LIBRARY SHELF:

Surface and Interface Properties

Title: "Use of periodic approximants in a dynamical LEED study of the quasicrystalline tenfold surface of decagonal Al-Ni-Co"

Authors: K. Pussi, N. Ferralis, M. Mihalkovic, M. Widom, S. Curtarolo, M. Gierer, C.J. Jenks, P. Canfield, I.R. Fisher, and R.D. Diehl

Contact:  I.R. Fisher

Journal:  Phys. Rev. B (2006) 73, 184203/1-184203/14.

Abstract:

The determination of quasicrystal (QC) surface structures is a challenge to current surface structure techniques. Low-energy electron diffraction (LEED) is the primary technique for the determination of periodic surface structures, but application of dynamical LEED to quasicrystals requires the use of many approximations. In this study, two different approaches were used to apply dynamical LEED to the structure of the tenfold surface of decagonal Al73Ni10Co17. One method (method 1) involves the use of a quasicrystalline model along with approximations that average over the composition and local geometries. The other method (method 2) uses periodic models that approximate the actual local QC structure (approximants) in more exact, atomistic calculations. Although the results using the two methods were consistent, the results of the approximant analysis (method 2) suggested a different way to apply the approximations in method 1, resulting in a better fit between experimental and calculated beams. Thus, periodic approximant structure models can provide a simpler and more efficient method for the determination of local geometries in QC surfaces, and may also facilitate analyses using quasicrystal models.

Title: "Ordering of Si atoms on the fivefold Al-Pd-Mn quasicrystal surface"

Authors: J. Ledieu, P. Unsworth, T.A. Lograsso, A.R. Ross, and R. McGrath

Contact:  J. Ledieu

Journal:  Phys. Rev. B (2006) 73, 012204/1-012204/4.

Abstract:

Silicon atoms are found to form an ordered overlayer on the fivefold surface of the icosahedral Al-Pd-Mn quasicrystal. Using scanning tunneling microscopy, the adsorption site is identified as the center of truncated clusters which are building blocks of the bulk structure. Comparison with theoretical models suggests that this site is favored because of bonding to Mn atoms.

Title: "Film growth arising from the deposition of Au onto an i-Al-Pd-Mn quasicrystal:  A medium energy ion scattering study"

Authors: T.C.Q. Noakes, P. Bailey, M. Draxler, C.F. McConville, A.R. Ross, T.A. Lograsso, L. Leung, J.A. Smerdon, and R. McGrath

Contact:  T.C.Q. Noakes

Journal:  J. Phys.: Condens. Matter (2006) 18, 5017-5027.

Abstract:

The room temperature deposition of 7 ML of Au onto the fivefold symmetric surface of icosahedral Al-Pd-Mn leads to the formation of a several monolayers thick Au-Al alloy film. An AlAu film with 1:1 stoichiometry is formed, which shows no evidence of ordered structure, being either amorphous or polycrystalline.  Annealing to 325°C causes more Al to diffuse into the film, producing Al2Au but still with no indication of structure.  Experiments using 0.5 ML of pre-deposited In demonstrated a surfactant effect as the In 'floated' on the surface during growth and produced a reduction in film roughness. However, contrary to previous findings the film was still either amorphous or polycrystalline, with no evidence of quasi-crystalline or aperiodic structure. Experiments were also conducted using smaller doses of Au to look for the formation of an epitaxial layer and, if formed, determine the registry with the substrate. However, no change in the Pd blocking curves for the surface could be seen, suggesting that the Au does not adsorb in well defined sites. This result is not surprising when considering that even for these low doses Al is drawn into the film, changing the composition and probably the structure of the topmost layers of the substrate, so that the potential adsorption sites on the clean surface may no longer exist.

Title: "Terrace-dependent morphology of thin Sn films deposited on the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal"

Authors: H.R. Sharma, M. Shimoda, A.R. Ross, T.A. Lograsso, and A.P. Tsai

Contact:  H.R. Sharma

Journal:  Philos. Mag. (2006) 86, 807-812.

Abstract:

We present an analysis of Sn film growth on the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal at multilayer coverage. Scanning tunnelling microscopy of the Sn-deposited surface revealed that film morphology differed from terrace to terrace. Most terraces exhibited flat-topped clusters with preferential growth at the step edges, while a minority of terraces showed a smooth film. The height, lateral size and d. of clusters also varied between terraces. Clusters have a preference for specific heights.

Title: "Island formation during Al deposition on 5-fold Al-Cu-Fe quasicrystalline surfaces: Kinetic Monte Carlo simulation of a disordered-bond-network lattice-gas model"

Authors: C. Ghosh, D.-J. Liu, K.J. Schnitzenbaumer, C.J. Jenks, P.A. Thiel, and J.W. Evans

Contact:  J.W. Evans

Journal:  Surf. Sci. (2006) 600, 2220-2230.

Abstract:

Scanning tunneling microscopy experiments have previously revealed the formation of pseudomorphic starfish-shaped islands during the initial stages of Al deposition on 5-fold icosohedral Al-Cu-Fe quasicrystal surfaces. To simulate this process, we first identify appropriate 5-fold surface terminations of Al-Cu-Fe from a model for bulk structure, and construct associated potential energy surfaces for the binding of Al adatoms on these terminations. We then identify a disordered-bond-network (DBN) connecting neighboring local adsorption sites for Al on Al-Cu-Fe, and determine site binding energies as well as activation barriers for Al adatom hopping between neighboring sites. Al-Al adsorbate interactions, which stabilize islands, are also prescribed. Then, within the framework of a DBN lattice-gas model, we simulate the deposition and diffusion of Al on Al-Cu-Fe. We explore the competition between starfish and incomplete starfish ensembles of sites (which provide traps in the form of deep potential energy wells for diffusing Al) and isolated trap sites, with regard to the heterogeneous nucleation and aggregation of Al into islands.

Title: "Hydrogen interactions with quasicrystalline Al-Pd-Mn surfaces"

Authors: R. Bastasz, J.A. Whaley, T.A. Lograsso, and C.J. Jenks

Contact:  R. Bastasz

Journal:  Philos. Mag. (2006) 86, 855-862.

Abstract:

In this work we examine the interaction of molecular and atomic deuterium with the fivefold surface of an icosahedral (i-) Al-Pd-Mn alloy using angle-resolved low-energy ion scattering under ultrahigh vacuum conditions. i-Al-Pd-Mn has been studied extensively and is known to form a laterally-bulk-terminated surface that is Al-rich. The density of Al atoms on the clean surface of fivefold i-Al-Pd-Mn is about that of Al(111) and hence we compare our results to studies of molecular and atomic hydrogen adsorbed on Al(111). We find that molecular deuterium does not adsorb or dissociate on i-Al-Pd-Mn, as on Al(111). Atomic D, however, readily adsorbs on both surfaces and on i-Al-Pd-Mn attenuates the scattering signals from Al, Pd and Mn substrate atoms. It thus appears that D chemisorbs on the outer layer of i-Al-Pd-Mn and bonds with Al atoms.

Title: "Adsorption of benzene on the five-fold surface of the i-Al70Pd21Mn9 quasicrystal"

Authors: J.T. Hoeft, J. Ledieu, S. Haq, T.A. Lograsso, A.R. Ross, and R. McGrath

Contact:  R. McGrath

Journal:  Philos. Mag. (2006) 86, 869-874.

Abstract:

The adsorption behavior of benzene (C6H6) on the five-fold surface of the icosahedral Al70Pd21Mn9 quasicrystal has been investigated. Reflection absorption IR spectroscopy (RAIRS) data taken at a temp. of 92K indicate molecular adsorption. The adsorption geometry of the molecules is found to change from parallel to the surface to oblique geometries as the dose is increased from monolayer to multilayer coverages. Scanning tunnelling microscopy (STM) images show that the molecuels adsorb in a disordered fashion. The adsorbate layer is found to completely desorb at 153K leaving the substrate unchanged.

Title: "Modelling of the energetics and kinetics of Al deposition on 5-fold Al-rich quasicrystal surfaces"

Authors: C. Ghosh, D.J. Liu, C.J. Jenks, P.A. Thiel, and J.W. Evans

Contact:  J.W. Evans

Journal:  Philos. Mag. (2006) 86, 831-840.

Abstract:

We determine potential energy surfaces for the binding of Al adatoms on 5-fold AlPdMn and AlCuFe quasicrystal surfaces. Appropriate geometric models are used to select physical surface terminations. The interaction between the Al adsorbate and the substrate is described using Lennard-Jones pair-wise interactions, with parameters chosen to fit ab initio energetics for Al on relevant single crystal substrates. We identify a ‘disordered-bond-network' (DBN) of local adsorption sites, which includes deep ‘starfish' and ‘incomplete starfish' ensembles. Our primary interest is in the kinetics of deposition and aggregation of Al atoms, and the possible formation of pseudomorphic starfish islands at starfish ensembles (observed for Al deposition on AlCuFe). The deposition process is modeled within the framework of a DBN lattice-gas model, wherein we specify rates for deposition and for hopping between neighboring sites of the DBN, as well as Al-Al adsorbate interactions, which stabilize islands. We, thus, present a picture for the formation of starfish islands subject to the competition between various deep sites for Al adatoms.

Title: "Adhesion properties of decagonal quasicrystals in ultrahigh vacuum"

Authors: J.Y. Park, D.F. Ogletree, M. Salmeron, R.A. Ribeiro, P.C. Canfield, C.J. Jenks, and P.A. Thiel

Contact:  M. Salmeron

Journal:  Philos. Mag. (2006) 86, 945-950.

Abstract:

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.

Title: "Voids and pits on sputter-annealed fivefold terraces of icosahedral Al-Pd-Mn quasicrystals"

Authors: B. Unal, T.A. Lograsso, A.R. Ross, C.J. Jenks, and P.A. Thiel

Contact:  B. Unal

Journal:  Philos. Mag. (2006) 86, 819-824.

Abstract:

Our scanning tunneling microscope studies on clean surfaces of fivefold icosahedral Al70.2Pd20.7Mn9.1 reveal that, after annealing at 900 K, two types of flat depressions are visible on terraces: voids (small, shallow, flat-bottomed depressions bordered by 2.5 Å steps) and pits (deeper, and often larger, depressions). They are different not only in their dimensions but in their behavior: voids preferentially nucleate on some of the terraces and, after annealing at 925 K, their coalescence and growth leads a new type of termination to be exposed. Pits, however, do not destroy the terraces on which they exist, under the experimental conditions. At the bottom of 4.1 Å-deep pits, 2.5 Å-deep voids nucleate, so that the total depth of the pit is 6.6 Å. We propose that 2.5 Å void-rich terraces are metastable terminations, and that these metastable terminations are also exposed at the bottom of the 4.1 Å-deep pits.

Title: "Characterization of aperiodic and periodic thin Cu films formed on the five-fold surface of i-Al70Pd21Mn9 using medium-energy ion scattering spectroscopy"

Authors: J.A. Smerdon, J. Ledieu, R. McGrath, T.C.Q. Noakes, P. Bailey, M. Draxler, C.F. McConville, T. Lograsso, and A. Ross

Contact:  J. Ledieu                 

Journal:  Phys. Rev. B (2006) 74, 035429/1-035429/7.

Abstract:

The elucidation of the local atomic structure of a pseudomorphic film of Cu deposited on the five-fold surface of i-Al70Pd21Mn9 using medium-energy ion scattering spectroscopy is reported. Monte Carlo calculations, using the VEGAS code, have been utilized to simulate the blocking of 100  keV He+ ions scattered from the overlayer. The coordinates of the Cu atoms in the overlayer derived from this procedure are consistent with a structure occurring in five rotational domains. Each domain consists of nanoscale strips of fcc Cu(100) with the <110> azimuth aligned along the five-fold directions of the quasicrystalline substrate. The strips are arranged according to a one-dimensional Fibonacci sequence with long and short widths related by the golden mean Τ. Upon annealing the film transforms to an alloyed structure composed of five orientational domains of fcc material with the (110) axis perpendicular to the surface.

Title: "Real-space observation of quasicrystalline Sn monolayer formed on the fivefold surface of icosahedral Al-Cu-Fe quasicrystal"

Authors: H.R. Sharma, M. Shimoda, A.R. Ross, T.A. Lograsso, and A.P. Tsai

Contact:  H.R. Sharma               

Journal:  Phys. Rev. B (2005) 72, 045428/1-045428/4.

Abstract:

We investigate a thin Sn film grown at elevated temperatures on the fivefold surface of an icosahedral Al-Cu-Fe quasicrystal by scanning tunneling microscopy (STM). At about one monolayer coverage, the deposited Sn is found to form a smooth film of height consistent with one-half of the lattice consistent of the bulk Sn. Analysis based on the Fourier transform and autocorrelation function derived from high-resolution. STM images reveals that Sn grows pseudomorphically and hence exhibits a quasicrystalline structure.

