Lead Halide Perovskite Nanocrystals Research Articles

The marriage of perovskite nanocrystals with lanthanide‐doped upconversion nanoparticles for advanced optoelectronic applications

AbstractThe exceptional optoelectronic properties of lead halide perovskite nanocrystals (PeNCs) in the ultraviolet and visible spectral regions have positioned them as a promising class of semiconductor materials for diverse optoelectronic and photovoltaic applications. However, their limited response to near‐infrared (NIR) light due to the intrinsic bandgap (>1.5 eV) has hindered their applications in many advanced technologies. To circumvent this limitation, it is of fundamental significance to integrate PeNCs with lanthanide‐doped upconversion nanoparticles (UCNPs) that are capable of efficiently converting low‐energy NIR photons into high‐energy ultraviolet and visible photons. By leveraging the energy transfer from UCNPs to PeNCs, this synergistic combination can not only expand the NIR responsivity range of PeNCs but also introduce novel emission profiles to upconversion luminescence with multi‐dimensional tunability (e.g., wavelength, lifetime, and polarization) under low‐to‐medium power NIR irradiation, which breaks through the inherent restrictions of individual PeNCs and UCNPs and thereby opens up new opportunities for materials and device engineering. In this review, we focus on the latest advancements in the development of PeNCs‐UCNPs nanocomposites, with an emphasis on the controlled synthesis and optical properties design for advanced optoelectronic applications such as full‐spectrum solar cells, NIR photodetectors, and multilevel anticounterfeiting. Some future efforts and prospects toward this active research field are also envisioned.

Selective detection of ascorbic acid by tuning the composition and fluorescence of the cesium lead halide perovskite nanocrystals

Lead halide perovskite nanocrystals (PNCs) have attracted intense attention due to their excellent optoelectronic properties. In this work, a series of water-stable CsPb(Br/I)3 PNCs fluorescent probes were prepared using an anion exchange method. It was found that the PNCs probes could be used to detect ascorbic acid (AA) in water, and interestingly, the FL spectra of the PNCs probes can be adjusted by controlling the concentration of KI in anion exchange to improve the detection selectivity of AA. The high sensitivity and selectivity make CsPb(Br/I)3 PNCs an ideal material for AA sensing. The concentration of AA can be linearly measured in the range from 0.01 to 50 μM, with a detection limit of 4.2 nM. The reason for the enhanced FL of CsPb(Br/I)3 PNCs was studied, and it is considered that AA causes the aggregation of CsPb(Br/I)3 PNCs. This strategy of improving the selectivity of the probe to the substrate by adjusting the spectrum will significantly expand the application of PNCs in the field of analysis and detection.

Control of Lateral Assembly and Vertical Stacking in Spin-Coated Lead Halide Perovskite Nanocrystal Films for Enhanced Photoluminescence Efficiency

Control of Lateral Assembly and Vertical Stacking in Spin-Coated Lead Halide Perovskite Nanocrystal Films for Enhanced Photoluminescence Efficiency

Impact of Bright-Dark Exciton Thermal Population Mixing on the Brightness of CsPbBr3 Nanocrystals.

Understanding the interplay between bright and dark exciton states is crucial for deciphering the luminescence properties of low-dimensional materials. The origin of the outstanding brightness of lead halide perovskites remains elusive. Here, we analyze temperature-dependent time-resolved photoluminescence to investigate the population mixing between bright and dark exciton sublevels in individual CsPbBr3 nanocrystals in the intermediate confinement regime. We extract bright and dark exciton decay rates and show quantitatively that the decay dynamics can only be reproduced with second-order phonon transitions. Furthermore, we find that any exciton sublevel ordering is compatible with the most likely population transfer mechanism. The remarkable brightness of lead halide perovskite nanocrystals rather stems from a reduced asymmetry between bright-to-dark and dark-to-bright conversion originating from the peculiar second-order phonon-assisted transitions that freeze bright-dark conversion at low temperatures together with the very fast radiative recombination and favorable degeneracy of the bright exciton state.

Photoluminescence Enhancement in Silica-Confined Ligand-Free Perovskite Nanocrystals by Suppression of Silanol-Induced Traps and Phase Impurities.

