Increase In Optical Band Gap Research Articles

Effect of γ-irradiation on adsorption and photocatalytic ability of MoS2nanomaterials

The ability of MoS2nanomaterials as adsorbent and photocatalytic materials of methylene blue (MB) dye after [Formula: see text]-ray irradiation is investigated. The MoS2nanomaterials are prepared by a simple hydrothermal route, and then irradiated with Co-60 gamma radioisotope at different doses of 1, 10, 100, 1000 kGy. All the samples are characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Diffusivity Reflectance Spectroscopy (DRS) and Brunauer–Emmet–Teller (BET). The XRD analysis shows no change in crystal structure of MoS2nanomaterials after irradiation. The DRS analysis indicates the optical bandgap increases from 1.73 to 1.82, 1.86, 1.94 and 2.00 eV, respectively. The performance of the dye-absorbing solutions and the photocatalytic dye solutions before and after irradiation is compared. After [Formula: see text]-ray irradiation, the adsorption capacity of the MoS2nanomaterials degrades, which can be attributed to the decreased specific surface area, from 77.060 to 48.812, 35.855, 38.789 and 27.137 m2/g, respectively. The photocatalytic degradation ability for the MB solution also decreases due to the increase of optical bandgap of the samples after [Formula: see text]-ray irradiation.

ZnO Nanorods Prepared by Ultrasonic Spray Pyrolysis: Effect of Deposition Time on the Structural Morphological and Optical Properties

Zinc oxide Nanorods (ZnO-NRs) were deposited onto glass substrates using zinc chloride by Ultrasonic Spray Pyrolysis (USP) method. The films were prepared in different deposition time at optimum deposition parameters. The effect of deposition time on the structural, morphological and optical properties of ZnO-NRs was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Vis spectrometry (UV-Vis). XRD and SEM measurements indicated that all films show a hexagonal wurtzite Nano rods (NRs) structure growing preferentially along c-axis perpendicular to the surface of the substrate. Optical transmission spectra showed high transmittance of 80-85% in the visible range for all thin films, and increase of optical band gap from 3.24 to 3.265 eV with deposition time. The high quality c-axis orientated ZnO thin films with minimum strain and tuneable optical properties could be used as a transparent conducting oxide (TCO) for optoelectronic applications.

Substrate Temperature Effect on Structural and Optical Properties of ZnO Thin Films Deposited by Spray Pyrolysis

In this work, ZnO thin films grown on heated glass substrates in a temperature range of 300 to 500 °C with a 50°C step. The prepared solution is composed of methanol and zinc acetate Zn(CH3COO)2.2H2O. ZnO thin films are deposited by pyrolysis spray technique, our work focuses on the study of the substrate temperature influence on the structural and optical properties of these layers. Therefore, The X-ray diffraction, showed a Wurtzit hexagonal structure of elaborated films, with (002) as a preferred orientation, and a grain size of 64 to 74 nm. The optical transmission spectroscopy UV-Visible, illustrated an increase of optical band gap from 3.19 to 3.25 eV, proportionally with the substrate temperature.

Investigation of Electrical and Magnetic Properties of La3+ Substituted M-Type Ba–Ni Nano-Ferrites

In this paper, La3+ substituted M-type Ba–Ni hexaferrite nanoparticles having composition Ba0.8Ni0.2Fe12-xLaxO19 (x = 0, 0.2, 0.4, 0.6, and 0.8) were prepared by co-precipitation route. The phase of the grown NPs was confirmed using X-ray diffraction analysis and results reveal the growth of M-type hexaferrite NPs as a major phase. The energy dispersive X-ray spectroscopy (EDX) was employed to determine the elemental composition of prepared samples. The FTIR spectra of synthesized samples confirm the bond formation of M-type hexaferrites. The optical bandgap was determined by UV-Vis spectroscopy and the results exhibit an increase in optical bandgap with decreasing crystallite size. The vibrating sample magnetometry (VSM) technique was used to analyze the effect of dopant on the magnetic properties like coercivity, saturation magnetization, and remanence of the synthesized samples. The electrical resistivity was measured by two probe method and it was observed that the activation energy shows a decreasing trend with increase in La content. The highly resistive nature of prepared nano ferrites indicates the potential to use these ferrites for high-frequency applications.

