Resolving the Emission Transition Dipole Moments of Single Doubly Excited Seeded Nanorods via Heralded Defocused Imaging.

Semiconductor nanocrystal emission polarization is a crucial probe of nanocrystal physics and an essential factor for nanocrystal-based technologies. While the transition dipole moment for the lowest excited state to ground state transition is well characterized, the dipole moment of higher multiexcitonic transitions is inaccessible via most spectroscopy techniques. Here, we realize direct characterization of the doubly excited-state relaxation transition dipole by heralded defocused imaging. Defocused imaging maps the dipole emission pattern onto a fast single-photon avalanche diode detector array, allowing the postselection of photon pairs emitted from the biexciton-exciton emission cascade and resolving the differences in transition dipole moments. Type-I1/2 seeded nanorods exhibit higher anisotropy of the biexciton-to-exciton transition compared to the exciton-to-ground state transition. In contrast, type-II seeded nanorods display a reduction of biexciton emission anisotropy. These findings are rationalized in terms of an interplay between the transient dynamics of the refractive index and the excitonic fine structure.

The surface chemistry of ionic liquid-treated CsPbBr3 quantum dots.

The power conversion efficiencies of lead halide perovskite thin film solar cells have surged in the short time since their inception. Compounds, such as ionic liquids (ILs), have been explored as chemical additives and interface modifiers in perovskite solar cells, contributing to the rapid increase in cell efficiencies. However, due to the small surface area-to-volume ratio of the large grained polycrystalline halide perovskite films, an atomistic understanding of the interaction between ILs and perovskite surfaces is limited. Here, we use quantum dots (QDs) to study the coordinative surface interaction between phosphonium-based ILs and CsPbBr3. When native oleylammonium oleate ligands are exchanged off the QD surface with the phosphonium cation as well as the IL anion, a threefold increase in photoluminescent quantum yield of as-synthesized QDs is observed. The CsPbBr3 QD structure, shape, and size remain unchanged after ligand exchange, indicating only a surface ligand interaction at approximately equimolar additions of the IL. Increased concentrations of the IL lead to a disadvantageous phase change and a concomitant decrease in photoluminescent quantum yields. Valuable information regarding the coordinative interaction between certain ILs and lead halide perovskites has been elucidated and can be used for informed pairing of beneficial combinations of IL cations and anions.

Effect of Surface Ligands in Perovskite Nanocrystals: Extending in and Reaching out.

ConspectusThe rediscovery of the halide perovskite class of compounds and, in particular, the organic and inorganic lead halide perovskite (LHP) materials and lead-free derivatives has reached remarkable landmarks in numerous applications. First among these is the field of photovoltaics, which is at the core of today's environmental sustainability efforts. Indeed, these efforts have born fruit, reaching to date a remarkable power conversion efficiency of 25.2% for a double-cation Cs, FA lead halide thin film device. Other applications include light and particle detectors as well as lighting. However, chemical and thermal degradation issues prevent perovskite-based devices and particularly photovoltaic modules from reaching the market. The soft ionic nature of LHPs makes these materials susceptible to delicate changes in the chemical environment. Therefore, control over their interface properties plays a critical role in maintaining their stability. Here we focus on LHP nanocrystals, where surface termination by ligands determines not only the stability of the material but also the crystallographic phase and crystal habit. A surface analysis of nanocrystal interfaces revealed the involvement of Brønsted type acid-base equilibrium in the modification of the ligand moieties present, which in turn can invoke dissolution and recrystallization into the more favorable phase in terms of minimization of the surface energy. A large library of surface ligands has already been developed showing both good chemical stability and good electronic surface passivation, resulting in near-unity emission quantum yields for some materials, particularly CsPbBr3. However, most of those ligands have a large organic tail hampering charge carrier transport and extraction in nanocrystal-based solid films.The unique perovskite structure that allows ligand substitution in the surface A (cation) sites and the soft ionic nature is expected to allow the accommodation of large dipoles across the perovskite crystal. This was shown to facilitate electron transfer across a molecular linked single-particle junction, creating a large built-in field across the junction nanodomains. This strategy could be useful for implementing LHP NCs in a p-n junction photovoltaic configuration as well as for a variety of electronic devices. A better understanding of the surface propeties of LHP nanocrystals will also enable better control of their growth on surfaces and in confined volumes, such as those afforded by metal-organic frameworks, zeolites, or chemically patterened surfaces such as anodic alumina, which have already been shown to significantly alter the properties of in-situ-grown LHP materials.

Heralded Spectroscopy Reveals Exciton-Exciton Correlations in Single Colloidal Quantum Dots.