The detriments of intrinsic silanol groups in mesoporous silica to the photoluminescence (PL) of lead halide perovskite nanocrystals (LHP NCs) confined in the template have never been determined and clearly elucidated. Here, we disclose that silanol-induced Cs+ and Br- deficiencies prompt the generation of traps and CsPb2Br5 impurities. The temperature-dependent PL spectra verify the higher energetic barrier of trap states in CsPbBr3 NCs confined in silanol-rich mesoporous silica. Femtosecond transient absorption spectra reveal the trapped state mediates a broadband photoinduced absorption and long-lived decay pathway of CsPbBr3 NCs in silanol-rich templates. A remarkable improvement (up to 160-fold) in PL quantum yields is realized by simple silanol elimination. This work demonstrates the detrimental effects of silanol sites on the PL properties of LHP NCs impregnated in mesoporous silica and provides a new perspective for the ligand-free synthesis of high-quality LHP NCs in mesoporous templates by facile impregnation for practical applications.

Enhanced Photostability of Lead Halide Perovskite Nanocrystals with Mn3+ Incorporation.

Recently, lead halide perovskite nanocrystals (NCs) have shown great potential and have been widely studied in lighting and optoelectronic fields. However, the long-term stability of perovskite NCs under irradiation is an important challenge for their application in practice. Mn2+ dopants are mostly proposed as substitutes for the Pb site in perovskite NCs synthesized through the hot-injection method, with the aim of improving both photo- and thermal stability. In this work, we employed a facile ligand-assisted reprecipitate strategy to introduce Mn ions into perovskite lattice, and the results showed that Mn3+ instead of Mn2+, even with a very low level of incorporation of 0.18 mol % as interstitial dopant, can enhance the photostability of perovskite binder film under the ambient conditions without emission change, and the photoluminescent efficiency can retain 70% and be stable under intensive irradiation for 12 h. Besides, Mn3+ incorporation could prolong the photoluminescent decay time by passivating trap defects and modifying the distortion of the lattice, which underscores the significant potential for application as light emitters.

Photoluminescence Enhancement in Silica‐Confined Ligand‐Free Perovskite Nanocrystals by Suppression of Silanol‐Induced Traps and Phase Impurities

AbstractThe detriments of intrinsic silanol groups in mesoporous silica to the photoluminescence (PL) of lead halide perovskite nanocrystals (LHP NCs) confined in the template have never been determined and clearly elucidated. Here, we disclose that silanol‐induced Cs+ and Br− deficiencies prompt the generation of traps and CsPb2Br5 impurities. The temperature‐dependent PL spectra verify the higher energetic barrier of trap states in CsPbBr3 NCs confined in silanol‐rich mesoporous silica. Femtosecond transient absorption spectra reveal the trapped state mediates a broadband photoinduced absorption and long‐lived decay pathway of CsPbBr3 NCs in silanol‐rich templates. A remarkable improvement (up to 160‐fold) in PL quantum yields is realized by simple silanol elimination. This work demonstrates the detrimental effects of silanol sites on the PL properties of LHP NCs impregnated in mesoporous silica and provides a new perspective for the ligand‐free synthesis of high‐quality LHP NCs in mesoporous templates by facile impregnation for practical applications.

Tuning the fluorescence emission of water-stable CsPb(Br/I)3 perovskite nanocrystal probes for naked-eye visual detection of mercury ion

All-inorganic cesium lead halide perovskite nanocrystals (PNCs) have rarely been reported in analytical chemistry, especially in water environment analysis, due to their high susceptibility to environmental conditions. Herein, we investigate and report a method of visual detection of Hg2+ concentration in aqueous solution using CsPb(Br/I)3 PNCs. In this strategy, three kinds of CsPb(Br/I)3 PNCs with FL emission peaks of 551 nm, 591 nm, and 617 nm were used to detect Hg2+, and it was found that Hg2+ can quench the fluorescence of CsPb(Br/I)3 PNCs and blue-shift the maximum FL emission peak. We also found that the Hg2+ concentration has a good linear relationship with the fluorescence quenching rate (I0/I) and FL emission peak shift (Δλ) of CsPb(Br/I)3 PNCs. With the red-shift of the probe FL emission peak, the detected concentration range of Hg2+ gradually expanded. Therefore, the possibility of detecting Hg2+ concentration without huge equipment was detailed by examining the red (R), green (G), and blue (B) values of photographs of CsPb(Br/I)3 PNCs with Hg2+. Under a 365 nm ultraviolet (UV) lamp, the solution color of CsPb(Br/I)3 PNCs emission peaks at 591 nm gradually changes from orange to green with the increase of Hg2+ concentration, which has an excellent visualization effect. The method presented here has the potential to establish a new analytical method for the visual detection of Hg2+ in aqueous environments, and may also facilitate the application of all-inorganic cesium lead halide perovskite nanocrystals for analytical detection in aqueous environments.

Oriented Assembly of Lead Halide Perovskite Nanocrystals.