Hybrid CdS nanowires/Si heterostructure photodetector fabricated by intense pulsed light assisted - laser ablation in liquid

Synthesis of CdS nanowires NWs by combined intense pulsed light (IPL) and pulsed laser ablation in liquid is demonstrated for the first time. An IPL source providing 3 ms duration pulses was used to assist the synthesis of CdS nanowires by 7 ns pulsed laser ablation of cadmium target in thiourea and CTAB solution. The results showed a small increase in optical energy band gap of CdS NWs and a decrease of the nanowires diameter from 50 to 40 nm after combining the IPL. The intensity of the photoluminescence emission peak located at 458 nm was doubled when the IPL combined laser ablation process. In addition the synthesized CdS nanowires showed single crystalline pure hexagonal wurtzite phase with decreased particles agglomeration and smaller particle size. Raman spectra were improved too by revealing sharp and shifted peaks at 293 and 585 cm−1. The figures of merit of hybrid n-CdSNWs/p-Si heterojunction photodetector namely responsivity and minority carrier lifetime were found to be improved remarkably after using IPL assisted laser ablation.

Nanostructured Fe,Co-Codoped MoO3 Thin Films

Molybdenum oxide (MoO3) and Fe,Co-codoped MoO3 thin films obtained by spray pyrolysis have been in-depth investigated to understand the effect of Co and Fe codoping on MoO3 thin films. The effect of Fe and Co on the structural, morphological and optical properties of MoO3 thin films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDAX), optical and photoluminescence (PL) spectroscopy, and electropyroelectric methods. The XRD patterns demonstrated the formation of orthorhombic α-MoO3 by spray pyrolysis. SEM characterization has shown an increase in roughness of MoO3 thin films by Fe and Co doping. Optical reflectance and transmittance measurements have shown an increase in optical band gap with the increase in Fe and Co contents. Thermal conductivity and thermal diffusivity of Fe,Co-doped MoO3 were 24.10–25.86 Wm−1K−1 and 3.80 × 10−6–5.15 × 10−6 m2s−1, respectively. MoO3 thin films have shown PL emission. Doping MoO3 with Fe and Co increases emission in the visible range due to an increase number of chemisorbed oxygen atoms. The photodegradation of an aqueous solution of methylene blue (MB) depended on the content of the codoping elements (Fe,Co). The results showed that a degradation efficiency of 90% was observed after 60 min for MoO3: Fe 2%-Co 1%, while the degradation efficiency was about 35% for the undoped MoO3 thin film.

Thermal annealing induced evolution of Bi3Se2 topological phase from Bi/As2Se3 thin film: Structural, optical and morphological study

In the present study, Bi/As2Se3 bilayer thin films (100 nm Bi as top layer and 800 nm As2Se3 as bottom layer) were prepared by thermal evaporation technique and were annealed at 165 °C for ½ h, 1 h and 2 h. The structural, micro-structural and optical studies of the films were characterized by X-ray diffractometer (XRD), Field emission scanning electron microscopy (FESEM), Atomic force microscopy (AFM) and Fourier transform infrared (FTIR) Spectrophotometer. The XRD study revealed the annealing induced diffusion of Bi into As2Se3 layer and its reaction with Se leading to the formation of Bi3Se2 phase. The as deposited Bi/As2Se3 bilayer film exhibited strain at the interface leading to localized states in the band gap of the bottom As2Se3 layer with a consequent reduction of its band gap. The increase in annealing time from 1/2 h–2 h led to strain relaxation and increased the band gap towards that of the pristine As2Se3 along with the precipitation of a second Bi3Se2 phase. The increase in optical band gap has been explained by the density of localized states in the gap. The change in refractive index, transmission, absorption coefficient, dielectric constant is the consequence of annealing effect. The FESEM images revealed the smooth surface for as deposited bilayer film and a large number of agglomerated grains of As2Se3 have been seen in the annealed films. The AFM study showed the grain growth stagnation or even decrease of grain size on prolonged annealing.