Title: "Copper adsorption on the fivefold Al70Pd21Mn9 quasicrystal surface"

Authors: J. Ledieu, J.T. Hoeft, D.E. Reid, J.A. Smerdon, R. D. Diehl, N. Ferralis, T.A. Lograsso, A.R. Ross, and R. McGrath

Contact:  J. Ledieu                

Journal:  Phys. Rev. B (2005) 72, 035420/1-035420/6.

Abstract:

Recently we reported the formation of a quasiperiodic Cu thin film on the fivefold icosahedral Al-Pd-Mn quasicrystal using scanning tunneling microscopy, LEED, and Auger electron spectroscopy. Here we provide details pertaining to the growth, stability, and structure of this film. Structural information has been gained by LEED measurements carried out at 85 K. Cu atoms are organized periodically with a nearest-neighbor distance of 2.5 ± 0.1 Å along the aperiodically spaced rows. Above 8 ML spontaneous mass transport resulting in island formation has been observed by STM. These observations point to ascending adatoms being responsible for the formation of 3D features. Finally, flashing the multilayer film to 570 K results in the desorption or diffusion of Cu into the bulk and the formation of five domains of a periodic structure.

Title: "First steps in the growth of Cu thin films on the five-fold surface of the icosahedral Al-Cu-Fe quasicrystal"

Authors: H.R. Sharma, M. Shimoda, V. Fournée, A.R. Ross, T.A. Lograsso, and A.P. Tsai

Contact:  H.R. Sharma                 

Journal:  Appl. Surf. Sci. (2005) 241, 256-260.

Abstract:

The growth of Cu on the five-fold surface of the icosahedral Al-Cu-Fe is investigated by scanning tunneling microscopy. The clean surface exhibits atomically flat terraces separated by steps of different heights. The steps are found to be frequently bunched. For a coverage close to a monolayer, Cu forms a nearly closed film of a height corresponding to a monolayer of Cu. The film, however, does not exhibit long-range order.

Title: "Atomic scale coexistence of periodic and quasiperiodic order in a 2-fold Al-Ni-Co decagonal quasicrystal surface"

Authors: J.Y. Park, D.F. Ogletree, M. Salmeron, R.A. Ribeiro, P.C. Canfield, C.J. Jenks, and P.A. Thiel

Contact:  M. Salmeron                 

Journal:  Phys. Rev. B (2005) 72, 220201/1-220201/4.

Abstract:

Decagonal quasicrystals are made of pairs of atomic planes with pentagonal symmetry periodically stacked along a 10-fold axis. We have investigated the atomic structure of the 2-fold surface of a decagonal Al-Ni-Co quasicrystal using scanning tunneling microscopy. The surface consists of terraces separated by steps of heights 1.9, 4.7, 7.8, and 12.6 Å containing rows of atoms parallel to the 10-fold direction with an internal periodicity of 4 Å. The rows are arranged aperiodically, with separations that follow a Fibonacci sequence and inflation symmetry. The results indicate that the surfaces are preferentially Al-terminated and in general agreement with bulk models.

Title: "Quantum size effects in metal thin films grown on quasicrystalline substrates"

Authors: V. Fournée, H.R. Sharma, M. Shimoda, A.P. Tsai, B. Unal, A.R. Ross, T.A. Lograsso, and P.A. Thiel

Contact:  P.A. Thiel                

Journal:  Phys. Rev. Lett. (2005) 95, 155504/1-155504/4.

Abstract:

We have investigated by scanning tunneling microscopy the growth of Bi and Ag thin films on the fivefold surface of Al63Cu24Fe13 and Al72Pd19.5Mn8.5 quasicrystal, respectively. For both systems, we observe the formation of islands with magic height, corresponding to the stacking of a specific number of atomic layers. We interpret this unusual growth morphology in terms of quantum size effects, arising from the confinement of the electron within the film. The magic island heights are thus a direct manifestation of the electronic structure of the quasicrystalline substrates.

Title: "Sensing dipole fields at atomic steps with combined scanning tunneling and force microscopy"

Authors: J.Y. Park, G.M. Sacha, M. Enachescu, D.F. Ogletree, R.A. Ribeiro, P.C. Canfield, C.J. Jenks, P.A. Thiel, J.J. Saenz, and M. Salmeron

Contact:  M. Salmeron                 

Journal:  Phys. Rev. Lett. (2005) 95, 136802/1-136802/4.

Abstract:

The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smoluchowski effect were measured from the electrostatic force exerted on the biased tip of a scanning tunneling microscope. By varying the tip-sample bias the contribution of the step dipole was separated from changes in the force due to van der Waals and polarization forces. Combined with electrostatic calculations, the method was used to determine the local dipole moment in steps of different heights on Au(111) and on the twofold surface of an Al-Ni-Co decagonal quasicrystal.

Title: "High frictional anisotropy of periodic and aperiodic directions on a quasicrystal surface"

Authors: J.Y. Park, D.F. Ogletree, M. Salmeron, R.A. Ribeiro, P. Canfield, C. Jenks, and P. Thiel

Contact: P.A. Thiel

Journal: Science (2005) 309, 1354 - 1356.

Abstract:
None

Title: "New phenomena in epitaxial growth: Solid films on quasicrystalline substrates"

Authors: V. Fournée and P.A. Thiel

Contact: P.A. Thiel

Journal: J. Phys. D: Appl. Phys. (2005) 38, R83-R106.

Abstract:
An overview is given of the research conducted in the field of solid film growth on quasiperiodic surfaces. An atomistic description of quasicrystalline surfaces is presented and discussed in relation to bulk structural models. The various systems for which thin film growth has been attempted so far are reviewed. Emphasis is placed on the nucleation mechanisms of the solid films, on their growth modes in relation to the nature of the deposited metals, on the possibility of intermixing or alloying at the interface and on the epitaxial relationships at the crystal-quasicrystal interfaces. We also describe situations where the deposited elements adopt a quasiperiodic structure, which opens up the possibility of extending our understanding of the relation between quasiperiodicity and the physical properties of such structurally and chemically complex solids.

Title: "Step structure on the fivefold Al-Pd-Mn quasicrystal surface, and on related surfaces"

Authors: J. Ledieu, E.J. Cox, R. McGrath, N.V. Richardson, Q. Chen, V. Fournée, T.A. Lograsso, A.R. Ross, K.J. Caspersen, B. Unal, J.W. Evans, and P.A. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (2005) 583, 4-15.

Abstract:
We compare step morphologies on surfaces of Al-rich metallic alloys, both quasicrystalline and crystalline. We present evidence that the large-scale step structure observed on Al-rich quasicrystals after quenching to room temperature reflects equilibrium structure at an elevated temperature. These steps are relatively rough, i.e., have high diffusivity, compared to those on crystalline surfaces. For the fivefold quasicrystal surface, step diffusivity increases as step height decreases, but this trend is not obeyed in a broader comparison between quasicrystals and crystals. On a shorter scale, the steps on Al-rich alloys tend to exhibit local facets (short linear segments), with different facet lengths, a feature which could develop during quenching to room temperature. Facets are shortest and most difficult to identify for the fivefold quasicrystal surface.

Title: "Elastic and inelastic deformations of ethylene-passivated tenfold decagonal Al-Ni-Co quasicrystal surfaces"

Authors: J.Y. Park, D.F. Ogletree, M. Salmeron, R.A. Ribeiro, P.C. Canfield, C.J. Jenks, and P.A. Thiel

Contact: P.A. Thiel

Journal: Phys. Rev. B (2005) 71, 144203/1-144203/6.

Abstract:
The adhesion and friction force properties between a tenfold Al-Ni-Co decagonal quasicrystal and a titanium nitride (TiN)-coated tip were investigated using an atomic force microscope in ultrahigh vacuum. To suppress the strong chemical adhesion found in the clean quasicrystal surfaces, the sample was exposed to ethylene that formed a protective passivating layer. We show that the deformation mechanism of the tip-substrate junction changes from elastic to inelastic at a threshold pressure of 3.8 to 4.0 GPa. Images of the indentation marks left above the threshold pressure indicate the absence of new steps, and indicate that surface damage is not accompanied by formation of slippage planes or dislocations, as found in plastically deforming crystalline materials. This is consistent with the lack of translational periodicity of quasicrystals. The work of adhesion in the inelastic regime is five times larger than in the elastic one, plausibly as a result of the displacement of the passivating layer. In the elastic regime, the friction dependence on load is accurately described by the Derjaguin-Müller-Toporov (DMT) model, consistent with the high hardness of both the TiN tip and the quasicrystal sample. Above the threshold pressure, the friction versus load curve deviates from the DMT model, indicating that chemical bond formation and rupture contribute to the energy dissipation.

Title: "Terrace selection during equilibration at an icosahedral quasicrystal surface"

Authors: B. Unal, T.A. Lograsso, A. Ross, C.J. Jenks, and P.A. Thiel

Contact: P.A. Thiel

Journal: Phys. Rev. B (2005) 71, 165411/1-165411/6.

Abstract:
We investigate the equilibration of a fivefold surface of the icosahedral Al-Pd-Mn quasicrystal at 900–915 and 925–950 K, using scanning tunneling microscopy. After annealing at the lower temperatures, there is a high density of shallow voids on some terraces but not on others; at 925–950 K, the void-rich terraces are much rarer. The terminations that are consumed by voids exhibit a distinctive local atomic configuration, called a "ring" by previous authors. Apparently, through growth and coalescence of the voids, a different termination becomes exposed on the host terraces, which also leads to a change in step heights at the edges of the terraces. We suggest that the shallow steps associated with the voids, and the ring configuration, signal a surface that is in an intermediate stage of structural equilibration.

Title: "RHEED and STM studies of the pseudo-tenfold surface of the ξ'-Al77.5Pd19Mn3.5 approximant crystal"

Authors: H.R. Sharma, M. Shimoda, V. Fournée, A.R. Ross, T.A. Lograsso, and A.P. Tsai

Contact: T. Lograsso

Journal: Phys. Rev. B (2005) 71, 224201/1-224201/8.

Abstract:
The pseudo-tenfold surface of the ξ'-Al77.5Pd19Mn3.5 crystal, an approximant of the icosahedral Al–Pd–Mn quasicrystal, is investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy. The observed RHEED patterns of the surface after sputtering are found to be consistent with those of a simple cubic lattice with (110) surface plane. The [001] and [110] axes of the surface plane are oriented along the principal low-index axes of the bulk. The RHEED patterns of the sputter-annealed surface consist of diffraction streaks with periodic spacings expected for the bulk truncated surface. The surface prepared under different preparation methods is found to exhibit different step-height distribution and terrace morphology. A longer annealing yields a high density of shallow pentagonal pits on terraces, separated predominantly by 0.80-nm high steps and occasionally by double steps. In contrast, the surface prepared with shorter annealing time exhibits highly perfect terraces with 0.80-nm-high steps and additional unusual steps of heights close to 0.40 nm. All step heights observed for both preparation methods are consistent with interlayer spacings of the bulk model.

Title: "Compositional and structural changes in i-AlPdMn quasicrystals induced by sputtering and annealing: A medium energy ion scattering study"

Authors: T.C.Q. Noakes, P. Bailey, C.F. McConville, C.R. Parkinson, M. Draxler, J. Smerdon, J. Ledieu, R. McGrath, A.R. Ross, and T.A. Lograsso

Contact: T.C.Q. Noakes

Journal: Surf. Sci. (2005) 583, 139-150.

Abstract:
The five-fold surface of an Al70Pd21Mn9 quasicrystal has been studied under a variety of surface preparation conditions using medium energy ion scattering. Argon ion sputtering of the surface was seen to give rise to Mn depletion and the formation of a thick five-domain AlPd(1 1 0) layer with ‹1 0 0› azimuths oriented close to, but not exactly on the five-fold symmetric directions of the substrate. Annealing to just 450 °C was sufficient to restore the Mn content and the icosahedral structure in the surface and near-surface region. Annealing to increasingly higher temperatures was seen to cause Mn enrichment in the surface region above the ideal composition for icosahedral structure and this was accompanied by increased amounts of disorder in the sample. For the highest temperature anneals the surface showed low apparent roughness and exhibited blocking patterns consistent with a contraction of the outermost layer of atoms in line with previous studies of this material.

Title: "SEM and AFM studies of a 5-fold surface of icosahedral AlPdMn"

Authors: F. Yu, M. Bischoff, A.R. Ross, T.A. Lograsso, H. Van Kempen, and T. Janssen

Contact: T. Janssen

Journal: J. Non-Cryst. Solids (2004) 334-335, 495-499.

