PhD defense - Olivia Bird (Dukovic group)

Title: Influence of Binding on Charge and Energy Transfer in Semiconductor Nanocrystal – Molecule Complexes

Abstact: Semiconductor nanocrystals are strong light absorbers with tunable optoelectronic properties. This makes them attractive candidates for photon energy conversion, such as directly converting photon energy into electricity in the form of quantum dot sensitized solar cells, or instead by converting photon energy into fuels via redox catalysis. This work explores two fundamental processes which underpin both of these applications: the extraction of charges and energy from an excited nanocrystal, and the extension of the charge separated state. In particular, this thesis seeks to understand how the binding between nanocrystals and molecular charge acceptors impacts the kinetics of charge transfer and recombination.
In Chapter 2 we modify methyl viologen, a well-studied electron acceptor, by adding a phosphonate group to one end, which chemically binds to the quantum dot surface. Using isothermal titration calorimetry and transient absorption spectroscopy we discovered that this modification created a “ligand-like” binding mode which slows electron transfer but also extends the charge separated lifetime. Kinetic Monte Carlo simulations reveal the regime where this trade-off would result in improved efficacy as a redox shuttle.
This concept was then extended in Chapter 3 which leveraged synthetic chemistry to further explore the factors affecting the binding and charge transfer in quantum dot–viologen complexes. By studying a series of aryl viologen derivatives we discovered that quantum dot–viologen binding is surprisingly robust to changes in steric hinderance and rotational geometry. We also provide more evidence as to the value of anionic binding groups as a handle to tune the binding and charge transfer as a carboxylate viologen was the only derivative to have substantially different behavior than unfunctionalized methyl viologen.
Finally, in Chapter 4, I employed kinetic Monte Carlo simulations to analyze the mechanisms underlying the photoluminescence signals of energy transfer between ZnO nanocrystals and AlexaFluor A555 dye. Single particle fluorescence microscopy measurements were used to examine the heterogeneity of binding in nanocrystal–acceptor complexes that cannot be observed in the ensemble data. Simulations revealed that the microfluctuations associated with energy transfer could be attributed to a competitive dye fluorescence quenching pathway.

Interested in attending via Zoom? Contact Olivia Bird for the meeting link.