Maximum Quantum Efficiency Values Research Articles

Growth and yield of Syagrus schizophylla (Mart.) Glass. in response to light gradients

This research studied growth and yield of Syagrus schizophylla, an extinction-endangered ornamental palm, grown under five light gradients. The treatments were: G1 - PAR=1234.10 μmol photons m-2 s-1, G2 - PAR=913.16 μmol photons m-2 s-1, G3 - PAR=666.34 μmol photons m-2 s-1, G4 - PAR=419.56 μmol photons m-2 s-1 and G5 - PAR=534.77 μmol photons m-2s-1. Before the experiment and at three, five and seven months of treatment, growth (plant height, collar diameter, number of leaves, petiole length, leaf length and width), gas exchange, chlorophyll a, and leaf green color intensity were assessed. The highest net photosynthetic rates were observed in plants under G2, G3 and G4. Values of maximum quantum efficiency (Fv/Fm) over 0.75 were observed under G2. At seven months, estimated SPAD values were 36 in G2 plants and 32 in G1 plants. According to the Dickson quality index (DQI), presented the highest growth and development rates. We conclude that G2 suits plants to be transplanted into the field, whereas G3 would be best for plants grown under shade environments. Highlights: Syagrus schizophylla is an extinction endangered ornamental palm. One layer of Red Chromatinet® 50% should be recommended for the first year of growth of schizophylla plants to be later transplanted to the field. Two Red Chromatinet® 50% layers would be best for plants meant to grow under shade environments. The highest net photosynthetic rates were observed in plants under G2, G3 and G4.

Downconversion mechanism in Er3+/Yb3+ codoped fluorotellurite glasses to enhance the efficiency of c-Si PV cells

Fluorotellurite glass codoped with erbium and ytterbium was elaborated through the melting-quenching route. Through the DSC and Raman measurements, the investigated glass samples presented high thermal stability and low phonon energy, respectively. By using the laser excitation at 488 nm, the downconversion mechanism in the Er3+/Yb3+ system was investigated and the energy transfers implicated in the emissions of erbium and ytterbium ions were discussed based on the different cross-relaxation processes. The addition of ytterbium resulted in an effective energy transfer from the excited 4S3/2/2H11/2 state of Er3+ ions to the emitting 2F5/2 state of Yb3+ ion, which considerably enhanced NIR emissions at 1000 nm and reduced experimental lifetimes of 4S3/2/2H11/2 states. Through luminescence decay analysis, the maximum values of quantum efficiency (ηQE) and energy transfer efficiency (ηET) were estimated as 61.2% and 161.2%, respectively. This result indicates that the proposed glasses can convert visible photons into NIR emissions at 1000 nm suitable to improve the spectral response of crystalline silicon PV cells.

Study on surface processes and photoemission properties of NEA-GaAs after repetitive sequence of thermal pretreatment and NEA activation

Photoemission properties of negative electron affinity (NEA)-GaAs(100) surface have been investigated in this work by repetitive sequence of the thermal pretreatment process and the NEA activation process with a combination of several temperature conditions and subsequent Cs and O2 supply procedures. After the specific thermal pretreatment cycles, the maximum quantum efficiency value increased, and two kinds of Cs-related adsorption structures were found. One of them enhances photoemission efficiency compared to an ordinary NEA activation process. The present results have established a remarkable improvement of photoemission.

Morphological and Physiological Stress Responses of Lettuce to Different Intensities of Continuous Light.

In this study, specific dynamic changes in growth, oxidative stress, ascorbate metabolism, and chlorophyll fluorescence were monitored during 12 days in lettuce plants exposed to continuous light (CL) of different intensities: low light (LL, 100 μmol·m−2·s−1), medium light (ML, 200 μmol·m−2·s−1), and high light (HL, 300 μmol·m−2·s−1). Lettuce plants grown under CL of higher light intensity gained greater biomass, dry weight ratio, root/shoot ratio, and specific leaf FW, but not leaf area. Both the reactive oxygen species (ROS) production and the lipid peroxidation degree, measured in terms of the malondialdehyde (MDA) levels, were progressively enhanced by increasing the light intensity of CL. Overall, the pool sizes of ascorbate (AsA) and glutathione, as well as the activities of enzymes involved in AsA metabolism, had positive correlations with light intensity under CL. Ascorbate peroxidase and dehydroascorbate reductase presented the maximal and minimal responses to light intensity, respectively, among all the studied enzymes. After 6 days under CL, ML and HL intensity caused reversible photoinhibition, represented by lower values of maximum quantum efficiency (F v /F m), effective quantum yield (ΦPSII), and photochemical quenching (qP) and a higher value of non-photochemical quenching (qN). However, this photoinhibition recovered on day 12 with increasing of F v /F m, ΦPSII, and qP. Taken together, under ML and HL conditions, greater AsA level could help maintain photosynthetic efficiency by elevating excess excitation energy dissipation, though ROS accumulation and lipid peroxidation could not be prevented in the long-term. Likewise, there was no dark period under LL condition, but no photooxidative stress was observed in lettuce. Thus, it is concluded that photooxidative stress induced by CL can be attributed to excessive daily light integral instead of circadian asynchrony.

