Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties

Tuning Hot-Carrier Temperature in CsPbBr3 Perovskite Nanoplatelets through Metal Halide Passivation

High carrier temperature and slow carrier cooling make perovskite nanostructures potential candidates for hot-carrier solar cells. Here, using time-resolved photoluminescence spectroscopy, hot-carrier dynamics is reported in strongly confined three-monolayer quasi-2D CsPbBr3 perovskite nanoplatelets characterized by sharp excitonic peaks in the absorption spectrum and narrow emission peaks in the blue region. Treatment with a PbBr2-ligand solution resulted in a remarkable seven-fold increase in photoluminescence intensity, attributed to effective passivation of surface defects due to lead(II) and bromide vacancies. Further investigations using time-resolved emission spectroscopy revealed consistent carrier cooling times of ∼300 fs for both pristine and treated nanoplatelets, indicating similar fundamental hot-carrier cooling processes. Notably, treated nanoplatelets exhibited higher carrier temperature (∼700 K), linked to increased radiative carrier density after defect passivation. This work demonstrates that treatment of quasi-2D CsPbBr3 perovskite nanoplatelets with metal halides substantially improves the optoelectronic properties. Notably, hot-carrier temperatures can be increased significantly while preserving the cooling time.

Energy Transfer from a Perovskite Nanocrystal to Cyanine Dyes Depending on Spectral Overlap Revealed by a Single-Particle Spectroscopy

Investigation of the energy transfer (ET) between perovskite nanocrystals (PNCs) and organic dyes is crucial because PNCs are suitable donor materials for creating photosensitizer systems. In this study, we investigated the relationship between a singlet ET mechanism and spectral overlap using CsPbBr3 PNC-cyanine dyes─Cy3 and Cy5. We prepared PNC-dye linkage systems and investigated the ET at the single-PNC levels. The results of our study reveal that efficient ET occurs via fluorescence resonance ET (FRET) in the PNC-Cy3 system. Even in the PNC-Cy5 system, where the spectral overlap integral is 1 order of magnitude smaller than in the PNC-Cy3 system, efficient ET is observed, suggesting the occurrence of Dexter-type ET (DET). These results clearly demonstrate the change in the ET mechanism between FRET and DET depending on the spectral overlap. These findings are important for understanding exciton dynamics in PNC-organic molecule hybrid systems.

Ligand-Solvent Library Design for Tailoring Interparticle Interactions in Colloidal Nanocrystals

This study explores the critical role of nonpolar ligand-solvent systems in modulating interparticle interactions in colloidal nanocrystals, profoundly affecting colloidal stability and enabling precision self-assembly. A library of 28 ligands with diverse molecular fragments─double bonds, branched chains, benzene rings, and naphthalene rings─and four solvents was developed to investigate how fragment types and positions affect ligand ordering and interparticle attraction. Explicit solvent simulations with enhanced sampling techniques reveal that fragments near the headgroup or midsection disrupt ligand ordering and weaken interparticle attraction, whereas terminal placement fosters ordered ligand packing and enhances attraction. Simulation predictions on the relationship between ligand structures and interparticle interactions were validated through self-assembly experiments using colloidal nanocrystals passivated by six representative ligands. Furthermore, the potential to control ligand ordering and interparticle interactions was demonstrated by tuning fragment types, positions, combinations, and solvent sizes. This work deepens the understanding of ligand-solvent dynamics and provides a theoretical framework for the molecular-level design of nanocrystal self-assembly.

Application of advanced quantum dots in perovskite solar cells: synthesis, characterization, mechanism, and performance enhancement

Quantum dots have garnered significant interest in perovskite solar cells (PSCs) due to their stable chemical properties, high carrier mobility, and unique features such as multiple exciton generation and excellent optoelectronic characteristics resulting from quantum confinement effects. This review explores quantum dot properties and their applications in photoelectronic devices, including their synthesis and deposition processes. This sets the stage for discussing their diverse roles in the carrier transport, absorber, and interfacial layers of PSCs. We thoroughly examine advances in defect passivation, energy band alignment, perovskite crystallinity, device stability, and broader light absorption. In particular, novel approaches to enhance the photoelectric conversion efficiency (PCE) of quantum dot-enhanced perovskite solar cells are highlighted. Lastly, based on a comprehensive overview, we provide a forward-looking outlook on advanced quantum dot fabrication and its impact on enhancing the photovoltaic performance of solar cells. This review offers insights into fundamental mechanisms that endorse quantum dots for improved PSC performance, paving the way for further development of quantum dot-integrated PSCs.

Identifying Lanthanide Energy Levels in Semiconductor Nanoparticles Enables Tailored Multicolor Emission through Rational Dopant Combinations

ConspectusThe unique photon emission signatures of trivalent lanthanide cations (Ln3+, where Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) enables multicolor emission from semiconductor nanoparticles (NPs) either through doping multiple Ln3+ ions of distinct identities or in combination with other elements for the creation of next-generation light emitting diodes (LEDs), lasers, sensors, imaging probes, and other optoelectronic devices. Although advancements have been made in synthetic strategies to dope Ln3+ in semiconductor NPs, the dopant(s) selection criteria have hinged largely on trial-and-error. This combinatorial approach is often guided by treating NP-dopant(s) energy transfer dynamics through the lens of spectral overlap. Over the past decade, however, we have demonstrated that the spectral outcomes correlate better with the placement of Ln3+ energy levels with respect to the band edges of the semiconductor, and oxide, host.In this Account, we describe how the Ln3+ energy level alignments affect the dopant emission intensities and dictate interdopant energy transfer processes in semiconductor nanoparticle hosts. This Account begins with a concise primer on the emission characteristics of trivalent lanthanides, the challenges that are associated with realizing meaningful lanthanide luminescence, and how semiconductor nanoparticles can act as a host to sensitize lanthanide emission. We then describe a semiempirical approach that can be used to place the lanthanide ground and luminescent energy levels with respect to the band edges of the host semiconductor nanoparticle. The ability of this model to track and predict the lanthanide sensitization efficiency is illustrated for singly doped zinc sulfide (ZnS), titanium dioxide (TiO2), and cesium lead chloride (CsPbCl3) perovskite hosts. Next, we discuss how knowledge of energy level offsets can be used to select dopant(s) for tunable multicolor emission by identifying different charge trapping processes for semiconductors doped with single and multiple lanthanides and discussing their impact on sensitization outcomes. Following this discussion, the Account lists viable Ln3+ combinations in ZnS NPs based on the charge trapping model and shows the limitations of spectral overlap models in predicting viable Ln3+ dopant combinations. Feasible f-f and d-f codopant combinations based on charge trapping are presented for TiO2 and CsPbCl3 NPs. The intricacies of interdopant energy migration and spin considerations that dictate the dopant(s) sensitization efficiencies are made known. Finally, we use these considerations to predict NP-dopant(s) combinations that should exhibit concerted emissions from the blue to the near-infrared (NIR) region, thereby enabling the design of bespoke optoelectronic properties. The Account ends with some forward-looking thoughts, arguing for the need to develop better quantitative models in order to explore the Ln3+ sensitization mechanisms and presenting ideas for applications of doped semiconductor NPs in energy and health that would be aided by interdopant energy transfer dynamics.

