Multilayer planar structures prepared from chalcogenide thin films of As–Se and Ge–Se systems and polymer thin films using thermal evaporation and spin-coating techniques

We report the preparation of multilayers based on polyamide–imide polymer and As–Se or Ge–Se chalcogenide thin films. Chalcogenide films of As–Se and Ge–Se systems were deposited using a thermal evaporation method periodically alternating with spin-coated Polyamide–imide films. Fifteen layers of PAI + As–Se system and nineteen layers of PAI + Ge–Se system were coated. Optical properties of prepared multilayers have been established using UV–vis–NIR and Ellipsometric spectroscopy. Both, PAI + As–Se and PAI + Ge–Se multilayer systems, exhibited the high-reflection bands centered around 830 nm and 1350 nm, respectively. The shift of the band position of PAI + Ge–Se multilayers to lower energies was caused by higher thickness of Ge–Se films. The bandwidth of reflection band of 8 PAI + 7 As–Se multilayer was ～90 nm while bandwidth of PAI + Ge–Se system decreased to ～70 nm because Ge–Se films have 0.1 lower refractive index against As–Se films. Design of 1D-photonic crystals based on alternating chalcogenide and polymer films is a new opportunity for application of chalcogenide thin films as optical materials for near-infrared region.     

The preferential orientation of the directionally solidified Si–TaSi2 eutectic in situ composite

The Si–TaSi₂ eutectic in situ composite is a favorable field emission material due to relatively low work function, good electron conductivity, and three-dimensional array of Schottky junctions grown in the composite spontaneously. The preferential orientation during directional solidification is determined by the growth anisotropy. In order to obtain the preferential direction of the steady-state crystal growth, the transmission electron microscopy (TEM) is used for analysis. It is found that the preferential orientation of the Si-TaSi₂ eutectic in situ composite prepared by Czochralski (CZ) technique is [3 3¯ 2¯] Si∥[0 0 0 1] TaSi₂, (2 2 0)Si∥(2 2¯ 0 0) TaSi₂. Whereas the preferential orientation of the Si–TaSi₂ eutectic in situ composite prepared by electron beam floating zone melting (EBFZM) technique is [0 1¯ 1¯ ]] Si∥[0 0 0 1] TaSi₂,(0 1¯ 1) Si∥(0 1¯ 1 1)TaSi₂. The preferential directions of the Si-TaSi₂ eutectic in situ composites prepared by two kinds of crystal growth techniques are distinctly different from each other, which results from different solid–liquid interface temperatures on account of the different crystal growth conditions, e.g. different solidification rate, different temperature gradient, different solid–liquid interface curvature and different kinetic undercooling.     

Effect of P incorporation on aggregation of nanocrystallites in amorphous and nanocrystalline mixed-phase silicon thin films

We report the effects of P incorporation on the nanometer-scale structural and electrical properties of amorphous and nanocrystalline mixed-phase Si:H films. In the intrinsic and weakly P-doped (3 × 10¹⁸ at/cm³) films, the nanocrystallites aggregate to cone-shaped structures. Conductive atomic force microscopy images showed high current flows through the nanocrystalline cones and a distinct two-phase structure in the micrometer range. Adding PH₃ into the processing gas moved the amorphous/nanocrystalline transition to a higher hydrogen dilution ratio required for achieving a similar Raman crystallinity. In a heavily P-doped (2 × 10²¹ at/cm³) film, the nanocrystalline aggregation disappeared, where isolated grains of nanometer sizes were distributed throughout the amorphous matrix. The heavily doped mixed-phase film with 5–10% crystal volume fraction showed a dramatic increase in conductivity. We offer an explanation for the nanocrystalline cone formation based on atomic hydrogen enhanced surface diffusion model, and propose that the coverage of P-related radicals on the existing nanocrystalline surface during film growth and the P segregation in grain boundaries are responsible for preventing new nucleation on the surface of the existing nanocrystallites, resulting in nanocrystallites dispersed throughout the amorphous matrix.     

Stability and phase changes in thin layers of rare-earth metals/iron and other binary compounds

The structure, thermal stability and phase transformations in thin Gd–Fe films have been studied. The films were obtained by means of thermal evaporation in vacuum. At room temperature of substrates (T_s) amorphous films were formed, at T_s = 300–500 K – amorphous–crystal condensates, and at T_s > 500K – polycrystalline films. The crystal structure of condensates was determined at various temperatures and crystallization of amorphous films was found to have a heterogeneous character. During crystallization of Gd₂Fe₁₇ amorphous films, formation of two phases, viz. Gd₆Fe₂₃ and a-Fe was observed. In the films obtained at substrates temperature > 500K, the presence of three phases was established: a hexagonal phase Gd₂Fe₁₇ of the Th₂Ni₁₇-type structure, a rhombohedral phase Gd₂Fe₁₇ of the Th₂Zn₁₇-type structure and a hexagonal phase GdFe₅ of the CaCu₅-type structure. GdFe₂ films had a face-centered cubic structure, expected in bulk GdFe₂.     

