The optical properties of organic semiconductors are almost exclusively described using the Frenkel exciton picture. In this description, the strong Coulombic interaction between an excited electron ...and the charged vacancy it leaves behind (a hole) is automatically taken into account. If, in an optical microcavity, the exciton-photon interaction is strong compared to the excitonic and photonic decay rates, a second quasiparticle, the microcavity polariton, must be introduced to properly account for this coupling. Coherent, laser-like emission from polaritons has been predicted to occur when the ground-state occupancy of polaritons left angle bracketngs right angle bracket, reaches 1 (ref. 3). This process, known as polariton lasing, can occur at thresholds much lower than required for conventional lasing. Polaritons in organic semiconductors are highly stable at room temperature, but to our knowledge, there has as yet been no report of nonlinear emission from these structures. Here, we demonstrate polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors.
A human-driven decline in global burned area Andela, N.; Morton, D. C.; Giglio, L. ...
Science (American Association for the Advancement of Science),
06/2017, Letnik:
356, Številka:
6345
Journal Article
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Fire is an essential Earth system process that alters ecosystem and atmospheric composition. Here we assessed long-term fire trends using multiple satellite data sets. We found that global burned ...area declined by 24.3 ± 8.8% over the past 18 years. The estimated decrease in burned area remained robust after adjusting for precipitation variability and was largest in savannas. Agricultural expansion and intensification were primary drivers of declining fire activity. Fewer and smaller fires reduced aerosol concentrations, modified vegetation structure, and increased the magnitude of the terrestrial carbon sink. Fire models were unable to reproduce the pattern and magnitude of observed declines, suggesting that they may overestimate fire emissions in future projections. Using economic and demographic variables, we developed a conceptual model for predicting fire in human-dominated landscapes.
White organic light‐emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid‐state lighting sources. Initially, ...WOLEDs were targeted towards display applications for use primarily as liquid‐crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid‐state lighting sources are also noted.
The efficiency of white organic light‐emitting devices (see Figure) has increased by orders of magnitude in the last twelve years. This review discusses the characterization, architectures, and development of these devices during that period. Photograph courtesy of Toyota Industries Corp.
To obtain the maximum luminous efficiency from an organic material, it
is necessary to harness both the spin-symmetric and anti-symmetric molecular
excitations (bound electron-hole pairs, or ...excitons) that result from
electrical pumping. This is possible if the material is phosphorescent, and
high efficiencies have been observed in phosphorescent organic
light-emitting devices. However, phosphorescence in organic
molecules is rare at room temperature. The alternative radiative process of
fluorescence is more common, but it is approximately 75% less efficient, due
to the requirement of spin-symmetry conservation. Here, we
demonstrate that this deficiency can be overcome by using a phosphorescent
sensitizer to excite a fluorescent dye. The mechanism for energetic coupling
between phosphorescent and fluorescent molecular species is a long-range,
non-radiative energy transfer: the internal efficiency of fluorescence can
be as high as 100%. As an example, we use this approach to nearly quadruple
the efficiency of a fluorescent red organic light-emitting device.
We demonstrate strong exciton-photon coupling in an optical microcavity containing a thermally evaporated polycrystalline organic thin film. Microcavity polaritons result from coupling between the ...0-0 excitonic transition of 3,4,7,8 napthalenetetracarboxylic dianhydride and a cavity photon. For thicker films, the 0-1 transition also couples to the cavity mode, as vibronic relaxation is overcome by the short Rabi period for strong coupling. To our knowledge, this is the first report of strong coupling between a cavity photon and multiple vibronic transitions in a single material, made possible by the pronounced vibronic absorption features characteristic of crystalline organic materials.
White stacked electrophosphorescent organic light‐emitting devices employing three metallorganic phosphors in a compound emission layer (see Figure) are demonstrated. They show an external efficiency ...of 51 % and power efficiency of 20.7 lm W–1 at practical illumination intensities. A transparent MoO3 film between two adjacent electrophosphorescent elements efficiently injects charge into the stacked elements.