Abstract:
A 5-fold surface of the icosahedral AlPdMn (i-AlPdMn) quasicrystal (QC) was studied by means of SEM (SEM) and atomic force microscopy (AFM) under ambient conditions. We show that a very complicated inhomogeneous surface morphology may develop on the 5-fold surface prepared by ion sputtering and annealing cycles in UHV. Sputtering and sublimation both contribute to material removal on the surface. The sublimation takes place at the annealing temperature at sites of surface defects induced by mechanical polishing and sputtering in UHV. The surface is characterized by nano-sized clusters of about 40 nm in size which form terraces. Results give further evidence for the cluster-based structure of the i-AlPdMn QC.

Title: "An introduction to the surface science of quasicrystals"

Authors: P.A. Thiel

Contact: P.A. Thiel

Journal: Prog. Surf. Sci. (2004) 75, 69-86.

Abstract:
None

Title: "Orientation relationship between metallic thin films and quasicrystalline substrates"

Authors: V. Fournée, A.R. Ross, T.A. Lograsso, and P.A. Thiel

Contact: P.A. Thiel

Journal: Mater. Res. Soc. Symp. Proc. (2004) 805, 281-286.

Abstract:
We present experimental results on the structure of Ag thin films grown on high-symmetry surfaces of both quasicrystals and approximants. For coverages above ten monolayers, Ag form fcc nanocrystals with (111) plane parallel to the surface plane. Depending on the substrate surface symmetry, the Ag nanocrystals exist in one, two or five different orientations, rotated by a multiple of 2p/30. The orientation relationship between crystalline films and substrates appears to be determined by the following principles: high atomic density rows of the adsorbate are aligned along high atomic density rows of the substrate.

Title: "Experimental evidence of the stability of net planes in decagonal quasicrystals"

Authors: C.J. Jenks, J. Bjergaard, P. Canfield, A.R. Ross, W. Steurer, and P.A. Thiel

Contact: C.J. Jenks

Journal: J. Non-Cryst. Solids (2004) 334-335, 486-490.

Abstract:
Because of the aperiodic nature of quasicrystals, lattice planes, in the traditional sense, do not exist for quasicrystals. For decagonal quasicrystals, it has been proposed, however, that one can link the aperiodic and periodic directions using what are termed net planes [Acta Crystallogr. A 57 (2001) 333]. These net planes are thought to play a critical role in the stability and growth of decagonal quasicrystals. To explore their potential role during growth and to shed light on their structural stability, we have studied single-grain surfaces of decagonal Al-Ni-Co by LEED and scanning tunneling microscopy. Our results suggest that these planes do indeed have special stability.

Title: "Friction and adhesion properties of clean and oxidized Al-Ni-Co decagonal quasicrystals: A UHV atomic force microscopy/scanning tunneling microscopy study"

Authors: J.Y. Park, D.F. Ogletree, M. Salmeron, C.J. Jenks, and P.A. Thiel

Contact: P.A. Thiel

Journal: Tribology Letters (2004) 17, 629-636.

Abstract:
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.

Title: "Low-energy electron diffraction study of Xe adsorption on the ten-fold decagonal Al-Ni-Co quasicrystal surface"

Authors: N. Ferralis, R.D. Diehl, K. Pussi, M. Lindroos, I. Fisher, and C.J. Jenks

Contact: C.J. Jenks

Journal: Phys. Rev. B (2004) 69, 075410/1-075410/6.

Abstract:
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±10 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.

Title: "Structure of the tenfold d-Al-Ni-Co quasicrystal surface"

Authors: N. Ferralis, K. Pussi, E.J. Cox, M. Gierer, J. Ledieu, I.R. Fisher, C.J. Jenks, M. Lindroos, R. McGrath, and R.D. Diehl

Contact: C.J. Jenks

Journal: Phys. Rev. B (2004) 69, 153404/1-153404/4.

Abstract:
The structure of the tenfold surface of decagonal Al-Ni-Co was studied using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface region is a relaxed truncated bulk structure, having the same composition as the bulk. The outermost layer spacing is contracted by 10% relative to the bulk interlayer spacing, while the next layer spacing is expanded by 5%. A small degree of intralayer rumpling was observed within each layer. There is a one-to-one correspondence between protrusions observed in the STM images and a subset of atoms in the model structure, indicating that in-plane reconstruction is minimal.

Title: "Probing the surface structure of quasicrystals via angle-resolved low-energy ion scattering"

Authors: C.J. Jenks and R. Bastasz

Contact: C.J. Jenks

Journal: Prog. Surf. Sci. (2004) 75, 147-160.

Abstract:
Angle-resolved low-energy ion scattering is a valuable technique for examining the topmost surface layers of materials. Using this technique, information about both composition and structure can be obtained. We discuss the physical basis of this technique and present our findings for the fivefold surface of icosahedral (i-) Al–Pd–Mn. Our results clearly show that the exposed surface has a higher Al content than the bulk and can have fivefold periodicity. Information about frequently occurring interatomic distances on the surface can also be obtained by this technique. We discuss the results and compare them to recent scanning tunneling microscopy studies and to bulk structure models.

Title: "Maximum density rule for bulk terminations of quasicrystals"

Authors: Z. Papadopolos, P. Pleasants, G. Kasner, V. Fournée, C.J. Jenks, J. Ledieu, and R. McGrath

Contact: C.J. Jenks

Journal: Phys. Rev. B (2004) 69, 224201/1-224201/7.

Abstract:
Bravais' rule, of wide validity for crystals, states that their surfaces correspond to the densest planes of atoms in the bulk. Comparing a theoretical model of icosahedral Al-Pd-Mn with experimental results on sputter-annealed surfaces, we find that this correspondence breaks down, i.e., the surfaces parallel to the densest planes in the model are not necessarily the most stable bulk terminations. The correspondence is restored by recognizing that there is a contribution to the surface not just from a single geometrical plane but from a layer of stacked atoms, possibly containing more than one plane. We find that not only does the stability of high-symmetry surfaces match the density of the corresponding layerlike bulk terminations but the exact spacings between surface terraces can be determined and the typical area of the terraces can be estimated by a simple analysis of the density of layers predicted by the bulk geometric model.

Title: "An STM and SXPS study of the interaction of C60 with the ten-fold surface of the Al72Ni11Co17 quasicrystal"

Authors: E.J. Cox, J. Ledieu, V.R. Dhanak, S.D. Barrett, C.J. Jenks, I. Fisher, and R. McGrath

Contact: C.J. Jenks

Journal: Surf. Sci. (2004) 566-568, 1200-1205.

Abstract:
The adsorption of C60 on the ten-fold surface of the decagonal Al–Ni–Co quasicrystal at room temperature has been investigated using scanning tunnelling microscopy (STM) and soft X-ray photoemission spectroscopy (SXPS). STM indicates disordered adsorption of intact C60 molecules on the surface up to a coverage of 1 ML, followed by the formation of a C60 multilayer. No step decoration is observed indicating that the molecules are immobile at room temperature. SXPS data of the Al 2p core level indicate a strong interaction of the C60 molecules with the Al atoms of the substrate in the sub-monolayer regime. Valence band data indicate that the C60 multilayer desorbs after annealing to 700 K leaving only the strongly bonded layer on the surface.

Title: "Structure and oxidation at quasicrystal surfaces"

Authors: P.A. Thiel

Contact: P.A. Thiel

Journal: Prog. Surf. Sci. (2004) 75, 191-204.

Abstract:
We have investigated the atomic and electronic structure, chemical composition, and oxidation characteristics of the surfaces of icosahedral, Al-rich quasicrystals, using a variety of surface-sensitive techniques (LEED, XPS, STM, AES). We have systematically investigated the way that these traits vary with preparation conditions (e.g. sputtering and then annealing to various temperatures, vs. fracture), with surface symmetry (e.g. 2f vs. 3f vs. 5f surfaces), and with bulk composition (e.g. i-Al–Pd–Mn vs. i-Al–Cu–Fe). We have also compared our results for the quasicrystals with results for crystalline approximants and other related crystalline phases. Our main conclusions are that, under specific conditions of sputter-annealing, the bulk atomic and electronic structures of the clean quasicrystal propagate to the surface. Also, the oxidation chemistry is dominated by that of the primary constituent, aluminum.

Title: "Photoemission studies of the sputter-induced phase transformation on the Al-Cu-Fe surface"

Authors: J.A. Barrow, V. Fournée, A.R. Ross, P.A. Thiel, M. Shimoda, and A.P. Tsai

Contact: P.A. Thiel

Journal: Surf. Sci. (2003) 539, 54-62.

Abstract:
The surface of a single grain icosahedral (i) Al–Cu–Fe quasicrystalline sample is studied as a function of annealing temperature using ultra-violet photoemission spectroscopy (UPS). Reflection high-energy electron diffraction and X-ray photoelectron spectroscopy are also performed to verify surface structure and composition. The sputtered surface shows structure and chemical composition consistent with that of β-Al–Cu–Fe cubic phase together with a sharp Fermi cut-off. With increasing annealing temperature, the surface structure and composition reverts to that of the quasicrystal. This transformation is correlated with a decrease of the spectral intensity at the Fermi level (EF). Analysis of the UPS spectra in the region near EF is performed by fitting the data with a pseudogap function.

Title: "Nucleation and growth of Ag films on a quasicrystalline AlPdMn surface"

Authors: V. Fournée, T.C. Cai, A.R. Ross, T.A. Lograsso, J.W. Evans, and P.A. Thiel

Contact: P.A. Thiel

Journal: Phys. Rev. B (2003) 67, 033406/1-033406/4.

Abstract:
Nucleation and growth of thin films of Ag on the fivefold surface of an Al72Pd19.5Mn8.5 icosahedral quasicrystal is studied with scanning-tunneling microscopy. For low coverages, flux-independent island nucleation is observed involving adatom capture at "traps." With increasing coverage, islands start growing vertically, but then spread, and ultimately form hexagonal nanocrystals. These have fcc symmetry and pyramidlike multilayer stacking along the <111> direction. The constituent hexagonal islands have five different orientations, rotated by 2π/5, thus reflecting the symmetry of the substrate.

Title: "Growth of Ag thin films on complex surfaces of quasicrystals and approximant phases"

Authors: V. Fournée, A.R. Ross, T.A. Lograsso, J.W. Evans, and P.A. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (2003) 537, 5-26.

Abstract:
The nucleation and growth of Ag thin films deposited on structurally complex alloy substrates is studied by scanning tunneling microscopy and low-energy electron diffraction. The substrates are high-symmetry surfaces of either icosahedral or decagonal quasicrystals or of related periodic crystals called approximants. At low coverage, completely different film morphologies are observed depending on the surface free energy of the substrate (relative to Ag). At high coverage, the formation of hexagonal nanocrystals is observed in all cases. These have fcc symmetry and pyramid-like multilayer stacking along the {1 1 1} direction. The constituent hexagonal islands have one, two or five different orientations depending on the rotational symmetry of the substrate on which the growth occurred. The films can be artificially smoothened by O2 induced coarsening, leading to a patchwork of Ag nanocrystals with a perfect texture, which are separated by grain boundaries.

Title: "Palladium clusteres formed on the complex pseudo-10-fold surface of the ξ'-Al77.5Pd19Mn3.5 approximant crystal"

Authors: V. Fournée, J.A. Barrow, M. Shimoda, A.R. Ross, T.A. Lograsso, P.A. Thiel, and A. P. Tsai

Contact: P.A. Thiel

Journal: Surf. Sci. (2003) 541, 147-159.

Abstract:
The growth and properties of a Pd thin film deposited on a structurally complex surface are studied by scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission spectroscopy. The substrate is the pseudo-10-fold surface of the ξ'-Al77.5Pd19Mn3.5 crystal, an approximant of the AlPdMn quasicrystalline phase. Spectroscopic data are consistent with a low density of electronic states around the Fermi level for the clean surface, in agreement with the poorly conducting nature of this intermetallic alloy. Deposited Pd atoms readily form small metal particles with relatively homogeneous size on the substrate, for coverages ranging from the submonolayer regime up to 7 ML. The Pd particles do not coalesce with time and are relatively stable upon annealing up to 920 K. Their average size increases slightly with film thickness. The local electronic structure of the Pd thin film has been studied by STS and appears to be particle-size dependent, with a transition from non-metal to metal, deduced from I(V) characteristics with increasing particle size. A shift of the Pd-3d core level is also measured by photoemission, and it can be correlated with a shift of the Pd-d band toward the Fermi level. The growth mode of the Pd film on the ξ′ approximant surface and its electronic structure are discussed in relation to Pd thin films grown on metal-oxides that are used as model catalysts.

Title: "Tribological properties of a B2-type Al-Pd-Mn quasicrystal approximant"

Authors: C. Mancinelli, C.J. Jenks, P.A. Thiel, and A.J. Gellman

Contact: C.J. Jenks

Journal: J. Mater. Res. (2003) 18, 1447-1456.