Site spectroscopy probing of Eu3+ incorporated into novel LiYxSryZrO3+α host matrix

In this work, we investigated the spectroscopic properties of LiYxSryZrO3+α: Eu3+, a red emitting nanophosphor based on SrZrO3 perovskite. The synthesis process was an auto-combustion process. X-ray diffractograms show the orthorhombic structure of SrZrO3. Photoluminescence (PL) excitation spectra display a split charge transfer band revealing the presence of two possible sites for the Eu3+ ions. The emission spectra at 231 nm excitation illustrate the dominance of the 5D0 - 7F1 transition, which is an indication that the smaller sized Eu3+ ions are mostly situated at the more ordered (symmetric) Sr2+ sites. The emission spectra at 292 nm & 397 nm excitations show the dominance of 5D0 - 7F2 transition which suggests some of the Eu3+ ions are also situated at the distorted Zr4+ sites. Both the intensity parameters, asymmetry ratio and the decay lifetimes of the nanophosphors show dependence on Y3+ concentration, signifying a modification in the host structure. Maximum quantum efficiency value of ≈ 46% was obtained for the nanophosphors which indicate the need for improvement for practical applications. CIE coordinates show the suitability of this phosphor for both red emission in LED and as a complementary colour for white LED applications.

Decoration of copper oxide nanoplatelets with gold nanoparticles by laser ablation in methanol for photodetection applications

In this work, CuO/n-Si and AuNPs-decorated CuO/n-Si heterojunction photodetectors were fabricated by deposition of CuO nanoplatelets and Au nanoparticles NPs decorated CuO nanoplatelets on silicon substrates by laser ablation in methanol. Atomic force microscope AFM, scanning electron microscope SEM and transmission electron microscope TEM were used to study the structural and surface morphology of CuO and AuNPs–CuO. The electrical properties showed that the CuO/Si and AuNPs decorated-CuO/Si showed rectifying behavior. The maximum values of quantum efficiency were about 41 and 78% at 700 nm for CuO/Si and AuNPs–CuO/Si photodetectors, respectively. The I–V characteristics of the photodetectors were measured under UV light.

Continuous high and low temperature induced a decrease of photosynthetic activity and changes in the diurnal fluctuations of organic acids in Opuntia streptacantha.

Opuntia plants grow naturally in areas where temperatures are extreme and highly variable in the day during the entire year. These plants survive through different adaptations to respond to adverse environmental conditions. Despite this capability, it is unknown how CAM photosynthetic activity and growth in Opuntia plantlets is affected by constant heat or cold. Therefore, the main objective of this research was to evaluate the short-term effect of high (40°C) and low (4°C) continuous temperatures on the photosynthetic efficiency, the organic acid content (malic acid) and the relative growth rate (RGR) in seven-month-old Opuntia streptacantha plantlets during 5, 10, and 15 days. Chlorophyll fluorescence analysis allowed us to determine that high temperatures negatively impact the photosynthetic efficiency of O. streptacantha plantlets, which exhibited the lowest values of maximum quantum efficiency of the photosystem II (Fv/Fm = 52%, Fv/F0 = 85%), operational quantum yield of PS (ΦPSII = 65%) and relative electron transport rate (rETR = 65%), as well as highest values of basal fluorescence (F0 = 226%) during 15 days of treatment. Similarly, low temperatures decreased Fv/Fm (16%), Fv/F0 (50%), ΦPSII and rETR (16%). High temperatures also decreased nocturnal acidification in approximately 34–50%, whereas low temperatures increased it by 30–36%. Additionally, both continuous temperatures affected drastically diurnal consumption of malic acid, which was related to a significant RGR inhibition, where the specific photosynthetic structure area component was the most affected. Our results allowed determining that, despite the high tolerance to extreme temperatures described for Opuntia plants, young individuals of O. streptacantha suffered photosynthetic impairment that led to the inhibition of their growth. Thus, the main findings reported in this study can help to predict the potential impact of climatic change on the establishment and survival of succulent species of arid and semiarid regions of Mexico.

