Insight into the Ligand-Mediated Synthesis of Colloidal CsPbBr3 Perovskite Nanocrystals: The Role of Organic Acid, Base, and Cesium Precursors

Blue-emitting and self-assembled thinner perovskite CsPbBr3 nanoplates: synthesis and formation mechanism

Low dimensional semiconductor nanomaterials show great promise for a variety of applications due to their size-dependent and excellent optoelectronic properties. In this work, we developed a strategy to synthesize uniform and very thin CsPbBr₃ perovskite nanoplates (NPls) by introducing additional metal bromides. The CsPbBr₃ NPls, self-assembled into a face-to-face stacked state, had a thickness of 4.4 nm (equal to only 2 monolayers, 2 MLs) and showed a maximum emission at 437 nm and a narrow FWHM of 14 nm. The formation mechanism of the CsPbBr₃ NPls by adding FeBr₃ was ascribed to the constrained growth of CsPbBr₃ nanocubes when the surface Cs⁺ ions were substituted by the protonated oleylammonium from the byproduct OLA-HBr.

Ammonium acetate passivated CsPbI3 perovskite nanocrystals for efficient red light-emitting diodes

Lead halide perovskite nanocrystals (PNCs) have very recently emerged as promising emitters for their superior optoelectronic properties. However, the defects in perovskite itself make it susceptible to the external environment and internal ion migration, resulting in low photoluminescence quantum yield (PLQY) and poor device efficiency. Herein, we developed a method to reduce the surface defects of PNCs by introducing ammonium acetate in the synthesis of CsPbI₃ PNCs. The addition of ammonium acetate can effectively eliminate undesired surface metallic lead cations and dangling bonds, resulting in an enhanced PLQY and stability. The passivated PNCs have an overall up-shift energy level, demonstrating better hole injection efficiency. As a result, the red light-emitting diodes (LEDs) fabricated with the passivated CsPbI₃ PNCs achieved an optimal EQE of 10.6% and a maximum brightness of 981 cd m^-2, which are 3.1 and 2.4 times that of unpassivated PNCs based devices, respectively.

Hollow metal halide perovskite nanocrystals with efficient blue emissions

Metal halide perovskite nanocrystals (NCs) have emerged as new-generation light-emitting materials with narrow emissions and high photoluminescence quantum efficiencies (PLQEs). Various types of perovskite NCs, e.g., platelets, wires, and cubes, have been discovered to exhibit tunable emissions across the whole visible spectrum. Despite remarkable advances in the field of perovskite NCs, many nanostructures in inorganic NCs have not yet been realized in metal halide perovskites, and producing highly efficient blue-emitting perovskite NCs remains challenging and of great interest. Here, we report the discovery of highly efficient blue-emitting cesium lead bromide (CsPbBr₃) perovskite hollow NCs. By facile solution processing of CsPbBr₃ precursor solution containing ethylenediammonium bromide and sodium bromide, in situ formation of hollow CsPbBr₃ NCs with controlled particle and pore sizes is realized. Synthetic control of hollow nanostructures with quantum confinement effect results in color tuning of CsPbBr₃ NCs from green to blue, with high PLQEs of up to 81%.

Composition-, Size-, and Surface Functionalization-Dependent Optical Properties of Lead Bromide Perovskite Nanocrystals

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on surface passivation. We present a study on the temperature-dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by surface passivation, and a postsynthesis ligand exchange with a quaternary ammonium bromide (QAB) results in more stable passivation over a larger temperature range. The PL intensity is highest from 50 to 250 K, which indicates that ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), surface passivation still needs to be improved to achieve maximum emission intensity in nanocrystal films.

Individual Cloud-Based Fingerprint Operation Platform for Latent Fingerprint Identification Using Perovskite Nanocrystals as Eikonogen

Fingerprint formed through lifted papillary ridges is considered the best reference for personal identification. However, the currently available latent fingerprint (LFP) images often suffer from poor resolution, have a low degree of information, and require multifarious steps for identification. Herein, an individual Cloud-based fingerprint operation platform has been designed and fabricated to achieve high-definition LFPs analysis by using CsPbBr₃ perovskite nanocrystals (NCs) as eikonogen. Moreover, since CsPbBr₃ NCs have a special response to some fingerprint-associated amino acids, the proposed platform can be further used to detect metabolites on LFPs. Consequently, in virtue of Cloud computing and artificial intelligence (AI), this study has demonstrated a champion platform to realize the whole LFP identification analysis. In a double-blind simulative crime game, the enhanced LFP images can be easily obtained and used to lock the suspect accurately within one second on a smartphone, which can help investigators track the criminal clue and handle cases efficiently.

Heating-up synthesis of cesium bismuth bromide perovskite nanocrystals with tailored composition, morphology, and optical properties

This study represents the heating-up synthesis of lead-free cesium bismuth bromide perovskite nanocrystals (NCs). CsBr and BiBr3 precursors are used to synthesize uniform and phase-pure cesium bismuth bromide NCs, and the reaction is performed via an injection-free, heating-up method in the presence of a solvent mixture with a high boiling point. The size and composition of cesium bismuth bromide NCs are readily controlled by changing the reaction time, temperature, and amount of surfactant added to the reaction mixture. Upon heating, sequential phase evolution occurs, resulting in the formation of kinetically stable BiOBr in the early reaction stages, which transformed into the thermodynamically stable Cs3BiBr6 and Cs3Bi2Br9 with an increase in either the reaction time or the reaction temperature. Furthermore, the absorption and photoluminescence properties of Cs3BiBr6 and Cs3Bi2Br9 NCs are characterized to investigate their composition-dependent optical properties. This work provides the potential to synthesize various types of lead-free perovskite NCs by tailoring the size and compositions.

