Terahertz Spectroscopic Probe of Hot Electron and Hole Transfer from Colloidal CsPbBr3 Perovskite Nanocrystals

A photoelectrochemical capacitor using polyoxometalates coupled with semiconductor nanocrystals as the photosensitizer

Solar energy storage technology ensures a sustainable and reliable energy supply. Herein, we show that electrons generated in semiconductor nanocrystals (NCs) of CsPbBr3 by visible light excitation can be stored in polyoxometalates (POMs) of W10O32, and extracted as an electric current using a photoelectrochemical cell.

Size Dependence of Ultrafast Electron Transfer from Didodecyl Dimethylammonium Bromide-Modified CsPbBr3 Nanocrystals to Electron Acceptors

Charge transfer efficiencies in all-inorganic lead halide perovskite nanocrystals (NCs) are crucial for applications in photovoltaics and photocatalysis. Herein, CsPbBr3 NCs with different sizes are synthesized by varying the ligand contents of didodecyl dimethylammonium bromide at room temperature. Adding benzoquinone (BQ) molecules leads to a decrease in the PL intensities and PL decay times in NCs. The electron transfer (ET) efficiency (ηET) increases with NC size in complexes of CsPbBr3 NCs and BQ molecules (NC-BQ complexes), when the same concentration of BQ is maintained, as investigated by transient photobleaching and photoluminescence spectroscopies. Controlling the same number of attached BQ acceptor molecules per NC induces the same ηET in NC-BQ complexes even though with different NC sizes. Our findings provide new insights into ultrafast charge transfer behaviors in perovskite NCs, which is important for designing efficient light energy conversion devices.

Unraveling the cation dependent carrier cooling and transient mobility in lead-free A3Sb2I9 perovskites

Lead halide perovskites (LHPs) have gained prominence for their exceptional photophysical properties, holding promise for applications in high-end optoelectronic devices. However, the presence of lead is one of the major obstacles to the commercialization of LHPs in the field of photovoltaics. To address this, researchers have explored environment friendly lead-free perovskite solar cells by investigating non-toxic perovskite materials. This study explores the enhancement of photophysical properties through chemical engineering, specifically cation exchange, focusing on the crucial photophysical process of hot carrier cooling. Employing femtosecond transient absorption spectroscopy and optical pump terahertz probe spectroscopy, we have probed the carrier relaxation dynamics in A3Sb2I9 with cesium and rubidium cations. This study unravels that the carrier relaxation is found to be slower in Rb3Sb2I9; along with this, the transient mobility decay is found to be retarded. Overall, this study suggests that an antimony-based Rb3Sb2I9 perovskite could be a substantial lead-free perovskite in photovoltaics. These findings provide valuable insights into cation engineering strategies, aiming to improve the overall performance of lead-free-based photovoltaic devices.

Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Interfacial Charge Transfer Regulates Photoredox Catalysis

Photoredox catalytic processes offer the potential for precise chemical reactions using light and materials. The central determinant is identified as interfacial charge transfer, which simultaneously engenders distinctive behavior in the overall reaction. An in-depth elucidation of the main mechanism and highlighting of the complexity of interfacial charge transfer can occur through both diffusive and direct transfer models, revealing its potential for sophisticated design in complex transformations. The fundamental photophysics uncover these comprehensive applications and offer a clue for future development. This research contributes to the growing body of knowledge on interfacial charge transfer in photoredox catalysis and sets the stage for further exploration of this fascinating area of research.

Structural Modification Strategies, Interfacial Charge-Carrier Dynamics, and Solar Energy Conversion Applications of Organic-Inorganic Halide Perovskite Photocatalysts

Over the past few decades, organic-inorganic halide perovskites (OIHPs) as novel photocatalyst materials have attracted intensive attention for an impressive variety of photocatalytic applications due to their excellent photophysical (chemical) properties. Regarding practical application and future commercialization, the air-water stability and photocatalytic performance of OIHPs need to be further improved. Accordingly, studying modification strategies and interfacial interaction mechanisms is crucial. In this review, the current progress in the development and photocatalytic fundamentals of OIHPs is summarized. Furthermore, the structural modification strategies of OIHPs, including dimensionality control, heterojunction design, encapsulation techniques, and so on for the enhancement of charge-carrier transfer and the enlargement of long-term stability, are elucidated. Subsequently, the interfacial mechanisms and charge-carrier dynamics of OIHPs during the photocatalytic process are systematically specified and classified via diverse photophysical and electrochemical characterization methods, such as time-resolved photoluminescence measurements, ultrafast transient absorption spectroscopy, electrochemical impedance spectroscopy measurements, transient photocurrent densities, and so forth. Eventually, various photocatalytic applications of OIHPs, including hydrogen evolution, CO2 reduction, pollutant degradation, and photocatalytic conversion of organic matter.

