Ultraviolet Emission Research Articles

Detailed cool star flare morphology with CHEOPS and TESS

Context. White-light stellar flares are proxies for some of the most energetic types of flares, but their triggering mechanism is still poorly understood. As they are associated with strong X and ultraviolet emission, their study is particularly relevant to estimate the amount of high-energy irradiation onto the atmospheres of exoplanets, especially those in their stars’ habitable zone. Aims. We used the high-cadence, high-photometric capabilities of the CHEOPS and TESS space telescopes to study the detailed morphology of white-light flares occurring in a sample of 130 late-K and M stars, and compared our findings with results obtained at a lower cadence. Methods. We employed dedicated software for the reduction of 3 s cadence CHEOPS data, and adopted the 20 s cadence TESS data reduced by their official processing pipeline. We developed an algorithm to separate multi-peak flare profiles into their components, in order to contrast them to those of single-peak, classical flares. We also exploited this tool to estimate amplitudes and periodicities in a small sample of quasi-periodic pulsation (QPP) candidates. Results. Complex flares represent a significant percentage (≳30%) of the detected outburst events. Our findings suggest that high-impulse flares are more frequent than suspected from lower-cadence data, so that the most impactful flux levels that hit close-in exoplanets might be more time-limited than expected. We found significant differences in the duration distributions of single and complex flare components, but not in their peak luminosity. A statistical analysis of the flare parameter distributions provides marginal support for their description with a log-normal instead of a power-law function, leaving the door open to several flare formation scenarios. We tentatively confirmed previous results about QPPs in high-cadence photometry, report the possible detection of a pre-flare dip, and did not find hints of photometric variability due to an undetected flare background. Conclusions. The high-cadence study of stellar hosts might be crucial to evaluate the impact of their flares on close-in exoplanets, as their impulsive phase emission might otherwise be incorrectly estimated. Future telescopes such as PLATO and Ariel, thanks to their high-cadence capability, will help in this respect. As the details of flare profiles and of the shape of their parameter distributions are made more accessible by continuing to increase the instrument precision and time resolution, the models used to interpret them and their role in star-planet interactions might need to be updated constantly.

Modeling Dust Production, Growth, and Destruction in Reionization-era Galaxies with the CROC Simulations. II. Predicting the Dust Content of High-redshift Galaxies

We model the interstellar dust content of the reionization era with a suite of cosmological, fluid-dynamical simulations of galaxies with stellar masses ranging from ∼105 to 109 M ⊙ in the first 1.2 Gyr of the Universe. We use a post-processing method that accounts for dust creation and destruction processes, allowing us to systematically vary the parameters of these processes to test whether dust-dependent observable quantities of galaxies at these epochs could be useful for placing constraints on dust physics. We then forward model observable properties of these galaxies to compare to existing data. We find that we are unable to simultaneously match existing observational constraints with any one set of model parameters. Specifically, the models that predict the largest dust masses D/Z ≳ 0.1 at z = 5—because of high assumed production yields and/or efficient growth via accretion in the interstellar medium—are preferred by constraints on total dust mass and infrared (IR) luminosities, but these models produce far too much attenuation in the ultraviolet (UV), preventing them from matching observations of β UV. To investigate this discrepancy, we analyze the relative spatial distribution of stars and dust as probed by IR and UV emission, which appear to exhibit overly symmetric morphologies compared to existing data, likely due to the limitations of the stellar feedback model used in the simulations. Our results indicate that the observable properties of the dust distribution in high redshift galaxies are a particularly strong test of stellar feedback.

