Crystal surface reconstruction

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. 

The nature of the surface promoter species has been debated for many years. As MoS2 crystals exist as layered structures, two models evolved. One proposed that the promoter was intercalated deep within the bulk of the MoS2 layers (51) and another proposed that bulk intercalation was not thermodynamically stable and a surface-intercalated structure was more likely (52) (see Fig. 16d). Both proposals related the promotion to crystal surface reconstruction and solid-state chemistry and are valid only for multilayered structures. 

Our low-temperature spectroscopic technique may be, as we have already indicated, very sensitive to surface-state modification. However, it provides only an overall and indirect observation of the crystal-surface reconstruction. For this reason, the conclusions we are proposing here need further confirmation by calculations and by complementary experimental techniques. 

Surface states can be divided into those that are intrinsic to a well ordered crystal surface with two-dimensional periodicity, and those that are extrinsic [25]. Intrinsic states include those that are associated with relaxation and reconstruction. Note, however, that even in a bulk-tenuinated surface, the outemiost atoms are in a different electronic enviromuent than the substrate atoms, which can also lead to intrinsic surface states. Extrinsic surface states are associated with imperfections in the perfect order of the surface region. Extrinsic states can also be fomied by an adsorbate, as discussed below. 

When atoms, molecules, or molecular fragments adsorb onto a single-crystal surface, they often arrange themselves into an ordered pattern. Generally, the size of the adsorbate-induced two-dimensional surface unit cell is larger than that of the clean surface. The same nomenclature is used to describe the surface unit cell of an adsorbate system as is used to describe a reconstructed surface, i.e. the synmietry is given with respect to the bulk tenninated (unreconstructed) two-dimensional surface unit cell. 

In 1985 Car and Parrinello invented a method [111-113] in which molecular dynamics (MD) methods are combined with first-principles computations such that the interatomic forces due to the electronic degrees of freedom are computed by density functional theory [114-116] and the statistical properties by the MD method. This method and related ab initio simulations have been successfully applied to carbon [117], silicon [118-120], copper [121], surface reconstruction [122-128], atomic clusters [129-133], molecular crystals [134], the epitaxial growth of metals [135-140], and many other systems for a review see Ref. 113. 

Pb also crystallizes in the fee system and therefore the same dependence of EamQ on the crystallographic orientation should be expected. Quite surprisingly, Ecm0 varies in the sequence (112) (110)> (100) >(111),135 i.e., exactly the other way round. Although the authors of the measurements do not remark on this apparent anomaly, a possible explanation can be sought in the surface mobility of Pb atoms at room temperature, which may lead to extensive surface reconstruction phenomena. It doesn t seem possible to clarify this aspect for the time being, since the most recent studies on the pzc of Pb single-crystal faces date back almost 20 years. 

From a theoretical point of view, the stability of nanocrystalline diamond was discussed by several authors. Badziag et al. [25] pointed out that, according to semi-empirical quantum chemistry calculations, sufficiently small nanocrystalline diamond (3-5 nm in diameter) may be more stable than graphite by forming C-H bonds at the growing surface. Barnard et al. [26] performed the ab initio calculations on nanocrystalline diamond up to approximately 1 nm in diameter. The results revealed that the surfaces of cubic crystals exhibit reconstruction and relaxations comparable to those of bulk diamond, and the surfaces of the octahedral and cubooctahedral crystals show the transition from sp to sp bonding. 

Van Hove MA, Koestner RJ, Stair PC, Biberian IP Kesmodel LL, Bartos I, Somorjai GA. 1981. The surface reconstructions of the (100) crystal faces of iridium, platinum and gold, 1. Experimental-observations and possible structural models. Surf Sci 103 189-217. 

Bhzanac BB, Arenz M, Ross PN, Markovic NM. 2004b. Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode. J Am Chem Soc 126 10130-10141. 

The significance and impact of surface science were now becoming very apparent with studies of single crystals (Ehrlich and Gomer), field emission microscopy (Sachtler and Duell), calorimetric studies (Brennan and Wedler) and work function and photoemission studies (M.W.R.). Distinct adsorption states of nitrogen at tungsten surfaces (Ehrlich), the facile nature of surface reconstruction (Muller) and the defective nature of the chemisorbed oxygen overlayer at nickel surfaces (M.W.R.) were topics discussed. 

Figure 1,4 The atomic arrangement in the missing row model of the reconstructed iridium (110) crystal surface. From G.A. Somorjai, Chemistry in Two Dimensions, Cornell University Press, London, 1981, p. 146. Used by permission of Cornell University Press.

Given the efforts in this group and others (Table 1) to form the Cd based II-VI compounds, studies of the formation of Cd atomic layers are of great interest. The most detailed structural studies of Cd UPD have, thus far, been published by Gewirth et al. [270-272]. They have obtained in-situ STM images of uniaxial structures formed during the UPD of Cd on Au(lll), from 0.1 M sulfuric acid solutions. They have also performed extensive chronocoulometric and quartz crystal microbalance (QCM) studies of Cd UPD from sulfate. They have concluded that the structures observed with STM were the result of interactions between deposited Cd and the sulfate electrolyte. However, they do not rule out a contribution from surface reconstructions in accounting for the observed structures. 

In summary, LEED is most often used to verify the structure and quality of single crystal surfaces, to study the structure of ordered adsorbates and to study surface reconstructions. In more sophisticated uses of LEED one also determines exact positions of atoms, the nature of defects and the morphology of steps, as well as Debye temperatures of the surface. 

A block model of defects on a single-crystal surface is depicted in Figure 2.4.17 The surface itself in reality is a two-dimensional defect of the bulk material. In addition, one-dimensional defects in the form of steps which have zero-dimensional defects in the form of kink sites. Terraces, which are also shown in the figure, have a variety of surface sites and may also exhibit vacancies, adatoms, and point defects. Surface boundaries may be formed as a result of surface reconstruction of several equivalent orientations on terraces. 

What is surface reconstruction and why is it difficult to prepare MgO (110) and (111) surfaces although MgO crystals having (111) faces occur naturally ... 

Tlie low index svirfaces of platinum single crystals were examined with cyclic voltammetry. As mentioned in Chapter 2, single crystal surfaces are reconstructed by Pt-OH formation and reduction at a higher potential. Therefore, it was impossible to clean the surface thoroughly by potential steps without disturbing the surface structure. 

Well-Defined Surface Structure and Reconstruction of Single-Crystal Surfaces... 

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