Boosting Tunable Blue Luminescence of Halide Perovskite Nanoplatelets through Postsynthetic Surface Trap Repair

Dielectrically Confined Stable Excitons in Few-Atom-Thick PbS Nanosheets

Two-dimensional colloidal PbS nanosheets exhibit more than one order of magnitude larger exciton binding energy than their bulk counterpart, making it possible to generate stable excitons at room temperature. It is experimentally revealed that the binding energy of the exciton increases from 26 to 68 meV as the thickness of the PbS nanosheet decreases from 4.7 to 1.2 nm. The dielectric confinement of the exciton plays a critical role in the binding-energy enhancement. The large binding energy results in a fast thickness-dependent exciton radiative recombination rate, confirmed experimentally.

Energy Transfer in Stability-Optimized Perovskite Nanocrystals

Outstanding optoelectronic properties and a facile synthesis render halide perovskite nanocrystals (NCs) a promising material for nanostructure-based devices. However, the commercialization is hindered mainly by the lack of NC stability under ambient conditions and inefficient charge carrier injection. Here, we investigate solutions to both problems, employing methylammonium lead bromide (MAPbBr₃) NCs encapsulated in diblock copolymer core-shell micelles of tunable size. We confirm that the shell does not prohibit energy transfer, as FRET efficiencies between these NCs and 2D CsPbBr₃ nanoplatelets (NPLs) reach 73.6%. This value strongly correlates to the micelle size, with thicker shells displaying significantly reduced FRET efficiencies. Those high efficiencies come with a price, as the thinnest shells protect the encapsulated NCs less from environmentally induced degradation. Finding the sweet spot between efficiency and protection could lead to the realization of tailored energy funnels with enhanced carrier densities for high-power perovskite NC-based optoelectronic applications.

Oleic acid/oleylamine ligand pair: a versatile combination in the synthesis of colloidal nanoparticles

A variety of colloidal chemical approaches has been developed in the last few decades for the controlled synthesis of nanostructured materials in either water or organic solvents. Besides the precursors, the solvents, reducing agents, and the choice of surfactants are crucial for tuning the composition, morphology and other properties of the resulting nanoparticles. The ligands employed include thiols, amines, carboxylic acids, phosphines and phosphine oxides. Generally, adding a single ligand to the reaction mixture is not always adequate to yield the desired features. In this review, we discuss in detail the role of the oleic acid/oleylamine ligand pair in the chemical synthesis of nanoparticles. The combined use of these ligands belonging to two different categories of molecules aims to control the size and shape of nanoparticles and prevent their aggregation, not only during their synthesis but also after their dispersion in a carrier solvent. We show how the different binding strengths of these two molecules and their distinct binding modes on specific facets affect the reaction kinetics toward the production of nanostructures with tailored characteristics. Additional functions, such as the reducing function, are also noted, especially for oleylamine. Sometimes, the carboxylic acid will react with the alkylamine to form an acid-base complex, which may serve as a binary capping agent and reductant; however, its reducing capacity may range from lower to much lower than that of oleylamine. The types of nanoparticles synthesized in the simultaneous presence of oleic acid and oleylamine and discussed herein include metal oxides, metal chalcogenides, metals, bimetallic structures, perovskites, upconversion particles and rare earth-based materials. Diverse morphologies, ranging from spherical nanoparticles to anisotropic, core-shell and hetero-structured configurations are presented. Finally, the relation between tuning the resulting surface and volume nanoparticle properties and the relevant applications is highlighted.

Tailoring CsPbBr3 Growth via Non-Polar Solvent Choice and Heating Methods

This study describes an investigation of the role of non-polar solvents on the growth of cesium lead halide (CsPbX₃ X = Br and I) nanoplatelets. We employed two solvents, benzyl ether (BE) and 1-octadecene (ODE), as well as two nucleation and growth mechanisms, one-pot, facilitated by microwave irradiation (MWI)-based heating, and hot-injection, using convection. Using BE and MWI, large mesoscale CsPbBr₃ nanoplatelets were produced, whereas use of ODE produced small crystallites. Differences between the products were observed by optical spectroscopies, which showed first band edge absorptions consistent with thicknesses of ∼9 nm [∼15 monolayer (ML)] for the BE-CsPbBr₃ and ∼5 nm (∼9 ML) for ODE-CsPbBr₃. Both products had orthorhombic crystal structures, with the BE-CsPbBr₃ revealing significant preferred orientation diffraction signals consistent with the asymmetric and two-dimensional platelet morphology. The differences in the final morphology were also observed for products formed via hot injection, with BE-CsPbBr₃ showing thinner square platelets with thicknesses of ∼2 ML and ODE-CsPbBr₃ showing similar morphologies and small crystallite sizes. To understand the role solvent plays in crystal growth, we studied lead plumbate precursor (PbBr_n^2-n) formation in both solvents, as well as solvent plus ligand solutions. The findings suggest that BE dissolves PbBr₂ salts to a higher degree than ODE, and that this BE to precursor affinity persists during growth.