Chronic intestinal inflammation and high dietary iron are associated with colorectal cancer development. The role of Stat3 activation in iron-induced colonic inflammation and tumorigenesis was ...investigated in a mouse model of inflammation-associated colorectal cancer. Mice, fed either an iron-supplemented or control diet, were treated with azoxymethane and dextran sodium sulfate (DSS). Intestinal inflammation and tumor development were assessed by endoscopy and histology, gene expression by real-time PCR, Stat3 phosphorylation by immunoblot, cytokines by ELISA and apoptosis by TUNEL assay. Colonic inflammation was more severe in mice fed an iron-supplemented compared with a control diet one week post-DSS treatment, with enhanced colonic IL-6 and IL-11 release and Stat3 phosphorylation. Both IL-6 and ferritin, the iron storage protein, co-localized with macrophages suggesting iron may act directly on IL-6 producing-macrophages. Iron increased DSS-induced colonic epithelial cell proliferation and apoptosis consistent with enhanced mucosal damage. DSS-treated mice developed anemia that was not alleviated by dietary iron supplementation. Six weeks post-DSS treatment, iron-supplemented mice developed more and larger colonic tumors compared with control mice. Intratumoral IL-6 and IL-11 expression increased in DSS-treated mice and IL-6, and possibly IL-11, were enhanced by dietary iron. Gene expression of iron importers, divalent metal transporter 1 and transferrin receptor 1, increased and iron exporter, ferroportin, decreased in colonic tumors suggesting increased iron uptake. Dietary iron and colonic inflammation synergistically activated colonic IL-6/IL-11-Stat3 signaling promoting tumorigenesis. Oral iron therapy may be detrimental in inflammatory bowel disease since it may exacerbate colonic inflammation and increase colorectal cancer risk.
The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (C ∧ N) ligands and a single monoanionic, ...bidentate ancillary ligand (LX), i.e., C ∧ N 2Ir(LX). The C ∧ N ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all β-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the C ∧ N 2Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1−0.6), and microsecond lifetimes (e.g., 1−14 μs). The strongly allowed phosphorescence in these complexes is the result of significant spin−orbit coupling of the Ir center. The lowest energy (emissive) excited state in these C ∧ N 2Ir(acac) complexes is a mixture of 3MLCT and 3(π−π*) states. By choosing the appropriate C ∧ N ligand, C ∧ N 2Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the C ∧ N ligands, which lead to bathochromic shifts of the free ligands, lead to similar bathochromic shifts in the Ir complexes of the same ligands, consistent with “C ∧ N 2Ir”-centered emission. Three of the C ∧ N 2Ir(acac) complexes were used as dopants for organic light emitting diodes (OLEDs). The three Ir complexes, i.e., bis(2-phenylpyridinato-N,C2‘)iridium(acetylacetonate) ppy 2Ir(acac), bis(2-phenyl benzothiozolato-N,C2‘)iridium(acetylacetonate) bt 2Ir(acac), and bis(2-(2‘-benzothienyl)pyridinato-N,C3‘)iridium(acetylacetonate) btp 2Ir(acac), were doped into the emissive region of multilayer, vapor-deposited OLEDs. The ppy 2Ir(acac)-, bt 2Ir(acac)-, and btp 2Ir(acac)-based OLEDs give green, yellow, and red electroluminescence, respectively, with very similar current−voltage characteristics. The OLEDs give high external quantum efficiencies, ranging from 6 to 12.3%, with the ppy 2Ir(acac) giving the highest efficiency (12.3%, 38 lm/W, >50 Cd/A). The btp 2Ir(acac)-based device gives saturated red emission with a quantum efficiency of 6.5% and a luminance efficiency of 2.2 lm/W. These C ∧ N 2Ir(acac)-doped OLEDs show some of the highest efficiencies reported for organic light emitting diodes. The high efficiencies result from efficient trapping and radiative relaxation of the singlet and triplet excitons formed in the electroluminescent process.
This study addresses entropic segregation effects at the surfaces of monodisperse and bidisperse melts. For the monodisperse melts, we focus on the segregation of chain ends to the surface, and for ...the bidisperse melts, we examine the segregation of short polymers to the surface. Universal shapes have been predicted for their concentration profiles, but the derivations rely on the mean-field approximation, which only treats the excluded-volume interactions in an approximate manner. To test whether or not the predictions hold up when the polymers are rigorously prevented from overlapping, we compare mean-field calculations with Monte Carlo simulations performed on the exact same model. Apart from a significant increase in the statistical segment length, the rigorous enforcement of excluded-volume interactions has a relatively small effect on the mean-field predictions. In particular, the universal profiles predicted by mean-field theory are found to be accurate.
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