Giant Shell Research Articles

Unlocking Invisible Defects of ZnSe Alloy Shells in Giant Quantum Dots with Near Unity Quantum Yield

AbstractPhotoluminescence quantum yield (PL QY) of colloidal quantum dots (QDs) can be improved by growing a shell, but it is rather limited if the shell thickness exceeds a threshold. Lattice mismatch between the core and shell is known to determine this critical shell thickness, securing QDs from defect formation through strain release. However, it cannot explain the recently reported high efficiency QDs with giant shells. Based on CdSe/ZnSe thick shell QDs, this study aims to identify the culprit limiting PL QY. In the shell growth process, the gradual reduction in PL QY is accompanied by a low‐energy tail emission, but the additional compressive strain by the outmost shell eliminates such abnormalities. It is revealed that the zinc vacancy in the shell provides shallow hole trap states. The computational study successfully explains the hole‐accepting zinc vacancy states above the CdSe 1Shh state, raised by compressive strain along radial direction. Additional hydrostatic compressive strain lifts the 1Shh state for this strained heterostructure to minimize the energetic gap with the zinc vacancy states. The finding suggests that critical shell thickness can be limited by atomic vacancy incorporated during shell growth, not by formation of misfit dislocation caused by strain release.

Two-dimensional CdSe-CdS heterostructures with thick shell grown at room temperature

In this work we adopted the one-pot two temperature synthesis originally proposed for CdS nanoplatelets in order to synthesize high quality CdSe nanoplatelets. Then, we demonstrate another approach to synthesizing flat CdSe/CdS core/shell nanoplatelets with giant shell grown at room temperature. TEM analysis confirms shell heaving up to 19 monolayer of CdS. Based on optical spectroscopy studies a high quality of the red-color emission was finally proven for obtained 2D-heterostructures.

Purifying single photon emission from giant shell CdSe/CdS quantum dots at room temperature.

Giant shell CdSe/CdS quantum dots are bright and flexible emitters, with near-unity quantum yield and suppressed blinking, but their single photon purity is reduced by efficient multiexcitonic emission. We report the observation, at the single dot level, of a large blueshift of the photoluminescence biexciton spectrum (24 ± 5 nm over a sample of 32 dots) for pure-phase wurtzite quantum dots. By spectral filtering, we demonstrate a 2.3 times reduction of the biexciton quantum yield relative to the exciton emission, while preserving as much as 60% of the exciton single photon emission, thus improving the purity from g2(0) = 0.07 ± 0.01 to g2(0) = 0.03 ± 0.01. At a larger pump fluency, spectral purification is even more effective with up to a 6.6 times reduction in g2(0), which is due to the suppression of higher order excitons and shell states experiencing even larger blueshifts. Our results indicate the potential for the synthesis of engineered giant shell quantum dots, with further increased biexciton blueshifts, for quantum optical applications requiring both high purity and brightness.

Giant Shell Quantum Dots for Color Conversion and as Active Material in QLED

Giant Shell Quantum Dots for Color Conversion and as Active Material in QLED

Elevated temperature and carbon dioxide levels alter growth rates and shell composition in the fluted giant clam, Tridacna squamosa

Giant clams produce massive calcified shells with important biological (e.g., defensive) and ecological (e.g., habitat-forming) properties. Whereas elevated seawater temperature is known to alter giant clam shell structure, no study has examined the effects of a simultaneous increase in seawater temperature and partial pressure of carbon dioxide (pCO2) on shell mineralogical composition in these species. We investigated the effects of 60-days exposure to end-of-the-century projections for seawater temperature (+ 3 °C) and pCO2 (+ 500 µatm) on growth, mineralogy, and organic content of shells and scutes in juvenile Tridacna squamosa giant clams. Elevated temperature had no effect on growth rates or organic content, but did increase shell [24Mg]/[40Ca] as well as [40Ca] in newly-formed scutes. Elevated pCO2 increased shell growth and whole animal mass gain. In addition, we report the first evidence of an effect of elevated pCO2 on element/Ca ratios in giant clam shells, with significantly increased [137Ba]/[40Ca] in newly-formed shells. Simultaneous exposure to both drivers greatly increased inter-individual variation in mineral concentrations and resulted in reduced shell N-content which may signal the onset of physiological stress. Overall, our results indicate a greater influence of pCO2 on shell mineralogy in giant clams than previously recognized.

