Microfacets

A kinked surface, like fee (10,8,7), can also be approximately expressed in this fomi the step plane (h k / ) is a stepped surface itself, and thus has higher Miller indices than tlie terrace plane. However, the step notation does not exactly tell us the relative location of adjacent steps, and it is not entirely clear how the terrace width M should be counted. A more complete microfacet notation is available to describe kinked surfaces generally [5]. 

Second, as their size increases, these adislands can be limited by two kinds of close-packed atomic rows corresponding to two different types of microfacets (001) for type A and (HI) for type B. In the above mentioned experiments, Wang and Ehrlich have shown that triangular Ir trimers with type B borders are energetically favoured. 

Pozniak B, Mo YB, Stefan IC, Mantey K, Hartmaim M, Scherson DA. 2001. In situ time-resolved second-harmonic generation from Pt(lll) microfacetted single-crystal platinum microspheres. J Phys Chem B 105 7874-7877. 

Figure 1.14 (a) STM image (lOnmx lOnm, tip bias +0.52V, tunneling current 0.5 nA) of a PVBA-induced sawtooth blade in a restructured Ag(l 1 0) surface terrace, (b) Structural model of the chiral kink arrangements induced by lateral interaction of molecular carboxylate end groups with Ag l 0 0 microfacets. (Reprinted with permission from Ref. [48]. Copyright 2004, American Institute of Physics.)... 

Fig. 1 shows a top and a side view of the missing-row reconstructed structure. Along the [001] direction the surface assumes a hill-and-valley profile, where the sides of the hills are actually (111) microfacets. In the (111) orientation, surface atoms are closely packed, therefore such orientations are energetically favored. 

Bilger and Pettinger ]17] have applied second-harmonic generation (SHG) method to study the anisotropy of Au(l 10) electrode in a wide potential range from —0.60 to 0.60 V, corresponding to the transition from the microfaceted to the unreconstructed surface. 

Details of the experiment have been described [77, 92, 106]. The authors examined Ir(110) microfacetted by Ir(lll) and a polycrystalline Pt foil by using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and thermal desorption (TD). 

The (110) surfaces of Au [24], Pt [25] and Ir [26] display (2 x 1) LEED patterns, which are described by missing row reconstructions, in which every other closed-packed atomic row along [110] is missing. The driving force in this case seems to be the formation of (111) microfacets with their lower surface energy [22]. The resulting ID channels have been used as a template for assembling molecular wires , e.g. of the amino acid cysteine [27]. 

Finally, in cases such as the (111) surface of MgO, alternating layers of anions and cations are obtained in the stacking sequence. Figure 8.lie. Here there are two choices of cut, one of which exposes a purely surface (Figure 8.Ilf) and one exposing purely Mg. Either will produce a dipolar slab. This is an example of a type 111 surface which, on a purely electrostatic basis, is unstable and so should not be observed experimentally unless stabiUzed by adsorbates or reconstruction. Indeed FEED analysis has been used to show that the apparent occurrence of the MgO(lll) surface under vacuum conditions can be explained by microfacets of 001 [84]. 

Figure 21.14 MgO (111) polar microfacet. (Reproduced from [45] with permission).

Ill) microfacets ( 3.9). Small ensembles of three-coordinated atoms with a p3Tamidal structure resembling a reconstructed (111) polar surface can in principle form and have been indeed been proposed as the centers where oxygen exchange reactions occur. 

In contrast, other low Miller index surfaces of Ti02 have not attracted so much attention in terms of adsorbate structure determinations. Only two such studies have been reported, both involving Ti02( 100). Fig. 11 shows simple schematic diagrams of the (1x1) and (1x3) phases of this surface, both of which will be mentioned below. The (1x3) reconstruction is known to consist of (110) microfacets from previous work (see Ref. 79 and Refs, therein), whereas the displayed (1x1) structure is merely that expected on the basis of Tasker s rule [80]. 

A model involving discrete bond breaking was proposed for the formation of the (1x3) surface [107,108]. Surfaces were prepared that exhibited both, the (1x1) termination and the ridges typical for the (1x3) surface. STM and non-contact AFM images showed an intermediate phase, which had a (1x3) symmetry, but did not possess the characteristics of the microfaceted structure. Raza et al. [107] suggested that the bonds labeled a in Fig. 15 are broken, which allows the Ti atoms to relax towards the other rows of bridging oxygen atoms. 

Despite the experimental results discussed in the previous section, it is presently not clear if the microfacet model for the (1x3) surface (Fig. 15b) is valid. In fact, recent evidence indicates that it may be a oversimplification. 

In the original literature the facets are referred to as microfacets. However, the length scales involved are in the nanometre range and we prefer the term nanofacet [246]. 

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