Synthesis of nanometric iron oxide films by RPLD and LCVD for thermo–photo sensors

Iron oxide films were deposited on <100> Si substrates by reactive pulsed laser deposition (RPLD) using a KrF laser (248 nm). These films were deposited too by laser (light) chemical vapor deposition (LCVD) using continuous ultraviolet photodiode radiation (360 nm). The deposited films demonstrated semiconducting properties. These films had large thermo-electromotive force (e.m.f.) coefficient (S) and high photosensitivity (F). For films deposited by RPLD the S coefficient varied in the range 0.8–1.65 mV/K at 205–322 K. This coefficient depended on the band gap (E _g ) of the semiconductor films, which varied in the range 0.43–0.93 eV. The largest F value found was 44 V_c/W for white light at power density I≅0.006 W/cm². Using LCVD, iron oxide films were deposited from iron carbonyl vapor. For these films, the S coefficient varied in the range −0.5 to 1.5 mV/K at 110–330 K. The S coefficient depended on E _g of the semiconductor films, which varied in the range 0.44–0.51 eV. The largest F value of these films was about 40 V_c/W at the same I≅0.006 W/cm². Our results showed that RPLD and LCVD can be used to synthesize iron oxide thin films with variable stoichiometry and, consequently, with different values of E _g . These films have large S coefficient and high photosensitivity F and therefore can be used as multi-parameter sensors: thermo–photo sensors.     

Photoactivity passivation of TiO<Subscript>2</Subscript> nanoparticles using molecular layer deposited (MLD) polymer films

Pigment-grade anatase TiO₂ particles (160 nm) were passivated using ultra-thin insulating films deposited by molecular layer deposition (MLD). Trimethylaluminum (TMA) and ethylene glycol (E.G) were used as aluminum alkoxide (alucone) precursors in the temperature range of 100–160 °C. The growth rate varied from 0.5 nm/cycle at 100 °C to 0.35 nm/cycle at 160 °C. Methylene blue oxidation tests indicated that the photoactivity of pigment-grade TiO₂ particles was quenched after 20 cycles of alucone MLD film, which was comparable to 70 cycles of Al₂O₃ film deposited by atomic layer deposition (ALD). Alucone films would decompose in the presence of water at room temperature and would form a more stable composite containing aluminum, which decreased the passivation effect on the photoactivity of TiO₂ particles.     

PLD and RF-PLD synthesis of Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> ferroelectric thin films for electrically controlled devices

Ba_0.6Sr_0.4TiO₃ (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance–voltage (C–V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.     

Synthesis and characterization of Zn<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript>S nanocrystalline films prepared on glass substrates

Nanocrystalline Zn_1-xMn_xS films (x=0.04, 0.08 and 0.12) were deposited on glass substrates at 400 K using a simple resistive thermal evaporation technique. All the deposited films were characterized by chemical, structural, morphological, optical and magnetic properties. Scanning electron microscopy and atomic force microscopy studies showed that all the films investigated were in nanocrystalline form with the grain size lying in the range 10–20 nm. All the films exhibited cubic structure and the lattice parameters increase linearly with composition. The absorption edge shifted from the higher-wavelength region to lower wavelengths with increase in Mn concentration. The magnetization increased sharply with increase of the Mn content up to x=0.08 and then decreased with further increase of the Mn content. Particularly, Zn_0.92Mn_0.08S concentration samples show a weak ferromagnetic nature, which might be the optimum concentration for optoelectronic and spintronic device applications. PACS 75.50.Pp; 78.66.Hf; 75.70.Ak; 75.75.+a     

Dielectric properties of thin-film ZrO<Subscript>2</Subscript> up to 50 GHz for RF MEMS switches

The dielectric properties of zirconium dioxide (ZrO₂) ceramic thin films were characterized up to 50 GHz using coplanar waveguides (CPWs) and metal–insulator–metal (MIM) capacitors with top circular electrodes. The ZrO₂ films were deposited using a chemical solution onto high-resistivity Si wafers and metal layers. The real part of the dielectric constant of approximately 22 and 26 was extracted at 50 GHz for CPW and MIM structures, respectively, and the loss tangent was approximately 0.09 at 50 GHz. C–V and I–V measurements were carried out to determine low-frequency and DC dielectric properties. The measurement results indicate that ZrO₂ is a promising material to be used as a dielectric layer for radio-frequency (RF) microelectromechanical systems (MEMS) capacitive switches.     

