Edge-driven transition in the surface structure of nanoscale silicon

We present an ab initio exploration of the phenomena that might become important for free-standing structures of silicon as they are realized on the nanoscale. We find that not only surface effects, but also edge effects are important considerations in structures of dimensions ∼3 nm. Specifically, for long nanoscale silicon bars, we find two competing low-energy reconstructions with a transition from one to the other as the cross section of the bar decreases. We predict that this size-dependent phase transition has a signature in the electronic structure of the bar but little effect on elastic properties.