First-principles Study of the Interactions of Electron Donor and Acceptor Molecules with Phosphorene

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.