Enhanced Photocurrent Owing to Shuttling of Charge Carriers across 4-Aminothiophenol-Functionalized MoSe2-CsPbBr3 Nanohybrids

Mixed-dimensional van der Waals nanohybrids (MvNHs) of two-dimensional transition-metal dichalcogenides (TMDs) and zero-dimensional perovskites are highly promising candidates for high-performance photonic device applications. However, the growth of perovskites over the surface of TMDs has been a challenging task due to the distinguishable surface chemistry of these two different classes of materials. Here, we demonstrate a synthetic route for the design of MoSe₂-CsPbBr₃ MvNHs using a bifunctional ligand, i.e., 4-aminothiophenol. Close contact between these two materials is established via a bridge that leads to the formation of a donor-bridge-acceptor system. The presence of the small conjugated ligand facilitates faster charge diffusion across MoSe₂-CsPbBr₃ interfaces. Density functional theory calculations confirm the type-II band alignment of the constituents within the MvNHs. The MoSe₂-CsPbBr₃ nanohybrids show much higher photocurrent (∼2 × 10⁴-fold photo-to-dark current ratio) as compared to both pure CsPbBr₃ nanocrystals and pristine MoSe₂ nanosheets owing to the synergistic effect of pronounced light-matter interactions followed by efficient charge separation and transportation. This study suggests the use of a bifunctional ligand to construct a nanohybrid system to tune the optoelectronic properties for potential applications in photovoltaic devices.

Origin of the Stability and Transition from Anionic to Cationic Surface Ligand Passivation of All-Inorganic Cesium Lead Halide Perovskite Nanocrystals

Recently, the structural stability of all-inorganic halide perovskite nanocrystals has been significantly enhanced. To understand the enhancement, we developed surface-passivation models for cubic CsPbBr₃ nanocrystals with anionic (oleate) and cationic (oleylammonium) organic ligands based on first-principles calculations and nuclear magnetic resonance investigations. We propose that the (100) surface is initially terminated with oleate ligand complexes on PbBr₂(100) surfaces. Also, the ligand transition to oleylammonium on the Pb-rich surfaces is expected due to the addition of metal halides (ZnBr₂) during colloidal synthesis. The significant improvement in the structural stability of the cationic ligand-passivated CsPbBr₃ nanocrystals was attributed to the suppressed exposure of the merging-vulnerable (110) surface, caused by the large difference in formation energy between the ligand-passivated (100) and Br-passivated (110) surfaces.

Design and synthesis of multigrain nanocrystals via geometric misfit strain

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials^1,2 is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations^3-5. Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a Co₃O₄ nanocube core that carries a Mn₃O₄ shell on each facet. The individual shells are symmetry-related interconnected grains⁶, and the large geometric misfit between adjacent tetragonal Mn₃O₄ grains results in tilt boundaries at the sharp edges of the Co₃O₄ nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase⁷. Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation^8-10. With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.

Insights into the role of the lead/surfactant ratio in the formation and passivation of cesium lead bromide perovskite nanocrystals

This study aims at rationalizing the effects of the lead/surfactant ratio on the structural evolution of cesium lead-bromide perovskite nanocrystals (NCs), ascertaining how their shape and surface composition can be modulated by suitably adjusting the ligand amount (an equivolumetric mixture of oleic acid and oleyl amine) relatively to lead bromide. The tailoring of the reaction conditions allows the obtainment of blue-emitting CsPbBr3 nanoplatelets in the presence of ligand excess, while green-emitting nanocubes are achieved under low-surfactant conditions. An insight into the NC's shape evolution dictated by the different reaction conditions suggests that the generation of CsPbBr3 nanoplatelets is controlled by the dimensions of [(RNH3)2(PbBr4)]n layers formed before the injection of cesium oleate. The growth step promoted by preformed layers is concomitant to (but independent from) the nucleation process of lead-based species, leading to centrosymmetric nanocubes (prevalent in low-surfactant regimes) or Cs4PbBr6 NCs (prevalent in high-surfactant regimes). The proposed NC growth is supported by the analysis of the optical properties of non-purified samples, which reveal the selective presence of structures endowed with four cell unit average thickness accompanying larger emissive nanocubes. By combining nuclear magnetic resonance (NMR) and UV-Vis spectroscopy techniques, it is ascertained that the lead/surfactant ratio also controls the relative proportion between lead-based species (PBr2, PbBr3-, PbBr42- and plausibly PbBr53- or PbBr64-) formed before cesium injection, which regulate the size of [(RNH3)2(PbBr4)]n layers as well as the formation of Cs4PbBr6 NCs during the nucleation stage. The surface chemistry of the differently structured perovskite NCs is investigated by correlating the elemental composition of the nanoparticles with specific NMR signals ascribable to the surface ligands. This level of investigation also sheds light on the stability of the time-dependent fluorescence exhibited by differently composed perovskite NCs before the loss of their colloidal integrity. Our findings can bring about a fine tuning of the synthetic methods currently employed for controlling the shape and surface chemistry of perovskite NCs.

Cyclodextrin-mediated colloidal synthesis of highly luminescent and stable CsPbBr3 perovskite nanocrystals

Strong interactions between cyclodextrins and perovskites endow CsPbBr₃ nanocrystals with high photoluminescence quantum yields and stability to moisture, heat and light.

Doping and ion substitution in colloidal metal halide perovskite nanocrystals

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Changing the shape and optical properties of CsPbBr3 perovskite nanocrystals with hydrohalic acids using a room-temperature synthesis process

The shape, size, and optical properties of the CsPbBr₃ nanocrystals could be tuned by the addition of various hydrohalic acids.

