Scaling from single-point sap velocity measurements to stand transpiration in a multispecies deciduous forest: Uncertainty sources, stand structure effect, and future scenarios

9 páginas.-- 5 figuras.-- 2 tablas.-- 58 referencias[EN] A major challenge in studies estimating stand water use in mixed-species forests is how to effectively scale data from individual trees to the stand. This is the case for forest ecosystems in the northeastern USA where differences in water use among species and across different size classes have not been extensively studied, despite their relevance for a wide range of ecosystem services. Our objectives were to assess the importance of different sources of variability on transpiration upscaling and explore the potential impacts of future shifts in species composition on the forest water budget. We measured sap velocity in five tree species (Fagus grandifolia Ehrh., Acer rubrum L., Acer saccharum Marsh., Betula alleghaniensis Britton, and Betula papyrifera Marsh.) in a mature stand and a young stand in New Hampshire, USA. Our results showed that the greatest potential source of error was radial variability and that tree size was more important than species in determining sap velocity. Total sapwood area was demonstrated to exert a strong controlling influence on transpiration, varying depending on tree size and species. We conclude that the effect of potential species shifts on transpiration will depend on the sap velocity, determined not only by radial variation and tree size, but also by the sapwood area distribution in the stand.[FR] Les études dont le but est d'estimer l'utilisation de l'eau a` l'échelle du peuplement dans les forêts mélangées font face a` un défi majeur : comment passer efficacement de l'échelle des arbres individuels a` l'échelle du peuplement. C'est le cas pour les écosystèmes forestiers dans le nord-est des États-Unis où les différences dans l'utilisation de l'eau entre les espèces et parmi les différentes catégories de taille n'ont pas fait l'objet d'études approfondies malgré leur pertinence pour une vaste gamme de services de l'écosystème. Nos objectifs consistaient a` évaluer l'importance des différentes sources de variation sur l'extrapolation de la transpiration et a` explorer les impacts potentiels des changements futurs dans la composition en espèces sur le bilan hydrique de la forêt. Nous avons mesuré la vitesse de la sève chez cinq espèces d'arbre (Fagus grandifolia Ehrh., Acer rubrum L., Acer saccharum Marsh., Betula alleghaniensis Britton et Betula papyrifera Marsh.) dans un peuplement mature et dans un jeune peuplement au New Hampshire (É.-U.). Nos résultats ont montré que la plus grande source potentielle d'erreur était la variation radiale et que la vitesse de la sève était davantage déterminée par la taille des arbres que par l'espèce. La surface totale de bois d'aubier avait un effet très déterminant sur la transpiration qui variait selon la taille et l'espèce d'arbre. Nous concluons que l'effet des changements potentiels dans la composition en espèces sur la transpiration dépendra de la vitesse de la sève qui est principalement déterminée par la variation radiale et la taille des arbres mais aussi de la distribution de la surface de bois d'aubier dans le peuplement.This work was funded by the University of New Hampshire and the New Hampshire Agricultural Experiment Station. The Bartlett Experimental Forest is operated by the USDA Forest Service Northern Research Station. S. Mcgraw, P. Pellissier, C. Breton, S. Alvarado-Barrientos, R. Snyder, and Z. Aldag assisted in the field and in the lab. The 2011 stand inventory was led by S. Goswami. Tree heights were measured and compiled by C. Blodgett, T. Fahey, and L. Liu. A. Richardson shared meteorology and solar radiation data from the Bartlett Amerflux tower. The stands used in this experiment are maintained and monitored by the MELNHE project under the direction of R. Yanai and M. Fisk, with funding from NSF grants DEB 0235650 and DEB 0949324Peer reviewe

Excited-State Dopant–Host Energy-Level Alignment: Toward a Better Understanding of the Photoluminescence Behaviors of Doped Phosphors

