Structural, optical and EPR studies on ZnO:Cu nanopowders prepared via low temperature solution combustion synthesis

Cu (0.1 mol%) doped ZnO nanopowders have been successfully synthesized by a wet chemical method at a relatively low temperature (300 °C). Powder X-ray diffraction (PXRD) analysis, scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transformed infrared (FTIR) spectroscopy, UV-Visible spectroscopy, Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR) measurements were used for characterization. PXRD results confirm that the nanopowders exhibit hexagonal wurtzite structure of ZnO without any secondary phase. The particle size of as-formed product has been calculated by Williamson-Hall (W-H) plots and Scherrer's formula is found to be in the range of ∼40 nm. TEM image confirms the nano size crystalline nature of Cu doped ZnO. SEM micrographs of undoped and Cu doped ZnO show highly porous with large voids. UV-Vis spectrum showed a red shift in the absorption edge in Cu doped ZnO. PL spectra show prominent peaks corresponding to near band edge UV emission and defect related green emission in the visible region at room temperature and their possible mechanisms have been discussed. The EPR spectrum exhibits a broad resonance signal at g ∼ 2.049, and two narrow resonances one at g ∼ 1.990 and other at g ∼ 1.950. The broad resonance signal at g ∼ 2.049 is a characteristic of Cu²⁺ ion whereas the signal at g ∼ 1.990 and g ∼ 1.950 can be attributed to ionized oxygen vacancies and shallow donors respectively. The spin concentration (N) and paramagnetic susceptibility (χ) have been evaluated and discussed.     

Synthesis and characterization of CaF2 nanocrystals

Calcium fluoride nanocrystals (CaF₂) were synthesized by two different techniques namely co-precipitation and hydrothermal. The synthesized nanocrystals were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared red spectroscopy (FTIR), scanning electron microscopy (SEM), optical absorption and photoluminescence (PL). The crystallite size estimated using Scherer's formula was found to be in the range 30–35 nm for nanocrystals synthesized by co-precipitation method where as in case of hydrothermally synthesized nanocrystals it is in the range 20–28 nm which is less compared to those obtained by co-precipitation method. The morphological features as studied using SEM revealed that the nanocrystals are agglomerated, crispy with porous. The SEM images of hydrothermally synthesized nanocrystals showed less agglomeration than those obtained by co-precipitation method and the images confirm the formation of nanoparticles. The optical absorption spectrum showed a strong absorption band peaked at 244 nm for nanocrystals synthesized by co-precipitation method and it is 218 nm peak in case of hydrothermally synthesized ones. The PL emission spectrum showed two prominent emission bands peaked at 330 and 600 nm when excited at 218 nm.     

Synthesis and characterization of Er doped CaF2 nanoparticles

Er³⁺ doped CaF₂ nanoparticles were synthesized by a chemical co-precipitation method. Effect of the dopant concentrations on the structure and optical properties of the CaF₂ nanoparticles was investigated. The X-ray powder diffraction and transmission electron microscopy analysis was used to characterize the structure and morphology of the nanoparticles. The nanoparticles with different dopant concentration exhibited a sphere-like morphology with diameters of about 8-36 nm. The incorporation of Er³⁺ ions into CaF₂ resulted in the decrease in grain size and deterioration of crystallinity, but enlarged the lattice constants of CaF₂. Additional annealing treatment at 400 °C to the prepared CaF₂ removed the NO₃⁻ and OH⁻ groups adsorbed on the particles’ surfaces, and improved the optical properties of the nanoparticles. The fluorescence intensity, with a maximum at approximately 0.4 mol%, decreased with the increase in doping concentration because of concentration quenching.     

Synthesis and characterization of spherical and rod like nanocrystalline Nd2O3 phosphors

Spherical and rod like nanocrystalline Nd₂O₃ phosphors have been prepared by solution combustion and hydrothermal methods respectively. The Powder X-ray diffraction (PXRD) results confirm that hexagonal A-type Nd₂O₃ has been obtained with calcination at 900 °C for 3 h and the lattice parameters have been evaluated by Rietveld refinement. Surface morphology of Nd₂O₃ phosphors show the formation of nanorods in hydrothermal synthesis whereas spherical particles in combustion method. TEM results also confirm the same. Raman studies show major peaks, which are assigned, to F_g and combination of A_g + E_g modes. The PL spectrum shows a series of emission bands at ∼326-373 nm (UV), 421-485 nm (blue), 529-542 nm (green) and 622 nm (red). The UV, blue, green and red emission in the PL spectrum indicates that Nd₂O₃ nanocrystals are promising for high performance materials and white light emitting diodes (LEDs).     

