Interplay of Bright Triplet and Dark Excitons Revealed by Magneto-Photoluminescence of Individual PbS/CdS Quantum Dots.

A low-temperature polarization-resolved magneto-photoluminescence experiment is performed on individual PbS/CdS core/shell quantum dots (QDs). The experiment enables a direct measurement of the exciton Landé g factor and the anisotropic zero-field splitting of the lowest emissive bright exciton triplet in PbS/CdS QDs. While anisotropic splittings of individual QDs distribute randomly in 104-325 µeV range, the exciton Landé g factors increase from 0.95 to 2.70 as the emission energy of the QD increases from 1.0 to 1.2 eV. The tight-binding calculations allow to rationalize these trends as a direct consequence of reducing a cubic symmetry of QD via addition/removal of a few (<70) atoms from the surfaces of the PbS core. Furthermore, it is observed that while right (σ  + ) and left (σ  - ) circularly polarized photoluminescence (PL) peaks split linearly with magnetic field as expected for Zeeman effect, the energy splitting between X and Y linearly polarized PL peaks remains nearly unchanged. The theoretical study reveals rich and complex magnetic field-induced interplay of bright triplet and dark exciton states explaining this puzzling behavior. These findings fill the missing gaps in the understanding of lead salt QDs and provide foundation for development of classical and quantum light sources operating at telecommunication wavelengths.

Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes.

Fluorination represents an important strategy in developing high-performance conjugated polymers for photovoltaic applications. Here, we use regioregular poly(3-ethylhexylthiophene) (P3EHT) and poly(3-ethylhexyl-4-fluorothiophene) (F-P3EHT) as simplified model materials, using single-molecule/aggregate spectroscopy and molecular dynamic simulations, to elucidate the impacts of backbone fluorination on morphology and excitonic coupling on the molecular scale. Despite its high regioregularity, regioregular P3EHT exhibits a rather broad distribution in polymer chain conformation due to the strong steric hindrance of bulky ethylhexyl side chains. This conformational variability results in disordered interchain morphology even between a few chains, prohibiting long-range effective interchain coupling. In stark contrast, the experimental and molecular dynamic calculations reveal that backbone fluorination of F-P3EHT leads to an extended rod-like single-chain conformation and hence highly ordered interchain packing in aggregates. Surprisingly, the ordered and close interchain packing in F-P3EHT does not lead to strong excitonic coupling between the chains but rather to dominant intrachain excitonic coupling that greatly reduces the molecular energetic heterogeneity.

Intrinsic Exciton Photophysics of PbS Quantum Dots Revealed by Low-Temperature Single Nanocrystal Spectroscopy.

With a tunable size-dependent photoluminescence (PL) over a wide infrared wavelength range, lead chalcogenide quantum dots (QDs) have attracted significant scientific and technological interest. Nevertheless, the investigation of intrinsic exciton photophysics at the single-QD level has remained a challenge. Herein, we present a comprehensive study of PL properties for the individual core/shell PbS/CdS QDs emissive near 1.0 eV. In contrast to the sub-meV spectral line widths observed for II/VI QDs, PbS/CdS QDs are predicted to possess broad homogeneous line widths. Performing spectroscopy at cryogenic (4 K) temperatures, we provide direct evidence confirming theoretical predictions, showing that intrinsic line widths for PbS/CdS QDs are in the range of 8-25 meV, with an average of 16.4 meV. In addition, low-temperature, single-QD spectroscopy reveals a broad low-energy side emission attributable to optical as well as localized acoustic phonon-assisted transitions. By tracking single QDs from 4 to 250 K, we were able to probe temperature-dependent evolutions of emission energy, line width, and line shape. Finally, polarization-resolved PL imaging showed that PbS/CdS QDs are characterized by a 3D emission dipole, in contrast with the 2D dipole observed for CdSe QDs.

The importance of the propagule-sediment-tide "power balance" for revegetation at the coastal frontier.