3D and 2D Metal Halide Perovskites for Blue Light-Emitting Diodes

Metal halide perovskites (MHPs) are emerging next-generation light emitters that have attracted attention in academia and industry owing to their low material cost, simple synthesis, and wide color gamut. Efficient strategies for MHP modification are being actively studied to attain high performance demonstrated by commercial light-emitting diodes (LEDs) based on organic emitters. Active studies have overcome the limitations of the external quantum efficiencies (EQEs) of green and red MHP LEDs (PeLEDs); therefore, the EQEs of PeLEDs (red: 21.3% at 649 nm; green: 23.4% at 530 nm) have nearly reached the theoretical limit for the light outcoupling of single-structured planar LEDs. However, the EQEs of blue PeLEDs (12.1% at 488 nm and 1.12% at 445 nm) are still lower than approximately half of those of green and red PeLEDs. To commercialize PeLEDs for future full-color displays, the EQEs of blue MHP emitters should be improved by approximately 2 times for sky-blue and more than 20 times for deep-blue MHP emitters to attain values comparable to the EQEs of red and green PeLEDs. Therefore, based on the reported effective approaches for the preparation of blue PeLEDs, a synergistic strategy for boosting the EQE of blue PeLEDs can be devised for commercialization in future full-color displays. This review covers efficient strategies for improving blue PeLEDs using fundamental approaches of material engineering, including compositional or dimensional engineering, thereby providing inspiration for researchers.

Controlling Intrinsic Quantum Confinement in Formamidinium Lead Triiodide Perovskite through Cs Substitution

Lead halide perovskites are leading candidates for photovoltaic and light-emitting devices, owing to their excellent and widely tunable optoelectronic properties. Nanostructure control has been central to their development, allowing for improvements in efficiency and stability, and changes in electronic dimensionality. Recently, formamidinium lead triiodide (FAPbI₃) has been shown to exhibit intrinsic quantum confinement effects in nominally bulk thin films, apparent through above-bandgap absorption peaks. Here, we show that such nanoscale electronic effects can be controlled through partial replacement of the FA cation with Cs. We find that Cs-cation exchange causes a weakening of quantum confinement in the perovskite, arising from changes in the bandstructure, the length scale of confinement, or the presence of δ_H-phase electronic barriers. We further observe photon emission from quantum-confined regions, highlighting their potential usefulness to light-emitting devices and single-photon sources. Overall, controlling this intriguing quantum phenomenon will allow for its suppression or enhancement according to need.

Insertion of metal cations into hybrid organometallic halide perovskite nanocrystals for enhanced stability: eco-friendly synthesis, lattice strain engineering, and defect chemistry studies

In this work, we developed a facile and environmentally friendly synthesis strategy for large-scale preparation of Cr-doped hybrid organometallic halide perovskite nanocrystals. In the experiment, methylammonium lead bromide, CH₃NH₃PbBr₃, was efficiently doped with Cr³⁺ cations by eco-friendly method at low temperatures to grow crystals via antisolvent-crystallization. The as-synthesized Cr³⁺ cation-doped perovskite nanocrystals displayed ∼45.45% decrease in the (100) phase intensity with an enhanced Bragg angle (2θ) of ∼15.01° compared to ∼14.92° of pristine perovskites while retaining their cubic (221/Pm-cm, ICSD no. 00-069-1350) crystalline phase of pristine perovskites. During synthesis, an eco-friendly solvent, ethanol, was utilized as an antisolvent to grow nanometer-sized rod-like crystals. However, Cr³⁺ cation-doped perovskite nanocrystals display a reduced crystallinity of ∼67% compared to pristine counterpart with ∼75% crystallinity with an improved contact angle of ∼72° against water in thin films. Besides, as-grown perovskite nanocrystals produced crystallite size of ∼48 nm and a full-width-at-half-maximum (FWHM) of ∼0.19° with an enhanced lattice-strain of ∼4.52 × 10^-4 with a dislocation-density of ∼4.24 × 10¹⁴ lines per m² compared to pristine perovskite nanocrystals, as extracted from the Williamson-Hall plots. The as-obtained stable perovskite materials might be promising light-harvesting candidates for optoelectronic applications in the future.

Morphology-Dependent One- and Two-Photon Absorption Properties in Blue Emitting CsPbBr3 Nanocrystals

The linear and nonlinear optical parameters and morphologic dependence of CsPbBr₃ nanocrystals (NCs) are crucial for device engineering. In particular, such information in asymmetric nanocrystals is still insufficient. We characterized the OPLA (σ¹) and TPA cross sections (σ²) of a series CsPbBr₃ nanocrystals with various aspect ratios (AR) using femtosecond transient absorption spectroscopy (TAS). The σ¹ presents a linear volume dependence of all the samples, which agrees with the previous behavior in CsPbBr₃ QDs. However, the σ² values do not exhibit conventional power dependency of the crystal volume but are also modulated by the shape-dependent local field factors. In addition, the local field effect in CsPbBr₃ NCs is contributed by their asymmetric morphologies and polar ionic lattices, which is more pronounced than in conventional semiconductor NCs. Finally, we revealed that the lifetimes of photogenerated multiexcitonic species of those nanocrystals feature identical morphology independence in both OPLA and TPA.

Post-doping induced morphology evolution boosts Mn2+ luminescence in the Cs2NaBiCl6:Mn2+ phosphor

As we know, defects caused in the synthetic process of metal halide perovskite are the most difficult to overcome, and greatly limit their photoelectric performances. Herein, a post-doped strategy was utilized to achieve an interesting morphology evolution from a standard octahedron to a snowflake-like sheet during the Mn²⁺-doped Cs₂NaBiCl₆ process, which realizes the obvious photoluminescence quantum efficiency (PLQY) enhancement of the Cs₂NaBiCl₆:Mn²⁺ phosphor. This surprising evolution is ascribed to the morphology collapse and reconstruction induced by Mn²⁺ exchange. The obtained phosphor exhibits enhanced 31.56% PLQY, which is two-fold higher than that synthesized by the traditional co-precipitation method, with broad emission spectrum and good PL color stability at 150 °C. Combined with the Cs₂SnCl₆ : 1mol%Bi³⁺ phosphor to fabricate the phosphor-converted light-emitting diode, bright white light emission with Ra = 88, CCT = 4320 K, CIE (0.36, 0.33) and a good application potential in high-resolution PL imaging agents was obtained. This work provides a possible effective strategy to improve the PL performance for impurity-doped lead-free metal halide perovskite.

