Synthesis and luminescent properties of Tb3+ activated AWO4 based (A = Ca and Sr) efficient green emitting phosphors

Optically efficient terbium activated alkaline earth metal tungstate nano phosphors (AWO₄ [A = Ca, Sr]) with different doping concentrations have been prepared by mechanochemically assisted solid state metathesis reaction at room temperature for the first time. The prepared phosphors were characterized by the X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), Fourier transform Raman (FT-Raman) spectroscopy, photoluminescence and diffuse reflectance spectroscopy measurements. The XRD and Raman spectra results showed that the prepared powders present a scheelite-type tetragonal structure. FTIR spectra exhibited a high absorption band situated at around 850 cm^?1, which was ascribed to the W–O antisymmetric stretching vibrations into the [WO₄]^2? tetrahedron groups and the SEM images reveal that the particle sizes were in the range of 20–60 nm. The excitation and the emission spectra were measured to characterize the luminescent properties of the phosphors. The excitation spectrum exhibits a charge transfer broad band along with some sharp peaks from the typical 4f–4f transitions of Tb³⁺. Under excitation of UV light, these AWO₄:xTb³⁺ (A = Ca, Sr) phosphors showed a strong emission band centered at 545 nm (green) which corresponds to ⁵ D ₄ → ⁷ F ₅ transition of Tb³⁺. Analysis of the emission spectra with different Tb³⁺ concentrations revealed that the optimum dopant concentration for CaWO₄:xTb³⁺ and SrWO₄:xTb³⁺ phosphors are about 8 and 6 mol% of Tb³⁺. The green emission intensity of the solid state meta-thesis prepared CaWO₄:0.08Tb³⁺ and SrWO₄:0.06Tb³⁺ phosphors are 1.5 and 1.2 times greater than that of the commercial LaPO₄:Ce, Tb green phosphor. All properties show that AWO₄:Tb³⁺ (A = Ca, Sr) is a very appropriate green-emitting phosphor for fluorescent lamp applications.     

Hydrothermal synthesis of Fe3O4 nanoparticles and its application in lithium ion battery

Well dispersed Fe₃O₄ nanoparticles with a mean diameter of about 160 nm were synthesized by a simple hydrothermal method in the presence of sodium sulfate. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectrum, and Fourier transform infrared spectra (FTIR). Electrochemical properties of the nanostructured Fe₃O₄ as cathode electrodes of lithium ion battery were studied by conventional charge/discharge tests, showing a high initial discharge capacity of 1267 mA h g^− 1 at a current density of 0.1 mA cm^− 2.     

Structural properties of lead vanadate glasses containing La3+ or Fe3+ ions

Structural properties of lead vanadate glasses containing La³⁺ or Fe³⁺ ions were investigated using X-ray diffraction, Fourier transform infrared spectroscopy and laser Raman spectroscopy. Crystalline Pb₂V₂O₇ was formed for the molar composition 66.7PbO-33.3V₂O₅. Incorporation of greater quantities of La³⁺ into lead metavanadate glass caused the crystallization of Pb₂V₂O₇. Fourier transform infrared and laser Raman spectra also suggested the presence of LaVO₄. Incorporation of Fe³⁺ ions into lead metavanadate glass, up to 20 wt% Fe₂O₃, did not cause crystallization inside the glass matrix. Changes in the vibrational spectra are discussed.     

Structural phase analysis,band gap tuning and fluorescence properties of Co doped TiO2 nanoparticles

