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Lead has been deposited on the 10-fold surface of decagonal Al72Ni11Co17 at room temperature to form an epitaxial quasicrystalline single-element monolayer. The overlayer grows through nucleation of nanometer-sized irregular-shaped islands and the coverage saturates at 1 MLE. The overlayer is well-ordered quasiperiodically as evidenced by LEED and Fourier transforms of STM images. LEED measurements also indicate that annealing the film to 600 K improves the structural quality, but STM shows that this causes the film to develop pores. Adsorption of C60 molecules on this surface showed that the pores are Pb-containing. Electronic structure measurements using X-ray photoemission spectroscopy indicate that the chemical interaction of the Pb atoms with the substrate is weak, and that Pb does not desorb from the surface upon annealing.

Scanning tunneling microscopy has been used to study the formation of a Bi monolayer deposited on the fivefold surface of i -Al70 Pd21 Mn9. Upon deposition of low submonolayer coverages, the nucleation of pentagonal clusters of Bi adatoms of edge length 4.9 is observed. The clusters have a common orientation leading to a film with fivefold symmetry. By inspection of images where both the underlying surface and the Bi atoms are resolved, the pentagonal clusters are proposed to nucleate on pseudo-Mackay clusters truncated such that a Mn atom lies centrally in the surface plane. The density of these sites is sufficient to form a quasiperiodic framework, and subsequent adsorption of Bi atoms ultimately leads to the formation of a quasicrystalline monolayer. The suggested nucleation site is different from that proposed on the basis of recent density-functional theory calculations.

We present an overview of experimental studies of the structure of adsorbed atomic and molecular species on quasicrystal surfaces with a particular focus on those systems where quasiperiodic ordering of the adsorbate structure occurs. Atomic adsorption is illustrated with examples from studies of Fe and Gd adsorption on the five-fold surface of i-Al-Pd-Mn. Overlayer structure is correlated with several physical parameters to identify trends. It is concluded that the chemical properties of the adsorbate (metallic radius, electronegativity, melting temperature) are important indicators of quasiperiodic structuring of the adsorbate system. These correlations allow predictions of those elements likely to yield pseudomorphic overlayers. Structural effects upon molecular adsorption on clean quasicrystal surfaces have been less successful. A new approach to ordered molecular adsorption is described and illustrated for C60 adsorption on a Pb monolayer film on the ten-fold surface of d-Al-Ni-Co.