Subpicosecond Hot Hole Transfer in a Graphene Quantum Dot Composite with High Efficiency

Dodecahedron CsPbBr3 Perovskite Nanocrystals Enable Facile Harvesting of Hot Electrons and Holes

This Letter reports the facile harvesting of hot carriers (HCs) in a composite of 12-faceted dodecahedron CsPbBr₃ nanocrystal (NC) and a scavenger molecule. We recorded ∼3.3 × 10¹¹ s^-1 HC cooling rate in NC when excited with ∼1.4 times the band gap energy (E_g), increasing to >3 × 10¹² s^-1 in the presence of scavengers at high concentration due to the HC extractions. Since the observed intrinsic charge transfer rate (∼1.7 × 10¹² s^-1) in our NC-scavenger complex is about an order of magnitude higher than the HC cooling rate (∼3.3 × 10¹¹ s^-1), carriers are harvested before their cooling. Further, a fluorescence correlation spectroscopy study reveals NC tends to form a quasi-stable complex with a scavenger molecule, ensuring charge transfer completed (τ_ct ≈ 0.6 ps) much before the complex breaks apart (>600 μs). The overall results of our study highlight the promise shown by 12-faceted NCs and their implications in modern applications, including hot carrier solar cells.

Triplet generation at the CdTe quantum dot/anthracene interface mediated by hot and thermalized electron exchange for enhanced production of singlet oxygen

Triplet energy transfer (TET) from semiconductor quantum dots (QDs) to molecular triplets has potential applications in photon up-conversion and singlet oxygen generation. Here, we have constructed a complex consisting of CdTe QDs as the donor and 9-anthracenecarboxylic acid (ACA) as the triplet acceptor, and studied the TET pathways and enhanced singlet oxygen generation properties. The results from steady-state and time-resolved spectroscopy demonstrate efficient TET with a total efficiency of over 80% from photoexcited CdTe QDs to ACA. Dynamical analysis clearly indicates two distinctive TET channels - hot electron exchange and thermalized electron exchange - mediating the TET process in the CdTe QDs-ACA complex. The TET efficiencies from hot electron exchange at high energetic levels and thermalized electron exchange on the lowest exciton state can reach ∼27% and ∼85%, respectively, following 530 nm excitation. This efficient TET endows the CdTe QDs-ACA complex with a good capability of generating singlet oxygen species with a yield of up to ∼59%. These findings contribute further insights to the mechanisms of interfacial TET processes and are significant in designing efficient TET systems based on semiconductor nanoparticles and triplet molecules.

Facet Engineering for Decelerated Carrier Cooling in Polyhedral Perovskite Nanocrystals

We report here the hot carrier (HC) cooling time scales within polyhedral CsPbBr₃ nanocrystals (NCs) characterized by different numbers of facets (6 to 26) utilizing a femtosecond upconversion setup. Interestingly, the observed cooling time scale slows many-fold (>10 times) upon opening the new facets on the NC surface. Furthermore, a temperature-dependent study reveals that cooling in multifaceted NCs is polaron mediated, where newly opened polar facets and the soft lattice of CsPbBr₃ NCs play pivotal roles. Our hallmark result of slow cooling in polyhedral NCs renders an excellent opportunity for harvesting high-energy carriers by a carefully chosen molecular system. To this end, employing the hole scavenger molecule aniline, we successfully extracted hot holes from optically pumped NCs. We believe that several intriguing properties of the polyhedral NCs, including rapid polaron formation, defect-tolerant nature, and the capability of soft lattice to support slow diffusion of charge carriers, resulted in decelerated cooling.