Shift Of Absorption Edge Research Articles

Influence of cobalt doping on structure, optical and magnetic properties of spray pyrolysed nano structured ZnO films

Zn1-xCoxO (x = 0–10 at%) films were prepared using the spray pyrolysis method at 723 K on glass substrates. The structural and transport properties of Zn1-xCoxO deposits were studied using X-ray Diffraction(XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), UV-VIS Spectrophotometer and Vibrating Sample Magnetometer (VSM). XRD analysis confirmed a hexagonal wurtzite-type structure with a preferential orientation in the (0 0 2) plane, indicating Co2+ ion in the ZnO host matrix, and absence of secondary phase of the metals in the ZnO host matrix. The surface morphology modified from fibrous to spherical grain shape with enrichment of cobalt in the deposits. The composition analysis indicates the existence of Zinc and Cobalt. The XPS analysis provides the dopant exists in Co2+ state. The optical absorption edge shift to a higher wavelength of three band transitions corresponds to the 4A2→2 E(G), 4A2→4T1(P) and 4A2→2A1(G) states. The reduction of energy band gap Eg1 in the UV region and Eg2 in the visible region was observed, which is due to the spin exchange interaction occurs in the sp-d transition state. At room temperature photoluminescence (PL) spectra showed emission in the UV and entire visible region due to the various defect states present in the deposits. All deposits show paramagnetic behavior even at low temperature.

Novel visible-light-responsive rGO-ZnO@Bi2MoO6 nanocomposite with enhanced light harvesting and Z-scheme charge transfer for photodegradation and detoxification of RhB

The visible light activated rGO-ZnO@Bi2MoO6 nanocomposites were prepared using an innovative two-step solvothermal process. XRD patterns, Raman and FT-IR spectra of rGO-ZnO@Bi2MoO6 samples confirmed the simultaneous presence of the ZnO and Bi2MoO6 components, as well as, a partial reduction of GO in the presence of ethylene glycol under solvothermal process. A significant red shift in absorption edge and remarkable reduction in PL peak intensity were detected by incorporating of Bi2MoO6 and rGO in prepared nanocomposites. The complete degradation and 79% mineralization of RhB dye was attained by rGO modified ZnO@Bi2MoO6 within 150 min under visible-light irradiation. The removal efficiency was increased from 36% for bare ZnO to 64% for ZnO@ Bi2MoO6 nanocompoite. In addition, the cytotoxicity of untreated and visible light/ZBMO-GO5-treated RhB solutions assessed using the skin fibroblast cells (Hu02) confirmed that treating wastewater in the developed photoreactor under visible light illumination could significantly improve the toxcity effect of RhB.

Enhanced visible light photocatalytic degradation of acetaminophen with Ag2S-ZnO@rGO core-shell microsphere as a novel catalyst: Catalyst preparation and characterization and mechanistic catalytic experiments

Ag2S-ZnO@rGO core-shell structured microspheres were prepared through a very simple, novel and green in-situ method using dimethyl sulfoxide as both the solvent and sulfur S source at low temperature (140 °C). The photocatalytic performance of synthesized samples was tested for degradation of acetaminophen (ACT) under visible light irradiation. ACT removal efficiency was directly depended on the GO-level in composite and the maximum photocatalytic yield was achieved for the sample with GO concentration of 0.2 g/L. Formation of Ag2S in the nanocomposite was confirmed using XRD, XPS and UV–Vis DRS and photoluminescence (PL) measurement. XRD patterns pointed to the existence of a wurtzite structure for all samples and by silver loading, Ag2S peaks were visualized in the XRD patterns, with increased intensity by rising the Ag content in the solution precursor. EDS and XPS data indicated the presence of sulfur and silver in the form of Ag+ and S2−. A considerable red shift in absorption edge and reduce in PL emission peak intensity was observed over the introduction of Ag in the precursor solution due to the existence of Ag2S. The Ag2S-ZnO@rGO composites indicated enhanced photocatalytic performance toward degradation of ACT under visible light compared to bare ZnO and ZnO@rGO. The underlying mechanism has been investigated based on the results of UV–Vis DRS, PL and reactive species scavenging experiments. The improved photocatalytic activity is mainly ascribed to better light harvesting and increased photogenerated electron-hole separation. Ag2S play an important role in the composite due to the synergistic effect of Ag2S and rGO, which not only improve the light harvesting but also reduce the charge recombination. The as-made catalyst indicated a great potential for recycling, suggesting the potentially large-scale applications of prepared catalyst for emerging water contaminants and wastewater treatment. The cytotoxicity of untreated and photocatalytic oxidation (PCO)-treated ACT solutions was evaluated using the cultured human embryonic kidney (HEK) cells, which revealed that ACT solution in the developed PCO system could be significantly detoxified.

