Discovery of the bi-layered structure in spin-coated polyacrylamide films on the gold surface

The reflection-absorption Fourier transform infrared spectroscopy (RA-FTIR) with the polarized lights was used to characterize polyacrylamide (PAL) films spin-coated on the gold (Au) surfaces. The PAL films spin-coated on the Au surface show the characteristics of a bi-layered structure. The first layer attached to the Au surface with a thickness of around 70 nm shows no chain orientation due to the dewetting effect of the Au surface. The upper layer is composed of the PAL molecules oriented parallel to the Au surface as the film thickness increases, which is believed to be due to the spin-coating effect. The bottom and the top layers are optically isotropic and anisotropic, respectively, as confirmed by the analysis of the RA-FTIR spectra. The thickness-threshold is of the order of the end-to-end distance (R_ee, 67 nm) of PAL molecules (M_w ∼ 10⁶) indicating the dewetting distance through which the Au surface functions.     

The spatial uniformity and electromechanical stability of transparent, conductive films of single walled nanotubes

We have prepared thin films of arc discharge single walled nanotubes by vacuum filtration. For film thicknesses greater than 40 nm, the films are of high optical quality; the optical transmission varies by <2% over the film area when measured with a spatial resolution of 4 μm. However, the films become spatially non-uniform for film thickness below 40 nm. The in-plane DC conductivity correlates with the uniformity, increasing from ∼3800 S/m for a 10 nm thick film to ∼2-2.5 × 10⁵ S/m for films of thickness >40 nm. Conductive atomic force microscopy maps show reasonably uniform current flow out of the plane of the film. For all thicknesses, the optical transmittance scales with film thickness as expected for a thin conducting film with optical conductivity of 1.7 × 10⁴ S/m (λ = 550 nm). For films with t > 40 nm the ratio of DC to optical conductivity was σ_DC/σ_Op = 13.0, leading to values of transmittance and sheet resistance such as T = 80% and R_s = 110 Ω/□ for the t = 40 nm film. Electromechanically, these films were very stable showing conductivity changes of <5% and <2% when cycled over 2000 times in compression and tension respectively.     

Kinetics of pressure-induced phase separation in polystyrene + acetone solutions at high pressures

The kinetics of pressure-induced phase separation in solutions of polystyrene (M_w = 129,200; PDI = 1.02) in acetone has been studied using time- and angle-resolved light scattering. A series of controlled pressure quench experiments with different quench depths were conducted at different polymer concentrations (4.0%, 5.0%, 8.2% and 11.4% by mass) to determine the binodal and spinodal boundaries and consequently the polymer critical concentration. The results show that the solution with a polymer concentration 11.4 wt% undergoes phase separation by spinodal decomposition mechanism for both the shallow and deep quenches as characterized by a maximum in the angular distribution of the scattered light intensity profiles. Phase separation in solutions at lower polymer concentrations (4.0, 5.0 and 8.2 wt%) proceeds by nucleation and growth mechanism for shallow quenches, but by spinodal decomposition for deeper quenches. These results have been used to map-out the metastable gap and identify the critical polymer concentration where the spinodal and binodal envelops merge.The time scale of new phase formation and growth as (accessed) from the time evolution of scattered light intensities is observed to be relatively short. The late stage of phase separation is entered within seconds after a pressure quench is applied. For the systems undergoing spinodal decomposition, the characteristic wave number qm corresponding to the scattered light intensity maximum Im was analyzed by power-law scaling according to qm∼t−α and Im∼tβ. The results show β≈2α. The domain size is observed to grow from 4 μm to 10 μm within 2 s for critical quench, but about 9 s for off-critical quenches. The domain growth displays elements of self-similarity.     

EIS analysis on low temperature fabrication of TiO2 porous films for dye-sensitized solar cells

A low temperature (<150 °C) fabrication method for preparation of TiO₂ porous films with high efficiency in dye-sensitized solar cells (DSSCs) has been developed. The Ti(IV) tetraisopropoxide (TTIP) was added to the paste of TiO₂ nanoparticles to interconnect the TiO₂ particles. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) and electron lifetime in the TiO₂ film (τ_e) under different molar ratios of TTIP/TiO₂ and also at various TiO₂ thicknesses. It was found that the R_ct2 decreased as the molar ratio increased from 0.02 to 0.08, however, it increased at a molar ratio of 0.2 due to the reduction in surface area for dye adsorption. In addition, the characteristic frequency peak shifted to lower frequency at a molar ratio of 0.08, indicating the longer electron lifetime. As for the thickness effect, TiO₂ film with a thickness around 17 μm achieved the best cell efficiency. EIS study also confirmed that, under illumination, the smallest R_ct2 was associated with a TiO₂ thickness of 17 μm, with the R_ct2 increased as the thickness of TiO₂ film increased. In the Bode plots, the characteristic frequency peaks shifted to higher frequency when the thickness of TiO₂ increased from 17.2 to 48.2 μm, indicating the electron recombination increases as the thickness of the TiO₂ electrode increases.Finally, to make better use of longer wavelength light, 30 wt% of larger TiO₂ particle (300 nm) was mixed with P25 TiO₂ as light scattering particles. It effectively increased the short-circuit current density and cell conversion efficiency from 7.44 to 8.80 mA cm⁻² and 3.75 to 4.20%, respectively.     