ASTRAEUS

Context. Observations with the James Webb Space Telescope (JWST) have revealed an abundance of bright z > 10 galaxy candidates, challenging the predictions of most theoretical models at high redshifts. Aims. Since massive stars dominate the observable ultraviolet (UV) emission, we explore whether a stellar initial mass function (IMF) that becomes increasingly top-heavy towards higher redshifts and lower gas-phase metallicities results in a higher abundance of bright objects in the early universe and how it influences the evolution of galaxy properties compared to a constant Salpeter IMF. Methods. We parameterised the IMF based on the findings from hydrodynamical simulations that track the formation of stars in differently metal-enriched gas clouds in the presence of the cosmic microwave background (CMB) at different redshifts. We incorporated this evolving IMF into the ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionisation in N-body dArk mattEr simUlationS) framework, which couples galaxy evolution and reionisation in the first billion years. Our implementation accounts for the IMF dependence of supernova (SN) feedback, metal enrichment, and ionising and UV radiation emission. We conducted two simulations: one with a Salpeter IMF and the other with the evolving IMF. In both, we adjusted the free model parameters to reproduce key observables. Results. Compared to a constant Salpeter IMF, we find that (i) the higher abundance of massive stars in the evolving IMF results in more light per unit stellar mass, resulting in a slower build-up of the stellar mass and lower stellar-to-halo mass ratio; (ii) due to the self-similar growth of the underlying dark matter (DM) halos, the evolving IMF’s star formation main sequence scarcely deviates from that of the Salpeter IMF; (iii) the evolving IMF’s stellar mass to gas-phase metallicity relation shifts to higher metallicities, while its halo mass to gas-phase metallicity relation remains unchanged; (iv) the evolving IMF’s median dust-to-metal mass ratio is lower due to its stronger SN feedback; and (v) the evolving IMF requires lower values of the escape fraction of ionising photons and exhibits a flatter median relation and smaller scatter between the ionising photons emerging from galaxies and the halo mass. However, the ionising emissivities of the galaxies mainly driving reionisation (Mh ∼ 1010 M⊙) are comparable to those of a Salpeter IMF, resulting in minimal changes to the topology of the ionised regions. Conclusions. These results suggest that a top-heavier IMF alone is unlikely to explain the higher abundance of bright z > 10 sources, since the lower mass-to-light ratio driven by the greater abundance of massive stars is counteracted by stronger stellar feedback.

Enhanced optoelectronic and catalytic properties of Sm doped SnO2 thin films

Herein, we report the optoelectronic and malachite green (MG) aqueous dye model pollutant degradation characteristics of samarium (Sm) doped tin oxide (SnO2:Sm) thin films deposited on glass substrate by chemical spray pyrolysis technique. The tetragonal crystal structure and polycrystalline nature were elucidated from X-ray diffraction (XRD) analysis, and the calculated crystallite size using Scherrer's relation was found to vary between 36 and 45 nm. A high optical transparency of 92 % was achieved for SnO2:Sm (3 wt%). The extrapolation of linear fit indicates a slight decrement in the band gap. Energy dispersive X-ray analysis (EDAX) confirmed the presence of Sn, O and Sm elements and their composition. The quantitative compositional percentage of Sn, O and Sm was determined by X-ray photoelectron spectroscopy (XPS) analysis. The scanning electron micrographs (SEM) showed polyhedron-like shaped grains with cracks and void-free film surface. The 3 wt% Sm:SnO2 thin film exhibited a higher average surface roughness (19.63 nm) than other films. Strong ultraviolet (365 nm), blue (493 nm), and green (520 nm) emissions were observed for all the films, as recorded by photoluminescence (PL) studies. High electron concentration (7.9 × 1020/cm3), low resistivity (0.356 × 10−3 Ω cm), and a high figure of merit (FOM, 5.11 × 10−2 Ω−1) were obtained for 3 wt % of SnO2:Sm thin films. Furthermore, a relatively high photocurrent (8.22 mA) was attained for the 3 wt% Sm:SnO2 thin film. Enhanced photocatalytic efficiency (95 %) was achieved for the 4 wt% Sm:SnO2 film against MG dye aqueous solution. The results obtained in the present study demonstrate high optical transparency, electrical conductivity, enhanced FOM, and photocatalytic degradation efficiency for 3 and 4 wt% Sm-doping, respectively. Therefore, Sm-doped SnO2 could be useful for developing and designing novel devices for optoelectronic and photocatalytic applications.

Variability of Atomic Hydrogen Brightness in the Martian Exosphere: Insights From the Emirates Ultraviolet Spectrometer on Board Emirates Mars Mission

