Absorption Of UV Photons Research Articles

Conformal zinc sulfide coating of vertically aligned ZnO nanorods by two-step hydrothermal synthesis on wide bandgap seed layers for lead-free perovskite solar cells

Vertically aligned ZnO nanorods (NRs) were grown hydrothermally on the wide bandgap (∼3.86 – 4.04 eV) seed layers (SLs) of grain size ∼162 ± 35 nm, prepared using ball-milled derived ZnO powder. The synthesized ZnO NRs were further decorated with ZnS nanocrystals to achieve a ZnO NR-ZnS core–shell (CS)-like nano-scaffolds by a subsequent hydrothermal synthesis at 70 °C for 1 h. UV-Vis-NIR spectroscopy, x-ray diffractometry (XRD), Raman spectroscopy and Field emission scanning electron microscopy (FESEM) coupled with Energy dispersive x-ray spectroscopy (EDX) analyses confirmed the formation of ZnS atop the vertically aligned ZnO NR arrays of ∼1.79 ± 0.17 µm length and ∼165 ± 27 nm diameter. Transmission electron microscopy (TEM)/EDX analyses revealed that vertically aligned ZnO NRs (core dia. ∼181 ± 12 nm) arrays are conformally coated by an ultrathin ZnS (∼25 ± 7 nm) shell layer with a preferential ZnS{111}/ZnO{10-10}-like partial epitaxy. The ZnO NRs exhibited a sharp band edge near ∼384 nm having optical bandgap energy (E g) of ∼3.23 eV. However, the ZnO NR-ZnS CS exhibited double absorption bands at E g ∼ 3.20 eV (ZnO-core) and E g ∼ 3.78 eV (ZnS-shell). The ZnS{111}/ZnO{10-10}-nano-scaffolds could be utilized to facilitate the enhanced absorption of UV photons as well as the radial junction formation between the Pb-free perovskite absorber and ZnS/ZnO NRs layers.

Determining Factors to Understand the External Quantum Efficiency Values: Study Carried Out with Copper(I)-I and 1,2-Bis(4-pyridyl)ethane Coordination Polymers as Downshifters in Photovoltaic Modules.

Downshifters refer to compounds with the capacity to absorb UV photons and transform them into visible light. The integration of such downshifters has the potential to improve the efficiency of commercial photovoltaic modules. Initially, costly lanthanide derivatives and organic fluorescent dyes were introduced, resulting in a heightened module efficiency. In a novel research direction guided by the same physicochemical principles, the utilization of copper(I) coordination compounds is proposed. This choice is motivated by its simpler and more economical synthesis, primarily due to copper being a more abundant and less toxic element. Our proposal involves employing 1,2-bis(4-pyridyl) ethane (bpe), an economically viable commercial ligand, in conjunction with CuI to synthesize coordination polymers: [CuI(bpe)]n(1), [Cu3I3(bpe)3]n(2), and [CuI(bpe)0.5]n(3). These polymers exhibit the ability to absorb UV photons and emit light within the green and orange spectra. To conduct external quantum efficiency studies, the compounds are dispersed on glass and then encapsulated with ethylene vinyl acetate through heating to 150 °C. Interestingly, during these procedural steps, the solvents and temperatures employed induce a phase transformation, which has been thoroughly examined through both experimental analysis and theoretical calculations. The outcomes of these studies reveal an enhancement in external quantum efficiency with [Cu3I3(bpe)3]n(2), at a cost significantly lower (between 340 and 350 times) than that associated with lanthanide DS complexes.

Photofragmentation of corannulene (C20H10) and sumanene (C21H12) cations in the gas phase and their astrophysical implications.

