Atomic geometry, surface structure

From the perspective of this symposium, analysis of the atomic dynamics and electronic structure of surfaces constitutes an even more exotic topic than surface atomic geometry. In both cases attention has been focused on a small number of model systems, e.g., single crystal transition metal and semiconductor surfaces, using rather specialized experimental facilities. General reviews have appeared for both atomic surface dynamics (21) and spectroscopic measurements of the electronic structure of single-crystal surfaces (, 22). An important emerging trend in the latter area is the use of synchrotron radiation for studying surface electronic structure via photoemission spectroscopy ( 23) Moreover, the use of the very intense synchrotron radiation sources also will enable major improvements in the application of core-level photoemission for surface chemical analysis (13). 

The atomic geometry of a surface or interface is, in certain respects, its most fundamental property. Since most surfaces and interfaces are metastable, especially those of technological interest, their composition and structure depends on their process history. Their structures determine, moreover, the "interesting" interfacial properties which are utilized in specific applications, e.g., reactivity and specificity in catalysis or Schottky barrier height in metal-semiconductor contacts. In addition, the interface structure is measurable by one or more of the techniques noted earlier. Therefore the structure of an interface is a measurable link between the process used to prepare it and the electronic and chemical properties which determine its utility. 

Over H, Wasserfall J, Ranke W, Ambiatello C, Sawitzki R, Wolf D, Moritz W (1997) Surface atomic geometry of Si(001) — (2 x 1) A low-energy electron-diffraction structure analysis Phys. 

One advantage of LEED is that the diffraction process filters out effects due to local defects or deviations from long-range order. The contribution of defects to I-V curves is proportional to the first power of the number of defects, while the contribution of the part of the surface with long-range order is proportional to the square of the number of atoms involved, so the LEED beam integrated intensity reflects the equilibrium geometry of the ordered surface structure. 

There are a number of other surface probes that are sensitive to the local geometry of the surface or give important information on surface composition, but which do not give direct information on atomic coordinates in the surface region. These techniques provide vital information needed to construct reasonable models of surface structure, which are needed to interpret data from quantitative techniques such as LEED and SEXAFS. 

Table 8 Mullikcn charges, dipole moments, and quadrupole moments ealeuiated for the Pd and Pt/Zrf) OI I) intetfaces, employing the Hatiree-Foek Hamiltonian, Values refer to the geometry optimised structure at the GGA level, Mulliken charges for the relaxed clean surface arc 0,(1,37 e) Zr (+2,76 e) and Zr, /, (+2,9,6 e). The quadrupole moment corresponds to the operator /, -x72 72, Zr refer to the Zr ion in the subsurface, while O, represents the surface oxygen on W hieh no metal atom is adsorbed.

The technique of low energy electron diffraction (LEED) has been the most widely used tool in the study of surface structure. LEED experiments involve the scattering of monoenergetic and collimated electrons from a crystal surface and detection of elastically diffracted electrons in a backscattering geometry (Figure 2). The characteristic diffraction pattern in LEED arises from constructive interference of electrons when scattered from ordered atomic positions. The diffraction pattern represents a reciprocal map of surface periodicities and allows access to surface unit cell size and orientation. Changes in the diffraction pattern from that of a clean surface can be indicative of surface reconstruction or adsorbed overlayers. 

Articles dealing with the structure and chemistry of solid and crystal surfaces include Tabor (1981) and Forty (1983), who discusses metals and catalysts in particular. The surface of diamond is discussed by Pate (1986), metal oxides by Henrich (1985), transition-metal compounds by Langell and Bernasek (1979), and transition-metal oxides by Henrich (1983). Some of these articles deal with the electronic structures of the surfaces as well as the surface atom geometry the volume edited by Rhodin and Ertl (1979) on the nature of the surface chemical bond and the review paper by Tsukada et al. (1983) on the electronic structure of oxide surfaces concentrate on this aspect. One of the few reviews directed specifically towards minerals is that of Berry (1985). 

Henrich, V. E., and R. L. Kurtz (1981). Surface electronic structure of TiO, atomic geometry, ligand coordination, and the effect of adsorbed hydrogen. Phys. Rev. B23, 6280-87. 

---