Hybrid Organic-Inorganic Perovskite Superstructures for Ultrapure Green Emissions.

All inorganic CsPbBr3 superstructures (SSs) have attracted much research interest due to their unique photophysical properties, such as their large emission red-shifts and super-radiant burst emissions. These properties are of particular interest in displays, lasers and photodetectors. Currently, the best-performing perovskite optoelectronic devices incorporate organic cations (methylammonium (MA), formamidinium (FA)), however, hybrid organic-inorganic perovskite SSs have not yet been investigated. This work is the first to report on the synthesis and photophysical characterization of APbBr3 (A = MA, FA, Cs) perovskite SSs using a facile ligand-assisted reprecipitation method. At higher concentrations, the hybrid organic-inorganic MA/FAPbBr3 nanocrystals self-assemble into SSs and produce red-shifted ultrapure green emissions, meeting the requirement of Rec. 2020 displays. We hope that this work will be seminal in advancing the exploration of perovskite SSs using mixed cation groups to further improve their optoelectronic applications.

Photosynthesis re-wired on the pico-second timescale.

Photosystems II and I (PSII, PSI) are the reaction centre-containing complexes driving the light reactions of photosynthesis; PSII performs light-driven water oxidation and PSI further photo-energizes harvested electrons. The impressive efficiencies of the photosystems have motivated extensive biological, artificial and biohybrid approaches to 're-wire' photosynthesis for higher biomass-conversion efficiencies and new reaction pathways, such as H2 evolution or CO2 fixation1,2. Previous approaches focused on charge extraction at terminal electron acceptors of the photosystems3. Electron extraction at earlier steps, perhaps immediately from photoexcited reaction centres, would enable greater thermodynamic gains; however, this was believed impossible with reaction centres buried at least 4 nm within the photosystems4,5. Here, we demonstrate, using in vivo ultrafast transient absorption (TA) spectroscopy, extraction of electrons directly from photoexcited PSI and PSII at early points (several picoseconds post-photo-excitation) with live cyanobacterial cells or isolated photosystems, and exogenous electron mediators such as 2,6-dichloro-1,4-benzoquinone (DCBQ) and methyl viologen. We postulate that these mediators oxidize peripheral chlorophyll pigments participating in highly delocalized charge-transfer states after initial photo-excitation. Our results challenge previous models that the photoexcited reaction centres are insulated within the photosystem protein scaffold, opening new avenues to study and re-wire photosynthesis for biotechnologies and semi-artificial photosynthesis.

Mixed Small-Molecule Matrices Improve Nanoparticle Dispersibility in Organic Semiconductor-Nanoparticle Films.

Controlling the dispersibility of nanocrystalline inorganic quantum dots (QDs) within organic semiconductor (OSC):QD nanocomposite films is critical for a wide range of optoelectronic devices. This work demonstrates how small changes to the OSC host molecule can have a dramatic detrimental effect on QD dispersibility within the host organic semiconductor matrix as quantified by grazing incidence X-ray scattering. It is commonplace to modify QD surface chemistry to enhance QD dispersibility within an OSC host. Here, an alternative route toward optimizing QD dispersibilities is demonstrated, which dramatically improves QD dispersibilities through blending two different OSCs to form a fully mixed OSC matrix phase.

InP/ZnS quantum dot photoluminescence modulation via in situ H2S interface engineering.

InP quantum dots (QDs) are attracting significant interest as a potentially less toxic alternative to Cd-based QDs in many research areas. Although InP-based core/shell QDs with excellent photoluminescence properties have been reported so far, sophisticated interface treatment to eliminate defects is often necessary. Herein, using aminophosphine as a seeding source of phosphorus, we find that H2S can be efficiently generated from the reaction between a thiol and an alkylamine at high temperatures. Apart from general comprehension that H2S acts as a S precursor, it is revealed that with core etching by H2S, the interface between InP and ZnS can be reconstructed with S2- incorporation. Such a transition layer can reduce inherent defects at the interface, resulting in significant photoluminescence (PL) enhancement. Meanwhile, the size of the InP core could be further controlled by H2S etching, which offers a feasible process to obtain wide band gap InP-based QDs with blue emission.

