Bulk-terminated surfaces

In summary, the alumina nanolayers formed by the high-temperature oxidation on NiAl alloy surfaces are structurally and chemically very different from the bulk-terminated surfaces of the various A1203 phases, and they thus provide very prototypical examples of oxide phases with novel emergent properties because of interfacial bonding and thickness confinement effects. 

All Fe oxide films on Pt have strongly relaxed, unreconstructed bulk-terminated surfaces, but while the Fe304 and Fe203 oxide layers are similar to their respective bulk compounds, the ultrathin FeO layers are true 2D oxide phases that are different from the FeO bulk and stabilized by the metal-oxide interface. 

Another way to describe surface reconstruction is that during a reconstruction the symmetry of the surface atoms change in some way relative to their symmetry in the (relaxed) bulk terminated surface. 

Fig. 1 shows the rocksalt lattice [15]. We will discuss MgO and NiO as limiting cases of oxides, one containing a simple metal ion and the other one a transition metal ion. The (100) surface of such a material represents a non-polar surface, the (111) surface represents a polar oxide surface. Since the lattice constants are very similar for both oxides (MgO 4.21 A, NiO 4.17 A) [15], we expect the surface structures to be similar. The non-polar surface exhibits a nearly bulk terminated surface as shown in Fig. 2a and it is very similar for both materials. We have put together information from FEED [16-21] and STM [22-25] analysis. There is very small interlayer relaxation and only a small rumpling of the surface atoms, whereby the larger anions move outwards and the small cations very slightly inward. A completely different situation is encountered for the polar (111) surfaces. Due to the divergent surface potential [13] on an ideally, bulk terminated polar surface, the surfaee reconstructs and exhibits a so... 

Fig. 16. (a) AES determined near-surface (average) concentration of A1 as function of annealing temperature for the FeAl(l 11) surface (three datasets). The dotted lines estimate the uncertainty introduced by the error in the matrix factor. The phases, which are observed in LEED after quenching the annealed sample to room temperature, are also shown, (b) Comparison of the segregation curves for all investigated surface orientations. Near-surface concentrations corresponding to bulk terminated surfaces are marked by open circles [77]. 

Figure 16. Surface structure the (0001) surface of sapphire (a-A Os) from Eng et al. (2000) with permission of the editor of Science, (a) Structure of vacuum-equihbrated diy surface. Al metal atoms sit on the surface, (b) Ideal bulk terminated surface with no relaxation or reconstraction. (c) Model for the wet-equihbrated surface at one atmosphere from surface scattering (ciystal truncation rod) diffraction measurements. Al metal atoms have shifted, the surface is oxygen terminated, and an organized water monolayer is required to accurately describe the observations, (d) Structure of gibbsite or y-Al(OH)3. The relaxed wet-equilibrated sapphire surface is intermediate between this structure and that of the bulk terminated stracture (B).

Much surface work is concerned with the local atomic structure associated with a single domain. Some surfaces are essentially bulk-terminated, 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 common, 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-terminated surface, figure A1.7.3(bl shows an oscillatory relaxation and figure AL7.3(c ) shows a reconstructed surface. 

Figure Al.7.3. Schematic illustration showing side views of (a) a bulk-terminated surface, (b) a relaxed surface with oscillatory behaviour, and (c) 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-terminated surface, the outermost atoms are in a different electronic environment 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 formed by an adsorbate, as discussed below. 

Figure Bl.21.1. Atomic hard-ball models of low-Miller-index bulk-terminated surfaces of simple metals with face-centred close-packed (fee), hexagonal close-packed (hep) and body-centred cubic (bcc) lattices (a) fee... 

In addition, molecular dynamics simulations at 298 K were used to study the interaction of adsorbed silane molecules (octyltrihydroxysilane, butyltrihydroxysi-lane, aminopropyltrihydroxysilane, and thiolpropyltrihydroxysilane) with the polar ZnO(OOOl) surface [505]. Komherrand coworkers [505] have compared a model of a reconstructed surface with a model of ideal bulk terminated surface. Surface morphology is shown to have little effect... 

A predictive theory of heterogeneous catalysis, therefore, should include the prediction of the restructuring phenomena that occur when a catalyst is brought into its reactive phase. This is especially important when surface roughening occurs with the creation of reactive steps and kinks. Surface reconstruction is driven by the surface s desire to minimize its surface energy. Sometimes a surface which is free of adsorbates will reconstruct its bulk terminated surface, as for instance the hexagonal reconstruction of the Pt(lOO) surface. Surface reconstruction of clean surfaces occurs predominantly on transition metals with spatially extended d-valence atomic orbitals and with high electron occupations such as Pt and Au. 

As already pointed out, the periodicity of a surface can deviate from that of the material s bulk termination. When (as usual) the unit cell is larger than that applying to the bulk-terminated surface, the scenario is called a supedattice. In many cases, this is due to an adsorbate accommodated on the surface (as an... 

For surfaces with chemically mixed layers, the unit mesh contains several different atoms in the bulk-terminated surface, that is, two in the example of CoAl(llO) as illustrated in Figure 4.28c. Even for ideal 1 1 stoichiometry, there is the freedom that the two atoms relax differently without breaking the lateral... 

Figure 4.34 Quasi-hexagonal reconstruction of the Ir(lOO) surface. The top-layer atoms of the bulk-terminated surface (a) rearrange to sixfold intralayer coordination (b). The resulting hexagon (c) is slightly distorted (quasi-hexagon (d)) so that a (5 x 1) coincidence superlattice results (e). By bonding to the quadratic substrate layer, the surface...

The Pt(lll) surface is unreconstructed at moderate temperatures and reconstructs reversibly at 1330 K into an isotropically compressed surface layer [97, 98]. The bulk-terminated surface is under tensile stress, but incorporation of extra atoms takes place only when these atoms are present as adatom gas on the terraces, which requires the high temperature. When they have to be taken from steps, their chemical potential is too high to be overcome below 1330 K, and the surface remains unreconstructed. This view is supported by the fact that the surface reconstructs already at T = 400 K when extra Pt adatoms are deposited [99], and already... 

In order to create a Si(klm) or Ge(klm) surface we have to truncate the crystal along a khn crystallographic plane breaking the covalent bonds. Figure 9.7 illustrates several such bulk-terminated surfaces with a unit cell periodicity of (1 X1), which is determined by the periodicity of the khn crystallographic plane. Such a bulk-terminated surface is called an ideal surface. 

Figure 9.7 Creation of the most important low-index bulk-terminated surfaces by virtual cleaving of a bulk sample along different planes. The (001) surface possesses a square unit cell, the (111) surface a hexagonal unit cell, and the (110) surface...

From each bulk band, at least one surface mode originates, as shown in Figure 9.46. According to Rayleigh s theorem [82], the number of localized surface modes from each bulk band is given by the number of degrees of freedom, which are affected by the surface perturbation. For an ideal bulk-terminated surface, as in the model calculation of Figure 9.46, no more than one surface phonon mode is expected for each bulk band. 

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