Aromatic infrared bands (AIBs) dominate the mid-infrared spectra of many galactic and extragalactic sources. These AIBs are generally attributed to fluorescence emission from aromatic molecules. Unified efforts from experimentalists and theoreticians to assign these AIB features have recently received additional impetus with the launch of the James Webb Space Telescope (JWST) as the Mid-InfraRed Instrument (MIRI) delivers a mid-IR spectrum with greatly increased sensitivity and spectral resolution. PAHs in space can exist in either neutral or ionic form, absorb UV photons and undergo fragmentation, becoming a rich source of small hydrocarbons. This top-down mechanism of larger PAHs fragmenting into smaller species is of utmost importance in photo-dissociation regions (PDR) in space. In this work, we experimentally and theoretically investigate the photo-fragmentation pathways of two astronomically significant PAH cations - corannulene (C20H10) and sumanene (C21H12), which are structural motifs of fullerene C60, to understand their sequential fragmentation pathways. The photo-fragmentation experiments exhibit channels that are significantly different from planar PAHs. The breakdown of the carbon skeleton is found to follow different pathways for C20H10 and C21H12 because of the number and positioning of pentagon rings, yet the most abundant low mass cations produced by these two species are found to be similar. The low mass cations showcased in this work could be of interest due to their possible astronomical detections. For completeness, the qualitative photofragmentation behaviour of dicationic corannulene and sumanene has also been investigated, but the potential energy surface of these dications is beyond the scope of this paper.

Stellar-wind variability in Cygnus X-1 from high-resolution excess variance spectroscopy with Chandra

Context. Stellar winds of massive stars are known to be driven by line absorption of UV photons, a mechanism that is prone to instabilities, causing the wind to be clumpy. The clumpy structure hampers wind mass-loss estimates, limiting our understanding of massive star evolution. The wind structure also impacts accretion in high-mass X-ray binary (HMXB) systems. Aims. We aim to analyse the wavelength-dependent variability of X-ray absorption in the wind to study its structure. Such an approach is possible in HMXBs, where the compact object serves as an X-ray backlight. We probe different parts of the wind by analysing data taken at superior and inferior conjunctions. Methods. We applied excess variance spectroscopy to study the wavelength-dependent soft (2–14 Å) X-ray variability of the HMXB Cygnus X-1 in the hard spectral state. Excess variance spectroscopy quantifies the variability of an object above the statistical noise as a function of wavelength, which allows us to study the variability of individual spectral lines. This technique was applied to high-resolution gratings spectra provided by Chandra, accounting for various systematic effects. The frequency dependence is investigated by changing the time binning. Results. The strong orbital phase dependence we observe in the excess variance is consistent with column-density variations predicted by a simple model for a clumpy wind. We identify spikes of increased variability with spectral features found by previous spectroscopic analyses of the same data set, most notably from silicon in over-dense clumps in the wind. In the silicon line region, the variability power is redistributed towards lower frequencies, hinting at increased line variability in large clumps. In prospect of the microcalorimetry missions that are scheduled to launch within the next decade, excess variance spectra present a promising approach to constraining the wind structure, especially if accompanied by models that consider changing ionisation.

Synthesis and Characterization of a Polystyrene-based Scintillator for Gamma Detection

This study aimed to create and examine plastic scintillators made from a polystyrene matrix, which are widely used in radiation detection due to their favorable properties such as rapid decay time, low cost, resistance to moisture, and ease of fabrication. The scintillators were doped with p-terphenyl (PTP) and 1,4-bis[2-(phenyloxazolyl)]-benzene (POPOP) using an injection method with an extruder machine. The materials were then characterized using various techniques. Fourier Transform Infrared Spectroscopy (FTIR) revealed the presence of aromatic chains which are essential for the scintillation process. Differential Scanning Calorimetry (DSC) analysis showed stable thermal properties with a glass transition temperature of approximately 100ºC. Scanning Electron Microscopy (SEM) showed that the surface of the polymer is amorphous with small bumps and protrusions likely caused by the PTP and POPOP dopants. The optical evaluation indicated that the sample could absorb UV photons up to 340 nm and emit photons in the wavelength range of 400-500 nm with a peak at 421 nm. Gamma spectra analysis indicated that the plastic scintillators performed well in gamma detection and could be used in a Radiation Portal Monitor (RPM).

