We compare step morphologies on surfaces of Al-rich metallic alloys, both quasicrystalline and crystalline. We present evidence that the large-scale step structure observed on Al-rich quasicrystals after quenching to room temperature reflects equilibrium structure at an elevated temperature. These steps are relatively rough, i.e., have high diffusivity, compared to those on crystalline surfaces. For the fivefold quasicrystal surface, step diffusivity increases as step height decreases, but this trend is not obeyed in a broader comparison between quasicrystals and crystals. On a shorter scale, the steps on Al-rich alloys tend to exhibit local facets (short linear segments), with different facet lengths, a feature which could develop during quenching to room temperature. Facets are shortest and most difficult to identify for the fivefold quasicrystal surface.

Direct observation of extrinsic Al atoms on the fivefold surface of icosahedral Al–Cu–Fe indicates that they form pseudomorphic islands resembling starfish. The starfish occupy specific types of sites on the laterally-bulk-terminated quasicrystalline surface. We postulate that the nucleation event consists of a diffusing Al atom dropping into an empty site at the centre of a substrate pentagon. Growth consists of the addition of five Al atoms (nearly) in lattice sites. These 6-atom starfish do not grow laterally as coverage increases, leading to islands of uniform size, and to early roughening.

Auger electron spectroscopy and low-energy electron diffraction (LEED) provide basic information about the structure and composition of the 5-fold surface of the quaternary quasicrystal, icosahedral Al67Ga4Pd21Mn8. Surface preparation techniques established previously for two of the icosahedral ternary alloys, Al–Pd–Mn and Al–Cu–Fe, appear to be similarly effective for Al–Ga–Pd–Mn. After annealing in the range 600–950 K, the surface concentration of Ga is constant and low. After annealing in the range 900–950 K, a good LEED pattern is obtained. LEED indicates that Ga changes the surface structure significantly.

We study the effect which exposure to molecular oxygen gas can have on the formation and aging of islands in a submonolayer Ag film on Ag(1 0 0). The technique used is high-resolution low-energy electron diffraction, and the surface temperature is 220–250 K. Oxygen has no discernible effect on the average separation of islands resulting from Ag deposition, implying that molecular oxygen does not interact with atomic Ag as it diffuses and nucleates islands on the terraces. However, oxygen serves to accelerate post-deposition coarsening, causing the average island separation to increase rapidly. We show that atomic, not molecular, oxygen is responsible for this effect. Dissociation of the molecule presumably takes place at kink sites, which are plentiful due to the abundance of island edges in the film, and can be triggered either by electron impact or by thermal activation.

We investigate the atomic structure of the fivefold surface of an icosahedral Al–Cu–Fe alloy, using scanning tunneling microscopy (STM) imaging and a special dynamical low energy-electron diffraction (LEED) method. STM indicates that the step heights adopt (primarily) two values in the ratio of τ, but the spatial distribution of these two values does not follow a Fibonacci sequence, thus breaking the ideal bulk-like quasicrystalline layer stacking order perpendicular to the surface. The appearance of screw dislocations in the STM images is another indication of imperfect quasicrystallinity. On the other hand, the LEED analysis, which was successfully applied to Al–Pd–Mn in a previous study, is equally successful for Al–Cu–Fe. Similar structural features are found for both materials, in particular for interlayer relaxations and surface terminations. Although there is no structural periodicity, there are clear atomic planes in the bulk of the quasicrystal, some of which can be grouped in recurring patterns. The surface tends to form between these grouped layers in both alloys. For Al–Cu–Fe, the step heights measured by STM are consistent with the thicknesses of the grouped layers favored in LEED. These results suggest that the fivefold Al–Cu–Fe surface exhibits a quasicrystalline layering structure, but with stacking defects.

We report the first STM study of a threefold surface of an icosahedral quasicrystal. We find that a rough, cluster-dominated structure evolves into a terrace-step morphology, with increasing temperature. The terraces display a fine structure whose long-range order is consistent with threefold symmetry. The fine structure includes small, deep holes. The steps can be very straight, serving to bound equilateral triangles (or portions thereof). These straight steps can cut directly across meandering step edges, superimposing a triangular ‘shadow’ upon the other terrace-step landscape. The data suggest that the triangles grow outward from a special type of central point. The triangles may represent the initial stages of facetting.

It is shown that low-energy electron diffraction (LEED) patterns of the three high-symmetry surfaces (fivefold, threefold and twofold) of icosahedral Al–Pd–Mn are all compatible with quasicrystallinity, under specific conditions of preparation. This conclusion results from comparing symmetries of experimental surface LEED patterns with bulk X-ray diffraction data which are converted to the conditions of the LEED experiment. This conclusion is also based upon an analysis of relative diffraction spot spacings in LEED. Hence, none of the three surfaces exhibits a massive lateral reconstruction, i.e. massive deviation from quasicrystallinity. The LEED pattern of the fivefold surface is distinct from the LEED pattern of the pseudo-tenfold surface of an orthorhombic approximant. We believe that this rules out the possibility that the fivefold surface of the icosahedral quasicrystal reconstructs to an approximant with tenfold or pseudo-10-fold symmetry. The twofold and threefold surfaces facet more readily, indicating qualitatively that they are less stable than the fivefold surface.