Phase-Stable and Highly Luminescent CsPbI3 Perovskite Nanocrystals with Suppressed Photoluminescence Blinking.

Despite their low band gap, the utility of CsPbI3 nanocrystals (NCs) in solar photovoltaic and optoelectronic applications is rather limited because of their phase instability and photoluminescence (PL) intermittency. Herein we show that phase-pure, monodispersed, stable and highly luminescent CsPbI3 NCs can be obtained by tweaking the conventional hot-injection method employing NH4I as an additional precursor. Single-particle studies show a significant suppression of PL blinking. Among all NCs studied, 60% exhibit only high-intensity ON states with a narrow distribution of intensity. The remaining 40% of NCs exhibit a much wider distribution of PL intensity with a significant contribution of low-intensity OFF states. Excellent characteristics of these CsPbI3 NCs are shown to be the result of NH4+ replacing some surface Cs+ of an iodide-rich surface of the NCs. These phase-stable and highly luminescent CsPbI3 NCs with significantly suppressed PL blinking can be useful single-photon emitters and promising materials for optoelectronic and solar photovoltaic applications.

On direct synthesis of high quality APbX3 (A = Cs+, MA+ and FA+; X = Cl-, Br- and I-) nanocrystals following a generic approach.

Direct synthesis of APbX3 [A = Cs+, methylammonium (MA+) or formamidinium (FA+) and X = Cl-, Br- or I-] perovskite nanocrystals (NCs) following a generic approach is a challenging task even today. Motivated by our recent success in obtaining directly high-quality red/NIR-emitting APbI3 NCs employing 1,3-diiodo-5,5-dimethylhydantoin (DIDMH) as an iodide precursor, we explore here whether violet/green-emitting APbCl3 and APbBr3 NCs can also be obtained using the chloro- and bromo-analog of DIDMH keeping in mind that a positive outcome will provide the generic protocol for direct synthesis of all APbX3 NCs using similar halide precursors. It is shown that green-emitting APbBr3 NCs with near-unity PLQY and violet-emitting CsPbCl3 NCs with an impressive PLQY of ∼70%, mixed-halide NCs, CsPb(Cl/Br)3 and CsPb(Br/I)3, emitting in the blue and yellow-orange region with PLQYs of 87-95% and 68-98%, respectively can indeed be obtained employing the bromo- and chloro-analog of DIDMH. These NCs exhibit remarkable stability under different conditions including the polar environment. Femtosecond pump-probe studies show no ultrafast carrier trapping in these systems. The key elements of the halide precursors that facilitated the synthesis and the factors contributing to the excellent characteristics of the NCs are determined by careful analysis of the data. The results are of great significance because a direct method of obtaining highly luminescent and stable APbX3 NCs (except violet-emitting hybrid NCs) is eventually identified and the work provides valuable insight into the selection of appropriate halide precursors for the development of superior systems.

Lack of Environmental Sensitivity of a Naturally Occurring Fluorescent Analog of Cholesterol.

Dehydroergosterol (DHE, Δ5,7,9(11),22-ergostatetraen-3ß-ol) is a naturally occurring fluorescent analog of cholesterol found in yeast. Since DHE has been shown to faithfully mimic cholesterol in a large number of biophysical, biochemical, and cell biological studies, it is widely used to explore cholesterol organization, dynamics and trafficking in model and biological membranes. In this work, we show that DHE, in spite of its localization at the membrane interface, does not exhibit red edge excitation shift (REES) in model membranes, irrespective of the membrane phase. These results are reinforced by semi-empirical quantum chemical calculations of dipole moment changes of DHE in ground and excited states, which show a very small change in the dipole moment of DHE upon excitation. We conclude that DHE fluorescence exhibits lack of environmental sensitivity, despite its usefulness in monitoring cholesterol organization, dynamics and traffic in model and biological membranes.

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.

Structural Stability and Conformational Dynamics of Cytochrome c in Hydrated Deep Eutectic Solvents.