A Giant Shell of Ionized Gas Discovered near M82 with the Dragonfly Spectral Line Mapper Pathfinder

Abstract We present the discovery of a giant cloud of ionized gas in the field of the starbursting galaxy M82. Emission from the cloud is seen in Hα and [N ii] λ6583 in data obtained though a small pathfinder instrument used to test the key ideas that will be implemented in the Dragonfly Spectral Line Mapper, an upcoming ultranarrow-bandpass imaging version of the Dragonfly Telephoto Array. The discovered cloud has a shell-like morphology with a linear extent of 0.°8 and is positioned 0.°6 northwest of M82. At the heliocentric distance of the M81 group, the cloud’s longest angular extent corresponds to 55 kpc and its projected distance from the nucleus of M82 is 40 kpc. The cloud has an average Hα surface brightness of 2 × 10−18 erg cm − 2 s − 1 arcsec − 2 . The [N ii] λ6583/Hα line ratio varies from [N ii]/Hα ∼ 0.2 to [N ii]/Hα ∼ 1.0 across the cloud, with higher values found in its eastern end. Follow-up spectra obtained with Keck LRIS confirm the existence of the cloud and yield line ratios of [N ii] λ6583/Hα = 0.340 ± 0.003 and [S ii] λλ6716, 6731/Hα = 0.64 ± 0.03 in the cloud. This giant cloud of material could be lifted from M82 by tidal interactions or by its powerful starburst. Alternatively, it may be gas infalling from the cosmic web, potentially precipitated by the superwinds of M82. Deeper data are needed to test these ideas further. The upcoming Dragonfly Spectral Line Mapper will have 120 lenses, 40× more than in the pathfinder instrument used to obtain the data presented here.

Transnational Experts Wanted: Nigerian Oil Spills before the Dutch Courts

Abstract This analysis recapitulates the Hague Court of Appeal’s decisions in a series of lawsuits brought by Nigerian farmers against Dutch oil giant Shell. The ruling upholds Shell’s civil liability for the pollution ensuing from several oil spills and has been widely hailed as a blueprint for similar claims before European courts. Beyond these headlines, the judgment charters unexplored legal territories and elicits burning questions about judicial and environmental expertise in complex transnational settings. I propose a three-pronged analysis to cut through this complexity. First, the case deals with the international ‘anchor’ jurisdiction of Dutch courts over claims against foreign defendants. Secondly, the ruling marks a significant contribution to the jurisprudence on tort liability across corporate structures. Finally, the verdict sheds a new light on the interplay between environmental rights and tort actions. The confluence of these three perspectives projects a kaleidoscopic vision of environmental liability across borders.

High Quantum Yield Water‐Dispersed Near‐Infrared In(Zn)As–In(Zn)P–GaP–ZnS Quantum Dots with Robust Stability for Bioimaging

AbstractFluorescent dyes with a high photoluminescence quantum yield (PLQY) and photostability are favored in bioimaging as they offer better imaging signals at lower dye loading, circumventing issues of cytotoxicity and biological pathway interference. Quantum dots (QDs) are efficient emitters with narrow spectral line‐widths, but often employ toxic heavy‐metals, and show significantly diminished PLQY and poor stability after phase‐transfer into aqueous media. Here, the preparation of nontoxic, water‐dispersed giant‐shell In(Zn)As–In(Zn)P–GaP–ZnS QDs that emit in the near‐infrared (NIR) at 828 nm is demonstrated. The QDs exhibit a remarkable PLQY of 75% when initially synthesized in a hydrophobic medium, and retains a high PLQY of 60% after ligand‐exchange and phase‐transfer into water. The water‐dispersed QDs show robust photostability with negligible photoluminescence (PL) attenuation after 3 h of laser irradiation, suggesting that the giant shell is effective in reducing nonradiative recombination that may be caused by photoinduced defects. The water‐dispersed QDs also exhibit high aqueous colloidal stability, retaining a high PLQY of 53% without QD agglomeration after 14 months of storage at 4 °C at a high loading concentration of 20 mg mL−1. The QDs are also successfully incorporated into HeLa cells for confocal bioimaging with no practical signs of cytotoxicity.

