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A generic method of obtaining APbX3 (A = Cs+, MA+ and FA+; X = Cl−, Br− and I−) perovskite NCs, emitting throughout the entire visible region (395–760 nm), has been developed employing 1,3-dihalo-5,5-dimethylhydantoin (DXDMH) as a halide precursor.

Cesium lead halide perovskite nanocrystals (NCs) are the most promising materials for photovoltaic and optoelectronic applications in recent years. However, lack of long-term stability and conseque...

Dehydroergosterol (DHE, Δ5,7,9(11),22-ergostatetraen-3β-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.

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.

The recent emergence of metal halide perovskites as triplet sensitizers has generated new possibilities in optoelectronics, the exploitation of which requires an understanding of the characteristic...

Doping is an important strategy for both introducing new properties and modifying the intrinsic properties of a semiconductor. In this work, we investigate Bi-doped CsPbBr3 nanocrystals (NCs), whic...

Despite their low band gap, the utility of CsPbI

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.

Transfer of the hot charge carriers prior to their cooling to the band-edge states can enhance the efficiency of a semiconductor-based solar cell much beyond its Shockley–Queisser (SQ) limiting val...

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.

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.

Stable green-emitting perovskite nanocrystals (NCs) with general formula APbBr3, exhibiting a high photoluminescence quantum yield (PLQY), narrow bandwidth, and adjustable PL in the 510–535 nm regi...

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.

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.

Defects have always been an integral part of semiconductor crystals, controlling their optical and electronic properties. Even though growing popularity of the CsPbX3 (X = Cl, Br, I, and their mixt...

Among the lead halide perovskites, photoluminescence quantum yield (PLQY) of violet-emitting CsPbCl3 nanocrystals (NCs) is the lowest (

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.

The lead halide-based hybrid and all-inorganic perovskites are currently the focus of intense investigation because of their great potential in solar photovoltaic and light emitting applications.1 As utilization of full potential of these substances in different applications requires a thorough understanding of the pathways through which the photo-generated charge carriers relax and the rate of their relaxation and capture by other substances prior to recombination, we have been studying these substances for the last few years addressing a variety of issues including those mentioned above.2-9 This talk will focus on femtosecond pump-probe and single-particle photoluminescence studies that provide important information on the nature and energy states of the photo-generated species, pathways and dynamics of relaxation of the charge carriers, location and nature of the trap states and their role in fluctuation of the photoluminescence of different perovskite nanocrystals. References Dey, A. et. al. ACS Nano 2021, 15, 10775. Mondal, N; De, A.; Das, S.; Paul, S.; Samanta, A. Nanoscale 2019, 4, 9796. Mondal, N; De, A.; Samanta, A. ACS Energy Lett. 2019, 4, 32. Seth, S.; Ahmed, T.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 7007. Mondal, N; De, A.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 3673. Ahmed, T; Seth. S.; Samanta, A. ACS Nano 2019, 13, 13537. Paul, S.; Samanta, A. ACS Energy Lett. 2020, 5, 64. Das, S.; De, A.; Samanta, A. J. Phys. Chem. Lett. 2020, 11, 1178. Das, S.; Samanta, A. ACS Energy Lett. 2021, 6, 3780.

Dehydroergosterol (DHE, Δ

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG 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. Ministerio de Ciencia e Innovación | Ref. RYC2018-026103-I Agencia Estatal de Investigación | Ref. CTQ2017-82711-P Ministerio de Economía y Competitividad | Ref. MDM-2015-0538 Agencia Estatal de Investigación | Ref. PID2019-107314RB-I00

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

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 CsPbX

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 Forster 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.

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 t...