Modulation of carrier conduction in CsPbBr3 perovskite quantum dots with band-aligned electron and hole acceptors

The lead halide perovskites have emerged as promising materials with intriguing photo-physical properties and have immense potential for photovoltaic applications. A comprehensive study on the kinetics of charge carrier (electron/hole) generation and transfer across the interface is key to realizing their future scope for efficient device engineering. Herein, we investigate the interfacial charge transfer (CT) dynamics in cesium lead halide (CsPbBr3) perovskite quantum dots (PQDs) with energetically favorable electron acceptors, anthraquinone (AQ) and p-benzoquinone (BQ), and hole acceptors such as pyrene and 4-(dimethylamino)pyridine (DMAP). With various steady-state and time-resolved spectroscopic and microscopic measurements, a faster electron transfer rate is estimated for CsPbBr3 PQDs with BQ compared to that of AQ, while a superior hole transfer for DMAP is divulged compared to pyrene. In concurrence with the spectroscopic measurements, conducting atomic force microscopic studies across the electrode-PQD-electrode junction reveals an increment in the conductance of the PQD in the presence of both the electron and hole acceptors. The variation of the density of states calculation in the presence of the hole acceptors offers strong support and validation for faster CT efficiency. The above findings suggest that a careful selection of simple yet efficient molecular arrangements can facilitate rapid carrier transfer, which can be designed as auxiliary layers for smooth CT and help in the engineering of cost-effective photovoltaic devices.

The Dark Side of Lead-Free Metal Halide Nanocrystals: Substituent-Modulated Photocatalytic Activity in Benzyl Bromide Reduction

We illustrate here the high photocatalytic activity of sustainable lead-free metal halide nanocrystals (NCs), namely, Cs₃Sb₂Br₉ NCs, in the reduction of p-substituted benzyl bromides in the absence of a cocatalyst. The electronic properties of the benzyl bromide substituents and the substrate affinity to the NC surface determine the selectivity in C-C homocoupling under visible light irradiation. This photocatalyst can be reused for at least three cycles and preserves its good performance with a turnover number of ca. 105,000.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Electronic effect of substituents on the charge-transfer dynamics at the CsPbBr3 perovskite-small molecule interface

To push the boundary of the efficiency of perovskite nanocrystal-based photovoltaics, understanding the charge transfer at the interface of these nanocrystals is necessary. In an effort to understand the electronic effects of the substituents in the charge acceptor moiety, three electronically different small molecules (namely, chloranilic acid (CA), p-benzoquinone (BQ), and duroquinone (DQ)) were chosen and their detailed charge transfer dynamics were studied at the CsPbBr₃ perovskite nanocrystal-small organic molecule interface using steady state and time-resolved spectroscopic methods. The steady-state absorption and time-resolved emission studies reveal that all three molecules interact with the NCs in the excited state. Femtosecond transient absorption experiments indicate a faster ground-state bleach recovery in the presence of the three acceptors, compared with the pristine NCs. Utilizing band alignment analysis, the faster bleach recovery of the NCs in presence of the acceptors was confirmed to be because of electron transfer from the photo-excited NCs to the acceptor molecules. Moreover, the electron transfer rates fall in the Marcus normal region and can be explained based on the electronic effects of the substituents present on the acceptor molecules.

Unraveling the Hole-Transport-Layer-Manipulated Carrier Transfer Dynamics in Perovskite Light-Emitting Diodes

Charge transfer dynamics is decisive for the performance of perovskite light emitting diodes (PeLEDs), and deep insight into the charge transfer process inside the working device is indispensable. Here, the influence of the hole transport layer on charge transport and recombination processes in PeLEDs is investigated via impedance spectroscopy. The results demonstrate that the rational interfacial energy level alignment can improve the radiative recombination by reducing the leakage current and carrier transport resistance. Shockley-Read-Hall recombination and Auger recombination enlarge the lifetime of carrier transfer in the working devices as determined from the electroluminescence spectrum. Our work provides a distinctive and reliable method to explore the charge transfer property and highlights the importance of interfaces to boost the performance of PeLEDs.

Multipulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites

Hot carriers in metal-halide perovskites (MHPs) present a foundation for understanding carrier-phonon coupling in the materials as well as the prospective development of high-performance hot carrier photovoltaics. While the carrier population dynamics during cooling have been scrutinized, the evolution of the hot carrier properties, namely mobility, remains largely unexplored. Here we introduce novel ultrafast visible pump-infrared push-terahertz probe spectroscopy to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide. We find a decrease in mobility upon optically re-exciting the carriers, as expected for band transport. Surprisingly, the conductivity recovery is incommensurate with the hot carrier population dynamics measured by infrared probe and exhibits a negligible dependence on the hot carrier density. Our results reveal the importance of localized lattice heating toward the hot carrier mobility. This collective polaron-lattice phenomenon may contribute to the unusual photophysics of MHPs and should be accounted for in hot carrier devices.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Probing the emissive behaviour of the lead-free Cs2AgBiCl6 double perovskite with Cu(ii) doping

Cu ion induced change in photoluminescence behaviour of Cs2AgBiCl6 double perovskite.

Optical control of terahertz plasmon-induced transparency based on hybrid CsPbBr3 quantum dot metasurfaces

Incorporating photosensitive material into structured metamaterials explores opportunities for dynamical operation across the terahertz functional devices, enabled by the efficient interaction between light and matter. In this work, the CsPbBr₃ quantum dots are incorporated into the metasurfaces, realizing the active control of the plasmon-induced transparency. In the experiment, the normalized modulation depth of transparency effect is up to 74%. Rigorous numerical and theoretical simulations verify that the variation of dynamic physical process is associated with the charge storage capacity in the capacitive metasurface. An observed phase advance and group delay indicate the hybrid metasurface is useful for slow light application. In addition, the simple process provides a convenient way for the development of terahertz functional devices.

Phonon-Mediated and Weakly Size-Dependent Electron and Hole Cooling in CsPbBr3 Nanocrystals Revealed by Atomistic Simulations and Ultrafast Spectroscopy

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ∼10 ps) of "warm" electrons in the vicinity (within ∼0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps-1 carrier-1, increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.

Photoexcited carrier dynamics in colloidal quantum dot solar cells: insights into individual quantum dots, quantum dot solid films and devices

The certified power conversion efficiency (PCE) record of colloidal quantum dot solar cells (QDSCs) has considerably improved from below 4% to 16.6% in the last few years. However, the record PCE value of QDSCs is still substantially lower than the theoretical efficiency. So far, there have been several reviews on recent and significant achievements in QDSCs, but reviews on photoexcited carrier dynamics in QDSCs are scarce. The photovoltaic performances of QDSCs are still limited by the photovoltage, photocurrent and fill factor that are mainly determined by the photoexcited carrier dynamics, including carrier (or exciton) generation, carrier extraction or transfer, and the carrier recombination process, in the devices. In this review, the photoexcited carrier dynamics in the whole QDSCs, originating from individual quantum dots (QDs) to the entire device as well as the characterization methods used for analyzing the photoexcited carrier dynamics are summarized and discussed. The recent research including photoexcited multiple exciton generation (MEG), hot electron extraction, and carrier transfer between adjacent QDs, as well as carrier injection and recombination at each interface of QDSCs are discussed in detail herein. The influence of photoexcited carrier dynamics on the physiochemical properties of QDs and photovoltaic performances of QDSC devices is also discussed.

Investigation of the Mn dopant-enhanced photoluminescence performance of lead-free Cs2AgInCl6 double perovskite crystals

The lead-free double perovskite Cs2AgInCl6 is a potential candidate for LEDs, the photoluminescence performance of which is reinforced greatly by Mn doping. Here, we analyzed the geometric, electronic and photoluminescence properties of Mn-doped Cs2AgInCl6 by means of first-principle calculations. We found that in the interior of Cs2AgInCl6, the Mn dopant formed defect complexes by substituting an Ag atom and generating an Ag vacancy (MnAgVAg) owing to the charge balance and the weak distortion of the metal octahedra. The MnAgVAg defect introduced two defect bands in the forbidden gap, which was contributed predominantly by the 3d orbitals of the Mn2+ ions. The electron transition of the Mn2+ ions from the first excited state to the ground state, i.e., from 4T1 to 6A1 states, gives rise to the PL spectrum that is lower than the bandgap. Therefore, we show that the Mn dopant indeed reinforces the PL performance of Cs2AgInCl6 greatly and is beneficial for its use as an LED material.

