Bonding dangling bonds, reactivity

Fig. 8.15. Model for etching mechanism of acidic and alkaline solutions on polar ZnO faces [109]. The dangling bonds at the surfaces exhibit partially positive (<5+ for Zn) or partially negative (

Silicon forms a diamond lattice. As Si single crystals are cut, some of the sp3 bonds that stabilize the diamond structure are cleaved, resulting in the creation of unsaturated, or dangling bonds at the surface. To reduce its surface energy, the system tends to minimize the number of dangling bonds. In this process, new bonds between adjacent Si atoms are formed, associated with geometric surface reconstruction. The reactivity of specific Si surface sites strongly depends on their... 

Quantum-chemical ab initio calculations have become an alternative to experiments for determining accurately structures, vibrational frequencies and electronic properties as well as intermolecular forces and molecular reactivity.28-31 Two specific approximations were developed to solve the problems of surface chemistry periodic approximation, where quantum-chemical method employs a periodic structure of the calculated system and cluster approximation, where a model of solid phase of finite size is created as a cutoff from the system of solid phase (it produces unsaturated dangling bonds at the border of the cluster). Cluster approximation has been widely used for studying interactions of molecules with all types of solids and their surfaces.32 This approach is powerful in calculating the systems with deviations from the ideal periodic structure like doping and defects. 

Dangling bonds at the surface make bare clusters highly reactive and hence experimentally difficult to generate and to study. Clusters with surfaces passivated by other elements are more natural and easier to handle. They are, however, more difficult to treat theoretically. As with mixed clusters, various different compositions have to be generated and checked, and suitable interparticle potentials for the different species have to be available. Here, accuracy requirements for these potentials may even be greater, since presence and structure of the outer passivation layer may subtly influence the structure preferences of the whole cluster. Therefore, only a few studies on such systems have appeared so far. 

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