Photogeneration of Spin Quintet Triplet-Triplet Excitations in DNA-Assembled Pentacene Stacks.

Singlet fission (SF), an exciton-doubling process observed in certain molecular semiconductors where two triplet excitons are generated from one singlet exciton, requires correctly tuned intermolecular coupling to allow separation of the two triplets to different molecular units. We explore this using DNA-encoded assembly of SF-capable pentacenes into discrete π-stacked constructs of defined size and geometry. Precise structural control is achieved via a combination of the DNA duplex formation between complementary single-stranded DNA and the local molecular geometry that directs the SF chromophores into a stable and predictable slip-stacked configuration, as confirmed by molecular dynamics (MD) modeling. Transient electron spin resonance spectroscopy revealed that within these DNA-assembled pentacene stacks, SF evolves via a bound triplet pair quintet state, which subsequently converts into free triplets. SF evolution via a long-lived quintet state sets specific requirements on intermolecular coupling, rendering the quintet spectrum and its zero-field-splitting parameters highly sensitive to intermolecular geometry. We have found that the experimental spectra and zero-field-splitting parameters are consistent with a slight systematic strain relative to the MD-optimized geometry. Thus, the transient electron spin resonance analysis is a powerful tool to test and refine the MD-derived structure models. DNA-encoded assembly of coupled semiconductor molecules allows controlled construction of electronically functional structures, but brings with it significant dynamic and polar disorders. Our findings here of efficient SF through quintet states demonstrate that these conditions still allow efficient and controlled semiconductor operation and point toward future opportunities for constructing functional optoelectronic systems.

Localization of deep brain stimulation trajectories via automatic mapping of microelectrode recordings to MRI.

Objective. Suboptimal electrode placement during subthalamic nucleus deep brain stimulation (STN DBS) surgery may arise from several sources, including frame-based targeting errors and intraoperative brain shift. We present a computer algorithm that can accurately localize intraoperative microelectrode recording (MER) tracks on preoperative magnetic resonance imaging (MRI) in real-time, thereby predicting deviation between the surgical plan and the MER trajectories.Approach. Random forest (RF) modeling was used to derive a statistical relationship between electrophysiological features on intraoperative MER and voxel intensity on preoperative T2-weighted MR imaging. This model was integrated into a larger algorithm that can automatically localize intraoperative MER recording tracks on preoperative MRI in real-time. To verify accuracy, targeting error of both the planned intraoperative trajectory ('planned') and the algorithm-derived trajectory ('calculated') was estimated by measuring deviation from the final DBS lead location on postoperative high-resolution computed tomography ('actual').Main results. MR imaging and MERs were obtained from 24 STN DBS implant trajectories. The cross-validated RF model could accurately distinguish between gray and white matter regions along MER trajectories (AUC 0.84). When applying this model within the localization algorithm, thecalculatedMER trajectory estimate was found to be significantly closer to theactualDBS lead when compared to theplannedtrajectory recorded during surgery (1.04 mm vs 1.52 mm deviation,p< 0.002), with improvement shown in 19/24 cases (79%). When applying the algorithm to simulated DBS trajectory plans with randomized targeting error, up to 4 mm of error could be resolved to <2 mm on average (p< 0.0001).Significance. This work presents an automated system for intraoperative localization of electrodes during STN DBS surgery. This neuroengineering solution may enhance the accuracy of electrode position estimation, particularly in cases where high-resolution intraoperative imaging is not available.

Soluble Diphenylhexatriene Dimers for Intramolecular Singlet Fission with High Triplet Energy.