Abstract:
The tribological properties of a B2-type Al-Pd-Mn quasicrystal approximant were investigated and compared with those of an Al-Pd-Mn icosahedral quasicrystal. The approximant was of the â phase, having a crystalline CsCl-type structure and nominal composition Al48Pd42Mn10. Friction coefficients measured in ultrahigh vacuum between a pair of Al48Pd42Mn10 samples having truly clean surfaces were found to be twice as high as those reported for the Al70Pd21Mn9 quasicrystal. When the surfaces were oxidized by exposure to O2 or H2O, the friction coefficients decreased by roughly a factor of two for both materials but the friction coefficient for the approximant remained roughly twice that of the quasicrystal. The rate of oxidation of the approximant was found to be one order of magnitude higher than that of the quasicrystal. This corroborates findings that suggest that quasicrystals exhibit an inherent resistance to oxidation and corrosion. Vickers hardness measurements show that the quasicrystal is roughly three times as hard as the approximant.

Title: "Surface structure of Al-Pd-Mn quasicrystals: Existence of supersaturated bulk vacancy concentrations"

Authors: Ph. Ebert, M. Yurechko, F. Kluge, T. Cai, B. Grushko, P.A. Thiel, and K. Urban

Contact: P.A. Thiel

Journal: Phys. Rev. B (2003) 67, 024208/1-024208/8.

Abstract:

We identified the presence and chemical nature of bulk vacancies in Al-Pd-Mn quasicrystals by measuring the structure and composition of two- and fivefold cleavage surfaces of different preannealed quasicrystals subjected to postcleavage heat treatments using scanning electron microscopy and Auger electron spectroscopy. A strong dependence of the surface structure from the preannealing is observed and explained by varying concentrations of bulk vacancies. The analysis of the data shows that as-grown Al-Pd-Mn quasicrystals contain Al, Pd, and Mn vacancies in supersaturated but near stoichiometric concentrations, while long-term preannealed samples contain a much lower vacancy concentration, with a tendency to have excess Al vacancies. We found that Al and Mn vacancies are more mobile at lower temperatures than Pd vacancies, and that the diffusion of Mn vacancies is directly coupled to the mobility of Al vacancies. The results demonstrate that the evolution of the surface structure is primarily affected by bulk vacancies migrating toward the surface during heat treatments and provide a new methodology to characterize the vacancies in intermetallic alloys.

Title: "Pseudomorphic starfish: Nucleation of extrinsic metal atoms on a quasicrystalline substrate"

Authors: T. Cai, J. Ledieu, R. McGrath, V. Fournée, T. Lograsso, A. Ross, and P. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (2003) 526, 115-120.

Abstract:

Direct observation of extrinsic Al atoms on the fivefold surface of icosahedral Al–Cu–Fe indicates that they form pseudomorphic islands resembling starfish. The starfish occupy specific types of sites on the laterally-bulk-terminated quasicrystalline surface. We postulate that the nucleation event consists of a diffusing Al atom dropping into an empty site at the centre of a substrate pentagon. Growth consists of the addition of five Al atoms (nearly) in lattice sites. These 6-atom starfish do not grow laterally as coverage increases, leading to islands of uniform size, and to early roughening.

Title: "Angle-resolved low-energy ion scattering studies of quasicrystalline Al-Pd-Mn"

Authors: C.J. Jenks, A.R. Ross, T.A. Whaley, and R. Bastasz

Contact: C.J. Jenks

Journal: Surf. Sci. (2002) 521, 34-42.

Abstract:

We present angle-resolved low-energy ion scattering data that provide information about the composition of the topmost surface layers and atomic arrangement for a clean, 5-fold surface of a single-grain of icosahedral Al–Pd–Mn. The results show that the surface of this material has 5-fold periodicity and has a surface composition with a higher Al content than the bulk material (Al-86, Pd-13, Mn-1 versus Al-71, Pd-20, Mn-9 atom%). A predominant neighbor atom distance of 7.6±0.5 Å is calculated from the results, consistent with scanning probe measurements and bulk models of Al–Pd–Mn quasicrystals. We calculate a nearest neighbor distance of 3.0±0.2 Å. We also find that a Pd-rich layer exists just below an Al-rich outer layer, in agreement with dynamical low-energy electron diffraction calculations.

Title: "Analysis of gas-phase clusters made from laser-vaporized icosahedral Al-Pd-Mn"

Authors: J.A. Barrow, D.J. Sordelet, M.F. Besser, C.J. Jenks, P.A. Thiel, E.F. Rexer, and S.J. Riley

Contact: P.A. Thiel

Journal: J. Phys. Chem. A (2002) 106(40), 9204-9208.

Abstract:

An icosahedral Al-Pd-Mn quasicrystal sample is laser vaporized to form metal clusters by gas aggregation. The clusters are subsequently laser ionized and mass analyzed in a time-of-flight mass spectrometer. The mass spectra show cluster compositions which are qualitatively similar to that of the sample. This is consistent with a kinetically controlled cluster growth process. Cluster thermodynamic stability is probed by multiphoton ionization/fragmentation, which induces primarily Al and Mn loss. The resulting spectra are composed of a series of Pd-rich Al-Pd clusters. The average cluster composition is 60 (±1)% Pd. This composition is close to a known eutectic in the Al-Pd system. When manganese is seen on these clusters, it is always in units of Mn3. These results are discussed in terms of relative binding strengths in the Al-Pd-Mn alloy system.

Title: "Sulphur adsorption on the fivefold surface of the i-Al-Pd-Mn quasicrystal"

Authors: J. Ledieu, V.R. Dhanak, R.D. Diehl, T.A. Lograsso, D.W. Delaney, and R. McGrath

Contact: C.J. Jenks

Journal: Surf. Sci. (2002) 512, 77-83.

Abstract:

The adsorption of sulphur on the fivefold flat-terraced surface of the icosahedral Al–Pd–Mn quasicrystal (i-Al–Pd–Mn) has been studied using extended X-ray absorption fine structure (EXAFS), Auger electron spectroscopy and low energy electron diffraction (LEED). Surface EXAFS data collected by Auger electron yield from the S saturated surface give average bond lengths for S–Al and S–Pd of 2.10±0.06 and 2.40±0.06 Å respectively. The data indicate that single-site adsorption on the unreconstructed surface may be ruled out; this is consistent with near-edge EXAFS data and the disappearance of the LEED pattern upon adsorption of sub-monolayer amounts of S. The implication of this finding is that adsorbate-induced reconstruction and/or multiple-site adsorption occurs in this system.

Title: "Morphological evolution of the fivefold surface of i-AlPdMn quasicrystals"

Authors: G. Cappello, J. Chevrier, F. Schmithusen, A. Stierle, V. Formoso, F. Comin, M. de Boissieu, M. Boudard, T. Lograsso, C. Jenks, and D. Delaney

Contact: C.J. Jenks

Journal: Phys. Rev. B (2002) 65, 245405/1-245405/7.

Abstract:

Morphology of the fivefold symmetric quasicrystal surface of AlPdMn was investigated by x-ray reflectivity and by x-ray diffraction. X-ray experiments revealed two different morphologies depending on the surface preparation. Sputtering and annealing up to 900 K, under UHV conditions, produced a rough and facetted quasicrystal surface. These features were confirmed by atomic force microscopy and scanning tunnel microscopy measurements. We also observed that an annealing above 900 K induces a rapid and irreversible transition toward a flat surface.

Title: "Quasicrystal Surfaces: Potential as Templates for Molecular Adsorption"

Authors: R. McGrath, J. Ledieu, E.J. Cox, S. Haq, R D. Diehl, C.J. Jenks, I. Fisher, A.R. Ross, and T.A. Lograsso

Contact: C. J. Jenks

Journal: J. Alloys Compd (2002) 342, 432-436.

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. CD4O adsorption leads to multilayers as in other crystals. C6H6 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 CD4O 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.

Title: "About the Al 3p density of states in Al-Cu-Fe compounds and its relation to the compound stability and apparent surface energy of quasicrystals"

Authors: E. Belin-Ferré, J.-M. Dubois, V. Fournée, P. Brunet, D.J. Sordelet, and L.M. Zhang

Contact: D.J. Sordelet

Journal: Mater. Sci. Eng., A (2000) 294-296, 818-821.

Abstract:

We report the characteristic parameters of the Al 3p densities of states gained from a thorough study of many Al–Cu(–Fe) intermetallics including quasicrystalline and approximant phases using X-ray emission spectroscopy. These parameters are the density of states at the Fermi energy EF, the average binding energy due to Al 3p states and the second moment of their distribution. Hence, we demonstrate that the contribution to the compound stability arising from localization of Al 3p states via the Hume-Rothery mechanism is maximum for the icosahedral state. We also show that Al-transition metal sp-d hybridization is enhanced in the presence of icosahedral order. Finally, we confirm the already pointed out relationship between depth of the pseudo-gap at EF and apparent surface energy of the samples..

Title: "STM study of the atomic structure of the icosahedral Al-Cu-Fe fivefold surface"

Authors: T. Cai, V. Fournée, T. Lograsso, A. Ross, and P.A. Thiel

Contact: P.A. Thiel

Journal: Phys. Rev. B (2002) 65, 140202/1-140202/4.

Abstract:

We use scanning tunneling microscopy (STM) to investigate the atomic structure of the icosahedral (i-) Al-Cu-Fe fivefold surface in ultra high vacuum (UHV). Studies show that large, atomically flat terraces feature many ten-petal “flowers” with internal structure. The observed flower patterns can be associated with features on Al rich dense atomic planes generated from two-dimensional cuts of bulk models based on x-ray and neutron diffraction experiments. The results confirm that the fivefold surface of i-Al-Cu-Fe corresponds to a bulk-terminated plane.

Title: "Electronic structure of quasicrystals deduced from auger and x-ray photoelectron spectroscopies"

Authors: V. Fournee, J.W. Anderegg, A.R. Ross, T.A. Lograsso, and P.A. Thiel

Contact: P.A. Thiel

Journal: J. Phys.: Condens. Matter (2002) 14, 2691-2703.

Abstract:

Specific features in the electronic structure of Al-transition metal quasicrystals are analysed by a combination of Auger and x-ray photoelectron spectroscopies. We first demonstrate that different degrees of asymmetry in the transition metals' 2p core-level lineshape observed across different types of surface structure correspond to variations in the density of states at the Fermi level, DOS(EF). Using this effect, we explore the controversial issue of whether the quasicrystalline, decagonal AlNiCo system is electronically stabilized. We find strong evidence for the presence of a reduced DOS(EF) in this system, as expected for electronically stabilized compounds, and as observed in the quasicrystalline, icosahedral AlPdMn and AlCuFe alloys. Finally, qualitative information on the nature of the electronic states in quasiperiodic structures extracted from the core-valence-valence Auger lines are presented and discussed.

Title: "Surface preparation and characterization of the icosahedral Al-Ga-Pd-Mn quasicrystal"

Authors: M. Heinzig, C.J. Jenks, M. Van Hove, I. Fisher, P. Canfield, and P.A. Thiel

Contact: P.A. Thiel

Journal: J. Alloys Compd. (2002) 338, 248-252.

Abstract:

Auger electron spectroscopy and low-energy electron diffraction (LEED) provide basic information about the structure and composition of the 5-fold surface of the quaternary quasicrystal, icosahedral Al67Ga4Pd21Mn8. Surface preparation techniques established previously for two of the icosahedral ternary alloys, Al–Pd–Mn and Al–Cu–Fe, appear to be similarly effective for Al–Ga–Pd–Mn. After annealing in the range 600–950 K, the surface concentration of Ga is constant and low. After annealing in the range 900–950 K, a good LEED pattern is obtained. LEED indicates that Ga changes the surface structure significantly.

Title: "Structural aspects of the fivefold quasicrystalline Al-Cu-Fe surface from STM and dynamical LEED studies"

Authors: T. Cai, F. Shi, Z. Shen, M. Gierer, A.I. Goldman, M.J. Kramer, C.J. Jenks, T.A. Lograsso, D.W. Delaney, P.A. Thiel, and M.A. Van Hove

Contact: P.A. Thiel

Journal: Surf. Sci. (2001) 495, 19-34.

Abstract:

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 τ, 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.

Title: "Formation of a stable decagonal quasicrystalline Al-Pd-Mn surfacelayer"

Authors: D. Naumovic, P. Aebi, L. Schlapbach, C. Beeli, K. Kunze, T.A. Lograsso, and D.W. Delaney

Contact: T.A. Lograsso

Journal: Los Alamos Natl. Lab., Prepr. Arch., Condens. Matter (2001) 1-5, arXiv:cond-mat/0107400.

Abstract:

We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn quasicrystal overlayer on the 5-fold symmetrical surface of an icosahedral Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by x-ray photoelectron diffraction, LEED and electron backscatter diffraction. This overlayer is also characterized by a reduced density of states near the Fermi edge as expected for quasicrystals. This is the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn is obtained, studied and compared to its icosahedral counterpart.

