Discovery of novel Si nanostructures would open up a new avenue for science and technology as the discoveries of C60 and carbon nanotubes did. With this expectation, we have explored novel Si nanostructures by combining empirical moleculardynamics simulations and structure optimizations with the density functional theory1,2,3,4. Our molecular-dynamics simulations demonstrate (1) an icosahedral Si nanodot forms by freezing a droplet in vacuum1, (2) Si-fullerene-linked nanowires, such as Si16- and Si20-linked nanowires, form by freezing liquid Si inside carbon nanotubes2, and (3) a polyicosahedral Si nanowire forms by freezing liquid Si inside a cylindrical nanopore3. The unique cage structure of the polyicosahedral Si nanowire allows us to tune the electronic properties by encapsulating guest atoms into its cages. Our density functional theory calculations reveal that a semiconducting hydrogen-terminated polyicosahedral Si nanowire becomes metallic by the sodium and iodine doping4.