Structural and spectroscopic characterization of poly(styrene sulfonate) films doped with neodymium ions

This work reports the structural and spectroscopy characterization of poly(styrene sulfonate) (PSS) films doped with neodymium (Nd) ions. Nd–PSS films were processed using the acid of poly(styrene sulfonate) – H–PSS and neodymium nitrate – Nd(NO₃)₃; the maximum incorporation of Nd ions in the polymeric matrix was equal 19.3%. The absorption in the UV–Vis–NIR spectral region presents typical electronic transitions of Nd³⁺ ions, with well resolved peaks. The infrared spectra present the transition bands of PSS with characteristic line shape broadening, and the presence of vibrational modes of N–O groups in the range of 1400–720 cm^?1, prove the permanence of Nd(NO₃)_x, with x = 1, 2 and/or 3, in the H–PSS matrix. UV–Vis site selective photoluminescence data indicate that the incorporation of Nd³⁺ introduces a blue shift in PSS emission (325–800 nm), decreasing the interaction between adjacent PSS lateral groups (aromatic rings). Nd³⁺ reabsorption and energy transfer effects between the PSS matrix and Nd³⁺ were also observed. The IR emission of Nd–PSS films at 1076 nm (⁴F_3/2 → ⁴I_11/2) present constant efficiency, independent on Nd³⁺ concentration. The Judd–Ofelt theory was employed to analyze radiative properties. The excitation spectra prove the energy transfer between the polymeric matrix and Nd³⁺. Complex impedance data was used to probe relaxation processes during the charge transport within the polymeric matrix.     

Vacuum-deposited poly(o-methoxyaniline) thin films: Structure and electronic properties

Thin poly(o-methoxyaniline) (POMA) films have been formed by thermovacuum deposition in the temperature range of 350–450 °C and at a pressure of 5 × 10^?5 Torr. The structure properties of vacuum deposited POMA films according to FTIR and UV–VIS spectra are similar to those observed for the emeraldine form of polyaniline. Current–voltage characteristics (I–V) of sandwichtype device ITO/POMA/A1 possess rectifying properties with the ideality factor ≈4 at room temperature. On the basis of the dependence of conductivity on frequency in the frequency range of 10 Hz to 1 MHz, it is shown that the Pollack–Pohl current flow hopping mechanism dominates in a polymer film; such mechanism is typical of non-ordered systems.     

Judd–Ofelt analysis of Nd3+ ions in poly(styrene sulfonate) films

In this work, we present the synthetic route and the optical characterization of poly(styrene sulfonate) (PSS) films doped with Neodymium ions (Nd³⁺). In the synthesis optimization we obtained the maximum incorporation of Nd³⁺ in the matrix about 14.0%. The UV–Vis–NIR curve presents an intense characteristic electronic transition ⁴I_9/2 → ⁴F_5/2 + ²H_9/2 at 800 nm. It was also shown the radiative transition ⁴F_3/2 → ⁴I_11/2 at about 1060 nm. Judd–Ofelt theory was used in order to obtain the near infrared Nd³⁺ radiative transition rate, emission cross-section and radiative lifetime.     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.     

The effect of Tb doped PbTiO3 inducing layer on texture and tunable property of sol–gel derived Pb0.4Sr0.6TiO3 thin films grown on ITO/glass substrate

Highly (1 0 0)-oriented Pb_0.4Sr_0.6TiO₃ (PST40) thin films have been prepared on the Tb doped PbTiO₃ (PTT) thin film coated ITO/glass substrate by sol–gel technique. The PTT inducing layers are (1 0 0)-oriented and can help to control the orientation of PST40 thin films. Crystallization of the PST40 thin film with the PTT inducing layer is more perfect than that without PTT layer due to less distortion in the thin film. The dielectric tunability of the PST40 thin film with PTT layer therefore reaches 65%, which is 85% higher than that without PTT layer. The dielectric loss of the PST thin film is only 0.05. These results indicate that (1 0 0)-oriented Tb doped PbTiO₃ can be used as an inducing layer for highly (1 0 0)-oriented tunable materials on ITO/glass substrate.     