Sink Status and Photosynthetic Rate of the Leaflet Galls Induced by Bystracoccus mataybae (Eriococcidae) on Matayba guianensis (Sapindaceae).

The galling insect Bystracoccus mataybae (Eriococcidae) induces green and intralaminar galls on leaflets of Matayba guianensis (Sapindaceae), and promotes a high oxidative stress in host plant tissues. This biotic stress is assumed by the histochemical detection of hydrogen peroxide, a reactive oxygen species (ROS), whose production alters gall physiology. Thus, we hypothesize that high levels of nutrients are accumulated during gall development in response to a local maintenance of photosynthesis and to the galling insect activity. Moreover, the maintenance of low levels of photosynthesis may guarantee O2 production and CO2 consumption, as well as may avoid hypoxia and hypercarbia in gall tissues. To access the photosynthesis performance, the distribution of chlorophyllous tissues and the photochemical and carboxylation rates in gall tissues were analyzed. In addition, histochemical tests for hydrogen peroxide and phenolic derivatives were performed to confirm the biotic stress, and set the possible sites where stress dissipation occurs. The contents of sugars and nitrogen were evaluated to quantify the gall sink. Currently, we assume that the homeostasis in gall tissues is ruptured by the oxidative stress promoted by the galling insect activity. Thus, to supply the demands of gall metabolism, the levels of water-soluble polysaccharides and starch increase in gall tissues. The low values of maximum quantum efficiency of PSII (Fv/Fm) indicate a low photosynthetic performance in gall tissues. In addition, the decrease of PSII operating efficiency, (F’m–F’)/F’m, and Rfd (instantaneous fluorescence decline ratio in light, to measure tissue vitality) demonstrate that the tissues of B. mataybae galls are more susceptible to damage caused by stressors than the non-galled tissues. Thus, the high oxidative stress in gall developmental sites is dissipated not only by the accumulation of phenolic derivatives in the protoplast, but also of lignins in the walls of neoformed sclereids.

Alterations in Chlorophyll a Fluorescence and Pigments Concentration in the Leaves of Cauliflower and Broccoli Transplants Subjected to Chilling

Chlorophyll a fluorescence parameters and photosynthetic pigments content in leaves of broccoli (Brassica oleracea L. var. italica) cv. ‘Monaco’ F1 and cauliflower (Brassica oleracea L. var. botrytis) cv. ‘Bruce’ F1 transplants were analyzed to investigate the effects of chilling temperatures (6 and 10 °C) and the exposure length (1 or 2 weeks) on the photosynthetic apparatus condition in the 3-year experiment. Data were compared to control plants exposed to 14 °C for 1 and 2 weeks. The lowest values of maximum quantum efficiency of photosystem II (Fv/Fm) and the performance index (PI) were observed in cauliflower chilled at 6 °C for 2 weeks. This was not accompanied by any substantial reduction in chlorophylls concentration, however, significant correlations between Fv/Fm or PI and chlorophylls content in cauliflower leaves were found. There were no negative effects of chilling on photosynthetic activity and chlorophyll content in broccoli leaves, the amount of carotenoids significantly increased in the broccoli chilled for 2 weeks at 6 °C. The chlorophyll a fluorescence indices and accelerated carotenoids synthesis showed that broccoli had less vulnerable photosynthetic apparatus to stress-induced temperatures and more efficient protection mechanisms in comparison to cauliflower transplants.