Intramolecular singlet fission (iSF) facilitates single-molecule exciton multiplication, converting an excited singlet state to a pair of triplet states within a single molecule. A critical parameter in determining the feasibility of SF-enhanced photovoltaic designs is the triplet energy; many existing iSF materials have triplet energies too low for efficient transfer to silicon via a photon multiplier scheme. In this work, a series of six novel dimers based upon the high-triplet-energy, SF-active chromophore, 1,6-diphenyl-1,3,5-hexatriene (DPH) [E(T1) ∼ 1.5 eV], were designed, synthesized, and characterized. Transient absorption spectroscopy and fluorescence lifetime studies reveal that five of the dimers display iSF activity, with time constants for singlet fission varying between 7 ± 2 ps and 2.2 ± 0.2 ns and a high triplet yield of 163 ± 63% in the best-performing dimer. A strong dependence of the rate of fission on the coupling geometry is demonstrated. For optimized iSF behavior, close spatial proximity and minimal through-bond communication are found to be crucial for balancing the rate of SF against the reverse recombination process.

Transmission-based charge modulation microscopy on conjugated polymer blend field-effect transistors.

Charge modulation microscopy (CMM) is an electro-optical method that is capable of mapping the spatial distribution of induced charges in an organic field-effect transistor (OFET). Here, we report a new (and simple) implementation of CMM in transmission geometry with camera-based imaging. A significant improvement in data acquisition speed (by at least an order of magnitude) has been achieved while preserving the spatial and spectral resolution. To demonstrate the capability of the system, we measured the spatial distribution of the induced charges in an OFET with a polymer blend of indacenodithiophene-co-benzothiadiazole and poly-vinylcarbazole that shows micrometer-scale phase separation. We were able to resolve spatial variations in the accumulated charge density on a length scale of 500 nm. We demonstrated through a careful spectral analysis that the measured signal is a genuine charge accumulation signal that is not dominated by optical artifacts.

Triplet transfer from PbS quantum dots to tetracene ligands: is faster always better?

Quantum dot-organic semiconductor hybrid materials are gaining increasing attention as spin mixers for applications ranging from solar harvesting to spin memories. Triplet energy transfer between the inorganic quantum dot (QD) and organic semiconductor is a key step to understand in order to develop these applications. Here we report on the triplet energy transfer from PbS QDs to four energetically and structurally similar tetracene ligands. Even with similar ligands we find that the triplet energy transfer dynamics can vary significantly. For TIPS-tetracene derivatives with carboxylic acid, acetic acid and methanethiol anchoring groups on the short pro-cata side we find that triplet transfer occurs through a stepwise process, mediated via a surface state, whereas for monosubstituted TIPS-tetracene derivative 5-(4-benzoic acid)-12-triisopropylsilylethynyl tetracene (BAT) triplet transfer occurs directly, albeit slower, via a Dexter exchange mechanism. Even though triplet transfer is slower with BAT the overall yield is greater, as determined from upconverted emission using rubrene emitters. This work highlights that the surface-mediated transfer mechanism is plagued with parasitic loss pathways and that materials with direct Dexter-like triplet transfer are preferred for high-efficiency applications.

Quantitative Singlet Fission in Solution-Processable Dithienohexatrienes.

Singlet fission (SF) is a promising strategy to overcome thermalization losses and enhance the efficiency of single junction photovoltaics (PVs). The development of this field has been strongly material-limited, with a paucity of materials able to undergo SF. Rarer still are examples that can produce excitons of sufficient energy to be coupled to silicon PVs (>1.1 eV). Herein, we examine a series of a short-chain polyene, dithienohexatriene (DTH), with tailored material properties and triplet (T1) energy levels greater than 1.1 eV. We find that these highly soluble materials can be easily spin-cast to create thin films of high crystallinity that exhibit ultrafast singlet fission with near perfect triplet yields of up to 192%. We believe that these materials are the first solution-processable singlet fission materials with quantitative triplet formation and energy levels appropriate for use in conjunction with silicon PVs.

