Influence of Ligand Structure on Excited State Surface Chemistry of Lead Sulfide Quantum Dots

Abstract

The ligand-nanocrystal boundaries of colloidal quantum dots (QDs) mediate the primary energy and electron transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We use mid-infrared transient absorption spectroscopy to reveal the influence that ligand structure and bonding to nanocrystal surfaces have on the changes of the excited state surface chemistry of this boundary in PbS QDs and the corresponding impact on charge transfer processes between nanocrystals. We demonstrate that oleate ligands undergo marked changes in their bonding to surfaces in the excitonic excited states of the nanocrystals, indicating that oleate passivated PbS surfaces undergo significant structural changes following photoexcitation. These changes can impact the surface mobility of the ligands and the ability of redox shuttles to approach the nanocrystal surfaces to undergo charge transfer in photocatalytic reactions. In contrast, markedly different transient vibrational features are observed in iodide/mercaptoproprionic acid passivated PbS QD films that result from charge transfer between neighboring nanocrystals and localization of holes at the nanocrystal surfaces near MPA ligands. This ability to distinguish the influence that excitonic excited states vs charge transfer processes have on the surface chemistry of the ligand-nanocrystal boundary lays the groundwork for exploration of how this boundary can be understood and controlled for the design of nanocrystalline materials tailored for specific applications in solar energy harvesting and photocatalytic reactions.

Exciton–Phonon Coupling and Carrier Relaxation in PbS Quantum Dots: The Case of Carboxylate Ligands

“…Our findings reveal that the distinct excited state surface chemistries of OA and MPA ligands (Figure ) , have no measurable influence on the exciton–phonon coupling or the electronic relaxation rates of PbS QDs of varying sizes. The results suggest that ligands bonding to PbS surfaces via carboxylate, thiol, or iodide moieties do so with sufficiently weak coupling that the phonon modes of the inorganic framework of the nanocrystals dominate exciton–phonon coupling and electronic relaxation. , It will be interesting to explore the influence that excited state surface chemistry of other ligand structures and functional groups has on exciton–phonon coupling and electronic relaxation rates in other quantum confined nanocrystalline systems in an effort to identify design rules to tailor materials for specific optoelectronic applications.…”

“…The transient vibrational spectra in the upper panels of Figures A and B reveal that OA ligands experience substantially different interactions with PbS QDs in their excitonic excited state in comparison to MPA ligands. When excited, QDs with OA ligands were found to undergo a reduction of the Pb–O bond order due to enhanced electron density at the surfaces of the nanocrystals, , while the carboxylate group of MPA ligands experienced a 25 cm –1 shift to lower frequency due to a combination of charge transfer between neighboring nanocrystals and localization of holes at the nanocrystal surfaces . We therefore measured broadband transient absorption spectra of I – /MPA and OA passivated PbS films from the mid-IR to the visible spectral region to assess the influence their different excited state surface chemistries have on exciton–phonon coupling in the nanocrystals …”

“…Recent time-resolved infrared spectroscopy studies have enabled investigations of the changes of excited state surface chemistry that occur on PbS QDs passivated with different carboxylate ligands. , Figure A presents a transient vibrational spectrum measured in a 4.9 nm oleate (OA) passivated PbS QD film at a 20 ns time delay following optical excitation at 532 nm . The negative-going ground state bleach feature reveals a decrease of carboxylate groups that bond to the nanocrystal surfaces via the chelating geometry, while the positive-going vibrational features indicate an increase in the population of carboxylates that adopt bridging and monodentate bonding geometries.…”

“…The negative-going ground state bleach feature reveals a decrease of carboxylate groups that bond to the nanocrystal surfaces via the chelating geometry, while the positive-going vibrational features indicate an increase in the population of carboxylates that adopt bridging and monodentate bonding geometries. These findings indicated a net reduction of the Pb–O coordination of oleate ligands in the excitonic excited states of the oleate passivated PbS QDs, possibly as a result of electron localization near their surfaces. , In contrast, Figure B depicts a transient vibrational spectrum of the carboxylate groups of mercapto­proprionic acid (MPA) ligands attached to 4.9 nm PbS QDs that exhibit different structural changes 30 ns after 532 nm optical excitation of the nanocrystalline film . The transient vibrational spectrum reveals a shift of the carboxylate vibrational modes of ∼25 cm –1 as a result of the localization of holes at the nanocrystal surfaces by MPA ligands …”

“…(B) In contrast, MPA ligands localize holes at the nanocrystal surfaces, leading to a shift of the vibrational modes of the carboxylate groups of the ligands caused by the change in charge distribution. Adapted with permission from ref .…”