Center for Functional Nanomaterials Seminar

Quasicrystals are non-periodic but well-ordered solids. Their unusual atomic structures cultivate peculiar surface properties such as low friction and enhanced oxidation resistance. In this talk, I will present scanning tunneling microscopy (STM) studies on clean fivefold surface of i-Al-Pd-Mn quasicrystals. These studies show that atomically-clean quasicrystal surfaces are different than surfaces of crystalline materials. They possess high degree of heterogeneity among different terraces, and even among structurally-identical adsorption sites. The origin of heterogeneity can be both structural and chemical.
In the second part of the talk, I will discuss the nucleation of Ag islands on the fivefold surface of i-Al-Pd-Mn quasicrystal monitored with STM. The density of Ag islands remains constant from 127 K to 300 K, but drops as temperature increases above 300 K. A mean field rate equation modeling, which takes into account the enhanced nucleation at traps relative to nucleation at regular terrace sites, is helpful to understand this behavior. The best fit to the model suggests that the critical sizes for the nucleation at both sites are large and binding between Ag atoms at the traps is stronger than at the regular terrace sites.
Finally, I will demonstrate the growth behavior of Ag islands captured by STM. On fivefold surfaces of i-Al-Pd-Mn quasicrystal, nucleation of single layer-high islands is followed by rapid formation of 2nd and 3rd layers and then subsequent lateral spreading while the height remains constant. The growth characteristic is elucidated by step dynamics modeling where several factors are incorporated such as: strain buildup for larger islands, enhanced binding in higher layers, and height selection due to quantum size effects. These unusual features observed on the surfaces of quasicrystals are consequences of the beautiful and complex - atomic structure of these metallic alloys.


Host: Peter Sutter