Regular Facets Research Articles

CO Adsorption on Pd Nanoparticles: Density Functional and Vibrational Spectroscopy Studies

Adsorption of CO on nanosize Pd particles was studied theoretically by density functional method and spectroscopically by means of infrared reflection absorption spectroscopy (IRAS) and sum frequency generation (SFG). A density functional approach was applied to three-dimensional crystallites of about 140 atoms. The model clusters were chosen as octahedral fragments of the face centered cubic (fcc) bulk, exhibiting (111) and (001) facets. Bare and adsorbate-decorated cluster models were calculated with Oh symmetry constraints. Various types of adsorption sites were inspected: 3-fold hollow, bridge, and on-top positions at (111) facets; 4-fold hollow and on-top sites at (001) facets; bridge positions at cluster edges; on-top positions at cluster corners; and on single Pd atoms deposited at regular (111) facets. Adsorption properties of the relatively small regular cluster facets (111) and (001) are calculated similar to those of corresponding ideal (infinite) Pd surfaces. However, the strongest CO bonding...

Catalytic activity and poisoning of specific sites on supported metal nanoparticles.

Catalytically active nanoparticle sites: Different catalytic activity of different active sites coexisting on a supported nanoparticle has been directly observed using molecular-beam techniques and in situ surface vibrational spectroscopy. For methanol decomposition on a supported Pd model catalyst (see picture), it is shown that carbon–oxygen-bond breakage and dehydrogenation occur as competing reaction pathways at particle defects and on regular facets, respectively.

Self-Assembled Structures of Gas-Phase Prepared FePt Nanoparticles

ABSTRACTWe report on a non-lithographic method for the preparation of self-assembled FePt nanoparticles via inert-gas condensation. Prior to deposition the particles can be sintered in flight at temperatures as high as TS = 1273 K. Whereas un-sintered particles have irregular shapes, particles sintered at elevated temperatures TS ≤ 793 K show a regular faceting. (High resolution) transmission electron microscopy ((HR)TEM) shows that these regularly faceted particles are of icosahedral structure. When being deposited onto amorphous carbon films, the gas-phase sintered particles are found to have a high mobility. In particular, for the high-temperature sintered FePt nanoparticles, we observe that this mobility leads to the formation of particle arrays with hexagonal close-packed arrangements. Within these ordered patches, the particles are separated from one another. Analytical investigations using energy filtered TEM (EFTEM) show that a carbon layer is formed between the particles. Magnetization analyses give results showing that the gas-phase sintered particles are superparamagnetic at room temperature with a blocking temperature of TB = 49K.

Self-Assembled Structures of Gas-Phase Prepared FePt Nanoparticles

AbstractWe report on a non-lithographic method for the preparation of self-assembled FePt nanoparticles via inert-gas condensation. Prior to deposition the particles can be sintered in flight at temperatures as high as TS = 1273 K. Whereas un-sintered particles have irregular shapes, particles sintered at elevated temperatures TS ≥ 793 K show a regular faceting. (High resolution) transmission electron microscopy ((HR)TEM) shows that these regularly faceted particles are of icosahedral structure. When being deposited onto amorphous carbon films, the gas-phase sintered particles are found to have a high mobility. In particular, for the high-temperature sintered FePt nanoparticles, we observe that this mobility leads to the formation of particle arrays with hexagonal close-packed arrangements. Within these ordered patches, the particles are separated from one another. Analytical investigations using energy filtered TEM (EFTEM) show that a carbon layer is formed between the particles. Magnetization analyses give results showing that the gas-phase sintered particles are superparamagnetic at room temperature with a blocking temperature of TB = 49K.

Tunisian Migrants and Their Experience of Racism in Modena

SummaryBased both on the author's personal testimony of living, working and studying in Italy for ten years, and on his more recent sociological fieldwork, this article explores various dimensions of racism in the north Italian city of Modena, paying special attention to the experiences of Tunisian immigrants. The methodology comprises interviews with Tunisian immigrants and their employers, observations in bars and other public places, analysis of racism in the workplace, monitoring of the local media and reference to the national political debate. The findings reveal that being on the receiving end of racism, hostility and violence are regular facets of the everyday lives of Tunisian immigrants in Modena, as well as being part of a wider local and national anti-immigrant discourse.