86‐5: Late‐News‐Paper: Giant Shell Quantum Dots for Color Conversion and as Active Material in QLEDs

We present an automated and up‐scaled synthesis of giant‐shell QDs (GSQD). For color conversion, these particles will be incorporated in a suitable matrix with high QD content and transferred onto a blue light LED. We will also show results of QLEDs with a thin film of GSQD as active material.

Role of shell composition and morphology in achieving single-emitter photostability for green-emitting "giant" quantum dots.

The use of the varied chemical reactivity of precursors to drive the production of a desired nanocrystal architecture has become a common method to grow thick-shell graded alloy quantum dots (QDs) with robust optical properties. Conclusions on their behavior assume the ideal chemical gradation and uniform particle composition. Here, advanced analytical electron microscopy (high-resolution scanning transmission electron microscopy coupled with energy dispersive spectroscopy) is used to confirm the nature and extent of compositional gradation and these data are compared with performance behavior obtained from single-nanocrystal spectroscopy to elucidate structure, chemical-composition, and optical-property correlations. Specifically, the evolution of the chemical structure and single-nanocrystal luminescence was determined for a time-series of graded-alloy "CdZnSSe/ZnS" core/shell QDs prepared in a single-pot reaction. In a separate step, thick (∼6 monolayers) to giant (>14 monolayers) shells of ZnS were added to the alloyed QDs via a successive ionic layer adsorption and reaction (SILAR) process, and the impact of this shell on the optical performance was also assessed. By determining the degree of alloying for each component element on a per-particle basis, we observe that the actual product from the single-pot reaction is less "graded" in Cd and more so in Se than anticipated, with Se extending throughout the structure. The latter suggests much slower Se reaction kinetics than expected or an ability of Se to diffuse away from the initially nucleated core. It was also found that the subsequent growth of thick phase-pure ZnS shells by the SILAR method was required to significantly reduce blinking and photobleaching. However, correlated single-nanocrystal optical characterization and electron microscopy further revealed that these beneficial properties are only achieved if the thick ZnS shell is complete and without large lattice discontinuities. In this way, we identify the necessary structural design features that are required for ideal light emission properties in these green-visible emitting QDs.

Suppression of Blinking and Enhancement of Optical Properties of Core-Shell Quantum Dots by Structural Formulation

Core-shell quantum dots (QDs) have gained immense significance in various technological applications due to their unique size-dependent optical properties. However, their efficiency in lightning and display applications can be limited by fluorescence intermittency or blinking which limits the viability of the QDs. In this work, a theoretical study on the causes and mitigation process of the blinking phenomenon in core-shell QDs has been carried out. An approach to eliminate blinking behavior is to modify the traditional core-shell structure where the interface between the core and shell materials is discontinuous and abrupt. A graded alloy core-shell QD where there is a gradual transition between pure core material and pure shell material shows suppressed blinking behavior. Another alternate strategy is the growth of core-shell QDs with giant shell structures and appropriate surrounding matrix considering the traps being few nanometers away from the core. The study of the effect of fluctuations in the local environment of the QDs helps in understanding the mechanisms of blinking and hence in curtailing its effect. These approaches have been applied and analyzed to suppress the blinking phenomenon and improve the optical properties of QDs.