Structural and electrical properties of pure and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> doped (PVA)<Subscript>0.7</Subscript>(NaI)<Subscript>0.3</Subscript> solid polymer electrolyte

Solid polymer electrolytes (SPE) based on poly-(vinyl alcohol) (PVA)_0.7 and sodium iodide (NaI)_0.3 complexed with sulfuric acid (SA) at different concentrations were prepared using solution casting technique. The structural properties of these electrolyte films were examined by X-ray diffraction (XRD) studies. The XRD data revealed that sulfuric acid disrupt the semi-crystalline nature of (PVA)_0.7(NaI)_0.3 and convert it into an amorphous phase. The proton conductivity and impedance of the electrolyte were studied with changing sulfuric acid concentration from 0 to 5.1 mol/liter (M). The highest conductivity of (PVA)_0.7(NaI)_0.3 matrix at room temperature was 10⁻⁵ S cm⁻¹ and this increased to 10⁻³ S cm⁻¹ with doping by 5.1 M sulfuric acid. The electrical conductivity (σ) and dielectric permittivity (ε′) of the solid polymer electrolyte in frequency range (500 Hz–1 MHz) and temperature range (300–400) K were carried out. The electrolyte with the highest electrical conductivity was used in the fabrication of a sodium battery with the configuration Na/SPE/MnO₂. The fabricated cells give open circuit voltage of 3.34 V and have an internal resistance of 4.5 kΩ.     

Characterization of TiO<Subscript>2</Subscript> nanoparticle suspensions for electrophoretic deposition

The rheology of suspensions is critically important for the successful achievement of defect-free TiO₂ deposits by electrophoretic deposition (EPD). The rheological behaviour of TiO₂ nanoparticle suspensions in acetylacetone with and without iodine was investigated over a broad solid-concentration range (0.3–2.5 wt.%) and at different shear rates ( = 10–250 s⁻¹). The influence of these parameters on the quality of TiO₂ films obtained by EPD on stainless steel substrates was assessed. The pure solvent and the 1 wt.% TiO₂ nanoparticles suspension without iodine exhibited shear-thickening flow behaviour. For other concentrations, the suspensions showed shear-thinning behaviour followed by an apparent shear-thickening effect at a critical shear rate (100 s⁻¹). For the suspension with 1 wt.% TiO₂ containing iodine, a shear-thickening flow behaviour was observed over the whole shear rate range investigated. The maximum solids fraction (ϕ_m) was experimentally determined from a linear relationship between solid concentration and viscosity. The estimated value was ϕ_m = 7.94 wt.% for this system. Using a suspension with 1 wt.% concentration, good-quality TiO₂ deposits on stainless steel planar substrates were obtained by EPD at constant voltage condition. The influence of pH on suspension stability was determined in the range pH = 1–9, being pH ≈ 5 the optimal value for this system in terms of EPD results.     

