Characteristics of electrohydrodynamically prepared titanium dioxide films

A titanium dioxide precursor sol flowing through a needle at a flow rate of 10^-10 m³ s^-1 was subjected to an electric field of 4.5 kV to generate droplets in the size range 0.3–6 μm. The droplets were collected on a silicon substrate to form uniformly thick, dense films. Raman spectroscopy, X-ray diffraction, field emission scanning electron microscopy and UV/Vis spectroscopy were used to characterize as-deposited and annealed films. Raman spectra show the annealed films were anatase phase with annealing converting it to the rutile phase. The energy bandgap of the titanium dioxide film annealed to 500 °C shows an indirect bandgap energy of 3.50 eV and a direct bandgap energy of 3.95 eV. PACS 81.15.Rs; 81.07.-b; 78.20.-e; 78.30.-j; 78.67.-n; 78.70.ck     

Electrical and optical properties of reactive dc magnetron sputtered silver-doped indium oxide thin films: role of oxygen

Silver-doped indium oxide thin films have been prepared on glass and quartz substrates at room temperature (300 K) by a reactive dc magnetron sputtering technique using an alloy target of pure indium and silver (80:20 at. %). During sputtering, the oxygen flow rates are varied in the range 0.00–2.86 sccm keeping the magnetron power constant at 40 W. The resistivity of these films is in the range 10⁰–10^-3 Ω cm and they show a negative temperature coefficient of resistivity. The films exhibit p-type conductivity at an oxygen flow rate of 1.71 sccm. The work function of these silver–indium oxide films has been measured by a Kelvin probe technique. The refractive index of the films (at 632.8 nm) varies in the range 1.13–1.20. Silver doping in indium oxide narrows the band gap of indium oxide (3.75 eV). PACS 73.30.+y; 81.15.Cd; 78.20.Ci; 73.61.Le     

Fabrication and characterization of fluorine-doped thin oxide thin films and nanorod arrays via spray pyrolysis

This paper reports the synthesis and characterization of fluorine-doped tin oxide (FTO) thin films via intermittent spray pyrolysis utilizing a solution mixture of tin chloride pentahydrate and ammonia fluoride. Utilizing the same solution, nanorod arrays were fabricated via template-based growth. Uniform and crack-free FTO films over 20×20 mm with a thickness up to 900 nm have been routinely achieved; such FTO films demonstrate electrical resistivity as low as 2.2×10^-4 Ω cm as well as good optical transparency ranging from 75 to 85%. In addition, FTO nanorods were fabricated using template-filling methods at a temperature of 440 °C. The nanorods have a diameter of ∼160–250 nm, appear to be comprised of small nanoparticles 5–10 nm in size, and have a resistivity value of 4×10^-1 Ω cm. PACS 81.07.-b; 73.61.-r; 81.16.Be     

Studies on PEO-based sodium ion conducting composite polymer films

A sodium ion conducting composite polymer electrolyte (CPE) prepared by solution-caste technique by dispersion of an electrochemically inert ceramic filler (SnO₂) in the PEO–salt complex matrix is reported. The effect of filler concentration on morphological, electrical, electrochemical, and mechanical stability of the CPE films has been investigated and analyzed. Composite nature of the films has been confirmed from X-ray diffraction and scanning electron microscopy patterns. Room temperature d.c. conductivity observed as a function of filler concentration indicates an enhancement (maximum) at 1–2 wt% filler concentration followed by another maximum at ∼10 wt% SnO₂. This two-maxima feature of electrical conductivity as a function of filler concentration remains unaltered in the CPE films even at 100 °C (i.e., after crystalline melting), suggesting an active role of the filler particles in governing electrical transport. Substantial enhancement in the voltage stability and mechanical properties of the CPE films has been noticed on filler dispersion. The composite polymer films have been observed to be predominantly ionic in nature with t _ion ∼ 0.99 for 1–2 wt% SnO₂. However, this value gets lowered on increasing addition of SnO₂ with t _ion ∼ 0.90 for 25 wt% SnO₂. A calculation of ionic and electronic conductivity for 25 wt% of SnO₂ film works out to be ∼2.34 × 10⁻⁶ and 2.6 × 10⁻⁷ S/cm, respectively.     