Tunable deep-level emission in ZnO nanoparticles via yttrium doping

Yttrium-doped ZnO nanoparticles (Zn_1−xY_xO, x = 0, 0.03, 0.05) were synthesized by sol-gel technique. The effects of yttrium doping concentration on the structures, morphologies and optical properties of as-synthesized Zn_1−xY_xO nanoparticles were investigated in detail. The results from structural characterizations clearly demonstrated that yttrium ions were successfully doped into the crystal lattice of ZnO matrix. Besides a UV emission centered at ∼383 nm, the PL spectra of all the samples exhibited a broad deep-level emission, which can be deconvoluted into two Gauss peaks centered at 539 nm (P1) and 598 nm (P2), respectively. As the concentration of Y doping increased from 0% to 5%, the peak position with maximum intensity in deep-level emission band was gradually tuned from 539 nm to 598 nm and the relative intensity ratio of I_P1/I_P2 also decreased step by step, which revealed a unique optical property of yttrium-doped ZnO nanoparticles.     

Influence of europium oxide doping on the structural and optical properties of pulsed laser ablated barium tungstate thin films

Nanostructured Eu₂O₃ doped Barium tungstate (BaWO₄) crystallites are successfully synthesized using pulsed laser deposition (PLD) technique. The influence of different Eu₂O₃ doping concentrations (1,2,3 & 5 wt.%) on the structural, surface morphological and optical properties are systematically studied using XRD, micro-Raman, SEM, AFM, UV-vis and photoluminescence spectroscopy. All the films are polycrystalline with tetragonal scheelite structure. The vibrational analysis of the atoms in BaWO₄ is studied by micro-Raman spectra using factor group analysis. The surface morphological analysis by SEM and AFM reveals the presence of fine nanoparticles with distinct grain boundaries in all the films. The band gap energy variation in the Eu₂O₃ doped BaWO₄ films is in accordance with the variation of the sizes of nano particles in the films. The films with higher Eu³⁺ doping concentrations (≥2 wt.%) show a PL emission peak centered around 614 nm when excited at 394 nm which can be attributed to the ⁵D₀ → ⁷F₂ (0-2) transition of Eu³⁺ ion.     

Novel photoluminescent properties of LiGaO2 nanoflakes

The luminescence properties of LiGaO₂ microflakes synthesized using the sol-gel process are investigated. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) and absorption spectra. The PL spectra excited at 325 nm have a broad and strong emission band with a peak at 383 nm, which corresponds to the self-activated luminescence of the tetrahedral gallium group. The optical absorption spectra of the sample annealed at 600 °C exhibited a band-gap energy of 3.38 eV.     

Influence of niobium doping on phase composition and defect-mediated photoluminescence properties of Eu-doped TiO2 nanopowders synthesized in Ar/O2 thermal plasma

Nb⁵⁺:Eu³⁺-codoped TiO₂ nanopowders for chemical composition adjustment have been synthesized via Ar/O₂ radio-frequency thermal plasma. X-ray diffraction (XRD) results reveal that all the resultant powders exhibited mixture polymorphs of anatase (mean size: ∼45 nm) as the major phase and rutile (mean size: ∼71 nm). Rutile formation was promoted by the Eu³⁺ doping but suppressed by the Nb⁵⁺ addition. Combined observation using FE-SEM and TEM indicates that all the plasma-synthesized powders had a majority of facet-shaped particles (several nanometers) and a small proportion of nearly spherical crystals (∼150 nm). For the defect-mediated photoluminescence (PL) emission through the energy transfer from the TiO₂ host to the Eu³⁺ activator, the PL intensity originating from the ⁵D₀ → ⁷F₂ electronic transition weakened but that from the ⁵D₀ → ⁷F₁ electronic transition strengthened with increasing Nb⁵⁺ content. This may be a result of the decrease in the oxygen vacancy defects in the TiO₂ host lattice, as revealed by the joint means of UV-vis absorption spectra and excitation and emission spectra.     

