Bright lead halide perovskite nanocrystals, which have been extensively studied in the past 5 years, are mostly confined to a six faceted hexahedron (cube/platelet) shape. With variations of ligand, precursor, reaction temperature, and surface modification, their brightness has been enhanced and phase became stable, but ultimate nanocrystals still retained the hexahedron cube or platelet shape in most of the hot injection reactions. In contrast, by exploration of α-halo ketone in amine as a halide precursor, different shaped nanocrystals without compromising the photoluminescence quantum yield (PLQY) are reported. Confining to orthorhombic CsPbBr3, the obtained nanocrystals are stabilized by 12 facets ({200}, {020}, {112}) and led to 12 faceted rhombic dodecahedrons. These facets are absolutely different from six ({110}, {002}) equivalent facets of widely reported orthorhombic cube shaped CsPbBr3 nanocrystals. These also retained the colloidal and phase stability, as well as showed near unity PLQY. With further annealing, these are transformed to 26 faceted rhombicuboctahedrons by dissolving all their vertices. Importantly, these 12 faceted nanocrystals showed wide area self-assembly in most of the reactions. It has also been concluded that primary ammonium ions led to six faceted nanocrystals, but tertiary ammonium ions obtained in this case stabilized different group of facets. While perovskite nanocrystals were broadly confined to only nanocubes, these new nanocrystals with intense emission would certainly provide a new avenue for continuing their further research.
Bringing together two nanostructures of different materials in a single building block can create a new platform where both can share their electronic and optical characteristics. While these have been extensively studied for metal nanoparticles and chalcogenide semiconductor nanocrystals, the literature is limited for perovskite nanocrystals. Because perovskite nanocrystals typically follow a fast formation process and also require a specific environment, their growth on other materials or vice versa indeed remains difficult. However, some success has been achieved in lead halide perovskites with several metal halides, oxides, and chalcogenides and also with noble metals. These findings indeed opened up new directions for designing perovskite nanocrystal heterostructures, investigating their heterojunctions, analyzing new properties, and exploring their possible utilizations. Summarizing all these developments, this Focus Review demonstrates challenges associated with their formation, changes in their optical properties, stability, and several applications. In addition, this work addresses a few possible future research directions regarding these perovskite heterostructures.
B-site doping with various metal ions in α-CsPbI3 has been proven to be a potential approach in bringing phase stability to these nanocrystals. However, while the doping of various homovalent ions in replacing Pb(II) has been extensively studied, heterovalent doping was observed to be limited. To understand the impact of heterovalent doping, Sb(III) was chosen here as an effective dopant for occupying the Pb(II) position in CsPbI3 nanocrystals. Importantly, it was observed that insertion of Sb(III) also stabilized the crystal phase of these red-emitting nanocrystals, but only with limited doping. However, with more intake, the cube shape turned to platelet and therefore also reduced the stability. Details of the insights of formation of these doped nanostructures are investigated, and further, these were implemented for photovoltaic application and comparable efficiency was recorded.
Pretreatment using metal chlorides during the formation of halide deficient free perovskite nanocrystals is reported. Among several metal chlorides, Cu(II)Cl2 was observed to be ideal for the synthesis of highly emitting CsPbCl3 nanocrystals at high reaction temperature. Because high temperature remained more favorable for the dopant insertion, doping of Mn(II) was carried out under this halide-rich system, and nearly 68% photoluminescence quantum yield was recorded. Analysis could not provide strong evidence of insertion of Cu(II) inside the nanocrystals; rather, it was established that Cu(II)Cl2 in the system helped to stabilize the reaction even at and above 260 °C and provided an adequate chloride source for obtaining the highly emitting host as well as doped nanocrystals. Details of the physical process involved for this metal ion-induced uplifting of the reaction temperature and the consequent impacts on the nanocrystal formation are studied in detail and reported in this Letter.
Red emitting α-CsPbI 3 nanocrystals are highly phase sensitive to ambient exposure, and B-site doping with suitable cations is adopted as one of the most feasible approaches for their phase stability. There are several reports herein: Ni(II) ions having the smallest transition metal Shannon radii were explored for doping in these nanocrystals. This successfully stabilized the cubic phase and retained the intense emission of nanocrystals for nearly 2 months. Being the smallest ion, the halide octahedra in the perovskite lattice were expected to provide high restraint ability toward δ-CsPbI 3 . Comparing with postsynthesis iodide treatments, the importance of doping in high temperature reaction was discussed. Finally, these doped nanocrystals were explored for photovoltaic devices and showed comparable efficiency (9.1%) to different other similar doped nanocrystals. Hence, the finding reported here is a step forward for understanding the insights of phase stability of α-CsPbI 3 perovskite nanocrystals.
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