Spectroscopic and dielectric studies of meso-tetrakis(p-sulfonatophenyl) porphyrin doped hybrid borate glasses

Hybrid borate glasses containing different concentrations of meso-tetrakis(p-sulfonatophenyl) porphyrin sodium salt (TPPS₄) were prepared. The obtained glass samples were found to be transparent and homogeneous. Formation of TPPS₄-J-aggregates in borate glass was investigated by means of optical absorption, steady state and time resolved fluorescence spectroscopy. The hybrid glasses exhibit a strong S₂→S₀ emission at ∼473 nm and J-aggregates show emission at ∼733 nm. Time resolved fluorescence show two exponential decay with lifetime of τ₁ = 65 ± 10 ps (∼80%) and τ₂ = 3.87 ± 0.1 ns (∼20%) respectively. Dielectric properties such as dielectric constant (?′), dielectric loss (tan δ) and ac conductivity (σ_ac) over a range of frequency and temperature of these glasses were studied. The ac conductivity was found to be proportional to ω^s (where s < 1). The observed change in dielectric parameters due to different concentrations of TPPS₄ has been analyzed in light of different polarization mechanisms.     

Polyaniline nanofibers synthesized by rapid mixing polymerization

Polyaniline (PANI) nanofibers were chemically synthesized by a rapid mixing polymerization with aniline concentration of 0.5 M. The time needed for the formation of PANI in the reaction medium decreased with increase of the molar ratio of ammonium peroxydisulphate (APS)/aniline and temperature. Morphological study showed at the end of polymerization, only the ones prepared with low molar ratio of APS/aniline (e.g. 0.25 and 0.50) and temperature (e.g. 0 and 20 °C) are nanofibers with diameters of ∼50 nm, though the initially formed products are all nanofibers, while with increasing of molar ratio of APS/aniline to 1 and temperature to 20 °C or higher, agglomerates of PANI nanofibers with diameters of ∼100 nm and larger sized irregular particles were formed. The yield of PANI nanofibers was in the range of 13.4–42.3%, which is favorable for mass production of PANI nanofibers. Conductivity measurement, UV–vis and FTIR spectra were performed to characterize the products. The conductivity of the PANI nanofibers increased with molar ratio of APS/aniline at low temperature, while decreased at higher temperature, which might be resulted from the degradation of PANI molecules in the presence of more APS molecules at higher temperature.     

Morphology characterization of polyaniline nano- and microstructures

Small-angle neutron scattering (SANS), nuclear magnetic resonance (NMR), wide-angle and small-angle X-ray scattering (WAXS and SAXS) measurements were carried out to investigate the three morphological forms of polyaniline emeraldine base (PANI-EB): unstructured, microtubes, and nanofibers. Although the chemical backbone between these two materials is quite similar, their solid structures are quite different, showing differences in the molecular conformation and supramolecular packing. Detailed solid-state ¹³C and ¹⁵N NMR characterization of PANI nanofibers (compared to the unstructured, granular form) revealed a slight variation in the structural features of the polymer that led to some differences in the chemical environments of the respective nuclei. The presence of two extra-sharp peaks at 96.5 and 179.8 ppm is a distinct feature found exclusively in the nanofiber spectra. Moreover, the crosspolarization (CP) dynamics study disclosed the presence of a complete set of sharp NMR peaks that are responsible for the presence of a more ordered morphology in the nanofiber. Small-angle neutron scattering indicated very sharp interfaces in the PANI fibers, which are well organized and have extremely sharp domains within the length scales probed (∼10–1 nm). Overall, the X-ray scattering and spectroscopy data suggest that the nanofiber form is structurally different from the unstructured, PANI-EB powder. These differences are manifested, in part, by the additional chemistry occurring during the synthesis of the nanofibers.     

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.     

