Tribological properties of a B2-type Al-Pd-Mn quasicrystal approximant

The tribological properties of a B2-type Al–Pd–Mn quasicrystal approximant were investigated and compared with those of an Al–Pd–Mn icosahedral quasicrystal. The approximant was of the _ phase, having a crystalline CsCl-type structure and nominal composition Al48Pd42Mn10. Friction coefficients measured in ultrahigh vacuum between a pair of Al48Pd42Mn10 samples having truly clean surfaces were found to be twice as high as those reported for the Al70Pd21Mn9 quasicrystal. When the surfaces were oxidized by exposure to O2 or H2O, the friction coefficients decreased by roughly a factor of two for both materials but the friction coefficient for the approximant remained roughly twice that of the quasicrystal. The rate of oxidation of the approximant was found to be one order of magnitude higher than that of the quasicrystal. This corroborates findings that suggest that quasicrystals exhibit an inherent resistance to oxidation and corrosion. Vickers hardness measurements show that the quasicrystal is roughly three times as hard as the approximant

Surface Reactivity of a Sputter-Annealed Al−Pd−Mn Quasicrystal

Quasicrystals are materials with unusual atomic structure, coupled with unusual physical properties. Surface properties are particularly important, because most proposed applications quasicrystals involve low-dimensional solidsscoatings, thin films, precipitates, or compositesswhere the ratio of surface area to volume is high. Perhaps the most fundamental surface property is chemical reactivity. Studies under various environmental conditions suggest that quasicrystals are relatively unreactive and/ or corrosion-resistan

Novel Self-Organized Structure of a Ag-S Complex on the Ag(111) Surface below Room Temperature

A well-ordered, self-organized dot-row structure appears after adsorption of S on Ag(111) at 200 K. This dot-row motif, which exhibits fixed spacing between dots within rows, is present over a wide range of coverage. The dots are probably Ag3S3 clusters with adsorbed S in the spaces between dots. Dynamic rearrangements are observed. Small domains of aligned dot-rows form during adsorption and grow quickly after adsorption ends. The domains also exhibit large equilibrium fluctuations after adsorption. The dot-row structure disappears reversibly upon heating above 200 K and transforms reversibly to an elongated island structure upon cooling below 200 K. DFT supports the assignment of the dots as Ag3S3 trimers and also lends insight into the possible origins of other structures observed in this complex system

Surface Studies of Oxidation of a Single-Grain Quasicrystal

We have used Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) to characterize the surface properties of a single-grain Al70Pd21Mn9 (APM) quasicrystal (QC) upon oxidation. When oxygen is adsorbed on this surface, a disordered layer is formed at low coverages. This chemisorbed oxygen destroys the five-fold quasiperiodicity completely. Further adsorption of oxygen leads to a thin layer (less than 20 A) of AI oxide which passivates the surface. At elevated temperatures (870 K), adsorption of oxygen induces an enrichment of AI on the surface. This is explained by the exothermicity of its oxide and the possibility of increased mobility of AI at higher temperatures. Al is the only element in this QC which can be oxidized. No evidence of oxidization for Pd and Mn is observed

Comment on “Sulfur-Induced Reconstruction of Ag(111) Surfaces Studied by DFT”

Reprinted with permission from The Journal of Physical Chemistry C 115, no. 47 (2011): 23651–23651, doi:10.1021/jp205888y. Copyright 2011 American Chemical Society.</p

Fine structure on flat surfaces of quasicrystalline Al-Pd-Mn

We have analyzed the fine structure revealed by scanning tunneling microscopy for a flat (within 0.8 Å) fivefold surface of i-Al-Pd-Mn. Even though features in the image appear to be arranged randomly, self-similar features are separated by distinct distances. The distribution of such distances is compatible with the separations between pseudo-Mackay icosahedra tangent to the topmost layer, and with separations between other cluster-based units. We propose that the fine structure is due to electronic structure imposed by the clusters

Reactivity and structure of CF3I on Ru(001)

