Showing papers by "Ronan McGrath published in 2006"

Surface vacancies at the fivefold icosahedral Al-Pd-Mn quasicrystal surface: A comparison of ab initio calculated and experimental STM images

Abstract: Scanning tunneling microscopy (STM) images of the fivefold surface of the icosahedral $(i)$ Al-Pd-Mn quasicrystal have been calculated using ab initio density functional methods. The STM signal is formed predominantly by the Al atoms in the surface plane. The Pd atoms are seen in the STM image as dark spots. The reason for their small contribution to the STM current is a small local density of states around the Fermi level and the localized $d$-character of the Pd states. On the other hand, the local paramagnetic density of states on Mn atoms is much higher than that on Pd atoms and therefore Mn atoms are seen in the STM images as bright spots. A direct comparison of the experimental images obtained from the STM with the ab initio calculated STM images reveals that the characteristic features of the images---the dark pentagonal holes---can also be created by surface vacancies and not only by truncated Bergman clusters as it has been supposed up to now.

Ordering of Si atoms on the fivefold Al-Pd-Mn quasicrystal surface

Abstract: Silicon atoms are found to form an ordered overlayer on the fivefold surface of the icosahedral $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Mn}$ quasicrystal. Using scanning tunneling microscopy, the adsorption site is identified as the center of truncated clusters which are building blocks of the bulk structure. Comparison with theoretical models suggests that this site is favored because of bonding to Mn atoms.

Adsorption of cobalt on the tenfold surface of d-Al72Ni11Co17 and on the fivefold surface of i-Al70Pd21Mn9

Abstract: The adsorption behaviour of Co on the tenfold surface of d-Al72Ni11Co17 and on the fivefold surface of i-Al70Pd21Mn9 has been studied using scanning tunnelling microscopy (STM), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). The analysis of a distinctive quasiperiodic LEED pattern for coverages from 3–30 ML of Co on d-Al72Ni11Co17 suggests that the Co forms in a pseudomorphic row structure composed of domains of Fibonacci spaced rows having a periodic lattice parameter along the rows of 2.5 ± 0.1 A. The same structure, though less well-ordered and with a larger lattice parameter, is formed on the fivefold surface of i-Al70Pd21Mn9.

Characterization of aperiodic and periodic thin Cu films formed on the five-fold surface of i-Al70Pd21Mn9 using medium-energy ion scattering spectroscopy

Abstract: The elucidation of the local atomic structure of a pseudomorphic film of Cu deposited on the five-fold surface of $i\text{\ensuremath{-}}{\mathrm{Al}}_{70}{\mathrm{Pd}}_{21}{\mathrm{Mn}}_{9}$ using medium-energy ion scattering spectroscopy is reported. Monte Carlo calculations, using the VEGAS code, have been utilized to simulate the blocking of $100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{He}}^{+}$ ions scattered from the overlayer. The coordinates of the Cu atoms in the overlayer derived from this procedure are consistent with a structure occurring in five rotational domains. Each domain consists of nanoscale strips of fcc $\mathrm{Cu}(100)$ with the $⟨110⟩$ azimuth aligned along the five-fold directions of the quasicrystalline substrate. The strips are arranged according to a one-dimensional Fibonacci sequence with long and short widths related by the golden mean $\ensuremath{\tau}$. Upon annealing the film transforms to an alloyed structure composed of five orientational domains of fcc material with the (110) axis perpendicular to the surface.

Adsorption of benzene on the five-fold surface of the i-Al70Pd21Mn9 quasicrystal

Abstract: The adsorption behaviour of benzene (C6H6) on the five-fold surface of the icosahedral Al70Pd21Mn9 quasicrystal has been investigated. Reflection absorption infrared spectroscopy (RAIRS) data taken at a temperature of 92 K indicate molecular adsorption. The adsorption geometry of the molecules is found to change from parallel to the surface to oblique geometries as the dose is increased from monolayer to multilayer coverages. Scanning tunnelling microscopy (STM) images show that the molecules adsorb in a disordered fashion. The adsorbate layer is found to completely desorb at 153 K leaving the substrate unchanged.

Film growth arising from the deposition of Au onto an i-Al–Pd–Mn quasicrystal: a medium energy ion scattering study

Abstract: The room temperature deposition of 7 ML of Au onto the fivefold symmetric surface of icosahedral Al–Pd–Mn leads to the formation of a several monolayers thick Au–Al alloy film. An AlAu film with 1:1 stoichiometry is formed, which shows no evidence of ordered structure, being either amorphous or polycrystalline. Annealing to 325 °C causes more Al to diffuse into the film, producing Al2Au but still with no indication of structure. Experiments using 0.5 ML of pre-deposited In demonstrated a surfactant effect as the In 'floated' on the surface during growth and produced a reduction in film roughness. However, contrary to previous findings the film was still either amorphous or polycrystalline, with no evidence of quasi-crystalline or aperiodic structure. Experiments were also conducted using smaller doses of Au to look for the formation of an epitaxial layer and, if formed, determine the registry with the substrate. However, no change in the Pd blocking curves for the surface could be seen, suggesting that the Au does not adsorb in well defined sites. This result is not surprising when considering that even for these low doses Al is drawn into the film, changing the composition and probably the structure of the topmost layers of the substrate, so that the potential adsorption sites on the clean surface may no longer exist.