Aging of Self-Assembled Lead Halide Perovskite Nanocrystal Superlattices: Effects on Photoluminescence and Energy Transfer

Abstract

Excitonic/electronic coupling and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a collective low energy emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by using CsPbBr3 nanocrystal superlattices under room temperature and cryogenic micro-photoluminescence spectroscopy. It is shown that keeping such structures under vacuum, a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days. As a result, a narrow, low energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr3 nanocrystals develop bulk-like CsPbBr3 particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays, resembling the features of superfluorescence. Overall, the reactivity of CsPbBr3 nanocrystals dramatically alters the emission of their assemblies, which should not be overlooked when studying collective optoelectronic properties nor confused with superfluorescence effects.

Electronic Coupling of Highly Ordered Perovskite Nanocrystals in Supercrystals

“…We do note that the energies of the absorption and PL peaks of the SBs are close to values typically found for CsPbBr 3 thin films, which is expected to be around 2.35 eV. 19 , 20 Furthermore, while the Stokes shift for NCs is ∼80 meV, for the SBs it decreases to ∼20 meV, which is in line with the value of bulk CsPbBr 3 . 21 A schematic of the differences in absorption and emission is given in Figure 1 e. Baranov et al observed a large red shift of an aged superlattice sample comprising 8 nm sized NCs as compared to the individual NCs, which they suggested to be related to bulk-like CsPbBr 3 particles formed by fused NCs in the superlattice.…”

“… 21 A schematic of the differences in absorption and emission is given in Figure 1 e. Baranov et al observed a large red shift of an aged superlattice sample comprising 8 nm sized NCs as compared to the individual NCs, which they suggested to be related to bulk-like CsPbBr 3 particles formed by fused NCs in the superlattice. 20 The small red shift that we observe in the freshly produced SBs in solution suggests there is no such coalescence or bulk formation due to such an aging effect. This is further supported by the larger photon energy, although moderately, than that reported for bulk perovskites.…”

“…This is further supported by the larger photon energy, although moderately, than that reported for bulk perovskites. 19 , 20 Also high-resolution SEM images ( Figure S2 ) show that the NCs in the SBs are clearly separated, displaying crystalline ordering and no coalescence of NCs. We have performed absorption and PL measurements for different dilutions (up to 100 times) of the SB suspensions in order to exclude any reabsorption effect ( Figure S4b,c ), as this could also give an apparent red shift.…”

Structural Diversity in Multicomponent Nanocrystal Superlattices Comprising Lead Halide Perovskite Nanocubes

“…2.3 eV, 535–540 nm) as well as a very strong quenching of the red aggregate band already at intermediate temperatures would hardly be reconcilable with an alternative assumption that the red-shifted emission peak could originate from the bulk material forming during the aggregation process. 78 The latter would imply an order of magnitude reduction of bulk emission quantum yield just within this temperature range.…”

Ultrafast Optical Properties of Cavity‐Enhanced Superfluorescence

“…The quantum dots (QDs) superlattice is based on QDs, which are self-assemble under the microscopic weak interaction force to form a long-range ordered 3D structure. [12][13][14][15][16] Its unique internal structure can trigger macroscopic quantum coupling effects, collective effects, and synergistic effects, [9][10][11][12]14,[17][18][19] which can also speed up the radiation rate. Therefore, the QDs superlattice microcavity reveals unique optical properties, cavity-enhanced superfluorescence (CESF) behavior.…”