Light Absorption Profile Research Articles

Boosting the Quantum Efficiency of Ionic Carbon Nitrides in Photocatalytic H2O2 Evolution via Controllable n → π* Electronic Transition Activation.

Hydrogen peroxide (H2O2) is a crucial chemical used in numerous industrial applications, yet its manufacturing relies on the energy-demanding anthraquinone process. Solar-driven synthesis of H2O2 is gaining traction as a promising research area, providing a sustainable method for its production. Herein, a controllable activation of n → π* electronic transition is presented to boost the photocatalytic H2O2 evolution in ionic carbon nitrides. This enhancement is achieved through the simultaneous introduction of structural distortions and defect sites (─C ≡ N groups and N vacancies) into the KPHI framework. The optimal catalyst (2%Ox-KPHI) reached an apparent quantum yield of 41% at 410nm without the need for any cocatalysts, outperforming most previously reported carbon nitride-based photocatalysts. Extensive experimental characterizations and theoretical calculations confirm that a corrugated configuration and the presence of defects significantly broaden the light absorption profile, improve carrier separation and migration, promote O2 adsorption, and lower the energy barriers for H2O2 desorption. Transient absorption spectroscopy indicates that the enhanced photocatalytic performance of 2%Ox-KPHI is largely attributed to the preferential migration of electrons at defect sites over extended timescales, following the diffusion of geminate carriers across the PHI sheets.

Effect of iron thicknesses on spin transport in a Fe/Au bilayer system

This paper is concerned with a theoretical analysis of the behavior of optically excited spin currents in bilayer and multilayer systems of ferromagnetic and normal metals. As the propagation, control, and manipulation of the spin currents created in ferromagnets by femtosecond optical pulses is of particular interest, we examine the influence of different thicknesses of the constituent layers for the case of electrons excited several electronvolts above the Fermi level. Using a Monte-Carlo simulation framework for such highly excited electrons, we first examine the spatiotemporal characteristics of the spin current density driven in a Fe layer, where the absorption profile of the light pulse plays an important role. Further, we examine how the combination of light absorption profile, spin-dependent transmission probabilities, and iron layer thickness affects spin current density in a Fe/Au bilayer system. For high-energy electrons studied here, the interface and secondary electron generation have a small influence on spin transport in the bilayer system. However, we find that spin injection from one layer to another is most effective within a certain range of iron layer thicknesses.

Packing-induced selectivity switching in molecular nanoparticle photocatalysts for hydrogen and hydrogen peroxide production.

Molecular packing controls optoelectronic properties in organic molecular nanomaterials. Here we report a donor-acceptor organic molecule (2,6-bis(4-cyanophenyl)-4-(9-phenyl-9H-carbazol-3-yl)pyridine-3,5-dicarbonitrile) that exhibits two aggregate states in aqueous dispersions: amorphous nanospheres and ordered nanofibres with π-π molecular stacking. The nanofibres promote sacrificial photocatalytic H2 production (31.85 mmol g-1 h-1) while the nanospheres produce hydrogen peroxide (H2O2) (3.20 mmol g-1 h-1 in the presence of O2). This is the first example of an organic photocatalyst that can be directed to produce these two different solar fuels simply by changing the molecular packing. These different packings affect energy band levels, the extent of excited state delocalization, the excited state dynamics, charge transfer to O2 and the light absorption profile. We use a combination of structural and photophysical measurements to understand how this influences photocatalytic selectivity. This illustrates the potential to achieve multiple photocatalytic functionalities with a single organic molecule by engineering nanomorphology and solid-state packing.