Electron-Hole Binding Governs Carrier Transport in Halide Perovskite Nanocrystal Thin Films

Two-dimensional halide perovskite nanoplatelets (NPLs) have exceptional light-emitting properties, including wide spectral tunability, ultrafast radiative decays, high quantum yields (QY), and oriented emission. Due to the high binding energies of electron-hole pairs, excitons are generally considered the dominant species responsible for carrier transfer in NPL films. To realize efficient devices, it is imperative to understand how exciton transport progresses therein. We employ spatially and temporally resolved optical microscopy to map exciton diffusion in perovskite nanocrystal (NC) thin films between 15 °C and 55 °C. At room temperature (RT), we find the diffusion length to be inversely correlated to the thickness of the nanocrystals (NCs). With increasing temperatures, exciton diffusion declines for all NC films, but at different rates. This leads to specific temperature turnover points, at which thinner NPLs exhibit higher diffusion lengths. We attribute this anomalous diffusion behavior to the coexistence of excitons and free electron hole-pairs inside the individual NCs within our temperature range. The organic ligand shell surrounding the NCs prevents charge transfer. Accordingly, any time an electron-hole pair spends in the unbound state reduces the FRET-mediated inter-NC transfer rates and, consequently, the overall diffusion. These results clarify how exciton diffusion progresses in strongly confined halide perovskite NC films, emphasizing critical considerations for optoelectronic devices.

Interfacial Manganese-Doping in CsPbBr3 Nanoplatelets by Employing a Molecular Shuttle

Mn-doping in cesium lead halide perovskite nanoplatelets (NPls) is of particular importance where strong quantum confinement plays a significant role towards the exciton-dopant coupling. In this work, we report an immiscible bi-phasic strategy for post-synthetic Mn-doping of CsPbX3 (X=Br, Cl) NPls. A systematic study shows that electron-donating oleylamine acts as a shuttle ligand to transport MnX2 through the water-hexane interface and deliver it to the NPls. The halide anion also plays an essential role in maintaining an appropriate radius of Mn2+ and thus fulfilling the octahedral factor required for the formation of perovskite crystals. By varying the thickness of parent NPls, we can tune the dopant incorporation and, consequently, the exciton-to-dopant energy transfer process in doped NPls. Time-resolved optical measurements offer a detailed insight into the exciton-to-dopant energy transfer process. This new approach for post-synthetic cation doping paves a way towards exploring the cation exchange process in several other halide perovskites at the polar-nonpolar interface.

Colloidal Metal-Halide Perovskite Nanoplatelets: Thickness-Controlled Synthesis, Properties, and Application in Light-Emitting Diodes

Colloidal metal-halide perovskite nanocrystals (MHP NCs) are gaining significant attention for a wide range of optoelectronics applications owing to their exciting properties, such as defect tolerance, near-unity photoluminescence quantum yield, and tunable emission across the entire visible wavelength range. Although the optical properties of MHP NCs are easily tunable through their halide composition, they suffer from light-induced halide phase segregation that limits their use in devices. However, MHPs can be synthesized in the form of colloidal nanoplatelets (NPls) with monolayer (ML)-level thickness control, exhibiting strong quantum confinement effects, and thus enabling tunable emission across the entire visible wavelength range by controlling the thickness of bromide or iodide-based lead-halide perovskite NPls. In addition, the NPls exhibit narrow emission peaks, have high exciton binding energies, and a higher fraction of radiative recombination compared to their bulk counterparts, making them ideal candidates for applications in light-emitting diodes (LEDs). This review discusses the state-of-the-art in colloidal MHP NPls: synthetic routes, thickness-controlled synthesis of both organic-inorganic hybrid and all-inorganic MHP NPls, their linear and nonlinear optical properties (including charge-carrier dynamics), and their performance in LEDs. Furthermore, the challenges associated with their thickness-controlled synthesis, environmental and thermal stability, and their application in making efficient LEDs are discussed.

Transformation of Metal Halides to Facet-Modulated Lead Halide Perovskite Platelet Nanostructures on A-Site Cs-Sublattice Platform

The conversion of metal halides to lead halide perovskites with B-site metal ion diffusion has remained a convenient approach for obtaining shape-modulated perovskite nanocrystals. These transformations are typically observed for materials having a common A-site Cs-sublattice platform. However, due to the fast reactions, trapping the interconversion process has been difficult. In an exploration of the tetragonal phase of Cs₇Cd₃Br₁₃ platelets as the parent material, herein, a slower diffusion of Pb(II) leading to facet-modulated CsPbBr₃ platelets is reported. This was expected due to the presence of Cd(II) halide octahedra along with Cd(II) halide tetrahedra in the parent material. This helped in microscopically monitoring their phase transformation via an epitaxially related core/shell intermediate heterostructure. The transformation was also derived and predicted by density functional theory calculations. Further, when the reaction chemistry was tuned, core/shell platelets were transformed to different facet-modulated and hollow CsPbBr₃ platelet nanostructures. These platelets having different facets were also explored for catalytic CO₂ reduction, and their catalytic rates were compared.

