Computational Study of the Resonance Enhancement of Raman Signals of Ligands Adsorbed to CdSe Clusters through Photoexcitation of the Cluster

Abstract

This paper describes density-functional-theory-based computations of resonance Raman (RR) spectra of ligand molecules adsorbed to the surface of a Cd16Se13 cluster. Signals from asymmetric vibrational modes of ligand binding groups, such as the asymmetric O–C–O stretching modes of carboxylates, are enhanced relative to the symmetric vibrational modes when the excitation energy is on-resonance with the excitonic energy of the cluster. Certain ligand molecules have frontier orbitals with the correct energies and symmetries to mix with the orbitals of the CdSe cluster, and as a result, the wave functions of the electron and the hole delocalize from the cluster onto the ligand molecules; experimentally, this delocalization results in a bathochromic shift of the band edge excitonic absorption. Increased excitonic delocalization results in greater vibronic coupling between the exciton and the ligand vibrations and, on average, preferential enhancements in the RR signals of those vibrations. This work suggests that the use of exciton-delocalizing ligands to optimize electronic coupling between neighboring CdSe nanoparticles may, at the same time, enhance the rates of nonradiative exciton decay by coupling the exciton and ligand vibrational modes.

Semiconductor quantum dots reveal dipolar coupling from exciton to ligand vibration

“…To conclude, the originality of this work relies on the direct investigation of NC/ligand vibroeletronic coupling with 2C-SFG spectroscopy that evidences without ambiguity a correlation between the electronic and the vibrational structures of functionalized QDs. As predicted with simulations in Raman spectroscopy 67 but not observed before now, pumping QDs in their excitonic states translates into fostering the vibrational response of their surrounding organic environment. Comparing the vibrational amplitudes of methyl and methylene modes of the ligands at the visible wavelengths 488 nm (first excitonic state) and 612 nm (out of excitonic resonance), we measured on average a gain factor of F 2 v ¼ 6:3 in terms of SFG intensity.…”

“…Thus, we succeeded in characterizing the NC/ligand coupling in a case which is not particularly favourable. Furthermore, the theoretical modelling that we developed to describe this coupling concludes here in favour of an energy transfer-based process, rather than a charge transfer 67,69 . Our formal calculations showed that a classical dipolar interaction can account for the results, considering that the polarizability of the molecules is subordinate to the local electric field induced by the excitons.…”

Sum-frequency generation through a unique Feynman diagram formalism: the case of bipartite organic/inorganic complexes

“…Consequently, the diagrammatic method we developed in the present article constitutes the proper way to compute second-order response functions for hybrid systems, and to resolve the deficiencies of classical approaches. We chose to illustrate the diagrammatic method with SFG spectroscopy in the case of hybrid organic/inorganic systems because the understanding of the vibroelectronic coupling occurring within such systems is a controversial issue [21,25,[32][33][34]. Given that our approach is very fundamental and general, this paves the way for the theoretical study of SHG (second harmonic generation), DFG (difference-frequency generation) and Raman spectroscopies.…”

Diagrammatic theory of linear and nonlinear optics for composite systems

“…Obviously, the loop-diagrammatic method can be enlarged to the case of systems made of more than two subunits and to all the other nonlinear optical processes. We have chosen to illustrate this formalism with SFG spectroscopy in the case of organic/inorganic systems because the understanding of the vibroelectronic coupling occurring within such systems is a controversial issue [37,38,[59][60][61]. Given that our approach is very fundamental and general, we expect interesting applications for the theoretical study of the second-order SHG (second harmonic generation), DFG (difference-frequency generation) and third-order Raman spectroscopies.…”