Transparent and flexible cellulose based luminescent film for multifunctional applications

Metal halide perovskite quantum dots (QDs) film shows significant potential in flexible optoelectronics, including applications in lighting, displays, wearable devices and non-planar X-ray imaging. However, developing highly luminescent, durable, and mechanical flexible film for practical use remains a challenge. In this study, we introduce a low-cost, environmentally friendly, biomass material - mixed cellulose esters (MCE) - as a novel encapsulation matrix. The CsPbBr3@MCE composite luminescent film was fabricated using a simple in-situ growth strategy, ensuring uniform distribution of QDs within the matrix. The chemical bond anchoring between MCE and CsPbBr3 QDs, combined with the effective isolation provided by in-situ encapsulation, resulted in exceptional luminescence properties, including a high photoluminescence quantum yield (PLQY = 67.73 %) and excellent color purity. Additionally, the film demonstrated enhanced stability against environmental, thermal, ultraviolet, and high-humidity stresses, thanks to the protective encapsulation of MCE. It also exhibited remarkable mechanical flexibility, transparency, the capability for large-area production. These findings suggest that the CsPbBr3@MCE composite holds great promise for various applications, including light-emitting diodes, flexible pattern display, hazardous chemical identification, information encryption, and anti-counterfeiting technologies.

Transfer printing of perovskite nanocrystal self-assembled monolayers via controlled surface wettability

Lead halide perovskite nanocrystals (LHP NCs) have attracted significant attention as next-generation semiconductor nanomaterials due to their near-unity photoluminescence quantum yields and tunable emission wavelengths. Despite their outstanding optical properties, their instability makes it difficult to apply conventional lithography techniques to LHP NC films, which hinders their application in nano-optoelectronics. To overcome this problem, in this work, we propose solvent- and heat-free contact printing technologies for the transfer and microfabrication of LHP NC self-assembled monolayers, employing viscoelastic stamps and wettability-controlled solid substrates. To proceed with multistep transfer of NC films, it is necessary to control the adhesion force between the NCs and the substrate at each step. There is also another requirement concerning the affinity between LHP NCs and substrates to fabricate a spatially uniform LHP NC self-assembled monolayer by spin-coating. To meet these two requirements, the initial substrates for spin-coating were treated with a mixture of fluoroalkyl and alkyl silanes (with a mixing ratio of 0.85 : 0.15), whereas those for transfer were treated with hexamethyldisilane (HMDS). The micropatterned LHP NC monolayers were successfully fabricated by employing patterned viscoelastic stamps. This approach using a back-to-basics technique provides a simple and reliable process for integrating LHP NCs into advanced nano-optoelectronic devices.

Tunable White Light Emission from Transparent Nanophosphor Films Embedding Perovskite Lead Halide Nanostructures

Exploring synergistic interactions between nanomaterials that can enhance their collective properties in ways that individual components cannot achieve represents an avenue for advancing beyond the current state of the art. This approach is particularly relevant in the context of ABX3 nanocrystals, where pursuing cooperation could help to overcome current challenges associated with light generation. Transparent photoluminescent coatings are developed by combining perovskite nanomaterials and porous scaffolds of high optical quality phosphor nanoparticles. Fine tuning of the spectral content of the emission is achieved with the photoexcitation wavelength, allowing the demonstration of white light emission with tunable hues.

Eco-friendly synthesis of Cs3Bi2Br9 perovskite quantum dots using castor oil as solvent and ligand for leather anti-counterfeiting

All-inorganic cesium bismuth bromide perovskite quantum dots(Cs3Bi2Br9 PQDs) have emerged as promising alternatives to lead-based perovskite quantum dots with excellent performance due to their low toxicity, drawing extensive attention over recent decades. However, challenges remain in terms of their stability and the reliance on organic solvents during the preparation process. Herein, a novel synthesis method for Cs3Bi2Br9 PQDs is introduced, utilizing eco-friendly castor oil as both the solvent and ligand (CO-Cs3Bi2Br9). These PQDs display a vivid blue emission at 430 nm, with an impressive photoluminescence quantum yield (PLQY) of 21.2%. Furthermore, they maintain 97.3% of their fluorescence intensity after 72 h of environmental exposure. The effects of various components of castor oil, including ricinoleic, oleic and linoleic acid, on crystal growth and properties of the Cs3Bi2Br9 are investigated. Significantly, the presence of conjugated double bonds in linoleic acid, when used as a solvent, results in a PLQY of up to 53% for the synthesized Cs3Bi2Br9 PQDs. Moreover, CO-Cs3Bi2Br9 PQDs are introduced into leather by a layer-by-layer self-assembly method, the bright blue fluorescent pattern can be observed in the CO-Cs3Bi2Br9/leather under ultraviolet irradiation, indicating the leather anti-counterfeiting potential of CO-Cs3Bi2Br9 PQDs. This study opens a novel pathway for the sustainable synthesis of PQDs through the utilization of castor oil-derived natural green solvents.

Studying the effect of dimensions and spacer ligands on the optical properties of 2D lead iodide perovskites

In recent years, metal-halide perovskites (MHPs) have emerged as highly promising optoelectronic materials based on their exceptional properties and versatility in applications such as solar cells, light-emitting devices, and radiation detectors. This study investigates the optical properties of two-dimensional (2D) MHPs, with the Ruddlesden-Popper structure, comparing three morphologies-bulk poly-crystals, colloidal nanoplatelets (NPs), and thin films, aiming to bridge between the bulk and nano dimensionalities. By synthesizing bulk 2D MHPs using long alkyl ammonium spacers, typically found in colloidal systems, and NPs using shorter ligands suitable for bulk growth, we elucidate the relationship between these materials' structural modifications and optical characteristics. We propose the existence of two regions in these 2D MHPs, which differ in their optoelectronic properties and are associated with "bulk" and "surface" regions. Specifically, for poly-crystals, we observe the appearance of a lower energy "bulk" phase associated with the stacking of many 2D sheets, apparent both in absorption and photoluminescence. For NPs, this stacking is hindered, and hence, only the "surface" phase exists. With the elongation of the spacer chain, the poly-crystal becomes more similar to the NPs. For thin films, an interesting phenomenon is observed - the rapid film formation mechanism forces a more colloid-like structure for the shorter ligands and a more poly-crystal-like structure for the longer ones. Overall, this study bridging the different dimensions of 2D MHPs may support new possibilities for future research and development in this innovative field.

Surface chemistry-engineered perovskite quantum dot photovoltaics

The discovery and synthesis of colloidal quantum dots (QDs) was awarded the Nobel Prize in Chemistry in 2023. Recently, the development of bulk metal halide perovskite semiconductors has generated intense interest in their corresponding perovskite QDs. QDs, more broadly known as nanocrystals, constitute a new class of materials that differ from both molecular and bulk materials. They have rapidly advanced to the forefront of optoelectronic applications owing to their unique size-, composition-, surface- and process-dependent optoelectronic properties. More importantly, their ultrahigh surface-area-to-volume ratio enables various surface chemistry engineering strategies to tune and optimize their optoelectronic properties. Finally, three-dimensional confined QDs, offering nearly perfect photoluminescent quantum yield, slow hot-carrier cooling time, especially their colloidal synthesis and processing using industrially friendly solvents, have revolutionized the fields of electronics, photonics, and optoelectronics. Particularly, in emerging perovskite QD-based PVs, the advancement of surface chemistry has boosted the record power conversion efficiency (PCE) to 19.1% within a five-year period, surpassing all other colloidal QD photovoltaics (PVs). Given the rapid enhancement of device performances, perovskite QD PVs have attracted significant attention. Further study of semiconducting perovskite QDs will lead to advanced surface structures, a deeper understanding of halide perovskites, and enhanced PCE. In this review article, we comprehensively summarize and discuss the emerging perovskite QD PVs, providing insights into the impact of surface chemical design on their electronic coupling, dispersibility, stability and defect passivation. The limitations of current perovskite QDs mainly arise from their "soft" ionic nature and dynamic surface equilibrium, which lead to difficulties in the large-scale synthesis of monodispersed perovskite QDs and conductive inks for high-throughput printing techniques. We present that the development of surface chemistry is becoming a platform for further improving PCE, aiming to reach the 20% milestone. Additionally, we discuss integrating artificial intelligence to facilitate the mass-production of perovskite QDs for large-area, low-cost PV technology, which could help address significant energy challenges.