Properties of light-induced degradation and the electronic properties of nanocrystalline silicon solar cells grown under functionally graded hydrogen dilutions

The electronic properties of hydrogenated nanocrystalline silicon (nc-Si:H) were studied using drive-level capacitance profiling (DLCP) to obtain defect density profiles as well as transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopies to study the spectra of defect related optical transitions. These measurements were performed on a series of n–i–p solar cell devices with intrinsic layer thickness of roughly 1 μm. The nc-Si:H intrinsic layers were deposited using RF or MVHF glow discharge with various hydrogen dilution profiles predominantly on specular stainless steel substrates (SS/n⁺/i nc-Si:H/p⁺/ITO), but also on textured back reflectors (SS/Ag/ZnO/n⁺/i nc-Si:H/p⁺/ITO) in some cases. Crystallite fractions were estimated using Raman spectroscopy. The electronic properties determined by our measurements could be correlated with variations in structural device parameters and with the degree of hydrogen dilution profiling during growth. We also found, depending on the growth conditions, that the devices exhibited markedly different behaviors after prolonged light exposure (100 h using light at 610 nm and 500 mW/cm² intensity). We discuss one specific microscopic mechanism that may be responsible for the light-induced changes that we have observed.     

Amorphous to crystalline transition and optoelectronic properties of nanocrystalline indium tin oxide (ITO) films sputtered with high rf power at room temperature

ITO thin films were deposited on quartz substrates by the rf sputtering technique using various rf power keeping the substrates at room temperature. The influence of rf power on the structural, electrical, optical and morphological properties was studied by varying the rf power in the range 50–350 W. X-ray diffraction results show an amorphous – crystalline transition with nano grains. At a power of 250 W, the ITO film showed preferential orientation along (4 0 0) peak. It is observed from the optical transmission studies that the optical band gap increased from 3.57 to 3.69 eV when the rf power was increased from 50 to 250 W. The resistivity value is minimum and grain size is maximum for the ITO film deposited at 250 W. The X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray (EDX) and Atomic force microscopy AFM results confirm that the ITO films are stoichiometric and the surface contained nano-sized grains distributed uniformly all over the surface. It can be concluded that the ITO film deposited at room temperature with 250 W rf power, can provide the required optical and electrical properties useful for developing optoelectronic devices at lower temperatures.     

Thermal,vibrational and Ac impedance studies on proton conducting polymer electrolytes based on poly(vinyl acetate)

Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO₄) have been prepared by solution casting technique with various compositions. The X-ray diffraction analysis confirms the polymer–HClO₄ complex formation. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). The shift in T_g towards the lower temperature indicates that the polymer salt interaction takes places in the amorphous phase of the polymer matrix. Ac impedance spectroscopy reveals that 75 mol% PVAc:25 mol% HClO₄ exhibits maximum conductivity, 3.75 × 10^? 3 S cm^? 1 at room temperature (303 K). The increase in conductivity with increase in dopant concentration and temperature may be attributed to the enhanced mobility of the polymer chains, number of charge carriers and rotations of side chains. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures.     

Efficient Bilayer Electrodes for Photosensitive Organic Heterostructure

The possibility of increasing the photosensitivity of organic heterostructures by using the bilayer transparent ITO electrodes with organic conducting polymer polyaniline, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) complex (PEDOT:PSS) and CuI has been verified experimentally. Photosensitive n-type organic semiconductor N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic acid diimide (МРР) and p-type pentacene were chosen as components of these heterostructures. Usage of ITO/PEDOT:PSS bilayer electrodes leads to increasing the photovoltaic sensitivity of these heterostructures by 2–3 orders and by 1–2 orders when using CuI due to decreasing the recombination of excitons and increasing the potential barrier heights on the interface between the organic semiconductor and layers of PEDOT:PSS and CuI.     

Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques

We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10 nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3 nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at T_g, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (T_g) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10 K as compared to the pure matrix).     

Fabrication and characterization of nanorod solar cells with an ultrathin a-Si:H absorber layer

In this paper, we present a three-dimensional nanorod solar cell design. As the backbone of the nanorod device, density-controlled zinc oxide (ZnO) nanorods were synthesized by a simple aqueous solution growth technique at 80 °C on ZnO thin film pre-coated glass substrate. The as-prepared ZnO nanorods were coated by an amorphous hydrogenated silicon (a-Si:H) light absorber layer to form a nanorod solar cell. The light management, current–voltage characteristics and corresponding external quantum efficiency of the solar cells were investigated. An energy conversion efficiency of 3.9% was achieved for the nanorod solar cells with an a-Si:H absorber layer thickness of 75 nm, which is significantly higher than the 2.6% and the 3.0% obtained for cells with the same a-Si:H absorber layer thickness on planar ZnO and on textured SnO₂:F counterparts, respectively. A short-circuit current density of 11.6 mA/cm² and correspondingly, a broad external quantum efficiency profile were achieved for the nanorod device. An absorbed light fraction higher than 80% in the wavelength range of 375–675 nm was also demonstrated for the nanorod solar cells, including a peak value of ~ 90% at 520–530 nm.     