Crystal Structure, Morphology, and Surface Termination of Cyan-Emissive, Six-Monolayers-Thick CsPbBr3 Nanoplatelets from X-ray Total Scattering

Highly anisotropic colloidal CsPbBr₃ nanoplatelets (NPLs) represent an appealing class of colloidal quantum wells with enhanced light emissivity. Strong quantum confinement imposed by the small platelet thickness and atomic flatness gives rise to enhanced oscillator strength, higher exciton binding energy, and narrow emission linewidth. While discrete thicknesses manifest themselves in discrete bandgap energies, fine-tuning of the emission energy can be achieved by compositional modulations. Here we address one of the most debated aspects of perovskite nanoplatelets: their crystal structure. Starting with the direct imaging by high-resolution electron microscopy (providing a clue on the pseudocubic faceting of the NPLs), we focus the study on X-ray total scattering techniques, based on the Debye scattering equation (DSE) approach, to obtain better atomistic insight. The nanoplatelets are six-monolayers thick and exhibit an orthorhombic structure. A thorough structure-morphology characterization unveils a specific orientation of the axial and equatorial bromides of the PbBr₆ octahedra versus the NPLs thickness; we found that {010} and {101} planes of the orthorhombic CsPbBr₃ lattice (Pnma space group) correspond to the six facets of the NPL, with basal planes being of {101} type. The NPLs undergo a lattice relaxation in comparison to cuboidal CsPbBr₃ NCs; the major deformation is observed in the axial direction, which suggests a structural origin of the higher compliance along the b axis. The DSE-based analysis also supports a CsBr surface termination model, with half Cs sites and a half (or slightly more) Br sites vacant.

Understanding the Ligand Effects on Photophysical, Optical, and Electroluminescent Characteristics of Hybrid Lead Halide Perovskite Nanocrystal Solids

There has been a tremendous amount of interest in developing high-efficiency light-emitting diodes (LEDs) based on colloidal nanocrystals (NCs) of hybrid lead halide perovskites. Here, we systematically investigate the ligand effects on EL characteristics by tuning the hydrophobicity of primary alkylamine ligands used in NC synthesis. By increasing the ligand hydrophobicity, we find (i) a reduced NC size that induces a higher degree of quantum confinement, (ii) a shortened exciton lifetime that increases the photoluminescence quantum yield, (iii) a lowering of refractive index that increases the light outcoupling efficiency, and (iv) an increased thin-film resistivity. Accordingly, ligand engineering allows us to demonstrate high-performance green LEDs exhibiting a maximum external quantum efficiency up to 16.2%. The device operational lifetime, defined by the time lasted when the device luminance reduces to 85% of its initial value, LT85, reaches 243 min at an initial luminance of 516 cd m^-2.

Solvothermal synthesis of Mn-doped CsPbCl3 perovskite nanocrystals with tunable morphology and their size-dependent optical properties

Doping metal ions in inorganic halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) endows the NCs with unique optical characteristics, and has thus attracted immense attention. However, controllable synthesis of high-quality doped perovskite NCs with tunable morphology still remains challenging. Here, we report a facile, effective and unified strategy for the controllable synthesis of Mn-doped CsPbCl3 quantum dots (QDs) and nanoplatelets (NPLs) via a single-step solvothermal method. The incorporation of Mn2+ into CsPbCl3 NCs introduces new broad photoluminescence (PL) emission from Mn2+ while maintaining the structure of host CsPbCl3 NCs nearly intact. The PL intensity, emission peak position and size of the NCs can be accurately adjusted by altering the experimental parameters such as Mn-to-Pb feed ratio and reaction time. Especially, by changing the amount of ligands, Mn-doped CsPbCl3 QDs, NPLs or their mixtures can be obtained. Both of the Mn-doped QDs and NPLs exhibit a size-dependent quantum confinement effect, which is confirmed by the relationship between the size of NCs and the exciton emission peaks. The solvothermal reaction condition plays an important role for the precise control of the structure, morphology and PL properties of the Mn-doped NCs. The as-prepared Mn-doped CsPbCl3 NPLs with thickness down to ∼2 nm exhibit a PL quantum yield (PLQY) of more than 22%. This work introduces a new strategy for the controllable synthesis of Mn-doped perovskite NCs, which provides ideas for the in-depth study of the dope-and-grow process and can be extended to approaches of doping other metal ions.

Role of Ammonium Derivative Ligands on Optical Properties of CH3NH3PbBr3 Perovskite Nanocrystals

Amines, ammonium salts, and their combination with organic acids are commonly employed ligands during the synthesis of colloidal perovskite nanocrystals (PNCs). However, the role of surface coordination, derived from different ammonium derivative ligands, on the optical properties of PNCs remains poorly understood. In this study, octylamine (OA), octylammonium bromide (OABr), and oleic acid (OAc) were applied, standing for amine, ammonium salt, and organic acid, respectively. The effects of four different types of ligands, including OA, OABr, OA-OAc, and OABr-OAc, on the surface coordination and subsequently optical properties of CH₃NH₃PbBr₃ PNCs were comparatively investigated. Compared to amine ligand, the ammonium salt could coordinate to both surface cations and anions of PNCs to passivate their surface defects more effectively, leading to enhanced optical properties including higher photoluminescence (PL) spectral intensities and PL quantum yield. Moreover, the combination of OAc with amine rather than ammonium salt ligand could trigger the protonation-deprotonation reaction to further improve their coordination effect on a PNC's surface, thus leading to significantly enhanced optical properties of PNCs. This study clarified the surface coordination of different ammonium derivative ligands and their role on the optical properties of PNCs, which could guide the design of ligands during the synthesis of PNCs.