Aluminum-doped zinc oxide thin films deposited by electrospray method

Aluminum-doped ZnO (AZO) thin films are deposited by economically-versatile fabrication method of electrospraying onto two types of substrates – glass and silicon wafer, at substrate temperature of 300 °C, voltage of 18 kV and emitter to collector distance of 6 cm, Zinc acetate dehydrate dissolved in a mixture of deionized water, ethanol and acetic acid is used for deposition of undoped ZnO film, while for Al-doping one two precursor materials are chosen - aluminum chloride and aluminum nitrate dissolved in ethanol. The doping levels are 5 and 10 wt% and the concentration dependence of films properties is investigated. All films exhibit polycrystalline structure with decreasing crystallites size from 21 nm for undoped films to 19 nm and 17 nm for AZO films doped with 5 and 10 wt% Al, respectively. The shift of the XRD peaks of doped samples towards higher diffraction angles reveals successful incorporation of Al into ZnO lattice. The optical properties are investigated by ellipsometric and photoluminescence measurements. A decrease of refractive index from 1.89 for undoped sample to 1.84 and 1.83 for 5 and 10% AZO films, respectively and an increase of optical band gap from 3.30 eV to 3.40 eV are observed. A substantial decrease of sheet resistance is measured for AZO films: 86 Ω/□ and 61 Ω/□ for 5 wt% and 10% doped samples, respectively. Simultaneously, the transmittance of the films is kept high. The average values in the visible range for films with thickness of 200 nm are 86% for undoped ones and 84% and 80% for 5 and 10% Al-doped films, respectively. Thus, the applicability of the studied films as transparent conductive electrodes is demonstrated.

Optical properties of La0.9Sr0.1Fe1-xMoxO3 (x = 0.1, 0.2, and 0.3) perovskite material prepared by sol-gel method

The perovskite-type oxide Lao.9Sro.1Fe1-xMoxO3 (LSFMO) with x = 0.1, 0.2, and 0.3 were successfully prepared by sol-gel method. The synthesized perovskite has been characterized by Fourier transformed infrared spectroscopy (FTIR) and UV-Vis spectroscopy for further studies. FTIR spectra confirm the presence of Fe/Mo—O stretching and Fe/Mo—O— Fe/Mo bending vibrations in lattice. UV-Vis analysis displayed the optical band gap increase with increasing of Mo-content.

Electron transport phenomena at the interface of Al electrode and heavily doped degenerate ZnO nanoparticles in quantum dot light emitting diode

ZnO nanoparticles (NPs) of 4–5 nm, widely adopted as an electron transport layer (ETL) in quantum dot light emitting diodes (QD-LEDs), were synthesized using the solution-precipitation process. It is notable that synthesized ZnO NPs are highly degenerate intrinsic semiconductors and their donor concentration can be increased up to ND = 6.9 × 1021 cm–3 by annealing at 140 °C in air. An optical bandgap increase of as large as 0.16–0.33 eV by degeneracy is explained well by the Burstein–Moss shift. In order to investigate the influence of intrinsic defects of ZnO NP ETLs on the performance of QD-LED devices without a combined annealing temperature between ZnO NP ETLs and the emissive QD layer, pre-annealed ZnO NPs at 60 °C, 90 °C, 140 °C, and 180 °C were spin-coated on the annealed QD layer without further post-annealing. As the annealing temperature increases from 60 °C to 180 °C, the defect density related to oxygen vacancy (VO) in ZnO NPs is reduced from 34.4% to 17.8%, whereas the defect density of interstitial Zn (Zni) is increased. Increased Zni reduces the width (W) of the depletion region from 0.21 to 0.12 nm and lowers the Schottky barrier (ФB) between ZnO NPs and the Al electrode from 1.19 to 0.98 eV. We reveal for the first time that carrier conduction between ZnO NP ETLs and the Al electrode is largely affected by the concentration of Zni above the conduction band minimum, and effectively described by space charge limited current and trap charge limited current models.