Graphitic Disks or Polygons? Faceting of Graphite Disks

Abstract The recent discovery of graphite cones [1,2] has raised some interesting questions about the nucleation and growth of curved graphitic structures. Here we report the structural peculiarities of one member of the ensemble, i.e. the flat graphite disks. Unlike graphite flakes seen in carbon soot which are irregularly shaped, the disks in this sample often show regular faceting which might give us additional insight into the growth mechanisms. The samples were examined in a Hitachi H9000 NAR TEM. Samples were sonicated in high purity methanol and dispersed on a 300 mesh Cu grid coated with a 20 nm thick amorphous carbon layer. Typical disk diameters range from 0.5 to 3.0 μm, with thickness ranging from 10 nm to 50 nm. A close examination reveals that the edges of the disks are faceted and typically have 12 sides. In most cases, the edges show a long facet alternating with a shorter facet.

鏡面膜のピロウ変形

We present in this paper an analytic solution for the pillow deformation of the deployable mesh antenna which is segmented into a regular convex polygon by the cable net. We treat this problem by using both the methods of power series expansion of the complex variables and of Fourier analysis, in which we take into account the self-consistent boundary condition of the mesh on the supporting cable. Using this solution, we find that the magnitude of the pillow deformation of the regular polygonal facets is directly proportional to the square of the side length of the facet and the reflector curvature, and is an increasing function of the ratio of the tension of the mesh and cable. We also derive the approximate formulas for the pillow deformation and the best-fit surface error from the ideal parabola for the case of triangular and rectangular facets. We performed the numerical evaluation of the solution, and discuss the applicability of above mentioned approximate formulas. The analytic solution here derived gives us the basic and quantitative ideas of the pillow deformation and is useful for the quick numerical evaluation of the pillow deformation.

Free Extensions of Chiral Polytopes

AbstractAbstract polytopes are discrete geometric structures which generalize the classical notion of a convex polytope. Chiral polytopes are those abstract polytopes which have maximal symmetry by rotation, in contrast to the abstract regular polytopes which have maximal symmetry by reflection. Chirality is a fascinating phenomenon which does not occur in the classical theory. The paper proves the following general extension result for chiral polytopes. If 𝒦 is a chiral polytope with regular facets 𝓕 then among all chiral polytopes with facets 𝒦 there is a universal such polytope 𝓟, whose group is a certain amalgamated product of the groups of 𝒦 and 𝓕. Finite extensions are also discussed.

Surface and interface ordering on non-(100)-oriented GaAs substrates

We have observed the formation of highly ordered surface structures on (211), (311), and (111) GaAs substrates. During molecular-beam epitaxy, the flat surfaces break up into regular facets that have nanometer-scale dimensions, which can be directly monitored by reflection high-energy electron diffraction (RHEED). The RHEED intensity dynamics reveals a unique growth mechanism on the stepped surfaces during the growth of GaAs/AlAs multilayer structures resulting in the formation of alternating narrow and wide regions of GaAs and AlAs. The observed energy shift of the luminescence is correlated with the height of the specific surface corrugation and with the average GaAs layer thickness. The additional lateral confinement introduced by the interface corrugation manifests itself in an increased exciton continuum energy, an enhanced exciton-phonon interaction, a pronounced optical anisotropy, and an increased integral luminescence intensity. The observed surface structures offer a unified picture to explain previously unclear results obtained from these non-(100)-oriented GaAs surfaces, and they provide a unique method to directly synthesize quantum-wire and quantum-dot structures.

TURING STRUCTURES IN CATALYTIC SURFACE REACTIONS: THE FACETTING OF Pt(110) IN CO + O2

Exposing a Pt(110) surface to catalytic CO oxidation causes the development of a regular facet pattern of microscopic dimensions which has been identified as a dissipative structure of the Turing type. The primary step in the facetting process is considered to be the [Formula: see text] phase transition which is also responsible for the occurrence of kinetic oscillations in the same system. This interpretation was confirmed in a Monte Carlo simulation which was based on the Langmuir-Hinshelwood mechanism of catalytic CO oxidation and the properties of the surface phase transition. Thus a detailed microscopic description is available for the process by which a Turing structure is formed during a surface reaction.

