Shift Of Edge Research Articles

Continuously broken ergodicity and reversible photo-darkening in As-Ch glass

The exact mechanism responsible for reversible photo-darkening in As-Ch glasses has been an open question. Many mechanisms have been suggested. Here I assume that self trapped excitons (STE) cause a local structural relaxation and end up in mid band states. I show that such an excitonic state produces a local frustration field which distorts the lattice. These states most probably decay non-radiatively through excitation of localized modes satisfying quantum statistics. I posit that these localized modes are the result of continuously broken ergodicity. These modes can channel to form local oscillations, which would act to raise the temperature, or couple to the valence electronic energy, which results in a local structural modification. I show the observed temperature dependence of both the band edge shift and the volume increase are predicted by the latter mechanism.

Amine Hole Scavengers Facilitate Both Electron and Hole Transfer in a Nanocrystal/Molecular Hybrid Photocatalyst

A well-known catalyst, fac-Re(4,4'-R2-bpy)(CO)3Cl (bpy = bipyridine; R = COOH) (ReC0A), has been widely studied for CO2 reduction; however, its photocatalytic performance is limited due to its narrow absorption range. Quantum dots (QDs) are efficient light harvesters that offer several advantages, including size tunability and broad absorption in the solar spectrum. Therefore, photoinduced CO2 reduction over a broad range of the solar spectrum could be enabled by ReC0A catalysts heterogenized on QDs. Here, we investigate interfacial electron transfer from Cd3P2 QDs to ReC0A complexes covalently bound on the QD surface, induced by photoexcitation of the QD. We explore the effect of triethylamine, a sacrificial hole scavenger incorporated to replenish the QD with electrons. Through combined transient absorption spectroscopic and computational studies, we demonstrate that electron transfer from Cd3P2 to ReC0A can be enhanced by a factor of ∼4 upon addition of triethylamine. We hypothesize that the rate enhancement is a result of triethylamine possibly altering the energetics of the Cd3P2-ReC0A system by interacting with the quantum dot surface, deprotonation of the quantum dot, and preferential solvation, resulting in a shift of the conduction band edge to more negative potentials. We also observe the rate enhancement in other QD-electron acceptor systems. Our findings provide mechanistic insights into hole scavenger-quantum dot interactions and how they may influence photoinduced interfacial electron transfer processes.

Influence of Ion Exchange on Photoresponsive Properties of Potassium Poly(heptazine imide)

Metal poly(heptazine imide) (MPHI) exhibits various optoelectronic functions by accommodating various metal ions in its heptazine-based graphite-like two-dimensional layer. Although potassium PHI (KPHI) is the most well-studied in MPHI series, there are unresolved issues regarding the mechanism responsible for its unique properties, particularly its high photocatalytic activity. The mechanisms underlying other diverse properties, such as photochromism and photoconduction, also remain unknown. Herein, we focused on elucidating the mechanisms of photochromism and photoconduction in KPHI. We developed a method to gradually replace K+ in KPHI with H+ and successfully prepare samples with tunable K+ concentrations. The structural, chemical, optical, electronic, and electrical properties of samples with continuously varying K+ concentrations were thoroughly investigated through various experiments and theoretical calculations. First, the gradual substitution of K+ for H+ in KPHI was investigated using X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy. XRD analysis revealed that K+ in KPHI was not randomly substituted with H+, but even small amounts of K+ substitution led to the formation of the long-range ordered region of HPHI. Absorbance measurements revealed a continuous blue shift of the absorption edge as the ratio of K+ decreased from that of KPHI. Energy band calculations revealed that the energy gap of HPHI was more than 14% larger than that of KPHI. The dependence of the K+ concentration measurements on the photochromism and photoconductive properties showed that in the K+-rich samples, the K+ released from the PHI layer upon white light irradiation had a significant effect on the electrical conduction properties. K+ conduction was dominant in the electrical conduction of KPHI, including photoconduction, under ambient conditions. However, the mobility of H+ in HPHI was very low, which was responsible for the lethargic return to the ground state after photoexcitation and minimal electrical conductivity.