This paper reports on structural and optical properties of Co (0, 3, 5 & 7 mol%) doped TiO₂ (titania) nanoparticles (NPs) synthesized by employing acid modified sol–gel method. The crystalline phase of the pure and doped NPs was observed with X-ray diffraction (XRD) followed by Raman scattering technique. Field emission scanning electron microscope and transmission electron microscopy give the morphological details. Fourier transform infrared spectra indicate the bonding interactions of Co ions with the titania lattice framework. Optical studies were attained with UV–visible absorption and fluorescence emission spectroscopy. XRD analysis reveals that all prepared samples have pure anatase phase with tetragonal symmetry devoid of any other secondary phase. The average crystallite size of all samples was calculated using Scherrer’s formula and was found to vary from 8 to 10 nm with doping concentration of Co. The Raman spectroscopy further confirmed the formation of TiO₂ in anatase structure in both pure and Co doped TiO₂ NPs. The most intense Raman active E_g peak of TiO₂ NPs shifted to higher energy on doping. Both UV–visible and fluorescence spectra show a blue shift in their absorption and band edge emission subsequently on increasing with Co percentage in titania host matrix, wherever there is an indication of quantum confinement effect with widening of band gap on decreasing in NPs size. There is also a possibility of strong Coulomb interaction effect on the optical processes involving the Co ions. However, the intensities of different emission spectra are not the same but decrease profoundly for doping samples due to concentration quenching effect.     

Effect of silane flow rate on structural,electrical and optical properties of silicon thin films grown by VHF PECVD technique

Hydrogenated silicon thin films deposited by VHF PECVD process for various silane flow rates have been investigated. The silane flow rate was varied from 5 sccm to 30 sccm, maintaining all other parameters constant. The electrical, structural and optical properties of these films were systematically studied as a function of silane flow rate. These films were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and UV–visible (UV–Vis) spectroscopy. Different crystalline volume fraction (22%–60%) and band gap (∼1.58 eV–∼1.96 eV) were achieved for silicon thin films by varying the silane concentration. A transition from amorphous to nanocrystalline silicon has been confirmed by Raman and FTIR analysis. The film grown at this transition region shows the high conductivity in the order of 10⁻⁴ Ω⁻¹ cm⁻¹.     

Preparation and characterization of Ba2TiSi2O8 ferroelectric films produced by sol–gel method

Fresnoite (Ba₂TiSi₂O₈, BTS) thin films were grown on polished Si(100) substrates by sol–gel method. The films were characterized using Fourier transform infrared spectroscopy (FTIR), Raman scattering spectroscopy, X-ray diffraction (XRD) and atom force microscopy (AFM). The results reveal that the crystallinity of fresnoite thin films increases and their structures become more compact as post-annealing temperature increases. Combined with XRD data, the strong FTIR peaks and Raman bands assigned to Ti–O and Si–O vibration indicate the formation of fresnoite phase in the films at a temperature of 750 °C. Besides, the AFM observation showed the films have a smooth surface, fine grains and dense structure.     

Effect of microwave power and C2 emission intensity on structural and surface properties of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films are synthesized using microwave plasma enhanced chemical vapour deposition technique at 2 × 10⁴ Pa and 600 °C with microwave power of 600-1600 W. Deposition is carried out on n-type (100) silicon wafer with Ar/H₂/CH₄ gas mixtures. The film properties are analyzed using micro Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy and atomic force microscopy. Raman spectra show two predominant peaks centered around 1335 cm⁻¹ and 1560 cm^− 1 and two humps around 1160 cm^− 1 and 1450 cm^− 1, respectively. FTIR spectra show C:H stretching modes around 3000 cm^− 1. XRD patterns show a peak at 44° (2θ). In situ diagnostic of plasma is carried out using Optical Emission Spectroscopy. It has been observed that C₂ dimer plays an important role in the nucleation of diamond crystals during NCD film deposition and the emission intensity of C₂ can be adjusted by varying the microwave power. It has also been observed that the structural properties like growth rate, surface morphology and grain size of the growing film are dependent on the C₂ intensity during deposition.     

Electrical and optical switching properties of ion implanted VO2 thin films

The metal-insulator transition in vanadium dioxide thin films implanted with O⁺ ions was studied. Ion implantation lowered the metal-insulator transition temperature of the VO₂ films by 12 °C compared to the unimplanted ones, as measured both optically and electrically. The lowering of the transition temperature was accomplished without significantly reducing the mid-wave infrared optical transmission in the insulating state for wavelengths > 4.3 μm. Raman spectroscopy was used to examine changes to the crystalline structure of the implanted films. The Raman spectra indicate that ion implantation effects are not annealed out for temperatures up to 120 °C.     

