Surface Structure and Structure-Property Relationships in Metal-Rich Solid-State Phase

Program Scope: The goal is to elucidate intrinsic structural and functional aspects of surfaces of complex intermetallics, notably quasicrystals, with particular attention to the structure-property relationships. These aspects include microscopic and mesoscopic morphology, atomic locations, electronic structure, interfacial growth, friction, and chemical reactivity.

Major Program Achievements: (1) Atomic and Electronic Structure:  Our work is at the forefront of understanding the nature of the clean surfaces of quasicrystals.  We use various ultrahigh vacuum surface-sensitive techniques including scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and angle-resolved low-energy ion scattering (LEIS) to probe these surfaces. By STM, we showed that temperature effects play a vital role in surface structure.  A temperature change from 900 to 925 K is sufficient to dramatically alter the quasicrystal terraces; at the lower temperature, a distinctive void-rich termination is common.  These results provided new insight into the way that a surface evolves toward a quasicrystalline structure.

(2) Interfacial Growth:  Recent work has focused on understanding the interface between crystalline and quasicrystalline materials.  In particular we have examined Ag deposited on quasicrystals.  We find that growth is temperature dependent.  Below 300 K intrinsic trap sites dominate nucleation and thus density remains constant and 2D islands form.  Above 300 K the film grows as 3D flat-topped mounds with preferred heights, thus showing quantum size effects.  This may be due to the electronic pseudogap of the bulk quasicrystal, and the fact that it is present up to the surface of the quasicrystal, as we have demonstrated previously using XPS.

(3) Friction and Wear: With the goal of examining the fundamental role of periodicity in friction, we realized that it is essential to measure friction under wear-free conditions. To that end, we established a collaboration with M. Salmeron and J.-Y. Park at LBNL. AFM measurements showed an 8-fold anisotropy in the friction force on a clean twofold surface of decagonal Al–Ni–Co. (The atoms are aperiodically spaced along one direction of this surface and periodically along the other.) The highest friction force was measured in the periodic direction, the lowest in the aperiodic direction, hence demonstrating that low friction forces are intrinsic to the aperiodic arrangement. This work was published in Science in 2005.  An offshoot of this work, published in Science in 2006, demonstrated that free charge carriers can play a critical role in frictional energy dissipation on semiconductors.

Program Impact: Our work to date has culminated in the acceptance in the field that atomically flat quasicrystalline surfaces can be produced and that these surfaces are laterally bulk terminated; this work has implications for the relative importance of three-dimensional clusters in stabilizing the icosahedral structure. It has also demonstrated that low friction is intrinsic to the aperiodic atomic structure of a clean quasicrystalline surface, laying to rest the possibility that low friction in these materials is due (solely) to hardness or oxide chemistries. In addition, our work has motivated and facilitated many new research projects around the world.

Contact Information:
Patricia Thiel

Selected Publications