The Excited-State Lifetime of Poly(NDI2OD-T2) Is Intrinsically Short.

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push-pull polymer poly(NDI2OD-T2) (N2200) is one such material. While reasonably efficient organic solar cells can be fabricated with N2200 as the acceptor, it generally fails to contribute as much photocurrent from its absorption bands as the donor with which it is paired. Moreover, transient absorption studies have shown N2200 to have a consistently short excited-state lifetime (∼100 ps) that is dominated by a ground-state recovery. In this paper, we investigate whether these characteristics are intrinsic to the backbone structure of this polymer or if these are extrinsic effects from ubiquitous solution-phase and thin-film aggregates. We compare the solution-phase photophysics of N2200 with those of a pair of model compounds composed of alternating bithiophene (T2) donor and naphthalene diimide (NDI) acceptor units, NDI-T2-NDI and T2-NDI-T2, in a dilute solution. We find that the model compounds have even faster ground-state recovery dynamics (τ = 45, 27 ps) than the polymer (τ = 133 ps), despite remaining molecularly isolated in solution. In these molecules, as in the case of the N2200 polymer, the lowest excited state has a T2 to NDI charge-transfer (CT) character. Electronic-structure calculations indicate that the short lifetime of this state is due to fast nonradiative decay to the ground state (GS) promoted by strong CT-GS electronic coupling and strong electron-vibrational coupling with high-frequency (quantum) normal modes.

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance.

High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices, due to their low cost, extensive tunability, easy fabrication, and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility, stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general), a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density, and by extension carrier mobility, in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model, and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density, a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.

Mediating Photochemical Reaction Rates at Lewis Acidic Rare Earths by Selective Energy Loss to 4f-Electron States.

Manifesting chemical differences in individual rare earth (RE) element complexes is challenging due to the similar sizes of the tripositive cations and the corelike 4f shell. We disclose a new strategy for differentiating between similarly sized Dy3+ and Y3+ ions through a tailored photochemical reaction of their isostructural complexes in which the f-electron states of Dy3+ act as an energy sink. Complexes RE(hfac)3(NMMO)2 (RE = Dy (2-Dy) and Y (2-Y), hfac = hexafluoroacetylacetonate, and NMMO = N-methylmorpholine-N-oxide) showed variable rates of oxygen atom transfer (OAT) to triphenylphosphine under ultraviolet (UV) irradiation, as monitored by 1H and 19F NMR spectroscopies. Ultrafast transient absorption spectroscopy (TAS) identified the excited state(s) responsible for the photochemical OAT reaction or lack thereof. Competing sensitization pathways leading to excited-state deactivation in 2-Dy through energy transfer to the 4f electron manifold ultimately slows the OAT reaction at this metal cation. The measured rate differences between the open-shell Dy3+ and closed-shell Y3+ complexes demonstrate that using established principles of 4f ion sensitization may deliver new, selective modalities for differentiating the RE elements that do not depend on cation size.

Optical Memory, Switching, and Neuromorphic Functionality in Metal Halide Perovskite Materials and Devices.

Metal halide perovskite based materials have emerged over the past few decades as remarkable solution-processable optoelectronic materials with many intriguing properties and potential applications. These emerging materials have recently been considered for their promise in low-energy memory and information processing applications. In particular, their large optical cross-sections, high photoconductance contrast, large carrier-diffusion lengths, and mixed electronic/ionic transport mechanisms are attractive for enabling memory elements and neuromorphic devices that are written and/or read in the optical domain. Here, recent progress toward memory and neuromorphic functionality in metal halide perovskite materials and devices where photons are used as a critical degree of freedom for switching, memory, and neuromorphic functionality is reviewed.

Nanoscale Photoexcited Carrier Dynamics in Perovskites.