Synthesis and characterization of direct Z-scheme CdS/TiO2 nanocatalyst and evaluate its photodegradation efficiency in wastewater treatment systems

In the present research, the photocatalytic degradation of dimethyl sulfoxide (DMSO), a reference substance for determination of photocatalytic efficiency of wastewater treatment systems, in the presence of CdS/TiO2 nanoparticles under visible light irradiation was investigated. For this purpose, an ultrasonic method combined by microemulsion was used to synthesize CdS/TiO2 core/shell nanoparticles in a mild condition without surfactant. The morphology, structure and properties of the nanoparticles were studied via X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible spectra (UV–vis), Fourier transform near IR (FTIR), energy-dispersive X-ray (EDAX) and dynamic light scattering (DLS). The TEM image clearly indicated the size of non-agglomerated nanoparticles in the absence of surfactant, are almost 10 nm in a spherical shape. UV–vis spectra for these particles revealed a red shift in absorption edge up to 570 nm which was in green area. The photodegradation studies were performed in a batch photoreactor with a sun simulator lamp. The degradation efficiency of nanoparticles was investigated by high-performance liquid chromatography (HPLC) device and high degradation efficiency up to 80% was recognized with this method. Also, kinetic study of photodegradation of nanoparticles was performed in an arbitrary point and the rate of DMSO degradation was fitted to Langmuir–Hinshelwood kinetic model and consequently, K parameter, has been determined.

The electronic structure and tunable emission of self-activated white-light-emitting Na2TiSiO5 phosphor

The exploration of rare-earth-free phosphors is desirable for white LEDs applications. In this work, Cr3+ doped Na2TiSiO5 self-activated phosphors were synthesized by conventional solid-state reaction. The electronic structure of Na2TiSiO5 was calculated using density functional theory (DFT) methods. The variations in crystal structure, luminescence properties and thermal stability of Na2Ti1-xSiO5: xCr3+ were exhaustively investigated by the analysis of XRD patterns, photoluminescence (PL) spectra, diffuse reflection spectra, PL decay curves and temperature-dependent PL spectra. Owing to the red shift of absorption edge and energy transfer between Cr3+ ions and Ti4+ ions, the emission intensity and thermal stability of Na2TiSiO5 were improved by the introduction of Cr3+ ions, and the peak position was red-shifted. The results indicate that the introduction of Cr3+ ions could be an effective method to regulate the luminescence of Ti4+-activated oxide phosphors.

Influence of Al, Ta Doped ZnO Seed Layer on the Structure, Morphology and Optical Properties of ZnO Nanorods