Preparation and luminescent characteristics of Sr3RE2(BO3)4:Dy (RE = Y, La, Gd) phosphors for white LED

Dysprosium-activated Sr₃RE₂(BO₃)₄ (RE = Y, La, Gd) phosphors were synthesized by a high temperature solid-state reaction method. The phase uniformity of the phosphors was characterized by X-ray powder diffraction (XRD) and the luminescence characteristics were investigated. The excitation spectra at 575 nm emission show strong spectral bands in the region of 300-500 nm. The emission spectra of the phosphors with 365 nm excitation show three bands centered at 484 nm, 575 nm and 680 nm, which originate from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 of Dy³⁺, respectively. The effect of Dy³⁺ concentration on the emission intensity of the phosphors was investigated. The fluorescence decay curves for ⁴F_9/2 → ⁶H_13/2 excited at 365 nm and monitored at λ_em of 575 nm were measured. The decay times decreased slowly with increasing Dy³⁺ doping concentration due to a trap capturing to resonance fluorescence transfer of the activated ions and due to the exchange interactions between activated ion pairs. In order to determine the type of interaction between activated ions, the concentration dependence curves (lg(I/x) versus lg x) of Sr₃RE₂(BO₃)₄:Dy³⁺ (RE = Y, La, Gd) were plotted. The concentration quenching mechanism of the ⁴F_9/2 → ⁶H_13/2 (575 nm) transition of Dy³⁺ is the d-d interaction. All results indicate these phosphors are promising white-color luminescent materials.     

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.     

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 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.     

Preparation and characterization of polyindole nanofibers by electrospinning method

Polyindole nanofibers with diameter ranging from 770 nm to 250 nm were firstly fabricated by an electrospinning method. Chemically synthesized polyindole was dissolved in acetonitrile to make polymer solution under ultrasonification. Electrospinning of polyindole was then carried out under electrical field strength of 1.0 kV/cm. The electrospun polyindole nanofibers exhibited smooth surface and the diameter of the fibers was ranged from 768 nm to 255 nm. The specific surface areas of polyindole nanofibers were ranged from 32 to 65 m²/g, which is significantly higher than that of the powder with same volume. The electrical conductivity of the polyindole nanofibers can reach 0.24 S/cm, which is much higher than that of the polyindole film. The polyindole nanofibers showed high thermal stability with glass transition temperature (T_g) around 132 °C and melting point (T_m) around 239 °C. The crystallinity of polyindole nanofibers was higher than that of polyindole film due to the formation of ordered molecule chains during the electrospinning. Cyclic voltammetry test results revealed that the doping and de-doping processes of BF₄⁻ ions were reversible and polyindole nanofibers had high electronically activity in the electrolytic solution containing LiBF₄.     

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).     

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.     

Photophysical change of poly(9,9-di(2-ethylhexyl)fluorene) and its copolymer with anthracene in solvent–non-solvent: Roles of interchain interactions on the formation of non-emissive and emissive aggregates

This contribution investigates photophysical change of poly(9,9-di(2-ethylhexyl)fluorene) (PF2/6) and its random copolymer with anthracene in solvent–non-solvent system. Isolated chains of the polymers in good solvent are driven to associate by addition of a poor solvent. The increase of interchain interactions upon decreasing solvent quality is found to cause the growth of a broad red-shift shoulder in absorption spectra of PF2/6. The measurements of photoluminescence (PL) spectra detect the increase of red-shift peaks at ∼440 nm and ∼525 nm. Detailed analyses of site-selective PL and PL excitation (PLE) spectra indicate the formation of emissive aggregates in this system. However, direct comparison of PLE and absorption spectra accompanied with the decrease of quantum yield suggests that different type of aggregates, the non-emissive one, also forms. The extent of interchain interactions dictates the existence of two types of aggregates. Our results signify the roles of interchain interactions on photophysical properties of PF2/6. Parallel studies of statistical copolymer of PF2/6 with anthracene groups (5 mol.%) observe drastic drop of the red-shift peak at ∼525 nm in PL spectra. The anthracenes exist as steric groups along the conjugated backbone, prohibiting the formation of emissive aggregates and/or excimers due to the increase of interchain proximity.     

