Physical properties of Icosahedral aluminum copper iron quasicrystal thin films

The research presented here deals with fundamental studies of physical properties of quasicrystal thin films. High quality Icosahedral Al62.5Cu25Fe12.5 quasicrystal thin films 1 nm to 1000 nm thick were developed for investigations of optical, electronic transport, structure, and surface properties of quasicrystals. High resistivities comparable to that of bulk Al62.5Cu25Fe12.5 quasicrystals have been achieved for film thicknesses of 300 nm and more. Both X-ray diffraction and transmission electron microscopy measurements showed diffraction peaks and patterns, respectively, that confirm the high quality of our films. Growing high quality thin films made it possible to probe the band structure of quasicrystals by direct optical transmission measurements. Optical studies showed a depression in the density of states with strong evidence for the existence of a pseudogap of 0.1 eV at the Fermi level. Moreover, the absorption coefficient is in agreement with theory that takes into account the degree of disorder. The density of states is proportional to the square root of the energy, which is in agreement with the cluster model of quasicrystals. The optical conductivity above the pseudogap increases almost linearly with frequency. In the near ultraviolet-visible region the absorption coefficient for i-Al62.5Cu25Fe12.5 quasicrystal thin films shows a resonance peak at a wavelength of ∼320 nm which is attributed to a plasma resonance in the basic Pseudo-Mackay Icosahedron clusters similar to resonance peaks in metallic clusters as explained by Mie scattering theory. Assuming this type of scattering here we have determined a cluster size of ∼1.3 nm. Surface studies showed results that characterize the quasicrystalline state in the films. X-ray photoemission spectroscopy and time of flight secondary ions mass spectroscopy measurements showed an average oxide, mainly aluminum oxide 2 nm thick on the surface of the films. Within the sample the distribution of Al, Cu and Fe was stoichiometrically uniform. Atomic force microscopy imaging of the surface showed an atomic structure with a fivefold symmetry for an oriented grain in the film. The image reveals a cluster-like structure of the quasicrystal with some overlap between adjacent clusters. The results presented here show that basic clusters are the building blocks of quasicrystals and that they are responsible for many of the properties of this materials.