Title: "Bulk termination of the quasicrystalline five-fold surface of Al70Pd21Mn9"

Authors: Z. Papadopolos, G. Kasner, R.D. Diehl, J. Ledieu, R. McGrath, T.A. Lograsso, and D.W. Delaney

Contact: T.A. Lograsso

Journal: Los Alamos Natl. Lab., Prepr. Arch. Condens. Matter (2001) 1-10, arXiv:cond-mat/0111479.

Abstract:

The structure of the Al70Pd21Mn9 surface has been investigated using high resolution scanning tunnelling microscopy (STM), and an atomically resolved surface has been imaged. The location of this surface-plane has been estimated in the bulk model M for icosahedral (i)-AlPdMn based on the three-dimensional tiling T*(2F) for an F-phase. The high resoln. STM image of the surface allows the position of the bulk terminations to be fixed within the layers of Bergman polytopes in the model M . They are 4:08 Å in the direction of the bulk from a surface of the most dense Bergman layers. The average atom density of terminations consisting of two plane-layers at the new positions is consistent with the results of Gierer et al. From the coding window of the model (M)-termination-plane we estimated the shortest edge of a possible tiling by Penrose (P1) prototiles to be 7.8 Å. The experimental derived tiling of the surface has edge length 8:0 ± 0:3 Å and hence matches edge-length expected from the model.

Title: "Surface oxidation of Al-Cr-Fe alloys characterized by x-ray photoelectron spectroscopy"

Authors: V. Demange, J.W. Anderegg, J. Ghanbaja, F. Machizaud, D.J. Sordelet, M. Besser, P.A. Thiel, and J.M. Dubois

Contact: P.A. Thiel

Journal: Appl. Surf. Sci. (2001) 173, 327-338.

Abstract:

We present X-ray photoelectron spectroscopy (XPS) measurements of several Al–Cr–Fe samples which are mixtures of approximants of the decagonal phase. Some samples also contain a hexagonal γ-brass phase. Our purpose is to evaluate the effect of chemical composition, particularly Cr content, on the response of the surface to oxidation. Under mild conditions only aluminium oxidizes, but under extreme conditions (water immersion at room temperature, or oxygen exposure at high temperatures), chromium oxidizes as well. XPS data also provide a measure of the oxide thickness. Cr has no discernible effect on oxide thickness when the oxidizing environment is the gas phase, but provides significant protection against water immersion, where high concentrations of Cr reduce the thickness by as much as 40%. These results for the Al–Cr–Fe samples are compared with results for approximants and quasicrystals in other systems.

Title: "Determination of auger sensitivity for Al-rich quasicrystals"

Authors: C.J. Jenks, T.E. Bloomer, M.J. Kramer, T.A. Lograsso, D.W. Delaney, A.R. Ross, D.J. Sordelet, M.F. Besser, and P.A. Thiel

Contact: C.J. Jenks

Journal: Appl. Surf. Sci. (2001) 180, 57-64.

Abstract:

We discuss and compare approaches to using Auger data for semi-quantitative determination of surface compositions of Al-based quasicrystals. We have examined two quasicrystalline phases: icosahedral Al–Cu–Fe and Al–Pd–Mn. We find that sensitivity factors obtained from pure elemental standards lead to underestimation of concentrations by as much as a factor of 2.5 for the minor constituents, Fe and Mn, and to overestimation by as much as a factor of 1.2 for the major constituent, Al. Surprisingly, calibration with a crystalline phase close in composition to the quasicrystal does not necessarily lead to more accurate results. Auger analyses of fractured surfaces, which are then compared to bulk analyses, is the best option for sensitivity factor calibration. Even without absolute calibration of sensitivity factors, however, surface analysis is extremely useful for detection of secondary phases, and is superior to common bulk analysis methods.

Title: "C60 adsorption on the quaiscrystalline surface of Al70Pd21Mn9"

Authors: J. Ledieu, C.A. Muryn, G. Thornton, R.D. Diehl, T.A. Lograsso, D.W. Delaney, and R. McGrath

Contact: T.A. Lograsso

Journal: Surf. Sci. (2001) 472, 89-96.

Abstract:

Room temperature adsorption of C60 on the flat quasicrystalline surface of Al70Pd21Mn9 has been investigated using scanning tunnelling microscopy. A dispersed overlayer is formed at low coverage, with avoidance of step-edges. There is no evidence of island formation or clustering. As the coverage is increased, a higher density layer is formed with no evidence of the formation of hexagonal ordered adsorbate structures seen on other substrates. This is followed by the onset of second layer formation. A range of bonding sites for C60 molecules is implied from measurements of apparent molecular heights and from thermal effects. Detailed analysis of the surface at a low coverage (~0.065 ML) provides evidence of adsorbate local order, with Fibonacci (τ-scaling) relationships between the C60 molecules. Where this occurs, the preferred adsorption site is tentatively identified as the pentagonal hollow. These local correlations however are not found to extend over larger regions of the surface.

Title: "Analysis of gas phase clusters made from laser-vaporized icosahedral Al-Pd-Mn"

Authors: J.A. Barrow, E.F. Rexer, D.J. Sordelet, M.F. Besser, C.J. Jenks, S.J. Riley, and P.A. Thiel

Contact: P.A. Thiel

Journal: MRS Symposium Proceedings (2001) 643, K5.4.1-K5.4.5.

Abstract:

Laser vaporization of an icosahedral Al-Pd-Mn sample with detection by time-of-flight mass spectrometry is used to probe metal clusters made from the alloy. After sample vaporization, clusters form by gas aggregation and may contain several to hundreds of atoms. Multi-photon ionization/fragmentation of these clusters yields mass spectra showing many cluster sizes with enhanced intensity. Clusters are identified at masses near those of pseudo-Mackay and Bergman clusters; however, these clusters do not appear special relative to neighboring clusters. Results of this study and its relationship to the proposed cluster structures in quasicrystalline materials are discussed.

Title: "Low-energy ion scattering measurements from an Al-Pd-Mn quasicrystal"

Authors: R. Bastasz, C.J. Jenks, T.A. Lograsso, A.R. Ross, P.A. Thiel, and J.A. Whaley

Contact: C.J. Jenks

Journal: MRS Symposium Proceedings (2001) 643, K11.1.1-K11.1.7.

Abstract:

Energy-angle distributions of low-energy inert-gas ions scattered from surfaces provide information about surface composition and structure. We have measured energy spectra of He+ scattered from an Al71Pd20Mn9 quasicrystal, which was oriented perpendicular to the 5-fold axis, along various azimuthal directions. Strong scattering signals are seen from Al and Pd, but only a weak Mn signal is observed. From measurements made of He+ at an oblique angle of incidence scattered in the forward direction, we observe a 72° periodicity in the azimuthal dependence of the scattering signal intensity from Al surface atoms. The effect arises from shadowing effects involving neighboring surface atoms and provides direct evidence that Al surface atoms exist in a local environment with 5-fold symmetry. In addition, measuring the variation of the signal intensity with incidence angle provides information about neighboring atom distances, which compare favorably with a model of the quasicrystal surface derived from the bulk structure.

Title: "Comparative study of the tribological and oxidative properties of AlPdMn quasicrystals and their cubic approximants"

Authors: C. Mancinelli, C.J. Jenks, P.A. Thiel, A.R. Ross, T.A. Lograsso, and A.J. Gellman

Contact: P.A. Thiel

Journal: MRS Symposium Proceedings (2001) 643, K8.2.1-K8.2.10.

Abstract:

An experimental comparison has been made between the properties of the surfaces of an Al70Pd21Mn9 quasicrystal and its Al48Pd42Mn10 approximant. The Al70Pd21Mn9 sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetry (000001) surface. The approximant was polycrystalline Β-phase Al48Pd42Mn10, which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. Both materials are oxidized by reaction with O2 to form a thin film of aluminum oxide that ultimately passivates their surfaces. The interesting difference between the two is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction between pairs of the approximant surfaces is significantly higher than that measured between the quasicrystal surfaces under all conditions of surface oxidation.

Title: "TDS study of potassium adsorption on Al(111) and fivefold Al-Pd-Mn"

Authors: B.I. Wehner, J.W. Anderegg, C.J. Jenks, A.R. Ross, T.A. Lograsso, R.D. Diehl, and P.A. Thiel

Contact: P.A. Thiel

Journal: MRS Symposium Proceedings (2001) 643, K11.4.1-K11.4.4.

Abstract:

We have used thermal desorption spectroscopy to carry out a comparative study of potassium adsorption on Al(111) and on the fivefold Al-Pd-Mn surface. Potassium adsorption on the quasicrystal was found to be different than on Al(111). The potassium monolayer desorbed from fivefold Al-Pd-Mn at lower temperatures than from Al(111). Potassium is known to form a dense monolayer on Al(111), with an ideal coverage of 0.33, but for the monolayer on fivefold Al Pd Mn we find that the saturation coverage is only one twelfth.

Title: "The 10-fold surface of the decagonal Al72Ni11Co17 quasicrystal studied by LEED, SPA-LEED, AES, and STM"

Authors: E.J. Cox, J. Ledieu, R. McGrath, R.D. Diehl, C.J. Jenks, and I. Fisher

Contact: C.J. Jenks

Journal: MRS Symposium Proceedings (2001) 643, K11.3.1-K11.3.6.

Abstract:

The ten-fold surface of the decagonal Al72Ni11Co17 (d-Al-Ni-Co) quasicrystal has been investigated using LEED, spot profile analysis LEED (SPA-LEED), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). This was done as a function of both annealing temperature and annealing time. The long-range order of the surface, as indicated by LEED, increases both as a function of annealing time and temperature. STM shows the surface to be rough and cluster-like at low annealing temperatures (≤ 725 K), while annealing to temperatures in excess of 725 K results in the formation of terraces. These terraces are small (≤ 100 Å width) at lower annealing temperatures and increase in size (100 Å ≤ x ≤ 500 Å) as the annealing temperature is increased (≥ 850 K). They are characterised by the presence of three-fold protrusions which align preferentially. STM images show single height steps as expected due to the periodicity of d-Al-Ni-Co in the z direction. To date it has not been possible to obtain atomic resolution, although this work is continuing.

Title: "Irreversible structural transformation of the five-fold i-AlPdMn quasicrystals after ion bombardment and annealing"

Authors: F. Schmithusen, G. Cappello, S. Decossas, G. Torricelli, T.-L. Lee, M. de Boissieu, Y. Calvayrac, T. Lograsso, F. Comin, and J. Chevrier

Contact: T.A. Lograsso

Journal: MRS Symposium Proceedings (2001) 643, K11.2.1-K11.2.11.

Abstract:

Five fold i-AlPdMn surface prepared under UHV by ion bombardment and annealing was so far considered to be bulk terminated. This result was substantially based on a quantitative LEED analyses [1]. Analysis of the specular rod in a X ray diffraction experiment at grazing incidence supported this result [2]. We present a new study of this surface by high resolution X ray diffraction at normal incidence. In this Bragg configuration the diffraction peak 18 - 29 for instance is at a photon energy of 2.873keV, the 72 - 116 reflection at 5.725keV. This results in an analyzed thickness of the sample surface of a few micrometers. The surface was cleaned by ion bombardment. During annealing (T>=880K), we clearly observed the progressive disappearance of the initial Bragg peak characteristic of the as cast bulk sample. Conversely a new Bragg peak grows at an energy position shifted by 1eV compared to the position of the original Bragg peak. This is a clear signature for an irreversible structural transformation which takes place on at least the micron thickness. On the transformed surface, both, a LEED pattern and a RHEED pattern, characteristic for a five fold surface were easily obtained. This high resolution experiment (the relative Bragg peak shift is 3.10^(-4) was reproduced on samples from different initial compositions. This shows that five fold i-AlPdMn surface changes after preparation by ion bombardment and annealing at 900K on a micrometer thickness. This is not consistent with the conclusion that the surface is simply terminated by a cut of the original bulk. We conclude that a reorganization process of the quasicrystalline structure during annealing proceeds in the surface vicinity (probed depth is close to a few microns).

Title: "Decomposition of the five-fold surface of Al70Pd21Mn9 at elevated temperature"

Authors: J. Ledieu, C.A. Muryn, G. Thornton, G. Cappello, J. Chevrier, R.D. Diehl, T.A. Lograsso, D.W. Delaney, and R. McGrath

Contact: T.A. Lograsso

Journal: Mater. Sci. Eng., A (2000) 294-296, 871-873.

Abstract:

Several surface phases have previously been identified on the five-fold surface of the Al70Pd21Mn9 quasicrystal. Two of these, the clustered and terraced phases, have been discovered by scanning tunnelling microscopy (STM) studies. In this paper, we report STM images showing the existence of a new phase on this surface. This new topography was obtained by annealing the sample close to its melting point for several minutes. A model is presented which exhibits the main features of the structure. The unit-cell dimensions match those of the Al3Mn orthorhombic phase. Mechanical polishing followed by sputtering and annealing restored a pentagonal quasicrystalline surface indicating that the decomposition is a near-surface phenomenon.