Background:Zinc oxide (ZnO) is one of the most attractive II-VI semiconductor oxide material, because of its direct wide band gap (3.37 eV) and large binding energy (60 meV). Zinc oxide (ZnO) is a promising semiconductor due to its optimised optical properties. Among semiconductor nanostructures, the vertically aligned one-dimensional ZnO nanorods are very important for nano device application.Methods:Vertically aligned ZnO nanorod arrays were grown on ZnO, aluminum doped ZnO (ZnO:Al), tantalum doped ZnO (ZnO:Ta) and aluminum and tantalum co-doped ZnO (ZnO:Al,Ta) seed layer by hydrothermal method.Results:The X-Ray Diffraction (XRD) investigation indicated the presence of hexagonal phase for the both seed layers and nanorods. The Scanning Electron Microscope (SEM) images of ZnO and doped ZnO seed layer thin-films show spherical shaped nanograins organized into wave like morphology. The optical absorption spectra revealed shift in absorption edge towards the shorter wavelength (blue shifted) for ZnO nanorods grown on ZnO:Al, ZnO:Ta and ZnO:Al,Ta seed layer compared to ZnO nanorods grown on ZnO seed layer.Conclusion:The increase in band gap value for the ZnO nanorods grown on doped ZnO seed layers due to the decrease in crystallite size and lattice constant as evidenced from XRD analysis. The unique property of Al, Ta doped ZnO can be used to fabricate nano-optoelectronic devices and photovoltaic devices, due to their improved optical properties.

Enhanced Visible Light Activity of Pr-TiO₂ Nanocatalyst in the Degradation of Dyes: Effect of Pr Doping and TiO₂ Morphology.

A facile hydrothermal method was adopted for the synthesis of bare TiO₂ and titania nanotubes (TNT). In an effort to increase the efficacy of the existing photocatalyst, different weight percentage (0.2, 0.4, 0.6, 0.8 and 1.0%) of praseodymium were deftly doped on to the synthesized titania scaffolds. The physicochemical characteristics of the architectured photocatalyst were thoroughly elucidated by various sophisticated techniques. The doping of Pr₂O₃ (Pr) on to titania nanotubes (TNT) resulted into a significant bathochromic/hyperchromic shift in the optical absorption edge (towards the visible region) as perceived from the DRS-UV spectra. The XRD and TEM analysis showed average crystallite size of the synthesised photocatalyst to be as small as 4-7 nm with well-formed nanotube framework. Photoluminescence spectra of Pr doped TNT catalyst clearly exhibited greater suppression of photogenerated electron-hole pair as compared to the undoped counterparts. The photocatalytic activity of the synthesized catalysts was evaluated towards the degradation of organic pollutants namely Rhodamine B (90%) and Crystal violet (93%) in the presence of solar light and its activity and durability was compared to that of commercial TiO₂ (Degussa P25). The observed enhanced photocatalytic activity of TNT and Pr-TNT can be unambiguously attributed to the inhibition of recombination of the electron-hole pairs due to doping of Pr into TNT. Among catalysts synthesized, 0.4 wt.% of Pr on to TNT yielded the highest photocatalytic activity under visible light irradiation.

InSb-ZnO:Ge nanocomposite thin films: One-step synthesis, structural, optical, and electrical properties

One-step synthesis of the composite thin film with InSb nanocrystals embedded in ZnO doped with Ge was investigated. The films were deposited on water-cooled substrate by radio-frequency sputtering using a target of ceramic ZnO disc with InSb and Ge chips and subsequently heat-treated in vacuum. The composites exhibited a shift in optical absorption edge due to a presence of InSb nanocrystals. Elemental mapping using energy dispersive X-ray microscopy revealed that the added Ge was selectively located in ZnO. The solubility limit of Ge in ZnO was 2 at.% at an annealing temperature of 873 K. Electrical resistivity of the composite was reduced to 6.6 × 10-3 Ω cm at an annealing temperature of 773 K due to doping with Ge. Simultaneous addition of InSb and Ge to ZnO therefore provided the different functionalities of the optical absorption shifts and relatively low electrical resistivity.

