Function Of Photon Flux Research Articles

Evaluation of a helicoidal flux photoreactor applied in the dicloxacillin degradation by UV-C/H2O2 and UV-A/photo-Fenton including the effect of photon absorption

The efficiency of a helical flow photoreactor in the oxidation of the antibiotic Dicloxacillin as a function of the recirculation flow, the local volumetric rate of photon absorption (LVRPA) and photonic efficiency using oxidation processes based on UV-C/H2O2 and UV-A/photo-Fenton were evaluated in this work. Mathematical modeling of the degradation process and experiments in lab-scale helicoidal reactor were done. Mathematical evaluation was supported in function of photon flux, mass balance and reaction rate law. The LVRPA was calculated by modified Beer-Lambert Equation. The effect of the concentration of hydrogen peroxide and iron (II) on the efficiency of the oxidation of the antibiotic Dicloxacillin was evaluated by means of the photo-Fenton. In 45 min, 100% degradation of dicloxacillin was achieved using 1.5 mM H2O2 and 7.32 μM ferrous ion in the photo-Fenton experiments with the helical flow photoreactor. The optical thickness of the reaction and distribution of energy absorption confirmed the kinetic and photonic efficiency performance. The prediction of the model presented a global error less to 2.4% between experimental and simulated data.

Photovoltaic Response and Charge Redistribution Processes in GaAs/AlGaAs Multiple‐Quantum Wells Structure

The impact of radiative and non‐radiative processes on the photovoltaic response of GaAs/AlGaAs multiple‐quantum wells is investigated by temperature, excitation power, and chopping‐frequency‐dependent surface photovoltage (SPV) measurements. It is shown that a systematic study of SPV amplitude along with phase spectra can provide important information on thermal escape, carrier localization, and spatial redistribution of charge carriers. Characteristic features in SPV‐phase spectra related to quantum well (QW) transitions and their variation under an external perturbation are explained by the electron–electron interaction. It is found that a many‐body interaction inside the QW is responsible for the capture of charges at the heterointerfaces, which builds up an interface electric field and hinders the drift/diffusion of charges. Such an effect becomes dominant under higher excitation powers, causing a sub‐linear enhancement of photovoltage amplitude and an increase in phase delay as a function of photon flux. From the chopping‐frequency‐dependent SPV measurements, it is concluded that carriers during the drift in barrier layers are repeatedly captured by point defects and other QWs, which enhances the effective time response of the photovoltage signal. Results obtained in this study are beneficial in the engineering of quantum structures for high‐efficiency photovoltaics and non‐invasive characterization of electro‐optical processes.

Harvesting multiple electron-hole pairs generated through plasmonic excitation of Au nanoparticles.

Multi-electron redox reactions, although central to artificial photosynthesis, are kinetically sluggish. Amidst the search for synthetic catalysts for such processes, plasmonic nanoparticles have been found to catalyse multi-electron reduction of CO2 under visible light. This example motivates the need for a general, insight-driven framework for plasmonic catalysis of such multi-electron chemistry. Here, we elucidate the principles underlying the extraction of multiple redox equivalents from a plasmonic photocatalyst. We measure the kinetics of electron harvesting from a gold nanoparticle photocatalyst as a function of photon flux. Our measurements, supported by theoretical modelling, reveal a regime where two-electron transfer from the excited gold nanoparticle becomes prevalent. Multiple electron harvesting becomes possible under continuous-wave, visible-light excitation of moderate intensity due to strong interband transitions in gold and electron-hole separation accomplished using a hole scavenger. These insights will help expand the utility of plasmonic photocatalysis beyond CO2 reduction to other challenging multi-electron, multi-proton transformations such as N2 fixation.

Optical Control of Internal Electric Fields in Band Gap-Graded InGaN Nanowires

InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowire-based photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps.

Photoelectrochemical Properties of LaFeO3 Nanoparticles

AbstractThe photoelectrochemical properties of LaFeO3 nanoparticles are discussed for the first time. Highly phase‐pure LaFeO3 particles prepared by a novel ionic‐liquid‐based method are characterized by a (60±14) nm mean size and a band gap of (2.56±0.07) eV. Thin films deposited by screen printing exhibit photocurrent responses associated with hydrogen generation at potential up to 1 V more positive than the formal hydrogen potential. Analysis of the photocurrent responses as a function of photon flux and potential bias suggest that the hole‐collection efficiency at the back contact is in competition with interfacial water oxidation.