Title: "Oxidation of Al-Cu-Fe and Al-Pd-Mn quasicrystals"

Authors: B.I. Wehner, U. Koster, A. Rudiger, C. Pieper, and D.J. Sordelet

Contact: D.J. Sordelet

Journal: Mater. Sci. Eng., A (2000) 294-296, 830-833.

Abstract:

Oxidation of Al–Cu–Fe and Al–Pd–Mn quasicrystals as compared to related crystalline phases was studied in air at high temperatures. Parabolic rate constants at 840°C were about 6×10-12 g2 cm-4 s-1 for icosahedral Al–Cu–Fe, but only 3×10-13 g2 cm-4 s-1 for cubic AlFe. The oxide layer formed on the icosahedral alloy was inhomogeneous with respect to the oxide growth morphology. Oxide compositions were studied by Auger electron spectroscopy (AES) and X-ray induced photo electron spectroscopy (XPS). The θ-Al2O3 layer on Al–Cu–Fe did not contain any quantities of Cu or Fe that were significant according to the analyses by AES and XPS. In an advanced oxidation stage the θ-Al2O3 layer was penetrated by α-Al2O3 nodules, which contained about 5% Cu.

Oxidation of Al–Pd–Mn quasicrystals was found to be strongly influenced by evaporation of Mn. Whereas the formation of δ-Al2O3 was observed at 800°C on Al–Pd–Mn quasicrystals followed by growth of needle-like θ-Al2O3, oxidation of hot isostatically pressed (HIPed) specimens at 850°C led to the formation of a θ-Al2O3 layer containing also extremely thin areas. The metallic Al(LMM) Auger peak observed in these areas was four times the oxidic Al(LMM) peak. Neither in the thick nor the thin oxide areas were Pd or Mn observed.

Title: "Characterization of surface morphologies at the Al-Pd-Mn fivefold surface"

Authors: G. Cappello, F. Schmithusen, J. Chevrier, F. Comin, A. Stierle, V. Formoso, M. de Boissieu, M. Boudard, T.A. Lograsso, C. Jenks, and D. Delaney

Contact: C.J. Jenks

Journal: Mater. Sci. Eng., A (2000) 294-296, 822-825.

Abstract:

Fivefold surfaces of Al–Pd–Mn quasicrystal have been prepared under ultra-high vacuum (UHV) by sputtering and annealing above 700 K and characterized by electron diffraction, Auger spectroscopy and X-ray photoemission spectroscopy (LEED, AES, XPS). X-ray reflectivity at grazing incidence reveals that an in situ temperature annealing at 900 K is necessary to irreversibly produce the signature of a flat and bulk terminated surface. Air-AFM investigations of surfaces prepared below or in the vicinity of this transformation temperature show a large scale structure in the micrometer range with high steps and parallel terraces. Terrace morphology is dominated by clusters with a typical size of about 10 nm. Nanometer steps can coexist with this cluster structure. With UHV-STM, terrace roughness based on cluster assembly is found to extend down to the nanometer scale.

Title: "Bulk and surface evidence for the long-range spatial modulation of x-ray absorption in Al-Pd-Mn quasicrystal at bragg incidence"

Authors: G. Cappello, A. Dechelette, F. Schmithusen, S. Decossas, J. Chevrier, F. Comin, V. Formoso, M. de Boissieu, T. Jach, R. Colella, T.A. Lograsso, C. Jenks, and D. Delaney

Contact: C.J. Jenks

Journal: Mater Sci. Eng., A (2000) 294-296, 863-866.

Abstract:

An X-ray standing waves experiment was performed at the ID32 beam line of the ESRF on an Al–Pd–Mn quasicrystal with the X-ray beam at normal incidence. The X-ray photoemission core levels for each element were recorded to probe the surface. The drain current, i.e. the electron current from ground needed to neutralize the irradiated sample and to compensate the photoemitted electrons was also recorded as a bulk probe that integrates over all atoms of the sample. Within the two-beam approximation applied to quasicrystals, a simple analysis of this effect is proposed. This provides a direct-space illustration of the diffraction process in a quasiperiodic structure.

Title: "Electronic structure of quasicrystalline surfaces: Effects of surface preparation and bulk structure"

Authors: V. Fournée, P.J. Pinhero, J.W. Anderegg, T.A. Lograsso, A.R. Ross, P.C. Canfield, I.R. Fisher, and P.A. Thiel

Contact: P.A. Thiel

Journal: Phys. Rev. B (2000) 62, 14049-14060.

Abstract:

We elucidate the nature of the surface electronic properties of quasicrystalline Al-Pd-Mn. We do this by using photoelectron and Auger electron spectroscopies, and by making a variety of comparisons—across types of bulk samples, and across methods of surface preparation. The main conclusions are these: (i) The narrow Mn 2p3/2 core-level line observed in the icosahedral phase is a fingerprint of a suppression in the density of states (a pseudogap) at the Fermi level and is not unique to the quasicrystalline phase. It is also independent of the symmetry of the quasicrystalline surface. The Auger line shape is also affected and may be used as a fingerprint of a pseudogap. (ii) A similarly narrow Fe 2p3/2 core-level line characterizes the icosahedral Al-Cu-Fe quasicrystal, consistent with the expectation that the electronic structure is of general importance in the stabilization of icosahedral phases. (iii) In icosahedral Al-Pd-Mn, the pseudogap of the bulk is not retained up to the surface immediately after fracture, but can be restored by annealing, or by sputter annealing to sufficiently high temperatures. Assuming that the pseudogap reflects an electronic stabilization of the atomic structure, these results suggest that the heat-treated surfaces are more stable than the surface obtained by fracturing at room temperature.

Title: "Structural aspects of the threefold surface of icosahedral Al-Pd-Mn"

Authors: D. Rouxel, T.-H. Cai, C.J. Jenks, T.A. Lograsso, A. Ross, and P.A. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (2000) 461, L521-L527.

Abstract:

We report the first STM study of a threefold surface of an icosahedral quasicrystal. We find that a rough, cluster-dominated structure evolves into a terrace-step morphology, with increasing temperature. The terraces display a fine structure whose long-range order is consistent with threefold symmetry. The fine structure includes small, deep holes. The steps can be very straight, serving to bound equilateral triangles (or portions thereof). These straight steps can cut directly across meandering step edges, superimposing a triangular ‘shadow’ upon the other terrace-step landscape. The data suggest that the triangles grow outward from a special type of central point. The triangles may represent the initial stages of facetting.

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

Authors: Z. Shen, W. Raberg, M. Heinzig, C.J. Jenks, V. Fournée, M.A. Van Hove, T.A. Lograsso, D. Delaney, T. Cai, P.C. Canfield, I.R. Fisher, A.I. Goldman, M.J. Kramer, and P.A. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (2000) 450, 1-11.

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.

Title: "Effect of sputtering gas on the surface composition of an Al-Pd-Mn quasicrystal"

Authors: C.J. Jenks, J.W. Burnett, D.W. Delaney, T.A. Lograsso, and P.A. Thiel

Contact: P.A. Thiel

Journal: Appl. Surf. Sci. (2000) 157, 23-28.

Abstract:

We demonstrate the effects of different inert sputtering gases on the surface composition of icosahedral (i-) Al–Pd–Mn. The sputtering gases used include He, Ne, Ar, Xe, and Kr. We demonstrate that the steady-state composition is independent of the inert gas chosen. This steady-state composition falls outside of the quasicrystalline region of the Al–Pd–Mn phase diagram. However, the fluence (ions/cm2) to achieve steady-state depends on the inert gas. Helium requires a much higher fluence than the other gases. Because of this, He can be used to clean the surface, if the time (fluence) for sputtering is kept to a minimum, without causing the large compositional shift induced with a relatively small fluence by the most common sputtering gas, Ar.

Title: "Fine structure on flat surfaces of quasicrystalline Al-Pd-Mn"

Authors: Z. Shen, C.R. Stoldt, C.J. Jenks, T. Lograsso, and P.A. Thiel

Contact: P.A. Thiel

Journal: Phys. Rev. B (1999) 60, 14688-14694.

Abstract:

We have analyzed the fine structure revealed by scanning tunneling microscopy for a flat (within 0.8 Å) fivefold surface of i-Al-Pd-Mn. Even though features in the image appear to be arranged randomly, self-similar features are separated by distinct distances. The distribution of such distances is compatible with the separations between pseudo-Mackay icosahedra tangent to the topmost layer, and with separations between other cluster-based units. We propose that the fine structure is due to electronic structure imposed by the clusters.

Title: "Surface oxidation of a quasicrystalline Al-Cu-Fe alloy: No effect of surface orientation and grain boundaries on the final state"

Authors: P.J. Pinhero, J.W. Anderegg, D.J. Sordelet, T.A. Lograsso, D.W. Delaney, and P.A. Thiel

Contact: P.A. Thiel

Journal: J. Mater. Res. (1999) 14(8), 3185-3188.

Abstract:

We have used x-ray photoelectron spectroscopy and Auger electron spectroscopy to examine the characteristics of oxides on two types of quasicrystalline Al ¾ Cu ¾ Fe samples. One type was formed by consolidation of powders, resulting in multiple grains with random surface orientations. The other was a single grain, oriented to expose a fivefold surface. Both were oxidized to saturation in a variety of environments at room temperature. We measured the elemental constituents that oxidized, the extent of oxygen-induced Al segregation, and the depth of the oxide. Under the conditions of our experiments, there was little, if any, significant difference between the two types of samples. Hence, surface orientation and bulk microstructure played little or no role on the final state of the oxide under these conditions.

Title: "Electrochemical pitting and repassivation on icosahedral Al-Cu-Fe and a comparison with crystalline phases"

Authors: N.A. Spurr, P.J. Pinhero, D.J. Sordelet, K.R. Hebert, and P.A. Thiel

Contact: P.A. Thiel

In Quasicrystals: Symposium Held November 30-December 2, 1998, Boston, MA, Ed. by J.-M. Dubois, P.A. Thiel, A.-P. Tsai and K. Urban, Materials Research Society, Warrendale (1999) 553, 275-280.

Abstract:

We report the electrochemical potentials at which localized pitting and repassivation occur on icosahedral Al-Cu-Fe, and on a series of related alloys and elemental metals. The electrochemical occurs in a buffered NaCl solution, pH 8.4. Under these conditions, pitting and repassivation appear to be controlled mainly by the chemical composition of the alloy, although the quasicrystalline phase displays an anomalous resistance to repassivation. Corrosion of this phase proceeds by dissolution of Al and Fe, leaving behind pits which are Cu-enriched.

Title: "Room temperature oxidation of Al-Cu-Fe and Al-Cu-Fe-Cr quasicrystals"

Authors: P.J. Pinhero, D.J. Sordelet, J.W. Anderegg, P. Brunet, J.-M. Dubois, and P.A. Thiel

Contact: P.A. Thiel

In Quasicrystals: Symposium Held November 30 to December 2, 1998, Boston, MA, Ed. by J.-M. Dubois, P.A. Thiel, A.-P. Tsai and K. Urban, Materials Research Society, Warrendale (1999) 553, 263-268.

Abstract:

We have investigated formation of oxides on quasicrystalline and crystalline alloy surfaces of similar composition, in different oxidizing environments. This includes a comparison between a quaternary orthorhombic approximate of Al-Cu-Fe quasicrystal and the ternary Al-Cu-Fe-Cr quasicrystalline and crystalline phases. We noted that each sample showed the following common trends: preferential oxidation of the Al, enrichment in the concentration of Al present at the surface upon oxidation, water concentration is directly related to oxide thickness, and the oxide thickness displays a strong correlation with the bulk concentration of Al in the sample.

Title: "Characterization and properties of the AlPdMn five-fold surface"

Authors: G. Cappello, A. Dechelette, F. Schmithusen, J. Chevrier, F. Comin, A. Stierle, V. Formoso, M. de Boissieu, T. Lograsso, C. Jenks, and D. Delaney

Contact: P.A. Thiel

In Quasicrystals: Symposium Held November 30 to December 2, 1998, Boston, MA, Ed. by J.-M. Dubois, P.A. Thiel, A.-P. Tsai and K. Urban, Materials Research Society, Warrendale (1999) 553, 243-250.

Abstract:

The surface of the AlPdMn quasicrystal perpendicular to a fivefold axis has been probed by LEED, XPS, Auger spectroscopy and surface EXAFS.

Title: "Surface characteristics of quasicrystalline materials: An overview of work conducted at Ames Laboratory"

Author: C.J. Jenks

Contact: C.J. Jenks

In Quasicrystals: Symposium Held November 30 to December 2, 1998, Boston, MA, Ed. by J.-M. Dubois, P.A. Thiel, A.-P. Tsai and K. Urban, Materials Research Society, Warrendale (1999) 553, 219-320.