Microstructure, mechanical, EPR and optical properties of lithium disilicate glasses and glass-ceramics doped with Mn ions

The microstructure, mechanical, EPR and optical properties of transparent MnO₂ doped lithium disilicate (LDS) glass-ceramics prepared by melt quenching and controlled crystallization, have been studied. The microstructure of the glass-ceramics has been characterized using FE-SEM, TEM, FT-IR and XRD techniques. FE-SEM micrographs show elongated, highly interlocked, dense (∼80 vol.%) nanocrystals of LDS with an average size ∼100 nm. XRD and FT-IR studies reveal that the only crystalline phase formed after heat-treatment at 700 °C for 1 h is LDS. A good combination of average microhardness ∼5.6 GPa, high fracture toughness ∼2.8 MPa m^1/2, 3-point flexural strength ∼250 MPa and moderate elastic modulus 65 GPa has been obtained. The EPR spectra of both LDS glasses and glass-ceramics exhibit resonance signals with effective g values at g = 4.73, g = 4.10, g = 3.3, and g = 1.98. The resonance signal at g = 1.98 is found to be more intense than the other signals and exhibits hyperfine structure at lower concentration of manganese. From the observed spectrum, the spin-Hamiltonian parameters have been evaluated. In glass samples the optical absorption spectrum exhibits a broad band around ∼20,320 cm⁻¹ which has been assigned to the transition ⁶A_1g(S) → ⁴A_1g(G) ⁴E_g(G)-of Mn²⁺ ions. The cerammed samples upon 394 nm excitation emit a green luminescence (565 nm, ⁴T_1g → ⁶A_1g(G) transition of Mn²⁺ ions), and a weak red emission (710 nm). From the ultraviolet absorption edges, the optical bandgap energies (E_opt) were evaluated and are discussed.     

Nanoscale synthesis and optical features of metallic nickel nanoparticles by wet chemical approaches

Nickel nanoparticles (Ni-NPs) were successfully synthesized and attached on indium tin oxide (ITO) substrate by two different methods: from solution reduction process by using sodium borohydride (NaBH₄) as reducing agent in the presence of poly(N-vinilpyrrolidone) (PVP) as protective and stabilizing agents and by polyol process under ethylene glycol EG as a solvent. The results indicated that the samples prepared in aqueous solution show the occurrence of face-centered cubic metallic nickel nanoparticles with a medium diameter of ∼31 nm and good size dispersion compared to the preparation in EG that revealed large size ∼150 nm. The dynamics of the nanoparticle's growth in the solvents and comparison with optical absorption is presented.     

An open aperture z-scan study of Sr2CeO4 blue phosphor

Sr₂CeO₄ blue phosphor has been prepared by the solid-state reaction method. The X-ray diffraction (XRD) study confirms the structure of the system to be orthorhombic. High resolution electron transmission microscopy reveals that Sr₂CeO₄ prepared by the solid state reaction method is composed of elongated spherical structures of length ∼0.2-0.6 μm and width ∼90-150 nm. The excitation spectrum shows a broad band which peaks at 275 nm. The emission spectrum shows a broad band which peaks at 467 nm when excited at 275 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer (CT) state of the Ce⁴⁺ ion. The Commission International de l’Eclairage (CIE) co-ordinates are x = 0.15, and y = 0.23. The nonlinear absorption behavior of Sr₂CeO₄ has been investigated using the open aperture z-scan technique. The calculated effective two-photon absorption coefficient shows that the Sr₂CeO₄ blue phosphor is a promising optical limiting material.     

Synthesis, structural and electrical characterization of Sb substituted spinel nickel ferrite (NiSbxFe2−xO4) nanoparticles by reverse micelle technique

Spinel nickel ferrite with nominal composition NiFe_2−xSb_xO₄ (where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1) has been synthesized by the reverse microemulsion method. The samples synthesized were characterized by XRD, FTIR, TGA/DTGA, SEM, AFM, Mossbauer Spectroscopy and DC electrical resistivity measurements. The XRD analysis confirmed the formation of single spinel phase and the crystallite size was found to be in the range of 8-38 nm. The particle size of the synthesized samples was also confirmed by the AFM and SEM which was found in the range of 5-45 nm and 10-45 nm, respectively and this size is small enough for obtaining the suitable signal-to-noise ratio in high density recording media. The Mossbauer spectra of samples showed two well-resolved Zeeman patterns corresponding to A and B sites and also observed a doublet at higher substitution. The DC electrical resistivity showed an interesting behavior with temperature and observed a metal-to-semiconductor transition temperature (T_M-S) which suggests that the material can be applied for switching applications. The resistivity increases with the increase in Sb-content and it suggests that the material can be fruitfully used for applications in microwave devices.     