Polarized spectral analysis of Er ions in Er:LiGd(MoO4)2 crystal

The Er³⁺:LiGd(MoO₄)₂ crystal with Ø21 × 33 mm³ was grown by the Czochralski technique, and the absorption spectra, the fluorescence spectra and the fluorescence decay curves were measured at room temperature. Some spectroscopic parameters, such as the parameters of oscillator strengths, the spontaneous transition probabilities, the fluorescence branching ratios, the radiative lifetimes and the emission cross-sections were estimated based on Judd-Ofelt theory and Füchtbauer-Ladenburg method. The infrared emission at 1450-1650 nm, due to ⁴I_13/2 → ⁴I_15/2 transition and the visible emission at 520-569 nm corresponding to ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition were observed in Er³⁺:LiGd(MoO₄)₂ crystals under 979 nm excitation at room temperature. The emission cross-sections are 4.37 × 10⁻²⁰ cm² at 553 nm and 0.584 × 10⁻²⁰ cm² at 1561 nm for π-polarization, and the following measured lifetimes are 4.57 ms and 10.74 μs. The upconversion emissions were attributed to energy transfer between Er³⁺ ions and the excited state absorption.     

One-step synthesis and properties of monolithic photoluminescent ruby colored cuprous oxide antimony oxide glass nanocomposites

Cuprous oxide (Cu₂O) antimony glass (K₂O-B₂O₃-Sb₂O₃) monolithic nanocomposites having brilliant yellow to ruby red color have been synthesized by a single-step melt-quench technique involving in situ thermochemical reduction of Cu²⁺ (CuO) by the reducing glass matrix without using any external reducing agent. The X-ray diffraction (XRD), infrared transmission and reflection spectra, and selected area electron diffraction analysis support the reduction of Cu²⁺ to Cu⁺ with the formation of Cu₂O nanoclusters along with Cu_ySb_2−x(O,OH)_6-7 (y ≤ 2, x ≤ 1) nanocrystalline phases while Cu⁰ nanoclusters are formed at very high Cu concentration. The UV-vis spectra of the yellow and orange colored nanocomposites show size-controlled band gap shift of the semiconductor (Cu₂O) nanocrystallites embedded in the glasses while the red nanocomposite exhibits surface plasmon resonance band at 529 nm due to metallic Cu. Transmission electron microscopic image advocates the formation of nanocystallites (5-42 nm). Photoluminescence emission studies show broad red emission band around 626 nm under various excitation wavelengths from 210 to 270 nm.     

Effects of iron catalyst on the formation of crystalline domain during carbonization of electrospun acrylic nanofiber

To investigate the effects of introducing the iron compound on the carbonization behavior polyacrylonitile (PAN)-based electrospun nanofibers were carbonized with or without iron(III) acetylacetonate (AAI) over the temperature range of 900–1500 °C in nitrogen atmosphere. The morphological characteristics of the carbon nanofibers were investigated using X-ray diffractometer (XRD), Raman spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. The electrical conductivity of the carbon nanofiber web was measured by four-point probe method. The iron catalyst had a profound effect on the crystal structure of the carbonized nanofiber. In the presence of AAI the nanofibers carbonized at 1300 °C developed graphite structure, which could be obtained at the temperature higher than 2000 °C in the absence of the catalyst. The in-plane size of the graphite crystals (L_a) was measured to be about 6.5 nm by Raman spectroscopy and the (0 0 2) spacing by XRD was 0.341 nm.     

Formation of ZnO nanorod arrays on polytetraflouroethylene (PTFE) via a seeded growth low temperature hydrothermal reaction

ZnO nanorod arrays were formed by a low temperature hydrothermal process on seeded polytetraflouroethylene (PTFE) sheets. The seed layer was formed using thermal oxidation of a thin evaporated Zn film on the PTFE sheet at 300 °C in air for 10 min. The formation of ZnO nanorod arrays in the hydrothermal reactive bath consisting of hexamethylamine (HMT) and Zn ions occurred via the reaction of hydroxyl ions released during the thermal degradation of HMT with the Zn ions. The seed layer provided a template for the nucleation of the ZnO and HMT which also acted as a chelating agent that promoted growth of the ZnO along the c-axis, leading to the formation of exclusively (0 0 2) ZnO nanorods. The effect of exposure time of the seeded PTFE to the reactive solution on the formation of the nanorods was investigated. Well aligned, relatively uniform tapered 300 nm long nanorods can be formed after 8 h of exposure. Longer exposure times to 24 h resulted in the formation of more uniform nanorods with base diameter averaged of ∼100 nm and the tip diameter of ∼50 nm. XRD analysis showed that the ZnO nanorod array had a hexagonal wurtzite structure. This result is in agreement with HR-TEM observations and Raman scattering analysis. Photoluminescence study showed that a strong UV emission peak was obtained at 380 nm and a small peak at 560 nm, which is associated with green emission. The optical band gap measured from these plots was at 3.2 eV on average.     