Revegetation of pioneer plants is a critical phase in community establishment for mudflats in seriously degraded coastal wetlands. We tested a hypothesis of the importance of a "power balance" among propagule resilience and sedimentary and tidal disturbances for vegetation reestablishment. Our experiment used three types of propagules (seeds, seedlings, and corms) of native Scirpus species in the fringing flats with similar tidal flows and varying sedimentary intensities in the Yangtze Estuary. Regardless of the initial planting densities, the seed germination rate was extremely low in the field situation. Although the incubated seedlings were planted directly on the bare flat, the wave movement easily flushed the seedlings, even at the site with moderate sedimentary accretion. Failure of the revegetation practice using the seed and seedling materials indicated that the combined "growing and anchoring power" of young seedlings and "stabilizing power" of the sediment were insufficient to withstand the "dislodging power" of the tidal energy. In contrast, the planting approach with underground propagules (corms) proved to be feasible for vegetation establishment at the sites with moderate and low-level sedimentary intensities. The successful practice improved the tipping point of plant survival and tussock formation could be surpassed when the combined growing and anchoring power of seedlings that developed from corms with the stabilizing power of the sediment was greater than the dislodging power of the wave energy. However, at the site with high-level sedimentary intensity, the excessive sediment converted to the burying stress power as seedlings developed from the corms, revealing a burial threshold for seedling survival. The risk of seedling establishment was high when the burying stress power of the sediment far outweighed the combination of the growing power of the seedlings and the sediment removal power of the tidal current and surpassed the tipping point of vegetation die-off. Additionally, we checked the practice cost of the different approaches to ensure a highly cost-effective revegetation planning based on site suitability. This study highlights that understanding of the propagule-sediment-tide power balance offers a tool for improvement of the revegetation and management of site-specific sedimentary and hydrological environments for many degraded coastal ecosystems.

Photoluminescence Enhancement of CuInS2 Quantum Dots in Solution Coupled to Plasmonic Gold Nanocup Array.

A strong plasmonic enhancement of photoluminescence (PL) decay rate in quantum dots (QDs) coupled to an array of gold-coated nanocups is demonstrated. CuInS2 QDs that emit at a wavelength that overlaps with the extraordinary optical transmission (EOT) of the gold nanocup array are placed in the cups as solutions. Time-resolved PL reveals that the decay rate of the QDs in the plasmonically coupled system can be enhanced by more than an order of magnitude. Using finite-difference time-domain (FDTD) simulations, it is shown that this enhancement in PL decay rate results from an enhancement factor of ≈100 in electric field intensity provided by the plasmonic mode of the nanocup array, which is also responsible for the EOT. The simulated Purcell factor approaches 86 at the bottom of the nanocup and is ≈3-15 averaged over the nanocup cavity height, agreeing with the experimental enhancement result. This demonstration of solution-based coupling between QDs and gold nanocups opens up new possibilities for applications that would benefit from a solution environment such as biosensing.

Excitonic energy migration in conjugated polymers: the critical role of interchain morphology.

Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in regioregular poly(3-hexylthiophene) (rr-P3HT) enables long-range interchain energy migration, while disordered packing in regiorandom poly(3-hexylthiophene) (rra-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to rr-P3HT, interchain energy migration in poly(3-(2'-methoxy-5'-octylphenyl)thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes.

Synthesis of a donor-acceptor diblock copolymer via two mechanistically distinct, sequential polymerizations using a single catalyst.

Treatment of a Ni-terminated poly(3-hexylthiophene) (P3HT), generated in situ from 5-chloromagnesio-2-bromo-3-hexylthiophene and Ni(1,3-bis(diphenylphosphino)propane)Cl2, with a perylene diimide-functionalized arylisocyanide monomer effects a chain-extension polymerization to afford a donor-acceptor diblock copolymer using a single catalyst and in a single reaction vessel. The two mechanistically distinct polymerizations proceed in a controlled, chain growth fashion, allowing the molecular weight of both the P3HT and poly(isocyanide) blocks to be tuned by adjusting the initial monomer-to-catalyst ratios. The resulting materials are found to self-assemble into crystalline, lamellar stacks of donor and acceptor components in the solid state, and also exhibit fluorescence quenching in thin films, properties which poise these materials for use in organic photovoltaic applications.

Green Light from Red-Emitting Nanocrystals: Broadband, Low-Threshold Lasing from Colloidal Quantum Shells in Optical Nanocavities.