Growth of Lead Halide Perovskite Nanocrystals: Still in Mystery

Lead halide perovskite nanocrystals with different halide ions can lead to color-tunable emissions in visible window with near-unity photoluminescence quantum yields. Extensive research has been carried out for optimizing the synthesis of these nanocrystals for the last 6 years, and thousands of research papers have been reported. However, due to the ionic nature, these nanocrystals formed instantaneously and hence, their growth kinetics could not be established yet. In most of the reactions, the formation mechanism typically followed one reaction for one size or shape principle, and their dimension tuning was achieved predominantly with thermodynamic control. There is no clear evidence yet on the decoupling growth from nucleations and monitoring their growth kinetics. Hence, the progress of understanding the fundamentals of crystal growth faced road blocks for these halide perovskite nanocrystals. Keeping eyes on all such reports on one reaction for one size and one reaction for tunable size of the most widely studied CsPbBr₃ nanocrystals, in this perspective, details of their size tunability are analyzed and reported. In addition, comparison of the classical mechanism, obstacles for establishing secondary growth, and possible road maps for controlling the kinetic parameters of formation of these nanocrystals are also discussed.

Efficient Pure Blue Light-Emitting Diodes Based on CsPbBr3 Quantum-Confined Nanoplates

Exploitation of next-generation blue light-emitting diodes (LEDs) is the foundation of the revolution in lighting and display devices. Development of high-performance blue perovskite LEDs is still challenging. Herein, 4-aminobenzenesulfonic acid (SA) is introduced to passivate blue CsPbBr₃ nanoplates (NPLs), reducing the ionic migration via a more stable Pb²⁺-SO₃^-- formation, and the trap state density of films shows a 50% reduction. The inevitable Br^- vacancy defects after the multistep washing process can be suppressed by a suitable MABr treatment, which can boost the external quantum efficiency (EQE) performance. It should be noted that the coverage of NPL films is another key factor to realize reproducible pure blue electroluminescence (EL). Therefore, we proposed an alternate droplet/spin coating method to improve the coverage and thickness of NPL layer to prevent hole transport layer emission and increase the reproducibility of LED performance and spectra. Furthermore, we designed hole transport layers to decrease the hole transport barrier and improve the energy-level alignment. According to SA passivation, MABr treatment, alternate droplet/spin coating method, and device structure optimization, a CsPbBr₃ NPL-based pure blue (0.138, 0.046) LED with 3.18% maximum EQE can be achieved, and the half-lifetime of EL can be enhanced 1.71 times as compared to that of the counterpart LED without SA. Both performance and stability of pure blue NPL LEDs can be greatly improved via ligand passivation, alternate droplet/spin coating method, and device structure optimization, which is a trend to promote the development of pure blue perovskite LEDs in future.

Highly Emissive Blue Quantum Dots with Superior Thermal Stability via In Situ Surface Reconstruction of Mixed CsPbBr3 -Cs4 PbBr6 Nanocrystals

Although metal halide perovskites are candidate high-performance light-emitting diode (LED) materials, blue perovskite LEDs are problematic: mixed-halide materials are susceptible to phase segregation and bromide-based perovskite quantum dots (QDs) have low stability. Herein, a novel strategy for highly efficient, stable cesium lead bromide (CsPbBr3 ) QDs via in situ surface reconstruction of CsPbBr3 -Cs4 PbBr6 nanocrystals (NCs) is reported. By controlling precursor reactivity, the ratio of CsPbBr3 to Cs4 PbBr6 NCs is successfully modulated. A high photoluminescence quantum yield (PLQY) of >90% at 470 nm is obtained because octahedron CsPbBr3 QD surface defects are removed by the Cs4 PbBr6 NCs. The defect-engineered QDs exhibit high colloidal stability, retaining >90% of their initial PLQY after >120 days of ambient storage. Furthermore, thermal stability is demonstrated by a lack of heat-induced aggregation at 120 °C. Blue LEDs fabricated from CsPbBr3 QDs with reconstructed surfaces exhibit a maximum external quantum efficiency of 4.65% at 480 nm and excellent spectral stability.

Dark and Bright Excitons in Halide Perovskite Nanoplatelets

Semiconductor nanoplatelets (NPLs), with their large exciton binding energy, narrow photoluminescence (PL), and absence of dielectric screening for photons emitted normal to the NPL surface, could be expected to become the fastest luminophores amongst all colloidal nanostructures. However, super-fast emission is suppressed by a dark (optically passive) exciton ground state, substantially split from a higher-lying bright (optically active) state. Here, the exciton fine structure in 2-8 monolayer (ML) thick Csn - 1 Pbn Br3n + 1 NPLs is revealed by merging temperature-resolved PL spectra and time-resolved PL decay with an effective mass model taking quantum confinement and dielectric confinement anisotropy into account. This approach exposes a thickness-dependent bright-dark exciton splitting reaching 32.3 meV for the 2 ML NPLs. The model also reveals a 5-16 meV splitting of the bright exciton states with transition dipoles polarized parallel and perpendicular to the NPL surfaces, the order of which is reversed for the thinnest NPLs, as confirmed by TR-PL measurements. Accordingly, the individual bright states must be taken into account, while the dark exciton state strongly affects the optical properties of the thinnest NPLs even at room temperature. Significantly, the derived model can be generalized for any isotropically or anisotropically confined nanostructure.