Hybrid halide perovskite quantum dots for optoelectronics applications: recent progress and perspective

Nanotechnology has transformed optoelectronics through quantum dots (QDs), particularly metal halide perovskite QDs (PQDs). PQDs boast high photoluminescent quantum yield, tunable emission, and excellent defect tolerance without extensive passivation. Quantum confinement effects, which refer to the phenomenon where the motion of charge carriers is restricted to a small region, produce discrete energy levels and blue shifts in these materials. They are ideal for next-generation optoelectronic devices prized for superior optical properties, low cost, and straightforward synthesis. In this review, along with the fundamental physics behind the phenomenon, we have covered advances in synthesis methods such as hot injection, ligand-assisted reprecipitation, ultrasonication, solvothermal, and microwave-assisted that enable precise control over size, shape, and stability, enhancing their suitability for LEDs, lasers, and photodetectors. Challenges include lead toxicity and cost, necessitating research into alternative materials and scalable manufacturing. Furthermore, strategies like doping and surface passivation that improve stability and emission control are discussed comprehensively, and how lead halide perovskites like CsPbBr3undergo phase transitions with temperature, impacting device performance, are also investigated. We have explored various characterization techniques, providing insights into nanocrystal properties and behaviors in our study. This review highlights PQDs' synthesis, physical and optoelectronic properties, and potential applications across diverse technologies.

Landé g-factors of electrons and holes strongly confined in CsPbI3 perovskite nanocrystals in glass

The Landé g-factor of charge carriers is a key parameter in spin physics controlling spin polarization and spin dynamics. In turn, it delivers information about the electronic band structure in the vicinity of the band gap and its modification in nanocrystals provided by strong carrier confinement. The coherent spin dynamics of electrons and holes are investigated in CsPbI3 perovskite nanocrystals with sizes varied from 4 to 16 nm by means of time-resolved Faraday ellipticity at the temperature of 6 K. The Landé g-factors of the charge carriers are evaluated through the Larmor spin precession in magnetic fields up to 430 mT across the spectral range from 1.69 to 2.25 eV, provided by variation of the nanocrystal size. The spectral dependence of the electron g-factor follows the model predictions when accounting for the mixing of the electronic bands with increasing confinement resulting from a decrease of the nanocrystal size. The spectral dependence of the hole g-factor, changing from -0.19 to +1.69, is considerably stronger than expected from the model. We analyze several mechanisms and conclude that none of them can be responsible for this difference. The renormalizations of the electron and hole g-factors roughly compensate each other, providing spectral independence for the bright exciton g-factor with a value of about +2.2.

Peptide-Perovskite Based Bio-Inspired Materials for Optoelectronics Applications

The growing demand for environmentally friendly semiconductors that can be tailored and developed easily is compelling researchers and technologists to design inherently bio-compatible, self-assembling nanostructures with tunable semiconducting characteristics. Peptide-based bioinspired materials exhibit a variety of supramolecular morphologies and have the potential to function as organic semiconductors. Such biologically or naturally derived peptides with intrinsic semiconducting characteristics create new opportunities for sustainable biomolecule-based optoelectronics devices. Affably, halide perovskite nanocrystals are emerging as potentially attractive nano-electronic analogs, in this vein creating synergies and probing peptide-perovskite-based bio-electronics are of paramount interest. The physical properties and inherent aromatic short-peptide assemblies that can stabilize, and passivate the defects at surfaces assist in improving the charge transport in halide perovskite devices. This review sheds light on how these peptide-perovskite nano-assemblies can be developed for optical sensing, optoelectronics, and imaging for biomedical and healthcare applications. The charge transfer mechanism in peptides along with as an outlook the electron transfer mechanism between perovskite and short peptide chains, which is paramount to facilitate their entry into molecular electronics is discussed. Future aspects, prevailing challenges, and research directions in the field of perovskite-peptides are also presented.

Synergistic Integration of Halide Perovskite and Rare-Earth Ions toward Photonics

Halide perovskites (HPs), emerging as a noteworthy class of semiconductors, hold great promise for an array of optoelectronic applications, including anti-counterfeiting, light-emitting diodes (LEDs), solar cells (SCs), and photodetectors, primarily due to their large absorption cross section, high fluorescence efficiency, tunable emission spectrum within the visible region, and high tolerance for lattice defects, as well as their adaptability for solution-based fabrication processes. Unlike luminescent HPs with band-edge emission, trivalent rare-earth (RE) ions typically emit low-energy light through intra-4f optical transitions, characterized by narrow emission spectra and long emission lifetimes. When fused, the cooperative interactions between HPs and REs endow the resulting binary composites not only with optoelectronic properties inherited from their parent materials but also introduce new attributes unattainable by either component alone. This review begins with the fundamental optoelectronic characteristics of HPs and REs, followed by a particular focus on the impact of REs on the electronic structures of HPs and the associated energy transfer processes. The advanced synthesis methods utilized to prepare HPs, RE-doped compounds, and their binary composites are overviewed. Furthermore, potential applications are summarized across diverse domains, including high-fidelity anticounterfeiting, bioimaging, LEDs, photovoltaics, photodetection, and photocatalysis, and conclude with remaining challenges and future research prospects.

Manipulation of Metal Halide Perovskite: Photoelectric Conversion or Light Emission?

Metal halide perovskites (MHPs) display a range of superior photophysical properties, rendering them promising as a candidate for the active medium of high-efficiency photovoltaic and electroluminescence devices. In order to maximize their efficacy in photoelectric conversion or light emission, it is essential to regulate the charge separation efficiency of MHPs in a desired manner. Herein, we demonstrate that the extent of charge separation can be effectively manipulated upon thermal annealing treatment on MHPs. As the annealing time is extended from 10 to 30 min, the accumulation of excess lead halides is observed at the boundaries of MHP grains, resulting in the construction of a quasi-Type II band alignment between the lead halide and the MHP. This facilitates the separation of electron-hole pairs, reducing the exciton binding energy from approximately 102 meV to a level comparable with kBT. Our findings elucidate the transition of MHPs from a light-emission material to a photoelectric-conversion material along with continuous heating treatment, which is anticipated to guide the flexible regulation of MHPs to meet the requirements of specific practical applications.

Perovskite-Molecular Photocatalyst Synergy and Surface Engineering for Superior Photocatalytic Performance

Metal halide perovskite nanocrystals (NCs), known for their strong visible-light absorption and tunable optoelectronic properties, show significant promise for photocatalytic applications. However, their efficiency is often hindered by rapid charge recombination and insufficient exciton dissociation, limiting effective catalysis. Excited-state interactions at the NC interface are critical in determining photocatalytic performance, underscoring the need for strategies that enhance charge separation and minimize recombination. To address these challenges, we developed a composite material by combining cesium lead bromide (CsPbBr3) nanocrystals with ferrocene carboxylic acid (FcA), a hole-extracting moiety. This integration enhances exciton dissociation through energy level alignment and recombination suppression, resulting in a 3-fold increase in the photocatalytic oxidation yield of benzylamine to N-benzylidenebenzylamine (35 ± 5% versus 12 ± 2% for pristine CsPbBr3). Additionally, thionyl bromide (SOBr2) surface modification strips off ligands and introduces bromide ions onto the CsPbBr3 NCs, further improving charge transfer and substrate accessibility, resulting in a 27 ± 5% yield within 3 h. While SOBr2 treatment enhances initial catalytic performance, its acidic nature may lead to reversible reactions and side products over extended reaction times. This study highlights the impact of molecular integration and surface engineering on optimizing interfacial charge dynamics, providing a pathway toward robust, high-efficiency perovskite photocatalysts for sustainable chemical transformations.

