Surface Ligand Vibrations Resolve Exciton Fine Structure in Quantum Dots

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.