The five-fold surface of the icosahedral Al70Pd21Mn9 quasicrystal

Quasicrystals represent a new class of material; fascinating due to their complexity and their rich potential in terms of technological applications. Investigations have been carried out on the five-fold surface of the i-Al70Pd21Mn9 sample using STM, LEED, AES, EXAFS and FTIR. The first experimental chapter reveals the work done on the clean surface where two main phases called the 'clustered phase' (rough) and the 'flat terraced phase' (terraces separated by steps) have been recognised. The rough surface was obtained after annealing to 810 K. Annealing the surface to 970 K gives rise to flat terraces where protrusions of height equal to ∼2 A are visible. Improvement in the sample preparation led to flatter terraces being obtained; this has led to better STM resolution images and to structural perfection. Therefore it has been possible to demonstrate using autocorrelation calculations and precise tiling of the surface that the surface structure is entirely consistent with a bulk termination of the model of Boudard et al. The decomposition of the surface at high temperature has also been observed along with the first quasicrystalline screw dislocation. The deposition of C60 molecules using STM, AES and SPA-LEED has been characterised and it was found that the molecules adsorb randomly on the surface, forming dispersed layers of increasing density as the coverage is increased. The C60 molecules have a range of bonding sites on this surface. Local τ-scaling relationships between molecules were identified at low coverages with bonding of these molecules in five-fold hollow sites. In order to passivate the surface from oxidation S has been adsorbed on the Al-Pd-Mn surface. Saturation coverage is obtained after 5 minutes dosing and the LEED pattern disappears after 5 seconds dosage. NEXAFS spectra reveal a disordered adsorption on such a surface. Bond lengths of 2.10 A for S-AI and 2.40 A for S-Pd have been estimated using SEXAFS. Finally the FTIR experiment shows that CO molecules do not stick intact on the surface. NO and HCOOH crack when reaching the quasicrystal surface and CD4O molecules behave as on other samples. However C6H6 molecules do not crack on the surface and do not affect the LEED pattern either. (author)