Stark Effect and Environment-Induced Modulation of Emission in Single Halide Perovskite Nanocrystals

Growth Control of InP/ZnSe Heterostructured Nanocrystals

The morphology of heterostructured semiconductor nanocrystals (h-NCs) dictates the spatial distribution of charge carriers and their recombination dynamics and/or transport, which are the main performance indicators of photonic applications utilizing h-NCs. The inability to control the morphology of heterovalent III-V/II-VI h-NCs composed of heavy-metal-free elements hinders their practical use. As a case study of III-V/II-VI h-NCs, the growth control of ZnSe epilayers on InP NCs is demonstrated here. The anisotropic morphology in InP/ZnSe h-NCs is attributed to the facet-dependent energy costs for the growth of ZnSe epilayers on different facets of InP NCs, and effective chemical means for controlling the growth rates of ZnSe on different surface planes are demonstrated. Ultimately, this article capitalizes on the controlled morphology of InP/ZnSe h-NCs to expand their photophysical characteristics from stable and pure emission to environment-sensitive one, which will facilitate their use in a variety of photonic applications.

Nanomaterials for Fluorescence and Multimodal Bioimaging

Bioconjugated nanomaterials replace molecular probes in bioanalysis and bioimaging in vitro and in vivo. Nanoparticles of silica, metals, semiconductors, polymers, and supramolecular systems, conjugated with contrast agents and drugs for image-guided (MRI, fluorescence, PET, Raman, SPECT, photodynamic, photothermal, and photoacoustic) therapy infiltrate into preclinical and clinical settings. Small bioactive molecules like peptides, proteins, or DNA conjugated to the surfaces of drugs or probes help us to interface them with cells and tissues. Nevertheless, the toxicity and pharmacokinetics of nanodrugs, nanoprobes, and their components become the clinical barriers, underscoring the significance of developing biocompatible next-generation drugs and contrast agents. This account provides state-of-the-art advancements in the preparation and biological applications of bioconjugated nanomaterials and their molecular, cell, and in vivo applications. It focuses on the preparation, bioimaging, and bioanalytical applications of monomodal and multimodal nanoprobes composed of quantum dots, quantum clusters, iron oxide nanoparticles, and a few rare earth metal ion complexes.

A Fano Cavity-Photon Interface for Directional Suppression of Spectral Diffusion of a Single Perovskite Nanoplatelet

Colloidal nanocrystals that are capable of mass production with wet chemical synthesis have long been proposed as color-tunable, scalable quantum emitters for information processing and communication. However, they constantly suffer from spectral diffusion due to being exposed to a noisy electrostatic environment. Herein we demonstrate a cavity-photon interface (CPI) which effectively suppresses the temperature-activated spectral diffusion (SD) of a single perovskite nanoplatelet (NPL) up to 40 K. The spectrally stabilized single-photon emission is achieved at a specific emission direction corresponding to an inhibited dipole moment of the NPL as the result of the Fano coupling between the two photon dissipation channels of the NPL. Our results shed light on the nature of the SD of perovskite nanocrystals and offer a general cavity quantum electrodynamic scheme that controls the brightness and spectral dynamics of a single-photon emitter.

Electrical control of biexciton Auger recombination in single CdSe/CdS nanocrystals

The Auger recombination effect is strongly enhanced in semiconductor nanocrystals due to the quantum confinement, and various strategies in chemical synthesis have been employed so far to suppress this nonradiative decay pathway of multiple excitons. Here we apply external electric fields on single CdSe/CdS giant nanocrystals at room temperature, showing that the biexciton Auger and single-exciton radiative rates can be averagely decreased by ∼40 and ∼10%, respectively. In addition to a reduced overlap of the electron-hole wavefunctions, the large decrease of biexciton Auger rate could be contributed by the enhanced exciton-exciton repulsion, while the electron-hole exchange interaction might be weakened to cause the relatively small decrease of the single-exciton radiative rate. The above findings have thus proved that the external electric field can serve as a post-synthetic knob to tune the exciton recombination dynamics in semiconductor nanocrystals towards their efficient applications in various optoelectronic devices.

