Nanoscale Charge Conservation and Transfer in Quantum Dot Assembly

Quantum dots (QDs) are nano-size building blocks that can be shaped and assembled to yield novel and tunable properties. They have drawn significant research interests in the last several decades due to the tremendous potential in engineering applications - their tunable optical and electronic properties as well as low-temperature and facile manufacturing processing conditions allow to compete with other materials in the fields of light-emitting diodes, photovoltaics, photodetectors, optical memories, etc.
In the first part of my thesis, I attempt to provide insights on understanding the impact of cation and anion interaction of an ionic ligand passivant to eradicate detrimental charge traps on perovskite (CsPbBr3) QDs, thus minimizing charge loss. We focus, in this study, on the pairing of cation – anion by varying cation choice on the degree of defect passivation efficacy on CsPbBr3 QD surface. In collaboration with computational calculations, we experimentally confirm that the photoluminescence quantum yield (PLQY) of CsPbBr3 QD is substantially impacted by the pairing between cation and anion, validating our hypothesis.
In the second part of my thesis, I experimentally tackle, using dielectric constant of an antisolvent, as a reasonable metric to study the nucleation and growth of perovskite thin film. Our results, supported by the absorbance and 207Pb-nuclear magnetic resonance (NMR) measurements, show that lower dielectric solvent environment promotes formation of methylammonium (MA+) – iodoplumbate complexes, resulting in smaller grain size.
Lastly, I report the first proof-of-concept study on quantum tunneling induced exciton dissociation by varying potential barrier height in the photochromic molecule (PCM) bridged PbS QD assembly. The potential barrier height, defined as highest-occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) difference between the QD and photochromic molecule, can be swiftly varied via light-induced configuration switch of the PCM. Supported by a series of experiments, I corroborate that the quantum tunneling induced exciton dissociation in the PbS QD assembly occurs predominantly when the potential barrier height decreases upon closing the photochromic molecule’s configuration. On the other hand, a radiative recombination acts as a dominant exciton relaxation pathway with larger potential barrier height, from the photochromic molecule’s open configuration.