Influence of the rate of radiation energy on the charge-carrier kinetics application of all-inorganic CsPbBr3 perovskite nanocrystals

In the field of optoelectronics, all-inorganic CsPbBr₃ perovskite nanocrystals (PNCs) have gained significant interest on account of their superb processability and ultra-high stability among all the counterparts. In this study, we conducted an in-depth analysis of CsPbBr₃ PNCs using joint transient optical spectroscopies (time-resolved photoluminescence and ultrafast transient absorption) in a very comprehensive manner. In order to understand the in-depth analysis of excited-state kinetics, the transient absorption spectroscopy has been performed. The structure of interest of CsPbBr₃ PNCs was subjected to the rates of the radiation energy of 0.10 mW (κ_r/κ_nr = ∼0.62) and 0.30 mW (κ_r/κ_nr = ∼0.64). With the rate of radiation energy 0.30 mW, it was observed that there was a significant increase in hot carrier relaxation together with high radiative recombination, resulting in a decrease in charge trappings. Herein, we demonstrate that the tuning of the rate of radiation energies helps to understand the charge-carrier kinetics of CsPbBr₃ PNCs, which would thus improve the manufacturing of efficient photovoltaic devices.

A mechanistic framework for hot carrier cooling process in CsPbBr₃ PNC is depicted via transient absorption spectroscopy.

Slow Hot-Carrier Cooling in Halide Perovskites: Prospects for Hot-Carrier Solar Cells

Rapid hot-carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot-carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot-carrier properties: slow hot-carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, long-range hot-carrier transport (up to ≈600 nm), and highly efficient hot-carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot-carrier cooling properties in halide perovskites and prospects for perovskite hot-carrier solar cells are also discussed.

Conductivity Tuning via Doping with Electron Donating and Withdrawing Molecules in Perovskite CsPbI3 Nanocrystal Films

Doping of semiconductors enables fine control over the excess charge carriers, and thus the overall electronic properties, crucial to many technologies. Controlled doping in lead-halide perovskite semiconductors has thus far proven to be difficult. However, lower dimensional perovskites such as nanocrystals, with their high surface-area-to-volume ratio, are particularly well-suited for doping via ground-state molecular charge transfer. Here, the tunability of the electronic properties of perovskite nanocrystal arrays is detailed using physically adsorbed molecular dopants. Incorporation of the dopant molecules into electronically coupled CsPbI₃ nanocrystal arrays is confirmed via infrared and photoelectron spectroscopies. Untreated CsPbI₃ nanocrystal films are found to be slightly p-type with increasing conductivity achieved by incorporating the electron-accepting dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄ TCNQ) and decreasing conductivity for the electron-donating dopant benzyl viologen. Time-resolved spectroscopic measurements reveal the time scales of Auger-mediated recombination in the presence of excess electrons or holes. Microwave conductance and field-effect transistor measurements demonstrate that both the local and long-range hole mobility are improved by F₄ TCNQ doping of the nanocrystal arrays. The improved hole mobility in photoexcited p-type arrays leads to a pronounced enhancement in phototransistors.

Ultrafast carrier dynamics of metal halide perovskite nanocrystals and perovskite-composites

Perovskite nanocrystals (NCs), especially those based on cesium lead halides, have emerged in recent years as highly promising materials for efficient solar cells and photonic applications. The key to realization of full potential of these materials lies however in the molecular level understanding of the processes triggered by light. Herein we highlight the knowledge gained from photophysical investigations on these NCs of various sizes and compositions employing primarily the femtosecond pump-probe technique. We show how spectral and temporal characterization of the photo-induced transients provide insight into the mechanism and dynamics of relaxation of hot and thermalized charge carriers through their recombination and trapping. We discuss how the multiple excitons including the charged ones (trions), generated using high pump fluence or photon energy, recombine through the Auger-assisted process. We discussed the harvesting of hot carriers prior to their cooling and band-edge carriers from these perovskite NCs to wide band-gap metal oxides, metal chalcogenide NCs and molecular acceptors. How perovskites can influence the charge carrier dynamics in composites of organic and inorganic semiconductors is also discussed.