Structural, optical and dielectric properties of chemical vapor transport based synthesis of rice-like nanostructured cadmium zinc oxide films

The cadmium zinc oxide (Cd-ZnO) films are synthesized on glass substrate at 200 sccm argon gas flow rate by chemical vapor transport technique. The synthesized Cd-ZnO films are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), Raman, Fourier transformed infrared spectroscopy (FTIR) and UV spectrometers. The XRD patterns exhibits the development of various diffraction planes related to Zn, ZnO and CdO phases thereby confirms the formation of polycrystalline Cd-ZnO films. The structural parameters like crystallinity, crystallite size, d-spacing, microstrains, dislocation density, degree of distortion, atomic packing fraction, lattice parameter, bond length and atomic packing factor are associated with increasing weights of Cd powder. The formation of chemical bonding, substitution of Cd+ into ZnO are confirmed by Raman, FTIR and SEM analysis. Red shift in absorption edge and increasing transmittance (> 90%) are associated with increasing weights of Cd powder. Refractive index (2.58–2.63) and energy band gap (3.11–2.87 eV) are increased/decreased with increasing weights of Cd powder. The change in optical, electrical and dielectric parameters of synthesized Cd-ZnO films are strongly associated with increasing weights of Cd powder. Results show that the synthesized Cd-ZnO films can be used for optoelectronics devices.

Promoted electron transfer in Fe2+/Fe3+ co-doped BiVO4/Ag3PO4 S-scheme heterojunction for efficient photo-Fenton oxidation of antibiotics

A Fe2+/Fe3+ co-doped BiVO4/Ag3PO4 S-scheme heterojunction catalyst was synthesized by a simple hydrothermal method and applied to achieve efficient photo-Fenton for antibiotics degradation (99.70 %). The co-doping of Fe3+/Fe2+ highlighted red shift of optical absorption edge (535–800 nm) and adjustable band structure of BiVO4/Ag3PO4. The collaborative system of photocatalytic and Fenton could maintain high catalytic activity and stability in the treat of simulated actual water and high efficiency within wide pH conditions (3–7). The degradation experiments were employed to show removal efficiency on tetracyclines and quinolones antibiotics including tetracycline (TC), ciprofloxacin (CIP), doxycycline (DO), levofloxacin (LEV), norfloxacin (NOR), and enrofloxacin (ENR). Mechanism exploration elaborated that the m-FBVO/APO composites with dominant Fe2+ preserved both high oxidation potential (<−0.33 eV vs NHE) and reduction potential (>2.45 eV vs NHE) through the constructed BiVO4/Ag3PO4 S-scheme heterojunction. Efficient e−/h+ pairs separation was further achieved through Fe3+/Fe2+ pairs cycling. And degradation pathway of TC was mainly investigated to verify its mineralization. This work provides a new sight towards catalytic mechanisms in heterogeneous photo-Fenton degradation systems.

Investigation of structural and electrical properties of ITO thin films and correlation to optical parameters extracted using novel method based on PSO algorithm

Thermally annealed DC sputtered indium tin oxide (ITO) thin films were investigated for improvement in properties. The structural and optoelectronic characteristics of as-grown and air annealed films were studied and correlated to the film deposition time. Raman spectroscopy analysis showed low crystalline quality films for as-grown films and were significantly improved after annealing. X-ray diffraction analysis confirmed the crystallinity of samples with (222) preferential orientation. The 30-min ITO films showed a peak at (400). The films optical study shows an increased transmittance (in the transparency region) with decreasing deposition time, yielding a high transparency of 90% for the 5- and 15-min ITO films annealed at 400°C. The films thickness and optical constants were determined from optical transmission only without interference fringe using a novel method based on particle swarm optimization (PSO) algorithm. The absorption coefficient and calculated refractive index decreased with increasing deposition time and their value reduced further after annealing treatment. The 30-min ITO films showed a comparable low resistivity of 4 × 10−3 Ω cm before and after annealing as determined by Hall effect measurements. This observation confirms their non-sensitivity to the oxygen post-contamination that resulted from (400) orientation. A shift of the absorption edge towards shorter wavelengths accompanied with an increase in the optical bandgap before and after annealing with decreasing thickness were observed. We have demonstrated that the optical parameters such as the optical gap depend mainly on the electrical parameters such as the carrier concentration.

