Cynthia J. Jenks

Oxygen adsorption on a single-grain, quasicrystal surface

Abstract Oxygen adsorbs on the five-fold surface of Al 70 Pd 21 Mn 9 . A chemisorbed phase, possibly in or below the surface plane, serves as a precursor to oxidation of Al. This chemisorbed phase destroys the quasiperiodicity of the surface. The major features of oxygen adsorption and oxidation are similar in the temperature range 105–500 K, but are different at 870 K, where Al segregates strongly to the surface. We postulate that Al segregation is driven by the exothermicity of its oxide, which is higher than that of the other constituents of this alloy. At all temperatures, the oxide layer is passivating (under the conditions of these experiments), is quite thin ( ⪅ 10 A ) and is thermally stable (at least upon heating to 870 K). The oxidation characteristics of this quasicrystal are broadly similar to those of its major constituent, Al, with the possible exception of the oxygen sticking coefficient.

Surface properties of quasicrystals

There is currently great interest in the surface reactivity of quasicrystalline materials, generated largely by a model, proposed by Janot, for the bulk atomic and electronic structure. This {open_quotes}hierarchical cluster{close_quotes} model predicts that quasicrystal surfaces should be intrinsically inert and rough, and is being used to rationalize their practical properties such as low friction coefficients and oxidation resistances Surface structure and surface preparation may play a role in the applicability of this model. In this talk, we examine these factors and present experimental measurements of the surface reactivity of Al-based icosahedral alloys. We make some comparisons with surface reactivity of pure, crystalline aluminum, and with that of crystalline alloys which are similar in composition to the quasicrystal.

Friction and Adhesion Properties of Clean and Oxidized Al-Ni-Co Decagonal Quasicrystals: A UHV Atomic Force Microscopy/Scanning Tunneling Microscopy Study

The tribological properties of adhesion and friction between 10-fold Al-Ni-Co decagonal quasicrystals and conductive W2C and TiN coated tips were studied in ultrahigh vacuum (UHV) with an atomic force microscope. Contacts between the tip and clean quasicrystals are dominated by strong adhesion forces, which result in irreversible deformations and material transfer. The irreversible adhesion was suppressed following an oxygen exposure of 10 Langmuir, which also reduced the adhesion force by a factor of two. An additional 2/3 reduction in adhesion force occurred after several 100 Langmuir exposure. A much larger decrease (by a factor 10) occurred by air-oxidation. The friction force decreased also with oxygen exposure although not as dramatically. A linear decrease by a factor two, was observed between the clean surface and the surface exposed to 200 Langmuir of oxygen. After that the friction force remained constant. Air-oxidation reduced friction by roughly another factor of two. In contrast with the clean surface, contacts with the air oxide substrate are well described by the Derjaguin-Müller-Toporov (DMT) model, while contacts with oxygen chemisorbed substrates are in the transition regime between Johnson-Kendall-Roberts (JKR) and DMT models.

Preparation of well-defined samples of AlPdMn quasicrystals for surface studies

Abstract We have developed a method for preparing single-grain, quasicrystalline AlPdMn samples for surface studies in ultrahigh vacuum. The main issues of concern are phase purity, the quality of the surface structure, and the surface, and the surface composition. Phase purity is enhanced by annealing the sample in ultra-pure Ar in a sealed quartz ampoule for several days before polishing. Polishing with colloidal silica allows secondary phases to be detected readily with an optical microscope. As a final precaution, phase purity can be checked sensitively with scanning Auger microscopy. After this stage, the sample can be cleaned in ultrahigh vacuum with ion bombardment. Annealing is required after bombardment to restore surface structure and to obtain a low-energy electron diffraction (LEED) pattern of an oriented sample. However, both ion bombardment and heating to temperatures above 870 K in vacuum, produce Pd-rich surfaces. As a final step, for the five-fold surface, we recommend heating briefly to 1050–1100 K and then annealing at 870 K for several hours. This produces both an excellent LEED pattern, and a surface composition close to that of the bulk.

