Palladium clusters formed on the complex pseudo-10-fold surface of the ξ′-Al77.5Pd19Mn3.5 approximant crystal

The growth and properties of a Pd thin film deposited on a structurally complex surface are studied by scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission spectroscopy. The substrate is the pseudo-10-fold surface of the ξ′-Al77.5Pd19Mn3.5 crystal, an approximant of the AlPdMn quasicrystalline phase. Spectroscopic data are consistent with a low density of electronic states around the Fermi level for the clean surface, in agreement with the poorly conducting nature of this intermetallic alloy. Deposited Pd atoms readily form small metal particles with relatively homogeneous size on the substrate, for coverages ranging from the submonolayer regime up to 7 ML. The Pd particles do not coalesce with time and are relatively stable upon annealing up to 920 K. Their average size increases slightly with film thickness. The local electronic structure of the Pd thin film has been studied by STS and appears to be particle-size dependent, with a transition from non-metal to metal, deduced from I(V) characteristics with increasing particle size. A shift of the Pd-3d core level is also measured by photoemission, and it can be correlated with a shift of the Pd-d band toward the Fermi level. The growth mode of the Pd film on the ξ′ approximant surface and its electronic structure are discussed in relation to Pd thin films grown on metal-oxides that are used as model catalysts.