Many deep eutectic solvents (DESs) are currently being explored as environment-friendly media for biorelated applications. As an understanding of the effect of these solvents on the structure of biomolecules is crucial for these applications, we study how two DESs comprising trimethylglycine (TMG) and ethylene glycol (EG) or glycerol (GL) influence the structural stability and conformational dynamics of cytochrome c (Cytc) using single-molecule-based fluorescence correlation spectroscopy (FCS) technique and several other ensemble-based biophysical methods. The FCS studies on A488-labeled Cytc enable an estimation of the size (20.5 ± 1.5 Å) of the protein and capture its conformational dynamics (54 ± 2 µs) in aqueous buffered solution. It is observed that both size and conformational dynamics of the protein are influenced in the presence of the DESs, but this effect is more pronounced in the case of TMG-EG. The ensemble measurements on both labeled and wild-type Cytc reveal that the protein structure is unfolded completely by TMG-EG, whereas the structure is slightly altered by TMG-GL. The results suggest that the behavior of Cytc in hydrated DESs is determined by the strength of interactions between the DES constituents as well as that between the constituents and the water molecules present in the system.

On the Stability and Conformational Dynamics of Cytochrome c in Ammonium Ionic Liquids.

Owing to their potential applications in the extraction, purification, and preservation of biomolecules and biocatalysis, ionic liquids (ILs) have gained great attention in biotechnology. Although it is known that the structure and dynamics of proteins in ILs depend on the nature of both proteins and ILs, the biophysical mechanism governing the protein-IL interaction, which determines the stability of proteins or the activity of an enzyme in these nonconventional media, is yet to be understood clearly. Herein, we study the effect of two ammonium ILs, triethylammonium dihydrogen phosphate (TEAP) and tributylammonium dihydrogen phosphate (TBAP), on the stability and conformational dynamics of cytochrome c (Cyt c) in its native and unfolded states, employing primarily the single molecule-based fluorescence correlation spectroscopy (FCS) technique. The results show that the native structure of Cyt c is not significantly altered by TEAP, but the tertiary structure is perturbed to a great extent by TBAP, which comprises a longer alkyl chain. Fluctuations of the fluorescence intensity of Alexa488 dye-labeled Cyt c in FCS measurements reveal conformational dynamics (67 ± 10 µs) in the native state of Cyt c that is accelerated in the presence of both ILs but not affected when Cyt c is in its unfolded state. The present findings demonstrate how the stability of this protein can be modulated by using ammonium ILs of different alkyl chain lengths.

Complete Solvation Dynamics of Coumarin 153 in Tetraalkylammonium Bromide-Based Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are novel environment-friendly media for a variety of applications. In order to obtain insight into the structure and dynamics of some less-explored DESs comprising ethylene glycol and tetraalkylammonium bromide salts with variable alkyl chain length, we have captured complete dynamics occurring in these solvents in a timescale of few femtoseconds to several nanoseconds by monitoring the time-dependent fluorescence Stokes shift of coumarin 153 employing a combination of time-correlated single-photon counting and fluorescence upconversion techniques. The solvent response function constructed from the measured data reveals a sub-picosecond component (∼0.8 ps, 20-35%) in addition to a slow component (180-475 ps) with a distribution of relaxation time. The slow time component is found to be strongly dependent on the viscosity of the medium, indicating that it arises from the diffusive motions of the solvent constituents into and out of the solvation shell, whereas the ultrafast time component, which is nearly independent of the solvent viscosity, arises from fast local motions of the constituents in the immediate vicinity of the solute molecule.

Ambient Condition Mg2+ Doping Producing Highly Luminescent Green- and Violet-Emitting Perovskite Nanocrystals with Reduced Toxicity and Enhanced Stability.

The lack of long-term stability, the presence of toxic lead, and a low photoluminescence (PL) efficiency are the major obstacles to the commercialization of lead-halide perovskite-based optoelectronic and photovoltaic devices. Herein we report a facile ambient condition doping protocol that addresses all three issues of the CsPbX3 perovskite nanocrystals (NCs) to a substantial extent. We show that the room-temperature treatment of these NCs with MgX2 results in the partial (18-23%) replacement of toxic lead, enhances the PL quantum yield of green-emitting CsPbBr3 (to ∼100% from ∼51%) and violet-emitting CsPbCl3 NCs (to ∼79% from ∼1%), and improves the stability under ambient conditions and in the presence of light and a polar solvent. Ultrafast pump-probe and temperature-dependent PL studies reveal that curing of the intrinsic structural disorder, introduction of some shallow energy levels close to the conduction band edge, and effective passivation of the halide deficiency contribute to the improved properties of the doped systems.

Insights into the Folding Pathway of a c-MYC-Promoter-Based i-Motif DNA in Crowded Environments at the Single-Molecule Level.

