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Nanostructured alloy surfaces present unique physical properties and chemical reactivities that are quite different from those of the close-packed low-index surfaces. This can be beneficial for the design of new catalysts and electronic and data-storage devices. However, the growth of such surface nanostructures is not straightforward at the atomic scale. The cluster-based bulk structure of intermetallic compounds presents an original alternative to build surfaces with specific morphologies, in comparison to more traditional methods based on mechanical, chemical, or plasma treatments. It relies on their specific electronic structures—built from a network of bonds with a combination of ionic, covalent-like, and metallic characters, and also depends on the experimental conditions. In this paper, a few surface structures of cluster-based intermetallics are reviewed, with a special emphasis on quasicrystals and clathrates. We show how the intrinsic electronic properties of such compounds, as well as the surface preparation conditions, impact their surface morphologies, which can further influence the growth of atomic and molecular thin films at their surface. [ABSTRACT FROM AUTHOR]

The formation of two-dimensional oxide dodecagonal quasicrystals as well as related complex approximant phases was recently reported in thin films derived from BaTiO3 or SrTiO3 perovskites deposited on (111)-oriented Pt single crystals. Here, we use an all-thin-film approach in which the single crystal is replaced by a 10 nm thick Pt(111) buffer layer grown by molecular beam epitaxy on an Al2O3(0001) substrate. An ultra-thin film of SrTiO3 was subsequently deposited by pulsed laser deposition. The film stacking and structure are fully characterized by diffraction and microscopy techniques. We report the discovery of two new complex phases obtained by reduction of this system through high temperature annealing under ultrahigh vacuum conditions. The formation of a new large square approximant with a lattice parameter equal to 44.4 Å is evidenced by low-energy electron diffraction and scanning tunneling microscopy (STM). Additionally, a new 2D hexagonal approximant phase with a lattice parameter of 28 Å has been observed depending on the preparation conditions. Both phases can be described by two different tilings constructed with the same basic square, triangle and rhombus tiles possessing a common edge length of about 6.7 Å. Using the tiling built from high resolution STM images, we propose an atomic model for each approximant which accounts for the experimental observations. Indeed, the STM images simulated using these models are found to be in excellent agreement with the experimental ones, the bright protrusions being attributed to the topmost Sr atoms. In addition our theoretical approach shows that the adhesion of the oxide layer is rather strong (−0.30 eV Å−2). This is attributed to charge transfer, from the most electropositive elements (Sr and Ti) to the most electronegative ones (Pt and O), and to hybridization with Pt-states. Density of states calculations indicate differences in the electronic structure of the two approximants, suggesting different chemical and physical properties. This all-thin-film approach may be useful to explore the formation of complex two-dimensional oxide phases in other metal-oxide combinations. [ABSTRACT FROM AUTHOR]

We have studied the growth and electronic structure of Ag thin films on the fivefold surface of the icosahedral (i)-Al-Cu-Fe quasicrystal using scanning tunneling microscopy, low energy electron diffraction (LEED), ultraviolet photoemission spectroscopy, and density functional theory. Upon deposition at 400 K, Ag islands grow to form crystallites with a preferred thickness for a given coverage. LEED patterns reveal five rotational domains of Ag crystallites with (111) orientation for coverages larger than approximately seven monolayers. Quantum well states are observed in the photoemission spectra of Ag/i-Al-Cu-Fe ranging from 5 to 35 monolayers, indicating electron confinement within the film thickness and, thus, confirming electronic growth of Ag thin films on quasicrystalline surfaces. Electronic structure calculations have been performed to discuss the possible origins of the confinement at the film-substrate interface. [ABSTRACT FROM AUTHOR]

The intermetallic compound InPd (CsCl type of crystal structure with a broad compositional range) is considered as a candidate catalyst for the steam reforming of methanol. Single crystals of this phase have been grown to study the structure of its three low-index surfaces under ultra-high vacuum conditions, using low energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). During surface preparation, preferential sputtering leads to a depletion of In within the top few layers for all three surfaces. The near-surface regions remain slightly Pd-rich until annealing to ~580 K. A transition occurs between 580 and 660 K where In segregates towards the surface and the near-surface regions become slightly In-rich above ~660 K. This transition is accompanied by a sharpening of LEED patterns and formation of flat step-terrace morphology, as observed by STM. Several superstructures have been identified for the different surfaces associated with this process. Annealing to higher temperatures (=750 K) leads to faceting via thermal etching as shown for the (110) surface, with a bulk In composition close to the In-rich limit of the existence domain of the cubic phase. The Pd-rich InPd(111) is found to be consistent with a Pd-terminated bulk truncation model as shown by dynamical LEED analysis while, after annealing at higher temperature, the In-rich InPd(111) is consistent with an In-terminated bulk truncation, in agreement with density functional theory (DFT) calculations of the relative surface energies. More complex surface structures are observed for the (100) surface. Additionally, individual grains of a polycrystalline sample are characterized by micro-spot XPS and LEED as well as low-energy electron microscopy. Results from both individual grains and "global" measurements are interpreted based on comparison to our single crystals findings, DFT calculations and previous literature. [ABSTRACT FROM AUTHOR]