While perovskite nanocrystals (NCs) have shown great promise as materials for efficient light-emitting diodes (LEDs), low photoluminescence quantum yield (PLQY) of the blue-emitting perovskites is an impediment to the development of white LEDs of which blue is an essential component. Herein, we report that room temperature postsynthetic treatment of weakly blue-violet-emitting (PLQY 3%) CsPbCl3 NCs with CdCl2 results in an instantaneous enhancement of the PLQY to near-unity without affecting the PL peak position (406 nm) and spectral width. The time-resolved PL and ultrafast transient absorption measurements confirm the removal of nonradiative defect states of the CsPbCl3 NCs in treated sample. The elemental composition and structural data of the treated sample reveal facile doping of Cd2+ into the crystal lattice without affecting the size and shape of the NCs. Extraordinary PLQY, high air stability and photostability and ease of preparation of this Cd-doped CsPbCl3 make it by far the most attractive blu...

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.

The lead halide perovskites have emerged as promising materials for photovoltaic applications within a very short period. The key to exploitation of the full potential of these substances by achieving even higher photovoltaic conversion efficiency lies in understanding of the charge (electron/hole) separation process and their transfer across the interface. As the latter depends on the nature of the electron/hole acceptor, quest for suitable carrier acceptor is an active area in this context. Herein, we explore the hole extraction ability of 1-ampinopyrene (AMP) from two very common all-inorganic perovskite nanocrystals (NCs), CsPbBr3 and CsPbI3, with different band gaps. The observation of efficient quenching of the photoluminescence (PL) of the NCs with little/negligible change in PL time-profile in the presence of AMP indicates static interaction of the two interacting species. The band alignment of the NCs relative to the HOMO–LUMO energy levels of AMP and ultrafast pump–probe measurements reveal rapi...

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.

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, ...

DNA dynamics, to which water, counterions, and DNA motions contribute, is a topic of considerable interest because it is closely related to the efficiency of biological functions performed by it. Simulation studies and experiments suggest that the counterion dynamics in DNA probed by a minor-groove binder are similar for various monovalent counterions. To date, the influence on DNA dynamics of higher-valence counterions, which are also present around DNA and are known to bind more strongly to it than monovalent ions, has not been studied. Herein we investigated DNA dynamics in the presence of Mg2+ and Ca2+ , chosen for their relative abundance in cells, by using minor-groove binder 4',6-diamidino-2-phenylindole (DAPI) as a fluorescence probe. The dynamics, as measured from the time-resolved fluorescence Stokes shifts of DAPI bound to calf thymus DNA on a subpicosecond-to-nanosecond timescale, were found to be very similar in the presence of both the divalent ions and Na+ ions. The observation is explained by considering the screening of the electric field of the divalent ion by its hydration shell, preferential binding of the ions to the phosphate groups, and displacement of ions from the minor groove by DAPI due to the stronger binding interaction of the latter. Furthermore, the similarity of our results in the presence of Na+ to those reported for smaller oligonucleotides suggests that the chain length of DNA does not influence the DNA dynamics.

Understanding the nature and dynamics of the photo-induced transients of all-inorganic perovskite nanocrystals (NCs) is key to their exploitation in potential applications. In order to determine the nature of charge carriers, their deactivation pathways and dynamics, the photo-induced transients of CsPbBr3, CsPbBr2I, CsPbBr1.5I1.5 and CsPbI3 NCs are spectrally and temporally characterized employing a combination of femtosecond transient absorption (TA) and photoluminescence (PL) up-conversion techniques and global analysis of the data. The results provide distinct identities of the excitons and free charge carriers and distinguish the hot charge carriers from the cold ones. The carrier trapping is attributed to the electrons and their dynamics is unaffected in mixed halide perovskites. The excitation energy dependence of the TA dynamics suggests that the trap states are shallow in nature and mainly limited near the band-edge level. In mixed halide perovskites, an increase in the iodine content leads to hole trapping in a short time scale (

Mn-Doped perovskite nanocrystals (NCs) are a new class of materials offering exciting opportunities to control over their optical and magnetic properties. Herein, we report a series of Mn-doped CsPbCl3 NCs exhibiting a tunable Mn photoluminescence (PL) band with a PL peak wavelength pushed up to 625 nm and tuned over a range of 40 nm, the largest achieved so far, by only varying the Mn content. The X-band EPR data and Mn PL decay behaviour of the NCs reveal that the exchange interaction between Mn2+ ions is mainly responsible for a large shift of the Mn PL band. Ultrafast pump–probe measurements show that exciton–dopant energy transfer in these NCs is slower (∼50–100 ps) than trapping of the carriers (∼8–10 ps) in the host lattice. The large PL tuning reported here along with the insights into the mechanism of tuning and carrier dynamics are expected to boost the potential of Mn-doped CsPbCl3 NCs in light-powered devices.