The Role of Alkyl Chain Length and Halide Counter Ion in Layered Dion-Jacobson Perovskites with Aromatic Spacers

Layered hybrid perovskites based on Dion-Jacobson phases are of interest to various optoelectronic applications. However, the understanding of their structure-property relationships remains limited. Here, we present a systematic study of Dion-Jacobson perovskites based on (S)PbX₄ (n = 1) compositions incorporating phenylene-derived aromatic spacers (S) with different anchoring alkylammonium groups and halides (X = I, Br). We focus our study on 1,4-phenylenediammonium (PDA), 1,4-phenylenedimethylammonium (PDMA), and 1,4-phenylenediethylammonium (PDEA) spacers. Systems based on PDA did not form a well-defined layered structure, showing the formation of a 1D structure instead, whereas the extension of the alkyl chains to PDMA and PDEA rendered them compatible with the formation of a layered structure, as shown by X-ray diffraction and solid-state NMR spectroscopy. In addition, the control of the spacer length affects optical properties and environmental stability, which is enhanced for longer alkyl chains and bromide compositions. This provides insights into their design for optoelectronic applications.

Remanent Polarization and Strong Photoluminescence Modulation by an External Electric Field in Epitaxial CsPbBr3 Nanowires

Metal halide perovskites (MHPs) have unique characteristics and hold great potential for next-generation optoelectronic technologies. Recently, the importance of lattice strain in MHPs has been gaining recognition as a significant optimization parameter for device performance. While the effect of strain on the fundamental properties of MHPs has been at the center of interest, its combined effect with an external electric field has been largely overlooked. Here we perform an electric-field-dependent photoluminescence study on heteroepitaxially strained surface-guided CsPbBr₃ nanowires. We reveal an unexpected strong linear dependence of the photoluminescence intensity on the alternating field amplitude, stemming from an induced internal dipole. Using low-frequency polarized-Raman spectroscopy, we reveal structural modifications in the nanowires under an external field, associated with the observed polarity. These results reflect the important interplay between strain and an external field in MHPs and offer opportunities for the design of MHP-based optoelectronic nanodevices.

Electrical Switching of Optical Gain in Perovskite Semiconductor Nanocrystals

Perovskite semiconductor nanocrystals are promising for optical amplification and laser applications benefiting from efficient optical gain generation. Nevertheless, the pump threshold is limited by more than one exciton per nanocrystal required to generate population inversion in neutral nanocrystals due to the level degeneracy. Here, we show that by charging nanocrystals with current injection, the level degeneracy can be lifted to generate charged exciton gain with markedly low excitation density. On the basis of the scenario, we have demonstrated electrical switching of amplified spontaneous emission in films of CsPbBr₃ nanocrystals sandwiched by two electrodes with over 50% threshold reduction owing to charged excitons. Our work provides an effective approach to electrically modulated optical gain in colloidal perovskite nanocrystals for potential applications in advanced laser and information technology.

State of the Art and Prospects for Halide Perovskite Nanocrystals

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

Photoinduced Rotation of Colloidal Semiconductor Nanocrystals in an Electric Field

We demonstrate that solution-phase semiconductor nanocrystals (NCs) undergo photoinduced rotation in an external electric field. Present measurements backed by theoretical calculations show that the rotation of colloidal NCs is driven by the excited-state dipole moment, which is counterbalanced by the solvent viscosity drag. Corresponding angular velocities range from 0.5°/ns for cubic CsPbBr₃ NCs to 3°/ns for nanoparticles with a large photoinduced charge separation (CdSe/CdS core-shell and dot-in-a-rod NCs). Because of photoinduced rotation, solution-phase semiconductor NCs exhibited an order-of-magnitude increase in the spectral changes caused by the quantum confined Stark effect (QCSE), compared to solid NC assemblies. The enhanced QCSE of colloidal NCs reflected their global alignment in solution, which could be retained in a solid environment by slow crystallization. Overall, we expect that the demonstrated phenomenon of the colloidal nanocrystal rotation in an electric field will open up new avenues for developing electro-optical and voltage-sensitive applications.

Probing Permanent Dipole Moments and Removing Exciton Fine Structures in Single Perovskite Nanocrystals by an Electric Field

Single perovskite nanocrystals have emerged as a novel type of semiconductor nanostructure capable of emitting single photons with rich exciton species and fine energy-level structures. Here we focus on single excitons and biexcitons in single perovskite CsPbI_{3} nanocrystals to show, for the first time, how their optical properties are modulated by an external electric field at the cryogenic temperature. The electric field can cause a blueshift in the photoluminescence peak of single excitons, from which the existence of a permanent dipole moment can be deduced. Meanwhile, the fine energy-level structures of single excitons and biexcitons in a single CsPbI_{3} nanocrystal can be simultaneously eliminated, thus preparing a potent platform for the potential generation of polarization-entangled photon pairs.

