Crystal reconstruction

It has been demonstrated that STM can operate not only in vacuum but also in air [37] and in electrolytes [38]. For example, Sonnenfeld et al. [37] monitored the in situ deposition of silver on graphite and Arvia et al. [39] monitored the effect of repetitive potential cycling on gold. On the other hand, Fan and Bard [40] studied the in situ STM measurements for the corrosion of stainless steel and the dissolution of nickel under potentiostatic conditions. Szklarczyk and Bockris [41] have published an account of the potential dependence of the crystal reconstruction on platinum in the sodium perchlorate solution at the angstrom scale. The atomic lattice of Al( 111) has been reported... 

In ionic crystals, reconstruction effects can also be involved in the stabilization of polar surfaces (Tasker s type 3). For instance, the (100) surface of the fluorite-type crystal of Li20 becomes stable if half of the Li atoms are moved from the bottom face of the slab to the top face above the oxygen atoms to produce a zero-dipole structure (Figure 39). In fact, this kind of surface has been observed experimentally. ... 

The electronegativity, the scale of lattice constant, and the geometrical orientation of the host surface determine the specific details of the adsorbate facilitation, bond ordering, and the orientation of the tetrahedron. This process gives rise to the versatile modes of crystal reconstruction and surface morphologies. For the analyzed representatives of transition metals, noble metals, and a... 

Knowing the lattice is usually not sufficient to reconstruct the crystal structure. A knowledge of the vectors (a, b, c) does not specify the positions of the atoms within the unit cell. The positions of the atoms withm the unit cell is given by a set of vectors i., = 1, 2, 3... u where n is the number of atoms in the unit cell. The set of vectors, x., is called the basis. For simple elemental structures, the unit cell may contain only one atom. The lattice sites in this case can be chosen to correspond to the atomic sites, and no basis exists. 

Much surface work is concerned with the local atomic structure associated with a single domain. Some surfaces are essentially bulk-temiinated, i.e. the atomic positions are basically unchanged from those of the bulk as if the atomic bonds in the crystal were simply cut. More coimnon, however, are deviations from the bulk atomic structure. These structural adjustments can be classified as either relaxations or reconstructions. To illustrate the various classifications of surface structures, figure A1.7.3(a ) shows a side-view of a bulk-temiinated surface, figure A1.7.3(b) shows an oscillatory relaxation and figure A1.7.3(c) shows a reconstructed surface. 

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 the discussion so far, an ideal termination of the bulk crystal has been assumed at the surface that is, the positions of atoms in the surface have been assumed to be the same as what they would have been in the bulk before the surface was created. This may not be true. Reconstruction, a rearrangement of atoms in the surfrce and near-surface layers, occurs frequently. It is caused by an attempt of the surface to lower its free energy by eliminating broken bonds. The atomic layers par-... 

Often (adsorption, reconstruction) the periodicity at the surface is larger than expected for a bulk-truncated surface of the given crystal this leads to additional (superstructure) spots in the LEED pattern for which fractional indices are used. The lattice vectors bi and b2 of such superstructures can be expressed as multiples of the (1 X 1) lattice vectors ai and Zx. ... 

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. 

The experimental investigation was performed by depositing copper films on the (100) -surface of a platinum single crystal. It was found that the reconstruction of the Pt surface was lifted upon Cu adsorption. The system was then heated to different temperatures and the formation of different ordered surface alloys was evidenced by... 

The main problem in the analysis of E vs. 0 plots is that the two quantities are usually measured independently on different samples. It may happen that the surface structure differs somewhat so that for the sample on which E is measured is different from that of the sample used in UHV experiments. This is especially the case with polycrystalline surfaces, whose structural reproducibility is occasional, but it is also the case with well-defined crystal faces if reconstruction phenomena are possible.60 The problem persists also in the absence ofreconstruction since the concentration and/or distribution of surface defects may be differ-... 

Over the past 10 years it has been demonstrated by a variety of in situ and ex situ techniques187,188 485 487 488 534 that flame-annealed Au faces are reconstructed in the same way as the surfaces of samples prepared in UHV,526-534 and that the reconstructed surfaces are stable even in contact with an aqueous solution if certain precautions are taken with respect to the potential applied and the electrolyte composition 485,487,488 A comprehensive review of reconstruction phenomena at single-crystal faces of various metals has been given by Kolb534 and Gao etal.511,513... 

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. 

Figure 2.3 3D-TEM reconstruction of (a) severely steamed and subsequently acid-leached Y zeolite [22] and (b) desilicated ZSM-5 zeolite crystal [24]. The mesopores in the crystal are shown as lighter gray tones.

Figure 4.25 shows an example LEED pattern that was obtained after exposing a dean single crystal of Fe(lll) to nitrogen under conditions where N2 dissociates and the surface becomes saturated with N atoms. The surface is believed to reconstruct significantly to accommodate these high coverages. 

---