Stepped faces

In this way two objectives ensue 1. We obtain the growth mechanism of the most important flat and stepped faces of the crystal. 2. We are able to foresee the global crystal morphology in a steady state for all temperature and supersaturation values. 

The idea that catalyst surfaces possess a distribution of sites of different energies has been around since the 1920s, but it has not been possible until fairly recently to show that adsorption sites on terraces, steps, and kinks differ in energy. For example, hydrogen shows stronger bonding to steps and kinks on platinum than on the 111 terraces. In addition, the activation energy for H2 dissociation is about zero on the step face and about 8.4 kJ mole-1 on the terrace plane. In addition, carbon monoxide is adsorbed with dissociation on the kinks of Pt, but in the molecular form on the steps and terraces. 

The closely related [Ni35(C)4(CO)39]6 (398), isolated from the products obtained by heating [Ni6(CO)i2]2 in CH2C12, has a structure formed by formal addition of the extra Ni atom to either of the two centrosymmet-rically related stepped faces in the Ni34 cluster.578 The CO stereochemistry is the same as found for the Ni34C4 cluster, except for the extra Ni(CO)... 

These faces are called stepped faces. For instance, the (221) face which is 15.79° from the (111) face on the zone from (111) to... 

Accepting this TLK model, a very convenient notation was proposed for stepped surfaces (see Appendix of Ref. 18). Thus a stepped surface is called n(/i,/c,/,) - (hskJs), where (fcfkf/f) represents the CO of the terraces, (Kksls) represents the step face, and n denotes the width of the terraces in number of atoms. For instance, (221)... 

It seems that for all clean metals the case of stepped faces is different from that of low-index faces. The monoatomic steps would be stabilized by repulsive forces therefore they would be less stable for a large width of terrace than for a small width of terrace. ... 

The two sisters were virtually clinging together at the bottom of the steps, faces heavy with doubt and dismay. "Yes, Mother," Louise said meekly. She started to tug on Genevieve s black riding jacket. 

Grind the step face to 28 mm thickness, taking care to avoid hitting the adjacent face with the side of the grinding wheel. 

Fig. 8.28. Plots of surface energy density versus biaxial mismatch strain for (105) vicinal surfaces with steps aligned with the [010] direction on (001) Si. The stepped edges are serrated in both cases by removing every other step-edge atom, which leaves two possibilities for dimer reconstruction on the terraces. In the configuration proposed by Khor and Das Sarma (1997), the dimer nearest the step edge rebonds directly into the step face, whereas the configuration proposed by Mo et al. (1990) presumes the alternate dimer reconstruction. The plot is based on compntations by Shenoy et al. (2002) using the Tersoff potential.

Between 900°C and 1100°C the overall morphology undergoes a marked change. Whereas crystallites produced at 900°C or below have nonequilibrium mean faces based largely on (111) and (110), those produced at 1100°C exhibit the influence of some more rapid and less precise transport process in crystallite shapes more closely approaching cubes but bounded by coarsely stepped faces vapor phase transport is likely. 

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