Cytosine-rich DNA sequences fold into secondary structures called i-Motifs, which are usually stable at acidic pH. However, molecular crowding agents, such as poly(ethylene glycol) (PEG), are known to facilitate the formation of these structures even at neutral pH. As crowding mimics the intracellular environment and not much is known about the folding pathway of i-Motifs in such constrained media, we have probed, in detail, the conformational changes of a 22-mer c-MYC-promoter-based C-rich sequence (Py22) in the presence of PEG, employing Förster resonance energy transfer and fluorescence lifetime measurements at the single-molecule level. We find that the folding process is not a simple two-state transition between a random coil and a folded i-Motif structure. Rather, it involves a partially folded conformation as an intermediate in which the bases are not as efficiently stacked as in the completely folded i-Motif form. The relative population of each species is governed by the size and concentration of PEG, and 30% (w/w) PEG6000 is the optimum condition for the folding of Py22. Under this condition, ∼80% of Py22 exists in the fully folded i-Motif form and ∼20% of it is in the partially folded state.

Mechanistic Investigation of the Defect Activity Contributing to the Photoluminescence Blinking of CsPbBr3 Perovskite Nanocrystals.

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.

Liquid Structure and Dynamics of Tetraalkylammonium Bromide-Based Deep Eutectic Solvents: Effect of Cation Chain Length.

Deep eutectic solvents (DESs) have emerged in recent years as environmentally sustainable media across several fields. However, knowledge of liquid structure, dynamics, and solute-solvent interactions in many DESs that is essential for exploiting their potential is still lacking. In this work, we make an attempt to obtain some insight into these aspects of a set of less-explored DESs comprising tetraalkylammonium bromide salts and ethylene glycol (EG) by monitoring the fluorescence response of some carefully chosen dipolar (C153 and 4-AP) and nonpolar (9-PA) solutes in these media. Specifically, we have studied the translational and rotational diffusion dynamics of these molecular systems using single-molecule-based fluorescence correlation spectroscopy technique and ensemble-based time-resolved fluorescence anisotropy measurements. These results point to spatial and dynamic heterogeneity of these DESs, which becomes prominent in systems comprising cations with a longer alkyl chain length. This study reveals that diffusion dynamics of the probe molecules is determined not only by the solvent bulk viscosity but also dependent on their microenvironments and solute-solvent interactions experienced in these media.

Ultrafast carrier dynamics of metal halide perovskite nanocrystals and perovskite-composites.

Perovskite nanocrystals (NCs), especially those based on cesium lead halides, have emerged in recent years as highly promising materials for efficient solar cells and photonic applications. The key to realization of full potential of these materials lies however in the molecular level understanding of the processes triggered by light. Herein we highlight the knowledge gained from photophysical investigations on these NCs of various sizes and compositions employing primarily the femtosecond pump-probe technique. We show how spectral and temporal characterization of the photo-induced transients provide insight into the mechanism and dynamics of relaxation of hot and thermalized charge carriers through their recombination and trapping. We discuss how the multiple excitons including the charged ones (trions), generated using high pump fluence or photon energy, recombine through the Auger-assisted process. We discussed the harvesting of hot carriers prior to their cooling and band-edge carriers from these perovskite NCs to wide band-gap metal oxides, metal chalcogenide NCs and molecular acceptors. How perovskites can influence the charge carrier dynamics in composites of organic and inorganic semiconductors is also discussed.

Interactions between a Bioflavonoid and c-MYC Promoter G-Quadruplex DNA: Ensemble and Single-Molecule Investigations.

Small molecules capable of stabilizing the G-quadruplex structure of the nuclease hypersensitivity element III1 (NHE III1) are useful in controlling the overexpression of the c-MYC oncogene. In this study, we have probed the interactions of a 22-mer c-MYC promoter quadruplex-forming sequence (Pu22) with a bioflavonoid 3,4',5,7-tetrahydroxyflavone, commonly known as kaempferol (KF). Ensemble fluorescence resonance energy transfer experiments on labeled Pu22 indicate that KF decreases the affinity of the former toward its complimentary strand, suggesting the stabilization of the quadruplex structure of Pu22. Considering that binding dynamics plays an important role in supramolecular interactions, there is hardly any information on this aspect for quadruplex-flavonoid systems; we have studied the kinetics of KF-Pu22 complexation and decomplexation processes on the single-molecule level by employing fluorescence correlation spectroscopy technique. The binding dynamics is characterized by a fast relaxation time of 10-50 µs. This leads to a high association rate constant ( k+) of ∼109 M-1 s-1, which is close to the pure diffusion controlled limit. However, it is the low dissociation rate constant ( k-) of ∼104 s-1 that is mainly responsible for the stability of the KF-Pu22 complex. Molecular docking study shows that KF binds near the 3'-end of Pu22 by forming several H-bonds with the bases. These findings suggest that KF is a potential binder of the c-MYC promoter quadruplex DNA and can be useful in anticancer therapies.