Spherical semiconductor nanoplatelets, known as quantum shells (QSs), have captured significant interest for their strong suppression of Auger recombination, which leads to long multiexciton lifetimes and wide optical gain bandwidth. Yet, the realization of benefits associated with the multiexciton lasing regime using a suitably designed photonic cavity remains elusive. Here, we demonstrate broadly tunable lasing from close-packed films of CdS/CdSe/CdS QSs deposited over nanopillar arrays on Si substrates. Wide spectral tuning of the stimulated emission in QSs with a fixed bandgap value was achieved by engaging single exciton (λX ∼ 634 nm), biexciton (λBX ∼ 627 nm), and multiple exciton (λMX ∼ 615-565 nm) transitions. The ensemble-averaged gain threshold of ∼ 2.6 electron-hole pairs per QS particle and the low photonic cavity fluence threshold of ∼4 µJ/cm2 were attributed to Auger suppression. The tuning of the lasing emission closely aligns with our model predictions achieved by varying the array period while preserving mode confinement and quality (Q) factors. These results mark a notable step toward the development of colloidal nanocrystal lasers.

Effect of the side-chain-distribution density on the single-conjugated-polymer-chain conformation.

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side-chain-distribution density on the conformation at the isolated single-polymer-chain level was investigated with regiorandom (rra-) poly(3-hexylthiophene) (P3HT) and poly(3-hexyl-2,5-thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single-molecule spectroscopy investigation. With single-molecule fluorescence excitation polarization spectroscopy, we found that rra-P3HTV single molecules form highly ordered conformations. In contrast, rra-P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side-chain-distribution density, that is, the spaced-out side-chain substitution pattern, in rra-P3HTV favors more ordered conformations compared to rra-P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer-chain conformation, even at the single-molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.

Combining bi-functional Pt/USY and electromagnetic induction for rapid in-situ adsorption-combustion cycling of gaseous organic pollutant.

By exploiting the superior adsorption capacity of ultra-stable Y-type zeolite (USY) and accurate input of energy by electromagnetic induction field (EMIF) technique, we successfully designed a highly energy-efficient system to eliminate gaseous toluene a common air pollutant. Pristine USY as adsorbent enriches gaseous toluene by a factor of fifteen, via room-temperature adsorption and then EMIF-driven thermal desorption. This operation model involving intermittent heating and mass transfer saves a lot of energy. Especially during temperature rising, 98.9% electric energy can be saved by the EMIF heating in comparison with conventional furnace approaches. In the bi-functional "adsorption-catalytic oxidation" 1Pt/USY, the concentrated toluene undergoes direct oxidation into CO2 rather than desorption when the EMIF heating starts, so one-step enrichment and mineralization are realized. In addition, the developed bi-functional system operates between adsorption and catalytic decomposition flexibly, which makes it ideal for cleaning VOCs emitted from intermittent sources.

Charge trapping and storage by composite P3HT/PC60BM nanoparticles investigated by fluorescence-voltage/single particle spectroscopy.

Fluorescence-voltage/single particle spectroscopy (F-V/SPS) was employed to study exciton-hole polaron interactions and interfacial charge transfer processes for pure poly(3-hexylthiophene) (P3HT) nanoparticles (NPs) and composite P3HT/PC(60)BM NPs in functioning hole-injection devices. F-V/SPS data collected on a particle-by-particle basis reveal an apparent bistability in the fluorescence-voltage modulation curves for composite NPs of P3HT and [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(60)BM) that is absent for pure P3HT NPs. A pronounced deep trapping of free electrons photogenerated from the composite P3HT/PC(60)BM NPs at the NP/dielectric interface and hole trapping by fullerene anions in composite P3HT/PC(60)BM NPs under photoexcitation lies at the basis of this finding. The deep electron trapping effect reported here for composite conjugated polymer/fullerene NPs presents an opportunity for future application of these NPs in nanoscale memory and imaging devices.

PbS/CdS Quantum Dot Room-Temperature Single-Emitter Spectroscopy Reaches the Telecom O and S Bands via an Engineered Stability.

We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.

Kinetics and Thermodynamics of Killing a Quantum Dot.