Cesium lead halide nanostructures have highly tunable optical and optoelectronic properties. Establishing precise control in forming perovskite single-crystal nanostructures is key to unlocking the full potential of these materials. However, studying the growth kinetics of colloidal cesium lead halides is challenging due to their sensitivity to light, electron beam, and environmental factors like humidity. In this study, in situ observations of CsPbBr3-particle dynamics were made possible through extremely low dose liquid cell transmission electron microscopy, showing that oriented attachment is the dominant pathway for the growth of single-crystal CsPbBr3 architectures from primary nanocubes. In addition, oriented assembly and fusion of ligand-stabilized cubic CsPbBr3 nanocrystals are promoted by electron beam irradiation or introduction of polar additives that both induce partial desorption of the original ligands and polarize the nanocube surfaces. These findings advance our understanding of cesium lead halide growth mechanisms, aiding the controlled synthesis of other perovskite nanostructures.

Atomic Imaging of Multi-Dimensional Ruddlesden-Popper Interfaces in Lead-Halide Perovskites.

Ruddlesden-Popper (RP) interface with defined stacking structure will fundamentally influence the optoelectronic performances of lead-halide perovskite (LHP) materials and devices. However, it remains challenging to observe the atomic local structures in LHPs, especially for multi-dimensional RP interface hidden inside the nanocrystal. In this work, the advantages of two imaging modes in scanning transmission electron microscopy (STEM), including high-angle annular dark field (HAADF) and integrated differential phase contrast (iDPC) STEM, are successfully combined to study the bulk and local structures of inorganic and organic/inorganic hybrid LHP nanocrystals. Then, the multi-dimensional RP interfaces in these LHPs are atomically resolved with clear gap and blurred transition region, respectively. In particular, the complex interface by the RP stacking in 3D directions can be analyzed in 2D projected image. Finally, the phase transition, ion missing, and electronic structures related to this interface are investigated. These results provide real-space evidence for observing and analyzing atomic multi-dimensional RP interfaces, which may help to better understand the structure-property relation of LHPs, especially their complex local structures.

Size dependence of luminescence decay process in CsPbBr3 nanocrystals

Lead halide perovskite nanocrystals (NCs) have superior luminescence properties compared to classical semiconductor NCs and they are considered promising materials for next-generation emitters. In this work, we measured the size and temperature dependence of the luminescence decay curve in CsPbBr3 NCs. To reproduce the experimental results, a simple model including one and two-phonon transitions between the exciton fine structures is introduced. It is found that the temperature dependence of the decay rates in small NCs is better reproduced by the two-phonon model rather than by the one-phonon model. However, the one-phonon process becomes more important for large NCs as the bright-dark splitting decreases.

Blue-Emitting Cs(Pb,Cd)Br3 Nanocrystals Resistant to Electric Field-Induced Ion Segregation.

High-performance solution-processed perovskite light-emitting diodes (PeLEDs) have emerged as a good alternative to the well-established technology of epitaxially grown AIIIBV semiconductor alloys. Colloidal cesium lead halide perovskite nanocrystals (CsPbX3 NCs) exhibit room-temperature excitonic emission that can be spectrally tuned across the entire visible range by varying the content of different halogens at the X-site. Therefore, they present a promising platform for full color display manufacturing. Engineering of highly efficient PeLEDs based on bromide and iodide perovskite NCs emitting green and red light, respectively, does not face major challenges except low operational stability of the devices. Meanwhile, mixed-halide counterparts demonstrating blue luminescence suffer from the electric field-induced phase separation (ion segregation) phenomenon described by the rearrangement (demixing) of mobile halide ions in the crystal lattice. This phenomenon results in an undesirable temporal redshift of the electroluminescence spectrum. However, to realize spectral tuning and, at the same time, address the issue of ion segregation less mobile Cd2+ ion could be introduced in the lattice at Pb2+-site that leads to the band gap opening. Herein, we report an original synthesis of CsPb0.88Cd0.12Br3 perovskite NCs and study their structural and optical properties, in particular electroluminescence. Multilayer PeLEDs based on the obtained NCs exhibit single-peak emission centered at 485 nm along with no noticeable change in the spectral line shape for 30 min which is a significant improvement of the device performance.

Continuous Nanomanufacturing of Inorganic Lead Halide Perovskite Nanocrystals with High-Concentration Precursors.

