Ligand Binding to Distinct Sites on Nanocrystals Affecting Energy and Charge Transfer

Energy Versus Electron Transfer: Managing Excited-State Interactions in Perovskite Nanocrystal-Molecular Hybrids

Energy and electron transfer processes in light harvesting assemblies dictate the outcome of the overall light energy conversion process. Halide perovskite nanocrystals such as CsPbBr₃ with relatively high emission yield and strong light absorption can transfer singlet and triplet energy to surface-bound acceptor molecules. They can also induce photocatalytic reduction and oxidation by selectively transferring electrons and holes across the nanocrystal interface. This perspective discusses key factors dictating these excited-state pathways in perovskite nanocrystals and the fundamental differences between energy and electron transfer processes. Spectroscopic methods to decipher between these complex photoinduced pathways are presented. A basic understanding of the fundamental differences between the two excited deactivation processes (charge and energy transfer) and ways to modulate them should enable design of more efficient light harvesting assemblies with semiconductor and molecular systems.

Direct Observation of Surface Polarons in Capped CuInS2 Quantum Dots by Ultrafast Pump-Probe Spectroscopies

Semiconductor nanocrystals are mostly prepared by colloid chemistry with organic surfactant molecules, and their surface polarization effect on the carrier relaxations are critical to their optoelectronic applications. Until now, the surface polarization effect and detailed photophysical processes of these capped quantum dots (QDs) are still unclear. Here, we studied the dynamics of the photoinduced carriers and capping molecule  vibrations of capped CuInS₂ quantum dots by using the femtosecond pump-probe system in both visible and IR zones. It is identified that the capping  molecular vibrations exhibit significant Fermion bleaching nature, whose relaxation profile is in good agreement with the radiative recombination dynamics of QDs in the visible region. These results demonstrate that the extrinsic surface polarons form by the coupling of photoinduced carriers and surface ligand vibrations, and take part in the photophysical processes of these QDs. This finding is helpful for the QD design and applications in photoelectronic devices.

Electronic Processes within Quantum Dot-Molecule Complexes

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.