Nickel oxide thin films synthesized by reactive pulsed laser deposition: characterization and application to hydrogen sensing

Transparent nickel oxide thin films were grown by reactive pulsed laser deposition. An ArF* (λ=193 nm, τ=12 ns) excimer laser source was used to ablate the Ni targets in a controlled pressure of ambient oxygen. The substrates were either kept at room temperature or heated to a selected temperature within the 200–400 °C range. Post-deposition heat treatment, which was applied to further promote crystallization and overcome any oxygen deficiency, yielded transparent thin films. The surface morphology and crystalline status of the synthesized thin structures were analyzed in correlation with their optical properties. A significant response to several concentrations of hydrogen was demonstrated when heating the nickel oxide films at 185 °C. PACS 78.66.Hf; 81.15.Fg; 82.47.Rs     

Fluorescence studies of confinement in polymer films and nanocomposites: Glass transition temperature, plasticizer effects, and sensitivity to stress relaxation and local polarity

Confinement effects in polystyrene and poly(methyl methacrylate) films and nanocomposites are studied by fluorescence. The ability to employ an intensive measurable, the excited-state fluorescence lifetime, in defining the glass transition temperature, T_g, of polymers is demonstrated and compared to the use of an extensive measurable, fluorescence intensity. In addition, intrinsic fluorescence from the phenyl groups in polystyrene is used to determine the T_g-confinement effect in films as thin as ~15 nm. The decrease in T_g with decreasing film thickness (below ∼60 nm) agrees well with results obtained by extrinsic pyrene fluorescence. Dye label fluorescence is used to quantify the enhancement in T_g observed with decreasing thickness (below ~90 nm) in poly(methyl methacrylate) films; addition of 2–4 wt% dioctyl phthalate plasticizer reduces or eliminates the T_g-confinement effect in films down to 20 nm thickness. Intrinsic polystyrene fluorescence, which is sensitive to local conformation, is used to quantify the time scales (some tens of minutes) associated with stress relaxation in thin and ultrathin spin-coated films at T_g + 10 K. Finally, the shape of the fluorescence spectrum of pyrene doped at trace levels in polystyrene films and polystyrene-silica nanocomposites is used to determine effects of confinement on microenvironment polarity.     

Patterning of PLZT and PSZT thin films by excimer laser

The patterning of lanthanum-doped lead zirconate titanate (PLZT) and strontium-doped lead zirconate titanate (PSZT) thin films has been examined using a 5-ns pulsed excimer laser. Both types of film were deposited by rf magnetron sputtering with in situ heating and a controlled cooling rate in order to obtain the perovskite-structured films. The depth of laser ablation in both PSZT and PLZT films showed a logarithmic dependence on fluence. The ablation rate of PLZT films was slightly higher than that of PSZT films over the range of fluence (10–150 J/cm²) and increased linearly with number of pulses. The threshold fluence required to initiate ablation was ∼ 1.25 J/cm² for PLZT and ∼ 1.87 J/cm² for PSZT films. Individual squares were patterned with areas ranging from 10×10 μm² up to 30×30 μm² using single and multiple pulses. The morphology of the etched surfaces comprised globules which had diameters of 200–250 nm in PLZT and 1400 nm in PSZT films. The diameter of the globules has been shown to increase with fluence until reaching an approximately constant size at ≤ 20 J/cm² in both types of film. The composition of the films following ablation has been compared using X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. PACS 79.20.Ds; 82.80.Pv; 82.80.Ej     

Laser nano-fabrication of large-area plasmonic structures and surface plasmon resonance tuning by thermal effect

A simple and flexible technique aimed to generate large-area periodic nano-dot array features on metal thin films by laser interference lithography (LIL) has been demonstrated. In this paper, gold nano-dot arrays with a period of ∼450 nm and a dot diameter of ∼100 nm on quartz substrates coated with a gold film of 50 nm thick were fabricated. Multiple enhanced transmission peaks were observed in this patterned film. In addition to the characteristic peak of the gold surface plasmon resonance around 500 nm, multiple shoulder peaks that range from 550 to 700 nm were also observed in the nano-chain array structures. These shoulder peaks disappeared after thermal annealing. It was found that the nano-dots became smaller and well-separated nano-balls under the high temperature annealing process. These nano-structures have potential applications in solar cell, nano-lithography and biosensing.     