Nitrogen doping of SiC thin films deposited by RF magnetron sputtering

Silicon carbide thin films (Si _x C _y ) were deposited in a RF (13.56 MHz) magnetron sputtering system using a sintered SiC target (99.5% purity). In situ doping was achieved by introducing nitrogen into the electric discharge during the growth process of the films. The N₂/Ar flow ratio was adjusted by varying the N₂ flow rate and maintaining constant the Ar flow rate. The structure, composition and bonds formed in the nitrogen-doped Si _x C _y thin films were investigated by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), Raman spectroscopy and Fourier transform infrared spectrometry (FTIR) techniques. RBS results indicate that the carbon content in the film decreases as the N₂/Ar flow ratio increases. Raman spectra clearly reveal that the deposited nitrogen-doped SiC films are amorphous and exhibited C–C bonds corresponding to D and G bands. After thermal annealing, the films present structural modifications that were identified by XRD, Raman and FTIR analyses.     

Structural and optical properties of Er implanted AlN thin films: Green and infrared photoluminescence at room temperature

In this work erbium ions were implanted into AlN films grown on sapphire with fluence range: (0.5-2) × 10¹⁵ at/cm⁻², ion energy range: 150-350 keV and tilt angle: 0°, 10°, 20°, 30°. The optical and structural properties of the films are studied by means of photoluminescence and Raman spectroscopy in combination with Rutherford backscattering/channeling (RBS/C) measurements. The photoluminescence spectra of the Er³⁺ were recorded in the visible and infrared region between 9 and 300 K after thermal annealing treatments of the samples. The emission spectrum of the AlN:Er films consists of two series of green lines centered at 538 and 558 nm with typical Er³⁺ emission in the infrared at 1.54 μm. The green lines have been identified as Er³⁺ transitions from the ²H_11/2 and ⁴S_3/2 levels to the ⁴I_15/2 ground state. Different erbium centers in the matrix are suggested by the change of infrared photoluminescence relative intensity of some of the emission lines when different excitation wavelengths are used. The relative abundances of these centers can be varied by using different implantation parameters. The Raman and RBS/C measurements show good crystalline quality for all the studied films.     

Fabrication and characterization of Ge nanocrystalline growth by ion implantation in SiO<Subscript>2</Subscript> matrix

Ge nanocrystallites (Ge-nc) have been formed by ion implantation of Ge⁺⁷⁴ into SiO₂ matrix, thermally grown on p-type Si substrates. The Ge-nc are examined by Raman spectroscopy, photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR). The samples were prepared with various implantation doses [0.5; 0.8; 1; 2; 3; 4] × 10¹⁶ cm⁻² with 250 keV energy. After implantation, the samples were annealed at 1,000 °C in forming gas atmosphere for 1 h. Raman intensity variation with implantation doses is observed, particularly for the peak near 304 cm⁻¹. It was found that the sample implanted with a doses of 2 × 10¹⁶ cm⁻² shows maximum photoluminescence intensity at about 3.2 eV. FTIR analysis shows that the SiO₂ film moved off stoichiometry due to Ge⁺⁷⁴ ion implantation, and Ge oxides are formed in it. This result is shown as a reduction of GeO_x at exactly the doses corresponding to the maximum blue-violet PL emission and the largest Raman emission at 304 cm⁻¹. This intensity reduction can be attributed to a larger portion of broken Ge–O bonds enabling a greater number of Ge atoms to participate in the cluster formation and at the same time increasing the oxygen vacancies. This idea would explain why the FTIR peak decreases at the same implantation doses where the PL intensity increases.     

