Effects of surface functionalization and size of MXene-based quantum dots on their optical properties: the exciton confinement matters

Abstract

In this study, we explore the influence of surface functionalization and quantum dot size on the electronic and optical properties of MXene quantum dots (MXQDs) using time-dependent density functional theory. Our findings demonstrate that surface terminations have a significant influence on the electronic and optical properties of Ti₂CT₂ (T = O, F, OH) MXQDs. Different functional groups induce notable shifts in both the energy gap and the absorption spectrum. The oxygen termination yields the highest stability and the largest energy gap, while hydroxyl and fluorine terminations shift absorption toward the visible and near-infrared regions. The study also reveals a pronounced blue shift in the absorption spectrum as the Ti₂CO₂ QDs shrink. In addition, small quantum dots with a lateral size of ∼1–2 nm exhibit strong quantum coupling effects, accompanied by an increase in exciton binding energy with delocalization across the quantum dot. The binding energy of the first exciton in MXQDs can achieve up to 75% of its energy gap (in corresponding 2D materials, the typical value is around 25%), which critically influences (increases and shifts) optical absorption.