  Scanning tunnelling microscopy has been used to study the formation of a Bi
monolayer deposited on the five-fold surface of i-Al70Pd21Mn9. Upon deposition
of low sub-monolayer coverages, the nucleation of pentagonal clusters of Bi
adatoms of edge length 4.9 A is observed. The clusters have a common
orientation leading to a film with five-fold symmetry. By inspection of images
where both the underlying surface and the Bi atoms are resolved, the pentagonal
clusters are found 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
initial nucleation site is different to that proposed on the basis of recent
density functional theory calculations.