Simultaneous improvements in power conversion efficiency and operational stability of polymer solar cells by interfacial engineering

This article addresses simultaneous improvements in the photovoltaic performance and operational stability of organic photovoltaic devices (OPVs) in the inverted configuration when nanostructured ZnO characterized by a lower density of localized surface atomic energy states is employed as an electron transport layer. Two sets of devices with the configuration ITO/ZnO/P3HT:PCBM/MoO3/Ag are employed in the present study. A difference in the density of localized energy states in the band gap of ZnO was produced by altering the crystallinity by annealing the ZnO at two temperatures, viz. 160 and 240 °C. The devices are characterized by scanning electron microscopy, X-ray diffractometry, current-voltage (I-V) measurements as functions of temperature and illumination intensity, incident photon to current conversion efficiency (IPCE) spectroscopy, and charge extraction by linearly increasing photovoltage (CELIV) spectroscopy. The devices fabricated using the ZnO nanostructures annealed at 240 °C have shown remarkably higher power conversion efficiency (PCE) and IPCE values than the other device. From I-V measured as a function of photon flux and temperature we show that the device with higher PCE is characterized by a lower depth of localized energy states by a factor of two than the other device. The implications of the lower trap depth was also evaluated using CELIV and the corresponding charge mobility obtained differed by a factor of three between the two sets of devices. The device with lower equilibrium concentration at the interface has three fold higher charge mobility and 40% enhanced photoconversion efficiency. The stability of the devices was evaluated with and without encapsulation under simulated sunlight (AM 1.5) following the ISOS-D-1 (shelf) and the ISOS-L-1 protocols; the device with higher PCE also showed higher operational stability. The findings in this study are expected to provide new directions in fabricating organic-inorganic heterojunction devices with high performance and stability.

Solar photocatalytic decomposition of estrogens over immobilized zinc oxide

The photocatalytic degradation of synthetic estrogen 17α-ethynylestradiol (EE2) in environmental samples was investigated. Zinc oxide immobilized onto a glass substrate was prepared and used as the photocatalyst, while radiation was provided by a solar simulator. EE2 in the range 50–200μg/L was treated in various matrices, i.e. ultrapure water, wastewater and drinking water, and treatment efficiency was assessed as a function of photon flux, ZnO loading and addition of hydrogen peroxide. Degradation follows apparent first-order kinetics and increases with increasing photon flux (4.93·10−7–5.8·10−7einstein/(Ls)) and H2O2 concentration (up to 100mg/L), while ZnO loading (1.2–16.3mg) has a marginal effect. Reaction in ultrapure water is twice as fast as in wastewater (e.g. the respective apparent rate constants are 17.3·10−3 and 9.4·10−3min−1 at maximum photon flux and 3.7mg ZnO) due to the competition for oxidants between EE2 and the wastewater components (organic matter and ions). The catalyst retained most of its activity upon repeated use (i.e. 21 consecutive runs of 31.5h duration) although it was partially dissolved in the liquid phase; leached zinc can trigger homogeneous reactions, thus contributing to the overall photocatalytic degradation.

Surface photovoltage effect at thep-WSe2:Rb surface: Photoemission experiment and numerical model

It is shown that a combined experimental and theoretical study of the surface photovoltage (SPV) effect can be utilized for the quantification of a number of material parameters in a semiconductor-adsorbate model system. At the Rb-adsorbed surface of the semiconducting layered transition metal dichalcogenide WSe${}_{2}$, a large SPV effect of $\ensuremath{\approx}$600 meV is observed already at a moderate photon flux. Using valence band PES and an auxiliary tunable light source, the surface potential and the SPV effect are traced as a function of the adsorbate density and absorbed light intensity. We find that a major part of the stationary band back bending as a function of photon flux is constrained to a highly sensitive region, that is, a large fraction (50%--80%) of the total SPV magnitude rises within less than 1 order of magnitude. The effect is reproduced by a numerical model involving semiclassical charge carrier dynamics in the surface space charge layer. Among the parameters determined from the simulation, we find an approximate bulk acceptor density between $0.9\ifmmode\times\else\texttimes\fi{}{10}^{17}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{17}$ cm${}^{\ensuremath{-}3}$, a broad distribution of adsorbate donor levels at $\ensuremath{\approx}$0.2 eV above the conduction band minimum, and electron and hole carrier mobilities ${\ensuremath{\mu}}_{e}=0.7$ cm${}^{2}$/(V s), ${\ensuremath{\mu}}_{h}=1.5$ cm${}^{2}$/(V s) in the crystallographic $c$ direction.

