  Recent reports on the fabrication of phosphorene, i.e., mono- or few-layer
black phosphorus, have raised exciting prospects of an outstanding
two-dimensional (2D) material that exhibits excellent properties for nanodevice
applications. Here we study by first-principles calculations the adsorption of
CO, CO2, NH3, NO and NO2 gas molecules on a mono-layer phosphorene. Our results
predict superior sensing performance of phosphorene that rivals or even
surpasses other 2D materials such as graphene and MoS2. We determine the
optimal adsorption positions of these molecules on the phosphorene and identify
molecular doping, i.e., charge transfer between the molecules and phosphorene,
as the driving mechanism for the high adsorption strength. We further
calculated the current-voltage (I-V) relation using a non-equilibrium Greens
function (NEGF) formalism. The transport features show large (one to two orders
of magnitude) anisotropy along different (armchair or zigzag) directions, which
is consistent with the anisotropic electronic band structure of phosphorene.
Remarkably, the I-V relation exhibits distinct responses with a marked change
of the I-V relation along either the armchair or the zigzag directions
depending on the type of molecules. Such selectivity and sensitivity to
adsorption makes phosphorene a superior gas sensor that promises wide-ranging
applications.