A variety of surface-sensitive techniques are used to elucidate the reaction pathways, as well as adsorbate structures, associated with thermal activation of CF3J following adsorption on Ru(001) at 100 K. XPS shows that the C-I bond of CF3I dissociates below 200 K to form CF3(ad) and I(ad); the subsequent reactions of CF2 are best viewed as being regulated by the availability of surface sites. CF3(ad) dissociates to CF2(ad) below 200 K. Further CF3 dissociation, some of which is activated by H(ad), occurs between 200 and 400 K until all available sites are filled. Desorption of the remaining CF3, peaking at 705 K, once again opens surface sites for decomposition. This is followed by recombination of the products to form CF3(g). No evidence for CF(ad) is ever observed. Hydrogen coadsorption studies explain interesting features associated with fluorine evolution. HREELS and ESDIAD results indicate that CF3 adopts a tilted configuration on Ru(001)

Evolution of far-from-equilibrium nanostructures on Ag(100) surfaces: Protrusions and indentations at extended step edges

Scanning tunneling microscopy is used to monitor the formation and relaxation of nanoprotrusions and nanoindentations at extended step edges following submonolayer deposition of Ag on Ag(100). Deposition of up to about 1/4 ML Ag produces isolated two-dimensional (2D) Ag clusters, which subsequently diffuse, collide, and coalesce with extended step edges, thus forming protrusions. Deposition of larger submonolayer amounts of Ag causes existing step edges to advance across terraces, incorporating 2D islands. The resulting irregular step structure rapidly straightens after terminating deposition, except for a few larger indentations. Relaxation of these far-from-equilibrium step-edge nanoconfigurations is monitored to determine rates for restructuring versus local geometry and feature size. This behavior is analyzed utilizing kinetic Monte Carlo simulations of an atomistic lattice-gas model for relaxation of step-edge nanostructures. In this model, mass transport is mediated by diffusion along the step edge (i.e., “periphery diffusion”). The model consistently fits observed behavior, and allows a detailed characterization of the relaxation process, including assessment of key activation energies

Terrace selection during equilibration at an icosahedral quasicrystal surface

We investigate the equilibration of a fivefold surface of the icosahedral Al‐Pd‐Mn quasicrystal at 900–915 and 925–950 K, using scanning tunneling microscopy. After annealing at the lower temperatures, there is a high density of shallow voids on some terraces but not on others; at 925–950 K, the void-rich terraces are much rarer. The terminations that are consumed by voids exhibit a distinctive local atomic configuration, called a “ring” by previous authors. Apparently, through growth and coalescence of the voids, a different termination becomes exposed on the host terraces, which also leads to a change in step heights at the edges of the terraces. We suggest that the shallow steps associated with the voids, and the ring configuration, signal a surface that is in an intermediate stage of structural equilibration

Adatom capture by arrays of two-dimensional Ag islands on Ag(100)

We examine the capture of diffusing Ag adatoms by arrays of two-dimensional Ag islands subsequent to deposition on Ag(100) at room temperature. This is achieved by a combination of scanning tunneling microscopy experiments, kinetic Monte Carlo simulations, and diffusion equation analyses. The dependence of the capture rates on Ag-island size is shown to reflect larger island-free regions surrounding the larger islands, i.e., a strong correlation between island sizes and separations. This feature, and the influence of the local environment of the islands on capture, are elucidated by introducing suitable tessellations of the surface into “capture zones” for each island. We show that a Voronoi-type tessellation based on the distance from the island edges accurately reflects adatom capture. However, a tessellation exactly describing adatom capture is only obtained from a solution of the steady-state equation describing adatom deposition, diffusion, and capture by an array of islands distributed as in experiment. The stochastic nature of adatom capture is also quantified by analysis of the dependence on the deposition location of the probability for diffusing adatoms to be captured by a specific island. The experimental island size dependence of adatom capture is found to be entirely consistent with that obtained from a “canonical” model for the irreversible nucleation and growth of square islands