Surface protection using an appropriate ligand is essential for controlling the size, stability and luminescence properties of the quantum dots (QDs). Though 3-mercaptopropanoic acid (3-MPA) is regarded as the most suitable protecting ligand among the mercapto acids for water soluble CdTe QDs, one receives a different picture from recent studies, which report a much higher luminescence efficiency of 3-mercaptobutyric acid (3-MBA) capped QDs compared with those capped by 3-MPA and attribute the observation to the influence of the side methyl group of mercapto acids. Herein we report the luminescence properties and carrier trapping dynamics of four different, but structurally related mercapto acid capped CdTe QDs prepared using a different method. The results show that these QDs are much more fluorescent than those prepared directly in an aqueous environment and surprisingly, no enhanced luminescence for the QDs capped by mercapto acids containing a side methyl group is observed. Ultrafast pump-probe measurements confirm these results in addition to providing insight into the carrier trapping dynamics of these systems. It is shown that our findings, which appear to be in conflict with the recent literature, can be rationalized and the exact role of the side methyl group of the mercapto acids can be understood by careful analysis of the results taking into consideration the difference in the methods of preparation of the QDs in the two cases.

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.

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 thi...

The lead halide perovskites have emerged in recent years as the most promising materials in photovoltaic and photonic applications.1,2 As utilization of the full potential of these materials in these applications requires a thorough understanding of the pathways through which the photo-generated charge carriers relax and the rate of their relaxation and capture by other substances prior to recombination, we have been investigating the dynamics of charge carrier recombination and extraction in a variety of caesium lead halide perovskite (CsPbX3) nanocrystals employing femtosecond pump-probe, fluorescence upconversion and single-particle fluorescence techniques. In this talk, some of our recent studies, which throw important insight on these topics, will be presented.3-8 References: Green, M.A.; Ho-Baillie, A.; Snaith, H.J. Nature Photonics 2014, 8, 506. Yakunin, S.; Protesescu, L.; Krieg, F.; Badnarchuk, M.I.; Nedelcu, G.; Humer, M.; Luca, G.D.; Fiebig, M.; Heiss, W.; Kovalenko, M.V. Nature Communications 2015 , 6, 8056. Mondal, N; De, A.; Das, S.; Paul, S.; Samanta, A. Nanoscale 2019, 4, 9796. Mondal, N; De, A.; Samanta, A. ACS Energy Letts. 2019, 4, 32. Seth, S.; Ahmed, T.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 7007. Mondal, N; De, A.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 3673. Ahmed, T; Seth, S; Samanta, A. ACS Nano 2019, 13, 13537. De, A.; Das, S.; Samanta, A. ACS Energy Letts. 2019, 4, 32.