Abstract:

Over the last few years, work in our laboratories in Ames has focussed on elucidating the surface characteristics of Al-based quasicrystalline materials, namely icosahedral (i-) Al-Cu-Fe and i-Al-Pd-Mn. Our work involves the study of the clean surfaces of these materials under ultrahigh vacuum conditions. We find that surfaces cleaned by Ar+ sputtering are depleted in Al relative to the bulk composition. Single grains, after sputtering, undergo a two-stage regrowth process as they are annealed. After heating to about 600 K, a crystalline overlayer is formed. This is rather abruptly replaced at about 750 K by a surface that appears quasicrystalline within the resolution of the experimental techniques used. Calculations based on LEED measurements of this higher temperature state indicate that the Al-rich layers in the bulk model of these materials are the favored surface terminations. Results of low-energy ion scattering (LEIS) corroborate this finding. Consistent with this, we find that the oxidation behavior and general reactivity of these materials are analogous to pure Al.

Title: "LEED investigations of a cubic Al-Pd-MN (110) alloy"

Authors: M.W. Heinzig, L.L. Warren, Z. Zhen, C.J. Jenks, T.A. Lograsso, and P.A. Thiel

Contact: C.J. Jenks

In Quasicrystals: Symposium Held November 30 to December 2, 1998, Boston, MA, Ed., by J.-M. Dubois, P.A. Thiel, A.-P. Tsai and K. Urban, Materials Research Society, Warrendale (1999) 553, 251-256.

Abstract:

An Al-Pd-Mn cubic alloy having a bulk chemical composition somewhat analogous to that of the icosahedral Al-Pd-Mn quasicrystal is studied. Our goal is to compare the surface structure and properties of the cubic alloy with those of the quasicrystalline alloy. In this paper, we report the first observations for the (110) surface of the cubic alloy using (primarily) LEED. The surface is prepared by sputtering and annealing in ultrahigh vacuum (UHV). In addition to the substrate LEED pattern, at least three superstructures evolve sequentially with annealing temperature.

Title: "Friction between single-grain Al70Pd21Mn9 quasicrystal surfaces"

Authors: J.S. Ko, A.J. Gellman, T.A. Lograsso, C.J. Jenks, and P.A. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (1999) 423, 243-255.

Abstract:

An ultra-high vacuum tribometer was used to make measurements of friction between two single-grain Al70Pd21Mn9(00001) quasicrystal surfaces. 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 ms=0.60+ 0.08. The friction coefficient of the perfectly clean quasicrystal surfaces is significantly lower than those reported for many perfectly clean metal surfaces 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 ms = 0.35+0.05. More subtle changes to the surfaces such as adsorption of sulfur or alteration of the surface symmetry produced no significant changes in the static friction coefficient.

Title: "Crystalline surface structures induced by ion sputtering of Al-rich icosahedral quasicrystals"

Authors: Z. Shen, M.J. Kramer, C.J. Jenks, A.I. Goldman, T. Lograsso, D. Delaney, M. Heinzig, W. Raberg, and P.A. Thiel

Contact: P.A. Thiel

Journal: Phys. Rev. B (1998) 58, 9961-9971.

Abstract:

Low-energy electron diffraction patterns, produced from quasicrystal surfaces by ion sputtering and annealing to temperatures below ∼700 K, can be assigned to various terminations of the cubic CsCl structure. The assignments are based upon ratios of spot spacings, estimates of surface lattice constants, bulk phase diagrams vs surface compositions, and comparisons with previous work. The CsCl overlayers are deeper than about five atomic layers, because they obscure the diffraction spots from the underlying quasicrystalline substrate. These patterns transform irreversibly to quasicrystalline(like) patterns upon annealing to higher temperatures, indicating that the cubic overlayers are metastable. Based upon the data for three chemically identical, but symmetrically inequivalent surfaces, a model is developed for the relation between the cubic overlayers and the quasicrystalline substrate. The model is based upon the related symmetries of cubic close-packed and icosahedral-packed materials. The model explains not only the symmetries of the cubic surface terminations, but also the number and orientation of domains.

Title: "Surface science of quasicrystals"

Authors: P.A. Thiel, A.I. Goldman, and C.J. Jenks

Contact: P.A. Thiel

Journal: Springer Ser. Solid-State Sci. (1999) 126, 327-359.

Abstract:

A review, with many references, is given on Al-rich alloy quasicrystals and their surface properties.

Title: "Surface oxidation of Al-Cu-Fe alloys: A comparison of quasicrystalline and crystalline phases"

Authors: P.J. Pinhero, J.W. Anderegg, D.J. Sordelet, M.F. Besser, and P.A. Thiel

Contact: P.A. Thiel

Journal: Philos. Mag. B (1999) 79, 91-110.

Abstract

We have used X-ray photoelectron spectroscopy and Auger electron spectroscopy to examine the characteristics of oxide surfaces on a family of Al-Cu-Fe alloys. These alloys are compositionally similar but structurally different: two are crystalline and one is quasicrystalline. The samples all are formed by consolidation of powders, resulting in multiple grains with random surface orientations. They are oxidized to saturation in a variety of environments at room temperature. Under the conditions of our experiments, there is no detectable difference in the oxidation characteristics of the three phases. That is, there is no difference in the elemental constituents which oxidize, nor in the relative extent of oxygen-induced Al segregation, nor in the depth of the oxide. Hence, the oxidation chemistry of these alloys is determined by their Al-rich chemical composition, and not by their atomic or electronic bulk structure, under these conditions.

Title: "Clustered, terraced and mixed surface phases of the Al70Pd21Mn9 quasicrystal"

Authors: J. Ledieu, A.W. Munz, T.M. Parker, R. McGrath, R.D. Diehl, D.W. Delaney, and T.A. Lograsso

Contact: T.A. Lograsso

In Proceedings of Quasicrystals: Symposium Held November 30 to December 2, 1998, Boston, MA, Ed. by J.-M. Dubois, P.A. Thiel, A.-P. Tsai and K. Urban, Materials Research Society, Warrendale (1999) 553, 237-242.

Abstract:

The five-fold surface of the Al70Pd21Mn9 quasicrystal has been studied using STM, LEED and AES. STM images from surfaces which have been sputtered and annealed to 875 K reveal 20-30 Å protrusions that have been identified by others as Mackay-type clusters. Higher-resolution images reveal substructures in these clusters having dimensions 2-3 Å. Longer annealing times at 875 K produced large areas having flat terraces which were imaged with atomic resolution. The LEED pattern from this surface has sharp spots on a low background, and AES indicates that the surface is deficient in Mn relative to the bulk. For surfaces annealed to 1050 K for less than 2 h, STM images indicate that cluster and terrace phases coexist, and a third phase having aligned arrays of clusters is identified which appears to be intermediate between the cluster and terrace phases.

Title: "Structural study of the five-fold surface of the Al70Pd21Mn9 quasicrystal"

Authors: J. Ledieu, A.W. Munz, T.M. Parker, R. McGrath, R.D. Diehl, D.W. Delaney, and T.A. Lograsso

Contact: T.A. Lograsso

Journal: Surf. Sci. (1999) 433-435, 666-671.

Abstract:

We report an investigation of the five-fold surface of the Al70Pd21Mn9 quasicrystal using STM, LEED and AES. Surfaces sputtered and annealed to 875 K showed excellent five-fold symmetric LEED patterns, and STM revealed cluster-like protrusions. Surfaces sputtered and annealed to 1050 K also showed sharp LEED patterns but here STM showed flat surfaces which could be imaged with atomic resolution. AES indicated these annealed surfaces were Mn deficient. Threshold analysis of images of the flat surfaces shows that the surface can be generated by tiling with pentagons. Large protrusions on the flat surfaces were found where the pentagonal tiling breaks down. Angles between these protrusions were found to match angles present in the stereographic projection of the icosahedral group m35.

Title: "Quasicrystalline nature of quasicrystal surfaces: A photoemission study"

Authors: D. Naumovic, P. Aebi, L. Schlapbach, C. Beeli, T.A. Lograsso, and D.W. Delaney

Contact: T.A. Lograsso

Journal: Phys. Rev. B (1999) 60, R16330-R16333.

Abstract:

Differently prepared surfaces of quasicrystalline i-Al-Pd-Mn are analyzed using angle-resolved photoemission in the x-ray and ultraviolet range of photon energies. Depending on the preparation, we find both surfaces with crystalline structure and metallic character, and surfaces with quasicrystalline structural fingerprints and a suppressed density of states at the Fermi level, compatible with a pseudogap.

Title: "The surface structure of a b-Al(Cu1-xFex)-(110) film formed on an AlCuFe quasicrystal substrate, analyzed by dynamical LEED"

Authors: F. Shi, Z. Shen, D.W. Delaney, A.I. Goldman, C.J. Jenks, M.J. Kramer, T. Lograsso, P.A. Thiel, and M.A. Van Hove

Contact: M.A. Van Hove

Journal: Surf. Sci. (1998) 411, 86-98.

Abstract:

In order to understand the unique properties of quasicrystalline surfaces, it is paramount to compare quasicrystalline and crystalline surfaces of similar chemical composition. To that end, the fivefold surface of an AlCuFe quasicrystal substrate was transformed, by ion bombardment and annealing, into a film with the b-Al(Cu1-xFex) bulk structure. This is a CsCl structure with Fe and Cu atoms randomly occupying the body-center sites, and Al atoms at the corner sites. The film exposes a (110) free surface: its external structure was analyzed by dynamical low-energy electron diffraction (LEED) in terms of atomic positions and layer-wise composition. The best fit between theory and experiment slightly favors a copper-rich composition in the surface layers (x=0), although the sensitivity to the value of x is low, because Cu and Fe are very similar electron scatterers. The Al atoms in the surface layer buckle outward by 0.12±0.04 Å relative to the Cu or Fe atoms in the same layer, while other relaxations from the bulk structure are small.

Title: "Comments on quasicrystals and their potential use as catalysts"

Authors: C.J. Jenks and P.A. Thiel

Contact: C.J. Jenks

Journal: J. Mol. Catal. (1998) 131, 301-306.

Abstract:

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.

Title: "Preparation of icosahedral AlPdMn and AlCuFe samples for LEED studies"

Authors: C.J. Jenks, P.J. Pinhero, Z. Shen, T.A. Lograsso, D.W. Delaney, T.E. Bloomer, S.-L. Chang, C.-M. Zhang, J.W. Anderegg, A.H.M.Z. Islam, A.I. Goldman, and P.A. Thiel

Contact: P.A. Thiel

Proceedings of the Sixth Inernational Conference on Quasicrystals (ICQ6), held in Tokyo, Japan, May 1997. S. Takeuchi and T. Fujiwara, Editors (World Scientific, Singapore, 1998) 761-764.

Abstract:

We describe conditions of sample preparation which give good LEED patterns for the fivefold and twofold surfaces of icosahedral AlPdMn, and for the fivefold surface of icosahedral AlCuFe. We also make some comments about the interpretation of the LEED patterns.

Title: "Study of the 5-fold and 2-fold i-AlPdMn surfaces by full-hemispherical x-ray photoelectron diffraction"

Authors: D.Naumovic, P.Aebi, L. Schlapbach, C. Beeli, T.A. Lograsso, and D.W. Delaney

Contact: T.A. Lograsso

Proceedings of the Sixth Inernational Conference on Quasicrystals(ICQ6), held in Tokyo, Japan, May 1997. S. Takeuchi and T. Fujiwara, Editors (World Scientific, Singapore, 1998) 749-756.

Abstract:

Using X-ray photoelectron diffraction the average, local, real-space environment of a given atomic species can be probed. The results of the Pd environment near the surface of a monograin icosahedral AlPdMn (i-AlPdMn) quasicrystals cut perpendicularly to either a fivefold or a twofold axis are presented. Changes of the local crystallography at the surface are found due to either preferential sputtering or annealing. An ordered, cubic surface alloy is formed with its (110) direction aligned parallel to the surface normal of the quasicrystal.

Title: "Oxidation of surfaces of icosahedral quasicrystals: Some key comparisons"

Authors: C.J. Jenks, P.J. Pinhero, T.E. Bloomer, S.-L. Chang, J.W. Anderegg, and P.A. Thiel

Contact: C.J. Jenks

Proceedings of the Sixth Inernational Conference on Quasicrystals(ICQ6), held in Tokyo, Japan, May 1997. S. Takeuchi and T. Fujiwara, Editors (World Scientific, Singapore, 1998) 741-748.

Abstract:

We compare oxide layers which form on quasicrystalline alloys of different compositions and in different environments, and also make comparison to a crystalline alloy of similar composition.

Title: "The 5-fold surface of quasicrystalline AlPdMn: Structure determination with low-energy electron diffraction"

Authors: M. Gierer and M.A. Van Hove, A.I. Goldman, Z. Shen, S.-L. Chang, P.J. Pinhero, C.J. Jenks, J.W. Anderegg, C.-M. Zhang, and P.A. Thiel

Contact: M.A. Van Hove

Journal: Phys. Rev. B (1998) 57, 7628-7641.

