The long-term aging of metal-halide perovskite nanocrystals is usually associated with uncontrolled degradation or transformation into unwanted phases. In contrast with this expected behavior, we report the spontaneous evolution of A3Cd2Cl7 (A = Cs, Rb) Ruddlesden-Popper nanoplatelets into crystalline multilayered stacks during prolonged storage in nonpolar solvents. This thermodynamically driven recrystallization is initiated by ligand interdigitation, which promotes face-to-face self-assembly, and is followed by Ostwald ripening within the stacks into a hybrid layered phase. These early assemblies are crucial in imposing the geometric constraints that make platelets converge toward the same morphology. This drives coordinated lateral growth and size-focusing over several months, yielding large, morphologically homogeneous nanosheet stacks. Multilayer X-ray diffraction reveals that these stacks consist of atomically precise inorganic slabs alternating with oleylammonium cation bilayers, corresponding to a stoichiometry of (oleylammonium)2Cs4Cd4Cl14, thereby identifying the assemblies as a new hybrid organic-inorganic Ruddlesden-Popper phase. Their persistence beyond one year without further evolution and their resistance to sonication demonstrate interplatelet interactions far more durable than typical colloidal assembly interactions. Nevertheless, stresses generated during solvent evaporation can induce lateral sliding, twisting and the collapse of stacks, generating characteristic card-deck patterns on transmission electron microscopy grids. Similar early assembly and lateral growth were observed for Cs2PbI2Cl2 nanoplatelets, suggesting that this pathway extends beyond Cd-based systems to other 2D-layered metal halides. These findings establish ligand interdigitation and aging-assisted ripening as a route to hybrid organic-inorganic phases and provide mechanistic insight into the self-organization of colloidal 2D nanocrystals into ordered superstructures inaccessible through direct synthetic routes.

The chemistry of sulfo-bromination of metals leading to dual-tapered 2D-shaped nanostructures is reported. This has been confined to the metal ion Bi(III), whose reduction followed by reionization in the presence of sulfide and bromide precursors leads to the desired bismuth sulfobromide nanocrystals. Initially Bi19S27Br3 seed rods are taken, where Bi(0) nanocrystals are connected via a redox chemistry, conjugating with secondary metal ions. Then these 1D-shaped Bi19S27Br3-Bi(0) heterostructures are transformed to dual-tapered 2D-shaped Bi19S27Br3-Bi(0) nanostructures following a solution-liquid-solid (SLS) growth mechanism. The sulfo-bromination process is initially nucleated on the metal(0) sites but grown on the surface of the substrate seed rods. The size of Bi(0) equilibrates the tapering, whereby the length of the rods is gradually reduced and the width of the rods is slowly widened, leading to the dual-tapered 1D to 2D shape-transferred Bi-chalcohalide nanostructures. Extensive electron microscopic analysis and stepwise synthesis have been carried out to investigate the growth mechanism and to understand the tapering during the shape evolution processes. In addition, these materials of seed rods, intermediate and final nanostructures obtained are further explored as photocatalysts for HER and their activities are compared. The reaction chemistry discussed here enables the controlled incorporation of two anions with Bi(0) ionization, where one end of the nanostructure initiates nucleation, while the other end promotes growth. Overall, the reaction chemistry here provides a pathway for the solution-processed 2D SLS growth process, leading to shape-controlled metal chalcohalide nanostructures.

The metal halide perovskite nanocrystals exhibit a remarkable tolerance to midgap defect states, resulting in high photoluminescence quantum yields. However, the potential of these nanocrystals for applications in display devices is hindered by the suppression of biexcitonic emission due to various Auger recombination processes. By adopting single-particle photoluminescence spectroscopy, herein, we establish that the biexcitonic quantum efficiency increases with the increase in the number of facets on cesium lead bromide perovskite nanocrystals, progressing from cube to rhombic dodecahedron to rhombicuboctahedron nanostructures. The observed enhancement is attributed mainly to an increase in their surface polarity as the number of facets increases, which reduces the Coulomb interaction of charge carriers, thereby suppressing Auger recombination. Moreover, Auger recombination rate constants obtained from the time-gated photon correlation studies exhibited a discernible decrease as the number of facets increased. These findings underscore the significance of facet engineering in fine-tuning biexciton emission in metal halide perovskite nanocrystals.

