The atomic scale adhesion properties of two high-symmetry surfaces of decagonal Al-Ni-Co quasicrystals have been investigated using atomic force microscopy (AFM) in ultrahigh vacuum. Imaging the surface allowed us to distinguish the plastic regime from the elastic (reversible) regime of tip-sample contact. The work of adhesion of the atomically clean quasicrystal surface in the plastic regime is smaller than that of single crystalline Pt(111) by a factor of 10, reflecting a lower surface energy for the quasicrystal surface. However, the adhesion force must be reduced even further, in order to make measurements outside of the plastic regime possible. We present a strategy for doing this that involves chemical modification of the surface or the tip, together with appropriate choice of mechanical contact parameters.
We investigated the nanoscale tribological properties of a decagonal quasicrystal using a combination of atomic force microscopy and scanning tunneling microscopy in ultrahigh vacuum. This combination permitted a variety of in situ measurements, including atomic-scale structure, friction and adhesion force, tip-sample current, and topography. We found that thiol-passivated tips can be used for reproducible studies of the tip-quasicrystal contact while nonpassivated probes adhere irreversibly to the clean quasicrystalline surface causing permanent modifications. The most remarkable results were obtained on the twofold surface of the $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Co}$ decagonal quasicrystal where atoms are arranged periodically along the tenfold axis and aperiodically in the perpendicular direction. Strong friction anisotropy was observed on this surface, with high friction along the periodic direction and low friction in the aperiodic direction.
The atomic structure of the fivefold symmetric quasicrystal surface of icosahedral AlPdMn has been investigated by means of a dynamical low-energy-electron diffraction (LEED) analysis. Approximations were developed to make the structure of an aperiodic, quasicrystalline surface region accessible to LEED theory. A mix of several closely similar, relaxed, bulklike lattice terminations is favored, all of which have a dense Al-rich layer on top followed by a layer with a composition of about 50% Al and 50% Pd. The interlayer spacing between these two topmost layers is contracted from the bulk value by 0.1 \AA{}, to a final value of 0.38 \AA{}, and the lateral density of the two topmost layers taken together is similar to that of an Al(111) surface. The LEED structural result is qualitatively consistent with data from ion scattering spectroscopy, which supports an Al-rich termination.
The adsorption of Xe onto the tenfold surface of decagonal Al-Ni-Co was studied using low-energy electron diffraction (LEED). LEED isobar measurements indicate that Xe grows in a layer-by-layer mode for at least the first two layers in the temperature range 60--80 K. The half-monolayer isosteric heat of adsorption was measured to be $250\ifmmode\pm\else\textpm\fi{}10\mathrm{meV}.$ No superlattice was observed for the first layer of Xe, which is therefore presumed either to have a quasicrystalline structure or to be disordered. Upon adsorption of the second layer, an ordered Xe bilayer forms, which has a structure consistent with domains of bilayer Xe(111) aligned along substrate symmetry directions. At higher Xe coverages (several Xe layers), the LEED pattern becomes more distinct and remains consistent with that from a Xe(111) surface.
Using scanning tunneling microscopy, we study the post-deposition coarsening of distributions of large, two-dimensional Ag islands on a perfect Ag(100) surface at 295 K. The coarsening process is dominated by diffusion, and subsequent collision and coalescence of these islands. To obtain a comprehensive characterization of the coarsening kinetics, we perform tailored families of experiments, systematically varying the initial value of the average island size by adjusting the amount of Ag deposited (up to 0.25 ML). Results unambiguously indicate a strong decrease in island diffusivity with increasing island size. An estimate of the size scaling exponent follows from a mean-field Smoluchowski rate equation analysis of experimental data. These rate equations also predict a rapid depletion in the initial population of smaller islands. This leads to narrowing of the size distribution scaling function from its initial form, which is determined by the process of island nucleation and growth during deposition. However, for later times, a steady increase in the width of this scaling function is predicted, consistent with observed behavior. Finally, we examine the evolution of Ag adlayers on a strained Ag(100) surface, and find significantly enhanced rates for island diffusion and coarsening.
Scanning tunneling microscopy experiments reveal the formation of a variety of geometrically exotic nanostructures following submonolayer deposition of Ag on Ag(100). These result from the diffusion of large Ag clusters, and their subsequent ``collision'' and coalescence with extended step edges, and with other clusters. Relaxation of these far-from-equilibrium step-edge configurations is monitored to determine rates for restructuring versus local geometry and feature size. This behavior is analyzed with lattice-gas model simulations to elucidate the underlying atomistic mass transport processes.