Interfacial polarization in piezoelectric fibre–polymer composites

Polymer composites of an epoxy resin matrix filled with PZT fibres were studied by means of dielectric spectroscopy in the frequency range 0.1 Hz to 100 kHz and temperature interval from 80 °C to 170 °C. An interfacial or a Maxwell–Wagner–Sillars relaxation process was revealed in the frequency range between 0.1 Hz and 10 Hz and temperatures above the glass transition. This interfacial relaxation was found to follow the Debye law for the distribution of relaxation times.     

Preparation of Cu2ZnSnS4 single crystals from Sn solutions

We investigated the phase diagrams of the Cu₂ZnSnS₄ (CZTS)–Sn pseudobinary system in order to obtain knowledge useful for the growth of high-quality CZTS single crystals using a solution-based method. For Sn solutions saturated with less than ～60 mol% CZTS, the solutes are separated into two phases (CZTS phase+SnS_x phase+liquid phase). On the other hand, for solutions with more than 60 mol% CZTS, the solutes are single phase (CZTS phase+liquid phase). The CZTS single crystals were obtained from a 70 mol% CZTS solution (liquid temperature 850 °C) at 900 °C. The powder X-ray diffraction (XRD) pattern of the CZTS single crystal shows preferred orientations of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The Raman spectrum shows three peaks at 287, 338, 371 cm^?1, which corresponded to CZTS peaks. The composition of the CZTS single crystal along the growth direction is found to be slightly Cu-poor, Zn-rich and S-rich. Therefore, it is assumed that the Cu vacancy is the dominant p-type conduction mechanism.     

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).     

Superconducting properties of an NbN–SiO2 disordered system obtained by ammonolysis of NbN–SiO2 sol–gel derived coatings

Studies in superconducting properties of NbN–SiO₂ films are reported. The films were obtained through nitridation of sol–gel derived Nb₂O₅–SiO₂ coatings at 1200 °C, a process leading to the formation of disordered structures with NbN metallic grains dispersed in the insulating SiO₂ matrix. Electrical resistivity was measured with the conventional four-terminal method in the temperature range from 5 to 280 K. The samples’ superconducting properties, examined with magnetically modulated microwave absorption (MMMA), depend on the NbN/SiO₂ molar ratio and the film’s thickness.     

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.     

Structure formation in Gd–Fe thin films

The structure, thermal stability and kinetics of phase transformations were explored for films of Gd₂Fe₁₇ compounds. The films were obtained by means of thermal evaporation in vacuum. Amorphous films were found to form at room temperature of substrates, amorphous-crystal condensates at T_s = 300–500 K, and polycrystalline films at T_s > 500 K. The crystal structure of condensates was determined at various temperatures and crystallization of amorphous films was found to be heterogeneous in character. Two phases, Gd₆Fe₂₃ and a-Fe, were observed in polycrystalline films, while three phases were found to exist in the films obtained at substrate temperatures >500 K: a hexagonal Gd₂Fe₁₇ phase of the Th₂Ni₁₇ structural type, a rhombohedral Gd₂Fe₁₇ phase of the Th₂Zn₁₇ structural type and a hexagonal GdFe₅ phase of the CaCu₅ structural type.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Melting and solidification behavior of laser-crystallized silicon thin-films studied by transient conductance measurements

In this paper we report on transient conductance measurements during melting and solidification of thin silicon films on foreign substrates, which were irradiated with an excimer laser. The silicon films were deposited on borosilicate float glass or single crystal silicon wafers that were coated with different intermediate layers. Our results show that the laser fluence required to melt the entire Si layer is mainly determined by the silicon–substrate interfacial thermal resistance and not by the heat conductivity of the bulk substrate. The solidification velocity, on the other hand, is strongly influenced by the heat conductivity of the bulk substrate and reaches a maximum value of 0.95 m/s for c-Si compared to 2.19 m/s for borosilicate float glass.     

2H NMR studies of supercooled and glassy aspirin

Acetyl salicylic acid, deuterated at the methyl group, was investigated using ²H NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations, studied using stimulated-echo spectroscopy, demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal and below 20 K they are indicating that rotational tunneling plays a role. Measurements of the spin–lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group motion is significantly lower than the well-defined barrier in the crystal.     

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.     

Insights into effects of annealing on microstructure from SnO2 thin films prepared by pulsed delivery

Tin dioxide thin films were prepared by pulsed laser deposition techniques on clean glass substrates, and the thin films were then annealed for 30 min from 50 to 550 °C with a step of 50 °C, respectively. The influence of the annealing temperature on the microstructural and morphological properties of the tin dioxide thin films was investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. The experimental results showed that the amorphous microstructure almost transformed into a polycrystalline tin dioxide phase exhibiting a preferred orientation related to the (1 1 0), (1 0 1) and (2 1 1) crystal planes with increased temperatures. The thin film annealed at 200 °C demonstrated the best crystalline properties, viz. optimum growth conditions. However, the thin film annealed at 100 °C revealed the minimum average root-mean-square roughness of 20.6 nm with average grain size of 26.6 nm. These findings indicate that the annealing temperature is very important parameter to determining the thin film quality, which involves the phase formation, microstructure and preferred orientation of the thin films.     