Microwave-assisted solvothermal synthesis and physical properties of Zn-doped MnS nanoparticles

Un- and Zn-doped Manganese Sulfide (MnS) nanoparticles (NPs) have been successfully synthesized through a microwave-assisted solvothermal process using MnCl2 and Na2S as the precursors with different Zn concentrations. The X-ray diffraction (XRD) patterns of the MnS NPs confirm their hexagonal structures. The results show that the average crystallite size of MnS decrease after doping MnS with Zn. Particles sizes of the Zn-doped MnS NPs is smaller than the un-doped sample and the samples include spherical like structures with negligible agglomeration. Optical studies demonstrate increase of optical band gap and, also, a blue shift in the emission band of MnS upon doping. UV-Vis and photoluminescence (PL) spectroscopies determine that each sample possesses one absorption peak and three emission bands in the visible light spectrum. Furthermore, the results reveal the effect of dopant on the electrical properties of MnS.

The evolution of PL properties of hydrogenated Si-rich silicon carbide/amorphous carbon nano-multilayer films grown by PECVD

Hydrogenated Si-rich silicon carbide (SixC1-x:H)/amorphous carbon (a-C:H) nano-multilayer films have been prepared by PECVD method with different silane mixture gas flow rates at 200 °C. The influence of the silane mixture gas flow rate on the chemical bonds, composition, microstructure, optical band gap and photoluminescence properties of the nano-multilayer films were studied by employing Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), ultraviolet-visible (UV–vis) and photoluminescence spectroscopy, respectively. The FTIR and UV–vis spectra demonstrated that the incorporation of more carbon atoms into the SixC1-x:H networks is the main reason for the increase of optical band gap of the nano-multilayer films with increase of silane mixture gas flow rate. The Raman and HRTEM showed that there is no any nano-particles in the nano-multilayer films. The photoluminescence mechanisms of the nano-multilayer films was shown to be mainly contributed by band-to-band recombination of the Si-rich SixC1-x:H layers.

Synthesis and dielectric characterisation of triiodide perovskite methylammonium lead iodide for energy applications

Impedance spectroscopic measurements on spin coated 550 nm thick perovskite films sandwiched between titanium oxide (TiO2) deposited on fluorine doped tin oxide (FTO) glass substrates and with a platinum (Pt) counter electrode have been performed to determine the influence of the percentage of PbI2 in methylammonium lead iodide (CH3NH3PbI3) compounds. These compounds with perovskite structure have been synthesized by weaving methylammonium iodide (CH3NH3I) and lead iodide (PbI2) in two different weight ratios of 1:4 and 3:7. The surface grains are found from the scanning electron microscoping images to have become relatively larger with increasing PbI2 content in spincoated perovskite film. Nearly 2% increase in optical band gap has been observed with increasing weight ratio of PbI2 content from 1:4 to 3:7.

Photoluminescence and structural analysis of wurtzite (ZnO)1−x(V2O5)x composite

This paper demonstrates the structural, vibrational and photoluminescence characteristics of (ZnO)1−x(V2O5)x (x = 0, 3, 6 and 9 mol%) composites semiconductor synthesized by using the solid state reaction method. X-ray diffraction (XRD) studies show that (ZnO)1−x(V2O5)x composites have the poly crystalline wurtzite structure of hexagonal ZnO. It is found from the XRD results that the lattice constants and the crystallite size increase while the dislocation density decreases with increase in doping concentration. The existence of E1 (TO) and E2 (high) Raman modes show that the ZnO still preserve wurtzite structure after doping vanadium oxide, which is in agreement with XRD results. Room temperature photoluminescence (PL) exhibit near band edge and broad deep level emission while indicating the suppression of deep level emission with the incorporation of V2O5 up to a certain concentration (x < 9). Moreover, the optical band gap increase with doping, which is accompanied by the blue shift of the NBE emission.