Luminescent materials, also known as phosphors, have been widely used for applications such as emissive displays, fluorescent lamps, light-emitting diodes, and X-ray scintillation detectors. The energy-level diagram of a phosphor is extremely important for understanding its photoluminescence behavior. Here, we demonstrate through a combined density functional theory and experimental study that excited-state energy-level alignment accounts for the photoluminescence behaviors much better than ground-state energy-level alignment. An efficient doped phosphor should exhibit a type I excited-state dopant–host energy-level alignment, regardless of whether its ground-state alignment is type I. A type II excited-state dopant–host energy-level alignment implies that exciton dissociation, resulting in photoluminescence quenching. Our results provide not only a better understanding of the photoluminescence behaviors of the reported phosphors but also critical guidance for designing prospective luminescent materials

Synergetic Effect of Silver Nanocrystals Applied in PbS Colloidal Quantum Dots for High-Performance Infrared Photodetectors

PbS colloidal quantum dot (CQD) photodetectors hold great potential for near-infrared detection due to their extremely high sensitivity and low-cost solution processing. In this paper we report that incorporation of 0.5% to 1% (by weight) Ag nanocrystals (NCs) into the PbS CQDs film could simultaneously enhance the photocurrent and suppress dark current and hence significantly boost device detectivity. A set of control experiments suggested that Ag NCs, once added to the PbS CQD film, could trap photogenerated electrons from neighboring PbS CQDs, extend carrier lifetime, and increase photocurrent. We further built a sensitive flexible photodetector using the optimized composite on stone paper, achieving an estimated detectivity as high as 1.5 × 10¹⁰ Jones. The synergetic effect found in our PbS CQD/Ag NC composite photodetectors is expected to be extendable to other binary NC systems for various applications

Low Noise and Fast Photoresponse of Few-Layered MoS₂ Passivated by MA₃Bi₂Br₉

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been widely used in electronic and optoelectronic devices. However, 2D TMDs suffer from surface defects and ambient gas absorption, which significantly degrade their electronic and optoelectronic properties. Here we revealed the passivation effect of methylamine (MA) halide on molybdenum disulfide (MoS₂) in the outstanding lead-based/MoS₂ hybrid structure. Lead-free MA₃Bi₂Br₉ with a high-crystalline quasi-layered structure was used to prolong the MABr passivation effect on MoS₂. As a result, MA₃Bi₂Br₉-coated MoS₂ photodetector achieved the fastest response time of 0.3 ms and the highest detectivity of 3.8 × 10¹² Jones among the reported MoS₂-based photodetectors so far. This photodetector also showed high photoresponse stability

Supplementary document for Event-based X-ray imager by ghosting-free scintillator film - 6902951.pdf

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High Quantum Yield Blue Emission from Lead-Free Inorganic Antimony Halide Perovskite Colloidal Quantum Dots

Colloidal quantum dots (QDs) of lead halide perovskite have recently received great attention owing to their remarkable performances in optoelectronic applications. However, their wide applications are hindered from toxic lead element, which is not environment- and consumer-friendly. Herein, we utilized heterovalent substitution of divalent lead (Pb²⁺) with trivalent antimony (Sb³⁺) to synthesize stable and brightly luminescent Cs₃Sb₂Br₉ QDs. The lead-free, full-inorganic QDs were fabricated by a modified ligand-assisted reprecipitation strategy. A photoluminescence quantum yield (PLQY) was determined to be 46% at 410 nm, which was superior to that of other reported halide perovskite QDs. The PL enhancement mechanism was unraveled by surface composition derived quantum-well band structure and their large exciton binding energy. The Br-rich surface and the observed 530 meV exciton binding energy were proposed to guarantee the efficient radiative recombination. In addition, we can also tune the inorganic perovskite QD (Cs₃Sb₂X₉) emission wavelength from 370 to 560 nm <i>via</i> anion exchange reactions. The developed full-inorganic lead-free Sb-perovskite QDs with high PLQY and stable emission promise great potential for efficient emission candidates

Double Perovskite Single Crystals with High Laser Irradiation Stability for Solid-State Laser Lighting and Anti-counterfeiting