Anomalously fresh Chukchi Sea surface salinity in summer-autumn 2021

ABSTRACT The Chukchi Sea is a marginal sea in the Arctic with a mixed layer that is a few psu units saltier than ambient open Arctic water. Such higher salinity is maintained by salty and warm Pacific water inflow through the Bering Strait, implying that changes in inflow characteristics should affect the thermohaline properties of the Chukchi Sea. Recently, two additional controlling factors have been highlighted – the strength of boundary currents along the Siberian coast, and meridional exchanges due to wind-driven transport. In this note, we illustrate that anomalous fresh Chukchi Sea surface salinity in summer-autumn 2021 may be related to the anomalous southward shift of the ice edge and its meltwater source. These anomalous ice conditions occur concurrently with anomalously low Beaufort High, anomalous westerly cyclonic winds over ice-covered and open water Chukchi Sea, and related southward Ekman transport of late season meltwater. The September 2021 ice expansion was the largest in 1981–2021 detrended ice records.

Improvement of third-order NLO properties of vacuum deposited Cd1-xPbxS nanostructured thin films for optoelectronic device applications

A polycrystalline nanostructured cadmium lead sulfide thin film was deposited using the thermal evaporation (PVD) technique (Cd1-xPbxS with x = 0.00, 0.01, 0.05 and 0.1 wt.% of Pb). Structural parameters of as-prepared Cd1-xPbxS thin films have been studied through X-ray diffraction. The optical investigation demonstrates that Cd1-xPbxS film's optical band gap (Eg) may be adjusted from the visible to the near-infrared region. (2.64 - 2.42 eV). The film is substantially more appropriate for absorbing layers in solar cells and optoelectronic applications due to the large decrease in ‘Eg.’ The enhanced Pb doping was found to have altered the surface morphology, verified by Field Emission Scanning Electron Microscopy (FESEM) images. The doped films also showed a significant red shift in the band edge and increased transmittance in the visible and NIR regions. The third-order nonlinear optical (TONLO) parameters of the samples were determined from the Q-switched Nd: YAG laser with 65-ps pulse duration at 1064 nm. The investigated TONLO components such as nonlinear absorption coefficient (β), nonlinear refractive index (n2) and the susceptibility χ(3)were found to be in the range from 1.16 × 10−3 to 4.12 × 10−3 (cmW−1), 1.06 × 10−8 to 3.32 × 10−8 (cm2 W−1) and 1.23 × 10−4 to 5.62 × 10−4 (esu) respectively. The results indicate that Pb-doping on CdS nanostructures on surface morphology can be used to modify NLO characteristics.Cd1-xPbxS thin film is a potential and able material for optoelectronic device applications, as seen by these encouraging NLO results.

The impact of Carbon Quantum Dots (CQDs) on the photocatalytic activity of TiO2 under UV and visible light

In this study, the impact of carbon quantum dots (CQDs) on the photocatalytic performance of TiO2 in aqueous medium was investigated under UV and visible light. CQDs were synthesized through a solvothermal procedure, and the effect of synthesis conditions on their optical properties were optimized. The CQDs/TiO2 nanocomposites were prepared via a hydrothermal method using CQDs with the extreme band-gaps (i.e. CQDs450 and CQDs550). The physicochemical properties of CQDs and CQDs/TiO2 samples were studied by various characterization techniques. According to the High-Resolution Tunneling Electron microscopy (HRTEM) micrographs, the synthesized spherical CQDs had diameters smaller than 5 nm. The Diffuse Reflectance Spectroscopy (DRS) analysis revealed that incorporation of CQDs into nanocomposite samples causes a remarkable shift in absorption edge to the visible region, in comparison with pure TiO2. According to the experimental data, the nanocomposite sample containing CQDs with the higher absorption edge, i.e. CQDs550/TiO2, showed a superior performance in photocatalytic removal of methylene blue (MB) under visible light. In this regard, the removal percent of MB on CQDs550/TiO2 and CQDs450/TiO2 nanocomposites, with 4%wt CQDs, under visible light (λ > 420 nm) were 92.1 % and 53.4 % after 120 min of irradiation at pH = 11 and 150 mgcat./L catalyst dosage, which are respectively 2.4 and 1.4 times higher than that on pure TiO2. However, under UV irradiation the observed photocatalytic performances were in the reverse order. Finally, two different reaction mechanisms have been proposed to explain the obtained results on CQDs/TiO2 samples, based on possible electron transfer between TiO2 and CQDs nanoparticles under UV and Visible lights.