Colloidal Perovskite Nanocrystal Superlattice Films with Simultaneous Polarized Emission and Orderly Electric Polarity via an In Situ Surface Cross-Linking Reaction

Superlattices (SLs) based on colloidal nanocrystals (NCs) represent a fascinating structure with long-range and ordered NCs inside the assembled superstructures, displaying great potential application in electronic devices because of the customizable arrangement of building blocks. It is a great challenge to achieve macroscopical SL films by a solution process due to the inherent sensitivity and difficulty in controlling colloidal NCs. In this study, we propose a controllable strategy to create perovskite CsPbBr3 NC SL films through a surface in situ cross-linking reaction incorporating conjugated linoleic acid (CLA), a naturally polymerizable small molecule. CLA enables the in situ cross-linking of adjacent NCs under polarity-triggered conditions, which effectively arranges the NCs in a solid form at a molecular level to achieve fcc SL structural films. Importantly, we report for the first time NC SL films that are simultaneous with outstanding intrinsically linearly polarized emission and orderly electric polarity, which are derived from consistent dipole alignment, thus showing promising potential for application in information storage and optoelectronics. This method provides a general bottom-up approach, expanding the assembly library for fundamental studies and technological applications.

Steady state and transient absorption spectroscopy in metal halide perovskites

Metal halide perovskites (MHPs) have emerged as the most promising materials due to superior optoelectronic properties and great applications spanning from photovoltaics to photonics. Absorption spectroscopy provides a broad and deep insight into the carrier dynamics of MHPs, and is a critical complement to fluorescence and scattering spectroscopy. However, absorption spectroscopy is often misunderstood or underestimated, being seen as UV-vis spectroscopy only, which can lead to various misinterpretations. In fact, absorption spectroscopy is one of the most important branches of spectroscopic techniques (others including fluorescence and scattering), which plays a critical role in understanding the electronic structure and optoelectrical dynamics of MHPs. In this tutorial, the basic principles of various types of absorption spectroscopy as well as their recent developments and applications in MHP materials and devices are summarized, covering comprehensive advances in steady state and transient absorption spectroscopy. Given the significance of absorption spectroscopy in directing the design of different optoelectronic applications of MHPs, this tutorial will comprehensively discuss absorption spectroscopy, covering wavelengths from optical to terahertz (THz) and microwave, and timescales from femtoseconds to hours, and it specifically focuses on time-dependent steady-state and transient absorption spectroscopy under light illumination bias to study MHP materials and devices, allowing researchers to select suitable characterization techniques.

Room-temperature synthesis of triple-cation green perovskite quantum dots for optoelectronic applications

The development of multi-cation perovskite quantum dots (PQDs) is limited by the low availability of fitting A-site cations due to the unsuitable radii of a large gamut of amine cations. The impact of oversized or undersized cations on the perovskite structure is detrimental to the structural stabilization and electroluminescence efficiency of the PQDs. Researchers are actively seeking suitable-sized cations to mitigate perovskite defect formation and optimize charge carrier confinement within the PQDs. In contrast to cesium (Cs) or formamidine (FA), which are exposed to degradation pathways, guanidinium (GA)-doping has been to provide a suitable radius and the lack a dipole moment. The triple nitrogen functionality of GA enables it to passivate both the PbBr6 octahedra and surface defects through vacant A-sites and entropically stabilize the perovskite. Furthermore, the insertion of GA into the PQD lattice is enthalpically facilitated by the presence and arrangement of smaller Cs and Br atoms. Herein, we have synthesized a Cs-FA PQD reference into which GA is doped via two chemical routes. Our triple-cation system exhibits substantially improved optical properties and was applied for the fabrication of a PeLED device. The optimized triple-cation PQDs-based PeLED device exhibited an external quantum efficiency of 5.87% and a luminescence of 13726 cd m-2.

Photon Reabsorption and Surface Plasmon Modulating Exciton-to-Dopant Energy Transfer Dynamics in Mn:CsPb(BrCl)3 Quantum Dots

Exciton-to-dopant energy transfer (ET) dynamics of Mn:CsPbX3 quantum dots (QDs), which is dominated by diverse physical factors, requires more comprehensive understanding. Here, the concentration-dependent photon reabsorption effect on ET dynamics has been meticulously analyzed in colloidal Mn:CsPb(BrCl)3 QDs. The results indicate that the photons emitted by the smaller QDs are absorbed by the larger QDs, effectively providing additional excitation light to the latter. The reabsorbed photons play a crucial role in significantly enhancing the ET process in the larger QDs. Additionally, the Mn:CsPb(BrCl)3 QDs/Poly(methyl methacrylate)/Ag/SiO2 multilayer films were fabricated to study the influence of the surface plasmon (SP) on ET dynamics. The results reveal that resonant energy transfer between excitons and SP via dipole interactions can regulate the ET process and Mn2+ emission intensity by controlling the distance between the SP and excitons. These findings provide insights into Mn:CsPbX3 QD ET dynamics and potential methods for controlling their luminescence performance in practical applications.

Role of Size and Shape in Photoluminescence and Ultra-Low-Frequency Raman of Methylammonium Lead Iodide Perovskite Quantum Dots

The photophysical properties of methylammonium lead iodide (MAPbI3) quantum dots (QDs) have not been systematically studied for size and shape dependence. Here, we synthesize MAPbI3 QDs using ligand-assisted reprecipitation, controlling the injection speed and reaction times to produce QDs with different sizes and shapes. Dropwise injection yields ∼5 nm spherical QDs, emitting photoluminescence (PL) at 2.06 eV. In contrast, swift injections yield larger (>10 nm) rectangular QDs with varying aspect ratios, supported by an infinite quantum well model. The PL lifetime of QDs increases with their size, and the size variation significantly influences the ultra-low-frequency Raman modes at 81, 107, and 127 cm-1, in contrast to what is observed in polymorphic MAPbI3 thin films. Our findings, supported by first-principles density functional theory, show that key PL and Raman properties are governed by the sizes and shapes of MAPbI3 QDs. This study contributes to the understanding of the optical behavior of these QDs, which is crucial for their potential applications and environmental implications.

Steering Energy Transfer Pathways through Mn-Doping in Perovskite Nanocrystals

Modulation of singlet and triplet energy transfer from excited semiconductor nanocrystals to attached dye molecules remains an important criterion for the design of light-harvesting assemblies. Whereas one can consider the selection of donor and acceptor with favorable energetics, spectral overlap, and kinetics of energy transfer as a means to direct the singlet and triplet energy transfer pathways, it is not obvious how to control the singlet and triplet characteristics of the donor semiconductor nanocrystal itself. By doping CsPb(Cl0.7Br0.3)3 nanocrystals with Mn2+, we have now succeeded in increasing the triplet characteristics of semiconductor nanocrystals. The singlet and triplet energy transfer between excited Mn-CsPb(Cl0.7Br0.3)3 nanocrystals and a cyanine dye (4,5-benzoindotricarbocyanine) show the participation of band gap states in singlet energy transfer and Mn2+-activated states in triplet energy transfer. By tracking donor and acceptor emission as well as transient absorption spectral features, we were able to distinguish the two independent energy transfer pathways. Whereas singlet energy transfer from the exciton emission band remains unchanged (2%), increasing the concentration of Mn2+ in perovskite nanocrystals results in an increase of triplet energy transfer yield up to 17.5%. The ability to enhance the triplet transfer yield in CsPb(Cl0.7Br0.3)3 nanocrystals through Mn-doping opens up new opportunities to develop optoelectronic and display devices.

A Brief Review of Perovskite Quantum Dot Solar Cells: Synthesis, Property and Defect Passivation

Perovskite quantum dot solar cells (PQDSCs), as the promising candidate for the next generation of solar cell, have garnered the significant attention over the past decades. However, the performance and stability of PQDSCs are highly dependent on the properties of interfaces between the perovskite quantum dots (PQDs) and the other layers in the device. This work provides a brief overview of PQDSCs, including the synthesis of PQDs, the characteristics and preparation methods of PQDs, the photoelectric properties as the light absorption layer and optimization methods for PQDSCs with high efficiency. Future directions and potential applications are also highlighted.

