Dimensionality Dependence of Radiative Recombination in Black Phosphorus from First-principles*

Monolayer black phosphorus is a unique anisotropic 2D material with a sizable direct band gap and high mobility that has promising optoelectronic applications. However, the origin of how excitonic effects and excited state lifetime change with dimensionality has not been well understood. In this work we studied the electronic excitations and the radiative recombination in black phosphorus monolayer, nanoribbons and quantum dots by employing GW approximation (GW) and solving Bethe-Salpeter Equation (BSE). We demonstrate that 1)the exciton wavefunctions in 0D, 1D and 2D nanostructures are similar, which extend more along the armchair direction than zigzag, and reducing the size of nanostructures along the armchair direction significantly affects the exciton energy while that along the zigzag direction does not. 2)The radiative lifetime increases dramatically when the dimension shrinks from 2D to 1D to 0D, because the constraint of energy and momentum conversation makes that the radiative lifetime increases approximately 10³ times for each dimension reduction. This study provides important insights on engineering excited state properties of nanostructure materials.