Charge Transfer and Chiral Imprinting on Nano Particles

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2023

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Quantum dots (QDs), being semiconductor particles small enough to exhibit quantum mechanical properties, are leveraged in numerous applications including energy harvesting, quantum computing, and biomedical imaging. Our research theoretically examined two functional QD systems: (1) first one undergoes the electron transfer (ET) between CdSe QDs facilitated by a solvent and linker molecule, and (2) a second performs charge transfer (CT) and triplet energy transfer (TET) between CdSe/CdS core/shell QDs and ligand acceptor. These processes were scrutinized based on the electronic coupling tunneling through the shell, reaction free energy change, and reorganization energy. Furthermore, we explored chiral imprinting mechanism in perovskite nanoplatelet and the ET dynamics of organic molecules with pathway interferences.We devised a discrete variable representation (DVR) method to simulate ET between CdSe QDs, mediated by a solvent and a linker molecule. Employing the effective mass approximation (EMA), we characterized the QDs, ligand, and solvent, studying the distance dependence of donor-acceptor coupling via the energy splitting method. We found that the ET coupling decreases exponentially with the interdot distance. The decay constant is dictated by the size of the linker and the tunneling barrier through the solvent and ligand. When the donor and acceptor sizes significantly iv surpass the diameter of the linker, such as in large QDs with an alkane chain linker, the ET is predominantly regulated by through-solvent tunneling. The DVR method was also applied to simulate the CdSe/CdS core/shell QD system. Notably, experiments observe a subtle TET rate decay with an increase in shell thickness. Simulating QDs of varying shell thickness, we found a large TET coupling decay constant. Marcus analysis finds that the QD TET operates in a deeply inverted regime, where an increase in shell thickness reduces the driving force, leading to a significant increase of the Franck-Condon factor. This in turn offsets the exponential decrease of the electronic coupling with shell thickness. Further, our findings demonstrated that variations in shell thickness could further decrease the TET rate decay constant. Applying density functional theory (DFT) calculations and a charge-perturbed- particle-in-box model, we investigated chiral imprinting of perovskite nanoplatelet by chiral ligands. We found that the imprinted CD signal is sensitive to the orientation of the chiral ligand. As the proportion of chiral surface ligands grows, our model calculations find that the intensity of the CD signal from the lowest energy exciton transition saturates. v

We also examined the effects of light polarization on the ET yield in the coherent limit, using a model Zinc porphyrin as the ET donor due to its near degenerate excited states and orthogonal transition dipole moments. These two donor excited states, coupling to the acceptor state, produce pathway interference that strongly impact the ET. Introducing dissipation due to system-environment interaction via the Lindblad equation, we found that the ET yield is influenced by the initial light polarization. In the DA system, linearly polarized light (LPL) is predicted to induce an ET yield difference of up to 85% with a 100 fs dephasing time, while the yield difference elicited by R- circularly polarized light (R-CPL) and L-circularly polarized light (L-CPL) was insignificant. The model Hamiltonian was subsequently simulated and the dynamics was predicted by our collaborators with a trapped ion qutrit system.

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Zhang, Zhendian (2023). Charge Transfer and Chiral Imprinting on Nano Particles. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/29198.

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