The use of quasicrystals as precursors to catalysts for the steam reforming of methanol is potentially one of the most important applications of these new materials. To develop application as a technology requires a detailed understanding of the microscopic behavior of the catalyst. Here, we report the effect of leaching treatments on the surface microstructure, chemical composition, and valence band of the icosahedral (i-) Al-Cu-Fe quasicrystal in an attempt to prepare a model catalyst. The high symmetry fivefold surface of a single grain i-Al-Cu-Fe quasicrystal was leached with NaOH solution for varying times, and the resulting surface was characterized by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The leaching treatments preferentially remove Al producing a capping layer consisting of Fe and Cu oxides. The subsurface layer contains elemental Fe and Cu in addition to the oxides. The quasicrystalline bulk structure beneath remains unchanged. The subsurface gradually becomes Fe3O4 rich with increasing leaching time. The surface after leaching exhibits micron sized dodecahedral cavities due to preferential leaching along the fivefold axis. Nanoparticles of the transition metals and their oxides are precipitated on the surface after leaching. The size of the nanoparticles is estimated by high resolution transmission microscopy to be 5-20 nm, which is in agreement with the AFM results. Selected area electron diffraction (SAED) confirms the crystalline nature of the nanoparticles. SAED further reveals the formation of an interface between the high atomic density lattice planes of nanoparticles and the quasicrystal. These results provide an important insight into the preparation of model catalysts of nanoparticles for steam reforming of methanol. [ABSTRACT FROM AUTHOR]

A combination of experimental methods was used to study the structure of In thin films deposited on the Pd(111) surface and the alloying behavior. X-ray photoelectron spectroscopy (XPS), lowenergy electron diffraction (LEED), and scanning tunneling microscopy results indicate that surface alloying takes place at room temperature. Below 2 monolayer equivalents (MLEs), the LEED patterns show the formation of three rotational domains of InPd(110) of poor structural quality on top of the Pd(111) substrate. Both core-levels and valence band XPS spectra show that the surface alloy does not yet exhibit the electronic structure characteristic of the 1:1 intermetallic compound under these conditions. Annealing the 1 MLE thin film up to 690 K yields to a transition from a multilayer InPd near-surface intermetallic phase to a monolayer-like surface alloy exhibiting a well ordered (√3 × √3) R30° superstructure and an estimated composition close to In2Pd3. Annealing above 690 K leads to further In depletion and a (1 x 1) pattern is recovered. The (√3 × √3) R30° superstructure is not observed for thicker films. Successive annealing of the 2 MLE thin film leads the progressive disappearance of the InPd diffraction spots till a sharp (1 x 1) pattern is recovered above 690 K. In the high coverage regime (from 4 to 35 MLE), the formation of three rotational domains of a bee-In7Pd3 compound with (110) orientation is observed. This In-rich phase probably grows on top of interfacial InPd(110) domains and is metastable. It transforms into a pure InPd(110) near-surface intermetallic phase in a temperature range between 500 and 600 K depending on the initial coverage. At this stage, the surface alloy exhibits core-level chemical shifts and valence band (VB) spectra identical to those of the 1:1 InPd intermetallic compound and resembling Cu-like density of states. Annealing at higher temperatures yields to a decrease of the In concentration in the near-surface region to about 20 at.% and a (1 x 1) LEED pattern is recovered. [ABSTRACT FROM AUTHOR]

The (110) surface of the InPd intermetallic compound and the In-Pd surface alloy properties are investigated in the framework of the density functional theory, within the projector augmented planewave method. Surface segregation is calculated to be energetically unfavorable at stoichiometric InPd(110) surfaces, while indium antisites are shown to segregate to the surface in off-stoichiometric InPd(110) systems. Concerning surface alloys obtained by burying In-doped Pd layers in Pd(111), we demonstrated that the most stable ones are those presenting atomic indium concentrations below 50 at. % (11 at. %, 25 at. %, 33 at. %). According to our calculations, the In-doped Pd layers with concentration above or equal to 50% lead to In-doped Pd multilayers, each presenting an atomic indium concentration below 50 at. %. Alloying and segregation effects in InPd intermetallic compound and In-Pd surface alloys clearly agree with the larger bonding strength of In-Pd ( - 0 . 4 4 eV) compared to In-In ( - 0 . 2 9 eV) and Pd-Pd ( - 0 . 3 1 eV). [ABSTRACT FROM AUTHOR]