Investigation of the Octahedral Network Structure in Formamidinium Lead Bromide Nanocrystals by Low-Dose Scanning Transmission Electron Microscopy.

Metal halide perovskites (MHP) are highly promising semiconductors. In this study, we focus on FAPbBr3 nanocrystals, which are of great interest for green light-emitting diodes. Structural parameters significantly impact the properties of MHPs and are linked to phase instability, which hampers long-term applications. Clearly, there is a need for local and precise characterization techniques at the atomic scale, such as transmission electron microscopy. Because of the high electron beam sensitivity of MHPs, these investigations are extremely challenging. Here, we applied a low-dose method based on four-dimensional scanning transmission electron microscopy. We quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb-Br-Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure. Additionally, this study provides an approach to prospectively investigating the fundamental degradation mechanisms of MHPs.

Nanocrystal Heterostructures Based on Halide Perovskites and Metal Sulfides.

We report the synthesis of nanocrystal heterostructures composed of CsPbCl3 and PbS domains sharing an epitaxial interface. We were able to promote the growth of a PbS domain (in competition with the more commonly observed Pb4S3Cl2 one) on top of the CsPbCl3 domain by employing Mn2+ ions, the latter most likely acting as scavengers of Cl- ions. Complete suppression of the Pb4S3Cl2 domain growth was then achieved by additionally selecting an appropriate sulfur source (bis(trimethylsilyl)sulfide, which also acted as a scavenger of Cl- ions) and reaction temperature. In the heterostructures, emission from the perovskite domain was quenched, while emission from the PbS domain was observed, pointing to a type-I band alignment, as confirmed by calculations. These heterostructures, in turn, could be exploited to prepare second-generation heterostructures through selective ion exchange on the individual domains (halide ion exchange on CsPbCl3 and cation exchange on PbS). We demonstrate the cases of Cl- → Br- and Pb2+ → Cu+ exchanges, which deliver CsPbBr3-PbS and CsPbCl3-Cu2-xS epitaxial heterostructures, respectively.

CsPbCl3 → CsPbI3 Exchange in Perovskite Nanocrystals Proceeds through a Jump-the-Gap Reaction Mechanism.

Halide exchange is a popular strategy to tune the properties of CsPbX3 nanocrystals after synthesis. However, while Cl → Br and Br → I exchanges proceed through the formation of stable mixed-halide nanocrystals, the Cl ⇌ I exchange is more elusive. Indeed, the large size difference between chloride and iodide ions causes a miscibility gap in the CsPbCl3-CsPbI3 system, preventing the isolation of stable CsPb(ClxI1-x)3 nanocrystals. Yet, previous works have claimed that a full CsPbCl3 → CsPbI3 exchange can be achieved. Even more interestingly, interrupting the exchange prematurely yields a mixture of CsPbCl3 and CsPbI3 nanocrystals that coexist without undergoing further transformation. Here, we investigate the reaction mechanism of CsPbCl3 → CsPbI3 exchange in nanocrystals. We show that the reaction proceeds through the early formation of iodide-doped CsPbCl3 nanocrystals covered by a monolayer shell of CsI. These nanocrystals then leap over the miscibility gap between CsPbCl3 and CsPbI3 by briefly transitioning to short-lived and nonrecoverable CsPb(ClxI1-x)3 nanocrystals, which quickly expel the excess chloride and turn into the chloride-doped CsPbI3 nanocrystals found in the final product.

Managing Excess Lead Iodide with Functionalized Oxo-Graphene Nanosheets for Stable Perovskite Solar Cells.

Stability issues could prevent lead halide perovskite solar cells (PSCs) from commercialization despite it having a comparable power conversion efficiency (PCE) to silicon solar cells. Overcoming drawbacks affecting their long-term stability is gaining incremental importance. Excess lead iodide (PbI2 ) causes perovskite degradation, although it aids in crystal growth and defect passivation. Herein, we synthesized functionalized oxo-graphene nanosheets (Dec-oxoG NSs) to effectively manage the excess PbI2 . Dec-oxoG NSs provide anchoring sites to bind the excess PbI2 and passivate perovskite grain boundaries, thereby reducing charge recombination loss and significantly boosting the extraction of free electrons. The inclusion of Dec-oxoG NSs leads to a PCE of 23.7 % in inverted (p-i-n) PSCs. The devices retain 93.8 % of their initial efficiency after 1,000 hours of tracking at maximum power points under continuous one-sun illumination and exhibit high stability under thermal and ambient conditions.