Vibrational spectroscopic characterization of lanthanide molybdates

The vibrational spectrum of lanthanide molybdates (materials with important optical, electrical and catalytic properties) was assigned by using sites symmetry and factor group analysis. Raman and infrared spectra were registered and the modes corresponding to the MoO ₄ ^2– and LnO_n polyhedra were specified. An interesting behaviour for the symmetric stretching mode of the molybdate ion was found in these condensed lattices. The lanthanide cation does not influence the MoO ₄ ^2– vibrational behaviour. The use of this spectroscopy to characterize these materials is thus shown.     

Preparation of Ag2S–Graphene nanocomposite from a single source precursor and its surface-enhanced Raman scattering and photoluminescent activity

Ag₂S–Graphene nanocomposite was prepared via a relatively facile hydrothermal method, using a single-source molecular (silver diethyldithiocarbamate [Ag(DDTC)]) as precursor and graphene sheets as a support material. The composite was characterized by X-ray power diffraction, X-ray photoelectron spectroscopy, Field-emission scanning electron microscope, transmission electron microscopy, Fourier transform infrared, Raman spectra and fluorescence spectroscopy. The experimental results show that the Ag₂S–Graphene nanocomposite displays surface-enhanced Raman scattering (SERS) activity for graphene oxide and reveals relatively better fluorescence property compared with pure Ag₂S.     

Structure and stability of monazite- and zircon-type LaVO4 under hydrostatic pressure

Pure monazite (m)- and zircon (t)-type LaVO₄ and LaVO₄:Eu³⁺ were successfully synthesized by a hydrothermal method. The high pressure behavior of m- and t-LaVO₄ nanoparticles has been investigated using Raman scattering techniques at room temperature. Raman measurements reveal a slight change for m-LaVO₄ at 11.2 GPa because of an isostructural phase transition. However, striking changes in Raman spectra indicate a pressure-induced irreversible phase transition from the zircon to monazite structure for t-LaVO₄ at around 5.9 GPa. The evolution of the luminescence spectra of t-LaVO₄:Eu³⁺ has also been studied during the pressure-induced phase transition. It is observed that pressure has a great influence on the fluorescence intensity and the energy levels, which allows a more in-depth understanding of the nature of the pressure-induced phase transition for t-LaVO₄. This result further confirms the conclusion that zircon-type RVO₄ compounds with larger rare-earth cations will experience zircon to monazite phase transition.     

High growth rate of a-SiC:H films using ethane carbon source by HW-CVD method

Hydrogenated amorphous silicon carbide (a-SiC:H) thin films were prepared using pure silane (SiH₄) and ethane (C₂H₆), a novel carbon source, without hydrogen dilution using hot wire chemical vapour deposition (HW-CVD) method at low substrate temperature (200 °C) and at reasonably higher deposition rate (19·5 Å/s < r _d < 3·2 Å/s). Formation of a-SiC:H films has been confirmed from FTIR, Raman and XPS analysis. Influence of deposition pressure on compositional, structural, optical and electrical properties has been investigated. FTIR spectroscopy analysis revealed that there is decrease in C–H and Si–H bond densities while, Si–C bond density increases with increase in deposition pressure. Total hydrogen content drops from 22·6 to 14·4 at.% when deposition pressure is increased. Raman spectra show increase in structural disorder with increase in deposition pressure. It also confirms the formation of nearly stoichiometric a-SiC:H films. Bandgap calculated using both Tauc’s formulation and absorption at 10⁴ cm^?1 shows decreasing trend with increase in deposition pressure. Decrease in refractive index and increase in Urbach energy suggests increase in structural disorder and microvoid density in the films. Finally, it has been concluded that C₂H₆ can be used as an effective carbon source in HW-CVD method to prepare stoichiometric a-SiC:H films.     