AbstractThe Emirates Mars Ultraviolet Spectrometer (EMUS), aboard the Emirates Mars Mission (EMM), has been conducting observations of ultraviolet emissions within the Martian exosphere. Taking advantage of the distinctive orbit of the EMM around Mars, EMUS utilizes a dedicated strafe observation strategy to scan the illuminated Martian exosphere at tangential altitudes ranging from 130 to over 20,000 km. To distinguish between emissions of Martian origin and those from the interplanetary background, EMUS conducts specialized background observations by looking away from the planet. This approach has allowed us to investigate the radial and seasonal variations in Martian coronal emission features at H Lyman‐α, β and γ wavelengths. Our analysis supports the previous studies indicating that Martian exospheric hydrogen Lyman emission brightness attains its highest levels around the southern summer solstice and reaches its lowest levels when Mars is near aphelion. Additionally, a secondary peak emission at all altitudes is observed after perihelion during Martian Year (MY) 36, which can be attributed to a Class C dust storm. Our study establishes a strong correlation between solar flux and coronal brightness for these emissions, highlighting the impact of solar activity on the visibility of Martian corona. In addition, we have examined interannual variability and found that emission intensities in MY 37 surpassed those in MY 36, primarily due to increased solar activity. These observations help to understand potential seasonal patterns of exospheric hydrogen, which is driven by underlying mechanisms in the lower atmosphere and solar activity, eventually suggesting an impact on water loss in the Martian atmosphere.

The ALPINE-ALMA [CII] survey: Dust emission effective radius up to 3 kpc in the early Universe

Aims. Measurements of the size of dust continuum emission are an important tool for constraining the spatial extent of star formation, and hence the buildup of stellar mass. Compact dust emission has generally been observed at cosmic noon (z ∼ 2 − 3). However, at earlier epochs, toward the end of the reionization (z ∼ 4 − 6), only the sizes of a handful of infrared (IR) bright galaxies have been measured. In this work, we derive the dust emission sizes of main-sequence (MS) galaxies at z ∼ 5 from the ALPINE survey. Methods. We measured the dust effective radius, re, FIR, in the uv-plane in Band 7 of ALMA for seven ALPINE galaxies with resolved emission and we compared it with rest-frame ultraviolet (UV) and [CII]158 μm measurements. We studied the re, FIR − LIR scaling relation by considering our dust size measurements and all the data in the literature at z ∼ 4 − 6. Finally, we compared our size measurements with predictions from simulations. Results. The dust emission in the selected ALPINE galaxies is rather extended (re, FIR ∼ 1.5 − 3 kpc), similar to [CII]158 μm but a factor of ∼2 larger than the rest-frame UV emission. Putting together all the measurements at z ∼ 5, spanning two decades in luminosity from LIR ∼ 1011 L⊙ to LIR ∼ 1013 L⊙, the data highlight a steeply increasing trend of the re, FIR − LIR relation at LIR < 1012 L⊙, followed by a downturn and a decreasing trend at brighter luminosities. Finally, simulations that extend up to the stellar masses of the ALPINE galaxies considered in the present work predict a subset of galaxies (∼25% at 1010 M⊙ < M⋆ < 1011 M⊙) with sizes as large as those measured.

Identification of a transition from stochastic to secular star formation around z = 9 with JWST

Star formation histories (SFHs) of early galaxies (6 < z < 12) have been found to be highly stochastic in both simulations and observations, while at z≲6 the presence of a main sequence (MS) of star-forming galaxies implies secular processes at play. In this work we characterise the SFH variability of early galaxies as a function of their stellar mass and redshift. We used the JADES public catalogue and derived the physical properties of the galaxies as well as their SFHs using the spectral energy distribution modelling code CIGALE. To this end, we implemented a non-parametric SFH with a flat prior allowing for as much stochasticity as possible. We used the star formation rate (SFR) gradient, an indicator of the movement of galaxies on the SFR–M* plane, linked to the recent SFH of galaxies. This dynamical approach of the relation between the SFR and stellar mass allows us to show that, at z > 9, 87% of massive galaxies (log(M*/M⊙)≳9) have SFR gradients consistent with a stochastic star formation activity during the last 100 Myr, while this fraction drops to 15% at z < 7. On the other hand, we see an increasing fraction of galaxies with a star formation activity following a common stream on the SFR–M* plane with cosmic time, indicating that a secular mode of star formation is emerging. We place our results in the context of the observed excess of UV emission as probed by the UV luminosity function at z ≳ 10 by estimating σUV, the dispersion of the UV absolute magnitude distribution, to be of the order of 1.2 mag, and compare it with predictions from the literature. In conclusion, we find a transition of star formation mode happening around z ∼ 9: Galaxies with stochastic SFHs dominate at z ≳ 9, although this level of stochasticity is too low to reach those invoked by recent models to reproduce the observed UV luminosity function.

