Atomic Structure and Chemical Composition

Consider the atomic structure and composition on the terraces. Most evidence indicates that on the surfaces of Al-rich QCs, there is no surface segregation and 

Even if the debate concerning the type of cut cluster that corresponds to the dark star motif is setded, the chemical decoration of dark stars is variable, within any given bulk structure model. Hence, one would expect different dark stars on a real surface to have different chemical compositions. Identical arguments can be made for the white flower and other types of motifs. This is important, as the properties of a given type of adsorption site form the basis for calculating and predicting chemisorption properties of QCs. Calculations that overlook the chemical diversity of sites may also overlook important aspects of QC surfaces. This issue has been addressed in detail [80, 174]. 

The foregoing discussion has been overly simplistic in implying that the bulk structure is always known. For i-Al-Pd-Mn, there is not one but several bulk structure models. Usually, when scientists have compared experimental surface data with bulk structure models, only a single model has been employed, perhaps because of problems with tractability or availability of the various models. In every case, excellent agreement between the (chosen) bulk model and the experimental data has been obtained. This is probably true because, as noted earlier, there is significant overlap between the models - it has been estimated that at least 80% of the atomic sites are identical or nearly so (but the chemical decorations at those sites differ much more significantly between models) [174]. Nonetheless, this approach overlooks the issue of uniqueness or optimiTation of fit 

Not only the surface atomic structure but also its electronic structure is important. In the bulk, QCs are characterized by a suppression in the density of states at the Fermi edge (Fp) known as a pseudogap. There is evidence that the pseudogap is preserved at the bulk-terminated flat surfaces (105, 109, 178-184] and that this strongly influences surface QC properties even in air, such as contact angles with polar liquids (wetting) [12,185,186). 

However, the electronic states close to the Fermi energy contributing to the density of states in QCs seem to have a mixed nature. Electronic states in periodic crystals can be described by the Bloch theorem, that is, yielding delocahzed bulk states along with surface and surface resonance states. Angle-resolved photoemission has been used to probe the details of surface electronic structure of low-index surfaces of d-Al-Ni-Co and i-Al-Pd-Mn [187-190]. These experiments revealed a nearly-free-electron-hke dispersion in the region of s-p-derived states, indicating a nonlocalized character. However, the experiments could not determine whether localized states were present in addition to the nonlocalized ones. 

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Basis sets and electron-ion interactions The primary computational task in a DFT calculation is the solution of the Kohn- Sham equations for a given atomic structure and chemical composition. Invariably, for a condensed phase system, this requires the use of a set of basis functions with which to expand the Kohn-Sham orbitals This choice of basis set is of critical importance to the... 

Basic information needed to understand the physical and chemical properties of solid surfaces and thin solid films include the atomic structures and the compositional variations across the surface and interface layers. The atomic structures can be studied with microscopies and with surface sensitive diffraction and particle scattering techniques. Compositions of surfaces and thin films can be studied with the atom-probe FIM. In general, however, compositional analyses are mostly done with surface sensitive macroscopic techniques, such as auger electron... 

Identification and study of the active sites where chemical bond scission or rearrangement occurs, so crucial to the working of a catalyst, requires that we investigate the structure and the chemical composition of the working catalyst on the atomic scale. Ideally, we would like to inspect each nonidentical surface site and determine its structure and chemical composition while the chemical reaction is taking place. 

A vast amount is now known, at the atomic level, about heterogeneous processes at gas-solid interfaces. Yet, three decades ago, research in solid surface science was mired in traditional thermodynamic and kinetic experiments that were not able to yield molecular level information. Fundamental issues, such as the molecular structure and chemical composition of the surface complex, could not be adequately addressed by the data generated from the classical methods. It was not until the development of highly surface-specific empirical tools that tangible gains in the study of the gas-solid interface were achieved. 

Since there is no detailed analysis at molecular level, kinetic modelling of complex mixtures raises a problem of methodology which clearly demands a scientific approach (Astarita and Sandler, 1991 Sapre and Krambeck, 1991). Petroleum cuts contain so many molecules and isomers, with such large variations in structure and chemical composition that their physico-chemical properties vary almost continuously with the number of carbon atoms. 

As indicated by this brief description, the process of adsorption of atoms and molecules on solid surfaces involves kinetic as well as static aspects. Obviously, the sequence of steps (l)-(5) above is a complex kinetic phenomenon. On the other hand, measuring the physical properties of an adsoibed atom, molecule or fragment concerns the static nature of that species. In both cases the structure and chemical composition of the clean surface is of importance, because the properties of the adsoibed species depend sensitively on the local structure and chemistry of the adsorprion site. Thus the description of adsorbed layers on surfaces is not thinkable without a detailed knowledge of clean surfaces. It is therefore no coincidence that the current volume of Adsorbed Layers follows the Landolt-Bomstein volume on Clean Surfaces. Important data characterizing clean surfaces of metals, semiconductors etc. are collected in the Landolt-Bomstein volumes III/24, subvolumes A-D. 

As later Gilbert Newton Lewis advanced the idea of atomic structure and chemical bonding, Mendeleev (1889) made the necessary passage from atomic periodicity feature to molecular composition by employing the atomic physicochemical features, eventually originating in atomic weights and number (latter with quantum mechanical relevance) ... 

Sen et al. investigated atomic structure and chemical order in GC first coordination shells of Ge and As atoms, chemical order is largely preserved in stoichiometric glasses and those with Se excess but not in heavily Se-deficient Ge fASySe y glasses, as shown in Fig. 4.2. 

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