Exploration on nano-composite fumed silica-based composite polymer electrolytes with doping of ionic liquid

Fumed silica (SiO₂)-based composite polymer electrolytes were prepared by means of solution casing technique. Horizontal attenuated total reflectance-Fourier Transform Infrared (HATR-FTIR) study shows the complexation between polymer matrix and SiO₂. The highest ionic conductivity of (4.11 ± 0.01) × 10^? 3 Scm^? 1 is achieved upon inclusion of 8 wt.% of SiO₂. Three different regions have been observed in the frequency dependence–ionic conductivity study. The conductivity rises sharply with frequency at low frequency regime. It is followed by a frequency independent plateau region and sharp decrease in the conductivity at high frequency range. The dielectric permittivity (ε^') and dielectric loss (ε^") are decreased with increasing the frequency. This phenomenon is mainly attributed to the electrode polarization effect. The formation of electrical double layer has been proven in these dielectric permittivity studies. This indicates the non-Debye properties of the nano-composite polymer electrolytes.     

Solid–liquid interface energies in the succinonitrile and succinonitrile–carbon tetrabromide eutectic system

The equilibrated grain boundary groove shapes for the commercial purity succinonitrile (SCN) and succinonitrile–carbon tetrabromide (CTB) eutectic system were directly observed. From the observed grain boundary groove shapes, the Gibbs–Thomson coefficients for the solid SCN–liquid SCN and solid SCN–liquid SCN CTB have been determined to be (5.43±0.27)×10⁻⁸ Km and (5.56±0.28)×10⁻⁸ Km, respectively, with numerical method. The solid–liquid interface energies for the solid SCN–liquid SCN and solid SCN–liquid SCN CTB have been obtained to be (7.86±0.79)×10⁻³ J m⁻² and (8.80±0.88)×10⁻³ J m⁻², respectively from the Gibbs–Thomson equation. The grain boundary energies in the SCN and SCN rich phase of the SCN–CTB system have been calculated to be (15.03±1.95)×10⁻³ J m⁻² and (16.51±2.15)×10⁻³ J m⁻², respectively, from the observed grain boundary groove shapes. The thermal conductivity ratios of the liquid phase to the solid phase for SCN and SCN–4 mol% CTB alloy have also been measured.     

Surface morphology of spin-coated As–S–Se chalcogenide thin films

Surface morphology and roughness of amorphous spin-coated As–S–Se chalcogenide thin films were determined using atomic force microscopy. Prepared films were coated from butylamine solutions with thicknesses d ∼ 100 nm and then annealed in a vacuum furnace at 45 °C and 90 °C for 1 h for their stabilization. The root mean square surface roughness analysis of surfaces of as-deposited spin-coated As–S–Se films indicated a very smooth film surface (with R_q values 0.42–0.45 ± 0.2 nm depending on composition). The nanoscale images of as-deposited films confirmed that surface of the films is created by domains with dimensions 20–40 nm, which corresponds to diameters of clusters found in solutions. The domain character of film surfaces gradually disappeared with increasing annealing temperature while the solvent was removed from the films. Middle-infrared transmission spectra recorded a decrease of intensities of vibration bands connected to N–H (at 3367 and 3292 cm⁻¹) and C–H (at 2965, 2935 and 2880 cm⁻¹) stretching vibrations. Temperature regions of solvent evaporation T = 60–90 °C and glass transformation temperatures T_g = 135–150 °C of spin-coated As–S–Se thin films were determined using a modulated differential scanning calorimetry.     

Polyaniline nanofiber–silica composite aerogels

Strong, electrically conducting aerogels were prepared by introducing polyaniline nanofibers to a silica sol just prior to gelation and drying through supercritical carbon dioxide processing. The addition of a few milligrams of polyaniline per cm³ increased the flexural strength of the cylindrical monoliths by 200%. Using preformed polymeric nanofibers avoided filling of microporosity often observed with polymer reinforcement of aerogels and allowed preparation of polyaniline–silica composite aerogels with surface areas over 900 m²/g. Despite the small amount of polyaniline nanofibers (1.3–16.5 wt.%), the composite aerogels were electrically conducting (8.0 × 10^? 8–1.83 × 10^? 5 S/cm) and it was possible to prepare chemiresistor sensors for detection of acidic (HCl) and basic (ammonia) gaseous molecules with response times similar to thin film sensors containing orders of magnitude more polyaniline.     

Structure of bioactive mixed polymer/colloid aerogels

The structure of polymer/colloid mixed silica sono-aerogels has been studied by SAXS, N₂ adsorption–desorption and Hg porosimetry. The system is described as a composite in which the polymeric phase (sonogel) is the matrix. The structure of this phase prepared with ultrasounds is very fine consisting in aggregates of ∼5 nm radius formed by elementary particles of 1 nm radius. Including Ca(II) into the silica atomic network causes enlarging the average size of the particle more than three times with respect to its size in its pure silica counterpart. The stiffness increases as well by 50%. The size of the particles and pores is also affected by the ultrasound dose applied; the higher for the larger the particles. On the other hand, a low dose produces a rough particle surface.