Synthesis and characterization of phosphorylated cellulose nanocrystals for enhanced UV stability of epoxy nanocomposite films

Epoxy resins offer excellent chemical resistance, low-temperature curing, and low shrinkage. Despite these excellent properties, they are susceptible to UV degradation, which compromises their durability in outdoor applications. Herein, we fabricated a nanocomposite epoxy film based on phosphorylated cellulose nanocrystal (P-CNC) to improve the UV stability of cured epoxy resin by incorporating liquid ammonia-treated P-CNC into an epoxy matrix using integral blending method. Ammonium phosphate groups ((NH4)2HPO4) were formed by nucleophilic attack of phosphates by ammonia on the P-CNC surface. The functional groups, morphology, crystallinity index, and surface charges of P-CNC were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrophoretic light scattering (ELS), respectively. The effect of P-CNC on the UV stability of the fabricated epoxy nanocomposite film was investigated using FTIR and UV–Vis before and after exposure to UV photons. The results showed that the incorporation of 1.0 wt.% of surface-treated P-CNC into the epoxy matrix maintained the UV stability of the nanocomposite film up to 800 h. Finally, a mechanism for UV stability of P-CNC epoxy nanocomposite film was proposed whereby the (NH4)2HPO4 groups grafted onto the P-CNC surface absorb UV photons and convert them into low-energy thermal energy or short wavelength electromagnetic waves, which are not sufficient to initiate photodegradation of the nanocomposite film. Thus, this new film provides resistance against UV degradation for extended service life for outdoor applications of epoxy resin.

Photothermoelectric AZO/SiO2/NiO Device

AbstractTransparent‐conductive‐oxide (TCO) materials and transparent devices combining photovoltage and thermoelectric effects are still scarce. Hence, a new transparent‐conductive‐oxide/insulating/transparent‐semiconductor‐oxide (TCO‐I‐TSO) structure combining such effects is developed. It is made of aluminum‐doped zinc oxide (AZO)/SiO2/NiO thin films sequentially deposited on glass substrates. AZO exhibits thermo and photovoltage in response to gradient temperature and absorption of UV photons, while NIR photons absorption in the NiO layer. Photovoltage appears in the plane between the AZO and NiO layer when the whole sample is irradiated with near infrared light, and it also depends on the thickness of the SiO2 layer. This photovoltage is continuously monitored on samples placed in a glass window facing south. Throughout the day, the photovoltage varies from 0 to 300 µV proportionally to the light intensity.

Nanoscale-Thick CuPc/β-Ga2O3 p–n Junctions for Harsh-Environment-Resistant Self-Powered Deep-UV Photodetectors

Due to their crucial role in ultraviolet communication and monitoring, deep-ultraviolet (DUV) photodetectors have garnered much interest. Recently, Ga2O3 has emerged as the best material for DUV photodetectors because of its ultrawide bandgap (4.5–4.9 eV), excellent UV photon absorption coefficient, high structural stability, and affordability. However, there are several difficulties in realizing high-performance Ga2O3-based DUV photodetectors with a high tolerance for harsh environments. In this work, nanoscale-thick CuPc/β-Ga2O3 p–n junctions were used to build high-performance DUV photodetectors by a straightforward solution-processing approach. The p–n junction photodetectors exhibit improved photoelectric performance compared to a single device made of β-Ga2O3 or CuPc, with a photo-to-dark current ratio of 3700 and a fast response time of ∼20 ms under a bias of 0 V. Due to the excellent stability of the nanoscale-thick CuPc film, the device can maintain a high photocurrent even at high temperatures or under long-term DUV irradiation. Our work provides an effective strategy toward highly harsh-environment-resistant DUV photodetectors.

Dusty Starbursts Masquerading as Ultra-high Redshift Galaxies in JWST CEERS Observations