Halide contents in the reaction system during the formation of lead halide perovskite nanocrystals remained a critical parameter for controlling their surface facets and optical properties. In addition, herein, the impact of halide ions has also been explored as a deciding factor for the formation of metal(0)-perovskite nanocrystal heterostructures. Using CsPbBr3 and Pt, it is observed that minimum bromide concentration is ideal for epitaxial connection of Pt particles, but with a greater amount, these are disconnected during annealing. In addition, CsPbBr3 perovskite nanocrystals also acted here as the catalyst for nucleating and shaping Pt particles, which on altering reaction parameters tuned their shape from small dots to cubes and also in rod shape. The control of obtaining one-to-one or one-to-many types of heterostructures, as well as excess Pt particles retained in bulk solution, are also established. Formation of all these shapes is further observed related to the host nanocrystals, as without CsPbBr3 no such structures of Pt are seen in control reactions. This catalytic nucleation and formation of Pt particles on the surface of CsPbBr3 and their retention or ejection from the surface of CsPbBr3 are correlated with the halide contents and the extent of epitaxial growth between these two materials. Hence, rather than using the traditional approach, herein, CsBr to CsPbBr3 transformation protocol is used for the formation of required excess halides in the reaction system, and phenacyl bromide is used for bringing and tuning shapes of the perovskite nanocrystals. Details of all these studies are investigated and the shaping of Pt particles are correlated to the facets, sizes, and shapes of CsPbBr3 perovskite nanocrystals.

Lead halide perovskite nanocrystals remained in the forefront of inorganic optical nanomaterials for a decade. The chemistry of their formation leading to bright and phase-stable nanocrystals is also largely understood. However, this study mostly focuses on isotropic nanostructures and is limited to anisotropic shapes. Again, the study of shape anisotropy is more explored for orthorhombic CsPbBr3 and limited for the high-bandgap cubic phase CsPbCl3 nanocrystals. Keeping this in mind, herein, the halide exchange is carried out in a specific cube-connected patterned vertex-oriented nanorod of CsPbBr3 intending for complete conversion to CsPbCl3. The host CsPbBr3 nanorods are prepared here by B-site cation exchange in CsCdBr3 following a Cs-sublattice platform, as no such traditional protocol has been developed for their direct synthesis. In addition, direct synthesis of CdCdCl3 nanorods having an appropriate Cs-sublattice framework leading to anisotropic CsPbCl3 nanorods is also not achieved in a similar pathway. Hence, cubic CsPbCl3 nanorods are targeted from CsPbBr3 through anion exchange. However, it is observed that such anion exchange has a limiting stage beyond which the anisotropic rod is dismantled into isotropic cube structures. The chemistry of control of such limiting parameters is investigated, and the phase-shape relationship during the anion exchange is established. The key observation here for such a unique shape of CsPbBr3 is related to their orthorhombic phase, which lost its sublattice structure when the phase slowly converted to a pure cubic phase. However, with optimum Cl insertion, the mixed halide perovskite nanorods are retained, and these are also further doped with Mn(II) for obtaining anisotropic doped nanorods, and their changes in optical features are reported. Hence, the shape-phase relationship in anisotropic halide perovskite nanorods matters and controls the limit of halide exchange leading to the mixed halide perovskite anisotropic nanorods.

Facet engineering in nanocrystals is typically controlled by the surface atoms of the crystal and the interaction of interface binding ligands. This has been extensively studied for covalent chalcogenide nanocrystals, but little progress has been made for ionic halide perovskite nanocrystals. Stabilizing the different surface facets, mostly, determines the nature of the polyhedral shapes and also helps in understanding the formation of their anisotropic structures. Recent progresses of different polyhedral halide perovskite nanocrystals suggest that their formations are mostly reagent specific. Two halide reagents, benzoyl bromide and phenacyl bromide, which are only different in one -CH2- group, showed drastic difference in the shapes of resulting CsPbBr3 nanocrystals. While former resulted in the cube shape, the later triggered the formation of dodecahedron-shaped nanocrystals. Keeping this in mind, the chemical activities of these reagents for the shape evolution of binary and ternary metal halides and their conversion to halide perovskites with B-site ions introduction are investigated in detail and reported in this perspective. In addition, the cross exchanges in these two reagents and other possible pathways are also proposed for possible dimension tunable halide perovskite nanocrystals.