Crystal structure of nanostructured PbS films at temperatures of 293–423 K

The crystal structure of lead sulfide films prepared by the hydrochemical deposition has been studied by X-ray diffraction. The thickness of the films synthesized is ∼100 nm, the size of coherent scattering regions is ∼70 nm, and the value of microstrains is ∼0.20%. It is established, for the first time, that the as-synthesized PbS films and the same films annealed in the temperature range 293–423 K have a cubic crystal structure (space group Fm3m) different from the B1-type structure. In the crystal lattice of the structure revealed, sulfur atoms are located not only in the 4(b) positions but also in the 8(c) positions. The occupancies of the 4(b) and 8(c) positions by the S atoms are ∼0.84 and 0.08, respectively.     

Determination of the optimal parameters for the fabrication of ZnO thin films prepared by spray pyrolysis method

In this work, ZnO thin films have been prepared by spray pyrolysis deposition method on the glass substrates. The effect of deposition parameters, such as deposition rate, substrate temperature and solution volume has been studied by X-ray diffraction (XRD) method, UV–Vis–NIR spectroscopy, scanning electron microscopy (SEM), and electrical measurements. The XRD patterns indicate polycrystalline wurtzite structure with preferred direction along (0 0 2) planes. Thin films have transparency around 90% in the visible range. The optical band gap was determined at 3.27 eV which did not change significantly. Evolution of electrical results containing the carriers’ density, sheet resistance and resistivity are in agreement with structural results. All the results suggest the best deposition parameters are: deposition rate, R = 3 ml/min, substrate temperature, T _s = 450°C and thickness of the thin films t = 110–130 nm.     

Stress, microstructure and mechanical properties of graded multilayer tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) films deposited using a filtered cathodic vacuum arc (FCVA) system, have high intrinsic stress which limits their application as protective coatings. To reduce the film stress and to improve the adhesion, a multilayer structure is deposited at a gradient substrate negative bias from 1500 V to 80 V. This paper investigates the stress, microstructure and nano-mechanical properties of graded multilayer ta-C film on Si substrates. Compared with that of single-layer films deposited at optimised bias, the graded multilayer film has low stress without a decline in hardness and Young’s modulus. Microstructural evaluation of the multilayer film using visible Raman spectra shows that the average content of the sp³ bonds of the multilayer film remain at a high level. Nanoscratch testing illustrates favorable scratch resistance and good adhesion of the multilayer film. Scanning electron microscope (SEM) observation confirms the collapse of the film surface along the scratching trace. Finally, deposition on single crystal germanium substrates of a durable coating ∼ 1100 nm thick, and composed of three graded multilayer films is demonstrated. PACS 81.05.Uw; 81.15.Jj; 68.65.Ac; 68.55.Nq; 68.60.Bs     

Effect of plasticizer on structural and electrical properties of nanocomposite solid polymer electrolytes

The solid polymer electrolyte films based on polyethylene oxide, NaClO₄ with dodecyl amine modified montmorillonite as filler, and polyethylene glycol as plasticizer were prepared by a tape casting method. The effect of plasticization on structural, microstructural, and electrical properties of the materials has been investigated. A systematic change in the structural and microstructural properties of plasticized polymer nanocomposite electrolytes (PPNCEs) on addition of plasticizer was observed in our X-ray diffraction pattern and scanning electron microscopy micrographs. Complex impedance analysis technique was used to calculate the electrical properties of the nanocomposites. Addition of plasticizer has resulted in the lowering of the glass transition temperature, effective dissociation of the salt, and enhancement in the electrical conductivity. The maximum value of conductivity obtained was ∼4.4 × 10⁻⁶ S cm⁻¹ (on addition of ∼20% plasticizer), which is an order of magnitude higher than that of pure polymer nanocomposite electrolyte films (2.82 × 10⁻⁷ S cm⁻¹). The enhancement in conductivity on plasticization was well correlated with the change in other physical properties.     

