Excitonic effect in black phosphorus oxides

Two-dimensional (2D) few-layer black phosphorus (BP) with extraordinary electronic and optical properties is an excellent candidate for optoelectronic applications. However, rapid surface oxidation under ambient environment significantly restricts its practicability. Here, we investigate excitonic effect in few-layer BP oxides via first-principle calculation and effective mass approximation. Influence of layer numbers and degree of oxidation on exciton binding energy (EBE) is discussed in detail for the first time. It is found that EBE in BP oxides decreases exponentially with increasing sample thickness and becomes almost oxygen independent over six layers with values similar to that of pristine BP. Instead, oxidation alters excitation probability of excitons in few-layer BP via a direct/indirect bandgap transition.