The influence of low-concentration γ-Fe2O3 magnetic nanoparticles in a poly(ether–b–ester) copolymer matrix on interfacial polarization and glass transition

A new poly(ester–b–ether) copolymer filled with magnetic γ-Fe₂O₃ nanoparticles of low concentration (0.1 and 0.3 wt%) has been synthesized, with poly(ethylene terephthalate) (PET) as a rigid segment and poly(tetraoxymethylene) (PTMO) as the flexible segment at the ratio of 50 wt%. The filler was used in the form of suspension or solid powder. The dielectric spectroscopy results for these materials, in a wide range of frequencies and temperatures, are presented and compared with similar results for a pure polymeric matrix. Two relaxation processes, α and β, have been observed clearly in all samples at about 253 K and 163 K, respectively. The α process was shifted towards the lower temperatures as the nanoparticle content increased, while the β-relaxation was unaffected by the doping. The interfacial polarization effect was observed in the samples with nanoparticles, less pronounced for the larger nanoparticles.     

Semi-interpenetrating polymer networks based on polyurethane and poly(2-hydroxyethyl methacrylate): Dielectric study of relaxation behavior

The study of molecular dynamics by broadband dielectric spectroscopy (BDS) is presented for polyurethane (PU), poly(2-hydroxyethyl methacrylate) (PHEMA) and for semi-IPNs based on PU and PHEMA synthesized by photopolymerization. The dielectric properties were performed in wide range of frequencies and temperatures with the goal to establish the relation between the relaxations and the structure. Five relaxation phenomena were finally detected for PHEMA : γ-, βsw-, β-relaxations at low temperatures and α-relaxation at 150 °C at high frequencies plus ionic conductivity relaxation which starts at 0 °C. For semi-IPNs the overlapping of γ- and βsw-relaxations of PHEMA (?125/?75 °C), then with increasing the temperature α-relaxation in PU (?75/0 °C), next ionic conductivity relaxation which starts at 0 °C, and finally the α-relaxation of PHEMA (+125/+170 °C) were detected. The α-relaxation of PHEMA in semi-IPNs shifts to lower temperatures and became broader with increasing amount of PU due to incomplete phase separation in the system and formation of interphases. The dielectric relaxation phenomena were fitted with Havriliak–Negami equation. Activation energy, τ_o and α parameters were calculated. For α-relaxations corresponding dielectric characteristics have been determined from Vogel–Fulcher–Tammann equation. The relaxation map for investigated PU, PHEMA and semi-IPNs was built.     

Effect of thermal treatment on high-frequency magneto-impedance in ferromagnetic/Cu/ferromagnetic trilayers

Ferromagnetic/Cu/ferromagnetic trilayers (FM/Cu/FM) have been produced by sputtering (FM = CoFeSiB or FeSiB, thickness 80–500 nm). The Cu layer, longer than the two FM layers, is connected to a strip line attached to a network analyzer (frequency interval 30 kHz–6 GHz). The variation of the characteristic impedance of the line with a static magnetic defines the giant magneto-impedance (GMI) ratio. Trilayers have been annealed in furnace, with and without a magnetic field (aimed to induce a magnetic anisotropy). The effect on GMI of the FM material composition, thickness and annealing has been studied. In general, transverse annealing improves the GMI response of the device, whereas longitudinal annealing on CoFeSiB-based trilayers does not improve the GMI response, but reduces the field value at which peak GMI effect is observed.     

Effects of thermal conversion on the metal/polymer interface of layer-by-layer PPV/DBS films

In this paper, the thermal conversion effects on the metal/polymer interface of poly(p-phenylene vinylene) (PPV) films were investigated. The substrates studied were: aluminum, indium–tin oxide, gold and glass (BK7). Layer-by-layer PPV films were processed from poly(xylylidene tetrahydrothiophenium chloride) and dodecylbenzenesulfonate with 5 and 20 layers. The thermal conversion treatment was performed at 110 °C and 230 °C. The films were investigated through emission spectra. Selectively, the emission occurred in large PPV segments and it showed a significant dependence on the film/metal interface. It was clear that this synthesis process reduced the metal degradation in the interface. The Huang–Rhys factor was estimated to demonstrate this effect.     

Molecular relaxations in radiationally crosslinked poly(vinyl methyl ether) hydrogels

Molecular relaxations in poly(vinyl methyl ether) hydrogels obtained by electron beam irradiation (range of doses: 13–50 kGy) of 10% polymer solution in water were investigated. Broadband dielectric spectroscopy (10⁻¹–10⁶ Hz) and differential scanning calorimetry results shown two relaxation processes of the polymer network origin – glass transition and polymer chains collapsing. Also detected were two relaxations connected with frozen water motions. At the lowest temperature there is a relaxation connected with the ice strongly defected by the polymer chains and at higher temperatures there is a relaxation connected with the hexagonal ice I.     