Alterations in Chlorophyll a Fluorescence and Pigments Concentration in the Leaves of Cauliflower and Broccoli Transplants Subjected to Chilling

Chlorophyll a fluorescence parameters and photosynthetic pigments content in leaves of broccoli (Brassica oleracea L. var.italica) cv. ‘Monaco’ F1 and cauliflower (Brassica oleracea L. var. botrytis) cv. ‘Bruce’ F1 transplants were analyzed to investigatethe effects of chilling temperatures (6 and 10 °C) and the exposure length (1 or 2 weeks) on the photosynthetic apparatuscondition in 3-year experiment. Data were compared to control plants exposed to 14 °C for 1 and 2 weeks. The lowest valuesof maximum quantum efficiency of photosystem II (Fv/Fm) and the performance index (PI) were observed in cauliflowerchilled at 6 °C for 2 weeks. This was not accompanied by any substantial reduction in chlorophylls concentration, however,significant correlations between Fv/Fm or PI and chlorophylls content in cauliflower leaves were found. There was no negativeeffects of chilling on photosynthetic activity and chlorophyll content in broccoli leaves, the amount of carotenoids significantlyincreased in the broccoli chilled for 2 weeks in 6 °C. The chlorophyll a fluorescence indices and accelerated carotenoidssynthesis showed that broccoli had less vulnerable photosynthetic apparatus to stress-induced temperatures and more efficientprotection mechanisms in comparison to cauliflower transplants.

Quality Improvement of GaN on Si Substrate for Ultraviolet Photodetector Application

GaN is grown on an Si substrate using metal-organic vapor-phase epitaxy. Compared with the full width at half maximum values from X-ray diffraction patterns and photoluminescence spectra of conventional GaN on the Si substrate, those of GaN on the Si substrate with the insertion of various-temperature AlN nucleation layers and Al0.3Ga0.7N/GaN superlattice intermediate layers are reduced by 34.9% and 25.6%, respectively. In addition, Raman spectra show that residual stress on the GaN epilayers decreased by 0.35 GPa. The c-lattice parameter of the GaN epilayer is 5.1844 Å, which is close to that of an unstrained GaN layer. Ultraviolet metal-semiconductor-metal photodetectors are fabricated on an almost-crack-free GaN surface. The dark current of a photodetector on the Si substrate is 2.4 ×10-11 A at a 9 V applied bias, which is one order of magnitude smaller than that of a photodetector on a conventional sapphire substrate. The maximum quantum efficiency value of a photodetector on the Si substrate is ~ 97% with an incident light wavelength of 360 nm and a 9 V applied bias.

Chlorophyll fluorescence imaging: a new method for rapid detection of herbicide resistance in Alopecurus myosuroides

SummaryDue to the steadily increasing number of putative herbicide‐resistant weed populations, the demand for rapid in‐season tests is rising. In this study, we introduce a new quantitative herbicide‐resistance test system based on chlorophyll fluorescence imaging analysis of photosynthesis‐related parameters. Susceptible and herbicide‐resistant populations of Alopecurus myosuroides (black‐grass) were cultivated in multiwell tissue culture plates containing nutrient agar and different dosages of fenoxaprop‐P‐ethyl and mesosulfuron+iodosulfuron. The maximum quantum efficiency of the PSII was measured 3 h after transplanting (HAT) and then for seven days every 24 h. Data of maximum quantum efficiency of the PSII were compared with standard whole‐plant pot tests and molecular tests for target‐site mutations. It was possible to fit dose‐response curves and calculate corresponding resistance factors for ED90 for all populations tested using the chlorophyll fluorescence imaging. It was possible to distinguish between resistant and susceptible populations. The results of the chlorophyll fluorescence imaging corresponded well with the standard whole‐plant pot tests in the glasshouse. However, populations with proved target‐site mutations did not differ from other herbicide‐resistant populations in the maximum quantum efficiency values of the PSII. We conclude that the chlorophyll fluorescence imaging provides reliable data on herbicide resistance for both modes of action tested in a shorter time and using less space, compared with standard whole‐plant pot tests in the glasshouse.

Temporal behaviour of photoemission for yttrium cathodes

In this work, the photoemission performance of rough and smooth cathodes is presented. The cathodes were made up of pure yttrium metal bulk. For evaluation of the photocathodes, a KrF excimer laser, operating at 248 nm wavelength, 5 eV photon energy and 23 ns full width at half maximum (FWHM) was used. The targets were tested in a vacuum photodiode cell at 10−7 mbar; they were irradiated at normal incidence and the anode–cathode distance was set to 5 mm. The measurements were performed after a series of electric breakdowns between anode and cathode, induced under high laser energy and high accelerating voltage. This operation was performed in order to restore/increase the surface roughness. For the rough cathode, the maximum output current was 10.5 A, reached at 25 kV and 12 mJ laser energy; the maximum quantum efficiency (QE) value was 1.3×10−4, whereas for the smooth cathode, the maximum output current was 8.4 A and the maximum QE value was 7.0×10−5, calculated in the same experimental conditions. The temporal behavior of photoemission from these cathodes was studied. Plasma production and Schottky effect played a fundamental role in this process.

