Erik J. Cox

Quasicrystal surfaces: structure and potential as templates

We present a review of recent progress in determining the surface structure of quasicrystals, with emphasis on their connections to mathematical tiling models. The review focusses in particular on the five-fold surface of icosahedral Al-Pd-Mn and the ten-fold surface of decagonal Al-Ni-Co. We also assess their potential as templates for the formation of two-dimensional quasicrystalline overlayers with reference to recent investigations of atomic and molecular adsorption.

Quasicrystal surfaces: potential as templates for molecular adsorption

Abstract We report on investigations of the potential for using quasicrystal surfaces as templates for molecular adsorption. The quasicrystalline surfaces of the i -Al–Pd–Mn and the d -Al–Ni–Co quasicrystals have been dosed with various molecules and the surface reactivity characterised by Fourier transform infrared spectroscopy (FTIR). The reflected signal intensities obtained are much weaker than those obtained from other crystals such as metals. Although both quasicrystals are Al-based their surface reactivities are seen to differ. For the Al–Pd–Mn surface, NO and HCOOH both dissociate at the surface and CO does not adsorb. CD 4 O adsorption leads to multilayers as in other crystals. C 6 H 6 molecules stick at low coverages and at low temperatures whilst not affecting the LEED pattern. For the Al–Ni–Co surface, CO molecules stick via activated adsorption to atop sites with vibrational frequencies very similar to CO adsorption on Ni. HCOOH forms multilayers at low temperatures which is reduced to a monolayer upon heating to 173 K; changes in the symmetry of the bonding are observed, which is also seen for HCOOH on Ni surfaces. NO and CD 4 O showed no characteristic spectra, which may be due to dissociation or a very low sticking probability. We summarise these observations and draw some conclusions as to the most favourable route for molecular overlayer formation.

Tilings and Coverings of Quasicrystal Surfaces

The tiling and covering of images derived from the experimental technique of high-resolution transmission electron microscopy (HRTEM) has been a fruitful avenue in studies of bulk quasicrystal structures. In particular, the quasi-unit-cell model has been successfully applied to the 2-dimensional decagonal AlNiCo (d-AlNiCo) quasicrystal (see Sect. 8.3.1). However, HRTEM is a bulk technique and can only be used to observe the average structure over the thickness of the sample. On the other hand, scanning tunneling microscopy (STM) is a purely surface technique. Hence, if high-quality images of quasicrystal surfaces can be obtained, then the possibility arises of applying tiling and covering methodologies to these surfaces, with a view to elucidating their structure and determining whether or not they are terminations of the bulk. A corollary of this approach is that such studies would provide independent verification of current bulk tiling and covering models. In this chapter, we describe our attempts to apply tilings and coverings to images of surfaces of quasicrystals obtained using STM [1, 2, 3, 4, 5, 6, 7].