Indirect Absorption Research Articles

Limitations of In2O3 as a transparent conducting oxide

Sn-doped In2O3 or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the process is relatively insensitive to temperature. The wavelength dependence of this indirect absorption process can be described by a power law. Indirect absorption mediated by charged defects or impurities is also unavoidable since doping is required to obtain conductivity. At high carrier concentrations, screening by the free carriers becomes important. We find that charged-impurity-assisted absorption becomes larger than phonon-assisted absorption for impurity concentrations above 1020 cm–3. The differences in the photon-energy dependence of the two processes can be explained by band structure effects.

Performance investigation of the direct absorption solar collector based on phase change slurry

The conventional solar collectors are usually indirect absorption solar collectors which convert solar energy into thermal energy through a coating surface with high optical absorbency. The direct absorption solar collector, which absorbs the incident radiation directly by the working fluid with high extinction coefficient, shows a better performance than that of the indirect absorption solar collector. The phase change slurry as a novel composite phase change material can work as both heat transfer fluid and energy storage medium due to the large heat capacity and fluidity. In the present study, the phase change slurry is applied in the direct absorption solar collector to further improve the solar collector efficiency, and a numerical model is built to investigate the performance of a parallel plate direct absorption solar collector. The optical properties of the particle of phase change material and the phase change slurry are analyzed based on the theories of the Mie scattering and modified independent scattering. The extinction coefficient of the phase change slurry increases with the increase of solid volume fraction and the absorption index of the particle. The effects of different parameters on the efficiency of the direct absorption solar collector are discussed. The collector efficiency increases first and then decreases with the increase of absorption index of the particle. The solid volume fraction shows different influences depending on the absorption index of particle. The melting of solid particle also plays an important role in the collector efficiency and the temperature distribution. The performances of different types of solar collectors are compared, and the phase change slurry based direct absorption solar collector shows the highest photothermal conversion efficiency. The results are helpful in extending the applications of phase change slurry and direct absorption solar collector as well as in improving the solar collector efficiency.

Phonon- and charged-impurity-assisted indirect free-carrier absorption in Ga2O3

Monoclinic $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ has a large band gap of 4.8 eV, and can therefore be used as a contact material that is transparent to visible and UV light. However, indirect free-carrier absorption processes, mediated by either phonons or charged impurities, will set a fundamental limit on transparency. We use first-principles calculations to accurately assess the absorption cross section and to elucidate the microscopic origins of these processes. Phonon-assisted absorption is dominated by the emission of phonons, and is therefore always possible. This indirect absorption is inversely proportional to the cube of the wavelength. The presence of charged impurities, whether intentional or unintentional, leads to additional absorption, but for realistic concentrations, phonon-assisted absorption remains the largest contribution. Direct free-carrier absorption also leads to below-gap absorption, with distinct peaks where optical transitions match energy differences to higher conduction bands. In contrast, indirect absorption uniformly reduces transparency for all sub-band-gap wavelengths.

Cloud point extraction of Cefixime drug by direct (UV- Vis) spectrophotometer and indirect(Flame Atomic Absorption) technique

A new method for the determination of the Cefixime drug in some Pharmaceuticals using ultra violet-visible (UV- Vis) and indirect Flame Atomic Absorption Spectrophotometry (FAAS) by Cloud point Extraction by using Triton X-114 as surfactant, the method based to form chelating complex CEF – Pd (II) at 456nm, variables parameters were studied such as the concentration of metal ion, effect of pH, Triton X-114 amount, equilibration temperature and incubation time. The best pH for the formation of chelating complex was (9). The best temperature on cloud-point extraction was 55 0C at 20 min. then complex extracted with ethanol. The mole-ratio method has been used to determine the structure of chelate CEF – Pd (II) and found to be 1:1 L:M (Ligand : Metal). Beer’s Law was obeyed in the range 2.5-30 and 2.5-32.5μg/mL for UVVis and FAAS respectively. Limit of Detection and Limit of Quantitation LOD and LOQ values for these methods were (0.456592, 0.33374) μg/mL (1.521976, 1.112471) μg/mL respectively.the method was validated and successfully applied to drug formulations like Cefix capsules marketed in Iraq. The results of analysis have been validated statistically and by recovery studies and were found satisfactory

Wrap-Around Core-Shell Heterostructures of Layered Crystals.

Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one-pot synthesis of layered tin chalcogenides spontaneously creates core-shell structures in which large orthorhombic SnS crystals are enclosed in a wrap-around shell of trigonal SnS2 , forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core-shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core-shell interfaces and extended long-wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core-shell structures attractive for energy-conversion applications.

Optical and electrical conductivity studies of VO2+ doped polyvinyl pyrrolidone (PVP) polymer electrolytes

Abstract Polymer electrolyte films of polyvinyl pyrrolidone (PVP) complexed with different concentrations (1, 2, 3, 4 and 5 mol%) of VO2+ ions were prepared by a solution casting technique. The formation of complexes between the VO2+ ions and the polymer was confirmed by the Fourier transform infrared spectroscopy (FTIR) and the UV-Vis spectroscopy. Room temperature impedance measurements in the frequency range 42 Hz to 5 MHz revealed that the ionic conductivity increased with the increasing the VO2+ ion concentration. The maximum ionic conductivity of 5.39 × 10−8 Scm−1 at 303 K was observed for the 5 mol% VO2+ ions doped PVP polymer electrolyte film. From the UV-Visible absorption spectra in the wavelength range of 200–800 nm the direct and indirect optical energy band gaps and optical absorption edges were found decreased with the increase in the VO2+ ion concentration. FTIR studies on pure and VO2+ doped PVP polymer films revealed the vibrational changes to occur due to the effect of the dopant VO2+ ions in the polymer. It is suggested that VO2+, as a dopant, is a good choice to improve the electrical properties of the PVP polymer electrolyte.

Supervisor Support and Affective Organizational Commitment: The Mediator Role of Work Engagement.

The aim of the study was to verify the mediator role of work engagement between supervisor social support and affective organizational commitment. A cross-sectional and correlational study using questionnaires was conducted. We obtained a convenience sample of 267 participants from 403 nurses from two public hospitals (66.25% response rate). Participants were required to be registered nurses without a supervisor position and to have worked for at least 1 year in the same ward in a public hospital. The mediator role of work engagement was examined using path analysis and bootstrapping method (bias-corrected confidence intervals). Results showed that affective organizational commitment was positively and significantly predicted by supervisor support, vigor, and absorption. Supervisor support had both a direct effect and an indirect effect, through vigor and absorption, on affective organizational commitment. Social support from supervisors allows an increase both in nurses' engagement and their desire to remain in the organization.

A zone-plate-based two-color spectrometer for indirect X-ray absorption spectroscopy.

X-ray absorption spectroscopy (XAS) is a powerful element-specific technique that allows the study of structural and chemical properties of matter. Often an indirect method is used to access the X-ray absorption (XA). This work demonstrates a new XAS implementation that is based on off-axis transmission Fresnel zone plates to obtain the XA spectrum of La0.6Sr0.4MnO3 by analysis of three emission lines simultaneously at the detector, namely the O 2p-1s, Mn 3s-2p and Mn 3d-2p transitions. This scheme allows the simultaneous measurement of an integrated total fluorescence yield and the partial fluorescence yields (PFY) of the Mn 3s-2p and Mn 3d-2p transitions when scanning the Mn L-edge. In addition to this, the reduction in O fluorescence provides another measure for absorption often referred to as the inverse partial fluorescence yield (IPFY). Among these different methods to measure XA, the Mn 3s PFY and IPFY deviate the least from the true XA spectra due to the negligible influence of selection rules on the decay channel. Other advantages of this new scheme are the potential to strongly increase the efficiency and throughput compared with similar measurements using conventional gratings and to increase the signal-to-noise of the XA spectra as compared with a photodiode. The ability to record undistorted bulk XA spectra at high flux is crucial for future in situ spectroscopy experiments on complex materials.

