Surface Ligand Vibrations Resolve Exciton Fine Structure in Quantum Dots

Abstract

Surface ligand vibrations govern charge delocalization and nonradiative relaxation in colloidal quantum dots, yet direct experimental access to ligand–exciton coupling remains elusive. In this study, we use polarization-selective two-dimensional electronic–vibrational spectroscopy (2D EV) to resolve coupled electronic and vibrational (vibronic) degrees of freedom in oleate-capped zincblende cadmium sulfide quantum dots. The 2D EV spectra map the vibronic couplings between the carboxylate stretching vibrations (1400–1600 cm–1) and nearly degenerate bright and dark excitonic fine structure states separated by 45 meV. The measurements reveal clear vibrational-coordinate dependence on the exciton resonance frequency, distinct polarization anisotropies between the coupled vibrations and excitonic states, and vibrational mode-specific vibronic coupling strengths that vary with different ligand binding geometries. Complementary electronic structure calculations on Cd34S34(O2CCH3)22H22 nanoclusters indicate substantial ligand participation in band-edge electronic excitations. These results establish 2D EV as a powerful method for interrogating coupled ligand–exciton degrees of freedom in complex nanomaterials.