Effect of erbium nanoparticles on structural and spectroscopic properties of bio-silica borotellurite glasses containing silver oxide

Er3+ NPs doped bio-silica borotellurite glasses containing silver oxide were prepared using melt quenching techniques. The silica used was extracted from rice husk by acid leaching process. Optical absorption and photoluminescence properties of the glass samples were studied. The samples were confirmed to be amorphous in nature from the XRD results. The FTIR results reveal the existing functional groups in the glass structure. XRF result shows that 98.6% silica was obtained from the rice husk. The optical absorption spectra revealed that fundamental absorption edge shifts to longer wavelength as the content of erbium NPs increases. The value of band gap had been calculated and shown to be increased with an increase content of erbium NPs. The Urbach energy was shown to be linearly decreased with an increase in content of erbium NPs oxides. The introduction of silver oxides modifies the optical properties of the glass. The emission cross-section (δe) and gain bandwidth (FWHM ×δe) for 4I13/2→4I15/2 transition were calculated from the McCumber theory. The photoluminescence spectra revealed three emission bands corresponding to 4H11/2-4I15/2 (470 nm), 4S3/2-4I15/2 (561 nm) and 4F9/2→4I15/2 (758 nm) transitions assigned to green and red emissions respectively. The results suggest that these glasses might be used as a promising candidate as well as gain media for solid state lasers.

Synthesis and characterization of undoped and Er-doped ZnO nano-structure thin films deposited by sol-gel spin coating technique

Pure and Erbium (Er3+)-doped ZnO (Er3+ = 0.0, 1.0 and 1.5 mol%) nano-structure thin films were prepared by modified sol-gel spin coating technique. The influence of Er doping on the composition, structure, morphology, optical was examined by energy dispersive x-ray analysis (EDS), x-ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy, field emission scanning electron microscopy (FESEM). The XRD measurement and selected area electron diffraction pattern (SEDA) confirmed the hexagonal wurtzite structure of ZnO for all samples and average crystallite size is about 9.7 nm. Moreover, Both XRD and Raman spectroscopy show that the incorporation of Er3+ in ZnO matrix resulted in improved crystallinity. FESEM plane images indicated that the ZnO thin film has interconnected spherical particles with the presence of pore, whilst Er-doped ZnO films have compact and uniform morphology. Analyses of optical data point out that value of band gap of Er doped ZnO have lower values than pure film, consistent with earlier works prepared by different methods. The absorption edge shifts towards lower photon energies with increasing Erbium content.

Stark‐Localization‐Limited Franz–Keldysh Effect in InAlAs Digital Alloys

The optical absorption characteristics of the InAlAs digital alloy semiconductor are investigated. The external quantum efficiency of the InAlAs digital alloy is compared with that of the In0.52Al0.48As random alloy. Unlike the random alloy, the digital alloy exhibits electric‐field‐induced Stark localization, which increases the optical absorption and suppresses the Franz–Keldysh red shift of the optical absorption edge.

Oxamide-modified g-C3N4 nanostructures: Tailoring surface topography for high-performance visible light photocatalysis

Pristine graphitic carbon nitride (g-C3N4) suffers from poor photocatalytic activity due to its limited light adsorption, small specific surface area and feasible recombination of photogenerated charge carriers. One practical approach to solve the problem of low solar-to-hydrogen efficiency is nanostructuring the morphology of g-C3N4 to enhance its light-harvesting property and boost the rate of charge-carrier transportation. Here, we present a strategy to optimize the textural properties and catalytic performance of g-C3N4 via using oxamide (OA) as a chemical modifier in the precursor solution, which significantly make the light absorption edge shift from 440 to 650 nm. Moreover, the nanostructure of the resultant g-C3N4 can be easily tuned with increasing the amount of oxamide. The morphological features of g-C3N4 materials change from sheet-like structure to tubular structure, porous nanotube and eventually interconnected porous sheet. More interestingly, the unique g-C3N4 nanostructure offers advantages of a broader solar absorption spectrum and accelerates charge carrier dynamics that are favorable for advancing photocatalytic performance. The findings provide insightful information that can be used to understand the structure-property relationship in g-C3N4 nanostructures for improved solar-to-fuel conversion.