Lyman-break galaxy (LBG) candidates at z ≳ 10 are rapidly being identified in James Webb Space Telescope (JWST)/NIRCam observations. Due to the (redshifted) break produced by neutral hydrogen absorption of rest-frame UV photons, these sources are expected to drop out in the bluer filters while being well detected in redder filters. However, here we show that dust-enshrouded star-forming galaxies at lower redshifts (z ≲ 7) may also mimic the near-infrared (near-IR) colors of z > 10 LBGs, representing potential contaminants in LBG candidate samples. First, we analyze CEERS-DSFG-1, a NIRCam dropout undetected in the F115W and F150W filters but detected at longer wavelengths. Combining the JWST data with (sub)millimeter constraints, including deep NOEMA interferometric observations, we show that this source is a dusty star-forming galaxy (DSFG) at z ≈ 5.1. We also present a tentative 2.6σ SCUBA-2 detection at 850 μm around a recently identified z ≈ 16 LBG candidate in the same field and show that, if the emission is real and associated with this candidate, the available photometry is consistent with a z ∼ 5 dusty galaxy with strong nebular emission lines despite its blue near-IR colors. Further observations on this candidate are imperative to mitigate the low confidence of this tentative submillimeter emission and its positional uncertainty. Our analysis shows that robust (sub)millimeter detections of NIRCam dropout galaxies likely imply z ∼ 4–6 redshift solutions, where the observed near-IR break would be the result of a strong rest-frame optical Balmer break combined with high dust attenuation and strong nebular line emission, rather than the rest-frame UV Lyman break. This provides evidence that DSFGs may contaminate searches for ultra-high redshift LBG candidates from JWST observations.

The evolution of circumstellar discs in the galactic centre: an application to the G-clouds

ABSTRACT The Galactic Centre is known to have undergone a recent star formation episode a few Myr ago, which likely produced many T Tauri stars hosting circumstellar discs. It has been suggested that these discs may be the compact and dusty ionized sources identified as ‘G-clouds’. Given the Galactic Centre’s hostile environment, we study the possible evolutionary pathways these discs experience. We compute new external photoevaporation models applicable to discs in the Galactic Centre that account for the subsonic launching of the wind and absorption of UV photons by dust. Using evolutionary disc calculations, we find that photoevaporation’s rapid truncation of the disc causes them to accrete onto the central star rapidly. Ultimately, an accreting circumstellar disc has a lifetime ≲ 1 Myr, which would fail to live long enough to explain the G-clouds. However, we identify a new evolutionary pathway for circumstellar discs in the Galactic Centre. Removal of disc material by photoevaporation prevents the young star from spinning down due to magnetic braking, ultimately causing the rapidly spinning young star to torque the disc into a ‘decretion disc’ state which prevents accretion. At the same time, any planetary companion in the disc will trap dust outside its orbit, shutting down photoevaporation. The disc can survive for up to ∼10 Myr in this state. Encounters with other stars are likely to remove the planet on Myr time-scales, causing photoevaporation to restart, giving rise to a G-cloud signature. A giant planet fraction of $\sim 10{{\ \rm per\ cent}}$ can explain the number of observed G-clouds.

Understanding the impact of Y2+ (Y[dbnd]Mn, Fe, and Co) cations on physical properties of SrYGe2O6 clinopyroxene: A DFT insight

Pyroxenes are one of the most important minerals in igneous rock with extremely varied compositions. Here, the structural, mechanical, magnetic, thermodynamic, and optoelectronic features of the SrYGe2O6 (Y= Mn, Fe, and Co) in the monoclinic phase is presented by the density functional theory (DFT) investigation. The generated structures using GGA-PBE, GGA-PBSOL, and LDA exchange-correlation functional in a ferromagnetic ordering have lattice parameters that agree with the existing experimental data. All of the three structures are found mechanically stable by Born stability criteria. The most ductile material is SrCoGe2O6, whereas Poisson's ratio (σ) predicts that SrMnGe2O6 is brittle. These have metallic nature for one spin state and semiconducting for another spin state, which confirms the half-metallic or half semiconducting nature. The direct type half-metallic gap for SrCoGe2O6, SrMnGe2O6 and SrFeGe2O6 are found as 1.76 eV, 1.252 eV, and 0.701 eV, respectively. A deep electronic structural configuration is analyzed by the bond population analysis. Effective masses of charge carriers and exciton binding energies revealed insight into the transport properties. The structures exhibit significant UV photon absorption, high visible photon reflectivity, and plasmon peaks of about 13 eV were seen in estimated loss function spectra. These characteristics collectively point to possible applications for these pyroxenes in optoelectronics and spintronics.