Real-Time Blinking Suppression of Perovskite Quantum Dots by Halide Vacancy Filling

Despite the excellent optoelectronic properties of halide perovskites, the ionic and electronic defects adversely affect the stability and durability of perovskites and their devices. These defects, intrinsic or produced by environmental factors such as oxygen, moisture, or light, not only cause chemical reactions that disintegrate the structure and properties of perovskites but also induce undesired photoluminescence blinking to perovskite quantum dots and nanocrystals. Blinking is also caused by the nonradiative Auger processes in the photocharged quantum dots or nanocrystals. Herein, we find real-time suppression of halide vacancy-assisted nonradiative exciton recombination and photoluminescence blinking in MAPbBr₃ and MAPbI₃ perovskite quantum dots by filling the vacancies using halide precursors (MABr and MAI). Also, halide vacancy filling increases the photoluminescence quantum efficiencies and lifetimes of the quantum dots. We estimate the rates of halide vacancy-assisted nonradiative recombination at 1 × 10⁸ s^-1 for MAPbBr₃ and 1.9 × 10⁹ s^-1 for MAPbI₃ quantum dots. The real-time blinking suppression using the halide precursors and statistical analysis of the ON/OFF blinking time reveal that the halide vacancies contribute to the type-A blinking through charging and discharging. Conversely, the blinking of the quantum dots after halide vacancy filling is dominated by the type-B mechanism.

Evolving Stark Effect During Growth of Perovskite Nanocrystals Measured Using Transient Absorption

Methylammonium lead triiodide (MAPbI3) nanocrystals (NCs) are emerging materials for a range of optoelectronic applications. Photophysical characterization is typically limited to structurally stable NCs owing to the long timescales required for many spectroscopies, preventing the accurate measurement of NCs during growth. This is a particular challenge for non-linear spectroscopies such as transient absorption. Here we report on the use of a novel single-shot transient absorption (SSTA) spectrometer to study MAPbI3 NCs as they grow. Comparing the transient spectra to derivatives of the linear absorbance reveals that photogenerated charge carriers become localized at surface trap states during NC growth, inducing a TA lineshape characteristic of the Stark effect. Observation of this Stark signal shows that the contribution of trapped carriers to the TA signal declines as growth continues, supporting a growth mechanism with increased surface ligation toward the end of NC growth. This work opens the door to the application of time-resolved spectroscopies to NCs in situ, during their synthesis, to provide greater insight into their growth mechanisms and the evolution of their photophysical properties.

High-Performance Green Light-Emitting Diodes Based on MAPbBr3 with π-Conjugated Ligand

The morphology, crystal size, and trap density of perovskite films significantly affect the luminescent properties of perovskite light-emitting diodes (PeLEDs). Recently, numerous studies have been conducted on ligands that surround the surface of perovskite crystals and passivate the trap sites to improve the performance of PeLEDs. In this study, a 4-aminobenzonitrile (ABN) ligand improved the performance of methylammonium lead bromide (MAPbBr₃)-based PeLEDs by reducing the MAPbBr₃ crystal size to the nanoscale and reducing the trap density. Moreover, the properties of PeLEDs with ABN were further improved using a surface-modified hole-transport layer (HTL) with a hydrophilic polymer. Finally, a bright green PeLED was fabricated, which exhibited the maximum luminance of 3350 cd/m² with an external quantum efficiency of 8.85%. Therefore, it is believed that the use of proper ligands for the perovskite layer and the optimization of the charge-transport layer have great potential for the development of high-performance PeLEDs.

Single-particle electroluminescence of CsPbBr3 perovskite nanocrystals reveals particle-selective recombination and blinking as key efficiency factors

Halide perovskites nanocrystals (NCs) are being explored as promising materials for optoelectronic applications, such as light-emitting devices or lasers. However, electroluminescence devices prepared from such NCs have long suffered from low efficiency and there has been no systematic study on the nanoscale origin of the poor efficiencies. Here, we use single-particle spectroscopy to compare electroluminescence and photoluminescence on the level of individual NCs of the perovskite CsPbBr₃. The NCs form aggregates in a conducting matrix used as an emission layer in an electroluminescence device. In electroluminescence, only a small fraction of the NCs within the aggregate is emitting as a result of efficient charge migration, accumulation and selective recombination on larger NCs, leading to pronounced blinking and decreased efficiency. Under the condition of comparable excitation rates in both electroluminescence and photoluminescence, the intrinsic quantum yield in electroluminescence is on average 0.36 of that in photoluminescence.

Halide perovskite nanocrystals show high photoluminescence (PL) but their electroluminescence (EL) devices suffered from low external quantum efficiency. Here Sharma et al. study both individual and statistical PL and EL behaviors of these nanocrystals and reveal the origin of the lower EL efficiency.