A compact photoreactor for automated H2 photoproduction: Revisiting the (Pd, Pt, Au)/TiO2 (P25) Schottky junctions

The configuration and geometry of chemical reactors underpins the accuracy of performance evaluation for photocatalytic materials and, accordingly, the development and validation of thermodynamic and kinetic model reactions. The lack of accurate photonic, mass, and heat transport profiles for photochemical reactors hinder standardization, scale-up, and ultimately comparison between different experiments. This work proposes two contributions at the interface between engineering of chemical process and materials science: (A) an automated compact stainless-steel photoreactor with 40 cm3 and 65 cm2 of volume and area, respectively, for hydrogen photoproduction as a model reaction and (B) the synthesis, characterization, and performance of TiO2 Schottky junctions, using Pd, Pt, or Au nanoparticles (ca. 0.5, 1, 2 wt% loadings each) to validate the operation of the reactor. A photonic profile methodology is implemented to the studied reactor to obtain the local light absorption profile, opening up for evaluation of the local quantum yield calculation for the selected materials. A combination of transmission electron microscopy, (X-ray/ultraviolet) photoelectron/electron, energy loss/infrared spectroscopies, X-ray scattering, inductively coupled plasma atomic emission spectroscopy, and ultraviolet–visible spectrophotometry is employed to determine the distinctive surface and bulk properties to build structure–function correlations. The (Pd, Pt, Au)/TiO2 Schottky junction exhibits H2 production rates slightly higher than previous studies, with quantum yields almost 2-fold higher than reported values. These results, demonstrate that the proposed novel geometry of the photoreactor improves the photonic, heat, and mass profiles. An in-depth analysis of the Au plasmon was investigated coupling electron energy loss spectroscopy, UV–vis, and transmission electron microscope, resulting in insightful information about the Au NP mode at the TiO2 interface.

A Photosynthetic Light Acclimation Model Accounting for the Effects of Leaf Age, Chlorophyll Content, and Intra-Leaf Radiation Transfer.

Mechanistic models of canopy photosynthesis usually upscale leaf photosynthesis to crop level. A detailed prediction of canopy microclimate with accurate leaf morphological and physiological model parameters is the pre-requisite for accurate predictions. It is well established that certain leaf model parameters (Vcmax, Jmax) of the frequently adopted Farquhar and Caemmerer photosynthesis model change with leaf age and light interception history. Previous approaches to predict Vcmax and Jmax focused primarily on light interception, either by cumulative intercepted photosynthetic photon flux density (PPFD) or by closely related proxy variables such as leaf nitrogen content per leaf area. However, for plants with monopodial growth, such as vertically grown tomatoes or cucumber crops, in greenhouse production, there is a strong relationship between leaf age and light interception, complicating the experimental and mathematical separation of both effects. We propose a modeling framework that separates age and light intensity-related acclimation effects in a crop stand: Improved approximation of intra-leaf light absorption profiles with cumulative chlorophyll content (Chl) is the basis, while parameters are estimated via Gaussian process regression from total Chl, carotenoid content (Car), and leaf mass per area (LMA). The model approximates light absorption profiles within a leaf and links them to leaf capacity profiles of photosynthetic electron transport. Published datasets for Spinacia oleracea and Eucalyptus pauciflora were used to parameterize the relationship between light and capacity profiles and to set the curvature parameter of electron transport rate described by a non-rectangular hyperbola on Cucumis sativus. Using the modified capacity and light absorption profile functions, the new model was then able to predict light acclimation in a 2-month period of a fully grown tomato crop. An age-dependent lower limit of the electron transport capacity per unit Chl was essential in order to capture the decline of Vcmax and Jmax over time and space of the investigated tomato crop. We detected that current leaf photosynthetic capacity in tomato is highly affected by intercepted light-sum of 3–5 previous days.

Measuring the carrier diffusion length in quantum dot films using graphene as photocarrier density probe.

The diffusion length of quantum dot (QD) films is a critical parameter to improve the performance of QD-based optoelectronic devices. The dot-to-dot hopping transport mechanism results in shorter diffusion lengths compared to bulk solids. Herein, we present an experimental method to measure the diffusion length in PbS QD films using single layer graphene as a charge collector to monitor the density of photogenerated carriers. By producing devices with different thicknesses, we can construct light absorption and photocarrier density profiles, allowing extracting light penetration depths and carrier diffusion lengths for electrons and holes. We realized devices with small (size: ∼2.5nm) and large (size: ∼4.8nm) QDs, and use λ = 532nm and λ = 635nm wavelength illumination. For small QDs, we obtain diffusion lengths of 180nm for holes and 500nm for electrons. For large QDs, we obtain diffusion lengths of 120nm for holes and 150nm for electrons. Our results show that films made of small QD films have longer diffusion lengths for holes and electrons. We also observe that wavelength illumination may have a small effect, with electrons showing a diffusion length of 500 and 420nm under λ = 532nm and λ = 635nm illumination, respectively, which may be due to increased interactions between photocarriers for longer wavelengths with deeper penetration depths. Our results demonstrate an effective technique to calculate diffusion lengths of photogenerated electrons and holes and indicate that not only QD size but also wavelength illumination can play important roles in the diffusion and electrical transport of photocarriers in QD films.