Ultrafast THz Probe of Photoinduced Polarons in Lead-Halide Perovskites

We study the nature of photoexcited charge carriers in CsPbBr_{3} nanocrystal thin films by ultrafast optical pump-THz probe spectroscopy. We observe a deviation from a pure Drude dispersion of the THz dielectric response that is ascribed to the polaronic nature of carriers; a transient blueshift of observed phonon frequencies is indicative of the coupling between photogenerated charges and stretching-bending modes of the deformed inorganic sublattice, as confirmed by DFT calculations.

Acid-Amine Equilibria for Formation and Long-Range Self-Organization of Ultrathin CsPbBr3 Perovskite Platelets

Impact of acid-amine equilibrium for triggering the formation of CsPbBr₃ platelets and subsequent long-range self-assembly as a function of acid concentration is reported. The study was performed by treating Cs precursor followed by oleic acid to the ongoing process of L₂PbBr₄ layered perovskites formation at room temperature, and this led to the thinnest possible platelets of CsPbBr₃ with one atomic layer of Cs insertion. These platelets form self-assembly in the micrometer range, and this ordering was tuned as a function of oleic acid concentration and reaction time. With evidence obtained from in situ live monitoring by optical microscopy of these assemblies during the ongoing reaction in solution and analyzing the collected samples ex situ by electron microscopy, it was established that these long-range 1D organizations of platelets took place in solution. Details of the insights of the formation, self-assembly, and impact of different reaction parameters in the formation of these platelets are investigated and reported in this Letter.

Flexible Polymer-Assisted Mesoscale Self-Assembly of Colloidal CsPbBr3 Perovskite Nanocrystals into Higher Order Superstructures with Strong Inter-Nanocrystal Electronic Coupling

Surface-passivating ligands, although ubiquitous to colloidal nanocrystal (NC) syntheses, play a role in assembling NCs into higher order structures and hierarchical superstructures, which has not been demonstrated yet for colloidal CsPbX₃ (X = Cl, Br, and I) NCs. In this work, we report that functional poly(ethylene glycols) (PEG₆-Y, Y = -COOH and -NH₂) represent unique surface-passivating ligands enabling the synthesis of near-uniform CsPbBr₃ NCs with diameters of 3.0 nm. The synthesized NCs are assembled into individual pearl necklaces, bundled pearl necklaces, lamellar, and nanorice superstructures, in situ. It is believed a variety of forces, including van der Waals attractions between hydrophilic PEG tails in a nonpolar solvent and dipole-dipole attraction between NCs, drive mesoscale assembly to form superstructures. Furthermore, postsynthetic ligand treatment strengthens the argument for polymer-assisted mesoscale assembly as pearl necklace assemblies can be successfully converted into either lamellar or nanorice structures. We observe an ∼240 meV bathochromic shift in the lowest energy absorption peak of CsPbBr₃ NCs when they are present in the lamellar and nanorice assemblies, representing strong inter-NC electronic coupling. Moreover, pearl necklace structures are spontaneously assembled into micrometer length scale twisted ribbon hierarchical superstructures during storage of colloidal CsPbBr₃ NCs. The results show that the self-assembled superstructures of CsPbBr₃ NCs are now feasible to prepare via template-free synthesis, as self-assembled structures emerge in the bulk solvent, a process that mimics biological systems except for the use of nonbiological surface ligands (PEG₆-Y). Taken together, emergent optoelectronic properties and higher order superstructures of CsPbBr₃ NCs should aid their potential use in solid-state devices and simplify scalable manufacturing.

Synthesis and luminescence of Mn-doped Cs2AgInCl6 double perovskites

Metal halide double perovskites (DPs) are being explored as stable and non-toxic alternatives of Pb-halide perovskites. Typically DPs exhibit a wide (>2.5 eV) and/or indirect bandgap, limiting their applications in the visible region. Here we impart the visible-light emission property in direct bandgap Cs2AgInCl6 DPs by doping Mn2+ ions. Synthesis, characterization and luminescence of metal halide double perovskites are reported.