Surface steps and defects

Real surfaces will always exhibit a certain number of defects at temperatures above 0 K (Fig. 8.13). This is true in spite of the fact that defects have a positive energy of formation compared to an ideal crystalline surface. What stabilizes these defects is the change in entropy connected with the induced disorder. Therefore, a certain average number of defects — that increases with temperature — will be present. 

As a consequence, real surfaces will not exhibit such evenly sized terrace or evenly spaced kinks as suggested by Fig. 8.12. The terrace-ledge-kink (TLK) model [332] can provide a more realistic description of vicinal surfaces. The distribution of the terrace widths is calculated taking into account the entropic repulsion between ledges. The confinement of a ledge between two neighboring steps (that cannot be crossed) leads to a reduction of the number 

Defects lead to a roughening of crystal surfaces with increasing temperature as already predicted by Burton et al. [335], However, calculations for low-index surfaces yield roughening transition temperatures well above the melting temperature. The reason is the high forma- 

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Finally, the symmetry constraint can be removed by considering a pair sum over substrate atoms as a single contribution to the many-body energy. For example, the periodic contribution of the substrate can be replaced by a sum of contributions from each individual substrate atom . This allows the study of the eflect of features such as amorphous surfaces, steps and defects on surface reactivity, while still retaining a potential derived from a rigid lattice. These types of potentials, however, can become time consuming in their evaluation, and can therefore be inconvenient for use in large-scale computer simulations. 

Figure Al.7.1. Schematic diagram illustrating terraces, steps, and defects, (a) Perfect flat terraces separated by a straight, monoatomic step, (b) A surface containing various defects.

The same theory, i.e. Eqs. (86) and (87), allows us to understand why CO and similar molecules adsorb so much more strongly on under-coordinated sites, such as steps and defects on surfaces. Since the surface atoms on these sites are missing neighbors they have less overlap and their d band wUl be narrower. Consequently, the d band shifts upwards, leading to a stronger bonding. 

CO oxidation is a highly structure-sensitive reaction that needs steps and defect sites. MobUity of CO on the electrode surface does not seem to play a role on... 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

The structure and dynamics of clean metal surfaces are also of importance for understanding surface reactivity. For example, it is widely held that reactions at steps and defects play major roles in catalytic activity. Unfortunately a lack of periodicity in these configurations makes calculations of energetics and structure difficult. When there are many possible structures, or if one is interested in dynamics, first-principle electronic structure calculations are often too time consuming to be practical. The embedded-atom method (EAM) discussed above has made realistic empirical calculations possible, and so estimates of surface structures can now be routinely made. 

The resolution of the atomic force microscope depends on the radius of curvature of the tip and its chemical condition. Solid crystal surfaces can often be imaged with atomic resolution. At this point, however, we need to specify what Atomic resolution is. Periodicities of atomic spacing are, in fact, reproduced. To resolve atomic defects is much more difficult and usually it is not achieved with the atomic force microscope. When it comes to steps and defects the scanning tunneling microscope has a higher resolution. On soft, deformable samples, e.g. on many biological materials, the resolution is reduced due to mechanical deformation. Practically, a real resolution of a few nm is achieved. 

Block and co-workers [35] modified the atom probe to develop a method called pulsed-field desorption mass spectrometry (PFDMS), whereby a short high-voltage pulse desorbs all species present on the tip during a catalytic reaction. The repetition frequency of the field pulse controls the time for which the reaction is allowed to proceed. Hence, by varying the repetition frequency between desorption pulses in a systematic way, one can study the kinetics of a surface reaction [35], In fact, this type of experiment - where one focuses on a facet of desired structure, which may include steps and defects - comes close to one of the fundamental goals of catalyst characterization, namely studying a catalytic reaction on substrates of atomically resolved structure with high time resolution. 

The kinetic and dynamical aspects of the dissociative adsorption of 02 on the Pt(l 1 1), and surfaces vicinal to Pt(l 11), has been investigated in some detail. It provides a good example of precursor mediated dissociation, but is complicated by the fact that both physisorbed and chemisorbed molecular precursor states are involved, and access to the chemisorbed precursor is activated. It is also a good example of the role of step and defect sites in the overall conversion of the precursor states. The adsorption system has the advantage that the characterisation of a number of molecular and atomic states has also been the subject of considerable attention. 

T. Bailey et al.. Step and flash imprint lithography Template surface treatment and defect analysis, J. Vac. Sci. TechnoL, B, 18, 3572, 2000. 

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