Quantitative adsorbate structure determination for quasicrystals using x-ray standing waves

Fullerene adsorption on intermetallic compounds of increasing structural complexity

Compared to elemental crystals (Al, Cu, Ag, etc.), the local atomic arrangement within Al-based complex intermetallics is usually best described by highly symmetric clusters decorating the unit cell. With the latter containing tens to several thousand atoms (or an infinite number for the case of quasicrystals), this translates to structurally complex surfaces exhibiting unique potential energy landscapes. This review will focus on the different studies reporting the adsorption of C60 molecules on such complex metallic alloy surfaces, aiming to benefit from this complexity to create exotic molecular nanostructures. First, we will recall the main adsorption mechanisms and surface phases that have been identified when fullerene adsorption is carried out on single crystal surfaces. Second, we will discuss how surfaces of increasing structural complexity impact the film properties. The presence of five-fold symmetric adsorption sites is another intrinsic property of these complex intermetallic surfaces. As will be presented in this review, this leads to specific molecular orientations to maximize substrate–adsorbate symmetry matching, hence introducing another degree of freedom to create new 2-D molecular architectures. The local electronic interactions at the adsorption site interface will also be introduced. Furthermore, the different fullerene structures formed upon adsorption on aperiodic surfaces of varying chemical composition and on Bi allotropes will be discussed. Finally, suggestions will be given for future work along with the foreseen area of interests.

Direct observation of solid-state reversed transformation from crystals to quasicrystals in a Mg alloy

Phase transformation of quasicrystals is of interest in various fields of science and technology. Interestingly, we directly observed unexpected solid-state epitaxial nucleation and growth of Zn₆Mg₃Y icosahedral quasicrystals in a Mg alloy at about 573 K which is about 300 K below the melting point of Zn₆Mg₃Y, in contrast to formation of quasicrystals through solidification that was usually found in many alloys. Maximizing local packing density of atoms associated with segregation of Y and Zn in Mg adjacent to Mg/Zn₃MgY interfaces triggered atomic rearrangement in Mg to form icosahedra coupled epitaxially with surface distorted icosahedra of Zn₃MgY, which plays a critical role in the nucleation of icosahedral clusters. A local Zn:Mg:Y ratio close to 6:3:1, corresponding to a valence electron concentration of about 2.15, should have been reached to trigger the formation of quasicrystals at Mg/Zn₃MgY interfaces. The solid-state icosahedral ordering in crystals opens a new window for growing quasicrystals and understanding their atomic origin mechanisms. Epitaxial growth of quasicrystals onto crystals can modify the surface/interface structures and properties of crystalline materials.