Study of ZnO and Ni-doped ZnO synthesized by atom beam sputtering technique

Zinc oxide (ZnO) and Ni-doped zinc oxide (ZnO:Ni) films are prepared by atom beam sputtering with an intent of growing transparent conducting oxide (TCO) material and understanding its physical properties. The crystalline phases of the films are identified by the grazing angle X-ray diffraction (GAXRD) technique. Thicknesses of the films are measured by ellipsometry. Chemical states of the elements present in the films are investigated by X-ray photoelectron spectroscopy (XPS), which indicates the presence of Ni in the ZnO environment in a divalent state. Average transmission across the ZnO:Ni film was determined to be ∼83% in the visible region, which is less than that (∼90%) of undoped ZnO films. The resistivity measured by van der Pauw technique of the ZnO:Ni film (∼9×10^-3 Ω cm) is two orders of magnitude smaller as compared to its undoped counterpart (1 Ω cm). For ZnO:Ni film an average carrier concentration of ∼1.4×10¹⁹ cm^-3 was observed by Hall measurements. Two important mechanisms reported in the literature viz. influence of d–d transition bands and electron scattering from crystallites/grains are discussed as the possible causes for the increase in conductivity on Ni doping in ZnO. PACS 73.50.Bk; 78.66.Li; 79.60.Dp; 61.05.cp     

Comparative study of ZnO thin films deposited by various methods for use as sensors

Good and adhesive semiconducting films of ZnO (∼ 100–1100 nm) were deposited over planar borosilicate glass by spray pyrolysis and dip & dry method. The films were characterized by X-ray diffraction and optical absorption measurements. The band gap of these films were found to be 3.21 eV and the films were randomly oriented having average crystallite sizes of 20 to 25 nm. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

High spectral resolution analysis of tunable narrowband resonant grating waveguide structures

A high spectral resolution analysis of narrowband reflection filters based on resonant grating waveguide structures is presented. A tunable high-performance dye laser with ∼ 0.15 cm^-1 line width and a beam analyzing system consisting of three simultaneously controlled CCD cameras were used to investigate grating waveguide resonances at wavelengths in the 694 nm and 633 nm ranges. A reflectivity of ∼ 91% and a line width of ∼ 0.55 nm were measured and theoretically modeled for a resonant reflection filter specifically designed for the ruby laser wavelength 694.2 nm. For a second grating waveguide structure, designed for the helium-neon laser emission wavelength 632.8 nm, we observed a thermal shift of its spectral resonance position of several nanometers, when increasing the sample temperature by some degrees. An inverse thermal shift was observed when the structure was subsequently cooled down to room temperature. Our results suggest implementation of grating waveguide devices combining a narrow line width with a tunability of the resonant response into innovative concepts for reflection filter and sensor applications. PACS 42.62.-b; 42.79.Dj; 42.79.Gn     

Preparation and characterization of La0.8Sr0.2MnO3 thin films by an excimer laser MOD process for a bolometer

La_0.8Sr_0.2MnO₃ (LSMO) films were prepared on LaAlO₃ substrates by excimer laser metal organic deposition (ELMOD) at 500 °C. The temperature dependence of resistance of the LSMO films was investigated by changing the laser fluence, irradiation time, and film thickness. It was found that the resistance of the LSMO films 80 nm in thickness that were irradiated by an ArF laser at a fluence of 100 mJ/cm² for 60 min showed a metallic temperature dependence, and the maximum temperature coefficient of resistance of the films (defined as 1/R×dR/dT) was 3.4% at 265 K. PACS 81.15.-z; 81.15.Fg; 81.15.Np; 73.61.-r; 71.30.+h     