Model for the formation of a microscopic Turing structure: The facetting of Pt(110) during catalytic oxidation of CO

A Monte Carlo simulation of the formation of regular facet patterns during catalytic CO oxidation on Pt(110) has been conducted. The simulation was based on the elementary steps of the surface reaction, the CO-induced 1\ifmmode\times\else\texttimes\fi{}1\ensuremath{\rightleftarrows}1\ifmmode\times\else\texttimes\fi{}2 phase transition, and the enhancement of ${\mathrm{O}}_{2}$ adsorption at step sites. The model can reproduce the formation of sawtoothlike facet patterns with a spacing of \ensuremath{\sim}100 to 200 \AA{}. The facets represent a Turing structure caused by the coupling of kinetic instabilities with the mass transport of Pt atoms.

Determination of the orientation of CuBi grain boundary facets using a photogrammetric technique

SEM examination of the fracture surfaces of CuBi bicrystals fractured intergranularly reveals a striking pattern of regular facet steps and striations. The morphology of the facet planes of two bicrystals is examined and, with the use of SEM photogrammetric technique, the orientation of the facet planes is determined. The results show that, despite the variety of facet morphologies observed within the same specimen, the facet planes belong to a small number of distinct families. However, their orientation is not related in a simple manner to the orientation of each single crystal, and it is also independent of the existence or lack of a coincidence site lattice (CSL) relationship.

Surface features of heavy mineral grains from Palaeocene sands of the central North Sea

Synopsis Scanning electron microscopy has revealed both mechanically and chemically produced surface textures on heavy mineral grains from Palaeocene sands of the central North Sea basin. Chemical activity initially modified irregular surfaces, producing mamillae. Further solution produced regular facets, the dimensions of which increase with intensity of etching. Variations in composition of the connate waters are thought responsible for slight variations in etching style. Mechanically-produced surface features similar to those seen on quartz grains, including breakage blocks, V-shaped indentations, grooves, upturned cleavage plates and conchoidal fractures, mostly originated in a pre-Palaeocene phase of sedimentation. The possibility of retention of pre-existing surface textures on grains during the sedimentation process must therefore be considered when using such textures in palaeoenvironmental interpretation.

Three dimensional organization of mammalian adrenal cortex

The adrenal cortex of different mammals was studied by SEM in order to demonstrate its actual three-dimensional organization. In the rat, as well as in the cat and pig, the adrenal cortex appeared as a "tunnelled continuum" of polyhedral cells arranged in plate-like structures (laminae). This laminar arrangement was more evident in the inner fasciculate and reticular zones where the cortex revealed a striking similarity to liver tissue. The polyhedral cells of all cortical zones possessed regular facets populated by small pits, larger invaginations and numerous microvilli with the exception of very short and smooth areas probably corresponding to attachment zones and/or gap junctions. This cellular architecture produced a labyrinthic system of intercellular channels or lacunae in which the capillaries were suspended. The pericapillary areas of this labyrinth contained microvilli, amorphous material, a delicate net of fibrils and occasional cells. The intercellular compartment of this lacunar system was mainly bordered by numerous microvilli arising from endocrine cells. The luminal surface of the capillary wall showed not only irregularly protruding margins (interpretable as endothelial junctions) but also clearly overlapping and flattened endothelial extensions. In all the animals and areas of the adrenal cortex examined, the endothelial wall was provided with abundant clusters of small fenestrations (about 50 nm in diameter) generally arranged in sieve plates. Larger fenestrations were noted mainly in the fasciculate and reticular zones of the cat and pig and occasionally in the rat. A final point related to the nature and significance of sinusoidal fenestrations was the occurrence of irregularly shaped and intracapillary located cells mainly noted in the deeper zones of the fasciculate and reticular zones of the gland. These elements--possessing the surface characteristics of macrophages--were observed, with their irregular and slender evaginations, in close proximity to the large fenestrations in a manner reminiscent of Kupffer cells within the lumen of liver sinusoids.

Approximations by polytopes with projectively regular facets

Approximations by polytopes with projectively regular facets