The ALPINE-ALMA [C ii] survey: characterization of spatial offsets in main-sequence galaxies at z ∼ 4–6

ABSTRACT The morphology of galaxies is shaped by stellar activity, feedback, gas and dust properties, and interactions with surroundings, and can therefore provide insight into these processes. In this paper, we study the spatial offsets between stellar and interstellar medium emission in a sample of 54 main-sequence star-forming galaxies at z ∼ 4–6 observed with the Atacama Large Millimeter/submillimeter Array (ALMA), and drawn from the ALMA Large Program to INvestigate C+ at Early times (ALPINE). We find no significant spatial offset for the majority (∼70 per cent) of galaxies in the sample among any combination of [C ii], far-infrared continuum, optical, and ultraviolet emission. However, a fraction of the sample (∼30 per cent) shows offsets larger than the median by more than 3σ significance (compared to the uncertainty on the offsets), especially between [C ii] and ultraviolet emission. We find that these significant offsets are of the order of ∼0.5–0.7 arcsec, corresponding to ∼3.5–4.5 kiloparsecs. The offsets could be caused by a complex dust geometry, strong feedback from stars and active galactic nuclei, large-scale gas inflow and outflow, or a combination of these phenomena. However, our current analysis does not definitively constrain the origin. Future, higher resolution ALMA and JWST observations may help resolve the ambiguity. Regardless, since there exist at least some galaxies that display such large offsets, galaxy models and spectral energy distribution fitting codes cannot assume co-spatial emission in all main-sequence galaxies, and must take into account that the observed emission across wavelengths may be spatially segregated.

Trivalent Gd incorporated Zn2SiO4 phosphor material for EPR and luminescence investigations

Abstract Zn2SiO4:Gd3+ powder was obtained by the sol–gel preparation technique and then characterized by using Fourier-transform infrared spectroscopy and X-ray diffraction techniques. The sample exhibited photoluminescence in the ultraviolet spectral zone with a maximum emission wavelength of 314 nm. Ultraviolet emission in the range of 305–320 nm is also called ultraviolet-B emission. This UV range emission is used in lamps for skin treatment and phototherapy. The electron paramagnetic resonance spectrum recorded for the Zn2SiO4:0.015Gd3+ phosphor revealed g values of 3.46, 2.64, and 2.20, confirming the presence of Gd3+ ions in the small ligand field. Furthermore, Gd3+ ions attain tetrahedral symmetry in the prepared host compound.

Computational study of K vacancy with an H interstitial defect in [formula omitted] crystal

The electronic structures, defect formations, defect transition levels and optical properties for VK+Hi defect models in KDP crystals have been studied based on DFT. Lattice dynamics methods give the most reasonable compensation mechanism for VK, namely compensation with the third-nearest Hi neighbor from VK for the paraelectric (PE) phase and compensation with the fourth-nearest neighbor Hi from VK for the ferroelectric (FE) phase. The defect formation energies indicate that the (VK+Hi)x (The superscript represents the charged state, the ‘x’ represents neutral, ‘’’ represents −1 charge state and ‘.’ represents +1 charge state.) is the main defect type in this kind of defect cluster and a self-trapped electron is located at Hi in the (VK + Hi)′ system. For (VK+Hi)x system, one electron is accommodated in VK. There is not a new defect state in the band gap. The Hi bonds with the O ion (0.99 Å) form a hydroxyl. For (VK + Hi)′ system, the hydroxyl is broken, the Hi exists in an atomic form and introduces new defect states in the band gap. As the large relaxation energy leads to a large Huang-Rhys factor, the Stokes red shifts will significantly affect the optical properties. A broad ultraviolet (UV) absorption band and emission band range from UV to visible are obtained originating from the defect cluster. We believe that the far-violet absorption peaks (167 nm and 179 nm) caused by defect cluster VK+Hi can significantly impact the optical damage threshold of the KDP.

Ultra-low threshold deep ultraviolet generation in a hollow-core fiber.

Tunable ultrashort pulses in the ultraviolet spectral region are in great demand for a wide range of applications, including spectroscopy and pump-probe experiments. While laser sources capable of producing such pulses exist, they are typically very complex. Notably, resonant dispersive-wave (RDW) emission has emerged as a simple technique for generating such pulses. However, the required pulse energy used to drive the RDW emission, so far, is mostly at the microjoule level, requiring complicated and expensive pump sources. Here, we present our work on lowering the pump energy threshold for generating tuneable deep ultraviolet pulses to the level of tens of nanojoules. We fabricated a record small-core antiresonant fiber with a hollow-core diameter of just 6 μm. When filled with argon, the small mode area enables higher-order soliton propagation and deep ultraviolet (220 to 270 nm) RDW emission from 36 fs pump pulses at 515 nm with the lowest pump energy reported to date (tens of nanojoules). This approach will allow the use of low-cost and compact laser oscillators to drive nonlinear optics in gas-filled fibers for the first time to our knowledge.

