(Invited) Photophysical Properties and Photodynamics in CdSe Quantum Dots as Photocatalysts and Photosensitizers

Interactions between dye molecules and the semiconductor quantum dots (QDs) has a strong effect on both solar-to-electrical and solar-to-chemical energy conversion processes. In both types of applications, the charge transfer from the photoexcited QD to the molecule play a crucial role. Applying DFT-based non-adiabatic molecular dynamics, we have revealed ultrafast hole transfer (~10 fs) from the CdSe QD to the Ru(II) dye, following by a slow component (~100 fs) associated with the redistribution of population throughout QD states. The ultrafast interfacial QD-to-dye hole transfer is rationalized by strong non-adiabatic couplings between the QD surface states and the high frequency vibrational modes of isocyanide ligands in the dye. We also have studied the effect of the complex heterostructures of Janus QDs, where a half of the QD is made from CdSe and another half is from PbSe forming an interface along a specific crystalline direction. Our calculations show that the Pb-enriched (111) crystalline direction of the interface enhances optical response of the QDs. The hole relaxation is much slower than the electron relaxation, followed by the hole transfer to the dye. The hole transfer is sensitive to the polarity of the environment, rather than the ways of dye’s attachment to the surface. We also found that in nonstoichiometric QDs, a loss of a passivating ligand affects the electronic structure in the same way as injection of an electron in Cd-rich QDs or hole injection in Se-rich QDs. Moreover, electron injection to the Se-rich QDs results in highly optically active redshifted transitions associated with the introduced charge. Overall, our calculations provide atomistic insights into QD-to-molecule charge transfer, giving researchers an additional handle for enhancing QD photophysical properties by means of the QD stoichiometry, interface structure, and QD-molecule engineering.