No abstract

No abstract

The structure of the Al70Pd21Mn9 surface has been investigated using high resolution scanning tunnelling microscopy (STM). From two large five-fold terraces on the surface in a short decorated Fibonacci sequence, atomically resolved surface images have been obtained. One of these terraces carries a rare local configuration in a form of a ring. The location of the corresponding sequence of terminations in the bulk model M of icosahedral i -AlPdMn based on the three-dimensional tiling T * (2F ) of an F-phase has been estimated using this ring configuration and the requirement from the LEED work of Gierer et al. that the average atomic density of the terminations is 0.136 atoms per A 2 . A termination contains two atomic plane layers separated by a vertical distance of 0.48Å. The position of the bulk terminations is fixed within the layers of Bergman polytopes in the model M: they are 4.08Å in the direction of the bulk from a surface of the most dense Bergman layers. From the coding windows of the top planes in terminations in M we conclude that a Penrose (P1) tiling is possible on almost all five-fold terraces. The shortest edge of the tiling P1, is either 4.8Å or 7.8Å. The experimentally derived tiling of the surface with the ring configuration has an edge-length of 8.0 ± 0.3Å and hence matches the minimal edge-length expected from the model.

No abstract

An ultrathin film with a periodic interlayer spacing was grown by the deposition of Cu atoms on the fivefold surface of the icosahedral Al70Pd21Mn9 quasicrystal. For coverages from 5 to 25 monolayers, a distinctive quasiperiodic low-energy electron diffraction pattern is observed. Scanning tunneling microscopy images show that the in-plane structure comprises rows having separations of S=4.5+/-0.2 A and L=7.3+/-0.3 A, whose ratio equals tau=1.618... within experimental error. The sequences of such row separations form segments of terms of the Fibonacci sequence, indicative of the formation of a pseudomorphic Cu film.

Low‐energy electron diffraction (LEED) is a common and powerful method for determining the geometric structure of solid surfaces. It has the advantage of being fast and inexpensive relative to many other surface techniques. LEED can provide quick information on the surface unit cell size and geometry of single crystal surfaces, and with more effort can be used to determine the complete surface geometry, i.e. composition, bond lengths and angles. Although LEED has been used primarily as a structural technique, it can be used to determine other surface properties such as atomic and molecular vibration and libration amplitudes and energies. LEED has dominated the study of surface geometries for relatively simple structures, and is expected to become increasingly important in the study of nanostructures, molecular adsorbates, and insulating surfaces.

This paper reviews the current situation in the understanding of alkali adsorption on metal surfaces, with particular emphasis on recent structural discoveries. We start by describing the classical `Langmuir - Gurney' model for alkali adsorption and the challenges to it which arose in the mid 1980s. We then describe the results of structural studies from the early 1990s which provide a whole new set of phenomena to be explicable within the framework of a new paradigm, and discuss whether calculations based on density-functional theory constitute such a paradigm.

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.