We use ultrafast infrared spectroscopy to examine resonant relaxation channels that bypass the predicted phonon-bottleneck in PbS quantum dots over a range of sizes and passivated with a variety of ligands. Direct measurements of higher energy 1P and 1D intraband states in the mid-infrared permits detailed analysis of the electronic overlap of the quantum confined states that influence their relaxation processes. In smaller PbS nanocrystals, where the phonon bottleneck is expected to be most pronounced, relaxation of parity selection rules combined with stronger electron-phonon coupling leads to greater spectral overlap of transitions among the quantum confined states. The effect is observed across a variety of ligands and likely reflects electron-phonon coupling of the nanocrystals and surfaces rather than their ligands. Larger but still quantum confined nanocrystals do not exhibit such resonant relaxation channels because their intraband states have more narrow energetic distributions. Furthermore, the generation of excitons in PbS quantum dots creates highly polarizable states that modulate the vibrational spectra of carboxylate groups of ligands attached to the nanocrystal surfaces through the formation of image dipoles. The symmetric vibrational mode with a transition dipole moment normal to the quantum dot surfaces is enhanced by the image dipoles while the antisymmetric vibrational mode with a transition moment parallel to the surfaces is suppressed. Analysis of the size dependent variation of the vibrational spectra reveals different chelating versus bridging bonding geometries that vary according to the edge-to-facet ratio of the nanocrystals.

Surface chemistry and image dipoles in PbS quantum dots during resonant relaxation of excited intraband states

2019 Fall National Meeting

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