Abstract:

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 Å, to a final value of 0.38 Å, 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.

Title: "Quasicrystals: A short review from a surface science perspective"

Authors: C.J. Jenks and P.A. Thiel

Contact: C.J. Jenks

Journal: Langmuir (1998) 14, 1392-1397.

Abstract: Quasicrystals are materials of both intellectual and practical importance. While some level of understanding can now be applied to their bulk electronic and atomic structure, the same cannot be said of their surface properties. In this article, we point out some of the key issues in the surface science of quasicrystals at present.

Title: "Surface properties of quasicrystals"

Authors: C.J. Jenks and P.A. Thiel

Contact: C.J. Jenks

Journal: MRS Bulletin (1997) 22(11), 55-58.

Abstract:

A variety of opportunities to develop technological appplications of quasicrystals is being considered. Several of these are discussed in this issue. A common feature of many proposed applications is that they involve surface or near-surface interactions. Information about the surface structure, composition, and chemistry is prerequisite to understanding the origin of the desirable traits of quasicrystals. For instance, basic issues of surface structure—whether quasicrystals are intrinsically rough or flat, quasicrystalline or not quasicrystalline—impact directly on our understanding of their surface thermodynamic, electronic, and chemical properties.

Title: "The fivefold surface of quasicrystalline AlCuFe: Preparation and characterization with LEED and AES"

Authors: Z. Shen, P.J. Pinhero, T.A. Lograsso, D.W. Delaney, C.J. Jenks and P.A. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (1997) 385, L923-L929.

Abstract:

We report the first surface characterization of a large, single-grain sample of quasicrystalline AlCuFe. The sample is oriented with its surface perpendicular to a five-fold axis, and has bulk composition 63.4-24.0-12.6 atom %. Following our method of preparation, the surface yields an exceptionally sharp and rich five-fold pattern in low-energy electron diffraction. The spot spacings and symmetries are consistent with expectations for this surface, based upon the unreconstructed structure.

Title: "Structure and stability of the twofold surface of icosahedral Al-Pd-Mn by low energy electron diffraction"

Authors: Z. Shen, C.J. Jenks, J. Anderegg, D.W. Delaney, T.A. Lograsso, P.A. Thiel, and A.I. Goldman

Contact: C.J. Jenks

Journal: Phys. Rev. Lett. (1997) 78, 1050-1053.

Abstract:

We have used low-energy electron diffraction and x-ray photoemission spectroscopy to investigate the structure of the twofold surface of icosahedral Al-Pd-Mn. The regrowth of the surface by annealing after sputtering took place in two distinct stages. The first stage was the appearance of a fine-grained surface phase with icosahedral, or near-icosahedral, symmetry. For higher annealing temperatures (above 800 K) a bulk terminated face-centered icosahedral surface was observed.

Title: "Surface carbon films on Al-Cu-Fe"

Authors: T.E. Bloomer, J. Flumerfelt, and M.J. Kramer

Contact: M.J. Kramer

Proceedings of the Conference New Horizons in Quasicrystals: Research and Applications, edited by A. I. Goldman, D. J. Sordelet, P. A. Thiel and J. M. Dubois (World Scientific, Singapore, 1997).

Abstract:

In order to study the unique properties of quasicrystals, it is necessary to form dense, homogenous monoliths of these alloys. Unfortunately, Al-Cu-Fe quasicrystalline alloy ingots prepared by conventional casting techniques result in large scale chemical inhomogenities which contain numerous cracks due to differential thermal contraction between the various phases during cooling. Thus a powder metallurgical approach using gas atomized (GA) powders is being pursued in order to form large samples of phase pure Al-Cu-Fe quasicrystal. GA samples of specific compositions and sizes are hot isostatic pressed (HIPed) to form dense monoliths. The effects of surface contamination of GA powders, which may inhibit particle-to-particle sintering and may also increase second phase contamination in the HIPed piece, is being studied by scanning Auger microprobe (SAM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Title: "Structural analysis of the fivefold symmetric surface of the Al70Pd21Mn9 quasicrystal by low energy electron diffraction"

Authors: M. Gierer, M.A. Van Hove, A.I. Goldman, Z. Shen, S.-L. Chang, C.J. Jenks, C.-M. Zhang, and P.A. Thiel

Contact: M.A. Van Hove

Journal: Phys. Rev. Lett. (1997) 78, 467-470.

Abstract:

The atomic structure of the fivefold symmetric quasicrystal surface of Al70Pd21Mn9 was investigated by means of a dynamical low energy electron-diffraction analysis. Approximations are developed to address the high structural complexity of quasicrystals, yielding average layer-by-layer structural and compositional information. A mix of several relaxed bulklike terminations is found, consistently favoring dense Al-rich outermost atomic layers. These results can be understood in terms of principles known to govern surface structures of simpler, crystalline metals.

Title: "Specular and diffuse scattering from the fivefold surface of AlPdMn"

Authors: T. Gu, A.I. Goldman, P. Pinhero, and D. Delaney

Contact: A.I. Goldman

Proc. Conference on New Horizons in Quasicrystals: Research and Applications, held in Ames, Iowa, Ed. By A. I. Goldman, D. J. Sordelet, P. A. Thiel and J. M. Dubois, World Scientific, Singapore, 165-168 (1997).

Abstract:

The AlPdMn quasicrystal surface perpendicular to a fivefold axis was studied by means of X-ray reflectivity and diffuse scattering. Surface and interface quantitites such as oxide layer thickness, rms roughness, lateral correlation length and roughness exponent were acquired by simultaneously fitting theoretical model to the specular and diffuse data.

Title: "Surface oxidation of Al-Pd-Mn and Al-Cu-Fe alloys"

Authors: C.J. Jenks, P.J. Pinhero, S.-L. Chang, J.W. Anderegg, M.F. Besser, D.J. Sordelet, and P.A. Thiel

Contact: C.J. Jenks

Proceedings of the Conference on New Horizons in Quasicrystals: Research and Applications, edited by A. I. Goldman, D. J. Sordelet, P. A. Thiel and J. M. Dubois (World Scientific), 157-164 (1997).

Abstract:

The application of ultrahigh vacuum surface analytical techniques to the study of the oxidation of Al-Pd-Mn and Al-Cu-Fe quasicrystals is presented. Comparison of the effects of various oxidation environments are made for each material as well as for a cubic phase of Al-Pd-Mn. We find that the oxidation of the two icosahedral alloys is similar. Exposure to oxygen in vacuum, normal air, humid air, and immersion in water represent progressively better oxidizing environments, resulting inincreasingly deeper oxides. Oxidation of Al occurs most readily, but oxidation of other metals also becomes apparent upon going from oxygen in vacuum to immersion in water, which also attests to the effectiveness of water as an oxidant. Oxide thicknesses for both quasicrystalline alloys are comparable for each oxidation environment. Comparison of quasicrystalline to cubic Al-Pd-Mn shows that the cubic phase is more readily oxidized. In addition, while the surfaces of both quasicrystalline alloys become significantly enriched in Al upon oxidation, such enrichment is not observed for the cubic alloy. This suggests that the structure of quasicrystals plays an important role in their oxidation chemistry.

Title: "Effect of water on surface oxidation of an Al-Pd-Mn quasicrystal"

Authors: P.J. Pinhero, S.-L. Chang, J.W. Anderegg, W.B. Chin, and P.A. Thiel

Contact: P.A. Thiel

Journal: Philos. Mag. B (1997) 75, 271-281.

Abstract:

X-ray Photoelectron Spectroscopy (XPS) is used to determine the effect of water on oxidation of a quasicrystalline alloy at room temperature. We use a single-grain sample, oriented with the five-fold axis perpendicular to the surface plane, and with nominal bulk composition Al70Pd21Mn9. We compare the oxide which forms upon exposure to four basic environments: pure oxygen in ultrahigh vacuum, ambient air, humid air, and liquid water. These environments are increasingly aggressive toward the alloy, as judged by the number of alloy constituents oxidized and by the thickness of the oxide layer.

Title: "Photoelectron spectra of an Al70Pd21Mn9 quasicrystal and the cubic alloy Al60Pd25Mn15 "

Authors: C.J. Jenks, S.-L. Chang, J.W. Anderegg, P.A. Thiel, and D.W. Lynch

Contact: C.J. Jenks

Journal: Phys. Rev. B (1996) 54, 6301-6306.

Abstract:

Photoelectron spectra of a five-fold quasicrystalline alloy Al70Pd21Mn9 and a related cubic alloy Al60Pd25Mn15 reveal two noteworthy features. The first is that the Pd 3d lines fall at binding energies which are 2.2 eV higher than in pure Pd. A similar shift is observed for Pd in other alloys. The second noteworthy feature is that the Mn 2p3/2 line is very sharp in the quasicrystal. Fitting the experimental peaks with a Doniach-Sunjic lineshape suggests that the position and density of Mn states near EF is very sensitive to the structural and/or chemical environment of Mn in the alloys, and that this accounts for the shape of the 2p3/2 Mn line. The sharpness of the Mn line may be a fingerprint of the quasicrystalline phase within the AlPdMn family.

Title: "Preparation of well-defined samples of AlPdMn quasicrystals for surface studies"

Authors: C.J. Jenks, D.W. Delaney, T.E. Bloomer, S.-L. Chang, T.A. Lograsso, Z. Shen, C.-M. Zhang, and P.A. Thiel

Contact: C. J. Jenks

Journal: Appl. Surf. Sci. (1996) 103(4), 485-493.

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 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.

Title: "Surface oxidation of an Al-Pd-Mn quasicrystal, characterized by x-ray photoelectron spectroscopy"

Authors: S.-L. Chang, J.W. Anderegg, and P.A. Thiel

Contact: P.A. Thiel

Journal: J. Non-Cryst. Solids (1996) 195, 95-101.

Abstract:

X-ray Photoelectron Spectroscopy (XPS) is used to determine the extent of oxidation of each of the three metals which comprise a quasicrystalline alloy. We use a single-grain sample, oriented with the five-fold axis perpendicular to the surface plane, and with nominal bulk composition Al70Pd21Mn9. We compare the oxide which results from exposure to ambient gas at room temperature, with that from exposure to pure oxygen in ultrahigh vacuum at temperatures up to 870 K. XPS probes the near-surface region (ca. top 100 Å), and shows that only the Al can be oxidized. The depth of the oxide layer depends systematically upon the conditions of treatment, but is always very thin--in the range of about 5 to 30 Å. Taken together, the data suggest that the surface forms a thin, passivating, surface layer of aluminum oxide.

Title: "Oxygen adsorption on a single-grain, quasicrystal surface"

Authors: S.-L. Chang, W.B. Chin, C.-M. Zhang, C.J. Jenks, and P.A. Thiel

Contact: P.A. Thiel

Journal: Surf. Sci. (1995) 337, 135-146.

Abstract:

Oxygen adsorbs on the five-fold surface of Al70Pd21Mn9. 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 Å) 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.

Title: "Surface studies of oxidation of a single-grain quasicrystal"

Authors: S.-L. Chang, C.M. Zhang, W.B. Chin, C.J. Jenks, J.W. Anderegg, and P.A. Thiel

Contact: P.A. Thiel

Proceedings of the 5th International Conference on Quasicrystals, Edited by C. Janot and R. Mosseri, World Scientific, Singapore, 786-789 (1995).

Abstract:

We have used Auger electron spectroscopy(AES), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) to characterize the surface properties of a single-grain Al70Pd21Mn9 (APM) quasicrystal (QC) upon oxidation. When oxygen is adsorbed on this surface, a disordered layer is formed at low coverages. This chemisorbed oxygen destroys the five-fold quasiperiiodicity completely. Further adsorption of oxygen leads to a thin layer (less than 20Å) of Al oxide which passivates the surface. At elevated temperatures (870 K), adsorption of oxygen induces an enrichment of Al on the surface. This is explained by the exothermicity of its oxide and the possibility of increased mobility of Al at higher tempertures. Al is the only element in this QC which can be oxidized. No evidence of oxidation for Pd and Mn is observed.

Title: "Electronic band dispersion and pseudogap in quasicrystals: Angular-resolved photoemission studies on icosahedral Al70Pd21.5Mn8.5"

Authors: X. Wu, S.W. Kycia, C.G. Olson, P.J. Benning, A.I. Goldman, and D.W. Lynch

Contact: D.W. Lynch

Journal: Phys. Rev. Lett. (1995) 75(4), 4540-4543.

Abstract:

Angular-resolved photoelectron spectra from the fivefold surface of ingle-grain icosahedral Al70Pd21.5Mn8.5 exhibit a quasiperiodic dispersion of 300 meV at 2.3 eV binding energy. Low energy electron diffraction studies confirm quasicrystalline order at the surface. A distinct pseudogap feature is observed with a density of states near the Fermi level decreasing as a square-root power law.


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