  Two dimensional few-layer black phosphorus crystal structures have recently
fabricated and demonstrated great potential in applications of electronics. In
this work, we employed first principles density functional theory calculations
to study the edge effects and quantum confinement on the electronic properties
of the phosphorene nanoribbons (PNR). Different edge functionalization groups,
such as H, F, Cl, OH, O, S, and Se in addition to a pristine case, were studied
for a series width of the ribbon up to 3.5 nm. It was found that the
armchair-PNRs (APNRs) are semiconductors for all edge groups considered in this
work. However, the zigzag-PNRs (ZPNRs) show either semiconductor or metallic
behavior in dependence on their edge chemical groups. Family I edges (H, F, Cl,
OH) form saturated bonds with P atoms and the edge states keep far away from
the band gap. However, Family II edges (pristine, O, S, Se) form weak
unsaturated bonds with the pz orbital of P atoms and bring edge states within
the band gap. These edge states of Family II ribbons present around the Fermi
level within the band gap, which close up the band gap of the ZPNRs. For the
APNRs, these edge states are at the bottom of the conduction band and result in
a reduced band gap.
