We report the results of molecular dynamics simulations for structures of pristine silicon nanowires and germanium nanowires with bulk cores oriented along the [ 110] direction and bounded by the ( 100) and ( 110) surfaces in the lateral direction. We found that the ( 100) surfaces for both silicon and germanium nanowires undergo 2 x 1 dimerization while their ( 110) surfaces do not show reconstruction. The direction of the dimer rows is either parallel or perpendicular to the wire axis depending on the orientation of the surface dangling bonds. The dimer length for Si is in good agreement with the result obtained by first-principles calculations. However, the geometry of Si dimers belongs to the symmetrical 2 x 1 reconstruction rather than the asymmetrical buckled dimers. We also show that surface reconstruction of a small nanowire induces significant change in the lattice spacing for the atoms not on the ( 100) surface, resulting in severe structural distortion of the core of the nanowire.
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