The microscale flow preparation scheme has been widely used in the preparation of inorganic perovskite nanocrystals (NCs). It is considered to be the most promising method for large-scale production. Recently, it has been suggested that increasing the precursor concentration can further improve efficiency, but there is still a lack of understanding of high-concentration synthesis. Here, we develop a microscale flow synthesis scheme using high-concentration precursors, and the typical concentration value in the reaction phase reaches 0.035 mol/L using cesium acetate. The CsPbBr3 NCs with sharp photoluminescence (PL) at 515.7 nm can be obtained, and their PL quantum yield after post-treatment exceeds 90%. The effect of the molar ratio of Pb/Cs (Rm), reaction time, reaction temperature, and excess ligands on this flow reaction is studied. Several new phenomena are observed in our experiment. At 120 °C, some Cs4PbBr6 NCs exist in addition to the usual CsPbBr3 nanoplatelets. Excess ligands lead to the formation of numerous Cs4PbBr6 NCs with a bright green PL, and these NCs will spontaneously transform into a nonemission form in the film. Moreover, mixed-halide CsPbBrxI3-x NCs and CsPbI3 NCs are also prepared in this scheme, and then they are used to obtain LEDs in a range of colors.

Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals, such as CsPbBr3, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr3 nanocrystals and attribute them to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL toward condensed phase optical refrigeration.

Hydrogen-Bonded Organic Cocrystal-Encapsulated Perovskite Nanocrystals as Coreactant-Free Electrochemiluminescent Luminophore for the Detection of Uranium.

Lead halide perovskite nanocrystals with excellent photophysical properties are promising electrochemiluminescence (ECL) candidates, but their poor stability greatly restricts ECL applications. Herein, hydrogen-bonded cocrystal-encapsulated CsPbBr3 perovskite nanocrystals (PeNCs@NHS-M) were synthesized by using PeNCs as nuclei for inducing the crystallization of melamine (M) and N-hydroxysuccinimide (NHS). The as-synthesized composite exhibits multiplicative ECL efficiencies (up to 24-fold that of PeNCs) without exogenous coreactants and with excellent stability in the aqueous phase. The enhanced stability can be attributed to the well-designed heterostructure of the PeNCs@NHS-M composite, which benefits from both moiety passivation and protection of the peripheral cocrystal matrix. Moreover, the heterostructure with covalent linkage facilitates charge transfer between PeNCs and NHS-M cocrystals, realizing effective ECL emission. Meanwhile, the NHS and M components act as coreactants for PeNCs, shortening the electron-transport distance and resulting in a significant increase in the ECL signal. Furthermore, by taking advantage of the specific binding effect between NHS-M and uranyl (UO22+), an ECL system with both a low detection limit (1 nM) and high selectivity for monitoring UO22+ in mining wastewater is established. The presence of UO22+ disrupted the charge-transfer effect within PeNCs@NHS-M, weakening the ECL signals. This work provides an efficient design strategy for obtaining stable and efficient ECLs from perovskite nanocrystals, offering a new perspective for the discovery and application of perovskite-based ECL systems.

Colloidal Aziridinium Lead Bromide Quantum Dots.

The compositional engineering of lead-halide perovskite nanocrystals (NCs) via the A-site cation represents a lever to fine-tune their structural and electronic properties. However, the presently available chemical space remains minimal since, thus far, only three A-site cations have been reported to favor the formation of stable lead-halide perovskite NCs, i.e., Cs+, formamidinium (FA), and methylammonium (MA). Inspired by recent reports on bulk single crystals with aziridinium (AZ) as the A-site cation, we present a facile colloidal synthesis of AZPbBr3 NCs with a narrow size distribution and size tunability down to 4 nm, producing quantum dots (QDs) in the regime of strong quantum confinement. NMR and Raman spectroscopies confirm the stabilization of the AZ cations in the locally distorted cubic structure. AZPbBr3 QDs exhibit bright photoluminescence with quantum efficiencies of up to 80%. Stabilized with cationic and zwitterionic capping ligands, single AZPbBr3 QDs exhibit stable single-photon emission, which is another essential attribute of QDs. In particular, didodecyldimethylammonium bromide and 2-octyldodecyl-phosphoethanolamine ligands afford AZPbBr3 QDs with high spectral stability at both room and cryogenic temperatures, reduced blinking with a characteristic ON fraction larger than 85%, and high single-photon purity (g(2)(0) = 0.1), all comparable to the best-reported values for MAPbBr3 and FAPbBr3 QDs of the same size.