Light-emitting nanocrystal quantum dots (QDs) are of high interest for use as down-conversion phosphors and direct emission sources in bulk solid-state devices and as reliable sources of single photons in quantum information science. However, these materials are prone to photooxidation that reduces the emission quantum yield over time. Current commercial applications use device architectures to prevent oxidation without addressing the underlying degradation reactions at the nanocrystal level. To instead prevent loss of functionality by better synthetic engineering of the nanoscale emitters themselves, the underlying properties of these reactions must be understood and readily accessible. Here, we use solid-state spectroscopy to obtain kinetic and thermodynamic parameters of photothermal degradation in single QDs by systematically varying the ambient temperature and photon pump fluence. We describe the resulting degradation in emission with a modified form of the Arrhenius equation and show that this reaction proceeds via pseudo-zero-order reaction kinetics by a surface-assisted process with an activation energy of 60 kJ/mol. We note that the rate of degradation is ∼12 orders of magnitude slower than the rate of excitonic processes, indicating that the reaction rate is not determined by electron or hole trapping. In the search for new robust light-emitting nanocrystals, the reported analysis method will enable direct comparisons between differently engineered nanomaterials.

Influence of morphology on the blinking mechanisms and the excitonic fine structure of single colloidal nanoplatelets.

Colloidal semiconductor nanoplatelets with a similar electronic structure as quantum wells have recently emerged as exciting materials for optoelectronic applications. Here we investigate how morphology affects important photoluminescence properties of single CdSe and core/shell CdSe/CdZnS nanoplatelets. By analyzing photoluminescence intensity-lifetime correlation and second-order photon correlation results, we demonstrate that, irrespective of the morphology, Auger recombination plays only a minor role in dictating the blinking behavior of the nanoplatelets. We find that a rough shell induces additional nonradiative channels presumably related to defects or traps of an imperfect shell. Furthermore, polarization-resolved spectroscopy analysis reveals exciton fine-structure splitting of the order of several tens of meV in rough-shell nanoplatelets at room temperature, which is attributed to exciton localization and is substantiated by theoretical calculations taking into account the nanoplatelet shape and electron-hole exchange interaction.

Effects of molecular architecture on morphology and photophysics in conjugated polymers: from single molecules to bulk.

Conjugated polymers (CPs) possess a wide range of desirable properties, including accessible energetic bandgaps, synthetic versatility, and mechanical flexibility, which make them attractive for flexible and wearable optoelectronic devices. An accurate and comprehensive understanding about the morphology-photophysics relations in CPs lays the groundwork for their development in these applications. However, due to the complex roles of chemical structure, side-chains, backbone, and intramolecular interactions, CPs can exhibit heterogeneity in both their morphology and optoelectronic properties even at the single chain level. This molecular level heterogeneity together with complicated intermolecular interactions found in bulk CP materials severely obscures the deterministic information about the morphology and photophysics at different hierarchy levels. To counter this complexity and offer a clearer picture for the properties of CP materials, we highlight the approach of probing material systems with specific structural features via single molecule/aggregate spectroscopy (SMS). This review article covers recent advances achieved through such an approach regarding the important morphological and photophysical properties of CPs. After a brief review of the typical characteristics of CPs, we present detailed discussions of structurally well-defined model systems of CPs, from manipulated backbones and side-chains, up to nano-aggregates, studied with SMS to offer deterministic relations between morphology and photophysics from single chains building up to bulk states.

Spectroscopic properties of Nd3+ doped transparent oxyfluoride glass ceramics.

In this paper, the spectroscopic properties of Nd(3+) doped transparent oxyfluoride glass ceramics containing LaF(3) nano-crystals were systematically studied. The formation and distribution of LaF(3) nano-crystals in the glass matrix were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Based on Judd-Ofelt theory, the intensity parameters Omega(t) (t=2, 4, 6), spontaneous emission probability, radiative lifetime, radiative quantum efficiency, width of the emission line and stimulated emission cross-section of Nd(3+) were evaluated. Particularly, the effect of Nd(3+) doping level on them was discussed. With the increase of Nd(3+) concentration in the glass ceramic, the experimental luminescence lifetime, radiative quantum efficiency and stimulated emission cross-section vary from 353.4 micros, 78.3% and 1.86 x 10(-20)cm(2) to 214.7 micros, 39.9% and 1.52 x 10(-20)cm(2), respectively. The comparative study of Nd(3+) spectroscopic parameters in different hosts suggests that the investigated glass ceramic system is potentially applicable as laser materials for 1.06 microm emission.

Using shape to turn off blinking for two-colour multiexciton emission in CdSe/CdS tetrapods.

Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance. In this limited class of nanocrystals, CdSe/CdS core/arm tetrapods exhibit the unusual trait of two-colour (red and green) multiexcitonic emission, with green emission from the CdS arms emerging only at high excitation fluences. Here we show that by synthetic shape-tuning, both this multi-colour emission process, and blinking and photobleaching behaviours of single tetrapods can be controlled. Specifically, we find that the properties of dual emission and single-nanostructure photostability depend on different structural parameters-arm length and arm diameter, respectively-but that both properties can be realized in the same nanostructure. Furthermore, based on results of correlated photoluminescence and transient absorption measurements, we conclude that hole-trap filling in the arms and partial state-filling in the core are necessary preconditions for the observation of multiexciton multi-colour emission.

An insight into non-emissive excited states in conjugated polymers.

Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes. Despite considerable research efforts, the underlying mechanism still remains controversial and elusive. Here, the nature and properties of excited states in the archetypal polythiophene are investigated via aggregates suspended in solvents with different dielectric constants (ɛ). In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2-5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ<∼ 3) they demonstrate much higher fluorescence QY up to 20-30%. A series of mixed quantum-classical atomistic simulations illustrate that dielectric induced stabilization of nonradiative charge-transfer (CT) type states can lead to similar drastic reduction in fluorescence QY as seen experimentally. Fluorescence lifetime measurement reveals that the CT-type states exist as a competitive channel of the formation of emissive exciton-type states.

Aggregation behavior of rod-coil-rod triblock copolymers in a coil-selective solvent.

Recent experiments have reported that the self-assembly of conjugated polymers mimicking rod-coil-rod triblock copolymers (BCPs) in selective solvents exhibits unique aggregate morphologies. However, the nature of the arrangement of the polymers within the aggregates and the spatial organization of the aggregates remain an unresolved issue. We report the results of coarse-grained Langevin dynamics simulations, which investigated the self-assembly behavior of rod-coil-rod BCPs in a coil-selective solvent. We observe a rapid formation of cylindrically shaped multichain clusters. The initial stages of formation of the aggregates was seen to be independent of the strength of the solvent selectivity. However, for higher solvent selectivities, the clusters were seen to merge into larger units at later stages. A reduction in rod to coil block ratio was observed to decrease the size and number of clusters. In the limit of a highly concentrated solution, we observe the formation of a networked system of distinct clusters, which however retain the cylindrical arrangement observed at lower polymer concentrations.

Influence of backbone rigidness on single chain conformation of thiophene-based conjugated polymers.

Structural order of conjugated polymers at different length scales directs the optoelectronic properties of the corresponding materials; thus it is of critical importance to understand and control conjugated polymer morphology for successful application of these materials in organic optoelectronics. Herein, with the aim of probing the dependence of single chain folding properties on the chemical structure and rigidness of the polymer backbones, single molecule fluorescence spectroscopy was applied to four thiophene-based conjugated polymers. These include regioregular poly(3-hexylthiophene) (RR-P3HT), poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-14), poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophene-2-yl)thiophen-2-ylthiazolo[5,4-d]thiazole) (PTzQT-12), and poly(3,3-didodecylquaterthiophene)] (PQT-12). Our previous work has shown that RR-P3HT and PBTTT-14 polymer chains fold in their nanostructures, whereas PQT-12 and PTzQT-12 do not fold in their nanostructures. At the single molecule level, it was found that RR-P3HT single chains almost exclusively fold into loosely and strongly aggregated conformations, analogous to the folding properties in nanostructures. PQT-12 displays significant chain folding as well, but only into loosely aggregated conformations, showing an absence of strongly aggregated polymer chains. PBTTT-14 exhibits a significant fraction of rigid polymer chain. The findings made for single molecules of PQT-12 and PBTTT-14 are thus in contrast with the observations made in their corresponding nanostructures. PTzQT-12 appears to be the most rigid and planar conjugated polymer of these four polymers. However, although the presumably nonfolding polymers PQT-12 and PTzQT-12 exhibit less folding than RR-P3HT, there is still a significant occurrence of chain folding for these polymers at the single molecule level. These results suggest that the folding properties of conjugated polymers can be influenced by the architecture of the polymer backbones; however, other factors such as intermolecular stacking interactions, solvent environment, and side chain interactions in corresponding materials should also be taken into account to predict conjugated polymer material morphology.