 meV/atom). Additionally, theoretical vibrational and electronic properties are provided without, however, related experiments being available.

The stable (5 x 1) surface reconstruction of Ir(1 0 0) is shown to restructure upon exposure to hydrogen as observed and determined by STM and quantitative LEED. The new phase, Ir(1 0 0)-(5 x 1)-H, consists of defect-free Ir nanowires of up to micrometre length with their lateral order depending sensitively on the details of preparation. The Ir wires can be decorated by Fe on both their sides, so that Fe-Ir-Fe sandwich nanowires are formed. The space between the Ir wires can also be filled completely equivalent to the formation of a Fe4Ir surface alloy. It is speculated how this lateral nanostructuring may be combined with one vertical to the surface. (C) 2004 Elsevier B.V. All fights reserved.

It is shown by scanning tunnelling microscopy (STM) that the Ir(100) surface can be used as a template to form other nanostructures after it has restructured upon exposure to hydrogen. Extremely long and defect-free Ir nanowires produced in the restructuring process can be decorated by, e.g., Fe so that FeIrFe nanochains develop as sandwiches of atomically thin Fe and Ir wires. Laterally they recur with 1.36 nm periodicity on average, i.e. two atomic rows are missing between two sandwiches. By additional deposition of Fe this space can be filled so that the vacancy spaced {FeIrFe} lateral superlattice transforms to a {Fe4Ir} superlattice. Their crystallography is determined by quantitative low-energy electron diffraction (LEED).

We show that the Ir(100) surface forms a new nanostructure in a self-organized way when its reconstructed equilibrium surface is exposed to hydrogen. Scanning tunneling microscopy and quantitative low-energy electron diffraction retrieve that a long-range ordered superlattice of defect-free Ir chains with average lateral spacing of 1.36 nm and micrometer lengths develops. This can be used as a template for the formation of other nanostructures as is demonstrated.