Resonance Raman (RR) spectroscopy is an effective tool for probing the extent of exciton-phonon coupling in colloidal semiconductor nanocrystals (NCs); however, there are conflicting reports in the literature of whether excitons also couple to covalently bound surface ligand vibrational modes.1-3 Recent theoretical and computational studies have posited that such couplings can occur through resonance with ligand-nanoparticle charge-transfer states, particularly in the case of asymmetric vibrational modes of ligands with states that lie within the semiconductor bandgap.4-5 Here we show that such predictions can be compared with experiment. We utilize a simple approach to prepare a set of 2-5 nm colloidal CdSe NCs capped with thiophenolate (PhS), which is a widely employed surface-enhanced Raman tag. The use of PhS also mitigates the strong background photoluminescence which generally impedes RR measurements of semiconductor nanocrystals. This system is further contrasted with obtained Raman spectra of an established studied set of molecular analogues (PhS, Cd(SPh)2, Cd4(SPh)10(MeN4), Se4Cd10(SPh)16(Me4N4), Se4Cd10(SPh)10). We will also present our recent efforts to expand this methodology to investigate sub 2 nm CdSe NCs. In this size regime, a secondary broad emissive feature appears, which we attribute to the formation of a radiative self-trapped exciton that is strongly coupled to the longitudinal optical phonon mode of CdSe.
1. Grenland, J. J.; Maddux, C. J. A.; Kelley, D. F.; Kelley, A. M., Charge Trapping versus Exciton Delocalization in CdSe Quantum Dots. The Journal of Physical Chemistry Letters 2017, 8, 5113-5118.
2. Wang, Y.; Zhang, J.; Jia, H.; Li, M.; Zeng, J.; Yang, B.; Zhao, B.; Xu, W.; Lombardi, J. R., Mercaptopyridine Surface-Functionalized CdTe Quantum Dots with Enhanced Raman Scattering Properties. The Journal of Physical Chemistry C 2008, 112, 996-1000.
3. Lifshitz, E., Evidence in Support of Exciton to Ligand Vibrational Coupling in Colloidal Quantum Dots. The Journal of Physical Chemistry Letters 2015, 6, 4336-4347.
4. Lombardi, J. R.; Birke, R. L., Theory of Surface-Enhanced Raman Scattering in Semiconductors. The Journal of Physical Chemistry C 2014, 118, 11120-11130.
5. Swenson, N. K.; Ratner, M. A.; Weiss, E. A., Computational Study of the Resonance Enhancement of Raman Signals of Ligands Adsorbed to CdSe Clusters through Photoexcitation of the Cluster. The Journal of Physical Chemistry C 2016, 120, 20954-20960.