Colloquium: Dr. Svetlana KilinaDr. Svetlana Kilina Department of Chemistry and Biochemistry North Dakota State University
Role of Surface Chemistry
An Interface on Photophysics of Quantum Dots
The significance of the surface passivation (protection) of colloidal quantum dots
(QDs) on their photophysical properties is discussed. Optically forbidden nature of
surface-associated states makes their direct measurements challenging. We present
several examples, where calculations based on time dependent density functional theory
(TDDFT) succeed in providing insights into these issues allowing for explanations
of experimental trends and observables sensitive to surface defects and ligand passivation.
Thus, our investigations of QD-ligand and QD-QD interactions provide an explanation
of experimentally detected enhancement of on-blinking times in closely packed Si QDs
and reveal the role of Cl ligands in formation of PbSe nanoplates growing along a
specific crystal lattice direction. Our calculations show that photophysics of stoichiometric,
magic-size CdnSen QDs is less sensitive to their passivation, compared to non-stoichiometric
CdnmSem. In the last, surface-driven optically inactive midgap states can be eliminated
by anionic ligands, such as carboxylates, thiolates, and even hydrides, explaining
better emission of metal-enriched than nonmetal-enriched QDs. Additionally, we found
that electron cooling is happening much faster in PbS/CdS than in PbSe/CdSe core/shell
QDs due to stronger electron coupling to ‘softer’ phonons in structures of lighter
elements (S vs. Se).
Overall, our calculations provide insights into the surface chemistry of QDs and offering guidance for controlling the optical response of nanostructures by means of core-shell, QD-QD and QD-ligand engineering.