Harnessing laser technology to create stable metal halide perovskite-rGO conjugates as promising electrodes for Zn-ion capacitors.

We report that the direct conjugation of metal halide perovskite nanocrystals on rGO sheets can provide high performance and stable electrodes for Zn-ion capacitors. It is the first time that metal halide nanocrystals have been used to enhance the energy storage of 2D materials in capacitors by introducing an additional pseudocapacitance mechanism. In particular, we present a simple, rapid and room temperature laser-induced method to anchor CsPbBr3 nanocrystals on rGO sheets without affecting the initial morphology and crystal structure of the two components. The flexible and high surface area of the rGO sheets enables the conjugation of individual metal halide perovskite nanocrystals, giving rise to new synergetic functionalities. As a result, the specific capacitance of the perovskite-rGO conjugated electrodes can be enhanced by 178- and 152-times compared to those of the plain rGO and perovskite electrodes respectively.

Ultrasmall CsPbBr3 Blue Emissive Perovskite Quantum Dots Using K-Alloyed Cs4PbBr6 Nanocrystals as Precursors.

We report a colloidal synthesis of blue emissive, stable cube-shaped CsPbBr3 quantum dots (QDs) in the strong quantum confinement regime via dissolution-recrystallization starting from pre-syntesized (K x Cs1-x )4PbBr6 nanocrystals which are then reacted with PbBr2. This is markedly different from the known case of Cs4PbBr6 nanocrystals that react within seconds with PbBr2 and get transformed into much larger, green emitting CsPbBr3 nanocrystals. Here, instead, the conversion of (K x Cs1-x )4PbBr6 nanocrystals to CsPbBr3 QDs occurs in a time span of hours, and tuning of the QD size is achieved by adjusting the concentration of the precursors. The QDs exhibit excitonic features in optical absorption that are tunable in the 420-452 nm range, accompanied by blue photoluminescence with quantum yield around 60%. Detailed spectroscopic investigations in both the single and multiexciton regime reveal the exciton fine structure and the effect of Auger recombination of these CsPbBr3 QDs, confirming theoretical predictions for this system.

Laser-Induced Morphological and Structural Changes of Cesium Lead Bromide Nanocrystals.

Metal halide perovskite nanocrystals, an emerging class of materials for advanced photonic and optoelectronic applications, are mainly fabricated with colloidal chemistry routes. On the quest for new properties according to application needs, new perovskite systems of various morphologies and levels of doping and alloying have been developed, often also involving post-synthesis reactions. Recently, laser irradiation in liquids has been utilized as a fast method to synthesize or transform materials and interesting laser-induced transformations on nanocrystals were induced. These studies in general have been limited to small nanocrystals (~15 nm). In the case of halide perovskites, fragmentation or anion exchange have been observed in such laser-based processes, but no crystal structure transformations were actually observed or deliberately studied. Nanocrystals are more sensitive to light exposure compared to the corresponding bulk crystals. Additional factors, such as size, morphology, the presence of impurities, and others, can intricately affect the photon absorption and heat dissipation in nanocrystal suspensions during laser irradiation. All these factors can play an important role in the final morphologies and in the time required for these transformations to unfold. In the present work, we have employed a 513 nm femtosecond (fs) laser to induce different transformations in large nanocrystals, in which two phases coexist in the same particle (Cs4PbBr6/CsPbBr3 nanohexagons of ~100 nm), dispersed in dichlorobenzene. These transformations include: (i) the exfoliation of the primary nanohexagons and partial anion exchange; (ii) fragmentation in smaller nanocubes and partial anion exchange; (iii) side-by-side-oriented attachment, fusion, and formation of nanoplatelets and complete anion exchange; (iv) side-by-side attachment, fusion, and formation of nanosheets. Partial or complete Br-Cl anion exchange in the above transformations was triggered by the partial degradation of dichlorobenzene. In addition to the detailed analysis of the various nanocrystal morphologies observed in the various transformations, the structure-photoluminescence relationships for the different samples were analyzed and discussed.