Sharp corners in the cross section of ultrathin Si nanowires.

We have determined stable geometries for pristine Si nanowires grown along their 100 axis through systematic density functional studies. Strikingly, Si nanowires with diameters smaller than 1.7 nm prefer a shape that has a square cross section. This stems from dimerization between corner atoms and also from benign reconstruction patterns that maximally saturate Si dangling bonds.