High-step edges

The monoatomic high-step edges, the microsteps, are required for continuous metal electrocrystallization. Possible sources of microsteps on a surface are shown in Figs. 2.7, 2. 8a, and 2.9a, i.e., the low-index planes, two-dimensional nuclei, emergent screw dislocations, and indestructible reentrant grooves [11, 32]. 

Figure 35. Atomic scale UHV STM image of the pyrite (100) surface, from Rosso et al. (2000). TTie tuimeling conditions were -0.2 V sample bias and 2 nA setpoint current. A face-centered cubic surface cell is outlined ( 5.4 A). Half unit cell high step edges are outlined by white dashed lines, and are observed to follow cubic [10] and diagonal [11] surface directions. Fe vacancies are indicated as sites A and B.

Figure 10.17 STM images of the changes in surface structure observed when meth-anethiol is adsorbed at a Cu(110) surface at room temperature, (a) Clean surface with terraces approximately lOnm wide separated by multiple steps, (b) After exposure to 2 L of methanethiol there has been considerable step-edge movement. On the terraces a local c(2 x 2) structure is evident, (c) After a further 7 L exposure, a view of a different area of the crystal shows rounded short terraces these still retain the c(2 x 2) local structure, (d) After 60 L gross changes to the surface are evident and the STM is unable to image at high resolution.

Au is an excellent electrode material. It is inert in most electrochemical environments, and its surface chemistry is moderately well understood. It is not, however, the substrate of choice for the epitaxial formation of most compounds. One major problem with Au is that it is not well lattice matched with the compounds being deposited. There are cases where fortuitous lattice matches are found, such as with CdSe on Au(lll), where the Vs times the lattice constant of CdSe match up with three times the Au (Fig. 63B) [115,125]. However, there is still a 0.6% mismatch. A second problem has to do with formation of a compound on an elemental substrate (Fig. 65) [384-387]. Two types of problems are depicted in Fig. 65. In Fig. 65A the first element incompletely covers the surface, so that when an atomic layer of the second element is deposited, antiphase boundaries result on the surface between the domains. These boundaries may then propagate as the deposit grows. In Fig. 65b the presence of an atomically high step in the substrate is seen to also promote the formation of antiphase boundaries. The first atomic layer is seen to be complete in this case, but when an atomic layer of the second element is deposited on top, a boundary forms at the step edge. Both of the scenarios in Fig. 65 are avoided by use of a compound substrate. 

Coluccia et al. (5) proposed a model of the MgO surface that shows Mg-O ion pairs of various coordination numbers (Fig. 1). MgO has a highly defective surface structure showing steps, edges, corners, kinks, etc., which provide sites of low... 

Figure 2. Simulated STM images of the non-rebonded Sb step edge, (a) the filled-state image and (b) the empty-state image, both under high-current conditions.

Etching preferentially at highly kinked step edges Au-TMTU complex... 

Waibel et al. [459] have studied deposition of Pt on unreconstructed Au(lll) and Au(lOO) applying CV and in situ STM. STM studies revealed that PtCh " complex is adsorbed on both surfaces and Pt is further deposited at rather high overpotential. Nucleation of Pt started mainly at the defects, like step edges, at low deposition rates. Due to the high overpotential, some nuclei also appeared on terraces at the random sites. 

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. 

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