24 research outputs found
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 ×
10<sup>10</sup> 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 MoS<sub>2</sub> Passivated by MA<sub>3</sub>Bi<sub>2</sub>Br<sub>9</sub>
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 (MoS<sub>2</sub>) in the outstanding lead-based/MoS<sub>2</sub> hybrid structure. Lead-free MA<sub>3</sub>Bi<sub>2</sub>Br<sub>9</sub> with a high-crystalline quasi-layered structure was used
to prolong the MABr passivation effect on MoS<sub>2</sub>. As a result,
MA<sub>3</sub>Bi<sub>2</sub>Br<sub>9</sub>-coated MoS<sub>2</sub> photodetector
achieved the fastest response time of 0.3 ms and the highest detectivity
of 3.8 × 10<sup>12</sup> Jones among the reported MoS<sub>2</sub>-based photodetectors so far. This photodetector also showed high
photoresponse stability
Efficient Top-Illuminated Organic-Quantum Dots Hybrid Tandem Solar Cells with Complementary Absorption
Organic and quantum
dots (QDs) semiconductors are promising to
build low-cost hybrid tandem solar cells since they are both fully
solution-processable, and have tunable bandgaps and absorption spectra.
The challenges for high-performance organic-QDs tandem solar cells
are to balance the photocurrent in subcells and construct an efficient
charge-recombination layer (CRL) to maximize the efficiency of the
whole tandem cell. In this work, we report a top illuminated organic-QDs
hybrid tandem solar cell that employs an organic-based front subcell
and a PbS QDs-based back subcell where the organic absorber complements
the absorption deficiency of QDs film in the range of 650–900
nm. The hybrid tandem solar cell is monolithically integrated and
electrically connected with a Spiro-MeOTAD/MoO<sub>3</sub>/Ag/PEIE
CRL. A conversion efficiency of 7.4% is achieved for the hybrid tandem
cells. The tandem solar cells exhibit an open-circuit voltage of 1.12
V, which is nearly the sum of the <i>V</i><sub>OC</sub> of
individual subcells, and a fill factor up to 56%, confirming the effectiveness
of CRL for building organic-QD hybrid tandem cells
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 (Pb<sup>2+</sup>) with trivalent antimony (Sb<sup>3+</sup>) to synthesize
stable and brightly luminescent Cs<sub>3</sub>Sb<sub>2</sub>Br<sub>9</sub> 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 (Cs<sub>3</sub>Sb<sub>2</sub>X<sub>9</sub>) 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
Postsurface Selenization for High Performance Sb<sub>2</sub>S<sub>3</sub> Planar Thin Film Solar Cells
Sb<sub>2</sub>S<sub>3</sub> 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. Sb<sub>2</sub>S<sub>3</sub> 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 Sb<sub>2</sub>S<sub>3</sub> 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 Sb<sub>2</sub>S<sub>3</sub>. TEM/EELS
mapping of treated Sb<sub>2</sub>S<sub>3</sub> film revealed that
only surface adjacent section was partly selenized and formed Sb<sub>2</sub>(S<sub><i>x</i></sub>Se<sub>1–<i>x</i></sub>)<sub>3</sub> alloy. In addition, XPS results further unfolded
that there was trace selenium doping in the bulk of Sb<sub>2</sub>S<sub>3</sub> film. The treated HTM-free Sb<sub>2</sub>S<sub>3</sub> based solar cells were fabricated and an improved efficiency of
4.17% was obtained. The obtained <i>V</i><sub>OC</sub> of
0.714 V was the highest and the power conversion efficiency also reached
the top value among HTM-free planar Sb<sub>2</sub>S<sub>3</sub> 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/Sb<sub>2</sub>S<sub>3</sub> 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3−<i>x</i></sub>Cl<sub><i>x</i></sub> 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>·H<sub>2</sub>O. Upon long-time exposure,
however, the resistance became irreversible due to the decomposition
into PbI<sub>2</sub>. 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
Vertically Aligned One-Dimensional Crystal-Structured Sb<sub>2</sub>Se<sub>3</sub> for High-Efficiency Flexible Solar Cells <i>via</i> Regulating Selenization Kinetics
Recently, antimony selenide (Sb2Se3) has
exhibited an exciting potential for flexible photoelectric applications
due to its unique one-dimensional (1D) chain-type crystal structure,
low-cost constituents, and superior optoelectronic properties. The
1D structure endows Sb2Se3 with a strong anisotropy
in carrier transport and a lasting mechanical deformation tolerance.
The control of the crystalline orientation of the Sb2Se3 film is an essential requirement for its device performance
optimization. However, the current state-of-the-art Sb2Se3 devices suffer from unsatisfactory orientation control,
especially for the (001) orientation, in which the chains stand vertically.
Herein, we achieved an unprecedented control of the (001) orientation
for the growth of the Sb2Se3 film on a flexible
Mo-coated mica substrate by balancing the collision rate and kinetic
energy of Se vapor particles with the surface of Sb film by regulating
the selenization kinetics. Based on this (001)-oriented Sb2Se3 film, a high efficiency of 8.42% with a record open-circuit
voltage (VOC) of 0.47 V is obtained for
flexible Sb2Se3 solar cells. The vertical van
der Waals gaps in the (001) orientation provide favorable diffusion
paths for Se atoms, which results in a Se-rich state at the bottom
of the Sb2Se3 film and promotes the in situ formation of the MoSe2 interlayer between
Mo and Sb2Se3. These phenomena contribute to
a back-surface field enhanced absorber layer and a quasi-Ohmic back
contact, improving the device’s VOC and the collection of carriers. This method provides an effective
strategy for the orientation control of 1D materials for efficient
photoelectric devices
Downregulation of LGR4 decreases ARPE-19 cell migration.
<p>ARPE-19 cells were transfected with LGR4 specific siRNA or a scrambled negative control (NC). Transwell (A) and <i>in vitro</i> scratch assays (B) were performed to evaluate the ARPE-19 cell migration. The number of cells that migrated in the transwell assay was quantified by counting the number of cells appearing in the lower chamber in five independent vision fields with a 20X microscope objective. (C) For <i>in vitro</i> scratch assay, cells that migrated into the gap were counted at 48 hours and 72 hours as indicated. Results were expressed as mean ± SD (n = 3, <i>*p</i> < 0.05). All images are representative of at least three independent experiments. Scale bar: 100 μm for transwell assay, and 200 μm for <i>in vitro</i> scratch assay.</p