Electronic Band Structure and Surface States

Surface electronic states have been identified in a few cases at the surface of epitaxial silicides. 

Iron Silicides Several metastable and stable iron siUcide phases can be epitaxially grown on Si, as described in Sections 14.2 and 14.3. Epitaxial thick films of FeSi(CsCl) exhibit different surface terminations, depending on the preparation conditions. The terminations are associated with characteristic electronic surface states and are obtained on stepwise annealing of an Fe film deposited at room temperature (RDE) to increasingly hi er temperatures. A first, a (1 x 1) structure 

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One group of techniques is sensitive to electronic structure at the surface, and can probe the electronic band structure and density of states near the Fermi level. This electronic information is useful for understanding the bonding mechanisms responsible for chemical process operating at the surface. Structural information can also be obtained by comparing experimentally observed electronic structure with theoretical calculations of electronic structure for model systems (see part 4). 

To evaluate whether trends in chemisorption energies on Pt nanoparticles are consistent with the d-band model, d-band densities of states were projected out for different adsorption sites to determine the corresponding d-band centers relative to the Fermi level. No correlation was observed between the adsorption energies and site-specific d-band centers. Even though metal nanoparticles possess a continuous electronic band structure and, thus, metal-like electronic properties, their catalytic surface properties are not controlled by band structure effects but by the local electronic structure of the adsorption sites. An important conclusion from this study is that... 

In order to understand how photoemission spectra relate to the electronic band structure and elementary excitation spectra of surfaces, we need to return to the discussion of the photoemission matrix element (Section 3.2.2.1.3) and of what is different when valence electronic states are involved. These states are characterized... 

From the experimental standpoint, the chemical and electronic structures of the surfaces of thin films of the conjugated materials are first studied in their pristine state, in order to generate surface electronic band structure parameters for input into device performance... 

The required 2D nearly free electron gas is realized in Shockley type surface states of close-packed surfaces of noble metals. These states are located in narrow band gaps in the center of the first Brillouin zone of the (lll)-projected bulk band structure. The fact that their occupied bands are entirely in bulk band gaps separates the electrons in the 2D surface state from those in the underlying bulk. Only at structural defects, such as steps or adsorbates, is there an overlap of the wave functions, opening a finite transmission between the 2D and the 3D system. The fact that the surface state band is narrow implies extremely small Fermi wave vectors and consequently the Friedel oscillations of the surface state have a significantly larger wave length than those of bulk states. 

The electronic properties of organic conductors are discussed by physicists in terms of band structure and Fermi surface. The shape of the band structure is defined by the dispersion energy and characterizes the electronic properties of the material (semiconductor, semimetals, metals, etc.) the Fermi surface is the limit between empty and occupied electronic states, and its shape (open, closed, nested, etc.) characterizes the dimensionality of the electron gas. From band dispersion and filling one can easily deduce whether the studied material is a metal, a semiconductor, or an insulator (occurrence of a gap at the Fermi energy). The intra- and interchain band-widths can be estimated, for example, from normal-incidence polarized reflectance, and the densities of state at the Fermi level can be used in the modeling of physical observations. The Fermi surface topology is of importance to predict or explain the existence of instabilities of the electronic gas (nesting vector concept see Chapter 2 of this book). Fermi surfaces calculated from structural data can be compared to those observed by means of the Shubnikov-de Hass method in the case of two- or three-dimensional metals [152]. 

The previous sections have shown that one can work back from band structures and densities of states to local chemical actions—electron transfer and bond formation. It may still seem that the qualitative construction of surface-adsorbate or sublattice-sublattice orbital interaction diagrams in the forward direction is difficult. There are all these orbitals. How to estimate their relative interaction ... 

Metal particles larger than about 100 atoms present an electronic band structure like in the bulk state, when the proportion of surface atoms, however, becomes non-negligible, several differences appear in the band structure. First, the width of the valence band is reduced and, second, its centre of gravity is shifted towards the Fermi level [55,56]. This evolution is a consequence of the reduction of the coordination that is equivalent to an increase in the localization of the valence electrons. This becomes more dramatic if we consider the local density of states on low-coordinated sites like edge and corner atoms. Figure 3.10 shows the calculated density of states on various atoms from a cubo-octahedron Pd cluster containing 3,871 atoms (equivalent to a radius of 5.7 nm) [40]. 

Thus, inter-atomic distances and the atomic state in Tetracarbon are fundamentally different from all the known forms of carbon. The differences between clear and hard diamond on the one hand, and soft and black graphite on the other hand, illustrate the differences among Tetracarbon and other forms of carbon. The distance between the neighboring sp -carbon atoms within the Tetracarbon chain is about 1.3 A, whereas the distance between the carbon chains is 4.80-5.03 A. It is interesting to note that in some respects Tetracarbon is similar to tubulenes, as it can be considered as tubulene in the limit when the diameter of the tube approaches the diameter of carbon atom. Nevertheless, in Tetracarbon the hybridization state of carbon atoms changes from sp to sp. It is basically a new purely one-dimensional sp -carbon modification with one-dimensional electron band structure, whereas tubulene is a quasi-one-dimensional material in which the number of one-dimensional electron bands increases with increasing tubulene diameter. Tetracarbon and tubulene are also similar in that the carbon chains in Tetracarbon are oriented normally to the surface of the film, similar to the orientation in tubulene. 

Although bimetallic catalysts did not represent a totally new area of research in the early 1960s, my research emphasized entirely new aspects of this subject. Earlier work on metal alloy catalysts was dominated by efforts to relate the catalytic activity of a metal to its electron band structure. Very little attention had been given to other aspects of metal alloy catalysts, such as the possibility of influencing the selectivity of chemical transformations on metal surfaces and of preparing metal alloys in a highly dispersed state. These aspects were the basis for my work on bimetallic catalyst systems. 

We summarize our survey as follows. The extraordinary instabilities and fluctuation effects characteristic of a hypothetical one-dimensional metal are quenched rapidly as the effective dimensionality is increased. Hence we should expect that the physics of a nearly one-dimensional conductor should be especially sensitive to the effective interchain coupling. This is particularly true in two-band organic systems based upon the prototype TIF-TCN3, where the one-electron band structure in the undistorted state is nominally seminetall.ic, and the shape of the Fermi surface and density of states at the Fermi level are dominated by interchain charge-transfer integrals. 

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