Advances in the Stability of Halide Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals are promising candidates for next-generation optoelectronics because of their facile synthesis and their outstanding and size-tunable properties. However, these materials suffer from rapid degradation, similarly to their bulk perovskite counterparts. Here, we survey the most recent strategies to boost perovskite nanocrystals stability, with a special focus on the intrinsic chemical- and compositional-factors at synthetic and post-synthetic stage. Finally, we review the most promising approaches to address the environmental extrinsic stability of perovskite nanocrystals (PNCs). Our final goal is to outline the most promising research directions to enhance PNCs' lifetime, bringing them a step closer to their commercialization.

Perovskite quantum dots for light-emitting devices

Perovskite quantum dots (QDs) have been hotly pursued in recent decades owing to their quantum confinement effect and defect-tolerant nature. Their unique optical properties, such as high photoluminescence quantum yield (PLQY) approaching unity, narrow emission bandwidth, tunable wavelength spanning the entire visible spectrum, and compatibility with flexible/stretchable electronics, render perovskite QDs promising for next-generation solid lighting sources and information displays. Herein, the advances in perovskite QDs and their applications in LEDs are reviewed. Strategies to fabricate efficient perovskite QDs and device configuration, including material composition design, synthetic methods, surface engineering, and device optimization, are investigated and highlighted. Moreover, the main challenges in perovskite QDs of instability and toxicity (lead-based) are identified, while the solutions undertaken with respect to composition engineering, device encapsulation, and lead-replacement QDs are demonstrated. Meanwhile, perspectives for the further development of perovskite QDs and corresponding LEDs are presented.

Ruddlesden-Popper Perovskites: Synthesis and Optical Properties for Optoelectronic Applications

Ruddlesden-Popper perovskites with a formula of (A')2(A) n -1B n X3 n +1 have recently gained widespread interest as candidates for the next generation of optoelectronic devices. The variations of organic cation, metal halide, and the number of layers in the structure lead to the change of crystal structures and properties for different optoelectronic applications. Herein, the different synthetic methods for 2D perovskite crystals and thin films are summarized and compared. The optoelectronic properties and the charge transfer process in the devices are also delved, in particular, for light-emitting diodes and solar cells.

Strategies to Improve Luminescence Efficiency of Metal-Halide Perovskites and Light-Emitting Diodes

Metal-halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light-emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence-limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next-generation displays and solid-state lightings are suggested.

Inorganic and Layered Perovskites for Optoelectronic Devices

Organic-inorganic halide perovskites are making breakthroughs in a range of optoelectronic devices. Reports of >23% certified power conversion efficiency in photovoltaic devices, external quantum efficiency >21% in light-emitting diodes (LEDs), continuous-wave lasing and ultralow lasing thresholds in optically pumped lasers, and detectivity in photodetectors on a par with commercial GaAs rivals are being witnessed, making them the fastest ever emerging material technology. Still, questions on their toxicity and long-term stability raise concerns toward their market entry. The intrinsic instability in these materials arises due to the organic cation, typically the volatile methylamine (MA), which contributes to hysteresis in the current-voltage characteristics and ion migration. Alternative inorganic substitutes to MA, such as cesium, and large organic cations that lead to a layered structure, enhance structural as well as device operational stability. These perovskites also provide a high exciton binding energy that is a prerequisite to enhance radiative emission yield in LEDs. The incorporation of inorganic and layered perovskites, in the form of polycrystalline films or as single-crystalline nanostructure morphologies, is now leading to the demonstration of stable devices with excellent performance parameters. Herein, key developments made in various optoelectronic devices using these perovskites are summarized and an outlook toward stable yet efficient devices is presented.

Low-dimensional perovskite nanoplatelet synthesis using in situ photophysical monitoring to establish controlled growth

Perovskite nanoparticles have attracted the attention of research groups around the world for their impressive photophysical properties, facile synthesis and versatile surface chemistry. Here, we report a synthetic route that takes advantage of a suite of soluble precursors to generate CsPbBr₃ perovskite nanoplatelets with fine control over size, thickness and optical properties. We demonstrate near unit cell precision, creating well characterized materials with sharp, narrow emission lines at 430, 460 and 490 nm corresponding to nanoplatelets that are 2, 4, and 6 unit cells thick, respectively. Nanoplatelets were characterized with optical spectroscopy, atomic force microscopy, scanning electron microscopy and transmission electron microscopy to explicitly correlate growth conditions, thickness and resulting photophysical properties. Detailed in situ photoluminescence spectroscopic studies were carried out to understand and optimize particle growth by correlating light emission with nanoplatelet growth across a range of synthetic conditions. It was found that nanoplatelet thickness and emission wavelength increase as the ratio of oleic acid to oleyl amine or the reaction temperature is increased. Using this information, we control the lateral size, width and corresponding emission wavelength of the desired nanoplatelets by modulating the temperature and ratios of the ligand.

Journey of Making Cesium Lead Halide Perovskite Nanocrystals: What's Next

Understanding physical insights of making different lead halide perovskite nanocrystals remains in limelight of current research because of their strong emission that is tunable in the entire visible spectrum. Optimizing reaction parameters, intensifying the emission, modulating A, B, and X sites to bring optical and phase stability, understanding the interface and ligand chemistry, investigating the growth kinetics, tuning the dimension, making heterostructures, etc. were intensively studied. In addition, several aspects of colloidal nanocrystals such as time-dependent growth and obtaining size-tunable nanocrystals as a function of time from one reaction, controlling anisotropic growth or stabilizing other than the six facets of cubes, formation of various heterostructures with epitaxial growths, etc. could not be established largely. Hence, while the field is emerging among efficient optically active materials, addressing these issues by summarizing different directions of research became important. Under these prospects, this Perspective focuses on top developments in the field of synthesis modulations where success has been achieved and also summarizes several directions of research where physical insights still could not be understood broadly.

