Evidence of carrier diffusion between emission states in CdSe/ZnS core-shell quantum dots: a comprehensive investigation combining fluorescence lifetime correlation spectroscopy (FLCS) and single dot photoluminescence studies.

Investigation of carrier dynamics in CdSe/ZnS core-shell quantum dots (QDs) is performed using fluorescence-lifetime-correlation-spectroscopy (FLCS) and single-dot PL blinking studies. The origin of an emitted photon from a QD in an FLCS study is assigned to either an exciton state or trap state based on its excited state lifetime (τfl). Subsequently, two intrastate autocorrelation functions (ACFs) representing the exciton and trap states and one cross-correlation function (CCF) coupling these two states are constructed. Interestingly, the timescales of carrier diffusion (τR) show striking similarities across all three correlation functions, which further correlate with τR of the conventional FCS. However, ACFs notably deviate from the CCF in their µs progression patterns, with the latter showing growth, whereas the former ones display decay. This implies inter-state carrier diffusions leading to the QD blinking. Further study of single particle PL blinking on a surface-immobilized QD indicates shallow trap states near the band edge cause the blinking at low excitation power, while trion recombination becomes an additional contributing factor at higher pump power. Overall, the results highlight not only an excellent correlation between these two techniques but also the potential of our approach for achieving an accurate and comprehensive understanding of carrier dynamics in CdSe/ZnS QDs.

Facet Dependent Photoluminescence Blinking from Perovskite Nanocrystals.

Photoluminescence (PL) blinking of nanoparticles, while detrimental to their imaging applications, may benefit next-generation displays if the blinking is precisely controlled by reversible electron/hole injections from an external source. Considerable efforts are made to create well-characterized charged excitons within nanoparticles through electrochemical charging, which has led to enhanced control over PL-blinking in numerous instances. Manipulating the photocharging/discharging rates in nanoparticles by surface engineering can represent a straightforward method for regulating their blinking behaviors, an area largely unexplored for perovskite nanocrystals (PNCs). This work shows facet engineering leading to different morphologies of PNCs characterized by distinct blinking patterns. For instance, examining the PL intensity trajectories of single PNCs, representing the instantaneous photon count rate over time, reveals that the OFF-state population significantly increases as the number of facets increases from six to twenty-six. This study suggests that extra-faceted PNCs, owing to their polar facets and expanded surface area, render them more susceptible to photocharging, which results in larger OFF-state populations. Furthermore, the fluorescence correlation spectroscopy (FCS) study unveils that the augmented propensity for photocharging in extra-faceted PNCs can also originate from their greater tendency to form complexes with neighboring molecules.

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 (Eg), 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 (τct ≈ 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.

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.