Multiply excited states in semiconductor quantum dots feature intriguing physics and play a crucial role in nanocrystal-based technologies. While photoluminescence provides a natural probe to investigate these states, room-temperature single-particle spectroscopy of their emission has proved elusive due to the temporal and spectral overlap with emission from the singly excited and charged states. Here, we introduce biexciton heralded spectroscopy enabled by a single-photon avalanche diode array based spectrometer. This allows us to directly observe biexciton-exciton emission cascades and measure the biexciton binding energy of single quantum dots at room temperature, even though it is well below the scale of thermal broadening and spectral diffusion. Furthermore, we uncover correlations hitherto masked in ensembles of the biexciton binding energy with both charge-carrier confinement and fluctuations of the local electrostatic potential. Heralded spectroscopy has the potential of greatly extending our understanding of charge-carrier dynamics in multielectron systems and of parallelization of quantum optics protocols.

Directing the Morphology, Packing, and Properties of Chiral Metal-Organic Frameworks by Cation Exchange.

We show that metal-organic frameworks, based on tetrahedral pyridyl ligands, can be used as a morphological and structural template to form a series of isostructural crystals having different metal ions and properties. An iterative crystal-to-crystal conversion has been demonstrated by consecutive cation exchanges. The primary manganese-based crystals are characterized by an uncommon space group (P622). The packing includes chiral channels that can mediate the cation exchange, as indicated by energy-dispersive X-ray spectroscopy on microtome-sectioned crystals. The observed cation exchange is in excellent agreement with the Irving-Williams series (MnZn) associated with the relative stability of the resulting coordination nodes. Furthermore, we demonstrate how the metal cation controls the optical and magnetic properties. The crystals maintain their morphology, allowing a quantitative comparison of their properties at both the ensemble and single-crystal level.

cSPARCOM: Multi-detector reconstruction by confocal super-resolution correlation microscopy.

Image scanning microscopy (ISM), an upgraded successor of the ubiquitous confocal microscope, facilitates up to two-fold improvement in lateral resolution, and has become an indispensable element in the toolbox of the bio-imaging community. Recently, super-resolution optical fluctuation image scanning microscopy (SOFISM) integrated the analysis of intensity-fluctuations information into the basic ISM architecture, to enhance its resolving power. Both of these techniques typically rely on pixel-reassignment as a fundamental processing step, in which the parallax of different detector elements to the sample is compensated by laterally shifting the point spread function (PSF). Here, we propose an alternative analysis approach, based on the recent high-performing sparsity-based super-resolution correlation microscopy (SPARCOM) method. Through measurements of DNA origami nano-rulers and fixed cells labeled with organic dye, we experimentally show that confocal SPARCOM (cSPARCOM), which circumvents pixel-reassignment altogether, provides enhanced resolution compared to pixel-reassigned based analysis. Thus, cSPARCOM further promotes the effectiveness of ISM, and particularly that of correlation based ISM implementations such as SOFISM, where the PSF deviates significantly from spatial invariance.

Measuring the optical properties of nanoscale biogenic spherulites.

Recent studies of optical reflectors as part of the vision apparatus in the eyes of decapod crustaceans revealed assemblies of nanoscale spherulites - spherical core-shell nanoparticles with radial birefringence. Simulations performed on the system highlighted the advantages of optical anisotropy in enhancing the functionality of these structures. So far, calculations of the nanoparticle optical properties have relied on refractive indices obtained using ab-initio calculations. Here we describe a direct measurement of the tangential refractive index of the spherulites, which corresponds to the in-plane refractive index of crystalline isoxanthopterin nanoplatelets. We utilize measurements of scattering spectra of individual spherulites and determine the refractive index by analyzing the spectral signatures of scattering resonances. Our measurements yield a median tangential refractive index of 1.88, which is in reasonable agreement with theoretical predictions. Furthermore, our results indicate that the optical properties of small spherulite assemblies are largely determined by the tangential index.

Optically functional isoxanthopterin crystals in the mirrored eyes of decapod crustaceans.

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.

Chiral and SHG-Active Metal-Organic Frameworks Formed in Solution and on Surfaces: Uniformity, Morphology Control, Oriented Growth, and Postassembly Functionalization.