Ligand-Mediated Release of Halides for Color Tuning of Perovskite Nanocrystals with Enhanced Stability

The rich chemistry of metal halide perovskites has enabled various methods of band structure control and surface passivation. Here we report a highly facile and efficient post-treatment approach for precise color tuning of cesium lead halide perovskite nanocrystals (NCs) with enhanced stability. By utilizing a special multifunctional organic ligand, triphenyl(9-phenyl-9H-carbazol-3-yl)phosphonium bromide (TPP-Carz), carbon-halide bond cleavage can be achieved to release halide ions from halogenated solvents in a controlled manner for color tuning of perovskite NCs via ion exchange. Besides controlled release of halide ions for anion exchange, TPP-Carz can effectively passivate the surfaces of perovskite NCs simultaneously. As a result, perovskite NCs prepared by this post-treatment method with tunable colors over the entire visible spectrum have shown significantly improved luminescence and stability in comparison to the ones prepared using reactive anion precursors without surface passivation by TPP-Carz.

High-Quality All-Inorganic Perovskite CsPbBr3 Quantum Dots Emitter Prepared by a Simple Purified Method and Applications of Light-Emitting Diodes

High-quality perovskite CsPbBr3 quantum dots (QDs-CsPbBr3) were prepared using the ultrasonic oscillation method, which is simple and provides variable yield according to requirements. The emission spectra over a large portion of the visible spectral region (450–650 nm) of QD-CsPbX3 (X = Cl, Br, and I) have tunable compositions that can be halide exchanged using the halide anion exchange technique and quantum size-effects. A strong peak with high intensity of (200) lattice plane of purified QDs-CsPbBr3 film is obtained, confirming the formation of an orthorhombic perovskite crystal structure of the Pnma space group. The photoluminescence of QDs-CsPbBr3 was characterized using a narrow line-width emission of 20 nm, with high quantum yields of up to 99.2%, and radioactive lifetime increasing to 26 ns. Finally, through the excellent advantages of QDs-CsPbBr3 mentioned above, purified perovskite QDs-CsPbBr3 as an active layer was utilized in perovskite quantum dot light-emitting diodes structure applications. As a result, the perovskite QDs-CsPbBr3 light-emitting diodes (LEDs) exhibits a turn-on voltage of 7 V and a maximum luminance of 5.1 cd/m2.

Synthetic Evolution of Colloidal Metal Halide Perovskite Nanocrystals

Metal halide perovskite semiconductor nanocrystals have emerged as a lucrative class of materials for many optoelectronic applications. By leveraging the synthetic toolboxes developed from decades of research into more traditional semiconductor nanocrystals, remarkable progress has been made across these materials in terms of their structural, compositional, and optoelectronic control. Here, we review this progress in terms of their underlying formation stages, synthetic approaches, and postsynthetic treatment steps. This assessment highlights the rapidly maturing nature of the perovskite nanocrystal field, particularly with regard to their lead-based derivatives. It further demonstrates that significant challenges remain around precisely controlling their nucleation and growth processes. In going forward, a deeper understanding of the role of precursors and ligands will significantly bolster the versatility in the size, shape, composition, and functional properties of these exciting materials.

Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals

Photon upconversion (UC) based on sensitized triplet-triplet annihilation (TTA), TTA-UC, can potentially alleviate the transmission loss of below-band-gap photons in solar energy conversion. TTA-UC across various spectral windows has been demonstrated, but efficient visible-to-ultraviolet (UV) UC remains a big challenge primarily due to the lack of suitable triplet sensitizers. Here we report a TTA-UC system sensitized by quantum-confined CsPbBr₃ perovskite nanocrystals (NCs) that simultaneously achieves a high photon energy gain of up to 0.7 eV (443-355 nm) and a high UC efficiency up to 10.2%. Time-resolved spectroscopy studies reveal that the performance is mainly enabled by ultrafast and efficient triplet energy transfer from the strongly confined NC sensitizers to triplet acceptors.

Chiral Perovskites: Promising Materials toward Next-Generation Optoelectronics

Halide perovskites have emerged as a type of extremely promising material for their diverse chemical and electronic structures along with their brilliant optoelectronic properties. The introduction of chirality into perovskite scaffolds, generating a novel concept of chiral perovskite materials, offers an immense step forward toward the development of smart optoelectronic and spintronic materials and devices. The present Review summarizes recent advances in such an emerging field regarding the design and construction of chiral perovskite materials, along with their optoelectronic performances. In addition, an outlook of future challenges as well as the potential significance of the chiral perovskite family on the optical communication is proposed.

Synergetic Effect of the Surfactant and Silica Coating on the Enhanced Emission and Stability of Perovskite Quantum Dots for Anticounterfeiting

Recently, the growing demand for optical anticounterfeiting technology has motivated intensive research in newly emerging halide perovskite quantum dots (QDs). However, the poor stability and unsatisfactory fluorescence efficiency of such materials are the main obstacles to the application of reliable anticounterfeiting. In this work, we performed a well-controlled investigation of the effect of the surfactant (l-α-phosphatidylcholine, LP) and silica encapsulation on the stability and emission of the CsPbBr₃ QDs. Because of the synergetic effect of the surfactant and core/shell configuration, the resulting CsPbBr₃/LP/SiO₂ QD composites demonstrated a higher photoluminescence quantum yield (>90%), a better color purity, and a significantly improved stability in heat, ultraviolet light, water, and ambient oxygen, which provide them the basic conditions as a high-tech security ink for anticounterfeiting. By inkjet printing technology, we demonstrated that our CsPbBr₃/LP/SiO₂ QD composites can act as a smart concealed ink for information encryption and decryption. More importantly, the anticounterfeiting effect can be efficiently sustained even though the paper with designable patterns was crudely treated by water-soaking, heating/cooling cycling, and continuous ultraviolet light switching (1500 cycles). The above results obtained provide effective strategies to improve emission efficiency and stability of perovskite QDs, thereby enduing them anticounterfeiting application potential.