Halide content of the reaction medium not only enhances the brightness of CsPbCl3 nanocrystals but also, control the shape modulations as well as doping Mn(II) in these host nanocrystals. Correlating both the shape effect and doping, herein, an in situ reaction of nucleophile-controlled halide release was explored for monitoring facets modulations and doping in CsPbCl3 nanocrystals. This was performed using alkyl amine as nucleophile which reacted with α-halo ketone, phenacyl chloride, to release chloride ions. Increase in amine concentration which released more Cl ions, reduced the possibility of shape transformation from perfect to truncated cubes during annealing. Similarly, for Mn(II) doping, the dopant photoluminescence intensity remained directly proportional to the amount of introduced amine nucleophiles. Quality of both doped and undoped nanocrystals obtained in this procedure remained unparallel and the method provided a strong correlation of rate of halide release with both facet modulations and doping in these nanocrystals.

The compositions of precursors during the syntheses of CsPbX3 nanocrystals typically determine the resulting crystal phase and stability. Different precursors providing single or multiple constituent ions have already been developed for tuning the reaction pathways for architecting different sizes and shapes of such nanocrystals. Among these, the CsPbBr3 dusters, composed of Cs, Pb, and Br, as well as limited ligands, employed as single-source precursors, provide an ideal pathway for obtaining a wide window of size/shape-tunable CsPbBr3 nanocrystals. Recent reports also revealed that this pathway can lead to the epitaxial growth of non-perovskite nanostructures on perovskite nanocrystals. Further, the limited ligands here helped in shape modulation by opening different facets and also triggering facet directed connections of the nanocrystals. Keeping those in mind, the importance of these unique cluster precursors for the development of halide perovskite nanocrystals and their heterostructures is discussed, and possibilities for their utilization in different unexplored areas of research are proposed in this Perspective.

Lead halide perovskite nanocrystal heterostructures have been extensively studied in the recent past for improving their photogenerated charge carriers mobility. However, most of such heterostructures are formed with random connections without having strong evidence of epitaxial relation. Perovskite-chalcohalides are the first in this category, where all-inorganic heterostructures are formed with epitaxial growth. Going beyond one facet, herein, different polyhedral nanocrystals of CsPbBr3 are explored for facet-selective secondary epitaxial sulfobromide growths. Following a decoupled synthesis process, the heterojunctions are selectively established along {110} as well as {200} facets of 26-faceted rhombicuboctahedrons, the {110} facets of armed hexapods, and the {002} facets of 12-faceted dodecahedron nanocrystals of orthorhombic CsPbBr3. Lattice matching induced these epitaxial growths, and their heterojunctions have been extensively studied with electron microscopic imaging. Unfortunately, these heterostructures did not retain the intense host emission because of their indirect band structures, but such combinations are found to be ideal for promoting photocatalytic CO2 reduction. The pseudo-Type-II combination helped here in the successful movement of charge carriers and also improved the rate of catalysis. These results suggest that facet-selective all-inorganic perovskite heterostructures can be epitaxially grown and this could help in improving their catalytic activities.

ADVERTISEMENT RETURN TO ISSUEPREVViewpointNEXTWhat Happens to Halide Perovskite Nanocrystals on TEM Grids upon Year-Long Ambient Storage? Surface Ligands versus Crystal StabilitySumit Kumar DuttaSumit Kumar DuttaSchool of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, IndiaMore by Sumit Kumar Duttahttps://orcid.org/0000-0002-9228-1916, Suman BeraSuman BeraSchool of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, IndiaMore by Suman Bera, Suvodeep SenSuvodeep SenSchool of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, IndiaMore by Suvodeep Sen, and Narayan Pradhan*Narayan PradhanSchool of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India*[email protected]More by Narayan Pradhanhttps://orcid.org/0000-0003-4646-8488Cite this: ACS Energy Lett. 2022, 7, 2, 773–777Publication Date (Web):January 21, 2022Publication History Received3 December 2021Accepted7 January 2022Published online21 January 2022Published inissue 11 February 2022https://pubs.acs.org/doi/10.1021/acsenergylett.1c02640https://doi.org/10.1021/acsenergylett.1c02640article-commentaryACS PublicationsCopyright © 2022 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views4757Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (5 MB) Get e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Crystals,Ligands,Nanocrystals,Phase transitions,Transmission electron microscopy Get e-Alerts