Preparation of multifunctional long-persistent photoluminescence cellulose fibres.

Simple preparation of flame-retardant, photoluminescent, and superhydrophobic smart nanocomposite coating was developed and applied onto cotton fibres using the simple pad-dry-cure technique. This novel strategy involved the immobilization of rare-earth-doped aluminium strontium oxide (ASO; SrAl2 O4 :Eu+2 ,Dy+3 ) nanoparticles, environmentally friendly room temperature vulcanizing silicone rubber (RTV) and environmentally friendly Exolet AP422 (Ex). The fabrics were also able to produce a char film in the fire-resistant assessment, providing fibres with a self-extinguishing characteristic. Furthermore, the fire-retardant performance of the coated cotton samples remained resistant to washing over 35 laundry cycles. The superhydrophobicity of the treated fabrics was monitored to improve by increasing the photoluminescent phosphor nanoparticles. The produced transparent photoluminescent film displayed an absorption at 360 nm and an emission at 526 nm. The photoluminescent fabrics were observed to generate different colorimetric shades, including white, green-yellow and bright white as monitored by Commission Internationale de l'Éclairage laboratory colorimetric coordinates. Slow emissions were detected for the treated cotton fabrics as monitored by emission, ultraviolet-visible light absorption, lifetime, and decay time spectral profiles to indicate glow in the dark phosphorescence effect. Both comfort and mechanical properties of the coated fibres were evaluated by measuring their bending length and air permeability.

Corrigendum: Conductivity-dependent absorption-free, gain-assisted superluminal propagation and rotary photon drag [Optical Materials 115 (2021) 111047

Unfortunately, some of the Figures were placed at wrong order (e.g. Figs. 1–6), in addition to the repetition of Fig. 6, in our published Article [ 1 ]. Besides correcting the order, the missing Fig. 1 (on the energy level scheme of the proposed quantum model) and Fig. 7 are added in the Corrigendum. As such, the correct orders of Figs. 1–6 in the Article should be Figs. 2–7 as in the Corrigendum. Figs. 8–9 are replaced with the correct ones. Figs. 10–12 are also correctly readjusted. Figures 13 and 14 in the Article are correct in all respects. However, description in the captions of all the Figures with the corrected order in the Article is same as in the corrigendum. The authors highly regret the inconvenience. • Study of absorption-free, gain-assisted propagation at positive and negative group velocity for positive and negative index refractive index of the conducting medium. • Phase and amplitude of conductivity influence largely the optical properties such as group index, group velocity, group delay and photon drag. • The maximum calculated values of positive and negative superluminal group velocities are of order 4 ×10 8 m/s at positive and negative group indices, respectively. • The maximum value of photon drag calculated is of order 6.4 ×10 −7 radian. • Probe detuning can be used as a tuning nob to switching the light absorption profile to the transparent window at resonance.

Self‐Driven Perovskite Dual‐Band Photodetectors Enabled by a Charge Separation Reversion Mechanism

AbstractNew mechanisms and modalities for multiband light detection are highly desirable for the ever‐growing developments in machine vision technologies. Demonstrated here is a unique talent of halide perovskites for making self‐driven dual‐band photodetectors with a negative broadband response in the above‐bandgap absorption region and a positive narrowband response in the sub‐bandgap absorption region. The device operates through a charge separation reversion mechanism by controlling the built‐in electric field and the thickness‐dependent light absorption profile in the active material. Impressively, the demarcation wavelengths between the negative broadband and the positive narrowband responses can be continuously adjusted, with the positive narrowband response easily tuned from 410 to 675 nm. The dual‐band photodetector exhibits an extremely low noise of 10−15 A Hz−1/2 and a high specific detectivity of >1012 Jones in both above‐bandgap and sub‐bandgap absorption regions, opening up application opportunities in intelligent recognition and biomedical diagnosis.