Broadband Optical Phase Modulation by Colloidal CdSe Quantum Wells

Two-dimensional (2D) semiconductors are primed to realize a variety of photonic devices that rely on the transient properties of photogenerated charges, yet little is known on the change of the refractive index. The associated optical phase changes can be beneficial or undesired depending on the application, but require proper quantification. Measuring optical phase modulation of dilute 2D materials is, however, not trivial with common methods. Here, we demonstrate that 2D colloidal CdSe quantum wells, a useful model system, can modulate the phase of light across a broad spectrum using a femtosecond interferometry method. Next, we develop a toolbox to calculate the time-dependent refractive index of colloidal 2D materials from widely available transient absorption experiments using a modified effective medium algorithm. Our results show that the excitonic features of 2D materials result in broadband, ultrafast, and sizable phase modulation, even extending to the near infrared because of intraband transitions.

Hexamethyldisilazane-Assisted Ambient Condition Mn2+ Doping Perovskite Nanocrystals

Doping Mn2+ ions into lead halide perovskite (LHP) nanocrystals (NCs) has attracted great attention in the optoelectronic fields due to the stability enhancement and unique dual-color emission characteristics arising from...

Laser-induced crystal growth observed in CsPbBr3 perovskite nanoplatelets

Benefiting from the easily adjustable optical properties of perovskite, CsPbBr3 nanocrystals (NCs) are considered to be able to show their advantages in the field of display. Here, we report that...

Power-Dependent Photoluminescence Efficiency in Manganese-Doped 2D Hybrid Perovskite Nanoplatelets

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn²⁺)-doped organic-inorganic hybrid perovskite nanoplatelets (chemical formula: L₂[APb_1-xMn_xBr₃]_n-1Pb_1-xMn_xBr₄; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb_1-xMn_xBr₆^4- octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn²⁺ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm²). Surprisingly, we find that the saturation of long-lived Mn²⁺ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

Equilibriums in Formation of Lead Halide Perovskite Nanocrystals

Physical insights related to ion equilibrium involved in the synthesis of lead halide perovskite nanocrystals remain key parameters for regulating the phase stability and luminescence intensity of these emerging materials. These have been extensively studied since the development of these nanocrystals, and different reaction processes controlling the formation of CsPbX₃ nanocrystals are largely understood. However, growth kinetics related to the formation of these nanocrystals have not been established yet. Hence, more fundamental understanding of the formation processes of these nanocrystals is urgently required. Keeping these in mind and emphasizing the most widely studied nanocrystals of CsPbBr₃, different equilibrium processes involved in their synthesis for phase and composition variations are summarized and discussed in this Perspective. In addition, implementations of these findings for shape modulations by growth are discussed, and several new directions of research for understanding more fundamental insights are also presented.

Photoinduced quasi-2D to 3D phase transformation in hybrid halide perovskite nanoplatelets

We present a photo-induced quasi-2D to 3D phase transition of MAPbBr₃ (MA = CH₃NH₃) perovskite nanoplatelets (NPLs). To begin with, we synthesized quasi-2D MAPbBr₃ NPLs (two octahedral layers thick, n = 2). A systematic increase in the thickness of the perovskite platelets is observed as a result of continuous photon irradiation leading to a 78 nm red shift in the emission spectra through different stages. Moreover, the bandgap of the compound decreases from 2.72 eV to 2.2 eV as we move from a quasi-2D to 3D phase. The excitonic Bohr radius of the MAPbBr₃ NPLs is found to be 1.8 nm, whereas the thickness of a single layer of PbBr₆^4- octahedra is 5.9 Å. As the layer thickness increases (>4-6 layers), MAPbBr₃ NPLs move out of the quantum confinement regime, governed by the red shift in the emission spectra. To complement the experimental results, density functional theory calculations were performed on MAPbBr₃ of various layer thicknesses. The van der Waals interaction and a more accurate Heyd-Scuseria-Ernzerhof functional were used to calculate the optical bandgap for MAPbBr₃ platelets of different layer thicknesses, which matches exceptionally well with the experimental results. Our findings disclose an interesting and meaningful phenomenon in the emerging hybrid perovskite NPLs and are beneficial for any future development of perovskite-based devices.

How Exciton-Phonon Coupling Impacts Photoluminescence in Halide Perovskite Nanoplatelets

Semiconductor nanocrystals are receiving increased interest as narrow-band emitters for display applications. Here, we investigate the underlying photoluminescence (PL) linewidth broadening mechanisms in thickness-tunable 2D halide perovskite (Cs_n-1Pb_nBr_3n+1) nanoplatelets (NPLs). Temperature-dependent PL spectroscopy on NPL thin films reveals a blue-shift of the PL maximum for thicker NPLs, no shift for three monolayer (ML) thick NPLs, and a red-shift for the thinnest (2 ML) NPLs with increasing temperature. Emission linewidths also strongly depend on NPL thickness, with the thinnest NPLs showing the smallest temperature-induced broadening. We determine the combined interaction of exciton-phonon coupling and thermal lattice expansion to be responsible for both effects. Additionally, the 2 ML NPLs exhibit a significantly larger Fröhlich coupling constant and optical phonon energy, possibly due to an inversion in the exciton fine structure. These results illustrate that ultrathin halide perovskite NPLs could illuminate the next generation of displays, provided a slightly greater sample homogeneity and improved stability.