The increase of the stability of perovskite nanocrystals with respect to exposure to polar media, layers growth, or shelling with different materials is in demand. While these are widely studied for metal chalcogenide nanocrystals, it has yet to be explored for perovskite nanocrystals. Even growth of a single monolayer on any facet or on the entire surface of these nanocrystals could not be established yet. To address this, herein, a secondary growth approach leading to creation of a secondary lattice with subsequent expansion on preformed CsPbBr3 perovskite nanocrystals is reported. As direct layer growth by adding precursors was not successful, Cs-lattice extension to preformed CsPbBr3 nanocrystals was performed by coupling CsBr to these nanocrystals. Opening both {110}/{002} and {200} facets of parent CsPbBr3 nanocrystals, CsBr was observed to be connected with lattice matching to the {200} facets. Further with Pb(II) incorporation, the Cs-sublattices of CsBr were expanded to CsPbBr3 and led to cube-couple nanocrystals. However, as cubes in these nanostructures were differently oriented, these showed lattice mismatch at their junctions. This lattice mismatch though restricted complete shelling but successfully favored the secondary growth on specific facets of parent CsPbBr3 nanocrystals. Details of this secondary growth via lattice extension and expansion are microscopically analyzed and reported. These results further suggest that lead halide perovskite nanocrystals can be epitaxially grown under proper reaction design and more complex as well as heterostructures of these materials can be fabricated to meet the current demands.

Nanocrystal heterostructures are one of the front-line energy materials widely known for enhancing the rate of photocatalysis and tuning optical properties and are also used in photovoltaics. These nanostructures where two crystalline nanomaterials are placed together in a single building block and share lattices are extensively studied for chalcogenide semiconductors and metal oxides but limited for recently emerged perovskite nanocrystals. Due to differences in the crystal bonding nature, interface chemistry, and also the formation mechanism, some constraints are present in designing common reaction pathways for the simultaneous formation of perovskites and chalcogenides or similar nanocrystals sharing common lattice planes. Hence, more fundamental understanding of both nucleation and growth of both materials is required for inducing heteronucleations of one on the surface of another. Literature reports also revealed that epitaxial growth of lead halide perovskites with nonhalide or chalcogenide colloidal 0D nanocrystals could not be established yet. Hence, the field remains challenging, and more investigations on both experimental as well as theoretical studies are timely required. Keeping these in mind, in this perspective the state-of-the-art issues related to the formation of all inorganic halides and nonhalide heterostructures formed with epitaxial relations are discussed. At the beginning, the chemistry of these nanocrystals with similarities and dissimilarities in their nature and examples of different heterostructures are summarized, and then, different synthetic possibilities for overtaking the hurdles and designing such heterostructures are proposed.

Lead halide hybrid perovskite nanocrystals are considered as one of the most efficient light-harvesting materials in current research. However, unlike all-inorganic perovskite nanocrystals, the quality optimization and synthetic progress for facet tuning of these hybrid nanocrystals have not moved forward at the same pace. To meet the current demand for high-quality, facet-tuned, and monodisperse nanocrystals, herein, surface A-site cations' population-controlled facet tuning of FAPbBr(3) nanocrystals is reported. Using specific alkylammonium ligands obtained from alpha-bromo ketones and comparing their concentration with FA(+), cubic, dodecahedral, and quasi-spherical shapes of nanocrystals in cubic phase are obtained having monodispersity comparable to that of their best reports. In addition, upon changing of surface ligands, spontaneous formation of self-assembled blue-emitting platelets was also observed. Because of the importance of these materials, these findings will aid the fundamental understanding of formation of hybrid perovskite nanocrystals and also support accelerating their widespread applications.

Lead halide perovskite nanocrystals, whether formed by their own nucleation and growth or by ion diffusion into the lattice of others, are still under investigation. Moreover, beyond isotropic nanocrystals, fabricating anisotropic perovskite nanocrystals by design has remained difficult. Exploring the lattice of orthorhombic-phase Cs2ZnBr4 with the complete replacement of Zn tetrahedra by Pb octahedra, dimension-tunable anisotropic nanocrystals of CsPbBr3 are reported. This B-site ion introduction led to CsPbBr3 nanorods having [100] as major axis, in contrast with all reports on rods/wires where the lengths were along the [001] direction. This was possible by using derivatives of α-bromo ketones, which helped in tuning the shape of Cs2ZnBr4 and also the facets of transformed CsPbBr3. While similar experiments are extended to orthorhombic Cs2HgBr4, standard nanorods with [001] as the major axis were observed. From these results, it is further concluded that anisotropic perovskite nanocrystals might not follow any specific rules for directional growth and instead might depend on the structure of the parent lattice.