Broadband near-infrared emission in Er-Tm co-doped bismuthate glasses

In this work, we report near infrared emission spectra of Er³⁺-Tm³⁺ co-doped Bi₂O₃-GeO₂-Na₂O (BGN) glasses with the excitation of 800 nm laser. A broad emission extending from 1300 to 1650 nm with a full width at half maximum (FWHM) of ∼160 nm is obtained in a 1.0 wt% Tm₂O₃ and 0.3 wt% Er₂O₃ co-doped BGN glass. The energy transfer processes between Tm³⁺ and Er³⁺ in BGN glasses are analyzed in detail. The temperature dependence of the broadband emission spectra in Er³⁺-Tm³⁺ co-doped BGN glass is also studied. The present work indicates that Er³⁺-Tm³⁺ co-doped BGN glasses can be promising materials for broadband light sources and broadband amplifiers for WDM transmission systems.     

Synthesis and magnetic properties of nickel nanoparticles deposited on the silicon nanowires

Nickel (Ni) nanoparticles with sizes of ∼35 nm were deposited on the surface of silicon nanowires (SiNWs) by electroless plating technique. The magnetic properties of Ni/SiNWs were investigated. The blocking temperature (T_B) of 370 K was obtained and confirmed by field-cooled (FC) and zero-field-cooled (ZFC) plots. The M-H hysteresis loops from 5 K to 400 K were measured. The saturation magnetization value was ∼4.5 emu/g and the coercivity was ∼375.3 Oe for the loop at 5 K, respectively. While for the loop at 400 K, these values were of ∼2.6 emu/g and ∼33.3 Oe, respectively. The temperature dependence of coercivity followed by the relation H_C(T) = H_C0[1 − (T/T_B)^1/2], indicating a superparamagnetic behavior. The magnetization of superparamagnetic grains in a magnetic field H was better described by Langevin function at 400 K. These novel magnetic properties of Ni/SiNWs were possibly attributed to the paramagnetic defects on the surface of SiNWs.     

Synthesis and upconversion luminescence properties of monodisperse Y2O3:Yb, Ho spherical particles

Monodispersed spherical Y₂O₃:Yb, Ho upconversion luminescence (UCL) particles with sizes of 40-200 nm are prepared using a homogeneous precipitation method. It is found that aging time, varying between 90 and 10 min, has a profound influence on the precursor size, which systematically decreases from 230 nm to 50 nm. The precursor shows poor stability when aging time is 10 min, and the stability of precursor can be improved by increasing the urea concentration. The UCL spectra of Y₂O₃:Yb, Ho with different particle sizes are investigated. The results indicate that the integrated emission intensity ratio of green to red (R_green/red) exhibits a gradual decrease from 2.7 to 0.45 when the particle size decreases from 200 nm to 40 nm, and the possible reasons are evaluated.     

Effect of sintering temperature and time on densification, microstructure and properties of the PZT/ZnO nanowhisker piezoelectric composites

Lead zirconate titanate (PZT) based piezoelectric composites embedded with ZnO nanowhiskers (ZnO_w) were investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. The samples are characterized by X-ray diffraction analysis and scanning electron microscopy. The results show that the increase of the sintering temperature and time is quite effective in improving the densification and piezoelectric properties of the PZT/ZnO_w composites. However, the relative density and piezoelectric properties deteriorate as the composites are sintered over the optimal sintering condition. Particularly, the PZT/ZnO_w composites sintered at 1150 °C for 2 h show excellent electrical properties of piezoelectric constant d₃₃ ∼ 471 pC/N, relative dielectric constant ? ∼ 3838, planar electromechanical coupling factor k_p ∼ 0.543, remnant polarization P_r ∼ 23.2 μC/cm² and coercive field E_c ∼ 9.2 kV/cm.