  Using first-principles density functional theory calculations, we investigate
the geometries, electronic structures, and thermodynamic stabilities of
substitutionally doped phosphorene sheets with group III, IV, V, and VI
elements. We find that the electronic properties of phosphorene are drastically
modified by the number of valence electrons in dopant atoms. The dopants with
even number of valence electrons enable the doped phosphorenes to have a
metallic feature, while the dopants with odd number of valence electrons keep
the semiconducting feature with a larger band gap than the undoped phosphorene.
This even-odd behavior is attributed to the peculiar bonding characteristics of
phosphorene and the strong hybridization of sp orbitals between dopants and
phosphorene. The calculated formation energies of various substitutional
dopants in phosphorene show that such doped systems can be thermodynamically
stable.