Synthesis and phosphorescence properties of Mn, La and Ho in MgAl2Si2O8

Mn⁴⁺, La³⁺ and Ho³⁺ doped MgAl₂Si₂O₈-based phosphors were first synthesized by solid state reaction. They were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X-ray powder diffraction (XRD), photoluminescence (PL) and scanning electron microscopy (SEM). The phosphors were obtained at about 1300 °C. They showed broad red and fuchsia-pink emission bands in the range of 610-715 nm and had a different maximum intensity when activated by UV illumination. Such a fuchsia-pink emission can be attributed to the intrinsic d-d transitions of Mn⁴⁺.     

Fe:ZnSe semiconductor nanocrystals: Synthesis, surface capping, and optical properties

Water-soluble Fe-doped ZnSe (Fe:ZnSe) nanocrystals (NCs) were synthesized by aqueous synthesis approach using thioglycolic acid (TGA) as capping agent. The undoped ZnSe and Fe:ZnSe NCs were well retained in the zinc blende structure, and the Fe dopants were well doped into the ZnSe NCs, as confirmed by X-ray photoelectron spectroscopy (XPS). The lattice constant of Fe:ZnSe NCs decreases slightly by the introduction of Fe, and Fe:ZnSe NCs exhibit a uniform size distribution with average grain size of ∼5 nm. The thioglycolic acid (TGA) was successfully capped on the surface of Fe:ZnSe NCs, confirmed by Fourier-transform-infrared (FT-IR) spectroscopy. The absorption edges of pure ZnSe and Fe:ZnSe NCs are blue-shifted compared to that of corresponding bulk ZnSe, indicating the quantum confinement effect, and the absorption edge of Fe:ZnSe NCs shows a slightly red shift with respect to the pure ZnSe NCs. The as-prepared Fe:ZnSe NCs exhibits an emission peak at ∼425 nm, and the photoluminescence (PL) intensity of the NCs has the maximum value when the Fe-doping concentration reaches 1.0 at%. It is of interest to note that the concentration quenching effect appears when the Fe-doping concentration is larger than 10.0 at%, and the underlying physical mechanisms were discussed.     

Structural and optical characterization of BaSnO3 nanopowder synthesized through a novel combustion technique

Nanocrystalline Barium stannate (BaSnO₃) was synthesized through auto-ignited combustion technique. The X-ray diffraction studies of BaSnO₃ nanoparticles reveals that the nanopowder is single phase, crystalline, and has a cubic perovskite structure with a lattice constant a = 4.115 Å. The average particle size calculated from full width half maximum (FWHM) using Scherer formula is ∼25 nm. The phase purity of the powder was further examined using Fourier transform infrared spectroscopy, Raman spectroscopy and transmission electron microscopic techniques. XRD pattern of BaSnO₃ was refined for atomic coordinates, lattice parameters and occupancies using Rietveld analysis. Vibrational analysis of sample shows that there is a phase transition from distorted cubic to ideal cubic structure during heat treatment. The thermal stability of BaSnO₃ nanopowder has been confirmed using thermo gravimetric analysis (TGA) and differential thermal analysis (DTA). The particle size of the as-prepared powder from transmission electron microscopy was found to be in the range 20-30 nm. The absorption spectra and photoluminescence spectra of the sample were also studied. The band gap determined was 2.887 eV and found to be a semiconductor.     

Template-free solvothermal synthesis of size-controlled yttria-stabilized-zirconia hollow spheres

Tetragonal yttria-stabilized-zirconia (YSZ) hollow spheres were prepared through a template-free solvothermal method in a homogeneous solution of butanol/acetylacetone or ethanol/acetylacetone with ZrOCl₂·8H₂O and Y(NO₃)₃·6H₂O as the reaction reagents. The hollow spheres were composed of nanometer-sized particles. The sizes of the hollow spheres could be tuned from 300 nm to 900 nm by changing the concentration of ZrOCl₂·8H₂O or altering the solvent. Time-dependent experiments showed that the hollow spheres transformed from amorphous solid spheres composed of nanoparticles. The Ostwald ripening process is proposed to explain the formation of the hollow structures. Er³⁺ doped zirconia (ZrO₂) hollow spheres were also synthesized using the same method. Characteristic emission patterns (515-565 nm) and (640-690 nm) were observed from Er³⁺ doped zirconia hollow spheres with 980 nm excitation.     