Modulated 3D light absorption profile in GaN nanorod arrays

The depth dependent UV-light absorption profile of GaN nanorods with different lattice arrays and filling factors was studied using finite-difference time-domain (FDTD) methods. By comparing to the results from Lambert-Beer's law with Maxwell-Garnett effective medium theory, we identified the quantitative contribution from nano-scattering effect on the light absorption in the nanorod arrays. The FDTD study of graphical 3D profile of light absorption and electric field intensity was parallelly conducted to investigate the origin of the nano-scattering. We found that the coupled electric field in the gap regions led to the larger absorption cross-section of the nanorod arrays, which is attributed to the distorted depth profile of the light absorption.

The Evolution of Icelandic Volcano Emissions, as Observed From Space in the Era of NASA's Earth Observing System (EOS)

AbstractVolcanoes are natural phenomena that have global environmental impacts. Satellite remote sensing can help classify volcanic eruptions and track the dispersion of emissions. We assess multiple volcanic eruptions in Iceland (Eyjafjallajökull 2010, Grímsvötn 2011, and Holuhraun 2014–2015), using space‐borne observations to infer information about the geological dynamics of each volcano and the properties and evolution of plume particles. We derive qualitative constraints on plume particle size, shape, and light‐absorption characteristics from Multiangle Imaging SpectroRadiometer (MISR) space‐borne imagery. With the MISR Research Algorithm (RA), we distinguish sulfate/water‐dominated volcanic plumes from Holuhraun and ash‐dominated plumes from Eyjafjallajökull and Grímsvötn, and even identify subtler changes in ash particle size and light‐absorption within plumes. Additionally, plume heights are retrieved geometrically from MISR. These are combined with surface thermal anomalies from the MODerate resolution Imaging Spectroradiometer (MODIS) and SO2 concentrations derived from the Ozone Monitoring Instrument (OMI) to synthesize eruption remote‐sensing chronologies. Signals related to differences in particle properties are identified and linked to evolving magma composition at Eyjafjallajökull. The results illustrate the potential to distinguish qualitative differences in eruptive magma composition based on particle light absorption and plume profile from remote‐sensing. For the sulfate‐rich Holuhraun plumes, the influence of aerosol hygroscopic growth during transport is inferred from such data. Three processes appear to dominate plume evolution in Iceland: downwind aerosol formation, particle hydration, and particle deposition. This work demonstrates enhanced MISR capabilities and, more generally, remote‐sensing analysis that can be applied globally, especially where suborbital volcano observations are limited or entirely absent.

Symmetry Breaking Induced by Growth Kinetics: One‐Pot Synthesis of Janus Au−AgBr Nanoparticles

AbstractThis study describes a one‐pot synthetic strategy consisting of concerted stabilization and photo and chemical reductions of metal ions to yield precise anisotropic nanostructures. Photoirradiation of an aqueous precursor solution containing Au3+, Ag+, Br−, and polyvinylpyrrolidone (PVP) forms Janus nanoparticles (NPs) composed of Au having a tiny Ag shell and AgBr. The Au NPs are initially generated by photoreduction of Au3+ in the precursor solution, in which the formation of silver halides is strongly inhibited by the coordination bond between Ag+ and the carbonyl groups in PVP. The coordinated PVP chains are adsorbed on the surfaces of the Au NPs, with the concomitant release of free Ag+ ions. The part of the surface of the Au NP is covered with a few Ag atoms via chemical reduction of Ag+ by PVP. AgBr nanocrystals grow at a bare site on the surface of each Au NP, resulting in the formation of nearly monodisperse Janus Au−AgBr NPs in high yield. A characteristic light absorption profile emerges due to the interface formed between Au and AgBr, which is demonstrated by finite‐difference time‐domain simulation. The precise nanostructures may be leveraged to elucidate mechanisms of photoelectrochemical processes and to construct advanced plasmonic photocatalysts.