Formation of a one-dimensional hole channel in MoS2 by structural corrugation

We have investigated the energetics and electronic structure of monolayer MoS2 with periodic structural corrugations by density functional theory. The total energy of corrugated MoS2 slightly increases with increasing corrugation height, which indicates that the MoS2 sheet intrinsically and extrinsically possesses nanometer scale structural corrugation. The corrugation causes an upward shift of the valence band edge and a downward shift of the conduction band edge owing to the local strain at the wrinkle peak. Accordingly, by injecting holes using the external electric field, the corrugation leads to a one-dimensional conducting channel in the MoS2 sheet. This indicates that corrugation is a plausible procedure to control the dimensionality of the electrons and holes in two-dimensional materials without implementing one-dimensional boundary conditions.

МОДЕЛЬ ПЛАСТИЧНОСТИ ЖЕЛЕЗОБЕТОННЫХ КОНСТРУКЦИЙ

A model of plasticity of reinforced concrete structures is considered, based on on the transformations of the intensity of the “stress-strain” connection by projecting the tensors of this connection, using special transitions for the main angle of deformations, total shear deformations, etc.). At the same time, the modulus of plasticity of concrete, the coefficient of transverse deformations are determined, and complex functions are constructed for linear and angular deformations in sections, taking into account deformation, gradients of deformations during the formation of cracks and stiffness changes. The hypotheses adopted for the calculation model determine the distribution of force flows - blocks for compressed and stretched concrete (first object), "main cracks" from the mechanics of destruction of reinforced concrete, complex functions and a two-cantilever element for modeling the deformation effect of reinforced concrete, developed by the author (second object). Tensile concrete resistance is transferred to the working reinforcement and is modeled using the sum of the average values of the longitudinal and transverse forces, as well as the average reduced coefficient of tension concrete. The "pin (nagel)" effect in the reinforcement crossed by a crack was obtained using the model of the second level of structural mechanics for a reinforcing bar with two pinched ends. The opening of the crack and the shift of the crack edges are simulated. The main force vector in the reinforcement is characterized by the values of longitudinal and transverse displacements (the third object).

Optical properties of multi-layers structured PEO/PVP solid polymer membranes doped with sodium perchlorate

In this present research work, fabrication of multi-layers structured (MLS) polymer membranes has been proposed to understand the variation of the optical bandgap and determine the types of electronic transitions as a function of sodium salt doping concentration. Solid MLS polymer membranes based on PEO/PVP have been prepared by the well-known solution cast technique. The recorded absorption, transmittance and reflectance spectra revealed that, incident energy absorption increased upon increase of salt dopant concentration and a shift of absorption edge towards lower energy region suggests the good complexation between the host polymers and dissociated salt (Na+, ClO4-) ions, which in turn the energy bandgap decrement expected. A detailed investigation has been carried out on the variation of refractive index of all the MLS polymer membranes as a function of salt doping concentration. The miscibility between the sodium salt and the individual polymers present in MLS polymer membranes can be understood from the linear relationship between the refractive index and the volume fraction of the added salt. The increase of extinction coefficient at high wavelengths was observed. The optical band gap measured from the plots of (αhυ)x versus photon energy (hυ) was compared to that determined from the optical dielectric loss.

