The Tuning of Exciton-Phonon Coupling in Colloidal Nanocrystals by a Dielectric Medium

Abstract

Here, a thorough study is reported that exposes the influence of surfactants’ dielectric screening on the exciton energy and exciton-phonon coupling in CdSe and CdSe/CdS colloidal quantum dots and nanoplatelets. The study follows the temperature dependence of the photoluminescence spectrum to monitor thermally activated exciton-phonon interactions. The dependence of the exciton energy on temperature exhibits an anomalous point at ≈250 K, which coincides with the melting point of surfactant ligands (oleate molecules), an effect that is accompanied by a change in the dielectric constant. Interestingly, this anomalous effect gradually fades away upon increasing the shell thickness in CdSe/CdS derivatives. Furthermore, inspecting the exciton band's full-width-at-half-maximum (FWHM) dependence on temperature reveals the tuning of the optical phonon frequency when in direct contact with surfactant ligands. However, this effect is replaced by a surplus coupling to the phonon mode of the shell (CdS or CdSeS) when the thickest shell solely isolates the ligand dielectric screening. The study uncovers the impact of the dielectric screening on the optical properties of semiconductor nanocrystals, with an added value in the implementation of those materials in various optoelectronic, quantum, and tagging devices.