Investigation of the initial growth of ultrananocrystalline diamond films by multiwavelength Raman spectroscopy

The initial growth phase of ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) has been investigated by scanning electron microscopy, atomic force microscopy and especially Raman spectroscopy. As due to resonance effects Raman spectra of carbon materials strongly depend on the excitation wavelength, a multiwavelength analysis has been performed with λ_exc ranging from the UV region (325 nm) over the visible range (488 and 514 nm) to the IR region (785 nm). In addition, a set of measurements has been performed with a confocal Raman microscope, i.e. depth resolved, with a wavelength of 532 nm. The samples investigated were deposited with constant parameters, the deposition time being the only parameter varied, resulting in film thicknesses from 100 to 500 nm. It turned out that the diamond fraction and also the grain boundary material do not vary during that stage whereas there are slight but distinct changes of the nature of the amorphous matrix which reflect, among others, in a shift of the graphite-related G band to higher wavenumbers and in an increase of the ratio of D and G bands with increasing film thickness. These changes are discussed in terms of the above mentioned resonance effects; the major changes are a transition of hydrogen containing sp² chains to hydrogen-free condensed sp² rings when the material is no longer in the surface region of the films but becomes incorporated within the film bulk.     

Morphology development in photopolymerization-induced phase separated mixtures of UV-curable thiol-ene adhesive and low molecular weight solvents

The influence of photopolymerization rate, solvent quality, and processing parameters on the photopolymerization-induced phase separated morphology of mixtures of thiol-ene based optical adhesive with mixed solvents of diglyme and water or acetone and isopropanol is described. Upon exposure to UV radiation (∼50 mW/cm², 365 nm) for periods of 10-90 s, homogeneous solutions of 5-10 wt% NOA65 and NOA81 adhesive formed phase separated structures with characteristic sizes ranging from 400 nm to 10 μm, with increased photopolymerization rates leading to smaller feature sizes. In the systems containing diglyme and water, morphologies formed by phase separation at a lower degree of photopolymerization were characteristic of spinodal decomposition, while morphologies formed by phase separation at a higher degree of photopolymerization exhibited characteristics of viscoelastic phase separation. In the systems containing acetone and isopropanol, interactions between evaporation and photopolymerization-induced phase separation led to the development of more complicated morphologies, including three-dimensional sparse networks. These morphologies provide a combination of connectivity and low overall volume fraction that can significantly enhance the performance of many multi-functional structures.     

Physical aging of thin glassy polymer films monitored by gas permeability

The physical aging at 35 °C of three glassy polymers, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide), has been tracked by measurement of the permeation of three gases, O₂, N₂, and CH₄, for over 200 days. Several techniques were used to accurately determine the thickness of films (∼400 nm-62 μm) in order to obtain absolute permeability coefficients and to study the effects of film thickness on the rate of physical aging. Each film was heated above the polymer T_g to set the aging clock to time zero; ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial pronounced aging response, attributed to a decrease in polymer free volume, was observed at temperatures more than 150 °C below T_g for thin films of each polymer compared to what is observed for the bulk polymers. The films with thicknesses of approximately 400 nm of the three polymers exhibit an oxygen permeability decrease by as much as two-fold or more and about 14-15% increase in O₂/N₂ selectivity at an aging time of 1000 h. The results obtained in this study were compared with prior work on thickness dependent aging. The effects of crystallinity on physical aging were examined briefly.     

Low temperature preparation and characterization of LSGMC based IT-SOFC cell by aerosol deposition

A low temperature (≤650 °C) process for fabricating nano-structured (La, Sr) (Ga, Mg, Co)O_3−δ (LSGMC) electrolyte/(La, Sr) (Co, Fe)O_3−δ-(Gd, Ce)O_2−δ (LSCF-GDC) cathode films, ∼7 and ∼25 μm in thickness, respectively, was developed using an aerosol deposition (AD) process for use in intermediate temperature solid oxide fuel cells (IT-SOFCs). NiO-GDC was used as an anode substrate in anode-supported type cells. The deposited LSGMC electrolyte and LSCF-GDC composite cathode film maintained good adhesion with the NiO-GDC anode, even though the coating processes were completed at the room temperature. The LSGMC electrolyte was composed of nano-sized grains smaller than 30 nm. The electrical conductivity of the LSGMC electrolyte fabricated at room-temperature was ∼30 mS/cm at 650 °C. The LSCF-GDC composite cathode showed ≥20% porosity with a grain size ≤100 nm. The peak power density of the LSGMC electrolyte-based, anode-supported-type cell with the nano-structured LSCF-GDC cathode produced at room-temperature by AD was 0.39 W/cm² at 650 °C.     

