Surfaces atomic structure

The three-dimensional synnnetry that is present in the bulk of a crystalline solid is abruptly lost at the surface. In order to minimize the surface energy, the themiodynamically stable surface atomic structures of many materials differ considerably from the structure of the bulk. These materials are still crystalline at the surface, in that one can define a two-dimensional surface unit cell parallel to the surface, but the atomic positions in the unit cell differ from those of the bulk structure. Such a change in the local structure at the surface is called a reconstruction. 

Aono M and Souda R 1985 Quantitative surface atomic structure analysis by low energy ion scattering spectroscopy Japan. J. Appl. Phys. Part 1 24 1249-62... 

Surface atomic structure. The integrated intensity of several diffracted beams is measured as a fimction of electron beam energy for different angles of incidence. The measurements are fitted with a model calculation that includes multiple scattering. The atomic coordinates of the surfiice atoms are extracted. (See also the article on EXAFS.)... 

J. B. Pendry. Low-Energy Electron Difraction. Academic Press, New York, 1974. Theoretical treatment, principally on surface atomic structure determination. 

Part of a 15-nm long, 10 A tube, is given in Fig. 1. Its surface atomic structure is displayed[14], A periodic lattice is clearly seen. The cross-sectional profile was also taken, showing the atomically resolved curved surface of the tube (inset in Fig. 1). Asymmetry variations in the unit cell and other distortions in the image are attributed to electronic or mechanical tip-surface interactions[15,16]. From the helical arrangement of the tube, we find that it has zigzag configuration. 

Surface relaxation thus has several effects. It modifies and reconstructs the surface atomic structure. Surface energies are reduced (possibly by as much as a factor of three in the above example - from 6.0 to 2.0 J m-2). More generally, it can reorder the relative stability of different surfaces and thus have a profound effect on the crystal morphology. 

Fig. 22. ETEM at 180°C in N2, illustrating the stability of gold nanorods, for nanoelectronics and catalysis applications. Gold atomic layers and surface atomic structures are visible. Surface of gold nanorod at room temperature showing twin defect lamellae on the atomic scale. They indicate interaction of the surfactant with the (110) surface forming twins to accommodate the shape misfit between the two.

Fig. 1. Principle of scanning tunneling microscopy. A sharp needlelike tip probes the surface atomic structure ofaspecimenby closely scanningthe surface, utilizing extreme sensitivity of the vacuum tunneling current to the tunneling gap. From Sakurai et al. (10) with permission.

Barth, J. V., Bume, H., Ertl, G., and Behm, R. J. (1990). Scanning tunneling microscopy observations on the reconstructed Au(lll) surface Atomic structure, long-range superstructure, rotational domains, and surface defects. Phys. Rev. B 42, 9307-9318. 

The tungsten (110) surface is one of the best studied of all surfaces, especially in field emission and field ion microscopy for many reasons. It is a very stable surface without surface reconstruction or phase transformation. It is also inert to contaminations. For the study of adatom-adatom interactions, it is a very smooth plane with the largest density of adsorption sites available of any W surface. Lesser restrictions are imposed on the adatom-adatom separation. As the surface is structurally very smooth, wave mechanical interference effects are least affected by the surface atomic structure. 

Fig. 4. 55 When few H3 ions are formed, the field dependence of the abundances of Hj and H+ is dependent neither on the atomic structure of the surface nor on the chemical species of the emitter surface, indicating that formation of these two ionic species depends only on the applied field. (b) Formation of H3", on the other hand, depends on both the surface atomic structure and the chemical species of the emitter tip, indicating that Hj is a surface and field catalyzed chemical...

We have seen that electron microscopy and scanning probe microscopies are very complementary techniques to characterize the structure and the morphology of supported clusters. The internal structure can only be resolved by HRTEM while the surface atomic structure can be only revealed by STM or AFM. TEM gives accurate diameter measurements and height can only be measured in profile view that needs special sample preparation. STM or AFM give accurate height measurements but diameters can be obtained only after correction from the tip-sample convolution effect. 

In this chapter, we first recall the basis of X-ray scattering under grazing incidence. The diffraction by a surface is next considered, with methods to quantitatively determine the surface atomic structure (roughness, relaxation or reconstruction). Experimental conditions are next discussed, in particular the sample requirements to obtain reliable quantitative measurements. 

Getting a precise knowledge of the MgO(OOl) surface atomic structure is also important because this surface has been chosen as a model system for... 

Low energy electron diffraction (LEED) is a commonly used probe of surface atomic structure [25-28]. In its simplest form, one determines the... 

Surface atomic structure of bulk PtsSn alloys... 

In most - but by no means all - studies of binary alloy systems reported so far, qualitative LEED data indicate that the surface unit mesh corresponds to what expected from truncation of the bulk lattice [5]. The observation of the expected pattern in LEED in itself is no proof that the surface atomic structure is actually the bulk truncation one. Furthermore, in the case of ordered intermetallic compounds, the bulk termination model is not normally univocal since the plcuies stacked along a specific crystallographic direction do not necessarily have all the same composition. In the case of fee CusAu (LI2) ordered compounds (Fig. 1) all the crystallographic directions, except the (111) have an. ..ABAB... stacking with - for instance in the case of PtsSn - a plane of pure Pt alternating to a plane of composition PtSn. Both terminations correspond to bulk truncation and in both cases the composition of the outermost plane is different from the average one of the bulk. 

Jakob and Chabal reasoned that the silicon atoms, such as those at the steps and kink sites, that are most physically accessible are preferentially attacked. The reactants and the dissolved complex have certain physical dimension and orientation so that certain pathways may be forbidden due to steric constraints. While this argument is intuitively sound, its verification requires information on the solvation structure of the involved species and their interaction with the surface atomic structures. 

Almost all the characterizations performed by us until now are ensemble characterizations (i.e. probing many nanostructures simultaneously). HRTEM and HRS EM do probe the structure (and elemental composition) of individual nanostructures, but they do not correlate this structure with a specific property. STM and STS measurements are real single-object measurements that reveal the size, shape, and surface atomic structure, as well as the electronic density of states (deduced the I-V characteristics). The STM/STS measurements offer a way to correlate the electronic properties of SiNWs with the nanostructure size. 

The surface atomic structure of silica gel was also simulated in Refs. [17, 18]. Silica gel is another form of amorphous silica which is formed not in the process of cooling of the liquid but as a result of coagulation at room temperature. Its surface is the surface of small microspheres which together form an irregular porous structure. We consider here only the simulation of the surface atomic structure presented in Refs. [17, 18]. The pore structure of such a material clearly depends on the arrangement of the microspheres in space. Together with the surface atomic structure, the pore structure determines the adsorption properties of silica gel and we consider it in this context in the next section. 

Structural sensitivity of the catalytic reactions is one of the most important problems in heterogeneous catalysis [1,2]. It has been rather thoroughly studied for metals, while for oxides, especially for dispersed ones, situation is far less clear due to inherent complexity of studies of their bulk and surface atomic structure. In last years, successful development of such methods as HREM and STM along with the infrared spectroscopy of test molecules has formed a sound bases for elucidating this problem in the case of oxides. In the work presented, the results of the systematic studies of the bulk/surface defect structure of the oxides of copper, iron, cobalt, chromium, manganese as related to structural sensitivity of the reactions of carbon monoxide and hydrocarbons oxidation are considered. 

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