Size Dependence of Trion and Biexciton Binding Energies in Lead Halide Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have attracted much attention as light-source materials for light-emitting diodes, lasers, and quantum light emitters. The luminescence properties of perovskite NCs and the performance of NC-based light-source devices depend on trion and biexciton dynamics. Here, we examined the size dependence of trion and biexciton binding energies by conducting low-temperature single-dot spectroscopy on three different perovskite NCs: CsPbBr3, CsPbI3, and FAPbBr3. While the photoluminescence spectral widths of the all-inorganic CsPbBr3 and CsPbI3 NCs were narrow, compared with those of the organic-inorganic hybrid FAPbBr3 NCs, the binding energies of trions and biexcitons of all three samples showed similar size dependences, independent of the A-site cation and halogen. The effective-mass approximation calculations implied the importance of dynamical dielectric screening on the formation of trions and biexcitons.

Highly luminescent polyfluorene-based composite with CsPbX3 perovskite nanocrystals for light-emitting devices

The rapid development of thin-film light emitting devices (LED) technologies has recently been associated with the superior optoelectronic properties of luminescent materials based on lead halide perovskite nanocrystals (NCs) due to their narrow emission line with high color purity. However, the large surface area of NCs leads to the need to use solvating ligands to prevent their agglomeration, which limits their use in optoelectronics. Here we develop a class of modular polyfluorene (PF) copolymer with 4-hydroxyphenyl-, diethylamino- and diethoxyphosphoryl- groups designed to stabilize perovskite NCs. We show that as-synthesized CsPbBr3 NCs can easily be mixed with custom-designed PFs resulting in polymer/NCs composite that shows efficient Förster energy transfer (FRET) from PF to NC with green photoluminescence (PL). We also found that the NCs composite studied here can be used as an effective emissive layer in LED due to the strong interaction between polymer host and perovskite NCs providing an efficient charge transfer from the PF matrice to the NC emitter. The fabricated LED show excellent performance with a highest current efficiency of ∼25.2 cd A–1. Our approach provides a low-cost and efficient way for light-emitting optoelectronic applications based on perovskite NCs.

Self‐Purification Affecting the Optical Performance of Mn‐Doped Halide Perovskite Nanocrystals

AbstractComponent doping is the fundamental topic for modulating the properties of semiconductor materials. The introduction of doping ions into lead halide perovskites (LHPs) can not only maintain the excellent photoelectric properties but also enhance the stability of LHPs in open air and thermal environments. However, due to the “self‐purification” effect in crystallography, there is an inherent trend to pop doping ions out of LHPs lattice. In this work, it is confirmed that in Mn2+ doped LHPs nanocrystals (NCs) the discharge of Mn2+ will be accelerated at higher temperatures. It is also proved that even at room temperature, the dopants in LHPs NCs will also actively “migrate”, resulting in declined optical performance. Therefore, for cation alloying/doping LHPs NCs, the migration of doping ions in the material should be considered in addition to the intrinsic halide migration characteristics. This work will provide a benign reference for application of doped LHPs NCs.

Tuning Energy Transfer Pathways in Halide Perovskite-Dye Hybrids through Bandgap Engineering.

Lead halide perovskite nanocrystals, which offer rich photochemistry, have the potential to capture photons over a wide range of the visible and infrared spectrum for photocatalytic, optoelectronic, and photon conversion applications. Energy transfer from the perovskite nanocrystal to an acceptor dye in the form of a triplet or singlet state offers additional opportunities to tune the properties of the semiconductor-dye hybrid and extend excited-state lifetimes. We have now successfully established the key factors that dictate triplet energy transfer between excited CsPbI3 and surface-bound rhodamine dyes using absorption and emission spectroscopies. The pendant groups on the acceptor dyes influence surface binding to the nanocrystals, which in turn dictate the energy transfer kinetics, as well as the efficiency of energy transfer. Of the three rhodamine dyes investigated (rhodamine B, rhodamine B isothiocyanate, and rose Bengal), the CsPbI3-rose Bengal hybrid with the strongest binding showed the highest triplet energy transfer efficiency (96%) with a rate constant of 1 × 109 s-1. This triplet energy transfer rate constant is nearly 2 orders of magnitude slower than the singlet energy transfer observed for the pure-bromide CsPbBr3-rose Bengal hybrid (1.1 × 1011 s-1). Intriguingly, although the single-halide CsPbBr3 and CsPbI3 nanocrystals selectively populate singlet and triplet excited states of rose Bengal, respectively, the mixed halide perovskites were able to generate a mixture of both singlet and triplet excited states. By tuning the bromide/iodide ratio and thus bandgap energy in CsPb(Br1-xIx)3 compositions, the percentage of singlets vs triplets delivered to the acceptor dye was systematically tuned from 0 to 100%. The excited-state properties of halide perovskite-molecular hybrids discussed here provide new ways to modulate singlet and triplet energy transfer in semiconductor-molecular dye hybrids through acceptor functionalization and donor bandgap engineering.