Vibronic coherences in semiconductor quantum dots with organic surface-capping ligands

Exciton relaxation in semiconductor quantum dots (QDs) is strongly dependent on the nature and extent of passivation of the surface by organic ligands. To determine how the ligands participate in nonradiative decay mechanisms, we characterized preparations of CdSe QDs capped with hexadecylamine or oleate ligands using broadband multidimensional spectroscopy with 6.7-fs laser pulses. Population transfer to the band edge and then to the photoluminescent state after optical preparation of the X3 exciton (1P_e state) is revealed in two-dimensional electronic spectra (2DES) by the time evolution of an off-diagonal cross peak. This process involves excited-state coherent wavepacket motions through a cascade of conical intersections between exciton potential-energy surfaces. Time-windowed oscillation maps allow us to distinguish between excited-state and ground-state (stimulated Raman) wavepacket motions at frequencies matching the vibrational modes of the organic ligands. These observations indicate that the ligand vibrations are quantum coherently mixed with the core electronic states of the QDs. These results raise new opportunities for engineering photoinduced electron transfer processes in QDs through control of electronic-vibrational coupling with organic ligands.