Electron–phonon effects and temperature-dependence of the electronic structure of monoclinic β-Ga2O3

Gallium oxide (Ga2O3) is a promising semiconductor for next-generation high-power electronics due to its ultra-wide bandgap and high critical breakdown field. To utilize its unique electrical properties for real-world applications, an accurate description of its electronic structure under device-operating conditions is required. Although the majority of first-principles models focus on the ground state, temperature effects govern the key properties of all semiconductors, including carrier mobility, band edge positions, and optical absorption in indirect gap materials. We report on the temperature-dependent electronic band structure of β-Ga2O3 in a wide temperature range from T = 0 to 900 K using first-principles simulations and optical measurements. Band edge shifts from lattice thermal expansion and phonon-induced lattice vibrations known as electron–phonon renormalization are evaluated by utilizing the quasi-harmonic approximation and the recently developed “one-shot” frozen phonon method, respectively. Electron–phonon effects and thermal expansion together induce a substantial temperature-dependence on the bandgap, reducing it by more than 0.5 eV between T = 0 and 900 K, larger than that observed in other wide bandgap materials. Key implications, including an increase in carrier concentrations, a reduction in carrier mobilities due to localization of band edge states, and an ∼20% reduction in the critical breakdown field, are discussed. Our prediction of temperature-dependent bandgap matches very well with experimental measurements and highlights the importance of accounting for such effects in first-principles simulations of wide bandgap semiconductors.

Further evidence in support of new empirical relation for chemical shift and effective charge in the X-ray absorption edge shifts

When a compound is formed, re-arrangement of electrons takes place. Consequently, the energies of core levels as well as outer states are altered. As a result, the position of the absorption edge undergoes shift in its energy. This shift in energy, when measured with respect to the pure metal, is called chemical shift, represented by ΔE. The concept of effective charge is found to be useful to understand chemical shift in X-ray absorption spectra. Validity of new empirical relation ΔE=aebq, between the chemical shift ΔE and effective charge ‘q’, has been demonstrated successfully using its linearized form ln|ΔE|=ln|a|+bq for a variety of binary and ternary cobalt compounds. As the proposed equation is empirical in nature, it is always desirable to examine its validity to as many systems as possible. The results obtained validate the correctness of exponential relation for chemical shift ΔE and effective charge ‘q’ proposed earlier.

First-principles investigation on structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2

The structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2 were studied by first-principles calculations based on density functional theory and the Hubbard U correction. The results demonstrated that hybridized states composed of Ta 5d and Nb 4d orbitals at the bottom of the conduction band enabled the Fermi level to enter the conduction band and facilitated (Nb, Ta) codoped anatase TiO2 to exhibit n-type metallic characteristics, and the conductivity of Ti0·875Nb0·0625Ta0·0625O2(I) is approximately 3.2 times that of Ti0·9375Nb0·0625O2. Noted that both conductivity and mobility decreased with the increase of the distance between impurity Nb and Ta atoms and the Nb doping concentration. Moreover, the calculated results of optical properties presented the shifts of the absorption edges towards the ultraviolet region, and (Nb, Ta) codoped anatase TiO2 possessed the high transmittance thanks to its low absorption and reflection of visible light.

Properties of (Cd, Zn)S films deposited by DC co-sputtering

CdS thin film is commonly used as a buffer layer in most thin film solar cells. However, the buffer layer with less absorption in the visible region of the solar spectrum as well as that which provides perfect lattice matching with the absorbers is required. The incorporation of Zn in the CdS films can affect the structural and optical properties of the buffer layer. In this work, films of CdS and (Cd,Zn)S films were deposited by DC sputtering utilizing CdS and Zn targets. The concentration of Zn in the CdS was varied by changing the sputtering power of the Zn target. The influence of Zn concentration on the structural and optical properties of CdS films was investigated using X-ray diffraction (XRD) and Ultraviolet-Visible-Near Infrared (UV-VIS-NIR) spectroscopy, respectively. The XRD patterns confirm the growth preferential orientation of all films along the (002) plane. UV-VIS spectroscopy results indicate absorption edge shifts toward lower wavelength with an increase of Zn concentration in the CdS. The band gap evaluated by the Tauc plot of these films varied from 2.40 eV to 2.50 eV. The increased bandgap induces the density of localized states on the edge of the bandgap which leads to the increase in the Urbach tail width.

Determination of optical constants and band gap variation of Zn0.98-xCu0.02MgxO thin films

Cu doped ZnO (ZnCuO) is a very important candidate for electronic applications, since it has been shown that it possesses p–type conductivity. In order to broaden its applications, it is crucial to tune optical and electronic properties. In this study, by doping ZnCuO with magnesium, variation of refractive index, extinction coefficient, and band gap of thin films were investigated. Optical constants were evaluated using a transmittance model which is derived from Fresnel equations. Refractive indices of thin films were expressed as a dispersion relation in a polynomial form, while extinction coefficients were modelled as a convolution by Lorentzian curves. It was observed that magnesium doping decreased the refractive index and also caused a blue shift in absorption edge which is a clear indicator of band gap widening.