Systematic Study of the Synthesis of Monodisperse CsPbI3 Perovskite Nanoplatelets for Efficient Color-Pure Light Emitting Diodes

Metal halide perovskite nanoplatelets (NPls) possess ultra-narrow photoluminescence (PL) bands tunable over the entire visible spectral range, which makes them promising for utilization in light-emitting diodes (LEDs) with spectrally pure emission colors. This calls for development of synthetic methods toward perovskite NPls with a high degree of control over both their thickness and lateral dimensions. A general strategy is developed to obtain such monodisperse CsPbI3 NPls through the control over the halide-to-lead ratio during heating-up reaction. The excess of iodine precursor changes the chemical equilibrium, thus yielding monodisperse (3 monolayers in thickness) CsPbI3 NPls whose PL width constitutes ≈22 nm, while the lateral dimensions of NPls are determined by choice of precursor and by the reaction temperature. Postsynthetic cation exchange on the A-site of the perovskite lattice allows for the tuning of the PL peak position, while simultaneous removal of the excess ligands and the surface passivation allows for improvement of the PL quantum yield to 96% and ensures superior stability of optical properties upon storage. Electroluminescent LEDs with the peak values are fabricated for the external quantum efficiency and luminance being 9.45% and 29800 cd m-2, respectively, and a narrow (≈26 nm) electroluminescence peak at 601 nm.

Tailorable Fluorescent Perovskite Quantum Dots for Multiform Manufacturing via Two-Step Thiol-Ene Click Chemistry

In practical applications, fluorescent perovskite quantum dots (PQDs) must exhibit high efficiency, stability, and processibility. So far, it remains a challenge to synthesize PQDs with stable dispersibility in tailorable monomers both before and after photocuring. In this work, a novel strategy of UV-induced two-step thiol-ene "click chemistry" is proposed to prepare PQDs with these attributes. The first step aims to epitaxially grow a shell around the PQD core to ensure stable dispersibility in a thiol-ene monomer solution. The second step is to achieve stable dispersibility in the photocured thiol-ene matrixes for multiform manufacturing processes. The tailorable PQDs (T-PQDs) not only have the highest photoluminescence quantum yield (PLQY) to ≈100% for green emission and over 96% for red emission, but also exhibit remarkable stability under severe conditions, including "double 85" aging, water exposure, and mechanical stress. Moreover, their exceptional processability allows for various processing techniques, including slot-die coating, inkjet printing, direct-laser writing, UV-light 3D printing, nanoimprinting, and spin coating. The high efficiency and stability of T-PQDs facilitate their multiform manufacturing for a wide range of applications.

Light-Emitting Diodes Based on Metal Halide Perovskite and Perovskite Related Nanocrystals

Light-emitting diodes (LEDs) based on halide perovskite nanocrystals have attracted extensive attention due to their considerable luminescence efficiency, wide color gamut, high color purity, and facile material synthesis. Since the first demonstration of LEDs based on MAPbBr3 nanocrystals was reported in 2014, the community has witnessed a rapid development in their performances. In this review, a historical perspective of the development of LEDs based on halide perovskite nanocrystals is provided and then a comprehensive survey of current strategies for high-efficiency lead-based perovskite nanocrystals LEDs, including synthesis optimization, ion doping/alloying, and shell coating is presented. Then the basic characteristics and emission mechanisms of lead-free perovskite and perovskite-related nanocrystals emitters in environmentally friendly LEDs, from the standpoint of different emission colors are reviewed. Finally, the progress in LED applications is covered and an outlook of the opportunities and challenges for future developments in this field is provided.

Patterning technologies of quantum dots for color-conversion micro-LED display applications

Quantum dot (QD) materials and their patterning technologies play a pivotal role in the full colorization of next-generation Micro-LED display technology. This article reviews the latest development in QD materials, including II-VI group, III-V group, and perovskite QDs, along with the state of the art in optimizing QD performance through techniques such as ligand engineering, surface coating, and core-shell structure construction. Additionally, it comprehensively covers the progress in QD patterning methods, such as inkjet printing, photolithography, electrophoretic deposition, transfer printing, microfluidics, and micropore filling method, and emphasizes their crucial role in achieving high precision, density, and uniformity in QD deposition. This review delineates the impact of these technologies on the luminance of QD color-conversion layers and devices, providing a detailed understanding of their application in enhancing Micro-LED display technology. Finally, it explores future research directions, offering valuable insights and references for the continued innovation of full-color Micro-LED displays, thereby providing a comprehensive overview of the potential and scope of QD materials and patterning technologies in this field.

Surface modification unleashes light emitting applications of APbX3 perovskite nanocrystals

Engineering the surface of metal halide perovskite nanocrystals (MHPNCs) is crucial for optimizing their optical properties, repairing surface defects, enhancing quantum yield, and ensuring long-term stability. These enhancements make surface-engineered MHPNCs ideal for applications in light-emitting devices (LEDs), displays, lasers, and photodetectors, contributing to energy efficiency. This article delves into an introduction to MHPNCs, their structure and types, particularly the ABX3 type (where A represents monovalent organic/inorganic cations, B represents divalent metal ions mainly Pb metal, and X represents halide ions), synthesis methods, unique optical properties, surface modification techniques using various agents (particularly inorganic molecules/materials, organic molecules, polymers, and biomolecules) to tune optical properties and applications in the aforementioned light-emitting technologies, challenges and opportunities, including advantages and disadvantages of surface-modified APbX3 MHPNCs, and a summary and future outlook. This article explores surface modification strategies to improve the optical performance of MHPNCs and aims to inspire advancements in light emitting applications. Importantly, the challenges and opportunities section of this article will illuminate the path to overcoming obstacles, providing invaluable insights for researchers in this field. This in-depth review explores the surface engineering of MHPNCs for light-emitting applications, highlighting their notable advantages and addressing ongoing challenges. By delving deep into various surface modification strategies, this article aims to revolutionize MHPNC-based light-emitting applications, setting a new benchmark in the field. This paves the way for revolutionary advancements, maximizing the capabilities of surface-engineered MHPNCs and heralding a transformative era in precise light-emitting research.

Band Gap Tuning in Mercaptoacetic Acid Capped Mixed Halides Perovskites and Effect of Solvents on Their Fluorescence Dynamics: A Potential Sensor for Polarity

From synthesis to application, there are always certain interactions between the polar solvents and perovskite nanocrystals (NCs). To explain the effect of solvent polarity especially on the photoluminescence (PL) properties of NCs is highly desirable, especially for sensing applications. Herein We have synthesized the methylammonium lead mixed halides (MAPbCl3 - nBrn, where n = 0, 0.5, 1, 1.5) perovskite nanocrystals (NCs) at room temperature by using ligand-assisted re-precipitation (LARP) method, by employing mercaptoacetic acid (MAA) as a capping ligand. Different techniques have been employed to get information regarding the structural and optical properties of the synthesized material. Powder X-ray diffraction (PXRD) confirms the orthorhombic crystal structure of the MAPbCl3 - nBrn perovskite NCs. FT-IR (Fourier-transform infrared) analysis confirms the successful interaction of capping ligands with NCs. By increasing MABr precursor concentration during the synthesis of perovskite NCs, a red shift in the UV-Vis absorption and PL spectra has been observed. The steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) techniques suggested that these perovskite NCs exhibit tunable PL relative to the substitution of Cl with Br in NCs. The comparative PL studies in non-polar (benzene) and polar (tetrahydrofuran (THF)) revealed that the PL properties are highly sensitive and selective toward the solvent chosen. All synthesized NCs possess longer PL lifetime in benzene than in THF. Relatively, perovskite NCs synthesized with 0.166 mM MABr precursor concentration show a longer PL lifetime (6.51 ns in benzene) as compared to other MABr concentrations. These studies not only propose that by controlling precursors concentration, one can synthesize NCs having tunable PL with a longer radiative PL lifetime, but also provide a comparative understanding of PL dynamics of NCs in different solvents.