Analysis of a chemotaxis model with indirect signal absorption

We consider the chemotaxis model{ut=Δu−∇⋅(u∇v),vt=Δv−vw,wt=−δw+u in smooth, bounded domains Ω⊂Rn, n∈N, where δ>0 is a given parameter.If either n≤2 or ‖v0‖L∞(Ω)≤13n we show the existence of a unique global classical solution (u,v,w) and convergence of (u(⋅,t),v(⋅,t),w(⋅,t)) towards a spatially constant equilibrium, as t→∞.The proof of global existence for the case n≤2 relies on a bootstrap procedure. As a starting point we derive a functional inequality for a functional being sublinear in u, which appears to be novel in this context.

Ultrafast bulk carrier dynamics in various GaN crystals at near-infrared wavelengths under one- and two-photon absorption

Femtosecond transient absorption (TA) as a probe of ultrafast carrier dynamics was conducted at near-infrared wavelengths in a series of GaN crystals. The TA kinetics in all the GaN crystals appeared to be single exponential under one-photon (1P) excitation but biexponential under two-photon (2P) excitation, which was inconsistent with previous experimental reports and model predictions. Surface recombination and carrier diffusion could be eliminated and the TA responses were identified as phonon-assisted indirect free-carrier absorption. Modelling the bulk carrier dynamics with a simplified model revealed that, at a 1P high carrier injection level, the carrier recombination was limited by the slow capture rate of electrons via deep defects, while at a 2P low carrier injection level, the initial carrier lifetime decreased remarkably due to fast hole capturing and could be further controlled by the inherent carrier and/or dislocation concentrations.

Phonon-assisted optical absorption in germanium

A comprehensive experimental and theoretical study of indirect gap optical absorption in bulk Ge is presented. While this topic has been the subject of intense studies from the early days of semiconductor physics, the resonant aspect of the absorption received very little attention until now. This is a unique property of Ge related to the proximity of the direct and indirect gaps. The absorption coefficient was measured over the entire spectral range between the two gaps for comparison with theory. It is shown that the standard textbook expressions, obtained by assuming intermediate states with constant energy, are in very poor agreement with experiment. A theory first proposed by Hartman [R. L. Hartman, Phys. Rev. 127, 765 (1962)], which takes into account the energy dependence of the intermediate states, provides a much better account of the photon-energy dependence of the absorption, but the prediction of the experimental absorption strength requires the incorporation of excitonic effects. The latter, however, have only been considered by Elliott [R. J. Elliott, Phys. Rev. 108, 1384 (1957)] in the limit of constant intermediate state energy. A generalization to the case of energy-dependent intermediate states, consistent with Hartman's theory, is presented here. The basic qualitative difference with the classical Elliott theory is that the excitonic character of the intermediate states affects the computed absorption, generating an additional resonant enhancement that is confirmed by the experimental data. The generalized theory presented here agrees very well with the experimental absorption using independently determined band structure parameters.

Bandgap Engineering in $$\hbox {TiO}_{2}$$ TiO 2 –Ge Nanocomposite Thin Films

Titanium dioxide–germanium ( $$\hbox {TiO}_{2}$$ –Ge) composite thin films were prepared by pulse laser deposition technique using third harmonic (355 nm) of Nd:YAG laser at fluence of $$12.73\,\hbox {J/cm}^{2}$$ . Films were grown on n-type Si (100) substrate using $$\hbox {TiO}_{2}$$ –Ge composite target. The bandgap was tailored by varying the composition of $$\hbox {TiO}_{2}$$ –Ge composite films based on the variable substrate–target distance from 5–8 cm. Ge concentration in $$\hbox {TiO}_{2}$$ matrix was measured through electron-dispersive X-ray spectroscopy and concentration variation from 0.9–25.3% was observed. Crystalline structure of thin films was analysed through X-ray diffraction, whereas micro-strains and dislocation density is also calculated through different X-ray diffraction peaks. A detailed study of the various polymorphs of $$\hbox {TiO}_{2}$$ and presence of Ge in crystalline form was carried out through Raman spectroscopy. Direct and indirect absorption transitions were observed through ultraviolet and visible spectroscopy. Absorption edge shifted to visible region of electromagnetic spectrum is associated with a possible quantum confinement effect in Ge or strain generated due to lattice mismatch of Si substrate and Ge with $$\hbox {TiO}_{2}$$ . Absorption transitions of thin film containing 2.6% Ge were observed in visible region along with the emission in the same spectral region observed through photoluminescence spectra.