Broadband ultrafast nonlinear optical studies revealing exciting multi-photon absorption coefficients in phase pure zero-dimensional Cs4PbBr6 perovskite films.

Lead halide perovskite nanocrystals (NCs) apart from their overwhelming optoelectronic applications have recently demonstrated promising nonlinear optical (NLO) properties such as strong two-photon absorption cross-sections (∼105 GM), two-photon fluorescence, and saturable absorption even at very high peak intensity. Zero-dimensional perovskite-related materials (0-D PRMs) are a new class of materials offering a high exciton binding energy (Eg ≥ 180 meV) with a strong photoluminescence (PL) quantum yield in few cases. Herein, we report the broadband third-order NLO properties of phase pure Cs4PbBr6 0-D PRM achieved using the Z-scan and degenerate four-wave mixing techniques in the femtosecond regime. Considering the growing content of the fluorescent and non-fluorescent forms of this material, we have performed our studies on both of them. These perovskite NCs exhibited strong multi-photon absorption properties in the near-infrared region with two-photon absorption (2PA) (cross-section, σ2 = 10-43-10-44 cm4 s equivalent to ∼106 GM) in the 500-800 nm region, three-photon absorption (3PA) (cross-section, σ3 ∼10-73 cm6 s2) in the 900-1200 nm region and four-photon absorption (4PA) (cross-section, σ4 ∼10-100 cm8 s3) in the 1300-1500 nm spectral region. These multi-photon absorption processes are explained using a simple band diagram. The measured NLO coefficients and cross-sections are fairly large when compared to some of the earlier reports on perovskite-based NCs. Cs4PbBr6 0-D PRM also demonstrated a large third-order NLO susceptibility χ(3) (∼10-7 esu), which can be attributed to the strong quantum confinement arising from spatially isolated, exciton containing individual [PbBr6]4- octahedron. These results clearly suggest the potential of 0D-PRMs in applications such as photonics and ultrafast all-optical switching devices.

Photoluminescence Flickering and Blinking of Single CsPbBr3 Perovskite Nanocrystals: Revealing Explicit Carrier Recombination Dynamics.

To obtain an in-depth understanding of the dynamics and mechanism of carrier recombination in CsPbBr3 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. Intensity-correlated lifetime analysis of the PL time trace reveals both trap-mediated 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 CsPbBr3 NCs.

How do the hydrocarbon chain length and hydroxyl group position influence the solute dynamics in alcohol-based deep eutectic solvents?

Deep eutectic solvents (DESs) have received considerable attention in recent years as new sustainable green media and some of their interesting properties have stimulated investigations on the microscopic solution structure, solute-solvent interactions and solute/solvation dynamics in these media. Even though the alcohol-based DESs, due to their low viscosity, serve as useful media in various applications, little is known about the structure and dynamics of these solvents. In order to obtain insight into the microscopic structure and interactions operating in these media, we have studied the rotational and translational diffusion dynamics of some carefully chosen molecular systems (both dipolar and nonpolar) using time-resolved fluorescence anisotropy and fluorescence correlation spectroscopy techniques in a series of choline chloride/alcohol based DESs differing in hydrocarbon chain length and positioning of the hydroxyl group on the hydrogen bond donor. The results reveal an increase of both spatial and dynamic heterogeneity upon an increase in chain length of one of the components of these solvents. No significant variation of heterogeneity, however, could be observed with the change in the hydroxyl group position. The analysis of the experimental results indicates that solute-solvent hydrogen-bonding interaction plays a dominant role in determining both rotational and translational diffusion dynamics of AP in these DESs.

Biexciton Generation and Dissociation Dynamics in Formamidinium- and Chloride-Doped Cesium Lead Iodide Perovskite Nanocrystals.

