Clean surface of solids

The clean siuface of solids sustains not only surface relaxation but also surface reconstruction in which the displacement of surface atoms produces a two-dimensional superlattice overlapped with, but different from, the interior lattice structure. While the lattice planes in crystals are conventionally expressed in terms of Miller indices (e.g. (100) and (110) for low index planes in the face centered cubic lattice), but for the surface of solid crystals, we use an index of the form (1 X 1) to describe a two-dimensional surface lattice which is exactly the same as the interior lattice. An index (5 x 20) is used to express a surface plane in which a surface atom exactly overlaps an interior lattice atom at every five atomic distances in the x direction and at twenty atomic distances in the y direction. 

The surface lattice plane of Pt (lOO)-(l x 1), created by cleavage along the close-packed cubic lattice plane (100) of platinum crystals, transforms into the 

The reconstructed surface (5 x 20) of platinum crystals contains as many atoms as 1.2 times the original surface (1 x 1) atoms, and hence the transformation of surface lattice in the reverse direction from (5 x 20) to (1 x 1) forces the excess siuface atoms to cohere in a striped pattern on the un-reconstructed (1 x 1) surface. 

Surface reconstruction on metal crystals depends on the interior lattice as well as on the nature of the metal, such as Au (100)-(5 x20), Au (lll)-(l x 23) and Pt (llOHl X 2) [Kolb, 1993]. In general, the activation energy of surface reconstruction is relatively great ( 1 eV) on clean metal surfaces so that the reconstruction is frequently suppressed at room temperature. Usually, surface adsorption changes the activation energy that catalyzes or inhibits surface reconstruction. 

Besides metallic crystals, covalent crystals also imdergo surface reconstruction as mentioned in Sec. 2.7 and dangling surface atoms reduce the number of their dangling bonds to stabilize the surface energy, thereby forming a reconstructed surface lattice different from the interior lattice. 

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METHODS OF OBTAINING atomically-clean surfaces of solids are listed with comments on their advantages and limitations. The method of argon-ion bombardment is reviewed with a discussion of the operating conditions and precautions necessary for successful results. The low-energy electron-diffraction method is used to determine the condition of the surface. Experimental results indicate that the relative positions of the atoms in the clean (100) surface planes of germanium and silicon are not the same as those of similar planes in the bulk crystals. 

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