No abstract

Defects have always been an integral part of semiconductor crystals, controlling their optical and electronic properties. Even though growing popularity of the CsPbX 3 (X = Cl, Br, I, and their mixture) nanocrystals (NCs) in various applications stems from their defect-tolerant nature, the properties, stability, and practical utility of these substances are still very much governed by the defects. A variety of methods, which are halide-specific, have been developed to regulate the activities of the defects for enhancing the photoluminescence and stability of these NCs. In this Perspective, we trace the origin and manifestation of different types of defects in photoluminescence properties and stability of the CsPbX 3 NCs, critically examine the rationale of various passivation strategies to obtain an in-depth understanding of the problem, and recommend the most effective strategy to be followed for tackling defects under any given condition.

Study of the emission behavior of all-inorganic perovskite nanocrystals CsPbBr3 and CsPbBr2I as a function of the excitation power employing fluorescence correlation spectroscopy and conventional techniques reveals fluorescence blinking in the microsecond time scale and photoinduced emission enhancement. The observation provides insight into the radiative and nonradiative deactivation pathways of these promising substances. Because both blinking and photoactivation processes are intimately linked to the charge separation efficiency and dynamics of the nanocrystals, these key findings are likely to be helpful in realizing the true potential of these substances in photovoltaic and optoelectronic applications.

Quantum-confined perovskites are a new class of promising materials in optoelectronic applications. In this context, a zero-dimensional perovskite-related substance, CsPbBr, having high exciton binding energy can be an important candidate, but its photoluminescence (PL) is a topic of recent debate. Herein, we report an ambient condition controlled synthesis of CsPbBr microdisks of different shapes and dimensions which exhibit fairly strong green PL (quantum yield up to 38%, band gap ∼2.43 eV) in the solid state. Using confocal fluorescence microscopy imaging of the single particles, we show that the fluorescence of CsPbBr microdisks is inherent to these particles. Fluorescence intensity and lifetime imaging of the microdisks reveals significant spatial heterogeneity with a bright central area and somewhat dimmer edges. This intensity and lifetime distribution is attributed to enhanced trap-mediated nonradiative deactivation at the edges compared to the central region of the microdisks. Our results, which unambiguously establish the PL of these CsPbBr and suggest its possible origin, brighten the potential of these materials in photon-emitting applications.

A facile and highly reproducible room temperature, open atmosphere synthesis of cesium lead halide perovskite nanocrystals of six different morphologies is reported just by varying the solvent, ligand and reaction time. Sequential evolution of the quantum dots, nanoplates and nanobars in one medium and nanocubes, nanorods and nanowires in another medium is demonstrated. These perovskite nanoparticles are shown to be of excellent crystalline quality with high fluorescence quantum yield. A mechanism of the formation of nanoparticles of different shapes and sizes is proposed. Considering the key role of morphology in nanotechnology, this simple method of fabrication of a wide range of high quality nanocrystals of different shapes and sizes of all-inorganic lead halide perovskites, whose potential is already demonstrated in light emitting and photovoltaic applications, is likely to help widening the scope and utility of these materials in optoelectronic devices.

To obtain an in-depth understanding of the dynamics and mechanism of carrier recombination in CsPbBr 3 nanocrystals (NCs), we have investigated the photoluminescence (PL) of this material at the single-particle level using the time-tagged−time-resolved method. The study reveals two distinct types of PL fluctuations of the NCs, which are assigned to flickering and blinking. The flickering is found to be due to excess surface trap on the NCs, and the flickering single particles are transformed into blinking ones with significant enhancement of PL intensity and stability on postsynthetic surface treatment. Intensitycorrelated lifetime analysis of the PL time trace reveals both trapmediated nonradiative band-edge carrier recombination and positive trion recombination in single NCs. Dynamical and statistical analysis suggests a diffusive nature of the trap states to be responsible for the PL intermittency of the system. These findings throw light on the nature of the trap states, reveal the manifestation of these trap states in PL fluctuation, and provide an effective way to control the dynamics of CsPbBr 3 NCs.

Zero-dimensional (0-D) perovskites and perovskite-related materials are an emerging class of optoelectronic materials exhibiting strong excitonic properties and, quite often, high photoluminescence (PL) in the solid state. Here we highlight two different classes of 0-D perovskites with contrasting structural and optical properties, focusing mainly on the less explored but rapidly growing bulk quantum materials termed as 0-D perovskite-related materials (0-D PRMs), whose PL properties are quite intriguing and a topic of recent debate. We attempt to present here a comprehensive picture to rationalize the contrasting properties of the 0-D PRMs and provide an understanding of the mechanism of exciton dynamics and PL of this class of materials. We hope that exciting PL and tunable composition of these systems will help design of new materials with versatile optical properties suited for practical applications.

Exploration of the full potential of the perovskite nanocrystals (NCs) for different applications requires a thorough understanding of the pathways of recombination of the photogenerated charge carriers and associated dynamics. In this work, we have tracked the recombination routes of the charge carriers by probing photoluminescence (PL) intermittency of the immobilized and freely diffusing single CsPbBr3 NCs employing a time-tagged-time-resolved method. The immobilized single CsPbBr3 NCs show a complex PL time-trace, a careful analysis of which reveals that nonradiative band-edge recombination through trap states, trion recombination, and trapping of the hot carriers contribute to the blinking behavior of any given NC. A drastically suppressed PL blinking observed for the NCs treated with a tetrafluoroborate salt indicates elimination of most of the undesired recombination processes. A fluorescence correlation spectroscopy (FCS) study on the freely diffusing single NCs shows that enhanced PL and suppressed blinking of the treated particles are the outcome of an increase in per-particle brightness, not due to any increase in the number of particles undergoing “off”–“on” transition in the observation volume. The mechanistic details obtained from this study on the origin of blinking in CsPbBr3 NCs provide deep insight into the radiative and nonradiative charge carrier recombination pathways in these important materials, and this knowledge is expected to be useful for better design and development of bright photoluminescent samples of this class for optoelectronic applications.