Electronic spectroscopy and energy transfer in cadmium selenide quantum dots and conjugated oligomers

The electronic excited state kinetics of CdSe quantum dots (QD) are studied through optical spectroscopy, by subjecting the quantum dots to different experimental conditions, as well as coupling them to phenylene-ethynylene oligomers. CdSe QDs feature a quantum-confined exciton state which pursues a variety of pathways once formed, such as band-edge recombination, charge separation by trapping at the dot surface, and electronic energy transfer (EnT). These phenomena are studied using different CdSe sizes, highlighting the effects of quantum confinement and surface energies on exciton decay.; The size dependence of the exciton lifetime is studied, and correlation of the radiative lifetime to theoretical expectations are found, as well as evidence that nonradiative relaxation through crystal vibrations follows the Energy Gap Law and Marcus Inverted Region kinetics. A detailed analysis of the lifetime decays using the Maximum Entropy Method (MEM) reveal the presence of distributed, dual excited states, which are assigned to band-edge recombination and charged exciton decay. Complementary time-resolved PL allows for direct measurement of excited state populations, which changes dramatically upon addition of an inorganic capping layer to the QD, reflecting the suppression of surface carrier trapping. A strong excitation power-dependence of the photo-activated photoluminescence (PL) is correlated to the established observation of PL intermittency.; Forming a hybrid nanocomposite of CdSe QDs and phenylene-ethynylene oligomers allows a detailed study of EnT between the organic phase and the inorganic phase, as well as complex energy migration kinetics within the organic phase. The size-dependent, and chain length-dependent EnT is found to arise from the spectral overlap dependence between the phases. Finally, CdSe QDs are mixed into phenylene-ethynylene oligomers at dopant-level concentrations to study the photo-induced phase transformations and subsequent electronic energy migration. A rudimentary example of using this material for all-optical memory devices is shown.