The optoelectronic properties of lead halide perovskite thin films can be tuned through compositional variations and strain, but the associated nanocrystalline structure makes it difficult to untangle the link between composition, processing conditions, and ultimately material properties and degradation. Here, we study the effect of processing conditions and degradation on the local photoconductivity dynamics in [(CsPbI3)0.05(FAPbI3)0.85(MAPbBr3)0.15] and (FA0.7Cs0.3PbI3) perovskite thin films using temporally and spectrally resolved microwave near-field microscopy with a temporal resolution as high as 5 ns and a spatial resolution better than 50 nm. For the latter FACs formulation, we find a clear effect of the process annealing temperature on film morphology, stability, and spatial photoconductivity distribution. After exposure of samples to ambient conditions and illumination, we find spectral evidence of halide segregation-induced degradation below the instrument resolution limit for the mixed halide formulation, while we find a clear spatially inhomogeneous increase in the carrier lifetime for the FACs formulation annealed at 180 °C.

Feasibility of conservative fluid administration and deresuscitation compared with usual care in critical illness: the Role of Active Deresuscitation After Resuscitation-2 (RADAR-2) randomised clinical trial.

PURPOSE: Fluid overload is common in critical illness and is associated with mortality. This study investigated the feasibility of a randomised trial comparing conservative fluid administration and deresuscitation (active removal of accumulated fluid using diuretics or ultrafiltration) with usual care in critical illness. METHODS: Open-label, parallel-group, allocation-concealed randomised clinical feasibility trial. Mechanically ventilated adult patients expected to require critical care beyond the next calendar day were enrolled between 24 and 48 h following admission to the intensive care unit (ICU). Patients were randomised to either a 2-stage fluid strategy comprising conservative fluid administration and, if fluid overload was present, active deresuscitation, or usual care. The primary endpoint was fluid balance in the 24 h up to the start of study day 3. Secondary endpoints included cumulative fluid balance, mortality, and duration of mechanical ventilation. RESULTS: One hundred and eighty patients were randomised. After withdrawal of 1 patient, 89 patients assigned to the intervention were compared with 90 patients assigned to the usual care group. The mean plus standard deviation (SD) 24-h fluid balance up to study day 3 was lower in the intervention group (- 840 ± 1746 mL) than the usual care group (+ 130 ± 1401 mL; P < 0.01). Cumulative fluid balance was lower in the intervention group at days 3 and 5. Overall, clinical outcomes did not differ significantly between the two groups, although the point estimate for 30-day mortality favoured the usual care group [intervention arm: 19 of 90 (21.6%) versus usual care: 14 of 89 (15.6%), P = 0.32]. Baseline imbalances between groups and lack of statistical power limit interpretation of clinical outcomes. CONCLUSIONS: A strategy of conservative fluid administration and active deresuscitation is feasible, reduces fluid balance compared with usual care, and may cause benefit or harm. In view of wide variations in contemporary clinical practice, large, adequately powered trials investigating the clinical effectiveness of conservative fluid strategies in critically ill patients are warranted.

Probing the Origin of the Open Circuit Voltage in Perovskite Quantum Dot Photovoltaics.

Perovskite quantum dots (PQDs) have many properties that make them attractive for optoelectronic applications, including expanded compositional tunability and crystallographic stabilization. While they have not achieved the same photovoltaic (PV) efficiencies of top-performing perovskite thin films, they do reproducibly show high open circuit voltage (VOC) in comparison. Further understanding of the VOC attainable in PQDs as a function of surface passivation, contact layers, and PQD composition will further progress the field and may lend useful lessons for non-QD perovskite solar cells. Here, we use photoluminescence-based spectroscopic techniques to understand and identify the governing physics of the VOC in CsPbI3 PQDs. In particular, we probe the effect of the ligand exchange and contact interfaces on the VOC and free charge carrier concentration. The free charge carrier concentration is orders of magnitude higher than in typical perovskite thin films and could be tunable through ligand chemistry. Tuning the PQD A-site cation composition via replacement of Cs+ with FA+ maintains the background carrier concentration but reduces the trap density by up to a factor of 40, reducing the VOC deficit. These results dictate how to improve PQD optoelectronic properties and PV device performance and explain the reduced interfacial recombination observed by coupling PQDs with thin-film perovskites for a hybrid absorber layer.

Linking optical spectra to free charges in donor/acceptor heterojunctions: cross-correlation of transient microwave and optical spectroscopy.