Synthesis of WO3 nanoparticles for photocatalytic O2 evolution by thermal decomposition of ammonium tungstate loading on g-C3N4

Tungsten trioxide (WO₃) nanoparticles have been successfully synthesized by thermal decomposition of ammonium tungstate loading on g-C₃N₄. The as prepared nanoparticles were characterized by XRD, UV-vis, photoluminescence spectra (PL) and TEM. The XRD results indicate that the g-C₃N₄ decomposed completely with WO₃ remaining at calcination temperature higher than 550 °C. The WO₃ prepared at temperature below 750 °C exhibits orthorhombic phase, and monoclinic phase at temperature higher than 850 °C. The UV-vis absorption onset wavelength of the obtained samples is approximately 470 nm, and the absorption intensity increases with calcination temperature, and reaches a maximum at 750 °C. The as prepared WO₃ powders, loaded with 0.5 wt% Pt as cocatalyst, were used as photocatalysts for O₂ evolution from an aqueous KIO₃ solution. The WO₃ nanoparticles prepared from ammonium tungstate loading on g-C₃N₄ showed photocatalytic activity in O₂ evolution up to 77 times higher than that of WO₃ samples prepared from ammonium tungstate without loading on g-C₃N₄.     

The synthesis of novel ZnO microcrystals through a simple solvothermal method and their optical properties

ZnO microcrystals with novel structures have been synthesized by a solvothermal method that is facile, low-cost and environment-friendly. Zn(NO₃)₂·6H₂O is the only precursor and absolute ethanol is the solvent. By controlling the reaction time, temperature and molarity of zinc nitrate, ZnO entities with the shape of flower, nut, hexagon-pillar, popcorn, brush and sphere can be synthesized in high selectivity. The ZnO micronuts (length ∼8 μm and width ∼5 μm) are uniform in morphology, displaying an open gap on the surface that divides the body into two. The investigation on the optical properties of the ZnO microcrystals reveals that all the ZnO samples exhibit an excitonic absorption edge around 376 nm, and compared to bulk ZnO, there is a modest red shift of ∼6 nm that can be ascribed to size effect as well as the unique morphologies of the ZnO microcrystals.     

Photoluminescence and thermoluminescence studies of Mg2SiO4:Eu nano phosphor

Nanoparticles of Eu³⁺ doped Mg₂SiO₄ are prepared using low temperature solution combustion technique with metal nitrate as precursor and urea as fuel. The synthesized samples are calcined at 800 °C for 3 h. The Powder X-ray diffraction (PXRD) patterns of the sample reveled orthorhombic structure with α-phase. The crystallite size using Scherer's formula is found to be in the range 50-60 nm. The effect of Eu³⁺ on the luminescence characteristics of Mg₂SiO₄ is studied and the results are presented here. These phosphors exhibit bright red color upon excitation by 256 nm light and showed the characteristic emission of the Eu³⁺ ions. The electronic transition corresponding to ⁵D₀ → ⁷F₂ of Eu³⁺ ions (612 nm) is stronger than the magnetic dipole transition corresponding to ⁵D₀ → ⁷F₁ of Eu³⁺ ions (590 nm). Thermoluminescence (TL) characteristics of γ-rayed Mg₂SiO₄:Eu³⁺ phosphors are studied. Two prominent and well-resolved TL glows with peaks at 202 °C and 345 °C besides a shoulder with peak at ∼240 °C are observed. The trapping parameters-activation energy (E), order of kinetics (b) and frequency factor (s) are calculated using glow curve shape method and the results obtained are discussed.     

Preparation and characteristics of Fe3O4@YVO4:Eu bifunctional magnetic-luminescent nanocomposites

A facile direct precipitation method has been developed for the synthesis of bifunctional magnetic-luminescent nanocomposites with Fe₃O₄ nanoparticles as the core and YVO₄:Eu³⁺ as the shell. Transmission electron microscopy (TEM) images revealed that the obtained bifunctional nanocomposites had a core-shell structure and a spherical morphology. The average size was ∼150 nm, and the thickness of the shell was ∼15 nm. The X-ray diffraction (XRD) patterns showed that a cubic spinel structure of Fe₃O₄ core and a tetragonal phase of YVO₄ shell were obtained. Fourier transform infrared (FT-IR) spectra confirmed that the YVO₄:Eu³⁺ had been successfully deposited on the surface of Fe₃O₄ nanoparticles. Photoluminescence (PL) spectra indicated that the nanocomposites displayed a strong red characteristic emission of Eu³⁺. Magnetic measurements showed that the obtained bifunctional nanocomposites exhibited superparamagnetic behavior at room temperature. Therefore, the bifunctional nanocomposites are expected to develop many potential applications in biomedical fields.