Direct Observation of Vibronic Coupling between Excitonic States of CdSe Nanocrystals and Their Passivating Ligands

Here, we report resonance Raman spectra of CdSe colloidal nanocrystals (NCs) passivated with organic ligands. In addition to the well-known longitudinal optical phonons, we observe ligand vibrations. The ligand vibrations are shown to be resonantly enhanced through electronic mixing with the states of the NC. These measurements were enabled by substituting the native ligands with thiophenol. Thiophenol serves as an ideal probe for exciton ligand coupling as it is a widely employed Raman molecular tag and quenches background luminescence in CdSe. The ligand vibrations are shown to be resonantly enhanced through exciting NC transitions. We show that vibronic coupling is observable in CdSe with diameters from 2 to 6 nm and for both phosphonic acid and amine native ligands. The coupling is evidenced by both asymmetric and symmetric mode enhancement through Herzberg-Teller or Franck-Condon and Herzberg-Teller mechanisms, repectively. The ligand exchange quenching strategy may be generally applicable to study exciton-ligand interactions in a variety of semiconductor NC materials and reveals information on the electronic and vibrational structure of the NC surface.