Reducing COVID vaccine hesitancy by inducing a comparative mindset.

OBJECTIVE: To investigate if a behavioral nudge comprising a vaccination opportunity that employs a comparative probe first (i.e., which vaccine to take) versus the more commonly-used deliberative probe (i.e., willingness to take a vaccine), reduces vaccine hesitancy, while controlling for political partisanship. METHODS: In a randomized study, conducted on Amazon Mechanical Turk and Prolific, we varied the manner in which the vaccination offer is posed. In one group, participants were asked to compare which vaccine they would like to take (i.e., the comparative probe), while, in another group, participants were asked to deliberate whether they would like to take the vaccine (i.e., the deliberative probe). Participants' political preferences were also measured. The primary outcome variable was vaccine hesitancy. RESULTS: A LOGIT regression (N = 1736), was conducted to test the research questions. Overall, the comparative probe yielded a 6% reduction in vaccine hesitancy relative to the typical deliberative probe. Additionally, while vaccine hesitancy varies due to individual political views, the comparative probe is effective at reducing vaccine hesitancy even among the most vaccine hesitant population (i.e., Pro-Trump Republicans) by almost 10% on average. CONCLUSIONS: Subtly changing the manner in which the vaccination offer is framed, by asking people to compare which vaccine to take, and not deliberate about whether they would like to take a vaccine, can reduce vaccine hesitancy, without being psychologically taxing or curtailing individuals' freedom to choose. The nudge is especially effective among highly vaccine hesitant populations such as Pro-Trump Republicans. Our results suggest a costless communication protocol in face-to-face interactions on doorsteps, in clinics, in Pro-Trump regions and in the mass media, that might protect 5 million Americans from COVID-19.

A mechanistic study of the dopant-induced breakdown in halide perovskites using solid state energy storage devices.

Doping halide perovskites (HPs) with extrinsic species, such as alkali metal ions, plays a critical, albeit often elusive role in optimising optoelectronic devices. Here, we use solid state lithium ion battery inspired devices with a polyethylene oxide-based polymer electrolyte to dope HPs controllably with lithium ions. We perform a suite of operando material analysis techniques while dynamically varying Li doping concentrations. We determine and quantify three doping regimes; a safe regime, with doping concentrations of <1020 cm-3 (2% Li : Pb mol%) in which the HP may be modified without detrimental effect to its structure; a minor decomposition regime, in which the HP is partially transformed but remains the dominant species; and a major decomposition regime in which the perovskite is superseded by new phases. We provide a mechanistic description of the processes mediating between each stage and find evidence for metallic Pb(0), LiBr and LiPbBr2 as final decomposition products. Combining results from synchrotron X-ray diffraction measurements with in situ photoluminescence and optical reflection microscopy studies, we distinguish the influences of free charge carriers and intercalated lithium independently. We find that the charge density is equally as important as the geometric considerations of the dopant species and thereby provide a quantitative framework upon which the future design of doped-perovskite energy devices should be based.

Direct observation of ultrafast singlet exciton fission in three dimensions.

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales.

Clues from the Pandora's Box: Frequency of Acute Abdominal Symptoms in COVID-19 and Its Association with Inflammatory Markers-a Cross-Sectional Study.