Photon-Induced Reshaping in Perovskite Material Yields of Nanocrystals with Accurate Control of Size and Morphology

Benefiting from morphology-/size-tunable optical features, nanocrystals have been considered promising candidates for display or lighting applications. To achieve selective characteristic emission, precise control in size and morphology is thus a prerequisite. Herein, we report that the nanosecond-pulsed laser irradiation induces CsPbBr₃ reshaping, yielding precise control of size and morphology. Under 532 and 355 nm laser irradiation, polydisperse CsPbBr₃ nanocrystals or raw micron powders can be reshaped into uniform sizes of 12 and 6 nm, respectively. Moreover, by tuning ligand composition, the morphology of reshaped nanocrystals can be manipulated, such as nanocubes, nanorods, or nanosheets. Results reveal that the reshaping process relies on striving for a delicate balance between energy deposition and heat dissipation under irradiation. A low dissipation rate leads to temperature rising and lattice breaking, which turn out to be the driving forces for reshaping. This feasible method provides a reliable, and scalable route toward preparation of perovskite functional nanocrystals.

Tuning from Quantum Dots to Magic Sized Clusters of CsPbBr3 Using Novel Planar Ligands Based on the Trivalent Nitrate Coordination Complex

We report the first demonstration of using trivalent metal hydrated nitrate coordination complexes (TMHNCCs) as novel passivation ligands to control the synthesis of magic sized clusters (MSCs) and quantum dots (QDs) of CsPbBr₃ perovskite at room temperature. We can easily tune from QDs to MSCs or produce a mixture of the two by changing the amount of TMHNCC ligands used, with more ligands favoring MSCs. The original TMHNCC introduced, aluminum nitrate nonahydrate [ANN, Al(NO₃)₃·9H₂O], led to the production of aluminum dihydroxide nitrate tetrahydrate {ADNT, [Al(OH)₂(NO₃)]·4H₂O}, with the assistance of oleic acid (OA) and oleylamine (OAm). Through several control experiments, we determined that ADNT is the primary ligand for effectively passivating the MSCs and QDs, with OAm being essential for deprotonating ANN and OA for adjusting the pH of the reaction system. We suggest that ADNT is planar on the surface of the MSCs or QDs with its NO₃^- and OH^- groups binding to the Cs⁺ and Pb²⁺ defect sites and Al³⁺ binding to the Br^- defect sites of the MSCs or QDs.

Surfactant-Induced Anion Exchange and Morphological Evolution for Composition-Controlled Caesium Lead Halide Perovskites with Tunable Optical Properties

Environmentally stable lead halide perovskite nanostructures with engineered composition and morphology are attractive because of their exotic optical properties. Here, we report the synthesis of monodispersed (∼20 nm) CsPbI₃ cubic perovskite nanocrystals (NCs) using edible olive oil as a solvent as well as a chelating reagent. Thereafter, bromide anion exchange reaction using the cetyl trimethyl ammonium bromide surfactant in hexane is carried out at relatively lower temperatures to synthesize caesium lead halide perovskites with variable halide compositions and tunable band gaps. Interestingly, because of the formation of micelles, continuous morphology evolution varying from NCs of different sizes to nanowires (NWs) and nanosheets is observed. The anion exchange temperature has a distinct effect on the morphology of the CsPbBr₃ nanostructure and the anion exchange reaction rate. Finally, an easy solution-processed photoconductive device is demonstrated using as-grown CsPbBr₃ NWs, indicating its potential for optoelectronic applications.

Low-dimensional iodide perovskite nanocrystals enable efficient red emission

We report herein a simple ligand-assisted reprecipitation method at room temperature to synthesize mixed-cation hybrid organic-inorganic perovskite nanocrystals with low structural dimensionality. The emission wavelength of iodide-based perovskites is thus tuned from the near-infrared to the red part of the visible spectrum. While this is mostly achieved in the literature by addition of bromide, we demonstrate here a controllable blueshift of the band gap by varying the chain length of the alkylammonium ligands. Furthermore, an antisolvent washing step was found to be crucial to purify the samples and obtain single-peak photoluminescence with a narrow linewidth. The so-formed nanocrystals exhibit high and stable photoluminescence quantum yields exceeding 90% over 500 hours, making these materials ideal for light-emitting applications.

Ligand-mediated synthesis of colloidal Cs2SnI6 three-dimensional nanocrystals and two-dimensional nanoplatelets

Cs₂SnI₆ is a variant on tin-iodide solution-processable materials and may lead to a lead-free material for use in next-generation photovoltaic cells and other optoelectronics. So far, only a few studies have been conducted where shape and geometry control of Cs₂SnI₆ nanocrystals is demonstrated. Here we report a general approach to directly synthesize Cs₂SnI₆ of two-dimensional (2D) layered nanoplatelets as well as three-dimensional (3D) nanocrystals. The shape of Cs₂SnI₆ nanocrystals could be engineered into 3D nanoparticles and different 2D nanoplatelets with well-defined morphology by choosing different organic acid and amine ligands via a hot injection process. Moreover, the thickness of layered 2D nanoplatelets could be adjusted by changing the amount of Cs-oleate present during the synthesis. The photoluminescence emission peaks changed from 643 to 742 nm based on nanomaterial shape. Our method provides a facile and versatile route to rationally control the shape of the Cs₂SnI₆ nanocrystals, which will create opportunities for applications in lead-free optoelectronics.