Recent studies show that perovskites (ABX3-type) comprising mixed A or B cation and/or mixed halide (X) are more stable and efficient materials for photovoltaic applications than their respective pure forms. Herein we report how doping of a small quantity of formamidinium and/or chloride ion influences the single and multiexciton dynamics of CsPbI3 nanocrystals (NCs). With the help of ultrafast pump-probe spectroscopic measurements, we show that chloride doping can enhance the biexciton lifetime of the system significantly by slowing down the Auger recombination (AR) process. The measured biexciton AR time scale (∼195-205 ps) in some of these NCs is the longest among those reported to date for any similar size perovskites. We further demonstrate that suppression of the AR rate and consequent lengthening of biexciton lifetime allow harvest of these species for their utilization through rapid (18-45 ps) electron transfer to fullerene. The insights obtained from this study are expected to help design more efficient doped perovskites for energy conversion purposes.

Broadband femtosecond nonlinear optical properties of CsPbBr3 perovskite nanocrystals.

We report the broadband nonlinear optical (NLO) properties of CsPbBr3 perovskite films achieved from colloidal nanocrystals prepared following a room temperature and open atmosphere anti-solvent precipitation method. The NLO studies were performed on the films of nanocubes (NCs) and nanorods (NRs) using the Z-scan technique with 1 kHz femtosecond pulses at 600, 700, and 800 nm. Large two-photon absorption cross sections (∼105 GM) were retrieved by fitting the open-aperture Z-scan data. Strong third-order NLO susceptibility (∼10-10 esu) was observed in these films. At higher peak intensities a switching of sign (in both NCs and NRs) in the real and imaginary parts of the NLO susceptibility was observed from the studies on these CsPbBr3 nanocrystals. The obtained NLO coefficients clearly suggest that these materials are promising for ultrafast photonic applications.

Ground- and Excited-State Interactions of a Psoralen Derivative with Human Telomeric G-Quadruplex DNA.

G-quadruplex DNA has been a recent target for anticancer agents, and its binding interactions with small molecules, often used as anticancer drugs, have become an important area of research. Considering that psoralens have long been studied in the context of duplex DNA but that very little is known about their potential as G-quadruplex binders and their excited-state interaction with the latter has not been explored, we have studied herein the binding of a planar water-soluble psoralen derivative, 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT), with the 22-mer human telomeric G-quadruplex-forming sequence, AGGG(TTAGGG)3, labeled here as (hTel22), and investigated the consequences of photoexcitation of AMT by calorimetric and spectroscopic techniques. The results show an enthalpy-driven 1:1 binding of AMT with hTel22 via end-stacking mode. Fluorescence quenching experiments on 6-fluorescein amidite-labeled oligomers indicate that the binding site is nearer to the 3' end of hTel22 in the diagonal loop region. Femtosecond time-resolved transient absorption measurements indicate electron transfer from the guanine moiety of hTel22 to photoexcited AMT, leading to the formation of a radical pair species (AMT•-G•+), which survives for 30 ps and is favored by a parallel/quasi-parallel orientation between the two. The findings reveal psoralens as a prospective class of compounds for the development of anticancer therapeutics by targeting the G-quadruplex DNA.

All-inorganic perovskite nanocrystal assisted extraction of hot electrons and biexcitons from photoexcited CdTe quantum dots.

Excitation of semiconductor quantum dots (QDs) by photons possessing energy higher than the band-gap creates a hot electron-hole pair, which releases its excess energy as waste heat or under certain conditions (when hν > 2Eg) produces multiple excitons. Extraction of these hot carriers and multiple excitons is one of the key strategies for enhancing the efficiency of QD-based photovoltaic devices. However, this is a difficult task as competing carrier cooling and relaxation of multiple excitons (through Auger recombination) are ultrafast processes. Herein, we study the potential of all-inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br) as harvesters of these short-lived species from photo-excited CdTe QDs. The femtosecond transient absorption measurements show CsPbX3 mediated extraction of both hot and thermalized electrons of the QDs (under a low pump power) and (under a high pump fluence) extraction of multiple excitons prior to their Auger assisted recombination. A faster timescale of thermalized electron transfer (∼2 ps) and a higher extraction efficiency of hot electrons (∼60%) are observed in the presence of CsPbBr3. These observations demonstrate the potential of all-inorganic perovskite NCs in the extraction of these short-lived energy rich species implying that complexes of the QDs and perovskite NCs are better suited for improving the efficiency of QD-sensitized solar cells.