Variations of the microstructure and the optical and electrical properties with sputtering power for direct-current-magnetron-sputtered indium-doped CuO thin films at room temperature

Preparation of single-phased indium-doped copper oxide (CuO:In) thin films with high quality at low temperature is crucial for the commercial application of CuO-based solar cells and light-emitting devices. Herein, direct-current magnetron sputtering assisted with high sputtering power and reactive pressure technique was used to prepare monoclinic CuO:In thin films at room temperature. The influence of sputtering power (Pspu) on the microstructure and the optical and electrical properties of the films were studied. The roles of indium in the CuO:In thin films were also proposed. CuO:In thin films are single-phased and monoclinic in structure. The crystallization and the preferred 〈−111〉 orientation of the films were enhanced with increased Pspu. The red shift of the film absorption edge was due to the decrease and increase of the lattice strains along crystal axes a and b with Pspu, respectively. A conducting transition from p type to n type of the films at 100 W Pspu was closely related to the increased substitution of copper by indium. The p- and n-type conductions of the films were mainly due to the copper vacancies and interstitials oxygen and to the effective substitution of copper by indium in the films, respectively. Increased Pspu increased the free carrier concentration and decreased the mobility of the films. The free carrier concentration reached the maxima of 9.333 × 1018 /cm3 at 100 W Pspu, whereas the mobility reached the maxima of 23.38 cm2/Vs at 60 W Pspu.

Tunable linear and non linear optical properties of GeSeSb chalcogenide glass with solute concentration and with silver doping

Exploration of new materials with improved, tunable linear and nonlinear optical property is of great interest in photonic device fabrication. Thermal, physical and optical properties of silver nanoparticle embedded GeSeSb bulk chalcogenide glasses, tunability of its linear and nonlinear properties in solution processed chalcogenide glasses with varying solute concentration are the main attractions of this study. A shift in absorption edge to higher wavelength with increased silver content in GeSeSb glasses is been observed from Tauc plots. High value of refractive index and high glass transition temperature observed in GeSeSbAg sample as compared with GeSeSb glasses are attributed to the difference in bonding statistics of these glasses. Third order optical nonlinearities of Ge Se Sb ChGs investigated using Z-scan technique shows enhancement on incorporating silver in the glass matrix which can be used to enhance desirable traits for application in nonlinear optical devices.

Effect of thickness on the structural and optical properties of the niobium‐doped β ‐Ga 2 O 3 films

To investigate the effect of thickness on the structural and optical properties of niobium-doped beta gallium oxide (β -Ga2O3 :Nb) thin films, a series of β -Ga2O3 :Nb thin films with different thicknesses were prepared by radio-frequency magnetron method. The crystalline quality is highly improved when the film thickness exceeds 145 nm. The surface exhibits different morphologies under different film thicknesses. The average transmittance of all the films are over 80% in visible range, and that the ultraviolet absorption edge shifts to longer wavelength indicates the bandgap shrinks with increasing the film thickness. Moreover, the photoluminescence spectrum measurements indicate that fewer defects were formed when the film thickness is around 145 nm.

Linear and nonlinear absorption, SHG and photobleaching behaviors of Dy doped GaSe single crystal

The linear absorption, nonlinear absorption (NA), second harmonic generation (SHG) and carrier mobilities of undoped and 0.1 at% Dy doped GaSe single crystals grown by modified Bridgman method have been studied by UV–vis, open aperture Z-scan, SHG and transmission ultrafast pump probe spectroscopy experiments. Both linear absorption and SHG measurements clearly showed that the doping of GaSe crystal with Dy3+ leads to a shift in the linear absorption edge. The mechanisms contributing to the NA behavior were explained and discussed. The photobleaching behaviors of the crystals were investigated by delaying of the probe signal in transmission pump-probe spectroscopy experiments.