Differential AC-chip calorimeter for glass transition measurements in ultra thin polymeric films

A differential AC-chip calorimeter capable to measure the glass transition in nanometer thin films is described. Due to the differential setup pJ/K sensitivity is achieved. Heat capacity can be measured for sample masses below one nanogram even above room temperature as needed for the study of the glass transition in nanometer thin polymeric films. The calorimeter allows for the frequency dependent measurement of complex heat capacity in the frequency range from 1 Hz to 1 kHz. The glass transition in thin films of polystyrene (PS) (100–4 nm) and polymethylmethacrylate (PMMA) (400–10 nm) was determined at well defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits (±3 K) - neither at constant frequency nor for the traces in the activation diagrams (1 Hz–1 kHz).     

Synthesis,characterization and magnetic properties of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanotubes

Well-aligned Co₃O₄ nanotubes were synthesized within the nanochannels of porous anodic alumina membranes using a single-source chemical vapor deposition method. Scanning electron microscopy and transmission electron microscopy showed that the Co₃O₄ nanotubes are highly ordered with uniform diameter in the range of 100–300 nm and length up to tens of microns. X-ray diffraction, the Raman spectrum, energy-dispersive spectroscopy and selected-area electron diffraction demonstrated that the nanotubes are composed of pure cubic phase polycrystalline Co₃O₄. Magnetic measurements using a SQUID magnetometer suggested the presence of a strong antiferromagnetic interaction with Weiss constant θ= -248 K. The real and imaginary parts of the ac susceptibility at f= 10 Hz had a maximum at 4.0 K, and the field dependence of the magnetization at 1.8 K showed a small hysteresis loop with a coercivity of ∼ 98 Oe. PACS 81.07.De; 81.15.Gh; 78.30.-j; 75.75.+a; 61.46.Np     

Structural and sensing properties of nanocrystalline SnO<Subscript>2</Subscript> films deposited by spray pyrolysis from a SnCl<Subscript>2</Subscript> precursor

We report the fabrication and characterization of tin dioxide gas sensing layers. The tin dioxide layers were synthesized using a convenient, simple and low-cost technique of spray pyrolysis. The formation of stoichiometric SnO₂ layers with fine-grain structure is revealed by Rutherford backscattering spectroscopy. The microstructure, phase, nanoparticle size distribution and surface morphology were studied by transmission electron microscopy, electron diffraction and atomic force microscopy. Most of the grains were of 10–20 nm size; however, some particles were up to 100 nm in size and had a microtwin lamellae structure of SnO₂ phase (cassiterite) with lattice parameters a= 0.474 nm and c= 0.319 nm. The sensitivity of the layers with respect to 1000–10000 ppm CH₄ in air was obtained from both resistivity (S_R) and capacity (S_C) measurements at 330 °C and values of S_R=5–7 and S_C=22–31 were extracted. PACS 68.43.-h; 68.55.-a; 81.05.Hd; 81.07.-b; 81.15.Rs     

Substrate effect on mechanical relaxation of polystyrene in ultrathin films

Mechanical relaxation behavior in ultrathin polystyrene (PS) films supported on silicon oxide (SiO_x) and gold (Au) substrates has been studied by dynamic viscoelastic measurement. Based on the method, effects of free surface and substrate interface on the segmental dynamics were discussed. In the case of thin PS films with a thickness of approximately 200 nm, α_a-relaxation process corresponding to the segmental motion did not show any deviation from the bulk behavior. In contrast, for the films thinner than about 50 nm, the relaxation time distribution for the α_a-process became broader, probably due to a mobility gradient in the surface and interfacial regions. When we sandwiched an ultrathin PS film between SiO_x layers, another relaxation process, in addition to the original α_a-process, appeared at a higher temperature side that we assigned to the interfacial α_a-relaxation process. However, this was never seen for an ultrathin PS film between Au layers, implying that restriction from the substrate interface might be weak in this case.