Excited-state Dynamics and Exciton-phonon Coupling of Colloidal Quantum Dots

Excited state dynamics govern the optical and electrical properties of materials, which are closely related to the material structure including atomic bonding and chemical compositions. Colloidal quantum dots are semiconducting materials with optical properties dictated by fundamental quantum mechanics. As a result of the quantum confinement, the exciton dynamics of quantum dots are different from corresponding bulk semiconductors, where the relationship among synthetic modifications, exciton and lattice dynamics remains to be elucidated. This thesis is centered on the interplay between the excited state dynamics and exciton-phonon coupling in colloidal III-V and perovskite quantum dots and the ultrafast spectroscopy techniques used. Collective vibrations in a solid, termed phonons, are important in relaxation processes of excitons as they provide pathways to dissipate potential energy. Therefore, the available phonon energies and the strength of exciton-phonon interactions impact the dynamics and coherent phenomena of the excited states. For quantum dots with rigid lattices and high crystal symmetry, such as InP, clearly observed optical phonon coherence and fast exciton relaxation on the femtosecond timescale are influenced by the core size and inorganic shell used for surface passivation. With the same ZnS shell thickness, InP quantum dots with smaller core size exhibit relatively larger exciton polarizability due to interfacial strain effect, resulting in a stronger exciton-phonon interactions between the bright excitons with energy corresponding to the photoluminescence and LO phonons. Alloying InP quantum dots with GaP changes not only the bandgap of materials, but also the excited state dynamics and exciton-phonon coupling. The interplay among electronic structure, exciton-phonon coupling and phonon frequencies can lead to a phenomenon of coherent exciton relaxation with a specific In-to-Ga ratio. On the other hand, inorganic metal halide perovskite quantum dots feature mixed ionic and covalent chemical bonding with molecular-like excited state dynamics. The bandgap of peroskite quantum dots is determined by the structure of the metal halide octahedra framework. The exciton-phonon coupling in cesium bismuth halide quantum dots depends strongly on the connectivity of bismuth halide octahedra. Specifically, by changing the connectivity from face-sharing to corner-sharing, the exciton-phonon interaction is significantly reduced. For lead halide perovskite nanocrystals, coherent lattice dynamics of metal halide octahedra also accompany the carrier relaxation following up-conversion. All these phenomena reveal the close relationship among excited state dynamics, excitonphonon coupling, and photophysical behavior. Through these findings, I aim to guide the control and design of excited state properties of colloidal quantum dots.