The primary photoexcited species in excitonic semiconductors is a bound electron-hole pair, or exciton. An important strategy for producing separated electrons and holes in photoexcited excitonic semiconductors is the use of donor/acceptor heterojunctions, but the degree to which the carriers can escape their mutual Coulomb attraction is still debated for many systems. Here, we employ a combined pump-probe ultrafast transient absorption (TA) spectroscopy and time-resolved microwave conductivity (TRMC) study on a suite of model excitonic heterojunctions consisting of mono-chiral semiconducting single-walled carbon nanotube (s-SWCNT) electron donors and small-molecule electron acceptors. Comparison of the charge-separated state dynamics between TA and TRMC photoconductance reveals a quantitative match over the 0.5 microsecond time scale. Charge separation yields derived from TA allow extraction of s-SWCNT hole mobilities of ca. 1.5 cm2 V-1 s-1 (at 9 GHz) by TRMC. The correlation between the techniques conclusively demonstrates that photoinduced charge carriers separated across these heterojunctions do not form bound charge transfer states, but instead form free/mobile charge carriers.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications.

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Metal chalcogenides for neuromorphic computing: emerging materials and mechanisms.

The approaching end of Moore's law scaling has significantly accelerated multiple fields of research including neuromorphic-, quantum-, and photonic computing, each of which possesses unique benefits unobtained through conventional binary computers. One of the most compelling arguments for neuromorphic computing systems is power consumption, noting that computations made in the human brain are approximately 106times more efficient than conventional CMOS logic. This review article focuses on the materials science and physical mechanisms found in metal chalcogenides that are currently being explored for use in neuromorphic applications. We begin by reviewing the key biological signal generation and transduction mechanisms within neuronal components of mammalian brains and subsequently compare with observed experimental measurements in chalcogenides. With robustness and energy efficiency in mind, we will focus on short-range mechanisms such as structural phase changes and correlated electron systems that can be driven by low-energy stimuli, such as temperature or electric field. We aim to highlight fundamental materials research and existing gaps that need to be overcome to enable further integration or advancement of metal chalcogenides for neuromorphic systems.

Fluid management and deresuscitation practices: A survey of critical care physicians.

Accumulation of a positive fluid balance is common in critically ill patients, and is associated with adverse outcomes, including mortality. However, there are few randomised clinical trials to guide clinicians as to the most appropriate fluid strategy following initial resuscitation and on the use of deresuscitation (removal of accumulated fluid using diuretics and/or renal replacement therapy). To inform the design of randomised trials, we surveyed critical care physicians with regard to perceptions of fluid overload in critical care, self-reported practice, acceptability of a variety of approaches to deresuscitation, appropriate safety parameters, and overall acceptability of a randomised trial of deresuscitation. Of 524 critical care specialists completing the survey, the majority practiced in mixed medical/surgical intensive care units in the United Kingdom. Most (309 of 363 respondents, 85%) believed fluid overload to be a modifiable source of morbidity; there was strong support (395 of 457, 86%) for a randomised trial of deresuscitation in critical illness. Marked practice variability was evident among respondents. In a given clinical scenario, self-reported practice ranged from the administration of fluid (N = 59, 14%) to the administration of a diuretic (N = 285, 67%). The majority (95%) considered it appropriate to administer diuretics for fluid overload in the setting of noradrenaline infusion and to continue to administer diuretics despite mild dysnatraemias, hypotension, metabolic alkalosis, and hypokalaemia. The majority of critical care physicians view fluid overload as a common and modifiable source of morbidity; deresuscitation is widely practiced, and there is widespread support for randomised trials of deresuscitation in critical illness.

Effect of nanotube coupling on exciton transport in polymer-free monochiral semiconducting carbon nanotube networks.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are attractive light-harvesting components for solar photoconversion schemes and architectures, and selective polymer extraction has emerged as a powerful route to obtain highly pure s-SWCNT samples for electronic applications. Here we demonstrate a novel method for producing electronically coupled thin films of near-monochiral s-SWCNTs without wrapping polymer. Detailed steady-state and transient optical studies on such samples provide new insights into the role of the wrapping polymer on controlling intra-bundle nanotube-nanotube interactions and exciton energy transfer within and between bundles. Complete removal of polymer from the networks results in rapid exciton trapping within nanotube bundles, limiting long-range exciton transport. The results suggest that intertube electronic coupling and associated exciton delocalization across multiple tubes can limit diffusive exciton transport. The complex relationship observed here between exciton delocalization, trapping, and long-range transport, helps to inform the design, preparation, and implementation of carbon nanotube networks as active elements for optical and electronic applications.