Solar-driven heterogeneous photocatalysis using a static mixer as TiO2-P25 support: Impact of reflector optics and material

Solar-driven heterogeneous photocatalysis using semiconductor thin-films immobilised on inert surfaces has restricted applicability, mainly due to the low catalyst surface per reactor volume and mass and photon transfer limitations. Furthermore, sunlight capture hardware presents some setbacks relating to high implementation and maintenance costs. Therefore, this work aims to overcome those limitations by using a stainless steel (SS) Kenics® static mixer (SM) coated with a thin-film of TiO2-P25 and assembled inside a borosilicate glass absorber tube (AT) placed above distinct solar collectors composed of different materials (uncoated [R85], coated [MS], and soiled anodised aluminium [R85s] and SS) and optics (traditional [DP] and [SP] simple double parabola, and flat [F]). The photon flux reaching/entering the AT (measured by 2-nitrobenzaldehyde actinometry) agreed with the theoretical light efficiency estimated for each reflective surface (based on material specular reflectance and geometry ray-wtrace analysis), increasing as follows: SS-F ≈ R85-F < R85s-DP ≈ SS-SP < SS-DP < MS-DP < R85-DP. The ability of each solar collector in combination with the Kenics® SM as TiO2-P25 support on promoting heterogeneous photocatalysis was compared using oxytetracycline (OTC) as the target pollutant. OTC degradation rates over time agreed with actinometric results, leading to similar apparent photonic efficiencies for all reflective surfaces, excepting R85-DP and MS-DP reflectors, which were slightly lower. Moreover, the semiconductor thin-film deposited on the Kenics® SM surface after calcination remained stable for at least seven consecutive cycles. Besides, the Kenics® SM geometry combined with flat reflectors allowed a more efficient absorption of solar UV photons by the catalyst when using two ATs, resulting in the highest volumetric treatment capacity per unit of time and collector surface. The enduring SS-F was found to be the most compact (37% fewer square meters) and cost-effective solution (50% less expensive).

Site-Specific Photochemical Reaction for Improved C=C Location Analysis of Unsaturated Lipids by Ultraviolet Photodissociation.

Unraveling the complexity of the lipidome requires the development of novel approaches to facilitate structural identification and characterization of lipid species with isomer-level discrimination. Ultraviolet photodissociation tandem mass spectrometry (UVPD MS/MS) is a promising tool for structure determination of lipids. The sensitivity of UVPD for lipid analysis however is limited mainly due to weak absorption of UV photons by a C=C. Herein, a C=C site-specific derivatization, the Paternò-Büchi (PB) reaction, was used to incorporate a chromophore to the C=C moiety in fatty acyls, leading to significantly improved UVPD efficiency and sensitivity for pinpointing C=C locations. The wavelength-dependent photodissociation of the PB products demonstrated 4-CF3-benzophenone as the best reagent for UVPD in terms of the efficiency of generating C=C diagnostic fragments and simplicity for C=C location assignments. We demonstrated the effectiveness of this approach for the shotgun profiling of C=C location isomers in different lipid classes from complex lipid extracts, highlighting its potential to advancing the identification of the C=C bond locations in unsaturated lipids.

Electron Holes in G-Quadruplexes: The Role of Adenine Ending Groups.

The study deals with four-stranded DNA structures (G-Quadruplexes), known to undergo ionization upon direct absorption of low-energy UV photons. Combining quantum chemistry calculations and time-resolved absorption spectroscopy with 266 nm excitation, it focuses on the electron holes generated in tetramolecular systems with adenine groups at the ends. Our computations show that the electron hole is placed in a single guanine site, whose location depends on the position of the adenines at the 3′ or 5′ ends. This position also affects significantly the electronic absorption spectrum of (G+)● radical cations. Their decay is highly anisotropic, composed of a fast process (<2 µs), followed by a slower one occurring in ~20 µs. On the one hand, they undergo deprotonation to (G-H2)● radicals and, on the other, they give rise to a reaction product absorbing in the 300–500 nm spectral domain.