Dielectric Confinement and Exciton Fine Structure in Lead Halide Perovskite Nanoplatelets

Owing to their flexible chemical synthesis and the ability to shape nanostructures, lead halide perovskites have emerged as high potential materials for optoelectronic devices. Here, we investigate the excitonic band edge states and their energies levels in colloidal inorganic lead halide nanoplatelets, particularly the influence of dielectric effects, in a thin quasi-2D system. We use a model including band offset and dielectric confinements in the presence of Coulomb interaction. Short- and long-range contributions, modified by dielectric effects, are also derived, leading to a full modelization of the exciton fine structure, in cubic, tetragonal and orthorhombic phases. The fine splitting structure, including dark and bright excitonic states, is discussed and compared to recent experimental results, showing the importance of both confinement and dielectric contributions.

Optical Probing of Crystal Lattice Configurations in Single CsPbBr3 Nanoplatelets

Quantum-confined nanostructures of CsPbBr₃ with luminescence quantum efficiencies approaching unity have shown tremendous potential for lighting and quantum light applications. In contrast to CsPbBr₃ quantum dots, where the fine structure of the emissive exciton state has been intensely discussed, the relationship among lattice orientation, shape anisotropy, and exciton fine structure in lead halide nanoplatelets has not yet been established. In this work, we investigate the fine structure of the bright triplet exciton of individual CsPbBr₃ nanoplatelets by polarization-resolved micro- and magnetophotoluminescence spectroscopy at liquid helium temperature and find a large zero-field splitting of up to 2.5 meV. A unique relation between the crystal structure and the photoluminescence emission confirms the existence of two distinct crystal configurations in such nanoplatelets with different alignments of the crystal axes with respect to the nanoplatelet facets. Polarization-resolved experiments eventually allow us to determine the absolute orientation of an individual nanoplatelet on the substrate purely by optical means.

Dimensionality Control of Inorganic and Hybrid Perovskite Nanocrystals by Reaction Temperature: From No-Confinement to 3D and 1D Quantum Confinement

This work focuses on the systematic investigation of the shape, size, and composition‐controlled synthesis of perovskite nanocrystals (NCs) under inert gas‐free conditions and using pre‐synthesized precursor stock solutions. In the case of CsPbBr₃ NCs, we find that the lowering of reaction temperature from ∼175 to 100 °C initially leads to a change of morphology from bulk‐like 3D nanocubes to 0D nanocubes with 3D‐quantum confinement, while at temperatures below 100 °C the reaction yields 2D nanoplatelets (NPls) with 1D‐quantum confinement. However, to our surprise, at higher temperatures (∼215 °C), the reaction yields CsPbBr₃ hexapod NCs, which have been rarely reported. The synthesis is scalable, and their halide composition is tunable by simply using different combinations of precursor solutions. The versatility of the synthesis is demonstrated by applying it to relatively less explored shape‐controlled synthesis of FAPbBr₃ NCs. Despite the synthesis carried out in the air, both the inorganic and hybrid perovskite NCs exhibit nearly‐narrow emission without applying any size‐selective separation, and it is precisely tunable by controlling the reaction temperature.

Copper-doping defect-lowered perovskite nanosheets for deep-blue light-emitting diodes

Mixed-halide blue perovskites CsPb(Br/Cl)₃ are considered promising candidates for developing efficient deep-blue perovskite light-emitting diodes (PeLEDs), but their low photoluminescence quantum yield (PLQY), environmental instability, and poor device performance gravely inhibit their future development. Here, we employ a heteroatomic Cu²⁺ doping strategy combined with post-treatment Br^- anion exchange to prepare high-performance deep-blue perovskites CsPb(Br/Cl)₃. The Cu²⁺ doping strategy significantly decreases the intrinsic chlorine defects, ensuring that the inferior CsPbCl₃ quantum dots are transformed into two-dimensional nanosheets with enhanced violet photoluminescence and increased exciton binding energy. Further, with the post-treatment Br^- anion exchange, the as-prepared CsPb(Br/Cl)₃ nanosheets with more radiation recombination and less ion migration present an enhanced PLQY of 94% and better humidity stability of 30 days. Based on the optimized CsPb(Br/Cl)₃, we fabricated deep-blue PeLEDs with luminescence emission at 462 nm, a maximum luminance of 761 cd m^-2, and a current density of 205 mA cm^-2. This work puts forward a feasible synthesis strategy to prepare efficient and stable mixed-halide blue perovskite CsPb(Br/Cl)₃ and related blue PeLEDs, which may promote the further application of mixed-halide perovskites in the blue light range.

Unusual Temperature Dependence of Bandgap in 2D Inorganic Lead-Halide Perovskite Nanoplatelets

Understanding the origin of temperature-dependent bandgap in inorganic lead-halide perovskites is essential and important for their applications in photovoltaics and optoelectronics. Herein, it is found that the temperature dependence of bandgap in CsPbBr3 perovskites is variable with material dimensionality. In contrast to the monotonous redshift ordinarily observed in bulk-like CsPbBr3 nanocrystals (NCs), the bandgap of 2D CsPbBr3 nanoplatelets (NPLs) exhibits an initial blueshift then redshift trend with decreasing temperature (290-10 K). The Bose-Einstein two-oscillator modeling manifests that the blueshift-redshift crossover of bandgap in the NPLs is attributed to the significantly larger weight of contribution from electron-optical phonon interaction to the bandgap renormalization in the NPLs than in the NCs. These new findings may gain deep insights into the origin of bandgap shift with temperature for both fundamentals and applications of perovskite semiconductor materials.