Mechanical and Durability Properties of Green Star Concretes

This paper presents mechanical and durability properties of green star concretes. Four series of concretes are considered. The first series is control concrete containing 100% ordinary Portland cement, 100% natural aggregates and fresh water. The other three series of concretes are green star concretes according to Green Building Council Australia (GBCA), which contain blast furnace slag, recycled coarse aggregates and concrete wash water. In all above concretes compressive strength, indirect tensile strength, elastic modulus, water absorption, sorptivity and chloride permeability are measured at 7 and 28 days. Results show that mechanical properties of green star concretes are lower than the control concrete at both ages with significant improvement at 28 days. Similar results are also observed in water absorption, sorptivity and chloride permeability where all measured durability properties are lower in green star concretes compared to control concrete except the higher water absorption in some green star concretes.

Model development and performance analysis of the nanofluid-based direct absorption parabolic trough collectors

In this paper, a simple one-dimensional heat transfer model for the nanofluid-based direct absorption parabolic trough collector (NDAPTC) was developed, which was quick to solve, easy to understand, and unnecessary to solve the complicated differential equations. In an actual system, the vacuum condition between the inner tube and glass cover may be changed gradually over time. Therefore, a continuous function for calculating the convective heat transfer in the annulus from near vacuum to atmospheric pressure was established. Combined with it, the derived heat transfer model was suited to evaluating the long-running thermal performance of a NDAPTC quickly. The heat transfer model for a conventional indirect absorption parabolic trough collector (CIAPTC) was also constructed, aiming at comparing the heat transfer processes of this two kinds of collectors. Both the heat transfer models were validated by the test data from the reference. Additionally, the effects of physical parameters, operational and weather conditions on the collector thermal performance were analyzed. It was shown that improvement of the glass transmittance and emittance was crucial to raise the collector efficiency, especially at a higher inlet temperature. The pressure change in the annulus affected the collector thermal performance greatly, thus it tried to avoid the vacuum damage. The collector efficiency was raised with the solar irradiance and ambient temperature increasing and wind velocity reducing, which was more sensible to these weather conditions at a higher inlet temperature and pressure in the annulus. The results in this paper were useful for guiding the collector improvements and engineering designs theoretically, and were beneficial for cleaner production in civil and industrial heat utilization further.

Preparation of Transparent N-Zno:Al / P-Cualcro2 Heterojunction Diodeby Sol-Gel Technology

Abstract This paper presents the study of oxide film heterojunction made of CuAlCrO2 and ZnO:Al (AZO) deposited on fused quartz by sol-gel spin coating. Transmission electronmicroscopy (TEM) analysis has showed that the solvent, which was used to prepare the AZO sol, affects the thickness and planarity of the applied layers and the grain size of the polycrystalline film. The formation of a fine-grained structure of AZO films (grain size is up to 12 nm) leads to the formation of a smoother p-CuAlCrO2 / n-AZO heterojunction. X-ray diffraction (XRD) analysis and TEM have revealed that the heterojunction was formed by AZO polycrystals, which orient axes [101] and [100] against the [101] and [006] directions in CuAlCrO2 polycrystals. The optical transmission of the heterojunction in the visible range reached 70 %. The current -voltage (I-V) characteristics of p-CuAlCrO2 / n-AZO corresponds to a diode one in the range from 4 up to 4 V. For the large values of the reverse bias, an increase in the leakage current is observed, which is associated with tunneling breakdown of heterojunction. The ideality factor of fabricated p-CuAlCrO2 / n-AZO heterojunction is more than 2, which indicates the mechanisms of conductivity by defect-levels. In additional, it has been observed the response of the current-voltage characteristics to the green illumination (532 nm) of the diode that is associated with indirect band-gap absorption in CuAlCrO2. The impact of the white LED illumination had no effect on the current-voltage characteristics of the diode.