Enabling Multicolor Information Encryption: Oleylammonium-Halide-Assisted Reversible Phase Conversion between Cs4PbX6 and CsPbX3 Nanocrystals

Recently, halide perovskites have been recognized for their thermochromic characteristics, showing significant potential in information encryption applications. However, the limited luminescence color gamut hinders the encryption of complex multicolor information. Herein, for the first time, multicolor thermochromic perovskites with luminescence covering the entire visible spectrum have been designed. Oleylammonium halide salts facilitate a reversible phase transformation between nonluminescent Cs4PbX6 nanocrystals (NCs) and luminescent CsPbX3 NCs upon heating or cooling. This process occurs without the need for external addition or removal of ligands or metal salts, enabling efficient and intelligent information encryption. A proof-of-concept demonstration successfully encrypts and decrypts multicolor digital information. This work not only advances the understanding of phase transformations in perovskites but also highlights their significant potential for information encryption applications.

Solvent-Free Ball Milling Synthesis of Water-Stable Tin-Based Pseudohalide Perovskites for Photocatalytic CO2 Reduction

A pseudohalide (SCN-) tin-based perovskite material using a solvent-free ball milling method is developed. The synthesized perovskite exhibits long-term water stability and demonstrated significant photocatalytic activity in reducing CO2 to CO under light irradiation. The structural transition from nanoparticles to planar perovskites is achieved by varying the ratios of dimethylammonium (DMA) and formamidinium (FA) cations, which is confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The surface elemental distribution, absorption spectra, band gap and energy levels estimations using energy-dispersive X-ray spectroscopy (EDS), Kubelka-Munk function, and ultraviolet photoelectron spectroscopy (UPS) are thoroughly investigated. These findings indicated that the incorporation of DMA cations increased the band gap and shifted the absorption spectra toward the blue region. The optimal photocatalytic performance is observed for the perovskite composition with a 50% DMA cation ratio (DMA0.5FA0.5SnI(SCN)2), achieving a CO production yield of 285 µmol g-1 with 12 hours irradiation in humid environment. The efficiency is critically dependent on the ball milling speed and duration, with 400 rpm and 1 hour being the optimal conditions. This research highlights the potential of environmentally friendly synthesis methods in developing stable and efficient lead-free perovskites as photocatalytic materials, contributing to the goal of net-zero carbon emissions.

Crystal structure of catena-poly[bis-(N,O-di-methyl-hydroxyl-ammonium) [di-μ-bromido-di-bromido-stannate(II)]]

The title compound, {(C2H8NO)2[SnBr4]} n , is a layered hybrid perovskite crystallizing in the monoclinic space group C2/c. The asymmetric unit consists of one H3C-O-NH2 +-CH3 cation (Me2HA+), one SnII atom located on a twofold rotation axis, and two Br atoms. The SnII atom has a distorted octa-hedral coordination environment formed by the bromido ligands. The {SnBr6} units corner-share their equatorial Br atoms, forming infinite mono-layers that extend parallel to the ab plane. These inorganic layers are sandwiched by the organic Me2HA+ cations organized in double-layers; stacking of the layers is along the c-axis direction. Consecutive inorganic layers, separated by the organic cations, are shifted relative to each other along the b-axis direction. Specifically, the SnII atom in one inorganic layer is offset by 3.148 Å along the b axis relative to the SnII atom in an adjacent inorganic layer. The N,O-di-methyl-hydroxyl-ammonium cation forms two hydrogen bonds with the axial bromide anions of the inorganic layers as acceptors, and leads to the cohesion of the crystal structure. According to Hirshfeld surface analysis, the highest contributions to the crystal packing are from H⋯H (46.2%), Br⋯H (38.5%), and H⋯O (14.8%) contacts.

Strategies for Controlling Emission Anisotropy in Lead Halide Perovskite Emitters for LED Outcoupling Enhancement

In the last decade, momentous progress in lead halide perovskite (LHP) light-emitting diodes (LEDs) is witnessed as their external quantum efficiency (ηext) has increased from 0.1 to more than 30%. Indeed, perovskite LEDs (PeLEDs), which can in principle reach 100% internal quantum efficiency as they are not limited by the spin-statistics, are reaching their full potential and approaching the theoretical limit in terms of device efficiency. However, ≈70% to 85% of total generated photons are trapped within the devices through the dissipation pathways of the substrate, waveguide, and evanescent modes. To this end, numerous extrinsic and intrinsic light-outcoupling strategies are studied to enhance light-outcoupling efficiency (ηout). At the outset, various external and internal light outcoupling techniques are reviewed with specific emphasis on emission anisotropy and its role on ηout. In particular, the device ηext can be enhanced by up to 50%, taking advantage of the increased probability for photons outcoupled to air by effectively inducing horizontally oriented emission transition dipole moments (TDM) in the perovskite emitters. The role of the TDM orientation in PeLED performance and the factors allowing its rational manipulation are reviewed extensively. Furthermore, this account presents an in-depth discussion about the effects of the self-assembly of LHP colloidal nanocrystals (NCs) into superlattices on the NC emission anisotropy and optical properties.

Factors influencing the chiral imprinting in perovskite nanoparticles

Chiral perovskites have emerged as a new class of nanomaterials for manipulation and control of spin polarized current and circularly polarized light for applications in spintronics, chiro-optoelectronics, and chiral photonics. While significant effort has been made in discovering and optimizing strategies to synthesize different forms of chiral perovskites, the mechanism through which chirality is imbued onto the perovskites by chiral surface ligands remains unclear. In this minireview, we provide a detailed discussion of one of the proposed mechanisms, electronic imprinting from a chiral ligand.

Internal Electric Field Manipulates Exciton-Phonon Couplings in Single Lead Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have attracted much attention as materials for light-emitting diodes and quantum light sources. A deep understanding of exciton-phonon couplings is essential for obtaining a narrow emission line, weak phonon-sideband photoluminescence (PL), and a long exciton coherence time, which are especially useful for high-color-purity quantum-light-source applications. Here, we report the PL spectra of single CsPbBr3 NCs at 5.5 K as a function of the applied electric field. The exciton peak energy shows an asymmetric parabolic shift for positive and negative biases, implying the presence of a spontaneously generated internal electric field in the NCs when no field is applied. Both the internal electric field and exciton-phonon couplings become larger in smaller NCs, and they have a positive correlation with each other. Our findings show that the exciton-phonon couplings can be manipulated with an electric field, which dominates the PL properties of perovskite NCs.

Temperature dependence of the electron and hole Landé g-factors in CsPbI3 nanocrystals embedded in a glass matrix

The coherent spin dynamics of electrons and holes in CsPbI3 perovskite nanocrystals in a glass matrix are studied by the time-resolved Faraday ellipticity technique in magnetic fields up to 430 mT across a temperature range from 6 K to 120 K. The Landé g-factors and spin dephasing times are evaluated from the observed Larmor precession of electron and hole spins. The nanocrystal size in the three studied samples varies from about 8 to 16 nm, resulting in exciton transition varying from 1.69 to 1.78 eV at a temperature of 6 K, allowing us to study the corresponding energy dependence of the g-factors. The electron g-factor decreases with increasing confinement energy in the NCs as a result of NC size reduction, and also with increasing temperature. The hole g-factor shows the opposite trend. Model analysis shows that the variation of g-factors with NC size arises from the transition energy dependence of the g-factors, which becomes strongly renormalized by temperature.