Stable and Efficient Blue-Emitting CsPbBr3 Nanoplatelets with Potassium Bromide Surface Passivation

Colloidal all-inorganic perovskites nanocrystals (NCs) have emerged as a promising material for display and lighting due to their excellent optical properties. However, blue emissive NCs usually suffer from low photoluminescence quantum yields (PLQYs) and poor stability, rendering them the bottleneck for full-color all-perovskite optoelectronic applications. Herein, a facile approach is reported to enhance the emission efficiency and stability of blue emissive perovskite nano-structures via surface passivation with potassium bromide. By adding potassium oleate and excess PbBr₂ to the perovskite precursor solutions, potassium bromide-passivated (KBr-passivated) blue-emitting (≈450 nm) CsPbBr₃ nanoplatelets (NPLs) is successfully synthesized with a respectably high PLQY of 87%. In sharp contrast to most reported perovskite NPLs, no shifting in emission wavelength is observed in these passivated NPLs even after prolonged exposures to intense irradiations and elevated temperature, clearly revealing their excellent photo- and thermal-stabilities. The enhancements are attributed to the formation of K-Br bonding on the surface which suppresses ion migration and formation of Br-vacancies, thus improving both the PL emission and stability of CsPbBr₃ NPLs. Furthermore, all-perovskite white light-emitting diodes (WLEDs) are successfully constructed, suggesting that the proposed KBr-passivated strategy can promote the development of the perovskite family for a wider range of optoelectronic applications.

Defect Passivation in Lead-Halide Perovskite Nanocrystals and Thin Films: Toward Efficient LEDs and Solar Cells

Lead-halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge carriers into light and vice versa. However, despite the defect-tolerance of LHPs, defects at the surface of colloidal NCs and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the PLQYs, device efficiency, and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes are critical for achieving advances in performance. This Review presents the current understanding of defects in halide perovskites and their influence on the optical and charge-carrier transport properties. Passivation strategies toward improving the efficiencies of perovskite-based LEDs and solar cells are also discussed.

Two-dimensional halide perovskites: synthesis, optoelectronic properties, stability, and applications

Halide perovskites are promising materials for light-emitting and light-harvesting applications. In this context, two-dimensional perovskites such as nanoplatelets or Ruddlesden-Popper and Dion-Jacobson layered structures are important because of their structural flexibility, electronic confinement, and better stability. This review article brings forth an extensive overview of the recent developments of two-dimensional halide perovskites both in the colloidal and non-colloidal forms. We outline the strategy to synthesize and control the shape and discuss different crystalline phases and optoelectronic properties. We review the applications of two-dimensional perovskites in solar cells, light-emitting diodes, lasers, photodetectors, and photocatalysis. Besides, we also emphasize the moisture, thermal, and photostability of these materials in comparison to their three-dimensional analogs.

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.

Lanthanide-doping enables kinetically controlled growth of deep-blue two-monolayer halide perovskite nanoplatelets

Impurity doping has been widely applied in nanomaterial synthesis for modulating the crystallographic phase, morphology, and size of nanocrystalline materials, but mostly by altering thermodynamic equilibria of final products. Here, we report the use of lanthanide dopants to manipulate the growing kinetics of halide perovskite nanocrystals to enable the preparation of highly anisotropic two-dimensional (2D) CsPbBr3-based nanoplatelets with precisely controlled thickness. We demonstrate that the incorporation of trivalent lanthanides increases the energy barrier in growing three-monolayer (3 ML) CsPbBr3 from a 2 ML intermediate. It enables the growth of thermodynamically unfavorable 2 ML CsPbBr3 products through kinetic control. This finding provides a novel approach for dimensional control of perovskite nanocrystals with strong quantum confinement. It offers opportunities to generate deep-blue emitting (at 430 nm) CsPbBr3:Lu3+ nanoplatelets with good structural- and photo-stabilities potentially useful for many applications including light-emitting, lasers, and photocatalysis.

Recent Advances in Ligand Design and Engineering in Lead Halide Perovskite Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) have recently garnered enhanced development efforts from research disciplines owing to their superior optical and optoelectronic properties. These materials, however, are unlike conventional quantum dots, because they possess strong ionic character, labile ligand coverage, and overall stability issues. As a result, the system as a whole is highly dynamic and can be affected by slight changes of particle surface environment. Specifically, the surface ligand shell of LHP NCs has proven to play imperative roles throughout the lifetime of a LHP NC. Recent advances in engineering and understanding the roles of surface ligand shells from initial synthesis, through postsynthetic processing and device integration, finally to application performances of colloidal LHP NCs are covered here.

Engineering the Bandgap and Surface Structure of CsPbCl3 Nanocrystals to Achieve Efficient Ultraviolet Luminescence

Herein, we report the design of novel ultraviolet luminescent CsPbCl₃ nanocrystals (NCs) with the emission peak at 381 nm through doping of cadmium ions. Subsequently, a surface passivation strategy with CdCl₂ is adopted to improve their photoluminescence quantum yield (PLQY) with the maximum value of 60.5 %, which is 67 times higher than that of the pristine counterparts. The PLQY of the surface passivated NCs remains over 50 % after one week while the pristine NCs show negligible emission. By virtue of density functional theory calculations, we reveal that the higher PLQY and better stability after surface passivation may result from the significant elimination of surface chloride vacancy (V_Cl ) defects. These findings provide fundamental insights into the optical manipulation of metal ion-doped CsPbCl₃ NCs.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

Understanding Discrete Growth in Semiconductor Nanocrystals: Nanoplatelets and Magic-Sized Clusters

