Anisotropic Behavior of Optical Properties in Edge-Modified Phosphorene Quantum Dots

Abstract

The evolution of optical properties in low-dimensional phosphorene quantum dots remains a largely unexplored area, with great potential for applications as optoelectronic devices. Herein, we present a comprehensive analysis of the optical properties of edge-passivated (H, NH2, Cl, OCN, CN) monolayer phosphorene quantum dots using density functional theory and time-dependent density functional theory calculations. An extensive characterization of absorptions, nonradiative relaxations, radiative emissions, radiative lifetimes, and the high-frequency Raman modes (Ag1, B2g, and Ag2) is undertaken, emphasizing the role of material directionality, quantum confinement, and edge passivation on the evolution of these properties. Our results indicate that optical absorptions and emissions are preferred from the armchair direction of phosphorene regardless of the type of edge functionalization or system size, rendering these systems as potential natural linear optical polarizers in the UV–vis region. Larger phosphorene quantum dots have smaller Stokes shifts and less geometric variation upon relaxation, leading to decreased radiative lifetimes. Additionally, we identify an uncharacteristic Raman response associated with dissimilar shifting trends of the Ag peaks that arise from the planar growth of phosphorene quantum dots in the armchair or the zigzag direction. The behavior is attributed to the competing effects of mass coupling and improved bond strength.