Back contacted a-Si:H/c-Si heterostructure solar cells

This paper shows how the amorphous/crystalline silicon technology can be implemented in the interdigitated back contact solar cell design. We have fabricated rear-junction, backside contact cells in which both the emitter and the back contact are formed by amorphous/crystalline silicon heterostructure, and the grid-less textured front surface is passivated by a double layer of amorphous silicon and silicon nitride, which also provides an anti-reflection coating. The entire self-aligned mask and photolithography-free process is performed at temperature below 300 °C with the aid of one metallic mask to create the interdigitated pattern. An open circuit voltage of 687 mV has been measured on a 0.5 Ωcm p-type monocrystalline silicon wafer. On the other hand, several technological aspects that limit the fill factor (50%) and the short circuit current density (32 mA/cm²) still need improvement. We show that the uniformity of the deposited amorphous silicon layers is not influenced by the mask-assisted deposition process and that the alignment is feasible. Moreover, this paper investigates the photocurrent limiting factors by one-dimensional modeling and quantum efficiency measurements.     

Molecular dynamics in glass-forming poly(phenyl methyl siloxane) as investigated by broadband thermal,dielectric and neutron spectroscopy

The molecular dynamics of glass-forming poly(methyl phenyl siloxane) (PMPS) is studied by thermal (10⁻³–5 × 10² Hz), dielectric (10⁻³–10⁹ Hz) and neutron (5 × 10⁸–10¹² Hz) spectroscopy. Because of the broad frequency range of 15 orders of magnitude the study provides a precise determination of glassy dynamics in a wide temperature range using different probes. The relaxation rates extracted from the different methods agree quantitatively in both their absolute values and in their temperature dependencies. A detailed analysis of the temperature dependence of the relaxation rate f_p by a derivative technique shows that the α-relaxation of PMPS has to be characterized by a high and a low temperature branch separated by a crossover temperature T_B = 250 K. In both temperature ranges the temperature dependence of f_p has to be described by Vogel/Fulcher/Tammann laws with different Vogel temperatures. Also the analysis of the dielectric strength in its temperature dependence gives a crossover behavior from a low to a high temperature region with a similar value of T_B. T_B can be interpreted as onset of cooperative fluctuations and the formation of dynamical heterogeneities. The dependence of the relaxation rate on the scattering vector Q extracted from neutron scattering obeys a power law τ ∼ Q^−Slope, where the power Slope varies between Slope = 2 and Slope = 3.5 with increasing temperature. This anomalous dependence of the relaxation time on the momentum transfer is discussed in terms of dynamic heterogeneities in the underlying motional processes even at temperatures above T_B. Besides the segmental dynamics the fast Methyl group rotation is considered as well. The relaxation rates of this process have an activated temperature dependence with an activation energy of 8.3 kJ/mol. The data were discussed in the framework of the threefold jump model were the incoherent elastic scattering from ‘fixed’ atoms which are frozen on the time scale of the Methyl group rotation was taken into account.     

X-ray absorption analysis of core/shell magnetic (Fe, Co)–B nanoparticles of amorphous and crystalline structure obtained by chemical reduction

Core/shell nanoparticles of M–B (M = Fe, Co) with amorphous and mixed amorphous + bcc-Fe core have been synthesized by chemical reduction of metallic salts by sodium borohydride. The XANES spectra show the existence of a surface layer consisting of metallic hydroxides in a percentage around 15 at.%. The EXAFS reveals the existence of short-range order in the amorphous core with a contribution from the M–O pairs of the shell and a variation of the coordination numbers with respect to bulk alloys. The existence of Fe-bcc in the mixed alloys is observed in the Fourier transform of the EXAFS data.     

Boron-containing poly(vinyl alcohol) as a ceramic precursor

In this paper, a preceramic precursor, prepared by condensation reaction from poly(vinyl alcohol) (PVA) and boric acid (B(OH)₃) in a B–OH: PVA-OH molar ratio of 1:1, was synthesized, as a potential precursor of lower-temperature route for ceramic material. The polymer-to-ceramic conversion was studied by thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The kinetics parameters, such as the activation energy (AE) involved in the polymer-to-ceramic conversion, were investigated by dynamic thermogravimetric analysis. The AE value for each step was correlated to the molecular architecture and degree of crosslinking of the precursor. The FTIR spectrum of the precursor showed B–O, C–H and B–O–C stretching modes. The polymer underwent a thermal degradation in three-steps, yielding 17.1 wt% of ceramic char at 980 °C. The XRD study showed the formation of B₂O₃ crystalline phase dispersed in the graphitic carbon phase, as the final product.