Facet Engineering for Decelerated Carrier Cooling in Polyhedral Perovskite Nanocrystals

Abstract

We report here the hot carrier (HC) cooling time scales within polyhedral CsPbBr 3 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 CsPbBr 3 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.

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

“…Carrier cooling in perovskite NCs has become the most exciting topic in the reported literature because of the tremendous implications in photovoltaic applications. − Harvesting HCs before their thermalization continues to be challenging due to the ultrashort time window available for their extractions. Extraction of HCs is even more difficult for cubic CsPbBr 3 perovskite NCs where the cooling time scale is reported to be as fast as ∼0.5 ps. , Researchers have invested tremendous efforts toward slowing down the hot carrier cooling time scale. , Many of them reported promising results of decelerated HC cooling in perovskite materials, which naturally opened an avenue for their extraction without losing the excess energy utilizing a carefully chosen scavenger. , It has been further reported that strategies involving large polaron formation, hot phonon bottlenecks, high carrier concentrations, , Auger-heating, etc.…”

“…are quite effective in achieving retarded cooling in perovskite materials. Recent reports highlighted that the ultrafast cooling mechanism in perovskites necessitates several steps occurring sequentially, starting from carrier–carrier scattering leading to a quasi-equilibrium state and then carrier–phonon/carrier–impurity scatterings, further guiding the cooling dynamics to a thermally equilibrated state. ,,, Recent reports explored several approaches, including dopant inclusion, size and composition variations, surface passivation, etc., that led them to observe slower cooling of HCs in perovskite materials. ,,− Recently, surface engineering has received considerable interest from the scientific community because of its vast potential for dramatic improvement of the photophysical properties of perovskite NCs. ,,− Pradhan and co-workers reported for the first time the facile synthesis of two unusual surface-engineered CsPbBr 3 NCs, characterized by, respectively, 12 (rhombic dodecahedron) and 26 (rhombicuboctahedron) facets. − Subsequently, they demonstrated how easily the gain threshold of amplified spontaneous emission from CsPbBr 3 NCs can be reduced upon opening new facets on the NC surface . In a very recent work, Pullerits and co-workers reported slower Auger recombination in 12-faceted dodecahedron NCs (CsPbBr 3 ) than in its cubic counterpart …”

Electron Transfer from an Optically Pumped Polyhedral Perovskite Nanocrystal to Pristine Fullerene

“…Powder XRD patterns depicted in Figure 1B reveal the presence of ( 112) and (200) facets, matching nicely with PDF reference lines for the orthorhombic phase (PDF-01-072-7929) and the previous reports. 1,20,21 Steady-state PL and UV−vis spectra of our 12-faceted PNCs exhibit peaks at ∼518 and ∼510 nm (first excitonic), respectively; both are slightly red-shifted compared to ones (∼515 and ∼504 nm) reported for cubic counterparts of similar size (Figure 1C). 1,22,23 Instruments Used.…”

“…1−5 For instance, our group and Ghosh and co-workers recently reported that the cooling of hot carriers gets sluggish as more facets are created on the nanocrystal surface. 1,3 Also, a more efficient biexciton formation (increased by 20−30% compared to cube PNCs) has been reported in 12faceted dodecahedron PNCs. 3 Pradhan and co-workers observed a dramatic reduction of the gain threshold of PNC's spontaneous emission with increasing facets, emphasizing the potential of multifaceted polyhedral PNCs in producing on-chip nanolasers.…”

Benzoyl Versus Phenacyl Bromide: The Strategic Differences in Halide Perovskite Nanocrystals Design

“…Nanocrystal engineering with facets and shape modulations in lead halide perovskites is important to tune their surface related functional properties. − However, due to their fast formation and ionic nature, , control over their facets and selective facet dependent anisotropic growth could not be successfully achieved yet. Unlike covalent chalcogenide nanocrystals, the chemistry of crystal modulations and surface engineering also could not be largely understood.…”