Device Engineering for All-Inorganic Perovskite Light-Emitting Diodes

Recently, all-inorganic perovskite light-emitting diodes (PeLEDs) have attracted both academic and industrial interest thanks to their outstanding properties, such as high efficiency, bright luminance, excellent color purity, low cost and potentially good operational stability. Apart from the design and treatment of all-inorganic emitters, the device engineering is another significant factor to guarantee the high performance. In this review, we have summarized the state-of-the-art concepts for device engineering in all-inorganic PeLEDs, where the charge injection, transport, balance and leakage play a critical role in the performance. First, we have described the fundamental concepts of all-inorganic PeLEDs. Then, we have introduced the enhancement of device engineering in all-inorganic PeLEDs. Particularly, we have comprehensively highlighted the emergence of all-inorganic PeLEDs, strategies to improve the hole injection, approaches to enhance the electron injection, schemes to increase the charge balance and methods to decrease the charge leakage. Finally, we have clarified the issues and ways to further enhance the performance of all-inorganic PeLEDs.

Solution-phase synthesis of CsPbI3 nanowire clusters via polymer-induced anisotropic growth and self-assembly

A facile solution-phase synthesis of black γ-phase CsPbI3 nanowire clusters was developed using poly(methyl methacrylate) (PMMA) as surfactant. PMMA was found to efficiently retard the crystal growth, thereby inducing anisotropic growth for formation of the nanowire structure, while the intermolecular hydrogen bonds of PMMA act as a driving force for self-assembly of the nanowires.

Ligand-free all-inorganic metal halide nanocubes for fast, ultra-sensitive and self-powered ozone sensors

Ligand-free all-inorganic lead halide nanocubes have been investigated as ozone sensing materials operating at room temperature. It is found that the nanocubes, crystallined in the orthorhombic CsPbBr₃ structure, can operate at room temperature, be self-powered and exhibit high sensitivity and remarkable repeatability. More importantly, they demonstrate higher sensitivity (54% in 187 ppb) and faster response and recovery times compared to hybrid lead mixed halide perovskite (CH₃NH₃PbI_3-x Cl _x ) layers, which is the only lead halide perovskite material tested for ozone sensing, to date. Following the exposure to an ozone environment, the stoichiometry and the morphology of the nanocubes remain unaltered. The facile and easy fabrication process together with the high responsivity and stability to the ozone environment makes the bare CsPbBr₃ nanocubes a promising material for sensing applications. The sensing properties of the nanoparticulate metal halides presented here provide new exciting opportunities towards engineering reliable and cheap sensing elements for room-temperature operated and self-powered sensors.

Stable Ligand Coordination at the Surface of Colloidal CsPbBr3 Nanocrystals

Ruling over the surface chemistry of metal halide perovskite nanocrystals (NCs) is crucial to access reliable luminophores. Here, we provide an atomic-level description of the surface of colloidal CsPbBr₃ NCs, achieving an effective passivation strategy that leads to near-unity photoluminescence quantum yield. To this end, we used two different types of CsPbBr₃ NCs, which had been synthesized with an outer shell of either oleylammonium bromide ion pairs or Cs-oleate complexes. We perturbed the dynamic equilibria at the NCs' surface with ligands from a comprehensive library, including amines (and their conjugated acids) with different basicities, chain lengths, and steric encumbrances. We demonstrate that control of both ligand binding affinity and ligand-to-NC molar ratio is essential to attain thermodynamically stable coordination of the NC surface. We thus present a reliable protocol for managing the surface chemistry of colloidal CsPbBr₃ NCs and for selectively addressing their ligand-induced morphological (and structural) transformations.

Spontaneous Self-Assembly of Cesium Lead Halide Perovskite Nanoplatelets into Cuboid Crystals with High Intensity Blue Emission

Colloidal all-inorganic perovskite nanocrystals have gained significant attention as a promising material for both fundamental and applied research due to their excellent emission properties. However, reported photoluminescence quantum yields (PL QYs) of blue-emitting perovskite nanocrystals are rather low, mostly due to the fact that the high energy excitons for such wide bandgap materials are easily captured by interband traps, and then decay nonradiatively. In this work, it is demonstrated how to tackle this issue, performing self-assembly of 2D perovskite nanoplatelets into larger size (≈50 nm × 50 nm × 20 nm) cuboid crystals. In these structures, 2D nanoplatelets being isolated from each other within the cuboidal scaffold by organic ligands constitute multiple quantum wells, where exciton localization on potential disorder sites helps them to bypass nonradiative channels present in other platelets. As a result, the cuboid crystals show an extremely high PL QY of 91% of the emission band centered at 480 nm. Moreover, using the same synthetic method, mixed-anion CsPb(Br/Cl)3 cuboid crystals with blue emission peaks ranging from 452 to 470 nm, and still high PL QYs in the range of 72-83% are produced.

Halide Perovskite Nanocrystals for Next-Generation Optoelectronics

Colloidal perovskite nanocrystals (PNCs) combine the outstanding optoelectronic properties of bulk perovskites with strong quantum confinement effects at the nanoscale. Their facile and low-cost synthesis, together with superior photoluminescence quantum yields and exceptional optical versatility, make PNCs promising candidates for next-generation optoelectronics. However, this field is still in its early infancy and not yet ready for commercialization due to several open challenges to be addressed, such as toxicity and stability. Here, the key synthesis strategies and the tunable optical properties of PNCs are discussed. The photophysical underpinnings of PNCs, in correlation with recent developments of PNC-based optoelectronic devices, are especially highlighted. The final goal is to outline a theoretical scaffold for the design of high-performance devices that can at the same time address the commercialization challenges of PNC-based technology.