The Franz-Keldysh effect and free carrier dispersion effect in germanium

We calculated the changes of refractive index and absorption coefficient in Ge induced by the Franz-Keldysh (FK) effect and free carrier dispersion (FCD) effect, taking advantage of reported absorption spectra.The FK effect is weak near the indirect absorption edge of Ge. FCD effect can induce a much larger change of refractive index. In the wavelength range of 2–10 μm, theoretical formulas about dependences of the changes of absorption coefficient and refractive index on the change of carrier concentrations are presented. In addition, FCD effects in N-type and P-type Ge are compared.

First-principles study of direct and indirect optical absorption in BaSnO3

We report first-principles results for the electronic structure and the optical absorption of perovskite BaSnO3 (BSO). BSO has an indirect fundamental gap, and hence, both direct and indirect transitions need to be examined. We assess direct absorption by calculations of the dipole matrix elements. The phonon-assisted indirect absorption spectrum at room temperature is calculated using a quasiclassical approach. Our analysis provides important insights into the optical properties of BSO and addresses several inconsistencies in the results of optical absorption experiments. We shed light on the variety of bandgap values that have been previously reported, concluding that the indirect gap is 2.98 eV and the direct gap is 3.46 eV.

Characteristics of copper sulfide nanoparticles obtained in the copper sulfate–sodium thiosulfate system

Stable hydrosols of copper sulfide nanoparticles are synthesized by heating aqueous solutions with different ratios of sodium thiosulfate and copper sulfate in the presence of polyvinylpyrrolidone and studied by a number of physicochemical methods in situ (optical spectroscopy, dynamic light scattering) and ex situ (transmission electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy). The main product is CuS covellite nanoparticles with some impurities of other phases (Cu2S, Cu1,8S, Cu7S4). With an increase in the initial molar ratio S2O 2−3 / Cu from 0.2 to 5 the nanoparticle size increases from 1-5 nm to 30-50 nm and then decreases to 4 nm at a ratio of 10. A substantial increase in the intensity of plasmon absorption within 800-1500 nm is observed during the formation of planar nanoparticles with a lateral size of about 30 nm at S2O 2−3:Cu = 5. A band gap obtained from both direct and indirect optical absorption spectra of sulfides (2.6 eV and 1.7 eV respectively) remains constant for all particles.

Phonon-assisted absorption of excitons in Cu2O

The basic theoretical foundation for the modelling of phonon assisted absorption spectra in direct bandgap semiconductors, introduced by Elliott 60 years ago using second order perturbation theory, results in a square root shaped dependency close to the absorption edge. A careful analysis of the experiments reveals that for the yellow S excitons in Cu$_2$O the lineshape does not follow that square root dependence. The reexamination of the theory shows that the basic assumptions of constant matrix elements and constant energy denominators is invalid for semiconductors with dominant exciton effects like Cu$_2$O, where the phonon assisted absorption proceeds via intermediate exciton states. The overlap between these and the final exciton states strongly determines the dependence of the absorption on the photon energy. To describe the experimental observed line shape of the indirect absorption of the yellow S exciton states we find it necessary to assume a momentum dependent deformation potential for the optical phonons.

Resonant indirect optical absorption in germanium

The optical absorption coefficient of pure Ge has been determined from high-accuracy, high-precision optical measurements at photon energies covering the spectral range between the indirect and direct gaps. The results are compared with a theoretical model that fully accounts for the resonant nature of the energy denominators that appear in perturbation-theory expansions of the absorption coefficient. The model generalizes the classic Elliott approach to indirect excitons, and leads to a predicted optical absorption that is in excellent agreement with the experimental values using just a single adjustable parameter: the average deformation potential ${D}_{\mathrm{\ensuremath{\Gamma}}L}$ coupling electrons at the bottom of the direct and indirect valleys in the conduction band. Remarkably, the fitted value, ${D}_{\mathrm{\ensuremath{\Gamma}}L}=4.3\ifmmode\times\else\texttimes\fi{}{10}^{8}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{cm}$, is in nearly perfect agreement with independent measurements and ab initio predictions of this parameter, confirming the validity of the proposed theory, which has general applicability.