  Density functional theory and molecular dynamics simulations have been used
to optimize the structure of nanowires of SiO2. The starting structures were
based on b-cristobalite, orthotridymite, b-tridymite, and rutile crystals. The
analysis of the electronic structure has been validated by many-body
perturbation calculations using the G0W0 and GW + Bethe-Salpeter equation
approximations, in order to account for quasi-particle and excitonic effects.
The calculations indicate that many of these nanowires have semiconducting
character, while the corresponding bulk solids are insulators. In the case of
thick rutile-like nanowires we found the gap congested with surface states.
Electronic charge is transferred from silicon atoms to oxygen atoms, and the
bonding in the nanowires is partly ionic and partly covalent. The magnitude of
the band gap can be engineered by changing the structure and the thickness of
the nanowires.