Low-macroscopic field emission from nanocrystalline Al doped SnO<Subscript>2</Subscript> thin films synthesized by sol–gel technique

We have observed low-macroscopic field electron emission from wide bandgap nanocrystalline Al doped SnO₂ thin films deposited on glass substrates. The emission properties have been studied for different anode-sample spacings and for different Al concentrations in the films. The turn-on field and approximate work function were calculated and we have tried to explain the emission mechanism from this. The turn-on field was found to vary in the range 5.6–7.5 V/μm for a variation of anode sample spacing from 80–120 μm. The turn-on field was also found to vary from 4.6–5.68 V/μm for a fixed anode-sample separation of 80 μm with a variation of Al concentration in the films 8.16–2.31%. The Al concentrations in the films have been measured by energy dispersive X-ray analysis. Optical transmittance measurement of the films showed a high transparency with a direct bandgap ∼3.98 eV. Due to the wide bandgap, the electron affinity of the film decreased. This, along with the nanocrystalline nature of the films, enhanced the field emission properties. PACS 81.20.Fw; 61.10.-i; 79.70.+q     

Lithiated c-V<Subscript>2</Subscript>O<Subscript>5</Subscript> thin-film as positive electrode for rocking-chair solid-state lithium microbattery

We report for the first time the use of lithiated crystalline V₂O₅ thin films as positive electrode in all-solid-state microbatteries. Crystalline Li_xV₂O₅ films (x ≈ 0.8 and 1.5) are obtained by vacuum evaporation of metallic lithium deposited on sputtered c-V₂O₅. An all-solid-state lithium microbattery of Li_1.5V₂O₅/LiPON/Li exhibited a typical reversible capacity of 50 μAh/cm² in the potential range 3.8/2.15 V which exceeds by far the results known on all-solid-state lithium batteries using amorphous V₂O₅ films and lithiated amorphous Li_xV₂O₅ thin films as positive electrode. Hence, the present work opens the possibility of using high performance crystalline lithiated V₂O₅ thin films in rocking-chair solid-state microbatteries.     

Synthesis and characterization of electron beam evaporated LiCoO<Subscript>2</Subscript> thin films

LiCoO₂ thin films were prepared by electron beam evaporation technique using LiCoO₂ target with Li/Co ratio 1.1 in an oxygen partial pressure of 5 × 10⁻⁴ mbar. The films prepared at substrate temperature T _s < 573 K were amorphous in nature, and the films prepared at T _s > 573 K exhibited well defined (104), (101), and (003) peaks among which the (104) orientation predominates. The X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) data revealed that the films prepared in the substrate temperature range 673–773 K are nearly stoichiometric. The grain size increases with an increase of substrate temperature. The Co–e_g absorption bands, are empty and their peak position lies at around 1.7 eV above the top to the Co–t_2g bands. The fundamental absorption edge was observed at 2.32 eV. The films annealed at 1,023 K in a controlled oxygen environment exhibit (104) out plane texture with large grains. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006     

Electrical and optical properties of pulse plated CdS<Subscript><Emphasis Type="BoldItalic">x</Emphasis></Subscript>Te<Subscript>1-<Emphasis Type="BoldItalic">x</Emphasis></Subscript> films

Thin films of CdS_xTe_1-x were deposited by the pulse electrodeposition technique using cadmium sulfate, sodium thiosulfate, and tellurium dioxide on titanium and conducting glass substrates. Structural studies indicated the formation of polycrystalline films possessing hexagonal structure. The resistivity varies from 53 Ω cm to 8 Ω cm as the stochiometric coefficient “x” value decreases from 1 to 0. The carrier concentration increases with CdTe concentration. It is observed that as the post-heat treatment temperature increases, the photosensitivity also increases. It is observed that a post-heat treatment temperature of 550 °C results in high photosensitivity as well as low light resistance. The optical constants, refractive index (n) and extinction coefficient (k) were evaluated from the transmission spectra of the films of different composition.     

