  Density functional theory calculations are performed on phosphorene quantum
dots having different shapes and edge terminations to investigate their
structure stability, electronic properties, and gas sensing ability. All the
selected phosphorene dots, namely hexagonal and triangular flakes with armchair
and zigzag terminations, have positive binding energies which insure their
stability even though the bond lengths are much longer than those in the
infinite phosphorene layer. It is found that all the selected hydrogen
passivated quantum dots have a wide energy gap. In contrast, the partial
passivation with sulfur decreases the gap. Moreover, it transforms the system
from antiferromagnetic to ferromagnetic state. The energy gap of hexagonal
zigzag cluster can be additionally tuned by electric field: narrowed by about
1.7 eV for hydrogenated or broadened by 0.25 eV for partially sulfurated edges.
It is shown that phosphorene quantum dots successfully adsorb H2S, CH4, CO, NH3
gas molecules either on their edge or surface. The highest adsorption energy is
obtained for NH3 molecule, when it is placed over the surface. This adsorption
is alleviated by in-plane electric field and hindered by perpendicular field.