Transformation of Bromine Species During Decomposition of Bromate under UV Light from Low Pressure Mercury Vapor Lamps

Bromate decomposition with low pressure mercury vapor lamps (LPMVL) was studied in buffer-free and buffered Milli-Q water by following the fate of bromine species BrO3 −, Br−, and free bromine. BrO3 − was converted over time to Br− with total free bromine (TFBr) as secondary reaction product. BrO3 − decay followed pseudo-first-order kinetics and was independent of [BrO3 −]o (0.06–0.6 mM), pHo (6.9–9.5) and HCO3 − (0.05–1.0 mM), slightly dependent on acetate (0.06–0.27 mM), highly dependent on and adversely affected by humic acids (HA) and an increasing function of photon flux I as measured by potassium ferrioxalate actinometry. Reaction pH dropped by as much as 2.2 units at pHo ≤ 8.5 in the buffer-free experiments, while it remained within 0.5 unit at pHo > 9. TFBr decay due to exposure to LPMVL resulted in formation of Br− (major) and BrO3 − (minor). Decay of HA in the presence of BrO3 − was highly augmented by photon emission of LPMVL at 185 nm and was found to substantially contribute to BrO3 − decay in the presence of HA. Correlations on the dependency of bromine species decay or formation rates as a function of photon flux are presented.

Comparison of centroid computation algorithms in a Shack-Hartmann sensor

Analytical theory is combined with extensive numerical simulations to compare different flavours of centroiding algorithms: thresholding, weighted centroid, correlation, quad cell (QC). For each method, optimal parameters are defined in function of photon flux, readout noise and turbulence level. We find that at very low flux the noise of QC and weighted centroid leads the best result, but the latter method can provide linear and optimal response if the weight follows spot displacements. Both methods can work with average flux as low as 10 photons per subaperture under a readout noise of three electrons. At high-flux levels, the dominant errors come from non-linearity of response, from spot truncations and distortions and from detector pixel sampling. It is shown that at high flux, centre of gravity approaches and correlation methods are equivalent (and provide better results than QC estimator) as soon as their parameters are optimized. Finally, examples of applications are given to illustrate the results obtained in the paper.

Observation of negative capacitance in a-SiC:H/a-Si:H UV photodetectors

UV photosensors based on a-Si:H/a-SiC:H were deposited by ultra high vacuum plasma enhanced chemical vapour deposition system (UHV-PECVD) over a large area (10 cm × 10 cm) in p–i–n configuration. The devices were characterized in the spectrum range 200–800 nm. They showed a good sensitivity in the UV range and a rejection of visible light. A relation was found between spectral response and the thickness of p- and i-layers. A linear dependence of the photocurrent as a function of photon flux at constant wavelength (365 nm) was found. The electrical properties were correlated with the deposition conditions. Negative values of capacity at high frequency were also found in all devices. These values appear at different critic frequencies and correspond to the maximum of the device conductance. The ability to obtain regular capacitive shifts simply by reversing the signal may open research for novel devices and applications.

Solar photooxidation of diphenylamine

Abstract Diphenylamine (DPA) on illumination with natural sunlight or artificial UV-light, in presence of air, transforms into N -phenyl-p-benzoquinonimine (PBQ). This direct photooxidation exhibits saturation kinetics with respect to DPA and air. Increase of the area of sunshine linearly improves the formation of PBQ. The UV-photooxidation rate is a linear function of photon flux and the quantum efficiency is low with illumination at 365 nm but is significant at 254 nm . Singlet oxygen quencher, azide ion, does not suppress the photooxidation. Sacrificial electron donors, vinyl monomers and micelles, both anionic and cationic, do not interfere in the photoreaction. The phototransformation of DPA to PBQ depends on the solvent employed; occurs in ethanol, n-butanol, ethyl acetate, ethyl methyl ketone, 1,4-dioxane, acetonitrile, chloroform and carbon tetrachloride, no reaction in n-hexane as well as in benzene and acetic acid, chlorobenzene, and toluene led to different products. The phototransformation is discussed in terms of a mechanism and kinetic law.