Photostriction of CH3NH3PbBr3 Perovskite Crystals

Organic–inorganic hybrid perovskite materials exhibit a variety of physical properties. Pronounced coupling between phonon, organic cations, and the inorganic framework suggest that these materials exhibit strong light–matter interactions. The photoinduced strain of CH₃NH₃PbBr₃ is investigated using high‐resolution and contactless in situ Raman spectroscopy. Under illumination, the material exhibits large blue shifts in its Raman spectra that indicate significant structural deformations (i.e., photostriction). From these shifts, the photostrictive coefficient of CH₃NH₃PbBr₃ is calculated as 2.08 × 10^?8 m² W^?1 at room temperature under visible light illumination. The significant photostriction of CH₃NH₃PbBr₃ is attributed to a combination of the photovoltaic effect and translational symmetry loss of the molecular configuration via strong translation–rotation coupling. Unlike CH₃NH₃PbI₃, it is noted that the photostriction of CH₃NH₃PbBr₃ is extremely stable, demonstrating no signs of optical decay for at least 30 d. These results suggest the potential of CH₃NH₃PbBr₃ for applications in next‐generation optical micro‐electromechanical devices.     

Structural, magnetic and optical behavior of pristine and Yb doped CoWO4 nanostructure

Nanocrystalline pure and Yb doped CoWO₄ nanostructures were synthesized successfully by single step chemical precipitation technique. The prepared sample was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermal analysis (TGA). XRD pattern reveals the pure and doped CoWO₄ nanoparticles belongs to the monoclinic structure with the space group of P_2/c. Electron microscopy studies clearly evidence the formation of round edged nanocubes with an average particle size of 60–80 nm, emerges in the polycrystalline nature. UV–Visible absorption spectra of Yb³⁺ doped CoWO₄ nanocrystals shows a strong absorption peak at 278 nm due to CoWO₄ metal to ligand charge transfer within the [WO₆]^6? complex. Photoluminescence spectra of pure and doped CoWO₄ nanostructures substantiate the effect of Yb on the wolframite structure and its response for optical behavior. These results suggest that the addition of Yb into the Co-site on CoWO₄ has no significant contribution for luminescent enhancement when compared to pure one up to 5 % Yb concentration. Typical magnetization curve shows the mixed ferromagnetic and diamagnetic transition of CoWO₄ with respect to the Yb doping concentration.     

UV Raman spectroscopic probing into nitrogen-doped hydrogenated amorphous carbon

From the viewpoint of hardness, chemical bonding states of nitrogen-doped hydrogenated amorphous carbon film were characterized by Ultra violet (UV) Raman spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. UV Raman spectra revealed that disorder of the structure in nitrogen-doped hydrogenated amorphous carbon was promoted by applying higher bias voltages in the preparation. FT-IR spectra showed that NH bonds decreased and sp³Csp³N bonds increased with the increase of the bias voltages. The content of sp³Csp³N bonds was maximized at the bias voltage of −800 V. We found out a guarantee that the content of sp³Csp³N bonds against bias voltages correlated with the hardness obtained by nanoindentation test. The structural disorder and the increase of sp³Csp³N bonds are possible source of the hardness in the case of nitrogen-doped hydrogenated amorphous carbon.     

Structural,magnetic, optical,and impedance properties of SrxCo1−xFe2O4 nanoparticles prepared by sol–gel method

Sr²⁺ ions-substituted cobalt ferrite having the composition formula Sr_xCo_1?xFe₂O₄ (x?=?0.05, 0.10, 0.15, and 0.20) was synthesized with sol–gel method. X-ray diffraction (XRD) patterns proved that the samples had a spinel structure. The Fourier-transform infrared (FTIR) spectra showed that the samples have two characteristic bands about the intrinsic vibrations of spinel ferrite. The morphology and chemical elements were tested by scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS). The elemental state of annealed samples was examined by X-ray photoelectron spectroscopy (XPS). The magnetic data showed the coercivity increased from 954.33 Oe to 1545.54 Oe when the Sr²⁺ ions content increased. The optical absorption properties of annealed samples were investigated by UV–Vis spectroscopy. Complex impedance spectroscopy showed that grain resistance of Sr_0.10Co_0.90Fe₂O₄ sample was 443.9 Ω.