MUSE crowded field 3D spectroscopy in NGC 300

Aims. As a new approach to the study of resolved stellar populations in nearby galaxies, our goal is to demonstrate with a pilot study in NGC 300 that integral field spectroscopy with high spatial resolution and excellent seeing conditions reaches an unprecedented depth in severely crowded fields. Methods. Observations by MUSE with seven pointings in NGC 300 have resulted in data cubes that are analyzed in four ways: (1) Point spread function-fitting 3D spectroscopy with PampelMUSE, as already successfully pioneered in globular clusters, yields de-blended spectra of individually distinguishable stars, thus providing a complete inventory of blue and red supergiants, and asymptotic giant branch (AGB) stars of type M and C. The technique is also applicable to emission line point sources and provides samples of planetary nebulae (PNe) that are complete down to m5007 = 28. (2) Pseudo-monochromatic images, created at the wavelengths of the most important emission lines and corrected for continuum light with the P3D visualization tool, provide maps of H II regions, supernova remnants (SNR), and the diffuse interstellar medium (ISM) at a high level of sensitivity, where also faint point sources stand out and allow for the discovery of PNe, Wolf–Rayet (WR) stars, etc. (3) The use of the P3D line-fitting tool yields emission line fluxes, surface brightness, and kinematic information for gaseous objects, corrected for absorption line profiles of the underlying stellar population in the case of Hα. (4) Visual inspection of the data cubes by browsing through the row-stacked spectra image in P3D is demonstrated to be efficient for data mining and the discovery of background galaxies and unusual objects. Results. We present a catalog of luminous stars, rare stars such as WR, and other emission line stars, carbon stars, symbiotic star candidates, PNe, H II regions, SNR, giant shells, peculiar diffuse and filamentary emission line objects, and background galaxies, along with their spectra. Conclusions. The technique of crowded-field 3D spectroscopy, using the PampelMUSE code, is capable of deblending individual bright stars, the unresolved background of faint stars, gaseous nebulae, and the diffuse component of the ISM, resulting in unprecedented legacy value for observations of nearby galaxies with MUSE.

The possible common origin of M 16 and M 17

Context. It has been suggested that the well-studied giant HII regions M 16 and M 17 may have had a common origin, being an example of large-scale triggered star formation. While some features of the distribution of the interstellar medium in the region support this interpretation, no definitive detection of an earlier population of massive stars responsible for the triggering has been made thus far. Aims. We have carried out observations looking for red supergiants in the area covered by a giant shell seen in HI and CO centered on galactic coordinates l ∼ 14°5, b ∼ +1° whose emission peaks near the same radial velocity as the bulk of the emission from both giant HII regions, which are located along the shell. Red supergiants have ages in the range expected for the parent association whose most massive members could have triggered the formation of the shell and of the giant HII regions along its rim. Methods. We have obtained spectroscopy in the visible of a sample of red stars selected on the basis of their infrared colors, whose magnitudes are consistent with them being red supergiants if they are located at the distance of M 16 and M 17. Spectroscopy is needed to distinguish red supergiants from AGB stars and RGB stars, which are expected to be abundant along the line of sight. Results. Out of a sample of 37 bright red stars, we identify four red supergiants that confirm the existence of massive stars in the age range between ∼10 and ∼30 Myr in the area. At least three of them have Gaia DR2 parallaxes consistent with them being at the same distance as M 16 and M 17. Conclusions. The evidence of past massive star formation within the area of the gaseous shell lends support to the idea that it was formed by the combined action of stellar winds and ionizing radiation of the precursors of the current red supergiants. These could be the remnants of a richer population, whose most massive members have already exploded as core-collapse supernovae. The expansion of the shell against the surrounding medium, perhaps combined with the overrun of preexisting clouds, is thus a plausible trigger of the formation of a second generation of stars currently responsible for the ionization of M 16 and M 17.