We demonstrate the formation of uniform and oriented metal-organic frameworks using a combination of anion effects and surface chemistry. Subtle but significant morphological changes result from the nature of the coordinative counteranion of the following metal salts: NiX2 with X = Br-, Cl-, NO3-, and OAc-. Crystals could be obtained in solution or by template surface growth. The latter results in truncated crystals that resemble a half structure of the solution-grown ones. The oriented surface-bound metal-organic frameworks (sMOFs) are obtained via a one-step solvothermal approach rather than in a layer-by-layer approach. The MOFs are grown on Si/SiOx substrates modified with an organic monolayer or on glass substrates covered with a transparent conductive oxide (TCO). Regardless of the different morphologies, the crystallographic packing is nearly identical and is not affected by the type of anion or by solution versus the surface chemistry. A propeller-type arrangement of the nonchiral ligands around the metal center affords a chiral structure with two geometrically different helical channels in a 2:1 ratio with the same handedness. To demonstrate the accessibility and porosity of the macroscopically oriented channels, a chromophore (resorufin sodium salt) was successfully embedded into the channels of the crystals by diffusion from solution, resulting in fluorescent crystals. These "colored" crystals displayed polarized emission (red) with a high polarization ratio because of the alignment of these dyes imposed by the crystallographic structure. A second-harmonic generation (SHG) study revealed Kleinman symmetry-forbidden nonlinear optical properties. These surface-bound and oriented SHG-active MOFs have the potential for use as single nonlinear optical (NLO) devices.

Evidence for laser-induced homogeneous oriented ice nucleation revealed via pulsed x-ray diffraction.

The induction of homogeneous and oriented ice nucleation has to date not been achieved. Here, we report induced nucleation of ice from millimeter sized supercooled water drops illuminated by ns-optical laser pulses well below the ionization threshold making use of particular laser beam configurations and polarizations. Employing a 100 ps synchrotron x-ray pulse 100 ns after each laser pulse, an unambiguous correlation was observed between the directions and the symmetry of the laser fields and that of the H-bonding arrays of the induced ice crystals. Moreover, an analysis of the x-ray diffraction data indicates that, in the main, the induced nucleation of ice is homogeneous at temperatures well above the observed and predicted values for supercooled water.

Tetragonal CH3NH3PbI3 is ferroelectric.

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that the material's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

Higher-Order Photon Correlation as a Tool To Study Exciton Dynamics in Quasi-2D Nanoplatelets.

Colloidal semiconductor nanoplatelets, in which carriers are strongly confined only along one dimension, present fundamentally different excitonic properties than quantum dots, which support strong confinement in all three dimensions. In particular, multiple excitons strongly confined in just one dimension are free to rearrange in the lateral plane, reducing the probability for multibody collisions. Thus, while simultaneous multiple photon emission is typically quenched in quantum dots, in nanoplatelets its probability can be tuned according to size and shape. Here, we focus on analyzing multiexciton dynamics in individual CdSe/CdS nanoplatelets of various sizes through the measurement of second-, third-, and fourth-order photon correlations. For the first time, we can directly probe the dynamics of the two, three, and four exciton states at the single nanocrystal level. Remarkably, although higher orders of correlation vary substantially among the synthesis' products, they strongly correlate with the value of second order antibunching. The scaling of the higher-order moments with the degree of antibunching presents a small yet clear deviation from the accepted model of Auger recombination through binary collisions. Such a deviation suggests that many-body contributions are present already at the level of triexcitons. These findings highlight the benefit of high-order photon correlation spectroscopy as a technique to study multiexciton dynamics in colloidal semiconductor nanocrystals.

Excitation and Emission Transition Dipoles of Type-II Semiconductor Nanorods.

The mechanisms of exciton generation and recombination in semiconductor nanocrystals are crucial to the understanding of their photophysics and for their application in nearly all fields. While many studies have been focused on type-I heterojunction nanocrystals, the photophysics of type-II nanorods, where the hole is located in the core and the electron is located in the shell of the nanorod, remain largely unexplored. In this work, by scanning single nanorods through the focal spot of radially and azimuthally polarized laser beams and by comparing the measured excitation patterns with a theoretical model, we determine the dimensionality of the excitation transition dipole of single type-II nanorods. Additionally, by recording defocused patterns of the emission of the same particles, we measure their emission transition dipoles. The combination of these techniques allows us to unambiguously deduce the dimensionality and orientation of both excitation and emission transition dipoles of single type-II semiconductor nanorods. The results show that in contrast to previously studied quantum emitters, the particles possess a 3D degenerate excitation and a fixed linear emission transition dipole.

Mineral Deposits in Ficus Leaves: Morphologies and Locations in Relation to Function.