Friction between single-grain Al70Pd21Mn9 quasicrystal surfaces

Abstract An UHV tribometer was used to make measurements of friction between the five-fold surfaces of two single-grain Al 70 Pd 21 Mn 9 quasicrystals. The surfaces of these quasicrystal samples were either truly clean or modified by controlled adsorption of desired species. The results of many (∼200) single-pass friction measurements using clean quasicrystal surfaces indicate that the average static friction coefficient is μ s =0.60±0.08. The friction coefficient of the perfectly clean quasicrystal surfaces is lower than those reported for the perfectly clean surfaces of many pure metals but is higher than those measured on quasicrystal surfaces exposed to air. Under shear the quasicrystal surfaces slide over one another without exhibiting stick-slip behavior or adhesion. This behavior differs from clean metal surfaces that often exhibit adhesion after being brought into contact in ultra-high vacuum. Studies were performed to determine the effects of surface oxidation, surface structure and surface composition on quasicrystal friction. During the early stages of oxidation the static friction coefficient decreased to a limit of μ s =0.35±0.05. More subtle changes to the quasicrystal surfaces such as adsorption of sulfur or formation of a thin Al–Pd–Mn alloy of CsCl structure produced no significant changes in the static friction coefficient.

Comments on quasicrystals and their potential use as catalysts

Recent findings suggest that quasicrystalline materials may make better catalysts than their crystalline counterparts. On the other hand, detailed surface science studies suggest that the surfaces of Al-based quasicrystals behave as if they are chemically similar to pure Al. In this paper, we discuss these results along with the unique thermodynamic and electronic properties of quasicrystals which could affect their catalytic nature.

A LEED comparison of structural stabilities of the three high-symmetry surfaces of Al-Pd-Mn bulk quasicrystals

Abstract It is shown that low-energy electron diffraction (LEED) patterns of the three high-symmetry surfaces (fivefold, threefold and twofold) of icosahedral Al–Pd–Mn are all compatible with quasicrystallinity, under specific conditions of preparation. This conclusion results from comparing symmetries of experimental surface LEED patterns with bulk X-ray diffraction data which are converted to the conditions of the LEED experiment. This conclusion is also based upon an analysis of relative diffraction spot spacings in LEED. Hence, none of the three surfaces exhibits a massive lateral reconstruction, i.e. massive deviation from quasicrystallinity. The LEED pattern of the fivefold surface is distinct from the LEED pattern of the pseudo-tenfold surface of an orthorhombic approximant. We believe that this rules out the possibility that the fivefold surface of the icosahedral quasicrystal reconstructs to an approximant with tenfold or pseudo-10-fold symmetry. The twofold and threefold surfaces facet more readily, indicating qualitatively that they are less stable than the fivefold surface.

Smoluchowski ripening of Ag islands on Ag(100)

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. Ho...

Structural aspects of the fivefold quasicrystalline Al–Cu–Fe surface from STM and dynamical LEED studies

We investigate the atomic structure of the fivefold surface of an icosahedral Al-Cu-Fe alloy, using scanning tunneling microscopy (STM) imaging and a special dynamical low energy-electron diffraction (LEED) method. STM indicates that the step heights adopt (primarily) two values in the ratio of tau, but the spatial distribution of these two values does not follow a Fibonacci sequence, thus breaking the ideal bulk-like quasicrystalline layer stacking order perpendicular to the surface. The appearance of screw dislocations in the STM images is another indication of imperfect quasicrystallinity. On the other hand, the LEED analysis, which was successfully applied to Al-Pd-Mn in a previous study, is equally successful for Al-Cu-Fe. Similar structural features are found for both materials, in particular for interlayer relaxations and surface terminations. Although there is no structural periodicity, there are clear atomic planes in the bulk of the quasicrystal, some of which can be grouped in recurring patterns. The surface tends to form between these grouped layers in both alloys. For Al-Cu-Fe, the step heights measured by STM are consistent with the thicknesses of the grouped layers favored in LEED. These results suggest that the fivefold Al-Cu-Fe surface exhibits a quasicrystalline layering structure, but with stacking defects.

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.