Temperature dependence of the photoconductivity of arsenic-doped hydrogenated amorphous germanium thin films

We report a study of the photoconductivity ( I PC) of As-doped a-Ge:H thin films as a function of temperature ( T) and photon-flux ( F). The experimental data discussed in this paper refer to the behavior of I PC at a fixed photon energy (1.3 eV) and at a fixed photon-flux (F≈2.4×10 16 cm −2 s −1) in the 14–380 K temperature range, as well as to the dependence of the exponent γ( I PC∝ F γ ) on T (55–380 K) and on As-doping level, in the 8×10 14–3×10 16 cm −2 s −1 photon-flux range. The measurement of I PC as a function of T indicates the presence of three distinct regions: (I) For T<40 K,I PC is poorly activated with T (activation energy ∼ few meV), (II) for 150 K<T<260 K,I PC is thermally activated with activation energies ranging between 122 and 167 meV, and (III) for T>260 K , a thermal quenching (TQ) of I PC is measured in all the As-doped a-Ge:H samples, the intensity of the TQ being enhanced with As doping. The measurement of I PC as a function of the photon-flux, at varying T, indicates the presence of a minimum for the exponent ( γ min) at a T min. Both γ min and T min have their extreme values in the undoped sample and vary as the Fermi energy shifts from mid-gap to the conduction band edge.

Large area and high sensitivity a-Si:H/a-SiC:H based detectors for visible and ultraviolet light

In this article we discuss the fabrication and performance of a-Si:H/a-SiC:H based ultraviolet sensitive devices. They were deposited over a large area (10 cm×10 cm) in p-i-n configuration using a new, multichamber, ultrahigh vacuum plasma enhanced chemical vapor deposition facility. The intrinsic layer thickness was 10–100 nm. The devices were characterized in the spectral range 365–660 nm and showed good sensitivity in the UV (365 nm). A good rejection of visible light was also measured. Responsivity as high as 0.30 A/W was measured at 365 nm for samples where the p layer and i layer were 5 and 10 nm thick, respectively. The linearity of the photogenerated current as a function of photon flux was measured. It was found that over an area of 25 cm2 the uniformity was within 15%. Using a laser (200 mW at 351 and 363 nm) the aging characteristics were measured and showed a 25% decrease in responsivity after the absorption of 105 J cm−2 under operating conditions.

Optically tuned superconductive-dielectric resonators with whispering-gallery modes

An analytical method is presented for calculating the resonant frequency andQ-factor of a superconducting dielectric disk resonator operating in millimeter-wave regime with whispering-gallery mode. Resonant frequency shift due to the optical generation of quasi-particles in superconducting film is investigated as a function of photon flux. An optically tunable resonant frequency of about 500 MHz is estimated, and good agreement is found between numerical results and experimental ones.

New spectroscopic tool for cluster science: Nonexponential laser fluence dependence of photofragmentation.

The photodestruction of Hg{sub 7}{sup ++} and Hg{sub 9}{sup ++} has been measured as a function of photon flux. A polarization dependent deviation from a purely exponential intensity decrease was observed in both cases. This effect, which in essence is an alignment phenomenon, can be used to characterize dissociating electronic transitions of molecules and clusters. For the clusters studied it is due to a one-dimensional transition dipole moment having a fixed direction within the cluster. The effect is expected to play a role in many photoabsorption experiments where molecule/cluster ionization or fragmentation is studied under high photon fluxes. {copyright} {ital 1996 The American Physical Society.}

Electrical and photovoltaic properties of indium-tin-oxide/p-InSe/Au solar cells

Conditions for efficiency improvement and optimization in indium-tin-oxide/p-indium-selenide solar cells are discussed in this paper. This aim is achieved by using low-resistivity p-indium-selenide and by incorporating a back-surface-field contact. This contact is insured by a p-indium selenide/gold barrier whose rectifying behavior is explained through the complex impurity structure of p-indium-selenide. Electrical and photovoltaic properties of the cells are also reported. The efficiency parameters under AM1 simulated conditions have been improved up to 32 mA/cm2 for the short-circuit current density, 0.58 V for the open-circuit voltage, and 0.63 for the filling factor. As a result, solar efficiencies larger than 10% in annealed cells and 8% in unannealed ones have been attained. The limitations of these devices are discussed by investigating the dependence of electrical and efficiency parameters in function of photon flux and temperature.

Pulse Excitation Studies of Gain and Trapping in Photoconductors

Excitation of photoconductors by single pulses of monoenergetic photons shows that the integrated gain over one pulse event is approximately equal to the steady-state gain, except in materials for which the steady-state gain itself is a function of photon flux. Measurement of the peak pulse current as a function of pulse length permits the determination of trap density in a simple manner, which agrees well with values measured independently by thermally stimulated conductivity. A model based on a quasiuniform trap distribution is shown to be capable of predicting the observed dependence of peak pulse current on pulse length without the use of any adjustable parameters. Changes in trap density by several orders of magnitude with variations in ambient atmosphere are demonstrated for certain cadmium sulfide crystals.