Ficus trees are adapted to diverse environments and have some of the highest rates of photosynthesis among trees. Ficus leaves can deposit one or more of the three major mineral types found in leaves: amorphous calcium carbonate cystoliths, calcium oxalates, and silica phytoliths. In order to better understand the functions of these minerals and the control that the leaf exerts over mineral deposition, we investigated leaves from 10 Ficus species from vastly different environments (Rehovot, Israel; Bologna, Italy; Issa Valley, Tanzania; and Ngogo, Uganda). We identified the mineral locations in the soft tissues, the relative distributions of the minerals, and mineral volume contents using microcomputed tomography. Each Ficus species is characterized by a unique 3D mineral distribution that is preserved in different environments. The mineral distribution patterns are generally different on the adaxial and abaxial sides of the leaf. All species examined have abundant calcium oxalate deposits around the veins. We used micromodulated fluorimetry to examine the effect of cystoliths on photosynthetic efficiency in two species having cystoliths abaxially and adaxially (Ficusmicrocarpa) or only abaxially (Ficuscarica). In F. microcarpa, both adaxial and abaxial cystoliths efficiently contributed to light redistribution inside the leaf and, hence, increased photosynthetic efficiency, whereas in F. carica, the abaxial cystoliths did not increase photosynthetic efficiency.

Light focusing through scattering media via linear fluorescence variance maximization, and its application for fluorescence imaging.

We demonstrate focusing and imaging through a scattering medium without access to the fluorescent object by using wavefront shaping. Our concept is based on utilizing the spatial fluorescence contrast which naturally exists in the hidden target object. By scanning the angle of incidence of the illuminating laser beam and maximizing the variation of the detected fluorescence signal from the object, as measured by a bucket detector at the front of the scattering medium, we are able to generate a tightly focused excitation spot. Thereafter, an image is obtained by scanning the focus over the object within the memory effect range. The requirements for applicability of the method and the comparison with speckle-correlation based focusing methods are discussed.

High-speed low-frequency chirped coherent anti-Stokes Raman scattering microscopy using an ultra-steep long-pass filter.

Coherent anti-Stokes Raman scattering (CARS) microscopy is becoming a more common tool in biomedical research. High-speed CARS microscopy has important applications in live cell imaging and in label-free pathology. However, only a few realizations exist of CARS imaging applied in the few terahertz spectral range (<300 cm-1), in which much is unknown to date. Although single-beam CARS microscopy proved to be robust in this low-frequency region, pixel-dwell time using presently available schemes is still relatively long, in the millisecond scale. Single-beam notch-shaped chirped-CARS (C-CARS) microscopy in the fingerprint region can be performed without using lock-in detection, yet it necessitates double-notch shaping, resulting in a relatively complex system. Here, we demonstrate that C-CARS in the low-frequency regime can be achieved using a sharp-edge, which is created by an ultra-steep long-pass filter (ULPF). Furthermore, we demonstrate that this variant of C-CARS spectroscopy can be performed without post-processing analyses. This is used to image collagen in a biological sample with a pixel dwell time of 200 microseconds. This sharp-edge C-CARS method may find important application in rapid low-frequency CARS imaging of live cells or for imaging of fast flowing objects such as in microfluidic channels.

Quantum correlation measurement with single photon avalanche diode arrays.

Temporal photon correlation measurement, instrumental to probing the quantum properties of light, typically requires multiple single photon detectors. Progress in single photon avalanche diode (SPAD) array technology highlights their potential as high-performance detector arrays for quantum imaging and photon number-resolving (PNR) experiments. Here, we demonstrate this potential by incorporating a novel on-chip SPAD array with 42% peak photon detection efficiency, low dark count rate and crosstalk probability of 0.14% per detection in a confocal microscope. This enables reliable measurements of second and third order photon correlations from a single quantum dot emitter. Our analysis overcomes the inter-detector optical crosstalk background even though it is over an order of magnitude larger than our faint signal. To showcase the vast application space of such an approach, we implement a recently introduced super-resolution imaging method, quantum image scanning microscopy (Q-ISM).

Optical properties of spherulite opals.

Spherulites are birefringent structures with spherical symmetry, typically observed in crystallized polymers. We compute the band structure of opals made of close-packed assemblies of highly birefringent spherulites. We demonstrate that spherulitic birefringence of constituent spheres does not affect the symmetries of an opal, yet significantly affects the dispersion of eigenmodes, leading to new pseudogaps in sections of the band structure and, consequently, enhanced reflectivity.

Simplified approach to low-frequency coherent anti-Stokes Raman spectroscopy using a sharp spectral edge filter.

Coherent anti-Stokes Raman scattering (CARS) has found wide applications in biomedical research. Compared with alternatives, single-beam CARS is especially attractive at low frequencies. Yet, currently existing schemes necessitate a relatively complicated setup to perform high-resolution spectroscopy. Here we show that the spectral sharp edge formed by an ultra-steep long-pass filter is sufficient for performing CARS spectroscopy, simplifying the system significantly. We compare the sensitivity of the presented methodology with available counterparts both theoretically and experimentally. Importantly, we show that this method, to the best of our knowledge, is the simplest and most suitable for vibrational imaging and spectroscopy in the very low-frequency regime (<200 cm-1).