Laser lighting devices, comprising an ultraviolet (UV) laser chip and a phosphor material, have emerged as a highly efficient approach for generating high-brightness light sources. However, the high power density of laser excitation may exacerbate thermal quenching in conventional polycrystalline or amorphous phosphors, leading to luminous saturation and the eventual failure of the device. Here, for the first time, we raise a single-crystal (SCs) material for laser lighting considering the absence of grain boundaries that scatter electrons and phonons, achieving high thermal conductivity (0.81 W m–1 K–1) and heat-resistance (575 °C). The SCs products exhibit a high photoluminescence quantum yield (89%) as well as excellent stability toward high-power lasers (>12.41 kW/cm2), superior to all previously reported amorphous or polycrystalline matrices. Finally, the laser lighting device was fabricated by assembling the SC with a UV laser chip (50 mW), and the device can maintain its performance even after continuous operation for 4 h. Double perovskite single crystals doped with Yb3+/Er3+ demonstrated multimodal luminescence with the irradiation of 355 and 980 nm lasers, respectively. This characteristic holds significant promise for applications in spectrally tunable laser lighting and multimodal anticounterfeiting

Postsurface Selenization for High Performance Sb₂S₃ Planar Thin Film Solar Cells

Sb₂S₃ has attracted great research interest very recently as a promising absorber material for thin film photovoltaics because of their unique optical and electrical properties, binary compound and easy synthesis. Sb₂S₃ planar solar cells from evaporation method without hole-transport layer (HTM) assistance suffer from sulfur deficit vacancy and high back contact barrier. Herein, we developed a postsurface selenization treatment to Sb₂S₃ thin film in order to improve the device performance. The XRD, Raman, and UV–vis spectra indicated the treated film kept the typical characters of Sb₂S₃. TEM/EELS mapping of treated Sb₂S₃ film revealed that only surface adjacent section was partly selenized and formed Sb₂(S_<i>x</i>Se_1–<i>x</i>)₃ alloy. In addition, XPS results further unfolded that there was trace selenium doping in the bulk of Sb₂S₃ film. The treated HTM-free Sb₂S₃ based solar cells were fabricated and an improved efficiency of 4.17% was obtained. The obtained <i>V</i>_OC of 0.714 V was the highest and the power conversion efficiency also reached the top value among HTM-free planar Sb₂S₃ solar cells. The nonencapsulated device exhibited high stability. After storing in ambient air for up to 100 days, the device could maintain 90% efficiency. Systematic materials and device characterizations were implemented to investigate the improvement mechanism for postsurface selenization. The back alloying could suppress the rear contact barrier to improve the fill factor and carrier extraction capability. The bulk Se-doping helped to passivate the interface and bulk defects so as to improve the CdS/Sb₂S₃ heterojunction quality and enhance the long-wavelength photon quantum yield. The robust treatment method with multifunctional effect holds great potential for new chalcogenide thin film solar cell optimization

Investigation of the Interaction between Perovskite Films with Moisture via in Situ Electrical Resistance Measurement

Organometal halide perovskites have recently emerged as outstanding semiconductors for solid-state optoelectronic devices. Their sensitivity to moisture is one of the biggest barriers to commercialization. In order to identify the effect of moisture in the degradation process, here we combined the in situ electrical resistance measurement with time-resolved X-ray diffraction analysis to investigate the interaction of CH₃NH₃PbI_3−<i>x</i>Cl_<i>x</i> perovskite films with moisture. Upon short-time exposure, the resistance of the perovskite films decreased and it could be fully recovered, which were ascribed to a mere chemisorption of water molecules, followed by the reversible hydration into CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>·H₂O. Upon long-time exposure, however, the resistance became irreversible due to the decomposition into PbI₂. The results demonstrated the formation of monohydrated intermediate phase when the perovskites interacted with moisture. The role of moisture in accelerating the thermal degradation at 85 °C was also demonstrated. Furthermore, our study suggested that the perovskite films with fewer defects may be more inherently resistant to moisture