Efficiency of Charge-Transfer Doping in Organic Semiconductors Probed with Quantitative Microwave and Direct-Current Conductance.

Although molecular charge-transfer doping is widely used to manipulate carrier density in organic semiconductors, only a small fraction of charge carriers typically escape the Coulomb potential of dopant counterions to contribute to electrical conductivity. Here, we utilize microwave and direct-current (DC) measurements of electrical conductivity to demonstrate that a high percentage of charge carriers in redox-doped semiconducting single-walled carbon nanotube (s-SWCNT) networks is delocalized as a free carrier density in the π-electron system (estimated as >46% at high doping densities). The microwave and four-point probe conductivities of hole-doped s-SWCNT films quantitatively match over almost 4 orders of magnitude in conductance, indicating that both measurements are dominated by the same population of delocalized carriers. We address the relevance of this surprising one-to-one correspondence by discussing the degree to which local environmental parameters (e.g., tube-tube junctions, Coulombic stabilization, and local bonding environment) may impact the relative magnitudes of each transport measurement.

Deresuscitation of Patients With Iatrogenic Fluid Overload Is Associated With Reduced Mortality in Critical Illness.

OBJECTIVES: To characterize current practice in fluid administration and deresuscitation (removal of fluid using diuretics or renal replacement therapy), the relationship between fluid balance, deresuscitative measures, and outcomes and to identify risk factors for positive fluid balance in critical illness. DESIGN: Retrospective cohort study. SETTING: Ten ICUs in the United Kingdom and Canada. PATIENTS: Adults receiving invasive mechanical ventilation for a minimum of 24 hours. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Four-hundred patients were included. Positive cumulative fluid balance (fluid input greater than output) occurred in 87.3%: the largest contributions to fluid input were from medications and maintenance fluids rather than resuscitative IV fluids. In a multivariate logistic regression model, fluid balance on day 3 was an independent risk factor for 30-day mortality (odds ratio 1.26/L [95% CI, 1.07-1.46]), whereas negative fluid balance achieved in the context of deresuscitative measures was associated with lower mortality. Independent predictors of greater fluid balance included treatment in a Canadian site. CONCLUSIONS: Fluid balance is a practice-dependent and potentially modifiable risk factor for adverse outcomes in critical illness. Negative fluid balance achieved with deresuscitation on day 3 of ICU stay is associated with improved patient outcomes. Minimization of day 3 fluid balance by limiting maintenance fluid intake and drug diluents, and using deresuscitative measures, represents a potentially beneficial therapeutic strategy which merits investigation in randomized trials.

Diameter-Dependent Optical Absorption and Excitation Energy Transfer from Encapsulated Dye Molecules toward Single-Walled Carbon Nanotubes.

The hollow cores and well-defined diameters of single-walled carbon nanotubes (SWCNTs) allow for creation of one-dimensional hybrid structures by encapsulation of various molecules. Absorption and near-infrared photoluminescence-excitation (PLE) spectroscopy reveal that the absorption spectrum of encapsulated 1,3-bis[4-(dimethylamino)phenyl]-squaraine dye molecules inside SWCNTs is modulated by the SWCNT diameter, as observed through excitation energy transfer (EET) from the encapsulated molecules to the SWCNTs, implying a strongly diameter-dependent stacking of the molecules inside the SWCNTs. Transient absorption spectroscopy, simultaneously probing the encapsulated dyes and the host SWCNTs, demonstrates this EET, which can be used as a route to diameter-dependent photosensitization, to be fast (sub-picosecond). A wide series of SWCNT samples is systematically characterized by absorption, PLE, and resonant Raman scattering (RRS), also identifying the critical diameter for squaraine filling. In addition, we find that SWCNT filling does not limit the selectivity of subsequent separation protocols (including polyfluorene polymers for isolating only semiconducting SWCNTs and aqueous two-phase separation for enrichment of specific SWCNT chiralities). The design of these functional hybrid systems, with tunable dye absorption, fast and efficient EET, and the ability to remove all metallic SWCNTs by subsequent separation, demonstrates potential for implementation in photoconversion devices.

Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents.

Unlike conventional spin-singlet Cooper pairs, spin-triplet pairs can carry spin1,2. Triplet supercurrents were discovered in Josephson junctions with metallic ferromagnet spacers, where spin transport can occur only within the ferromagnet and in conjunction with a charge current. Ferromagnetic resonance injects a pure spin current from a precessing ferromagnet into adjacent non-magnetic materials3,4. For spin-singlet pairing, the ferromagnetic resonance spin pumping efficiency decreases below the critical temperature (Tc) of a coupled superconductor5,6. Here we present ferromagnetic resonance experiments in which spin sink layers with strong spin-orbit coupling are added to the superconductor. Our results show that the induced spin currents, rather than being suppressed, are substantially larger in the superconducting state compared with the normal state; although further work is required to establish the details of the spin transport process, we show that this cannot be mediated by quasiparticles and is most likely a triplet pure spin supercurrent.

Carbon-Nanotube-Based Thermoelectric Materials and Devices.

Conversion of waste heat to voltage has the potential to significantly reduce the carbon footprint of a number of critical energy sectors, such as the transportation and electricity-generation sectors, and manufacturing processes. Thermal energy is also an abundant low-flux source that can be harnessed to power portable/wearable electronic devices and critical components in remote off-grid locations. As such, a number of different inorganic and organic materials are being explored for their potential in thermoelectric-energy-harvesting devices. Carbon-based thermoelectric materials are particularly attractive due to their use of nontoxic, abundant source-materials, their amenability to high-throughput solution-phase fabrication routes, and the high specific energy (i.e., W g-1 ) enabled by their low mass. Single-walled carbon nanotubes (SWCNTs) represent a unique 1D carbon allotrope with structural, electrical, and thermal properties that enable efficient thermoelectric-energy conversion. Here, the progress made toward understanding the fundamental thermoelectric properties of SWCNTs, nanotube-based composites, and thermoelectric devices prepared from these materials is reviewed in detail. This progress illuminates the tremendous potential that carbon-nanotube-based materials and composites have for producing high-performance next-generation devices for thermoelectric-energy harvesting.

Conservative fluid management or deresuscitation for patients with sepsis or acute respiratory distress syndrome following the resuscitation phase of critical illness: a systematic review and meta-analysis.

BACKGROUND: It is unknown whether a conservative approach to fluid administration or deresuscitation (active removal of fluid using diuretics or renal replacement therapy) is beneficial following haemodynamic stabilisation of critically ill patients. PURPOSE: To evaluate the efficacy and safety of conservative or deresuscitative fluid strategies in adults and children with acute respiratory distress syndrome (ARDS), sepsis or systemic inflammatory response syndrome (SIRS) in the post-resuscitation phase of critical illness. METHODS: We searched Medline, EMBASE and the Cochrane central register of controlled trials from 1980 to June 2016, and manually reviewed relevant conference proceedings from 2009 to the present. Two reviewers independently assessed search results for inclusion and undertook data extraction and quality appraisal. We included randomised trials comparing fluid regimens with differing fluid balances between groups, and observational studies investigating the relationship between fluid balance and clinical outcomes. RESULTS: Forty-nine studies met the inclusion criteria. Marked clinical heterogeneity was evident. In a meta-analysis of 11 randomised trials (2051 patients) using a random-effects model, we found no significant difference in mortality with conservative or deresuscitative strategies compared with a liberal strategy or usual care [pooled risk ratio (RR) 0.92, 95 % confidence interval (CI) 0.82-1.02, I 2 = 0 %]. A conservative or deresuscitative strategy resulted in increased ventilator-free days (mean difference 1.82 days, 95 % CI 0.53-3.10, I 2 = 9 %) and reduced length of ICU stay (mean difference -1.88 days, 95 % CI -0.12 to -3.64, I 2 = 75 %) compared with a liberal strategy or standard care. CONCLUSIONS: In adults and children with ARDS, sepsis or SIRS, a conservative or deresuscitative fluid strategy results in an increased number of ventilator-free days and a decreased length of ICU stay compared with a liberal strategy or standard care. The effect on mortality remains uncertain. Large randomised trials are needed to determine optimal fluid strategies in critical illness.