Absorption edge, urbach tail, and electron-phonon interactions in topological insulator Bi2Se3 and band insulator (Bi0.89In0.11)2Se3

We employ infrared transmission spectroscopy to explore the temperature-dependent absorption edge and electron-phonon (e-ph) interaction in topological insulator Bi2Se3 and band insulator (Bi0.89In0.11)2Se3 films. Upon heating from 5 K to 300 K, the absorption edge shifts from 262 to 249 meV for Bi2Se3 and from 367 to 343 meV for (Bi0.89In0.11)2Se3. By analyzing the temperature dependence of the Urbach tail, the significant role of Raman-active phonon mode Eg2 in e-ph interaction is identified, which agrees well with the ab initio calculation.

Al2O3:Cr2O3:CuO (1:1:1) thin film prepared by radio frequency magnetron sputtering technique: a promising material for high sensitive room temperature ammonia sensor

In the present work, we report the room temperature gas sensing behavior of Al2O3:Cr2O3:CuO (1:1:1) thin films prepared by radio frequency (RF) magnetron sputtering technique for the first time. X-ray diffraction study confirmed the presence of tetragonal phase Al2O3:Cr2O3:CuO in the deposited film. X-ray photoelectron spectroscopic data confirmed the predominant presence of Al3+, Cr3+ and Cu2+ states in the films. The annealing temperature induced shift in absorption edge may be due to the morphological change and the improvement in crystallinity of the films. The gas sensing study shows the excellent sensitive behavior of the films to NH3 gas at room temperature. Interestingly, the annealed film was very sensitive to NH3 gas even at a very small concentration of 1 ppm, while the as-deposited one was responded at 10 ppm level. The maximum response of 99.17% was observed for 75 ppm of NH3 gas at room temperature. The sensor uphold its initial response without any decrease upon 10 repeated cyclic operation and also retains 94% and 91% of its initial response after 6 months duration for as-deposited and annealed films, respectively, which confirms the high operational stability and repeatability of Al2O3:Cr2O3:CuO (1:1:1) thin film in the detection of ammonia gas.

Opto-electronics properties of starch capped CdSe nanoparticles

The nanoparticles of a prominent luminescent material, cadmium selenide (CdSe) have been prepared by simple wet chemical route using starch as capping agent. The X-ray diffraction of the synthesized CdSe nanocrystals exhibits hexagonal phase with average crystallite size decreasing with capping agent concentration. Surface morphology of CdSe nanoparticles has been studied using scanning electron microscopy. The UV–Vis absorption shows blue shift in absorption edge indicating increased band gap with decreasing the particle size due to quantum confinement. The photoluminescence and electroluminescence (EL) measurement show single peak at about 650 nm with increasing intensity for smaller nanocrystals, however emission peak is quite broad in case of EL indicating involvement of impurity states. The luminescence intensity increases because of increased oscillator strength and the better passivation of the surface defects responsible for non-radiative relaxation process. The light emission starts at lower threshold voltage when the concentration of capping agent is increased.

A study on optical properties of Sb2Se3 thin films and resistive switching behavior in Ag/Sb2Se3/W heterojunctions

Abstract In this study, optical phase change property and bipolar memristive switching behavior were systematically characterized in Sb2Se3 thin films with different annealing temperatures and Ag/Sb2Se3/W heterojunction, respectively. The structural investigations reveal that the crystallinity of Sb2Se3 sample improves with increasing annealing temperature. Anomalous changes of both structure and optical properties are attributed to the increase of defect states at an excessively high annealing temperature. Additionally, the absorption edge shift towards longer wavelength after thermal treatment, indicating a decrease in the optical band gap. In the aspect of electrical characteristics, the Ag/Sb2Se3/W cell has a bipolar memristive switching behavior and shows reversible switching property. The study based on the optical properties and memristive switching behaviors of Sb2Se3 thin films offers an important understanding to the applications of Sb2Se3 and other chalcogenide semiconductor as promising materials for integrated optical system and chalcogenide-based memory devices.