A First-Principles Study on Electron Donor and Acceptor Molecules Adsorbed on Phosphorene

Abstract

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 tetrathiafulvalene (TTF) as electron donor. The TCNQ molecule introduces shallow acceptor states in the gap of phosphorene close to the valence band edge, which makes the doped system become 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 out-of-plane electric field with direction from phosphorene to TTF, or an in-plane tensile strain, or their combination, under which the conduction band edge of the phosphorene moves closer to the TTF-derived donor states, and then the TTF-adsorbed phosphorene system becomes an n-type semiconductor. It is also noted that the out-of-plane electric field and in-plane strain can modulate the band gap of the TTF-adsorbed phosphorene markedly. The effective bipolar doping of single-layer phosphorene via molecular adsorption, especially n-doping against its native p-doping propensity, and the good response of band gap in the infrared waveband of the TTF-adsorbed phosphorene to the out-of-plane electric field and in-plane strain would broaden the way to the application of this new type of two-dimensional material in nanoelectronic and optoelectronic devices.