ConspectusSemiconductor nanocrystals (NCs) fluoresce with a color that strongly depends on their size and shape. Thus, to obtain homogeneous optical properties, researchers have strived to synthesize particles that are uniform. However, because NCs typically grow through continuous, incremental addition of material, slight differences in the growth process between individual crystallites yield statistical distributions in size and shape, leading to inhomogeneities in their optical characteristics. Much work has focused on improving synthetic protocols to control these distributions and enhance performance. Interestingly, during these efforts, several syntheses were discovered that exhibit a different type of growth process. The NCs jump from one discrete size to the next. Through purification methods, one of these sizes can then be isolated, providing a different approach to uniform NCs. Unfortunately, the fundamental mechanism behind such discrete growth and how it differs from the conventional continuous process have remained poorly understood.Discrete growth has been observed in two major classes of NCs: semiconductor nanoplatelets (NPLs) and magic-sized clusters (MSCs). NPLs are quasi-two-dimensional crystallites that exhibit a precise thickness of only a few atomic layers but much larger lateral dimensions. During growth, NPLs slowly appear with an increasing number of monolayers. By halting this process at a specific time, NPLs with a desired thickness can then be isolated (e.g., four monolayers). Because the optical properties are primarily governed by this thickness, which is uniform, NPLs exhibit improved optical properties such as narrower fluorescence line widths.While NPLs have highly anisotropic shapes and show discrete growth only in one dimension (thickness), MSCs are isotropic particles. The name "magic" arose because a specific set of NC sizes appear during synthesis. They have been believed to represent special atomic arrangements that possess enhanced structural stability. Historically, they were very small, hence molecular-scale "clusters." Isolation of one of the MSC sizes can then, in principle, provide a uniform sample of NCs. More recently, MSC growth has been extended to larger sizes, beyond what is commonly considered to be the "cluster" regime, challenging the conventional explanation for these materials.This Account summarizes recent work by our group to understand the mechanism that governs discrete growth in semiconductor NCs. We begin by describing the synthesis of NPLs. Next, we discuss the mechanism behind the highly anisotropic shape of NPLs. We build on this by examining the ripening process in NPLs. We show that NPLs slowly appear with increasing thickness, counterintuitively through lateral growth. Then, we turn to the synthesis of MSCs, in particular focusing on their growth mechanism. Our findings indicate a strong connection between NPLs and MSCs. Finally, we review several remaining challenges for the growth of NPLs and MSCs and give a brief outlook on the future of discrete growth. By understanding the underlying process, we believe that it can be exploited more broadly, potentially moving us toward more uniform nanomaterials.

Metal Halide Perovskites for Optical Parametric Modulation

Metal halide perovskites (MHPs) have attracted considerable academic and industrial attention because of their remarkable optoelectronic properties. The development of optical parametric modulation is urgently needed because it plays an important role in display applications and optical communication. Perovskites can become the bridge between materials and optics. Through changing the composition and nanostructure of perovskites, we can modulate optical parameters, including optical intensity, frequency, polarization, and phase. This Perspective provides a brief introduction to this field and summarizes the methods of modulating optical parameters. It is instructive for building a relationship between perovskite nanostructures and optics, which is meaningful for display technologies and optical communication.

Momentarily trapped exciton polaron in two-dimensional lead halide perovskites

Two-dimensional (2D) lead halide perovskites with distinct excitonic feature have shown exciting potential for optoelectronic applications. Compared to their three-dimensional counterparts with large polaron character, how the interplay between long- and short- range exciton-phonon interaction due to polar and soft lattice define the excitons in 2D perovskites is yet to be revealed. Here, we seek to understand the nature of excitons in 2D CsPbBr₃ perovskites by static and time-resolved spectroscopy which is further rationalized with Urbach-Martienssen rule. We show quantitatively an intermediate exciton-phonon coupling in 2D CsPbBr₃ where exciton polarons are momentarily self-trapped by lattice vibrations. The 0.25 ps ultrafast interconversion between free and self-trapped exciton polaron with a barrier of ~ 34 meV gives rise to intrinsic asymmetric photoluminescence with a low energy tail at room temperature. This study reveals a complex and dynamic picture of exciton polarons in 2D perovskites and emphasizes the importance to regulate exciton-phonon coupling.

Two-dimensional perovskite shows potential for optoelectronic applications due to its large exciton binding energy, yet the exciton-phonon interaction with the polar soft lattice is not well-understood. Here, the authors reveal the intermediate coupling regime where exciton polarons are momentarily trapped by lattice vibrations.

Nonlinear Photonics Using Low-Dimensional Metal-Halide Perovskites: Recent Advances and Future Challenges

Low-dimensional metal-halide perovskites have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor counterparts, thanks to their peculiar physical and electronic structures. Their exceptional nonlinear optical characteristics make them excellent candidates for revolutionizing widespread applications. However, the research of nonlinear photonics based on low-dimensional metal-halide perovskites is in its infancy. There is a lack of comprehensive and in-depth summary of this research realm. Here, the state-of-the-art research progress related to third-and higher-order nonlinear optical properties of low-dimensional metal-halide perovskites with diverse crystal structures from 3D down to 0D, together with their practical applications, is summarized comprehensively. Critical discussions are offered on the fundamental mechanisms beneath their exceptional nonlinear optical performance from the physics viewpoint, attempting to disclose the role of intrinsic attributes (e.g., composition, bandgap, size, shape, and structure) and external modulation strategies (e.g., developing core-shell structures, transition metal ion doping, and hybridization with dielectric microspheres) in tuning the response. Additionally, their potential applications in nonlinear photonics, nonlinear optoelectronics, and biophotonics are systematically and thoroughly summed up and categorized. Lastly, insights into the current technical challenges and future research opportunities of nonlinear photonics based on low-dimensional metal-halide perovskites are provided.