Mechanistic Description of Surfactant-Induced Exciton Delocalization in Semiconductor Quantum Dots

This dissertation describes the mechanistic description of strong coupling between organic surfactants and inorganic quantum dots (QDs) that results in a decrease in the confinement energy of the excitonic carriers of the QDs. The decrease in confinement energy is observed as a decrease in the optical band gap of the QDs; upon exchange of a native insulating for a strong-coupling ligand, the excitonic absorption features shift to lower energy. This work focuses on the chelating ligand phenyldithiocarbamate, PTC, which produces a shift in the optical bandgaps of cadmium and lead chalcogenide QDs that is larger, by more than a factor of six, than produced by any reported chemical treatment of QDs. Studies of the dependence of the bathochromic shift of the bandgap on the physical size of the inorganic core of the QDs show that this shift occurs through stabilization of the excitonic state through delocalization of the excitonic hole out of the inorganic core, and into the interfacial states of the QD-organic complex. In order to explore the mechanism of PTC-induced delocalization further, PTC was synthesized with electron donating or withdrawing surfactants at the para position of the phenyl ring. This modification results in a series of X-PTCs with highest occupied molecular orbital (HOMO) energies that span ∼0.7 eV. When the PTC HOMO energetically aligns with the highest density-of-states region of the QD valence band, the QD-organic coupling increases, the delocalization volume provided by PTC increases, and the barrier for the excitonic hole to tunnel outside the inorganic core (i.e., the confinement energy of the hole) decreases.