Reconstruction solid surfaces

Fundamental information from vibrational spectra is important for understanding a wide range of chemical and physical properties of surfaces, e.g., chemical reactivity and forces involved in the atomic rearrangement (relaxation and reconstruction) of solid surfaces. Practical applications of HREELS include studies of ... 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

One important aspect not discussed above is the change in atomic structure at a surface. Contrary to the schematic picture of the Si(lll) surface shown in Fig. 14.6, a solid surface is usually not just the end of a perfect crystal. Surfaces reconstruct in response to the changes in the electronic distribution caused by the surface itself. Again, all these changes occur selfconsistently, and in principle, if the total energy for various configurations of atomic structures at a surface could be evaluated, the shifts in the positions of the atoms and the electronic structures of the surface could be determined theoretically. This approach will be discussed in the next section, but the first calculations for reconstructed surfaces were done using experimental determinations of the atomic positions. 

A basic concept in the reconstruction theory of solid surfaces is the soft phonon approach of displacive structural transitions. An essential property of these structural phase transitions is the existence of an order parameter which... 

Figure 3.15. Rearrangement of atomic positions at a solid surface (a) the bulk exposed plane (b) relaxation of the surface plane outward (c) reconstruction (hypothetical) of the outer four atomic planes. (From Ref. 12, with permission from Oxford University Press.)...

The first successful first-principle theoretical studies of the electronic structure of solid surfaces were conducted by Appelbaum and Hamann on Na (1972) and A1 (1973). Within a few years, first-principles calculations for a number of important materials, from nearly free-electron metals to f-band metals and semiconductors, were published, as summarized in the first review article by Appelbaum and Hamann (1976). Extensive reviews of the first-principles calculations for metal surfaces (Inglesfeld, 1982) and semiconductors (Lieske, 1984) are published. A current interest is the reconstruction of surfaces. Because of the refinement of the calculation of total energy of surfaces, tiny differences of the energies of different reconstructions can be assessed accurately. As examples, there are the study of bonding and reconstruction of the W(OOl) surface by Singh and Krakauer (1988), and the study of the surface reconstruction of Ag(llO) by Fu and Ho (1989). 

Determinations of the surface structure by computing the diffraction beam intensities from low energy electron diffraction are concentrated in two frontier areas at present. One is the determination of the surface structures of adsorbed molecules of ever bigger size and the other is the determination of the atomic locations in reconstructed clean solid surfaces. 

It is now well established that the structure of a thermally equilibrated surface may be quite different from that obtained by simply truncating a solid.10 Surface atoms may rearrange to form a structure according to the lowest surface free energy of that surface. The determination of the atomic structure of solid surfaces is one of the important endeavors in surface science, as the atomic structure will decide both the physical and the chemical properties of the surfaces. Surface reconstruction will also play an important role in the crystal growth and epitaxy of thin films. 

FIGURE 4.28 Typical adsorption isotherms of ionic surfactants on a solid surface. [Graph reconstructed from data by Day et al., Adv. Chem. Ser., 79, 135 (1968).]... 

FIGURE 4.29 Adsorption isotherms of nonionic surfactants on the polar and nonpolar solid surfaces. [Graph reconstructed from data by T. F. Tadros, Solid/Liquid Dispersions, Academic Press, New York, 1987.]... 

Again, we assume that the solid surface in question is untarnished. Even so, most surfaces are not ideal. They undergo energy-lowering processes known as relaxation or reconstruction. The former process does not alter the symmetry, or structural periodicity, of the surface. By contrast, surface reconstruction is a surface symmetry-lowering process. With reconstruction, the surface unit cell dimensions differ from those of the projected crystal unit cell. It will be recalled that a crystal surface must possess one of 17 two-dimensional space group symmetries. The bulk crystal, on the other hand, must possess one of 230 space group symmetries. 

Ti(>2 is a basic and simple material, and more than thirty years have passed since the discovery of the Ti(>2 photocatalytic reaction. The wettability of solid surfaces is also a basic and familiar property. It is amazing that the excellent characteristics (high hydrophilicity among other properties) of the already well understood TiC>2 were discovered only recently. In this paper, we introduced the surface reconstruction model of hydroxyl groups as the mechanism for the highly hydrophilic conversion. However, areas for further research still remain. 

The most important difference between particles inside the bulk and in the interfacial layer comes from the surrounding environment of the particles the particles inside the bulk are in an isotropic environment, while those in the interface are in an anisotropic environment thus, in the interlayer, the forces between the particles are unbalanced. To reduce the resulting surface pressure, some additional processes occur that must be taken into account. On clean surfaces (for example, on a solid surface in vacuum), these processes are the bond-length contraction or relaxation and reconstruction of the surface particles (Somorjai 1994). It results in significantly reduced spacing between the first and second layers compared to the bulk. The perturbation caused by this movement propagates a few layers into the bulk. The other effect is that the equilibrium position of the particles changes that is the outermost layers can have different crystal structure than the bulk. This phenomenon is the reconstruction. 

Nanoscopies supplemented with conventional techniques will allow the rational handling of the catalyst/reactive system based on its knowledge at the atoniic/molecular level. The application of nanoscopies in surface chemistry offers the possibility for determining the nanostructure of solid surfaces, surface reconstruction phenomena, to identify the structure of ionic and molecular adlayers, to study the dynamics of these adlayers in their adsorption and desorption at the submonolayer and monolayer (ML) level. Likewise, they are important tools to follow reactions at sohd surfaces in real time in different environments. The reader can get acquainted with the state of the art on these topics in Refs [5-12]. 

Fifth, while little attention has been paid to the accuracy of the force model for the lattice, it is clear that simple primary zone interactions will not suffice as the dynamical simulations become increasingly more accurate. This will be especially true of simulations of reactions at finite surface coverages, and for processes that lead to reconstruction of the solid surface during the course of the reaction (Ladas et al. 1988 Sobyanin and Zhdanov... 

Keywords Oxide surfaces. First Principles, Surfece energy. Structure of clean surfeces. Reconstructions, Solid-gas/vacuum interfaces, Density functional theory. Crystal stoichiometry. Adsorption... 

Two mport mt tmc[uoil changes ihai occur ai solid surfaces are unique and are associatc d with the iwo-diiriensional and anisotropic cnvironnient to wtiich the surface atoms must adjust. These structural changes arc bond-length coniraciitin or relaxation and reconstruct ion- They arc discussed below... 

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