Smart integration of cold plasma stream and surface discharge with ns laser ablation for composite nanomaterial.

In this paper, smart integration of cold dielectric barrier discharge (DBD) plasma in various geometrical arrangements with laser ablation at atmospheric pressure for nanomaterial was described. A composite Co:ZnO target was ablated in an airflow by a nanosecond (ns) laser (wavelength: 1064 nm, pulse duration: 30 ns) using fluence of 5 J-cm-2 at a repetition rate of 10 Hz. The nanomaterial produced under vertical and oblique plasma streams, surface discharge and gas flow, were compared. Utilization surface discharge markedly improved the material adhesion by altering surface intrinsic behavior, inducing anticipated surface energy activation, chemical changes, and the formation of a densely packed solid structure. Under all conditions, the material consistently retained its crystalline nature, elemental composition, and ultraviolet emission characteristics. These preliminary findings hold promise for additional research, suggesting avenues for making complex materials in a flexible environment. Such new advancements could facilitate applications in the biomedical, catalysis, pharmaceutical, and surgical device domains.

Synthesis of 2D Gallium Sulfide with Ultraviolet Emission by MOCVD.

Two-dimensional (2D) materials exhibit the potential to transform semiconductor technology. Their rich compositional and stacking varieties allow tailoring materials' properties toward device applications. Monolayer to multilayer gallium sulfide (GaS) with its ultraviolet band gap, which can be tuned by varying the layer number, holds promise for solar-blind photodiodes and light-emitting diodes as applications. However, achieving commercial viability requires wafer-scale integration, contrasting with established, limited methods such as mechanical exfoliation. Here the one-step synthesis of 2D GaS is introduced via metal-organic chemical vapor deposition on sapphire substrates. The pulsed-mode deposition of industry-standard precursors promotes 2D growth by inhibiting the vapor phase and on-surface pre-reactions. The interface chemistry with the growth of a Ga adlayer that results in an epitaxial relationship is revealed. Probing structure and composition validate thin-film quality and 2D nature with the possibility to control the thickness by the number of GaS pulses. The results highlight the adaptability of established growth facilities for producing atomically thin to multilayered 2D semiconductor materials, paving the way for practical applications.

FT-UV emission spectroscopy of the [formula omitted] progression bands of the [formula omitted] system of [formula omitted]C[formula omitted]O[formula omitted]

We report an experimental investigation of the 0−ν′′ progression bands of the First-Negative (B2Σ+−X2Σ+) system of 12C16O+. The ultraviolet wavelength emission spectrum of CO+ was generated using an air-cooled stainless steel hollow-cathode discharge lamp filled with a mixture of CO2 and Ne. The study includes four bands, i.e. 0−0, 0−1, 0−2 and 0−3, observed in emission with a high-resolution Fourier-transform spectrometer in the 38500–46000 cm−1 region. In total, 541 ro-vibronic transitions have been measured with an absolute accuracy of 0.005–0.010 cm−1. The experimental data were analyzed using the PGOPHER program, which resulted in a set of high accuracy molecular constants and ro-vibronic level positions for the X2Σ+, ν=0−3 and B2Σ+, ν=0 levels. The current values of the band origins have been compared with historical data, including the most recent results obtained by Ventura et al. (2020) and the discrepancies pointed out by the authors were explained.

Absence of nebular He ii λ4686 constrains the UV emission from the Ultraluminous X-ray pulsar NGC 1313 X–2