Nonlinear optical transmission of silk/green fluorescent protein (GFP) films

In this initial work, we demonstrate a technique for preparing thin (10-20 μm) films of silk doped with green fluorescent protein (GFP) by casting/annealing at 20 °C and describe the resulting film characteristics. Notably, the GFP molecules maintain their nonlinear optical properties as evidenced by two-photon fluorescence microscopy and two-photon absorption measurements using near-infrared femtosecond pulses. The fractional transmission of focused near-infrared pulses of 775 nm wavelength, 140 fs pulsewidth was observed to decrease as the incident pulse energy is increased and/or the incident spot size is decreased, indicating that nonlinear absorption is taking place. Visible damage from the pulses is observed in a ∼10 μm film at the highest peak incident fluences, which were in the range of 0.1-0.2 J/cm². Variations in thickness, morphology and GFP concentration of the films make precise specification of the two-photon absorption coefficient difficult. Since these films have potential applications in photonics, we suggest techniques for improving these properties in future generations of films. The suggestions present opportunities for future work.     

On some properties of PZT-NZF composite films manufactured by hybrid synthesis route

Composite magnetoelectric films using ferroelectric lead zirconate titanate (PZT) and ferromagnetic nickel zinc ferrite (NZF) were prepared using the combination of sol-gel and hydrothermal process on Pt/Ti/SiO₂/Si substrates. The thickness was estimated ∼2 μm using cross-sectional SEM. Structure, morphology and electro-magnetic characterization were assessed using XRD, XPS, SEM, dielectric, leakage current, ferroelectric, and magnetic property analyze. The composite films exhibit coexistence of ferroelectric and ferromagnetic ordering at room temperature with a remnant polarization (P_r), and coercive field (E_c) of 1.2 μC/cm² and 7.8 kV/cm, respectively, and saturation magnetization (M_s) ∼20 emu/cm³. Polarization improved ∼5.2% upon poling the composite film using a magnetic field of 1 T.     

Formation of porous anodic titanium oxide films in hot phosphate/glycerol electrolyte

The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide film of 12 μm thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5 V for 3.6 ks in 0.6 mol dm⁻³ K₂HPO₄ + 0.2 mol dm⁻³ K₃PO₄/glycerol electrolyte containing only 0.04% water at 433 K. The growth efficiency is reduced by increasing the formation voltage to 20 V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20 V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10 V, during the initial anodizing at a constant current density of 250 A m⁻². The pore diameter is not significantly dependent upon the formation voltage, being ∼10 nm.     

Synthesis of nitrogen-doped horn-shaped carbon nanotubes by reduction of pentachloropyridine with metallic sodium

Nitrogen-doped horn-shaped carbon nanotubes (CNTs) have successfully been prepared by reducing pentachloropyridine with metallic sodium at 350 °C. A typical CNT has an open-end diameter of ∼2 μm, a close-end diameter of ∼0.3 μm, a wall thickness of ∼30 nm, and a length up to 8 μm. TEM observation indicates that the CNTs account for ∼30% of the products, and the rest is solid and hollow carbon nanospheres (CNSs) with a diameter of about 50-290 nm. Elemental analysis shows that the N/C atomic ratio of the carbon nanostructures is about 0.0208. XRD and HRTEM measurements reveal that the CNTs are amorphous. To understand the growth process and refine the growth condition, various control experiments have been finished. At last, a sodium-catalysis-reduction solid-liquid-solid growth mechanism of the CNTs has been suggested on the basis of the experiments.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.     

Electrochemical characterization of a new binary oxide of Mo with Co for O2 evolution in alkaline solution

The α-CoMoO₄ oxide has been obtained by a precipitation method and investigated for the first time for electrocatalysis of the oxygen evolution reaction (oer) in alkaline medium. This method produced the pure crystalline CoMoO4 monoclinic phase with crystallite size ∼46 nm and lattice constants: a = 9.666 Å, b = 8.854 Å, c = 7.755 Å and β = 113.82°. The average particle size (based on area density) and the BET surface area of powders of the oxide were 11.58 μm and 9.4 m² g⁻¹, respectively. Results show that the new oxide is quite active for the oer. Values of the Tafel slope and the reaction order with respect to OH⁻ concentration are observed to be ∼60 mV and ∼1, respectively.     

