Surface chemistry governs ultrafast charge polarization in CdSe quantum dots: A real-time TDDFT study

Abstract

Ligand chemistry plays an important role in tuning the optoelectronic response of cadmium selenide (CdSe) quantum dots, yet the microscopic mechanisms linking ligand–core interactions to charge separation and exciton dynamics remain elusive. In this work, we employ real-time time-dependent density functional theory (rt-TDDFT) simulations to investigate the ultrafast electronic response of (CdSe)33 nanocrystals functionalized with methylamine, acetate, and 1-propanethiol ligands under resonant optical excitation. The time evolution of the dipole moment, Mulliken charge distribution, and orbital populations indicates that ligand identity modulates the amplitude and rate of charge separation as well as the degree of exciton coherence and stabilization. Methylamine ligands promote enhanced charge polarization and exciton-like delocalization through weak Cd–N coupling and field-induced Stark effects, while thiol passivation introduces deep trap states that favor back-transfer and suppress sustained charge separation on the simulated ultrafast time scale. Acetate ligands exhibit intermediate behavior, with shallow O 2p-derived traps and moderate stabilization of the excitonic charge distribution. Within the coherent electronic regime accessed by rt-TDDFT, these results provide atomistic insight into how ligand-induced fields and covalency influence the electronic dynamics in colloidal CdSe, helping to bridge the experimental observations of ligand-dependent photophysics with microscopic charge-transfer mechanisms.