Behavior of photocathodes on superficial modification by electrical breakdown

In this work, we present the experimental study and a theoretical approach of the photoemission of rough and smooth photocathodes. The cathode was made of pure yttrium while its surface was morphologically modified. The cathode surface was irradiated by a KrF excimer at normal incidence. The measurements were performed after electric breakdowns between anode and cathode. For the rough cathode the maximum output current was 10.5A, with a maximum quantum efficiency value of 1.3×10−4. Instead, the smooth cathode provided an output current of 8.4A with a maximum quantum efficiency value of 7.0×10−5. In this process, the plasma production and the Schottky effect play a fundamental role.

Light microhabitats, growth and photosynthesis of an epiphytic bromeliad in a tropical dry forest

In the tropical dry forest of Dzibilchaltun, Yucatan, Mexico, light microhabitats, tissue acidity changes, chlorophyll fluorescence and growth were investigated for the epiphytic bromeliad Tillandsia brachycaulos Schltdl. Most individuals occurred in sites between 20 and 60% of the height of the tree, and directly on the main trunk or branches closed to the main trunk. During the dry season, individuals received about nine times more photon flux density (PFD) than during the rainy season. Individuals that occurred under 30-59% of the daily ambient PFD showed greater leaf length increase and monthly leaf production, and produced more flowers than individuals in the brighter ( > 60% of ambient PFD) and the darker (11-29% of ambient PFD) light microhabitats. Well-watered plants acclimated to deep shade (6% of ambient PFD) showed values of tissue acidity similar to plants under higher light treatments. Well-watered plants under 60% of daily ambient PFD showed low values of maximum quantum efficiency at midday during most of the year but showed similar values of tissue acidity than plants under 6 and 30% of ambient PFD. Leaf temperatures of plants in low sites within the canopy and on the forest floor were significantly higher than the air during about 1 h, indicating that those plants could not dissipate the excess of heat. Individuals located in the most shaded microhabitats could also receive less amount of water from rainfall and dew. Therefore, it would be less damaging for this epiphytic bromeliad to occur in the most exposed micro habitats than in the most shaded microhabitats in this tropical dry forest.

Optical Response Study of the Al/a‐Sic:H Schottky Diode for Different Substrate Temperatures of the R.F. Sputtered a‐Sic:H Thin Film

In the present work, Schottky diodes of Al/a‐SiC:H included in the structure Al/a‐SiC:H/c‐Si(n)/Al were fabricated and their optical response was studied in the wavelength region from 350 nm up to 1000 nm, for different substrate temperatures, Ts (from 30°C up to 290°C) of the r.f. sputtered a‐SiC:H thin film. The spectral response of these structures exhibits two maximum values of quantum efficiency. The first maximum is presented at wavelength λ≅525 nm and the other at λ = 850 850 nm, which are attributed to the Aa‐SiC:H Schottky junction and the a‐SiC:H/c‐Si(n) isotype heterojunction, respectively. The position of the first maximum owing to the Al/a‐SiC:H junction as well as the values of quantum efficiency, for the range of wavelengths from 350 nm up to 700 nm, depends on the substrate temperature, Ts, of the a‐SiC:H thin film. In the case that Ts is 120°C, the spectral response of the Al/a‐SiC:H/c‐Si(n)/Al structure for a reverse bias voltage V = ‐2V exhibits high values of quantum efficiency (up to 40%) with slight variation in the range of wavelength from 475 nm up to 925 nm, making this structure interesting as a wide band optical sensor device. Finally, the minority carrier (holes) diffusion length of a‐SiC:H for TS = 120°C was calculated and it was found to be ≅2.200 Å.

Composite photocathodes for visible photon imaging with gaseous photomultipliers

We report on the results of coating K–Cs–Sb visible photocathodes, with thin CsBr and CsI protective films, in view of their possible combination with gas electron multipliers. Absolute quantum efficiency, photoelectron transport through the coating film, the effect of exposure to oxygen and to air and the photocathode behavior under intense photon flux are presented. The coated K–Cs–Sb photocathodes have typically maximum quantum efficiency values of 5–6% at 312nm. They can withstand exposure to 150Torr of oxygen for more than half an hour. Appropriate electron multipliers and possible applications are briefly discussed.