Intermediate-size fullerenes as degradation products of interstellar polycyclic aromatic hydrocarbons

The high interstellar abundances of polycyclic aromatic hydrocarbons (PAHs) and their size distribution are the result of complex chemical processes implying dust, UV radiation, and the main gaseous components (H, C+, and O). These processes must explain the high abundance of relatively small PAHs in the diffuse interstellar medium (ISM) and imply the continuous formation of some PAHs that are small enough (number of carbon atoms NC &lt; ~35–50) to be completely dehydrogenated by interstellar UV radiation. The carbon clusters Cn thus formed are constantly exposed to the absorption of ~10–13.6 eV UV photons, allowing isomerization and favoring the formation of the most stable isomers. They might tend to form irregular carbon cages. The frequent accretion of interstellar C+ ions could favor further cage isomerization, as is known in the laboratory for C60, possibly yielding most stable fullerenes, such as C40, C44, and C50. These fullerenes are expected to be very stable in the diffuse ISM because C2 ejection is not possible by single UV photon absorption, but could need rare two-photon absorption. It is possible that at least one of these fullerenes or its cation is as abundant as C60 or C60+ in the diffuse ISM, although this abundance is limited by the lack of observed matching features in observed mid-infrared spectra. B3LYP calculations of the visible spectrum for a number of fullerene isomers with 40 ≤NC ≤ 50 show that they generally have a few spectral bands in the visible range, with f-values in the range of a few 10−2. This could make such fullerenes interesting candidates for the carriers of some diffuse interstellar bands.

Wireless UV-A LEDs-driven AOP in the treatment of agro-industrial wastewaters

A wireless UV-A LEDs lab-scale reactor powered by a resonant inductive coupling (RLC) system was built to maximize the UV photon absorption of agro-industrial wastewaters. The UV-A LEDs (λ = 365 nm) energy was supplied through a magnetic field generated inside of the photoreactor by induction coils placed on the external wall made of polyvinyl chloride. Immersing the light sources in the wastewater increases the photon transfer efficiency and the reaction rate. Maximum magnetic field and optical irradiance were obtained at 26.8 and 27.0 kHz, respectively.As proof-of-concept, elderberry wastewater (EW), olive washing wastewater (OWW) and white and red winery wastewaters (WWW and RWW) were treated combining the wireless UV-A LEDs with the Advanced Oxidation Process (AOP) - Fenton reagent.Fenton experiments were performed using [Fe2+] = 10 mg L−1, [H2O2] = 500 mg L−1, pH = 3 and a reaction time of 4 h. With EW a DOC removal of 35% (k = 0.0696 h−1) was achieved, whereas adding the wireless UV-A LEDs (f = 26.8 kHz) 53% was attained (k = 0.1722 h−1). The Electric Energy per Order (EEO) for the wireless UV-A LEDs consumption was calculated (EEO LEDs = 48.7 kWh m−3 order−1) and for all the remain equipment (air pump, RC box and power amplifier), EEO total = 495 kWh m−3 order−1. Experiments with OWW presented a DOC removal of 62% and a EEO LEDs = 40.5 kWh m−3 order−1; RWW shown 40% of DOC removal and a EEO LEDs = 68.4 kWh m−3 order−1, while with WWW 35% of DOC removal and a EEO LEDs = 79.8 kWh m−3 order−1 were obtained. This work shows that wireless UV-A LEDs can be a promising alternative to conventional UV lamps and wired LEDs in the treatment of real wastewaters. However, optimization of the induction system is still needed, as well as the number and wavelength of the LEDs (e.g. UV-C LEDs) to reduce the overall treatment costs.

Cover Feature: Energetics of Formation of Cyclacenes from 2,3‐Didehydroacenes and Implications for Astrochemistry (Chem. Eur. J. 14/2021)

Rolling up: Larger acenes are potential carriers of diffuse interstellar bands, and should undergo dehydrogenation reactions upon absorption of interstellar UV photons. This could result in 2,3-didehydroacenes that have a reactive aryne functionality at one of the short ends of the long molecule. The intramolecular Diels-Alder reaction to dihydroethenocyclacenes with the other end of the acene and the subsequent cycloreversion of acetylene were studied computationally and were shown to have energy requirements that are compatible with cyclacene formation under the extreme conditions of interstellar space. More information can be found in the Full Paper by H. F. Bettinger et al. on page 4605.

