Step edge mobility

Iodine and bromine adsorb onto Au(l 11) from sodium iodide or sodium bromide solutions under an applied surface potential with the surface structure formed being dependent on the applied potential [166]. The iodine adsorbate can also affect gold step edge mobility and diffusion of the Au surface. Upon deposition of a layer of disordered surface iodine atoms, the movement of gold atoms (assisted by the 2-dimensional iodine gas on the terrace) from step edges out onto terraces occurs. However, this diffusion occurs only at the step edge when an ordered adlayer is formed [167]. 

Step-mobility-limited models can be further separated into two limits conserved and non-conserved [20]. This terminology refers to the local conservation of mass transport is said to be conserved if a surface defect generated at a step edge eventually annihilates at the same step or at one of the two adjacent steps. Thus, the motion of adjacent steps is coupled. The 1-D conserved model of Nozieres [21] predicts T a L, independent of Zo. On the other hand, in a non-conserved model the motion of adjacent steps is uncorrelated surface defects generated at a step edge can annihilate at any step edge on the surface. Uwaha [22] has considered this case and found x a L L/zay. In the discussion below, we will use these two limiting cases of step-mobility-limited models [21, 221 to extract the step-mobilities on Si(OOl) and Ge(OOl) surfaces from experiments on relaxation kinetics. 

Fig. 12 Looking from scans (a) to (d) one can observe induced mobility of Pt away from the step edges. The Pt forms islands shown by the right arrow in (d). The formation of the island results in leaving a single atom deep hole as shown by the left arrow in (d). ... 

The selectivity of the nickel(l 1 1) surface may thus be controlled by modification of the number of free step sites, and this notion was tested experimentally by blocking the steps with small amounts of silver (84). In other STM investigations it was found that when silver was deposited on nickelfl 1 1) at room temperature, the silver preferentially nucleated and grew as islands at the step edges. When this system was post-annealed to 800 K, the silver atoms were observed to become highly mobile and decorate all the step edges of nickelfl 1 1), as shown in Fig. 6b. 

Adsorption Due to their mobility, most C6H5I molecules are adsorbed at the Cu step-edges even at 20K. Therefore, they are easy to locate. 

Recently, monatomic steps, which look frazzled in STM images, were observed by several authors [2.19-2.22]. An example for a monatomic step on Ag(lll) is shown in Fig. 2.10. This phenomenon was attributed to a mobility of kink sites on step edges. 

The reactivity of surface material depends, to some extent, on its crystallographic environment. The mobility of a species may vary from one set of crystal faces to another. Individual crystal constituents tend to coordinate with a maximum number of nearest neighbours. Coordination on surfaces is necessarily unsymmetrical and isolated constituents tend to migrate to sites such as step edges where stability can be enhanced by increased coordination. Surface mobility is thus often appreciable at temperatures well below the melting point. Such behaviour is intermediate between solid and liquid states and is important both in sintering and as a transfer mechanism in solid-solid reactions. Migration across surfaces may be complicated by concurrent adsorption and desorption processes. The surface is thus a zone of... 

After application of a single potential step/scan towards negative potentials, the ordered 2,2 -BP adlayer dissolves, and the entire substrate surface appears very mobile. Fractal, monatomic high gold features grow at step edges (Fig. 38c), their surface... 

Step (6.49) preferably occurs at the step edges and is fast. This implies that OHads builds up concentration at the step edges. The CO binds to strong on step edges to react. The more weakly bonded, mobile CO adsorbed to terrace, therefore, reacts first with adsorbed OH. 

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