Comparison between the use of uniform and non-uniform light absorption profiles in modelling organic photovoltaics

To avoid complexity, a uniform light absorption profile is usually used when modelling organic photovoltaic cells (OPVs). However, the actual light absorption profile is not uniform. It is found that a uniform light absorption profile can be used as a replacement for the actual non-uniform light absorption profile in modelling an OPV provided that the actual light absorption profile inside the OPV has a peak absorption value that is roughly less than twice its average absorption value. Nevertheless, the use of a uniform light absorption profile in investigating the effect of a certain parameter (e.g. the active layer thickness) on the performance of OPVs should still be used with care if variations in the value of the said parameter lead to different light absorption profiles.

Color Determination from a Single Broadband Organic Photodiode

AbstractEstablishing the color of incident light is essential for many applications, such as machine vision, but generally requires either a dispersive component or multiple spectrally selective photodetectors. In contrast, here an incident spectrum is parametrized using a single broadband organic photodiode (OPD). This is achieved by exploiting the incident wavelength dependence of charge extraction caused by optically induced trap states in a metal oxide electron extraction layer, which results in an atypical spectral dependence of the reverse bias photocurrent density vs voltage (J–V) characteristics. Such dependence is augmented by confining the active layer within an optical microcavity to influence the light absorption profile and thus metal oxide trap state density. The average wavelength of an (approximately normally distributed) incident spectrum is then calculated to within ≈5 nm by algorithmically minimizing the difference between a measured J–V curve and one determined from the overlap integral of a trial spectrum with previously acquired voltage bias dependent external quantum efficiency (EQE) spectra.

Insights on the impact of doping levels in oxygen-doped gC3N4 and its effects on photocatalytic activity

In recent years, graphitic carbon nitride (gC3N4) has hit the limelight as a bright, metal-free and visible-light responsive photocatalyst. Pristine gC3N4, however, suffers from inadequate light absorption profile, limited surface area and significant recombination of photo-induced electron-hole pairs, thus requiring reformative strategies. Herein, this work has employed elemental doping of oxygen to gC3N4 (O-gC3N4) and uncovered the evolution of properties that arises in conjunction with increasing doping level. These intrusive changes in properties by increasing oxygen doping levels are then evaluated based on the trends observed from photocatalytic hydrogen (H₂) evolution. It is revealed that oxygen doping had a competing dual effect to the photocatalytic activity. On one hand, oxygen doping introduced more porosity and added sub-gap impurity states in its electronic band structure, which resulted in enhanced light harvesting capabilities. On the other hand, impurity levels from high density oxygen groups behave detrimentally as potent electron-holes recombination centers. This became the governing factor for higher doping levels O-gC3N4, which thereby resulted in a weakened photoactivity compared to pristine gC3N4. In overall, it is hoped that the findings of this study can provide a new understanding of the rational design and oxygen-doping strategy for gC3N4 for its photocatalytic enhancement.

Illuminating Photosynthesis in the Mesophyll of Diverse Leaves.

Since [Terashima and Saeki (1983)][1] demonstrated that light attenuation through the leaf was accompanied by spectral changes, there has been increasing interest in how light absorption profiles align with photosynthetic capacity. Progress in our understanding has come from painstaking work

Structure and optical properties of a new AgBiW2O8 polymorph

In this work, we report a novel polymorph of AgBiW2O8, which was synthesized by a simple arc melting method. The structural details of this compound were determined by X-ray powder diffraction, and elemental composition was confirmed with ICP-AES. β-AgBiW2O8 adopts the scheelite structure type with tetragonal space group I41/a and lattice parameters a = b = 5.3019(3) and c = 11.678(2) Å. Structural details and optical properties are discussed in detail. Diffuse reflectance spectroscopy (DRS) measurements of β-AgBiW2O8 showed light absorption profile extended to the visible region, which suggests the material has a smaller band gap energy than Bi2WO6, as expected due to contribution of Ag d-orbitals to the electronic structure.