Investigation on optical properties of Bi<Subscript>2.85</Subscript>La<Subscript>0.15</Subscript>TiNbO<Subscript>9</Subscript> thin films by prism coupling technique

Layered-perovskite ferroelectric Bi_2.85La_0.15TiNbO₉ (LBTN) optical waveguiding thin films were grown on fused silica substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) revealed that the film is highly (00l) textured. We observed sharp and distinct transverse electric (TE) and transverse magnetic (TM) multimodes and measured the refractive indices of LBTN thin films at 632.8 nm. The ordinary and extraordinary refractive indices were calculated to be n _TE=2.358 and n _TM=2.464, respectively. The film homogeneity and the film-substrate interface were analyzed using an improved version of the inverse Wentzel–Kramer–Brillouin (iWKB) method. The refractive index of the film remains constant at n ₀ within the waveguiding layer. The average transmittance of the film is 70% in the wavelength range of 400–1400 nm and the optical waveguiding properties were evaluated by the optical prism coupling method. Our results showed that the LBTN films are very good electro-optical active material.     

Electromagnetic wave absorption properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> octahedral nanocrystallines in gigahertz range

Large-scale octahedral Fe₃O₄ nanocrystallines with crystalline size of 100−500 nm were synthesized by a facile solvent-thermal method for electromagnetic wave application. The Fe₃O₄ nanocrystallines showed a higher saturation magnetization (M _s ) value of 86.8 emu/g and larger coercivity (H _cj ) value of 255 Oe than that of magnetite polycrystallines because of their good crystallization and dispersion. The epoxy resin composites with 40 vol% Fe₃O₄ powders provided good electromagnetic wave absorption performance (RL < −20 dB) in the range of 2.0–4.3 GHz over the absorber thicknesses of 3.5–6.8 mm. A minimum RL value of −47 dB was observed at 3.1 GHz with a thickness of 4.8 mm.     

Carbon-free SiO<Subscript>x</Subscript> films deposited from octamethylcyclotetrasiloxane (OMCTS) by an atmospheric pressure plasma jet (APPJ)

The deposition of carbon-free, silicon oxide (SiO_x) films with a non-thermal, RF capillary jet at 27.12 MHz at normal pressure is demonstrated. The gas mixture for film deposition is constituted of argon, oxygen and small admixtures of octamethylcyclotetrasiloxane (Si₄O₄C₈H₂₄, 0.4 ppm). Surface analysis of the deposited films reveals their exceptionally low carbon content. The XPS atom percentage stays at 2% and less, which is near detection limit. The parametric study reported here focuses on the optimization of the deposition process with regard to the chemical and morphological surface properties of the coating by varying oxygen feed gas concentration (0–0.2%) and substrate temperature (10–50 °C).     

Influence of a dopant source on the structural and optical properties of Mn doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films

Thin films of ZnGa₂O₄:Mn²⁺ were deposited on quartz substrates using an rf magnetron sputtering technique. The sputtering target, ZnGa₂O₄ doped with 2 at. % manganese, was synthesized by a high temperature solid state reaction. Two different dopant sources were used to incorporate the dopant ions into the target, namely, manganese acetate and manganese oxide. The structural and optical properties of the thin films were studied using XRD, PL and transmission spectra. Polycrystalline ZnGa₂O₄:Mn with a spinel structure could be grown at an optimized substrate–target distance even at room temperature. No luminescence was observed in the as-deposited films grown using (CH₃COO)₂Mn as the dopant source in the target. Substrate heating or post-deposition annealing in the reducing ambient didn’t impart any luminescence to the films, ruling out the possibility of Mn²⁺ incorporation in the films. However, when using MnO as the manganese source in the target, the as-deposited films exhibited green photoluminescent emission (peak maximum at 508 nm) for substrate temperatures at and above 500 °C. This suggests that, in thin films, Mn incorporation and subsequent luminescent outcome is strongly influenced by the dopant source, which is quite different from the bulk phosphor behavior. PACS 81.15.Cd; 78.55.-m; 85.60.-q     

Efficient and widely step-tunable terahertz generation with a dual-wavelength CO<Subscript>2</Subscript> laser

Coherent terahertz radiation in a widely step-tunable range of 72.3–2706 μm (0.11–4.15 THz) has been generated in GaAs crystal by difference-frequency generation using one CO₂ laser with dual-wavelength output. The peak power of THz pulse reaches 35 W at the wavelength of 236.3 μm, which corresponds to a pulse energy of 2.1 μJ. An average power of 10 μW has been achieved when working repetitively. This efficient terahertz radiation source is more compact and widely tunable than other THz sources pumped by CO₂ laser.     

