Are Shockley-Read-Hall and ABC models valid for lead halide perovskites?

Metal halide perovskites are an important class of emerging semiconductors. Their charge carrier dynamics is poorly understood due to limited knowledge of defect physics and charge carrier recombination mechanisms. Nevertheless, classical ABC and Shockley-Read-Hall (SRH) models are ubiquitously applied to perovskites without considering their validity. Herein, an advanced technique mapping photoluminescence quantum yield (PLQY) as a function of both the excitation pulse energy and repetition frequency is developed and employed to examine the validity of these models. While ABC and SRH fail to explain the charge dynamics in a broad range of conditions, the addition of Auger recombination and trapping to the SRH model enables a quantitative fitting of PLQY maps and low-power PL decay kinetics, and extracting trap concentrations and efficacies. However, PL kinetics at high power are too fast and cannot be explained. The proposed PLQY mapping technique is ideal for a comprehensive testing of theories and applicable to any semiconductor.

Highly efficient energy transfer from a water soluble zinc silver indium sulphide quantum dot to organic J-aggregates.

The present work has been carried out with the aim to design and develop an efficient light harvesting inorganic-organic hybrid nanoscale material by employing a less toxic, environment friendly inorganic substance and also to understand the mechanism of inter-particle electronic interaction between the inorganic and organic components of the nanomaterial. Specifically, the inorganic-organic hybrid associate has been made by integrating water soluble semiconductor (zinc-silver-indium-sulfide (ZAIS)) QDs and organic J-aggregates of a cyanine dye (S2165). The fabrication of the present nano-hybrid system has been achieved via electrostatically driven self-assembly of organic dyes over ZAIS QDs. The interaction between QD and J-aggregates has been investigated by using steady state and time resolved fluorescence measurements. Zeta potential measurements have also been performed to understand the role of electrostatic interaction and thermodynamic feasibility of the association process. The investigations have revealed that the energy transfer (ET) process between QD and J-aggregates was mediated through a dipole-dipole mechanism. Interestingly, data analysis based on Förster theory has further revealed that the ET from QD to J-aggregates is very high, indicating efficient electronic coupling between the inorganic QD and the organic J-aggregates. Zeta potential measurements and thermodynamic calculations have demonstrated that the interaction between QD and organic dye is electrostatically driven and the association of organic dyes over QDs is thermodynamically feasible. The outcome of the present study is expected to be helpful in designing efficient nanoscale light harvesting devices. Additionally, fluorescence microscopy and toxicity studies on the QDs have also shown their suitability for biological applications.

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.

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.

Differences in the behavior of dicationic and monocationic ionic liquids as revealed by time resolved-fluorescence, NMR and fluorescence correlation spectroscopy.

With an aim to understand the behavior in terms of the intermolecular interactions, structure and dynamics of dicationic and monocationic ionic liquids (ILs), two imidazolium-based dicationic ionic liquids (DILs), 1,8-bis-(3-methylimidazolium-1-yl)octane bis-(trifluoromethylsulfonyl)amide ([C8(mim)2][NTf2]2), 1,9-bis-(3-methylimidazolium-1-yl)nonane bis-(trifluoromethylsulfonyl)amide ([C9(mim)2][NTf2]2), and one monocationic ionic liquid (MIL), 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C4(mim)][NTf2]), have been investigated through combined fluorescence, electron paramagnetic resonance (EPR), NMR and fluorescence correlation spectroscopy (FCS). The DILs were synthesized by following a standard synthetic protocol and subsequently characterized by different analytical techniques. Steady state absorption, emission and EPR spectroscopic data reveal that DILs are less polar compared to MIL. The polarities of the DILs and MIL were found to be close to those of acetonitrile and short chain alcohols, respectively. The excitation wavelength dependent emission data reveals that DILs are more micro-heterogeneous in nature than MIL. The rotational diffusion of two organic solutes, perylene and 8-methoxypyrene-1,3,6-sulfonate (MPTS), were examined in the DILs and MIL. The rotational diffusion data for perylene and MPTS were analyzed in light of the Stokes-Einstein-Debye (SED) hydrodynamic theory. The rotation of perylene in the DILs was observed to be relatively faster to that in the MIL, and it goes beyond the limit predicted by the SED theory. In order to explain the rotational motion of perylene in DILs, the data was analyzed further by invoking quasi-hydrodynamic theory. The observed rotational behavior of perylene has been explained by considering the fact that perylene is located in the nonpolar region of ILs, and larger solvent molecules (DILs) induce a lower friction to the rotating solute. Interestingly, unlike perylene, rotations of MPTS in both of the ILs were observed to be much hindered indicating a relatively stronger MPTS-IL interaction than perylene-IL interaction. More interestingly, rotation of MPTS was observed to be faster in the DILs than that in the MIL despite the fact that DILs are more viscous than MILs. Relatively faster rotation of MPTS in DILs has been explained by resorting to NMR and FCS studies. The outcomes of the NMR and FCS studies revealed that DILs in the experimental condition exist in their folded form and because of this structural restriction of DILs it becomes difficult for the bulky MPTS to make stronger hydrogen bonding interactions with DILs, which eventually makes the rotation of MPTS in DILs faster. Essentially, the outcomes of all of these studies have demonstrated that the behavior of DILs is quite different to that of the usual MILs.

