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The conversion of metal halides to lead halide perovskites with B-site metal ion diffusion has remained a convenient approach for obtaining shape-modulated perovskite nanocrystals. These transformations are typically observed for materials having a common A-site Cs-sublattice platform. However, due to the fast reactions, trapping the interconversion process has been difficult. In an exploration of the tetragonal phase of Cs 7 Cd 3 Br 13 platelets as the parent material, herein, a slower diffusion of Pb(II) leading to facet-modulated CsPbBr 3 platelets is reported. This was expected due to the presence of Cd(II) halide octahedra along with Cd(II) halide tetrahedra in the parent material. This helped in microscopically monitoring their phase transformation via an epitaxially related core/shell intermediate heterostructure. The transformation was also derived and predicted by density functional theory calculations. Further, when the reaction chemistry was tuned, core/shell platelets were transformed to different facet-modulated and hollow CsPbBr 3 platelet nanostructures. These platelets having different facets were also explored for catalytic CO 2 reduction, and their catalytic rates were compared.

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 CsPbBr 3 perovskite nanocrystals is reported. As direct layer growth by adding precursors was not successful, Cs-lattice extension to preformed CsPbBr 3 nanocrystals was performed by coupling CsBr to these nanocrystals. Opening both {110}/{002} and {200} facets of parent CsPbBr 3 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 CsPbBr 3 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 CsPbBr 3 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.

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 Cs 2 ZnBr 4 with the complete replacement of Zn tetrahedra by Pb octahedra, dimension-tunable anisotropic nanocrystals of CsPbBr 3 are reported. This B-site ion introduction led to CsPbBr 3 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 Cs 2 ZnBr 4 and also the facets of transformed CsPbBr 3 . While similar experiments are extended to orthorhombic Cs 2 HgBr 4 , 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.

Herein, we report a complexation reaction between Zn2+ ions present on the surface of an orange-red-emitting environmentally sustainable Mn2+-doped ZnS QD and a non-emitting copper quinolate (CuQ2) complex, which leads to the formation of a greenish blue-emitting surface zinc quinolate (ZnQ2) complex. The synchronous contribution of the surface ZnQ2 complex and Mn2+-doped ZnS QD is directed towards the generation of photostable bright white light (at λex - 355 nm) with chromaticity coordinates of (0.34, 0.42), color rendering index (CRI) of 71 and color-correlated temperature (CCT) of 5046 K. The ZnQ2 complexed Mn2+-doped ZnS QD is herein called as quantum dot complex (QDC). The excitation- and time-dependent tunability in emission, chromaticity, CRI and CCT of QDC revealed their futuristic applications in light-emitting devices with an anticipated color output. The current work also shows the catalytic behavior of Mn2+-doped ZnS QDs towards facilitating the formation of surface ZnQ2 from CuQ2, which is not feasible with regard to the reactivity of CuQ2 under normal conditions according to the Irving-William series. The rate of the reaction was observed to be first order with respect to CuQ2 at 20 °C, and the complexation constant for the formation of ZnQ2 was estimated to be 8.3 × 105 M-1. This is important for understanding the surface chemistry of metal chalcogenide QDs towards complexation reactions.

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 CsPbBr 3 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.

Highly emissive isotropic CsPbX 3 (X = Cl, Br, and I) perovskite nanocrystals are typically observed in a six-faceted cube shape. When a unique approach is adopted and the reaction medium is enriched with halides, arm growth on all six facets was carried out and reported. Analysis suggested that these armed nanostructures were obtained from intermediate polyhedron shaped structures having 26 facets, and these were formed under halide-deficient conditions. Surface energy calculations further supported the possible existence of all facets for both of these structures under different halide composition environments. The entire study was first explored for CsPbBr 3 and then extended to CsPbCl 3 ; however, for CsPbI 3 nanocrystals, Sr(II) dopant was used for obtaining stable emission. Arm lengths could also be tuned with a function of reaction temperature for CsPbBr 3 . Formation of stable facets in polyhedron shaped nanostructures and their transformation to respective hexapods under halide-deficient and halide-rich conditions add new fundamental concepts for these nanostructures and their shape evolutions.