Annealing induced AgInSe2 formation from Ag/In/Ag/In multilayer film for solar cell absorbing layer

In the present study, the AgInSe2 (AIS) thin films 500 nm in thickness were prepared by selenization of the Ag/In/Ag/In multilayer deposited by DC magnetron sputtering. The selenization was carried out at 250 °C and was followed by annealing at 450 °C, 475 °C and 500 °C. The selenized and annealed films were characterized by grazing incidence X-ray diffraction (GIXRD), UV-Visible-NIR spectroscopy, Field-Emission Scanning Electron Microscopy (FESEM) and Raman spectroscopy. The XRD analysis revealed the formation of the desired AgInSe2 phase along with Ag2Se (AS) impurity phase. The impurity Ag2Se phase is being diminished at higher annealing temperature (500 °C) resulting the formation of AgInSe2 phase. The super ionic nature of α-Ag2Se phase facilities the diffusion of Ag ion with its neighbouring tetrahedral sites. The low band gap Ag2Se impurity phase suppression by annealing at 500 °C lead to optical band gap increase. The Raman spectrum shows individual AgInSe2 and Ag2Se peaks and also the overlapping of these two phases. The Ag2Se peaks are vanished after annealing. The EDXRF study revealed that the film is off-stoichiometric and exhibit n-type conductivity. The RBS analysis indicates the diffusion of the film material to the glass substrate. From Hall measurement it is confirmed that the film shows n-type conductivity and the resistivity decreases with increasing the annealing temperature.

(Invited) Variable Doping of MoS2 By Atomic Layer Deposition of Molybdenum Oxides of Controlled Stochiometry

Two dimensional (2D) materials are extraordinarily sensitive to their surroundings, and several schemes have been demonstrated to dope 2D semiconductors using nearby ions, molecules or compounds without introducing chemical or structural defects that would degrade charge carrier mobility. Of these, oxides are attractive due to their stability and ability to function as dielectrics or passivation layers. Molybdenum oxide in particular has been demonstrated as a dopant for multiple 2D semiconductors. A practical oxide doping scheme requires not only deposition without damage to the 2D substrate, but also quantitative control over the doping density (e.g. via oxide Fermi level), which is currently lacking. Oxide work functions are sensitive to composition in general, and particularly so for molybdenum oxides. Thus, control of oxide stoichiometry could enable precise control of the doping level in 2D semiconductors, an approach which we demonstrate here. By using a low-oxidation-state molybdenum precursor (Mo(NMe2)4) for atomic layer deposition(ALD), we demonstrate two new low-temperature ALD processes to produce a series of molybdenum oxide compositions. The MoOx optical bandgap and resistivity increase with increasing oxygen content, following the same trends as crystalline molybdenum oxides. We demonstrate controllable doping of MoS2 by depositing a MoOx overlayer and tuning the threshold voltage shift in thin film transistors (TFTs) from positive (p-type) to negative (n-type) by tuning the oxide composition. Low hysteresis in sweeps of the transistor gate voltage, the absence of strain, and reversibility upon oxide etching together suggest that this process maintains the van der Waals interface without significant chemical changes, and should thus be generally applicable as a doping strategy for 2D systems. Furthermore, the sequential deposition of spatially patterned ALD layers enables the fabrication of lateral p-n junction diodes.

Electronic and optical properties of CH3NH3Pb1-xAgxI3 from the first-principles calculations

The optical and electronic properties of CH3NH3Pb1−xAgxI3 (MAPb1−xAgxI3) (x = 0, 0.025, 0.050, 0.075, 0.100) are researched using first-principles calculations. The formation energy of the system increases with the increase in the doping concentration. When the doping concentration reaches 5%, it can be easily found that the valence band shifts toward high energy levels and the Fermi energy enters the valence band, and this indicates that MAPb0.95Ag0.05I3 has a p-type conductive behavior. The optical band gap of MAPbI3 is about 1.56 eV and the optical band gap increase of MAPb1−xAgxI3 is about 3.11 eV with Ag doping concentrations from 5% to 30%. The high-frequency dielectric constant of MAPbI3 is 7.19.