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.

Photoluminescence of Zero-Dimensional Perovskites and Perovskite-Related Materials.

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.

Fluorescent Phase-Pure Zero-Dimensional Perovskite-Related Cs4PbBr6 Microdisks: Synthesis and Single-Particle Imaging Study.

Quantum-confined perovskites are a new class of promising materials in optoelectronic applications. In this context, a zero-dimensional perovskite-related substance, Cs4PbBr6, 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 Cs4PbBr6 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 Cs4PbBr6 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 Cs4PbBr6 and suggest its possible origin, brighten the potential of these materials in photon-emitting applications.

A Facile Methodology for Engineering the Morphology of CsPbX3 Perovskite Nanocrystals under Ambient Condition.

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.

Probing the interactions of structurally similar but chemically distinguishable organic solutes with 1-ethyl-3-methylimidazolium alkyl sulfate (alkyl = ethyl, hexyl and octyl) ionic liquids through fluorescence, NMR and fluorescence correlation spectroscopy (FCS) studies.

The rotational diffusion of two organic solutes, 4-(azitidine-1-yl)-7-nitrobenzo[1,2,5]oxadi-azole (ANBD) and 7-nitrobenzo[1,2,5]oxadiazole-4-amine (HNBD), has been examined in 1-ethyl-3-methylimidazolium alkyl sulfate (alkyl = ethyl, hexyl and octyl) ionic liquids with the aim of understanding intermolecular interactions among solute and solvent molecules. Solute-solvent interaction has also been investigated by employing fluorescence, NMR and fluorescence correlation spectroscopy (FCS). The ionic liquids (ILs) having a fixed cationic moiety are chosen to monitor the roles of the alkyl chain length as well as the solute-anion (sulfate) interactions in the rotational diffusion of the solutes in the given ILs. In the present study, two chemically distinguishable solutes with structural similarity in their fluorophore unit are employed so that the difference in the rotational behavior of the solutes, if any, can be correlated with the solute-solvent interactions, and not with other factors like the size of the solutes etc. The steady state absorption and emission data indicate that HNBD and ANBD exist in different microenvironments in both ILs. The rotational diffusion data for both the solutes have been analyzed in light of hydrodynamic and quasi hydrodynamic theories. Interestingly, the rotational dynamics of ANBD is observed to be relatively fast and shows slip hydrodynamics, whereas HNBD exhibits hindered rotation with superstick behavior. The hindered rotation of HNBD as compared to that of ANBD in both ILs has been explained by considering the strong hydrogen bonding interactions between the two N-H groups of HNBD and sulfate anions of ILs. The faster rotation of ANBD with the increasing anion chain length has been explained by invoking quasi hydrodynamic theory. The presence of a specific hydrogen bonding interaction between solute (HNBD) and solvent (ILs) molecules has been confirmed by (1)H-NMR experiments. FCS measurements have independently demonstrated the relatively strong association of HNBD with the present ILs as compared to that of ANBD.

Fluorescence Blinking and Photoactivation of All-Inorganic Perovskite Nanocrystals CsPbBr3 and CsPbBr2I.

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.

Effect of Controlled Deposition of ZnS Shell on the Photostability of CdTe Quantum Dots as Studied by Conventional Fluorescence and FCS Techniques.

The effect of one and two monolayers of ZnS shells on the photostability of CdTe quantum dots (QDs) in aqueous and nonaqueous media has been studied by monitoring the fluorescence behavior of the QDs under ensemble and single-molecule conditions. ZnS capping of the CdTe QDs leads to significant enhancement of the fluorescence brightness of these QDs. Considerable enhancement of the photostability of the shell-protected QDs, including the suppression of photoactivation, is also observed. Fluorescence correlation spectroscopy measurements reveal an increase in the number of particles undergoing reversible fluorescent on-off transitions in the volume under observation with increasing excitation power; this effect is found to be more pronounced in the case of core-only QDs than for core-shell QDs.

Contrasting Response of Two Dipolar Fluorescence Probes in a Leucine-Based Organogel and Its Implications.

The microenvironments of a leucine-based organogel are probed by monitoring the fluorescence behavior of coumarin 153 (C153) and 4-aminophthalimide (AP). The steady-state data reveals distinctly different locations of the two molecules in the gel. Whereas AP resides close to the hydroxyl moieties of the gelator and engages in hydrogen-bonding interactions, C153 is found in bulk-toluene-like regions. In contrast to C153, AP exhibits excitation-wavelength-dependent emission, indicating that the environments of the hydrogen-bonded AP molecules are not all identical. A two-component fluorescence decay of AP in gel, unlike C153, supports this model. A time-resolved fluorescence anisotropy study of the rotational motion of the molecules also reveals the strong association of only AP with the gelator. That AP influences the critical gelation concentration implies its direct involvement in the gel-formation process. The results highlight the importance of guest-gelator interactions in gels containing guest molecules.