Excited-state vibronic coherences with mixing of core–ligand character promote hot-carrier cooling in oleate-capped CdSe quantum dots

Abstract

We present herein a multidimensional electronic spectroscopy study (2DES and 3DES) of vibronic coherences in CdSe quantum dots (QDs) showing that mid-frequency vibrations of the surface-capping oleate ligands promote hot-carrier cooling on the <50 fs time scale via a vibrationally coherent mechanism. Vibronic progressions in oscillation maps assigned to stimulated Raman coherences indicate that the LO phonon of the QD core is mixed with vibrations of the alkylcarboxylate moiety of the oleate ligands. Excited-state vibronic coherences, including a 375 cm−1 vibration assigned to a bending or wagging motion of the alkylcarboxylate (CCO) or carboxylate (OCO) group and a 126 cm−1 vibration assigned to a mixed, core–ligand mode, are rapidly damped on the same time scale as the nonradiative relaxation to the band edge and photoluminescence states. The results support the hypothesis that the rapidly damped vibrations serve as branching modes in a coherent nonadiabatic mechanism for hot-carrier cooling. The 375 cm−1 vibration may be acting as a tuning mode for the CIs along the relaxation pathway to the band-edge state because it modulates the π-electron donation properties of the alkylcarboxylate moiety of the oleate ligand.