Tailoring the Structural, Optical and Electrical Properties of Zinc Oxide Nanostructures by Zirconium Doping

Owing to its low resistivity, high transmittance, and tunable optical band gap, ZnO is of great interest for optoelectronic applications. Herein, the sol–gel technique was used to synthesize un-doped and zirconium-doped zinc oxide (ZZO) nanostructures with different concentrations of Zirconium (Zr). X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), UV-Vis spectroscopy, and photoluminescence (PL) measurements were used to investigate the influence of Zr doping on the structural, optical, and electrical properties of developed nanostructures. XRD and SEM confirmed the increase in crystallite size with increasing concentrations of Zr. Raman analysis indicated the presence of oxygen vacancies in synthesized nanostructures. UV-Vis spectroscopy illustrated the blue shift of band gap and red shift of the absorption edge for ZZO nanostructures with increasing concentrations of Zr. For the measurement of electrical properties, the spin-coating technique was used to deposit un-doped and Zr-doped ZnO layers of ~165 nm thickness. The four-probe-point (4PP) method illustrated that the doping of Zr caused a reduction in electrical resistance. Hall Effect measurements showed a high value, 3.78 × 1020 cm−3, of the carrier concentration and a low value, 10.2 cm2/Vs, of the carrier mobility for the Zr-doped layer. The high optical transmittance of ~80%, wide band gap of 3.51 eV, low electrical resistivity of 1.35 × 10−3 Ω·cm, and maximum carrier concentration of 3.78 × 1020 cm−3 make ZZO nanostructures one of the most promising candidates for the application of transparent conductive oxide (TCO) in optoelectronic devices.

Triphenylphosphine assisted phosphorization of g-C3N4 for enhanced photocatalytic activity

It is an effective strategy to regulate the structure of photocatalysts for enhanced photocatalytic hydrogen evolution. In this work, a facile method was developed to introduce P atoms into g-C3N4via the thermal calcination treatment. In contrast to pristine g-C3N4, the obtained P-CN(PPh3) exhibited a negative shift of conduction band edge with enhanced light absorption after the introduction of P atoms into g-C3N4, thus benefiting the photocatalytic hydrogen evolution. Moreover, the introduction of P element in g-C3N4 can also improve the charge separation during the photocatalytic hydrogen reaction, significantly accelerating the photocatalytic hydrogen production rate of P-CN(PPh3).

Cation doped approach for photodegradation of 4-chlorophenol by highly efficient solar active NiS photocatalyst: The case of Cu2+ doping

This study reports an efficient photodegradation of 4-chlorophenol using Cu2+ doped nickel sulfide (NiS) nanoparticles. Our study provides a facile method to synthesize Cu2+ doped NiS nanoparticles by partial polyol method and structurally validates the formation of single phased random-shaped particles. Cu2+ doping in NiS matrix results in a red shift of the absorption band edge, thus lowering the band gap from 3.62 to 2.98 eV and creating electron trapping centers that prevent electron-hole recombination. The presence of such electron trapping centers was attributed to oxygen vacancies which were justified from the appearance of g-signal in electron paramagnetic resonance analysis. Photodegradation study of 4-chlorophenol by Cu2+ doped NiS nanoparticles under ambient solar light showed significant improvement in degradation efficiency up to 90 % as compared to 19 % for undoped NiS nanoparticles. Furthermore, increased degradation efficiency was also backed by total organic carbon (TOC) analysis of the mineralization ability of Cu2+ doped NiS nanoparticles. Moreover, scavenging trapping analysis showed holes, ⋅OH and superoxide radicals as reactive oxidative species (ROS) primarily responsible for increased photodegradation efficiency. Thus, the synthesized Cu doped NiS nanoparticles proved to be an efficient catalyst, which could be used as a potential candidate for industrial photocatalytic applications under ambient solar light.