Abstract While much has been learned in recent decades about the X-ray emission of the extragalactic Ultraluminous X-ray sources (ULXs), their radiative output in the UV band remains poorly constrained. Understanding of the full ULX spectral energy distribution (SED) is imperative to constrain the accretion flow geometry powering them, as well as their radiative power. Here we present constraints on the UV emission of the pulsating ULX (PULX) NGC 1313 X–2 based on the absence of nebular He ii λ4686 emission in its immediate environment. To this end, we first perform multi-band spectroscopy of the ULX to derive three realistic extrapolations of the SED into the unaccessible UV, each predicting varying levels of UV luminosity. We then perform photo-ionization modelling of the bubble nebula and predict the He ii λ4686 fluxes that should have been observed based on each of the derived SEDs. We then compare these predictions with the derived upper limit on He ii λ4686 from MUSE data, which allows us to infer a UV luminosity LUV ≲ 1 × 1039 erg/s in the PULX NGC 1313 X–2. Comparing the UV luminosity inferred with other ULXs, our work suggests there may be an intrinsic difference between hard and soft ULXs, either related to different mass-transfer rates and/or the nature of the accretor. However, a statistical sample of ULXs with inferred UV luminosities is needed to fully determine the distinguishing features between hard and soft ULXs. Finally, we discuss ULXs ionising role in the context of the nebular He ii λ4686 line observed in star-forming, metal-poor galaxies.

Band Gap Modulation in Zn2TiO4 Spinels for Efficient UV-A Persistent Luminescence

Spinels are important materials for an application in bioimaging. The key advantage with spinel-type hosts is the presence of antisite defects, which act as charge reservoirs for trapping electrons and holes at complementary defect sites. This makes them a host system similar to a molecular system. Herein, we present a systematic approach to modulating the band gap of an inverse Zn2TiO4 spinel. With a change in ZnO concentration, the absorption band at 375 nm diminishes and disappears at a ZnO:TiO2 concentration of 1.40:1.00. The band gap of the material is modified from 3.30 to 4.40 eV. The crystal structure of the sample does not change drastically as determined using X-ray diffraction and Rietveld refinement. The Zn2TiO4 emits in the UV-A region with a lifetime in the time domain of `ns’. The sample also shows persistent luminescence of at least 15 min upon excitation with 254 nm with prominent emission in the UV-A region (300–390 nm). The present results open a new avenue for the synthesis of spinel hosts where the band gap can be modified with ease. The UV emission thus observed is expected to find usage in interesting applications like photocatalysis, anti-counterfeiting, water disinfecting, etc.

Measurement report: Optical characterization, seasonality, and sources of brown carbon in fine aerosols from Tianjin, North China: year-round observations

Abstract. To investigate the optical characteristics and sources of brown carbon (BrC) in North China, where the atmospheric aerosol loadings are high and have severe impacts on Earth's climate system, we collected fine aerosols (PM2.5) at an urban site in Tianjin over a 1-year period. We measured the ultraviolet (UV) light absorption and excitation emission matrix (EEM) fluorescence of the water-soluble BrC (WSBrC) and the water-insoluble but methanol-soluble BrC (WI-MSBrC) in the PM2.5 using a three-dimensional fluorescence spectrometer. The average light absorption efficiency of both WSBrC (Abs365, WSBrC) and WI-MSBrC (Abs365, WI-MSBrC) at 365 nm was found to be highest in winter (10.4 ± 6.76 and 10.0 ± 5.13 Mm−1, respectively) and distinct from season to season. The averages of the fluorescence index (FI) and the biological index (BIX) of WSBrC were lower in summer than in the other seasons and opposite to that of the humification index (HIX), which implied that the secondary formation and further chemical processing of aerosols were more intensive during the summer period than in the other seasons. However, in winter, the higher HIX together with the higher FI and BIX of WI-MSBrC suggested that the BrC loading was mainly influenced by primary emissions from biomass burning and coal combustion. Based on the EEM, the types of fluorophores in WSBrC were divided into humic-like substances (HULIS), including low-oxygenated and high-oxygenated species and protein-like compounds (PLOM), though mostly PLOM in the WI-MSBrC. The direct radiation absorption by both WSBrC and WI-MSBrC in the range of 300–400 nm accounted for ∼ 40 % of that (SFEAbs, 4.97 ± 2.71 and 7.58 ± 5.75 W g−1, respectively) in the range 300–700 nm.