Tunable Blue-Light-Emitting Organic-Inorganic Zinc Halides with Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence

Hybrid metal halides have received great interests in the field of solid-state lighting technologies due to their diverse structures and excellent emission properties. In this work, we report the synthesis and characterization of four blue-emitting zero-dimensional hybrid metal halides, namely, (2HP)2ZnCl2, (2HP)2ZnBr2, (2TP)2ZnCl2, and (2TP)2ZnBr2 (2HP = 2-hydroxypyridine, 2TP = pyridine-2-thiol). By changing the ligands and halides, a remarkable increase in the photoluminescence quantum yield of (2HP)2ZnCl2 (44.7%) compared to (2TP)2ZnBr2 (1.8%) is realized. The 2HP series features excitation-dependent emission characteristics, whereas the 2TP series does not due to the effect of a different organic ligand. Utilizing time-resolved and temperature-dependent photoluminescence spectroscopies, all four compounds exhibit both thermally activated delayed fluorescence and room-temperature phosphorescence properties. These materials have excellent ambient and thermal stabilities and are solution-processable. Our work shows the importance of carefully incorporating organic ligands with the appropriate inorganic metal center to achieve tunable emission properties.

Micrometer-Resolution Fluorescence and Lifetime Mappings of CsPbBr3 Nanocrystal Films Coupled with a TiO2 Grating

Enhancing light emission from perovskite nanocrystal (NC) films is essential in light-emitting devices, as their conventional stacks often restrict the escape of emitted light. This work addresses this challenge by employing a TiO2 grating to enhance light extraction and shape the emission of CsPbBr3 nanocrystal films. Angle-resolved photoluminescence (PL) demonstrated a 10-fold increase in emission intensity by coupling the Bloch resonances of the grating with the spontaneous emission of the perovskite NCs. Fluorescence lifetime imaging microscopy (FLIM) provided micrometer-resolution mapping of both PL intensity and lifetime across a large area, revealing a decrease in PL lifetime from 8.2 ns for NC films on glass to 6.1 ns on the TiO2 grating. Back focal plane (BFP) spectroscopy confirmed how the Bloch resonances transformed the unpolarized, spatially incoherent emission of NCs into polarized and directed light. These findings provide further insights into the interactions between dielectric nanostructures and perovskite NC films, offering possible pathways for designing better performing perovskite optoelectronic devices.

CsPbBr3-PbSe Perovskite-Chalcogenide Epitaxial Nanocrystal Heterostructures and Their Charge Carrier Dynamics

Lead halide perovskite and chalcogenide heterostructures which share the ionic and covalent interface bonding may be the possible materials in bringing phase stability to these emerging perovskite nanocrystals. However, in spite of significant successes in the development of halide perovskite nanocrystals, their epitaxial heterostructures with appropriate chalcogenide nanomaterials have largely remained unexplored. Keeping the importance of these materials in mind, herein, epitaxial nanocrystal heterostructures of CsPbBr3-PbSe are reported. The shape remained rhombic dodecahedral-tetrahedral, and the phase retained orthorhombic-cubic for CsPbBr3 and PbSe nanocrystals, respectively. These are synthesized following the standard classical approach of heteronucleations of chalcogenide PbSe with CsPbBr3 perovskite nanostructures and characterized with high-resolution electron microscopic imaging. With an ultrafast study, the hot charge transfer from CsPbBr3 to PbSe is also established. As these are first of its kind nanostructures which are obtained with heteronucleation and growth of chalcogenides on halide perovskites, this finding is expected to open the roadmap for designing other heterostructures which are important for catalysis and photovoltaic applications.

Photoelectrochemical transistors based on semiconducting polymers: an emerging technology for future bioelectronics

In recent years, organic electrochemical transistors (OECTs) have attracted widespread attention due to their significant advantages such as low-voltage operation, biocompatibility, and compatibility with flexible substrates. Organic photoelectrochemical transistors (OPECTs) are OECTs with photoresponse capabilities that achieve photoresponse and signal amplification in a single device, demonstrating tremendous potential in multifunctional optoelectronic devices. In this mini-review, we briefly introduce the channel materials and operation mechanisms of OECTs/OPECTs. Then different types of OPECTs are discussed depending on their device-architecture-related photoresponse generation. Following this, we summarize recent advances in OPECT applications across various fields including biomedical sciences, optoelectronics, and sensor technologies. Finally, we outline the current challenges and explore future research prospects, aiming at extending their further development and applications.

Caesium-Iodide-Assisted Synthesis of High-Quality, Stable, and Robust Lead-Free Perovskite Quantum Dots

The poor morphology, and susceptibility to oxidation of tin-based perovskite quantum dots (TQDs) have posed significant challenges, limiting their application potential. This study presents a straightforward method for synthesizing high-quality CsSnI3-based perovskite quantum dots (TQDs) by incorporating a mixed Cs source of Cs2CO3 and CsI. The addition of CsI increased the I:Sn ratio while maintaining Sn:Cs, resulting in TQDs with smaller size and improved uniformity. X-ray photoelectron spectroscopy (XPS), and Nuclear magnetic resonance (NMR) analyses confirmed enhanced crystallinity, photoluminescence intensity, and antioxidation ability of CsI-TQDs. Remarkably, these TQDs exhibit exceptional stability, enduring over 1 h in air and more than 24 h before complete oxidation, surpassing the previously reported longest lifetime in air for TQDs with conventional oleic acid (OA) and oleylamine (OAm) ligands. Furthermore, these TQD films retain robustness after ligand exchange with methyl acetate (MeOAc) and formamidinium iodide (FAI), representing the first successful short-ligand exchange of TQDs and enabling further electronic device applications. These findings suggest that CsI in the Cs source plays a crucial role in facilitating the formation of surface complexes, regulating TQD growth and suppressing iodine vacancies.

Revealing Anion Exchange in Two-Dimensional Nanocrystals

Ion exchange is a powerful postsynthesis tool for the design of functional nanomaterials. However, achieving anion exchange while maintaining the original morphology and crystal structure, as well as elucidating the mechanism, remains challenging. Here, we developed an anion-exchange strategy under mild conditions and revealed an unusual ion-exchange mechanism in the semiconductor nanoplatelets. Kinetic studies have demonstrated that the transformation follows first-order kinetics, with the ligand restricting the guest anion from diffusing only in one-dimensional directions. By monitoring the reaction process, we demonstrated that the anion exchange reaction occurs selectively on the polar surface of the NPLs and exhibits asymmetry at the two polar end faces. Theoretical simulations further confirmed that anion exchange began from the chalcogenide-dominated facet. The thermodynamic data suggest that guest ions diffuse into the crystal interior via a direct exchange mechanism. This study provides a pathway for anion exchange and the construction of functional nanocrystals and a platform for studying the optoelectronic behavior of single-sheet heterojunctions.

Self-passivation of Halide Interstitial Defects by Organic Cations in Hybrid Lead-Halide Perovskites: Ab Initio Quantum Dynamics

Halide interstitial defects severely hinder the optoelectronic performance of metal halide perovskites, making research on their passivation crucial. We demonstrate, using ab initio nonadiabatic molecular dynamics simulations, that hydrogen vacancies (Hv) at both N and C atoms of the methylammonium (MA) cation in MAPbI3 efficiently passivate iodine interstitials (Ii), providing a self-passivation strategy for dealing with the Hv and Ii defects simultaneously. Hv at the N site (Hv-N) introduces a defect state into the valence band, while the state contributed by Hv at the C site (Hv-C) evolves from a shallow level at 0 K to a deep midgap state at ambient temperature, exhibiting a high environmental activity. Both Hv-N and Hv-C are strong Lewis bases, capable of capturing and passivating Ii defects. Hv-C is a stronger Lewis base, bonds with Ii better, and exhibits a more pronounced passivation effect. The charge carrier lifetimes in the passivated systems are significantly longer than in those containing either Hv or Ii, and even in pristine MAPbI3. Our demonstration of the Hv and Ii defect self-passivation in MAPbI3 suggests that systematic control of the relative concentrations of Hv and Ii can simultaneously eliminate both types of defects, thereby minimizing charge and energy losses. The demonstrated defect self-passivation strategy provides a promising means for defect control in organic-inorganic halide perovskites and related materials and deepens our atomistic understanding of defect chemistry and charge carrier dynamics in solar energy and optoelectronic materials.