Efficient and Stable CsPbI3 Solar Cells via Regulating Lattice Distortion with Surface Organic Terminal Groups

All-inorganic cesium lead iodide perovskites (CsPbI₃ ) are promising wide-bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner-sharing octahedral framework ([PbI₆ ]^4- ). Here, a novel strategy to suppress the octahedral tilting of [PbI₆ ]^4- units in cubic CsPbI₃ by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X-ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted-structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.

Ultrastable CsPbBr3 Perovskite Quantum Dot and Their Enhanced Amplified Spontaneous Emission by Surface Ligand Modification

The poor stability and aggregation problem of CsPbBr₃ quantum dots (QDs) in air are great challenges for their future practical application. Herein, a simple and effective ligand-modification strategy is proposed by introducing 2-hexyldecanoic acid (DA) with two short branched chains to replace oleic acid (OA) with long chains during the synthesis process. These two short branched chains not only maintain their colloidal stability but also contribute to efficient radiative recombination. The calculations show that CsPbBr₃ QDs with DA modification (CsPbBr₃ -DA QDs) have larger binding energy than CsPbBr₃ QDs with OA (CsPbBr₃ -OA QDs), resulting in significantly enhanced stability. Due to the strong binding energy between DA ligands and QDs, CsPbBr₃ -DA QDs exhibit no aggregation phenomenon even after stored in air for more than 70 d, and CsPbBr₃ -DA QDs films can maintain 94.3% of initial PL intensity after 28 d, while in CsPbBr₃ -OA QDs films occurs a rapid degradation of PL intensity. Besides, the enhanced amplified spontaneous emission (ASE) performance of CsPbBr₃ -DA QDs films has been demonstrated under both one- and two-photon laser excitation. The ASE threshold of CsPbBr₃ -DA QDs films is reduced by more than 50% and their ASE photostability is also improved, in comparison to CsPbBr₃ -OA QDs films.

Single-Particle Organolead Halide Perovskite Photoluminescence as a Probe for Surface Reaction Kinetics

Photoluminescence (PL) of organolead halide perovskites (OHPs) is sensitive to OHPs' surface conditions and is an effective way to report surface states. Literature has reported that at the ensemble level, the PL of photoexcited OHP nanorods declines under an inert nitrogen (N₂) atmosphere and recovers under subsequent exposure to oxygen (O₂). At the single-particle level, we observed that OHP nanorods photoblink at rates dependent on both the excitation intensity and the O₂ concentration. Combining the two sets of information with the charge-trapping/detrapping mechanism, we are able to quantitatively evaluate the interaction between a single surface defect and a single O₂ molecule using a new kinetic model. The model predicts that the photodarkening of OHP nanorods in the N₂ atmosphere has a different mechanism than conventional PL quenching, which we call photo-knockout. This model provides fundamental insights into the interactions of molecular O₂ with OHP materials and helps design a suitable OHP interface for a variety of applications in photovoltaics and optoelectronics.

Ligand-Induced Tunable Dual-Color Emission Based on Lead Halide Perovskites for White Light-Emitting Diodes

Cesium lead halide perovskites (CsPbX₃, X = Cl, Br, and I) have emerged as an important class of color-tunable light-emitting materials in the past 4 years. However, single-CsPbX₃ nanostructures with dual-color emission remain scarce. Here, we demonstrate dual-color emission from lead halide perovskite nanowires induced by the surface ligands, that is, oligomeric methoxypolyethylene glycol (MEOPEG). In addition to the characteristic emission from the host lattice, an unprecedented emission from the expanded band gap caused by MEOPEG is observed. The ratio of the two emission intensities can be easily adjusted by changing the concentration of the surface ligands. Moreover, the band gaps of CsPbX₃-MEOPEG can be further fine-tuned by a simple postsynthetic anion exchange process. As a result, white light-emitting diodes (WLEDs) with high-quality CIE coordinates of (0.33, 0.29) and a high color rendering index value (84) are realized. These CsPbX₃-MEOPEG materials, with tunable dual-color emission, may serve as ideal model systems for WLEDs, which will undoubtedly expand the applications of cesium lead halide perovskites.

Engineering the Photoluminescence of CsPbX3 (X = Cl, Br, and I) Perovskite Nanocrystals Across the Full Visible Spectra with the Interval of 1 nm

Fluorescent CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals (NCs) are compelling candidates for illumination and display applications because of the high photoluminescence quantum yields (PLQYs) narrow PL emission spectra, and in particular, the potential to tune the emission spectra in the entire visible range. However, limited by the current preparation strategy, the successive adjustment of PL emission across the full visible spectral range with very small interval, like conventional semiconductor quantum dots, is still challenging. In this work, we demonstrate the capability to tune the PL emission of CsPbX₃ NCs in the full visible range with the interval of 1 nm on the basis of a modified anion-exchange route. Highly luminescent CsPbCl₃ NCs with PLQY up to 34.2% are foremost prepared using alkanoyl chlorides as the chlorine source and further employed to perform anion exchange. A successive and accurate adjustment of the PL emission is achieved with the addition of ZnX₂ (X = Br and I) aqueous solution and assisted by ultrasound to improve the reactivity of halogens in the anion exchange. Besides the accurately tunable PL emission position, the as-prepared CsPbX₃ NCs exhibit good phase/chemical stability, high PLQY, and narrow PL emission spectra.

Manipulating the Transition Dipole Moment of CsPbBr3 Perovskite Nanocrystals for Superior Optical Properties

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14° and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.