Dislocations and the reconstruction of (111) fcc metal surfaces

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Abstract
It is shown that a pair of hexagonal dislocation structures can be used as a basis for understanding the herringbone pattern and the node networks that have been observed in the structure of several (111) metal surfaces and overlayers. The hexagons are composed of the dissociation patterns of perfect edge dislocations, which are derived from the well-known concepts of the Thompson tetrahedron. At each corner there is a unit threading dislocation; the actual location of this dislocation alternates between the inner and outer Shockley partial dislocation. One of the hexagons will form when there is an excess of vacancies in the surface layer, the other when there are excess atoms. The hexagons can each be reduced to give a corresponding threefold node also terminated by threading dislocations. From such a model, structural characteristics of (111) surface reconstructions such as the presence of perfect edge dislocations on only one side of the stacking fault, can be understood. Whether the herringbone pattern or the node array will be observed in a particular case will depend on the details of the specific systems. In particular, the balance between the mechanisms of the dislocation pattern formation and the equilibrium configuration may be important. The threading dislocations play a critical role in these processes.