Antisolvent controls the shape and size of anisotropic lead halide perovskite nanocrystals

Colloidal lead halide perovskite nanocrystals have potential for lighting applications due to their optical properties. Precise control of the nanocrystal dimensions and composition is a prerequisite for establishing practical applications. However, the rapid nature of their synthesis precludes a detailed understanding of the synthetic pathways, thereby limiting the optimisation. Here, we deduce the formation mechanisms of anisotropic lead halide perovskite nanocrystals, 1D nanorods and 2D nanoplatelets, by combining in situ X-ray scattering and photoluminescence spectroscopy. In both cases, emissive prolate nanoclusters form when the two precursor solutions are mixed. The ensuing antisolvent addition induces the divergent anisotropy: The intermediate nanoclusters are driven into a dense hexagonal mesophase, fusing to form nanorods. Contrastingly, nanoplatelets grow freely dispersed from dissolving nanoclusters, stacking subsequently in lamellar superstructures. Shape and size control of the nanocrystals are determined primarily by the antisolvent's dipole moment and Hansen hydrogen bonding parameter. Exploiting the interplay of antisolvent and organic ligands could enable more complex nanocrystal geometries in the future.

Tutorial on Describing, Classifying, and Visualizing Common Crystal Structures in Nanoscale Materials Systems

Crystal structures underpin many aspects of nanoscience and technology, from the arrangements of atoms in nanoscale materials to the ways in which nanoscale materials form and grow to the structures formed when nanoscale materials interact with each other and assemble. The impacts of crystal structures and their relationships to one another in nanoscale materials systems are vast. This Tutorial provides nanoscience researchers with highlights of many crystal structures that are commonly observed in nanoscale materials systems, as well as an overview of the tools and concepts that help to derive, describe, visualize, and rationalize key structural features. The scope of materials focuses on the elements and their compounds that are most frequently encountered as nanoscale materials, including both close-packed and nonclose-packed structures. Examples include three-dimensionally and two-dimensionally bonded compounds related to the rocksalt, nickel arsenide, fluorite, zincblende, wurtzite, cesium chloride, and perovskite structures, as well as layered perovskites, intergrowth compounds, MXenes, transition metal dichalcogenides, and other layered materials. Ordered versus disordered structures, high entropy materials, and instructive examples of more complex structures, including copper sulfides, are also discussed to demonstrate how structural visualization tools can be applied. The overall emphasis of this Tutorial is on the ways in which complex structures are derived from simpler building blocks, as well as the similarities and interrelationships among certain classes of structures that, at first glance, may be interpreted as being very different. Identifying and appreciating these structural relationships is useful to nanoscience researchers, as it allows them to deconstruct complex structures into simpler components, which is important for designing, understanding, and using nanoscale materials.

Leveraging ordered voids in microporous perovskites for intercalation and post-synthetic modification

We report the use of porous organic layers in two-dimensional hybrid organic-inorganic perovskites (HOIPs) to facilitate permanent small molecule intercalation and new post-synthetic modifications. While HOIPs are well-studied for a variety of optoelectronic applications, the ability to manipulate their structure after synthesis is another handle for control of physical properties and could even enable use in future applications. If designed properly, a porous interlayer could facilitate these post-synthetic transformations. We show that for a series of copper-halide perovskites, a crystalline arrangement of designer ammonium groups allows for permanently porous interlayer space to be accessed at room temperature. Intercalation of the electroactive molecules ferrocene and tetracyanoethylene into this void space can be performed with tunable loadings, and these intercalated perovskites are stable for months. The porosity also enables reactivity at the copper-halide layer, allowing for facile halide replacement. Through this, we access previously unobserved reactivity with halogens to perform halide substitution, and even replace halides with pseudohalides. In the latter case, the porous structure allows for stabilization of new phases, specifically a novel copper-thiocyanate perovskite phase, only accessible through post-synthetic modification. We envision that this broad design strategy can be expanded to other industrially relevant HOIPs to create a new class of highly adjustable perovskites.

Exsolution of Ni nanoparticles in A-site excess STO films

Exsolution is a technique to create metal nanoparticles embedded within a matrix. The phenomenon has previously predominantly been studied in A-site deficient and stoichiometric perovskite powders. Here, we present a systematic study of an A-site excess perovskite oxide based on SrTiO3 thin films, doped with nickel and exsolved under different conditions. The study aims to shed light on particle formation in these novel systems, including the effects of (i) the thin film thickness, (ii) pre-exsolution annealing in an oxidative atmosphere, (iii) a reductive atmosphere during the exsolution step, and (iv) exsolution time on the particle size and particle density. Our results indicate that exsolution occurs quickly, forming nanoparticles both on the surface and in the bulk of the host perovskite. The findings indicate that pre-annealing in an ambient atmosphere leads to fewer but larger exsolved particles compared to samples without pre-annealing. Consequently, while crystallization of the thin film occurs in both atmospheres, the simultaneous crystallization of the thin film and formation of the nanoparticles leads to a smaller apparent average radius. Moreover, we present evidence that metal particles can be found beyond the originally doped region. These findings are a step towards realizing tunable functional materials using exsolution to create metallic nanostructures within a thin film in a predictable manner.

Large-Area Perovskite Nanocrystal Metasurfaces for Direction-Tunable Lasing

Perovskite nanocrystals (PNCs) are attractive emissive materials for developing compact lasers. However, manipulation of PNC laser directionality has been difficult, which limits their usage in photonic devices that require on-demand tunability. Here we demonstrate PNC metasurface lasers with engineered emission angles. We fabricated millimeter-scale CsPbBr3 PNC metasurfaces using an all-solution-processing technique based on soft nanoimprinting lithography. By designing band-edge photonic modes at the high-symmetry X point of the reciprocal lattice, we achieved four linearly polarized lasing beams along a polar angle of ∼30° under optical pumping. The device architecture further allows tuning of the lasing emission angles to 0° and ∼50°, respectively, by adjusting the PNC thickness to shift other high-symmetry points (Γ and M) to the PNC emission wavelength range. Our laser design strategies offer prospects for applications in directional optical antennas and detectors, 3D laser projection displays, and multichannel visible light communication.

Lattice Engineering via Transition Metal Ions for Boosting Photoluminescence Quantum Yields of Lead-Free Layered Double Perovskite Nanocrystals

Lead-free layered double perovskite nanocrystals (NCs), i.e., Cs4M(II)M(III)2Cl12, have recently attracted increasing attention for potential optoelectronic applications due to their low toxicity, direct bandgap nature, and high structural stability. However, the low photoluminescence quantum yield (PLQY, <1%) or even no observed emissions at room temperature have severely blocked the further development of this type of lead-free halide perovskites. Herein, two new layered perovskites, Cs4CoIn2Cl12 (CCoI) and Cs4ZnIn2Cl12 (CZnI), are successfully synthesized at the nanoscale based on previously reported Cs4CuIn2Cl12 (CCuI) NCs, by tuning the M(II) site with different transition metal ions for lattice tailoring. Benefiting from the formation of more self-trapped excitons (STEs) in the distorted lattices, CCoI and CZnI NCs exhibit significantly strengthened STE emissions toward white light compared to the case of almost non-emissive CCuI NCs, by achieving PLQYs of 4.3% and 11.4% respectively. The theoretical and experimental results hint that CCoI and CZnI NCs possess much lower lattice deformation energies than that of reference CCuI NCs, which are favorable for the recombination of as-formed STEs in a radiative way. This work proposes an effective strategy of lattice engineering to boost the photoluminescent properties of lead-free layered double perovskites for their future warm white light-emitting applications.