Optical Properties and Dielectric Functions of Grain Boundaries and Interfaces in CdTe Thin-Film Solar Cells

CdTe thin-film solar cells have complex microstructures, such as grain boundaries within the absorber layer, as well as CdS window, and Au back contact interfaces, where the local structure and chemistry undergo significant changes. The optical properties at these nanoscale defects are unknown, but their accurate measurement is required in order to identify potential losses in device efficiency. Here monochromated electron energy loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM) is used to measure the complex dielectric function for the CdTe1–xSx interdiffusion layer at the CdS–CdTe interface, high angle CdTe grain boundaries and Au–CdTe interface. CdTe1–xSx is shown to have a lower absorption coefficient than CdTe, but its refractive index is more closely matched to CdS. Grain boundaries have a negligible effect on the light absorption profile within CdTe, despite significant changes in the local structure and chemistry (i.e., Te depletion) at the grain boundary. Delocalization in inelastic scattering is the dominant systematic error in the above measurements. Finally a light backscattering mechanism via surface plasmon polaritons at the Au–CdTe interface is uncovered, which could potentially increase the photocurrent extracted from incident light at energies just above the CdTe band gap.

Optoelectronic modeling of the Si/α-Fe2O3 heterojunction photoanode

In photoelectrochemical (PEC) water-splitting systems, photoelectrode configuration is a key ingredient to obtain the high conversion efficiency. To effectively guide the device fabrication, the intrinsic multi-domain physics and mechanisms (e.g., light absorption in optical domain and carrier generation/separation/transport/collection in electrical domain) have to be uncovered, which requires the advanced optoelectronic simulation. In this study, focusing on the Si/α-Fe2O3 heterojunction photoanodes, we present a comprehensive optoelectronic study on the microscopic photoelectric processes inside the dual-absorbable photoanode. We systematically simulate the carrier generation, separation, recombination, and collection of the electron-hole pairs so that the complete optoelectronic responses of this kind of electronic devices can be obtained. The systems under consideration include n-Si/α-Fe2O3/electrolyte, p-Si/α-Fe2O3/electrolyte, and α-Fe2O3/electrolyte systems in order to uncover the intrinsic physics as well as find the optimal device designs. We obtain the spectra of light absorption, output- and recombination-photocurrent profiles, along with the current-voltage characteristics under dark and illumination conditions. Moreover, the energy band diagram under various system configurations are calculated and compared. The optoelectronic simulation and investigation provide a convenient methodology for the design of PEC-related systems for improved performance.

Chlorophyll Can Be Reduced in Crop Canopies with Little Penalty to Photosynthesis

The hypothesis that reducing chlorophyll content (Chl) can increase canopy photosynthesis in soybeans was tested using an advanced model of canopy photosynthesis. The relationship among leaf Chl, leaf optical properties, and photosynthetic biochemical capacity was measured in 67 soybean (Glycine max) accessions showing large variation in leaf Chl. These relationships were integrated into a biophysical model of canopy-scale photosynthesis to simulate the intercanopy light environment and carbon assimilation capacity of canopies with wild type, a Chl-deficient mutant (Y11y11), and 67 other mutants spanning the extremes of Chl to quantify the impact of variation in leaf-level Chl on canopy-scale photosynthetic assimilation and identify possible opportunities for improving canopy photosynthesis through Chl reduction. These simulations demonstrate that canopy photosynthesis should not increase with Chl reduction due to increases in leaf reflectance and nonoptimal distribution of canopy nitrogen. However, similar rates of canopy photosynthesis can be maintained with a 9% savings in leaf nitrogen resulting from decreased Chl. Additionally, analysis of these simulations indicate that the inability of Chl reductions to increase photosynthesis arises primarily from the connection between Chl and leaf reflectance and secondarily from the mismatch between the vertical distribution of leaf nitrogen and the light absorption profile. These simulations suggest that future work should explore the possibility of using reduced Chl to improve canopy performance by adapting the distribution of the "saved" nitrogen within the canopy to take greater advantage of the more deeply penetrating light.