  Density functional theory calculations have been carried out to investigate
single-layer phosphorene functionalized with two kinds of organic molecules,
i.e. an electrophilic molecule tetracyano-p-quinodimethane (TCNQ) as electron
acceptor and a nucleophilic molecule tetrathia-fulvalene (TTF) as electron
donor. The TCNQ molecule introduces shallow acceptor states in the gap of
phosphorene close to the valence band edge (VBE), which makes the doped system
a p-type semiconductor. However, when the TTF molecule is adsorbed on the
phosphorene, the occupied molecular states introduced into the gap are of deep
donor states so that effective n-doping for transport cannot be realized. This
disadvantageous situation can be amended by applying an external electric field
perpendicular to the phosphorene surface with direction from the phosphorene to
the TTF molecule, under which the TTF-introduced donor states move closer to
conduction band edge (CBE) of the phosphorene and then the TTF-doped
phosphorene system becomes an n-type semiconductor. The effective bipolar
doping of single-layer phosphorene via molecular adsorption predicted above,
especially n-doping against its native p-doping propensity, would broaden the
way to the application of this new type of two-dimensional material in
nanoelectronic and optoelectronic devices.