In situ growth of ultrasmall cesium lead bromine quantum dots in a mesoporous silica matrix and their application in flexible light-emitting diodes

Recently, CsPbX₃ (X = Cl, Br, and I) perovskite quantum dots (QDs) have exhibited significant potential for application in the field of lighting. However, their self-absorption and agglomeration significantly decrease their photoluminescence when their solution is centrifuged to form a powder; this hinders their applications in the field of solid-state lighting. Currently, there is lack of efficient solutions to overcome the self-absorption issue for CsPbX₃ QDs. Thus, herein, an effective strategy is proposed via the in situ growth of CsPbBr₃ (CPB) QDs in a mesoporous silica (m-SiO₂) matrix, where self-absorption originating from the agglomeration of the QD powder is distinctly suppressed in the m-SiO₂ matrix. Furthermore, due to its higher transmissivity, some photons can transport along the channels of m-SiO₂ with less light loss. As a result, the photoluminescence quantum yield (PLQY) of 68% for the CsPbBr₃/m-SiO₂ (CPB/MS) powder is distinctly higher than that of the discrete CPB powder (36%). In addition, the chemical stability, thermal quenching and luminous decay were evidently improved for the CPB/MS nanocomposite. Finally, a remote flexible light-emitting diode with ultrahigh stability and arbitrary bending angle was achieved, which presented a pathway for the application of CPB QDs in solid-state lighting.

Insights on heterogeneity in blinking mechanisms and non-ergodicity using sub-ensemble statistical analysis of single quantum-dots

Photo-luminescence (P-L) intermittency (or blinking) in semiconductor nanocrystals (NCs), a phenomenon ubiquitous to single-emitters, is generally considered to be temporally random intensity fluctuations between "bright" ("On") and "dark" ("Off") states. However, individual quantum-dots (QDs) rarely exhibit such telegraphic signals, and yet, a vast majority of single-NC blinking data are analyzed using a single fixed threshold which generates binary trajectories. Furthermore, while blinking dynamics can vary dramatically over NCs in the ensemble, the extent of diversity in the exponents (m_On/Off) of single-particle On-/Off-time distributions (P(t_On/Off)), often used to validate mechanistic models of blinking, remains unclear due to a lack of statistically relevant data sets. Here, we subclassify an ensemble of QDs based on the emissivity of each emitter and subsequently compare the (sub)ensembles' behaviors. To achieve this, we analyzed a large number (>1000) of blinking trajectories for a model system, Mn⁺² doped ZnCdS QDs, which exhibits diverse blinking dynamics. An intensity histogram dependent thresholding method allowed us to construct distributions of relevant blinking parameters (such as m_On/Off). Interestingly, we find that single QD P(t_On/Off)s follow either truncated power law or power law, and their relative proportion varies over subpopulations. Our results reveal a remarkable variation in m_On/Off amongst as well as within subensembles, which implies multiple blinking mechanisms being operational amongst various QDs. We further show that the m_On/Off obtained via cumulative single-particle P(t_On/Off) is distinct from the weighted mean value of all single-particle m_On/Off, evidence for the lack of ergodicity. Thus, investigation and analyses of a large number of QDs, albeit for a limited time span of a few decades, are crucial to characterize the spatial heterogeneity in possible blinking mechanisms.

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

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

High-performance and stable CsPbBr3 light-emitting diodes based on polymer additive treatment

Because of their high efficiency and sharp emission, perovskite light-emitting diodes are a promising candidate for next-generation lighting techniques. However, the relatively poor stability of perovskite light-emitting diodes lowers their utility. Therefore, a highly stable perovskite light-emitting diode has to be developed to meet the commercial demand. Herein, we report a highly stable CsPbBr₃ light-emitting diode via simple polymer treatment. The addition of 2-methyl-2-oxazoline in perovskite film assists the formation of CsPbBr₃ nanocrystals, improving the quality and photoluminescence property of perovskite film. Based on such CsPbBr₃ nanocrystals and polymer hybrid film, our device presents a high external quantum efficiency and luminance of around 3.0% and 16 648 cd m⁻², respectively. Moreover, an excellent device half-lifetime of more than 2.4 hours has been achieved, under continuous operation at a relatively high initial luminance of 1000 cd m⁻², representing one of the most stable PeLEDs operated at such high initial luminance.

The 2-methyl-2-oxazoline additive induced the formation of high-quality CsPbBr₃ nanocrystals and a stable PeLED with a half-lifetime of 2.4 hours at high initial luminance of 1000 cd m⁻² was demonstrated, representing one of the most stable PeLEDs.