Bright emitting lead halide perovskite nanocrystals reported to date are mostly obtained in a six-faceted perfect cube shape. While extensive research has been carried out for intensifying and stabilizing the bright emission of these nanocrystals, little attention has been paid to obtaining isotropic (sphere-like) nanocrystals in other shapes. Using a nanocrystal-to-nanocrystal dissolution formation protocol and under limited ligands and insufficient halides in reaction medium, herein, dimension-variable faceted isotropic orthorhombic phase CsPbBr3 nanocrystals beyond the cubic shape are reported, and their optical properties are studied. The size and facets of these nanocrystals were also tuned as a function of injection temperature. However, unlike cube-shaped samples, because these nanocrystals were prepared under ligand- and halide-deficient conditions, they showed poor emission efficiency. Because these were colloidally stable, beyond light-emitting devices, these might be useful as a potential material in other applications where suppression of charge carrier recombination is essential.

Connecting nanocrystals with removal of interface ligand barriers is one of the key steps for efficient carrier transportation in optoelectronic device fabrication. Typically, ion migration for crystal deformation or connection with other nanocrystals needs a solvent as medium. However, on the contrary, this has been observed for CsPbBr3 perovskite nanocrystals in film where nanocrystals were swollen to get wider and fused with adjacent nanocrystals in self-assembly on film during solvent evaporation. Depending on precursor composition and exposed facets, again these connections could be programmed for tuning their connecting directions leading to different shapes. Aging further on solid substrate, these were also turned to continuous film of nanostructures eliminating all interparticle gaps on the film. This transformation could be ceased at any point of time, simply by heating or adding sufficient ligands. Analysis suggested that these unique and controlled connections were only observed with polyhedron shaped nanostructures with certain compositions and not with traditionally cubes. Details of this solid-surface transformation during solvent evaporation were analyzed, and an interparticle material transfer type mechanism was proposed. As these observations were not seen in chalcogenide and oxide nanocrystals and exclusively observed in perovskite nanocrystals, this would add new fundamentals to the insights of crystal growths of nanocrystals and would also help in obtaining films of connecting nanocrystals.

Light-emitting lead halide perovskite nanocrystals are typically obtained in a six-faceted cube shape. However, for applications such as catalysis, more active facets for the adsorption/desorption of reactants/products and the suppression of carrier recombination are essentially required. To meet these challenges, herein CsPbBr3 perovskite nanocrystals in cube and faceted noncube shapes were explored for photocatalytic reductions of CO2. Importantly, halide-deficient dim multifaceted noncube emitters having less than 1% photoluminescence quantum yields showed superior catalytic activity compared to that of bright halide-rich cube nanocrystals. Beyond these, hexapod-shaped nanocrystals were also explored, and these remained in an intermediate state. With the support of density functional theory, the adsorption and desorption probabilities of reactants/products on different facets were also calculated and correlated with experimental findings. These results indicated that facets and defects of perovskite nanocrystals are equally important for carrying out catalytic reactions.

While solution-processed doping of Mn ions in 3D lead halide perovskites is extensively studied, the chemistry for Mn doping in 2D layered perovskites is limited. Following a generic solution-phase colloidal approach in the presence of alkylammonium salts, formation of single-crystalline microcrystals of Mn-doped layered perovskites (L2PbX4, X = Cl, Br, I) is reported. While Mn was present in all microstructures, only L2PbBr4 led to Mn d-d emission with high quantum yield (similar to 6I%). These doped layered structures showed robust stability and even retained the original emission in continuous thermocycling or constant heating at 200 degrees C in air for more than 24 h. Moreover, these materials also showed solid-state thermal annealing induced 2D agglomeration leading to a larger structure, which was reflected from optical microscopic images and the enhanced intensity of powder XRD peaks that originated from the layered structure. Apart from the generic synthesis, these results also provided several new fundamental insights on doping and doped 2D perovskites, which were timely required for the advancement of the materials property and understanding the growth mechanism of these materials.

Lead halide perovskite nanocrystals have recently emerged as an efficient optical material for light harvesting. While these have been extensively studied for obtaining bright emissions, their use as catalysts for enhancing the rate of chemical reactions has been explored little. Considering their importance in catalysis, herein, Fe(II)-doped CsPbBr3 perovskite nanocrystals have been explored for photocatalytic reduction of CO2. In comparison to undoped CsPbBr3, doped nanocrystals showed enhanced catalytic activity and also predominantly led to evolution of CH4 instead of CO. The observation of a reverse trend of predominated CH4 evolution in doped nanocrystals rather than CO observed for undoped nanocrystals was correlated to the adsorption/desorption energy of respective products established theoretically earlier. This selective evolution of major products on doping remained unique and also a step forward for understanding more regarding light to chemical energy conversions using perovskite nanocrystals.