Structural, mechanical, electronic and optical properties of half-metallic XFeSi2O6 (X = Li, Na, K) clinopyroxenes: A first-principles investigation

Pyroxene structures are ferromagnetic silicates that are found in abundance in the Earth’s crust and upper mantle, which crystallizes in orthorhombic and monoclinic systems. Here, we present a study using first-principles density functional theory approach in analyzing the structural, physical, optical and magnetic properties of XFeSi2O6 (X = Li, Na, K) pyroxenes in a monoclinic phase. The lattice parameters and lattice angles of the simulated structures in a ferromagnetic structural ordering were consistent with the available experimental data. The optical properties suggested that the structures have a high absorption of UV photons, high reflectivity for visible photons, and calculated loss function spectra show the presence of plasmon peaks near 15 eV–17 eV. The NaFeSi2O6 structure was found to be ductile and the KFeSi2O6 structure demonstrated an auxetic nature. All three compounds were found as ferromagnetic half-metals with a magnetic moment of 5.0μB. These properties suggest potential applications of such pyroxenes in optoelectronics and plasmonics.

Energetics of Formation of Cyclacenes from 2,3-Didehydroacenes and Implications for Astrochemistry.

The carriers of the diffuse interstellar bands (DIBs) are still largely unknown although polycyclic aromatic hydrocarbons, carbon chains, and fullerenes are likely candidates. A recent analysis of the properties of n‐acenes of general formula C4n+2H2n+4 suggested that these could be potential carriers of some DIBs. Dehydrogenation reactions of n‐acenes after absorption of an interstellar UV photon may result in dehydroacenes. Here the reaction energies and barriers for formation of n‐cyclacenes from 2,3‐didehydroacenes (n‐DDA) by intramolecular Diels–Alder reaction to dihydro‐etheno‐cyclacenes (n‐DEC) followed by ejection of ethyne by retro‐Diels–Alder reactions are analyzed using thermally assisted occupation density functional theory (TAO‐DFT) for n=10–20. It is found that the barriers for each of the steps depend on the ring strain of the underlying n‐cyclacene, and that the ring strain of n‐DEC is about 75 % of that of the corresponding n‐cyclacene. In each case, ethyne extrusion is the step with the highest energy barrier, but these barriers are smaller than CH bond dissociation energies, suggesting that formation of cyclacenes is an energetically conceivable fate of n‐acenes after multiple absorption of UV photons.

Exceptional Responsivity (&gt;6 kA/W) and Dark Current (&lt;70 fA) Tradeoff of n-Ga2O3/p-CuO Quasi-Heterojunction-Based Deep UV Photodetector

Reduction of high dark current has been a challenge for high responsivity photodetector (PD). In this context, the present article demonstrates an ultralow dark current of Ga2O3-based deep ultraviolet (UV)-photodetectors (UV-PDs) with enhanced photoresponses by tailoring a p/n heterojunction. An n- Ga2O3/p-CuO quasi-heterostructure-based deep UV-PDs has been fabricated on a sapphire (0001) substrate using an inexpensive electrospraying technique. After Ga2O3 deposition, platinum (Pt) electrodes ( $\sim 50$ nm) are fabricated as a metal–semiconductor–metal (MSM) device using sputtering. The device exhibits a very low dark current in the order of few fA $(6.94\times 10^{-14}$ A) at 5 V because of the enhanced depletion width at p/n heterojunction and Pt/ Ga2O3 metal–semiconductor contacts, which provides a narrow path for free carriers to travel from one metal contact to other under dark condition. The depletion layers get thinner due to the absorption of UV-photons in the UV-illumination condition. Hence, the photogenerated carriers get a wider channel to get collected at Pt-electrodes. Thus, in addition to ultralow dark current, the device exhibits an extraordinary photodetection characteristics, such as a high responsivity ( $\sim 6.33 \times 10^{3} \mathrm{AW}^{-1}$ ), remarkable phototo-dark current ratio ( $\sim 2.99 \times 10^{6}$ ), very high detectivity ( $\sim 4.44 \times 10^{14} \mathrm{mHz}^{0.5} \mathrm{~W}^{-1}$ ), and exceptional external quantum efficiency of $\sim 3.1 \times 10^{6}$ % at 5 V.