Luminescent properties of Y2LaCaGa3ZrO12: Bi3+, Al3+ cyan phosphors for application in white light emitting diodes

High-quality white light can be obtained from a mixture of blue, green, and red phosphors excited by ultraviolet chips. However, this approach suffers from the presence of a cyan gap in the emission spectra, which hinders further improvement in the color rendering index (CRI). The obvious solution to this problem should be the development of efficient ultraviolet-excited cyan phosphors to fill this gap. Therefore, a series of Y2LaCaGa3ZrO12: xBi3+ (x = 0–0.1) cyan phosphors were synthesized via high-temperature solid-state reaction method, and their properties were systematically characterized. The Bi3+ ion in Y2LaCaGa3ZrO12 exhibits dual-mode luminescence with emission bands peaking at 356 nm and 483 nm, originating from the 3P1→1S0 transitions of Bi3+ at different sites. The ultraviolet emission is attributed to Bi3+ occupying 8 coordination sites, while the cyan emission arises from Bi3+ in 6 coordination sites. It is noteworthy that the emission intensity and thermal stability of the phosphors can be further improved by partially substituting Ga3+ with Al3+. By incorporating Y2LaCaGa3ZrO12: 0.02Bi3+ or Y2LaCaGa2.7Al0.3ZrO12: 0.02Bi3+ with commercial phosphors and depositing them onto a 365 nm ultraviolet chip, the prototype white light emitting diodes (w-LEDs) exhibits white light with high CRI of 89.6 and 90.4, respectively. These results validate the potential of Bi3+ activated Y2LaCaGa3-yAlyZrO12 (y = 0–1) cyan phosphor as a promising candidate material for application in UV-excited w-LEDs.

Efficient Ultraviolet Circularly Polarized Luminescence in Zero-Dimensional Hybrid Cerium Bromides.

Ultraviolet circularly polarized luminescence (UV-CPL) with high photon energy shows great potential in polarized light sources and stereoselective photopolymerization. However, developing luminescent materials with high UV-CPL performance remains challenging. Here, we report a pair of rare earth Ce3+-based zero-dimensional (0D) chiral hybrid metal halides (HMHs), R/S-(C14H24N2)2CeBr7, which exhibits characteristic UV emissions derived from the Ce 5d-4f transition. The compounds show simultaneously high photoluminescent quantum yields of (32-39)% and large luminescent dissymmetry factor (|glum|) values of (1.3-1.5)×10-2. Thus, the figures of merits of R/S-(C14H24N2)2CeBr7 are calculated to be (4.5-5.8)×10-3, which are superior to the reported UV-CPL emissive materials. Additionally, nearly 91 % of their PL intensities at 300 K can be well preserved at 380 K (LED operating temperature) without phase transition or decomposition, demonstrating the excellent structural and optical thermal stabilities of R/S-(C14H24N2)2CeBr7. Based on these enantiomers, the fabricated UV-emitting CP-LEDs exhibit high polarization degrees of ±1.0 %. Notably, the UV-CPL generated from the devices can significantly trigger the enantioselective photopolymerization of diacetylene with remarkable stereoselectivity, and consequently yield polymerized products with the anisotropy factors of circular dichroism (gCD) up to ±3.9×10-2, outperforming other UV-CPL materials and demonstrating their great potential as UV-polarized light sources.

Efficient Ultraviolet Circularly Polarized Luminescence in Zero‐Dimensional Hybrid Cerium Bromides

AbstractUltraviolet circularly polarized luminescence (UV‐CPL) with high photon energy shows great potential in polarized light sources and stereoselective photopolymerization. However, developing luminescent materials with high UV‐CPL performance remains challenging. Here, we report a pair of rare earth Ce3+‐based zero‐dimensional (0D) chiral hybrid metal halides (HMHs), R/S‐(C14H24N2)2CeBr7, which exhibits characteristic UV emissions derived from the Ce 5d–4f transition. The compounds show simultaneously high photoluminescent quantum yields of (32–39)% and large luminescent dissymmetry factor (|glum|) values of (1.3–1.5)×10−2. Thus, the figures of merits of R/S‐(C14H24N2)2CeBr7 are calculated to be (4.5–5.8)×10−3, which are superior to the reported UV‐CPL emissive materials. Additionally, nearly 91 % of their PL intensities at 300 K can be well preserved at 380 K (LED operating temperature) without phase transition or decomposition, demonstrating the excellent structural and optical thermal stabilities of R/S‐(C14H24N2)2CeBr7. Based on these enantiomers, the fabricated UV‐emitting CP‐LEDs exhibit high polarization degrees of ±1.0 %. Notably, the UV‐CPL generated from the devices can significantly trigger the enantioselective photopolymerization of diacetylene with remarkable stereoselectivity, and consequently yield polymerized products with the anisotropy factors of circular dichroism (gCD) up to ±3.9×10−2, outperforming other UV‐CPL materials and demonstrating their great potential as UV‐polarized light sources.