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

Abstract

Some ligand−nanocrystal combinations exhibit rapid cooling of highly excited electronic states while other nanocrystal/shell combinations do not appear to have this effect. There remains a need to identify the distinguishing properties of ligand−nanocrystal interactions that avoid such rapid relaxation processes to guide the design of colloidal quantum dots (QDs) that take advantage of multiple exciton generation or hot-carrier extraction processes. Here, we use mid-infrared transient absorption spectroscopy to investigate the influence that carboxylate ligands with distinct excited state surface chemistries has on exciton− phonon coupling and hot exciton relaxation in PbS quantum dot (QD) films. Our findings reveal that despite significant differences in the excited state surface chemistry of oleate (OA) and iodide/mercaptopropionic acid (I − /MPA) ligands, PbS QD films passivated with both ligand types exhibit identical electronic relaxation rates and exciton−phonon coupling strengths within experimental precision. The data suggest that the inorganic lattice is the principal source of exciton−phonon coupling that influences hot exciton relaxation, rather than the vibronic modes of carboxylate ligands. The size-dependent nature of the exciton−phonon coupling strength is consistent with the localization of charge on the QD surfaces, which enhances the mixing of electronic and nuclear coordinates particularly when the electronic states are more quantum confined in smaller nanocrystals.

Quantized Exciton Motion and Fine Energy-Level Structure of a Single Perovskite Nanowire

“…As such, the anticorrelated intensity variations of the two sets of doublet PL peaks in Figure e strongly suggests that there exists a dynamic relaxation channel connecting the higher- and lower-energy doublet peaks, which renders the higher-energy doublet peaks sometimes nonemissive. Compared to single LNWRs, the single SNWRs possess a larger surface-to-volume ratio to make them more vulnerable to environmental charge fluctuations, which could affect the crystal structure and the strength of exciton–phonon coupling. − …”

Intra- and Inter-Molecular Charge Transfer Dynamics of Carbene-Metal-Amide Photosensitizers

“…pico-→nanosecond and nano-→millisecond transient absorption (psTA and nsTA, respectively), intersystem crossing (ISC) rates and lifetimes have been determined through time correlated single photon counting (TCSPC), and characterization of PS + and PSanalyzed by spectroelectrochemistry (SEC), we have generated a full picture of the charge transfer dynamics of these cMa complexes. 8,[14][15][16] We also report a nsTA study of intermolecular electron and hole transfer from the excited cMa to oxidants and reductants in solution, respectively.…”

Direct Synthesis of Ultrasmall PbS Nanocrystals Passivated with a Metal-Halide-Perovskite-like Monolayer

“…Lead sulfide nanocrystals (PbS NCs) are employed in a variety of applications including photovoltaics, infrared sensing, and photon conversion, − and they have served as a model system for understanding both physical − and chemical − structure/function relationships in infrared-active nanomaterials. There exist two broad classes of PbS NC syntheses that give rise to differing surface chemistriescontrol of which is a longstanding handle to understand the intrinsic photophysics of many NC materials and ultimately advance optoelectronic and photonic applications. ,− The first class of PbS NC syntheses employs lead oleate precursors (Hines-type), − and the second uses lead halide salts (Cademartiri-type). − When employing a lead oleate precursor, an intrinsic reaction intermediate can be circumvented by process additives to achieve the convenient synthesis of ultrasmall diameter (ø < 2 nm) PbS NCs at low temperatures (<50 °C) . In contrast, employing a lead halide salt precursor generally requires higher reaction temperatures (>80 °C) and pervasively yields larger PbS NCs (ø > 3.5 nm) with a perovskite-like lead halide monolayer on the NC surface. − , Cademartiri-type NCs display desirable ensemble photophysics but have rarely been employed in optoelectronic applications ,, in part because their surface chemistry was only recently understood .…”