Ground penetrating radar in the medieval oyster shell middens of Saint-Michel-en-l'Herm (Vendée, France)

This study presents the integrated results of GPR (Ground Penetrating Radar) and sedimentological analysis, performed on the giant shell middens of the medieval site of Saint-Michel-en-l'Herm (Vendée, France). Avoiding the use of destructive methods like large-scale digging, the purpose of this study was to determine the internal structure of the northern midden and the links with its substrate, and to obtain insight on the manner of its construction. Four GPR profiles were obtained along the main axis of the midden, on the shell mound, and at its foot, in an area leveled by the former extraction of shells. Sedimentological analysis is also presented from study of the extant parts of the midden and with hand auger cores taken along one of the GPR profiles. The integrated results show that each sedimentological facies is related to a specific radar facies, allowing the mapping of lateral and vertical facies variations along the GPR sections. Within these deposits, the major tilted reflectors correspond to thin soils and trampled surfaces. Their geometry and the stacking pattern highlight multiphase deposition triggered by successive inputs of large volumes of shells (in the range of 100m3), from west to east. Below the exploited parts of the midden, the data are interpreted to show the presence of undisturbed shells, although, an ancient dug area was also identified by GPR. Finally, sedimentological and GPR data both show evidence of a midden established on the west bank of a channel, progressively infilling the channel eastward. These results illustrate the relevance of GPR in answering major questions common to all large shell middens.

HI emission from the red giant Y CVn with the VLA and FAST

Imaging studies with the Very Large Array (VLA) have revealed HI emission associated with the extended circumstellar shells of red giants. We analyze the spectral map obtained on Y CVn, a J-type carbon star on the Asymptotic Giant Branch. The HI line profiles can be interpreted with a model of a detached shell resulting from the interaction of a stellar outflow with the local interstellar medium. We reproduce the spectral map by introducing a distortion along a direction corresponding to the star’s motion in space. We then use this fitting to simulate observations expected from the Five-hundred-meter Aperture Spherical radio Telescope (FAST), and discuss its potential for improving our description of the outer regions of circumstellar shells.

Shell’s SADAF exit raises $820 million

Oil giant Shell has agreed to sell its 50% stake in Jubail, Saudi Arabia-based Saudi Petrochemical Co. (SADAF) to Saudi Basic Industries, SADAF’s co-owner, for $820 million. SADAF has a total output of more than 4 million metric tons per year of chemical products that include chlorine, ethanol, ethylene, ethylene dichloride, methyl tert-butyl ether, and styrene. The move is driven by Shell’s goal to narrow the range of its chemicals portfolio. The joint venture agreement would have expired in 2020.

Atomic Structure of Wurtzite CdSe (Core)/CdS (Giant Shell) Nanobullets Related to Epitaxy and Growth.

Heteronanocrystals consisting of a CdSe core and a giant CdS shell have shown remarkable optical properties which are promising for applications in opto-electrical devices. Since these properties sensitively depend on the size and shape, a morphological characterization is of high interest. Here, we present a high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) study of CdSe (core)/CdS (giant shell) heteronanocrystals. Electron tomography reveals that the nanocrystals have a bullet shape, either ending in a tip or a small dip, and that the CdSe core is positioned closer to the tip (or dip) than to the hexagonal base. Based on a high resolution HAADF-STEM study, we were able to determine all the surface facets. We present a heuristic model for the different growth stages of the CdS crystal around the CdSe core.

Combating environmental irresponsibility of transnational corporations in Africa: an empirical analysis

ABSTRACTEnvironmental irresponsibility is one of the most prominent issues confronting host communities of transnational corporations (TNCs) engaged in the production of economic goods and, sometimes, services. Drawing mainly on stakeholder theory, combined with legitimacy theory, this article addresses how host communities in Africa combat the challenge of environmental irresponsibility of TNCs. To illustrate the dimensions and dynamics of the challenge, this paper examines the experience of despoliation of Ogoniland by the oil giant Shell in Nigeria. The analysis draws attention to the significance of the role of individuals and civil society groups in securing accountability of one of the most formidable fronts of economic globalisation. The analysis is particularly relevant to the experience of environmental irresponsibility in the context of weak governance structures.