Coronavirus disease 2019 (COVID-19) is primarily considered to be a respiratory ailment. Hitherto, abdominal symptoms have been reported with variable frequency in acute COVID-19. The purpose of this study was to estimate the frequency of abdominal symptoms at presentation among patients hospitalised with COVID-19 infection, and to determine their association with disease severity. This was a single-centre cross-sectional observational study conducted at a COVID-19 tertiary care hospital (CTRI/2021/10/037195, registered on 08/10/2021). Consecutive patients hospitalised with acute COVID-19 illness during the study period were included in the study. Their demographic information, abdominal symptoms, comorbidities and category of COVID-19 illness were elicited. All patients had serum inflammatory markers tested on the day of hospitalisation. Among the 685 participants, 214 patients had mild-to-moderate category illness whereas the rest 471 had severe COVID-19 illness. Abdominal complaints were present among 132/685 (18.3%) patients with distension of abdomen (8.03%) being the most common symptom, followed by vomiting (6.72%) and abdominal pain (3.94%). At admission to the hospital, abdominal complaints were commoner among patients with severe disease than in those with mild-to-moderate disease (101/471 vs. 31/214; p=0.029). Abdominal symptoms were associated with a higher neutrophil to lymphocyte ratio (p=0.029). The mortality among COVID-19 patients with abdominal symptoms was higher (9.09 vs. 3.25%; p = 0.007). This study demonstrates the spectrum of abdominal symptoms that can be a part of acute COVID-19 at hospitalisation and also highlights their prognostic potential in acute COVID-19 infection.

Dielectric control of reverse intersystem crossing in thermally activated delayed fluorescence emitters.

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination.

I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm-1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

Operando monitoring of single-particle kinetic state-of-charge heterogeneities and cracking in high-rate Li-ion anodes.

To rationalize and improve the performance of newly developed high-rate battery electrode materials, it is crucial to understand the ion intercalation and degradation mechanisms occurring during realistic battery operation. Here we apply a laboratory-based operando optical scattering microscopy method to study micrometre-sized rod-like particles of the anode material Nb14W3O44 during high-rate cycling. We directly visualize elongation of the particles, which, by comparison with ensemble X-ray diffraction, allows us to determine changes in the state of charge of individual particles. A continuous change in scattering intensity with state of charge enables the observation of non-equilibrium kinetic phase separations within individual particles. Phase field modelling (informed by pulsed-field-gradient nuclear magnetic resonance and electrochemical experiments) supports the kinetic origin of this separation, which arises from the state-of-charge dependence of the Li-ion diffusion coefficient. The non-equilibrium phase separations lead to particle cracking at high rates of delithiation, particularly in longer particles, with some of the resulting fragments becoming electrically disconnected on subsequent cycling. These results demonstrate the power of optical scattering microscopy to track rapid non-equilibrium processes that would be inaccessible with established characterization techniques.

A New Frontier in Exciton Transport: Transient Delocalization.

Efficient exciton transport is crucial to the application of organic semiconductors (OSCs) in light-harvesting devices. While the physics of exciton transport in highly disordered media is well-explored, the description of transport in structurally and energetically ordered OSCs is less established, despite such materials being favorable for devices. In this Perspective we describe and highlight recent research pointing toward a highly efficient exciton transport mechanism which occurs in ordered OSCs, transient delocalization. Here, exciton-phonon couplings play a critical role in allowing localized exciton states to temporarily access higher-energy delocalized states whereupon they move large distances. The mechanism shows great promise for facilitating long-range exciton transport and may allow for improved device efficiencies and new device architectures. However, many fundamental questions on transient delocalization remain to be answered. These questions and suggested next steps are summarized.

Fast A-Site Cation Cross-Exchange at Room Temperature: Single-to Double- and Triple-Cation Halide Perovskite Nanocrystals.

We report here fast A-site cation cross-exchange between APbX3 perovskite nanocrystals (NCs) made of different A-cations (Cs (cesium), FA (formamidinium), and MA (methylammonium)) at room temperature. Surprisingly, the A-cation cross-exchange proceeds as fast as the halide (X=Cl, Br, or I) exchange with the help of free A-oleate complexes present in the freshly prepared colloidal perovskite NC solutions. This enabled the preparation of double (MACs, MAFA, CsFA)- and triple (MACsFA)-cation perovskite NCs with an optical band gap that is finely tunable by their A-site composition. The optical spectroscopy together with structural analysis using XRD and atomically resolved high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast (iDPC) STEM indicates the homogeneous distribution of different cations in the mixed perovskite NC lattice. Unlike halide ions, the A-cations do not phase-segregate under light illumination.