A kinetic study of electrochemical lithium insertion into oriented V2O5 thin films prepared by rf sputtering

The kinetics of the electrochemical lithium insertion reaction in crystalline V₂O₅ thin films in liquid electrolyte has been investigated using ac impedance spectroscopy. The experimental data are obtained for sputtered films characterized by a common morphology corresponding to an arrangement of V₂O₅ platelets perpendicular to the substrate (h 0 0 or 1 1 0 preferred orientation). The results are discussed as a function of the Li content for 0 < x < 1 in Li_xV₂O₅, the film thickness in the range of 0.6-3.6 μm and temperature 15-55 °C. The moderate evolution of the chemical diffusion coefficient D vs. the lithium content is related with the specific structural response of these pure thin film materials which exhibit a single phase behavior. A comparison of the kinetic parameters for different thickness values allows to indicate the same Li diffusion rate whatever the film thickness and the diffusion pathway does not correspond to the thickness but to the length of the edge (≈1 μm) of V₂O₅ platelets. For the first time, an experimental evaluation of the activation energy for Li diffusion in crystalline V₂O₅ is obtained. A value of 0.98 eV is found for a diffusion phenomenon along the b direction. This work demonstrates the excellent capacity-rate performance as well as the efficient and homogeneous behavior of these oriented films can be explained by their specific microstructure.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

Effect of decreasing film thickness on the electrochemical responses of cations adsorbed in clay-modified electrodes

Clay-modified electrodes ranging in thickness from 3.4 μm to 8 nm, as estimated from the clay loadings, were prepared using three different smectites by spin-coating, solvent evaporation or electrophoretic deposition. For all three clays, the voltammetic waves obtained for [Ru(bpy)₃]²⁺ or [Os(bpy)₃]²⁺ adsorbed in these CMEs were independent of the film thickness for all films thicker than 100 nm. Only in very thin films, ≤40 nm were significant decreases in the peak currents observed. However, when the contributions to the peak currents from the electroactive concentrations, C* and effective diffusion coefficients, D_eff were separated, the values of C* were found to increase with decreasing film thickness, while D_eff decreased by several orders of magnitude. This was attributed to increase contributions to the electrochemical responses from less mobile electrostatically bound cations in the thinner films. Similar variations in C* and D_eff were obtained in films prepared by solvent evaporation. However, C* obtained in 20 nm thick electrodeposited films were significantly lower than in 40 nm spin-coated films. For [Ru(NH₃)₆]³⁺, the peak currents increased rapidly with the film thickness. However, no significant changes in the values of C* and D_eff with film thickness were found for this ion. This is consistent with the greater mobility of [Ru(NH₃)₆]³⁺ in clays films that allows a larger fraction of the adsorbed ions to remain electroactive even in thicker films. Results obtained for [Fe(bpy)₃]²⁺ were intermediate. While, the peak currents were independent of film thickness, the values of C* or D_eff obtained for this ion were also independent of the clay loadings.     

Enhanced visible light photocurrent generation at surface-modified TiO2 nanotubes

Photoelectrodes consisting of TiO₂ nanotube layers with different thicknesses (0.5 μm, 1.7 μm, 3 μm, 6 μm, 9 μm, and 18 μm) were prepared by anodization of titanium substrates and subsequent surface modification by a heat treatment at 400 °C in the presence of urea pyrolysis products. In contrast to unmodified TiO₂ nanotubes, the modified photoelectrodes exhibit photocurrents under visible light irradiation down to 750 nm. Photocurrent transients indicate enhanced recombination unless a suitable hole-scavenger, like iodide, is present since the photogenerated holes do not oxidize water efficiently. In the visible light the photoconversion efficiency increases significantly with nanotube length. The maximum incident photon-to-current efficiency (IPCE) was observed for tubes with the length of 6-9 μm (IPCE ∼4.5% and 1.4% at 450 nm and 550 nm, respectively) and the photocurrent enhancement with increasing tube length is found to be stronger at longer irradiations wavelengths.     

Synthesis and characterization of carbon fibrils formed by stacking graphite sheets of nanometer thickness

Carbon fibrils with length of 2-3 μm formed by stacking graphite sheets of ∼10-40 nm in thickness along the 〈0 0 1〉 direction were prepared by pyrolysis of tetrahydrofuran at 600 °C in the existence of metallic nickel. The spacing of adjacent sheets concentrated on ∼3.8 nm and ∼50 nm. Metallic nickel played a catalytic role in the formation of carbon fibrils, and a possible formation process was proposed.     

Near-infrared photorefractive polymer composites based on carbon nanotubes

A photorefractive effect at the wavelength of 1064 nm is demonstrated for a composite consisting of an aromatic polyimide and carbon single wall nanotubes. The two-beam gain coupling coefficient and the net gain coefficient are equal to 90 and 65 cm⁻¹, respectively, at 80 V/μm for a nanocomposite containing 0.25 wt% crude nanotube material. The refractive index modulation measured at E₀ = 50 V/μm is close to Δn = 0.004.