Atomic real surfaces

For many studies of single-crystal surfaces, it is sufficient to consider the surface as consisting of a single domain of a unifonn, well ordered atomic structure based on a particular low-Miller-mdex orientation. However, real materials are not so flawless. It is therefore usefril to consider how real surfaces differ from the ideal case, so that the behaviour that is intrinsic to a single domain of the well ordered orientation can be distinguished from tliat caused by defects. 

Fig. 1.7 Variation of the value of (pg as the centre of the adsorbed atom moves along a straight line parallel to the surface of a solid and distant Co from it. (---------) For a real surface (-----) for an ideal surface.

All real surfaces will contain defects of some kind. A crystalline surface must at the very least contain vacancies. In addition, atomic steps, facets, strain, and crystalline subgrain boundaries all can be present, and each will limit the long-range order on the surface. In practice, it is quite difficult to prepare an atomically flat surface. 

What gives rise to streaks in a RHEED pattern from a real surface For integral-order beams, die explanation is atomic steps. Atomic steps will be present on nearly all crystalline surfaces. At the very least a step density sufficient to account for any misorientation of the sample from perfeedy flat must be included. Diffraction is sensitive to atomic steps. They will show up in the RHEED pattern as streaking or as splitdng of the diffracted beam at certain diffraction conditions that depend on the path difference of a wave scattered from atomic planes displaced by an atomic step height. If the path difference is an odd muldple of A./2, the waves scattered... 

The comparison of continuum and atomistic models by Luan and Robbins demonstrates that the atomic details of this contact can have a significant influence on the calculated friction. However, those calculations did not explore atomically rough surfaces, which are most likely found in real engineering contacts. The effect of roughness has been investigated recently by Qi et al. in a study of the friction at the interface between two Ni(100) surfaces.85 Two models were considered in that work. In the first model, both surfaces were atomically flat i.e., the rms roughness was 0.0 A. In the... 

Also, a real surface has atomic structures associated with roughness and defects and thus the atoms at these lattice structures have different bonding conditions, some are similar to those on a (100) surface in terms of bonding characteristics some are similar to a (111) surface and others are in between. Thus, a real surface may have varying degrees of reactivity determined by the concentration of the active atoms, which is a function of lattice structure determined by orientation, roughness, and type and density of defects. 

The term surface of a metal usually means the top layer of atoms (ions). However, in this book the term surface means the top few (two or three) atomic layers of a metal. Surfaces can be divided into ideal and real. Ideal surfaces exhibit no lattice defects (vacancies, impurities, grain boundaries, dislocations, etc.). Real surfaces have all types of defects. For example, the density of metal surface atoms is about 10 and the density of dislocations is on the order of magnitude 10 cm . ... 

The structure of real surfaces differs from the structure of ideal surfaces by the surface roughness. Whereas an ideal surface is atomically smooth, a real surface has defects, steps, kinks, vacancies, and clusters of adatoms (Fig. 3.16). 

States due to different biographical structural defects existing on any real surface and playing the part of local disturbances in the strictly periodic structure of the surface (Sec. IX,A). These include vacant lattice sites in the surface layer of the lattice, atoms or ions of the lattice ejected onto the surface, and foreign atomic inclusions in the surface of the lattice (surface impurities). 

One must distinguish between macroscopic and microscopic imperfections existing on a real surface. Macroscopic imperfections are perturbations of the periodic structure covering a region of dimensions considerably greater than the lattice constant. They include cracks on the surface of the crystal, pores, and various macroscopic inclusions. We shall not deal with such imperfections here. Microscopic imperfections are perturbations of dimensions of the order of a crystallc raphic cell. Microscopic imperfections include vacancies in the surface layer of the crystal, foreign atoms or lattice atoms on the surface, different groups of such atoms (ensembles), etc. We shall limit ourselves to a consideration of this kind of imperfection. 

Figure 7.8 shows the current density of H2 evolution on Pt microcrystals [46]. It is intriguing that the activity increases as the particle size decreases, although the current is referred to unit real surface area. The excess increase in activity is definitely to be attributed to especially active surface atoms emerging in very small particles. 

For the atomistic simulation of the kaolinite interface it is assumed that surfaces are planar. Irregularities such as steps, kinks and ledges, which are present on real surfaces, are omitted for the present treatment. For kaolinite the energy of the 001 basal surface was evaluated using a suitable cell containing 425 atoms. 

The determination of the atomic structure of surfaces is the cornerstone of surface science. Before the invention of STM, various diffraction methods are applied, such as low-energy electron diffraction (LEED) and atom beam scattering see Chapter 4. However, those methods can only provide the Fourier-transformed information of the atomic structure averaged over a relatively large area. Often, after a surface structure is observed by diffraction methods, conflicting models were proposed by different authors. Sometimes, a consensus can be reached. In many cases, controversy remains. Besides, the diffraction method can only provide information about structures of relatively simple and perfectly periodic surfaces. Large and complex structures are out of the reach of diffraction methods. On real surfaces, aperiodic structures such as defects and local variations always exist. Before the invention of the STM, there was no way to determine those aperiodic structures. 

There is a convenient mathematical idealization which asserts that a cube of edge length, / cm, possesses a surface area of 6 f cm and that a sphere of radius r cm exhibits 4nr cm of surface. In reality, however, mathematical, perfect or ideal geometric forms are unattainable since under microscopic examinations all real surfaces exhibit flaws. For example, if a super microscope were available one would observe surface roughness due not only to the atomic or molecular orbitals at the surface but also due to voids, steps, pores and other surface imperfections. These surface imperfections will always create real surface area greater than the corresponding geometric area. 

Another area where nanocrystalUnity is important is that of surface effects due to the high real surface areas of such films (often tens of percent of all atoms... 

Diffusion into the electrode. If the surface radical is H, there may be diffusion into the electrode and this may cause a change in the character of the surface and the atoms immediately beneath it. Hence, for surface-catalyzed reactions on real surfaces, finding the steady state in the i—t curve at constant potential may show complexities (Fig. 7.44). Where is the steady state in Fig. 7.44(b) It becomes a matter of judgment The best plan is to take the first time-invariant section and to reject the further variations, which simply indicate a nonconstant surface.44... 

The diameter of the Pt atom can be calculated from the density as 2.5 x 10 cm postulating that the adsorption following the metal packing (rather than forming a compact layer on the surface, with each adsorbate atom touching another) gave 1.6 x 1 O 5 H atoms/real cm2. To remove this number of atoms from the surface according to ... 

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