Nanostructural dependence of photoluminescence and photosensing properties in hydrothermally synthesized Mg-doped ZnO nanorod arrays

In this study, the Mg doping effects on the nanostructural dependence of photoluminescence and the photosensing properties in Zn1−xMgxO (x = 0.01, 0.03, and 0.05) nanorod arrays were systematically examined. The vertical-aligned nanorod arrays were separately grown on ZnO-seeded n-Si substrates via a low-temperature hydrothermal process. X-ray diffraction analysis revealed that all Zn1−xMgxO nanorod arrays exhibited the same preferentially c-oriented wurtzite structure. Images of all the Zn1−xMgxO nanorod arrays provided by field emission scanning electron microscope showed that both the length and diameter of the Zn1−xMgxO nanorod arrays increased with increasing Mg doping concentration. Also, an enhanced blue-shifted ultraviolet (UV) emission and a reduced visible emission were observed from the photoluminescence spectra of the Zn1−xMgxO nanorod arrays. The former indicated the decrease of defects and the latter indicated the enhancement of the crystallinity and the increase in optical bandgap. Current–voltage curves were separately measured in the dark and under UV illumination. Finally, the mechanism of the Mg doping effects on the corresponding defects, bandgap, and UV photosensing characteristics of the Zn1−xMgxO nanorod arrays was investigated in detail.

Amorphous to crystalline phase transformation and band gap refinement in ZnSe thin films

Zinc selenide (ZnSe) thin films are deposited by physical vapor deposition on indium tin oxide (ITO) substrates with different thicknesses. The crystalline phase refinement is studied by carrying out post-deposition annealing in air at various temperatures. The composition analyses carried out by the Rutherford backscattering spectroscopy have shown the ratios of Zn:Se to be 1:1. As-deposited samples have shown predominant crystalline phase with a small inclusion of the amorphous phase. The contribution of amorphous phase diminishes after the post-deposition annealing of the samples in air. However, the peaks of tin oxide and ITO begin to appear in the X-ray diffraction patterns after post-deposition annealing, showing that substrate material has started reacting with ZnSe films. The crystallinity of the ZnSe samples increases with annealing temperature, which results in the increase of conductivity and optical bandgap of the samples. It is proposed that as-prepared un-annealed samples grow with a large population of trapping centers filled with electrons near the top edge of the valance band. These electrons are elevated to the conduction band when exposed to the light, resulting in a very high photo current in as-prepared samples in comparison with post-annealed samples. The population of the localized trapping centers near the top edge of the valence band of ZnSe diminishes significantly with the post-annealing that refines the band gap; optimize their electrical and optical properties.

Structural, optical and electrical characterization of CBD synthesized CdO thin films: influence of deposition time

AbstractCadmium oxide (CdO) thin films were grown on glass substrates by chemical bath deposition (CBD) method for different deposition times using cadmium acetate as cationic precursor. The structural and optical characterization was carried out using XRD, TEM, and UV-Vis spectrophotometer measurements. Structural analyses with XRD confirmed cubic structure of the CdO. Average particle size estimated from Rietveld refinement method of XRD pattern corresponded well with TEM measurement. The optical band gap varied between 2.35 eV to 2.48 eV with deposition time and an increase in optical band gap with decreasing film thickness was observed. The AC electrical conduction behavior of the CdO film was investigated as a function of temperature as well as frequency. The conductivity measurements indicated localized conduction and hopping of carriers between localized states. The value of real part of dielectric constant was found to decrease with frequency and increase with temperature. The Nyquist plots at different temperatures showed the existence of both grains and grain boundaries contributing to conduction mechanism.