  Atomic and electronic structures of phosphorene nanoribbons are studied
within density functional theory. These novel materials present different
physical phenomena expected in two very different physical systems: one
dimensional metallic chains and semiconductor surfaces. While `rugged'
nanoribbons are semiconducting in their layer-terminnated structures, pure
`linear' and `zigzag' nanoribbons are metallic due to metallic edge states.
Linear nanoribbons undergo edge reconstruction and zigzag nanoribbons beyond a
certain width undergo Peierls transition leading to opening of a band gap in
the electronic structure and lowering of total energy. Mixed nanoribbons with
linear and zigzag edges on the two sides turn out to be a curious case that
display both edge reconstruction and Peierls transition simultaneously. Most
phosphoeren nanoribbons turn out to be semiconductors having important
implications for their application.
