Abstract
Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding. The influence of the molecular surroundings on QDs optoelectronic properties is therefore intensively studied. Here, from the converse point of view, we analyse and model the influence of QDs optoelectronic properties on their ligands. As revealed by sum-frequency generation spectroscopy, the vibrational structure of ligands is critically correlated to QDs electronic structure when these are pumped into their excitonic states. Given the different hypotheses commonly put forward, such a correlation is expected to derive from either a direct overlap between the electronic wavefunctions, a charge transfer, or an energy transfer. Assuming that the polarizability of ligands is subordinate to the local electric field induced by excitons through dipolar interaction, our classical model based on nonlinear optics unambiguously supports the latter hypothesis.
Highlights
Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding
The experimental evidence is adduced through twocolour sum-frequency generation (2C-SFG) spectroscopy combining visible and infrared laser beams on commercial ligand-conjugated CdTe NCs, as it is properly dedicated to characterizing vibroelectronic interactions at interfaces and probing the surface chemistry[10,45,46,47]
From a chemical point of view, 2C-SFG reveals the presence of unexpected residual hydrophobic ligands at the surface of these commercial NCs, suggesting the incompleteness of ligand exchange during their synthesis and functionalization
Summary
Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding. Contrary to the excitonic behaviour of the core, which is realistically well understood, the coupling of the QD states to their direct chemical environment, and to the ligand states, is still under discussion[44] Within this framework, this article demonstrates experimentally and theoretically the correlation between the electronic structure of NCs and the vibrational structure of ligands, and brings significant information about the surface and ligand chemistry of commonly used QDs, addressing thereby fundamental and practical issues. We demonstrate that a classical dipolar interaction between NCs and molecules is sufficient to describe their coupling as unveiled by nonlinear optics, which is evidence in support of an energy transfer-based process Within this theoretical model, the predicted ωvis-dependence of the ligand vibration amplitudes fits very well the measurements obtained with 2C-SFG spectroscopy. This work enhances the understanding of the coupling between QDs and ligands and ensures that the vibrational properties of the molecular surroundings is well correlated to NCs’ excitons
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