Fabrication and characterization of CdS-sensitized TiO<Subscript>2</Subscript> nanotube photoelectrode

CdS quantum dot (Qd)-sensitized TiO₂ nanotube array photoelectrode is synthesised via a two-step method on tin-doped In₂O₃-coated (ITO) glass substrate. TiO₂ nanotube arrays are prepared in the ethylene glycol electrolyte solution by anodizing titanium films which are deposited on ITO glass substrate by radio frequency sputtering. Then, the CdS Qds are deposited on the nanotubes by successive ionic layer adsorption and reaction technique. The resulting nanotube arrays are characterized by scanning electron microscopy, X-ray diffraction (XRD) and UV–visible absorption spectroscopy. The length of the obtained nanotubes reaches 1.60 μm and their inner diameter and wall thickness are around 90 and 20 nm, respectively. The XRD results show that the as-prepared TiO₂ nanotubes array is amorphous, which are converted to anatase TiO₂ after annealed at 450 °C for 2 h. The CdS Qds deposited on the TiO₂ nanotubes shift the absorption edge of TiO₂ from 388 to 494 nm. The results show that the CdS-sensitized TiO₂ nanotubes array film can be used as the photoelectrode for solar cells.     

Magnetotransport and magnetocaloric effect in Ho<Subscript>2</Subscript>In

The magnetotransport and magnetic properties of the binary intermetallic compound Ho₂In have been investigated. Clear signature of long range ferromagnetic order in the resistivity and the magnetization data at T_C = 85 K is observed. A further spin reorientation type transition is also apparent in our measured data at around T_t = 32 K. The sample exhibits negative magnetoresistance (peak value of –14% at 5 T) over a wide temperature range that extends well above T_C. Substantially large magneto-caloric effect is also observed in the sample (maximum value of –8.5 J kg^-1K^-1 for 0 → 5 T), which peaks around the T_C of the sample. The observed magnetoresistance and magnetocaloric effect are related to the suppression of spin disorder by an external magnetic field. Ho₂In can be an interesting addition to the list of rare-earth based magnetic refrigerant materials showing magneto-caloric effect across a second order phase transition.     

Structure and electrical properties of HfO<Subscript>2</Subscript> high-<Emphasis Type="Italic">k</Emphasis> films prepared by pulsed laser deposition on Si (100)

High-k gate dielectric hafnium dioxide films were grown on Si (100) substrate by pulsed laser deposition at room temperature. The as-deposited films were amorphous and that were monoclinic and orthorhombic after annealed at 500°C in air and N₂ atmosphere, respectively. After annealed, the accumulation capacitance values increase rapidly and the flat-band voltage shifts from −1.34 V to 0.449 V due to the generation of negative charges via post-annealing. The dielectric constant is in the range of 8–40 depending on the microstructure. The I–V curve indicates that the films possess of a promising low leakage current density of 4.2×10⁻⁸ A/cm² at the applied voltage of −1.5 V.     

InTaO<Subscript>4</Subscript>-based nanostructures synthesized by reactive pulsed laser ablation

Nanostructured Ni-doped indium–tantalum–oxides (InTaO₄) were synthesized by a reactive pulsed laser ablation process, aiming at the final goal of direct splitting of water under visible sunbeam irradiation. The third harmonics beam of a Nd:YAG laser was focused onto a sintered In_0.9Ni_0.1TaO_4−δ target in pure oxygen background gases (0.05–1.00 Torr). Increasing the oxygen gas pressure, via thin films having nanometer-sized strong morphologies, single-crystalline nanoparticles were synthesized in the reactive vapor phases. The nanostructured deposited materials have the monoclinic layered wolframite-type structure of bulk InTaO₄, without oxygen deficiency.