MILLIMETER OBSERVATIONS OF CS, HCO+, AND CO TOWARD FIVE PLANETARY NEBULAE: FOLLOWING MOLECULAR ABUNDANCES WITH NEBULAR AGE

Millimeter and sub-millimeter observations of CO, CS, and HCO{sup +} have been conducted toward five planetary nebulae (PNe: K4-47, NGC 6537 (Red Spider), M2-48, NGC 6720 (Ring), and NGC 6853 (Dumbbell)), spanning an age range of 900-10,000 yr, using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. The J = 5 → 4, J = 3 → 2, and J = 2 → 1 transitions of CS at 245, 147, and 98 GHz, as well as the J = 3 → 2 and J = 1 → 0 lines of HCO{sup +} at 268 and 89 GHz, were detected toward each source. At least three rotational transitions of CO have also been observed, including the J = 6 → 5 and J = 4 → 3 lines at 691 and 461 GHz. CS had not been definitively identified previously in PNe, and new detections of HCO{sup +} were made in four of the five nebulae. From a radiative transfer analysis of the CO and CS data, kinetic temperatures of T {sub K} ∼ 10-80 K and gas densities of n(H{sub 2}) ∼ 0.1-1 × 10{sup 6} cm{sup –3} were determined for the molecular material inmore » these sources. Column densities for CO, CS, and HCO{sup +} were N {sub tot} ∼ 0.2-5 × 10{sup 16} cm{sup –2}, N {sub tot} ∼ 0.4-9 × 10{sup 12} cm{sup –2}, and N {sub tot} ∼ 0.3-5 × 10{sup 12} cm{sup –2}, respectively, with fractional abundances, relative to H{sub 2}, of f ∼ 0.4-2 × 10{sup –4}, f ∼ 1-4 × 10{sup –8}, and f ∼ 1 × 10{sup –8}, with the exception of M2-48, which had f(HCO{sup +}) ∼ 10{sup –7}. Overall, the molecular abundances do not significantly vary over a duration of 10,000 yr, in contrast to predictions of chemical models. The abundances reflect the remnant asymptotic giant branch shell material, coupled with photochemistry in the early PN phase. These observations also suggest that PNe eject substantial amounts of molecular material into the diffuse interstellar medium.« less

MOND prediction of a new giant shell in the elliptical galaxy NGC 3923

Context. Stellar shells, which form axially symmetric systems of arcs in some elliptical galaxies, are most likely remnants of radial minor mergers. They are observed up a radius of $\sim$100 kpc. The stars in them oscillate in radial orbits. The radius of a shell depends on the free-fall time at the position of the shell and on the time since the merger. We previously verified the consistency of shell radii in the elliptical galaxy NGC 3923 with its most probable MOND potential. Our results implied that an as~yet undiscovered shell exists at the outskirts of the galaxy. Aims. We here extend our study by assuming more general models for the gravitational potential to verify the prediction of the new shell and to estimate its position. Methods. We tested the consistency of the shell radial distribution observed in NGC 3923 with a wide variety of MOND potentials of the galaxy. The potentials differed in the mass-to-light ratio and in distance to the galaxy. We considered different MOND interpolation functions, values of the acceleration constant $a_0$, and density profiles of the galaxy. We verified the functionality of our code on a Newtonian self-consistent simulation of the formation of a shell galaxy. Results. Our method reliably predicts that exactly one new outermost shell exists at a galactocentric radius of about 1900$^{\prime\prime}$ ($\sim$210 kpc) on the southwestern side of the galaxy. Its estimated surface brightness is about 28 mag arcsec$^{-2}$ in $B$ - a value accessible by current instruments. This prediction enables a rare test of MOND in an elliptical down to an acceleration of $a_0/10$. The predictive power of our method is verified by reconstructing the position of the largest known shell from the distribution of the remaining shells.