Biexciton Emission in CsPbBr3 Nanocrystals: Polar Facet Matters.

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.

Facets-Directed Epitaxially Grown Lead Halide Perovskite-Sulfobromide Nanocrystal Heterostructures and Their Improved Photocatalytic Activity.

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.

Nucleophile-Controlled Halide Release from the Substitution Reaction of Haloketone for Facet Tuning and Manganese Doping in CsPbCl3 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.

Cs-Lattice Extension and Expansion for Inducing Secondary Growth of CsPbBr3 Perovskite Nanocrystals.

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.

Introducing B-Site Cations by Ion Exchange and Shape Anisotropy in CsPbBr3 Perovskite Nanostructures.

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.

Facets and Defects in Perovskite Nanocrystals for Photocatalytic CO2 Reduction.

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.

Facets Directed Connecting Perovskite Nanocrystals.

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.

Doping Iron in CsPbBr3 Perovskite Nanocrystals for Efficient and Product Selective CO2 Reduction.

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.

Coupled Halide-deficient and Halide-rich Reaction System for Doping in Perovskite Armed Nanostructures.

Insights of Mn(II) doping in CsPbCl3-armed hexapod nanostructures is reported. These complex structures were typically formed in halide concentration tuned modulated reactions. Cores were first formed under halide deficient condition and with enriching halides; these were transformed to armed structures. Doping of Mn(II) was observed facilitated during the arm growth in the second stage of the reaction. These observations were supported with decoupled reactions with minimized and maximized halide concentrations carried out in separate reactions. However, less interference for the exciton to dopant energy transfer was noticed for the defect states created in halide-deficient medium, and the intensity of the dopant emission remained proportional to the amount of dopant inserted in the nanocrystals. Being this is an in situ observation in the coupled reactions of both poor and rich halide reaction systems, the finding would strengthen the understanding of doping in perovskite host nanocrystals.

Arm Growth and Facet Modulation in Perovskite Nanocrystals.

Highly emissive isotropic CsPbX3 (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 CsPbBr3 and then extended to CsPbCl3; however, for CsPbI3 nanocrystals, Sr(II) dopant was used for obtaining stable emission. Arm lengths could also be tuned with a function of reaction temperature for CsPbBr3. 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.

Blue-Emitting CsPbCl3 Nanocrystals: Impact of Surface Passivation for Unprecedented Enhancement and Loss of Optical Emission.

High-energy-emitting CsPbCl3 nanocrystals have shown significant loss and enhancement of their emission intensity (∼40-50 folds) during purification and surface treatments, respectively. This confirms that the surfaces of these nanocrystals are very sensitive. In this Letter, physical insights of the interface bindings on the surface of these blue-emitting CsPbCl3 nanocrystals with different passivating agents and their consequential impact on purification are investigated. Using various metal chlorides irrespective of the charge and size of the metal ions, metal acetates, and nonmetal chloride, the predominant influence of chloride ions in helping retrieve/intensify the blue emission is established. The purification processes are observed to be very delicate, and successive purifications with introduction of polar nonsolvents led to the transformation of an emitting cubic CsPbCl3 phase to nonemitting tetragonal CsPb2Cl5 phase nanocrystals irreversibly. The impact of various salt additions only temporarily helped in enhancing the emission, but the phase change remained inevitable upon successive purification. However, as a remedy, by in situ use of alkylammonium chloride salt in high-temperature reactions, the surface binding was improved, and significant emission as well as the phase could be retained with successive purifications.

Annealing CsPbX3 (X = Cl and Br) Perovskite Nanocrystals at High Reaction Temperatures: Phase Change and Its Prevention.

Annealing perovskite nanocrystals at high reaction temperature changes their crystal phase, shape, and optical properties. Carrying out reactions between 180 and 250 °C, the impact of thermal annealing for CsPbCl3 and CsPbBr3 nanocrystals in a reaction flask was investigated here. At higher temperature, a phase change was observed instantly, which could not be trapped even with ice-bath cooling. Interestingly, using a calculated amount of preformed alkylammonium halides as dual passivating agents, the nanocrystals of both CsPbCl3 and CsPbBr3 could even be stabilized for hours of annealing at 250 °C. CsPbCl3, which was reported to be a poor emissive nanocrystal in comparison to CsPbBr3, could sustain even more than 5 h of annealing at 250 °C and recorded ∼51% absolutely quantum yield. Details of the interface chemistry and the role of the used dual passivating agent for providing thermal stability are studied and reported in this Letter.

Metal Semiconductor Heterostructures for Photocatalytic Conversion of Light Energy.

For fast separation of the photogenerated charge carriers, metal semiconductor heterostructures have emerged as one of the leading materials in recent years. Among these, metal Au coupled with low bandgap semiconductors remain as ideal materials where both can absorb the solar light in the visible region. It is also established that on excitation, the plasmonic state of gold interacts with excited state of semiconductor and helps for the delocalization of the photogenerated electrons. Focusing these materials where electron transfer preferably occurs from semiconductor to metal Au on excitation, in this Perspective, we report the latest developments in the synthetic chemistry in designing such nano heterostructures and discuss their photocatalytic activities in organic dye degradation/reduction and/or photocatalytic water splitting for generation of hydrogen. Among these, materials such as Au-CZTS, Au-SnS, Au-Bi2S3, Au-ZnSe, and so forth are emphasized, and their formation chemistry as well as their photocatalytic activities are discussed in this Perspective.