Two novel phenanthro [9,10-d]imidazole substituted metal phthalocyanines, ImCuPc and ImZnPc, have been synthesized and their potential as photosensitizers has been investigated through optical, electrochemical, and ultrafast nonlinear optical (NLO) studies, including their excited state dynamics. The absorption as well as emission spectra of both the title compounds exhibited negligible Stokes shift in different solvents. The molecules depicted a broad Soret band due to the incorporation of phenanthro[9,10-d]imidazole moiety while the Q-bands were characteristic to their metal phthalocyanine group. DFT studies elucidated the HOMO-LUMO (4.99–2.90 eV) levels of these molecules with their energy optimized structures generated using DFT, TD-DFT analysis. The fs-TAS spectra of both the molecules exhibited contribution from singlet-singlet excited state absorption followed by transition to triplet states via intersystem crossing, ISC. The lifetimes calculated from obtained kinetics at specific wavelengths using global fitting demonstrated slow singlet to triplet state intersystem crossing in ImCuPc (decay rate of 1.6 × 108 s−1) and ImZnPc (decay rate of 1.4 × 108s−1). The ns-TAS studies established long lived triplet states in both the molecules with ImCuPc depicting a slightly higher triplet lifetime of ~1.4 μs over ImZnPc with a triplet lifetime of ~1.2 μs The triplet quantum yields (φT) were calculated to be 0.51 for ImCuPc while it was 0.27 for ImZnPc suggesting ImCuPc to be a better photosensitizer (PS) over ImZnPc. We believe the intramolecular D-π-A interaction is stronger in ImCuPc resulting in higher triplet yield. Third-order NLO studies performed at a non-resonant excitation of 800 nm (~50 fs, 1 kHz pulses) demonstrated ImZnPc had a larger two-photon absorption cross-section (994 GM), possibly due to heavy atom effect, while ImCuPc was found to possess superior n2 (3.7 × 10−15 cm2/W). The slightly superior nonlinear optical (NLO) performance of ImCuPc over ImZnPc could also possibly be due to lower energy gap and Copper's open 3d shells as opposed to full 3d shell in Zinc which overlaps with Pc 2p shells making it more active.

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.

In order to explore the potential of nanocomposites comprising semiconductor quantum dots (QDs) and metal nanoclusters (NCs) in photovoltaic and catalytic applications, the interaction between CdTe QDs and gold NCs, Au10 and Au25, stabilized by histidine, bovine serum albumin (BSA) and glutathione, is studied by an ultrafast transient absorption (TA) technique. Temporal and spectral studies of the transients reveal photoinduced 2-way electron transfer between the two constituents of the nanocomposites, where Au NCs, which generally act as electron donors when used as photosensitizers, perform the role of the efficient electron acceptor. Interestingly, it is found that the electron transfer dynamics in these composites is governed not by the distance of separation of the constituents but by the nature of the surface capping ligands. Despite a large separation between the QDs and NCs in a giant BSA-capped system, a higher electron transfer rate in this composite suggests that unlike other smaller capping agents, which act more like insulators, BSA allows much better electron conduction, as indicated previously.

Because of their potential in photovoltaic and photonic applications, great attention is being paid to both hybrid and all-inorganic lead halide perovskite nanocrystals.1,2 However, despite growing interests on these substances, our understanding of different pathways of relaxation of the photo-generated charge carriers and their dynamics, which is key to utilization of the full potential of these materials in various applications, is still quite limited. In this talk, we plan to discuss some of our recent works on charge carrier recombination dynamics of caesium lead halide perovskite (CsPbX3) nanocrystals highlighting how femtosecond pump-probe and single-particle fluorescence based techniques can be effectively employed to obtain information on the identity and energy states of the photogenerated species, pathways and dynamics of relaxation of the charge carriers (including their recombination), location and nature of the trap states and their role in fluctuation of the photoluminescence of different CsPbX3 perovskite nanocrystals.3-7 References: 1. Green, M.A.; Ho-Baillie, A.; Snaith, H.J. Nature Photonics 2014, 8, 506. 2. Yakunin, S.; Protesescu, L.; Krieg, F.; Badnarchuk, M.I.; Nedelcu, G.; Humer, M.; Luca, G.D.; Fiebig, M.; Heiss, W.; Kovalenko, M.V. Nature Communications 2015 , 6, 8056. 3. Mondal, N; De, A.; Das, S.; Paul, S.; Samanta, A. Nanoscale 2019, 4, 9796. 4. Mondal, N; De, A.; Samanta, A. ACS Energy Letts. 2019, 4, 32. 5. Seth, S.; Ahmed, T.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 7007. 6. Mondal, N; De, A.; Samanta, A. J. Phys. Chem. Lett. 2018, 9, 3673. 7. Ahmed, T; Seth, S; Samanta, A. ACS Nano DOI: 10.1021/acsnano.9b07471.